Galanin and
galanin receptors are widely distributed within the central nervous system, but historically much research has been focused on hypothalamic
galanin systems including those in the preoptic area, paraventricular nucleus (PVN), supraoptic nucleus (SON), and median eminence. In early studies,
galanin mRNA, immunoreactivity, and binding sites were detected in neurons of the SON and both the magnocellular and parvocellular regions of the PVN, all of which also contain
vasopressin,
oxytocin, and several other
peptides. This article briefly reviews some important recent studies of the electrophysiologic effects of
galanin on magno-cellular neurons in vitro; regulation of
galanin expression by the physiologic stimulus of lactation; the role of parvocellular
galanin systems in energy balance,
body weight, and
obesity; and the regional and cellular localization of
galanin and
galanin receptor mRNAs in the PVN/SON. In relation to the latter issue, two distinct
galanin receptor subtypes, GalR1 and GalR2, have now been cloned and characterized. In situ hybridization histochemical studies of rat brain by several groups have consistently demonstrated GalR1
mRNA in the SON and PVN, in the magnocellular and parvocellular regions. By contrast, our recent experiments using [35S]-labeled
oligonucleotide probes detected GalR2
mRNA enriched in the parvocellular, not the magnocellular regions of the PVN, and the transcripts were not detected in the SON, whereas studies by other using a
digoxigenin-labeled
RNA probe have detected GalR2
mRNA in the SON (and PVN). Nonetheless, given the known effects of hyperosmotic stimuli, changes in metabolic status, and various
hormones on
galanin synthesis and release and the ability of
galanin to regulate the electrical and secretory activity of magnocellular neurons, it will be of interest to determine any possible (differential) regulation of
galanin receptor subtype expression and the pre- and postsynaptic roles of GalR1 and GalR2 receptors in magnocellular and parvocellular neurons.