Effects of physostigmine, paraoxon and soman on brain GABA level and metabolism

Differential expression of glutamate transporters in cerebral cortex of paraoxon-treated rats

Glutamatergic system is involved in pathological effects of organophosphorus (OP) compounds. We aimed to determine in vivo effects of paraoxon, the bioactive metabolite of parathion, on the expression of glutamate transporters as well as Bax and Bcl2 in rat cerebral cortex. Male Wistar rats received an intraperitoneal (i.p.) injection of one of three doses of paraoxon (0.3, 0.7, or 1mg/kg) or corn oil as vehicle (1ml/kg). After 4 or 18h, cerebral cortices were dissected out and used for quantitative reverse transcriptase polymerase chain reaction (qRT-PCR) and western blot assays to measure mRNA and protein levels, respectively. The cortical glial glutamate transporters (GLAST and GLT-1) were up-regulated in animals treated with 0.7mg/kg of paraoxon, but down-regulated in 1mg/kg group. Neuronal glutamate transporter (EAAC1) was unchanged in 0.7mg/kg treated rats, while reduced in 1mg/kg group. No significant difference was found in the mRNA and protein expression of EAAC1 in animals intoxicated with 0.3mg/kg of paraoxon. Paraoxon (1mg/kg) resulted in an up-regulation of Bax and down-regulation of Bcl2 mRNA levels in the rat cerebral cortex. These results indicate that paraoxon can differentially regulate expression of glutamate transporters at mRNA and protein levels in the cerebral cortex. Changes in the expression of glutamate transporters are closely related to paraoxon-induced seizure activity.

In vitro assessment of paraoxon effects on GABA uptake in rat hippocampal synaptosomes

Treating organophosphate poisoning is achieved mainly using compounds with anticholinergic characteristics. Nevertheless currently the focus of attention is aimed at examining their interference with other neurotransmitter systems. The present investigation studied the potential interactions between paraoxon and GABA uptake in hippocampal synaptosomes. Wistar rats weighing 200-250 g were used. Hippocampal synaptosomes were prepared and incubated with [(3)H] GABA in the presence of different doses of paraoxon for 10 min at 37 degrees C; and were then layered in chambers of a superfusion system and the [(3)H] GABA uptake was measured. Our finding revealed that mean GABA uptake decreased by 21%, 42%, 37%, 20%, and 8% of the corresponding control values in the presence of paraoxon concentrations of 0.01, 0.1, 1, 10, and 100 microM, respectively which was significant at 0.1 and 1 microM of paraoxon (P<0.05). In conclusion, micromolar concentrations of paraoxon were shown to interfere with GABA uptake in hippocampal synaptosomes, which indicates the GABA transporters may play a role in organophosphate-induced convulsions.

Synaptosomal GABA uptake decreases in paraoxon-treated rat brain

A synaptosomal model was used to evaluate in vivo effects of paraoxon on the uptake of [(3)H]GABA in rat cerebral cortex and hippocampus. Male Wistar rats were given a single intraperitoneal injection of one of three doses of paraoxon (0.1, 0.3, or 0.7 mg/kg) and acetylcholinesterase (AChE) activity in the plasma, cerebral cortex, and hippocampus was measured at 30 min, 4h, and 18 h after exposure. [(3)H]GABA uptake in synaptosomes was also studied in another series of animals. Paraoxon administration (0.3 and 0.7 mg/kg) caused significant inhibition of AChE activity in the plasma and both brain areas at all time points. 0.1 mg/kg paraoxon significantly inhibited AChE activity but only in the plasma for 4h, the activity was completely recovered at 18 h. GABA uptake was significantly (p<0.001) reduced in both cerebral cortex (18-32%) and hippocampal (16-23%) synaptosomes at all three time points after administering 0.7 mg/kg of paraoxon, a dose that seems to be sufficient to induce seizure activity. L-DABA, an inhibitor of neuronal GABA transporter, allowed us to conclude that the uptake was mediated primarily by neuronal GABA transporter GAT-1. In conclusion, present data suggests that GABA uptake by synaptosomes decreases probably secondary to paraoxon-induced seizure activity.

The effect of acetylcholinesterase-inhibition on depolarization-induced GABA release from rat striatal slices

The severity of poisoning after intoxication with the acetylcholinesterase (AChE) inhibitor soman has been shown to be positively correlated with GABA release in rat striatum. Since most of the neurons in striatum and striatal projection regions use GABA as transmitter, it is still unclear, whether an increase of extracellular GABA in this region results from enhanced activation of these projections or is due to the local effect of AChE inhibition. In this study, the modulation of depolarization-induced increase in GABA concentration by soman was determined in the superfusate of rat striatal slices. Soman and neostigmine increased GABA concentration in the superfusate dose dependently. This increase was exerted through M-cholinoceptors as it could be blocked by atropine and enhanced by application of the muscarinic agonists pilocarpine or oxotremorine. These results clearly indicate that AChE inhibition by soman in rat striatum can directly lead to enhanced release of GABA through M-cholinoceptors.

Nasal absorption of procyclidine in rats and dogs

Nasal absorption of procyclidine, a synthetic anticholinergic compound, was investigated in Wistar rats and Beagle dogs. The dosing solution was prepared by dissolving 14C-procyclidine in 50% ethanolic saline. The dosing solution was administered intravenously and intranasally to rats at a dose of 0.6 mg/kg (i.e., 60 microl/kg in the form of a 1% w/v solution), and intravenously, orally and intranasally to dogs at a dose of 0.3 mg/kg (i.e., 6 microl/kg in the form of a 5% w/v solution). Blood samples were taken from an artery of the animals through the catheter for periods of 1200 (for rats) and 1,440 min (for dogs), and the radioactivity in the samples was determined by liquid scintillation counting. The nasal bioavailability of procyclidine in rats and dogs, based on the radioactivity, was calculated to be 81.1 and 98.6%, respectively. In both rats and dogs, the plasma profiles of procyclidine following nasal administration were very close to those following intravenous administration, leading to nearly superimposable profiles between the two protocols. In dogs, nasal administration resulted in significantly higher plasma concentrations during the first 30 min period compared to oral administration, suggesting the superiority of the nasal route over the oral route in terms of a prompt expression of the pharmacological effect of the drug. The results obtained in this study indicate that procyclidine is rapidly and nearly completely absorbed via the nasal route. In conclusion, nasal administration represents a viable alternative to intravenous administration in the case of procyclidine.

Comparison of aldicarb and methamidophos neurotoxicity at different ages in the rat: behavioral and biochemical parameters

Young organisms are often more sensitive to the toxic effects of pesticides, and this finding has spurred research on further characterization of this susceptibility. The neurotoxic effects of cholinesterase (ChE)-inhibiting pesticides are of particular concern for human health risk assessment due to the widespread exposure potential in children. This study evaluated age-related differences in susceptibility for a carbamate (aldicarb) and an organophosphorus pesticide (methamidophos). Comparisons were made between preweanling (Postnatal Day 17, PND17), postweanling (PND27), and adult (approximately PND70) male and female rats. All were acute studies using oral administration. Sensitivity was quantified by (1) determination of maximally-tolerated doses (MTDs); (2) measurement of brain and blood ChE inhibition; and (3) neurobehavioral evaluation using end points known to be sensitive indicators of exposure to anticholinesterases. MTD data showed that preweanling rats were twice as sensitive as adults to aldicarb, but there was no differential sensitivity to methamidophos. The dose-response data for brain ChE inhibition followed a similar pattern of age-related differences, and similar levels of inhibition were measured at the MTD regardless of age. Dose-response and time course studies of neurobehavioral end points indicated that differential effects due to age depend on the behavioral end point examined. Following aldicarb administration, the dose-response curves for a few end points overlapped; however, the young rats otherwise showed fewer signs of toxicity than did the adults despite similar levels of brain ChE inhibition. Motor activity assessment showed that aldicarb did not produce any activity depression in PND17 rats, whereas the data for the PND27 and adult rats overlapped. With methamidophos, the dose-response curves for most end points for preweanling and adult rats were quite similar. Aldicarb-induced ChE inhibition was readily reversible in all age groups, whereas with methamidophos, enzyme activity recovered more rapidly in the young. Most behavioral alterations had recovered by 24 h with either pesticide. The results of these studies indicate that (1) ChE-inhibiting pesticides are not all the same regarding relative sensitivity of the young; (2) age-related differences were reflected in both the MTDs and degree of ChE inhibition; and (3) age-related differences in neurobehavioral measures depended on the pesticide and on the end points examined.

Behavioral changes and cholinesterase activity of rats acutely treated with propoxur

Early assessment of neurological and behavioral effects is extremely valuable for early identification of intoxications because preventive measures can be taken against more severe or chronic toxic consequences. The time course of the effects of an oral dose of the anticholinesterase agent propoxur (8.3 mg/kg) was determined on behaviors displayed in the open-field and during an active avoidance task by rats and on blood and brain cholinesterase activity. Maximum inhibition of blood cholinesterase was observed within 30 min after administration of propoxur. The half-life of enzyme-activity recovery was estimated to be 208.6 min. Peak brain cholinesterase inhibition was also detected between 5 and 30 min of the pesticide administration, but the half-life for enzyme activity recovery was much shorter, in the range of 85 min. Within this same time interval of the enzyme effects, diminished motor and exploratory activities and decreased performance of animals in the active avoidance task were observed. Likewise, behavioral normalization after propoxur followed a time frame similar to that of brain cholinesterase. These data indicate that behavioral changes that occur during intoxication with low oral doses of propoxur may be dissociated from signs characteristic of cholinergic over-stimulation but accompany brain cholinesterase activity 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.

Transient impairment of the gabaergic function during initiation of soman-induced seizures

The changes in extracellular gamma-aminobutyric acid (GABA) levels, the modifications in binding capacities of GABA-receptor subtypes A and B and of the Cl- ionophore sites localized in the ionic-channel associated to the GABAA receptors were studied in hippocampus of rats subjected to a convulsive dose of the acetylcholinesterase inhibitor soman. Whereas extracellular GABA levels, just as binding on GABAA and GABAB receptors, were not modified under soman, a significant transient decrease in the binding capacities of the Cl- ionophore site of the GABAA receptor complex occurred within the first 10 min of seizures in CA1, CA3 areas, and in the dentate gyrus with return to basal values after 30 min. Accordingly, a transient decrease of the brain muscimol-gated Cl- influx was observed after 10 min of seizures. An increased ability of diazepam to potentiate the GABAA gated Cl- influx occurred at the same time. Altogether, these data demonstrated that an impairment of the GABAA receptor function occurs at the beginning of seizures. This suggests that a temporary decrease of GABAAergic function may contribute to the onset of seizures.

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.

Pharmacological nature of soman-induced hypothermia in mice

The object of the study was to determine the pharmacological nature of pinacolyl methylphosphonofluoridate (soman)-induced hypothermia in mice. This was accomplished by examining the soman hypothermia dose response and the effect of various pharmacological antagonists in comparison to the hypothermia responses of muscarinic and nicotinic cholinergic agonists such as oxotremorine and nicotine and another anticholinesterase, physostigmine. Core temperature in mice was monitored by telemetry. In general, atropine antagonized oxotremorine, physostigmine, and soman hypothermia but not nicotine hypothermia whereas mecamylamine antagonized nicotine hypothermia but not that produced by the other agonists. Soman hypothermia was not affected significantly by various pharmacological antagonists, suggesting that other neurotransmitters were not involved in the expression of soman hypothermia. Soman hypothermia appears to be due to muscarinic receptor stimulation and can be effectively antagonized, but not completely, by the use of atropine. Acetylcholinesterase oxime reactivators, such as HI-6 and toxogonin, were ineffective in antagonizing soman-induced hypothermia and reactivating hypothalamic acetylcholinesterase, whereas HI-6 was effective in reactivating soman-inhibited diaphragm acetylcholinesterase when administered up to 10 min after soman, indicating that aging of the soman-inhibited acetylcholinesterase had not occurred. Soman hypothermia appears to be primarily a muscarinic receptor-related event.

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 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.

Neurotransmitter changes in guinea-pig brain regions following soman intoxication

The effects of the organophosphate acetylcholinesterase (AChE) inhibitor soman (31.2 micrograms/kg s.c.) on guinea-pig brain AChE, transmitter, and metabolite levels were investigated. Concentrations of acetylcholine (ACh) and choline (Ch), noradrenaline (NA), dopamine (DA), 5-hydroxytryptamine (5-HT), and their metabolites, and six putative amino acid transmitters were determined concurrently in six brain regions. The brain AChE activity was maximally inhibited by 90%. The ACh content was elevated in most brain areas by 15 min, remaining at this level throughout the study. This increase reached statistical significance in the cortex, hippocampus, and striatum. The Ch level was significantly elevated in most areas by 60-120 min. In all regions, levels of NA were reduced, and levels of DA were maintained, but those of its metabolites increased. 5-HT levels were unchanged, but those of its metabolites showed a small increase. Changes in levels of amino acids were restricted to those areas where ACh levels were significantly raised: Aspartate levels fell, whereas gamma-aminobutyric acid levels rose. These findings are consistent with an initial increase in ACh content, resulting in secondary changes in DA and 5-HT turnover and release of NA and excitatory and inhibitory amino acid transmitters. This study can be used as a basis to investigate the effect of toxic agents and their treatments on the different transmitter systems.

Evidence that alterations in gamma-aminobutyric acid and acetylcholine in rat striata and cerebella are not related to soman-induced convulsions

Many reports have suggested that gamma-aminobutyric acid (GABA) may play a role in organophosphate-induced convulsions. The balance between GABA and acetylcholine (ACh) in the brain also has been suggested by some investigators to be related to brain excitability. We examined these questions by studying the levels of GABA and ACh and the ratios of GABA to ACh in rat striata and cerebella (two major motor control areas in the CNS) after the administration of soman, an organophosphate acetylcholinesterase inhibitor also known as nerve gas. Male Sprague-Dawley rats weighing 250-300 g were injected subcutaneously with three different doses of soman: a subconvulsive dose of 40 micrograms/kg (approximately 30% of the ED50 for convulsions in rats), a convulsive dose of 120 micrograms/kg (approximately one ED50 for convulsions), and a higher convulsive dose of 150 micrograms/kg (approximately 120% of the ED50 for convulsions). The incidence and severity of convulsions were monitored in individual rats until they were sacrificed by focused microwave irradiation of the head at the following time points after soman administration: 4 min, a time prior to the onset of convulsions; 10 min, the time of onset of convulsions; 1 h, the time of peak convulsive activity; and 6 h, a time at which rats were recovering from convulsions. Results showed that in rat striata and cerebella, neither changes in levels of GABA and ACh nor changes in ratios of GABA to ACh were related to soman-induced convulsions, i.e., none of the changes in either levels or ratios of these two neurotransmitters were related to the initiation of, maintenance of, or recovery from soman-induced convulsions.

Concomitant increases in the levels of choline and free fatty acids in rat brain: evidence supporting the seizure-induced hydrolysis of phosphatidylcholine

The main objective of this study was to determine whether the excitotoxic cholinesterase inhibitor soman increases the catabolism of phospholipids in rat brain. Injections of soman (70 micrograms/kg, s.c.), at a dose that produced toxic effects, increased the levels of both free fatty acids (175-250% of control) and free choline (250% of control) in rat cerebrum 1 h after administration. All fatty acids contained in brain phosphatidylcholine were elevated significantly including palmitic (16:0), stearic (18:0), oleic (18:1), arachidonic (20:4), and docosahexaenoic (22:6) acids. The changes observed were consistent with those reported to occur following ischemia and the administration of other convulsants. Pretreatment of rats with the anticonvulsant diazepam (4 mg/kg, i.p.) prevented both the signs of soman toxicity and the soman-induced increase of choline and free fatty acids. Diazepam alone did not affect the levels of choline or free fatty acids, cholinesterase activity, or soman-induced cholinesterase inhibition, suggesting that soman toxicity involves a convulsant-mediated increase in phosphatidylcholine catabolism. In addition, administration of the convulsant bicuculline, at a dose that produces seizures and increases the levels of free fatty acids in brain, significantly increased the levels of choline. Results suggest that excitotoxic events enhance the hydrolysis of phosphatidylcholine in brain as evidenced by a concomitant increase in the levels of choline and free fatty acids.

Cortical acetylcholine release is increased and gamma-aminobutyric acid outflow is reduced during morphine withdrawal

The effects of naloxone on acetylcholine (ACh) and gamma-aminobutyric acid (GABA) outflow from the cerebral cortex of freely moving, morphine-dependent guinea-pigs was studied. The cortical efflux of ACh in chronically-treated guinea-pigs was about half of that of normal animals. GABA efflux was unaffected. During opioid withdrawal precipitated by naloxone (0.1-10 mg kg-1, i.p.) the guinea-pigs showed jumping, hyperactivity and wet dog shakes, the intensity of which was directly related to the dose of naloxone. The withdrawal syndrome was accompanied by a dose-dependent increase in ACh release and reduction in GABA outflow; ACh release was increased by naloxone at doses lower (0.1-3 mg kg-1) than those acting on GABA efflux (1-10 mg kg-1). Atropine (10 mg kg-1) and diazepam (5 mg kg-1) did not prevent GABA and ACh changes.

Effects of chronic intake of diazinon on blood and brain monoamines and amino acids

Male rats were treated bi-weekly by gavage with the equivalent of 0.5 mg X kg-1 X day-1 technical diazinon for up to 28 weeks. The animals were sacrificed at specific time intervals (7, 14 and 28 weeks) and compared with age matched controls. Blood and brain tissues were analysed for cholinesterase activity and for concentrations of catecholamines and amino acids. Only Plasma cholinesterase was significantly reduced by the low level pesticide treatment. Erythrocyte acetyl cholinesterase and brain acetyl cholinesterase were unchanged while during the same period several putative brain neurotransmitters aspartate, glutamate (excitatory) and taurine as well as GABA (inhibitory) were significantly reduced in experimental vs control animals whereas no significant changes occurred between weeks in similarly fed animals. Blood serotonin was significantly elevated but no other blood or brain monoamine was significantly altered. Overt manifestations of brain toxicity observed were not apparent in experimental compared with control animals save for a significant decrease in growth observed in experimental animals. It was concluded that oral administration of low doses of diazinon exerts significant effects other than as an anticholinesterase on important brain neurotransmitters even at the low dose levels administered in this study.