Voltage‐dependent κ‐opioid modulation of action potential waveform‐elicited calcium currents in neurohypophysial terminals

Abstract

Release of neurotransmitter is activated by the influx of calcium. Inhibition of Ca2+ channels results in less calcium influx into the terminal and presumably a reduction in transmitter release. In the neurohypophysis (NH), Ca2+ channel kinetics, and the associated Ca2+ influx, is primarily controlled by membrane voltage and can be modulated, in a voltage-dependent manner, by G-protein subunits interacting with voltage-gated calcium channels (VGCC). In this series of experiments we test whether the κ- and μ-opioid inhibition of Ca2+ currents in NH terminals is voltage-dependent. Voltage-dependent relief of G-protein inhibition of VGCC can be achieved with either a depolarizing square pre-pulse or by action potential waveforms. Both protocols were tested in the presence and absence of opioid agonists targeting the κ- and μ-receptors in neurohypophysial terminals. The κ-opioid VGCC inhibition is relieved by such pre-pulses, suggesting that this receptor is involved in a voltage-dependent membrane delimited pathway. In contrast, μ-opioid inhibition of VGCC is not relieved by such pre-pulses, indicating a voltage-independent diffusible second-messenger signaling pathway. Furthermore, relief of κ-opioid inhibition during a physiological action potential burst stimulation indicates the possibility of activity-dependent modulation in vivo. Differences in the facilitation of Ca2+ channels due to specific G-protein modulation during a burst of action potentials may contribute to the fine-tuning of Ca2+-dependent neuropeptide release in other CNS terminals, as well.

A Predictive, Quantitative Model of Spiking Activity and Stimulus-Secretion Coupling in Oxytocin Neurons

“…The terminals express a variety of Ca 2+ channels ( 44 ), and there is evidence that Ca 2+ release from intracellular stores also has a role ( 45 ). Exocytosis is also modulated by activity-dependent secretion of several modulators, including adenosine triphosphate ( 46 ), adenosine ( 47 ), and endogenous opioids ( 48 , 49 ). The terminals do not contain clearly separate pools of readily releasable and reserve vesicles, but rather a heterogeneous population differing in releasability ( 50 ).…”

Modulation/physiology of calcium channel sub-types in neurosecretory terminals

“…Kappa-opioid effects are seen on all channels in both types of terminals, however. Furthermore, in contrast to μ-opioids, κ-opioid effects are via a membrane-delimited pathway [112]. …”

“…(iii) Accumulation of endogenously released Dynorphin A also directly inhibits, via the κ-opioid receptor (KOR), VGCC and the subsequent release of AVP in the later portion of such a burst of action potentials. (iv) Since both adenosine and Dynorphin A act via a voltage-regulated membrane-delimited pathway [112], both would be inactive or “knocked off”(“k.o.”) during the higher frequencies at the beginning of bursts. (v) Interburst silent periods are necessary for the clearance of both the accumulated purines and opioids.…”

μ-Opioid Inhibition of Ca2+Currents and Secretion in Isolated Terminals of the Neurohypophysis Occurs via Ryanodine-Sensitive Ca2+Stores

“…Corelease of dynorphin-A, an endogenous -opioid receptor (KOR) agonist, with vasopressin from dendrites facilitates activity-dependent modulation of vasopressinergic neurons (Brown and Bourque, 2004;Brown et al, 2004;Roper et al, 2004;Brown et al, 2006;Sabatier and Leng, 2007). Isolated HNS terminals demonstrate inhibition of release in the presence of either MOR or KOR agonists for both oxytocin and vasopressin (Sumner et al, 1990;Kato et al, 1992;Russell et al, 1993); I Ca is similarly inhibited (Rusin et al, 1997;Ortiz-Miranda et al, 2003;Ortiz-Miranda et al, 2005). The signaling mechanism and modulatory importance of MOR and KOR activation at these presynaptic terminals and subsequent I Ca inhibition, however, is not well understood.…”

“…MOR and KOR are G-protein-coupled receptors that could potentially mediate inhibitory effects of opioids/opiates on I Ca through either a membrane-delimited or diffusible second messenger pathway (Wilding et al, 1995;Soldo and Moises, 1998;Kaneko et al, 1998;Connor and Christie, 1999;Chen et al, 2000). The MOR-signaling pathway seems to contrast sharply with that documented for the KOR in the same isolated neurohypophysis (NH) terminals (Velázquez-Marrero et al, 2010).…”