The effects of neurotransmitter antagonists and glucose on luteinizing hormone secretion in growth-restricted wethers

Chronic undernutrition results in reduced secretion of luteinizing hormone (LH). Two experiments were conducted in wethers whose LH secretion was suppressed by growth restriction caused by feeding a maintenance ration. The first study examined neurotransmitters that may be actively inhibiting LH secretion during growth restriction. The effects of various neurotransmitter antagonists were investigated: pimozide (PIM, dopamine), cyproheptadine (CYP, serotonin), pyrilamine (PYR, H1), cimetidine (CMT, H2) and propranolol (PRO, β-adrenergic) in wethers specifically fed to maintain a body weight of 28 kg (GR, n = 7) and in full fed control wethers (n = 5). Blood samples were taken at 15 min intervals for 4 h before and after drug administration. Serum LH concentrations were determined by radioimmunoassay (RIA). Only PIM increased (P < 0.01) serum LH pulse frequency in the GR wethers (pre 0.5 ± 0.2 pulses per 4 h vs. post 2.6 ± 0.9 pulses per 4 h). None of the drugs tested had an effect on the control wethers. Experiment 2 examined the effect of glucose administration (50, 100, or 150 mg i.v.) on LH secretion in GR wethers. Only the 150 mg dose significantly (P < 0.05) increased LH pulse frequency compared to the pre-injection period (1.1 ± 0.3 vs. 2.0 ± 0.4 pulses per 4 h). After refeeding, LH pulse frequency and serum glucose concentrations increased. Proglumide, a cholecystokinin (CCK) antagonist, did not block this increase (2.1 ± 0.4 vs. 2.2 ± 0.3 pulses per 4 h). These data suggest that dopamine inhibits LH secretion in GR wethers and that increasing serum glucose concentrations increased LH secretion.