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Lordosis behaviour was induced in immature 20-day-old male rats by sequential treatment with oestradiol benzoate (OB) and progesterone, but prepubertal male rats were behaviourally less sensitive to the OB and progesterone treatment than were female rats. Thus, the sex difference in the lordosis response was present early during development. Castration at various times after birth showed that the capacity of immature rats to show lordosis is normally inhibited by an action of testicular secretions exerted during the first 10 days of life. Treatment of day 0 castrated rats with OB, either as a single injection given on the day of birth or as daily injections given on the first 10 days after birth, was much more effective in inhibiting the display of lordosis behaviour at 30 and 37 days of age than was treatment with testosterone benzoate (TB). Treatment with dihydrotestosterone benzoate neonatally had no inhibitory effect. Treatment of intact male rats or day 0 castrated OB-or TB-treated rats with the anti-oestrogen ethamoxytriphetol (MER-25) during the first 10 days of life antagonized the inhibitory effect of the testes and of the OB or TB treatment on the development of the lordosis response. It is suggested that during normal development oestradiol formed in the brain from testosterone in the circulation acts during the first 10 days of life to inhibit the capacity of male rats to show lordosis when adult.
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Male rats were treated daily with 100 μg of the anti-oestrogen ethamoxytriphetol (MER-25) or oil during the first 10 days of life and tested for lordosis behaviour and mounting behaviour as intact adults, after castration and after castration and oestradiol benzoate or testosterone propionate treatment. The MER-25-treated rats showed higher levels of lordosis behaviour than oil-treated rats in all four treatment groups. Under each of these endocrine conditions, except after castration alone, the MER-25-treated rats showed a reduced capacity to ejaculate. Treatment of the neonatal rat with MER-25 reduced body weight in adulthood but did not change the weight of the accessory sexual glands, the testes, the number of cornified papillae on the glans penis or plasma testosterone concentrations during development. The response of the accessory sexual glands and cornified papillae on the glans penis to treatment with oestradiol benzoate or testosterone propionate after castration in adulthood was unaffected by treatment with MER-25. It is suggested that formation of oestrogen in the neonatal male rat brain from testosterone in the circulation inhibits the capacity to show lordosis behaviour and facilitates the capacity to ejaculate in response to gonadal hormone treatment in adulthood.
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SUMMARY
Sex differences in the lordosis response of adult rats to ovarian hormones were studied in a series of experiments. Male rats were less sensitive to oestradiol benzoate (OB, a single injection of 10, 100 or 1000 μg/kg or seven daily injections of 2, 10 or 50 μg/kg) than were female rats. Oestradiol benzoate-primed (10 μg/kg) female, but not male, rats showed dose-dependent responses to progesterone (0·4, 2·0 or 10·0 mg/kg). Male rats responded clearly to progesterone (2 mg/rat) only when primed with a high dose of OB (100 μg/rat). Display of the whole pattern of female sexual behaviour was induced in male rats by treatment with 100 μg OB and 2 mg progesterone. Female rats treated with 1 mg testosterone propionate (TP) on day 4 of life, ovariectomized as adults and tested under the same endocrine conditions as the rats described above, retained behavioural OB sensitivity but responded poorly to progesterone. Evidence is presented that ovarian secretions during development significantly modify the response of neonatally TP-treated and normal female rats to OB in adulthood.
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ABSTRACT
Ovariectomy and treatment with oestradiol benzoate (10 μg OB) on the day before behavioural oestrus eliminated the preovulatory surge of LH and reduced the level of sexual receptivity on the following day. Sexual behaviour, but not the LH surge, was restored by progesterone (0·5 mg) given 18 h later. Injection of OB on the day after behavioural oestrus induced a small release of LH and normal sexual behaviour on the following day. Ovariectomy on the day after behavioural oestrus reduced the stimulatory effect of OB on sexual behaviour and eliminated its weakly stimulatory effect on LH release. Sexual behaviour, but not the small LH surge, was restored in these animals by progesterone (0·5 mg) given 18 h later. Treatment of rats ovariectomized 2 days before the day of the LH surge with implants containing oestradiol or injections of oestradiol (1 μg) induced LH surges but the amplitudes of these LH surges were much smaller than those of the normal LH surge. Treatment of intact rats with OB increased serum progesterone levels 24 h later, an effect which was eliminated by ovariectomy. Injections of LH (20 μg) into intact rats on the day after behavioural oestrus also increased serum progesterone concentrations but failed to stimulate sexual behaviour. It is suggested that OB treatment of intact rats on the day after behavioural oestrus stimulates sexual behaviour by inducing a surge of LH secretion which activates ovarian secretion of progesterone. Thus, oestrogen and progesterone but not the LH surge are essential for sexual behaviour. Whereas oestradiol and progesterone restore normal sexual behaviour in ovariectomized rats, additional ovarian factors may be required for induction of normal LH surges.
J. Endocr. (1987) 112, 133–138
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ABSTRACT
Male rats showed maternal behaviour within 72 h after the onset of continuous exposure to newborn rat pups. The latency of the behavioural response could be reduced by daily treatment with the dopamine receptor antagonist domperidone (2 × 2·5 mg/rat), which increased serum prolactin concentrations (241·4 ± 26·5 (s.e.m.) μg/l) above those of vehicle-treated males exposed to pups (25·3 ± 11·7 μg/l). Male rats did not respond to exposure to pups by secreting prolactin; keeping endogenous prolactin concentrations at a minimum (2·8±0·1 μg/l) by daily treatment with the dopamine receptor agonist bromocriptine (0·5 mg/rat) did not affect the behavioural response of male rats to newborn pups. Neither exposure to pups nor the modest hyperprolactinaemia induced by daily domperidone treatment affected the display of male sexual behaviour by male rats.
J. Endocr. (1984) 102, 115–119
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ABSTRACT
The frequency of pup sucking behaviour was related to serum concentrations of prolactin and LH in rats during various phases of lactation. Sucking frequency and prolactin levels decreased and LH levels increased as lactation progressed. There was no clear relationship between sucking frequency and either prolactin or LH levels. Serum prolactin concentrations were highest when the rats spent most of their time away from their pups and lowest when the rats spent most of their time with the pups attached to their nipples. Prolactin was secreted episodically during prolonged continuous nipple stimulation. Removal of the pups in late lactation and replacement with a newborn litter increased sucking frequency but did not affect serum LH levels and only marginally increased serum prolactin levels. Injection of the dopamine receptor antagonist domperidone produced a far more pronounced release of prolactin from the pituitary gland in early than in late lactation. A circadian control mechanism and an episodic pattern of release may contribute to the great variation in serum prolactin concentrations seen in early lactation; decreased pituitary sensitivity to dopamine receptor blockade may be related to the low concentration of serum prolactin found in late lactation.
J. Endocr. (1984) 102, 251–256
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Serum levels of oestradiol-17β fluctuated markedly during the oestrous cycle of rats. The onset of sexual receptivity occurred in close correlation with increasing serum levels of progesterone. The serum levels of oestradiol and progesterone in ovariectomized rats implanted with constant-release implants filled with oestradiol or progesterone were related to the amount of hormone in the implants. Constant low serum levels of oestradiol stimulated sexual behaviour in ovariectomized rats, but progesterone stimulation was required for maximum behavioural responses. Peak levels of progesterone in the serum during the oestrous cycle were much higher than those needed for induction of the behaviour in ovariectomized, oestradiol-treated rats. Progesterone, administered in physiological doses, inhibited the induction of sexual receptivity caused by oestradiol and progesterone and the inhibition depended on the strength of the stimulation with oestradiol.
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Progesterone-filled constant-release implants facilitated the induction of sexual receptivity in ovariectomized rats given implants of oestradiol-17β precisely 32 h before testing, irrespective of the time of implantation. Inhibition by progesterone implants of the behavioural response to an injection of progesterone occurred after the facilitation 32 h after oestradiol implantation. Sexual receptivity could be induced in pseudopregnant rats in the absence of progesterone treatment by injection of 1 μg oestradiol 32 and 16 h before testing at a time when endogenous serum levels of oestradiol were low and progesterone levels were high. The behavioural response of ovariectomized rats implanted with oestradiol and tested daily was unaffected by implantation of progesterone at the time of oestradiol implantation, although serum levels of progesterone varied with the number of progesterone implants inserted. Inhibition by progesterone implants of the behavioural response to an injection of progesterone 6 h before behavioural testing occurred only if the progesterone implants were present for at least 32 h of a 48 h period. Serum levels of progesterone were raised within 1 h of progesterone implantation and declined within a 6 h period after implant removal. It is concluded that progesterone does not inhibit the behavioural effect of oestradiol and that progesterone does not play an inhibitory role in the regulation of the behavioural oestrous cycle in our strain of rats.
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Male rats were given daily injections of the antioestrogen ethamoxytriphetol (MER-25, 100 μ/day) or oil during the first 10 days of life. Rats treated with MER-25 showed a more pronounced diurnal rhythm in both mounting behaviour and lordosis behaviour than did oil-treated rats when tested as intact adults and after castration together with treatment with testosterone- or oestradiol-filled constant release implants. Serum levels of androgen varied markedly in samples obtained at four different times of the light: darkness (LD) cycle in both neonatal treatment groups and no significant LD-dependent pattern was obvious. Castration and treatment with testosterone implants produced stable androgen levels which showed little individual variation and did not vary with the LD cycle. The results supported the hypothesis that perinatal androgen stimulation affects the development of sexual behaviour in rats primarily by decreasing the diurnal rhythmicity of the behaviour of the adult.
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Ovariectomized rats, pre-implanted with elastomer capsules containing oestradiol, became sexually receptive after exposure to progesterone (implanted in elastomer capsules) for 4–6 h. Implantation of progesterone capsules facilitated receptivity in oestradiol-implanted rats independently of both previous exposure to progesterone implants and the presence of progesterone at the time of implantation.
The duration of sexual receptivity in ovariectomized rats implanted with oestradiol and progesterone capsules was dependent upon the length of both the oestradiol and progesterone capsules, but the decline in sexual behaviour of receptive rats was independent of the continued presence of either oestradiol or progesterone. Repeated implantation of progesterone capsules at 6 hourly intervals prevented the decline of sexual receptivity.
