Galanin-receptor ligand M40 peptide distinguishes between putative galanin-receptor subtypes

The galanin-receptor ligand M40 [galanin-(1-12)-Pro3-(Ala-Leu)2-Ala amide] binds with high affinity to [mono[125I]iodo-Tyr26]galanin-binding sites in hippocampal, hypothalamic, and spinal cord membranes and in membranes from Rin m5F rat insulinoma cells (IC50 = 3-15 nM). Receptor autoradiographic studies show that M40 (1 microM) displaces [mono[125I]iodo-Tyr26]Galanin from binding sites in the hippocampus, hypothalamus, and spinal cord. In the brain, M40 acts as a potent galanin-receptor antagonist: M40, in doses comparable to that of Galanin, antagonizes the stimulatory effects of Galanin on feeding, and it blocks the galaninergic inhibition of the scopolamine-induced acetylcholine release in the ventral hippocampus in vivo. In contrast, M40 completely fails to antagonize both the galanin-mediated inhibition of the glucose-induced Insulin release in isolated mouse pancreatic islets and the inhibitory effects of Galanin on the forskolin-stimulated accumulation of 3',5'-cAMP in Rin m5F cells; instead M40 is a weak agonist at the Galanin receptors in these two systems. M40 acts as a weak antagonist of Galanin in the spinal flexor reflex model. These results suggest that at least two subtypes of the Galanin receptor may exist. Hypothalamic and hippocampal Galanin receptors represent a putative central galanin-receptor subtype (GL-1-receptor) that is blocked by M40. The pancreatic Galanin receptor may represent another subtype (GL-2-receptor) that recognizes M40, but as a weak agonist. The Galanin receptors in the spinal cord occupy an intermediate position between these two putative subtypes.