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Spatial working and reference memory in rats bred for autonomic sensitivity to cholinergic stimulation: acquisition,
P J Bushnell1, E D Levin, D H Overstreet
1Neurotoxicology Division, United States Environmental Protection Agency Research Triangle Park, NC 27711, USA.
Abstract:
Rat lines were selected by breeding for sensitivity to signs of autonomic stimulation (hypotherma, loss of body weight, and reduced water intake) induced by the cholinesterase inhibitor diisopropyl fluorophosphate (DFP). These lines have since been maintained for 10 generations by continued selection for hypothermic responsiveness to the muscarinic agonist oxotremorine. The sensitive rats (Flinders Sensitive Line, FSL) differ from the resistant rats (Flinders Resistant Line, FRL) both neurochemically and behaviorally, particularly in aversively motivated test situations in which response speed is assessed. This study was conducted to determine whether the selected differences in cholinergic autonomic sensitivity would be expressed as differences in cognitive ability based on choice accuracy in appetitive tasks. The working and reference memory of rats of these two strains was thus assessed using operant delayed matching-to-position/visual discrimination (DMTP/VD) and the radial-arm maze. A Long-Evans (L-E) reference group was included in the DMTP/VD study. FSL rats responded more slowly than the other rats during acquisition of both tasks, but showed no differences in response accuracy either during acquisition or during asymptotic performance of either task. In addition, challenges with muscarinic and nicotinic antagonists and agonists [scopolamine (0.06-1.0 mg/kg), pilocarpine (1.0-4.0 mg/kg), mecamylamine (1.0-10.0 mg/kg), and nicotine (0.1-0.3 mg/kg)] demonstrated predicted differences in sensitivity among the lines only on performance measures such as response latency and trial completion. Counter to prediction, the sensitivity of the FRL rats to the ability of scopolamine to reduce matching accuracy was lower than those of the L-E and FSL rats. Thus selection based upon physiological endpoints related to cholinergic autonomic homeostasis did not produce analogous differences in cognitive function in rats.