GABA(A), GABA(C), AND NMDA RECEPTOR SUBUNIT EXPRESSION IN THE SUPRACHIASMATIC NUCLEUS AND OTHER BRAIN-REGIONS

Identification of the neurotransmitter receptor subtypes within the su prachiasmatic nuclei (SCN) will further understanding of the mechanism of the biological clock and may provide targets to manipulate circadi an rhythms pharmacologically. We have focused on the ionotropic GABA a nd glutamate receptors because these appear to account for the majorit y of synaptic communication in the SCN. Of the 15 genes known to code for GABA receptor subunits in mammals we have examined the expression of 12 in the SCN, neglecting only the alpha 6, gamma 3, and rho 2 subu nits. Among glutamate receptors, we have focused on the five known gen es coding for the NMDA receptor subunits, and two subunits which help comprise the kainate-selective receptors. Expression was characterized by Northern analysis with RNA purified from a large number of mouse S CN and compared to expression in the remaining hypothalamus, cortex an d cerebellum. This approach provided a uniform source of RNA to genera te many replicate blots, each of which was probed repeatedly. The most abundant GABA receptor subunit mRNAs in the SCN were alpha 2, alpha 5 , beta 1, beta 3, gamma 1 and gamma 2. The rho 1 (rho1) subunit, which produces GABA(C) pharmacology, was expressed primarily in the retina in three different species and was not detectable in the mouse SCN des pite a common embryological origin with the retina. For several GABA s ubunits we detected additional mRNA species not previously described. High expression of both genes coding for glutamic acid decarboxylase ( GAD(65) and GAD(67)) was also found in the SCN. Among the NMDA recepto r subunits, NR1 was most highly expressed in the SCN followed in order of abundance by NR2B, NR2A, NR2C and NR2D. In addition, both GluR5 an d GluR6 show clear expression in the SCN, with GluRS being the most SC N specific. This approach provides a simple measure of receptor subtyp e expression, complements in situ hybridization studies, and may sugge st novel isoforms of known subunits.