The aim of the present study was to correlate the number of prolactin and LH receptors in the ovary with the changes in sexual behaviour that occur within a few days following parturition in rabbits. Multiparous New Zealand white rabbits at days 0, 3 and 10 of lactation were tested for their receptivity upon presentation to a male. Rabbits were classed as either receptive or nonreceptive at each stage of lactation; half of the animals in each class were treated with bromocryptine to examine the effects of prolactin deprivation. Ovarian receptors for LH and prolactin, as well as the concentration of their corresponding mRNA, were measured at each stage of lactation in every group. Results indicate that receptive behaviour is correlated with significantly more follicles on the rabbit ovary (diameter > 1 mm; P < 0.05) and an increase in the concentration of LH receptor mRNA (P < 0.001) and prolactin receptors (P < 0.05). In addition, on day 4 of lactation, there were significantly fewer follicles in nonreceptive rabbits (P < 0.05). LH receptor content remained constant on days 1 and 4 of lactation but increased on day 11 (P < 0.05). Bromocryptine treatment had no effect on the number of follicles or on the amount of LH receptor mRNA in does, but it significantly increased LH receptors (P < 0.01), and the concentration of prolactin receptor mRNA (P < 0.001), particularly on day 11 of lactation (P < 0.05), and prolactin receptor content (P < 0.001). Receptive rabbit ovaries therefore display more follicles that can respond to an LH surge via newly transcribed LH receptors than do nonreceptive. Bromocryptine treatment seems to relieve some repressive activity exerted by prolactin on the number of LH receptors in the rabbit ovary.

The ontogeny of testicular LH and FSH receptors was studied in New Zealand rabbits from 20 to 180 days postpartum. The concentrations of free receptors (per mg total proteins) were very low at day 20. They increased steeply at day 30 for the LH receptor and at day 50 for the FSH receptor. Three RNA bands (1.2, 2.5 and 3 kb) were repeatedly detected on northern blots for the LH receptor and two bands (1.2 and 2.2 kb) were detected for the FSH receptor. The 1.2 kb band (which cannot give rise to full-length, membrane-anchored receptor) was present throughout the 20–180 day period for each receptor. However, the higher molecular mass bands were nearly undetectable at day 20. The 2.5 and 3 kb bands of the LH receptor increased twofold between day 20 and day 120, while the 2.2 kb band of the FSH receptor increased fivefold between day 20 and day 75. Thus the very low concentrations, or even absence, of the larger transcripts of both LH and FSH receptors were correlated with the inability to detect their cognate protein until 20 days of age. Subsequently, coordinated increases in high molecular mass transcripts and protein were observed for both receptors. Total LH receptor content increased in parallel to the previously reported increase in plasma testosterone between day 65 and day 100. FSH receptor density began to increase steeply at day 50, just at the onset of spermatogenesis. Thus, postnatal testicular development in the rabbit seems to entail the transcription of high molecular mass, translatable transcripts of the gonadotrophin receptors.