Modulation of a 5-HT1A receptor-mediated behavioral response by the neuropeptide galanin

Injection of galanin into the dorsal hippocampus impairs emotional memory independent of 5-HT1A receptor activation

There is evidence that interaction between the neuropeptide galanin and the 5-HT1A receptor represents an integrative mechanism in the regulation of serotonergic neurotransmission. Thus, in rats intracerebroventricular (i.c.v.) galanin did not impair retention in the passive avoidance (PA) test 24 h after training, but attenuated the retention deficit caused by subcutaneous (s.c.) administration of the 5-HT1A receptor agonist 8-OH-DPAT. This impairment has been linked to postsynaptic 5-HT1A receptor activation. To confirm these results in mice, galanin was infused i.c.v. (1 nmol/mouse) in C57BL/6/Bkl mice 30 min prior to training followed by s.c. injection (0.3 mg/kg) of 8-OH-DPAT or saline 15 min before PA training. In line with previous results, i.c.v. galanin significantly attenuated the PA impairment caused by 5-HT1A receptor activation in mice. To study if the galanin 5-HT1A receptor interaction involved the dorsal hippocampus, galanin (1 nmol/mouse) was directly infused into this brain region alone or in combination with s.c. 8-OH-DPAT. However, unlike i.c.v. galanin, galanin infusion into the dorsal hippocampus alone impaired PA retention and failed to attenuate the 8-OH-DPAT-mediated PA impairment. These results indicate that the ability of i.c.v. galanin to modify 5-HT1A receptor activation is not directly mediated via receptor interactions in the dorsal hippocampus. Instead, the galanin-mediated PA impairment suggests an important inhibitory role of galanin receptors in the dorsal hippocampus for acquisition (encoding) and/or consolidation of emotional memory. In addition, the interaction between galanin and 5-HT1A receptors probably involves a wide serotonergic network that is important for the integration of emotional and cognitive behaviors.

Anxiolytic- and antidepressant-like profiles of the galanin-3 receptor (Gal3) antagonists SNAP 37889 and SNAP 398299

The neuropeptide galanin mediates its effects through the receptor subtypes Gal(1), Gal(2), and Gal(3) and has been implicated in anxiety- and depression-related behaviors. Nevertheless, the receptor subtypes relevant to these behaviors are not known because of the lack of available galanin-selective ligands. In this article, we use behavioral, neurochemical, and electrophysiological approaches to investigate the anxiolytic- and antidepressant-like effects of two potent small-molecule, Gal(3)-selective antagonists, SNAP 37889 and the more soluble analog SNAP 398299. Acute administration of SNAP 37889 or SNAP 398299 enhanced rat social interaction. Furthermore, acute SNAP 37889 was also shown to reduce guinea pig vocalizations after maternal separation, to attenuate stress-induced hyperthermia in mice, to increase punished drinking in rats, and to decrease immobility and increase swimming time during forced swim tests with rats. Moreover, SNAP 37889 increased the social interaction time after 14 days of treatment and maintained its antidepressant effects during forced swim tests with rats after 21 days of treatment. In microdialysis studies, SNAP 37889 partially antagonized the galanin-evoked reduction in hippocampal serotonin (5-hydroxytryptamine, 5-HT), as did the 5-HT(1A) receptor antagonist WAY100635. Their combination produced a complete reversal of the effect of galanin. SNAP 398299 partially reversed the galanin-evoked inhibition of dorsal raphe cell firing and galanin-evoked hyperpolarizing currents. These results indicate that Gal(3)-selective antagonists produce anxiolytic- and antidepressant-like effects, possibly by attenuating the inhibitory influence of galanin on 5-HT transmission at the level of the dorsal raphe nucleus.