Days Of Chronic Treatment Research Articles

Mechanisms of tolerance to the anticholinesterase, DFP: Acetylcholine levels and dynamics in the rat brain

The effect of diisopropylfluorophosphate (DFP) on total and regional brain acetylcholine (ACh) levels and dynamics was investigated in rats as they developed tolerance to DFP. Microwave irradiation and integrated gas chromatography mass spectrometry (GCMS) were used in all assays. Significant increases in total brain ACh was seen at 4 through to 48 hr after an acute injection of DFP and after 1 through 22 days of chronic administration of DFP. Total brain choline levels were significantly decreased at 4 through 24 hr after DFP administration. Choline levels then became significantly greater than control values after 10 and 22 days of chronic treatment with DFP. No changes were seen in ACh synthesis or choline uptake. Regional brain levels of ACh or choline were not significantly changed 24 hr after acute administration of DFP. These results indicate that behavioral tolerance to DFP is not due to end-product inhibition of ACh synthesis. It is concluded that the probable mechanisms of tolerance may involve changes in presynaptic muscarinic receptors, as well as the changes in postsynaptic receptors which have already been verified.

The influence of disulfiram and other inhibitors of oxidative metabolism on the formation of 2-hydroxyethyl-mercapturic acid from 1,2-dibromoethane by the rat

The mercapturic acid derivative, N- acetyl-S-2- hydroxyethyl- l - cysteine , is a major metabolite of 1,2-dibromoethane in vivo. This compound can be formed via two pathways, both involving a potentially dangerous reactive intermediate. One way involves the intermediacy of bromoacetaldehyde, formed by microsomal oxidation, followed by loss of hydrogen bromide. The second pathway, direct conjugation of 1,2-dibromoethane with glutathione, gives rise to S-2-bromoethyl glutathione. Using several inhibitors of microsomal mixed function oxidases, it was found that under these conditions about 10% of the mercapturic acid derivative formed via direct conjugation. Disulfiram, an inhibitor of aldehyde dehydrogenases, but also of microsomal oxidation, also markedly inhibits the excretion of the mercapturic acid, after administration of a single high dose (1 g/kg) or upon chronic treatment with a low dose (50 mg/kg). The inhibitory effect is maximal after 10 days of chronic treatment. Administration of large amounts of 1,2-dibromoethane (>0.20 nmole/rat) following a single lower dose of disulfiram (125 mg/kg) also leads to a lower excretion of mercapturic acid metabolite a phenomenon associated with a decrease in cytochrome P-450 levels. From these results it is concluded that the enhanced carcinogenic effect of the combination disulfiram (chronic)/1,2-dibromoethane is not caused by bromoacetaldehyde, since its formation is completely inhibited under these conditions, but by S-2-bromoethyl-glutathione, although a role for 1,2-dibromoethane itself cannot be excluded.

Development of transient myasthenia-like symptoms in chickens injected with acetylcholine receptor from Torpedo marmorata [proceedings

Tyrosine hydroxylase activity was determined in eight brain regions: cerebral cortex, tuberculum olfactorium, nucleus accumbens, hypothalamus, amygdala, striatum, pons and medulla. Tyrosine hydroxylase activity was measured by the radiometric method of Hendry & Iversen (1971). After 30 days of chronic treatment, the specific activity of tyrosine hydroxylase was decreased t o a value below the sensitivity of the assay in all eight brain regions analysed. Within 36 h after withdrawing treatment, the activity of this enzyme had returned to control values in three brain regions, but was still significantly below control in another four (Fig. 1). Only the activity in the cerebral cortex remained undetectable, but this could be misleading, since control enzyme activity in this region was only 3 % of the striatal activity, and in control samples of cortex radioactivity was only twice blank values. The results of Fig. I suggest that the regions with the highest control activity recover more rapidly on withdrawal of the drug. Our results on the changes in tyrosine hydroxylase activity have been compared with catecholamine concentrations in the same regions. The results indicate that there appears to be no significant cell loss in the striatum, amygdala and nucleus accumbens after chronically treating rats with methamphetamine on the aforementioned drug regime. However, a direct comparison with the observations of Seiden e t a / . (1976) has not been made, since there may be species differences between rat and monkey, and until it has been determined whether the activity of tyrosine hydroxylase returns to control values in all regions after a longer period of withdrawal.