THE INVOLVEMENT OF DOPAMINERGIC AND OPIOIDERGIC NEURONAL SYSTEMS IN THE CONTROL OF THE EARLY RISE IN LH-SECRETION IN BULL CALVES

There is an early rise in secretion of luteinizing hormone (LH) in bul l calves between 6 and 20 weeks of age. This study was designed to exa mine the involvement of opioidergic and dopaminergic neuronal systems in the control of this early rise in gonadotrophin secretion. Four gro ups of five bull calves were bled every 15 min for 10 h and administer ed one of the following treatments i.v. at 6, 14 and 24 weeks of age: a) saline; b) 1 mg naloxone/kg body weight every hour for 10 h; c) 0.8 mg pimozide/kg body weight as a single injection, after the first blo od sample; d) 1 mg naloxone + 0.8 mg Pimozide/kg body weight. The earl y rise in LH secretion had started by 6 weeks of age, was at a peak at 14 weeks of age and was complete by 24 weeks of age (P<0.05). Naloxon e (an opioid antagonist) treatment resulted in increased basal and mea n serum concentrations of LH and LH pulse frequency at 14 weeks of age , increased mean serum LH concentrations of LH pulse frequency at 24 w eeks of age, decreased FSH pulse amplitude at 14 weeks of age and incr eased mean serum concentrations of FSH at 24 weeks of age compared to control calves (P<0.05). Pimozide (a dopamine antagonist) decreased LH pulse amplitude at 24 weeks of age compared to all other groups of ca lves (P<0.05). Naloxone and pimozide in combination resulted in LH sec retory patterns similar to naloxone treated calves or intermediate to naloxone and control calves at 14 weeks of age but similar to control calves at 24 weeks of age. In calves given the combination treatment, parameters of FSH secretion resembled those in the other groups of cal ves except for basal and mean serum concentrations of FSH at 24 weeks of age, which were similar to control or pimozide treated calves, but lower than in naloxone treated calves (P<0.05). At 14 weeks of age mea n serum concentrations of testosterone were greater in calves given na loxone and pimozide in combination compared to controls, but at 24 wee ks of age they were greater in the naloxone treated calves and lower i n pimozide treated calves compared to the control calves (P<0.05). We concluded that an opioidergic inhibition of LH secretion was seen from the middle of the early rise in LH secretion (14 weeks of age) onward s but inhibition of FSH secretion was only seen at 24 weeks of age. A dopaminergic drive for LH and FSH secretion developed after the comple tion of the early rise in gonadotrophin secretion (24 weeks of age). O pioidergic inhibition of gonadotrophin secretion largely involved inhi bition of a dopaminergic drive at the end of the early rise in LH secr etion (24 weeks of age) but not at earlier ages.