Localization of 5‐HT2A receptor mRNA by in situ hybridization in the olfactory bulb of the postnatal rat

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Abstract
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 input or whether the cells express markers for the input throughout life. These issues need resolution in order for the mechanisms of serotonergic interactions to be better understood. The mRNA for the 5‐HT2A receptor 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 months) and aged (22‐30 month) Fisher 344 rats by in situ hybridization using an 35S‐labelled 5‐HT2A specific oligonucleotide probe mixture. In all animals, hybridization was observed in mitral cells which are the major output cells of the main olfactory bulb. Tufted cells, located in the external plexiform layer and juxtaglomerular region, were readily 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 external division of the anterior olfactory nucleus in all animals. 5‐HT2A receptor mRNA could not be detected either qualitatively or quantitatively 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.

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