Interactions of cholinesterase inhibitors and glucocorticoids with ketamine and pentobarbitone-induced general anaesthesia in the rat: possible effects on central cholinergic activity.

Doses of 100, 150 and 200 micrograms kg-1 of the cholinesterase inhibitor neostigmine reverse the anaesthetic action of ketamine. The antagonistic effect is increased as the dose is increased. The duration of anaesthesia induced by pentobarbitone is reversed by the cholinesterase inhibitor in doses of 150, 200 and 250 micrograms kg-1. Choline, in a dose of 50 mg kg-1, significantly antagonizes the action of the two anaesthetics, whereas hemicholinium-3, an inhibitor of the uptake of choline and the synthesis of acetylcholine, markedly potentiates their action. Dexamethasone induces a significant reduction of the duration of anaesthesia produced by ketamine and pentobarbitone. The potentiation of the anaesthetic effect caused by hemicholinium-3 is also reversed by dexamethasone. The acetylcholine content in rat cerebral cortex is increased after treatment with ketamine and pentobarbitone. Measurements of the course of the plasma level of pentobarbitone do not reveal alterations in the pharmacokinetic profile by either neostigmine or dexamethasone. These results indicate that central cholinergic systems may somehow be involved in the anaesthesia induced by ketamine and pentobarbitone and that the interactions described in this paper may be the result of modification by neostigmine and dexamethasone of the alterations in cholinergic activity caused by the two anaesthetics.

Effect of 3-aminobenzamide on antigenic variation of Trypanosoma brucei.

African trypanosomes, like Trypanosoma brucei, depend on antigenic variation to evade the immune response of the vertebrate host. An antigenic switch corresponds to the activation of a variable surface glycoprotein (VSG) gene from a large silent repertoire. Most switches require the duplicative transposition of a VSG gene, which involves strand breaks in DNA and subsequent repair. The nuclear enzyme adenosine-diphosphoribosyl transferase (ADPRT), which is dependent on the presence of DNA strand breaks for its activity, might be involved in this process because it has a regulatory role in DNA repair in all eukaryotic cells studied so far. In previous work, the presence of ADPRT activity was demonstrated in T. brucei. Moreover, it was also shown in isolated trypanosomes the ADPRT activity, which is stimulated by the induction of DNA strand breaks, could be blocked by the competitive inhibitor 3-aminobenzamide. Here we report experiments using rats which were infected with small numbers of T. brucei expressing VSG gene 118. After two days, the rats were coupled to a continuous intraperitoneal infusion system administrating 3-aminobenzamide in 0.9% NaCl (81.4 mM) at a rate of 0.65 ml/hr/rat for a period of up to five days. Control rats received only a 0.9% NaCl infusion. At days 1, 3 and 5, 250 microliters blood was obtained from a tail artery. Plasma 3-aminobenzamide was determined using a new high performance liquid chromatography method, developed for these experiments. In most rats the plasma concentrations were maintained between 0.8 and 1.2 mM. The rate of antigenic switching was determined by quantitating the fraction of trypanosomes that had lost their VSG 118 coat, using antibody against VSG 118 and a limiting dilution in mice. The average switching rate found was 2.0 X 10(-6) in controls and 1.3 X 10(-7) in drug-treated rats (15-fold reduction). This suggests that ADPRT is required for completing most antigenic switching events. We discuss the possibility that drug-resistant switching only involves non-duplicative VSG gene activation.

Block of locust muscle glutamate receptors by delta-philanthotoxin occurs after receptor activations.

One component (delta-philanthotoxin (delta-PTX) of the venom from the wasp Philanthus triangulum blocks transmission postsynaptically at excitatory synapses on locust muscle. delta-PTX depresses both the iontophoretic glutamate potential and the excitatory junctional current (e.j.c.) in a glutamate receptor activation-dependent manner. The rate of recovery from the effects of the toxin is reduced following either prolonged application of L-glutamate or repetitive iontophoretic application of this amino acid or high frequency neural stimulation of the muscle in the presence of delta-PTX. The decay phase of the e.j.c. is shortened by delta-PTX. The effects of delta-PTX on the e.j.c. are not voltage dependent. The open-close kinetics of glutamate channels in extrajunctional muscle membrane are modified by delta-PTX as shown by patch clamp analysis. The mean life time of the glutamate channel is reduced, whilst the mean interval between single opening events is increased with the events often occurring in bursts. These data are consistent with glutamate channel blocking by this toxin. It is proposed that the toxin blocks open channels gated by both junctional and extrajunctional glutamate receptors on locust muscle. It is further proposed that delta-PTX enters a compartment of the muscle through the glutamate open channels and that it can also block the open channels from this site.

Characterization of two paralysing protein toxins (A-MTX and B-MTX), isolated from a homogenate of the wasp Microbracon hebetor (Say).

Two paralysing toxins (A-MTX and B-MTX) from extracts of Microbracon hebetor (Say) wasps were isolated and purified by gel chromatography, ion exchange chromatography and gel electrophoresis. Both toxins are labile proteins with molecular weights of 43,700 and 56,700, respectively, as estimated by gel chromatography, and with isoelectric pH of 6.85 and 6.62, respectively. Both toxins are inactivated by proteolytic enzymes and by dithiothreitol, but A-MTX appears to be more resistant than B-MTX. The relative amino acid compositions of both toxins show great similarity. The biological effects of the two toxins were identical to those previously found for crude toxin preparations.

Acetylcholine and an unidentified, muscle-contracting factor in the venom of the bumblebee, Bombus terrestris L.

Extracts of venom reservoirs as well as dilutions of venom droplets collected from the tip of the sting of Bombus terrestris were tested for biological activity on the isolated guinea-pig ileum, the frog rectus abdominis, and the rat phrenic nerve-hemidiaphragm preparation. The venom contains a cholinergic factor, apparently identical to acetylcholine, at a concentration of about 30 micrograms per venom reservoir. This was confirmed using radioimmunoassay of acetylcholine. The venom also contains a component causing a slow contraction of guinea-pig ileum and rat diaphragm. This component is heat stable, non dialysable, and is slowly destroyed in the venom preparation at about 0 degrees C.