DEVELOPMENTAL-CHANGES IN IMMUNOREACTIVE INHIBIN AND FSH IN PLASMA OF CHICKENS FROM HATCH TO SEXUAL MATURITY

1. The relationship between immunoreactive inhibin and follicle-stimul ating hormone (FSH) was studied in male and female chickens from hatch to sexual maturity. Plasma inhibin was estimated by a heterologous ra dioimmunoassay validated for use in the chicken. FSH was measured by a recently developed homologous radioimmunoassay. 2. In a cross-section al study, blood samples and gonads were collected from chickens of bot h sexes at 1, 3, 5, 7, 14, 21 and 28 d after hatching and subsequently at 14-day intervals until 182 d of age. 3. In the female, plasma prog esterone concentration (P-4) progressively increased during sexual dev elopment. The plasma luteinising hormone (LH) concentration rose durin g the first week after hatching, and fluctuated thereafter, with troug hs at 6 and 14 weeks and peaks at weeks 10 and 18. The plasma inhibin and FSH concentrations remained low until the start of puberty and inc reased simultaneously thereafter. However, from week 18 on, plasma inh ibin continued to rise while plasma FSH fell. Hence, FSH and inhibin w ere positively correlated before puberty, but developed a negative cor relation during sexual maturation. 4. In the male, plasma testosterone and LH concentrations increased 38- and 3.7-fold respectively over th e period studied. Inhibin and FSH followed similar time courses and we re consequently positively correlated. 5. These results suggest sex di fferences in the role of inhibin in regulating FSH secretion during de velopment. The FSH-inhibin feedback loop may become operational at the onset of sexual maturity in the hens. In male chickens, the similar p attern of inhibin and FSH secretion suggests that inhibin secretion is driven by FSH.