Topology of the Shaker potassium channel probed with hydrophilic epitope insertions.

The structure of the Shaker potassium channel has been modeled as passing through the cellular membrane eight times with both the NH2 and COOH termini on the cytoplasmic side (Durrell, S.R., and H.R. Guy. 1992. Biophys. J. 62:238-250). To test the validity of this model, we have inserted an epitope consisting of eight hydrophilic amino acids (DYKDDDDK) in predicted extracellular and intracellular loops throughout the channel. The channels containing the synthetic epitope were expressed in Xenopus oocytes, and function was examined by two-electrode voltage clamping. All of the mutants containing insertions in putative extracellular regions and the NH2 and COOH termini expressed functional channels, and most of their electrophysiological properties were similar to those of the wild-type channel. Immunofluorescent staining with a monoclonal antibody against the epitope was used to determine the membrane localization of the insert in the channels. The data confirm and constrain the model for the transmembrane topology of the voltage-gated potassium channel.

Toxicodynamic modeling of highly toxic organophosphorus compounds.

Although the in vitro effect of organophosphorus (OP) compounds on acetylcholine-esterase (AChE) has been studied extensively, the hypothesis that OP inhibition of AChE is the primary mechanism of acute in vivo OP toxicity has been controversial. For example, a recent review (Pope and Liu, 2004) suggested that OP compounds have direct toxic effects on other enzymes, ACh receptors, and receptor/ channel complexes that are independent of AChE inhibition. The purpose of this report is to examine the hypothesis that AChE inhibition is the mechanism of acute toxicity of OP compounds by mathematically modeling the in vivo lethal effects of highly toxic OP compounds and determining the amount of variation in OP toxicity that is explained by AChE inhibition.

Neuropharmacological mechanisms of nerve agent-induced seizure and neuropathology.

This paper proposes a three phase "model" of the neuropharmacological processes responsible for the seizures and neuropathology produced by nerve agent intoxication. Initiation and early expression of the seizures are cholinergic phenomenon; anticholinergics readily terminate seizures at this stage and no neuropathology is evident. However, if not checked, a transition phase occurs during which the neuronal excitation of the seizure per se perturbs other neurotransmitter systems: excitatory amino acid (EAA) levels increase reinforcing the seizure activity; control with anticholinergics becomes less effective; mild neuropathology is occasionally observed. With prolonged epileptiform activity the seizure enters a predominantly non-cholinergic phase: it becomes refractory to some anticholinergics; benzodiazepines and N-methyl-D-aspartate (NMDA) antagonists remain effective as anticonvulsants, but require anticholinergic co-administration; mild neuropathology is evident in multiple brain regions. Excessive influx of calcium due to repeated seizure-induced depolarization and prolonged stimulation of NMDA receptors is proposed as the ultimate cause of neuropathology. The model and data indicate that rapid and aggressive management of seizures is essential to prevent neuropathology from nerve agent exposure.

Pharmacological modulation of soman-induced seizures.

Anticholinergics, benzodiazepines and N-methyl-D-aspartate (NMDA) antagonists have been shown to modulate the expression of nerve agent-induced seizures. This study examined whether the anticonvulsant actions of these drugs varied depending on the duration of prior seizure activity. Rats implanted with electrodes to record electroencephalographic (EEG) activity were pretreated with the oxime HI-6 (125 mg/kg, IP) to prolong survival, and then challenged with a convulsant dose of the nerve agent soman (180 micrograms/kg, SC); treatment compounds (scopolamine, diazepam, MK-801, atropine, benactyzine, and trihexyphenidyl) were delivered IV at specific times after seizure onset. Both diazepam and MK-801 displayed a similar profile of activity: At both short or long times after seizure initiation the anticonvulsant efficacy of each drug remained the same. Diazepam, and especially MK-801, enhanced the lethal actions of soman by potentiating the respiratory depressant effects of the agent; scopolamine given prior to diazepam or MK-801 protected against the respiratory depression. Scopolamine and atropine showed a dose- and time-dependent effectiveness; the longer the seizure progressed the higher the dose of drug required to terminate the seizure, with eventual loss of anticonvulsant activity if the seizure had progressed for 40 min. In contrast, benactyzine and trihexyphenidyl showed a third profile of activity: There was a smaller increase in drug dosage required for anticonvulsant activity as seizure duration increased, and both drugs could terminate seizures that had progressed for 40 min. The early anticonvulsant action of anticholinergics is interpreted as a specific effect that blocks the primary cholinergic excitatory drive that initiates, and first maintains, nerve agent seizures. If allowed to progress, the seizure activity itself recruits excitatory neurotransmitter systems (i.e., NMDA) that eventually maintain the seizure independent of the initial cholinergic drive. This is indicated by the eventual ineffectiveness of scopolamine and atropine as the duration of the seizure progresses. Diazepam and MK-801 appear to act to moderate nerve agent seizures by enhancing inhibitory activity (diazepam) or dampening the secondarily activated noncholinergic excitatory system (MK-801). Benactyzine and trihexyphenidyl represent compounds that possibly have both anticholinergic and NMDA antagonistic properties.

Anticonvulsants for poisoning by the organophosphorus compound soman: pharmacological mechanisms.

Exposure to high doses of organophosphorus nerve agents such as soman, even with carbamate pretreatment, produces a variety of toxic cholinergic signs, including secretions, convulsions and death. Evidence suggests that soman-induced convulsions may be associated with postexposure brain neuropathology. The purpose of this study was to investigate the pharmacologic mechanism of action of soman-induced convulsions and of anticonvulsant drugs. Various classes of compounds were evaluated for their efficacy in preventing soman-induced convulsions in rats pretreated with the oxime HI-6 to increase survival time, along with various doses of the test compounds (IM) either in the absence or presence of atropine sulfate (16 mg/kg, IM) 30 minutes prior to a soman challenge dose (180 micrograms/kg, SC; equivalent to 1.6 x LD50) that produced 100% convulsions. Without atropine sulfate, only tertiary anticholinergics (scopolamine, trihexyphenidyl, biperiden, benactyzine, benztropine, azaprophen and aprophen), caramiphen, carbetapentane and MK-801 were effective anticonvulsants. In the presence of atropine sulfate, the benzodiazepines (diazepam, midazolam, clonazepam, loprazolam and alprazolam), mecamylamine, flunarizine, diphenylhydantoin, clonidine, CGS 19755 and Organon 6370 studied were effective. We have examined the possibility that diazepam may exert some of its anticonvulsant effects through cholinergic mechanisms and found that a reduced release of ACh into synapses after diazepam and atropine treatment may account for diazepam's anticonvulsant activity against soman. We also found that at anticonvulsant doses biperiden and trihexyphenidyl each significantly reversed the effects of soman on striatal levels of DOPAC and HVA, the metabolites of dopamine, and have concluded that in addition to actions on muscarinic receptors, the anticonvulsant effects of these anticholinergics in soman poisoning may be partially related to their actions on the striatal dopaminergic system. These findings allow us to postulate that central muscarinic cholinergic mechanisms are primarily involved in eliciting the convulsions following exposure to soman and that subsequent recruitment of other excitatory neurotransmitter systems and loss of inhibitory control may be responsible for sustaining the convulsions and for producing the subsequent brain damage. Future studies to confirm these neuropharmacological mechanisms are proposed.

Cholinergic mechanisms of analgesia produced by physostigmine, morphine and cold water swimming.

This study concerns the cholinergic involvement in three experimental procedures which produce analgesia. Rats were given one of seven treatments: saline (1.0 ml/kg, i.p.); morphine sulfate (3.5, 6.0 or 9.0 mg/kg, i.p.); physostigmine salicylate (0.65 mg/kg, i.p.); warm water swim (3.5 min at 28 degrees C); and cold water swim (3.5 min at 2 degrees C). Each rat was tested on a hot plate (59.1 degrees C) once prior to and 30 min after treatment. Immediately after the last test the rats were killed with focussed microwave radiation. Levels of acetylcholine (ACh) and choline (Ch) in six brain areas (brain stem, cerebral cortex, hippocampus, midbrain, cerebellum and striatum) were analyzed by gas chromatograph-mass spectrometer. Morphine (9.0 mg/kg), physostigmine and cold water swimming caused significant analgesia. Morphine elevated the levels of ACh in the cerebellum and striatum, cold water swimming--in the cerebellum, striatum and cortex, and physostigmine--in the striatum and hippocampus. Levels of choline were elevated by morphine in the cerebellum, cortex and hippocampus, while cold water swimming elevated levels of choline in the cerebellum, cortex, striatum and hippocampus. Physostigmine did not change levels of choline in any of the brain areas studied. These data suggest that the analgetic effects of morphine or cold water swimming may be mediated by components of the cholinergic system that differ from those involved in the analgetic effects of physostigmine.

Time course effects of soman on acetylcholine and choline levels in six discrete areas of the rat brain.

The time course of changes in rat brain levels of acetylcholine (ACh) and choline (Ch) was investigated following a single SC injection of soman (0.9 LD50, 120 micrograms/kg) to understand the relationship between central neurotransmitter alteration and soman toxicity. Of the animals exposed to the dose of soman, 46% died within 24 h, with maximum mortality occurring during the first 40 min following soman administration. In a second group, surviving rats were killed at various times after treatment by a beam of focused microwave radiation to the head, and ACh and Ch levels were determined by gas chromatography-mass spectrometry. Soman produced a maximal ACh elevation in the brain stem at 20 min (34.4%), in cerebellum at 40 min (51.9%), in cortex and striatum at 2 h (320.3% and 35.2%, respectively), and in hippocampus and midbrain at 3 h (94.5% and 56.8%, respectively). ACh levels remained above normal approximately 30 min in the brain stem; 2 h in the midbrain, cerebellum, and striatum; 8 h in the cortex; and 16 h in the hippocampus. Ch levels were elevated in all areas except the striatum. Ch maxima occurred at 10-40 min and returned to control levels approximately 3 h after injection. Results suggest that perturbation of ACh levels due to soman was not uniform throughout the brain and that soman toxicity may reflect ACh changes in multiple areas, rather than changes in any given area. These data further suggest a possible relationship between elevated Ch levels and soman toxicity.

Effects of chronic lead exposure on levels of acetylcholine and choline and on acetylcholine turnover rate in rat brain areas in vivo.

Rats were exposed to lead acetate from birth, and were killed at the age of 44--51 days for analysis of levels and turnover rates of acetylcholine (ACh). Steady-state levels of ACh were not altered in midbrain, cortex, hippocampus, or striatum of lead-exposed rats. Similarly, no changes in choline (Ch) concentrations were found in cortex, hippocampus, or striatum. In the midbrain, however, a 30% reduction in Ch levels was observed. Changes in specific activity of Ch and ACh were measured as a function of time in selected brain areas of rats infused with a radio-labeled precursor of Ch. Specific activities of ACh were not altered. Ch specific activities were, however, significantly elevated in all brain areas examined, as compared with age-matched control rats. The in vivo ACh turnover rate in cortex, hippocampus, and striatum was diminished by 35%, 54%, 51%, and 33%, respectively. These findings provide direct evidence for an inhibitory effect of lead exposure from birth on central cholinergic function in vivo. Since a significant reduction of body weight was found in those animals treated with lead acetate, the alteration of central cholinergic function may partially be attributed to malnutrition observed in the lead-exposed animals.

The effect of adrenalectomy and dexamethasone on the antinociceptive effects of physostigmine.

The tail-flick procedure was used to study the antinociceptive effects of physostigmine in adrenalectomized and sham-operated rats. At 5 days after surgery, they were tested 30 min after either 0.32 or 0.45 mg/kg IP physostigmine. Adrenalectomized animals showed significantly greater elevation of TF scores from predrug latencies than the sham controls at both doses of physostigmine. Following 3 days of dexamethasone replacement therapy on days 18, 19, and 20 post-surgery the antinociceptive effects of physostigmine were uniformly attenuated across doses or surgical groups. On the other hand, animals receiving saline injection instead of dexamethasone did not manifest any reduction of the physostigmine antinociceptive effect. The potentiation by adrenalectomy and the reduction following dexamethasone of the antinociceptive effects of physostigmine suggest that these effects may be mediated through hypothalamic-pituitary-adrenal mechanisms and are consistent with beta-endorphin-induced sensitization of opiate or cholinergic receptors.

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.

Cytophotometric analyses of mesencephalic reticular formation neuronal RNA in soman intoxicated rats.

Quantitative azure B cytophotometry was used to monitor ribonucleic acid (RNA) responses of individual neurons within the nucleus cuneiformis (NC) and ventrotegmental nucleus (VTN) of the rat mesencephalic reticular formation following single subcutaneous soman (pinacolyl methylphosphonofluoridate) injections (0.5, 0.9 or 1.5 LD(50)). The sub-lethal (0.5 LD(50)) dosage of soman produced RNA accumulation in NC neurons, but VTN-RNA levels were not significantly altered. In contrast, both reticular nuclei exhibited prominent RNA depletion with higher soman dosages, the severity of which was greater with lethal (1.5 LD(50)) than near-lethal (0.9 LD(50)) dosages. These data indicate that metabolic correlates of enhanced activation of cholinergic reticular nuclei are present only with sub-lethal dosages, and that higher dosages produce characteristics of impaired activation of ascending cholinergic pathways. At present, mechanisms underlying soman-induced metabolic and neurologic deficits remain speculative.

Protective effect of diazepam pretreatment on soman-induced brain lesion formation.

Histopathological analyses of brains of rats receiving a single 0.9 LD50 injection of soman, a potent anticholinesterase neurotoxin, revealed massive widespread lesions in the cerebral cortex and thalamus 4 weeks post-injection. Such lesions were not evidenced in rats receiving diazepam (2.2 mg/kg, i.m.) 10 min prior to soman treatment. Thus, anticonvulsant antidotes may aid in preventing extensive or permanent brain damage in rats surviving near-lethal soman dosages.

Thyrotropin-releasing hormone (TRH) is markedly increased in the rat brain following soman-induced convulsions.

Soman is an organophosphorus (OP) compound which irreversibly inhibits acetylcholinesterase (AChE), the primary synaptic inactivator of acetylcholine. Resultant excessive cholinergic activity elicits generalized convulsions and brain lesions. Recent evidence suggests that other neurotransmitter/neuromodulator systems may be affected by the OP compounds as well. Since we have shown that both electrically and chemically induced seizures cause significant and prolonged increases in the neuropeptide thyrotropin-releasing hormone (TRH) in epileptogenic sites, we examined soman-induced convulsion effects on CNS TRH. Rats were injected with either soman (100 microg/kg SC; equivalent to 0.9 LD50) or saline and observed for convulsive activity. Forty-eight hours post injection, dramatic increases of TRH over control levels were seen in frontal cortex (30-fold), pooled cortex (24-fold), hippocampus (16-fold), piriform cortex (14-fold), entorhinal cortex (11-fold), and amygdala (2-fold). No change was observed in either hypothalamus or pituitary. Our results demonstrate, for the first time, a substantial effect of an OP on a specific neuropeptide system in vivo. The neurochemical and behavioral consequences of the soman-induced increases in TRH, especially in the frontal cortex, are presently unknown. Clearly, much more work is required to discern the exact role TRH has following soman exposure.

Cholinergic actions of diazepam and atropine sulfate in soman poisoning.

The effectiveness of diazepam alone or in the presence of atropine sulfate in reversing soman-induced convulsions, inhibition of blood and brain cholinesterase (ChE) activity, and elevation of brain acetylcholine (ACh) and choline (Ch) concentrations in rats was studied. Diazepam (5 mg/kg, IM) blocked the convulsive activity of soman (100 micrograms/kg, SC) whereas atropine sulfate (12 mg/kg, IM) did not. Inclusion of atropine sulfate enhanced the anticonvulsant effects of diazepam. Neither diazepam nor atropine sulfate alone affected ChE activity in the blood and brain of rats, nor did they alone, or in combination, reverse the ChE inhibition induced by soman. Diazepam by itself caused an increase in ACh concentrations in the striatum and a decrease in Ch concentrations in the cortex and striatum. On the other hand, atropine sulfate produced a decrease in ACh and an increase in Ch concentrations in these two brain regions. With combined treatment, diazepam reversed the effect of atropine sulfate on brain ACh and Ch concentrations. Diazepam attenuated the soman-induced elevation of ACh and Ch concentrations in most of the brain regions studied, while atropine sulfate did not. Only when diazepam was given concurrently with atropine sulfate did the elevated brain ACh or Ch concentrations induced by soman return to normal. These results suggest that the anticonvulsant activity of diazepam in soman poisoning may be partially related to its action on presynaptic cholinergic mechanism.

Cerebral blood flow and metabolism in soman-induced convulsions.

Regional cerebral blood flow (CBF) and regional cerebral glucose utilization (CGU) were studied by quantitative autoradiographic techniques in rats. Animals were treated either with a toxic dose of soman, an irreversible organophosphorus cholinesterase inhibitor, that produced convulsions or with saline as controls. An increased arterial blood pressure (mean increase = 41% of control) always preceded onset of convulsions. Convulsive activity was associated with an increase of plasma glucose concentration and marked increases over controls of CGU [average of all regions: control = 75 +/- 5 mumol.100 g-1.min-1, n = regions/animals (304/8); seizures = 451 +/- 20 mumol.100 g-1.min-1, n = 190/5] and CBF [average of all regions: control = 135 +/- 6 ml.100 g-1.min-1, n = 190/5; seizures = 619 +/- 29 ml.100 g-1.min-1, n = 190/5). Regional distribution of these effects revealed a greater proportional increase of CBF over CGU in cingulate, motor, and occipital cortex and caudate-putamen. In contrast, a lower proportional increase of CBF over CGU in CA3 region of hippocampus, dentate gyrus, medial thalamus, and substantia nigra was observed, implying the existence of a relative ischemia in these brain areas. These findings may be relevant to the pathogenesis of brain lesions associated with soman-induced convulsions.

Effects of soman and sarin on high affinity choline uptake by rat brain synaptosomes.

Synaptosomes were incubated at various time intervals following injection of 120 micrograms/kg SC of soman or sarin or with various concentrations (10(-8) to 10(-2) M) of soman or sarin in vitro. Total cholinesterase (ChE) activities in each brain region were also measured. Following soman injection, sodium-dependent, high affinity choline uptake (SDHACU) was decreased from 1 to 4 hr in the cortex and from 1 to 2 hr in the hippocampus, but increased from 2 to 24 hr in the striatum. Similarly, following sarin injection SDHACU was decreased at 0.5 hr in the cortex and from 1 to 4 hr in the hippocampus, but increased at 1 hr in the striatum. Injection of soman severely inhibited (83-99%) total ChE activity in the cortex, hippocampus and striatum from 1 to 24 hr. In contrast, sarin did not severely inhibit ChE activity in these regions and maximal inhibition (40-60%) did not occur until 24 hr after injection. With both compounds, by 168 hr ChE activity in all regions had partially recovered. Incubation of synaptosomes with soman or sarin in vitro at concentrations below 10(-4) M did not affect SDHACU in any of the brain regions. These data demonstrated that acute soman and sarin injection produced similar effects upon SDHACU in different brain regions, although the time-course of these effects was different for the two compounds. These effects were probably neither due to a direct action of these compounds on the uptake process nor dependent on ChE inhibition.

Cerebral blood flow-metabolism coupling after administration of soman at nontoxic levels.

The effect of soman, an irreversible organophosphorus cholinesterase inhibitor, on regional cerebral blood flow and glucose utilization were studied with a double-tracer, autoradiographic technique in rats. Soman was given at a subtoxic dose of 55 micrograms/kg SC and variables were measured 45 min later. No changes in arterial blood pressure or signs of toxicity were present in the animals studied. Soman induced a pronounced increase in cerebral blood flow. This change was not accompanied by an increase in cerebral metabolism, with exception of superior colliculi. Brain regions showing the more pronounced (greater than 200% over control) increases in blood flow were motor, sensory and temporal cortex, area 18a of the occipital cortex, claustrum, inferior colliculus and cerebellum. These findings differ from those previously reported for the carbamate cholinesterase inhibitor, physostigmine, and constitute the first demonstration of cerebrovascular effects for an organophosphorus cholinesterase inhibitor, soman, at nonsymptomatic doses.

Mapping of cortical metabolic activation in soman-induced convulsions in rats.

The metabolic activation of the cerebral cortex during convulsions induced by the organophosphorus cholinesterase inhibitor soman was studied in detail. Soman was given at a dose equivalent to 0.9 LD50 (100 microgram/kg SC after pretreatment with 26 microgram/kg pyridostigmine, IM, to decrease lethality) to examine separately the metabolic effects of severe acetylcholinesterase inhibition, present always with this dose, and convulsions, present only in some of the animals. Cerebral glucose utilization (CGU) values of cortex divided by CGU of brain stem (nCGU) were calculated for 96 locations in nine coronal slices. Animals injected with pyridostigmine-soman and that developed convulsions (n = 7) showed statistically significant increases of nCGU with regard to animals injected with saline (n = 5) in 33 locations, 27 of which were in a single cluster, with the piriform cortex at its center. Perirhinal cortex, and insular cortex also showed significantly higher nCGU in convulsing rats. Other foci of elevated nCGU were found in frontal and parietal locations. In animals injected with pyridostigmine-soman and that did not develop convulsions (n = 5) in spite of severe cholinesterase inhibition, a single location (piriform cortex) showed significantly higher nCGU than controls. Neuropathology evaluation showed a significant decrease in viable cells only in animals that developed convulsions. This effect correlated with enhanced nCGU. It is concluded that the presence of convulsions, and not exposure to pyridostigmine-soman, determined the pattern of nCGU cortical activation, which correlated closely with the structural changes.

Regional sensitivity of neuroleptic receptors to sub-acute soman intoxication.

Sub-acute exposure to anti-cholinesterase organophosphorous compounds induces, in humans, cognitive and emotional deficits which include depression, anxiety, emotional lability, and schizophrenic-like symptoms. Neuroleptic drugs used to treat similar clinical conditions bind to serotonergic (S2) and dopaminergic (D2) receptors, suggesting that these sites are involved in the psychiatric consequences of organophosphate exposure. Rats were given saline or soman (50 micrograms/kg, SC) on a sub-acute regimen (three times weekly for four weeks) and killed 1 hr, 1 day or 3 days after the final injection. Response of regional neuroleptic receptors to soman intoxication was assessed using 3H-spiperone as ligand. Initial high affinity binding experiments using mianserine, haloperidol, or both to identify specific cortical binding revealed that mianserine displaceable binding sites showed the greatest down-regulation in response to soman. Subsequent kinetic analyses of mianserine displaceable 3H-spiperone binding indicated a dramatic decrease in the number of hippocampal binding sites and a decrease in the affinity of cortical binding sites. These S2 sites, considered to be involved with neural excitation, have the ability to self-regulate and appear to be involved in the expression of soman neurotoxicity.

Age-related differences in soman toxicity and in blood and brain regional cholinesterase activity.

The toxicity (lethality, acute toxic signs and body weight loss) of the irreversible ChE inhibitor soman was assessed in four groups of male rats differing in age: 30, 60, 120 and 240 days old. Plasma and brain regional ChE activity profiles were also studied in these groups. All measures of the toxicity of soman were found to increase with age. The calculated 24-hr LD50s were 110, 87, 66 and 59 micrograms/kg, IM, for 30-, 60-, 120- and 240-day-old rats, respectively. A significant and positive age-related effect on toxic sign rating scores was observed at one hr following soman injection. Furthermore, during a 14-day postsoman observation period, it was observed that young rats had less initial weight loss and more rapid, sustained recovery of growth than older animals. Survivors from the two oldest age groups did not recover to baseline body weights by the end of the 14-day observation period. Basal level of plasma ChE activity did not change significantly with age, while brain regional ChE showed two distinct age-dependent patterns: a linear decrease in the brainstem, midbrain and cerebellum and an inverted U-shaped change in the cortex, hippocampus and striatum. Our data suggest a relationship between soman toxicity and the aging process, but fails to demonstrate a definite relationship between soman toxicity and basal ChE activity in blood and brain of rats.