Endogenous prostaglandins affect growth hormone and thyrotropin release at a hypothalamic, not a pituitary level

The role of prostaglandins (PGs) in the physiological secretion of the adenohypophyseal hormones, growth hormone (GH) and thyrotropin (TSH), in the unanesthetized, freely behaving male rat was investigated using pharmacological agents. Confirming previous observations, PG synthesis-inhibiting drugs, salicylate and indomethacin (INDO), reduced GH and TSH secretion. Another PG synthesis-inhibiting drug, acetaminophen, also reduced GH and TSH secretion. Antisomatostatin serum administered to INDO-treated rats indicated that somatostatin has a relatively small role in the GH and TSH suppression caused by PG synthesis inhibitors. Stimulation of GH secretion by the alpha 2-adrenergic receptor agonist, clonidine, and by morphine was similar in control and INDO-treated rats whereas PGE2 evoked a significantly greater release of GH in INDO- than in DDC- (dopamine beta-hydroxylase inhibitor-)treated rats. Stimulation of TSH secretion was similar in response to thyrotropin-releasing hormone (TRH) in INDO-treated and control rats and was also similar in response to PGE2 in INDO- and DDC-treated rats. However, clonidine evoked a significantly greater rise in TSH secretion in control than in INDO-treated rats. These results confirm the observation that PGs are important in physiological GH and TSH secretion in the rat and suggest that PGs are involved in GH secretion by interaction proximally to the adrenergic synapse on growth hormone-releasing factor (GRF) neurons and are involved in TSH secretion by interaction postsynaptically to the adrenergic synapse on TRH neurons.