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Abstract
Fasting and malnutrition affect the hypothalamus-pituitary-thyroid axis causing secondary hypothyroidism. The aim of this study was to determine the effect of 48h fasting in rats of 4 and 12 weeks old, and of 7 weeks of food restriction, in the activity of adenohypophysial thyrotropin-releasing hormone (TRH)-degrading ectoenzyme; also, to study if these conditions altered peptide levels and its messenger ribonucleic acid (mRNA) in brain areas of the limbic system. Male Wistar rat brains were excised, frozen and dissected for measurement of: (TRH) content by radioimmunoassay, its mRNA by reverse transcriptase-polymerase chain reaction (RT-PCR), the activity of its degrading ectoenzyme, pyroglutamyl peptidase II, by a fluorometric assay, serum thyrotropin and corticosterone by radioimmunoassay. Malnutrition increased median eminence TRH and decreased thyrotropin serum levels as did fasting in adults, that also diminished hypothalamic peptide content; post-weaned rats in contrast, only had a decreased pyroglutamyl peptidase II activity in adenohypophysis. Serum corticosterone levels increased in all treatments. Brain TRH content varied differently among groups: a decrease in nucleus (n.) accumbens and an increment in hippocampus of 4 weeks old; a decrease in amygdala of adults (with a concomitant diminution in its mRNA levels). No variations were observed in food restricted rats. These results show a differential regulation of TRH metabolism by fasting dependent on the age of the animals.