Influence of the gonad on the degree of suppression induced by an LHRH agonist implant in the marmoset monkey

Abstract

In a previous study pituitary-testicular function was shown to be maintained in a New World primate after the administration of an LHRH agonist implant. In the present study the mechanism of action of the same LHRH agonist (buserelin) on pituitary-gonadal function in the marmoset was investigated and a comparison made between the effects of treatment in three intact males, six adult cyclic females with regular ovulatory cycles, and six long-term ovariectomized animals. These were injected s.c. with an LHRH agonist implant (1.5 mg buserelin in a rod 0.5 cm long). In both the males and intact females, basal plasma LH concentrations were maintained within the normal range throughout the expected duration of agonist action (at least 3 months). Despite this, an absence of response to an LHRH challenge indicated that pituitary desensitization had occurred. In the intact females, ovulation was inhibited in five of six animals, plasma progesterone concentrations initially being maintained but subsequently remaining suppressed until 136 +/- 18 (S.E.M.) days after treatment. Responsiveness to administered LHRH returned prior to onset of return to ovarian cycles. In contrast, in ovariectomized marmosets, plasma LH was markedly suppressed to concentrations which were at or below the limit of detection of the assay and were therefore less than those observed in the buserelin-implanted intact animals. These results show that apparently normal pituitary-gonadal function in this species disguises an underlying pituitary desensitization to LHRH. This allows continuation of testosterone secretion in the male, but in the female ovulation is prevented, presumably as a result of failure of the desensitized pituitary to produce an LH surge.(ABSTRACT TRUNCATED AT 250 WORDS)

Social suppression of cortisol in female marmosets: Role of luteinizing hormone/chorionic gonadotropin

“…4), as previously reported for this species (Abbott et al, 1988;Lunn et al, 1992;Saltzman et al, 1998). Basal CG concentrations of ovx females were signiWcantly higher than those of intact females in the follicular phase …”

Effect of deslorelin implants on follicular development, parturition and post-partum oestrus in the tammar wallaby (Macropus eugenii)

“…Gonadotrophin releasing hormone (GnRH) agonist treatment has been shown to temporarily suppress ovarian cycles in dogs (Lacoste et al 1989, ewes (McNeilly & Fraser 1987), marmoset monkeys (Lunn et al 1992), stumptailed macaques (Fraser & Sandow 1985) and women (Fraser et al 1990). In these species, suppression of ovarian cycles is characterised by an inhibition of follicular development, with a resultant decline in the concentrations of oestradiol and progesterone and an inhibition of oestrus.…”

Long-term effects of deslorelin implants on reproduction in the female tammar wallaby (Macropus eugenii)

“…These basal progesterone concentrations were presumably of adrenal, rather than ovarian, origin, as proposed by Sernia et al (1980) for ovariectomised animals. Extended periods of acyclicity during long-term GnRH agonist treatment have previously been reported in marmosets (Lunn et al 1992) and heifers . In marmosets, buserelin treatment resulted in a gradual decline in progesterone concentrations, which then remained at low, follicular phase concentrations until 136^18 days after treatment was initiated (Lunn et al 1992).…”

“…Extended periods of acyclicity during long-term GnRH agonist treatment have previously been reported in marmosets (Lunn et al 1992) and heifers . In marmosets, buserelin treatment resulted in a gradual decline in progesterone concentrations, which then remained at low, follicular phase concentrations until 136^18 days after treatment was initiated (Lunn et al 1992). In heifers, progesterone concentrations were maintained at basal levels for 203^28 and 170^28 days during treatment with one or two deslorelin implants respectively, thus indicating the absence of a functional CL .…”

“…This may be accompanied by a reduction or maintenance of basal LH concentrations. Basal LH concentrations are reduced in women (Shaw et al 1985) and macaques , while in marmosets (Lunn et al 1992) and heifers (Gong et al 1995, Bergfeld et al 1996 basal LH concentrations remain unchanged. Despite the variable effects on basal LH, the preovulatory LH surge appears to be universally inhibited, as is ovulation.…”