Effect of hypothalamus lesions on the presence of CRF-immunoreactive nerve terminals in the median eminence and on the pituitary-adrenal response to stress.

By use of an antiserum raised against conjugated ovine corticotropin releasing factor (CRF1-41), nerve fibres can be stained immunocytochemically in the external zone of the median eminence of rats. The presence of CRF-immunoreactive (CRFi) nerve fibres and the plasma corticosterone response to ether stress were studied in rats 6-7 days after making various types of lesions in the hypothalamus. Complete anterolateral deafferentation of the mediobasal hypothalamus caused complete disappearance of CRFi fibres from the median eminence and blocked the corticosterone response to stress. Incomplete anterolateral hypothalamic deafferentation did not prevent the stress-induced increase of corticosterone and in these rats, part of the CRFi nerve fibres remained intact. A horizontal cut placed ventral to the paraventricular nuclei, completely prevented the corticosterone response in those rats that showed a complete disappearance of CRFi nerve fibres from the median eminence. Some rats however, still exhibited CRFi nerve fibres and these animals responded to stress with increased corticosterone levels. A similar horizontal cut made just dorsal to the paraventricular nuclei affected neither the corticosterone response to stress nor the appearance of CRFi nerve fibres in the median eminence. We conclude that the presence of CRFi nerve fibres in the median eminence is a prerequisite for rats to show a pituitary-adrenal response to ether stress and therefore represents the first functional evidence for the role of these hypothalamic CRFi-neurons.