A role for catecholaminergic neurons in the suppression of pulsatile luteinizing hormone secretion in the prepubertal ewe lamb.

These experiments tested the hypothesis that inhibitory catecholaminergic neural systems suppress tonic luteinizing hormone (LH) secretion in the prepubertal ewe lamb, as they do in the adult anestrous ewe. This was done by determining the effects of the alpha-adrenergic antagonist, phenoxybenzamine, and the dopaminergic antagonist, pimozide. In the first experiment, pimozide at 0.24 mg/kg increased LH plus frequency in 28-week-old lambs, but not in 17-week-old animals. Phenoxybenzamine (0.8 mg/kg) had no effect at either age. In experiment 2, a dose-response curve for pimozide (0.008, 0.08, 0.8 mg/kg) and a high dose of phenoxybenzamine were tested at two ages. The highest dose of pimozide and phenoxybenzamine stimulated LH release at both 22 and 28 weeks, although the magnitude of the response was blunted in older lambs. In the third experiment, we tested whether age or time of year determined the response to pimozide by using two groups of lambs born 5 weeks apart. Two doses of pimozide (0.24 and 0.8 mg/kg) were tested at the same time of year in these lambs. The highest dose of pimozide again stimulated LH secretion in 22-week-old lambs, but not in 17-week-old animals; the lower dose produced a partial (not statistically significant) response only in older lambs. These data suggest that inhibitory catecholaminergic (probably dopaminergic) neurons contribute to the suppression of tonic LH secretion prior to puberty in the ewe lamb, but that this neural system only becomes functional between 17 and 22 weeks of age.