Photolysis of caged inositol 1,4,5-trisphosphate induces action potentials in frog vomeronasal microvillar receptor neurones

To study the effect of inositol 1,4,5-trisphosphate (IP3) in isolated frog vomeronasal microvillar receptor neurones, whole-cell recordings were performed with 0.5 muM caged IP3 dissolved in the pipette solution. IP3 was released by photolysis of caged IP3 initiated by a 0.8-ms ultraviolet flash from a xenon flash lamp 70 s after the start of dialysis of caged IP3 into the cell. Flash illuminating the whole receptor neurone with caged IP3 triggered action potentials when the current was clamped at zero and a series of transient inward currents of 12-55 pA at a holding potential of -70 mV. The average number of spikes during the first 40 s after release of IP3 was 7.2 +/- 2.5 (n=6, mean +/- S.E.M.). The average maximum current and the total inward transport of charge during the first 40 s after photolysis of caged IP3 were -24 +/- 8.0 pA and -1.7 +/- 0.8 pC, respectively (n=5, mean +/- S.E.M.). Inward membrane currents of 12-55 pA after release of IP3 were not observed with 50 muM La3+ in the bath. Notably, flash focused on the terminal vesicle also triggered action potentials. No action potentials were observed following flash focused on the soma or outside the dendrite. The average number of spikes during the first 40 s after release of IP3 initiated by flash spatially restricted to the terminal vesicle was 5.0 +/- 2.0 (n=4, mean S.E.M.). The present study indicates that local release of IP3 in the terminal vesicle of the vomeronasal neurones triggers transient depolarizations and induces action potentials. We suggest that IP3 might be a second messenger in the vomeronasal microvillar receptor neurones. (C) 2003 IBRO. Published by Elsevier Science Ltd. All rights reserved.