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 t he prostate gland during ejaculation in eutherians, although functiona l OT receptors in this tissue have not been identified. Male marsupial s secrete mesotocin (MT) from the pituitary and have relatively large, muscular prostate glands, so we examined Mi receptors (MTRs) in the r eproductive tract of the male tammar wallaby at the mRNA and protein l evel. We first obtained a partial (588 base pair) sequence of the tamm ar MTR cDNA that showed high homology to eutherian OT receptors (74-77 %) and low homology to vasopressin receptors (38-52%). Analysis by rev erse 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 surr ounding the urethral lumen and in the fibromuscular capsule. There was a single high-affinity I-125-D(CH2)(5)[Tyr(Me)(2), Tyr(4), Orn(8), Ty r-NH29]-vasotocin (I-125-OTA) binding site in the adult prostate. Comp etitive binding assays revealed identical ligand-binding profiles to t he myometrium MTR (OTA > OT = MT > arginine vasopressin [AVP] antagoni st > AVP). A lower-affinity I-125-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, bu t not the testis, of the tammar. Localization of MTRs to the smooth mu scle fibers in the capsule and surrounding the urethral lumen suggests a contractile function for MT during ejaculation.