Effects of Nimodipine, Calcium-Free Medium and Colchicine on Electrogenesis of Neurosecretory Retzius Cells in the Leech Hirudo medicinalis

Retzius neuron (RN) in the medicinal leech is known to function as a typical neurosecretory cell. This study addresses the relationship between its two functions, neural and paracrine. It was shown that synaptic activation of RN at 710-Hz frequencies causes the neurophysiological process of habituation. Experiments conducted in calcium-free solution and in that containing nimodipine and colchicine (which block somatic exocytosis of serotonin under these experimental conditions) demonstrate alterations in the electrophysiological characteristics of RN: the rate of spontaneous impulse activity (IA), action potential (AP) amplitude, and synaptic stimulation threshold. Under these conditions, RN generates AP to every stimulus even at frequencies of 7 to 10 Hz. Thus, while somatic exocytosis of serotonin is blocked, habituation does not develop. It is suggested that habituation of RN to high-frequency synaptic stimulation is mediated by the concurrent effects of two factors — stimulatory (via synaptic activation) and inhibitory (via autoinhibition).