Detection of dopaminergic modulators in a tier I screening battery for identifying endocrine-active compounds (EACs)

Apomorphine (APO; D-2 receptor agonist), haloperidol (HAL; D-2 receptor ant agonist), and reserpine (RES; a dopamine depletor that acts to lower brain dopamine levels by depleting central nervous system monoamines via disrupti ng storage vesicle function) have been examined in a Tier I screening batte ry, which has been designed to detect endocrine-active compounds (EACs). Th e Tier I battery incorporates two short-term in vivo tests (a 5-day ovariec tomized female battery and a 15-day intact male battery using Sprague-Dawle y rats) and an in vitro yeast transactivation system (YTS). Tn addition, tw o blood collection procedures were evaluated for their utility in detecting HAL-induced increases in serum prolactin (PRL) levels (i.e., the stress as sociated with each procedure). In the in vivo female battery, both HAL and RES increased serum PRL concentrations as expected, although the increase c aused by RES was marginal. Increases in serum PRL levels are enhanced when daily dosages are administered via multiple-daily dosing of the test compou nd, which results in higher sustained blood levels of the test compounds. A PO failed to decrease serum PRL concentrations in the female battery. In th e in vivo male battery, HAL increased serum PRL concentrations as expected. However, APO and RES failed to affect serum PRL concentrations. The blood collection comparison experiment demonstrated that possible confounding of the data can occur with serum PRL concentrations when animals are exposed t o stress. Basal levels of PRL were approximately fourfold higher in animals that were bled via the tail vein procedure when compared to PRL levels fro m animals that were bled under CO2 anesthesia at euthanization. As a result of the higher basal PRL levels, the HAL-induced increase in serum PRL conc entrations was completely attenuated in the tail-vein bled animals (1.3-fol d). In contrast, HAL produced a fivefold increase in serum PRL in animals w here blood was collected under CO2 anesthesia at euthanization. Hence, coll ection of blood from animals under CO2 anesthesia at euthanization is an ac ceptable approach for detection of compounds that increase PRL. In summary, HAL-like compounds would be identified in the Tier I male and female batte ry primarily via increased serum PRL concentrations. RES-like compounds wou ld be identified in the Tier I male battery via decreased gonadotropins and steroids and possibly in the Tier I female battery by a minimal increase i n serum PRL concentrations. Compounds that produce a marginal increase in s erum PRL when administered using single daily dosing can also be confirmed in an in vivo female battery with multiple dosing because this regimen incr eases the magnitude of the PRL increase. APO, a D-2 receptor agonist, was n ot detected in the in vivo male or female batteries, but in both instances the top dosage produced minimal decreases in body weight (99 to 96% of cont rol). Hence, the proposed Tier I battery needs to be further evaluated with higher dosages of APO and other D-2 receptor agonists to determine whether it is capable of detecting such agents. (C) 2000 Elsevier Science Inc. All rights reserved.