Synaptic Chloride Channels Generating Hyperpolarizing On-Responses in Monopolar Neurones of the Blowfly Visual System

ABSTRACT The large monopolar neurones LI and L2 in the first optic ganglion of dipteran flies are postsynaptic to short-fibre photoreceptors (Trujillo-Cenòz, 1965; Boschek, 1971 ; Burkhardt & Braitenberg, 1976). Each neurone is contacted by six photoreceptor axons resulting in 1200 synaptic terminations (Nicol & Meinertzhagen, 1982). The photoreceptor response to light is a graded depolarization which, by synaptic transmission, is converted with considerable gain to a postsynaptic hyperpolarization (Järvilehto & Zettler, 1971). When presynaptic photoreceptors are stimulated with light of constant intensity, the postsynaptic potential (PSP) typically consists of a fast initial hyperpolarization (on-response) followed by a slower decay to a constant level about 100 ms after stimulus onset. Termination of the light stimulus results in an off-transient depolarization (the complete, normal PSP shape is shown in Fig. 1). All phases of the PSP are intensity-dependent. The present report is concerned with the hyperpolarizing on-response in the blowfly monopolar neurones.