Development and disappearance of subsensitivity to pilocarpine following a single administration of the irreversible anticholinesterase angent, DFP.

The present study examined the possibility that subsensitivity to pilocarpine might occur following a single injection of the irreversible anticholinesterase agent, DFP. In one experiment male Sprague-Dawley rats were trained to drink from experimental drinking chambers for 1/2 h per day. After establishment of baselines, pilocarpine hydrochloride (8 mg/kg) was injected i.p. 5 min before the drinking session. One week later DFP or the arachis oil vehicle (1 mg/kg) was injected intramuscularly and injections of pilocarpine were given at varying times thereafter. The suppression of water intake by this dose of pilocarpine was unaffected by pretreatment with arachis oil, but was markedly attenuated by pretreatment with DFP. This subsensitivity was first observed on the second day but had largely disappeared by the 14th day. DFP was found to have comparable effects on water intake and brain acetylcholinesterase activity when the injections were separated by 20 days. In a second experiment the hypothermic effects of pilocarpine were found to be reduced in rats acutely treated with DFP. These data establish that subsensitivity to pilocarpine occurs following a single administration of DFP. This subsensitivity could reflect a reduced sensitivity of postsynaptic receptors to acetylcholine, which may partially account for the behavioural recovery of the rats while acetylcholinesterase activity is still markedly depressed.