The effects of bretylium and guanethidine on catecholaminergic transmission in an invertebrate.

1 Secretory potentials produced by stimulation of the salivary duct nerves were recorded intracellularly from cockroach isolated salivary glands. The secretory potential normally consisted of a 40-80 mV hyperpolarization of the gland cell.2 Bretylium (0.1-1 mM) reduced the amplitude of the secretory potential without affecting the response of the gland to dopamine (0.25-1 muM). In addition, bretylium caused an increase in the frequency of miniature secretory potentials and the appearance of nerve terminal action potentials.3 Guanethidine (1 mM) reduced the response to nerve stimulation without depressing the sensitivity of the salivary gland to dopamine (0.25-0.5 muM) and without causing an increase in the occurrence of miniature potentials. Higher concentrations (4-5 mM) completely eliminated secretory potentials but also reduced the sensitivity of the gland cell to dopamine.5 These results indicate a presynaptic depression of the secretory potential by both bretylium and guanethidine. It is suggested that, in this system, bretylium acts by depolarizing the nerve terminal while guanethidine does not.