Decreased muscarinic binding sites in small intestine from mice treated with neostigmine

Effects of single or repeated administration of a carbamate, propoxur, and an organophosphate, DDVP, on jejunal cholinergic activities and contractile responses in rats

Wistar rats were injected once or repeatedly for 10 days with dichlorvos (DDVP, 5 mg kg-1), propoxur (10 mg kg-1), oxotremorine (0.1 mg kg-1) or atropine (5 mg kg-1). Animals were killed 20 min or 24 h after single or consecutive injections, respectively, for determinations of cholinergic activities and contractile responses to acetylcholine (ACh) of the jejunum. Single treatments: while DDVP and propoxur decreased acetylcholinesterase (AChE) activity, oxotremorine and atropine did not. Although DDVP, propoxur and oxotremorine increased levels of ACh, atropine decreased them. Contractile responses to ACh were enhanced by DDVP and reduced by oxotremorine and atropine. The Bmax value of binding of [3H]quinuclidinyl benzylate (QNB) to muscarinic ACh receptors was decreased by atropine. Consecutive treatments: DDVP and oxotremorine decreased AChE activity markedly and slightly, respectively. Although DDVP and oxotremorine increased levels of ACh, propoxur decreased them. Without affecting the contractile responses, DDVP caused a reduction and propoxur and atropine caused an increase in the Bmax value for binding of [3H]QNB. Both the contractile responses and the value of Bmax for binding of [3H]-QNB were decreased by oxotremorine. In summary, propoxur and DDVP showed similar effects mainly through their anticholinesterase properties in the case of single injection, but DDVP had similar effects to those of oxotremorine and propoxur had similar effects to those of atropine in the case of repeated injection.

Mathematical modelling of the enteric nervous network. II: Facilitation and inhibition of the cholinergic transmission

The pharmacokinetic responses of the cholinergic enteric neurone to treatment with acetylcholinesterases, tetrodotoxin, some chloride salts of divalent cations, botulinum toxin, beta-bungarotoxin and changes in the concentration of calcium ions in the external medium and repetitive stimulation are presented. The numerical results obtained reproduce quantitatively the effects of toxins and salts of divalent cations acting at different levels of acetylcholine release from the nerve-terminal. The addition of cholinergic agonists potentiates the action of acetylcholine and increases the amplitude of the generated excitatory postsynaptic potential. A decrease in the concentration of extracellular Ca2+ ions reduces the amplitude of the excitatory postsynaptic potential and significantly increases synaptic transmission time. The effect of tetrodotoxin is the blockade propagation of the action potential along the nerve axon and, as a consequence, acetylcholine release from the vesicular store. All these effects have been shown to be dose-dependent. The repetitive stimulation of the neurone reproduces the effects of accumulation and potentiation. The possible applications of the model for the analysis of the enteric nervous system function are discussed.

Biochemical and pharmacological evidence for central cholinergic regulation of shock-induced aggression in rats

Acetylcholinesterase (AChE) activity was estimated in brain and heart homogenates and plasma of 'aggressive' and 'nonaggressive' rats. Brain homogenates of 'nonaggressive' rats hydrolyzed significantly more substrate when compared to the 'aggressive' rats. Such differences were not seen in the heart homogenates or plasma of these two groups of rats. Acute DFP (0.1, 0.3 and 1.0 mg/kg) attenuated shock-induced aggression (SIA) 2 hr after treatment but facilitated SIA 24 hr and 48 hr after drug administration. Long-term DFP (0.3 mg/kg x 10 days), on the other hand, induced a significant enhancement in the SIA score, whereas atropine (1.0 and 5.0 mg/kg) produced a dose-related attenuation of the same. Pretreatment of rats with atropine (5 mg/kg) antagonized the long-term DFP-induced facilitation of SIA. These results are discussed in the light of an inhibitory central cholinergic mechanism in the regulation of SIA.

Contractile responses of tracheal smooth muscle in organophosphate-treated swine: 2. Effects of antagonists

1. Swine tracheal smooth muscles (TSM) developed spontaneous contractions following the acute administration of DFP in vivo and/or in vitro which could be blocked pharmacologically using atropine (2 x 10(-7) M), pirenzepine (3 x 10(-7) M), or hemicholinium 3 (HC3, 5 x 10(-6) M). 2. Treatment of TSM in vitro with DFP caused them to become responsive to ACh concentrations as low as 10(-10) M. 3. Atropine and pirenzepine (at 2 and 3 x 10(-7) M respectively) increased the EC50 concentrations for ACh approximately 300- and 8-fold respectively. The shifts caused by atropine and pirenzepine in the dose-response curves for ACh were not parallel after in vitro treatment of the muscle with DFP. By contrast, the shifts in the dose-response curves were parallel when muscles from swine injected for 7 days with DFP were used. 4. HC3 had no effect on the control dose-response curve for ACh, but steepened the dose-response curve after in vitro treatment of the muscle with DFP. The ACh dose-response curve obtained in the presence of HC3 and DFP was identical to that obtained using muscles from swine treated for 7 days with DFP. 5. McN-A-343, a partial agonist at muscarinic receptors, also induced contraction although the maximal tension induced was 56% of the maximal tension obtained using ACh. In vitro treatment of the muscle with DFP caused a leftward shift in the dose-response curve for McN-A-343. The muscles from animals treated for 7 days with DFP did not respond to McN-A-343 at doses up to 10(-3) M. 6. McN-A-343 competition for [3H]QNB binding suggested that the loss of the contractile response can be correlated with the loss of a high affinity site for McN-A-343 from the muscle. 7. We conclude that tolerance to DFP with subacute treatment results in part from the reduction in sensitivity of neural elements associated with swine tracheal smooth muscle to ACh. In addition the response to the partial agonist McN-A-343 is lost after subacute DFP treatment.

Tolerance to oxotremorine's effects on schedule-controlled behavior in physostigmine-tolerant rats

Tolerance to the effects of physostigmine and oxotremorine in rats was evaluated using a multiple fixed-ratio 10, extinction schedule of food presentation. Physostigmine was administered either once daily or three times daily for 18 consecutive days. Tolerance to physostigmine's response decreasing effects was observed under both administration regimens. Cumulative dose-effect functions for oxotremorine (0.0056-0.562 mg/kg) were determined before and after chronic physostigmine administration. Oxotremorine's potency to produce response rate suppression decreased in rats receiving physostigmine three times daily but did not substantially change in rats receiving single daily injections. These results demonstrate that the dose or duration of action of physostigmine can determine whether tolerance to physostigmine's effects is accompanied by cross-tolerance to oxotremorine's effects.

Contraindication for osmotic mini-pump in the abdominal cavity to study muscarinic cholinergic control of pancreatic enzyme secretion and muscarinic receptors

Alzet mini-pumps were used to study modulation of rat pancreatic muscarinic receptors and the acinic secretory response to long-term N-methylscopolamine (NMS) treatment. Infused intraperitoneally (i.p.), NMS, 25 mg.kg-1.day-1 for 14 days, resulted in a shift to the left in the amylase dose-response curve to carbamylcholine (Cch) and an increase in receptor concentration compared to saline-infused group. Compared to previously published control data [4,5] the saline-infused group exhibited desensitization with increased EC50 for secretion. An alternative mode of saline treatment, subcutaneous (s.c.) infusion showed normal pancreatic secretory response. Desensitization by i.p. saline infusion was prevented by NMS infusion. In conclusion, a foreign body such as a mini-pump in the abdominal cavity can cause desensitization of the pancreatic secretory function under neural cholinergic control.

The muscarinic receptor adenylate cyclase complex of rat striatum: desensitization following chronic inhibition of acetylcholinesterase activity

Chronic inhibition of acetylcholinesterase activity by treatment with diisopropylfluorophosphate (DFP) decreased the capacity of acetylcholine (ACh) acting at a muscarinic receptor to inhibit basal adenylate cyclase activity in homogenates from rat striatum. There was also a loss of the capacity of ACh to inhibit the activation of adenylate cyclase by dopamine. The desensitization of the muscarinic receptor adenylate cyclase complex was associated with a marked attenuation of the capacity of ACh to stimulate a high-affinity GTPase activity present in striatal membranes. The EC50 value of ACh for inhibiting adenylate cyclase and for stimulating GTPase activity increased following treatment with DFP, while the Hill coefficient for both responses was unaltered.

Chronic administration of an organophosphorus insecticide to rats alters cholinergic muscarinic receptors in the pancreas

Male rats were treated for 10 days with the organophosphorus insecticide, acetylcholinesterase inhibitor, O,O-diethyl S-[2-(ethylthio)ethyl] phosphorodithioate (disulfoton, 2 mg/kg/day by gavage). At the end of the treatment, binding of [3H]quinuclidinyl benzilate ( [3H]QNB) to cholinergic muscarinic receptors and cholinesterase (ChE) activity were assayed in the pancreas. Functional activity of pancreatic muscarinic receptor was investigated by determining carbachol-stimulated secretion of alpha-amylase in vitro. ChE activity and [3H]QNB binding were significantly decreased in the pancreas from disulfoton-treated rats. The alteration of [3H]QNB binding was due to a decrease in muscarinic receptor density with no change in the affinity. Basal secretion of amylase from pancreas in vitro was not altered, but carbachol-stimulated secretion was decreased. The effect appeared to be specific since pancreozymin was able to induce the same amylase release from pancreases of control and treated rats. The results suggest that repeated exposures to sublethal doses of an organophosphorus insecticide lead to a biochemical and functional alteration of cholinergic muscarinic receptors in the pancreas.

Topographical distribution of down-regulated muscarinic receptors in rat brains after repeated exposure to diisopropyl phosphorofluoridate

Quantitative receptor autoradiography demonstrated that muscarinic receptors were down-regulated in Wistar rats after repeated exposure to diisopropyl phosphorofluoridate. The density of receptors was decreased to 60-85% of the controls. Reductions in muscarinic receptor binding were observed in cortex, caudate-putamen, lateral septum, hippocampal formation, superior colliculus, and pons. The density of muscarinic receptors was unchanged in thalamic and hypothalamic nuclei, periaqueductal grey, cerebellum, inferior colliculus and reticular formation of the brain stem. The down-regulation of muscarinic receptors in forebrain structures, such as cortex, caudate-putamen and hippocampus, may be important in the adaptation to the behavioral effects of organophosphate poisons.

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.

Muscarinic receptor alterations following neostigmine treatment

Male mice administered neostigmine in the drinking water a daily increasing concentrations (20-100 mg/l) for four days became tolerant to its toxicity and presented a reduced binding of [3H]quinuclidinyl benzilate ([3H]QNB) in the small intestine. An increased binding of [3H]QNB was found in the forebrains of neostigmine-treated animals. This was due to an increase in muscarinic cholinergic receptor density. Neostigmine-treated animals were also more sensitive than control to the hypothermic effect induced by oxotremorine. Dopaminergic and alpha- and beta-adrenoceptor were not affected. Administration of methylatropine together with neostigmine prevented the decrease of [3H]QNB binding in the small intestine as well as the increase in the forebrain.

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.