Concentrations of immunoreactive luteinizing hormone releasing hormone in discrete brain regions of the cockerel: effects of castration and testosterone replacement therapy

Abstract

The validity of using a radioimmunoassay employing an antiserum raised against synthetic luteinizing hormone releasing hormone (LH-RH) for the quantification of luteinizing hormone releasing factor (LH-RF) in birds was investigated. Extracts of avian hypothalamus yielded displacement curves which were parallel to that of the synthetic LH-RH standard and the immunoreactive potencies of a number of extracts assayed concurrently using two different anti-LH-RH sera were found to be similar. Moreover, after chromatography of cockerel hypothalamic extract on carboxymethyl–cellulose. immunoreactive and biologically active LH-RF were found in the same eluate fractions. Immunoreactive LH-RH was shown to be widely distributed in cockerel hypothalamus with the highest concentrations present in the mediobasal hypothalamus (MBH; 6·55± 1·86 pg/μg protein, n = 6) and medial preoptic region (POR; 0·95 ±0·07 pg/μg protein, n = 6). The postcastration rise in plasma LH in the cockerel was accompanied by significant (P <0·05) increases in the concentration of LH-RH in five hypothalamic areas including the POR; testosterone replacement therapy completely reversed these effects. Although castration raised the mean concentration of LH-RH in four other hypothalamic areas including the MBH, these differences were not significant. However, testosterone replacement therapy depressed LH-RH in all four regions to levels significantly (P <0·05) less than those in castrated cockerels. These findings constitute the first direct evidence that the negative feedback action of testosterone on LH secretion in the cockerel is mediated, at least in part, by an action on hypothalamic LH-RF-producing neurones.