Galanin-R1 and-R2 receptor mRNA expression during the development of rat brain suggests differential subtype involvement in synaptic transmission andplasticity

The present study employed S-35-labelled oligonucleotides and in situ hybri dization to examine the distribution in the developing rat brain of mRNA en coding two galanin receptor subtypes, i.e. Gal-R1 and Gal-R2. Gal-R1 and/or Gal-R2 mRNA was detected at embryonic day (E) 20 and from postnatal day (P ) 0-70. Gal-R1 mRNA was highly expressed in olfactory regions, ventral hipp ocampal CA fields, dorsomedial thalamic areas and many hypothalamic nuclei at all ages studied. In adult brain, Gal-R2 mRNA was most abundant in the d entate gyrus, anterior and posterior hypothalamus, raphe and spinal trigemi nal nuclei, and in the dorsal motor nucleus of the vagus. At P0-P7, Gal-R2 mRNA was more widely distributed and abundant than at other ages, with high est levels of expression detected throughout the neocortex and thalamus. Th us, Gal-R2 transcripts had a more restricted distribution than Gal-R1 and w ere differentially abundant at different ages, while the distribution and r elative abundance of Gal-R1 mRNA did not alter substantially during postnat al development. In general, Gal-R1 and -R2 mRNAs were localized in regions previously shown to contain [I-125]-galanin binding sites and galanin-posit ive terminals in adult brain. Galanin-immunostaining was assessed in postna tal brain to determine whether peptide innervation correlated with observed transient receptor expression, but was not particularly enriched in Gal-R2 mRNA-positive areas of P4 or P7 brain. These results, together with earlie r findings [e.g. Burazin, T. C. D. & Gundlach, A. L. (1998) J. Neurochem., 71, 879-882], suggest that Gal-R1 receptors have a broad role in normal syn aptic transmission, while Gal-R2 receptors, in addition to a similar role i n particular pathways, may be involved in processes prominent during the es tablishment and maturation of synaptic connections in developing brain and during neural damage and repair in the mature nervous system.