The localization of oxytocin and mesotocin in the reproductive tract of the male marsupial bandicoot Isoodon macrourus

Oxytocin in the Male Reproductive Tract; The Therapeutic Potential of Oxytocin-Agonists and-Antagonists

In this review, the role of oxytocin and oxytocin-like agents (acting via the oxytocin receptor and belonging to the oxytocin-family) in the male reproductive tract is considered. Previous research (dating back over 60 years) is revised and connected with recently found aspects of the role oxytocin plays in male reproductive health. The local expression of oxytocin and its receptor in the male reproductive tract of different species is summarized. Colocalization and possible crosstalk to other agents and receptors and their resulting effects are discussed. The role of the newly reported oxytocin focused signaling pathways in the male reproductive tract, other than mediating contractility, is critically examined. The structure and effect of the most promising oxytocin-agonists and -antagonists are reviewed for their potential in treating male disorders with origins in the male reproductive tract such as prostate diseases and ejaculatory disorders.

Oxytocin and the human prostate in health and disease

Oxytocin is a peptide hormone produced by the neurohypophysis. The discovery that the peptide is produced locally within the male and female reproductive tracts has raised the possibility that oxytocin may have paracrine and autocrine actions outside of the nervous system. Oxytocin and its receptor have been identified in the human prostate. The prostate is an androgen-dependent organ whose function is to secrete components of the seminal fluid. Oxytocin has been shown to modulate contractility of prostate tissue and also to regulate local concentrations of the biologically active androgens. Oxytocin has also been shown to regulate cell growth. Prostate disease is common and results from abnormal growth of the gland. Oxytocin concentrations are altered in both benign and malignant prostate diseases and in vitro studies suggest that the peptide may be involved in the pathophysiology of these diseases.

Seasonal Changes in Mesotocin and Localization of Its Receptor in the Prostate of the Brushtail Possum (Trichosurus vulpecula)

The prostate gland in the brushtail possum grows and regresses seasonally. It has similarities to the human prostate and may therefore provide a unique model for investigating prostatic hyperplasia. Oxytocin has been implicated in the regulation of prostate growth in eutherian mammals, and the initial aim of this study was to identify and localize the marsupial equivalent, mesotocin, and its receptor in the prostate of the brushtail possum. Seasonal changes in prostatic mesotocin concentrations and receptor localization were then assessed and related to prostate growth. Mesotocin and mesotocin receptor gene transcripts with high sequence homology to eutherian oxytocin/oxytocin receptors were demonstrated, and mesotocin, neurophysin, and the receptor were all localized predominantly in the epithelial cells of the glandular acini. Western blot analysis confirmed the presence of a single immunoreactive receptor protein of approximately 60 Mr-3. Prostatic mesotocin concentrations were highest immediately before the increases in prostate weight associated with the autumn and spring breeding periods. At this time, mesotocin receptors were also present in the prostatic capsule in addition to those present in the glandular tissue. Mesotocin concentrations proceeded to decrease in association with the regression of prostate size toward the end of the breeding periods. No significant differences were present in serum testosterone or dihydrotestosterone throughout the year. The identification of mesotocin and its receptor in the possum prostate and the demonstration of seasonal changes in local mesotocin concentrations preceding changes in prostate size suggests that mesotocin may play a physiological role in regulating prostate growth and regression.

Mesotocin receptor gene and protein expression in the prostate gland, but not testis, of the tammar wallaby, Macropus eugenii

Evidence suggests that systemic oxytocin (OT) causes contractions of the prostate gland during ejaculation in eutherians, although functional OT receptors in this tissue have not been identified. Male marsupials secrete mesotocin (MT) from the pituitary and have relatively large, muscular prostate glands, so we examined MT receptors (MTRs) in the reproductive tract of the male tammar wallaby at the mRNA and protein level. We first obtained a partial (588 base pair) sequence of the tammar MTR cDNA that showed high homology to eutherian OT receptors (74-77%) and low homology to vasopressin receptors (38-52%). Analysis by reverse transcription-polymerase chain reaction demonstrated MTR mRNA in the adult, juvenile, and pouch young prostate and epididymis, but not testis. MTR transcripts were observed in the smooth muscle layers surrounding the urethral lumen and in the fibromuscular capsule. There was a single high-affinity 125I-D(CH2)5[Tyr(Me)2, Tyr4, Orn8, Tyr-NH29]-vasotocin (125I-OTA) binding site in the adult prostate. Competitive binding assays revealed identical ligand-binding profiles to the myometrium MTR (OTA > OT = MT > arginine vasopressin [AVP] antagonist > AVP). A lower-affinity 125I-OTA-binding site was present in the testis, with ligand-binding profiles indicating binding to vasopressin receptors. MTR concentrations in the prostate were 8-fold lower than concentrations in the myometrium. Our data demonstrate the presence of an MTR gene and functional receptor protein in the prostate gland, but not the testis, of the tammar. Localization of MTRs to the smooth muscle fibers in the capsule and surrounding the urethral lumen suggests a contractile function for MT during ejaculation.

Posterior pituitary of the newborn marsupial possum, Trichosurus vulpecula

The fetal anterior pituitary-adrenal axis is thought to be involved in the initiation of birth in both eutherian and marsupial mammals. Little is known about the structure and function of the posterior pituitary at birth in the marsupial. Immunocytochemistry, high pressure liquid chromatography, and radioimmunoassay were used to identify vasopressin and mesotocin in the posterior pituitary of a newborn marsupial, the brushtail possum, Trichosurus vulpecula. The concentrations of vasopressin and mesotocin in the head of the newborn possum were 0.34 and 0.28 ng, respectively. The concentration of vasopressin was always greater than that of mesotocin, and the amounts of neuropeptides present in the head increased as the possum developed.

Arginine vasopressin- and oxytocin-like peptides in the testis of two Australian marsupials

High performance liquid chromatography (HPLC) and specific radioimmunoassay (RIA) for arginine vasopressin (AVP), mesotocin (MT), and oxytocin (OT) were used to identify and quantify these peptides in the testis of the brushtail possum (Trichosurus vulpecula) and the northern brown bandicoot (Isoodon macrourus). Arginine vasopressin (0.092 +/- 0.041 ng/g) and MT (0.198 +/- 0.089 ng/g), but not OT, were found in the possum testis, while the bandicoot testis contained AVP (0.061 ng/g), MT (0.108 +/- 0.024 ng/g), and OT (0.114 +/- 0.053 ng/g). The values correlate well with those reported for AVP- and OT-like peptides in the testis of eutherian mammals. It was concluded that there are neurohypophysial peptides present in the marsupial testis.

Brain content and plasma concentrations of arginine vasopressin in an Australian marsupial, the brushtail possum Trichosurus vulpecula

Arginine vasopressin (AVP) has been identified and quantified in the brain and plasma of the possum using a highly specific radioimmunoassay and high-performance liquid chromatography. Large amounts of AVP were found in the pituitary (16.3 +/- 0.56 micrograms/pituitary, n = 5) and hypothalamus (398 +/- 82.5 ng/hypothalamus), and significant amounts of AVP were also present in the cerebral cortex (26.8 +/- 11.5 ng/cortex). Plasma AVP concentrations were significantly lower (2.2 +/- 0.45 pg/ml, n = 10) during anesthesia compared to concentrations while conscious (4.5 +/- 1.19 pg/ml). Severe hemorrhage markedly increased plasma concentrations to 1091 +/- 225 pg/ml (n = 8). It was concluded that AVP is present in the possum brain, pituitary, and plasma, and that its secretion is stimulated by hypovolemia and inhibited by surgical stress.

Immunohistochemical evidence for the presence of oxytocin in the opossum corpus luteum

Corpora lutea from opossums late in pregnancy were examined by immunohistochemistry for the presence of oxytocin. Oxytocin-immunoreactivity was observed in all corpora lutea examined but not elsewhere in ovarian tissue. The immunoreactive staining observed was confined primarily to the perinuclear cytoplasm of reactive luteal cells. Not all luteal cells showed oxytocin-immunoreactivity. The immunohistochemical localization of oxytocin in the pregnant opossum corpus luteum demonstrates for the first time this peptide in a metatherian ovary. Its presence in this primitive species suggests that oxytocin has a fundamental role in the physiology of the mammalian ovary.