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

There is evidence that interaction between the neuropeptide galanin and the 5-HT 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-HT receptor agonist 8-OH-DPAT. This impairment has been linked to postsynaptic 5-HT 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-HT receptor activation in mice. To study if the galanin 5-HT 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-HT 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-HT receptors probably involves a wide serotonergic network that is important for the integration of emotional and cognitive behaviors.