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This study was carried out to test the hypothesis that sustained hyperprolactinaemia in patients with prolactinomas stimulates hypothalamic dopaminergic activity via a short loop positive feedback effect of prolactin (PRL). The intensity of dopamine (DA) effects on the pituitary around the adenoma was evaluated by measuring thyroid stimulating hormone (TSH) responses to intravenous injection of domperidone (10 mg) a new DA receptor blocking drug that does not penetrate the blood-brain barrier. TSH responses have been compared with those of PRL to the same agent. Eight females with prolactinomas showed greater TSH release after domperidone than nine normal females (sum of TSH increments over 20 min 17.5 +/- 1.7 v. 8.9 +/- 1.5 mu/l, P less than 0.001) whilst PRL release was reduced (sum of PRL increments over 120 min 5.9 +/- 2.4 v. 21.8 +/- 3.8 mu/l x 10(-3), P less than 0.01). Amongst nineteen hyperprolactinaemic females with apparently normal pituitary fossae (plain skull X-ray), ten showed an exaggerated TSH response (delta TSH, 4.2 +/- 0.6 mu/l, range 2.5-9.0 mu/1) and reduced PRL response to domperidone, comparable with established tumor cases. In the remaining nine normal fossa hyperprolactinaemic females, the TSH and PRL responses to dopaminergic were similar to normal females. These results support the initial hypothesis and indicate the coexistence of a defect in the dopaminergic inhibition of PRL release and increased dopaminergic inhibition of TSH release in patients with prolactinomas. The presence of an exaggerated TSH response to DA antagonism in a euthyroid, radiologically normal (plain skull X-ray), hyperprolactinaemic patient is compatible with the presence of an autonomously-functioning, PRL secreting, pituitary microadenoma and the TSH changes seen in these patients after DA antagonist administration can be readily detected by sensitive TSH radioimmunoassay.

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SUMMARY The interaction between the mechanisms involved in the LH, FSH, growth hormone, prolactin, ACTH and TSH responses to the synthetic LH and FSH releasing hormone, thyrotrophin releasing hormone and insulin induced hypoglycaemia was studied in twelve normal male volunteers. Each subject acted as his own control and the test procedures were performed individually and in combination. The simultaneous administration of one releasing hormone with another or with insulin in no way modified the hormonal responses to either releasing hormone or to hypoglycaemia. Clinical testing with these procedures may therefore be performed simultaneously, so that the pituitary reserve for the five anterior pituitary hormones may be assessed together in under 2 hr. In addition it has been shown that TRH releases a small amount of FSH but not LH in male subjects.

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In order to establish the influence of dopaminergic, alpha-adrenergic and cholinergic pathways on GRF-mediated GH release we have studied the GH responses to GRF 1-29 (100 or 50 micrograms as i.v. bolus) alone and in combination with metoclopramide (MCP, 10 mg, i.v.), thymoxamine (THYM, 210 micrograms/min, 150 min infusion), and atropine (1.2 mg, i.v.). We have also investigated any possible interaction between TRH and GRF in view of the reported inhibitory effects of TRH infusion on stimulated GH release. Dopaminergic and alpha-adrenergic blockade with MCP and THYM respectively, did not have any effect on the GH responses to GRF. This lack of effect strongly suggests that any action which these neurotransmitters may exert on GH secretion is not at a pituitary level. TRH did not modify the GH response to GRF suggesting that the inhibitory effect on stimulated GH secretion is exerted at a hypothalamic level. In contrast, GH responses to GRF were significantly reduced by prior administration of atropine. These data support the view that cholinergic pathways play an important role in the regulation of GH secretion and such control may be exerted at both hypothalamic and pituitary levels.