Dexmedetomidine Diminishes Halothane Anesthetic Requirements in Rats Through a Postsynaptic Alpha2 Adrenergic Receptor

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Abstract
The effect of 4(5)-[1-(2,3-dimethylphenyl)ethyl]imidazole (medetomidine), the alpha2 adrenergic agonist, on anesthetic requirements was investigated in rats anesthetized with halothane. Halothane MAC was determined before and after either dexmedetomidine (d-enantiomer) or levomedetomidine (l-enantiomer) 10, 30, and 100 .mu.g/kg or vehicle ip. There was a dose-dependent decrease in MAC with the d-, but not the l-, stereoisomer. At the highest dose of dexmedetomidine (100 .mu.g/kg), halothane could be discontinued for up to 30 min with no response to tail clamping. To determine whether alpha2 adrenoreceptors mediated this effect of dexmedetomidine on MAC, cohorts of rats were pretreated with idazoxan, 10 mg/kg ip, a highly selective alpha2 antagonist. This completely prevented the reduction of MAC caused by dexmedetomidine. To determine whether the reduction of MAC caused by dexmedetomidine was mediated in part through either opiate or adenosine receptors, groups of rats pretreated with either naltrexone, 5 mg/kg ip, an opiate antagonist, or 8-phenyltheophylline, 2.5 mg/kg ip, an A1 adenosine antagonist. These two pretreatments did not alter the reduction of MAC by dexmedetomidine. To determine whether postsynaptic mechanisms mediate the anesthetic effect of dexmedetomidine, rats were depleted of central catecholamine stores with either n-(2-chloroethyl)-n-ethyl-2-bromobenzylamine (DSP-4) or reserpine and alpha-methyl-para-tyrosine and MAC was determined before and after each dose of dexmedetomidine. While the catecholamine-depleted rats had a lower basal MAC than the vehicle controls, there was still a profound reduction in halothane MAC after administration of dexmedetomidine. The reduction of MAC by dexmedetomidine was blocked with idazoxan in the catecholamine depleted rats. These data indicate that the reduction of MAC caused by dexmedetomidine is mediated through alpha2 adrenoreceptors with no apparent involvement of either opiate or A1 adenosine receptors. Data from catecholamine-depleted rats suggest that the mediating mechanism must involve site(s) other than or in addition to the presynaptic alpha2 adrenergic receptors on noradrenergic neurons. The authors conclude that central postsynaptic alpha2 adrenergic receptors mediate a significant part of the reduction of anesthetic requirements caused by dexmedetomidine.