Hypothyroidism and Specific Phase Relation of Circadian Serotonergic and Dopaminergic Oscillators in Gonadal Development of Japanese Quail (Coturnix coturnix japonica)

Abstract

Sexually mature male Japanese quail were used to study thyroid-gonad relationship and reproductive effects of temporal phase relation (8-hr and 12-hr) of circadian serotonergic and dopaminergic oscillators in hypothyroidic condition. To induce hypothyroidism, birds were injected with an antithyroid drug (2-thiouracil - TU). Quail were divided into six groups. First group quail received two daily injections of normal saline (Control), second group quail received one daily injection of 2-Thiouracil (an antithyroid drug-TU group), third group received serotonin precursor (5-Hydroxytryptophan, 5-HTP) at 5.00 A.M. and dopamine precursor (L-dihydroxyphenylalanine, L-DOPA) at 1.00 P.M. (8-hr group), fourth group received 5-HTP and L-DOPA injections at the interval of 8 hours as in group third as well as one daily injection of TU (8-hr + TU group), fifth group received daily injections of LDOPA12 hours after the injections of 5-HTP (12-hr group) and sixth group received 5-HTP and L-DOPA injections at the interval of 12 hour as well as one daily injections of TU (12-hr + TU group). Quail received daily injections of 5-HTP and L-DOPA 8 hours and 12 hours apart respectively over a period of eleven days only. However, TU injections alone or along with 5-HTP and L-DOPA continued till the termination of study (i.e. 7 weeks). Findings indicate that cloacal gland volume; testicular activity and testosterone level along with thyroxin was significantly low in TU treated quail compared to control suggesting parallel thyroid-gonad relationship in this species. Quail of both 8-hr and 8-hr+TU treated group also showed reproductively regressed condition. On the other hand, testes of 12-hr quail showed full breeding condition, which was also maintained in 12-hr +TU quail. Obviously, gonadostimulatory effect of 12-hr temporal phase relation of 5-HTP and L-DOPA injections were maintained even in hypothyroidic condition suggesting that inhibitory effect of TU on gonadal development was overpowered by stimulatory effect of 12-hr phase relation, it is concluded that gonadal growth in Japanese quail requires the presence of optimal level of thyroid hormones and 12-hr temporal relation of circadian serotonergic and dopaminergic activities is a potent stimulator of neuroendocrine gonadal axis even in hypothyroidic condition, while 8-hr temporal phase relation of these neurotransmitter precursors inhibits gonadal activity. It is suggested that circadian temporal phase relation of neurotransmitters may modulate the activity of neuroendocrine axis.