The overall objective of this project is to understand the physiological and pathological mechanisms of opioid-mediated plasticity in the neural circuitry controlling serotonin (5-HT) release in the mammalian forebrain, and thereby provide a sound basis for understanding and treatment of psychiatric disorders such as depression and drug addiction. The experimental approach combines behavioral measures with microdialysis in the CNS of freely behaving rats. We propose that mu, delta, kappa and ORL-1 opioids have different roles in regulation of 5-HT release. By reverse microdialysis infusion of selective opioid receptor agonists into the dorsal raphe nucleus (DRN) we will test the specific hypothesis that mu- and delta-opioids produce increases, and kappa- and ORL-1 decreases in 5-HT release. We further hypothesize that opioids have regionally selective effects on 5-HT. Preliminary data indicate that mu- and delta- opioids act in the DRN to increase 5-HT release in specific forebrain projection sites. In contrast, kappa-opioids may inhibit 5-HT release by acting on terminals in widespread areas of the brain including the DRN, median raphe nucleus, nucleus accumbens and dorsal hippocampus. The hypothesis that the increase in 5-HT elicited by mu-opioids is mediated indirectly by neural circuitry will also be examined. Specifically, mu-opioids may inhibit both GABAergic and glutamatergic inputs to 5-HT neurons in the DRN. This hypothesis will be investigated by using selective GABA and glutamate receptor antagonists to characterize the tonic activity of inhibitory and excitatory influences on 5-HT neurons. Preliminary data indicate a strong tonic influence of GABA compared with a relatively weak effect of glutamate in the DRN. We propose experiments to further examine the hypothesis that the effect of mu-opioids on 5-HT can be attributed to direct inhibition of both GABA and glutamate neurons. Because, GABA tone is predominant, the net effect of inhibiting both inputs is an increase in 5-HT release. In contrast, kappa- and ORL-1-opioids may directly inhibit 5-HT neurons. Prolonged opioid treatment may produce plasticity in the strength of synaptic connections in the raphe and thus affect the regulation of 5-HT release. To examine this hypothesis, opioid dependence will be induced by either direct infusion of opioids into the DRN for 4 days or by subcutaneous implantation of morphine pellets. Based on our previous observations, we will test the hypothesis that mu-opioids cause adaptations in 5-HT release by enhancement of GABA inputs. Similarly, the possibility of enhanced glutamate synaptic transmission will be tested. By determining changes in 5-HT during withdrawal, we will test the specific hypothesis that the enhancement in GABA exceeds the change in glutamate. This may result in a more pronounced net inhibitory effect of afferent inputs, thus decreased 5-HT release after tolerance to opioids develops. Further experiments will test the hypothesis that the excitability of 5-HT neurons is increased after prolonged treatment with ORL-1 opioids. Finally, we will test the hypothesis that 5-HT release is enhanced after regular exercise and this may ameliorate the adverse consequences of prolonged opioid treatment. Recent data suggest that the increase in 5-HT release associated with treadmill locomotion is enhanced after regular exercise. To examine the role of exercise in alleviating behavioral depression, opioid-dependent rats will be exercised regularly on a treadmill. We hypothesize that regular exercise results in an increase in the strength of glutamate relative to GABA connections with 5-HT neurons in the DRN, and that this may normalize the enhancement in net inhibitory associated with prolonged exposure to opioids.
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