Effect of Continous Light and Darkness Exposures on the Pituitary-gonadal Axis and Thyroid Activity in Male Rats

The effects of constant light and dark exposure of pubertal male rats on the pituitary-gonadal axis and thyroid activity were studied. The indices of thyroid activity were weight of the thyroid gland, estimation of serum levels of thyroxine (T4) by radioimmunoassay (RIA) technique and histological studies of the thyroid gland. The indices of gonadal activity chosen were weight and histological study of the testes and determination of serum levels of Luteinizing hormone (LH) and Follicle Stimulating Hormon (FSH) by RIA technique. In the rats exposed to continuous light, the weight of the thyroid gland increased significantly (P<0.02) and the serum level of T4 also increased significantly (P<0.02). The histological observations revealed thyroid follicles that were sparsely filled with colloid while the epithelial cells were more columnar, evidence of increased glandular activity. The size of the testes increased significantly (P<0.001). The serum level of FSH was increased significantly (P<0.001) while there was a slight rise in the serum level of LH. Histological observations in the testes showed that the seminiferous tubules were well spaced out and appeared to contain more spermatozoa when compared with the control. In the rats exposed to constant darkness, the weight of the thyroid gland was significantly lower (P<0.02) when compared with that of the control. The serum level of T4 remained unchanged while histological examination of the thyroid gland showed the follicles containing less colloid and the epithelial cells appearing more columnar when compared with the control. The weight of the testes was markedly reduced (P<0.001). The serum level of LH was reduced significantly (P<0.05) while there was a slight fall in the serum level of FSH. The histological examination of the testes showed that the, seminiferous tubules clustered together and contained little or no spermatozoa. When compared with the rats that were exposed to normal lighting rhythm, continuous darkness exposure resulted in a decrease in the thyroid activity as well as a marked decrease in gonadal activity in male rats.