Abstract
1. 1. The median eminence content of immunoreactive somatostatin (IRS) was measured by radioimmunoassay in 107 male albino rats, who were either hypothyroid after surgical thyroidectomy ( N = 38), hyperthyroid following a subcutaneous implant of 5 mg of l-thyroxine ( N = 36), or otherwise untreated ( N = 33). 2. 2. Thyroid function was assessed by determining plasma levels of T 4 and TSH from trunk blood obtained at the time of decapitation. Subgroups of animals from the 3 groups were killed either before (1800 hr), during (2200, 0200, 0400 hr), or after the dark portion of their 14:10LD photoperiod. 3. 3. Although no changes in median eminence IRS content were found throughout the period of study within any of the 3 groups, hypothyroid animals (297.23 ± 13.47 ng per ME; 620.41 ±58 ngIRS/mg protein) had a significantly lower median eminence IRS concentration than untreated rats (355.86 ± 16.55 ng of IRS per ME, P < 0.01; 906.86 ± 96.38 ng IRS/mg protein, P <0.05) and hyperthyroid animals (384.12 ± 14.67 ng per ME, P < 0.001; 874.1 ± 104.5 ng IRS//mg protein, P < 0.05). 4. 4. It is concluded, that the feedback of thyroid hormones on the hypothalamic-pituitary axis during thyroid hormone excess in vivo, contrary to what occurs in hypothyroidism, is probably independent of hypothalamic somatostatin.
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