Differential Involvement of Ventral Tegmental Mu, Delta and Kappa Opioid Receptors in Modulation of Basal Mesolimbic Dopamine Release: in Vivo Microdialysis Studies

In vivo microdialysis was used to assess the involvement of ventral tegmental area (VTA) mu, delta, and kappa opioid receptors in modulation of basal extracellular ventral striatal dopamine (DA) and DA-metabolite concentrations. Independent groups of chloral hydrate-anesthetized rats were given VTA microinjections of selective opioid agonists, and extracellular ventral striatal DA and DA-metabolite concentrations were assayed using HPLC. VTA microinjections of [D-Ala2, N-Me-Phe4-Gly5-ol]-enkephalin (DAMGO; a mu agonist) and [D-Pen2, D-Pen5]-enkephalin (DDDPE; a delta agonist) each caused dose-orderly increases in ventral striatal DA and DA-metabolite concentrations. The effective concentrations of DPDPE were 100- to 1000-fold higher than the effective concentrations of DAMGO. VTA microinjections of (trans-(dl)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)cyclo-hexyl]- benzeneacetamide) methane sulfonate hydrate (U-50,488H); a kappa agonist) failed to alter ventral striatal DA concentrations at any dose tested, but subsequent systemic injections significantly decreased DA and DA-metabolite concentrations. Pretreatment with VTA microinjections of 17-cyclopropylmethyl-6,7-dehydro-4,5-epoxy-3,14-dihydroxy-6,7,2',3'- indolmorphinan hydrochloride (naltrindole; a delta antagonist) (delta antagonist) antagonized VTA DPDPE-mediated increases in ventral striatal DA and DA-metabolite concentrations but failed to antagonize VTA DAMGO-mediated increases. Pretreatment with D-Pen-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 (CTOP; a mu antagonist) antagonized VTA DAMGO-mediated increases but failed to antagonize VTA DPDPE-mediated increases. Thus both mu and delta receptor agonist appear capable of increasing ventral striatal DA and DA-metabolite concentrations through selective actions on their preferred class of opioid receptors in the VTA. The increases in ventral striatal DA and DA-metabolite concentrations that are seen after systemic treatment with kappa opioid agonists appear not to involve VTA opioid receptors.