| The involvement of the hypothalamus in territorial aggression between conspecifics is well documented. However, very little is known regarding its role in other innate defense behaviors. The purpose of this study was to examine the involvement of the lateral hypothalamus (LH) and anterior hypothalamus (AHN) on two defensive behaviours by rats using the elevated plus-maze and shock-probe burying tests. In Experiment 1, anesthetized rats were implanted with bilateral guide cannulae aimed at either the LH or the AHN. Infusions of muscimol, a GABAA agonist, into the LH selectively increased rats' open-arm avoidance in the elevated plus-maze, but had no effect on rats' burying behaviour in the shock-probe burying test. The opposite behavioural pattern was observed following infusions of muscimol into the AHN. At that site, muscimol infusions selectively decreased burying without affecting open-arm avoidance. This double dissociation supports the contention that the lateral and anterior hypothalamic nuclei are part of two functionally distinct limbic pathways, each of which regulate specific defensive responses. In Experiment 2, the focus was to replicate that LH infusions of muscimol selectively increased open-arm avoidance and to examine whether such infusions alter neuronal activation following exposure to the plus-maze. Anaesthetized rats were implanted with bilateral guide cannulae aimed at the LH. Following recovery, rats were infused with muscimol, tested in the plus-maze. Two hours later their brains were excised, fixed and processed for cFos-immunoreactivity. As in our prior study, infusions of muscimol into the LH selectively increased rats' open-arm avoidance. Relative to home-cage and saline-treated rats, muscimol-treated rats displayed altered neuronal activity in several interconnected structures implicated in behavioral defense regulation (i.e., anterior hypothalamus, ventromedial hypothalamus, dorsal premammillary nucleus, paraventricular hypothalamic nucleus, lateral septum, and basolateral amygdala). Muscimol also altered cFos levels in several lateral hypothalamic efferent structures that are implicated in both feeding responses and defensive behaviours (i.e., shell of the nucleus accumbens, dorsomedial hypothalamus, and ventromedial hypothalamus). Overall, these results support the idea that the LH regulates rats' exploration of potentially dangerous environments via its connections with several structures implicated in defense regulation. |
