CHOLINERGIC AND GABAERGIC RECEPTORS ON FLY TANGENTIAL CELLS AND THEIRROLE IN VISUAL-MOTION DETECTION

1. To identify some of the neurotransmitters involved in the processin g of visual motion information the pharmacology of transmitter recepto rs on motion-sensitive visual interneurons (VS and HS cells) was inves tigated in an in vitro preparation of the blowfly (Calliphora erythroc ephala) brain. Cholinergic and GABAergic drugs were applied in the bat h and iontophoretically while recording intracellularly from HS and VS cells. 2. Bath-applied carbachol (10 and 100 mu M) leads to a depolar ization in HS and VS cells. One micromolar nicotine also has a depolar izing effect. Both agonists are effective in O Ca2+/high Mg2+-saline, too, which isolates the cells synaptically. The muscarinic agonists pi locarpine and oxotremorine have no effects on the membrane potential. 3. Iontophoretic application of acetylcholine, carbachol, and nicotine depolarizes VS and HS cells. The iontophoretic carbachol response is antagonized by alpha-bungarotoxin (EC(50) = 0.19 mu M), mecamylamine ( EC(50) = 0.32 mu M), d-tubocurarine (EC(50) = 9.5 mu M), and bicuculli ne but not by decamethonium and scopolamine. 4. Bath application of mu scimol strongly hyperpolarizes VS cells in normal fly saline. The gamm a-aminobutyric acid-C (GABA(C))-receptor agonist cis-4-aminocrotonic a cid (CACA) has no effects. The hyperpolarizing response to iontophoret ic applied muscimol is present in O Ca2+/high Mg2+ saline as well as i n Co2+-containing saline. The muscimol response is reduced in low chlo ride saline and thus chloride sensitive. The muscimol response is bloc ked by picrotoxinin (EC(50) = 3.4 mu M) but not by the GABA(A) recepto r antagonist bicuculline. 5. Taken together the primary responses of t he lobula plate tangential cells appear to be nicotinic cholinergic an d GABAergic. 6. The pharmacology of natural synaptic input to VS cells was investigated by extracellular electrical stimulation of the medul la. Such evoked excitatory postsynaptic potentials (EPSPs) are blocked reversibly in O Ca2+/high Mg2+ saline. The nicotinic antagonists meca mylamine (1 mu M) and d-tubocurarine (50-100 mu M) abolish or diminish the EPSPs, respectively. 7. The pharmacological data are incorporated into a semicellular model of a visual motion detector favoring a role of lobula plate tangential cells in certain steps of visual motion pr ocessing. Cholinergic and GABAergic inputs are an ideal cellular imple mentation of a linear subtraction of the signals arising from local mo tion-sensitive elements with opposite preferred direction. Such a mech anism enhances direction-selectivity and, together with dendritic inte gration, increases the sensitivity of the tangential cells for wide-fi eld motion.