Effects of induced fescue toxicosis on plasma and tissue catecholamine concentrations in sheep

Abstract

Sixteen crossbred wethers were distributed among four treatments and fed a control ration based on annual rye-orchardgrass (R-O) for 8 days. Indwelling jugular cannulae were installed and experimental regimes begun the following day (experimental day 1). One-half of the wethers were fed a ration based on endophyte-infected Kentucky-31 fescue while the remainder continued to receive the R-O control diet for 10 days. Spiperone, a dopamine antogonist, was administered to one-half of the wethers receiving each ration on days 8 and 9. Plasma prolactin (PRL), dopamine (DA), norepinephrine (NE) and epinephrine (E) were measured in jugular venous blood on days 1,3,5 and 7–10 of the trial. On day 10, the animals were decapitated; and DA, NE, E and 3,4-dihydroxyphenylacetic acid (DOPAC) and monoamine oxidase (MAO) were determined in hypothalamic and pituitary tissue. Plasma DA was elevated (P<.05) following day 8 in wethers fed infected fescue over those fed (R-O), while plasma PRL was reduced (P=.08). Wethers receiving Spiperone had lowered (P<.05) plasma DA and elevated (P<.01) plasma PRL. Plasma DA was negatively correlated (P<.01) with plasma PRL (r = −0.50) following day 8. Plasma NE and E levels and NE, E, DA and DOPAC tissue concentrations were not affected by Spiperone administration or diet. MAO levels in pituitaries were higher (P<.01) for Spiperone-treated wethers. Wethers receiving the toxic fescue ration exhibited elevated plasma DA concentrations which was associated with depressed prolactin secretion in wethers. The modification of DA secretion may in part be responsible for the diverse symptoms associated with fescue toxicity.