Interactions among mu- and delta-opioid receptors, especially putative delta1- and delta2-opioid receptors, promote dopamine release in the nucleus accumbens.

The effect of interactions among mu- and delta-opioid receptors, especially the putative delta(1)- and delta(2)-opioid receptors, in the nucleus accumbens on accumbal dopamine release was investigated in awake rats by in vivo brain microdialysis. In agreement with previous studies, perfusion of the nucleus accumbens with the mu-, delta(1)- and delta(2)-opioid receptor agonists [D-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin (DAMGO), [D-Pen(2,5)]-enkephalin (DPDPE) and [D-Ser(2)]Leu-enkephalin-Thr(6), respectively, significantly enhanced the extracellular amount of accumbal dopamine in a dose-related manner (5.0 nmol and 50.0 nmol). However, the highest concentration tested (50.0 nmol) of DAMGO induced a biphasic effect, i.e. a rapid onset increase lasting for 75 min followed by a slower onset gradual and prolonged increase. The mu-opioid receptor antagonist D-Phe-Cys-Tyr-d-Trp-Orn-Thr-Phe-Thr-NH(2) (0.15 nmol) primarily reduced the DAMGO-induced second component. The delta(1)-opioid receptor antagonist (E)-7-benzylidenenaltrexone (0.15 nmol) significantly reduced the first component and abolished the second component induced by DAMGO, while the delta(2)-opioid receptor antagonist naltriben (1.5 nmol) significantly reduced only the first component. The DPDPE (50.0 nmol)-induced dopamine increase was almost completely abolished by (E)-7-benzylidenenaltrexone, but only partially reduced by D-Phe-Cys-Tyr-d-Trp-Orn-Thr-Phe-Thr-NH(2) and naltriben. The [D-Ser(2)]Leu-enkephalin-Thr(6) (50.0 nmol)-induced dopamine increase was almost completely abolished by naltriben, but not at all by D-Phe-Cys-Tyr-d-Trp-Orn-Thr-Phe-Thr-NH(2) and (E)-7-benzylidenenaltrexone. The non-selective opioid receptor antagonist naloxone (0.75 and 1.5 nmol) dose-dependently reduced the effects of DAMGO, DPDPE and [D-Ser(2)]Leu-enkephalin-Thr(6) but only to about 10-25% of the control values. Moreover, perfusion with the sodium channel blocker tetrodotoxin (0.1 nmol) reduced the DAMGO-induced dopamine increase by 75%, while it almost completely abolished the increase induced by DPDPE or [D-Ser(2)]Leu-enkephalin-Thr(6). The results show that stimulation of mu-opioid receptors or, to a lesser degree, delta(1)-opioid receptors results in a large naloxone-sensitive increase and a small naloxone-insensitive increase of extracellular dopamine. It is suggested that the naloxone-insensitive component is also tetrodotoxin-insensitive. Furthermore, it is hypothesized that stimulation of mu-opioid receptors activates delta(1)-receptors, which in turn activate delta(2)-opioid receptors, thereby giving rise to a rapid onset increase of extracellular dopamine. In addition, it is hypothesized that stimulation of another group of mu-opioid receptors activates a second group of delta(1)-opioid receptors that is not coupled to delta(2)-opioid receptors and mediates a slow onset increase of extracellular dopamine. Finally, it is suggested that stimulation of delta(1)- or delta(2)-opioid receptors inhibits mu-opioid receptors involved in the slow onset increase in extracellular dopamine, whereas stimulation of delta(1)-, but not delta(2)-, opioid receptors is suggested to activate mu-opioid receptors involved in the rapid increase in extracellular dopamine.