D-2 DOPAMINE-RECEPTOR RESPONSE TO ENDOPHYTE-INFECTED TALL FESCUE AND AN ANTAGONIST IN THE RAT

Effects on rat brain D-2 dopamine receptors by endophyte-infected tall fescue seed consumption and antagonist injection were characterized. Forty-eight male Wistar rats (225 g) in three separate trials were exposed to either 22 or 32 degrees C. Diets, to maintain similar concentrations of ergovaline, contained 10% (Trial 1) or 15% (Trials 2 and 3) endophyte-infected (E+; 325 average ppb of ergovaline) or uninfected (E-; 0 ppb of ergovaline) tall fescue seed. Rats were injected i.p. daily with either placebo (PL) or an experimental D-2 dopamine antagonist (DA, .0375 mg/kg BW). No effects (P > .10) on diet DM intake by E+ ingestion or DA injection were detected at 22 degrees C. However, ingestion of E+ reduced (P < .01) and injection of DA improved (P < .05) DM intake of rats housed in 32 degrees C (11.1 vs 15.4 g of DM/d for E+ vs E-, respectively). Whole brain D-2 dopamine receptor density (B-max) and mRNA were reduced (P < .05) by E+ and increased (P < .05) by DA in Trial 1. No treatment effects (P > .10) on cerebral cortex alpha(1)- and alpha(2)-adrenergic or striatal D-2 dopamine receptor B-max were measured in Trials 2 and 3. In summary, dietary E+ reduced whole brain D-2 dopamine mRNA and B-max, whereas injection of DA increased D-2 dopamine mRNA. Thus, long-term regulation of monoamine receptors seems to be affected by E+ ingestion or DA injection.