Differential sensitivity of rods and cones in Xenopus retina to hemicholinium-3

Hemicholinium-3 (HC-3), a drug which prevents synthesis of acetylcholine in neurons, when injected intraperitoneally in doses as low as 2 X 5 mg/kg produces marked ultrastructural changes and damage in rod but not in cone photoreceptors. In rods it causes a reduction in cytoplasmic background density, swelling of the endoplasmic reticulum, ballooning of the outer membrane of the nucleus, leaching of the nucleoplasm and clumping of the nuclear chromatin. In dark-adapted rods HC-3 produces some loss of cytoplasmic synaptic vesicles but no reduction in numbers of those vesicles which lie adjacent to the synaptic ribbons. In light-adapted rods the drug does not cause such an apparent reduction of synaptic vesicles but does induce a considerable reduction in numbers of vesicles associated with the ribbons. These structural changes are discussed in the light of what is known about the pharmacology of HC-3 and neurotransmitter release from vertebrate photoreceptors.