The possible role of the neural connections of the medial-basal hypothalamus (MBH) in the maintenance of GH releasing activity of the pituitary stalk median eminence (SME) was investigated. Male rats, subjected to sham-operation and to complete and anterolateral cuts around the MBH were used 7–8 days after surgery. Electrical stimulation of neural structures within the MBH caused an increase of plasma GH in pentobarbitone- as well as in urethane-anaesthetized animals. In sham-operated rats the rise of plasma GH levels was apparent only after completion of 10 min of electrical stimulation, while in animals with complete or anterolateral cuts an increase was already evident during electrical stimulation. The results suggest that depolarization of somatostatin secreting fibres in the median eminence may be responsible for the delay in the rise of GH levels in sham-operated rats, while the increment can be attributed to a GH releasing principle in the hypothalamus. Acidic extracts of the SME of rats with complete or anterolateral cuts stimulated the release of GH by primary cultures of rat anterior pituitary cells. Microinjection of 0·05 SME equivalents of SME extract into the anterior pituitary gland of urethane-anaesthetized rats produced a rise in plasma GH levels within 3 min of injection. These data favour the existence of a GH releasing factor, and suggest that the ventromedial and arcuate hypothalamic nuclei are major sites of production of this releasing hormone.
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