Carnosine-like immunoreactivity is associated with synaptic vesicles in photoreceptors of the frog retina.

The distribution of carnosine-like immunoreactivity in photoreceptors of the frog retina was studied by post-embedding electron microscope immunocytochemistry. Different fixation and embedding procedures were applied and the best results were achieved on sections from tissue embedded in the hydrophilic resin LR White. All photoreceptor types present in the frog retina (red and green rods, single and double cones) were intensely reactive for the carnosine antiserum. The immunoreactivity was particularly prominent in the synaptic terminal, nucleus and myoid, but decreased in the ellipsoid and in the outer segment. Müller glial cells and horizontal and bipolar cell processes in the outer plexiform layer were completely unstained. At the level of the photoreceptor terminals, the presence of gold particles within the synaptic vesicles could be demonstrated. This study is the first to provide the anatomical evidence for the presence of carnosine in the synaptic vesicles of a nervous cell. The present data may be relevant to the understanding of the biological functions of carnosine (and related dipeptides) and offer an exploitable system in which to verify its proposed role as a neurotransmitter or neuromodulator in sensory systems.