PHOSPHOLIPASES AND ARACHIDONIC-ACID CONTRIBUTE INDEPENDENTLY TO SENSORY TRANSDUCTION AND ASSOCIATIVE NEURONAL FACILITATION IN HERMISSENDA TYPE-B PHOTORECEPTORS

During contiguous pairings of light and rotation, B photoreceptors in the Hermissenda eye undergo an increase in excitability that contribut es to a modification of several light-elicited behaviors. This excitab ility increase requires a light-induced rise in intracellular Ca2+ in the photoreceptor concomitant with transmitter binding to G protein-co upled receptors as a result of presynaptic vestibular hair cell stimul ation. Phospholipases and arachidonic acid (ArA) are here reported to be involved in independent signal transduction pathways that underlie both receptor function and activity-dependent facilitation of the B ph otoreceptor. 4-Bromophenacyl bromide (BPB), an inhibitor of phospholip ases A(2) (PLA(2)) and C (PLC), blocked the generation of light-induce d depolarizing generator potentials, but had no affect on the inhibito ry postsynaptic potential (IPSP) in the B cell that results from hair cell stimulation. Quinacrine, which predominantly blocks the activity of PLA(2) in neurons, had no affect on either the light response or th e IPSP, but did block increases in excitability (i.e. increased input resistance and elicited spike rate) of the B cell that results from pa irings of light and presynaptic vestibular stimulation (i.e., in vitro associative conditioning). Neither nordihydroquararetic acid (NDGA), which inhibits metabolism of ArA by cyclooxygenase, nor indomethacin, which inhibits lipoxygenase metabolism of ArA, affected the light resp onse or IPSP, but both blocked the increases in excitability in the B cell that accompanied in vitro conditioning. In combination with earli er results, these data suggest that ArA activates PKC in a synergistic fashion with Ca2+ and diacylglycerol in the B cell, and suggest that PLA(2)-induced ArA release, though not necessary for transduction of l ight or the hair cell-induced IPSP in the B cell, is a critical compon ent of the convergence of signals that precipitates associative facili tation in this system. (C) 1997 Elsevier Science B.V.