Pertussis toxin-mediated ribosylation of G proteins blocks the hypnotic response to an α2-agonist in the locus coeruleus of the rat

Abstract

Biologic responses mediated by adrenoceptors are transduced by a receptor-effector mechanism that involves a guanine nucleotide binding protein (G protein). Recently, we determined that the transduction mechanism for the hypnotic response to dexmedetomidine, a highly selective α 2-agonist, is located in the locus coeruleus (LC) of the rat. In this study, we examined the role of pertussis toxin-sensitive (PTX) G proteins in the LC for the hypnotic response to dexmedetomidine. The LC of rats were stereotactically cannulated and treated with PTX, 0.34 μg, or vehicle. Five days later, the hypnotic response to dexmedetomidine, 7 μg into the LC or 50 μg · kg −1 IP, was tested. On the following day, the LC was harvested and assayed to determine whether the G proteins had been ribosylated by pretreatment with PTX in vivo. Quantitative immunoblotting of G 0α, G iα1,2, and G iα3, the α-subunit of three PTX-sensitive proteins, was also performed. In vivo treatment with PTX into the LC blocked the hypnotic response to LC-administered desmedetomidine and, to a lesser extent, IP-administered dexmedetomidine. The in vivo PTX treatment effectively ribosylated the G proteins. No alteration in the amount of the different species of PTX-sensitive α-subunit was produced by in vivo PTX treatment. These data suggest a pivotal role for PTX-sensitive G proteins in the LC in the hypnotic response to α 2-agonists in the rat.