REGIONAL CHOLINESTERASE ACTIVITY IN WHITE-THROATED SPARROW BRAIN IS DIFFERENTIALLY AFFECTED BY ACEPHATE (ORTHENE(R))

Effects of a 14-day dietary exposure to an organophosphorus pesticide, acephate (acetylphosphoramidothioic acid O,S-dimethyl ester), were de termined on cholinesterase activity in three regions (basal ganglia, h ippocampus, and hypothalamus) of the white-throated sparrow, Zonotrich ia albicollis, brain. All three regions experienced depressed cholines terase activity between 0.5-2 ppm acephate. The regions exhibited chol inesterase recovery at 2-16 ppm acephate; however, cholinesterase acti vity dropped and showed no recovery at higher dietary levels (>16 ppm acephate). Evidence indicates that the recovery is initiated by the ma gnitude of depression, not the duration. In general, as acephate conce ntration increased, differences in ChE activity among brain regions de creased. Three terms are introduced to describe ChE response to acepha te exposure: 1) ChE resistance threshold, 2) ChE compensation threshol d, and 3) ChE depression threshold. It is hypothesized that adverse ef fects to birds in the field may occur at pesticide exposure levels cus tomarily considered negligible.