Male Sprague-Dawley rats administered with a sublethal acute dose of carbofuran (1.5 mg/kg, sc) developed the observable toxic signs of anticholinesterase nature within 5-7 min. The toxic signs with increasing propensity to maximal severity including tremors, generalized muscle fasciculations, and convulsions were evident during 15 min to 1 h and lasted for 2 h. Thereafter, signs were seen up to 3 h with reduced intensity. By the end of 3.5 h toxic signs were completely subsided. Maximal acetylcholinesterase (AChE) inactivation occurred at 1 h in discrete brain regions (cortex, stem, striatum, and hippocampus) and hemidiaphragm muscle when most severe signs of toxicity were also evident. A single sc dose of memantine HCl (MEM, 18 mg/kg) and atropine sulfate (ATS, 16 mg/kg) 60 and 15 min, respectively, prior to carbofuran administration completely prevented the expected gross toxic signs and significantly (p less than .01) attenuated the carbofuran-induced inhibition of AChE activity. When given therapeutically, this combined treatment completely reversed the clinical evidence of carbofuran toxicity within 15 min and also markedly reduced AChE inactivation. Memantine or atropine when given alone was less effective compared to their combined administration. The results of this study suggested that, in addition to cholinolytic effects of atropine, memantine may prevent and antagonize the acute toxicity of carbofuran by (a) protection of AChE activity and its rapid reactivation from inhibition and (b) rapid elimination of carbofuran.

Pretreatment of rats with the nonspecific esterase inhibitor iso-OMPA (1 mg/kg sc) 1 h prior to carbofuran (2,3-dihydro-2,2-dimethyl-7-benzofuranyl N-methylcarbamate, 0.5 mg/kg sc) administration potentiated carbofuran toxicity by more than threefold. Neither iso-OMPA nor carbofuran in the given doses produced any gross toxic signs. Rats receiving combined treatment, however, showed severe hypercholinergic signs (salivation, tremors, muscle fasciculations, and convulsions) within 5-10 min following carbofuran administration, and the severity was comparatively greater than that observed with an acute dose of carbofuran (1.5 mg/kg sc). Rats pretreated with iso-OMPA (0.5 mg/kg) died within 10-15 min following the acute dose of carbofuran (1.5 mg/kg). Each drug when given alone (1.0 mg/kg iso-OMPA, 0.5 mg/kg carbofuran) caused a significant (p less than .01) inhibition of carboxylesterase (CarbE) activity in brain structures (cortex, stem, striatum, and hippocampus), skeletal muscle (hemidiaphragm), liver, and plasma, whereas acetylcholinesterase (AChE) activity remained significantly (p greater than .01) unchanged. The maximal CarbE inactivation in plasma (less than 14% remaining activity) following either drug indicated a tremendous nonspecific binding to non-AChE serine-containing enzymes. iso-OMPA pretreatment markedly potentiated carbofuran's anticholinesterase activity both in neuronal and in nonneuronal tissues. It can be concluded that iso-OMPA pretreatment potentiates carbofuran toxicity either by preventing nonspecific binding of carbofuran to CarbE and/or possibly by inhibiting its detoxification.

Intoxication with an acute dose of carbofuran (1.5 mg/kg, sc) in male Sprague-Dawley rats evoked severe toxic manifestations characteristic of hypercholinergic preponderance with profound muscle fasciculations and convulsions during 30-60 min, lasting for about 2 h. Lactate dehydrogenase, a "biomarker" cytoplasmic enzyme catalyzing the reversible reaction of lactate-pyruvate, was represented by five electrophoretically distinct isoenzymes in the serum and tissues. The amounts of each isoenzyme in different tissues were widely varied and consequently the patterns were tissue specific. A 24-h time-course following carbofuran administration indicated a two-fold increase in the activity of total LDH in serum and more than 30% in hemidiaphragm and liver. The patterns of LDH isoenzymes in serum revealed a significant (P less than 0.01) increase in all the isoenzymes except LDH-4 (64% decrease). Analysis of each tissue revealed characteristic changes in LDH isoenzyme patterns indicating organ-specific tissue damage. These alterations in LDH and its isoenzymes, in addition to acetylcholinesterase inhibition, may be directly or indirectly related to the mechanism(s) of the toxic action, and also provide insight into the site/organ(s) of toxic injury, thus providing an early prognostic indicator.

The objective of this investigation was to determine the distribution of cholinergic (acetyl-cholinesterase, AChE) and noncholinergic markers in slow-, fast-, and mixed-fiber containing muscles (soleus, SOL; extensor digitorum longus, EDL; and diaphragm, DIA, respectively). Noncholinergic markers included high-energy phosphates (adenosine triphosphate, ATP; phosphocreatine, PCr; and their metabolites), and the activity of creatine kinase (CK) and lactate dehydrogenase (LDH) and their isoenzymes and subforms. All three types of muscles had only one CK isoenzyme, CK-MM, which totally consisted of MM3 subform. Levels of these determinants were highest in EDL followed by DIA and least in SOL. Another objective was to determine alterations of these markers under the influence of acute carbofuran (1.5 mg/kg) or methyl parathion (MPTH, 5 mg/kg) toxicity. Rats receiving either insecticide showed cholinergic signs with maximal severity including muscle fasciculations and convulsions within 15-30 min that lasted for about 2 h. At 1 h postinsecticide injection, when AChE was maximally inhibited (81-96%), significant depletion of ATP and PCr was evident in muscles (DIA > SOL > EDL), and activities of CK-MM and LDH were elevated in muscles and consequently in serum. Serum CK-MM3 activity was markedly reduced with sequential increase in MM2 and MM1 subforms, probably due to induced higher carboxypeptidase activity. These findings suggested that (1) the differences in levels of biochemical constituents in muscles depend upon the fiber type, (2) anticholinesterase insecticide-induced increased muscle activity produces characteristic changes in CK and LDH isoenzymes patterns, and (3) leakage of these enzymes/isoenzymes into serum is due to depletion of ATP and PCr, which are required to maintain the cell membrane permeability.

Gupta, R.C., and W.L. Kadel: Subacute toxicity of aldicarb: Prevention and treatment with memantine and atropine. Drug Dev. Res. 24:343-353, 1991. Daily administration (i.p.) of aldicarb in male Sprague-Dawley rats at various dosage levels for 21 days revealed 1) 0.1 mg/kg, nontoxic dose: 2) 0.2 mg/kg, moderately toxic dose; and 3) 0.4 mg/kg, severely toxic dose. Inhibition of acetylcholinesterase (AChE) in discrete brain regions and diaphragm muscle was dose dependent. Toxic signs were predominantly peripheral even though AChE inhibition was significantly higher in the brain (except striatum). Besides AChE inhibition, marked inactivation of carboxylesterases (false targets) was observed, suggesting a protective mechanism especially against low dosage by reducing free concentration of aldicarb. After 30 min following the 7th, 14th, or 21st dose of aldicarb, the degree of inhibition of esterase(s) remained the same, and consequently no tolerance developed to aldicarb. On day 21, administration of memantine HCI (18 mg/kg, i.p.) and atropine sulfate (16 mg/kg, i.p.) 30 min and 15 min, respectively, prior to aldicarb (0.4 mg/kg) injection provided complete protection. Therapeutic administration of these antidotes completely reversed the clinical manifestations of intoxication. The present findings indicated that memantine provided protection and reversal of AChE from inhibition in addition to reversible blockage of hyperneuromuscular activity.