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Immunocytochemical methods were used to visualize carnosine (β-alanyl- l -histidine)-like immunoreactivity (-LI) in the frog retina and to compare its localization with that of glutamate. Carnosine-LI was conspicuous in photoreceptors and bipolar cells. The axon terminals of labelled bipolar cells formed five bands in the inner plexiform layer. A few presumed amacrine and ganglion cells, as well as Muller cell endfeet, were also labelled. Post-embedding immunocytochemistry revealed particularly high levels of glutamate-LI in the synaptic axon terminals of bipolar cells, with a mean gold particle density 5 × higher than that of amacrine cells. Photoreceptor terminals were also labelled, but with a labelling intensity about half that of bipolar cells. Labelling of serial semithin sections showed co-localization of carnosine and glutamate in photoreceptors and bipolar cells. These findings are consistent with the notion that glutamate is the neurotransmitter of neuronal elements that transfer information vertically through the retina. We propose that carnosine may modulate GABA and/or glutamate receptors by virtue of its ability to chelate Zn 2+ and other ions.

We demonstrate that both glutamate-like and carnosine-like immunoreactivities are present in hair cells and in fibers of the vestibular organ of the frog inner ear. Comparison of the two immunoreactivity patterns indicates that glutamate and carnosine might be colocalized in some hair cells. The presence of glutamate-like immunoreactivity in hair cells is consistent with biochemical and pharmacological data indicating glutamate as the excitatory neurotransmitter in these sensory receptors. There is also evidence that carnosine might have a neuromodulatory function.