Opioid Receptor-Mediated Inhibition of v-Conotoxin GVIA-Sensitive Calcium Channel Currents in Rat Intracardiac Neurons

Adams, David J. and Carlo Trequattrini. Opioid receptor-medinized the effects of these substances in rabbit (Weitzell et ated inhibition of v-conotoxin GVIA-sensitive calcium channel al. 1984) and canine heart (Musha et al. 1989). currents in rat intracardiac neurons. J. Neurophysiol. 79: 753–762, Endogenous opioid peptides were found in rat and guinea 1998. Modulation of depolarization-activated ionic conductances pig heart associated with cardiac nerves (Hughes et al. 1977; by opioid receptor agonists was investigated in isolated parasympaWeihe et al. 1985). Immunohistochemical studies had localthetic neurons from neonatal rat intracardiac ganglia by using the ised enkephalin-immunoreactive axons that were predomiwhole cell perforated patch clamp technique. Met-enkephalin (10 nantly perivascular within the guinea pig heart (Reinecke mM) altered the action potential waveform, reducing the maximum and Forssmann 1984; Weihe et al. 1985). The dramatic amplitude and slowing the rate of rise and repolarization but the reduction in the levels of enkephalin and prodynorphinafterhyperpolarization was not appreciably altered. Under voltage derived peptides after chemical sympathectomy suggested clamp, 10 mM Met-enkephalin selectively and reversibly inhibited that enkephalin was present in sympathetic nerves (Lang et the peak amplitude of high-voltage–activated Ca channel currents elicited at 0 mV by Ç52% and increased threeto fourfold al. 1983). Immunoreactivity for prodynorphinand proenthe time to peak. Met-enkephalin had no effect on the voltage kephalin-derived peptides was also recently observed in popdependence of steady-state inactivation but shifted the voltage deulations of parasympathetic postganglionic neurons in pendence of activation to more positive membrane potentials guinea pig cardiac ganglia and their axons (Steele et al. whereby stronger depolarization was required to open Ca chan1994, 1996). Proenkephalin mRNA was also found in rat nels. Half-maximal inhibition of Ba current (IBa) amplitude was atria and ventricles and the enkephalin peptides were shown obtained with 270 nM Met-enkephalin or Leu-enkephalin. The to be synthesized and secreted from cardiac myocytes opioid receptor subtype selective agonists, DAMGO and DADLE, (Springhorn and Claycomb 1992). but not DPDPE, inhibited IBa and were antagonized by the opioid Although the release of opioid peptides from cardiac receptor antagonists, naloxone and naltrindole with IC50s of 84 nM nerves and myocytes and the presence of high affinity saturaand 1 mM, respectively. The k-opioid receptor agonists, bremazocble opioid binding were shown in rat atria, the mechanisms ine and dynorphin A, did not affect Ca channel current amplitude or kinetics. Taken together, these data suggest that enkephalinof action of enkephalin peptides in the mammalian heart induced inhibition of Ca channels in rat intracardiac neurons is remain largely unknown. Leu-enkephalin was reported to mediated primarily by the m-opioid receptor type. Addition of Metdecrease the intracellular Ca transient and contraction amenkephalin after exposure to 300 nM v-conotoxin GVIA, which plitudes in individual cardiac ventricular cells, in part, by blocked Ç75% of the total Ca channel current, failed to cause reducing L-type Ca channel currents (Xiao et al. 1993). a further decrease of the residual current. Met-enkephalin inhibited Local administration of enkephalin to the right atrial ganthe v-conotoxin GVIA-sensitive but not the v-conotoxin-insensiglion plexus was also shown to increase the neuronal activity tive IBa in rat intracardiac neurons. Dialysis of the cell with a generated by in situ canine intracardiac neurons (Armour et GTP-free intracellular solution or preincubation of the neurons in al. 1993). Pertussis toxin (PTX) abolished the attenuation of IBa by MetOpioids were previously shown to inhibit neuronal, voltenkephalin, suggesting the involvement of a PTX-sensitive Gage-dependent Ca currents and activate inwardly rectifyprotein in the signal transduction pathway. The activation of mopioid receptors and subsequent inhibition of N-type Ca channels ing K currents (see review by North 1993). In the present in the soma and terminals of postganglionic intracardiac neurons study, the effects of enkephalin peptides on depolarizationis likely to inhibit the release of ACh and thereby regulate vagal activated ionic currents were investigated in isolated paratransmission to the mammalian heart. sympathetic neurons of rat intracardiac ganglia by using the amphotericin B, perforated-patch configuration of the whole cell patch clamp technique. A preliminary report of some of I N T R O D U C T I O N these results was presented (Adams and Trequattrini 1995).