Effect of continous light and darkness exposures on the petuitary-gonadal axis and thyroid activity in male rats

Abstract

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 (T 4) 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 T 4 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 T 4 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.

Can constant light exposure affect the thyroid gland in prepubertal male albino rats? Histological and ultrastructural study

“…In the present work, light microscopic findings in rats exposed to constant light for short period (1 month) revealed that the thyroid gland was stimulated, as evidenced by microfollicular thyroid structure and little or no colloid in the thyroid follicles. Similar results were detected by Olatunji and Sofola [30]. Belviranli and Baltaci [3] reported that the activity of the pineal gland increased and thyroid growth was inhibited in male rats under light restriction.…”

“…It has given rise to the concept that melatonin is the chemical expression of darkness. Continuous exposure to light leads to a complete suppression of melatonin release [9,30].…”

Effects of long-term light, darkness and oral administration of melatonin on serum levels of melatonin

“…These data indicate that any study involving hypothalamic TRH determination should be consider the diurnal variation and the effect of environmental light exposure [7] . When compared with the rats exposed to normal lighting rhythm, continuous darkness exposure resulted in a decrease in the thyroid activity as well as a remarkable decrease in gonadal activity in male rats [8] .However, what is the influence of continuous light, darkness and exogenous melatonin on serum levels of melatonin? Is it similar to circadian rhythm or not?…”

The Effects of Prolonged Darkness Phase of Photoperiodic Cycle on Serum Levels of Creatine Kinase and Alkaline Phosphatase in Female Rats

“…Previous studies also show that prolonged darkness phase can change the serum level of some hormones in body [15]. For example several studies show that prolonged darkness phase in photoperiodic cycle causes decreased serum level of T3 [16], [17], whereas there is a study indicating that prolonged darkness phase has no effect on secretion of T3 hormone [18]. The probable mechanism of action could be referred to the effect of prolonged darkness phase on systems such as immunity and gonadal system [19]- [21], which in turn, may result in increased serum level of creatine kinase and alkaline phosphatase.…”