Mechanism of α₂-adrenergic inhibition of neuroeffector transmission in the mouse vas deferens

Summary

• 1. The process by which the activation of presynaptic α₂-adrenoceptors inhibits the release of noradrenaline from terminals of postganglionic sympathetic nerves was studied in the mouse isolated vas deferens. Clonidine was used as a prototypic agonist. Field stimulation-evoked excitatory junction potentials (e.j.p.s) were recorded from individual muscle cells. The e.j.p. amplitudes were taken as a measure of transmitter release.

• 2. Changes in the external Ca²⁺ concentration from 2.5 to 1.25 or 5 mM caused corresponding changes in the size of e.j.p.s. When the normal Ca²⁺ concentration of the medium (2.5 mM) was substituted by equimolar quantities of Ba²⁺ or Sr²⁺, the e.j.p. amplitudes decreased considerably.

• 3. Clonidine (0.3–30 nM) inhibited the nerve stimulation-evoked e.j.p. amplitudes in a concentration-dependent manner, without altering appreciably the frequency of spontaneous e.j.p.s. Procedures known to enhance Ca²⁺ entry into nerve terminals, like a high Ca²⁺ medium (Ca²⁺ 5 mM) or 4-aminopyridine 30 μM reduced the effect of clonidine. Repetitive nerve stimulation at 3 Hz, which is supposed to lead to an accumulation of free Ca²⁺ inside nerve terminals, similarly counteracted the effect of clonidine 10 nM. Whereas the α₂-adrenergic inhibition of the first e.j.p. in a train was unaffected, the inhibition of all successive e.j.p.s was gradually decreased. At 5 mM Ca²⁺ only the time-course of facilitation became faster, the decrease in α₂-adrenergic inhibition proceeded with the same pulse-dependent rate as at a normal external Ca²⁺ concentration, although from a lower initial level.

• 4. Procedures known to decrease the efflux of K⁺ from the nerve terminal, like a high K⁺ medium (K⁺ 12 mM) or Ba²⁺ 100 μM, depolarized the smooth muscle and enhanced the effect of clonidine on the e.j.p.s. In a low Ca²⁺ high K⁺ medium (Ca²⁺ 1.25 mM, K⁺ 12 mM) the potentiation of the clonidine effect was slightly smaller than at a normal Ca²⁺ concentration of 2.5 mM.

• 5. It is suggested that α₂-adrenoceptor activation may decrease Ca²⁺ entry through voltage-sensitive channels into terminals of postganglionic sympathetic nerves and in consequence inhibit nerve stimulation-evoked transmitter release. Our results do not agree with an indirect modulation of axoplasmic Ca²⁺ or a blockade of action potential propagation by a potassium permeability increase in the terminal axon.