IDENTIFICATION OF THE OPIOID RECEPTOR TYPES MEDIATING BETA-ENDORPHIN-INDUCED ALTERATIONS IN DOPAMINE RELEASE IN THE NUCLEUS-ACCUMBENS

In the present study we used in vivo microdialysis to examine the influence of β-endorphin on dopamine (DA) release in the nucleus accumbens of anesthetized rats and to identify the opioid receptor types mediating its effects. Microdialysis probes were inserted into the nucleus accumbens and perfusates were analysed for DA and its metabolites, dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), using a reversed phase HPLC system with electrochemical detection for separation and quantification. Intracerebroventricular (i.c.v.) administration of β-endorphin resulted in a dose-dependent increase in DA and its metabolites. Pretreatment with the selective δ-antagonist ICI 174,864 significantly attenuated the β-endorphin-induced increase in DA release and metabolism whereas pretreatment with the selective μ-antagonist CTOP resulted abolition of the β-endorphin effect. These data demonstrate that the blockade of either μ- or δ-opioid receptors is sufficient to antagonize the stimulatory effects of β-endorphin on DA release and metabolism. As such these findings suggest that the concomitant activation of both μ-and δ-receptors underlies the effects of β-endorphin on DA release in the nucleus accumbens. © 1990.