Perspectives in Pharmacology In Vivo Gene Modification Elucidates Subtype-Specific Functions of a2-Adrenergic Receptors

Mice with altered a2-adrenergic receptor genes have become important tools in elucidating the subtype-specific functions of the three a2-adrenergic receptor subtypes because of the lack of sufficiently subtype-selective pharmacological agents. Mice with a deletion (knockout) of the a2A-, a2B-, or a2C-gene as well as a point mutation of the a2A-gene (a2A-D79N) and a 3-fold overexpression of the a2C-gene have been generated. Studies with these mice indicate that most of the classical functions mediated by the a2-adrenergic receptor, such as hypotension, sedation, analgesia, hypothermia, and anesthetic-sparing effect, are mediated primarily by the a2A-subtype. The a2B-subtype is the principal mediator of the hypertensive response to a2-agonists, appears to play a role in salt-induced hypertension, and may be important in developmental processes. The a2C-subtype appears to be involved in many central nervous system processes such as the startle reflex, stress response, and locomotion. Both the a2Aand a2C-subtypes are important in the presynaptic inhibition of norepinephrine release and appear to have distinct regulatory roles. The ability to study subtype-specific functions in different mouse strains by altering the same a2-adrenergic receptor in different ways strengthens the conclusions drawn from these studies. Although these genetic approaches have limitations, they have significantly increased our understanding of the functions of a2-adrenergic receptor subtypes. Adrenergic receptors mediate the physiological responses to the catecholamines, norepinephrine and epinephrine. They belong to the superfamily of G protein-coupled receptors containing seven putative transmembrane domains and are divided pharmacologically into a1-, a2-, and b-adrenergic receptor types (Bylund, 1988). a2-Adrenergic receptors are implicated in diverse physiological functions particularly of the cardiovascular system and the central nervous system. a2-Adrenergic receptor agonists are used clinically in the treatment of hypertension, glaucoma, and attention-deficit disorder, in the suppression of opiate withdrawal, and as adjuncts to general anesthesia. a2-Adrenergic receptors have been divided into three subtypes (a2A, a2B, and a2C) on the basis of pharmacological and molecular cloning evidence (Lomasney et al., 1991; Bylund et al., 1994; Hein and Kobilka, 1995). Understanding the role of a2-adrenergic receptor subtypes in these diverse functions is clearly important particularly from a pharmacological point of view. One line of evidence supporting differential functions of the subtypes is differences in their characteristics, such as their tissue distributions throughout development (Winzer-Serhan et al., 1997), and in the adult, their coupling to G proteins and regulation in response to agonist stimulation. Although in situ hybridization studies of a2adrenergic receptor subtype expression in mice during development and in adults (Wang et al., 1996) and rats (Nicholas et al., 1993; Rosin et al., 1993; Scheinin et al., 1994) can reveal where a2-adrenergic receptor subtypes are expressed, these findings cannot definitively link particular subtypes to physiological functions. Furthermore, assigning the physiological functions of a2-adrenergic receptors to specific subtypes in vivo has been difficult because of the lack of sufficiently subtype-selective agonists and antagonists. The ability to genetically manipulate a2-subtypes provides an alternative approach to elucidating subtype-specific functions as demonstrated in recent experiments using mice with deletions, mutations, or overexpression of specific a2-adrenergic receptor subtypes (MacDonald et al., 1997). Received for publication October 25, 1999. 1 This work was supported by National Institutes of Health Grant NS33194. 2 Current address: Department of Neuroscience, University of Pennsylvania, Philadelphia, PA 19104-6074. ABBREVIATIONS: KO, knockout; OE, overexpression; PPI, prepulse inhibition. 0022-3565/00/2931-0001$03.00/0 THE JOURNAL OF PHARMACOLOGY AND EXPERIMENTAL THERAPEUTICS Vol. 293, No. 1 Copyright © 2000 by The American Society for Pharmacology and Experimental Therapeutics Printed in U.S.A. JPET 293:1–7, 2000