DOPAMINE-OPIOID RECEPTOR INTERACTIONS IN BASAL FOREBRAIN

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Simulation of opioid and dopamine receptors in the nucleus accumbens are, at least partly, responsible for increases in locomotor activity and the reinforcing properties of systemically administered opioid and psychostimulant drugs of abuse. Although the opioid system does not depend upon the dopamine system for its actions, the depletion of dopamine in the nucleus accumben augments opioid-induced locomotor activity. The biochemical basis for this augmentation is not known. During the tenure of the previous proposal, the mu opioid receptor subtype was identified as most important in the behavioral augmentation, but there was no change in the number of mu-agonist binding sites or affinity in the lesioned accumbens. Furthermore, these was no alteration in the capacity of mu-opioids to inhibit adenylyl cyclase in lesioned accumbens. In this continuation proposal, the first series of experiments will characterize the opioid receptor subtype involved in behavioral augmentation to endogenous opioids and the time course of this augmentation. The second series of experiments will test the first major hypothesis that the augmentation to mu-opioids in the dopamine-depleted nucleus accumbens results from an alteration in the postsynaptic neuron, either in mu1 opioid receptor binding or transduction mechanisms other than adenylyl cyclase. A role for G proteins or protein kinases will be analyzed. Preliminary data revealing an increase in phorbol ester binding in the dopamine-depleted nucleus accumbens supports the involvement of protein kinase C. The second major hypotheses to be tested is that the opioids affect the GABAergic projection from the nucleus accumbens to the ventral pallidum, which may be augmented in the dopamine-depleted rats. To evaluate this hypothesis, two general experiments will be conducted. In the first, neurons in the nucleus accumbens will be double-labeled with Fluoro-Gold, retrogradely transported from the ventral pallidum, and mRNA for preproenkephalin or glutamic acid decarboxylase (synthetic enzyme for GABA). These mRNA's were upregulated after unilateral lesions. This experiment will determine if these mRNAs are upregulated after bilateral lesions, and if so, whether the ventral pallidal-projecting neurons show upregulation in mRNA. The second experiment will employ in vivo microdialysis to measure extracellular GABA in the ventral pallidum in response to opioid stimulation in the nucleus accumbens. This study will determine if lesions after the capacity of opioids in the nucleus accumbens to modulate GABA in the ventral pallidum. These studies are important because the nucleus accumbens is involved in the reinforcing properties of both opioids and indirect dopamine agonists. Therefore, understanding the mechanisms mediating the augmentation of opioid action following accumbal dopamine depletions may provide mechanistic information on the neurobiology and plasticity in the response of the nucleus accumbens to the abuse of opioid and indirect dopamine agonists.