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Biomedical subjects

A F Dixson

Publications and source records attributed to A F Dixson.

At least 37 records · Page 2Linked to original sources
PubMed1992

Development from birth to sexual maturity in a semi-free-ranging colony of mandrills (Mandrillus sphinx) in Gabon.

This report presents information collected over 7 years (1983-1990) in Gabon, on a breeding group of 14, increasing to 45, mandrills maintained in a rainforest enclosure. Under these conditions, a seasonal cycle of mating (June-October) and birth (January-May) occurred. Females began to exhibit sexual skin swellings at age 2.75-4.5 years (3.6 +/- 0.6 years, mean +/- SD; n = 10) and first delivered offspring when 3.25-5.5 years old (4.4 +/- 0.8 years; n = 9). Gestation periods ranged from 152 to 176 days (167 +/- 9 days; n = 6 accurately dated pregnancies) and interbirth intervals from 11 to 15 months (12.4 +/- 1.3 months; n = 15). Females could reproduce 2 years before attaining adult body weight (10-15 kg) and complete dental eruption by 5.0-5.5 years. Males, by contrast, developed more slowly, reaching adult body weight (30-35 kg) and testicular volume (volume of left testis: 25-30 ml) at 8 years. Consistently high circulating testosterone concentrations (8.17 +/- 2.0 ng ml-1) could be measured by 9 years of age. Fully developed males exhibited fatting of the rump and flanks, as well as striking sexual skin coloration and an active sternal cutaneous gland; little expression of these features was evident during pubertal development. Marked individual age differences occurred with regard to the onset and complete development of these features, suggesting possible interactions between social environment and physical maturation.

Animals↗
PubMed1991

Penile spines affect copulatory behaviour in a primate (Callithrix jacchus).

Androgen-dependent, keratinized "spines" occur on the glans penis in many rodents, primates and other mammals. Since penile spines overlie dermal tactile receptors, they may play a role in copulatory behaviour. An experiment was conducted to test this hypothesis. Sixteen sexually experienced adult male marmosets were paired with ovariectomized females before, and after, removal of penile spines (using thioglycollate cream applied to the glans under anaesthesia) or a sham operation. Spine removal resulted in an increased duration of preintromission pelvic thrusting (mean +/- s.e.m. from 6.87 +/- 1.09 to 14.94 +/- 3.32 s, p = 0.05) and of intromitted thrusting (from 1.73 +/- 0.11 to 2.0 +/- 0.11 s, p less than 0.05). Three males exhibited partial intromissions during some postspinectomy tests, an effect which had not been observed prior to the operation. Sham operations had no behavioural effects. Results indicate that penile spines play a significant (but not indispensible) role in sensory feedback during copulation in this primate species.

Androgens↗
PubMed1991

Sexual selection, natural selection and copulatory patterns in male primates.

Complex copulatory patterns, involving multiple brief intromissions or prolonged single intromissions (PI) occur most frequently among primate species in which females mate with a number of partners (multimale and dispersed mating systems). However, the PI pattern is also confined almost exclusively to arboreal primates - to either smaller-bodied, cryptic, nocturnal species or much larger diurnal forms. Predation pressures may have limited the evolution of the PI pattern, particularly where small-bodied diurnal primates or terrestrial forms are concerned. The available evidence indicates that both sexual selection and natural selection may have influenced the evolution of copulatory patterns.

Animals↗
PubMed1990

Gonadotropin-releasing hormone analogs inhibit primate granulosa cell steroidogenesis via a mechanism distinct from that in the rat.

Gonadotropin-releasing hormone (GnRH) and related peptides are implicated in the local control of rat ovarian function, but evidence to date for direct effects of such peptides on primate ovarian cells is equivocal. In contrast to rat ovaries, where GnRH action is mediated through specific, high-affinity GnRH receptors, no such binding sites have been identified in primate tissue. Using undifferentiated granulosa cells from immature follicles in cyclic (luteal phase) marmoset ovaries, we have observed direct suppression of human (h) FSH-induced steroidogenesis by GnRH analogs in vitro. Granulosa cells from immature (less than 1 mm diameter) follicles were incubated for 4 days in the presence of hFSH and testosterone (aromatase substrate) to stimulate cyclic AMP (cAMP) production and steroidogenesis. The additional presence of GnRH alone (up to 10 microM) had no effect on FSH action. However, the GnRH agonist, [D-Ser(But)6]GnRH 1-9)-ethylamide (Buserelin, 0.1 microM-10 microM), caused time- and dose-dependent inhibition of estradiol (maximum inhibition = 79%; ED50 = 0.55 microM) and progesterone production (maximum inhibition = 93%; ED50 = 0.1 microM). Accumulation of cAMP was also inhibited by up to 54%. Paradoxically, a GnRH antagonist [( N-Ac-D-Nal(2)1,D-pCl-Phe2, D-Trp3, D-hArg(Et2)6, D-Ala10]-GnRH; 10 microM) alone also inhibited hFSH-stimulated cAMP and steroid production by 40% and 70%, respectively. Moreover, the suppressive effects of the GnRH agonist on granulosa cell functions were augmented by the presence of the GnRH antagonist (10 microM).(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1990

Medial hypothalamic lesions and sexual receptivity in the female common marmoset (Callithrix jacchus).

Medial hypothalamic lesions cause major deficits in sexual receptivity in a variety of mammalian species. Effects of such lesions upon sexual receptivity in female primates have not been investigated. Two experiments are described in which thermal lesions were made in the hypothalamus in 11 ovariectomized or ovariectomized/adrenalectomized, oestradiol-treated common marmosets. Proceptivity (sexual initiating behaviour) was reduced or abolished by lesions in the dorsomedial or ventromedial hypothalamus. Sexual receptivity was unaffected except in 3 females in which damage to the ventromedial area was associated with a 25-42% increase in terminations of males' mounts. However, this effect was reversed by treating females with oestradiol. Therefore, medial hypothalamic lesions have pronounced effects upon proceptivity in the marmoset, whereas effects upon receptivity are much less pronounced or absent. These results are discussed in relation to the evolution of the hormonal control of receptivity in anthropoid primates as compared to other mammals.

Animals↗
PubMed1990

Pituitary-testicular function is suppressed by an LHRH antagonist but not by an LHRH agonist in the marmoset monkey.

The use of pituitary desensitization by an LHRH agonist (buserelin) to examine pituitary-testicular function was investigated in a New World primate. Six adult male marmoset monkeys were injected s.c. with an LHRH agonist implant (1.5 mg in a rod 0.5 cm long). Pharmacokinetics, determined by radioimmunoassay of urinary buserelin, revealed a rapid initial release of the agonist followed by a steady decline during a 200-day period. The LHRH agonist treatment resulted in a rapid initial rise in plasma LH followed by a return to mean values similar to those seen in the control samples by 7 days after implantation. Using the present protocol, no evidence of subsequent pituitary desensitization or suppression of testicular function was observed, plasma concentrations of LH and testosterone remaining within the normal range during the 200-day study period. In contrast, pituitary-testicular function was suppressed in the male marmoset after blockade of pituitary LHRH receptors by an LHRH antagonist. Five adults were treated with a single s.c. injection of the antagonist [Ac-D-Nal(2)1,D-pCl-Phe2,D-Trp3,D-Ser(Rha)6,AzGlyNH2(10)]-LHRH at a dose of 300 micrograms/kg. The LHRH antagonist induced a marked suppression of plasma LH and testosterone by 6-8 h, the low levels being maintained for 24-48 h. These results show that, whereas treatment with an LHRH antagonist can inhibit pituitary-testicular function in the male marmoset, it may be that desensitization cannot be induced by the LHRH agonist used.

Animals↗
PubMed1989

Effects of male partners upon proceptivity in ovariectomized estradiol-treated marmosets (Callithrix jacchus).

Effects of male partners upon the expression of female proceptivity were examined in two experiments using 16 ovariectomized marmosets. Experiment 1 showed that the female's proceptive tongue-flicking display (PTF) is triggered specifically by eye contact with the male. Stimulation of PTF by administration of estradiol-17 beta (E2) to ovariectomized females depends in part upon the male's responsiveness to female displays, such as "staring" and "freezing," which may serve to attract his attention and to establish eye contact. Experiment 2 provided evidence that females' proceptivity decreases if their male partners are lesioned in the preoptic area/anterior hypothalamus. Such males are sexually hypoactive and less responsive to the females' visual displays. However, E2 still activates PTF if females succeed in initiating eye contact with males. Results indicate that variability in effects of E2 upon proceptivity in marmosets may be influenced by subtle aspects of facial communication between the sexes as well as by individual differences in hormonal sensitivity. Copulatory activity in males is not essential for E2 to exert its stimulatory action upon proceptivity in female marmosets.

Animals↗
PubMed1989

Naloxone reversal of stress-induced suppression of LH release in the common marmoset.

There is considerable evidence suggesting that endogenous opioid peptides (EOP) might play a role in the defensive response to stressful stimuli. The stress associated with receipt of aggression from a female conspecific and physical restraint for collection of serial blood samples (at 10-min intervals) resulted in a significant (p less than 0.05) suppression of LH secretion in the oestradiol primed ovariectomized marmoset. Administration of the opioid receptor antagonist, Naloxone (1 mg/kg) immediately before and after receipt of aggression completely prevented the stress-induced decrease in LH release. This suggests that EOP may mediate, at least in part, this stress induced decline in LH release in the marmoset.

Aggression↗
PubMed1989

Control of inhibin production by primate granulosa cells.

In-vitro data from experiments on rats implicate granulosa cells as primary sites of hormone-dependent ovarian inhibin biosynthesis, but no equivalent data exist for primates. We have used the common marmoset (Callithrix jacchus) to investigate inhibin biosynthesis in primate granulosa cells in vitro and to determine its relationship to preovulatory follicular development. To relate the production of immunoactive inhibin to follicular maturity, we studied primary granulosa cell cultures from follicles at progressive stages of preovulatory development. Granulosa cells from 'large' (greater than or equal to 2.0 mm diameter) follicles expressed high rates of inhibin production and steroidogenesis (progesterone), and were positively regulated by human (h)LH in vitro. Less mature granulosa cells from 'medium' (1.1-1.9 mm) and 'small' (less than or equal to 1.0 mm) follicles expressed proportionately lower rates of inhibin production and steroidogenesis, but each parameter was stimulated in a dose- and time-dependent manner by hFSH in vitro. The stimulatory action of hFSH on immunoreactive inhibin was augmented by the presence of testosterone or oestradiol; testosterone (but not oestradiol) also augmented the steroidogenic response to hFSH. Marmoset luteal tissue also produced inhibin in vitro and expressed an approximately 1.5 kb inhibin alpha-subunit mRNA, confirming the corpus luteum as a source of ovarian inhibin in primates. These results provide direct experimental evidence that primate granulosa cells produce inhibin. They suggest that production of inhibin by immature granulosa cells is initially induced by FSH and subject to modulation by follicular steroids.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1988

Effects of hypothalamic lesions upon the sexual and social behaviour of the male common marmoset (Callithrix jacchus).

The sexual and associated behaviour of 10 adult male marmosets was recorded during pair tests with ovariectomized females, before and after bilateral thermal lesions of the hypothalamus. Four sham-lesioned males served as controls. Lesions varied in volume from 1.49 to 3.28 mm3 and extended from the ventromedial hypothalamus to the diagonal band of Broca. Precopulatory behaviours (anticipatory erections, tongue flicking and anogenital investigations of the female) as well as frequencies of mounting, intromission and ejaculation decreased in lesioned males. The greatest suppression of sexual behaviour occurred after lesioning the anterior hypothalamus (AH), beneath the anterior commissure, or at the junction of the AH with the preoptic area (POA). Lesions confined to the POA had less profound behavioural effects. Treating ovariectomized females with estradiol stimulated their proceptivity but had no consistent effects upon the males' behaviour. Lesioned males did not exhibit signs of social withdrawal and frequencies of allogrooming or grooming invitations increased post-operatively. Preliminary studies on intermale aggression indicated that lesions which had the greatest effect upon sexual behaviour also tended to decrease aggressive interactions with other males. Hypothalamic lesions did not affect plasma testosterone levels, except in one male and only one animal showed signs of ill health (weight loss and hypothermia) post-operatively. These results show that damage to the AH or AH-POA junction in male marmosets causes a profound suppression of sexual 'arousal' and copulatory behaviour and that such affects are not due to androgen insufficiency or other, non-specific, side effects of neural damage.

Aggression↗
PubMed1988

Effects of dorsal penile nerve transection upon the sexual behaviour of the male marmoset (Callithrix jacchus).

The sexual behaviour of 7 adult male marmosets and their ovariectomized partners was observed during pair-tests before and after bilateral dorsal penile nerve transection (n = 4) or a sham operation (n = 3). After transection of the dorsal penile nerves, males mounted more frequently and the initial mount of each test was more prolonged. Males continued to orientate correctly during mounts and to make rapid pelvic thrusts, with the erect penis close to the vaginal orifice. However, the deep pelvic thrust required to attain intromission was abolished. Intromission was rarely observed postoperatively and ejaculation occurred only twice, in one male. These effects were not observed in sham-operated males. The results show that afferent impulses from the penis, travelling through the dorsal penile nerves, play a vital role in the coordination of intromission and ejaculation in this primate species.

Animals↗
PubMed1988

Cellular aspects of pre-ovulatory folliculogenesis in primate ovaries.

According to current concepts of pre-ovulatory folliculogenesis in primate ovaries, each growing follicle has a 'threshold' requirement for stimulation by FSH which must be met if it is to enter the oestrogen-secretory phase of pre-ovulatory development. Until recently, our understanding of the intra-follicular mechanisms underlying FSH action on granulosa cells was based largely on information from non-primate laboratory animals, mainly polyovulators such as rats. The present paper describes studies on FSH-regulated granulosa cell function in relation to pre-ovulatory development in vivo and in vitro using a laboratory primate, the common marmoset (Callithrix jacchus). Measurement of aromatase activity is used as an index of granulosa cell cytodifferentiation to verify three major tenets of the 'threshold' hypothesis: (i) that granulosa cells acquire increased responsiveness to FSH and LH during pre-ovulatory growth; (ii) that these developmentally related changes are directly induced by FSH; and (iii) that intrafollicular steroids modify FSH action and thereby contribute to the establishment of follicular FSH thresholds. The results obtained highlight the value of this experimental animal model for studies of cellular and molecular aspects of pre-ovulatory folliculogenesis which are relevant to human ovaries.

Animals↗
PubMed1988

Effects of acute stress on the patterns of LH secretion in the common marmoset (Callithrix jacchus).

Stressful stimuli associated with aggressive encounters and low social rank may affect female fertility in a variety of mammalian species. In these experiments we examined the effects of aggressive encounters and physical restraint in a primate chair on the patterns of LH secretion in ovariectomized, oestrogen-primed female marmosets. Receipt of aggression from a female conspecific, followed by physical restraint for collection of blood samples (at 10-min intervals for 4 h), resulted in marked declines in LH concentrations during oestradiol-induced LH surges in five animals (from 112 +/- 24 micrograms/l to 45 +/- 12 micrograms/l; group means +/- S.E.M.; P less than 0.05). This was due to reductions in LH pulse amplitude rather than to changes in pulse frequency. Decreases in plasma concentrations of LH were reversed by treating females with exogenous LH-releasing hormone (LHRH). Cortisol treatment had no effect on LH levels during oestrogen-induced LH surges. Effects of aggressive encounters and physical restraint on plasma LH were not therefore due to reduced pituitary responsiveness to LHRH or to increased plasma concentrations of cortisol. In separate experiments it was found that physical restraint alone had no effect on plasma LH in habituated subjects, and that decreases in plasma LH after receipt of aggression only occurred if animals were subsequently placed in the restraint chair. A summation of stressful effects is therefore required to produce the fall in circulating LH. A summation of social and other environmental stressors may also underlie the reduced fertility seen in free-living animals.

Acute Disease↗
PubMed1987

Post-partum changes in hormones and sexual behaviour in captive groups of marmosets (Callithrix jacchus).

Video-tape recordings were made of behavioural interactions in eight family groups of marmosets for 24-30 days after parturition. Blood samples were collected from females for measurement of plasma luteinizing hormone (LH) and progesterone. A pre-ovulatory LH surge occurred in six females between days 10-18 (mean +/- sem, 13.8 +/- 1.3 days). In these groups, sexual interactions increased in frequency during the post-partum period and males began to initiate mounts on days 1, 2, 2, 6, 8 and 10 respectively. Mount and ejaculation frequencies increased significantly during the peri-ovulatory phase as compared to the pre-ovulatory phase and females were more proceptive and receptive than in the early post-partum period. Peri-ovulatory increases in sexual activity were more pronounced in 3 groups in which females had failed to conceive. In the remaining 3 groups, sexual interactions tended to continue at higher levels into the post-ovulatory phase when the females were pregnant. In captive marmoset groups, copulatory behaviour during the post-partum period is stimulated by changes in non-behavioural cues from the female (increased sexual attractiveness to the male) as well as by changes in her behaviour (proceptivity, receptivity) which vary in importance between individuals.

Aggression↗
PubMed1987

Effects of adrenalectomy upon proceptivity, receptivity and sexual attractiveness in ovariectomized marmosets (Callithrix jacchus).

The sexual behaviour of seven pairs of marmosets was observed during blocks of 30 minute tests before and after the ovariectomized partners were bilaterally adrenalectomized. Adrenalectomized females were maintained by intramuscular injections of deoxycorticosterone pivalate (DC) and hydrocortisone hemisuccinate (HC). Twice daily injections of HC were required to maintain the high levels of plasma cortisol typical of marmosets and some other New World primates. Adrenalectomy had no significant effects upon proceptivity, receptivity or female sexual attractiveness. Increasing the replacement dose of DC had no effects upon sexual behaviour. Oestradiol treatment caused a marked increase in proceptivity (tongue-flicking, staring and immobile displays) and in receptive tongue-flicking behaviour. Males ejaculated more frequently during tests with oestradiol-treated partners. These results indicate that during "pair tests" adrenocortical sex steroids are not essential for maintenance of sexual behaviour in female marmosets and that activation of proceptivity by oestradiol 17 beta can occur in the absence of the adrenal glands.

Adrenalectomy↗
PubMed1986

Anteromedial hypothalamic lesions block proceptivity but not receptivity in the female common marmoset (Callithrix jacchus).

The sexual behavior of 8, estradiol-treated, ovariectomized common marmosets was recorded prior to and after bilateral radiofrequency lesions of the hypothalamus. Lesions of the anterior hypothalamus, which extended to varying degrees into the medial hypothalamus, virtually abolished proceptive tongue-flicking and staring displays. Tongue-flicking during copulation also decreased, but the females did not show any significant increase in the number of mounts which they refused or terminated with the exception of one animal which had received more extensive damage to the medial hypothalamus. Significant increases in the frequencies of allogrooming and grooming invitations also occurred after the females had been lesioned. Sexual behavior was not significantly altered in 3 females which received sham lesions. These results provide the first direct evidence for a neuroanatomical distinction between hypothalamic mechanisms which regulate proceptivity and receptivity in primates.

Animals↗
PubMed1986

Genital sensory feedback and sexual behaviour in male and female marmosets (Callithrix jacchus).

Local anaesthesia (using lidocaine) of the penis, or of the vagina and cervix caused marked changes in the sexual behaviour of marmosets. Penile desensitization did not affect sexual arousal or erectile capacity. However, intromission latencies increased and 7 out of 10 males failed to intromit during some post-lidocaine tests. Females failed to show several behavioural patterns which normally occur during intromission (shrugging body movements, head turning and opening the mouth) or exhibited them at reduced frequencies after intra-vaginal application of lidocaine. The tendency of females to terminate mounts after ejaculation had occurred was also reduced with resultant increases in intromission duration in all cases. Desensitization of the females' external genitalia had no effect upon sexual behaviour. Co-ordination of copulatory behaviour in this primate species depends, in part, upon afferent stimuli from the penis and from the vagina or cervix.

Animals↗