Peptidases in the rat hypothalamus inactivating thyrotrophin-releasing hormone (TRH)

Peptidases capable of inactivating thyrotrophin-releasing hormone (TRH) have been demonstrated in the hypothalamus. With the development of a specific radioimmunoassay for TRH, this method was used to further study the enzymes acting on the releasing hormone. Whole hypothalamic homogenates from male and female rats inactivated TRH, with greater peptidase activity being found in the female animals. Separation of the homogenates into particulate (microsomal and mitochondrial) and supernatant (soluble/cytoplasmic) fractions showed approximately the same amounts of enzyme activity in both fractions, while dialysis of the fractions slightly reduced the TRH peptidase activity present, suggesting that a diffusible co-factor might be partially involved in the releasing hormone's degradation. These results confirm the presence of TRH-inactivating peptidases in the rat hypothalamus and suggest that the enzymes may be involved in some way in the mechanisms by which the brain controls thyrotrophin release by the anterior pituitary.