μ-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 of chick embryos at stages 39-40. 2. Excitatory postsynaptic currents (EPSCs) were recorded by nystatin-perforated patch clamp at low [Ca²⁺](o) and high [Mg²⁺](o). [Leu⁵]enkephalin (L-ENK, 1-10 μM) reduced the quantal content (m) without changing the quantal size (q). This effect was antagonized by naloxone (1 μM). Similar results were observed under conventional whole-cell clamp of the postsynaptic neuron. 3. A specific agonist of the μ-opioid receptor, [D-Ala²,M-Me-Phe⁴,Gly⁵]enkephalin-ol (DAMGO) reduced m without changing q. A specific agonist of the δ-opioid receptor, [D-Pen²,D-Pen⁵]enkephalin (DPDPE) also reduced m without changing q. 4. Both L-ENK and [Met⁵]enkephalin (M-ENK) reduced the stimulus-dependent increment of the intraterminal Ca²⁺ concentration (Δ[Ca²⁺](t)) without affecting the decay time constant of the intraterminal Ca²⁺ concentration and basal Ca²⁺ level. This effect was antagonized by naloxone. DAMGO reduced Δ[Ca²⁺](t) more effectively than DPDPE. 5. When extracellular Ca²⁺ was replaced by Ba²⁺, the stimulus-dependent increment of the intraterminal Ba²⁺ concentration (Δ[Ba²⁺](t)) was also reduced by L-ENK or DAMGO. 6. L-ENK reduced Δ[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 ω-conotoxin GVIA (ω-CgTx(GVIA)), the Δ[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 μ-opioid receptor may suppress presynaptic Ca²⁺ influx by selectively inhibiting N-type Ca²⁺ channels.