Noradrenaline modulates transmitter release by enhancing the Ca²⁺ sensitivity of exocytosis in the chick ciliary presynaptic terminal

1. The giant presynaptic terminal of chick ciliary ganglion was used to examine how noradrenaline (NA) modulates neurotransmitter release. The cholinergic excitatory postsynaptic currents (EPSCs) were recorded under whole-cell voltage clamp of the postsynaptic neuron. 2. Although the EPSC was potentiated by NA, the current directly activated by acetylcholine (I_ACh) was unaffected. NA also increased the quantal contents without changing the quantal size. 3. The NA-dependent potentiation was antagonized by neither phentolamine nor propranolol. The EPSC was also potentiated by adrenaline and dopamine but not by normetanephrine, phenylephrine or isoprenaline. The EPSC was attenuated by clonidine. Therefore, NA potentiated the transmitter release through a receptor pharmacologically different from both α- and β-adrenergic receptors. 4. The Ca²⁺ increment produced by an action potential (Δ[Ca²⁺]_pre) was reduced by NA through an α₂-adrenergic receptor. However, when α2-adrenergic receptors were blocked, neither α[Ca²⁺]_pre nor resting Ca²⁺ were changed by NA. 5. The [Ca²⁺]₀-EPSC relation was shifted by NA, decreasing the half-saturating [Ca²⁺J₀, without changing the maximum. 6. It is concluded that NA-dependent potentiation of transmitter release was due to an increase in the Ca²⁺ sensitivity of the exocytotic process. The enhancement of the fusion probability is suggested.