The effect of testosterone on the responsiveness of the quail's pituitary to luteinizing hormone-releasing hormone (LH-RH) during photoperiodically induced testicular growth

Experiments were undertaken to determine whether the responsiveness of the pituitary to luteinizing hormone-releasing hormone (LH-RH) changed during photoperiodically induced testicular growth in the Japanese quail ( Coturnix coturnix japonica). LH release from the pituitary was induced either by the intravenous administration of 0.5 μg LH-RH/100 g body weight, or by elevating the animal's own level of endogenous LH-RH by electrical stimulation in the posterior basal hypothalamus. Groups of both intact and castrated males were tested while still on short days and on the seventh, twenty-eighth, fifty-sixth day following transfer to long daylengths. Pituitary responsiveness to LH-RH was increased by the seventh long day in the intact birds (max. incremental LH change: 11.6 ± 2.2 (20) ng/ml and 25.8 ± 4.0 (20) ng/ml, respectively) but this heightened sensitivity had disappeared by the twenty-eighth and fifty-sixth long day (max. incremental LH change: 17.5 ± 1.9 (19) ng/ml and 12.6 ± 1.3 (18) ng/ml, respectively). No significant changes were observed in the responsiveness of the castrates during photostimulation. The role of testosterone in modifying pituitary responsiveness was investigated by implanting Silastic capsules containing testosterone into castrated males. After 7 long days it was observed that in the group of birds where testosterone had very little negative feedback effect on resting plasma LH levels the incremental change induced by LH-RH was significantly augmented compared with the untreated castrates (27.6 ± 6.2 (7) ng/ml versus 15.0 ± 1.6 (13) ng/ml), suggesting a positive sensitizing effect of testosterone on the pituitary. In addition to this positive effect testosterone also causes negative feedback, which operates to maintain a low plasma LH concentration throughout the life of the animal both under short days, when the birds are immature, and when they are sexually mature on long days. It would appear, therefore, that testosterone has a dual role in modifying plasma LH concentration. A negative feedback, probably acting at the level of hypothalamus dampens LH secretion while superimposed upon, but independent of this negative effect, there is a transient positive feedback action on the pituitary to increase its responsiveness to LH-RH.