Further evidence of the dual role of noradrenaline in regulation of thyrotrophin secretion in male rats.

The dual role previously suggested for noradrenergic neurons in TRH-TSH regulation was studied in male rats. Alpha-Methyl-p-tyrosine (alpha MPT) significantly decreased brain noradrenaline (NA) and dopamine (DA) levels 0.5-6 h after the injection. Maximum inhibition of TSH cold-response occurred at 4 h. One h after administration of L-dopa the brain DA (but not NA) level was considerably elevated. TSH cold-response was not significantly changed. The effects of alpha MPT and Ca-fusarate on the cold-induced TSH secretion were antagonized by neither dihydroxyphenylserine (DOPS; this drug restores NA stores) nor by L-dopa (which restores DA levels). DOPS even potentiated the depressive effect of alpha MPT and Ca-fusarate. When given into the third ventricle, NA did not modify basal TSH levels but decreased the cold-induced TSH secretion. When infused directly into the median eminence of the medial basal hypothalamus, NA reduced both basal and cold-induced TSH secretion. Pinealectomy decreased the TSH cold-response but NA seemed to retain its ability to inhibit TSH secretion. These results demonstrate that besides the possible stimulatory action on TSH secretion, noradrenergic neurons also have an inhibitory effect outside the blood-brain barrier, possibly in the median eminence.