Mesotocin Receptor Gene and Protein Expression in the Prostate Gland, but Not Testis, of the Tammar Wallaby, Macropus eugenii1

Abstract

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.

The role of oxytocin and regulation of uterine oxytocin receptors in pregnant marsupials

“…the hypothalamus, there is a relatively high degree of mesotocin transcription in the hypothalamic region at the end of pregnancy (Parry et al 2000). Our data show that mesotocin is synthesised and probably stored in the neurohypophysis in increasing concentrations throughout pregnancy.…”

“…As yet, comparative analysis on the nucleotide sequence encoding the various marsupial oxytocin-like precursors has not been conducted. However, we recently obtained the nucleotide and the derived amino acid sequences of the mesotocin precursor Publication of The Physiological Society (Parry et al 2000).…”

“…The first exon encodes a 32 amino acid signal peptide followcd by the nonapeptide, a Gly-Lys-Arg C-terminal amidation and cleavage signal and the first nine amino acids of the neurophysin molecule. The second exon contains the highly conserved region of the neurophysin and the third exon encodes the C-terminus of the neurophysin, with the dipeptide Phe-Ser replacing the Arg or His residues in eutherian oxytocin genes (Parry et al 2000). The open reading frame of the tammar wallaby mesotocin cDNA molecule encodes a precursor polypeptide of 136 amino acids.…”

“…The presence of oxytocin/mesotocin receptors in the tammar wallaby uterus has also been demonstrated at the m W A level. We obtained a 588 base pair partial nucleotide sequence of a putative mesotocin receptor cDNA using primers based on conserved sequences in the I1 and IV transmembrane domains of known oxytocin receptors (Parry & Bathgate, 1998). The 196 amino acid derived sequence of this fragment shows high homology (74-77 YO) to eutherian oxytocin receptors but low homology to vasopressin receptors (38-52 YO).…”

“…Mesotocin receptor mRNA has been detected in the gravid myometrium as well as the ovary, prostate gland and epididymis (Parry & Bathgate, 1998;Parry et al 1998).…”