Co-localization of neurotensin-like immunoreactivity and 3H-glycine uptake system in sustained amacrine cells of turtle retina.

Amacrine cells are axonless intrinsic neurones of the vertebrate retina which have cell bodies in the proximal inner nuclear layer and processes contributing to the synaptic network of the inner plexiform layer. They receive input from bipolar, interplexiform and other amacrine cells, and synapse onto these and ganglion cells. Amino acid and monoamine transmitters are found in most retinal neurones, but peptide transmitters are exclusively located in amacrine cells. Only one neuropeptide, amino acid or monoamine transmitter exists in any single amacrine cell population. Coexistence of neuropeptides with classical transmitters has been demonstrated histologically in many neurones of the central nervous system, but the physiological relevance of these findings is unknown. We report here evidence of such coexistence in retinal amacrine cells of the turtle, Pseudemys scripta elegans. Using combined immunocytochemistry and autoradiography, we have localized both neurotensin-like immunoreactivity and a high affinity uptake system for 3H-glycine to the same amacrine cell, implying that this cell type may use both substances as neurotransmitters. We also present electrophysiological evidence that this type of amacrine cell responds to photic stimulation with a sustained and graded membrane depolarization.