Effects of varying photoperiods on the morphometry and hormonal activity of the thyroid gland of the african giant rats (Cricetomys gambianus, Waterhouse)
This study aimed to investigate the effects of continuous exposure to light and darkness on the thyroid gland of the captive African giant rats (Cricetomys gambianus). A total of thirty-six (36) apparently healthy African giant rats (AGR) of both sexes were randomly divided into three different photoperiodic groups (n = 12/group); vis; Group I: rats were exposed to continuous darkness. Group II: were exposed to continuous (24h) artificial light and group III: (Control) rats were exposed to a 12h light:12h dark cycles for a period of four (4) weeks of experimentation. The effects of continuous light and darkness exposures on the thyroid activity were studied. Serum concentration of free triiodothyronine (FT₃), free tetrathyronine (FT₄), and thyroid - stimulating hormones (TSH) were determined by radioimmunoassay (RIA) technique. In the rats exposed to continuous darkness the weight of the thyroid gland increased significantly (P<0.001) and the serum level of T₃, T₄ and TSH was higher than that of the control. The weight and width of the thyroid gland also increased significantly (P<0.001) and (P<0.05) respectively in rats exposed to light when compared with the control. However, the serum level of T₃, T₄ and TSH was higher than the control with no significant difference (P>0.05). The organo-somatic thyroid index for the control and rats exposed to darkness and lighting were 3.772 ± 0.03, 4.398 ± 0.04 and 4.085 ± 0.04 respectively, with no significant (P > 0.05) difference. In conclusion, this study has shown that exposure of AGR for four (4) weeks of continuous lighting resulted in significant increase in thyroid activity.
Keywords: African giant rat, Continuous artificial light, Continuous darkness, Thyroid gland hormones, Thyroid gland parameters
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