Direct Inhibitory Action of Galanin on Hypothalamic Arcuate Nucleus Neurones Expressing Galanin Receptor Gal-R1 mRNA

Abstract

Galanin may act in the regulation of reproduction and feeding behaviour through a direct action on arcuate nucleus (ARC) neurones but evidence for a postsynaptic action is absent. The effects of rat galanin(1–29) on rat ARC neurones were examined during recordings with biocytin-filled sharp microelectrodes from brain slices. Neurones were subsequently visualised and hybridized for the mRNA of the galanin receptor Gal-R1. Current-clamp recordings revealed that ARC neurones displayed a functional heterogeneity in their firing properties. A great majority of neurones showed low-threshold spikes, instantaneous and time-dependent inward rectifications and an outward rectification. Two types of bursting behaviours were recorded, depending on the level of the membrane potential. Galanin (500 nM) elicited a reversible hyperpolarisation with a decrease in the resistance or a decrease in resistance only in 9 of 34 tested neurones, independent of the electrophysiological phenotype. The reversal potential for the galanin response was about –94 mV, near the equilibrium potential for K⁺. Galantide (10 nM) antagonised the effects which were not blocked by tetrodotoxin, indicating a postsynaptic action. In situ hybridisation revealed that Gal-R1 mRNA occurred in the neurones that responded to galanin, whereas the unresponsive neurones never exhibited the hybridisation signal. This suggests that the responsive neurones possess functional postsynaptic Gal-R1. The study demonstrates for the first time a direct inhibitory action of galanin on ARC neurones, probably mediated by activation of Gal-R1. This action may have important implications for the functioning of neurones involved in the regulation of reproduction and feeding behaviour, such as proopiomelanocortin neurones.