Differential distribution of inositol 1,4,5‐triphosphate receptors in the rat olfactory bulb

The inositol 1,4,5‐triphosphate receptor (IP 3 R) regulates the release of calcium from intracellular stores. In the present study, the distribution of IP 3 R in the rat main olfactory bulb was determined by immunohistochemistry. Immunofluorescence was used to double label for IP 3 R and for γ‐aminobutyric acid (GABA) or for projection neurons, which were retrogradely labeled following dextran injection into the lateral olfactory tract (LOT). The expression profile of IP 3 R changes dramatically during development. In the glomerular layer of adults, many juxtaglomerular neurons are IP 3 R immunoreactive [IP 3 R(+)]; the majority of these cells are also GABA immunoreactive [GABA(+)]. Scattered sparsely throughout the external plexiform layer are small numbers of IP 3 R(+) neurons, a small number of which are LOT‐projecting tufted cells. Significant numbers of IP 3 R(+) neurons are in the granule cell layer; however, most of these cells are GABA(−). The vast majority of mitral cells contain little or no IP 3 R immunoreactivity. These findings indicate that, in the olfactory bulb of adult rats, IP 3 R is preferentially localized in specific classes of intrinsic neurons and that it is rarely expressed in projection neurons. In contrast, during the first postnatal week, the receptor is detected almost exclusively in mitral cells. Expression of IP 3 R in subclasses of intrinsic neurons begins during the second and third weeks, concomitant with a decrease in immunostaining of mitral cells. Adult patterns of IP 3 R immunostaining are apparent by the fourth postnatal week. These observations raise the possibility that the expression of IP 3 R in specific classes of neurons during development is activity dependent. J. Comp. Neurol. 389:224–234, 1997. © 1997 Wiley‐Liss, Inc.