Rat clonidine mydriasis model:Imidazoline receptors are not involved

The clonidine mydriasis model in rats has been widely applied in preclinical research to characterize a -adrenoceptor antagonistic properties of drugs. The present study was undertaken to pharmacologically determine if imidazoline I receptors are also involved in this model system. Sigmoid dose-response curves for pupillary dilation were produced in pentobarbital anesthetized rats by intravenous administration of increasing doses of agonists (guanabenz for a -adrenoceptors, clonidine for both a - adrenoceptors and imidazoline I receptors, and rilmenidine for imidazoline I receptors). Two antagonists (RS 79948 for a -adrenoceptors and efaroxan for imidazoline I receptors) were used to antagonize the mydriasis elicited by those three agonists, with antagonistic potencies calculated. In additional experiments, we examined the effect of the selective imidazoline I receptor antagonist, AGN 192403, on clonidine-induced mydriasis. The results showed that pupillary response curves elicited by guanabenz, clonidine and rilmenidine were competitively antagonized by both RS 79948 (0.03-1 mg/kg) and efaroxan (0.03-1 mg/kg) in a dose-related fashion. The potencies of either antagonist against the three agonists were not significantly different. AGN 192403 (5 mg/kg) did not significantly shift the clonidine mydriasis curve. These results suggest that imidazoline I receptors are not functionally involved in the rat clonidine mydriasis model and support this in vivo system as a useful model for studies of a -adrenoceptors. © 2004 Elsevier B.V. All rights reserved.

Targeted cytoplasmic irradiation induces bystander responses.

The observation of radiation-induced bystander responses, in which cells respond to their neighbors being irradiated, has important implications for understanding mechanisms of radiation action particularly after low-dose exposure. Much of this questions the current dogma of direct DNA damage driving response in irradiated systems. In this study, we have used a charged-particle microbeam to target individual helium ions ((3)He(2+)) to individual cells within a population of radioresistant glioma cells cultured alone or in coculture with primary human fibroblasts. We found that even when a single cell within the glioma population was precisely traversed through its cytoplasm with one (3)He(2+) ion, bystander responses were induced in the neighboring nonirradiated glioma or fibroblasts so that the yield of micronuclei was increased by 36% for the glioma population and 78% for the bystander fibroblast population. Importantly, the yield of bystander-induced micronuclei was independent of whether the cytoplasm or nucleus of a cell was targeted. The bystander responses were fully eliminated when the populations were treated with 2-(4-carboxyphenyl)-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxide or filipin, which scavenge nitric oxide (NO) and disrupt membrane rafts, respectively. By using the probe 4-amino-5-methylamino-2',7'-difluorofluorescein, it was found that the NO level in the glioma population was increased by 15% after 1 or 10 cytoplasmic traversals, and this NO production was inhibited by filipin. This finding shows that direct DNA damage is not required for switching on of important cell-signaling mechanisms after low-dose irradiation and that, under these conditions, the whole cell should be considered a sensor of radiation exposure.