Abstract

Opioid mu, kappa and delta receptors are present in significant densities in the ventral pallidum (VP). To examine their contribution to VP neuronal activity, changes in firing rate during microiontophoresis of the receptor-selective agonists [D-Ala2,N-Me-Phe4,Gly-ol5]-enkephalin (DAMGO) (mu), [D-Pen2,5]-enkephalin (DPDPE) (delta) and trans-(+/-)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl) cy-clohexyl]-benzene-acetamide methane sulfonate (U50488H) (kappa), and the antagonists D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 (CTOP) (mu) and norbinaltorphimine (kappa) were determined in chloral hydrate-anesthetized rats. A majority of the neurons demonstrated ejection current-dependent decreases in neuronal activity to DAMGO and U50488H. The rate suppressions were attenuated by coiontophoresis of the homotypic antagonist, indicating receptor subtype-specificity of the responses. In contrast, DPDPE decreased firing in only 24% of the recorded neurons. In those neurons tested with all three agonists, nearly 70% were sensitive to at least one. Among responding neurons, approximately one-quarter was influenced by activation of all three receptor subtypes while another quarter was sensitive to only mu activation. Thus, subpopulations of VP neurons may exist according to the influence of particular opioid receptor subtypes. These findings were compared to the nonselective opioid, morphine. Morphine iontophoresis elicited both excitations and inhibitions whereas DAMGO exclusively inhibited the same VP neurons. Responses to both were antagonized by naloxone and CTOP, indicating mu receptor-specific actions. The results are discussed in terms of differential direct and indirect effects of morphine and DAMGO. In summary, mu, delta and kappa opioid receptors can independently alter neuronal activity within the VP, and direct and indirect effects are most likely involved.