chapter  8
8 Pages

Herbicides

ByDonaldJ.Ecobichon

The toxicity associated with exposure to phenoxy herbicides, particularly 2,4D, bas been extensively reviewed,4" and, by request of the U.S. EPA, the entire toxicological data base for 2,4-D bas been re-examined by conducting a complete spectrum of tests in accordance with current regulatory requirements.6 With the exception of a low incidence of brain tumors, astrocytomas, found in male F344 rats (which subsequent review of the material concluded was not treatment-related), little new toxicological evidence about 2,4-D was revealed. 7•1 There is little evidence of neuropathy associated with either the central or peripheral nervous systems even at dosages exceeding levels that can be efficiently eliminated by animals. 4-6 Signs and symptoms of central nervous system disturbances in exposed animals have been attributed to damage to the blood-brain barrier and capillary vessels by high concentrations of absorbed agent found in the bloodstream with a subsequent accumulation in the brain. 9 •10 Investigators have reported myotonia, spasms in the bindlimbs, paralysis in the extremities, and vomition, at least some of these signs suggesting possible neurological involvement. 11•12 Desi et al., 13 following acute intraperitoneal administration of 2,4-D (200 mglkg) to rats, noted a transient shortening of the duration of the desynchronization phenomenon in the brain following stimulation, abolition by 25 minutes but recovery by SO minutes post-treatment. Placing granules of crystalline 2,4-D on the exposed cortex of rats and cats, the electroencephalograms showed a marked decrease in amplitude and an increase in frequency only in regions close to the site of application, with recovery by 35 minutes post-treatment. More recently, 2,4-dicbloropbenoxyacetic butyl ester administered to nulliparous female rats elicited no deleterious effects on open field or rotarod performance, but, when given to male rats or to females during pregnancy, it caused an impairment in open field and rotarod tests and improved active avoidance learning. 14 Parallel experiments revealed an apparent involvement of testosterone with the toxicity of the ester, however unclear the mechanism.