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

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.