Immunohistochemical distribution of the somatostatin receptor subtype 5 inthe adult rat brain: Predominant expression in the basal forebrain

Somatostatin exerts its actions by means of a family of G protein-coupled r eceptors, five of which have so far been cloned. Whereas mRNAs for receptor subtypes sst(1)-sst(4) have been unequivocally localized in the brain, the data concerning the fifth subtype, sst(5), are contradictory. Moreover, wh ereas sst(1) and sst(2A) receptor proteins have been localized by immunohis tochemistry, the distribution of sst(3)-sst(5) receptor proteins and/or sub type-specific binding remains to be determined in the central nervous syste m. In the present study, we investigated the distribution of immunoreactive sst(5) in adult rat brain and pituitary and demonstrated the presence of t his receptor protein in the central nervous system by using an affinity-pur ified antibody generated against the C-terminus of the receptor. The specif icity of the antibody for sst(5) was established by immunoblotting experime nts on membranes prepared from cells transfected with cDNA encoding differe nt somatotropin release inhibiting (SRIF) receptor subtypes as well as from anterior pituitary. In both systems, the antibody specifically recognized a band at approximately 50 kDa molecular mass, corresponding well to the re ported size of the cloned receptor (48 kDa). Immunofluorescence in COS-7 ce lls transfected with individual SRIF receptor subtypes as well as in sectio ns of rat pituitary demonstrated the antibody's applicability to the immuno histochemical detection of sst(5) receptors. In rat brain sections, sst(5) immunoreactivity was predominantly associated with neuronal perikarya and p rimary dendrites. Immunolabeling was most prominent in the olfactory tuberc le, islands of Calleja, diagonal band of Broca, substantia innominata, and magnocellular preoptic nucleus of the basal forebrain as well as in the ret icular nucleus of the thalamus. Other, less intensely labeled areas include d the cerebral cortex, hippocampus, amygdala, preoptic area as well as the lateroanterior nucleus of the hypothalamus. The present findings provide th e first characterization of the anatomic distribution of sst5 receptors in the rat brain. They demonstrate a prominent expression of these receptors i n the basal forebrain, suggesting that they may be involved in the mediatio n of somatostatin effects on the sleep-wake cycle through their association with cortically projecting subcortical systems. (C) 1999 Wiley-Liss, Inc.