Mechanisms of tolerance to the anticholinesterase, DFP: acetylcholine levels and dynamics in the rat brain

The Neuroinflammatory Phenotype in a Mouse Model of Gulf War Illness is Unrelated to Brain Regional Levels of Acetylcholine as Measured by Quantitative HILIC-UPLC-MS/MS

Many veterans of the 1991 Persian Gulf War (GW) returned with a chronic multisymptom illness that has been termed Gulf War Illness (GWI). Previous GWI studies have suggested that exposure to acetylcholinesterase inhibitors (AChEIs) in theater, such as sarin and/or pesticides, may have contributed to the symptomatology of GWI. Additionally, concomitant high physiological stress experienced during the war may have contributed to the initiation of the GWI phenotype. Although inhibition of AChE leading to accumulation of acetylcholine (ACh) will activate the cholinergic anti-inflammatory pathway, the signature symptomatology of GWI has been shown to be associated with neuroinflammation. To investigate the relationship between ACh and neuroinflammation in discrete brain regions, we used our previously established mouse model of GWI, which combines an exposure to a high physiological stress mimic, corticosterone (CORT), with GW-relevant AChEIs. The AChEIs used in this study were diisopropyl fluorophosphate (DFP), chlorpyrifos oxon (CPO), and physostigmine (PHY). After AChEI exposure, ACh concentrations for cortex (CTX), hippocampus (HIP), and striatum (STR) were determined using hydrophilic interaction liquid chromatography with ultraperformance liquid chromatography-tandem-mass spectrometry (MS/MS). CORT pretreatment ameliorated the DFP-induced ACh increase in HIP and STR, but not CTX. CORT pretreatment did not significantly alter ACh levels for CPO and PHY. Further analysis of STR neuroinflammatory biomarkers revealed an exacerbated CORT + AChEI response, which does not correspond to measured brain ACh. By utilizing this new analytical method for discrete brain region analysis of ACh, this work suggests the exacerbated neuroinflammatory effects in our mouse model of GWI are not driven by the accumulation of brain region-specific ACh.

Cerebral acetylcholine and choline contents and turnover following low-dose acetylcholinesterase inhibitors treatment in rats

Male Sprague-Dawley rats were treated for 3 weeks with (1) regular tap drinking water plus subcutaneous (s.c.) saline (0.5 ml/kg) injections three times/week, (2) pyridostigmine bromide (PB) in drinking water (80 mg/L) plus s.c. saline injections three times/week, (3) regular tap drinking water plus s.c. sarin (0.5 x LD(50)) injections three times/week, or (4) PB in drinking water plus s.c. sarin injections three times/week. Repeated doses of sarin, in the presence or absence of PB, were devoid of acute toxicity during the three-week treatment period. Two, 4, and 16 weeks post-treatment, animals were given an intravenous pulse injection of choline labeled with 4 deuterium atoms (D4Ch) followed, after 1 min, by microwave fixation of the brain in vivo. Tissue levels of endogenous acetylcholine (D0ACh), endogenous choline (D0Ch), D4Ch, and ACh synthesized from D4Ch (D4ACh) were measured by gas-chromatography mass-spectrometry in hippocampus, infundibulum, mesencephalon, neocortex, piriform cortex, and striatum. Ch uptake from blood and ACh turnover were estimated from D4Ch and D4ACh concentrations in brain tissue, respectively. Statistically significant differences among brain regions were found for D0Ch, D4Ch, D0ACh and D4ACh at 2, 4 and 16 weeks post-treatment. However, differences in the values of these parameters between control and drug treatments were found only for D0ACh and D0Ch at 2 and 4 weeks, but not at 16 weeks post-treatment. In conclusion, the results from these experiments do not support a delayed or persistent alteration in cholinergic function after exposure to low doses of PB and/or sarin.

Regulation of muscarinic acetylcholine receptor function in acetylcholinesterase knockout mice

Acetylcholinesterase (AChE) hydrolyzes acetylcholine to terminate cholinergic neurotransmission. Overstimulation of cholinergic receptors by excess acetylcholine is known to be lethal. However, AChE knockout mice live to adulthood, although they have weak muscles, do not eat solid food, and die early from seizures. We wanted to know what compensatory factors allowed these mice to survive. We had previously shown that their butyrylcholinesterase activity was normal and had not increased. In this report, we tested the hypothesis that AChE-/- mice adapted to the absence of AChE by downregulating cholinergic receptors. Receptor downregulation is expected to reduce sensitivity to agonists and to increase sensitivity to antagonists. Physiological response to the muscarinic agonists, oxotremorine (OXO) and pilocarpine, showed that AChE-/- mice were resistant to OXO-induced hypothermia, tremor, salivation, and analgesia, and to pilocarpine-induced seizures. AChE+/- mice had an intermediate response. The muscarinic receptor binding sites measured with [3H]quinuclinyl benzilate, as well as the protein levels of M1, M2, and M4 receptors measured with specific antibodies on Western blots, were reduced to be approximately 50% in AChE-/- brain. However, mRNA levels for muscarinic receptors were unchanged. These results indicate that one adaptation to the absence of AChE is downregulation of muscarinic receptors, thus reducing response to cholinergic stimulation.

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.

Chronic, low-level exposure to the cholinesterase inhibitor DFP. II. Time course of behavioral state changes in rats

Rats were repeatedly administered with low doses of diisopropylfluorophosphate (DFP; 0.2 mg/kg/day, SC), an irreversible cholinesterase (ChE) inhibitor. Control rats received a daily injection of oil vehicle or of saline. Recordings of the sleep-wake states were obtained in the 6 h following 1, 3, 6, 9, 13, 17, and 21 injections, as well as 2, 4, and 19 days after 9-day treatment. DFP administration increased waking at the expense of slow-wave sleep (SWS), but not of paradoxical sleep (PS); as a result, the PS/SWS ratio was strongly enhanced. These changes developed across days, were maximal after six to nine injections, and were then maintained at that level until cessation of treatment. This time course of behavioral state alterations paralleled the time course of ChE inhibition in the mesopontine cholinergic nuclei and the pontine reticular formation described in the companion article. In contrast, after DFP withdrawal, behavioral states returned to control values more rapidly (in 2-4 days) than did ChE activity. These results are discussed regarding the promoting role of cholinergic neurotransmission in brain-activated states.

Evidence for homeostatic adjustments of rat somatosensory cortical neurons to changes in extracellular acetylcholine concentrations produced by iontophoretic administration of acetylcholine and by systemic diisopropylfluorophosphate treatment

We describe the responses of single units in the awake (24 cells) or urethane-anesthetized (37 cells) rat somatosensory cortex during repeated iontophoretic pulses (1.0 s, 85 nA) of acetylcholine, both before and after systemic treatment with the irreversible acetylcholinesterase inhibitor diisopropylfluorophosphate (i.p., 0.3-0.5 LD50). The time-course of the response to acetylcholine pulses differed among cortical neurons but was characteristic for a given cell. Different time-courses included monophasic excitatory or inhibitory responses, biphasic (excitatory-inhibitory, inhibitory-excitatory, excitatory-excitatory, and inhibitory-inhibitory), and triphasic (excitatory-excitatory-inhibitory, inhibitory-inhibitory-excitatory, and inhibitory-excitatory-inhibitory) responses. Although the sign and time-course of the individual responses remained consistent, their magnitude fluctuated across time; most cells exhibited either an initial increase or decrease in response magnitude followed by oscillations in magnitude that diminished with time, gradually approaching the original size. The time-course of the characteristic response to an acetylcholine pulse appeared to determine direction and rate of change in response magnitude with successive pulses of acetylcholine. Diisopropylfluorophosphate treatment, given 1 h after beginning repeated acetylcholine pulses, often resulted in a gradual increase in spontaneous activity to a slightly higher but stable level. Superimposed on this change in background activity, the oscillations in the response amplitude reappeared and then subsided in a pattern similar to the decay seen prior to diisopropylfluorophosphate treatment. Our results suggest that dynamic, homeostatic mechanisms control neuronal excitability by adjusting the balance between excitatory and inhibitory influences within the cortical circuitry and that these mechanisms are engaged by prolonged increases in extracellular acetylcholine levels caused by repeated pulses of acetylcholine and by acetylcholinesterase inhibition. However, this ability of neurons in the cortical neuronal network to rapidly adjust to changes in extracellular levels of acetylcholine questions the potential efficacy of therapeutic treatments designed to increase ambient levels of acetylcholine as a treatment for Alzheimer's disease or to enhance mechanisms of learning and memory.

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.

Regulation of central muscarinic receptors after cholinesterase inhibition: effect of clonidine

In rats, the injection of soman (70 micrograms/kg, SC) resulted in a 90% inhibition of the cholinesterase (ChE) activities in three brain regions. The density (Bmax) for muscarinic acetylcholine receptors (mAChRs) following a single injection of soman was significantly reduced at 2 h after injection in the cortex and hindbrain. Bmax values, however, returned to baseline within 24 h. Subacute (repeated injection every 15 min) treatment with a sublethal dose of soman over 2 h also decreased the density of mAChRs. In both cases the density of mAChRs was reduced by about 15% for the cortex and 17% for the hindbrain (the midbrain was also reduced by 18% for subacute injections). Chronic administration (once daily for 7 days) of soman (20 micrograms/kg, SC) produced maximal inhibition of ChE activity but did not significantly downregulate mAChRs. Clonidine pretreatment reversed the soman-induced mAChR downregulation in cortex and hindbrain produced by acute soman administration. Thus, marked reduction in the levels of brain ChE is not the only factor involved in the production of mAChR downregulation to cholinesterase inhibitors.

Determination of serum cholinesterase activity by liquid chromatography with electrochemical detection

A sensitive enzymatic assay to measure cholinesterase activity in serum using liquid chromatography with electrochemical detection has been devised and used to examine cholinesterase inhibition in mice treated with diisopropyl phosphorofluoridate. Acetylcholine was used as substrate, and a postcolumn reactor containing immobilized choline oxidase converted the enzymatic product, choline, and the internal standard, ethylhomocholine, into the electrochemically active H2O2. The postcolumn reactor also contained acetylcholinesterase to allow the indirect detection of the substrate. Assay optimization included investigations of substrate concentration, buffer pH and ionic strength, enzyme concentration, incubation time, and reaction termination method. The optimized procedure is applicable to samples with activities of 0.11 to 269 mmol/ml/h. Intrasample coefficient of variation for mouse serum samples was 1.7% (n = 12), while intersample coefficient of variation was 8.0% (n = 5). The mean +/- SE serum cholinesterase activity found for controls and mice treated with diisopropyl phosphofluoridate (6.3 mg/kg, ip, 24 h prior) was 158.7 +/- 5.7 mumol/ml/h and 36.6 +/- 3.1 mumol/ml/h, respectively (P less than 0.001).

Effects of repeated injection of sublethal doses of soman on behavior and on brain acetylcholine and choline concentrations in the rat

The effects of repeated exposure to a sublethal dose (60 micrograms/kg; 0.4 LD50) of soman on brain regional acetylcholine (ACh) and choline (Ch) levels, spinal cord cholinesterase (ChE) activity and on water consumption, body weight and gross behavioral changes were examined. Male rats were dosed once a week or three times a week and at 24 h after 2, 4 or 6 weeks of dosing, selected brain tissues and behavior were examined. During the 6-week period, there was no difference between control and soman-dosed rats in water consumption or body weight under either treatment regimen. The animals treated once a week adapted to this exposure regimen well. They exhibited no change in the levels of ACh or Ch in any of the brain areas when examined at the end of 2, 4 or 6 weeks, nor did they show any obvious signs of poisoning. The total ChE activity fluctuated between 70 and 100% of control. When treated three times a week, however, survivors (90%) of the soman-treated rats developed signs that progressed in severity to a hyper-reactivity syndrome which consisted of an exaggerated reaction to mild tactile stimuli. Brain ACh levels did not change and ChE activity showed inhibition of 40, 58 and 75% when measured at 2, 4 and 6 weeks, respectively. At the end of 6 weeks, the levels of Ch, except in the striatum, were significantly elevated in brainstem, cerebral cortex, hippocampus, midbrain, and cerebellum (52%, 147%, 68%, 46%, and 91%, respectively), indicating that Ch metabolism in neuronal membranes may be altered following more frequent low-dose soman exposures.

Comparison of the effects of diisopropylfluorophosphate, sarin, soman, and tabun on toxicity and brain acetylcholinesterase activity in mice

The LD50s and ED50s for inhibition of acetylcholinesterase (AChE) in whole mouse brain by DFP (diisopropylfluorophosphate), sarin (methylphosphonofluoridic acid 1-methyl ethyl ester), soman (methylphosphonofluoridic acid 1,2,2-trimethyl propyl ester), and tabun (dimethylphosphoramidocyanidic acid ethyl ester) were compared after iv administration. The LD50s of DFP, sarin, soman, and tabun in ICR (Institute for Cancer Research) mice were 3.40, 0.109, 0.042, and 0.287 mg/kg, respectively. The recovery of AChE activity in whole mouse brain after sub-LD50 doses of these agents was slow and did not reach control values by 14 d after iv administration. AChE activity was inhibited in a dose-dependent manner in whole mouse brain, as well as in six brain regions (cortex, hippocampus, striatum, midbrain, medulla-pons, and cerebellum). None of these brain areas appeared to be particularly sensitive to AChE inhibition. The ED50s for DFP, sarin, soman, and tabun for inhibition of AChE in whole mouse brain were approximately 19, 38, 69, and 66% of their respective LD50s. Because of the differential potencies between lethality and inhibition of AChE, it is concluded that the lethality of these agents is due to more factors than simply the inhibition of AChE within the brain.

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.

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 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.

Acetylcholine content in rat brain is elevated by status epilepticus induced by lithium and pilocarpine

The effects of status epilepticus on the concentration, synthesis, release, and subcellular localization of acetylcholine, the concentration of choline, and the activity of acetylcholinesterase in rat brain regions were studied. Generalized convulsive status epilepticus was induced by the administration of pilocarpine to lithium-treated rats. The concentration of acetylcholine in the cortex, hippocampus, and striatum decreased prior to the onset of spike activity or status epilepticus. Once status epilepticus began, the concentration of acetylcholine increased over time in the cortex and hippocampus, reaching peak levels that were 461% and 304% of control levels, respectively, after 2 h of seizures. Such high in vivo levels of acetylcholine had not been reported previously following any treatment. During status epilepticus, the concentration of acetylcholine in the striatum returned to control levels after the initial depression, but did not accumulate to high levels as it did in the other two regions. The in vivo cortical efflux of acetylcholine was also increased during the seizures. Choline levels were increased by status epilepticus in all three brain regions. Inhibition of seizures by pretreatment with atropine blocked the increases of acetylcholine and choline. Synaptosomes prepared from the cortex and from the hippocampus of rats with status epilepticus had elevated concentrations of acetylcholine: in the hippocampus the acetylcholine was principally in the cytoplasmic fraction, whereas in the cortex the acetylcholine was elevated in both the cytoplasmic and the vesicular fractions. The extra acetylcholine was in a releasable compartment, since increased K+ in the media or ouabain increased the release of acetylcholine from cortical slices to a greater extent in tissue from seized rats than from controls.(ABSTRACT TRUNCATED AT 250 WORDS)

Interaction of cycloheximide and diisopropylphosphorofluoridate (DFP) during subchronic administration in rat

Male Sprague-Dawley rats daily treated with DFP (0.5 mg/kg/day, sc) exhibited signs of cholinergic toxicity such as tremors and muscle fasciculations between Days 3 and 5 comparable to those observed 15 min after a single acute signs-producing dose (1.5 mg/kg, sc). Further administration of DFP (0.5 mg/kg/day, sc) for 6-14 days led to tolerance development as evidenced by disappearance of the described toxicity signs. The protein synthesis inhibitor cycloheximide, when given in a nontoxic dose (0.5 mg/kg/day, sc) 1 hr before DFP (0.5 mg/kg/day, sc) administration, potentiated the DFP toxicity and rats died after the fifth injection. DFP-tolerant rats developed toxicity signs when subsequently treated with cycloheximide (0.5 mg/kg/day, sc) and DFP (0.5 mg/kg/day, sc). Each drug when given alone for 4 days caused 30-50% reduction of [14C]valine uptake in vivo into the free amino acids pool as well as its incorporation into proteins of brain and skeletal muscles. A combination of these drugs caused a significantly greater inhibitory effect on [14C]valine incorporation into proteins. Cycloheximide (0.5 mg/kg/day, sc) administered for 4 days did not significantly alter the levels of acetylcholinesterase (AChE), butyrylcholinesterase (BuChE), or carboxylesterase (CarbE) activities but potentiated the DFP-induced inhibition of the activities of these enzymes. It is concluded that the cycloheximide pretreatment potentiates DFP toxicity by a mechanism that is related to inhibition of the synthesis of proteins such as AChE, BuChE, and CarbE.

Dissociation of decreased numbers of muscarinic receptors from tolerance to DFP

Several studies have demonstrated that chronic treatment with organophosphates, such as DFP, elicits a decreased number of brain muscarinic receptors (measured by the binding of QNB) which has been presented as an explanation for tolerance to the organophosphates. The purpose of the studies presented here was to assess whether graded changes in QNB binding could be attained following different methods of chronic DFP treatment, and whether tolerance to DFP paralleled these changes. Male DBA mice were injected with DFP every 4 days or 2 days for 30 days or daily for 14 days. The animals were subsequently challenged with DFP or the muscarinic agonist, oxotremorine, and respiratory rate, heart rate, body temperature, Y-maze activity and rearing were recorded. Chronic DFP-treated animals were supersensitive to the effects of DFP on respiratory rate, heart rate, and body temperature whereas a modest tolerance to the effects of oxotremorine on respiratory rate, heart rate, and body temperature was seen. Neither tolerance nor supersensitivity were observed for the effects of DFP and oxotremorine on the Y-maze measures. Chronic DFP treatment elicited reduced binding of QNB in striatum, cortex, and hippocampus with the group that had been treated every other day exhibiting the greatest changes. The changes in drug response did not parallel changes in QNB binding which raises questions as to the cause of the reduction in binding.

Cholinergic mechanisms in affective disorders. Future directions for investigation

Advances in clinical and basic research methodology combined with clearly articulated concepts create new opportunities for researching the roles of cholinergic mechanisms in the pathophysiology of affective disorders. Areas for study include: roles of cholinergic mechanisms in mediating effects of stress and cholinergic mechanisms linking the pathophysiologies of affective and panic disorders, use of pharmacologic agents to produce cholinergic system supersensitivity in modeling biologic aspects of affective illness, use of multigenerational intrapedigree studies of cholinergic markers associated with affective disease, research into the neurobiology of lithium and ECT as they pertain to muscarinic cholinergic mechanisms, study of the interrelationship of sodium, calcium and lithium ion metabolism and their relationship to cholinergic-monoaminergic interaction, the development of brain imaging strategies and techniques, e.g., positron emission tomography (PET), to measure changes in cholinergic receptor density and affinity as a function of clinical state, identification and validation of a peripheral model of the central muscarinic receptor, study of the pharmacology of abusable substances and its relationship to mechanisms regulating mood, affect, psychomotor function and other variables related to the affective disorders, and development of in vitro and in vivo models useful in studying the physiology and biochemistry of the interaction of cholinergic and monoaminergic neurons. These models may allow us to bridge the traditional cholinergic and monoamine hypotheses of affective disorders.

Elevated choline levels in brain. A non-cholinergic component of organophosphate toxicity

The role of cholinergic and non-cholinergic mechanisms in mediating organophosphate cholinesterase (ChE) inhibitor-induced elevations in choline levels in brain was investigated. The nerve agents soman and sarin, when administered to rats at doses greater than the IC50 for acetylChE inhibition, significantly increased the levels of choline and acetylcholine in both the striatum and hippocampus. The elevation in choline levels was evident 1 hr after injection with a maximal increase at 2 hr. Levels of choline returned to control by 4 hr. In contrast, the administration of diisopropyl phosphorofluoridate at doses greater than the IC50 for acetylChE inhibition increased the levels of acetylcholine, but did not alter the concentration of choline during the first 3 hr. Between 4 and 24 hr after injection, however, a significant decrease in choline levels was apparent. This effect persisted for 48 hr. When rats were pretreated with the anticonvulsant diazepam, the sarin- and soman-induced increases in choline levels were attenuated significantly. Results indicate that the organophosphates differentially alter the levels of choline in brain and suggest that the effect of soman and sarin to elevate choline levels is not a reflection of excessive cholinergic activity, but rather may be a consequence of the excitotoxic actions of these compounds.

Pathophysiology of "cholinoceptor supersensitivity" in affective disorders

Phenomenological and physiological variables demonstrate supersensitive changes to cholinergic challenge in affective disorder subjects. Theorists generally assume the primary defect is the postsynaptic muscarinic receptor. However, in addition to defectiveness or up-regulation of this receptor, the appearance of postsynaptic "cholinoceptor supersensitivity" can result from abnormal presynaptic mechanisms, membrane "pathology," derangement of intrasystolic mechanisms that amplify effects of receptor-agonist coupling, or aberrant cholinergic-monoaminergic interaction. This article discusses abnormalities of the postsynaptic receptor, regulation of postsynaptic receptor density, the presynaptic muscarinic receptor, and other mechanisms regulating the release of acetylcholine, membrane dynamics, and "cascade" mechanisms-specifically the phosphatidylinositol (PI) cycle, Ca2+ mobilization, and cyclic guanosine monophosphate (GMP) generation-as causes of cholinergic system "supersensitivity." It is suggested that an approach to the topic emphasizing site of abnormality will encourage greater clarity of thought in the study of the cholinergic component of the pathophysiology of affective illness.

Mechanisms of toxicity and tolerance to diisopropylphosphorofluoridate at the neuromuscular junction of the rat

Diisopropylphosphorofluoridate (DFP), an irreversible inhibitor of acetylcholinesterase (AChE) activity, when given as an acute dose (1.5 mg/kg, sc) caused fasciculations and induced necrosis in rat skeletal muscle fibers. No adaptation was seen to daily dosing of DFP (1.5 mg/kg, sc) since all rats died after the second or third injection. Daily dosing of DFP in a concentration (0.5 mg/kg, sc) that as a single dose did not cause symptoms, produced onset of fasciculations on the third day associated with a reduced number of muscle fiber lesions. Further administration of DFP (14 days) caused disappearance of fasciculations and loss of sensitivity to the necrotizing actions in all muscles tested (diaphragm, soleus, and extensor digitorum longus). Activity of all molecular forms of AChE was reduced to 20-24% of control when symptoms of cholinergic hyperactivity appeared. Continuous injections of DFP (0.5 mg/kg/day, sc) up to 14 days did not cause greater inhibition of AChE activity. Instead, recovery of enzyme activity, especially of the 4S and 10S forms, was seen. During this period choline acetyltransferase activity (ChAT) was increased in muscle (intramuscular nerves) while the postsynaptic nicotinic acetylcholine receptor (nAChR) density (Bmax) was decreased to 44% without a change in the affinity constant (KD). It is concluded that neuromuscular adaptation to DFP is caused by recovery of AChE activity due to de novo synthesis and reduction in the number of nAChR.

Effects of insecticidal carbamates on brain acetylcholine content, acetylcholinesterase activity and behavior in mice

Mice showed no toxic signs after a single injection of o-sec-butylphenyl methylcarbamate (BPMC, 10 mg/kg) or 2-isopropoxyphenyl-N-methylcarbamate (propoxur, 2 mg/kg). Each dose of BPMC or propoxur caused an increase in acetylcholine content and a decrease in acetylcholinesterase activity in the forebrain of mice at 10 min, followed by an almost complete recovery in the content at 60 min. Spontaneous motor activity was depressed 10 min after and recovered 60 min after, injection of BPMC or propoxur. Neither rotarod performance nor rectal temperature showed any change after injection of BPMC or propoxur. Spontaneous motor activity may therefore be a simple method for assessing small changes in the cholinergic system.

In vivo regulation of [3H]acetylcholine recognition sites in brain by nicotinic cholinergic drugs

The in vivo regulation of [3H]acetylcholine [( 3H]ACh) recognition sites on nicotinic receptors in rat brain was examined by administering drugs that increase stimulation of nicotinic cholinergic receptors, either directly or indirectly. After 10 days of treatment with the cholinesterase inhibitor diisopropyl fluorophosphate, [3H]ACh binding in the cortex, thalamus, striatum, and hypothalamus was decreased. Scatchard analyses indicated that the decrease in binding in the cortex was due to a reduction in the apparent density of [3H]ACh recognition sites. In contrast, after repeated administration of nicotine (5-21 days), the number of [3H]ACh recognition sites was increased in the cortex, thalamus, striatum, and hypothalamus. Similar effects were observed in the cortex and thalamus following repeated administration of the nicotinic agonist cytisin. The nicotinic antagonists mecamylamine and dihydro-beta-erythroidine did not alter [3H]ACh binding following 10-14 days of administration. Further, concurrent treatment with these antagonists and nicotine did not prevent the nicotine-induced increase in these binding sites. The data indicate that [3H]ACh recognition sites on nicotinic receptors are subject to up- and down-regulation, and that repeated administration of nicotine results in a signal for up-regulation, probably through protracted desensitization at the recognition site.

Changes in presynaptic release of acetylcholine during development of tolerance to the anticholinesterase, DFP

The rat myenteric plexus was used as a peripheral model for studying muscarinic modulation of acetylcholine (ACh) release from presynaptic muscarinic neurons during development of tolerance to the anticholinesterase agent, diisopropylfluorophosphate (DFP). DFP in arachis oil was administered subcutaneously to intact animals according to both acute and chronic regimens, with arachis oil injections serving as controls. Post-mortem analyses showed that the mean AChE activity level in whole brain was reduced under all DFP conditions to 18.0 +/- 1.4% when compared with the control level. After 10 days of DFP treatment, the AChE level was 22.3 +/- 2.1% of control in the myenteric plexus. There were no significant differences among the treatment groups in resting ACh release. Release evoked by electrical stimulation (difference between stimulated and resting release) in the absence of atropine, i.e., "basal rate," for strips taken at various times after a single injection of DFP did not differ from that for strips from animals receiving arachis oil only. However, basal release for strips from chronically treated subjects was significantly greater than that of controls (p less than 10(-3), although not different from each other. Analysis of variance (ANOVA) for repeated measures showed that there existed a highly significant atropine dependency in strips from all treatments when they were stimulated in concentrations of atropine from 10(-9) to 10(-5) M (p less than 10(-10). Further analyses established that the increases in rates of evoked ACh release as concentrations of atropine increased were similar for strips from chronically treated DFP and arachis oil animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect on striatal dopamine metabolism and differential motor behavioral tolerance following chronic cholinesterase inhibition with diisopropylfluorophosphate

Rats were treated with diisopropylfluorophosphate (DFP), using 1 or 2 mg/kg acutely, or with 1 mg/kg daily for 4, 14 or 28 days. Their behaviors and striatal dopamine (DA) and dihydroxyphenylacetic acid (DOPAC) levels were studied. The behaviors: tremors, chewing-movements and hind-limb abduction induced by DFP increased in a steeply dose-dependent manner. Chronic treatment for up to 28 days produced biphasic patterns of change for all the behavioral parameters. Tremor occurred in a complex spectrum of slow to intense fast types. Except for chewing, tolerance developed for these parameters, but at different rates. After acute treatment striatal DA and DOPAC levels were altered and the DOPAC/DA ratios were consistently increased within about the first two hr, suggesting an increased turnover of DA. After chronic treatment for 4 and 14 but not 28 days, both DA and DOPAC levels were decreased without a change in their ratios. It is suggested that the changes in DA metabolism arose secondarily to an elevation of brain acetylcholine following cholinesterase inhibition. A prolonged change in the levels or turnover of DA could be responsible for increase of postsynaptic DA receptor density previously found by us [36], which might then partly mediate the behavioral tolerance to DFP.

Selective breeding for differences in cholinergic function: pre- and postsynaptic mechanisms involved in sensitivity to the anticholinesterase, DFP

To determine the contribution of presynaptic cholinergic mechanisms to the increased sensitivity of a genetically selected line of Sprague-Dawley rats (Flinders S-line) to the anticholinesterase, diisopropyl fluorophosphate (DFP), rats were sacrificed by focused microwave irradiation of the head 1 min after a pulse injection of deuterium-labeled choline into the tail vein. The S-line rats exhibited higher concentrations of labeled acetylcholine (ACh) in the cortex than the rats bred for resistance to DFP (Flinders R-line). To determine the contribution of postsynaptic cholinergic mechanisms the concentration of brain muscarinic ACh receptors (mAChR) was determined. The S-line rats exhibited higher concentrations of striatal and hippocampal mAChR than the R-line rats. Thus, both pre- and postsynaptic cholinergic mechanisms may contribute to the increased sensitivity to DFP but their relative importance varies with brain region: increased ACh synthesis in the cortex and increased concentrations of mAChR in the striatum and hippocampus.

Antidepressant withdrawal-induced activation (hypomania and mania): mechanism and theoretical significance

Electrocortical and behavioral arousal are separate phenomena subserved by different neural substrata operating in parallel. A comprehensive theory of 'activation' must take into account the relationships between the electrical and behavioral activating systems. In pathological or experimentally induced states paradoxes, resolvable by a theory positing functional interaction between these systems, arise. EEG arousal is directly mediated, in both the waking and sleeping state, by cholinergic mechanisms. Antidepressant withdrawal precipitates cholinergic overdrive; this would account for the apparent disturbances of REM sleep occurring when antidepressants are stopped. Generally, cholinergic overdrive would produce behavioral inhibition but in particular instances it triggers marked psychomotor arousal by mobilizing a 'limbic activating system'. The existence of a monoaminergic 'limbic activating system', system 'A', with the properties attributed to it in this paper, is supported by both clinical and laboratory observations. System 'A' theory provides a parsimonious means of adequately explaining many phenomena. This theory also has in its favor explanatory power and scope. The Cholinergic-Monoaminergic Interaction Theory of antidepressant withdrawal induced activation and of rapidly-cycling manic-depressive illness maintains that system 'A' and a cholinergic inhibitory system interact dynamically, and that excessive monoaminergic function can precipitate excessive cholinergic function and a dearth of monoaminergic function (due to autoregulation) and hence depression. Likewise, excessive cholinergic function is posited to activate monoaminergic systems and hence to secondarily cause behavioral activation. Rapidly-cycling manic-depressive patients, according to the model, develop alternating cholinergic and monoaminergic overdrive states because the homeostatic mechanisms which should serve to maintain, within normal limits, the composite of cholinergic inhibitory and monoaminergic activating influences are defective. Consequently, rather than reaching a reasonable balance compatible with adaptive function there is oscillation between extremes. Each oscillatory movement is actually a move towards the 'golden mean' and is induced by deviation from this ideal but the defective homeostatic mechanisms promote ' perpetual ' overshooting. Lithium and ECT may be useful in the treatment of rapidly-cycling patients as both treatments may down-regulate muscarinic receptors, and otherwise modify cholinergic and monoaminergic systems in ways promoting homeostasis.(ABSTRACT TRUNCATED AT 400 WORDS)

Behavioural plasticity and the cholinergic system

Evidence for the involvement of the cholinergic system in behavioural plasticity is reviewed by considering three forms of behavioural plasticity: habituation, learning and memory, and tolerance development. Although the cholinergic system may modulate the response tendencies of an animal, it does not appear to be involved in the process of habituation. A number of studies have indicated that the cholinergic system may be involved in learning and memory processes in infrahuman animals. In general, cholinergic antagonists tend to disrupt memory while agonists may, under the appropriate conditions, facilitate memory. Recent studies have pointed to a relation between dysfunctions of the cholinergic system and dysfunctions of memory in aged animals. Studies of tolerance development suggest that the cholinergic system may undergo plastic changes which may underlie the development of tolerance to some drugs, with receptor alterations being the most reproducible finding. However, more work is necessary to establish the degree of plasticity. The cholinergic system also appears to be involved in learning and memory processes in humans. However, attempts to correct the memory deficits in aged humans by manipulating the cholinergic system have met with limited success. The reasons for this lack of success are briefly considered.

Effect of acute and chronic cholinesterase inhibition with diisopropylfluorophosphate on muscarinic, dopamine, and GABA receptors of the rat striatum

The effects of acute and chronic administration of diisopropylfluorophosphate (DFP) to rats on acetylcholinesterase (AChE) activity (in striatum, medulla, diencephalon, cortex, and medulla) and muscarinic, dopamine (DA), and gamma-aminobutyric acid (GABA) receptor characteristics (in striatum) were investigated. After a single injection of (acute exposure to) DFP, striatal region was found to have the highest degree of AChE inhibition. After daily DFP injections (chronic treatment), all brain regions had the same degree of AChE inhibition, which remained at a steady level despite the regression of the DFP-induced cholinergic overactivity. Acute administration of DFP increased the number of DA and GABA receptors without affecting the muscarinic receptor characteristics. Whereas chronic administration of DFP for either 4 or 14 days reduced the number of muscarinic sites without affecting their affinity, the DFP treatment caused increase in the number of DA and GABA receptors only after 14 days of treatment; however, the increase was considerably lower than that observed after the acute treatment. The in vitro addition of DFP to striatal membranes did not affect DA, GABA, or muscarinic receptors. The results indicate an involvement of GABAergic and dopaminergic systems in the actions of DFP. It is suggested that the GABAergic and dopaminergic involvement may be a part of a compensatory inhibitory process to counteract the excessive cholinergic activity produced by DFP.

Selective breeding for sensitivity to DFP: generalization of effects beyond criterion variables

The degree of generalization to the effects of DFP, an organophosphate anticholinesterase, was studied in two lines of Sprague-Dawley derived rats selectively bred for varying sensitivities to DFP. In the S13, S14, S15, and S16 generations the Flinders S-line of rats were still more sensitive to the effects of DFP on the criterion variables upon which selection was based: core body temperature, body weight and a simple operant response for water reward. The flinders S-line were also more sensitive to the effects of DFP on locomotor activity, FR5 responding for a water reward, and analgesia, indicating some degree of generalization. However, diarrhea, a symptom of peripheral effects of DFP, occurred at a similar incidence in the two lines, although males of both lines had higher incidences than the females. Neither of the two lines was affected by DFP for variables in which aversive (i.e. shock) motivation was used: The number of discriminative escape responses and the escape times were similar. These findings indicate that while the effects of DFP do generalize beyond the criterion variables upon which selection was based, the generalization is relatively specific. The data are consistent with the hypothesis that the changes in sensitivity have arisen because of changes in the functioning of a central cholinergic system(s).

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.