Galanin Modulates 5‐Hydroxytryptamine Functions: Focus on Galanin and Galanin Fragment/5‐Hydroxytryptamine1A Receptor Interactions in the Braina

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The reciprocal interactions between galanin and 5-HT1A receptors in the rat brain are presented. Galanin and its NH2-terminal fragments antagonize 5-HT1A receptor-mediated transmission at the postjunctional level, whereas galanin receptor activation mimics the inhibitory action of 5-HT1A receptor activation at the soma-dendritic level, leading to reductions of 5-HT metabolism and release. These interactions have been shown in both receptor binding studies and functional studies. In view of the present findings, galanin antagonists may represent a new type of anti-depressant drug, based on the 5-HT hypothesis of depression, by enhancing 5-HT release and postjunctional 5-HT1A-mediated transmission. Moreover, following intracerebroventricular injection galanin was found to be internalized in a population of hippocampal nerve cells mainly representing GABA, somatostatin, and/or NPY-immunoreactive nerve cells. The relevance of these findings is discussed in relation to the concept of volume transmission.