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

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Abstract
The dual role previously suggested for noradrenergic neurons in thyrotropin releasing hormone-TSH [thyrotropin] regulation was studied in male rats. .alpha.-Methyl-p-tyrosine (.alpha.MPT) significantly decreased brain noradrenaline [norepinephrine] (NA) and dopamine (DA) levels 0.5-6 h after the injection. Maximum inhibition of TSH cold-response occurred at 4 h. One hour after L-dopa administration 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 3rd 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. 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.