POSTTRANSLATIONAL PROCESSING OF PROGONADOTROPIN-RELEASING HORMONE (PROGNRH) IN EWES

Gonadotropin-releasing hormone (GnRH) is released from hypothalamic neurons into the hypophyseal-portal blood system following enzymatic cleavage of the decapeptide from a large precursor (proGnRH) molecule. The purpose of this study was to determine whether the ability of GnRH-producing neurons to synthesize and/or process proGnRH differed during physiological states associated with a suppressed and enhanced release of GnRH in ewes. Tissues were collected from ovariectomized ewes (OVX, n=4), OVX-estradiol treated ewes (OVX-E, n=5), and ewes (n=7) slaughtered 5 d after parturition (PP). Following euthanasia and exsanguination, stalk-median eminence (SME), medial-basal hypothalamus (MBH) and preoptic areas (POA) were collected. Concentrations of GnRH and proGnRH were determined by radioimmunoassay using specific antisera. Concentrations of GnRH in the SME did not differ (P>.05) between OVX-E and OVX ewes, but both groups contained less (P<.05) GnRH than the SME from PP ewes (4.4 +/- 0.7, 12.1 +/- 3.8 vs 24.3 +/- 5.1 fmol/mg tissue, respectively). Concentrations of proGnRH in SME mimicked those of GnRH and were less (P<.05) in OVX-E ewes than PP ewes, but were not different (P>.05) from those in OVX ewes (.34 +/- .34 vs 3.76 +/- 1.53 and 1.7 +/- .78 fmol/mg, respectively). In the MBH, OVX-E ewes had greater (P<.05) concentrations of GnRH than PP ewes (0.76 +/- 0.29 vs 0.24 +/- 0.04 fmol/mg) and OVX ewes were intermediate (0.41 +/- 0.13 fmol/mg). No differences (P>.05) in concentrations of GnRH in the POA were detected among groups. Concentrations of proGnRH in MBH and POA were not different (P>.05) among groups. In summary, proGnRH is present in the SME which contains nerve terminals of GnRH-producing cells. Although concentrations of proGnRH and GnRH in the SME and MBH were affected by physiological state, ratio's of the prohormone:mature decapeptide remained constant. Therefore, alterations in posttranslational processing of the prohormone leading to formation of the mature GnRH-decapeptide were not demonstrated.