The hypothalamus: cross-roads of endocrine and behavioural regulation in grooming and aggression.

Anatomical and functional studies show that the hypothalamus is at the junction of mechanisms involved in the exploratory appraisal phase of behaviour and mechanisms involved in the execution of specific consummatory acts. However, the hypothalamus is also a crucial link in endocrine regulation. In natural settings it has been shown that behavioural challenges produce large and fast increases in circulating hormones such as testosterone, prolactin, corticotropin and corticosterone. The behavioural function and neural mechanisms of such fast neuroendocrine changes are not well understood. We suggest that behaviourally specific hypothalamic mechanisms, at the cross-roads of behavioural and endocrine regulation, play a role in such neuroendocrine changes. Mild stimulation of the hypothalamic aggressive area, produces stress levels of circulating prolactin, corticotropin, and corticosterone. Surprisingly luteinizing hormone does not change. This increase in stress hormones is due to the stimulation itself, and not caused by the stress of fighting. Similar increases in corticosterone are observed during electrical stimulation of the hypothalamic self-grooming area. The corticosterone response during self-grooming-evoking stimulation is negatively correlated with the amount of self-grooming observed, suggesting that circulating corticosterone exerts a negative feedback control on grooming. Earlier literature, and preliminary data form our laboratory, show that circulating corticosterone exerts a fast positive feedback control over brain mechanisms involved in aggressive behaviour. Such findings suggest that the hormonal responses caused by the activity of behaviourally specific areas of the hypothalamus may be part of a regulation mechanism involved in facilitating or inhibiting the very behavioural responses that can be evoked from those areas. We suggest that studying such mechanisms may provide a new approach to behavioural dysfunctions associated with endocrine disorders and stress.