Light-dependent repetitive Ca2+ spikes induced by extracellular application of neomycin in honeybee drone photoreceptors

Photoreceptor cells of the honeybee drone fire, in the presence of the poly cationic aminoglycoside neomycin, repetitive slow spike-like potentials sup erimposed on the receptor potential plateau phase. We have used conventiona l intracellular recordings and micro-fluorometric intracellular Ca2+ measur ements to characterize these spike potentials. We have shown that the spike frequency increases in a light-intensity-dependent manner. The spikes are fired only when light stimuli depolarize the cell from a resting potential of -50 to -60 mV to at least -40 to --45 mV; they are tetrodotoxin insensit ive and blocked by the Ca2+ channel blockers Ni2+, Cd2+, omega-agatoxin TK, verapamil and methoxyverapamil. Depolarization of the photoreceptors with high extracellular K+ in the presence of neomycin in darkness does not gene rate spikes. Small intracellular Ca2+ oscillations superimposed on the plat eau phase of the light-induced increase in intracellular free Ca2+ concentr ation have a similar temporal pattern as the spike-like potentials. We conc lude that the spike-like potentials require stimulation by light and are ge nerated by voltage-dependent Ca2+ channels localized on the soma of the pho toreceptors, distal to the basal lamina.