Pharmacological properties of presynaptic muscarinic receptors in the sixth abdominal ganglion of the cockroach,Periplaneta americana L.

The effects of muscarinic agonists on acetylcholine (ACh) release in the cercal-afferent giant-interneuron synapses of the cockroach have been studied using the single-fibre oil-gap method. Decrease in amplitude of the cEPSP was induced by pressure ejection of arecoline (ARE), carbachol (CCh) and oxotremorine free base (OXO) within the sixth abdominal (A6) ganglion. This depressive effect was dose-dependent without any effect at the post-synaptic site in the range of concentrations used. The concentration for 50% of cEPSP inhibition of the most commonly used muscarinic agonists was determined. The rank order of potencies was: ARE > CCh > OXO > McN-A-343 > bethanechol (BET). Plateau levels of 56%, 54% and 68% were reached when increasing the concentration of ARE, CCh and OXO respectively. McN-A-343 and BET had an inhibitory effect on ACh release but higher doses also acted at the postsynaptic level. Furthermore, muscarinic agonists were able to evoke uIPSP and a smaller inhibitory effect of ARE and OXO was observed in presence of picrotoxin. It is concluded that the ACh release in the cockroach A6 ganglion is modulated by presynaptic muscarinic receptors which are possible target sites for insecticides. The presence of putative muscarinic receptors on inhibitory GABAergic interneurons, involved in pre- and postsynaptic inhibition is suspected.