Involvement of Amygdalar Galanin in Coping with Emotional Stress

Abstract

Proceedings of the 9th International Multidisciplinary Conference «Stress and Behavior» Saint-Petersburg, Russia, 16–19 May 2005.Galanin is a neuropeptide with inhibitory action to neuromediators release. Recent investigations showed the galanin participation in CNS response to stress. The influence of galanin on aversive emotional learning suggests its involvement in adaptation to stressful stimuli and habituation to repeated stresses. Amygdala plays a key role in processing of biological relevance of events and forming an appropriate behavior response. Together with hippocampus, it participates in brain plasticity during stress-coping, and certain affective stress-related disorders are associated with increased excitability of amygdala.Methods. The model of acute psychological stress — a single exposure rats to predator (python) was used. Males Wistar rats (250–300 g) were exposed to python for 25 min, and their behavior was recorded by a video camera. Rats were returned to their regular cages thereafter. During a several days, the dynamics of emotional state of rats ware evaluated by free exploration paradigm test in our modification in combination with standard behavioral tests: open field and elevated plus maze. In the second part of experiment, after acute psychological stress, the rats were separated on two experimental groups, one of them exposed to a 5-min forced swim stress 6 days later. To study possible changes in reactivity of amygdalar galanin to stress in rats after acute exposure to predator, we assessed changes in pre-progalanin mRNA expression 10-min after termination of swimming stress. The rats were sacrificed by decapitation; the brain structures were dissected, immediately frozen in liquid nitrogen and stored at –70 °C up to RNA extraction. The control animals were sacrificed without stress procedure. The level of mRNA expression was estimated by RT-PCR.Results and discussion. Preliminary experiments were carried out to study the normal behavior of Wistar rats. The following tests were used: the open field, elevated plus maze, forced swimming, social exploration and free exploration paradigm. Data showed that Wistar rats demonstrated a passive behavior style in standard behavioral tests. It appears during severe stress conditions (psychological stress — exposure to predator) too. Thus, most of Wistar rats demonstrated «freezing» behavior during the predator exposure. However, after a severe psychological stress, Wistar rats demonstrated agitation lasting for several days. We observed significant changes in the free exploration paradigm: 1.5 fold increasing of time spent in unknown part of box, and increasing of number of entrances to it with a significant decrease of latent period after stress (the same day) compared to control day. 24 h after stress, these changes were retained, besides that, in «open field» we observed signs of arousal — increased locomotor activity and impulsive behavior (increased number + lower time of holes exploration; appearance of short grooming). On the 3rd day after psychological stress, a significant decrease in anxiety level in Wistar rats was revealed in the «elevated plus-maze» test. Time spent in the open arm and number of head dipping increased. The investigation of pre-progalanin mRNA expression in amygdala revealed the changes in reactivity of amygdalar galanin to subsequent stress in rats survived the acute psychological stress. Six days after predator exposure, a forced swimming induced the increase of pre-progalanin mRNA expression in amygdala only in stressed rats, while it was not shown by naive rats. We showed that the natural psychological stress (single exposure rats to predator) induced longtime changes in behavior and promoted the hyperactivity of amygdalar galanin expression to subsequent stress. We suggest that observed prolonged post-stress motor arousal in Wistar rats may be important for stress-related pathogenesis such as anxiety and depression. Now there are evidences that galanin release in response to stress is in close correlation with level of NE-activation (Khoshbouei et al., 2002). Several mechanisms of galanin action in amygdala during psychological stress may be proposed. At first, the inhibitory action of galanin to NE release. Second, modulation of GABA-ergic system. Finally, since both galanin-producing neurons and galanin receptors there are in amygdala, galanin may act in paracrine mode, decreasing activity of amygdala. In any case, it is clear that activation of galanin expression in amygdala may lead to pooper receptivity to subsequent stimulation, therefore having a protective role. Study was supported by RFBR, grant # 03–04-49459