The authors would like to thank Anni Lee and Maureen Byrne for their help in establishing the extended conditioned place preference protocol.

All procedures are conducted in accordance with the Weill Cornell Medical College Institutional Animal Care and Use Committee rules.
1. Preparation and Setup for Stereotaxic Delivery of Viral Vectors
<ol>
	<li>If an instrument, sterile swab, or hand wearing sterile glove is contaminated by coming into contact with a non-sterile surface, discard or re-sterilize the instrument using a hot bead sterilizer, discard the swab or change to new sterile gloves.</li>
	<li>Place a mouse cage with bedding on a heat block to warm for post-operative recovery.</li>
	<li>Set up electric razor for shaving head and ethanol and iodine to sterilize the scalp.</li>
	<li>Wipe down ear bars and mouth hold of the stereotax with 70% ethanol.</li>
	<li>Using a 5 &mu;l Hamilton syringe, draw up 5 &mu;l of viral vector (1 x 106 viral particles/&mu;l) and set up in syringe holder on stereotax.</li>
	<li>Microwave heating pad or turn electric pad on and place on stereotax. Cover with a clean absorbent pad to reduce the risk of burns. Monitor mouse body temperature (between 97-99.5 &deg;F) frequently throughout procedure. </li>
	<li>Inject mouse (weighing between 22-35 g) with a mixture of ketamine (100 mg/kg) and xylazine (20 mg/kg). Once a mouse is anesthetized, pinch tail tip to check the mouse's reflexes and note any movement or response to ensure that the mouse is adequately anesthetized.</li>
	<li>Shave head between ears and eyes.</li>
	<li>Clean shaved head with alternating ethanol and iodine sterile cotton applicators.</li>
	<li>Transfer mouse to heated pad on stereotax.</li>
</ol>
2. Stereotaxic Microinjection of Viral Vectors
<ol>
	<li>Wear sterile gloves throughout the surgery. If gloves are contaminated at any point by touching non-sterile surfaces, change gloves.</li>
	<li>Zero tooth bar and ear bar scales.</li>
	<li>Set teeth in tooth bar.</li>
	<li>Screw in muzzle. If wiggling or whisker movement is noted at any time, administer an i.p. booster dose (~0.05 cc) of a mixture of ketamine (100 mg/kg) and xylazine (20 mg/kg).</li>
	<li>Screw in ear bars.</li>
	<li>Lubricate eyes with PuraLube.</li>
	<li>Cut open scalp to just behind ears with a scalpel.</li>
	<li>Place bulldog clips on the corners of the scalp to peel the skin away from skull. Bregma and lambda should be easily visible.</li>
	<li>Make sure suture from bregma to lambda is aligned straight. If the suture is crooked readjust positioning of the head.</li>
	<li>Check to make sure all settings on the stereotax are zeroed.</li>
	<li>Put needle tip in place at bregma.</li>
	<li>Measure all coordinates for both bregma and lambda. This includes dorsal/ventral, medial/lateral and anterior/posterior coordinates.</li>
	<li>Adjust the head until the ventral coordinates for bregma and lambda are within 0.02 mm of each other.</li>
	<li>Once aligned, from bregma go medially out to the medial/lateral (M/L) coordinates. Make sure the left and right sides are aligned within 0.02 mm of each other.</li>
	<li>Re-center needle tip on bregma and from there move in the anterior/posterior (A/P) direction to the desired anterior/posterior coordinate.</li>
	<li>If necessary, angle needle in one direction and make sure the stereotax is locked. Go out to the left M/L coordinate (+/- 1.52)</li>
	<li>Measure the ventral position of the top of the skull to use as a reference after the borehole is drilled.</li>
	<li>Drill a borehole using a sterile drill bit at the desired coordinate. Check that the needle can enter uninterrupted through the skull.</li>
	<li>Lower the needle to the desired ventral coordinate.</li>
	<li>Once the coordinate is reached, dip 0.015 mm below for ~10 sec to create a small pocket for the viral vector.</li>
	<li>Inject the desired volume of viral vector at a rate of ~0.1 &mu;l/min. Wait 3 min for viral vector to diffuse completely. Raise the needle tip 0.015 mm and wait an additional 2 min.</li>
	<li>Dip the needle (if doing angled injections) in the opposite direction. Again, record the ventral position as a reference for the top of the skull.</li>
	<li>Repeat the same steps as on the first side to inject viral vector on the opposite side (if doing bilateral injections).</li>
	<li>Apply bone wax to both holes to seal them using the wooden end of a sterile cotton applicator tip.</li>
	<li>Suture the scalp.</li>
	<li>Apply nerve block to the wounded area.</li>
	<li>Remove mouse from stereotax and place in warmed cage on heat block to recover for 45 min.</li>
	<li>Monitor for signs of pain including: decreased overall activity or restlessness, decreased food and water consumption or increased vocalization. Buprenorphine (0.05 - 0.2 mg/kg) can be administered twice daily for a week if necessary for pain. Acceptable alternatives include: meperidine (20-60 mg/kg), pentazocine (10 mg/kg), nalbuphine (4-8 mg/kg).</li>
</ol>
3. Conditioned Place Preference: Acquisition, Extinction, and Reinstatement
<ol>
	<li>Place mice into the central chamber of a three-chamber place preference apparatus (Med Associates Inc., St. Albans, VT, USA) for a 60 sec habituation period with guillotine doors closed.</li>
	<li>After habituation, open guillotine doors and allow mice free exploration of all 3 chambers for 1,200 sec, with time spent in each chamber being recorded using MedPC IV software (Med Associates, Inc.).</li>
	<li>Assign mice to conditioning chambers based on their performance during the pre-test. Using a biased design, pair drug administration during the subsequent 3 days with the compartment that was least-preferred during the baseline pre-test.</li>
	<li>During the next 3 conditioning days, inject mice with cocaine (10 mg/kg; i.p.) during the morning session and confine them in their assigned chamber for 1,200 sec immediately following the injection. Return mice to their home cages after the session. After 4 hr, inject mice with saline (0.01 ml/g body weight) and confine them to the opposite chamber for the 1,200 sec afternoon session.</li>
	<li>To test for acquisition, place mice in the central chamber with guillotine doors closed for a 60 sec habituation period. Open guillotine doors and allow free exploration for 1,200 sec, with time spent in all chambers recorded by MedPC IV software (Med Associates, Inc.). Calculate preference by subtracting the amount of time spent in the saline-paired chamber from the amount of time spent in the cocaine-paired chamber.</li>
	<li>To extinguish the place preference, expose mice to two 20 min sessions of free exploration daily in the same apparatus, separated by at least 2 hr, beginning 24 hr following the test for acquisition. Time spent in all chambers should again be recorded and the magnitude of the initial CPP should be compared with that of each extinction session.</li>
	<li>Determine whether mice have extinguished the drug-induced preference by assessing whether preference for a particular chamber across two consecutive extinction days is significantly lower compared to the acquisition preference score.</li>
	<li>Reinstate mice with a priming dose of cocaine (5 or 10 mg/kg; i.p.) and re-expose to the apparatus allowing free exploration and record the time spent in each compartment.</li>
	<li>Administer Euthasol euthanasia solution (150 mg/kg). Transcardially perfuse mice with 4% paraformaldehyde (PFA) and perform a histological examination to validate correct microinjection placement.</li>
</ol>

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The authors have nothing to disclose.

Regionally- and temporally-specific gene ablation via stereotaxic microinjection of viral vectors combined with CPP that includes extinction and reinstatement phases allows for investigation of the specific contributions of genes to three distinct phases of addictive-like behavior. While conditional knockout that is achieved utilizing the traditional Cre-LoxP system provides for spatio-temporally restricted gene ablation, stereotaxically microinjecting Cre-recombinase into discrete brain areas of floxed mice allows for even tighter control of when and where a gene is knocked out. Additionally, this circumvents the possibility of compensatory changes occurring during development, which sometimes accounts for limited phenotypic variation observed in knockout mice. Moreover, using cohorts of mice that were microinjected at various time points during the behavior can allow for causal links to be made between a change in gene expression and a change in behavior. Importantly, this technique can be applied to several different cocaine behavioral paradigms including self-administration and psychomotor sensitization.
It is important to wait at least 10-14 days following surgeries to ensure that maximal activation of Cre recombinase has occurred 20. Beginning behavioral testing prior to this can result in the use of mice with an unknown amount of sub-maximal Cre expression, making behavioral findings difficult to interpret. Maximal activation of Cre results in complete knockout of a gene; however, this only occurs in cells that are in fact infected. Depending on the volume and the titer of the viral vector, approximately 50-70% of cells are infected for a given region resulting in the corresponding amount of mRNA and protein knockout which can lead to some residual gene expression (Figure 1C). However, we have demonstrated that a 50-60% mRNA this type of knockout is sufficient to alter expression of psychomotor sensitization14. Moreover, this method is ideal for longer behavioral paradigms as once Cre activation occurs in vivo, its expression and the corresponding gene knockout lasts for at least 6 months21.
If one is interested in examining the role of a gene over a more discrete period of time however, that is not possible with this particular method. For shorter-term expression, herpes simplex viral (HSV) vectors can be used, which preferentially infect neurons22. Compared to rAAV expression of Cre recombinase, which persists for longer periods as the promoter is not silenced after days or weeks in vivo 23, HSV vectors are preferred for transient expression as they express in vivo for days. HSV vectors are also advantageous in that they demonstrate broad transduction efficiency and that helper virus-free stocks exist 22. Moreover, HSV vectors can also express Cre recombinase and mediate the excision of floxed sequences both in culture and in vivo 24. However, occasional cytotoxicity has been found.
Almost any region within the brain can be targeted using this microinjection technique. When attempting to target smaller subregions within a larger region, one may need to optimize the amount of viral vector used. For example, we microinjected 0.5 &mu;l of rAAV-Cre-GFP to target the NAc and ventral tegmental area (VTA) 14, whereas when targeting larger structures such as the hippocampus and cortex, Ahmed and colleagues 20 used 0.8 &mu;l of rAAV-Cre. Alternatively, exploring various serotype options that each have differential spread and infectivity properties (see Choi et al. (2005) 25 for review) can aid in targeting a smaller, more defined area. Conversely, if attempting to target a larger region, one can increase the volume of viral vector microinjected. This is more efficacious than utilizing higher vector titers as they likely yield additional infections per cell as opposed to a higher number of total cells infected 22. Additionally, one can perform multiple injections within a region, or across multiple regions, to more effectively infect a larger area 26.
Use of rAAV vectors is also advantageous when compared with other viral vectors that are used to examine brain function and disease. Specifically, traditional adenoviral vectors proved problematic in that they induced severe inflammatory responses, while other vectors such as retroviruses and lentiviruses can involve issues with insertional mutagenesis. rAAV is a benign parvovirus and there is no known disease associated with rAAV infection. Moreover, unlike other viral vectors, transduction with rAAV occurs without the expression of native AAV viral proteins 27, making secondary effects on target cells much less of a concern. rAAV vectors can be produced in high titers and can transduce post-mitotic neurons in the brain in a highly spatially localized manner 21,28. Moreover, rAAV vectors are now being generated that can target specific subtypes of neurons in the mouse brain 29 and Cre-dependent rAAV expression systems are also becoming more widely used (see Zhang et al. (2010) 30 for review).
In summary, cocaine CPP serves as a simple yet informative behavioral tool to study cocaine contextual learning relevant to the acquisition, extinction, and reinstatement of cocaine-seeking behavior. A combination of utilizing rAAV expressing Cre recombinase in mice carrying a floxed allele with CPP provides a powerful strategy for exploring the role of target genes in specific brain regions that mediate cocaine seeking behavior.

Cocaine is a highly reinforcing psychostimulant. Following repeated exposure, several molecular and cellular adaptations occur in reward-relevant brain circuitry that are believed to result in compulsive drug-seeking behavior, prompting high rates of relapse which pose a serious clinical problem 1. Cocaine exerts these long-lasting behavioral effects by regulating gene expression. To study the adaptations that arise from chronic cocaine use, preclinical rodent models have been used extensively. One such model is the conditioned place preference (CPP) paradigm. This model involves the development of a learned association between a previously neutral environment and the rewarding properties of cocaine. After several pairings of cocaine with a particular chamber, animals are allowed to freely explore the cocaine-paired and non-cocaine-paired environments and if they prefer the drug-paired compartment, they are said to have acquired a cocaine-induced place preference. Moreover, following an extinction training period, this paradigm can be used to study context-specific reinstatement of cocaine-seeking behavior.
Compared to other behavioral models of addictive-like behavior, such as psychomotor sensitization, which is used to study long-term cocaine-induced behavioral and molecular plasticity 2,3 and self-administration (SA), that is thought to more accurately mimic addictive-like behavior found in humans, the CPP paradigm is a simple procedure to study cocaine contextual learning 4. CPP protocols can be conveniently extended to include extinction and reinstatement phases, similarly to SA, which allow for investigation of the mechanisms underlying drug craving and relapse 5-7 and which are believed to recapitulate aspects of what occurs in human drug-seeking behavior and drug- and cue-induced relapse 8-10.
One mechanism that underlies the cocaine-induced behavioral plasticity that is characteristic of each of the distinct phases of CPP, including acquisition, extinction, and reinstatement, is activation of unique signatures of gene expression within different brain regions. To directly test which genes within reward-relevant brain regions mediate cocaine-induced behavioral changes, it is useful to be able to manipulate them selectively in a regionally-specific manner. One way of achieving this is to microinject recombinant adenoassociated viral (rAAV) vectors that express Cre recombinase using stereotaxic surgery, into distinct brain areas of mice that have target genes flanked by LoxP sites (floxed mice). This method allows for very precise temporal and regional control over when and where genes are ablated, in both dividing and non-dividing neurons, without inducing immune responses 11-13. This level of control represents an important advantage over traditional Cre-LoxP technology of breeding floxed mice with mice expressing Cre recombinase under the control of an endogenous gene promoter, in that the timing and distribution of gene deletion can be more tightly regulated. Additionally, viral vector-mediated gene knockout circumvents potential developmental compensatory effects that may occur using traditional knockout strategies.
In addition to cocaine CPP, which is described here, microinjection of rAAV-Cre into the brain of floxed mice to assess the role of specific genes can be applied ubiquitously to behavioral paradigms that involve separate phases, including self-administration and psychomotor sensitization. For example, our laboratory has utilized the rAAV-Cre technology to study the role of Cav1.2 L-type Ca2+ channels in cocaine psychomotor sensitization 14. Specifically, rAAV-Cre was microinjected into the nucleus accumbens (NAc) of mice with the gene encoding Cav1.2 floxed, to demonstrate that Cav1.2 acting in this region mediates the expression phase of psychomotor sensitization 14,15. However, the rAAV-Cre strategy cannot be used if mice with a floxed gene of interest do not exist, as was our experience when assessing the role of Cav1.3 L-type Ca2+ channels in psychomotor sensitization. Hence, a limitation of using rAAV-Cre presents itself if conditional mice do not exist for a particular gene of interest. However, rAAVs that express siRNA can be used to knockdown target genes, as we have done to examine the role of Cav1.3 channels 14,15.
Microinjecting rAAV-Cre into discrete brain regions of floxed mice and then testing them in the CPP paradigm allows for investigation into the specific genes that mediate the various phases of addictive-like behavior and where they are acting. Use of this paradigm has aided in our understanding of how repeated cocaine administration in essence hijacks the brains reward circuitry causing maladaptive changes in molecular signal transduction pathways and gene expression that lead to the addicted state 1,16,17.

<table border="1">
	<tbody>
		<tr>
			<td>Name of Reagent/Material</td>
			<td>Company</td>
			<td>Catalog Number</td>
			<td>Comments</td>
		</tr>
		<tr>
			<td>Conditioned place preference activity chambers</td>
			<td>Med Associates, Inc., St. Albans, VT, USA</td>
			<td>MED-CPP-MS</td>
			<td>&nbsp;</td>
		</tr>
		<tr>
			<td>Stereotaxic alignment system for mouse</td>
			<td>David Kopf Instruments, Tujunga, CA, USA</td>
			<td>model 900</td>
			<td>&nbsp;</td>
		</tr>
		<tr>
			<td>Hamilton syringes</td>
			<td>Hamilton Company, Reno, Nevada, USA</td>
			<td>7634-01</td>
			<td>&nbsp;</td>
		</tr>
		<tr>
			<td>rAAV2-Cre-GFP</td>
			<td>Vector BioLabs, Philadelphia, PA, USA</td>
			<td>7016</td>
			<td>&nbsp;</td>
		</tr>
		<tr>
			<td>rAAV2-GFP</td>
			<td>Vector BioLabs, Philadelphia, PA, USA</td>
			<td>7004</td>
			<td>&nbsp;</td>
		</tr>
	</tbody>
</table>

stereotaxic microinjection of viral vectors expressing cre recombinase to study the role of target genes in cocaine conditioned place preference

Microinjecting recombinant adenoassociated viral (rAAV) vectors expressing Cre recombinase into distinct mouse brain regions to selectively knockout genes of interest allows for enhanced temporally- and regionally-specific control of gene deletion, compared to existing methods. While conditional deletion can also be achieved by mating mice that express Cre recombinase under the control of specific gene promoters with mice carrying a floxed gene, stereotaxic microinjection allows for targeting of discrete brain areas at experimenter-determined time points of interest. In the context of cocaine conditioned place preference, and other cocaine behavioral paradigms such as self-administration or psychomotor sensitization that can involve withdrawal, extinction and/or reinstatement phases, this technique is particularly useful in exploring the unique contribution of target genes to these distinct phases of behavioral models of cocaine-induced plasticity. Specifically, this technique allows for selective ablation of target genes during discrete phases of a behavior to test their contribution to the behavior across time. Ultimately, this understanding allows for more targeted therapeutics that are best able to address the most potent risk factors that present themselves during each phase of addictive behavior.

CPP
After performing CPP on microinjected mice, one should verify that the cohort of control-injected (rAAV-GFP) mice have normally acquired preference for the drug-paired chamber (Figure 1A, 1B). Mice are considered to have acquired preference for a particular chamber when cocaine preference (time spent in the cocaine-paired chamber minus time spent in the saline-paired chamber) is significantly higher compared to baseline preference score (Figure 1B, A vs. B). If, as a cohort, control-injected mice do not show a preference for the cocaine-paired chamber, or show an aversion, placement should be verified immediately via histology (see rAAV Injections subsection) and mice with incorrect placement should be discarded.
If preference has been normally acquired by the control-injected cohort, one can subsequently extend the paradigm by beginning extinction (Figure 1A ; Day 6 onwards). Mice are considered to have extinguished the drug-induced preference for a particular chamber when cocaine preference across two consecutive extinction days is significantly lower compared to the acquisition preference score (Figure 1B, E3, E4, E5 vs. A). At this time, one can reinstate with a drug prime (Figure 1A, 1B). If the preference score following administration of the drug prime is statistically indistinguishable from that at acquisition, then that cohort is said to have reinstated the drug-induced preference for the drug-paired chamber (Figure 1B, R vs. A).
rAAV microinjections
Prior to pursuing the rAAV-Cre experimental strategy within any behavioral paradigm one should verify that rAAV-Cre has in fact knocked out the gene of interest in microinjected mice by qPCR (Figure 1C) and/or western blot or immunohistochemical analysis 18. For example, following microinjection of rAAV-Cre-GFP into the NAc of mice that had the L-type calcium channel isoform Cav1.2 floxed, rAAV-Cre injected punches show a significant decrease in gene expression (Figure 1C). Additionally, we have shown by immunohistochemical analysis that rAAV-Cre can knockout Cav1.2 protein in the mouse brain 18. Following all behavioral analyses, mice should be transcardially perfused with 4% paraformaldehyde and fluorescent immunohistochemistry should be performed for green fluorescent protein (GFP). If bilateral structures were targeted, proper bilateral placement should be verified by fluorescent immunohistochemical staining (Figure 1D, representative targeting of the NAc). If the correct area has indeed been targeted, staining should be visualized exclusively in the intended region and not outside of it. If one or both of the injections are off-target, mice should be retroactively excluded from the CPP data set. Another strategy for visualization of accurate targeting of viral vector-infected brain regions and subsequent dissection of the infected area for biochemical analyses, can be achieved using a GFP-illuminating flashlight as described in Li and Wolf (2011) 19.
<img alt="Figure 1" fo:content-width="6in" fo:src="/files/ftp_upload/50600/50600fig1highres.jpg" src="/files/ftp_upload/50600/50600fig1.jpg" /> 
Figure 1A. Experimental timeline of rAAV-Cre microinjection and conditioned place preference acquisition, extinction and reinstatement phases. rAAV-Cre is microinjected into the region of interest 2-3 weeks prior to the initiation of behavioral testing. Conditioned place preference begins with a pre-test to ascertain any initial biases, followed by 3 days of conditioning with cocaine (10 mg/kg, i.p.). Four hours after each cocaine conditioning session, mice are treated with saline and confined to the opposite chamber. The following day mice are tested for acquisition of a CPP. Extinction training begins 24 hr later and involves 2 daily sessions, that are separated by at least 2 hr, that allow free access to the entire apparatus. Once mice have reached the extinction criterion, they are administered a priming injection of cocaine (10 mg/kg, i.p.) and their time spent in all chambers of the apparatus is assessed to test for reinstatement of cocaine-seeking behavior. Figure 1B. Representative results of baseline, acquisition, extinction training, and reinstatement. WT C57BL/6J mice were treated with cocaine during days 2, 3 and 4 of conditioning. Mice exhibited acquisition, extinction and cocaine-induced reinstatement (10 mg/kg i.p. cocaine prime). ***p&lt;0.001 versus baseline (B); #p&lt;0.05 versus acquisition (A). Baseline, B; Acquisition, A; Extinction, E; Reinstatement, R. Figure 1C. Stereotaxic delivery of rAAV-Cre-GFP into the NAc of Cav1.2 floxed mice significantly decreased Cav1.2 mRNA. Homozygous Cav1.2 floxed mice were microinjected with control (rAAV-GFP) or experimental (rAAV-Cre-GFP) viral vectors. Following maximal Cre activation, mice were sacrificed and mRNA was isolated and relative expression levels were examined via quantitative polymerase chain reaction using Cav1.2 specific primers. Stereotaxic delivery of rAAV-Cre-GFP into the NAc of Cav1.2 floxed mice significantly decreased Cav1.2 mRNA levels, compared to mice injected with AAV-GFP. &dagger;&dagger;p&lt;0.001 versus rAAV-GFP. Figure 1D. Neurons microinjected with rAAV-GFP in the mouse NAc. Homozygous Cav1.2 floxed mice were microinjected with rAAV-GFP to check for proper bilateral placement in the NAc. Mice were perfused with 4% PFA and immunohistochemistry was performed. GFP staining demonstrates proper bilateral targeting of the NAc. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/50600/50600fig1large.jpg" target="_blank">Click here to view larger figure</a>.

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Stereotaxic Microinjection of Viral Vectors Expressing Cre Recombinase to Study the Role of Target Genes in Cocaine Conditioned Place Preference

This article describes how to microinject viral vectors into mouse brain and then test in a conditioned place preference paradigm that includes an acquisition, extinction and reinstatement phase.

Microinjecting recombinant adenoassociated viral (rAAV) vectors expressing Cre recombinase into distinct mouse brain regions to selectively knockout genes ...

stereotaxic-microinjection-viral-vectors-expressing-cre-recombinase

Nanyang Technological University

Research

JoVE Journal

Behavior

18.5K Views.  Weill Cornell Graduate School of Biomedical Sciences. This article describes how to microinject viral vectors into mouse brain and then test in a conditioned place preference paradigm that includes an acquisition, extinction and reinstatement phase.

Video: Stereotaxic Microinjection of Viral Vectors Expressing Cre Recombinase to Study the Role of Target Genes in Cocaine Conditioned Place Preference

Stereotactic Injection of MicroRNA-expressing Lentiviruses to the Mouse Hippocampus CA1 Region and Assessment of the Behavioral Outcome

MicroRNAs (miRNAs) are small regulatory single-stranded RNA molecules around 22 nucleotides long that may each target numerous mRNA transcripts and dim an entire gene expression pathway by inducing destruction and/or inhibiting translation of these targets. Several miRNAs play key roles in maintaining neuronal structure and function and in higher-level brain functions, and methods are sought for manipulating their levels for exploring these functions. Here, we present a direct in vivo method for examining the cognitive consequences of enforced miRNAs excess in mice by stereotactic injection of miRNA-encoding virus particles. Specifically, the current protocol involves injection into the hippocampal CA1 region, which contributes to mammalian memory consolidation, learning, and stress responses, and offers a convenient injection site. The coordinates are measured according to the mouse bregma and virus perfusion is digitally controlled and kept very slow. After injection, the surgery wound is sealed and the animals recover. Lentiviruses encoding silencers of the corresponding mRNA targets serve to implicate the specific miRNA/target interaction responsible for the observed effect, with na&iuml;ve mice, mice injected with saline and mice injected with "empty" lentivirus vectors as controls. One month post-injection, the animals are examined in the Morris Water Maze (MWM) for assessing their navigation learning and memory abilities. The MWM is a round tank filled with colored water with a small platform submerged 1 cm below the water surface. Steady visual cues around the tank allow for spatial navigation (sound and the earth's magnetic field may also assist the animals in navigating). Video camera monitoring enables measuring the route of swim and the time to find and amount the platform. The mouse is first taught that mounting the hidden platform offers an escape from the enforced swimming; it is then tested for using this escape and finally, the platform is removed and probe tests examine if the mouse remembers its previous location. Repeated tests over several consecutive days highlight improved performance of tested mice at shorter latencies to find and mount the platform, and as more direct routes to reach the platform or its location. Failure to show such improvement represents impaired learning and memory and/or anxiety, which may then be tested specifically (e.g. in the elevated plus maze). This approach enables validation of specific miRNAs and target transcripts in the studied cognitive and/or stress-related processes.

MicroRNAs have significant roles in brain structure and function. Here we describe a method to enforce hippocampal miRNA over-expression using stereotactic injection of an engineered miRNA-expressing lentivirus. This approach can serve as a relatively rapid way to assess the in vivo effects of over-expressed miRNAs in specific brain regions.

MicroRNAs (miRNAs) are small regulatory single-stranded RNA molecules around 22 nucleotides long that may each target numerous mRNA transcripts and dim an ...

A Procedure to Study the Effect of Prolonged Food Restriction on Heroin Seeking in Abstinent Rats

In human drug addicts, exposure to drug-associated cues or environments that were previously associated with drug taking can trigger relapse during abstinence. Moreover, various environmental challenges can exacerbate this effect, as well as increase ongoing drug intake.
The procedure we describe here highlights the impact of a common environmental challenge, food restriction, on drug craving that is expressed as an augmentation of drug seeking in abstinent rats.
Rats are implanted with chronic intravenous i.v. catheters, and then trained to press a lever for i.v. heroin over a period of 10-12 days. Following the heroin self-administration phase the rats are removed from the operant conditioning chambers and housed in the animal care facility for a period of at least 14 days. While one group is maintained under unrestricted access to food (sated group), a second group (FDR group) is exposed to a mild food restriction regimen that results in their body weights maintained at 90% of their nonrestricted body weight. On day 14 of food restriction the rats are transferred back to the drug-training environment, and a drug-seeking test is run under extinction conditions (i.e.&#160;lever presses do not result in heroin delivery).
The procedure presented here results in a highly robust augmentation of heroin seeking on test day in the food restricted rats. In addition, compared to the acute food deprivation manipulations we have used before, the current procedure is a more clinically relevant model for the impact of caloric restriction on drug seeking. Moreover, it might be closer to the human condition as the rats are not required to go through an extinction-training phase before the drug-seeking test, which is an integral component of the popular reinstatement procedure.

A procedure that allows a demonstration of robust augmentation of drug seeking in food-restricted rats is described. Following heroin self-administration training, rats go through an abstinence period, in a drug-free environment, during which they are mildly food restricted. Drug seeking is then tested in the drug-associated environment.

In human drug addicts, exposure to drug-associated cues or environments that were previously associated with drug taking can trigger relapse during ...

Fat Preference: A Novel Model of Eating Behavior in Rats

Obesity is a growing problem in the United States of America, with more than a third of the population classified as obese. One factor contributing to this multifactorial disorder is the consumption of a high fat diet, a behavior that has been shown to increase both caloric intake and body fat content. However, the elements regulating preference for high fat food over other foods remain understudied.
To overcome this deficit, a model to quickly and easily test changes in the preference for dietary fat was developed. The Fat Preference model presents rats with a series of choices between foods with differing fat content. Like humans, rats have a natural bias toward consuming high fat food, making the rat model ideal for translational studies. Changes in preference can be ascribed to the effect of either genetic differences or pharmacological interventions. This model allows for the exploration of determinates of fat preference and screening pharmacotherapeutic agents that influence acquisition of obesity.

Dietary fat content influences both energy intake and body fat composition in mammals. By examining rats&#8217; preference for high fat food in a series of choice experiments, it is possible to test genetic differences and pharmacological interventions on their preference for high fat food.

Obesity is a growing problem in the United States of America, with more than a third of the population classified as obese. One factor contributing to ...

A Procedure to Observe Context-induced Renewal of Pavlovian-conditioned Alcohol-seeking Behavior in Rats

Environmental contexts in which drugs of abuse are consumed can trigger craving, a subjective Pavlovian-conditioned response that can facilitate drug-seeking behavior and prompt relapse in abstinent drug users. We have developed a procedure to study the behavioral and neural processes that mediate the impact of context on alcohol-seeking behavior in rats. Following acclimation to the taste and pharmacological effects of 15% ethanol in the home cage, male Long-Evans rats receive Pavlovian discrimination training (PDT) in conditioning chambers. In each daily (Mon-Fri) PDT session, 16 trials each of two different 10 sec auditory conditioned stimuli occur. During one stimulus, the CS+, 0.2 ml of 15% ethanol is delivered into a fluid port for oral consumption. The second stimulus, the CS-, is not paired with ethanol. Across sessions, entries into the fluid port during the CS+ increase, whereas entries during the CS- stabilize at a lower level, indicating that a predictive association between the CS+ and ethanol is acquired. During PDT each chamber is equipped with a specific configuration of visual, olfactory and tactile contextual stimuli. Following PDT, extinction training is conducted in the same chamber that is now equipped with a different configuration of contextual stimuli. The CS+ and CS- are presented as before, but ethanol is withheld, which causes a gradual decline in port entries during the CS+. At test, rats are placed back into the PDT context and presented with the CS+ and CS- as before, but without ethanol. This manipulation triggers a robust and selective increase in the number of port entries made during the alcohol predictive CS+, with no change in responding during the CS-. This effect, referred to as context-induced renewal, illustrates the powerful capacity of contexts associated with alcohol consumption to stimulate alcohol-seeking behavior in response to Pavlovian alcohol cues.

A procedure to study the capacity of an alcohol associated environmental context to trigger the renewal of alcohol-seeking behavior in rats is described.

Environmental contexts in which drugs of abuse are consumed can trigger craving, a subjective Pavlovian-conditioned response that can facilitate ...

Rodent Brain Microinjection to Study Molecular Substrates of Motivated Behavior

Brain microinjection can aid elucidation of the molecular substrates of complex behaviors, such as motivation. For this purpose rodents can serve as appropriate models, partly because the response to behaviorally relevant stimuli and the circuitry parsing stimulus-action outcomes is astonishingly similar between humans and rodents. In studying molecular substrates of complex behaviors, the microinjection of reagents that modify, augment, or silence specific systems is an invaluable technique. However, it is crucial that the microinjection site is precisely targeted in order to aid interpretation of the results. We present a method for the manufacture of surgical implements and microinjection needles that enables accurate microinjection and unlimited customizability with minimal cost. Importantly, this technique can be successfully completed in awake rodents if conducted in conjunction with other JoVE articles that covered requisite surgical procedures. Additionally, there are many behavioral paradigms that are well suited for measuring motivation. The progressive ratio is a commonly used method that quantifies the efficacy of a reinforcer to maintain responding despite an (often exponentially) increasing work requirement. This assay is sensitive to reinforcer magnitude and pharmacological manipulations, which allows reinforcing efficacy and/ or motivation to be determined. We also present a straightforward approach to program operant software to accommodate a progressive ratio reinforcement schedule.

Rodents are an appropriate model to investigate the molecular substrates of behavior and complex psychiatric disorders. Brain microinjection in awake rodents can be used to elucidate disease substrates. An efficient and customizable brain microinjection method as well as the execution of an operant paradigm that quantifies motivation is presented.

Brain microinjection can aid elucidation of the molecular substrates of complex behaviors, such as motivation. For this purpose rodents can serve as ...

Assessment of Cocaine-induced Behavioral Sensitization and Conditioned Place Preference in Mice

It is thought that rewarding experiences with drugs create strong contextual associations and encourage repeated intake. In turn, repeated exposures to drugs of abuse make lasting alterations in the brain function of vulnerable individuals, and these persistent alterations likely serve to maintain the maladaptive drug seeking and taking behaviors characteristic of addiction/dependence2. In rodents, reward experience and contextual associations are frequently measured using the conditioned place preference assay, or CPP, wherein preference for a previously drug-paired context is measured. Behavioral sensitization, on the other hand, is an increase in a drug-induced behavior that develops progressively over repeated exposures. Since sensitized behaviors can often be measured after several months of drug abstinence, depending on the dose and length of initial exposure, they are considered observable correlates of lasting drug-induced plasticity. Researchers have found these assays useful in determining the neurobiological substrates mediating aspects of addiction as well as assessing the potential of different interventions in disrupting these behaviors. This manuscript describes basic, effective protocols for mouse CPP and locomotor behavioral sensitization to cocaine.

This protocol is intended to enable researchers to conduct experiments designed to test these aspects of addiction using the conditioned place preference and locomotor behavioral sensitization assays.

It is thought that rewarding experiences with drugs create strong contextual associations and encourage repeated intake. In turn, repeated exposures to ...

Place and Response Learning in the Open-field Tower Maze

This protocol describes how the Open-field Tower Maze (OFTM) paradigm is used to study spatial learning in rodents. This maze is especially useful for examining how rats learn to use a place- or response-learning to successfully navigate in an open-field arena. Additionally, this protocol describes how the OFTM differs from other behavioral maze paradigms that are commonly used to study spatial learning in rodents. The OFTM described in this article was adapted from the one previously described by Cole, Clipperton, and Walt (2007). Specifically, the OFTM was created to test spatial learning in rodents without the experimenter having to consider how &#8220;stress&#8221; might play a role as a confounding variable. Experiments have shown that stress-alone can significantly affect cognitive function1. The representative results section contains data from an experiment that used the OFTM to examine the effects of estradiol treatment on place- and response-learning in adult female Sprague Dawley rats2. Future studies will be designed to examine the role of the hippocampus and striatum in place- and response-learning in the OFTM.

The Open-field Tower Maze (OFTM) is a maze created to study the behavioral and neural mechanisms of spatial learning (e.g., place- or response-learning) in rats. This maze is especially useful for experimenters who want to use a non-stressful maze paradigm to investigate spatial learning in their research.

This protocol describes how the Open-field Tower Maze (OFTM) paradigm is used to study spatial learning in rodents. This maze is especially useful for ...

Reversible Cooling-induced Deactivations to Study Cortical Contributions to Obstacle Memory in the Walking Cat

On complex, naturalistic terrain, sensory information about an environmental obstacle can be used to rapidly adjust locomotor movements for avoidance. For example, in the cat, visual information about an impending obstacle can modulate stepping for avoidance. Locomotor adaptation can also occur independent of vision, as sudden tactile inputs to the leg by an expected obstacle can modify the stepping of all four legs for avoidance. Such complex locomotor coordination involves supraspinal structures, such as the parietal cortex. This protocol describes the use of reversible, cooling-induced cortical deactivation to assess parietal cortex contributions to memory-guided obstacle locomotion in the cat. Small cooling loops, known as cryoloops, are specially shaped to deactivate discrete regions of interest to assess their contributions to an overt behavior. Such methods have been used to elucidate the role of parietal area 5 in memory-guided obstacle avoidance in the cat.

Complex locomotion in naturalistic environments requiring careful coordination of the limbs involves regions of the parietal cortex. The following protocol describes the use of reversible cooling-induced deactivation to demonstrate the role of parietal area 5 in memory-guided obstacle avoidance in the walking cat.

On complex, naturalistic terrain, sensory information about an environmental obstacle can be used to rapidly adjust locomotor movements for avoidance. For ...

Reinstatement of Drug-seeking in Mice Using the Conditioned Place Preference Paradigm

The present protocol describes the Conditioned Place Preference (CPP) as a model of relapse in drug addiction. In this model, animals are first trained to acquire a conditioned place preference in a drug-paired compartment, and after the post-conditioning test, they perform several sessions to extinguish the established preference. The CPP permits the evaluation of the conditioned rewarding effects of drugs related to environmental cues. Then, the extinguished CPP can be robustly reinstated by the non-contingent administration of a priming dose of the drug, and by exposure to stressful stimuli. Both methods will be explained here. When the animal reinitiates the behavioral response, a reinstatement of the conditioned reward is considered to have taken place.
The main advantages of this protocol are that it is non-invasive, inexpensive, and simple with good validity criteria. In addition, it allows the study of different environmental manipulations, such as stress or diet, which can modulate relapse into drug seeking behaviors. However, one limitation is that if the researcher aims to explore the motivation and primary reinforcing effects of the drug, it should be complemented with self-administration procedures, as they involve operant responses of animals.

This protocol describes the Conditioned Place Preference (CPP) as a model of relapse. This procedure permits the measurement of relapse in laboratory animals, considering the impact of drug-associated environmental cues as craving and relapse in abstaining addicts is currently the focus of drug-abuse treatment programs.

The present protocol describes the Conditioned Place Preference (CPP) as a model of relapse in drug addiction. In this model, animals are first trained to ...

A Conditioned Place Preference Protocol for Measuring Incubation of Craving in Rats

A major cause of repeated relapses is a craving for the drug. Drug craving increases progressively during the abstinence period, a phenomenon termed incubation of drug craving. Here, we describe a morphine conditioned place preference (CPP) protocol for measuring the incubation of craving in rats. In this protocol, a CPP paradigm mainly employing somatosensory cues is used to establish a long-term reward memory of morphine. A three-chamber CPP box that differs in the texture of the chamber floor is constructed. First, the animals are tested for their baseline preference to the two side chambers for three consecutive days. Then, they are injected intraperitoneally with morphine/saline and put into their non-preferred/preferred chamber for 45 min. After 6 days of conditioning, their preference to the side chambers is tested for 15 min at different time points after the last conditioning session. With this paradigm, the reward memory of morphine could last for at least 18 days. To test whether the above-mentioned protocol can model increased craving, the number of entrances into the two side chambers are counted during the abstinence period. The results show that the entrances increased, suggesting that the CPP paradigm could mimic the incubation of craving. Future studies can employ this model to study neural mechanisms underlying long-term memory and incubation of craving.

Here, a morphine conditioned place preference (CPP) protocol is described to measure incubation of craving in rats.

A major cause of repeated relapses is a craving for the drug. Drug craving increases progressively during the abstinence period, a phenomenon termed ...