κ-Opioid Receptor Activation Modulates Ca²⁺Currents and Secretion in Isolated Neuroendocrine Nerve Terminals

Whole-cell patch-clamp recordings were performed together with time-resolved measurements of membrane capacitance (Cm) in nerve terminals acutely dissociated from neurohypophysis of adult rats to investigate modulation of Ca2+ currents and secretion by activation of opioid receptors. Bath superfusion of the kappa-opioid agonists U69,593 (0.3-1 microM), dynorphin A (1 microM), or U50,488H (1-3 microM) reversibly suppressed the peak amplitude of Ca2+ currents 32. 7 +/- 2.7% (in 41 of 56 terminals), 37.4 +/- 5.3% (in 5 of 8 terminals), and 33.5 +/- 8.1% (in 5 of 10 terminals), respectively. In contrast, tests in 11 terminals revealed no effect of the mu-opioid agonist [D-Pen2,5]-enkephalin (1-3 microM; n = 7) or of the delta-agonist Tyr-D-Ala-Gly-N-Me-Phe-Gly-ol (1 microM; n = 4) on Ca2+ currents. Three components of high-threshold current were distinguished on the basis of their sensitivity to blockade by omega-conotoxin GVIA, nicardipine, and omega-conotoxin MVIIC: N-, L-, and P/Q-type current, respectively. Administration of U69,593 inhibited N-type current in these nerve terminals on average 32%, whereas L-type current was reduced 64%, and P/Q-type current was inhibited 28%. Monitoring of changes in Cm in response to brief depolarizing steps revealed that the kappa-opioid-induced reductions in N-, L-, or P/Q-type currents were accompanied by attenuations in two kinetically distinct components of Ca2+-dependent exocytotic release. These data provide strong evidence of a functional linkage between blockade of Ca2+ influx through voltage-dependent Ca2+ channels and inhibitory modulation of release by presynaptic opioid receptors in mammalian central nerve endings.