Dissociation of decreased numbers of muscarinic receptors from tolerance to DFP

ROLES OF .MU.-OPIOID RECEPTORS IN DEVELOPMENT OF TOLERANCE TO DIISOPROPYLFLUOROPHOSPHATE (DFP)

Anatomical evidence indicates that cholinergic and opioidergic systems are co-localized and acting on the same neuron. However, the regulatory mechanisms between cholinergic and opioidergic system have not been well characterized. In the present study, the potential involvement of mu-opioid receptors in mediating the changes of toxic signs and muscarinic receptor binding after administration of irreversible anti-acetylcholinesterase diisopropylfluorophosphate (DFP) was investigated. DFP (1 mg/kg/day, subcutaneous injection, s.c.)-induced tremors and chewing movements were monitored during the 28-day treatment period in mu-opioid receptor knockout and wild type mice. Autoradiographic studies of total, M1, and M2 muscarinic receptors were conducted using [(3)H]-quinuclidinyl benzilate, [(3)H]-pirenzepine, and [(3)H]-AF-DX384 as ligands, respectively. DFP-induced tremors in both mu-opioid receptor knockout and wild type mice showed tolerance development. However, DFP-induced tremors in mu-opioid receptor knockout mice showed delayed tolerance development than that of DFP-treated wild type controls. DFP-induced chewing movements in both mu-opioid receptor knockout and wild type mice failed to show development of tolerance after four weeks of treatment. M2 muscarinic receptor binding of DFP-treated mu-opioid receptor knockout mice was significantly decreased than that of the DFP-treated wild type controls in the striatum, but not in the cortex and hippocampus. However, there were no significant differences in total and M1 muscarinic receptor binding between DFP-treated mu-opioid receptor knockout and wild type mice in the cortex, striatum and hippocampus. These studies indicate that mu-opioid receptors play an important role through the striatal M2 muscarinic receptors to regulate the development of tolerance to DFP-induced tremors.

Pharmacological adaptations and muscarinic receptor plasticity in hypothalamus of senescent rats treated chronically with cholinergic drugs

Receptor plasticity is an important compensatory process by which the central nervous system adapts to pathological insult or long-term exposure to drugs. Senescent animals may show an age-related impairment of muscarinic receptor up- or down-regulation after chronic exposure to cholinergic drugs. The purpose of this study was to assess biochemical and pharmacological endpoints of muscarinic receptor plasticity in young, adult and senescent animals. Male, Fischer 344 rats (ages 3, 9, and 27 months) were administered methylatropine or oxotremorine intracerebroventricularly (IVT) for 3 weeks and tested for their functional response to a muscarinic agonist. The density of hypothalamic, muscarinic receptors was also estimated from analysis of 3H-QNB binding isotherms. In young rats, parallel changes in muscarinic receptors and response were noted, but chronic administration of cholinergic drugs to senescent animals had no effect. Thus, 3H-QNB binding in hypothalamus of young and adult rats was increased (31% and 17%) after chronic IVT methylatropine and decreased (20% and 15%) after IVT oxotremorine. Also, young rats treated with IVT methylatropine were supersensitive to the hypothermic effects of a muscarinic agonist (oxotremorine), while young and adult animals administered chronic IVT oxotremorine exhibited marked tolerance. In contrast, identically treated senescent rats showed no changes in 3H-QNB binding or oxotremorine-induced hypothermia. These results demonstrate the impaired ability of senescent rats to up- or down-regulate brain muscarinic receptors and to exhibit functional adaptations seen in young animals treated chronically with cholinergic drugs.

Down-regulation of muscarinic receptors and the m3 subtype in white-footed mice by dietary exposure to parathion

The effect of ad libitum dietary exposure (as occurs in the field) to parathion for 14 d was investigated on the muscarinic acetylcholine receptor (mAChR) in brains and submaxillary glands of adults of a field species, the white-footed mouse Peromyscus leucopus. Immunoprecipitation using subtype selective antibodies revealed that the relative ratios of the m1-m5 mAChR subtypes in Peromyscus brain were similar to those in rat brain. There was little variability in acetylcholinesterase (AChE) activity in control mice brains but large variability in 39 exposed mice, resulting from differences in food ingestion and parathion metabolism. Accordingly, data on radioligand binding to mAChRs in each mouse brain were correlated with brain AChE activity in the same mouse, and AChE inhibition served as a biomarker of exposure reflecting in situ paraoxon concentrations. Exposure to parathion for 14 d reduced maximal binding (Bmax) of [3H]quinuclidinyl benzilate ([3H]QNB), [3H]-N-methylscopolamine ([3H]NMS), and [3H]-4-diphenylacetoxy-N-methylpiperidine methiodide ([3H]-4-DAMP) by up to approximately 58% without affecting receptor affinities for these ligands. Maximal reduction in Bmax of [3H]QNB and [3H]-4-DAMP binding occurred in mice with highest AChE inhibition, while equivalent maximal reduction in Bmax of [3H]NMS occurred in mice with only approximately 10% AChE inhibition, without further change at higher parathion doses. This is believed to be due to the hydrophilicity of [3H]NMS, which limits its accessibility to internalized desensitized receptors. In submaxillary glands (mAChRs are predominantly m3 subtype), there were significant dose-dependent reductions in [3H]QNB binding and m3 mRNA levels in exposed mice, revealed by Northern blot analyses. The reduction in m3 receptors is suggested to result mostly from reduced synthesis at the transcription level, rather than from translational or posttranslational events. The data suggest that down-regulation of mAChRs occurs after dietary exposure for 14 d to sublethal concentrations of parathion in a field rodent species, and that significant though incomplete recovery in AChE and mAChRs occurs in 7 d following termination of exposure.

Effect of repeated organophosphate administration on carbachol-stimulated phosphoinositide hydrolysis in the rat brain

The effects of repeated exposure to two organophosphates on the turnover of phosphoinositides, the second messenger system coupled to the M1 and M3 subtypes of muscarinic receptors, were examined in the rat hippocampus. Repeated diisopropylfluorophosphate (DFP) exposure (0.2-0.8 mg/kg, SC) decreased brain acetylcholinesterase activity and muscarinic receptor density. The incorporation of [3H]myoinositol into brain slices was also decreased. Phosphoinositide turnover was measured as the accumulation of [3H]inositol phosphates (IP) in the presence of lithium. DFP did not affect basal IP accumulation, but decreased carbachol-stimulated IP accumulation in the hippocampus after 0.4 and 0.8 mg/kg. The effects of repeated disulfoton administration (2.0 mg/kg, IP) were also examined in the hippocampus. Similar to DFP, repeated disulfoton exposure decreased acetylcholinesterase activity, receptor density, and carbachol-stimulated IP accumulation. The incorporation of myoinositol, however, was increased in disulfoton-treated rats. These data indicate that repeated organophosphate exposure results in a functional decrease in muscarinic receptor activity, as well as changes in myoinositol incorporation into phospholipids.

Species differences in diisopropylfluorophosphate-induced decreases in the number of brain nicotinic receptors

DBA and C3H mice were injected chronically with 2.0 mg/kg diisopropylfluorophosphate (DFP) every other day for 2 or 4 weeks. Although acetylcholinesterase (AChE) activity and muscarinic receptor numbers ([3H] quinuclidinyl benzilate (QNB) binding) were decreased in DFP-treated DBA and C3H mice, the number of nicotinic receptors (L-[3H]nicotine and alpha-[125I]bungarotoxin (BTX) binding) was unchanged by chronic DFP treatment. Sprague-Dawley rats injected chronically with lower doses of DFP than were used in mice exhibited a greater reduction in AChE activity, as well as accompanying decreases in [3H]QNB and [3H]nicotine binding. Neither species exhibited changes in alpha-[125I]BTX following chronic DFP injection. The effects of chronic DFP treatment on sensitivity to DFP and to nicotine were also assessed in the two mouse strains using a battery of behavioral and physiological tests that included rotarod performance, Y-maze crossing and rearing activity, heart rate, and body temperature. No tolerance to DFP was observed in either mouse strain after 2 weeks of treatment. Following 4 weeks of treatment, DFP-treated DBA mice exhibited modest tolerance to the effect of DFP on body temperature. C3H mice did not survive the 4-week treatment. Some evidence for reduced sensitivity to nicotine's effects was detected in the DFP-treated DBA mice, but cross-tolerance to nicotine was not observed in the DFP-injected C3H mice. Because chronic DFP treatment did not evoke a change in the number of brain nicotinic receptors, the reduced sensitivity to some of nicotine's effects seen in DBA mice must be due to some factor other than receptor downregulation.

Effect of a single dose of an acetylcholinesterase inhibitor on oxotremorine- and nicotine-induced hypothermia in mice

Downregulation of cholinergic receptors is a consequence of subchronic exposure to an organophosphate anticholinesterase. The purpose of this investigation was to determine if there was a downregulation of the cholinergic receptors in mice following administration of a single dose of soman (pinacolyl methylphosphonofluoridate) or physostigmine. The change in the temporal response (mean minimum temperature and area under the curve) of core temperature following administration of either a muscarinic or nicotinic agonist such as oxotremorine (156 micrograms/kg, IP) or nicotine hydrogen tartrate (15 mg/kg, SC) was used as an indicator of downregulation of muscarinic or nicotinic receptors, respectively. Twenty-four h following soman (100 micrograms/kg, SC) administration, there was a significant decrease (p less than 0.05) in oxotremorine- but not nicotine-induced hypothermia. The significant differences in the mean minimum temperature and AUC were still present 4 days after exposure to the soman. Neither lower doses of the organophosphate anticholinesterase, soman (50 and 70 micrograms/kg), nor the carbamate anticholinesterase, physostigmine (500 micrograms/kg), produced a significant change in either oxotremorine- or nicotine-induced hypothermia. The results of this study suggest that receptor downregulation observed after subchronic administration of soman is also evident following administration of a single, sublethal dose of an organophosphate anticholinesterase, soman, but not after administration of a carbamate anticholinesterase, physostigmine. The in vivo assessment of the muscarinic receptor using oxotremorine hypothermia may be a sensitive indicator of the functionality of the drug-receptor coupling and indicate a physiological consequence of receptor downregulation.

Hypothermia: limited tolerance to repeated soman administration and cross-tolerance to oxotremorine

The effect of repeated administration of the organophosphate anticholinesterases, soman (pinacolyl methylphosphonofluoridate) and DFP (diisopropylfluorophosphate) on core temperature was investigated in mice. Mice were implanted with telemetry transmitters for the monitoring of core temperature. Following repeated administration of soman (3-10 injections), tolerance (as defined by a decrease in the organophosphate-induced hypothermia upon subsequent administration) to the organophosphate-induced hypothermia was evident after the 5th injection; however, there was cross-tolerance to oxotremorine hypothermia as early as after the 3rd injection of soman. Following repeated administration of DFP, there was no tolerance to the DFP-induced hypothermia following 5 injections, whereas cross-tolerance to oxotremorine was evident following the 5th injection. The organophosphate-induced hypothermia may have another component which contributes to the response. It is proposed that the cross-tolerance to oxotremorine hypothermia after subchronic administration of an anticholinesterase is representative of the functionality of muscarinic cholinergic receptor coupling.

Atropine treatment induced cholinergic supersensitivity at receptor and second messenger levels in the rat salivary gland

Physiological, biochemical and morphological correlates of chronic treatment of rats with the classical muscarinic antagonist atropine for 14 days (20 mg kg-1 day-1 s.c.) were studied in submandibular salivary glands. The amount of saliva collected from submandibular glands following a single injection of isoproterenol (30 mg kg-1 i.p.) was significantly larger and had higher protein concentration in rats treated with atropine than in saline-treated animals. In the glands of atropine-treated rats a conspicuous increase in the amount of rough endoplasmic reticulum (RER) along with a decrease in the mucous volume was observed in the acinus when examined by light microscopy. Several biochemical changes were observed in an enriched plasma membrane fraction from the submandibular gland of the atropine-treated rats: (1) an increase in the number of muscarinic antagonist binding sites (31 + 3.4%), (2) a decrease in the specific activity of basal adenylate cyclase, (3) a significantly lower Vmax of the adenylate cyclase in the presence of GTP (10 microM) and varying concentrations of Mg2+ (0-22.5 mM) with no apparent change in affinity of the enzyme for Mg2+ but (4) higher magnitude of stimulation in the presence of GTP (100 microM), vasoactive intestinal polypeptide (5 microM), isoproterenol (100 microM), NaF (10 microM) and forskolin (10 microM). There was however no change in the density of beta-adrenergic receptors upon atropine treatment. In tissue slices from the submandibular glands of atropine-treated rats we found lower basal cAMP levels (decrease 29 +/- 6.9%) and no significant change in the phosphatidylinositol breakdown stimulated by carbachol (10(-6) to 10(-4) M). It appears that chronic blockade of an inhibitory muscarinic input to the adenylate cyclase system is compensated by lowered adenylate cyclase activity. Phosphoinositide metabolism is not subject to the same adaptation, suggesting that cAMP may be the pivotal second messenger in the supersensitive salivary response.

The effects of chronic oxotremorine treatment on spatial learning and tolerance development in mice

C57BL mice were treated with 0.5 mg/kg/hr oxotremorine through an implanted subcutaneous cannula for 6 days. Tolerance to oxotremorine was evaluated after treatment by constructing cumulative dose-response curves and measuring body temperature and rotarod performance. At 2 hr after removal, mice exhibited a 15-fold tolerance as measured by body temperature and a 4-fold tolerance as measured by rotarod performance. This tolerance as measured by body temperature was lost by two days after removal from treatment. Immediately after treatment, 3H-QNB binding was reduced in cortex, hippocampus, midbrain, hindbrain, and hypothalamus. Receptors returned to normal within 4 to 8 days after cessation of treatment depending on the brain region. Spatial learning was examined using the Morris water task. Mice that began their training in this task 1 day after they were removed from oxotremorine treatment were impaired in their spatial ability as evidenced by a lack of preference for the trained site during a probe trial. Mice that began their training 2 days after cessation of oxotremorine treatment showed no evidence of impairment in spatial learning. These results suggest that a loss of muscarinic receptors after oxotremorine treatment can be dissociated from tolerance loss and spatial learning deficits.

Inhibition of phosphoinositide turnover by selective muscarinic antagonists in the rat striatum. Correlation with receptor occupancy

In the rat corpus striatum, receptor occupancy and the inhibition of phosphoinositide turnover by muscarinic antagonists have been examined under very similar conditions with respect to tissue preparation and buffer composition. The results suggest a good correlation between receptor occupancy and inhibition by muscarinic antagonists, of the carbachol-stimulated turnover of inositol phospholipids, measured by the accumulation of [3H]inositol phosphates in the presence of 5 mM LiCl. In the presence of 10 mM carbachol (CCh), the accumulation of labeled inositol phosphates was increased 8-fold above basal levels (EC50 = 95 microM). Inclusion of antagonists resulted in a dose-dependent inhibition of the 0.1 mM CCh-stimulated inositol phosphate accumulation, with a rank order of potency of atropine greater than trihexyphenidyl greater than pirenzepine greater than or equal to gallamine. Radioligand binding studies with [3H]-l-quinuclidinyl benzilate [( 3H]QNB) in a cell aggregate preparation revealed a single class of saturable, high affinity [3H]QNB binding sites exhibiting a Kd of 74 pM and a Bmax of 2.85 pmol/mg protein. The antagonists examined were able to inhibit the binding of [3H]QNB with the same rank order of potency as for the inhibition of carbachol-stimulated phosphoinositide turnover (atropine greater than trihexyphenidyl greater than pirenzepine greater than or equal to gallamine). Although the inhibition of phosphoinositide turnover and [3H]QNB binding by the nonselective antagonist atropine was best described by interaction at a single site, inhibition of phosphoinositide turnover and [3H]QNB binding by both pirenzepine, which is selective for M1 receptors, and gallamine, which is selective for M2 receptors, is complex. Pirenzepine was much more potent than gallamine for both binding to receptors and inhibiting phosphoinositide turnover. Nonlinear curve-fitting analysis indicated that slope factors for inhibition of phosphoinositide turnover (analogous to Hill coefficient for binding) by only subtype selective antagonists were significantly less than unity. The above-mentioned antagonist interactions together with the apparently multicomponent stimulation of phosphoinositide turnover by carbachol suggest that phosphoinositide turnover may be coupled to more than one muscarinic receptor subtype in the corpus striatum.

Cross-tolerance between muscarinic agonists: role of muscarinic receptors

In order to explore the relationship between response to muscarinic agonists and brain muscarinic receptors, two mouse strains that differ in acute sensitivity (DBA and C3H) were injected chronically with DFP or infused with oxotremorine. Chronic DFP-treated DBA mice were not tolerant to DFP's effects on any measure, but they were cross-tolerant to the effects of oxotremorine on heart rate and body temperature. DFP-treated C3H mice were not tolerant to DFP or cross-tolerant to oxotremorine on any measure. Oxotremorine infusion resulted in tolerance to oxotremorine in both mouse strains, and chronically infused DBA mice were cross-tolerant to DFP on five of the six measures. Oxotremorine-infused C3H mice were cross-tolerant to DFP on two of the measures. These results suggest that genetic factors influence the development of tolerance or cross-tolerance. These genetic factors do not seem to be related to changes in brain QNB binding. Both mouse strains showed comparable changes in QNB binding following chronic DFP and oxotremorine with DFP eliciting reductions in QNB binding in striatum and hippocampus and oxotremorine eliciting reductions in nearly every brain region. However, tolerance and cross-tolerance did not seem to correlate with changes in binding which suggests that the relationship between receptor changes and responses to muscarinic agonists must be examined further.