Modulation of α2-Adrenoceptor Functions by Heterotrimeric Gαi Protein Isoforms

Subtype diversity of heterotrimeric G proteins and G protein-coupled receptors enables a wide spectrum of signal transduction. However, the significance of isoforms within receptor or G protein subfamilies has not been fully elucidated. In the present study, we have tested whether alpha(2)-adrenoceptors require specific G alpha isoforms for their function in vivo. In particular, we analyzed the role of the highly homologous G alpha(i) isoforms, G alpha(i1), G alpha(i2), and G alpha(i3), in typical alpha(2)-adrenoceptor-controlled functions. Mice with targeted deletions in the genes encoding G alpha(i1), G alpha(i2), or G alpha(i3) were used to test the effects of alpha(2)-adrenoceptor stimulation by the agonist medetomidine. The alpha(2)-adrenoceptor agonist medetomidine inhibited [H-3]norepinephrine release from isolated prefrontal brain cortex or cardiac atria tissue specimens with similar potency and efficacy in tissues from wildtype or G alpha(i)-deficient mice. In vivo, bradycardia, hypotension, induction of sleep, antinociception, and hypothermia induced by alpha(2)-adrenoceptor activation did not differ between wild-type and G alpha(i)-knockout mice. However, the effects of the alpha(2)-agonists medetomidine or 5-bromo-6-(2-imidazolin-2-ylamino)quinoxaline tartrate (UK14,304) on spontaneous locomotor activity or anesthetic sparing were reduced or absent, respectively, in mice lacking G alpha(i2). In microdissected locus coeruleus neurons or postganglionic sympathetic neurons from stellate ganglia, all three G alpha(i) subunits were expressed as determined by quantitative reverse transcription-polymerase chain reaction, with G alpha(i1) and G alpha(i2) dominating over G alpha(i3). Functional redundancy of the highly homologous G alpha(i) isoforms may predominate over specificity to regulate distinct intracellular pathways downstream of alpha(2)-adrenoceptors in vivo. In contrast, inhibition of locomotor activity and anesthetic sparing may be elicited by a specific coupling of alpha(2A)-adrenoceptors via the G alpha(i2) isoform to intracellular pathways.