Relations between prolonged hypothyroidism and electroneuro-physiological events in trout, Salmo gairdnerii. Effects of replacement dosages of thyroxine

Abstract

A total of sixty 2.5-year-old rainbow trout ( Salmo gairdnerii) were used. About half of them had been radiothyroidectomized by I 131 injections that began when the animals weighed 2–3 g, and continued for 6 months. Accordingly, they were without thyroids for more than two years, and fed a low iodine diet. Heart rate was 26% lower in thyroidectomized than in control trout. It was increased in both groups by low dosages of thyroxine, but decreased by a higher dose. Thyroidectomized trout had a differentiated (from normal) pattern of spontaneous EEG from the olfactory bulb, they had a higher stimulus threshold for evoking an electrical response in the olfactory bulb, and the magnitude of the response at any particular stimulus strength was inferior to the control. Thyroxine lowered the threshold in thyroidectomized trout and increased the size of the electrical response. Most of the these measured parameters of nervous function varied biphasically with thyroxine dose. That is, they were stimulated at lower doses, but beyond a certain dose most often they were progressively inhibited. Measuring the evoked olfactory response in these animals after section of the olfactory tract or of the midbrain-diencephalon connection showed that a complex series of inhibitory or stimulatory (depending on thyroid state) regulatory influences move centrifugally from the hindbrain and from the forebrain to modify the olfactory response.