Neuroendocrinology of gonadotrophin secretion in prepubertal heifers
The pattern of gonadotrophin secretion and the mechanisms involved in the regulation of gonadotrophin secretion in prepubertal heifers were studied. Methods involved periods of frequent blood sampling (with or without concomitant treatments) and weekly blood sampling to characterize the temporal pattern of gonadotrophin secretion and/or responses to treatments. The developmental pattern of regulation of gonadotrophin release by opioid peptides, dopaminergic and adrenergic neuronal systems and possible interactions were studied in prepubertal heifers using bolus administration of receptor blockers, alone or in combination. It was concluded that although endogenous opioids suppress gonadotrophin secretion in the early postnatal period, inhibitory effects were stronger in the mid- to late-prepubertal period. It was also evident that dopaminergic inhibition negated the effects of naloxone (an opioid antagonist), suggesting that the suppressive effects of opioids are in part exerted through the inhibition of a dopaminergic neuronal system. Alpha-adrenergic neuronal systems had stimulatory effects on LH release, especially during the late prepubertal period. There were no evidence for á-adrenergic mediation of opioidergic suppression of LH release in prepubertal heifers. The role of excitatory amino acid (EAA) neurotransmitters in the regulation of gonadotrophin secretion during prepubertal development was examined using N-methyl-D,L-aspartic acid (NMA, an excitatory amino acid agonist) at different ages from birth to puberty. The results showed that excitatory amino acids seem to be involved in the regulation of LH and FSH release and that their effects were age-related, developing in the early postnatal period and reaching their maximum in the mid- to late-prepubertal period, sometime before first ovulation. Involvement of nitric oxide in the control of gonadotrophin secretion and possible mediation of the stimulatory effects of GnRH and NMA was studied. nitric oxide is involved in the regulation of LH, and possibly FSH, secretion and that nitric oxide may mediate, at least in part, the stimulatory effects of excitatory amino acids on LH, and to some extent FSH, release. The responses to GnRH led us to suggest that nitric oxide may have inhibitory effects on the pituitary and NMA may have increased pituitary sensitivity to GnRH. Finally, the effects of season of birth on the patterns of gonadotrophin secretion as well as age and weight at puberty were investigated by comparing spring- and autumn-born heifers. It was shown that an early postnatal increase in LH secretion (10 to 20 weeks of age) occurred in spring-born heifers, whereas in autumn-born heifers LH secretion decreased during this period. In spite of these differences, age and weight at puberty did not differ between spring- and autumn-born heifers although autumn-born heifers showed a wider range in age at puberty.