Dopamine reward dysfunction and cocaine-seeking in a rat model of PTSD

We hypothesize that posttraumatic stress disorder (PTSD) results in reduced dopaminergic neurotransmission which may contribute to deficient reward function and vulnerability to drug-seeking behavior. We combined single-prolonged stress (SPS) with a series of behavioral and neuropharmacological assays to assess dopaminergic reward function and cocaine intake.;A modified SPS procedure consisting of 2 hours of restraint, 20 minutes of group swimming, isoflurane exposure until loss consciousness, and 7 days of isolation was used to induce severe stress in our studies. Initial studies were conducted to examine the effect of SPS on cocaine-conditioned reward and anhedonia-like behavior in adult male Sprague-Dawley rats. Using a biased conditioned place preference paradigm, unstressed controls demonstrated a significant preference for the cocaine-paired context following four pairings with cocaine (5--20 mg/kg, i.p.). Preference for the cocaine-paired side was significantly lower in rats exposed to SPS, suggesting a deficit in the rewarding properties of cocaine following exposure to severe stress. Anhedonia-like behavior was assessed by a two-bottle choice sucrose preference test. Robust consumption of sucrose solution (0.25--1%) was observed in rats that underwent control handling, however, SPS significantly reduced sucrose intake compared to controls. Finally, basal behavioral activity in SPS rats was compared to unstressed controls in a 24-hour test. Results indicate a significant reduction in spontaneous nocturnal activity following SPS versus control handling. In contrast, hyperlocomotion induced by an acute cocaine injection (5--20 mg/kg, i.p.) was unaltered between rats that underwent SPS or control handling.;Intravenous cocaine self-administration was conducted to examine the effect of SPS on the acquisition, motivation, and escalation of cocaine intake. Acquisition of cocaine self-administration was studied using an escalating dose regimen in which rats had sequential access to 0.1875, 0.375, and 0.75 mg/kg/infusion on a fixed-ratio 1 schedule of reinforcement. Rats exposed to SPS did not significantly differ from control handled animals in the latency to meet acquisition criteria or the general pattern and level of cocaine intake at each dose. A subsequent study assessing the breakpoint for cocaine self-administration using a progressive-ratio schedule of reinforcement determined a dose-dependent increase in motivation to work for cocaine (0--1.5 mg/kg/infusion) across both experimental groups. However, motivation to obtain cocaine was similar between SPS and unstressed rats, as there was no significant difference in breakpoint for cocaine self-administration at any dose of cocaine tested. To evaluate potential differences in the transition to escalated cocaine intake, self-administration was measured using an extended-access procedure in which unlimited cocaine (0.375 mg/kg/infusion) was available for six hours daily. Upon extended-access to cocaine, SPS significantly attenuated cocaine intake compared to control handling over 14 sessions. Despite a significant reduction in cocaine intake, rats exposed to SPS still significantly escalated their cocaine intake over the course of 14 days.;To understand the mechanisms of reduced reward and behavior in the SPS model of PTSD, a series of neurochemical assays was used to assess the ability of SPS to induce dysfunction of dopaminergic neurotransmission. Using high performance liquid chromatography, tissue levels of dopamine and the dopamine metabolites DOPAC and HVA were measured immediately and one week following SPS or control handling. Tissue obtained from SPS rats demonstrated significant decreases in dopamine, DOPAC, and HVA content in both the nucleus accumbens and caudate putamen immediately following SPS and one week later. Quantitative autoradiography was used measure the density of dopamine transporters and dopamine D1 and D2 receptors. [3H]WIN35428 binding to dopamine transporters was higher in the nucleus accumbens of SPS rats compared to controls. The level of [3H]WIN35428 binding in the caudate putamen was not different between groups. [3H]Raclopride binding to D2 receptors was significantly reduced in both the nucleus accumbens and caudate putamen following SPS versus control handling.;A preliminary functional assessment of dopamine transporters revealed a significant increase in dopamine uptake in the nucleus accumbens of SPS rats compared to controls, whereas uptake in the caudate putamen was unaltered between groups. Enhanced dopamine uptake following SPS is consistent with the increase in dopamine transporter density observed in the nucleus accumbens of SPS rats. Activation of D1 receptors and G-protein mediated transduction was assessed using an adenylyl cyclase assay with the D1 agonist SKF82958. In the caudate putamen, a significant decrease in D1 receptor-stimulated cAMP production was revealed in SPS rats compared to controls, whereas SKF82958-induced cAMP was unchanged in the nucleus accumbens. Finally, the function of D2 dopamine receptors was assessed by D2 receptor-stimulated [35S]GTPgammaS binding using quinpirole. In the caudate putamen, [35S]GTPgammaS binding following stimulation of D2 receptors was enhanced by SPS compared to control handling, whereas no difference was observed between groups in the nucleus accumbens. (Abstract shortened by UMI.).