Characterization of Activation of the Hypothalamic-Pituitary-Adrenal Axis by the Herbicide Atrazine in the Female Rat

Abstract

Atrazine (ATR) is a commonly used pre-emergence and early postemergence herbicide. Rats gavaged with ATR and its chlorometabolites desethylatrazine (DEA) and deisopropylatrazine (DIA) respond with a rapid and dose-dependent rise in plasma corticosterone, whereas the major chlorometabolite, diaminochlorotriazine (DACT), has little or no effect on corticosterone levels. In this study, we investigated the possible sites of ATR activation of the hypothalamic-pituitary-adrenal (HPA) axis. ATR treatment had no effect on adrenal weights but altered adrenal morphology. Hypophysectomized rats or rats under dexamethasone suppression did not respond to ATR treatment, suggesting that ATR does not directly stimulate the adrenal gland to induce corticosterone synthesis. Immortalized mouse corticotrophs (AtT-20) and primary rat pituitary cultures were treated with ATR, DEA, DIA, or DACT. None of the compounds induced an increase in ACTH secretion or potentiated ACTH release in conjunction with CRH on ACTH release. In female rats gavaged with ATR, pretreatment with the CRH receptor antagonist astressin completely blocked the ATR-induced rise in corticosterone concentrations, implicating CRH release in ATR-induced HPA activation. Intracerebroventricular infusion of ATR, DEA, and DIA but not DACT at concentrations equivalent to peak plasma concentrations after gavage dosing resulted in an elevation of plasma corticosterone concentrations. However, ATR did not induce c-Fos immunoreactivity in the paraventricular nucleus of the hypothalamus. These results indicate that ATR activates the HPA axis centrally and requires CRH receptor activation, but it does not stimulate cellular pathways associated with CRH neuronal excitation.

The Herbicide Atrazine Potentiates Angiotensin II-Induced Aldosterone Synthesis and Release From Adrenal Cells

“…Through our previous investigation, the zona glomerulosa (zG) of the adrenal cortex was found to be reduced in thickness after 4 days of atrazine treatment (11). As mentioned, atrazine treatment results in elevated concentrations in circulating ACTH similar to those found in restraint stressed animals (11,13). ACTH treated or restraint stressed rats have similar reductions in zG thickness to atrazine treated animals (14,15).…”

“…Animals were allowed 7 to 10 days to recover before additional procedures were conducted. Ovarectomized-female rats were used in the current experiments to be consistent with our previous findings of atrazine-induced reduction in zG morphology (11). To reduce nonspecific stress associated with handling and gavage, animals used for all experiments were habituated for 4 to 7 days before treatment was initiated.…”

“…The in vivo no effect level for the atrazine-induced activation of the HPA axis is 5 mg/kg in male rats (13), and the lowest observed effect level reported for female rats is 12.5 mg/kg (16). However, to characterize the effects of atrazine on adrenal morphology and activation, doses at the higher end of the known effective range to elicit a rise in stress hormone concentrations were used (11).…”

“…Previous reports involving repeated stress and changes to adrenal morphology used 30 mins or 2 hours of daily restraint stress (14,15). In the present studies, 30 mins of restraint stress was used because the hormone response (amplitude and duration of ACTH and corticosterone) to 30 mins of restraint is similar to that of atrazine (100mg/kg) treatment (11,13). Animals were weighed daily.…”

“…Likewise, the concurrent rise in plasma ACTH and corticosterone concentrations after atrazine treatment, and the lack of atrazine-induced corticosterone in animals under dexamethasone suppression or in animals without a pituitary, does not support a direct action of atrazine on adrenal gland steroidogenesis. However, we have termed atrazine's effects on the HPA axis as "stress-like" since, although CRH receptor activation is required for atrazine-induced ACTH release, CRH cells of the hypothalamic paraventricular nucleus fail to display markers of activation after atrazine treatment (cFos and P-CREB) (11). Likewise, animals do not perceive atrazine as a stressor.…”