Muscarinic receptor subsensitivity in the longitudinal muscle of the rat ileum following chronic anticholinesterase treatment with diisopropylfluorophosphate

Adaptive processes of the central and autonomic cholinergic neurotransmitter system: age-related differences

Potential age-related differences in the response of the ileum strip longitudinal and circular muscle to repeated treatment with diisopropyl fluorophosphate (DFP) were evaluated in Sprague-Dawley rats. The response was measured in terms of both biochemical parameters (acetylcholinesterase-AChE inhibition, muscarinic acetylcholine receptor binding sites-mAChRs, choline acetyltransferase-ChAT) and functional responsiveness (contractility of the isolated ileum stimulated by cholinergic agonists). The biochemical data were compared with those obtained for the cerebral cortex. Male 3- and 24-month old rats were s.c. injected with DFP on alternate days for 2 weeks (doses in mg/kg: first 1.1, two of 0.7 and four of 0.35). They were killed 48 hr and 7, 14, 21, 28 and 35 days after the last treatment. In the ileum strip of control rats there was a significant age-related decline of AChE, maximal density of 3H-QNB binding sites (Bmax) and ChAT. During the first week of DFP treatment the cholinergic syndrome was more pronounced in aged than in young rats, resulting in 35% and 10% mortality, respectively; subsequently the syndrome attenuated. At the end of DFP treatment ileal AChE were inhibited by about 30%; the down-regulation of mAChRs was about 50% in young and 35% in aged rats. No significant differences in the recovery rate of AChE were noted between young and aged rats (normalization within 7 days). On the contrary, mAChRs normalized within 5 weeks in young and 3 weeks in aged rats. This was probably due to more adaptive decline in the former group. There was a post-treatment increase of ChAT, transitory in young and persistent in aged rats. In spite of age-related marked loss of ileal mAChRs there were only little, although significant, changes in the contractile responsiveness of the isolated ileum to cholinergic agonists. Considerable DFP-induced down-regulation of mAChRs was not accompanied by changes in contractility stimulated by the agonists. The overall data indicate that age- and treatment-induced changes of AChE, mAChRs and ChAT in the ileum strip differ considerably from those observed in the cerebral cortex of the same rats.

Down-regulation of muscarinic receptors in the striatum of organophosphate-treated swine

Subacute (daily) administration of diisopropylfluorophosphate (DFP) to male swine (Yorkshire white) resulted in a 97% inhibition of cholinesterase and a decrease of [3H]quinuclidinyl benzilate [( 3H]QNB) binding sites in homogenates of striata by approximately 50% after 14 days. The maximal density of receptors (Bmax) decreased from 2.1 +/- 0.3 to 1.0 +/- 0.2 pmole/mg protein. There was no significant change in the dissociation constant (Kd) for [3H]QNB binding (control: 52.6 +/- 10.7 pM; 7-day: 57 +/- 2.8 pM). Carbachol displacement of [3H]QNB binding yielded data best fit by a two-binding site model. The dissociation constants were KiL = 115 +/- 62 microM (55 +/- 3%) and KiH = 1.8 +/- 0.7 microM (45 +/- 3%), respectively, for the low- and high-affinity states. Seven-Day treatment with DFP reduced the percentage of high-affinity receptors to 22 +/- 8.6%, but affected neither the low- nor the high-affinity Kd (100 +/- 20 and 2 +/- 0.6 microM). With the addition of Mg2+, striatal homogenates had low- and high-affinity receptors in the proportion of approximately 1 to 1. In the presence of Gpp(NH)p + Mg2+ the ratio of high- to low-affinity receptors was 3:1 in homogenates of control tissue (to 26 +/- 5%). This treatment had no effect on this ratio in homogenates of tissue from 7-day DFP-treated swine (3:1) since it was already 3:1. Pirenzepine displacement of [3H]QNB binding was best described by a two-binding site model, with Ki values of 38 +/- 14 and 201 +/- 78 nM, which represent 74 and 26% of the binding sites, respectively. The high affinity Kd value was unchanged following 7 days of DFP treatment (24 +/- 5 nM). There appears to be little change in the displacement curves for pirenzepine inhibition of [3H]QNB binding. This suggests that about 75% of the receptors are of the M1 subtype. Thus, subacute administration of DFP causes not only a decrease in the number of receptors, but also a change in the proportion of agonist affinity states which is related to the interaction of the guanine nucleotide binding protein and the muscarinic receptor.

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.

Contractile responses of tracheal smooth muscle in organophosphate-treated swine: 1. Agonist changes

1. Male weanling swine were injected daily for up to 14 days with the organophosphate cholinesterase inhibitors, diisopropylfluorophosphate (DFP) or sarin. The clinical signs of poisoning disappeared or were attenuated by 7 days after starting the DFP treatment, indicating the development of tolerance to DFP toxicity. 2. A significant decrease in acetylcholinesterase activity (85-98%) occurred over the course of this treatment followed by a decrease in the maximal density (Bmax) of [3H] quinuclidinyl benzilate ([3H]QNB) and [3H] N-methylscopolamine ([3H]-NMS) binding sites in isolated cells. The affinity of the muscarinic receptors (KD) for [3H]QNB and [3H]NMS binding, however, remained unaffected. 3. Dose-response curves for ACh-induced increase in isometric tension of tracheal smooth muscle (TSM) showed a leftward shift from control, 2 h after DFP injection. Twenty-four hours after the last DFP treatment, for animals receiving 1 or up to 14 daily injections of DFP, all the dose-response curves were shifted to the left to approximately the same ACh sensitivity when compared with that for control tissue. 4. In vitro treatment of the muscle with 10(-4) M DFP shifted the dose-response curves leftward, in both control and injected animals, and rendered the muscles from control, 1- and 3-day injected animals sensitive to ACh concentrations as low as 10(-10) M. Sensitivity to 10(-10) M ACh was eliminated by carefully cleaning the smooth muscle of adherent connective tissue containing nerves and ganglia and after subacute treatment of swine for 7 days with DFP. DFP-induced spontaneous contractions were also eliminated by careful cleaning. 5. Subacute DFP treatment caused a small leftward shift in the dose-response curve for bethanechol at 2 h and a rightward shift at 1,3 and 7 days, compared to controls. 6. Dose-response curves for K+ were shifted to the right after 1 and 3 days of DFP treatment, but shifted back towards the control after 7 days of treatment. The muscle cells were hyperpolarized by approximately 5 mV after 7 days of DFP or sarin injections. The membrane potential was slightly more sensitive to changes in K+ concentration after 7 days of sarin injection. 7. Subacute treatment of swine with organophosphates modifies the response of neural elements in swine TSM to ACh. Chronic cholinesterase inhibition causes a reduction in the sensitivity of the neural elements to ACh. The decrease in muscarinic receptor density which occurs with chronic cholinesterase inhibition is not sufficient to explain tolerance to organophosphates since TSM maintains an almost normal responsiveness to ACh.

Changes in affinity states during down-regulation of muscarinic receptors in tracheal smooth muscle of organophosphate-treated swine

1. Subacute (daily) treatment of male swine with the organophosphate acetylcholinesterase inhibitor diisopropylfluorophosphate (DFP) resulted in tolerance to the effects of DFP within 5-6 days. 2. Subacute administration of DFP resulted in a 98% inhibition of tissue cholinesterase after 7 days and in a decrease of [3H]quinuclidinyl benzilate [( 3H]QNB) binding sites in homogenates of tracheal smooth muscle by 77%. The maximal density of receptors (Bmax) decreased from 1.8 +/- 0.4 to 0.5 +/- 0.1 pmole mg-1 protein. There was no significant change in the dissociation constant (Kd) for [3H]QNB binding. 3. Pirenzepine displacement of [3H]QNB binding was best described by a single binding site model, with a Ki of 230 +/- 40 nM. This value was unchanged following seven days of DFP treatment (250 +/- 30 nM). The low affinity for this M1 antagonist suggests that there is predominantly a single population of [3H]QNB binding sites of the M2 subtype in tracheal smooth muscle. 4. Carbachol displacement of [3H]QNB binding yielded data best fit by a two-binding site model. The dissociation constants were KiL = 210 +/- 60 microM (61 +/- 1%) and KiH = 1.2 +/- 0.4 microM (39 +/- 1%) respectively (n = 7) for the low and high affinity states. Seven-day treatment with DFP reduced the percent of high affinity receptors to 25 +/- 4%. 5. Addition of Mg++ to the incubation medium prevented this shift in the proportion of low and high affinity receptors. Gpp(NH)p and Mg++ together decreased the proportion of the high affinity receptors when added to the incubation medium in control tissue (to 25%), but not tissue from 7-day DFP-treated swine. NEM increased the proportion of muscarinic receptors in the high affinity state both for controls and for the DFP-treated swine, in both cases yielding receptors with identical binding properties. 6. Thus, subacute administration of DFP causes not only a decrease in the number of receptors, but also a change in the affinity of the receptors for agonists which is related to the interaction of the guanine nucleotide binding protein and the muscarinic receptor.

The toxic effect of an AChE-inhibitor on the cholinergic nervous system in airway smooth muscle

Excessive cholinergic stimulation of presynaptic muscarinic cholinergic receptors, due to complete inhibition of acetylcholinesterase (AChE) by O-(1,2,2-trimethylpropyl)-methyl-phosphonofluoridate (soman), reduced the release of acetylcholine (ACh) from cholinergic nerves in rat bronchi by almost 25%. Furthermore, long-term (40 h) exposure by inhalation of soman (0.45-0.63 mg/m3) reduced the contraction of bronchi induced by ACh by approximately 70%. This is probably due to reduction of the number of muscarinic cholinergic receptors, since there was a reduction in the binding capacity (Bmax) of [3H]QNB by 40%, without any changes in the dissociation constant (Kd).

Paraoxon-induced desensitization and its reversal by tetrodotoxin

1. Paraoxon (10 microM for 20 min) induced a desensitization of the taenia caecum of the guinea-pig for contractions produced by a number of cholinomimetics. 2. Tetrodotoxin (0.3 microM) reversed the desensitization suggesting involvement of Na+ channels.

Genetic influences on tolerance development with chronic oxotremorine infusion

Mice of four inbred strains (BALB, C57BL, DBA and C3H) were administered either saline or oxotremorine, a muscarinic agonist, at a dose of 0.5 mg/kg/hr by constant infusion through cannulas implanted in the right jugular veins. Chronic treatment resulted in the development of tolerance to the effects of oxotremorine both on rotarod performance and on body temperature. For drug-treated BALB mice, the dose-response curves for both measures were parallel to those for saline-treated mice, while for DBA and C3H mice the slopes of the dose-response curves were significantly less for treated mice than they were for controls. The equi-effective doses for the drug-treated animals were at least 8-fold greater than those for saline-treated mice. Drug treatment resulted in a significant decrease in the total number of muscarinic receptors in cortex as measured by the binding of [3H]quinuclidinyl benzilate (QNB) without effect on the KD for this ligand. Similarly, drug treatment did not affect the affinity of carbamylcholine as an inhibitor of QNB binding, but did significantly decrease the levels of both the high- and low-affinity agonist binding sites in cortex. The number of M1 muscarinic receptors measured by high affinity [3H]pirenzepine (PZ) binding was also significantly decreased in cortex without effect on the KD. The experiments were extended to five other brain regions. Full saturation curves were not constructed, however. Oxotremorine treatment significantly reduced QNB binding in every brain region. While the binding to agonist affinity states measured by carbamylcholine inhibition of QNB binding and M1 receptor levels measured by high affinity PZ binding tended to decrease with oxotremorine treatment not all changes were statistically significant. The changes in muscarinic receptor subtype levels induced by oxotremorine infusion did not differ among the strains. The results demonstrate that chronic treatment with a muscarinic agonist results in substantial tolerance to the effects of the drug in all four mouse strains. Although some differences in tolerance development exist, these differences are not readily explained by differences in the number or affinity states of brain muscarinic receptors.

Acute and sub-acute inhalation of an organophosphate induce alteration of cholinergic muscarinic receptors

Acute and sub-acute inhalation exposure of rats to the organophosphorus compound soman (O-[1,2,2-trimethylpropyl]-methylphosphonofluoridate) reduced the contraction of the bronchial smooth muscle induced by cholinergic stimulation. Acute exposure to 8.51 mg/m3 of soman for 45 min (total dose of 383 mg X min/m3) inhibited the acetylcholinesterase (AChE) activity of the bronchial smooth muscle by 85% and reduced the contraction induced by ACh and carbachol by 70% and 80% respectively. In spite of the extensive inhibition of AChE and reduction in the contraction following cholinergic stimulation, there was no alteration of the binding capacity (Bmax) or the equilibrium dissociation constant (Kd) to [3H]-quinuclidinyl benzilate ([3H]-QNB) in the rat bronchi following such an acute exposure. After sub-acute exposure (40 hr) to 0.45-0.63 mg/m3 of soman (total dose of 1080-1519 mg X min/m3) there was a reduction in AChE-activity of 94% and in the contraction of the bronchial smooth muscle induced by ACh and carbachol of 70%. Furthermore, also a reduction of the binding capacity to [3H]-QNB of approximately 40% was observed. Following exposure to soman by both acute and sub-acute inhalation exposure there was an increase in the apparent affinity (pD2) to ACh in the bronchial smooth muscle, due to the extensive inhibition of the AChE-activity. Inhalation of soman also induced a substantial inhibition of the AChE-activity in the lung (86%), but somewhat smaller inhibition in the hippocampus (70%) and almost no inhibition in the neostriatum (19%). Moreover, it was only in the lung where sub-acute exposure to soman produced a reduction of the binding capacity to [3H]-QNB and the reduction was approximately 50%. The results therefore show that after sub-acute inhalation of a relatively low concentration of the AChE-inhibitor soman, alterations in the number of cholinergic receptors are only observed in the peripheral cholinergic nervous system.

Tolerance, cross-tolerance, and receptors after chronic nicotine or oxotremorine

Saline, 8.0 mg/kg/hr nicotine, or 1.0 mg/kg/hr oxotremorine was continuously infused into the jugular veins of DBA female mice. After 10 days of treatment, respiratory rate, Rotarod performance, Y-maze crossings, Y-maze rears, heart rate, and body temperature were measured after challenge with 2.0 mg/kg nicotine or saline or 0.2 mg/kg oxotremorine. Nicotine-infused mice were tolerant to the effects of nicotine for all six tests and oxtremorine-infused mice were tolerant to the effects of oxotremorine for all six tests and to the effects of nicotine on heart rate and body temperature. Oxotremorine infusion reduced the Bmax for [3H]-L-QNB binding, but had no effect on Bmax for either [3H]-DL-nicotine or [125I]-alpha-BTX binding. Conversely nicotine infusion did not alter the Bmax for [3H]-L-QNB binding, but increased the Bmax for both [3H]-DL-nicotine and [125I]-alpha-BTX binding. These results indicate that tolerance developed to the effects of two cholinergic agents, nicotine and oxotremorine, and that some cross-tolerance to the effects of nicotine occurred in oxotremorine-treated mice. Treatment with oxotremorine caused down-regulation of muscarinic receptors, while treatment with nicotine caused up-regulation of nicotinic receptors. Although some cross-tolerance to the effects of nicotine occurred in oxotremorine-treated mice, this did not appear to result from changes in nicotinic receptors.

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.

Modulation of rat pancreatic amylase secretion and muscarinic receptor populations by chronic bethanechol treatment

F8P6effect of a chronic bethanechol treatment (12 mg X kg-1 X day-1 i.p., for 14 days) was investigated on pancreatic amylase secretion and muscarinic cholinergic receptors in the rat. Dispersed pancreatic acini were used to evaluate enzyme secretion and binding of [3H]N-methylscopolamine, [( 3H]NMS). The bethanechol treatment caused a 4 fold decrease in sensitivity of the pancreas for amylase release in the presence of carbamylcholine, the EC50 being shifted from 0.69 microM to 2.9 microM. Receptor concentration was reduced by 42%, from 3360 to 1930 fmol/mg DNA. The equilibrium dissociation constant (KD) of the receptors for the ligand remained unchanged at 0.17 nM. Binding analysis of carbachol on the muscarinic receptors in terms of two classes of binding sites indicated that the shift in the dose-response curve of amylase secretion was accompanied by modifications to the high and low affinity forms. The maximal number of high affinity sites remained the same while their affinity was greatly decreased from 0.24 to 6.1 microM. The low affinity form showed a moderate decrease in affinity from 34 to 150 microM but a large drop in their numbers from 2620 to 890 fmol/mg DNA. These results suggest that the shift in the amylase dose-response curve to carbamylcholine, noted following bethanechol treatment in vivo, could be coupled with the observed change in affinity of the two agonist forms of muscarinic receptor in the pancreas. However, gradual occupancy or formation of the high affinity form of muscarinic receptors by cholinergic agonist corresponds well with the gradual stimulation of amylase release.

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.

Quantitation of tolerance development after chronic oxotremorine treatment

A new procedure was developed to quantitate the tolerance which develops as mice are chronically infused with the muscarinic agonist, oxotremorine. Cumulative dose-response curves were constructed for the effects of oxotremorine on body temperature and rotarod performance by administering sequential injections to individual animals. These dose-response curves compare favorably to those constructed by injecting individual animals with one of several doses. The sequential injection technique was used to assess the magnitude of tolerance development to oxotremorine. A linear relationship between oxotremorine infusion rate (dose) and magnitude of change of the ED50 value for impairment of rotarod performance was observed, with animals receiving an infusion rate of 1.0 mg/kg/hr showing a 24-fold increase in ED50. Dose-response curves for tolerant animals were parallel to those constructed for naive animals. The oxotremorine dose required to decrease body temperature to 35 degrees C (ED35 degrees) was 80-fold greater than control in the group treated with 1.0 mg/kg/hr. The dose-response curves for tolerant animals were not parallel to those seen in naive animals. Time courses of recovery from a challenge dose of oxotremorine suggest little change in metabolism occurred during chronic infusion. Chronic oxotremorine infusion resulted in a decrease in the total number of QNB binding sites. Both high- and low-affinity sites were reduced in number. Since no change in K1 for the muscarinic agonist, carbamylcholine, was observed, it seems unlikely that a change occurs in the affinity of the muscarinic receptor for agonists. Significant change in receptor number was detected only in animals that received higher doses of oxotremorine. Chronic oxotremorine treatment had no effect on choline uptake by synaptosomes prepared from any of five brain regions.

Increase in muscarinic acetylcholine receptors in mouse intestine by hexamethonium treatment

The effects of hexamethonium (C6) administration on muscarinic acetylcholine receptors (mACh-R) in the intestine and brain of mice were investigated. Mice were treated with C6 with an osmotic mini-pump (330 mg/kg/day) for one week and then the binding of 3H-quinuclidinylbenzilate (3H-QNB) in the intestine and brain were assayed. This treatment increased the maximum specific binding (Bmax) of 3H-QNB from 160 to 320 fmoles/mg protein in the ileum and from 190 to 340 fmoles/mg protein in the rectum, without affecting the KD values in these regions. On the contrary, C6 treatment did not change the Bmax or KD value in brain tissues. This C6 treatment increased the sensitivity of the contractile response of the intestine to muscarinic agonists, possibly by increasing mACh-R.

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.

Desensitization of the alpha adrenergic receptor system in guinea pig vas deferens

Desensitization induced by alpha adrenergic (alpha-Ad) stimulation was investigated in organ cultured vas deferens of guinea pig. Brief exposure (1-2 min) of the muscle to noradrenaline (NA) caused short-term desensitization to both NA and acetylcholine (ACh), but not to high K+. After removing the agonist this desensitization completely disappeared within 15 min. Prolonged exposure to NA (i.e., cultured with NA for 3-24 hr) elicited long-term desensitization to NA, ACh and K+ (50 mM), but it did not change the maximal contraction by high K+ (154 mM). After removing NA from the culture medium the response to the agonist was restored to normal within 24 hr, but not within 15 min. The number and affinity of alpha-Ad and muscarinic ACh receptors, which were measured by the binding of 3H-WB4101 and 3H-QNB, respectively, were not changed in the muscle during these treatments. Moreover, long-term desensitization, but not short-term desensitization, was depressed by the concomitant presence of cycloheximide. The possible mechanisms of desensitization were discussed in comparison with those of various receptor systems.

Binding of [3H]quinuclidinyl benzilate to intestinal mucus. An artifact in identification of epithelial cell muscarinic receptors

The widely used muscarinic receptor ligand [3H]quinuclidinyl benzilate ([3H]QNB) was found to bind in a site-specific but artifactual manner to rat intestinal mucus, obscuring specific binding to muscarinic receptors on intestinal epithelial cells. Atropine inhibited [3H]QNB binding to mucus with an apparent IC50 of 2.1 x 10(-7) M, compared to an IC50 of 1.4 x 10(-8) M obtained with a homogenate of intestinal epithelial cells. Unlabeled QNB also inhibited binding of [3H]QNB to mucus but the apparent IC50 (4 x 10(-7) M) was about 300-fold greater than the IC50 determined with a control tissue, heart muscle (IC50, 1.2 x 10(-9) M). [3H]QNB binding was saturable over the concentration range of 1-7 nM in the heart, with an apparent KD of 0.76 nM. As expected from the high IC50 for QNB in the mucus binding experiments, binding to mucus was not saturable over the 1-15 nM concentration range. Based on pH profiles and temperature dependency of binding, it seems unlikely that mucin, the primary component of mucus, was responsible for [3H]QNB binding to the mucus. The findings have implications for studies which involve binding of [3H]QNB in particular and other ligands in general to mucus-secreting epithelial tissues.

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.

Tolerance to the carbamate insecticide propoxur

Male mice were given the carbamate insecticide propoxur (2-isopropoxy phenyl methylcarbamate; Baygon) in the drinking water at weekly increasing concentrations (from 50 to 2000 ppm), for a period of 6 weeks. At the end of the treatment the LD50 for propoxur was significantly higher in the treated animals as compared with controls. Propoxur-treated animals were also resistant to the hypothermic effect of an acute administration of the same compound. Groups of mice were challenged with the cholinergic agonist carbachol at intervals during the drinking water dosing and at its end. No differences in sensitivity to carbachol acute toxicity were found between control and treated animals. Propoxur-tolerant animals were also not resistant to the hypothermic effect of oxotremorine, another cholinergic agonist. [3H]Quinuclidinyl benzilate ([3H]QNB) binding (a measure of muscarinic receptor density and affinity) in forebrain, hindbrain and ileum never differed in control and treated mice. The possibility that repeated administrations of propoxur induced increased metabolic inactivation was tested by measuring hexobarbital sleeping time and carboxylesterase activity in treated and control mice. No changes in tissue carboxylesterase activities occurred but hexobarbital sleeping time was significantly reduced in propoxur treated animals suggesting an induction of hepatic microsomal enzymes. These results suggest that tolerance to propoxur is not mediated by a decrease of cholinergic receptors, as reported for other acetylcholinesterase inhibitors, but possibly by an enhancement of its metabolism.