Galanin (GAL) plays a role in mood regulation. In this study we analyzed the action of the active N-terminal fragment [GAL(1–15)] in anxiety- and depression-related behavioral tests in rats. The effect of GAL(1–15) was analyzed in the forced swimming test, tail suspension test, open field test, and light/dark test. The proximity of GAL₁ and GAL₂ receptors was examined with the proximity ligation assay (PLA). We tested the GAL receptors involved in GAL(1–15) effects with the GAL₂ receptor antagonist M871 and with an in vivo model of siRNA GAL₂ receptor knockdown or siRNA GAL₁ receptor knockdown rats. The effects of GAL(1–15) were also studied in the cell line RN33B. GAL(1–15) induced strong depression-like and anxiogenic-like effects in all the tests. These effects were stronger than the ones induced by GAL. The involvement of the GAL₂ receptor was demonstrated with M871 and with the siRNA GAL₂ receptor knockdown rats. The PLA indicated the possible existence of GAL₁ and GAL₂ heteroreceptor complexes in the dorsal hippocampus and especially in the dorsal raphe nucleus. In the siRNA GAL₁ receptor knockdown rats the behavioral actions of GAL(1–15) disappeared, and in the siRNA GAL₂ receptor knockdown rats the reductions of the behavioral actions of GAL(1–15) was linked to a disappearance of PLA. In the cell line RN33B, GAL(1–15) decreased 5-HT immunoreactivity more strongly than GAL. Our results indicate that GAL(1–15) exerts strong depression-related and anxiogenic-like effects and may give the basis for the development of drugs targeting GAL₁ and GAL₂ heteroreceptor complexes in the raphe-limbic system for the treatment of depression and anxiety.

Due to the binding to a number of proteins to the receptor protomers in receptor heteromers in the brain, the term “heteroreceptor complexes” was introduced. A number of serotonin 5-HT1A heteroreceptor complexes were recently found to be linked to the ascending 5-HT pathways known to have a significant role in depression. The 5-HT1A–FGFR1 heteroreceptor complexes were involved in synergistically enhancing neuroplasticity in the hippocampus and in the dorsal raphe 5-HT nerve cells. The 5-HT1A protomer significantly increased FGFR1 protomer signaling in wild-type rats. Disturbances in the 5-HT1A–FGFR1 heteroreceptor complexes in the raphe-hippocampal 5-HT system were found in a genetic rat model of depression (Flinders sensitive line (FSL) rats). Deficits in FSL rats were observed in the ability of combined FGFR1 and 5-HT1A agonist cotreatment to produce antidepressant-like effects. It may in part reflect a failure of FGFR1 treatment to uncouple the 5-HT1A postjunctional receptors and autoreceptors from the hippocampal and dorsal raphe GIRK channels, respectively. This may result in maintained inhibition of hippocampal pyramidal nerve cell and dorsal raphe 5-HT nerve cell firing. Also, 5-HT1A–5-HT2A isoreceptor complexes were recently demonstrated to exist in the hippocampus and limbic cortex. They may play a role in depression through an ability of 5-HT2A protomer signaling to inhibit the 5-HT1A protomer recognition and signaling. Finally, galanin (1–15) was reported to enhance the antidepressant effects of fluoxetine through the putative formation of GalR1–GalR2–5-HT1A heteroreceptor complexes. Taken together, these novel 5-HT1A receptor complexes offer new targets for treatment of depression.