Effects of hypothyroidism on the enzymes for biogenic amines in the developing rat brain

Abstract

The effects of neonatal radiothyroidectomy on the development of tyrosine hydroxylase (TH), DOPA-decarboxylase (DOPAdC), 5-hydroxytryptophan-decarboxylase (5-HTPdC), catechol-O-methyl transferase (COMT), and monoamine oxidase (MAO) in cerebral hemispheres (CH) and brainstem (BS) of rats have been studied in homogenates and subcellular fractions at two critical developmental stages. At 13 days of age, DOPAdC, 5-HTPdC, COMT and MAO activities were decreased in the CH of hypothyroid animals whereas TH activity tended to be increased. These changes were particularly evident in the subcellular compartments displaying the highest relative specific activity (RSA) of the individual enzymes. Less severe changes were observed in 32 day old rats perhaps because the brain had adapted to the hormonal deficiency. The RSA of the various enzymes was little affected by hypothyroidism and age. The preponderance of the synthesis over catabolism, as reflected by increased TH and decreased MAO and COMT activities, suggests an enzymatic compensatory response to hypothyroidism to normalize the levels of catecholamines, and probably serotonin, available to the brain.