mu-Opioid receptor inhibits N-type Ca2+ channels in the calyx presynaptic terminal of the embryonic chick ciliary ganglion

1. A study was made on the mechanisms by which enkephalins inhibit synaptic transmission at calyx-type presynaptic terminals in the ciliary ganglion o f chick embryos at stages 39-40. 2. Excitatory postsynaptic currents (EPSCs) were recorded by nystatin-perfo rated patch clamp at low [Ca2+](o) and high [Mg2+](o). [Leu(5)]enkephalin ( L-ENK, 1-10 mu M) reduced the quantal content (m) without changing the quan tal size (q). This effect was antagonized by naloxone (1 mu M). Similar res ults were observed under conventional whole-cell clamp of the postsynaptic neuron. 3. A specific agonist of the mu-opioid receptor, [D-Ala(2), M-Me-Phe(4),Gly (5)]enkephalin-ol (DAMGO) reduced m without changing q. A specific agonist of the delta-opioid receptor, [D-Pen(2),D-Pen(5)]-enkephalin (DPDPE) also r educed m without changing q. 4. Both L-ENK and [Met(5)]enkephalin (M-ENK) reduced the stimulus-dependent increment of the intraterminal Ca2+ concentration (Delta[Ca2+](t)) without affecting the decay time constant of the intraterminal Ca2+ concentration and basal Ca2+ level. This effect was antagonized by naloxone. DAMGO reduce d Delta[Ca2+](t) more effectively than DPDPE. 5. When extracellular Ca2+ was replaced by Ba2+, the stimulus-dependent inc rement of the intraterminal Ba2+ concentration (Delta[Ba2+](t)) was also re duced by L-ENK or DAMGO. 6. L-ENK reduced Delta[Ca2+](t) even in the presence of 4-aminopyridine (4- AP), which blocks the transient K+ conductance during the falling phase of the presynaptic action potential. When N-type Ca2+ channels were blocked by omega-conotoxin GVIA (omega-CgTx(GVIA)), the Delta[Ca2+)(t) was no longer sensitive to L-ENK and DAMGO. 7. It is suggested that enkephalins reduce the transmitter release through presynaptic opioid receptors. The mu-opioid receptor may suppress presynapt ic Ca2+ influx by selectively inhibiting N-type Ca2+ channels.