DIURNAL MODULATION OF PHOTORECEPTOR POTASSIUM CONDUCTANCE IN THE LOCUST

Single electrode clamp techniques demonstrated diurnal changes in phot oreceptor membrane conductance, recorded intracellularly in the intact , dark-adapted retina of the locust Schistocerca gregaria. In the day, locust photoreceptors exhibited the membrane properties of fast cells , as previously defined in rapidly moving diurnal Diptera. Depolarizat ion activated a powerful potassium conductance with two kinetic compon ents, one rapidly activating close to resting potential and the other activating more slowly when further depolarized, giving a pronounced d elayed rectification. There was little inactivation. At night, locust photoreceptors resembled slow cells, as defined in weakly flying crepu scular and nocturnal Diptera. Depolarization rapidly activated an outw ard current which then inactivated over 100 ms to reduce rectification . The change from day to night state was mimicked by applying 10 mM se rotonin extracellularly to the retina. We conclude that the potassium conductances of locust photoreceptor membranes are modulated according to a diurnal rhythm, possibly by serotonin. This neuromodulation is u sed to match photoreceptor membrane properties to photic habitat. Our findings suggest a definite and potentially widespread function for se rotonin as a mediator of diurnal changes in the insect visual system.