M-2-LIKE PRESYNAPTIC RECEPTORS MODULATE ACETYLCHOLINE-RELEASE IN THE COCKROACH (PERIPLANETA-AMERICANA) CENTRAL-NERVOUS-SYSTEM

The effects of muscarinic agonists on the acetylcholine release in the cercal-afferent giant-interneurone synapses of the cockroach (Periplaneta americana) were investigated using the single-fibre oil-gap method. Pressure-ejection of arecoline, carbachol and oxotremorine within the sixth abdominal ganglion reduced the cEPSP amplitude in a concentration-dependent manner without any effect at the postsynaptic site in the range of concentrations used. A maximal inhibitory effect of 56, 54 and 68% was estimated for arecoline, carbachol and oxotremorine, respectively. The rank order of potencies for the agonists tested was: arecoline > carbachol > oxotremorine. McN-A-343 and bethanechol had an inhibitory effect on acetylcholine release but higher doses also acted at the postsynaptic level. The depressive effect of arecoline was only counteracted by AF-DX 116 and methoctramine. In presence of picrotoxin the inhibitory effect of arecoline and oxotremorine was reduced by about 15%. It is concluded that acetylcholine release in the cockroach central nervous system is modulated by functional M2-like presynaptic receptors. The influence of presynaptic electrical activity on this modulatory effect is discussed.