Potential of some monoterpenoids and their new N-methyl carbamate derivatives against Schistosomiasis snail vector, Biomphalaria alexandrina

Some monoterpenoids and their corresponding new N-methyl carbamate derivatives were used to study their molluscicidal effect on Biomphalaria alexandrina, the snail-vector of Schistosoma mansoni in Egypt. Improving the efficacy of the tested monoterpenoids and/or their corresponding carbamates by either piperonyl butoxide (PBO) or triton X-100 (TX) was also performed. Thymol, β-citronellol, carvacrol, and geraniol exhibited high molluscicidal activity against the snails. Geraniol, β-citronellol, and carvacrol were strongly synergized by PBO but, an opposite trend was found with TX. The molluscicidal activity of geraniol or β-citronellol when mixed with PBO was as potent as copper sulfate. Another attempt to improve the bioactivity of monoterpenoids was through their structure modifications. Thus, conversion of the tested monoterpenoids into their corresponding carbamates led to enhancement in the activity of aliphatic monoterpenoids and reduction in the aromatics.

Actions of the insecticide 2(nitromethylene)tetrahydro-1,3-thiazine on insect and vertebrate nicotinic acetylcholine receptors

The nitromethylene heterocyclic compound 2(nitromethylene)tetrahydro)1,3-thiazine (NMTHT) inhibits the binding of [125I]alpha-bungarotoxin to membranes prepared from cockroach (Periplaneta americana) nerve cord and fish (Torpedo californica) electric organ. Electrophysiological studies on the cockroach fast coxal depressor motorneuron (Df) reveal a dose-dependent depolarization in response to bath-applied NMTHT. Responses to ionophoretic application of NMTHT onto the cell-body membrane of motorneuron Df are suppressed by bath-applied mecamylamine (1.0 x 10(-4) M) and alpha-bungarotoxin (1.0 x 10(-7) M). These findings, together with the detection of a reversal potential close to that estimated for acetylcholine, provide evidence for an agonist action of this nitromethylene on an insect neuronal nicotinic acetylcholine receptor. The binding of [3H]H12-histrionicotoxin to Torpedo membranes was enhanced in the presence of NMTHT indicating an agonist action at this vertebrate peripheral nicotinic acetylcholine receptor. NMTHT is ineffective in radioligand binding assays for rat brain GABAA receptors, rat brain L-glutamate receptors and insect (Musca domestica) L-glutamate receptors. Partial block of rat brain muscarinic acetylcholine receptors is detected at millimolar concentrations of NMTHT. Thus nitromethylenes appear to exhibit selectivity for acetylcholine receptors and exhibit an agonist action at nicotinic acetylcholine receptors.

Interaction of narcotic antagonist naltrexone with nicotinic acetylcholine receptor

The interactions of naltrexone with the nicotinic acetylcholine receptor were studied electrophysiologically using the frog sciatic nerve-sartorius muscle and biochemically using membranes from the electric organ of Torpedo ocellata. At nanomolar concentrations naltrexone increased the peak amplitude of endplate currents with little change in the decay time constant. At micromolar concentrations there was a concentration-dependent depression of endplate current and miniature endplate current amplitudes and decay time constants. Decay time constant depression was enhanced with hyperpolarization. Only marginal curvature was induced in peak endplate current amplitude versus membrane voltage plots by naltrexone. Naltrexone had no effect on single channel conductance but decreased open channel lifetime, according to fluctuation analysis. Naltrexone alone (less than or equal to 3 microM) did not impair binding of [125I]alpha-bungarotoxin to the receptor in a fast pre-equilibration assay, but increased the ability of acetylcholine to displace [125I]alpha-bungarotoxin. The drug displaced the agonist-stimulated binding of [3H]perhydrohistrionicotoxin to the channel site. Biphasic functional changes in neuromuscular transmission can be attributed to an allosteric mechanism with increased agonist binding to the nicotinic receptor at nanomolar concentrations and caused a non-competitive blockade of the ionic channel at micromolar concentrations.

Pharmacology of putative glutamate receptors from insect skeletal muscles, insect central nervous system and rat brain

Binding of [3H]glutamate to housefly brain and honeybee brain and thoracic muscle membranes as well as to the American cockroach nerve cord was measured in Na+-free Tris-citrate buffer, 2.5 mM CaCl2, pH 7.4. The dissociation constants (KDS) ranged from 0.16 to 1.36 microM, and thoracic muscles had 2-4-fold higher density of receptors than brain tissue. The potent inhibitors of housefly brain binding were in decreasing order of effectiveness: L-glutamate greater than L-aspartate = L-cysteate = ibotenate greater than quisqualate greater than L-homocysteate greater than L-APB greater than L-APV greater than NMDA greater than D-APB greater than D-glutamate, with no inhibition by 100 microM of GDEE, dihydrokainate, D-APV, D-homocysteate or D-aspartate. The drug specificity of [3H]glutamate binding sites in housefly brain was generally similar to that of binding sites in housefly muscle, except that the former had a slightly higher affinity for L-APB, L-homocysteate and NMDA. [3H]Glutamate binding to insect tissues differed in its drug sensitivity from binding to rat brain. Binding to insect membranes was much less sensitive to L-APB, D-APB, APV, homocysteate, L-cysteate, quisqualate and ibotenate. However, the insect binding site was much more stereoselective for the L than D isomers of glutamate and aspartate, while the rat brain site was more stereoselective for APB. It is suggested that the observed [3H]glutamate binding to insect tissue is not to NMDA or kainate receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison of the actions of carbamate anticholinesterases on the nicotinic acetylcholine receptor

Neostigmine (Neo), pyridostigmine (Pyr), and physostigmine (Phy) at low concentrations inhibited acetylcholine (ACh) esterase, thereby indirectly potentiating ACh enhancement of [3H]perhydrohistrionicotoxin (H12-HTX) binding to the channel sites of the nicotinic ACh receptor of Torpedo membranes. However, at higher concentrations, they inhibited ACh action due to their direct binding to the ACh receptor. They displaced binding of [3H]ACh and 125I-alpha-bungarotoxin (alpha-BGT) to the receptor sites with the following order of decreasing potency: Neo greater than Phy greater than Pyr. Furthermore, Neo and Pyr potentiated [3H] H12-HTX binding to the receptor's channel sites. Preincubation of ACh receptors with any of the three carbamates reduced the rate of binding of 125I-alpha-BGT and increased the potency of carbamylcholine in inhibiting 125I-alpha-BGT binding, suggesting that the three carbamates act as partial agonists and potentiate receptor desensitization. Although none of the three carbamates inhibited [3H]H12-HTX binding to the receptor's closed channel conformation, only Phy was a potent inhibitor of [3H]H12-HTX binding to the carbamylcholine-activated conformation. The potency of Phy was not due to the absence of positive charge since Phy methiodide acted similarly. The data suggest that the major action of the three carbamates at nicotinic cholinergic synapses is inhibition of ACh-esterase. Their interactions with the nicotinic ACh receptor are with its "receptor" as well as allosteric "channel" sites, but they differ in their effects. Neo and Pyr act mainly as partial agonists, while Phy is mostly an inhibitor of the channel in the activated receptor conformation.