[Immunohistochemical study on the primary olfactory neuron of rat and suncus]

Carnosine (beta-alanyl-L-histidine), a putative neurotransmitter, was identified immunohistochemically in the primary olfactory neuron, using newly developed anti-carnosine antiserum. Similar results were obtained in the rat and suncus. Carnosine-like immunoreactivity was observed in the olfactory cells and its apical dendrites of the olfactory epithelium. The olfactory nerve and glomerular layer of the olfactory bulb showed positive reaction. At electron microscopic level, carnosine-immunoreactive end products widely spread out in the cytoplasm of olfactory nerve cells and also on microtubules of olfactory cilia. In the glomerular layer, reaction products were found diffusely in many axon terminals. These terminals had small spherical vesicles and often made asymmetric synaptic contacts to the second neuron. Unilateral closure of the olfactory naris resulted lower immunoreactivity of tyrosine hydroxylase in periglomerular cells, which are dopaminergic interneurons and are thought to regulate neurotransmission of olfactory input. In contrast no remarkable changes were seen on carnosine immunoreactivity in olfactory bulb. The present results suggest that carnosine may play some important roles in the olfactory mucosa. The functional role of carnosine remarks to be further examined.