Joro spider venom: glutamate agonist and antagonist on the rod retina of the dogfish.

The venom of the Joro spider, reported to block glutamate action selectively at some glutaminergic synapses, was applied focally to rod horizontal and on-bipolar cells in dark-adapted retinal slices. The venom hyperpolarized horizontal cells in the dark and blocked their responses to light. Depolarization of horizontal cells by ionophoretic pulses of L-glutamate, L-aspartate or kainate was antagonized by the toxin in a slowly reversible manner. These results are consistent with other evidence for glutamate as the rod neurotransmitter acting on a single postsynaptic receptor type on horizontal cells. In contrast, the venom, like glutamate, closed the same ion channels of on-bipolar cells as the transmitter, thereby blocking light responses of on-bipolars. Joro spider toxin distinguishes the glutamate receptor-channel complexes of rod horizontal and on-bipolar cells.