CONICET | Buscador de Institutos y Recursos Humanos

The effects of 2,4-Dichlorophenoxyacetic acid (2,4-D) on brain monoamines and the serum level of hormones involved in milk synthesis and on the milk ejection reflex in rats, were evaluated in this work. Dams were treated with 2.5, 5, 15, 25, 50 or 70 mg 2,4-D/kg bw according to two experimental designs: a) through food from post partum day 1 (PPD1) to PPD 16 and the respective control groups or b) an unique ip injection on PPD11. To measure milk ejection, the litters were separated from the mother at the 11th day of lactation during 8 h, returned to their mothers and allowed to suckle for a period of 15 min. The procedure was repeated on 3 consecutive days until the end of treatment. The change in litter weight during the suckling period was taken as a measure of the amount of milk ejected during this period. The dams serum prolactin (PRL), oxytocin (OT) and growth hormone levels were determined by radioimmunoassay. Both treatment regimens produced a dose dependent decrease in the amount of milk ejected and circulating PRL and OT secreted in response to the suckling stimulus. Administration of OT before returning the pups restored the milk ejection, indicating no impairment in the capacity of the mammary gland to produce and secrete milk. In addition, dopamine levels were increased by the 2,4-D treatments in Arcuate Nucleus (ArN) and anterior lobe of pituitary gland (AL), while serotonin level was drastically decreased in ArN. 2,4-D treatment increased both calcium dependent and calcium independent Nitric Oxide Synthase (NOS) activities in ArN. These results suggest that 2,4-D inhibits the suckling induced hormone release, milk production and ejection at the central level, through a stimulation of hypothalamic NOS and dopamine and by an inhibition of hypothalamic serotonin transmission.