Nathan S. Pentkowski

Effects of lesions to the dorsal and ventral hippocampus on defensive behaviors in rats

This study investigated the role of the hippocampus in both unconditioned and conditioned defensive behaviors by examining the effects of pretraining ibotenic acid lesions to the dorsal and ventral hippocampus in male Long–Evans hooded rats exposed to three types of threat stimuli: cat‐odor, a live cat and footshock. Defensive behaviors were assessed during exposure to cat‐odor and a live cat, and immediately following the presentation of footshock. Conditioned defensive behaviors were also assessed in each context 24 h after initial threat exposure. During both unconditioned and conditioned trials, dorsal hippocampal lesions failed to significantly alter any behavioral measure in each test of defense. In contrast, ventral hippocampal lesions significantly reduced unconditioned defensive behaviors during exposure to cat‐odor without producing any observable effects during cat exposure. Furthermore, ventral lesions significantly attenuated conditioned defensive behaviors following the administration of footshock and during re‐exposure to each context. These results suggest a specific role for the ventral, not dorsal, hippocampus in modulating anxiety‐like behaviors in certain animal models of defense.

Lesions of structures showing FOS expression to cat presentation: effects on responsivity to a Cat, Cat odor, and nonpredator threat.

Exposure of rats to a cat elicits Fos activity in a number of brain areas or structures. Based on hodological relationships of these, Canteras has proposed a medial hypothalamic defense system, with input from several forebrain sites. Both electrolytic and neurotoxic lesions of the dorsal premammillary nucleus, which shows the strongest Fos response to cat exposure, produce striking decrements in a number of defensive behaviors to a cat or to cat odor stimuli, but do not have a major effect on either postshock freezing, or responsivity to the odor of a female in estrus. Neurotoxic lesions of the medial amygdala produce decrements in defensiveness to predator stimuli, particularly odor stimuli, that are consistent with a view of this structure as involved with allomonal cues. While dorsal hippocampal lesions had little effect on responsivity to predator stimuli, neurotoxic lesions of the ventral hippocampus reduced freezing and enhanced a variety of nondefensive behaviors to both cat odor and footshock, with similar reductions in defensiveness during context conditioning tests for cat odor, cat exposure and footshock. These results support the view that the dorsal premammillary nucleus is strongly and selectively involved in control of responsivity to predator stimuli. Structures with important input into the medial hypothalamic defense system appear also to be functionally involved with antipredator defensive behaviors, and these lesion studies may suggest specific hypotheses as to the particular defense functions of different areas.

Anti-craving effects of environmental enrichment

We hypothesized that environmental enrichment in rats may reduce cocaine-seeking behaviour elicited by cocaine-priming injections and by cocaine-associated cues. Rats trained to self-administer cocaine while housed in isolated conditions were then assigned to live in isolation or an enriched environment for 21 d of forced abstinence. Subsequently, extinction and reinstatement of cocaine-seeking behaviour (operant responses without cocaine available) were assessed. Expt 1 showed that enrichment resulted in less cocaine-seeking behaviour during extinction and cue-elicited reinstatement compared to continued isolation housing, but had no effect on cocaine-primed reinstatement. A subsequent experiment, which included a pair-housed group to control for potential isolation stress, again demonstrated that enrichment attenuated cocaine seeking during extinction, but not cocaine-primed reinstatement, relative to both isolation and pair-housed conditions. The findings suggest that enrichment reduces the impact of cocaine-associated environmental stimuli, and hence it may be a useful intervention for attenuating cue-elicited craving in humans.

Limited physical contact through a mesh barrier is sufficient for social reward-conditioned place preference in adolescent male rats

Adolescence is a period of enhanced sensitivity to social influences and vulnerability to drug abuse. Social reward in adolescent rats has been demonstrated with the conditioned place preference (CPP) model, but it is not clear whether limited contact with another rat without play is sufficient to produce reward. We investigated this issue using an apparatus containing two main compartment, each with a wire mesh barrier that allowed rats placed on either side of the barrier to have limited physical contact. Adolescent male rats were given two conditioning sessions/day for 2 or 8 days following baseline preference tests. Rats were placed into their preferred side alone for one daily 10-min session and into their initially non-preferred side (i.e., CS) for the other session during which they either had restricted or unrestricted physical access to another rat (Rat/Mesh or Rat/Phys, respectively) or to a tennis ball (Ball/Mesh or Ball/Phys, respectively) unconditioned stimulus (US). Only the Rat/Phys group exhibited CPP after 2 CS-US pairings; however, after 8 CS-US pairings, the Rat/Mesh and Ball/Phys groups also exhibited CPP. During conditioning, the rat US elicited more robust approach and contact behavior compared to the ball, regardless of physical or restricted access. The incidence of contact and/or approach increased as the number of exposures increased. The results suggest that the rank order of US reward efficacy was physical contact with a rat>limited contact with a rat>physical contact with a ball, and that rough-and-tumble play is not necessary to establish social reward-CPP. The findings have important implications for emerging drug self-administration models in which two rats self-administering drug intravenously have limited physical contact via a mesh barrier shared between their respective operant conditioning chambers.

Emotional valence and context of social influences on drug abuse-related behavior in animal models of social stress and prosocial interaction

RationaleSocial factors are important determinants of drug dependence and relapse.ObjectivesWe reviewed pre-clinical literature examining the role of social experiences from early life through the development of drug dependence and relapse, emphasizing two aspects of these experiences: (1) whether the social interaction is appetitive or aversive and (2) whether the social interaction occurs within or outside of the drug-taking context.MethodsThe models reviewed include neonatal care, isolation, social defeat, chronic subordination, and prosocial interactions. We review results from these models in regard to effects on self-administration and conditioned place preference established with alcohol, psychostimulants, and opiates.ResultsWe suggest that in general, when the interactions occur outside of the drug-taking context, prosocial interactions are protective against drug abuse-related behaviors, whereas social stressors facilitate these behaviors. By contrast, positive or negative social interactions occurring within the drug-taking context may interact with other risk factors to enhance or inhibit these behaviors.ConclusionsDespite differences in the nature and complexity of human social behavior compared to other species, the evolving animal literature provides useful models for understanding social influences on drug abuse-related behavior that will allow for research on the behavioral and biological mechanisms involved. The models have contributed to understanding social influences on initiation and maintenance of drug use, but more research is needed to understand social influences on drug relapse.

Stimulation of Medial Prefrontal Cortex Serotonin 2C (5-HT 2C ) Receptors Attenuates Cocaine-Seeking Behavior

Serotonin 2C receptor (5-HT2CR) agonists administered systemically attenuate both cocaine-primed and cue-elicited reinstatement of extinguished cocaine-seeking behavior. To further elucidate the function of these receptors in addiction-like processes, this study examined the effects of microinfusing the 5-HT2CR agonist MK212 (0, 10, 30, 100 ng/side/0.2 μl) into the medial prefrontal cortex (mPFC) on cocaine self-administration and reinstatement of extinguished cocaine-seeking behavior. Male Sprague–Dawley rats were trained to self-administer cocaine (0.75 mg/kg, i.v.) paired with light and tone cues. Once responding stabilized, rats received MK212 microinfusions before tests for maintenance of cocaine self-administration. Next, extinction training to reduce cocaine-seeking behavior, defined as responses performed without cocaine reinforcement available, occurred until low extinction baselines were achieved. Rats then received MK212 microinfusions before tests for reinstatement of extinguished cocaine-seeking behavior elicited by cocaine-priming injections (10 mg/kg, i.p.) or response-contingent presentations of the cocaine-associated cues; operant responses during cocaine-primed reinstatement tests produced no consequences. MK212 microinfusions into the prelimbic and infralimbic, but not anterior cingulate, regions of the mPFC dose-dependently attenuated both cocaine-primed and cue-elicited reinstatement of extinguished cocaine-seeking behavior, but did not reliably affect cocaine self-administration. A subsequent experiment showed that the effects of MK212 (100 ng/side/0.2 μl) on reinstatement of extinguished cocaine-seeking behavior were blocked by co-administration of the 5-HT2CR antagonist SB242084 (200 ng/side/0.2 μl). MK212 administered alone into the mPFC as a drug prime produced no discernable effects on cocaine-seeking behavior. These findings suggest that stimulation of 5-HT2CRs in the mPFC attenuates the incentive motivational effects produced by sampling cocaine or exposure to drug-paired cues.

Stimulation of 5-HT(1B) receptors enhances cocaine reinforcement yet reduces cocaine-seeking behavior.

Paradoxically, stimulation of 5‐HT1B receptors (5‐HT1BRs) enhances sensitivity to the reinforcing effects of cocaine but attenuates incentive motivation for cocaine as measured using the extinction/reinstatement model. We revisited this issue by examining the effects of a 5‐HT1BR agonist, CP94253, on cocaine reinforcement and cocaine‐primed reinstatement, predicting that CP94253 would enhance cocaine‐seeking behavior reinstated by a low priming dose, similar to its effect on cocaine reinforcement. Rats were trained to self‐administer cocaine (0.75 mg/kg, i.v.) paired with light and tone cues. For reinstatement experiments, they then underwent daily extinction training to reduce cocaine‐seeking behavior (operant responses without cocaine reinforcement). Next, they were pre‐treated with CP94253 (3–10 mg/kg, s.c.) and either tested for cocaine‐primed (10 or 2.5 mg/kg, i.p.) or cue‐elicited reinstatement of extinguished cocaine‐seeking behavior. For reinforcement, effects of CP94253 (5.6 mg/kg) across a range of self‐administered cocaine doses (0–1.5 mg/kg, i.v.) were examined. Cocaine dose‐dependently reinstated cocaine‐seeking behavior, but contrary to our prediction, CP94253 reduced reinstatement with both priming doses. Similarly, CP94253 reduced cue‐elicited reinstatement. In contrast, CP94253 shifted the self‐administration dose‐effect curve leftward, consistent with enhanced cocaine reinforcement. When saline was substituted for cocaine, CP94253 reduced response rates (i.e. cocaine‐seeking behavior). In subsequent control experiments, CP94253 decreased open‐arm exploration in an elevated plus‐maze suggesting an anxiogenic effect, but had no effect on locomotion or sucrose reinforcement. These results provide strong evidence that stimulation of 5‐HT1BRs produces opposite effects on cocaine reinforcement and cocaine‐seeking behavior, and further suggest that 5‐HT1BRs may be a novel target for developing medications for cocaine dependence.

Environmental enrichment counters cocaine abstinence-induced stress and brain reactivity to cocaine cues but fails to prevent the incubation effect.

Environmental enrichment (EE) during a period of forced abstinence attenuates incentive motivational effects of cocaine‐paired stimuli. Here we examined whether EE during forced abstinence from cocaine self‐administration would prevent time‐dependent increases in cue‐elicited cocaine‐seeking behavior (i.e. the incubation effect). Rats were trained to self‐administer cocaine, which was paired with light/tone cues, for 15 days while living in isolated conditions (IC). Controls received yoked saline infusions. Subsequently, rats were assigned to live in either continued IC or EE for either 1 or 21 days of forced abstinence prior to a test for cocaine‐seeking behavior. During testing, responding resulted only in presentation of the light/tone cues. Contrary to our prediction, cocaine‐seeking behavior increased over time regardless of living condition during abstinence; however, EE attenuated cocaine‐seeking behavior relative to IC regardless of length of abstinence. Brains were harvested and trunk blood was collected immediately after the 60‐minute test and later assayed. Results indicated that short‐term EE elevated hippocampal brain‐derived neurotrophic factor and reduced plasma corticosterone compared with IC. Furthermore, 21 days of EE during forced abstinence prevented increases in the cue‐elicited amygdala phosphorylated extracellular signal‐regulated kinase expression that was observed in IC rats. These findings suggest that EE attenuates incentive motivational effects of cocaine cues through a mechanism other than preventing the incubation effect, perhaps involving reduction of stress and neural activity in response to cocaine‐paired cues during acute withdrawal.

Protracted withdrawal from cocaine self-administration flips the switch on 5-HT1B receptor modulation of cocaine abuse-related behaviors

BACKGROUND The role of serotonin-1B receptors (5-HT(1B)Rs) in modulating cocaine abuse-related behaviors has been controversial due to discrepancies between pharmacological and gene knockout approaches and opposite influences on cocaine self-administration versus cocaine-seeking behavior. We hypothesized that modulation of these behaviors via 5-HT(1B)Rs in the mesolimbic pathway may vary depending on the stage of the addiction cycle. METHODS To test this hypothesis, we examined the effects of increasing 5-HT(1B)R production by microinfusing a viral vector expressing either green fluorescent protein and 5-HT(1B)R or green fluorescent protein alone into the medial nucleus accumbens shell of rats either during maintenance of cocaine self-administration (i.e., active drug use) or during protracted withdrawal. RESULTS 5-HT(1B)R receptor gene transfer during maintenance shifted the dose-response curve for cocaine self-administration upward and to the left and increased breakpoints and cocaine intake on a progressive ratio schedule, consistent with enhanced reinforcing effects of cocaine. In contrast, following 21 days of forced abstinence, 5-HT(1B)R gene transfer attenuated breakpoints and cocaine intake on a progressive ratio schedule of reinforcement, as well as cue- and cocaine-primed reinstatement of cocaine-seeking behavior. CONCLUSIONS This unique pattern of effects suggests that mesolimbic 5-HT(1B)Rs differentially modulate cocaine abuse-related behaviors, with a facilitative influence during periods of active drug use, in striking contrast to an inhibitory influence during protracted withdrawal. These findings suggest that targeting 5-HT(1B)Rs may lead to a novel treatment for cocaine dependence and that the therapeutic efficacy of these treatments may vary depending on the stage of the addiction cycle.

Stimulation of dopamine D2/D3 but not D1 receptors in the central amygdala decreases cocaine-seeking behavior

Alterations in dopamine output within the various subnuclei of the amygdala have previously been implicated in cocaine reinforcement, as well as cocaine-seeking behavior. To elucidate the potential for increased stimulation of D1- and D2-like receptors (D1Rs and D2Rs, respectively) specifically in the central nucleus of the amygdala (CeA) to modulate cue- and cocaine-elicited reinstatement of cocaine-seeking behavior, we infused either the D1R agonist, SKF-38393 (0-4.0 microg/side) or the D2R agonist, 7-OH-DPAT (0-4.0 microg/side) into the CeA immediately prior to tests for cue and cocaine-primed reinstatement. We also examined the effects of 7-OH-DPAT on cocaine self-administration as a positive behavioral control. 7-OH-DPAT decreased cue-and cocaine-primed reinstatement, and reduced the number of cocaine infusions obtained during self-administration; SKF-38393 produced no discernable effects. The results suggest that enhanced stimulation of D2Rs, but not D1Rs, in the CeA is sufficient to inhibit expression of the incentive motivational effects of cocaine priming and cocaine-paired cues. Together with previous findings that D1R blockade attenuates reinstatement of cocaine-seeking behavior, the results suggest that D1R stimulation may be necessary, but not sufficient, to modulate the incentive motivational effects of cues and cocaine priming.