MELATONIN AND PINEAL NEUROCHEMICALS IN STEERS GRAZED ON ENDOPHYTE-INFECTED TALL FESCUE - EFFECTS OF METOCLOPRAMIDE

Plasma and pineal melatonin (MEL) and selected pineal neurochemicals ( 5-hydroxytryptophan, serotonin, N-acetylserotonin, dopamine, norepinep hrine) associated with MEL synthesis were determined in steers grazing Acremonium coenophialum (endophyte)-infected 'Kentucky-31' tall fescu e paddocks. Paddock treatments included low (LE, 33 %) or high (H E, 7 4 %) endophyte at either low (134 kg.ha-1.yr-1, LN) or high (335 kg.h a-1.yr-1, HN) nitrogen fertilization. Twelve pairs of yearling Angus s teers were randomly assigned to three replications of the paddock trea tments (LEHN, HEHN, LELN, and HELN). One steer in each of the 12 paddo cks received per os either a dopamine antagonist, metoclopramide (MC; 15 mg/kg), or sucrose (S; 15 mg/kg) three times weekly for 10 wk. Bloo d was collected via jugular cannulas during the day and night for plas ma MEL analysis and pineal glands were collected at termination. Day a nd night plasma MEL in the S/HEHN steers was reduced by 31.7 and 35.4% (P < .05), respectively, compared with that in S/LEHN steers. Mean ni ght plasma MEL in the S/HELN steers was reduced by 26.7% (P < .05) com pared with that in S/LELN steers. Metoclopramide reduced mean day and night plasma MEL by 22.9 and 38.3% (P < .05), respectively, in the LEH N steers and increased night MEL in the HELN animals by 35.1% (P < .05 ). During the day and night, there was a MC x pasture treatment intera ction (P < .05). No differences were observed in either pineal MEL or the pineal neurochemicals. Acremonium coenophialum-infected fescue red uced plasma concentrations of MEL in steers, whereas treatment with MC altered plasma MEL biphasically. Dopaminergic mechanisms apparently p lay a role in MEL secretions and warrant further investigation.