TRP, TRPL and Cacophony Channels Mediate Ca2+ Influx and Exocytosis in Photoreceptors Axons in Drosophila

Abstract

In Drosophila photoreceptors Ca2+-permeable channels TRP and TRPL are the targets of phototransduction, occurring in photosensitive microvilli and mediated by a phospholipase C (PLC) pathway. Using a novel Drosophila brain slice preparation, we studied the distribution and physiological properties of TRP and TRPL in the lamina of the visual system. Immunohistochemical images revealed considerable expression in photoreceptors axons at the lamina. Other phototransduction proteins are also present, mainly PLC and protein kinase C, while rhodopsin is absent. The voltage-dependent Ca2+ channel cacophony is also present there. Measurements in the lamina with the Ca2+ fluorescent protein G-CaMP ectopically expressed in photoreceptors, revealed depolarization-induced Ca2+ increments mediated by cacophony. Additional Ca2+ influx depends on TRP and TRPL, apparently functioning as store-operated channels. Single synaptic boutons resolved in the lamina by FM4-64 fluorescence revealed that vesicle exocytosis depends on cacophony, TRP and TRPL. In the PLC mutant norpA bouton labeling was also impaired, implicating an additional modulation by this enzyme. Internal Ca2+ also contributes to exocytosis, since this process was reduced after Ca2+-store depletion. Therefore, several Ca2+ pathways participate in photoreceptor neurotransmitter release: one is activated by depolarization and involves cacophony; this is complemented by internal Ca2+ release and the activation of TRP and TRPL coupled to Ca2+ depletion of internal reservoirs. PLC may regulate the last two processes. TRP and TRPL would participate in two different functions in distant cellular regions, where they are opened by different mechanisms. This work sheds new light on the mechanism of neurotransmitter release in tonic synapses of non-spiking neurons. © 2012 Astorga et al.