Calcium influx into dendrites of the leech Retzius neuron evoked by 5-hydroxytryptamine

Abstract

5-Hydroxytryptamine (5-HT) is a ubiquitous neurotransmitter and neuromodulator that affects neural circuits and behaviours in vertebrates and invertebrates. In the present study, we have investigated 5-HT-induced Ca2+ transients in subcellular compartments of Retzius neurons in the leech central nervous system using confocal laser scanning microscopy, and studied the effect of 5-HT on the electrical coupling between the Retzius neurons. Bath application of 5-HT (50mM) induced a Ca2+ transient in axon, dendrites and cell body of the Retzius neuron. This Ca2+ transient was significantly faster and larger in dendrites than in axon and cell body, and was half-maximal at a 5-HT concentration of 5–12mM. The Ca2+ transient was suppressed in the absence of extracellular Ca2+ and by methysergide (100mM), a non-specific antagonist of metabotropic 5-HT receptors, and was strongly reduced by bath application of the Ca2+ channel blocker Co2+ (2mM). Injection of the non-hydrolysable GTP analogue GTPγS increased and prolonged the dendritic 5-HT-induced Ca2+ transient. The non-selective protein kinase inhibitor H7 (100mM) and the adenylate cyclase inhibitor SQ22536 (500mM) did not affect the Ca2+ transient, and the membrane-permeable cAMP analogue dibutyryl-cAMP (500mM) did not mimic the effect of 5-HT application. 5-HT reduced the apparent electrical coupling between the two Retzius neurons, whereas suppression of the Ca2+ influx by removal of external Ca2+ improved the transmission of action potentials at the electrical synapses which are located between the dendrites of the adjacent Retzius neurons. The results indicate that 5-HT induces a Ca2+ influx through calcium channels located primarily in the dendrites, and presumably activated by a G protein-coupled 5-HT receptor. The dendritic Ca2+ increase appears to modulate the excitability of, and the synchronization between, the two Retzius neurons.