Am J Physiol Regulatory Integrative Comp Physiol 283: R287–R295, 2002;
`10.1152/ajpregu.00123.2002.
`
`invited review
`
`Physiological significance of ␣2-adrenergic receptor
`subtype diversity: one receptor is not enough
`
`MELANIE PHILIPP, MARC BREDE, AND LUTZ HEIN
`Institut fu¨ r Pharmakologie und Toxikologie, Universita¨ t Wu¨ rzburg, 97078 Wu¨ rzburg, Germany
`
`Philipp, Melanie, Marc Brede, and Lutz Hein. Physiological sig-
`nificance of ␣2-adrenergic receptor subtype diversity: one receptor is not
`enough. Am J Physiol Regulatory Integrative Comp Physiol 283:
`R287–R295, 2002; 10.1152/ajpregu.00123.2002.—␣2-Adrenergic recep-
`tors mediate part of the diverse biological effects of the endogenous
`catecholamines epinephrine and norepinephrine. Three distinct sub-
`types of ␣2-adrenergic receptors, ␣2A, ␣2B, ␣2C, have been identified from
`multiple species. Because of the lack of sufficiently subtype-selective
`ligands, the specific biological functions of these receptor subtypes were
`largely unknown until recently. Gene-targeted mice carrying deletions in
`the genes encoding for individual ␣2-receptor subtypes have added im-
`portant new insight into the physiological significance of adrenergic
`receptor diversity. Two different strategies have emerged to regulate
`adrenergic signal transduction. Some biological functions are controlled
`by two counteracting ␣2-receptor subtypes, e.g., ␣2A-receptors decrease
`sympathetic outflow and blood pressure, whereas the ␣2B-subtype in-
`creases blood pressure. Other biological functions are regulated by syn-
`ergistic ␣2-receptor subtypes. The inhibitory presynaptic feedback loop
`that tightly regulates neurotransmitter release from adrenergic nerves
`also requires two receptor subtypes, ␣2A and ␣2C. Similarly, nociception
`is controlled at several levels by one of the three ␣2-receptor subtypes.
`Further investigation of the specific function of ␣2-subtypes will greatly
`enhance our understanding of the relevance of closely related receptor
`proteins and point out novel therapeutic strategies for subtype-selective
`drug development.
`adrenergic receptors; transgenic mice; gene targeting
`
`ADRENERGIC RECEPTORS FORM the interface between the
`endogenous catecholamines epinephrine and norepi-
`nephrine and a wide array of target cells in the body to
`mediate the biological effects of the sympathetic ner-
`vous system. To date, nine distinct adrenergic receptor
`subtypes have been cloned from several species: ␣1A,
`␣1B, ␣1D, ␣2A, ␣2B, ␣2C, ␤1, ␤2, and ␤3 (11). For many of
`these receptors, their precise physiological functions
`and their therapeutic potential have not been fully
`elucidated. Only for ␤-adrenergic receptors have suffi-
`ciently subtype-selective ligands been developed that
`have helped to identify the physiological significance of
`
`Address for reprint requests and other correspondence: L. Hein,
`Institut fu¨ r Pharmakologie und Toxikologie, Universita¨t Wu¨ rzburg,
`Versbacher Strasse 9, 97078 Wu¨ rzburg, Germany (hein@toxi.uni-
`wuerzburg.de).
`
`␤1-, ␤2-, and ␤3-receptors, some of which have entered
`clinical medicine. Selective agonists for the ␤2-adren-
`ergic receptor play an important role in asthma ther-
`apy, whereas ␤1-receptor antagonists are first-line
`medication for patients with hypertension, coronary
`heart disease, or chronic heart failure (8, 20, 50). For
`␣1-receptors, subtype-selective ligands that can dimin-
`ish the symptoms of benign prostate hyperplasia with-
`out causing hypotension have just entered clinical
`therapy (33). Despite the fact that ␣2-adrenergic recep-
`tors serve a number of physiological functions in vivo
`and have great therapeutic potential, no sufficiently
`subtype-selective ligands are clinically available yet.
`Despite this fact, non-subtype-selective ␣2-receptor ago-
`nists like clonidine, medetomidine, and brimonidine are
`being used to treat patients with hypertension, glau-
`coma, tumor pain, postoperative pain, and shivering or
`
`http://www.ajpregu.org
`
`0363-6119/02 $5.00 Copyright © 2002 the American Physiological Society
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`to block the symptoms of sympathetic overactivity dur-
`ing drug withdrawal (66). Unfortunately, the fields of
`therapeutic application and unwanted side effects are
`overlapping, e.g., ␣2-receptor-mediated sedation is an
`important problem for treatment of hypertension. Se-
`vere side effects are one reason why ␣2-receptor ago-
`nists are only second-line antihypertensive agents. It is
`tempting to speculate that ␣2-receptor-mediated ther-
`apy could be greatly improved and advanced if receptor
`subtype-selective ligands were available. However, be-
`fore developing specific ligands, the therapeutic targets
`have to be identified. Recently, transgenic and gene-
`targeted mouse models have added considerable infor-
`mation about individual adrenergic receptor subtypes
`(15, 25, 37, 39, 53, 54). This review focuses on the
`specific functions of the three ␣2-adrenergic receptor
`subtypes in mouse models carrying targeted deletions
`in the genes encoding for ␣2-receptors.
`
`␣2-ADRENERGIC RECEPTOR GENES
`
`So far, three distinct genes have been identified from
`several species that encode for separate subtypes of
`␣2-adrenergic receptors (11). From these genes, three
`␣2-receptors are synthesized, termed ␣2A, ␣2B, and ␣2C.
`The pharmacological ligand binding characteristics of
`the ␣2A-subtype differ significantly between different
`species, thus giving rise to the pharmacological sub-
`types ␣2A in humans, rabbits, and pigs and ␣2D in rats,
`mice, and guinea pig (11). This species variation is at
`least in part due to a single amino acid variation in the
`fifth transmembrane domain of the ␣2A-receptor that
`renders this receptor less sensitive to yohimbine bind-
`ing (34).
`
`GENE-TARGETED MICE LACKING INDIVIDUAL
`␣2-RECEPTOR SUBTYPES
`
`Several mouse lines have been established by gene
`targeting that do not express functional ␣2-adrenergic
`receptors (2, 35, 36). All of these mice developed appar-
`ently normally, although mice lacking ␣2B-adrenergic
`receptors were not born at the expected Mendelian
`ratios, indicating that this receptor may play a role
`during embryonic development (13, 35).
`In addition, a point mutation has been introduced
`into the ␣2A-receptor gene (␣2-D79N) to evaluate the
`physiological role of separate intracellular signaling
`pathways of this receptor in vivo (38). The D79N mu-
`tation substitutes asparagine for an aspartate residue
`at position 79, which is predicted to lie within the
`second transmembrane region of the ␣2A-receptor and
`is highly conserved among G protein-coupled receptors.
`In vitro, the ␣2A-D79N receptor has been shown to be
`deficient in coupling to K⫹ channel activation (76).
`However, in vivo this point mutation was found to be
`deficient in K⫹ current activation and Ca2⫹ channel
`inhibition (32). Surprisingly, the density of ␣2A-D79N
`receptors in the mouse brain was decreased to ⬃20% of
`the normal level (38). Thus, in most (but not all) func-
`tional tests, the ␣2A-D79N receptor had characteristics
`resembling a functional “knockout” of the ␣2A-receptor
`
`(40). One important exception was the observation that
`the presynaptic inhibitory function of the ␣2A-D79N
`receptor was normal or only slightly blunted in intact
`tissues (2). Most likely, the decreased expression of
`␣2A-D79N receptors in vivo rather than a selective
`defect in receptor signaling seems to be important for
`the “functional knockout.” At the presynaptic side, a
`high number of spare receptors is characteristic for
`␣2-receptor function, i.e., activation of very few ␣2-
`receptors results in maximal presynaptic inhibition of
`transmitter release (1). Thus the reduced number of
`presynaptic ␣2A-D79N receptors may still be sufficient
`for presynaptic control, whereas the decreased receptor
`density may compromise receptor signal transduction
`at other sites with a smaller receptor reserve.
`
`WHICH ␣2-RECEPTOR SUBTYPE IS THE
`PRESYNAPTIC REGULATOR?
`␣2-Adrenergic receptors were initially characterized
`as presynaptic receptors that serve as parts of a nega-
`tive feedback loop to regulate the release of norepi-
`nephrine (71). Soon it was shown that ␣2-receptors are
`not restricted to presynaptic locations but also have
`postsynaptic functions (Fig. 1A). With the use of an
`array of pharmacological antagonists, the ␣2A-receptor
`was predicted to be the major inhibitory presynaptic
`receptor regulating release of norepinephrine from
`sympathetic neurons as part of a feedback loop (82).
`However, in some tissues, the ␣2C-receptors were con-
`sidered to be in the inhibitory presynaptic receptor
`(55).
`With the genetic deletion of individual ␣2-receptor
`genes in mice, this classification of the presynaptic
`autoreceptor subtype was challenged. In mice lacking
`the ␣2A-subtype, presynaptic feedback regulation was
`severely impaired but not abolished, indicating that
`indeed the ␣2A-receptor is the major autoreceptor in
`sympathetic neurons (Fig. 1A) (2, 26). Most surpris-
`ingly, the ␣2C-receptor turned out to function as an
`additional presynaptic regulator in all central and pe-
`ripheral nervous tissues investigated (Fig. 1A) (2, 9, 26,
`70, 79, 80). However, the relative contributions of ␣2A-
`and ␣2C-receptors differed between central and periph-
`eral nerves, with the ␣2C-receptor being more promi-
`nent in sympathetic nerve endings than in central
`adrenergic neurons. ␣2A- and ␣2C-receptors differ in
`their time course of expression after birth (65). While
`␣2A-mediated autoinhibition of neurotransmitter re-
`lease is already operative immediately after birth, the
`␣2C-receptor function is established later in mice (65).
`Furthermore, the ␣2-autoreceptor subtypes could
`be distinguished functionally: ␣2A-receptors inhibited
`transmitter release significantly faster and at higher
`action potential frequencies than the ␣2C-receptors
`(Fig. 1B) (9, 26, 62). When ␣2A- and ␣2C-receptors were
`stably expressed together with N-type Ca2⫹ channels
`or with G protein-coupled inwardly rectifying K⫹
`(GIRK) channels, no differences in the activation ki-
`netics of these two receptor subtypes were detected at
`identical levels of receptor expression (10). However,
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`Fig. 1. Presynaptic ␣2-adrenergic receptor sub-
`types. A: in sympathetic or central adrenergic
`nerves, ␣2A- and ␣2C-receptors operate as inhib-
`itory autoreceptors to control neurotransmitter
`release. ␣2B-Receptors are located on postsynap-
`tic cells to mediate the effects of catecholamines
`released from sympathetic nerves, e.g., vasocon-
`striction. B: presynaptic ␣2A- and ␣2C-receptors
`can be distinguished functionally. In intact tis-
`sue slices from mouse heart atria, ␣2A-receptors
`inhibit norepinephrine release from sympathetic
`nerves primarily at high stimulation frequen-
`cies, whereas the ␣2C-receptor can also operate
`at very low frequencies to control basal norepi-
`nephrine release. WT, wild type. Data adapted
`from Ref. 26.
`
`when receptor GIRK channel deactivation after re-
`moval of norepinephrine was followed, the ␣2C-receptor
`was found to be active for a significantly longer time
`than the ␣2A-subtype irrespective of the level of recep-
`tor expression. This difference in ␣2-receptor deactiva-
`tion kinetics could be explained by the higher affinity of
`norepinephrine for the ␣2C- than for the ␣2A-receptor
`subtype (10). This property makes the ␣2C-receptor
`particularly suited to control neurotransmitter release
`at low action potential frequencies (Fig. 1) (26). In
`contrast, the ␣2A-receptor seems to operate primarily
`at high stimulation frequencies in sympathetic nerves
`and may thus be responsible for controlling norepi-
`nephrine release during maximal sympathetic activa-
`tion.
`␣2-Adrenergic receptors not only inhibit release of
`their own neurotransmitters (autoreceptors) but can
`
`also regulate the exocytosis of a number of other neu-
`rotransmitters in the central and peripheral nervous
`system. In the brain, ␣2A- and ␣2C-receptors can inhibit
`dopamine release in basal ganglia (9) as well as sero-
`tonin secretion in mouse hippocampal or brain cortex
`slices (61). In contrast, the inhibitory effect of ␣2-
`agonists on gastrointestinal motility was mediated
`solely by the ␣2A-subtype (63).
`Part of the functional differences between ␣2A- and
`␣2C-receptors may be explained by their distinct sub-
`cellular localization patterns (Fig. 2) (14, 47, 86, 87). In
`cultured sympathetic neurons from newborn mice,
`functional presynaptic ␣2-receptors develop to inhibit
`voltage-dependent Ca2⫹ channels and norepinephrine
`release (77, 78). In sympathetic neurons, only the ␣2A-
`subtype but not the ␣2C-receptor contributed to inhibi-
`tion of neurotransmitter release (81). Remarkably, in-
`
`Fig. 2. ␣2-Adrenergic receptors differ
`in their trafficking itineraries in cells.
`When expressed in rat1 fibroblasts,
`␣2A- and ␣2B-receptors are targeted to
`the plasma membrane (immunofluo-
`rescence images). On stimulation with
`agonist, only ␣2B-receptors are revers-
`ibly internalized into endosomes. ␣2C-
`Receptors are primarily localized in an
`intracellular membrane compartment,
`from where the ␣2C-receptors can be
`translocated to the cell surface after
`exposure to cold temperature (29). Bot-
`tom: murine ␣2-receptor subtypes after
`transient transfection into rat1 fibro-
`blasts as described previously (14). Ar-
`rows point to ␣2-receptors residing in
`the plasma membrane; arrowhead
`marks ␣2C-receptors in an intracellular
`compartment.
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`hibition of Ca2⫹ channels located on neuronal cell
`bodies and dendrites was mediated by both ␣2A- and
`␣2C-receptors. Thus ␣2C-receptors in neurons may re-
`quire a specific itinerary to guide their expression to
`axonal termini.
`
`BLOOD PRESSURE REGULATION
`␣2-Receptors are involved in the control of blood
`pressure homeostasis at a number of locations (Fig. 3).
`Nonselective activation of ␣2-receptors usually leads to
`a biphasic blood pressure response: after a short hy-
`pertensive phase that is more pronounced after rapid
`intravenous injection, arterial pressure falls below the
`baseline. After oral application of ␣2-agonists, the hy-
`potensive action prevails and is being used to treat
`elevated blood pressure in hypertensive patients. In-
`terestingly, the two phases of the pressure response
`are mediated by two different ␣2-receptor subtypes in
`vivo: ␣2B-receptors are responsible for the initial hy-
`pertensive phase, whereas the long-lasting hypoten-
`sion is mediated by ␣2A-receptors (2, 35, 38). Thus the
`␣2A-receptor is a therapeutic target for subtype-selec-
`tive antihypertensive agents. The blockade of ␣2-recep-
`tors may be of therapeutic benefit in patients with
`atherosclerotic coronary arteries (3), whereas it is still
`unknown which ␣2-receptor subtype is responsible for
`the vasoconstriction in humans. An insertion/deletion
`polymorphism with decreased receptor desensitization
`of the ␣2B-receptor subtype is associated with an in-
`creased risk for acute coronary events (69).
`Some evidence indicates that ␣2A-receptors also par-
`ticipate to a smaller degree in the vasoconstrictor ac-
`tion of ␣2-agonists in mice (38). Bolus injection of nor-
`epinephrine caused transient hypertension in wild-
`type mice and in ␣2B- and ␣2C-deficient mice but not in
`mice lacking the ␣2A-receptor (16). Vascular ␣2-recep-
`tor subtypes may be differentially distributed between
`vascular beds. When ␣2-agonists were injected into the
`carotid artery, most of the hypertensive response to
`␣2-activation was mediated by the ␣2B-receptor (35),
`whereas injection into the femoral artery showed a
`blunted hypertensive effect in mice with the ␣2A-D79N
`receptor (38). In some arteries, ␣2-mediated vasocon-
`
`striction may even predominate over ␣1-receptor-in-
`duced contraction, and decreased ␣-receptor respon-
`siveness may contribute to elevated blood flood in
`tissue inflammation, e.g., arthritis (45).
`In addition to its role as a vasoconstrictor, the ␣2B-
`receptor seems to be required for the development of
`salt-sensitive hypertension (Fig. 3) (22, 41–43). Ne-
`phrectomy followed by Na⫹ loading has been estab-
`lished as a model of hypertension in mice (22). In this
`system, the development of hypertension depends on
`increased vasopressin release and sympathetic activa-
`tion (21). Bilateral nephrectomy and saline infusion
`raised blood pressure in wild-type and in ␣2A- and
`␣2C-receptor-deficient mice. However, in ␣2B-deficient
`animals a small fall in arterial pressure was observed
`(41). Recent experiments with ␣2B-antisense oligonu-
`cleotide injection into the lateral brain ventricle sug-
`gest that a central ␣2B-adrenergic receptor is necessary
`for induction of salt-dependent hypertension (31).
`Under certain conditions, even the ␣2C-receptor sub-
`type may contribute to vascular regulation: when kept
`below 37°C for a while, cutaneous arteries of the mouse
`tail show an ␣2C-receptor-dependent vasoconstriction
`that could not be observed when the vessel segments
`were incubated at body temperature (12). This finding
`may be of great therapeutic interest for the treatment
`of Raynaud’s disease. Patients with Raynaud’s phe-
`nomenon suffer from severe periods of vasoconstriction
`of their fingers and toes that are usually triggered by
`exposure to cold. Treatment of these patients with
`␣2-adrenergic antagonists diminished the vasocon-
`striction (19). Interestingly, silent ␣2C-receptors may
`be translocated from an intracellular receptor pool to
`the cell surface on cooling (Fig. 2) (29). This phenome-
`non has been observed in human embryonic kidney
`(HEK-293) cells transfected with recombinant ␣2C-re-
`ceptors: cooling of cells to 28°C evoked a redistribution
`of ␣2C-receptors from the Golgi apparatus to the
`plasma membrane within 1 h (29). Thus inhibition of
`␣2C-receptors may prove an effective treatment for
`Raynaud’s phenomenon.
`In addition to these vascular and central neuronal
`mechanisms, renal ␣2-receptors may be involved in the
`
`Fig. 3. Integrative regulation of blood
`pressure by different ␣2-adrenergic re-
`ceptor subtypes. Activation of ␣2A-re-
`ceptors leads to a decrease in blood
`pressure by inhibiting central sympa-
`thetic outflow as well as norepineph-
`rine release from sympathetic nerves
`(2, 38). ␣2B-Receptors may counteract
`this effect by causing direct vasocon-
`striction and salt-induced hyperten-
`sion (22, 35). ␣2C-Receptors participate
`in ␣2-mediated vasoconstriction after
`exposure to cold temperature (12).
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`long-term regulation of blood pressure and fluid and
`electrolyte homeostasis (48, 49). Activation of renal
`vascular ␣2B-receptors may lead to an increase in med-
`ullary NO production and thus counteract the vasocon-
`strictor effects of norepinephrine in the renal medulla
`(90). Via this mechanism, ␣2B-receptors may be essen-
`tial in the regulation of renal medullary blood flow and
`oxygen supply.
`
`ANALGESIA
`␣2-Agonists are potent analgesics, and they can po-
`tentiate the analgesic effect of opioids (75, 85, 88).
`Recent data indicate that all three ␣2-receptor sub-
`types are involved in the regulation of pain perception
`in the mouse (Fig. 4).
`The ␣2A-receptor mediates the antinociception in-
`duced by systemically applied ␣2-agonists, including
`clonidine and dexmedetomidine (18, 74). Compared
`with control mice, ␣2-agonists were completely ineffec-
`tive as an antinociceptive agent in the tail immersion
`or substance P test in ␣2A-D79N mice (27). The ␣2A-
`D79N mutation also blocked the synergy seen in wild-
`type mice between ␣2-agonists and delta-opioid ago-
`nists (74). Interestingly, ␣2A-receptor-deficient mice
`showed a reduced antinociceptive effect to isoflurane
`(30). However, not all ␣2-receptor agonists required
`functional ␣2A-receptors for their antinociceptive effect
`(Fig. 4). The imidazoline/␣2-receptor ligand mox-
`onidine caused spinal antinociception that was at least
`partially dependent on ␣2C-receptors (17).
`Surprisingly, nitrous oxide, which is used as a potent
`inhalative analgesic during anesthesia, requires the
`␣2B-subtype for its antinociceptive effect (Fig. 4) (23,
`60). Supraspinal opioid receptors and spinal ␣2B-recep-
`tors are involved in the analgesic pathway for nitrous
`oxide. Activation of endorphin release in the periaque-
`ductal gray by nitrous oxide stimulates a descending
`noradrenergic pathway that releases norepinephrine
`
`onto ␣2B-receptors in the dorsal horn of the spinal cord
`(89). In mice lacking ␣2B-receptors, the analgesic effect
`of nitrous oxide was completely abolished (60).
`
`SEDATION
`␣2-Agonists are used in the postoperative phase or in
`intensive care as sedative, hypnotic, and analgesic
`agents (44, 66). The sedative effects of ␣2-agonists in
`mice are solely mediated by the ␣2A-receptor subtype
`(32). ␣2A-D79N mice showed no sedative response to
`the ␣2-agonist dexmedetomidine (32). In contrast, mice
`lacking the ␣2B- or␣ 2C-receptors did not differ in their
`sedative response from wild-type control mice (27, 59).
`Similarly, the anesthetic-sparing effect of ␣2-agonists
`was completely abolished in ␣2A-D79N mice (32).
`The hypnotic effect of ␣2-agonists is most likely me-
`diated in the locus ceruleus. Neurons of the locus
`ceruleus express ␣2A-adrenergic receptors at very high
`density (84). Furthermore, ␣2A-antisense oligonucleo-
`tide injection into the locus ceruleus in rats attenuated
`the sedative effects of exogenous ␣2-agonists (46).
`BEHAVIOR
`
`Because of their widespread distribution in the cen-
`tral nervous system, ␣2-receptors affect a number of
`behavioral functions (5, 56, 57, 67). In particular, the
`␣2C-receptor subtype has been demonstrated to inhibit
`the processing of sensory information in the central
`nervous system of the mouse (for a recent review, see
`Ref. 64). Activation of ␣2-receptors also resulted in
`locomotor inhibition. While direct activation of ␣2-re-
`ceptors by dexmedetomidine did not alter spontane-
`ous motor activity in ␣2C-receptor-deficient mice (59),
`D-amphetamine stimulated locomotor activity to a
`greater extent in ␣2C-deficient mice than in wild-type
`mice (58).
`Mice overexpressing ␣2C-receptors were impaired in
`spatial and nonspatial water maze tests, and an ␣2-
`
`Fig. 4. Three ␣2-adrenergic receptor
`subtypes are involved in the control of
`pain perception in mice. A: schematic
`representation of ␣2-receptor subtypes
`controlling spinal nociception. B: dis-
`tribution of ␣2-receptors in the mouse
`spinal cord by autoradiography with a
`non-subtype-selective ␣2-receptor an-
`tagonist (9). In the spinal cord, the
`highest density of ␣2-adrenergic recep-
`tors was observed in the superficial
`layers of the dorsal horns (B, arrows).
`Here, all 3 ␣2-receptor subtypes control
`incoming nociceptive impulses: ␣2A-re-
`ceptors are required for the analgesic
`effect of systemically applied ␣2-ago-
`nists, spinal ␣2C-receptors contribute
`to the moxonidine-mediated analgesia,
`and ␣2B-receptors are required for the
`spinal antinociceptive effect of nitrous
`oxide. See text for references. The au-
`toradiogram shown in B was kindly
`provided by K. Hadamek, Wu¨ rzburg,
`Germany.
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`antagonist fully reversed the water maze escape defect
`in these mice (4–6). The ␣2-agonist dexmedetomidine
`increased swimming distance more effectively in wild-
`type mice than in ␣2C-receptor-deficient mice (4). Acti-
`vation of ␣2C-receptors disrupts execution of spatial
`and nonspatial search patterns, whereas stimulation
`of ␣2A- and/or ␣2B-receptors may actually improve spa-
`tial working memory in mice (7). It may be concluded
`that novel agonists devoid of ␣2C-receptor affinity can
`modulate cognition more favorably than non-subtype-
`selective drugs.
`Altered startle reactivity and attenuation of the in-
`hibition of the startle reflex by an acoustic prepulse
`have been observed in schizophrenia, and disrupted
`prepulse inhibition has frequently been used as an
`animal model for drug antipsychotic drug develop-
`ment. Interestingly, ␣2C-receptor-deficient mice had
`enhanced startle responses, diminished prepulse inhi-
`bition, and shortened attack latency in the isolation-
`aggression test (57). Thus drugs acting via the ␣2C-
`receptor may have therapeutic value in disorders
`associated with enhanced startle responses and senso-
`rimotor gating deficits, such as schizophrenia, atten-
`tion deficit disorder, posttraumatic stress disorder, and
`drug withdrawal. In addition to the ␣2C-subtype, the
`␣2A-receptor has an important role in modulating be-
`havioral functions. Experiments using gene-targeted
`mice indicate that the ␣2A-receptor may play a protec-
`tive role in some forms of depression and anxiety, and
`this receptor may mediate part of the antidepressant
`effects of imipramine (67). Thus ␣2A- and ␣2C-receptors
`complement each other to integrate central nervous
`system function and behavior.
`
`OTHER PHYSIOLOGICAL FUNCTIONS AND
`PHARMACOLOGICAL TARGETS
`␣2-Receptors are involved in the regulation of body
`temperature as well as seizure threshold. Activation of
`central ␣2A-receptors causes a powerful antiepilepto-
`genic effect in mice (28). Two receptor subtypes, ␣2A
`and ␣2C, may be involved in the hypothermic action of
`␣2-agonists (27, 59). Another important function of
`␣2-agonists is their inhibitory effect on intraocular
`pressure. The ␣2-agonists apraclonidine and bri-
`monidine are currently being used to lower intraocular
`pressure in patients with glaucoma (52, 68). In adipose
`tissue, ␣2-receptors inhibit lipolysis (72, 73) and ␣2-
`receptors are potential targets for the treatment of
`obesity. Mice expressing human ␣2-receptors in fat
`tissue in vivo, in the absence of ␤3-adrenergic recep-
`tors, developed high-fat diet-induced obesity (83). How-
`ever, the precise role of individual ␣2-receptor subtypes
`in the control of lipolysis is unknown at present.
`
`CONCLUSIONS
`Genetic deletion of ␣2-adrenergic receptor subtype
`genes in mice has greatly enhanced our understanding
`of the physiological functions and therapeutic potential
`of individual ␣2-receptor subtypes. ␣2-Adrenergic re-
`ceptors are important regulators of sympathetic tone,
`
`neurotransmitter release, blood pressure, and intraoc-
`ular pressure. ␣2-Receptor activation causes sedation
`and potent analgesia. Further potential therapeutic
`functions may be unraveled with the help of mouse
`models with deleted ␣2-receptor genes. Before these
`genetic animal models were available, it was hypothe-
`function of ␣2-receptors
`sized that each biological
`would be mediated by one receptor subtype. Thus it
`was reasonable to assume that novel subtype-specific
`pharmacological agonists or antagonists would be of
`great therapeutic value because of their reduced poten-
`tial for ␣2-receptor-mediated side effects. However,
`with more and more studies of the ␣2-receptor physi-
`ology in gene-targeted mice being published, the situ-
`ation became more complicated than initially antici-
`pated. Indeed, only a few biological functions of ␣2-
`receptors were found to be mediated by one single
`␣2-adrenergic receptor subtype. Examples are the hy-
`potension or sedation caused by ␣2A-receptor activa-
`tion.
`For other ␣2-receptor-mediated functions, two differ-
`ent strategies seem to have emerged to regulate adren-
`ergic signal transduction: some biological functions are
`controlled by two counteracting ␣2-receptor subtypes,
`e.g., ␣2A-receptors decrease sympathetic outflow and
`blood pressure, whereas the ␣2B-subtype increases
`blood pressure by direct vasoconstriction. In contrast,
`the inhibitory presynaptic feedback loop that tightly
`regulates neurotransmitter release from adrenergic
`nerves requires two receptor subtypes, ␣2A and ␣2C,
`with similar but complementary effects. Similarly,
`pain perception is controlled at several levels of by one
`of the three ␣2-receptor subtypes.
`The fact that more than one receptor subtype may be
`involved in regulating one particular physiological
`function does not limit the therapeutic potential of
`novel subtype-selective drugs for ␣2-adrenergic recep-
`tors. However, it emphasizes that knowledge of the
`spectrum of in vivo biological effects is mandatory
`before making precise predictions about the in vivo
`effects of subtype-specific drugs. For treatment of hy-
`pertension, a selective ␣2A-receptor agonist without
`affinity for the ␣2B-receptor might be advantageous. As
`␣2B-receptors counteract the hypotensive effect of ␣2A-
`receptor activation, a selective ␣2A-agonist could be
`given at a lower dose to achieve similar blood pressure
`lowering with reduced sedative side effects. In addi-
`tion, a combination of agonistic and antagonistic prop-
`erties may become desirable, for instance, for antihy-
`pertensives, e.g., ␣2A-agonist and ␣2B-antagonist. The
`primary target for ␣2-mediated pain modulation would
`be the ␣2B-receptor. As illustrated by the potent anal-
`gesic effect of nitrous oxide, ␣2B-receptor activation
`might be a very promising analgesic strategy. Whether
`␣2C-receptors are equally effective in inhibiting pain
`pathways in the spinal cord has to be tested in future
`studies (17). The main advantage of ␣2B- or␣ 2C-recep-
`tor-specific agonists for antinociception would be their
`lack of sedative side effects compared with nonselective
`␣2-agonists that also stimulate ␣2A-receptors. In addi-
`tion, they would not cause respiratory depression and
`
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`addiction, which are two major problems associated
`with opioid therapy. In anesthesia and intensive care,
`the availability of pairs of subtype-selective agonists
`and antagonists might be of great benefit (66). ␣2A-
`Receptor-mediated sedation that can be rapidly re-
`versed by a selective ␣2-antagonist may be used in
`future human anesthesia (as it is already being used
`with non-subtype-selective agonists/antagonists in vet-
`erinary anesthesia). Finally, mice lacking all ␣2-recep-
`tor subtypes will also be essential tools to determine
`the function of imidazoline receptors and the potential
`of future imidazoline receptor drugs (24, 51).
`Further investigation of the specific function of ␣2-
`subtypes will greatly enhance our understanding of the
`relevance of closely related receptor proteins and point
`out novel therapeutic strategies for subtype-selective
`drug development.
`
`Our work has been supported by the Deutsche Forschungsgemein-
`schaft.
`
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