Serotonin-induced changes in the activity of single Ca2+ channels inHelix pomatia neurons

The influence of the neurotransmitter serotonin (5-hydroxytryptamine, 5-HT) on single Ca²⁺ channel activity was studied on unidentified neurons of the snailHelix pomatia. Only one type of Ca²⁺ channels with the unitary conductance of 5 pS was identified using 100 mM Ca²⁺ in the patch pippette under patch-clamp in a cell-attached configuration. The amplitude histogram showed only one peak with the mean value of 0.5 pA at the testing potential of –30 mV. The distribution of channel open times monotonically declined with the mean time constant of 0.2 msec. The distribution of channel closed times could be fitted by a double-exponential curve with time constants of 1 and 12 msec. The study of the effect of 5-HT on Ca²⁺ single channel activity showed that 5-HT influenced the channel molecule indirectly, as the transmitter could exert its effect by being added to the bath solution, which did not come into contact with the tested membrane fragment under the micropipette tip. 5-HT prolonged the mean channel open time (up to 0.3 msec) and proportionally decreased both channel closed time constants to 0.4 and 6.0 msec, respectively. A conclusion is made that enhancement of Ca²⁺ macrocurrent by 5-HT is determined by three factors: (i) changes in kinetics of aiready existing channels, (ii) an increase in the number of active channels of the same type, and (iii) an increase in probability of a channel being open. At the same time, the unitary channel conductance was not affected by the transmitter.Neirofiziologiya/Neurophysiology, Vol. 28, No. 2/3, pp. 132–140, March–June, 1996.