Effect of reduced cholinesterase activity on the maintenance of an operant response

Change in atropine dose effect curve after subacute soman administration

Thirty-one adult male rats were trained on a two component (FR10-extinction) operant task. After establishment of an atropine sulfate dose effect curve (vehicle, 1.0, 1.8, 3.2, 5.6 or 10.0 mg/kg), all rats were injected SC with 35.0 micrograms/kg soman three times per week for four weeks. One, twenty-eight and fifty-six days after the last soman injection, additional atropine tests were given. Atropine produced significant dose related decreases in FR10 responding at 3.2, 5.6 and 10.0 mg/kg during the initial dose effect curve with no effect on extinction responding. Although four rats died during the soman exposure period, the remaining rats exhibited neither signs of organophosphate toxicity nor significant disruptions in FR responding. However, a significant increase in extinction component responding was seen on soman injection days. The atropine dose effect curve for FR10 responding showed a significant shift to the left on the day after the last dose of soman. However, the dose effect curves at 4 and 8 weeks were the same as the initial curve. These results provide behavioral evidence that chronic soman exposure results in a supersensitivity to the antimuscarinic drug, atropine, possibly due to a down-regulation in the number of muscarinic receptors.

Effect of repeated intraperitoneal injections of soman on schedule-controlled behavior in the rat

Intraperitoneal (IP) administration of the acetylcholinesterase inhibitor, soman (10-40 micrograms/kg), suppressed in a dose-related manner response rates in rats maintained under a multiple fixed-interval 50-s fixed-ratio 25 schedule of food delivery. Chronic administration of soman at weekly intervals resulted in tolerance to the response. When soman administration was separated by 2-5 weeks in individual rats, the suppressive effects of the agent again became apparent. Analysis of acetylcholinesterase activity revealed that enzyme inhibition was limited to gastrointestinal areas near the site of injection. There was no significant effect on brain acetylcholinesterase even following IP injection of doses which completely suppressed responding. The IP route may be useful for studying tolerance and other chronic effects of soman without producing generalized toxicity.

Effects of PAM, proPAM, and DFP on behavior, thermoregulation, and brain AChE in rats

The effects of pyridine-2 aldoxime methyl iodide (PAM), N-methyl-1,6-dihydro-pyridine-2-carbaldoxime hydrochloride (proPAM), and diisopropyl phosphorofluoridate (DFP) on performance of a conditioned avoidance response (CAR), body temperature, and in vivo acetylcholinesterase (AChE) activity in five brain regions in the rat were examined. Sublethal doses of DFP (1.5 to 2.5 mg/kg, IP) markedly degraded CAR performance. This effect was antagonized by 5 mg/kg, subcutaneously injected (SC) atropine. A 50 mg/kg, SC dose of PAM had no effect on the CAR, but an equal dose of proPAM caused a transient deterioration of performance. Given 10 min or 2 hr after DFP, 50 mg/kg proPAM initially exacerbated the behaviorally toxic effects of DFP. Neither PAM nor proPAM antagonized DFP-induced hypothermia. PAM did not reactivate DFP-inhibited brain AChE, and proPAM reactivated it by only 6 to 12% of control activity.