Opioid receptor-mediated inhibition of conotoxin GVIA-sensitive calcium channel currents in rat intracardiac neurons. David J. Adams and Carlo Trequattrini. Department of Molecular and Cellular Pharmacology, University of Miami School of Medicine, Miami, FL 33101, USA and Department of Physiology and Pharmacology, University of Queensland, Brisbane, Queensland 4072, Australia.
APStracts 4:281N, 1997.
ABSTRACT
Modulation of depolarization-activated ionic conductances by opioid receptor agonists was investigated in isolated parasympathetic neurons from neonatal rat intracardiac ganglia using the whole-cell perforated patch clamp technique. Met-enkephalin (10 æM) altered the action potential waveform, reducing the maximum amplitude and slowing the rate of rise and repolarization but the after-hyperpolarization was not appreciably altered. Under voltage- clamp, 10 æM Met-enkephalin selectively and reversibly inhibited the peak amplitude of high voltage-activated Ca2+ channel currents elicited at 0 mV by 52% and increased 3-4-fold the time to peak. Met-enkephalin had no effect on the voltage-dependence of steady-state inactivation but shifted the voltage- dependence of activation to more positive membrane potentials whereby stronger depolarization was required to open Ca2+ channels. Half-maximal inhibition of Ba2+ current (IBa) amplitude was obtained with 270 nM Met-enkephalin or Leu- enkephalin. The opioid receptor subtype selective agonists, DAMGO and DADLE, but not DPDPE, inhibited IBa and were antagonized by the opioid receptor antagonists, naloxone and naltrindole with IC50's of 84 nM and 1 æM, respectively. The _-opioid receptor agonists, bremazocine and dynorphin A, did not affect Ca2+ channel current amplitude or kinetics. Taken together, these data suggest that enkephalin-induced inhibition of Ca2+ channels in rat intracardiac neurons is mediated primarily by the æ-opioid receptor type. Addition of Met-enkephalin following exposure to 300 nM _-conotoxin GVIA, which blocked 75% of the total Ca2+ channel current, failed to cause a further decrease of the residual current. Met-enkephalin inhibited the conotoxin GVIA-sensitive but not the _-conotoxin-insensitive IBa in rat intracardiac neurons. Dialysis of the cell with a GTP-free intracellular solution or preincubation of the neurons in Pertussis toxin (PTX) abolished the attenuation of IBa by Met-enkephalin, suggesting the involvement of a PTX- sensitive G-protein in the signal transduction pathway. The activation of æ- opioid receptors and subsequent inhibition of N-type Ca2+ channels in the soma and terminals of postganglionic intracardiac neurons is likely to inhibit the release of ACh and thereby regulate vagal transmission to the mammalian heart. 

Received  1997; accepted in final form  1997.
APS Manuscript Number J.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 29 October 1997