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

Abstract

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 Ca 2ϩ currents and secretion by activation of opioid receptors. Bath superfusion of the -opioid agonists U69,593 (0.3-1 M), dynorphin A (1 M), or U50,488H (1-3 M) reversibly suppressed the peak amplitude of Ca 2ϩ currents 32.7 Ϯ 2.7% (in 41 of 56 terminals), 37.4 Ϯ 5.3% (in 5 of 8 terminals), and 33.5 Ϯ 8.1% (in 5 of 10 terminals), respectively. In contrast, tests in 11 terminals revealed no effect of the -opioid agonist [D-Pen 2,5 ]-enkephalin (1-3 M; n ϭ 7) or of the ␦-agonist Tyr-D-Ala-Gly-N-Me-PheGly-ol (1 M; n ϭ 4) on Ca 2ϩ currents. Three components of high-threshold current were distinguished on the basis of their sensitivity to blockade by -conotoxin GVIA, nicardipine, and -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 -opioid-induced reductions in N-, L-, or P/Q-type currents were accompanied by attenuations in two kinetically distinct components of Ca 2ϩ -dependent exocytotic release. These data provide strong evidence of a functional linkage between blockade of Ca 2ϩ influx through voltagedependent Ca 2ϩ channels and inhibitory modulation of release by presynaptic opioid receptors in mammalian central nerve endings.

κ-Opioid Receptor Inhibition of Calcium Oscillations in Spinal Cord Neurons

“…From both colocalization and subcellular fractionation studies, it is evident that KORs are present in spinal glutamatergic neurons with a relative abundance in the presynaptic compartment. This localization of KORs in embryonic spinal neurons is consistent with the ability of KOR ligands to modulate neurotransmitter release via a presynaptic mechanism of action (Rusin et al, 1997;Ford et al, 2007;Iremonger and Bains, 2009).…”

Retrograde Opioid Signaling Regulates Glutamatergic Transmission in the Hypothalamus

“…Many of these studies have implicated an opioid-mediated inhibition of presynaptic Ca 2ϩ channels (Rusin et al, 1997;Hjelmstad and Fields, 2003) or the activation of presynaptic K ϩ channels (Simmons and Chavkin, 1996;Vaughan et al, 1997). Other studies, however, rule out Ca 2ϩ channels and/or K ϩ channels (Castillo et al, 1996;Ford et al, 2007), indicating that the nature of a unifying mechanism remains unresolved.…”

Burst Initiation and Termination in Phasic Vasopressin Cells of the Rat Supraoptic Nucleus: A Combined Mathematical, Electrical, and Calcium Fluorescence Study

“…However, a similar functional form to p ϱ has been determined for the calcium dependence of a K ϩ leak conductance in rat hippocampal pyramidal cells (Selyanko and Sim, 1998).…”

“…current to calcium. However, dynorphin is also known to directly inhibit high-voltage-activated calcium channels, and the activation of -opioid receptors in neuroendocrine terminals is known to reduce Ca 2ϩ influx (Rusin et al, 1997). It is therefore possible that dynorphin could instead terminate the burst by manipulating calcium rather than by a downstream modulation of the DAP.…”