LOCALIZATION OF CALCIUM-ENTRY THROUGH CALCIUM CHANNELS IN OLFACTORY RECEPTOR NEURONS USING A LASER-SCANNING MICROSCOPE AND THE CALCIUM INDICATOR DYES FLUO-3 AND FURA-RED

The intracellular calcium concentration [Ca2+](i) in olfactory receptor neurones of Xenopus laevis was imaged with high spatial and temporal resolution. A new method using a mixture of the calcium indicator dyes Fluo-3 and Fura-Red was employed. The fluorescence patterns in two wavelength bands were measured on the emission side of a confocal laser scanning microscope, and the ratio R of the fluorescence Intensities was taken as an estimate of [Ca2+](i). When the neurones were depolarized by elevating the extracellular potassium concentration [K+]o they showed one of three types of responses: a fast increase In [Ca2+](i), a stow increase in [Ca2+](i), or no change in [Ca2+](i). The fast increase in [Ca2+](i) took place in the soma compartment. For at least 4 s after the onset of depolarization the calcium distribution in the dendrite remained essentially unchanged. To study the fast increase with high time resolution, line scan images were taken. The neurones were depolarized for brief periods applying a solution containing high [K+] onto the soma from an application pipette. The fast increase in [Ca2+](i) began with a delay of about 200 ms and went from the resting concentration to about 110 nM above resting concentration. Following the depolarization, recovery from elevated [Ca2+](i) to resting levels had a time constant of about 15 s. The slow response seemed to depend on the removal of [Na+] from the bath rather than on the elevated [K+] In the bath. The response was also observed with Cd2+, Ni2+, and Co2+ (1.5 mM each) in the bath. The fast increase in [Ca2+](i) upon depolarization was never seen if R > 0.8 ([Ca2+](i) > 300 nM). For R < 0.8, 45% of the cells showed a fast response. Cells that responded with a fast increase in [Ca2+](i) at low resting [Ca2+](i) did not do so for R > 0.8. We suggest that the physiological role of calcium entry through calcium channels on the soma of olfactory cells Is to decrease the membrane impedance in an activity dependent way by activating a calcium dependent potassium conductance.