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

Abstract

Posttraumatic stress disorder (PTSD) co-occurs with substance use disorders at high rates, but the neurobiological basis of this relationship remains largely unknown. Identifying mechanisms that underlie this association is necessary, and recognizing pathologies shared by these disorders may provide pertinent information in understanding their functional relationship. Separate lines of evidence suggest that PTSD and drug addiction may share a common feature, that is, dysregulation of the brain's reward circuitry. We hypothesize that PTSD results in reduced dopaminergic neurotransmission which may contribute to deficient reward function and vulnerability to drug-seeking behavior. To address this hypothesis, we combined single-prolonged stress (SPS), a rodent model of PTSD, with a series of behavioral and neuropharmacological assays to assess dopaminergic reward function and cocaine intake. The results of the studies presented herein extend our understanding of the effects of severe stress on drug reinforcement and consumption, and establish a potential mechanism by which PTSD produces deficient reward function through alterations in the dopamine system. 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. These results suggest an increase in anhedonia-like behavior or a reduction in the rewarding effects of sucrose as a non-drug reinforcer. 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. These data suggest that deficient behavioral activity may be specific to voluntary movements or behavior, and support an increase in anhedonia following exposure to SPS. 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 (consumption of 6.75 mg/kg/day for 3 consecutive days) 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. These results suggest that escalation of cocaine intake occurred in the presence of lower total doses of cocaine in the SPS exposed animals compared to controls. In addition, SPS rats demonstrated a greater percent increase in cocaine consumption compared to controls. This finding suggests that rats exposed to SPS compensated for a decrease in cocaine reinforcement by escalating their intake to a greater magnitude than controls. These studies indicate that SPS may not alter the acquisition of cocaine self-administration or motivation for cocaine. However, the finding of reduced cocaine intake upon extended-access in SPS rats is consistent with a deficit in cocaine-induced reward. The ability of SPS rats to escalate cocaine intake in the presence of less cocaine, or a greater magnitude of escalated cocaine intake than controls, may reflect mechanisms leading to enhanced vulnerability to cocaine abuse. 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, suggesting a potential deficit in dopaminergic tone. 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, suggesting an increase in dopamine transporter density following severe stress. 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. These results suggest a decrease in the density of striatal D2 receptors. D1 receptor expression was not significantly altered by SPS, as no significant difference in [3H]SCH23390 binding was detected in SPS rats compared to controls. 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]GTPγS binding using quinpirole. In the caudate putamen, [35S]GTPγS 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. These results indicate increased D2 receptor-mediated activation of G-proteins in the caudate putamen following SPS. In summary, the studies described herein tested the hypothesis that reduced dopaminergic function may be a mechanism for deficient reward and heightened susceptibility to drug use in PTSD. Results demonstrated a significant reduction in cocaine-conditioned reward, as well as attenuated sucrose preference and spontaneous activity in rats exposed to SPS. These findings are consistent with the presence of a dysfunctional reward system which may contribute to anhedonia-like behavior in PTSD. Furthermore, reward deficits may promote altered patterns of cocaine taking behavior and vulnerability to substance abuse. Results demonstrated significant escalation of drug intake following exposure to SPS, which occurred in the presence of less cocaine than controls. A greater increase in cocaine intake was observed in SPS rats over the course of escalation, which may reflect a mechanism for enhanced vulnerability to the development of a substance use disorder in PTSD. Dopaminergic dysfunction may contribute to deficient reward capacity and an altered pattern of cocaine intake in SPS. SPS-induced alterations in dopamine function included a reduction in striatal dopamine content alongside enhanced dopamine transporter levels and function. Mild alterations in D2 receptor density and the function of D1 and D2 receptors were also observed. These findings support the hypothesis that PTSD results in reduced dopaminergic neurotransmission, which may contribute to deficient reward function and altered drug-seeking behavior. Identifying the pathology of PTSD, such as altered dopamine neurotransmission, may lead to enhanced treatment strategies and interventions to prevent substance abuse in persons with PTSD.