Hypothalamic substrates for brain stimulation-induced grooming, digging and circling in the rat

Despite a great number of studies concerned with the induction of specific behavioural responses from the rat hypothalamus by electrical brain stimulation, hypothalamic response areas and underlying neural substrates have never been determined accurately. In this study the boundaries of the hypothalamic response areas for grooming, digging and circling were delimited using moveable electrodes, an enriched environment containing a variety of goal objects, and an appropriate statistical technique. A total of 641 hypothalamic sites in 71 male CPB/WU Wistar rats were electrically stimulated. Results are plotted on a detailed stereotaxic brain atlas of the rat hypothalamus. Positive sites for any behavioural response cluster into restricted hypothalamic areas. Discriminant analysis of both positive and negative electrode localizations yields areas with high, intermediate or low probabilities of inducing the behavioural response concerned. Each response has its own response area where probabilities are high, although there may be overlap. Even within response areas a distinction can be made between areas in which the response can be induced at relatively high or low threshold current intensities. Lowest threshold sites within electrode tracks are often clustered. In search of neuroanatomical correlates, grooming is related to the distribution of ACTH-immunoreactive neural elements, digging is related to the distribution of efferent fibres from the bed nucleus of the stria terminalis, and circling is related to the distribution of dopaminergic fibres of the nigrostriatal pathway. The results clearly point to the stimulation site being the most important determinant of the evoked behavioural response. Evidently behavioural specificity does exist within the hypothalamus.