Reduction in open field-induced hyperthermia in the rat exposed to chlorpyrifos, an anticholinesterase pesticide

Abstract

Rodents develop a marked elevation in core temperature ( T c) when they are placed in an open field environment (OFE). Open field-induced hyperthermia (OFH) involves activation of thermoregulatory systems that are quiescent under baseline conditions. Cholinergic stimulation arising from exposure to organophosphates (OP) is expected to modulate OFH; subtle effects of OPs may be observed during OFH that would not occur in the absence of stress. We have found that methyl scopolamine (MS), a peripheral muscarinic antagonist, attenuates stress-induced hyperthermia. To this end, we assessed if a low dose of the OP pesticide chlorpyrifos (CHP) would alter OFH with and without administration of MS. T c and motor activity (MA) of male, Sprague-Dawley rats were monitored by telemetry while housed in their home cage at an ambient temperature of 22°C. At 9 AM the rats were gavaged with corn oil or 10 mg/kg CHP that resulted in a ∼40% inhibition in plasma cholinesterase activity. T c and MA were the same in the control and CHP groups prior to OFE. The rats were then dosed IP with saline or 1.0 mg/kg MS at 12 noon and subjected to OFE (61×61×61 cm box) for 1 h while T c and MA were monitored every 2 min. T c of the corn oil/saline group increased by 1.2°C during OFE, whereas T c of the CHP/saline group was significantly attenuated. Administration of MS attenuated the OFH in rats treated with corn oil and CHP. Overall, exposure to OFE exacerbated the effects of a relatively low dose of CHP that had no effect in the unstressed animal. In addition, the OFH response to MS suggests a peripheral (i.e., outside of CNS) cholinergic pathway is operative in the control of OFH.