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Biomedical subjects

Tony M Plant

Publications and source records attributed to Tony M Plant.

24 records · Page 2Linked to original sources
PubMed2003

Inhibitory and stimulatory regulation of testicular inhibin B secretion by luteinizing hormone and follicle-stimulating hormone, respectively, in the rhesus monkey (Macaca mulatta).

This study examined the relative role of FSH and LH in governing testicular inhibin B secretion in the rhesus monkey. Adult male monkeys, rendered hypogonadotropic and hypogonadal by administration of a GnRH receptor antagonist (acyline), were implanted with testosterone (T)-filled or empty capsules. Following T-induced restoration of spermatogenesis, both groups received recombinant human FSH and vehicle for 12 d. Juvenile male monkeys received an 11-d infusion of single-chain recombinant human LH and recombinant human FSH, either alone or in combination. In adults, chronic hypogonadotropism resulted in a modest reduction of circulating inhibin B levels, which was more than fully reversed by FSH. In the presence of T, which exerted a marked suppression in inhibin B secretion, FSH restored inhibin B levels only to those observed before acyline treatment. In juveniles, treatment with single-chain recombinant human LH led to a suppression of inhibin B secretion and curtailed the FSH-induced stimulation of this testicular hormone. The T-induced decrease in inhibin B secretion was associated with suppression in inhibin-beta(B) mRNA levels, but FSH stimulation of inhibin B secretion occurred in the absence of clear changes in expression of this subunit gene. These findings indicate that inhibin B secretion by the monkey testis is governed by the inhibitory and stimulatory action of LH and FSH, respectively. The action of LH is presumably indirect and likely mediated by T inhibition of inhibin-beta(B) gene expression. The molecular basis of the stimulatory action of FSH on inhibin B secretion requires further study.

Age Factors↗
PubMed2003

Sertoli cell proliferation during prepubertal development in the rhesus monkey (Macaca mulatta) is maximal during infancy when gonadotropin secretion is robust.

Although a marked pubertal increase in Sertoli cell number is a hallmark of testicular development in the rhesus monkey, the ontogeny of this somatic cell type before puberty is less clear. To clarify this issue, groups (n = 4) of neonate (1-2 d old), infant (4-5 months old), juvenile (14-17 months old), and adult male rhesus monkeys were injected with 5-bromo-2'-deoxyuridine (BrdU) 2 h before castration. Tissue was fixed in Bouin's fluid, and the percentage of BrdU-labeled Sertoli cells at each developmental stage was calculated. In addition to the labeling index, Sertoli cell number per testis for the neonate and infant groups was enumerated using standard histomorphometry and compared with that previously reported by this laboratory for juvenile and adult rhesus monkeys. The number of Sertoli cells per testis in infants (156 +/- 49 x 10(6), mean +/- SD) was 4-fold greater than that in neonates (42 +/- 12 x 10(6)). The previously established value for this parameter in juvenile monkeys was 286 +/- 121 x 10(6). Incorporation of BrdU into nuclei of Sertoli cells indicated that these cells were mitotically active at all three stages of prepubertal development. The labeling index in the neonate and infant groups (1.33% in both cases), however, was significantly greater than that in juveniles (0.25%). From the foregoing results, we conclude that Sertoli cell proliferation during prepubertal development in the rhesus monkey occurs predominantly during infancy, when gonadotropin secretion is elevated, and to a lesser extent during the juvenile phase of development, when circulating gonadotropin concentrations are undetectable.

Age Factors↗
PubMed2002

Neuroendocrine mechanisms that delay and initiate puberty in higher primates.

This paper highlights a series of studies using the male rhesus monkey that has led to a model for the control of the onset of puberty in higher primates. The model proposes that the timing of puberty in these species is governed by the duration of a central brake that, during juvenile development, holds in check the hypothalamic network of gonadotropin-releasing hormone (GnRH) neurons, which, in the adult, drive the pituitary-gonadal axis. The neurobiology of this hypothalamic brake, and the physiological mechanisms that time its application and removal, are incompletely understood. Nevertheless, the pubertal resurgence of pulsatile GnRH release, which terminates the juvenile phase of primate development and triggers the initiation of puberty in man and monkeys, is associated with structural and molecular remodeling of the hypothalamus. A major component of this developmental plasticity appears to involve neuropeptide Y (NPY). NPY inhibits GnRH release, and NPY gene expression in the hypothalamus is elevated during juvenile development when GnRH release is restrained. Since the changes in hypothalamic function and morphology that trigger primate puberty unfold in the absence of gonadal steroid feedback, the possibility is raised that, in addition to activating the pituitary-gonadal axis at this stage of development, they may also contribute directly to the causation of behaviors and affective states that emerge at adolescence.

Animals↗
PubMed2002

Leptin and pubertal development.

Sexual development after birth in rodents, nonhuman primates, and humans is driven by the gonadotropin-releasing hormone (GnRH) pulse generator. During the neonatal period in primates, pulsatile GnRH discharge from the medial basal hypothalamus drives an active period of pituitary gonadotropin and gonadal hormone secretion. During the transition from the neonatal to the juvenile period, however, the activity of the GnRH pulse generator is restrained or arrested and gonadotropin and gonadal hormone secretion enters a quiescent period that continues until the onset of puberty. As puberty approaches the GnRH pulse generator is reactivated, resulting in enhanced gonadotropin secretion, accelerated growth, maturation of the gonads, and the achievement of sexual competence. Rodents do not appear to exhibit a developmental phase analogous to the quiescent juvenile period in primates when the GnRH pulse generator is held in check. Instead, progressive maturational changes in the pattern of GnRH pulsatility appear to drive sexual development in rodents. The role that leptin plays in sexual development has not been fully defined, but the balance of current evidence appears to support the idea that, in both rodents and primates, leptin plays a permissive rather than a causal role in timing this process. When body energy reserves rise above a critical level, blood leptin increases to a threshold concentration signaling to the central nervous system that the body can support sexual function. Puberty can apparently occur over a wide range of concentrations above this critical leptin threshold. Leptin does not appear to act as a trigger to time the initiation of puberty but, instead, once leptin reaches this threshold pubertal development may proceed if, and only if, other critical control mechanisms are operational.

Animals↗
PubMedDate not supplied

Neurobiological mechanisms of puberty in higher primates.

Puberty in humans is comprised of two developmental processes; namely, gonadarche and adrenarche. Of the two, gonadarche is fundamentally the most important, and this review examines the neurobiological mechanisms that first prevent, and later trigger, progression into this critically important phase of human development when the ability to first reproduce is established. The review draws extensively upon results obtained by studies of the rhesus monkey (Macaca mulatta), a representative higher primate which, like man, exhibits a postnatal pattern in activity of the hypothalamic-pituitary-gonadal axis that is characterized by a prolonged period of relative quiescence from late infancy until the initiation of the pubertal process. The proximate cause of the prepubertal quiescence in this neuroendocrine axis is the arrest or restraint of the pulsatile mode of hypothalamic GnRH release by a neurobiological brake that holds in check release of this decapeptide, without seeming to down-regulate the transcriptional activity of the gene encoding GnRH (GnRH-I). Thus, if neurogenomes control the onset of gonadarche, they must reside upstream from that of the GnRH neuron. The genetic and physiological factors (with a particular emphasis on leptin) that time the application and duration of the prepubertal brake on GnRH release are also considered.

Animals↗