Endocannabinoid signaling as an intrinsic component of the circuits mediating adaptive responses to repeated stress exposure in adult male sprague dawley rats.

Evidence implicates the endocannabinoid (eCB) system as a negative modulator of neural and endocrine responses to acute stressors. Recently, eCB signaling was also reported to contribute to habituation of hypothalamo-pituitary-adrenal (HPA) axis responses to repeated homotypic stress. The present studies were initiated to distinguish a potential role of eCB signaling in the expression vs. the acquisition of habituation of the HPA axis response to repeated stress. In each of three experiments, adult male Sprague Dawley rats were exposed to daily, 30-minute sessions of loud white noise (95 dB), which resulted in a progressive decrease in HPA axis response over successive days. Cannabinoid receptor 1 (CB1) antagonist AM251 (0.5, 1.0 or 2.0 mg/kg, i.p.) was used to examine the role of eCB signaling in homotypic stressor habituation and heterotypic (novel) stressor cross-sensitization of neuroendocrine activity. Pretreatment with high dose (2.0 mg/kg) AM251 before each of 7 consecutive, daily loud noise exposures (acquisition of habituation) resulted in potentiation of stress-induced HPA axis activation and disruption of habituation. After an 8th loud noise exposure without AM251 pretreatment, the same group of rats displayed a habituated plasma corticosterone (CORT) level similar to that of controls, indicating that CB1 receptor antagonist pretreatments did not disrupt the acquisition of habituation. In two additional experiments, rats acquired habituation to loud noise drug free, then lower doses of AM251 (0.5 and 1.0 mg.kg) were administered before a final exposure (expression of habituation) to the homotypic stressor and/or a novel heterotypic stressor. CB1 receptor antagonism disrupted the expression of CORT response habituation and some of the c-fos mRNA reduction associated with it and facilitated novel stressor sensitization in doses that did not potentiate acute responses to these stressors. Collectively, these data suggest a progressive intensification of neural eCB signaling at CB1 receptors with repeated stress exposures.

Role of dopamine D2-like receptors and their modulation by adenosine receptor stimulation in the reinstatement of methamphetamine seeking.

RATIONALE AND OBJECTIVE: Previous work has demonstrated that dopamine and adenosine receptors are involved in drug-seeking behaviors, yet the pharmacological interactions between these receptors in methamphetamine (MA) seeking are not well characterized. The present studies examined the role of the dopamine D2-like receptors in MA seeking and identified the interactive effects of adenosine receptor stimulation. METHODS: Adult male Sprague-Dawley rats were trained to lever press for MA in daily 2-h self-administration sessions on a fixed-ratio 1 schedule for 10 consecutive days. After 1 day of abstinence, lever pressing was extinguished in six daily extinction sessions. Treatments were administered systemically prior to a 2-h reinstatement test session. RESULTS: An increase in MA seeking was observed following the administration of the dopamine D2-like agonist, quinpirole, or the D3 receptor agonist, 7-OH-DPAT. Stimulation of D2 or D4 receptors was ineffective at inducing MA seeking. Quinpirole-induced MA seeking was inhibited by D3 receptor antagonism (SB-77011A or PG01037), an adenosine A1 agonist, CPA, and an adenosine A2A agonist, CGS 21680. MA seeking induced by a MA priming injection or D3 receptor stimulation was inhibited by a pretreatment with the adenosine A1 agonist, CPA, but not the adenosine A2A agonist, CGS 21680. CONCLUSIONS: These results demonstrate the sufficiency of dopamine D3 receptors to reinstate MA seeking that is inhibited when combined with adenosine A1 receptor stimulation.

Methamphetamine self-administration reduces alcohol consumption and preference in alcohol-preferring P rats.

Subclinical levels of polysubstance use are a prevalent and understudied phenomenon. Alcohol is a substance commonly co-used with other substances of other drug classes. These studies sought to determine the consumption effects of combining alcohol drinking and methamphetamine (MA) self-administration. Male alcohol-preferring P rats had continuous access to a two-bottle alcohol drinking procedure in the home cage. Control rats remained alcohol naïve. Rats were also surgically implanted with intra-jugular catheters and trained to self-administer saline (control) or MA in daily 2-hour sessions. We first measured the acquisition and maintenance of MA intake in alcohol-consuming or control rats. MA intake was initially enhanced by alcohol consumption on a fixed ratio 1 schedule of reinforcement, but this effect did not prevail as the difficulty of the schedule (FR5 and progressive ratio) was increased. We next measured both alcohol consumption and preference before, during and after MA (or saline) self-administration. MA self-administration significantly reduced alcohol intake and preference ratios, a robust effect that persisted across several experimental variations. Interestingly, alcohol consumption rebounded following the cessation of MA self-administration. The effects of MA self-administration were specific to alcohol intake because it did not alter total fluid consumption or consumption of sucrose. MA self-administration did not impact blood-alcohol concentrations or alcohol-induced loss of righting reflex suggesting no effect of MA intake on the alcohol metabolism or sensitivity. Together, the results suggest that MA intake disrupts alcohol consumption and preferences but not the reverse in alcohol-preferring P rats.

Adolescent caffeine consumption increases adulthood anxiety-related behavior and modifies neuroendocrine signaling.

Caffeine is a commonly used psychoactive substance and consumption by children and adolescents continues to rise. Here, we examine the lasting effects of adolescent caffeine consumption on anxiety-related behaviors and several neuroendocrine measures in adulthood. Adolescent male Sprague-Dawley rats consumed caffeine (0.3g/L) for 28 consecutive days from postnatal day 28 (P28) to P55. Age-matched control rats consumed water. Behavioral testing for anxiety-related behavior began in adulthood (P62) 7 days after removal of caffeine. Adolescent caffeine consumption enhanced anxiety-related behavior in an open field, social interaction test, and elevated plus maze. Similar caffeine consumption in adult rats did not alter anxiety-related behavior after caffeine removal. Characterization of neuroendocrine measures was next assessed to determine whether the changes in anxiety were associated with modifications in the HPA axis. Blood plasma levels of corticosterone (CORT) were assessed throughout the caffeine consumption procedure in adolescent rats. Adolescent caffeine consumption elevated plasma CORT 24h after initiation of caffeine consumption that normalized over the course of the 28-day consumption procedure. CORT levels were also elevated 24h after caffeine removal and remained elevated for 7 days. Despite elevated basal CORT in adult rats that consumed caffeine during adolescence, the adrenocorticotropic hormone (ACTH) and CORT response to placement on an elevated pedestal (a mild stressor) was significantly blunted. Lastly, we assessed changes in basal and stress-induced c-fos and corticotropin-releasing factor (Crf) mRNA expression in brain tissue collected at 7 days withdrawal from adolescent caffeine. Adolescent caffeine consumption increased basal c-fos mRNA in the paraventricular nucleus of the hypothalamus. Adolescent caffeine consumption had no other effects on the basal or stress-induced c-fos mRNA changes. Caffeine consumption during adolescence increased basal Crf mRNA in the central nucleus of the amygdala, but no additional effects of stress or caffeine consumption were observed in other brain regions. Together these findings suggest that adolescent caffeine consumption may increase vulnerability to psychiatric disorders including anxiety-related disorders, and this vulnerability may result from dysregulation of the neuroendocrine stress response system.