Assessment of behavioral, neurochemical and developmental effects in developing rats following in utero exposure to non-teratogenic levels of 2,4-D and 2,4,5-T

Abstract

The safe or non-teratogenic levels following prenatal exposure of animals to chlorophenoxy herbicides are currently based on offspring survival, growth and absence of anatomical defects or overt neurological impairment. No adequate consideration is given to possible subtle neurological consequences. Pregnant rats were gavaged with 2 ml/kg/day of peanut oil vehicle (control, G1), 50 (G2), 100 (G3) and 125 (G4) mg/kg/dayof 1:1 2,4-D/ 2,4,5-T mixture (as found in Agent Orange) on gestational days 6 to 15. Group 3 and 4 mothers gained significantly (P<0.05) less weight during pregnancy and delivered fewer offspring than control. Neonatal mortality on postnatal day (PND) 1 significantly increased only in G4. Treatments did not significantly affect gestational lengths, sex ratios or birth weights, and no anatomical defects were seen. With the exception of a significant 3% reduction in body weights of the progeny in G4 on PND 60, treatments did not affect progeny growth and maturational landmarks of development. Surface righting (PND 2-5), negative geotaxis at 450 angle (PND 15-17) and swimming performance (PND 7-21) were significantly delayed in all the treated groups. Olfactory discrimination of home-nest odor (PND 9-11) and negative geotaxis at 250 angle (PND 7-11) were significantly delayed in 63 and 64. In the females of 62, the negative geotaxis response at 250 was significantly delayed on PND 9. Higher running wheel activity was seen only in the male progeny of 64 on PND 22 and 23 and following d-amphetamine sulfate challenges. In utero exposure to 2,4-D/2,4,5-T altered neurochemical development of the progeny. In 62 and 64 glutamate contents in the cerebrum and cerebellum were significantly reduced on PND 1. 6ABA levels were unaffected by treatments. Examination of regional brain protein, DNA and RNA contents and protein:DNA and RNA:DNA ratios did not indicate severe of generalized developmental deficits. However, on PND 22 RNA levels in the neocortex of 64 and protein:DNA ratios (cell size) in the thalamus-hypothalamus of 62 and 64 were significantly reduced. Whole brain DA but not NE turnover of 62 and 63 was signficantly reduced on PND 3. Treatment (62 and 63) also delayed ontogeny of DA but not NE levels in the thalamus-hypothalamus, pons-medulla on PND 7, 9 and 15 and olfactory bulb on PND 9. On PND 25 5-HT levels were significantly reduced in pons-medulla of 63, whereas its metabolite 5-HIAA levels decreased in thalamus-hypothalamus and pons-medulla of 62 and 63. The data indicated that prenatal exposure to non-teratogenic levels of 2,4-D/2,4,5-T mixture produced adverse behavioral and neurological consequences in the overtly normally developing rat progeny lasting well past cessation of herbicides exposure. The neurochemical findings provided possible neurochemical basis for some of the 2,4-0/ 2,4,5-T induced neurobehavioral effects. The data also indicated the values of behavioral and neurochemical ontogenic assessment of low dose exposures which result in functional alterations in the absence of overt toxic signs.