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POTENTIAL ROLE OF TUBERO-INFUNDIBULAR DOPAMINERGIC NEURONS IN THE DISRUPTION OF PITUITARY HORMONE SECRETION BY ATRAZINE
Citation:
COOPER, R. L., A. R. BUCKALEW, A. E. MURR, S. C. LAWS, AND T. E. STOKER. POTENTIAL ROLE OF TUBERO-INFUNDIBULAR DOPAMINERGIC NEURONS IN THE DISRUPTION OF PITUITARY HORMONE SECRETION BY ATRAZINE. Presented at Society of Toxicology, San Diego, CA, March 05 - 09, 2006.
Description:
Previously, we demonstrated that atrazine suppressed the ovulatory surge of luteininzing hormone and disrupted estrous cycles in the female rat. We also reported that this disruption of ovulation is likely the result of atrazine's effect on hypothalamic gonadotropin hormone releasing hormone (GnRH ) release into the pituitary portal system. In addition, we reported that atrazine inhibits prolactin secretion. In the present study, we examined the concentrations of hypothalamic GnRH as well as the concentration of selected neurotransmitters [norepinephrine (NE), dopamine (DA)] and neuropeptides [neuropeptide-Y (NPY) and B-endorphin (-endo)] known to regulate GnRH release. Female rats displaying regular (4-day) estrous cycles were gavaged with atrazine daily (25 & 75 mg/kg at 1200 h) beginning on the day of vaginal estrus for 4 days. On the morning of the next proestrus, groups of rats were killed at 2h intervals between 1000-2000h. The peak amplitude of the LH surge was decreased at both doses. The concentration of GnRH was increased in the median eminence region [MER, the area immediately above the pituitary stalk), but not in the rostral hypothalamus (i.e., medial preoptic area). Similarly, the concentration of DA was increased in the MER and arcuate nucleus. No changes in hypothalamic NE, NPY or B-endo were identified. Based on these observations, we hypothesize that atrazine decreases GnRH release because it enhances the synthesis and release of DA from the tubero-infundibular dopminergic (TIDA) neurons within the arcuate-MER region of the hypothalamus. The TIDA neurons have been reported to inhibit GnRH via presynaptic (axo-axonal synapses) and the release of TIDA DA into the pituitary portal system is known to decrease prolactin release from the pituitary. Thus, a single change in hypothalamic DA activity may explain the change in secretion of both LH and prolactin from the pituitary. This abstract does not necessarily reflect EPA policy.