Enhanced psychostimulant response, but not social avoidance, depends on GluA1 AMPA receptors in VTA dopamine neurons following intermittent social defeat stress in rats.

Evidence from both human and animal studies demonstrates the importance of social stress in the development of addiction-related behaviour. In rats, intermittent social defeat stress causes long-lasting psychostimulant cross-sensitization. Our recent data reveal heightened expression of AMPA receptor (AMPAR) GluA1 subunit in rat ventral tegmental area (VTA), which occurs concurrently with social stress-induced amphetamine (AMPH) cross-sensitization. In addition, social stress in rats induced social avoidance behaviour. The present study evaluated the effects of intermittent social defeat stress on GluA1 expression in VTA dopamine (DA) neurons, then utilized Cre-dependent virus-mediated gene transfer to determine the functional role of homomeric GluA1-AMPARs in these neurons. Social defeat stress exposure induced GluA1 expression in VTA DA neurons, as demonstrated by a greater density of GluA1/tyrosine hydroxylase (TH) double-labelling in VTA neurons in stressed rats. Additionally, functional inactivation of VTA GluA1 AMPARs in DA neurons prevented stress-induced cross-sensitization, or augmented locomotor response to low dose AMPH challenge (1.0 mg/kg, i.p.), but had no effect on social stress-induced social avoidance behaviour. Furthermore, wild-type overexpression of GluA1 in VTA DA neurons had the opposite effect; locomotor-activating effects of AMPH were significantly augmented, even in the absence of stress. Taken together, these results suggest that stress-induced GluA1 expression in VTA DA neurons is necessary for psychostimulant cross-sensitization, but not for social avoidance. This differential effect suggests that different neural pathways are implicated in these behaviours. These findings could lead to novel pharmacotherapies to help prevent stress-induced susceptibility to substance abuse.

Effects of Repeated Ropinirole Treatment on Phencyclidine-Induced Hyperlocomotion, Prepulse Inhibition Deficits, and Social Avoidance in Rats.

Phencyclidine (PCP), a noncompetitive N-methyl d-aspartate (NMDA) receptor antagonist, provides the most complete pharmacologic model of schizophrenia in humans and animals. Acute PCP causes hyperlocomotion, disrupts prepulse inhibition (PPI), and increases social avoidance in rats. We have previously shown that repeated treatment with the dopamine (DA) D2-like receptor agonists, quinpirole or ropinirole, prevents agonist-induced PPI disruption. In the present study, we examined whether repeated ropinirole treatment similarly attenuates the effects of PCP in a more complete model of schizophrenia symptoms and examined the effect of repeated D2-like agonist treatment on locomotion, PPI, and social interaction after acute PCP challenge. The acute effect of PCP (3.0 or 6.0 mg/kg) on locomotor activity was examined to establish a minimum effective dose. Thereafter, the effect of PCP challenge (3.0 mg/kg) on locomotor activity, PPI, and social interaction was assessed in adult male rats before or 7-10 days after termination of repeated daily treatment with ropinirole (0.1 mg/kg) or saline vehicle (0.1 ml/kg) for 28 days. Repeated ropinirole treatment attenuates PCP-induced hyperlocomotion, PPI deficits, and social avoidance. These findings suggest that repeated ropinirole treatment might affect a final common pathway that is vulnerable to both PCP- and dopamine agonist-induced behavioral disruption, thereby providing an alternative approach to block the effects of PCP.

Cannabinoid receptor subtype 2 (CB2R) agonist, GW405833 reduces agonist-induced Ca(2+) oscillations in mouse pancreatic acinar cells.

Emerging evidence demonstrates that the blockade of intracellular Ca(2+) signals may protect pancreatic acinar cells against Ca(2+) overload, intracellular protease activation, and necrosis. The activation of cannabinoid receptor subtype 2 (CB2R) prevents acinar cell pathogenesis in animal models of acute pancreatitis. However, whether CB2Rs modulate intracellular Ca(2+) signals in pancreatic acinar cells is largely unknown. We evaluated the roles of CB2R agonist, GW405833 (GW) in agonist-induced Ca(2+) oscillations in pancreatic acinar cells using multiple experimental approaches with acute dissociated pancreatic acinar cells prepared from wild type, CB1R-knockout (KO), and CB2R-KO mice. Immunohistochemical labeling revealed that CB2R protein was expressed in mouse pancreatic acinar cells. Electrophysiological experiments showed that activation of CB2Rs by GW reduced acetylcholine (ACh)-, but not cholecystokinin (CCK)-induced Ca(2+) oscillations in a concentration-dependent manner; this inhibition was prevented by a selective CB2R antagonist, AM630, or was absent in CB2R-KO but not CB1R-KO mice. In addition, GW eliminated L-arginine-induced enhancement of Ca(2+) oscillations, pancreatic amylase, and pulmonary myeloperoxidase. Collectively, we provide novel evidence that activation of CB2Rs eliminates ACh-induced Ca(2+) oscillations and L-arginine-induced enhancement of Ca(2+) signaling in mouse pancreatic acinar cells, which suggests a potential cellular mechanism of CB2R-mediated protection in acute pancreatitis.

Overexpression of BDNF in the ventral tegmental area enhances binge cocaine self-administration in rats exposed to repeated social defeat.

Stress is a major risk factor for substance abuse. Intermittent social defeat stress increases drug self-administration (SA) and elevates brain-derived neurotrophic factor (BDNF) expression in the ventral tegmental area (VTA) in rats. Intra-VTA BDNF overexpression enhances social defeat stress-induced cross-sensitization to psychostimulants and induces nucleus accumbens (NAc) ΔFosB expression. Therefore, increased VTA BDNF may mimic or augment the development of drug abuse-related behavior following social stress. To test this hypothesis, adeno-associated virus (AAV) was infused into the VTA to overexpress either GFP alone (control) or GFP + BDNF. Rats were then either handled or exposed to intermittent social defeat stress before beginning cocaine SA training. The SA acquisition and maintenance phases were followed by testing on a progressive ratio (PR) schedule of cocaine reinforcement, and then during a 12-h access "binge" cocaine SA session. BDNF and ΔFosB were quantified postmortem in regions of the mesocorticolimbic circuitry using immunohistochemistry. Social defeat stress increased cocaine intake on a PR schedule, regardless of virus treatment. While stress alone increased intake during the 12-h binge session, socially-defeated rats that received VTA BDNF overexpression exhibited even greater cocaine intake compared to the GFP-stressed group. However, VTA BDNF overexpression alone did not alter binge intake. BDNF expression in the VTA was also positively correlated with total cocaine intake during binge session. VTA BDNF overexpression increased ΔFosB expression in the NAc, but not in the dorsal striatum. Here we demonstrate that VTA BDNF overexpression increases long-access cocaine intake, but only under stressful conditions. Therefore, enhanced VTA-BDNF expression may be a facilitator for stress-induced increases in drug abuse-related behavior specifically under conditions that capture compulsive-like drug intake.

Chronic stress enhanced fear memories are associated with increased amygdala zif268 mRNA expression and are resistant to reconsolidation.

The chronically stressed brain may present a vulnerability to develop maladaptive fear-related behaviors in response to a traumatic event. In rodents, chronic stress leads to amygdala hyperresponsivity and dendritic hypertrophy and produces a post traumatic stress disorder (PTSD)-like phenotype that includes exaggerated fear learning following Pavlovian fear conditioning and resistance to extinction. It is unknown whether chronic stress-induced enhanced fear memories are vulnerable to disruption via reconsolidation blockade, as a novel therapeutic approach for attenuating exaggerated fear memories. We used a chronic stress procedure in a rat model (wire mesh restraint for 6h/d/21d) to create a vulnerable brain that leads to a PTSD-like phenotype. We then examined freezing behavior during acquisition, reactivation and after post-reactivation rapamycin administration (i.p., 40mg/kg) in a Pavlovian fear conditioning paradigm to determine its effects on reconsolidation as well as the subsequent functional activation of limbic structures using zif268 mRNA. Chronic stress increased amygdala zif268 mRNA during fear memory retrieval at reactivation. Moreover, these enhanced fear memories were unaffected by post reactivation rapamycin to disrupt long-term fear memory. Also, post-reactivation long term memory processing was also associated with increased amygdala (LA and BA), and decreased hippocampal CA1 zif268 mRNA expression. These results suggest potential challenges for reconsolidation blockade as an effective approach in treating exaggerated fear memories, as in PTSD. Our findings also support chronic stress manipulations combined with fear conditioning as a useful preclinical approach to study a PTSD-like phenotype.

Knockdown of ventral tegmental area mu-opioid receptors in rats prevents effects of social defeat stress: implications for amphetamine cross-sensitization, social avoidance, weight regulation and expression of brain-derived neurotrophic factor.

Social defeat stress causes social avoidance and long-lasting cross-sensitization to psychostimulants, both of which are associated with increased brain-derived neurotrophic factor (BDNF) expression in the ventral tegmental area (VTA). Moreover, social stress upregulates VTA mu-opioid receptor (MOR) mRNA. In the VTA, MOR activation inhibits GABA neurons to disinhibit VTA dopamine neurons, thus providing a role for VTA MORs in the regulation of psychostimulant sensitization. The present study determined the effect of lentivirus-mediated MOR knockdown in the VTA on the consequences of intermittent social defeat stress, a salient and profound stressor in humans and rodents. Social stress exposure induced social avoidance and attenuated weight gain in animals with non-manipulated VTA MORs, but both these effects were prevented by VTA MOR knockdown. Rats with non-manipulated VTA MOR expression exhibited cross-sensitization to amphetamine challenge (1.0 mg/kg, i.p.), evidenced by a significant augmentation of locomotion. By contrast, knockdown of VTA MORs prevented stress-induced cross-sensitization without blunting the locomotor-activating effects of amphetamine. At the time point corresponding to amphetamine challenge, immunohistochemical analysis was performed to examine the effect of stress on VTA BDNF expression. Prior stress exposure increased VTA BDNF expression in rats with non-manipulated VTA MOR expression, while VTA MOR knockdown prevented stress-induced expression of VTA BDNF. Taken together, these results suggest that upregulation of VTA MOR is necessary for the behavioral and biochemical changes induced by social defeat stress. Elucidating VTA MOR regulation of stress effects on the mesolimbic system may provide new therapeutic targets for treating stress-induced vulnerability to substance abuse.

Congo red modulates ACh-induced Ca(2+) oscillations in single pancreatic acinar cells of mice.

AIM: Congo red, a secondary diazo dye, is usually used as an indicator for the presence of amyloid fibrils. Recent studies show that congo red exerts neuroprotective effects in a variety of models of neurodegenerative diseases. However, its pharmacological profile remains unknown. In this study, we investigated the effects of congo red on ACh-induced Ca(2+) oscillations in mouse pancreatic acinar cells in vitro. METHODS: Acutely dissociated pancreatic acinar cells of mice were prepared. A U-tube drug application system was used to deliver drugs into the bath. Intracellular Ca(2+) oscillations were monitored by whole-cell recording of Ca(2+)-activated Cl(-) currents and by using confocal Ca(2+) imaging. For intracellular drug application, the drug was added in pipette solution and diffused into cell after the whole-cell configuration was established. RESULTS: Bath application of ACh (10 nmol/L) induced typical Ca(2+) oscillations in dissociated pancreatic acinar cells. Addition of congo red (1, 10, 100 µmol/L) dose-dependently enhanced Ach-induced Ca(2+) oscillations, but congo red alone did not induce any detectable response. Furthermore, this enhancement depended on the concentrations of ACh: congo red markedly enhanced the Ca(2+) oscillations induced by ACh (10-30 nmol/L), but did not alter the Ca(2+) oscillations induced by ACh (100-10000 nmol/L). Congo red also enhanced the Ca(2+) oscillations induced by bath application of IP3 (30 µmol/L). Intracellular application of congo red failed to alter ACh-induced Ca(2+) oscillations. CONCLUSION: Congo red significantly modulates intracellular Ca(2+) signaling in pancreatic acinar cells, and this pharmacological effect should be fully considered when developing congo red as a novel therapeutic drug.

Knockdown of tropomyosin-related kinase B receptor expression in the nucleus accumbens shell prevents intermittent social defeat stress-induced cross-sensitization to amphetamine in rats.

The nucleus accumbens (NAc) is a critical brain region for the rewarding effects of drugs of abuse. Brain-derived neurotrophic factor (BDNF) can facilitate stress- and drug-induced neuroadaptation in the mesocorticolimbic system. BDNF-containing projections to the NAc originate from the ventral tegmental area (VTA) and the prefrontal cortex, and BDNF release activates tropomyosin-related kinase B (TrkB). In this study, we examined the necessity for BDNF-TrkB signaling in the NAc shell during social defeat stress-induced cross-sensitization to amphetamine. Adeno-associated virus expressing short hairpin RNA directed against TrkB (AAV-shTrkB) was infused bilaterally into the NAc shell to knock down TrkB, whereas AAV-GFP (green fluorescent protein) was used as the control virus. Rats were exposed to intermittent social defeat stress or handling procedures; amphetamine challenge was given at 10 days after the last defeat and locomotor activity was measured. Stressed rats that received the control virus showed cross-sensitization to amphetamine compared with the handled rats. In contrast, NAc TrkB knockdown prevented social defeat stress-induced cross-sensitization. TrkB knockdown in the NAc was found to reduce the level of phospho-extracellular signal-regulated kinase 1 in this region. NAc TrkB knockdown also prevented stress-induced elevation of BDNF and the glutamate receptor type 1 (GluA1) subunit of AMPA receptor in the VTA, as well as ΔFosB expression in the NAc. These findings indicated that BDNF-TrkB signaling in the NAc shell was required for social defeat stress-induced cross-sensitization. NAc TrkB-BDNF signaling also appeared to be involved in the regulation of GluA1 in the VTA, as well as in the NAc ΔFosB accumulation that could trigger cross-sensitization after social defeat stress.

Experience-dependent effects of context and restraint stress on corticolimbic c-Fos expression.

Stressors are typically multidimensional, comprised of multiple physical and sensory components that rarely occur as single isolated events. This study used a 2-day stress exposure paradigm to assess functional activation patterns (by Fos expression) in key corticolimbic structures following repeated context, repeated restraint, context followed by restraint or restraint followed by context. On day 1, rats were transported to a novel context and either restrained for 6 h or left undisturbed. On day 2, these two groups were either restrained or not in the same context, then processed for Fos immunohistochemistry. Regardless of prior stress experience, rats exposed to context only on day 2 expressed more Fos-like immunoreactive (IR) labeling in CA1 and CA3 of dorsal hippocampus, basolateral amygdala and central amygdala than those that were not. This pattern was reversed in the dentate gyrus infrapyramidal blade. In contrast, in the infralimbic region of the medial prefrontal cortex (mPFC), the experience of a single restraint on either day 1 or day 2 rats elevated Fos-like IR relative to rats that had been exposed to context alone. These data show that exposure to context produces robust Fos induction in the hippocampus and amygdala, regardless of prior experience with restraint and compared to the immediate experience of restraint, with prior experience modulating Fos expression within the mPFC.

BDNF overexpression in the ventral tegmental area prolongs social defeat stress-induced cross-sensitization to amphetamine and increases ΔFosB expression in mesocorticolimbic regions of rats.

Social defeat stress induces persistent cross-sensitization to psychostimulants, but the molecular mechanisms underlying the development of cross-sensitization remain unclear. One candidate is brain-derived neurotrophic factor (BDNF). The present research examined whether ventral tegmental area (VTA) BDNF overexpression would prolong the time course of cross-sensitization after a single social defeat stress, which normally produces transient cross-sensitization lasting <1 week. ΔFosB, a classic molecular marker of addiction, was also measured in mesocorticolimbic terminal regions. Separate groups of intact male Sprague-Dawley rats underwent a single episode of social defeat stress or control handling, followed by amphetamine (AMPH) challenge 3 or 14 days later. AMPH cross-sensitization was apparent 3, but not 14, days after stress. Intra-VTA infusion of adeno-associated viral (AAV-BDNF) vector resulted in a twofold increase of BDNF level in comparison to the group receiving the control virus (AAV-GFP), which lasted at least 45 days. Additionally, overexpression of BDNF in the VTA alone increased ΔFosB in the nucleus accumbens (NAc) and prefrontal cortex. Fourteen days after viral infusions, a separate group of rats underwent a single social defeat stress or control handling and were challenged with AMPH 14 and 24 days after stress. AAV-BDNF rats exposed to stress showed prolonged cross-sensitization and facilitated sensitization to the second drug challenge. Immunohistochemistry showed that the combination of virally enhanced VTA BDNF, stress, and AMPH resulted in increased ΔFosB in the NAc shell compared with the other groups. Thus, elevation of VTA BDNF prolongs cross-sensitization, facilitates sensitization, and increases ΔFosB in mesocorticolimbic terminal regions. As such, elevated VTA BDNF may be a risk factor for drug sensitivity.

Viral depletion of VTA BDNF in rats modulates social behavior, consequences of intermittent social defeat stress, and long-term weight regulation.

Mesolimbic brain-derived neurotrophic factor (BDNF) is implicated in sustained behavioral changes following chronic social stress, and its depletion may reduce susceptibility to such behavioral alterations. Enhanced mesolimbic BDNF is proposed as pro-depressive and anhedonic, while depleting ventral tegmetal area (VTA) BDNF increases weight by enhancing hedonic eating. Here, we questioned whether depletion of VTA BDNF would alleviate social defeat stress-induced deficits in weight regulation, or affect social behavior in the presence or absence of social stress. Male Sprague-Dawley rats received bilateral intra-VTA infusions of adeno-associated virus (AAV) vectors containing shRNA against BDNF or a control virus. Three weeks later, rats underwent 4 episodes of social defeat stress involving exposure to an aggressive Long-Evans resident rat, or control handling every third day. Depleted VTA BDNF conferred resistance to the deficient weight regulation normally observed during intermittent social defeat stress, and enhanced long-term weight gain regardless of stress history. In addition, social approach and avoidance behavior towards a novel social target were measured 7 weeks after stress. Social defeat stress chronically reduced social behavior, whereas depletion of VTA BDNF chronically increased social behavior. Our results reveal that depletion of VTA BDNF alleviates some consequences of intermittent social defeat stress, enhances social behavior, and may contribute to weight gain. These data implicate VTA BDNF in protracted behavioral responses to stress, social stimuli, and weight regulation.

Sensitized activation of Fos and brain-derived neurotrophic factor in the medial prefrontal cortex and ventral tegmental area accompanies behavioral sensitization to amphetamine.

Behavioral sensitization, or augmented locomotor response to successive drug exposures, results from neuroadaptive changes contributing to addiction. Both the medial prefrontal cortex (mPFC) and ventral tegmental area (VTA) influence behavioral sensitization and display increased immediate-early gene and BDNF expression after psychostimulant administration. Here we investigate whether mPFC neurons innervating the VTA exhibit altered Fos or BDNF expression during long-term sensitization to amphetamine. Male Sprague-Dawley rats underwent unilateral intra-VTA infusion of the retrograde tracer Fluorogold (FG), followed by 5 daily injections of either amphetamine (2.5 mg/kg, i.p.) or saline vehicle. Four weeks later, rats were challenged with amphetamine (1.0 mg/kg, i.p.) or saline (1.0 mL/kg, i.p.). Repeated amphetamine treatment produced locomotor sensitization upon drug challenge. Two hours later, rats were euthanized, and mPFC sections were double-immunolabeled for either Fos-FG or Fos-BDNF. Tissue from the VTA was also double-immunolabeled for Fos-BDNF. Amphetamine challenge increased Fos and BDNF expression in the mPFC regardless of prior drug experience, and further augmented mPFC BDNF expression in sensitized rats. Similarly, more Fos-FG and Fos-BDNF double-labeling was observed in the mPFC of sensitized rats compared to drug-naïve rats after amphetamine challenge. Repeated amphetamine treatment also increased VTA BDNF, while both acute and repeated amphetamine treatment increased Fos and Fos-BDNF co-labeling, an effect enhanced in sensitized rats. These findings point to a role of cortico-tegmental BDNF in long-term amphetamine sensitization.

cAMP response element binding protein phosphorylation in nucleus accumbens underlies sustained recovery of sensorimotor gating following repeated D2-like receptor agonist treatment in rats.

BACKGROUND: Prepulse inhibition (PPI) is a cross-species measure of sensorimotor gating. PPI deficits are observed in humans and rats upon acute treatment with dopamine D2-like receptor agonists and in patients with schizophrenia. Repeated treatment with a D2-like agonist, however, reverses PPI deficits and increases cyclic adenosine monophosphate (cAMP) signaling in the nucleus accumbens (NAc). This study examined the short- and long-term effects on PPI of treatment with quinpirole and ropinirole, dopamine D2/D3 receptor agonists, and the molecular mechanism by which they occur. METHODS: PPI was assessed in adult male Sprague-Dawley rats following acute and chronic treatment with quinpirole or ropinirole and 1, 2, 3, and 4 weeks after termination of repeated ropinirole treatment. Finally, the effect of dominant negative mutant cAMP response element binding protein (CREB) overexpression in the NAc on PPI following chronic quinpirole treatment was assessed. RESULTS: Acute quinpirole produced dose-dependent PPI deficits, whereas ropinirole caused consistent PPI reduction at all but the highest dose. Repeated ropinirole treatment significantly increased PPI compared with acute treatment, and increased CREB phosphorylation in NAc neurons. Subsequent ropinirole challenge had no effect as long as 28 days later, at which time NAc CREB phosphorylation had normalized. Overexpression of dominant negative mutant CREB prevented PPI recovery induced by chronic quinpirole treatment. CONCLUSIONS: Chronic quinpirole or ropinirole treatment produces sustained PPI recovery; CREB activity in the NAc is required to induce PPI recovery but not to maintain it. The results suggest that transcriptional regulation by CREB mediates long-lasting changes occurring within NAc circuits to promote recovery of sensorimotor gating.

Long-lasting alteration in mesocorticolimbic structures after repeated social defeat stress in rats: time course of mu-opioid receptor mRNA and FosB/DeltaFosB immunoreactivity.

Social defeat stress is a salient stressor that induces neuroadaptive changes in the mesocorticolimbic dopaminergic system. Substantial evidence indicates that mu-opioid receptors (MORs) modulate dopamine transmission in the ventral tegmental area (VTA). FosB/DeltaFosB protein accumulation in dopaminergic projections during repeated treatments is thought to be involved in long-term neuroplasticity. In this study we characterize the magnitude and time-course of MOR mRNA expression and FosB/DeltaFosB immunoreactivity in mesocorticolimbic regions following repeated social defeat stress. Effects of brief repeated social defeat stress or control handling procedures were studied in rats either 2 h after the last exposure, or 3, 7, 14, 21 and 28 days later. We found that MOR mRNA expression in the VTA doubled after the last stress compared with handling, and remained 30-70% higher until day 21. The number of FosB/DeltaFosB-labeled neurons in regions of the frontal cortex, nucleus accumbens (NAc) shell and core, and in the medial, central and basolateral amygdala increased significantly immediately after the last stress episode, and remained enhanced for 21 days. Another group of rats received bilateral intra-VTA infusion of the MOR agonist, DAMGO, 7 days after the last stress. Prior social defeat stress augmented DAMGO-induced Fos expression in the NAc shell, suggesting that Fos expression in this region might be the direct result of MOR activity in the VTA. Social defeat stress leads to an increased capacity for MOR activation in the VTA, which may be relevant to enduring FosB/DeltaFosB expression in mesocorticolimbic areas and to the behaviorally sensitized response to psychostimulant drugs.

Heritable differences in the dopaminergic regulation of behavior in rats: relationship to D2-like receptor G-protein function.

We reported heritable differences between Sprague-Dawley (SD) and Long Evans (LE) rats in their sensitivity to the disruption of prepulse inhibition of startle (PPI) by dopamine (DA) agonists, and in their basal levels and turnover of forebrain DA. In an effort to better understand these differences, we assessed strain patterns in the efficacy of D2-like receptor-G-protein coupling using [35S]GTPgammaS binding in brain regions that contribute to the dopaminergic regulation of PPI. Sensitivity to the PPI-disruptive effects of apomorphine (APO) was examined in SD, LE, and F1 (SD x LE) rats. Basal and DA-stimulated [35S]GTPgammaS binding were then assessed in these rats using conditions that preferentially exclude Gs proteins to favor visualization of D2-like receptors. To explore the behavioral specificity of these strain differences, locomotor responses to APO and amphetamine (AMPH) were also assessed in SD, LE, and F1 rats. Strain differences were evident in the PPI-disruptive effects of APO (SD>F1>LE), and in the locomotor responses to AMPH (LE>F1>SD) and APO (SD exhibited motor suppression, LE exhibited motor activation). Compared to SD rats, LE rats exhibited greater DA-stimulated [35S]GTPgammaS binding in nucleus accumbens and caudatoputamen, while F1 progeny had intermediate levels. In conclusion, SD and LE rats exhibit heritable differences in D2-mediated behavioral and biochemical measures. Conceivably, genes that regulate heritable differences in forebrain D2 function may contribute to heritable differences in PPI in patients with specific neuropsychiatric disorders, including schizophrenia and Tourette Syndrome.

Two quantitative trait loci for prepulse inhibition of startle identified on mouse chromosome 16 using chromosome substitution strains.

Prepulse inhibition (PPI) of acoustic startle is a genetically complex quantitative phenotype of considerable medical interest due to its impairment in psychiatric disorders such as schizophrenia. To identify quantitative trait loci (QTL) involved in mouse PPI, we studied mouse chromosome substitution strains (CSS) that each carry a homologous chromosome pair from the A/J inbred strain on a host C57BL/6J inbred strain background. We determined that the chromosome 16 substitution strain has elevated PPI compared to C57BL/6J (P = 1.6 x 10(-11)), indicating that chromosome 16 carries one or more PPI genes. QTL mapping using 87 F(2) intercross progeny identified two significant chromosome 16 loci with LODs of 3.9 and 4.7 (significance threshold LOD is 2.3). The QTL were each highly significant independently and do not appear to interact. Sequence variation between B6 and A/J was used to identify strong candidate genes in the QTL regions, some of which have known neuronal functions. In conclusion, we used mouse CSS to rapidly and efficiently identify two significant QTL for PPI on mouse chromosome 16. The regions contain a limited number of strong biological candidate genes that are potential risk genes for psychiatric disorders in which patients have PPI impairments.

Prolonged effects of repeated social defeat stress on mRNA expression and function of mu-opioid receptors in the ventral tegmental area of rats.

Social defeat stress alters the activity of mesocorticolimbic dopamine projections from the ventral tegmental area (VTA), a process that has been implicated in the development of sensitization and drug-seeking behavior. We showed previously that acute brief social defeat stress increased short-term expression of mu-opioid receptor mRNA in the VTA. The present study assessed the presence and functional significance of mu-opioid receptor mRNA expression 1 week after the last episode of social defeat stress. Social defeat stress was induced in intruder rats during short confrontations with an aggressive resident rat, and subsequent exposures behind a protective screen once a day for 5 days. Regional mu-receptor mRNA levels were assessed by in situ hybridization histochemistry, and the amount of mRNA labeling was measured in the VTA and the substantia nigra (SN). Expression of mu-opioid receptor mRNA was significantly higher in defeated rats relative to handled control animals in the VTA, but not in the SN. In an additional group of rats, bilateral local intra-VTA injection of the selective mu-opioid receptor agonist DAMGO (1.0 microg per side) was performed 7-10 days after the last defeat stress or handling control procedure. Baseline motor activity did not differ between control and stressed rats. Intra-VTA DAMGO significantly increased locomotor activity in stressed rats compared to handled control rats. These results suggest that repeated social stress upregulates VTA mu-opioid receptors and can produce locomotor activation via stimulation of these receptors. This locomotor effect is probably the consequence of enhanced disinhibition of mesolimbic dopamine neurons.

Brief social defeat stress: long lasting effects on cocaine taking during a binge and zif268 mRNA expression in the amygdala and prefrontal cortex.

Social stress can engender behavioral and neural sensitization and this process appears to enhance the transition to compulsive drug abuse. Exposures to brief social defeat stress in rats have significant consequences on cocaine-reinforced behavior and on the level of functional activation within regions of the mesocorticolimbic dopamine system. The objectives of the current study were to examine the enduring consequences of brief episodes of social defeat stress on cocaine bingeing (during 24 h of continuous access) and on the emergence of neural adaptations as revealed by zif268 immediate early gene expression. Adult, male Long-Evans rats were subjected to four 25 min episodes of social defeat (once every 72 h). After 2 months, cocaine binges or zif268 mRNA gene expression were studied after confirming behavioral cross-sensitization to stimulant challenge. Sensitization to social defeat increased cocaine intake during a 24 h binge, effectively abolishing the typical circadian pattern of intake. Furthermore, 60 days after exposure to the sensitizing regimen of social defeat, levels of functional activation, measured by zif268 mRNA expression, in the central and medial amygdala were increased, while levels of activation in the medial prefrontal cortex were decreased. Persistent stress-induced levels of zif268 in the central and medial amygdala were attenuated by an injection of amphetamine (1.0 mg/kg). Divergent changes in zif268 within the amygdala and cortex 2 months after social defeat stress indicate the vulnerability of distinct cellular populations in networks that modulate the behavioral actions of psychomotor stimulants.

Repeated quinpirole treatment increases cAMP-dependent protein kinase activity and CREB phosphorylation in nucleus accumbens and reverses quinpirole-induced sensorimotor gating deficits in rats.

Sensorimotor gating, which is severely disrupted in schizophrenic patients, can be measured by assessing prepulse inhibition of the acoustic startle response (PPI). Acute administration of D2-like receptor agonists such as quinpirole reduces PPI, but tolerance occurs upon repeated administration. In the present study, PPI in rats was reduced by acute quinpirole (0.1 mg/kg, s.c.), but not following repeated quinpirole treatment once daily for 28 days. Repeated quinpirole treatment did not alter the levels of basal-, forskolin- (5 microM), or SKF 82958- (10 microM) stimulated adenylate cyclase activity in the nucleus accumbens (NAc), but significantly increased cAMP-dependent protein kinase (PKA) activity. Phosphorylation of cAMP response element-binding protein (CREB) was significantly greater in the NAc after repeated quinpirole treatment than after repeated saline treatment with or without acute quinpirole challenge. Activation of PKA by intra-accumbens infusion of the cAMP analog, Sp-cAMPS, prevented acute quinpirole-induced PPI disruption, similar to the behavioral effect observed following repeated quinpirole treatment. Thus, repeated quinpirole treatment increases NAc PKA activity and CREB phosphorylation, and this neuroadaptive response might facilitate the recovery of sensorimotor gating in schizophrenia.

Aggression and defeat: persistent effects on cocaine self-administration and gene expression in peptidergic and aminergic mesocorticolimbic circuits.

The question of how ostensibly aversive social stress experiences in an aggressive confrontation can persistently increase intense drug taking such as cocaine 'bingeing' needs to be resolved. The biology of social conflict highlights distinctive behavioral, cardiovascular and endocrine profiles of dominant and subordinate animals, as seen also in rodents and primates under laboratory conditions. In contrast to continuous subordination stress that produces chronic pathophysiological consequences and often is fatal, animals adapt to brief episodes of social defeat stress, but show enduring functional activation in mesocorticolimbic microcircuits. Uncontrollable episodes of social defeat stress produce long-lasting tolerance to opiate analgesia and, concurrently, behavioral sensitization to challenges with either amphetamine or cocaine. One week after a single social defeat stress, cross-sensitization to cocaine is evident in terms of enhanced motor activity as well as in terms of increased Fos labeling in the periaqueductal grey area, the locus coeruleus, and the dorsal raphe nuclei. When challenged with a low amphetamine dose, the behavioral and neural effects of repeated brief episodes of social defeat stress persist for months. Previous exposure to social defeat stress can (1). significantly shorten the latency to acquire cocaine self-administration, (2). maintain this behavior at low cocaine unit doses, (3). significantly increase the levels of cocaine taking during a 24 h binge of continuous drug availability, (4). dysregulate the timing of consecutive infusions, and (5). abolish the circadian pattern of self-administration. Amygdaloid modulation, especially originating from central and basolateral nuclei, of dopaminergic pathways via peptidergic and glutamatergic neurons appears to be a key mechanism by which social defeat stress affects cocaine self-administration. Social stress alters the feedback from prefrontal cortex and thereby may contribute to the dysregulation of dopaminergic activity that is necessary for cocaine self-administration.