Altered sensitivity to drugs following repeated injections of a cholinesterase inhibitor to rats

Changes in ACh levels in the rat brain during subacute administration of diisopropylfluorophosphate

Rats were treated with diisopropylfluorophosphate (DFP) acutely and daily for 14 days. The total, free, and bound acetylcholine (ACh) levels were monitored in striatum, hippocampus, and frontal cortex after DFP administration. Thirty minutes after daily administration of DFP, the total and free ACh levels were significantly increased and remained constant after each successive dose. The bound ACh levels in striatum and frontal cortex were also significantly increased; however, they were comparable to control levels after the 14th injection of DFP. The total ACh levels 30 min after a challenge dose of 2 mg/kg DFP in saline and DFP subacutely treated rats were significantly increased in hippocampus (34 and 76%) and frontal cortex (49 and 64%) and were not significantly different between the two groups. However, the level of total ACh in striatum was increased less in the tolerant rats (10%) than in the acutely treated rats (36%). The levels of free and bound ACh after acute administration of 2 mg/kg DFP were markedly increased in three brain regions. After subacute administration, the levels of bound ACh were significantly increased in hippocampus (84%) and frontal cortex (40%); however, that in striatum did not change. The increase in the bound ACh level in the subacute treatment group was less than that in acutely treated rats in all three brain regions; however, the duration of the elevation of the free ACh in striatum was shorter in subacutely treated rats. These results suggest that the presynaptic cholinergic storage sites for ACh might be changed during subacute administration of DFP.

Evidence for the involvement of presynaptic cholinergic functions in tolerance to diisopropylfluorophosphate

Rats were treated with diisopropylfluorophosphate (DFP) acutely or daily for 14 days. The involvement of various presynaptic and postsynaptic functions of the cholinergic system in the development of tolerance to DFP was studied. Receptor density and affinity of both muscarinic and nicotinic receptors, high-affinity choline uptake, and [K+]-evoked release of acetylcholine (ACh) by atropine were not changed after acute administration of 2 mg/kg DFP. Both muscarinic and nicotinic receptors were down-regulated to the same extent (40-50%) after subacute administration of DFP (1 mg/kg) without changes in their affinities. Binding sites of muscarinic receptors were maximally decreased after 7 days of DFP administration. Thereafter, they remained constant throughout 14 days of administration. One hour after the last injection of 2 mg/kg DFP to subacutely treated rats, the maximum velocity of high-affinity choline uptake was significantly decreased in the striatum (33%) and hippocampus (53%) without changes in Km values. Twenty-four hours after the last injection of DFP, only a higher dose of DFP (2 mg/kg) significantly inhibited choline uptake. Potassium-evoked release of ACh by slices of striatum was not different between acutely and subacutely treated rats. However, the release of ACh by slices of striatum and hippocampus was significantly increased by atropine in subacutely treated rats. It is suggested that along with the down-regulation of the postsynaptic receptors, subsensitivity of presynaptic functions of the cholinergic synapse also develops during subacute administration of DFP.

Quantitative assessment of tolerance development to diisopropylfluorophosphate

Rats were treated with diisopropylfluorophosphate (DFP) acutely or daily for 14 days. The quantitative assessment of tolerance development after a challenge dose of DFP, 2 mg/kg, was studied. The subacutely-treated rats developed tolerance to DFP-induced tremors. However, the severity of tremors in DFP-tolerant animals was not significantly different from that of the controls after the challenge dose of DFP was administered. Hind-limb abduction was significantly lower in the subacutely-treated group than in the acutely-treated group. The recovery of body weights in subacutely-treated rats (3.5%/day) was significantly higher than that in acutely-treated rats (2.0%/day). The consummatory behaviors (food and water consumption) recovered faster in subacutely-treated rats than in the acutely-treated group. Body temperatures were decreased to the same extent in both groups, but the subacutely-treated group recovered faster. The total mortality was significantly lower in subacutely-treated rats (10%) than in acutely-treated rats (35%). The results further substantiate the finding that tolerance develops to various DFP-induced signs of toxicity.

Muscarinic receptor alterations as a mechanism of anticholinesterase tolerance

Tolerance to the toxic signs of the organophosphorus ester acetylcholinesterase inhibitor, O,O-diethyl S-[2-(ethylthio)ethyl] phosphorodithioate (disulfoton), was induced in rats by giving 10 doses of 2.0 mg/kg/day. Concurrent with the induction of tolerance, decreased sensitivity to the cholinergic agonists carbachol and oxotremorine could be demonstrated in studies of heart rate in vivo and in isolated preparations of ileum and atria. A significant decrease in the binding of the muscarinic antagonist [3H]quinuclidinyl benzilate could be demonstrated in ileum from disulfoton-tolerant animals. However, no alterations in the binding of [3H]quinuclidinyl benzilate, [3H]oxotremorine-M, or oxotremorine were evident in atria from tolerant animals. The results suggest that, in addition to receptor loss, other mechanisms distal to ligand recognition sites or removed from the receptor complex may contribute to the subsensitivity of tissues to muscarinic cholinergic agonists.

Differential alterations of cholinergic muscarinic receptors during chronic and acute tolerance to organophosphorus insecticides

Male mice treated for 2 weeks with the anticholinesterase insecticide disulfoton (O,O-diethyl S-[2-(ethylthio)-ethyl] phosphorodithioate; 10 mg per kg per day) became tolerant to the hypothermic and antinociceptive effects of disulfoton itself and of oxotremorine, a muscarinic cholinergic agonist. Homogenates of brain and ileum from tolerant animals exhibited reduced binding of the specific muscarinic antagonist [3H]quinuclidinyl benzilate ([3H]QNB). In forebrains of tolerant animals, the number of receptors (Bmax) was decreased 40% with no change in the affinity constant. Acetylcholinesterase (AChE) activity was 15% of control. Forty-eight hours after a single injection of disulfoton (10 mg/kg) mice were more resistant than their controls to the hypothermic and antinociceptive effects of a second administration of the same insecticide and of oxotremorine. Tolerance was not present 96 hr after a single administration of disulfoton. A single injection of disulfoton produced 74, 65 and 27% inhibition of AChE activity after 4, 48 and 96 hr respectively. Four hours after a second injection at 49 and 96 hr, 73 or 72% inhibition was found. [3H]QNB binding of animals treated with a single injection of disulfoton and of controls did not differ at either time point. An increase in the Ki for inhibition of [3H]QNB binding by unlabeled oxotremorine was observed in forebrain from mice killed 48 hr after a single injection of disulfoton, indicating a decreased affinity of the muscarinic receptor for agonists. Binding of [3H]oxotremorine-M was decreased significantly 48 hr after a single injection of disulfoton and after chronic treatment. It is suggested that a differential down-regulation of muscarinic receptors occurs in acute and chronic tolerance, involving agonist and antagonist binding sites and depending on duration of exposure.

Tolerance to anticholinesterase compounds in mammals

Administration of multiple, sublethal doses of organophosphorus insecticides induces the development of tolerance to their toxicity. Among the different hypotheses investigated to explain the mechanism of this phenomenon, the one which has received the greatest experimental support is a downregulation of the muscarinic cholinergic receptors. Subsensitivity to cholinergic agonist has been demonstrated in vivo and in vitro in isolated organ preparations. Receptor binding experiments using muscarinic antagonists and agonists revealed a decrease of cholinergic receptors in central and peripheral tissues. Tolerance to another class of acetylcholinesterase inhibitors, carbamates has also been demonstrated. Differences from and similarities to organophosphate tolerance are discussed.

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.