Inositol 1,4,5-trisphosphate receptor localization in the brain of a weakly electric fish (Apteronotus leptorhynchus) with emphasis on the electrosensory system.

Inositol 1,4,5-trisphosphate is a widespread intracellular second messenger that mobilizes intracellular Ca2+ stores. The inositol 1,4,5-trisphosphate receptor involved is associated with the endoplasmic reticulum in neurons. In mammalian brain, inositol 1,4,5-trisphosphate receptor-containing neurons are found in many diverse regions, with cerebellar Purkinje cells containing the highest density of these receptors. We used immunohistochemical methods to identify the distribution of inositol 1,4,5-trisphosphate receptor-containing neurons in the brain of the weakly electric fish and Western blotting to confirm that a protein similar to the inositol 1,4,5-trisphosphate receptor of mammalian brain was recognized in the fish brain. In the telencephelon, the dorsal forebrain regions had low amounts of inositol 1,4,5-trisphosphate receptor. In the diencephalon, only the nucleus tuberis posterior was moderately immunoreactive. In the mesencephalon, only the optic tectum contained cells with intense immunoreactivity, similar to our findings for the ryanodine receptor (G.K.H. Zupanc, J.A. Airey, L. Maler, J. Sutko, and M.H. Ellisman, 1992, J. Comp. Neurol. 325:135-151), which also mobilizes intracellular calcium. In the rhombencephalon, a subset of the pyramidal cells of the electrosensory lateral line lobe contained inositol 1,4,5-trisphosphate receptor. These cells have been shown to contain ryanodine receptor (Zupanc et al., 1992). However, unlike the ryanodine receptor, the distribution of inositol 1,4,5-trisphosphate receptor in these cells is constrained to the soma and proximal dendrites. This compartmentalization may indicate the limit of the range of second-messenger action. Other regions containing immunoreactive cells were the nucleus praeminentialis dorsalis (multipolar and boundary cells), nucleus medialis and crista cerebellaris, and the cerebellum, whose Purkinje cells were the most intensely labeled. The functional implications of inositol 1,4,5-trisphosphate receptor localization in the electrosensory lateral line lobe are discussed.