Localization of neuropeptide precursor-synthesizing neurons in the rat olfactory bulb: a hybridization histochemical study.

The distribution of seven kinds of neuropeptide precursor mRNA-containing neurons was investigated in the rat main and accessory olfactory bulbs, where various peptides have previously been identified immunohistochemically, by means of in situ hybridization using [35S]cRNA probes. In the glomerular layer, numerous preprothyrotropin-releasing hormone mRNA-expressing neurons, moderate numbers of preprosomatostatin and preproenkephalin A neurons, and a small number of preprocholecystokinin neurons were detected. In the external plexiform layer, numerous medium sized preprocholecystokinin and preprocorticotropin-releasing hormone neurons, and a small number of beta-preprotachykinin A neurons were observed. In addition, small preprovasoactive intestinal polypeptide and preprothyrotropin-releasing hormone neurons were evenly distributed in the external plexiform layer. Medium to large sized beta-preprotachykinin A neurons formed a thin layer in the mitral cell layer. In the granule cell layer, in addition to numerous small preproenkephalin A neurons, moderate numbers of small beta-preprotachykinin A and preprocorticotropin-releasing hormone neurons, and a small number of preprothyrotropin-releasing hormone neurons, were identified. Large sized preprosomatostatin neurons were located in the deep layer of the granule cell layer. The distribution patterns of these neurons, as a whole, confirmed previous studies based on immunohistochemistry, although peptide precursor mRNA-expressing neurons were far more numerous than those immunoreactive to the respective neuropeptides. Moreover, mRNA-expressing neurons were observed in areas where no immunoreactive neurons had been observed (e.g. preprovasoactive intestinal polypeptide and preprosomatostatin neurons in the mitral cell layer of the assessory olfactory bulb). The distribution patterns were generally similar in the main and accessory olfactory bulbs.