Gene expression and in vitro release of galanin in rat hypothalamus during development

The expression and distribution of the mRNA coding for galanin precursor, preprogalanin (ppGAL), were analysed in several rat hypothalamic nuclei (periventricular, paraventricular, supraoptic, dorsomedial and arcuate nuclei and the lateral hypothalamic area) during development by an in situ hybridization technique and computer-assisted grain counting over individual cells. ppGAL mRNA (expressed as number of grains/100 microns2) was detectable from postnatal day (PD) 1 in all the nuclei considered, and the amount of transcript per cell was 6-11 times less than in the adult. ppGAL mRNA progressively increased from PD8 to 14 to 21. The level of ppGAL mRNA in all the nuclei at PD21 was about half that in adulthood, except in the dorsomedial nucleus, where the difference was no more than 20%. As an index of the activity of galanin-containing neurons, we measured the basal and K(+)-evoked in vitro release of galanin-like immunoreactivity from hypothalamic slices of PD14, 21 and 90 rats by radioimmunoassay. Basal release of galanin-like immunoreactivity remained at the same level from PD14 to PD90, but the response to KCl (50 mM) stimulation was lower at PD14 (approximately 90%) and PD21 (> 200%) than at PD90 (350%). Basal and K(+)-evoked release was sensitive to tetrodotoxin, indicating a neuronal origin. This study provides the first evidence that the increase in ppGAL mRNA during the ontogeny of hypothalamic nuclei is associated with an increase in galaninergic neuronal function.