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 (C(m)) 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 κ-opioid agonists U69,593 (0.3-1 μM), dynorphin A (1 μM), or U50,488H (1-3 μM) 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 μ-opioid agonist [D-Pen^2,5]-enkephalin (1-3 μM; n = 7) or of the δ- agonist Tyr-D-Ala-Gly-N-Me-Phe-Gly-ol (1 μM; n = 4) on Ca²⁺ currents. Three components of high-threshold current were distinguished on the basis of their sensitivity to blockade by ω-conotoxin GVIA, nicardipine, and ω- 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 C(m) in response to brief depolarizing steps revealed that the κ-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.