Gene targeting — homing in on α2-adrenoceptor-subtype function

Yohimbine inhibits firing activities of rat dorsal root ganglion neurons by blocking Na+ channels and vanilloid VR1 receptors

Yohimbine, an indole alkaloid, is a natural alpha(2)-adrenoceptor antagonist and is frequently used to assess the mechanism of a drug's effect on alpha-adrenoceptors. Recently, several studies showed that yohimbine exhibited analgesic effects in in vivo animal models. However, the underlying mechanism is not known. We investigated the effects of yohimbine on Na(+) channels and vanilloid VR1 receptors in dorsal root ganglion cells. We found that yohimbine inhibited tetrodotoxin-sensitive Na(+) channels (Na(V)1.2), the tetrodotoxin-resistant Na(+) channels, including both slow inactivating (Na(V)1.8) and persistent (Na(V)1.9) Na(+) channels, and capsaicin-sensitive vanilloid VR1 receptors. Action potential firing activities of dorsal root ganglion neurons evoked by current injection or capsaicin were eliminated by yohimbine. The blocking effects of yohimbine on nociceptive-related ion channels and firing activities of dorsal root ganglion neurons may underlie the ionic mechanism of yohimbine's analgesic effects observed in in vivo studies.

Identification of duplicated fourth alpha2-adrenergic receptor subtype by cloning and mapping of five receptor genes in zebrafish

The alpha(2)-adrenergic receptors (alpha(2)-ARs) belong to the large family of rhodopsinlike G-protein-coupled receptors that share a common structure of seven transmembrane (TM) alpha-helices. The aims of this study were (1) to determine the number of alpha(2)-AR genes in a teleost fish, the zebrafish (Danio rerio), (2) to study the gene duplication events that generated the alpha(2)-AR subtypes, and (3) to study changes in receptor structure that have occurred since the divergence of the mammalian and fish lineages. Here, we report the cloning and chromosomal mapping of fish orthologs for all three mammalian alpha(2)-ARs. In addition, we identified a fourth alpha(2)-AR subtype with two duplicates in zebrafish. Chromosomal mapping showed that the zebrafish alpha(2)-AR genes are located within conserved chromosomal segments, consistent with the origin of the four alpha(2)-AR subtypes by two rounds of chromosome or block duplication before the divergence of the ray fin fish and tetrapod lineages. Thus, the fourth subtype has apparently been present in the common ancestor of vertebrates but has been deleted or is yet to be identified in mammals. The overall percentage identity between the fish and mammalian orthologs is 53% to 67%, and in the TM regions 80% to 87%. These values are clearly lower than what is observed between mammalian orthologs. Still, all of the residues thought to be important for alpha(2)-adrenergic ligand binding are conserved across species and subtypes, and even the most divergent regions of the fish receptors show clear "molecular fingerprints" typical for orthologs of a given subtype.

Regulation of alpha(2)-adrenoceptors in human vascular smooth muscle cells

This study analyzed the regulation of alpha2-adrenoceptors (alpha2-ARs) in human vascular smooth muscle cells (VSMs). Saphenous veins and dermal arterioles or VSMs cultured from them expressed high levels of alpha2-ARs (alpha2C > alpha2A, via RNase protection assay) and responded to alpha2-AR stimulation [5-bromo-N-(4,5-dihydro-1H-imidazol-2-yl)-6-quinoxalinamine (UK-14,304, 1 microM)] with constriction or calcium mobilization. In contrast, VSMs cultured from aorta did not express alpha2-ARs and neither cultured cells nor intact aorta responded to UK-14,304. Although alpha2-ARs (alpha2C >> alpha2A) were detected in aortas, alpha2C-ARs were localized by immunohistochemistry to VSMs of adventitial arterioles and not aortic media. In contrast with aortas, aortic arterioles constricted in response to alpha2-AR stimulation. Reporter constructs demonstrated higher activities for alpha2A- and alpha2C-AR gene promoters in arteriolar compared with aortic VSMs. In arteriolar VSMs, serum increased expression of alpha2C-AR mRNA and protein but decreased expression of alpha2A-ARs. Serum induction of alpha2C-ARs was reduced by inhibition of p38 mitogen-activated protein kinase (MAPK) with 2 microM SB-202190 or dominant-negative p38 MAPK. UK-14,304 (1 microM) caused calcium mobilization in control and serum-stimulated cells: in control VSMs, the response was inhibited by the alpha2A-AR antagonist BRL-44408 (100 nM) but not by the alpha2C-AR antagonist MK-912 (1 nM), whereas after serum stimulation, MK-912 (1 nM) but not BRL-44408 (100 nM) inhibited the response. These results demonstrate site-specific expression of alpha2-ARs in human VSMs that reflects differential activity of alpha2-AR gene promoters; namely, high expression and function in venous and arteriolar VSMs but no detectable expression or function in aortic VSMs. We found that alpha2C-ARs can be dramatically and selectively induced via a p38 MAPK-dependent pathway. Therefore, altered expression of alpha2C-ARs may contribute to pathological changes in vascular function.

α-2 agonists induce amnesia through activation of the Gi-protein signalling pathway

The post-receptorial mechanism of the amnesic action of the alpha2-agonists clonidine and guanabenz was investigated in the mouse passive avoidance test. Animals were i.c.v. injected with pertussis toxin (PTX) or with antisense oligonucleotides, complementary to the sequence of the alpha-subunit mRNA of Gi1, Gi2, Gi3, Go1 and Go2 proteins. The administration of PTX (0.25 microg per mouse i.c.v.) reversed the amnesia induced by both alpha2-agonists. Similarly, anti-Gialpha1 (6.25-12.5 microg per mouse i.c.v.), anti-Gialpha3 (3.12-12.5 microg per mouse i.c.v.), anti-Goalpha1 (12.5-25 microg per mouse i.c.v.) antagonised the detrimental effect induced by clonidine and guanabenz. By contrast, pretreatment with anti-Gialpha2 (3.12-25 microg per mouse i.c.v.) and anti-Goalpha2 (12.5-25 microg per mouse i.c.v.) never modified the impairment of memory processes induced by the alpha2-agonists. At the highest effective doses, none of the compounds used impaired motor coordination (rota rod test), nor modified spontaneous motility and inspection activity, (hole board test). These results indicate the involvement of Gi1, Gi3, and Go1, but not Gi2 and Go2, protein subtypes in the transduction mechanism responsible for the induction of amnesia by clonidine and guanabenz.

Functional expression and direct visualization of the human α2B-adrenergic receptor and α2B-AR-green fluorescent fusion protein in mammalian cell using Semliki Forest virus vectors

The alpha 2B -adrenergic receptor ( alpha 2B -AR), a member of the G protein-coupled receptor (GPCR) superfamily, was expressed at high levels from Semliki Forest virus (SFV) vectors in mammalian cells. Constructs were engineered by fusing enhanced green fluorescent protein (eGFP) and the SFV capsid to opposite ends of the alpha 2B -AR. The receptor fusions alpha 2B -AR-eGFP and CAP- alpha 2B -AR expressed in CHO-K1 cells generated alpha 2B values of 176 and 122pmol/mg of membrane protein, respectively, and showed similar ligand binding characteristics, alpha 2B -AR subtype-selectivity, and G protein activation as reported for stable expression in CHO-K1 cells. Cryo-electron microscopy and eGFP-based fluorescence indicated the same subcellular receptor distribution. SFV expression is well suited for studies on the pharmacology, biochemistry, and cell biology of GPCRs, and for large-scale recombinant protein production in mammalian suspension culture to generate sufficient receptor quantities for structural biology.

alpha 2B-Adrenoceptor levels govern agonist and inverse agonist responses in PC12 cells

Receptor density is an important determinant of cellular effector responses to receptor activation. We analysed cytosolic Ca(2+) responses to alpha(2)-adrenergic agents in PC12 cells expressing human alpha(2B)-adrenergic receptors (AR) at two densities (3.8 and 1.3 pmol/mg protein). The efficacy (E(max)) of agonists was greater in cells with higher receptor expression; while the potency (EC(50)) of norepinephrine and oxymetazoline was independent of alpha(2B)-AR levels. Several classical alpha(2)-AR antagonists behaved as either partial or inverse agonists in a receptor density-dependent fashion. No apparent structural similarities were found among the inverse agonists, precluding simple predictions of inverse agonist activity. Transfected PC12 cells expressing alpha(2B)-AR at relatively high density would be a useful approach to screen inverse agonists for this class of receptors. Our results further indicate that receptor density significantly influences the properties of ligands, not only of partial agonists as predicted by classical receptor theory, but also of antagonists and full agonists.

Regional distribution of alpha(2C)-adrenoceptors in brain and spinal cord of control mice and transgenic mice overexpressing the alpha(2C)-subtype: an autoradiographic study with [(3)H]RX821002 and [(3)H]rauwolscine

Behavioral studies on gene-manipulated mice have started to elucidate the neurobiological functions of the alpha(2C)-adrenoceptor (AR) subtype. In this study, we applied quantitative receptor autoradiography to investigate the potential anatomical correlates of the observed functional effects of altered alpha(2C)-AR expression. Labeling of brain and spinal cord sections with the subtype non-selective alpha(2)-AR radioligand [(3)H]RX821002 and the alpha(2C)-AR-preferring ligand [(3)H]rauwolscine revealed distinct binding-site distribution patterns. In control mice, [(3)H]rauwolscine binding was most abundant in the olfactory tubercle, accumbens and caudate putamen nuclei, and in the CA1 field of the hippocampus. A mouse strain with overexpression of alpha(2C)-AR regulated by a gene-specific promoter showed approximately two- to four-fold increased levels of [(3)H]rauwolscine binding in these regions. In addition, dramatic increases in [(3)H]rauwolscine binding were seen in the nerve layer of the olfactory bulb, the molecular layer of the cerebellum, and the ventricular system of alpha(2C)-AR-overexpressing mice, representing "ectopic" alpha(2C)-AR expression. Competition-binding experiments with several alpha(2)-AR ligands confirmed the alpha(2C)-AR identity of these sites. Our results provide quantitative evidence of the predominance of the alpha(2A)-AR subtype in most regions of the mouse CNS, but also disclose the wide distribution of alpha(2C)-AR in the normal mouse brain, although at relatively low density, except in the ventral and dorsal striatum and the hippocampal CA1 area. alpha(2C)-AR are thus present in brain regions involved in the processing of sensory information and in the control of motor and emotion-related activities such as the accumbens and caudate putamen nuclei, the olfactory tubercle, the lateral septum, the hippocampus, the amygdala, and the frontal and somatosensory cortices. The current results may help in specifying an anatomical framework for the functional roles of the alpha(2A)- and alpha(2C)-AR subtypes in the mouse CNS.

Alpha2-adrenergic drug effects on brain monoamines, locomotion, and body temperature are largely abolished in mice lacking the alpha2A-adrenoceptor subtype

alpha(2)-ARs regulate brain monoaminergic function by inhibiting neuronal firing and release of monoamine neurotransmitters, noradrenaline (NA), serotonin (5-HT) and dopamine (DA). Both alpha(2A)- and alpha(2C)-AR inhibit monoamine release in vitro in brain slices, but the in vivo roles of individual alpha(2)-AR subtypes in modulating monoamine metabolism have not been characterised. Metabolism of brain monoamine neurotransmitters, locomotor activity and body temperature were investigated in mice with targeted inactivation of the gene encoding alpha(2A)-AR (alpha(2A)-knockout, alpha(2A)-KO) and wild-type (WT) mice after treatment with the alpha(2)-AR agonist dexmedetomidine and the antagonist atipamezole. Dexmedetomidine caused profound hypothermia (up to 14.7 degrees C mean reduction in rectal temperature) and locomotor inhibition in WT mice, and inhibited the turnover of NA, 5-HT and DA, but increased NA turnover in alpha(2A)-KO mice. alpha(2)-AR agonist-induced hypothermia and locomotor inhibition were attenuated, but not totally abolished, in alpha(2A)-KO mice. These results suggest that alpha(2A)-ARs are principally responsible for the alpha(2)-AR mediated inhibition of brain monoamine metabolism, but other alpha(2)-ARs, possibly alpha(2C)-ARs, are also involved, especially in the striatum. However, secondary effects of the physiological alterations caused by drug administration, especially hypothermia, may have contributed to the observed neurochemical changes in WT mice.

The alpha2A-adrenoceptor subtype is not involved in inflammatory hyperalgesia or morphine-induced antinociception

The purpose of the present study was to investigate the role of the alpha(2A)-adrenoceptor subtype in inflammatory hyperalgesia, and in adrenergic-mu-opioid interactions in acute pain and inflammatory hyperalgesia. Behavioral responses to mechanical and thermal stimuli were studied in alpha(2A)-adrenoceptor knockout mice and their wild-type controls. Thermal nociception was evaluated as paw withdrawal latencies to radiant heat applied to the hindpaws. Mechanical nociception was measured using von Frey monofilament applications to the hindpaws. Mechanical and thermal hyperalgesia, induced with intraplantar carrageenan (1 mg/40 microl) were compared in alpha(2A)-adrenoceptor knockout and wild-type mice. The effects of the systemically administered mu-opioid receptor agonist morphine (1-10 mg/kg) were evaluated on mechanical withdrawal responses under normal and inflammatory conditions in knockout and wild-type mice. Withdrawal responses to radiant heat and von Frey monofilaments were similar in alpha(2A)-adrenoceptor knockout and wild-type mice before and after the carrageenan-induced hindpaw inflammation. Also, the antinociceptive effects of morphine in mechanical nociceptive tests were similar before and after carrageenan-induced hindpaw inflammation. Our observations indicate that alpha(2A)-adrenoceptors are not tonically involved in the modulation of inflammation-induced mechanical and thermal hyperalgesia. In addition, alpha(2A)-adrenoceptors do not appear to play an important role in mu-opioid receptor-mediated antinociception or antihyperalgesia.

Central alpha2B-adrenergic receptor antisense in plasmid vector prolongs reversal of salt-dependent hypertension

OBJECTIVE

Previous studies have shown that a fully functional alpha(2B)-adrenergic receptor (AR) is necessary for the development of salt-induced hypertension. The current studies were designed to explore the effect of prolonged inhibition of central alpha(2B)-AR gene expression by antisense (AS) DNA on this hypertension.

METHODS

We developed a plasmid vector driven by a cytomegalovirus promoter, containing a green fluorescent protein reporter gene and AS for rat alpha(2B)-AR protein. Subtotally nephrectomized, salt-loaded hypertensive rats received intracerebroventricular injection of 500 microg of either the AS plasmid (n = 9) or sense plasmid (containing cDNA for alpha(2B)-AR), as control (n = 7).

RESULTS

The AS injection produced a fall in SBP from 201 +/- 4 to 171 +/- 5 mmHg within 12 h. The level of BP in the 3 days post-injection was 174 +/- 6, 181 +/- 4 and 184 +/- 6 mmHg on day 1, day 2 and day 3, respectively (P < 0.05), and returned gradually towards baseline in subsequent days, although it remained significantly lower for the 8 days of observation. The control sense plasmid injections produced no significant changes in blood pressure (BP). Neither group had histological evidence of neural tissue disruption.

CONCLUSIONS

These results indicate that protracted translational inhibition of the alpha(2B)-AR gene in the central nervous system can be obtained by AS DNA delivered via plasmid vector and lead to decreased generation of alpha(2B)-AR protein, which can partly reverse salt-induced hypertension for several days.

The vasculopathy of Raynaud's phenomenon and scleroderma

The scleroderma (SSc) disease process involves dramatic dysfunction in acute and chronic vascular regulatory mechanisms; it presents initially with heightened vasoconstrictor or vasospastic activity and progresses to structural derangement or vasculopathy of the microcirculation. This article discusses the regulatory mechanisms that contribute to this dysfunction and the vascular changes in the context of the other aspects of the SSc disease process in a novel attempt to integrate the individual pathologies of the disease process.

Alpha2-adrenoceptor activation increases a cationic conductance and spontaneous GABAergic synaptic activity in dopaminergic neurones of the rat substantia nigra

Noradrenaline (NA) plays an important role in compensating for the loss in dopaminergic (DA) function following lesions of the DA neurones of the substantia nigra (SN). Alpha2-adrenoceptors are largely expressed in these neurones, but the cellular response to their activation is unknown. Whole-cell patch-clamp recordings were made from DA neurones of rat SN. At a holding potential of -60 mV, bath application of NA (50 microM) induced an inward current (-20.3+/-10.0 pA) in 50% of the recorded neurones. This effect was mimicked by UK-14304 (50 microM), a specific alpha2-adrenoceptor agonist, whereas alpha1-adrenoceptor and beta-adrenoceptor agonists failed to induce a response. Surprisingly, alpha2-adrenoceptor antagonists (idazoxan, RX-811059, SKF-86466 and yohimbine) also induced an inward current that could occlude the one induced by UK-14304, suggesting that they may act as alpha2-adrenoceptor agonists. The inward current results from an increase in cationic conductance identical to the one previously described in these neurones, as neurotensin (1 microM), known to activate it, occluded the inward current induced by UK-14304. In addition, GABAergic miniature inhibitory postsynaptic current frequency was increased by activation of presynaptic alpha2-adrenoceptors. We conclude that the effects of NA on alpha2-adrenoceptors can contribute to the previously described composite action of NA on DA neurone firing and can be pharmacologically differentiated from the effect of NA on DA and neighbouring neurones known to be mediated through alpha1-adrenoceptors.

Yohimbine dimers exhibiting selectivity for the human alpha 2C-adrenoceptor subtype

Yohimbine is a potent and selective alpha2- versus alpha1-adrenoceptor antagonist. To date, drugs with high specificity for the alpha2-adrenoceptor show marginal selectivity among the three alpha2-adrenoceptor subtypes. Initial studies showed that yohimbine was about 4- and 15-fold more selective for the human alpha2C-adrenoceptor in comparison with the alpha2A- and alpha2B-adrenoceptors, respectively. To improve on this alpha2-adrenoceptor subtype selectivity, a series of yohimbine dimers (varying from n = 2 to 24 spacer atoms) were prepared and evaluated for receptor binding on human alpha2-adrenoceptor subtypes expressed in Chinese hamster ovary cells. Each dimeric analog showed higher affinities for alpha2A- and alpha2C-adrenoceptor versus the alpha2B-adrenoceptor; and yohimbine dimers with spacers of n = 2, 3, 4, 18, and 24 exhibited selectivity for the alpha2C-adrenoceptor. The yohimbine dimers n = 3 and n = 24 showed the highest potency and selectivity (32- and 82-fold. respectively) for the alpha2C-adrenoceptor in receptor binding and in functional studies (42- and 29-fold, respectively) measuring cAMP changes using a cell-based luciferase reporter gene assay. The dimers (n = 3 and n = 24) had high selectivity (>1000-fold) for the alpha2C-adrenoceptor compared with the three alpha1-adrenoceptor subtypes. These findings demonstrate that the addition of spacer linkages to bivalent yohimbine molecules provides a successful approach to the development of ligands that are potent and highly selective for the alpha2C-adrenoceptor.

Ligand modulation of [35S]GTPgammaS binding at human alpha(2A), alpha(2B) and alpha(2C) adrenoceptors

Affinities and efficacies of chemically diverse ligands--some of them used as clinical agents--were examined, employing [3H]RX821,002 and [35S]GTPgammaS binding assays, respectively, at human (h) cloned, halpha(2A), halpha(2B) and halpha(2C) adrenoceptors (AR) expressed in Chinese hamster ovary (CHO) cells. As compared to noradrenaline (NA, efficacy defined as 100%), the majority of the 13 agonists tested generally behaved as partial agonists. Amongst 18 antagonists, pK(B) and pK(i) values, which were highly correlated for each alpha(2)-AR subtype, failed to reveal any strikingly selective agents. Inverse agonist properties were not detected for any antagonist, consistent with a lack of constitutive activity suggested by the monophasic inhibition of [35S]GTPgammaS binding by GTPgammaS. These data should facilitate interpretation of experimental and clinical actions of adrenergic agonists. Moreover, they emphasize the continuing need for alpha(2)-AR subtype-selective antagonists in order to define further the roles and therapeutic relevance of halpha(2A)-, halpha(2B)-, and halpha(2C)-AR.

Behavioral and neurochemical characterization of alpha(2A)-adrenergic receptor knockout mice

Genetic manipulation of mice now provides new tools to evaluate the biological functions of the alpha(2)-adrenergic receptor (alpha(2)-AR) subtypes (alpha(2A), alpha(2B), and alpha(2C)). To investigate the role of the alpha(2A)-AR in the modulation of mouse primary behavioral characteristics and brain neurochemistry, mice with targeted inactivation of the gene for the alpha(2A)-AR were compared with wild-type C57BL/6 control animals. First, a comprehensive behavioral screen was employed to provide a detailed characterization of basic neurologic functions. Thereafter, the mice were analyzed in three models of anxiety, i.e. the elevated-plus maze test, the marble burying test and the open field test. The diurnal activity pattern of the mice was assessed in a 24-h locomotor activity test. Furthermore, receptor autoradiography of the brain was performed using the subtype-non-selective alpha(2)-AR antagonist radioligand [(3)H]RS-79948-197. Lack of the alpha(2A)-AR was associated with alterations in autonomic functions, including increased heart rate and piloerection. The mutant mice also exhibited impaired motor coordination skills, increased anxiety-like behavior and an abnormal diurnal activity pattern. In addition, neurochemical analysis of monoamine neurotransmitters revealed a considerable increase in brain norepinephrine turnover in mice lacking alpha(2A)-AR. Our results provide further support for the crucial role of the alpha(2A)-AR in modulating brain noradrenergic neurotransmission and many aspects of mouse behavior and physiology.

Regulated expression of ?2B adrenoceptor during development

There are three subtypes of alpha2 adrenoceptor, i.e., alpha2A, alpha2B, and alpha2C, mediating the specific effect of epinephrine and norepinephrine in various tissues by means of G protein-coupled signal transduction pathways. In an attempt to delineate the regulatory mechanism of the alpha2B receptor subtype (encoded by subtype gene Adra2b) expression in the central nervous system (CNS), we have established transgenic (Tg) mice lines in which the transgene (NN-lacZ) was composed of the promoter region of Adra2b (NcoI fragment, 4.7 kb immediately upstream from receptor coding region) and a reporter gene lacZ (encoding beta-galactosidase). The selective expression of alpha2B in brain as indexed by beta-galactosidase, under the direction of this promoter region, may be traced in situ by using X-gal staining. The expression pattern of Adra2b-NN-lacZ in CNS of Tg mice during development was examined. The temporal course of examination was from gestation day 9.5 (E9.5) to postnatal day 28 (P28). Significant X-gal staining was detected in the dorsal root ganglion and cranial nerves V and VII at E12.5. By E18.5, expression was noted in the cerebral cortex, anterior olfactory nucleus, hypothalamus, brainstem, and cerebellar Purkinje cells, among others, and persisted through postnatal development. Adra2b-NN-directed reporter expression was detected in the hippocampal dentate gyrus first at P4. The temporal course of expression up to P28 in this area is in accordance with the developmental profiles of granule neurons of dentate gyrus. From P7 on, transgene expression was detected in additional brain areas, including the septum and thalamus. The expression correlates well with the noradrenergic innervations as evidenced by colocalization by using tyrosine hydroxylase or dopamine-beta-hydroxylase immunocytochemistry.

Norepinephrine transporter and alpha(2c) adrenoceptor allelic variants and personality factors

It has been suggested that reward dependence, as measured by the Tridimensional Personality Questionnaire (TPQ), is related to central noradrenergic activity, a proposition supported by two studies of urinary norepinephrine metabolite. In the current investigation, 190 normal young Han Chinese were examined, with genetic polymorphisms determined for the norepinephrine transporter (1287G/A) and the alpha(2c)-adrenoceptor (Del322-325) to test the association with TPQ personality traits. No significant association was demonstrated for these two polymorphisms and any of the TPQ personality-factor scores, including reward dependence and its subscales. Our negative findings suggest that the investigated polymorphisms of the norepinephrine transporter and the alpha(2c) adrenoceptor do not play a major role in the reward-dependence personality trait as assessed by TPQ.

Gender differences in hypothalamic tyrosine hydroxylase and alpha(2)-adrenoceptor subtype gene expression in cafeteria diet-induced hypertension and consequences of neonatal androgenization

This study investigated the incidence of cafeteria-diet induced hypertension on hypothalamic tyrosine hydroxylase (TH) and alpha(2)-adrenoceptor subtype gene expression in male, female, and neonatally testosterone-imprinted female rats. After 10 weeks of cafeteria diet, all these rats were hyperleptinemic. In contrast, males and testosterone-treated females developed hypertension, whereas intact females remained normotensive. In these rats, cafeteria diet up-regulated TH gene expression only in males and testosterone-treated females. On the other hand, cafeteria diet differentially affected hypothalamic gene expression of alpha(2)-adrenoceptor subtypes. In fact, this diet increased alpha(2A)-adrenoceptor mRNA levels only in intact normotensive females. In contrast, gene expression of the alpha(2B)-adrenoceptor was up-regulated only in male and testosterone-treated female cafeteria-fed rats. Furthermore, an alpha(2C)-adrenoceptor gene over-expression was also induced, but only in male cafeteria-fed rats. If one assumes that the up-regulations in TH and alpha(2B)-adrenoceptor gene expression are indicative of increased sympathetic nervous activity, then, these altered gene expressions could be responsible for the maintenance of high blood pressure in male and testosterone-treated female cafeteria-fed rats. Conversely, in intact females, the absence of these over-expressions and the up-regulation of the alpha(2A)-adrenoceptor gene expression could reflect an adaptive response to the diet and, consequently, could be protective against cafeteria diet-induced hypertension. Moreover, neonatal testosterone imprinting in females could have induced an irreversible android susceptibility to the cafeteria diet, leading to the onset of hypertension.

Physiological significance of alpha(2)-adrenergic receptor subtype diversity: one receptor is not enough

Alpha(2)-adrenergic receptors mediate part of the diverse biological effects of the endogenous catecholamines epinephrine and norepinephrine. Three distinct subtypes of alpha(2)-adrenergic receptors, alpha(2A), alpha(2B), alpha(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 alpha(2)-receptor subtypes have added important 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 alpha(2)-receptor subtypes, e.g., alpha(2A)-receptors decrease sympathetic outflow and blood pressure, whereas the alpha(2B)-subtype increases blood pressure. Other biological functions are regulated by synergistic alpha(2)-receptor subtypes. The inhibitory presynaptic feedback loop that tightly regulates neurotransmitter release from adrenergic nerves also requires two receptor subtypes, alpha(2A) and alpha(2C). Similarly, nociception is controlled at several levels by one of the three alpha(2)-receptor subtypes. Further investigation of the specific function of alpha(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.

Alpha-2 adrenoceptor activity affects propofol-induced sleep time

alpha(2) Adrenoceptor activity is involved in the mechanism of anesthesia. Clonidine, a alpha(2) adrenoceptor agonist, and yohimbine, a alpha(2) adrenoceptor antagonist, increase and decrease barbiturate-induced sleep times. In this study, we examined the effects of these drugs on propofol-induced sleep time. One-hundred-eighteen male Wistar rats weighing 320-400 g were used. Rats received saline, yohimbine (1, 0.1, or 0 mg/kg), or clonidine (300, 30, 3, or 0 microg/kg) intraperitoneally followed by 60 mg/kg of propofol in various combinations. In two series of experiments, either sleep time or prefrontal cortex norepinephrine release (microdialysis) was measured. One milligram/kilogram of yohimbine decreased propofol-induced sleep time to approximately 70% of control, and this was accompanied by an increase in perfusate norepinephrine of approximately 240% of control. Clonidine increased sleep time approximately 260% (300 microg/kg) and approximately 170% (30 microg/kg), and this was accompanied by a decrease (approximately 60% in both doses) in perfusate norepinephrine. In the present study, we show that the alpha(2) antagonist, yohimbine, decreased and the alpha(2) agonist, clonidine, increased propofol-induced sleep times. These changes were essentially mirrored in both groups by changes in norepinephrine release in the prefrontal cortex.

IMPLICATIONS

Central alpha(2) adrenoceptor is thought to be involved in several IV anesthetics-induced sleep. In this study, activation of the receptor increased the propofol-induced sleep time, whereas its inhibition decreased the sleep time. The results provide further evidence that the alpha(2) receptor is a good tool to elucidate the mechanism of anesthetics-induced sleep.

Alpha-2 adrenergic agonists as analgesics in horses

Administration of alpha-2 agonists to horses produces a variety of behavioral effects (sedation, somnolence, analgesia), and physiological effects. One of the most significant beneficial effects of administering alpha-2 agonists is the degree of analgesia they provide. Alpha-2 agonists have been the mainstay of analgesia for colic pain in horses since their introduction to clinical veterinary medicine. The increased potency of the more recently introduced alpha-2 agonists allows the provision of analgesia for conditions not previously relieved by other drugs. Unfortunately, there are significant side effects associated with alpha-2 agonist administration. Studies are underway to identify the physiologic effects associated with the stimulation of each alpha-2 receptor subtype, in hopes of developing subtype-specific alpha-2 agonists and antagonists.

Evidence for synergy between alpha(2)-adrenergic and nonadrenergic mechanisms in central blood pressure regulation

BACKGROUND

Both alpha(2)-adrenergic and non--alpha(2)-adrenergic mechanisms seem to be involved in the hypotensive effect of imidazoline-like drugs. This study aimed at investigating how these 2 mechanisms work together to modify blood pressure (BP).

METHODS AND RESULTS

LNP 509, which appeared in this study to be devoid of alpha(2A)-adrenergic activity, was administered to anesthetized rabbits and wild-type (WT) mice into the cisterna magna and into the fourth ventricle, respectively. Mean arterial pressure decreased by a maximum of 46 +/- 4% and 16 +/- 2%, respectively. In D79N mice, which lack functional alpha(2A)-adrenergic receptors, LNP 509 also reduced mean arterial pressure by 17 +/- 2%. The hypotension induced by LNP 509 (100 microg/kg intracisternally) was prevented by S23757 (1 mg/kg intracisternally), an antagonist highly selective for I(1)-imidazoline binding sites (I(1)BS). A synergy between LNP 509 and the alpha(2)-adrenergic agonist alpha-methylnoradrenaline (alpha-MNA) was observed in rabbits (cisterna magna injection) and in WT mice (fourth ventricle injection) but not, as expected, in D79N mice. Similar to LNP 509 alone, rilmenidine (fourth ventricle injection), which binds both to alpha(2)-adrenergic receptors and to I(1)BS, decreased BP in D79N mice. In WT animals, rilmenidine had a significantly greater effect. Microinjections performed in rabbits showed that the synergism occurred at least in part in the nucleus reticularis lateralis of the brain stem.

CONCLUSIONS

These results demonstrate that a central imidazoline-sensitive, but non--alpha(2)-adrenergic, mechanism can modify BP by itself. This mechanism, which may involve I(1)BS, interacts synergistically with an alpha(2)-adrenergic mechanism to decrease BP.

alpha(2C)-Adrenergic receptors mediate spinal analgesia and adrenergic-opioid synergy

The alpha(2A)-adrenergic receptor (AR) subtype mediates antinociception induced by the alpha(2)AR agonists clonidine, dexmedetomidine, norepinephrine, and 5-bromo-N-(4,5-dihydro-1H-imidazol-2-yl)-6-quinoxalinamine (UK-14,304) as well as antinociceptive synergy of UK-14,304 with opioid agonists [D-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin and deltorphin II. Differential localization of alpha(2)-adrenergic (alpha(2A)-, alpha(2B)-(,) alpha(2C)-) and opioid (mu-, delta-, kappa-) subtypes suggests differential involvement of subtype pairs in opioid-adrenergic analgesic synergy. The present study applies a novel imidazoline(1)/alpha(2)-adrenergic receptor analgesic, moxonidine, to test for involvement of alpha(2B)- and alpha(2C)ARs in antinociception and antinociceptive synergy, because spinal antinociceptive activity of moxonidine shows minimal dependence on alpha(2A)AR. Intrathecal administration of moxonidine produced similar (2-3-fold) decreases in both mutant mice with a functional knockout of alpha(2A)AR (D79N-alpha(2A)AR) and alpha(2C)AR knockout (KO) mice. The potency of moxonidine was not altered in alpha(2B)KO mice, indicating that this subtype does not participate in moxonidine-induced spinal antinociception. Moxonidine-mediated antinociception was dose dependently inhibited by the selective alpha(2)-receptor antagonist SK&F 86466 in both D79N-alpha(2A) mice and alpha(2C)KO mice, indicating that alpha(2)AR activation is required in the absence of either alpha(2A)- or alpha(2C)AR. Spinal administration of antisense oligodeoxynucleotides directed against the alpha(2C)AR decreased both alpha(2C)AR immunoreactivity and the antinociceptive potency of moxonidine. Isobolographic analysis demonstrates that moxonidine-deltorphin antinociceptive synergy is present in the D79N-alpha(2A) mice but not in the alpha(2C)AR-KO mice. These results confirm that the alpha(2C)AR subtype contributes to spinal antinociception and synergy with opioids.

Activation and deactivation kinetics of alpha 2A- and alpha 2C-adrenergic receptor-activated G protein-activated inwardly rectifying K+ channel currents

Although G protein-coupled receptor-mediated signaling is one of the best studied biological events, little is known about the kinetics of these processes in intact cells. Experiments with neurons from alpha(2A)-adrenergic receptor knockout mice suggested that the alpha(2A)-receptor subtype inhibits neurotransmitter release with higher speed and at higher action potential frequencies than the alpha(2C)-adrenergic receptor. Here we investigated whether these functional differences between presynaptic alpha(2)-adrenergic receptor subtypes are the result of distinct signal transduction kinetics of these two receptors and their coupling to G proteins. alpha(2A)- and alpha(2C)-receptors were stably expressed in HEK293 cells at moderate ( approximately 2 pmol/mg) or high (17-24 pmol/mg) levels. Activation of G protein-activated inwardly rectifying K(+) (GIRK) channels was similar in extent and kinetics for alpha(2A)- and alpha(2C)-receptors at both expression levels. However, the two receptors differed significantly in their deactivation kinetics after removal of the agonist norepinephrine. alpha(2C)-Receptor-activated GIRK currents returned much more slowly to base line than did alpha(2A)-stimulated currents. This observation correlated with a higher affinity of norepinephrine at the murine alpha(2C)- than at the alpha(2A)-receptor subtype and may explain why alpha(2C)-adrenergic receptors are especially suited to control sympathetic neurotransmission at low action potential frequencies in contrast to the alpha(2A)-receptor subtype.

The alpha2 -adrenergic receptors in hypertension and heart failure: experimental and clinical studies

This is a brief overview of experimental and clinical studies exploring the hemodynamic functions of the alpha2A and alpha2B adrenergic receptor (AR) subtypes in animals submitted to genetic manipulations or gene treatment, as well as the clinical effects of central sympathetic suppression with the alpha2-AR agonist clonidine in patients with ischemic heart disease and/or heart failure. The animal experiments have led us to conclude that the sympathetic outflow is regulated by activation of the presynaptic alpha2A-AR subtype, which is the predominant alpha2-AR subtype in the central nervous system and exerts a sympathoinhibitory (hypotensive) action; on the contrary, activation of the central alpha2B-AR elicits a sympathoexcitatory response (such as seen in salt-induced hypertension, which requires functionally intact alpha2B-AR). Since there are no selective pharmacologic agents yet capable of discriminating among alpha2-AR subtypes, clinical studies utilize clonidine, the central sympathetic suppressant effect of which has been used for 35 years to treat hypertension. In small clinical trials, clonidine was used successfully for treatment of acute or chronic heart failure, acute myocardial infarct or hypertensive cardiomyopathy with subclinical diastolic dysfunction. We speculate that future development of agents capable of selectively activating the alpha2A-AR or blocking the alpha2B-AR may further improve our capability to treat hypertension, ischemic heart disease and heart failure.

Cooling evokes redistribution of alpha2C-adrenoceptors from Golgi to plasma membrane in transfected human embryonic kidney 293 cells

Cold-induced vasoconstriction in cutaneous blood vessels is mediated by increased constrictor activity of vascular alpha2-adrenoceptors (alpha2-ARs). In mouse cutaneous arteries, alpha2-AR constriction at 37 degrees C is mediated by alpha2A-ARs, whereas after cold exposure (28 degrees C), alpha2C-ARs are no longer silent and mediate the remarkable cold-induced augmentation of alpha2-AR responsiveness. The goals of the present study were to develop a cell model of cutaneous thermoregulation and to determine the mechanisms underlying the thermosensitivity of alpha2C-ARs. Human embryonic kidney 293 cells were transiently transfected with the mouse alpha2A- or alpha2C-AR. In cells expressing alpha2A-ARs, UK-14,304 (5-bromo-N-(4,5-dihydro-1H-imidazol-2-yl)-6-quinoxalinamine), an alpha2-AR agonist, inhibited (10 pM) and stimulated (1-10 nM) the accumulation of cAMP evoked by forskolin. Similar responses were obtained at 37 degrees C and 28 degrees C. In contrast, in cells expressing alpha2C-ARs, UK-14,304 did not affect forskolin-stimulated cAMP accumulation at 37 degrees C but did cause a concentration-dependent inhibitory effect at 28 degrees C. Subcellular fractionation revealed that at 37 degrees C alpha2C-ARs were localized predominantly to Golgi compartments, whereas alpha2A-ARs localized predominantly to the plasma membrane. After cooling (28 degrees C), alpha2C-ARs relocated from Golgi compartments to the plasma membrane, whereas the alpha2A-AR remained at the plasma membrane. Immunofluorescence microscopy confirmed that, at 37 degrees C, alpha2A-ARs were localized to the cell surface, whereas alpha2C-ARs colocalized with a trans-Golgi marker. Cooling did not affect localization of alpha2A-ARs, but shifted alpha2C-ARs to the cell surface. Moderate cooling, therefore, caused a selective redistribution of alpha2C-ARs from the Golgi compartments to the cell surface, allowing the rescue of the alpha2C-adrenergic functional response. This mechanism may explain the role of alpha2-ARs in thermoregulation of the cutaneous circulation.

Spatial working memory improvement by an alpha2-adrenoceptor agonist dexmedetomidine is not mediated through alpha2C-adrenoceptor

1. Aged alpha2C-adrenoceptor knockout and wild type mice were used to investigate whether alpha2C-adrenoceptors are involved in mediating the beneficial effects of alpha2-adrenoceptor agonist, dexmedetomidine, on spatial working memory. 2. A win-stay task in the radial arm maze was used to dissociate the effects of dexmedetomidine on working vs. reference memory. In addition, the animals were tested in simple response habit learning in the T-maze. 3. Knockout mice made more working memory errors after the change of the baited arm in radial arm maze, but after training reached again as accurate level of performance as wild type controls. Dexmedetomidine 5 and 10 microg/kg alleviated the increase in spatial working memory errors after the change of the baited arm in knockout mice. Knockout and wild type mice performed equally well in T-maze, and dexmedetomidine had no effect on this simple response learning. 4. The present results indicate that alpha2-adrenoceptor agonists have a selective effect on spatial working memory not only in monkeys but also in mice. Further, this study confirms our earlier finding that the presence of alpha2C-adrenoceptors is not necessary for the spatial working memory enhancing effect of alpha2-adrenoceptor agonists.

Role of alpha-2 adrenergic receptors in neuroprotection and glaucoma

The loss of retinal ganglion cells (RGCs) in glaucoma occurs progressively over many years. A neuroprotective drug should enhance survival of RGCs in the presence of chronic stress/injury. Four criteria are proposed for assessing the likely therapeutic utility in human glaucoma of drugs that have demonstrated neuroprotective activity in animal models: 1) A specific receptor target must be in the retina/optic nerve; 2) Activation of the target must trigger pathways that enhance a neuron's resistance to stress/injury and/or suppresses toxic insults; 3) The drug must reach the retina/vitreous at pharmacologic doses; and 4) The neuroprotective activity should be demonstrated in clinical trials. Data are presented that illustrate how the specific and potent alpha-2 agonist, brimonidine, meets these criteria. The alpha-2A receptor was localized in the inner rat retina by immunohistochemistry. Brimonidine reduced the rate of RGC loss in the calibrated rat optic nerve injury model even when dosed 12 and 24 hours before injury, consistent with a long-term enhancement of RGC resistance to stress. Brimonidine was also neuroprotective in the lasered chronic hypertensive rat model, reducing RGC loss over three weeks from 33% to 15%. A clinical trial has been initiated to determine brimonidine's neuroprotective activity in patients with non-arteritic ischemic optic neuropathy.

Interaction of formamidine pesticides with the presynaptic alpha(2)-adrenoceptor regulating

The effects of the formamidine pesticides amitraz and chlordimeform on the alpha(2)-adrenergic receptor subtype that mediates the release of [(3)H]noradrenaline by synaptosomes from rat hypothalami were studied. We initially characterized the presynaptic autoreceptor on noradrenergic nerve endings using selective antagonists. Yohimbine (a nonselective alpha(2) antagonist) and BRL 44408 (selective for subtypes alpha(2A)/alpha(2D)) diminished the inhibitory effect of xylazine on K(+)-evoked release of [(3)H]noradrenaline; the K(B) values were 481 and 154 nM, respectively. In contrast, prazosin (a selective alpha(2B)/alpha(2C) antagonist) did not modify the inhibitory effect of xylazine. These results indicate that the release of noradrenaline by noradrenergic nerve endings in the rat hypothalamus is regulated by alpha(2D)-adrenoceptors, a species variation of the human alpha(2A) subtype. We then assessed the effects of the two pesticides on the K(+)-evoked release of [(3)H]noradrenaline. Amitraz reduced release in a dose-dependent manner; the effect observed at the maximal concentration tested (10 microM) was 13.0 +/- 2.0% and it was reversed by yohimbine. Amitraz also diminished the inhibitory effects of the alpha(2)-adrenergic agonists clonidine and xylazine. Chlordimeform displayed no effects, possibly because the true active compound of this insecticide is its demethylated metabolite. Based on these findings we conclude that the formamidine pesticides act as partial agonists of presynaptic alpha(2D)-adrenergic receptors in the rat hypothalamus. This interaction may be responsible for the in vivo alterations in catecholaminergic regulation of cyclic variations in gonadotropin-releasing hormone (GnRH) secretion, which can have grave functional repercussions on the reproductive system of mammals exposed to these xenobiotics.

Molecular mechanism for agonist-promoted alpha(2A)-adrenoceptor activation by norepinephrine and epinephrine

We present a mechanism for agonist-promoted alpha(2A)-adrenergic receptor (alpha(2A)-AR) activation based on structural, pharmacological, and theoretical evidence of the interactions between phenethylamine ligands and alpha(2A)-AR. In this study, we have: 1) isolated enantiomerically pure phenethylamines that differ both in their chirality about the beta-carbon, and in the presence/absence of one or more hydroxyl groups: the beta-OH and the catecholic meta- and para-OH groups; 2) used [(3)H]UK-14,304 [5-bromo-N-(4,5-dihydro-1H-imidazol-2-yl)-6-quinoxalinamine; agonist] and [(3)H]RX821002 [2-(2-methoxy-1,4-benzodioxan-2-yl)-2-imidazoline; antagonist] competition binding assays to determine binding affinities of these ligands to the high- and low-affinity forms of alpha(2A)-AR; 3) tested the ability of the ligands to promote receptor activation by measuring agonist-induced stimulation of [(35)S]GTPgammaS binding in isolated cell membranes; and 4) used automated docking methods and our alpha(2A)-AR model to predict the binding modes of the ligands inside the alpha(2A)-AR binding site. The ligand molecules are sequentially missing different functional groups, and we have correlated the structural features of the ligands and ligand-receptor interactions with experimental ligand binding and receptor activation data. Based on the analysis, we show that structural rearrangements in transmembrane helix (TM) 5 could take place upon binding and subsequent activation of alpha(2A)-AR by phenethylamine agonists. We suggest that the following residues are important in phenethylamine interactions with alpha(2A)-AR: Asp113 (D(3.32)), Val114 (V(3.33)), and Thr118 (T(3.37)) in TM3; Ser200 (S(5.42)), Cys201 (C(5.43)), and Ser204 (S(5.46)) in TM5; Phe391 (F(6.52)) and Tyr394 (Y(6.55)) in TM6; and Phe411 (F(7.38)) and Phe412 (F(7.39)) in TM7.

Alpha2C-adrenoceptor overexpression disrupts execution of spatial and non-spatial search patterns

We investigated the role of alpha2C-adrenoceptors in the modulation of spatial and non-spatial navigation behaviour. Alpha2C-adrenoceptor overexpressing mice developed an ineffective thigmotaxic search pattern characterized by swimming close to the pool walls during both spatial and non-spatial water maze training. A subtype-non-selective alpha2-adrenoceptor antagonist, atipamezole (1000 microg/kg, s.c.), fully reversed this impairment in their search strategy. Withdrawal of atipamezole at the end of spatial training resulted in an immediate disruption of the search pattern in alpha2C-adrenoceptor overexpressing mice. The swimming pattern of alpha2C-adrenoceptor overexpressing mice during a five day free swimming period was normal, when no cognitive component was required. Diazepam (0.3 and 1.0 mg/kg, i.p.), neither improved the accuracy in finding the platform nor decreased thigmotaxis. These results suggest that alpha2C-adrenoceptors may modulate the execution of complex navigation patterns.

alpha(2C)-Adrenoceptors modulate the effect of methylphenidate on response rate and discrimination accuracy in an operant test

The present study investigated the role of alpha(2C)-adrenoceptors in the regulation of activity and discrimination accuracy in an operant chamber test. We trained food deprived control and alpha(2C)-adrenoceptor knockout mice to collect liquid food rewards in an operant chamber during the light (20 s) period. No food reward was delivered during the dark period (40 s). The alpha(2C)-adrenoceptor knockout mice tended to make fewer total responses and collect less rewards than their controls after saline treatment. However, only response accuracy of alpha(2C)-adrenoceptor knockout mice was significantly lower than that of the control mice. Methylphenidate, a drug blocking dopamine re-uptake and increasing dopamine release, dose-dependently decreased the number of total responses and collected food rewards in control mice but increased those measures in alpha(2C)-adrenoceptor knockout mice. In addition, the effect of methylphenidate on discrimination accuracy differed between knockout and control mice. Our results indicate that alpha(2C)-adrenoceptors may regulate dopamine-mediated functions.

Heterogeneous control of blood flow amongst different vascular beds

The control and maintenance of vascular tone is due to a balance between vasoconstrictor and vasodilator pathways. Vasomotor responses to neural, metabolic and physical factors vary between vessels in different vascular beds, as well as along the same bed, particularly as vessels become smaller. These differences result from variation in the composition of neurotransmitters released by perivascular nerves, variation in the array and activation of receptor subtypes expressed in different vascular beds and variation in the signal transduction pathways activated in either the vascular smooth muscle or endothelial cells. As the study of vasomotor responses often requires pre-existing tone, some of the reported heterogeneity in the relative contributions of different vasodilator mechanisms may be compounded by different experimental conditions. Biochemical variations, such as the expression of ion channels, connexin subtypes and other important components of second messenger cascades, have been documented in the smooth muscle and endothelial cells in different parts of the body. Anatomical variations, in the presence and prevalence of gap junctions between smooth muscle cells, between endothelial cells and at myoendothelial gap junctions, between the two cell layers, have also been described. These factors will contribute further to the heterogeneity in local and conducted responses.

Cell-type specific targeting of the alpha 2c-adrenoceptor. Evidence for the organization of receptor microdomains during neuronal differentiation of PC12 cells

We have previously shown differences in the intracellular targeting of alpha2a (alpha(2A))- and alpha2c (alpha(2C))-adrenoreceptors expressed in the same cell line (von Zastrow, M., Link, R., Daunt, D. , Barsh, G., and Kobilka, B. (1993) J. Biol. Chem. 268, 763-766; Daunt, D. A., Hurt, C., Hein, L., Kallio, J., Feng, F., and Kobilka, B. K. (1997) Mol. Pharmacol. 51, 711-720). alpha(2A)-Adrenoreceptors reside primarily in the plasma membrane in HEK 293 cells, while co-expressed alpha(2C)-adrenoreceptors are found mainly in an intracellular compartment. Since alpha(2c)-adrenoreceptors are expressed primarily in the brain, we compared the intracellular targeting of alpha(2C)-adrenoreceptors in two neuroendocrine cell lines with the targeting in three epithelial cell lines and one fibroblast cell line. In transiently transfected COS7 cells, and in stably transfected normal rat kidney cells, Madin-Darby canine kidney cells, and Rat1 fibroblasts, a significant proportion of alpha(2C)-adrenoreceptor detected by immunocytochemistry co-localized with markers for both the endoplasmic reticulum and the cis/medial Golgi compartments. In contrast, both PC12 cells and AtT20 cells efficiently targeted alpha(2C)-adrenoreceptors to the plasma membrane. Ligand binding and Western blot analyses indicate that intracellular receptor in normal rat kidney cells is functional and undergoes normal post-translational processing. In PC12 cells the expressed alpha(2C)-adrenoreceptors become concentrated in neurite outgrowths in discrete regions of the plasma membrane having a high density of F-actin following treatment with nerve growth factor. These findings provide evidence for cell-type specific factors that facilitate the targeting of the G protein-coupled receptors to the plasma membrane.

Cardiorespiratory effects of four alpha2-adrenoceptor agonist-ketamine combinations in captive red wolves

OBJECTIVE

To evaluate the cardiopulmonary effects of immobilizing doses of xylazine-ketamine (XK), medetomidine-ketamine (MK), medetomidine-ketamine-acepromazine (MKA), and medetomidine-butorphanol-ketamine (MBK) in captive red wolves.

DESIGN

Prospective study.

ANIMALS

32 adult captive red wolves.

PROCEDURE

Wolves were randomly assigned to 1 of 4 treatment groups: XK, MK, MKA, or MBK. Physiologic variables measured included heart rate, blood pressure, respiratory rate, tidal volume, oxygen-hemoglobin saturation (Spo2), end-tidal CO2, arterial blood gases, and rectal temperature. Induction time, muscle relaxation, and quality of recovery were assessed.

RESULTS

Heart rates were lower in wolves in the MBK group than for the other groups. All 4 drug combinations induced considerable hypertension, with diastolic pressures exceeding 116 mm Hg. Blood pressure was lowest in wolves receiving the MBK combination. Respiratory rate was significantly higher in wolves receiving XK, MK, and MKA. Tidal volumes were similar for all groups. Wolves receiving XK, MK, and MKA were well-oxygenated throughout the procedure (SPo2 > 93%), whereas those receiving MBK were moderately hypoxemic (87% < Spo2 < 93%) during the first 20 minutes of the procedure. Hyperthermia was detected initially following induction in all groups.

CONCLUSIONS AND CLINICAL RELEVANCE

The alpha2-adrenoceptor agonist-ketamine combinations provide rapid reversible anesthesia for red wolves but cause severe sustained hypertension. Such an adverse effect puts animals at risk for development of cerebral encephalopathy, retinal hemorrhage, pulmonary edema, and myocardial failure. Although the MBK combination offers some advantages over the others, it is advised that further protocol refinements be made to minimize risks associated with acute hypertension.

Silent alpha(2C)-adrenergic receptors enable cold-induced vasoconstriction in cutaneous arteries

Cold constricts cutaneous blood vessels by increasing the reactivity of smooth muscle alpha(2)-adrenergic receptors (alpha(2)-ARs). Experiments were performed to determine the role of alpha(2)-AR subtypes (alpha(2A)-, alpha(2B)-, alpha(2C)-ARs) in this response. Stimulation of alpha(1)-ARs by phenylephrine or alpha(2)-ARs by UK-14,304 caused constriction of isolated mouse tail arteries mounted in a pressurized myograph system. Compared with proximal arteries, distal arteries were more responsive to alpha(2)-AR activation but less responsive to activation of alpha(1)-ARs. Cold augmented constriction to alpha(2)-AR activation in distal arteries but did not affect the response to alpha(1)-AR stimulation or the level of myogenic tone. Western blot analysis demonstrated expression of alpha(2A)- and alpha(2C)-ARs in tail arteries: expression of alpha(2C)-ARs decreased in distal compared with proximal arteries, whereas expression of the glycosylated form of the alpha(2A)-AR increased in distal arteries. At 37 degrees C, alpha(2)-AR-induced vasoconstriction in distal arteries was inhibited by selective blockade of alpha(2A)-ARs (BRL-44408) but not by selective inhibition of alpha(2B)-ARs (ARC-239) or alpha(2C)-ARs (MK-912). In contrast, during cold exposure (28 degrees C), the augmented response to UK-14,304 was inhibited by the alpha(2C)-AR antagonist MK-912, which selectively abolished cold-induced amplification of the response. These experiments indicate that cold-induced amplification of alpha(2)-ARs is mediated by alpha(2C)-ARs that are normally silent in these cutaneous arteries. Blockade of alpha(2C)-ARs may prove an effective treatment for Raynaud's Phenomenon.

Overexpression of alpha2C-adrenoceptors impairs water maze navigation

We investigated the role of overexpression of alpha2C-adrenoceptors in water maze navigation in mice transgenically manipulated to have a threefold overexpression of the alpha2C-adrenoreceptors. Alpha2C-adrenoreceptors overexpressing mice swam more in the peripheral annulus of the pool and did not find the hidden escape platform as well as the wild type control mice. A subtype-nonselective alpha2-adrenoreceptor antagonist, atipamezole (ATI, 1000 microg/kg, s.c.), fully reversed the deficit in platform finding and search strategy in overexpressing mice. Noradrenaline depletion (-95%) induced by N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4) did not impair platform finding of wild type or overexpressing mice. The DSP-4 lesion slightly increased swimming in the peripheral annulus in wild type mice, but not in overexpressing mice. The DSP-4 lesion produced a dissociable effect on the action of atipamezole to improve platform finding and search strategy in overexpressing mice: atipamezole did not alleviate the platform finding deficit in DSP-4 lesioned overexpressing mice, but normalized their abnormal search strategy. These results suggest that the abnormal search pattern and deficit in the accuracy of platform finding are mediated by constitutive activity of overexpressed alpha2C-adrenoreceptors.

Role of the postsynaptic alpha(2)-adrenergic receptor subtypes in catecholamine-induced vasoconstriction

Catecholamines induce direct vasoconstriction mediated by postsynaptic alpha-adrenergic receptors (alpha-ARs) of both the alpha(1) and alpha(2) type. To evaluate the contribution of each alpha(2)-AR subtype (alpha(2A), alpha(2B), and alpha(2C)) to this function, we used groups of genetically engineered mice deficient for the gene to each one of these subtypes and compared their blood pressure (BP) responses to their wild-type counterparts. Blood pressure responses to a bolus of norepinephrine (NE) were assessed before and after sequential blockade of alpha(1)-ARs with prazosin and alpha(2)-ARs with yohimbine. The first NE bolus elicited a brief 32 to 44 mm Hg BP rise (p < 0.001 from baseline) in all six groups. Prazosin decreased BP by 23 to 33 mm Hg in all groups, establishing a new lower baseline. Repeat NE at that point elicited lesser but still significant (p < 0.001) brief pressor responses between 32% and 45% of the previous BP rise in five of the six groups. Only the alpha(2A)-AR gene knockouts differed, responding instead with a 20-mm Hg fall in BP, a significant change from baseline (p < 0.001) and different from the pressor response of their wild-type counterparts (p < 0.001). The addition of yohimbine produced no further BP change in the five groups, but it did produce a small 7. 5-mm Hg fall (p < 0.05) in the alpha(2A)-AR knockouts. Norepinephrine bolus during concurrent alpha(1) and alpha(2)-AR blockade produced significant (p < 0.001) hypotensive responses in all subgroups, presumably attributable to unopposed stimulation of beta(2)-vascular wall ARs. We conclude that the alpha(2)-AR-mediated vasoconstriction induced by catecholamines is attributable to the alpha(2A)-AR subtype because mice deficient in any one of the other subtypes retained the capacity for normal vasoconstrictive responses. However, the alpha(1)-ARs account for the major part (as much as 68%) of catecholamine-induced vasoconstriction.

Role of alpha(2)-adrenergic receptor subtypes in the acute hypertensive response to hypertonic saline infusion in anephric mice

Experimental evidence suggests that the acute hypertensive response induced in anephric animals by infusion of a hypertonic saline solution is mediated by disinhibition of the presynaptic sympathoinhibitory alpha(2)-adrenergic receptors (alpha(2)-AR) of the central nervous system. The purpose of the present experiments was to dissect the role of the 3 distinct alpha(2)-AR subtypes (alpha(2A)-, alpha(2B), - and alpha(2C)-AR) in this response. Groups of genetically engineered mice deficient in each one of these alpha(2)-AR subtype genes were submitted to bilateral nephrectomy followed by a 0.4-mL infusion of 4% saline over a 2-hour period, with constant direct blood pressure (BP) monitoring. The alpha(2A)-AR-deficient and alpha(2C)-AR-deficient mice responded with significant BP elevations (by 11.8+/-2.5 and 16.7+/-1.7 mm Hg, respectively), and so did their wild-type counterparts (17.8+/-2.5 and 11.8+/-2.0 mm Hg, respectively) and the wild-type alpha(2B) +/+ (13.1+/-2.4 mm Hg). However, the alpha(2B)-AR-deficient mice were unable to raise their BP and had a slightly lowered BP (by -3.0+/-4. 0 mm Hg) at the end of the infusion period. All 6 groups exhibited elevated plasma norepinephrine levels ranging between 0.8 and 1.8 ng/mL at the end of the infusion. In all cases, the alpha(2)-AR-deficient groups tended to have higher norepinephrine levels than their wild-type counterparts. Surprisingly, this difference was significant only in the alpha(2B)-AR-deficient mice, which, despite the elevated norepinephrine, were unable to raise their BP. The data suggest that a full complement of the alpha(2B)-AR is needed to mediate the hypertensive response to acute saline load, even though its absence does not prevent the release of norepinephrine under these conditions.

Ligand-receptor interactions as controlled by wild-type and mutant Thr(370)Lys alpha2B-adrenoceptor-Galpha15 fusion proteins

Fusion proteins were constructed between either a wild-type or mutant Thr370Lys alpha2B-adrenoceptor (alpha2B AR) and a mouse Galpha15 protein to analyze ligand-receptor interactions at a receptor/Galpha15 protein density ratio of 1. Activation of the wild-type alpha2B AR-Galpha15 fusion protein in CHO-K1 cells by (-)-adrenaline induced a time- and concentration-dependent (pEC50 = 7.37+/-0.13) increase in the intracellular Ca2+ concentration, which could be antagonized by RX 811059 (pK(B) = 7.55+/-0.15). Whereas d-medetomidine and oxymetazoline were as efficacious agonists as (-)-adrenaline, the following ligands displayed partial agonist properties: BRL 44408 < atipamezole < clonidine < UK 14304 < BHT 920. A comparison with the mutant Thr370Lys alpha2B AR-Galpha15 fusion protein displayed similar Ca2+ kinetics and a ligand-mediated receptor activation profile characterized by higher potencies and greater maximal Ca2+ responses for the ligands being investigated, including the putative antagonists dexefaroxan and idazoxan. RX 811059 and RX 821002 remained silent. Similar conclusions could be made on enhancement of the ligands' intrinsic activities by coexpression of the mutant Thr370Lys alpha2B AR with either a Galpha15 or Galphao Cys351Ile protein. The Thr370Lys alpha2B AR-Galpha protein interactions may modify the tertiary structure of the mutant receptor in such a way that some putative alpha2 AR antagonists are capable of stabilizing an active receptor conformation, thereby generating positive efficacy.

Comparison of dopamine and noradrenaline release in mouse prefrontal cortex, striatum and hippocampus using microdialysis

In vivo release of dopamine (DA) and noradrenaline (NA) in mouse medial prefrontal cortex, medial striatum and hippocampus was characterized using in vivo microdialysis. Basal release of NA was similar in these areas, but DA in striatum was 13-30 times higher than in other areas. Unconditioned stimuli (handling, novelty) induced strong increases, except for striatal DA. Striatal NA was more sensitive to handling than NA in other areas.

Regulation of monoamine receptors in the brain: dynamic changes during stress

Monoamine receptors are membrane-bound receptors that are coupled to G-proteins. Upon stimulation by agonists, they initiate a cascade of intracellular events that guide biochemical reactions of the cell. In the central nervous system, they undergo diverse regulatory processes, among which are receptor desensitization, internalization into the cell, and downregulation. These processes vary among different types of monoamine receptors. alpha 2-Adrenoceptors are often downregulated by agonists, and beta-adrenoceptors are internalized rapidly. Others, such as serotonin1A-receptors, are controlled tightly by steroid hormones. Expression of these receptors is reduced by the "stress hormones" glucocorticoids, whereas gonadal hormones such as testosterone can counterbalance the glucocorticoid effects. Because of this, the pattern of monoamine receptors in certain brain regions undergoes dynamic changes when there are elevated concentrations of agonists or when the hormonal milieu changes. Stress is a physiological situation accompanied by the high activity of brain monoaminergic systems and dramatic changes in peripheral hormones. Resulting alterations in monoamine receptors are considered to be in part responsible for changes in the behavior of an individual.

An Asp79Asn mutation of the alpha2A-adrenoceptor interferes equally with agonist activation of individual Gialpha-family G protein subtypes

The quantitative effects of an Asp79Asn mutation in the porcine alpha2A-adrenoceptor on adrenaline-mediated stimulation of the alpha subunit of individual members of the Gi family of G proteins were assessed by measuring GTP turnover number for fusion proteins between the wild type or mutated receptor and pertussis toxin-resistant forms of each of Gi1, Gi2 and Gi3. In each case the receptor mutation limited activation of the G protein to 8-14% of that produced by the wild type receptor. Previous demonstration that in a single cell this mutation selectively interferes with alpha2A-adrenoceptor regulation of distinct effector end points transduced by Gi family members must therefore reflect differential requirements for amplification or the cellular location of individual, co-expressed, G proteins.

Method for comparison of the hemodynamic effects of equi-antinociceptive oral doses of drugs in anesthetized rats

In a typical flowchart for discovery of novel analgesic (or other) agents, a critical path often involves maximization of the separation of the therapeutic endpoint from known adverse-effect (AE) endpoint(s). Although strategies can easily be designed for in vitro paradigms such as high-throughput screening, extension to in vivo testing can represent a major obstacle to the rapid progression to the next step in development. The problem can be particularly acute when the assessment is required for oral dosing, and when it is not known if the therapeutic and AE mechanism(s) of action are the same. As a case in point, alpha(2)-adrenoceptor (alpha(2)-AR) agonists have potential therapeutic use as analgesics, but they also produce cardiovascular (CV) effects. However, whether the two effects are inexorably linked has not been resolved, particularly for oral administration. The present study used a novel method for comparing the CV effects produced by alpha(2)-AR agonists given by intraduodenal administration to anesthetized rats at fixed ratios of the oral antinociceptive ED(50) dose of each agonist. The technique provided a useful screen of compounds. In addition,there was no correlation between CV endpoints and alpha(2A)-AR affinity, suggesting that oral alpha(2)-AR-mediated analgesia and CV effects might be separable or that other mechanisms might be involved.

Adrenergic alpha2C-receptors reside in rat striatal GABAergic projection neurons: comparison of radioligand binding and immunohistochemistry

We have investigated the distribution of alpha2c-adrenergic receptors in the rat striatum and characterized the striatal neuron types expressing these receptors. Sequential double-labelled immunocytochemistry was performed with a polyclonal antibody against rat alpha2c-adrenoceptors and antibodies against GABA, Calbindin-D28k, parvalbumin and calretinin. The subregional distribution of alpha2c-adrenoceptor binding sites in the striatum was also quantitatively investigated using selective radioligands. Almost all lightly stained striatal GABAergic neurons, with the morphological characteristics of medium-sized spiny projection neurons (94% of GABAergic cells counted), contained alpha2c-adrenoceptor-immunoreactive structures. Intensely labelled GABAergic inteneurons (6%) were devoid of alpha2c-adrenoceptor immunoreactivity. The co-localization of calbindin- and alpha2c-adrenoceptor immunoreactivity in the majority of the cells confirmed the presence of alpha2c-adrenoceptors in the population of medium-sized spiny neurons. Furthermore, the alpha2c-adrenoceptor/calbindin double-labelling disclosed the existence of three neuronal subsets in the matrix compartment of the striatum: a large proportion (83%) of double-labelled neurons, a population of neurons (8%) that exhibited only alpha2c-adrenoceptor immunoreactivity without calbindin immunoreactivity, and a population of neurons (9%) immunoreactive for calbindin, but lacking alpha2c-adrenoceptors. In addition, alpha2c-adrenoceptor immunolabelled neurons were observed in calbindin-free striatal patches. Parvalbumin- and calretinin-positive neurons never displayed alpha2c-adrenoceptor immunoreactivity, confirming that striatal GABAergic interneurons are devoid of alpha2c-adrenoceptors. The present findings indicate that alpha2c-adrenoceptors are localized in GABAergic medium-sized spiny projection neurons but not in interneurons of the rat striatum, and that they may modulate both the direct and indirect pathways of the basal ganglia, as well as participate in the regulation of mesencephalic dopaminergic neurons.