LOCALIZATION OF 5-HT2A RECEPTOR MESSENGER-RNA BY IN-SITU HYBRIDIZATION IN THE OLFACTORY-BULB OF THE POSTNATAL RAT

The olfactory bulb receives a dense serotonergic input and appears to require serotonergic input in early olfactory associational learning. However, it is not known which cell types receive the serotonergic inp ut or whether the cells express markers for the input throughout life. These issues need resolution in order for the mechanisms of serotoner gic interactions to be better understood. The mRNA for the 5-HT2A rece ptor was localized in the olfactory bulb of postnatal day 1, 2, 14 and 9-month-old Sprague-Dawley rats as well as in the bulb of adult (6 mo nths) and aged (22-30 month) Fisher 344 rats by in situ hybridization using an S-35-labelled 5-HT2A-specific oligonucleotide probe mixture. In all animals, hybridization was observed in mitral cells which are t he major output cells of the main olfactory bulb. Tufted cells, locate d in the external plexiform layer and juxtaglomerular region, were rea dily observed in adult and aged animals and were also observed, albeit not as readily, in neonate pups. Quantitative analysis of the silver grain density over cells confirmed qualitative observations and showed that mitral and tufted cells were labelled in the neonate as well as in adult and aged animals. Labelled cells were also numerous in the ex ternal division of the anterior olfactory nucleus in all animals. 5-HT 2A receptor mRNA could not be detected either qualitatively or quantit atively by in situ hybridization in the accessory olfactory bulb, nor could it be observed in the olfactory epithelium. The results suggest that the output cells of the main olfactory bulb receive serotonergic input via 5-HT2A receptors and that the serotonergic input onto these cells could influence olfactory functioning at all postnatal ages. (C) 1995 Wiley-Liss, Inc.