[3H]-MK 912 binding delineates two alpha 2-adrenoceptor subtypes in rat CNS one of which is identical with the cloned pA2d alpha 2-adrenoceptor

Scientific rationale for the use of α2A-adrenoceptor agonists in treating neuroinflammatory cognitive disorders

Neuroinflammatory disorders preferentially impair the higher cognitive and executive functions of the prefrontal cortex (PFC). This includes such challenging disorders as delirium, perioperative neurocognitive disorder, and the sustained cognitive deficits from "long-COVID" or traumatic brain injury. There are no FDA-approved treatments for these symptoms; thus, understanding their etiology is important for generating therapeutic strategies. The current review describes the molecular rationale for why PFC circuits are especially vulnerable to inflammation, and how α2A-adrenoceptor (α2A-AR) actions throughout the nervous and immune systems can benefit the circuits in PFC needed for higher cognition. The layer III circuits in the dorsolateral PFC (dlPFC) that generate and sustain the mental representations needed for higher cognition have unusual neurotransmission and neuromodulation. They are wholly dependent on NMDAR neurotransmission, with little AMPAR contribution, and thus are especially vulnerable to kynurenic acid inflammatory signaling which blocks NMDAR. Layer III dlPFC spines also have unusual neuromodulation, with cAMP magnification of calcium signaling in spines, which opens nearby potassium channels to rapidly weaken connectivity and reduce neuronal firing. This process must be tightly regulated, e.g. by mGluR3 or α2A-AR on spines, to prevent loss of firing. However, the production of GCPII inflammatory signaling reduces mGluR3 actions and markedly diminishes dlPFC network firing. Both basic and clinical studies show that α2A-AR agonists such as guanfacine can restore dlPFC network firing and cognitive function, through direct actions in the dlPFC, but also by reducing the activity of stress-related circuits, e.g. in the locus coeruleus and amygdala, and by having anti-inflammatory actions in the immune system. This information is particularly timely, as guanfacine is currently the focus of large clinical trials for the treatment of delirium, and in open label studies for the treatment of cognitive deficits from long-COVID.

Guanfacine Extended Release: A New Pharmacological Treatment Option in Europe

Children/adolescents with attention-deficit/hyperactivity disorder (ADHD) may have a poor or inadequate response to psychostimulants or be unable to tolerate their side-effects; furthermore, stimulants may be inappropriate because of co-existing conditions. Only one non-stimulant ADHD pharmacotherapy, the noradrenaline transporter inhibitor atomoxetine, is currently approved for use in Europe. We review recent advances in understanding of the pathophysiology of ADHD with a focus on the roles of catecholamine receptors in context of the α2A-adrenergic receptor agonist guanfacine extended release (GXR), a new non-stimulant treatment option in Europe. Neuroimaging studies of children/adolescents with ADHD show impaired brain maturation, and structural and functional anomalies in brain regions and networks. Neurobiological studies in ADHD and medication response patterns support involvement of monoaminergic neurotransmitters (primarily dopamine and noradrenaline). Guanfacine is a selective α2A-adrenergic receptor agonist that has been shown to improve prefrontal cortical cognitive function, including working memory. The hypothesized mode of action of guanfacine centres on direct stimulation of post-synaptic α2A-adrenergic receptors to enhance noradrenaline neurotransmission. Preclinical data suggest that guanfacine also influences dendritic spine growth and maturation. Clinical trials have demonstrated the efficacy of GXR in ADHD, and it is approved as monotherapy or adjunctive therapy to stimulants in Canada and the USA (for children and adolescents). GXR was approved recently in Europe for the treatment of ADHD in children and adolescents for whom stimulants are not suitable, not tolerated or have been shown to be ineffective. GXR may provide particular benefit for children/adolescents who have specific co-morbidities such as chronic tic disorders or oppositional defiant disorder (or oppositional symptoms) that have failed to respond to first-line treatment options.

Plasticity of α2-adrenergic spinal antinociception following nerve injury: selective, bidirectional interaction with the delta opioid receptor

Interactions of opioid receptors with other receptor families can be made use of to improve analgesia and reduce adverse effects of opioid analgesics. We investigated interactions of the α2-adrenergic receptor (α2AR) with opioid receptors of the mu (MOR) and delta (DOR) types in the spinal dorsal horn in an animal model of neuropathic pain induced by spinal nerve ligation. Nine days after nerve injury, immunoreactivity for the α2AR subtype A (α2AAR) was increased both in tissue homogenates and at pre- and post-synaptic sites in transverse sections. The efficacy of spinally administered α2AAR agonist guanfacine at reducing C-fiber-evoked field potentials was increased in nerve-ligated rats. This reducing effect was impaired by simultaneous administration of DOR antagonist naltrindole, but not MOR antagonist CTOP, suggesting that concurrent DOR activation was required for α2AAR-mediated inhibition. While DOR agonist deltorphin II and MOR agonist DAMGO both effectively depressed C-fiber-evoked spinal field potentials, DOR- but not MOR-mediated depression was enhanced by subclinical guanfacine. In conscious, nerve-ligated rats, chronically administered deltorphin II produced stable thermal and mechanical antinociception over the 9 following days after nerve injury without apparent signs of habituation. Such an effect was dramatically enhanced by co-administration of a low dose of guanfacine, which reversed thermal and mechanical thresholds to levels near those prior to injury. The results suggest that spinal, α2AAR-mediated antinociception is increased after nerve injury and based on DOR co-activation. We demonstrate in vivo that α2AAR/DOR interaction can be exploited to provide effective behavioral antinociception during neuropathic pain.

Pharmacological identification of α1- and α2-adrenoceptor subtypes involved in the vasopressor responses induced by ergotamine in pithed rats

Ergotamine has been used in clinical practice for the acute treatment of migraine for over 90 years. So far, it is known that ergotamine interacts with diverse receptors (including α1/2-adrenoceptors, 5-HT1, 5-HT2 and D2-like receptors) and that produces increases in mean blood pressure which are significantly blocked by yohimbine, a classical α2-adrenoceptor antagonist with a moderate affinity for α1-adrenoceptors. Since α1/2-adrenoceptors mediate vasopressor and vasoconstrictor responses in the cardiovascular system, this study was designed to identify the α-adrenoceptor subtypes (α1A, α1B, α1D, α2A, α2B and α2C) involved in ergotamine-induced vasopressor responses in pithed rats. In male Wistar pithed rats baseline heart rate and blood pressure were recorded. Then, the vasopressor responses to intravenous (i.v.) bolus injections of ergotamine were determined after administration of vehicle or several α1⧸2-adrenoceptor antagonists. I.v. administration of the antagonists prazosin (α1, 0.1-30 µg/kg), rauwolscine (α2, 0.3-300 µg/kg), prazosin (0.1 µg/kg) plus rauwolscine (0.3 µg/kg), 5-methylurapidil (α1A, 100 and 300 µg/kg), L-765,314 (α1B, 100 and 300 µg/kg), BMY 7378 (α1D, 100 and 300 µg/kg), BRL44408 (α2A, 300 and 1000 µg/kg) and JP-1302 (α2C, 300 µg/kg), significantly blocked the vasopressor responses to ergotamine, whereas imiloxan (α2B, 1000 and 3000 µg/kg), JP-1302 (100 µg/kg) or the corresponding vehicles (saline 0.9%, propylene glycol 20% or dimethyl sulfoxide 10%; 1ml/kg) failed to modify the responses to ergotamine. The above results suggest that the vasopressor responses to ergotamine in pithed rats are mainly mediated by α1A-, α1B-, α1D-, α2A- and α2C-adrenoceptors and may explain its adverse/therapeutic effects.

alpha2A-adrenergic receptors heterosynaptically regulate glutamatergic transmission in the bed nucleus of the stria terminalis

Stress is a major driving force in reinstatement of drug-seeking behavior. The bed nucleus of the stria terminalis (BNST) has been identified as a key brain region in this behavior, and receives a dense input of the stress-neurotransmitter norepinephrine through the ventral noradrenergic bundle. Activation of alpha(2)-adrenergic receptors (alpha(2)-ARs) in the BNST blocks stress-induced reinstatement of drug-seeking, indicating a potentially important role for these receptors. Currently, it is unclear how alpha(2)-AR agonists elicit this behavioral action, or through which alpha(2)-AR subtype. Activation of alpha(2)-ARs decreases glutamatergic transmission in the BNST, an effect which is nearly absent in the alpha(2A)-AR knockout mouse. Here, we take advantage of a knock-in mouse in which a hemagglutinin-tagged alpha(2A)-AR was inserted into the endogenous locus, along with the alpha(2A)-AR selective agonist guanfacine, to further study the role of the alpha(2A)-AR subtype in modulation of neurotransmission in the BNST. Using immunohistochemistry, we find that alpha(2A)-ARs are highly expressed in the BNST, and that this expression is more similar in distribution to the vesicular glutamate transporters than to either norepinephrine transporter or tyrosine hydroxylase positive terminals. Using whole cell patch-clamp recordings, we show that guanfacine causes a depression of evoked excitatory and, to a more limited extent, inhibitory fast synaptic transmission. In total, these data support a prominent heterosynaptic role for alpha(2A)-ARs in modulating fast synaptic transmission in the BNST.

Inhibition of opioid release in the rat spinal cord by alpha2C adrenergic receptors

Neurotransmitter receptors that control the release of opioid peptides in the spinal cord may play an important role in pain modulation. Norepinephrine, released by a descending pathway originating in the brainstem, is a powerful inducer of analgesia in the spinal cord. Adrenergic alpha2C receptors are present in opioid-containing terminals in the dorsal horn, where they could modulate opioid release. The goal of this study was to investigate this possibility. Opioid release was evoked from rat spinal cord slices by incubating them with the sodium channel opener veratridine in the presence of peptidase inhibitors (actinonin, captopril and thiorphan), and was measured in situ through the internalization of mu-opioid receptors in dorsal horn neurons. Veratridine produced internalization in 70% of these neurons. The alpha2 receptor agonists clonidine, guanfacine, medetomidine and UK-14304 inhibited the evoked mu-opioid receptor internalization with IC50s of 1.7 microM, 248 nM, 0.3 nM and 22 nM, respectively. However, inhibition by medetomidine was only partial, and inhibition by UK-14304 reversed itself at concentrations higher than 50 nM. None of these agonists inhibited mu-opioid receptor internalization produced by endomorphin-2, showing that they inhibited opioid release and not the internalization itself. The inhibitions produced by clonidine, guanfacine or UK-14304 were completely reversed by the selective alpha2C antagonist JP-1203. In contrast, inhibition by guanfacine was not prevented by the alpha2A antagonist BRL-44408. These results show that alpha2C receptors inhibit the release of opioids in the dorsal horn. This action may serve to shut down the opioid system when the adrenergic system is active.

Characterization of the postjunctional alpha 2C-adrenoceptor mediating vasoconstriction to UK14304 in porcine pulmonary veins

BACKGROUND AND PURPOSE

In terms of postjunctional alpha(2)-adrenoceptors in the pulmonary circulation, no evidence is available with regard to the receptor subtypes mediating vasoconstriction. Therefore, we characterized the alpha(2)-adrenoceptor subtypes mediating contraction in isolated porcine pulmonary veins.

EXPERIMENTAL APPROACH

alpha-adrenoceptor-mediated vasoconstriction was studied using a tissue bath protocol. mRNA profile and relative quantification of alpha(2)-adrenoceptor subtypes were determined in porcine pulmonary veins using reverse-transcriptase polymerase chain reaction (RT-PCR) and real-time PCR.

KEY RESULTS

In porcine pulmonary veins, noradrenaline, phenylephrine (alpha(1)-adrenoceptor agonist), UK14304 and clonidine (alpha(2)-adrenoceptor agonists) caused concentration-dependent contractions. The rank order of agonist potency was: NA approximately UK14304 approximately clonidine > phenylephrine. UK14304 responses were antagonised by MK912 (noncompetitive antagonist parameter pD'(2): 10.1), rauwolscine (pK(B): 9.5), yohimbine (pK(B): 9.1), WB4101 (pK(B): 8.7), ARC239 (pK(B): 7.5), prazosin (pK(B): 7.1) and BRL44408 (pK(B): 7.0). Antagonist potencies fitted best with radioligand binding data (pK(i)) at the human recombinant alpha(2C)-adrenoceptor (r(2)=0.96, P=0.0001). Correlation with alpha(2B)-adrenoceptors was lower (r(2)=0.74, P>0.01) and no correlation was obtained with alpha(2A)-adrenoceptors. Moreover, RT-PCR studies in porcine pulmonary veins showed mRNA signals for alpha(2A)- and alpha(2C)-adrenoceptors, but not for alpha(2B)-adrenoceptors, whilst real-time PCR studies indicated a prominent expression of alpha(2C)-adrenoceptor mRNA.

CONCLUSIONS AND IMPLICATIONS

Postjunctional alpha(2C)-adrenoceptors mediated contraction in porcine pulmonary veins. alpha(1)-Adrenoceptors also seem to be present in this tissue. Since alpha(2)-adrenoceptor responsiveness is increased when pulmonary vascular tone is elevated, alpha(2C)-adrenoceptor antagonists may be beneficial in diseases such as pulmonary hypertension or congestive heart failure.

Alpha2A adrenergic receptor activation inhibits epileptiform activity in the rat hippocampal CA3 region

Norepinephrine has potent antiepileptic properties, the pharmacology of which is unclear. Under conditions in which GABAergic inhibition is blocked, norepinephrine reduces hippocampal cornu ammonis 3 (CA3) epileptiform activity through alpha(2) adrenergic receptor (AR) activation on pyramidal cells. In this study, we investigated which alpha(2)AR subtype(s) mediates this effect. First, alpha(2)AR genomic expression patterns of 25 rat CA3 pyramidal cells were determined using real-time single-cell reverse transcription-polymerase chain reaction, demonstrating that 12 cells expressed alpha(2A)AR transcript; 3 of the 12 cells additionally expressed mRNA for alpha(2C)AR subtype and no cells possessing alpha(2B)AR mRNA. Hippocampal CA3 epileptiform activity was then examined using field potential recordings in brain slices. The selective alphaAR agonist 6-fluoronorepinephrine caused a reduction of CA3 epileptiform activity, as measured by decreased frequency of spontaneous epileptiform bursts. In the presence of betaAR blockade, concentration-response curves for AR agonists suggest that an alpha(2)AR mediates this response, as the rank order of potency was 5-bromo-N-(4,5-dihydro-1H-imidazol-2-yl)-6-quinoxalinamine (UK-14304) >or= epinephrine >6-fluoronorepinephrine > norepinephrine >>> phenylephrine. Finally, equilibrium dissociation constants (K(b)) of selective alphaAR antagonists were functionally determined to confirm the specific alpha(2)AR subtype inhibiting CA3 epileptiform activity. Apparent K(b) values calculated for atipamezole (1.7 nM), MK-912 (4.8 nM), BRL-44408 (15 nM), yohimbine (63 nM), ARC-239 (540 nM), prazosin (4900 nM), and terazosin (5000 nM) correlated best with affinities previously determined for the alpha(2A)AR subtype (r = 0.99, slope = 1.0). These results suggest that, under conditions of impaired GABAergic inhibition, activation of alpha(2A)ARs is primarily responsible for the antiepileptic actions of norepinephrine in the rat hippocampal CA3 region.

Pharmacological evidence that alpha2A- and alpha2C-adrenoceptors mediate the inhibition of cardioaccelerator sympathetic outflow in pithed rats

It has been suggested that the alpha(2)-adrenoceptors mediating cardiac sympatho-inhibition in pithed rats closely resemble the pharmacological profile of the alpha(2A)-adrenoceptor subtype. However, several lines of evidence suggest that more than one subtype may be involved. Thus, the present study has pharmacologically re-evaluated the receptor subtype(s) involved in the inhibitory effect of the alpha(2)-adrenoceptor agonist, B-HT 933, on the tachycardic responses elicited by selective cardiac sympathetic stimulation (0.03, 0.1, 0.3, 1 and 3 Hz) in desipramine-pretreated pithed rats. I.v. continuous infusions of B-HT 933 (30 microg/kg min), which failed to modify the tachycardic responses to exogenous noradrenaline, inhibited those induced by preganglionic (C(7)-T(1)) stimulation of the cardiac sympathetic outflow at all frequencies of stimulation (0.03-3 Hz). This cardiac sympatho-inhibitory response to B-HT 933 was: (1) unaltered by saline (1 ml/kg) or the antagonists BRL44408 (100 microg/kg; alpha(2A)) or imiloxan (3000 and 10,000 microg/kg; alpha(2B)); (2) partially antagonized by BRL44408 (300 microg/kg) or MK912 (10 microg/kg; alpha(2C)) given separately; and (3) completely antagonized by rauwolscine (300 microg/kg; alpha(2)), MK912 (30 microg/kg) or the combination of BRL44408 (300 microg/kg) plus MK912 (10 microg/kg). Moreover, the above doses of antagonists, which are high enough to block their respective receptors, failed to block per se the tachycardic responses to sympathetic stimulation. These results suggest that the cardiac sympatho-inhibition induced by B-HT 933 in pithed rats is mainly mediated by stimulation of alpha(2A)- and alpha(2C)-adrenoceptors.

α2-Adrenoceptor-mediated modulation of the release of GABA and noradrenaline in the rat substantia nigra pars reticulata

The control of movement by the basal ganglia is influenced by inputs from diverse brain structures. Unfortunately, the mechanisms of modulation are poorly defined. Based on neuroanatomical evidence for alpha2A and alpha2C subtypes of alpha2 adrenergic receptors within this region, we hypothesize that noradrenergic alpha2-receptors can influence transmitter release in the SNr. To test this hypothesis we examined the effect of the alpha 2 adrenergic agonist, clonidine, and antagonist, rauwolscine, on the efflux of [3H]-GABA and [3H]-noradrenaline from brain slices of the rat substantia nigra pars reticulata. At low concentrations (10 nM), rauwolscine caused an 84.2 +/- 18.51% (p < 0.01) increase in KCl-evoked GABA release. At higher concentrations, rauwolscine caused a dose-dependent return to basal levels. Rauwolscine also enhanced basal GABA efflux after KCl washout with a similar biphasic concentration-dependence. Surprisingly, clonidine also enhanced [3H]-GABA release but had no effect on KCl-evoked [3H]-GABA release at concentrations which inhibited [3H]-NA efflux. These effects were potentiated by the GABA re-uptake inhibitor nipecotic acid. Together, our data indicate an important role for noradrenergic modulation in the SNr. The enhancing effect of both the alpha2 adrenoceptor agonist and antagonist on GABA release, while appearing paradoxical, can be rationalised by actions at distinct subsets of alpha2 adrenoceptors, using a simple model where alpha2A adrenoceptors are localized on the terminals of noradrenergic afferents impinging upon alpha2C adrenoceptor-containing GABAergic striato-nigral neurones.

Ultrasonic vocalization production of preweanling rats: effects of central and peripheral administration of alpha2-adrenoceptor agonists

Stimulation of alpha2-adrenoceptors increases the ultrasonic vocalization production of preweanling rats, however it is not known whether these critical alpha2-adrenoceptors are located peripherally or centrally. In a series of three experiments, ultrasonic vocalizations were measured after 11-day-old rats had been administered clonidine or 2-[2,6-diethylphenylamino]-2-imidazole (ST-91) either systemically (i.p.) or into the third ventricle (i.c.v.). These particular alpha2-adrenoceptor agonists were chosen because clonidine is lipophilic and enters the central nervous system, while ST-91 is hydrophilic and does not readily cross the blood-brain barrier. In the third experiment, clonidine- (1 microg, i.c.v.) and ST-91-induced (15 microg, i.c.v.) ultrasonic vocalizations were measured after systemic injection of the alpha2-adrenoceptor antagonist yohimbine (0.5 or 1 mg/kg, i.p.). Results showed that central administration of both clonidine and ST-91 increased the ultrasonic vocalization production of 11-day-old rats, whereas peripheral administration of only clonidine, and not ST-91, increased ultrasonic vocalizations. These results indicate that the alpha2-adrenoceptors mediating ultrasonic vocalization production are located in the central nervous system. Yohimbine fully attenuated clonidine-induced ultrasonic vocalizations but only partially attenuated ST-91-induced vocalizations. This pattern of results may have been due to the differential selectivity of clonidine and ST-91 for alpha2-adrenoceptor subtypes (alpha2A, alpha2B, and alpha2C) or imidazoline receptors. When combined with past research, the present results are consistent with the hypothesis that centrally located alpha2-adrenoceptors are a component of a neural system that mediates ultrasonic vocalization production.

Alpha-2 agonist-induced memory impairment is mediated by the alpha-2A-adrenoceptor subtype

The activation of alpha2-adrenoceptors has been reported to impair memory functions in both rats and humans. The alpha2-adrenoceptor subtype responsible for this detrimental effect is still unknown. The effect of the alpha2-agonists clonidine and guanabenz on memory processes, in dependence to the time of administration, was evaluated in the mouse passive avoidance test. Clonidine (0.02-0.2 mg kg(-1) i.p.) and guanabenz (0.1-0.3 mg kg(-1) i.p.) induced amnesia in a dose-dependent manner. From time-course experiments emerged that the impairment of memory function was detectable only when clonidine and guanabenz were administered 60 min before or immediately after the training test, respectively. This detrimental effect was prevented by pretreatment with the alpha2-antagonist yohimbine (1-3 mg kg(-1) i.p.) and by the alpha2A-antagonist BRL-44408 (0.3-1 mg kg(-1) i.p.). By contrast, the alpha(2B,C) antagonists ARC-239 (10 mg kg(-1) i.p.) and prazosin (1 mg kg(-1) i.p.) did not revert the amnesia induced by both clonidine and guanabenz. At the highest effective doses, clonidine and guanabenz were devoid of behavioral side-effects as well as maintained unaltered the motor coordination, as revealed by the rota-rod test. Furthermore, none of the compounds used modified the spontaneous motility as indicated by the Animex apparatus. These results indicate that clonidine and guanabenz impaired memory processes in a mouse passive avoidance paradigm through the selective activation of the alpha2A-adrenoceptor subtype.

Alpha2-adrenergic receptor subtype specificity of intrathecally administered tizanidine used for analgesia for neuropathic pain

Object. Intrathecally administered α2-adrenergic receptor subtype—specific antagonists were used to determine which α2-adrenergic receptor subtype mediates the analgesic effect of intrathecally administered tizanidine in a chronic constriction injury (CCI) rat model of neuropathic pain.

Methods. Seven days after CCI and intrathecal catheter surgeries had been performed in Sprague—Dawley rats, baseline neuropathic pain tests including cold-floor ambulation and paw pinch were performed. Either the dimethyl sulfoxide vehicle (seven rats) or one of the antagonists—5, 23, or 46 µg yohimbine (22 rats); 5, 25, 50, or 100 µg prazosin (25 rats); or 5, 45, or 90 µg WB4101 (11 rats)—were intrathecally administered to the animals, followed in 30 minutes by 50 µg intrathecally administered tizanidine. The neuropathic pain tests were repeated 30 minutes later. The resulting profile showed a descending order of antagonist efficacy for yohimbine, prazosin, and WB4101 for the cold-floor ambulation test and for the paw-pinch test of the affected paw. As expected given tizanidine's lack of analgesic effect on the contralateral, normal paw, there were no effects of antagonists on contralateral paw responses. The results of the paw-pinch test on the affected side were compared with binding data cited in the existing literature for the three different α2-adrenergic receptor subtypes (α2A, α2B, and α2C) with yohimbine, prazosin, and WB4101. The antagonist response profile for the paw-pinch test of the affected paw most closely approximated the α2B receptor binding profile.

Conclusions. The antagonist profile from the current study is most consistent with the theory that the α2B-adrenergic receptor subtype mediates the analgesic effect of intrathecally administered tizanidine on CCI-associated neuropathic pain.

Dexmedetomidine produces its neuroprotective effect via the α2A-adrenoceptor subtype

Which of the three alpha2-adrenoceptor subtypes of alpha2A, alpha2B, or alpha2C mediates the neuroprotective effect of dexmedetomidine was examined in cell culture as well as in an in vivo model of neonatal asphyxia. Dexmedetomidine dose-dependently attenuated neuronal injury (IC50=83+/-1 nM) in neuronal-glial co-cultures derived from wild-type mice; contrastingly, dexmedetomidine did not exert neuroprotection in injured cells from transgenic mice (D79N) expressing dysfunctional alpha2A-adrenoceptors. An alpha2A-adrenoceptor subtype-preferring antagonist 2-[(4,5-Dihydro-1H-imidazol-2-yl)methyl]-2,3-dihydro-1-methyl-1H-isoindole maleate (BRL44408) completely reversed dexmedetomidine-induced neuroprotection, while other subtype-preferring antagonists 2-[2-(4-(2-Methoxyphenyl)piperazin-1-yl)ethyl]-4,4-dimethyl-1,3-(2H,4H)-isoquinolindione dihydrochloride (ARC239) (alpha2B) and rauwolscine (alpha2C) had no significant effect on the neuroprotective effect of dexmedetomidine in neuronal-glial co-cultures. Dexmedetomidine also protected against exogenous glutamate induced cell death in pure cortical neuron cultures assessed by flow cytometry and reduced both apoptotic and necrotic types of cell death. Likewise this neuroprotective effect was antagonised by BRL44408 but not ARC239 or rauwolscine. Dexmedetomidine exhibited dose-dependent protection against brain matter loss in vivo (IC50=40.3+/-6.1 microg/kg) and improved the neurologic functional deficit induced by the hypoxic-ischemic insult. Protection by dexmedetomidine against hypoxic-ischemic-induced brain matter loss was reversed by the alpha2A-adrenoceptor subtype-preferring antagonist BRL44408; neither ARC239 nor rauwolscine reversed the neuroprotective effect of dexmedetomidine in vivo. Our data suggest that the neuroprotective effect of dexmedetomidine is mediated by activation of the alpha2A adrenergic receptor subtype.

[Ethyl-3H]RS-79948-197 α2-adrenoceptor autoradiography validation in α2-adrenoceptor knockout mice

[Ethyl-(3)H][8aR,12aS,13aS]-5,8,8a,9,10,11,12,12a,13,13a-decahydro-3-methoxy-12-(ethylsulfonyl)-6H-isoquino[2,1-g][1,6]naphthyridine ([ethyl-(3)H]RS-79948-197) was evaluated for alpha(2)-adrenoceptor autoradiography in brain sections from wild-type mice and alpha(2A)- and alpha(2ABC)-adrenoceptor knockout mice. Receptor numbers were 83% lower in cortex and 28% lower in caudate putamen of alpha(2A)-knockout mice than in wild-type mice. No specific binding was seen in alpha(2ABC)-knockout mice. [Ethyl-(3)H]RS-79948-197 saturation binding parameters were compared to those of [(3)H]2-(2,3-dihydro-2-methoxy-1,4-benzodioxan-2-yl)-4,5-dihydro-1H-imidazoline ([(3)H]RX821002) and [methyl-(3)H]17alpha-hydroxy-20alpha-yohimban-16beta-carboxylic acid methyl ester ([methyl-(3)H]rauwolscine). [Ethyl-(3)H-]RS-79948-197 detected a larger number of both alpha(2A)- and alpha(2B/C)-adrenoceptors than [(3)H]RX821002, while [methyl-(3)H]rauwolscine only underestimated the number of alpha(2A)-adrenoceptors. Oxymetazoline and prazosin competed for [ethyl-(3)H]RS-79948-197 binding with the expected rank order of affinities. Higher than necessary [ethyl-(3)H]RS-79948-197 concentrations resulted in a rapid increase in non-specific binding. Slow dissociation kinetics, high specific radioactivity and high alpha(2)-adrenoceptor affinity (slightly lower for the alpha(2A)-adrenoceptor than for the other subtypes) confer [ethyl-(3)H]RS-79948-197 distinct advantages compared to [(3)H]RX821002 for detection of alpha(2)-adrenoceptor subtypes in a mixed alpha(2)-adrenoceptor population.

Effect of alpha 2-adrenoceptor blockade on lithium action in the rat brain

The inhibitory effect of different concentrations of lithium (0.15-10 x 10(-3) M) on cAMP production induced by isoprenaline (1 x 10(-4) M) after the blockade of alpha(2)-adrenoceptors in the rat cerebral cortex was investigated. Low lithium concentrations (0.3-0.6 x 10(-3) M) exerted a significant inhibitory effect after yohimbine (1 x 10(-5) M) addition, but had no effect when isoprenaline alone or prazosin (1 x 10(-7) M) was added. The recovery of [3H]yohimbine binding after irreversible inactivation by N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) was evaluated in cortical membranes to study how alpha(2)-adrenoceptor repopulation affects the action of lithium on the adenylyl cyclase-cAMP system. When the density of alpha(2)-adrenoceptors was lower than 21%, lithium showed a significant inhibitory effect at all concentrations tested. However, at higher densities, increased concentrations of lithium were required to inhibit cAMP production. Our results suggest that the inhibitory effect of lithium on cAMP levels in the rat brain is conditioned by alpha(2D)-adrenoceptors.

The anticonvulsant and proconvulsant effects of α2-adrenoreceptor agonists are mediated by distinct populations of α2a-adrenoreceptors

The alpha2-adrenoreceptor (AR) is the most investigated noradrenergic receptor with regard to modulation of seizure activity. However, because of the complexity of multiple alpha2-AR subtypes and their distribution, the exact role of this receptor in modulating seizure activity is not clear. alpha2A- and alpha2C-ARs function as both autoreceptors (presynaptic) on noradrenergic neurons, where they regulate norepinephrine (NE) release, and as postsynaptic receptors on neurons that receive noradrenergic innervation, where they regulate the release of other neurotransmitters (heteroreceptor). The nonselective alpha2-AR agonist clonidine produced a proconvulsant effect on seizure susceptibility, while the selective alpha2A-AR agonist guanfacine was anticonvulsant. The effects of both alpha2-AR agonists were absent in alpha2a knockout mice, suggesting that the alpha2A-AR mediates the proconvulsant and anticonvulsant effect of alpha2-AR agonists on seizure susceptibility. To determine whether the alpha2-AR agonists were acting on inhibitory presynaptic autoreceptors to decrease NE release or on postsynaptic receptors on NE target neurons, the effects of clonidine and guanfacine were determined in dopamine beta-hydroxylase knockout (Dbh -/-) mice that lack NE. The anticonvulsant effect of guanfacine persisted in Dbh -/- mice, suggesting that guanfacine may act preferentially on alpha2A-postsynaptic receptors that regulate the action of NE on target neurons. In contrast, the proconvulsant effect of clonidine was lost in Dbh -/- mice, suggesting that clonidine may act on presynaptic autoreceptors to decrease NE release. We hypothesize that the alpha2A-presynaptic autoreceptor is responsible for the proconvulsant effect of alpha2-AR agonists, while the alpha2A-postsynaptic receptor is responsible for the anticonvulsant effect of alpha2-AR agonists. These data help to clarify the inconsistent effects of alpha2-AR agonists on seizure activity.

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.

Interactions of xylazine and detomidine with alpha2-adrenoceptors in brain tissue from cattle, swine and rats

Xylazine is an alpha2-adrenoceptor agonist sedative with a much higher interspecies variability in effect than detomidine, another alpha2-agonist used in veterinary practice. In the present study, we have used radioligand binding in brain tissue to investigate if the high species variation in sensitivity to xylazine could be explained in terms of receptor interactions. Species known to be more (cattle) or less (swine and rats) sensitive to xylazine were used. There was no variation in the density or the subtype pattern of the alpha2-adrenoceptors that could explain the species variation recorded in vivo, as a homogenous population of the alpha2A/D-subtype (200-300 fmol/mg protein) was found in all species. The species differences in the affinities of xylazine and detomidine were minor and similar for the two drugs. The only parameter investigated where a significant species difference was found for xylazine but not for detomidine was the slope of the inhibition binding curve when the G-protein coupling was diminished. For xylazine this slope was considerably lower than unity (i.e. 0.77 +/- 0.075) using cattle preparations compared with 0.92 +/- 0.037 (mean +/- SE) and 0.90 +/- 0.028, respectively for swine and rats, while for detomidine this parameter was close to unity in all species (cattle, swine, rat). This finding indicates that the species variation in effect for xylazine could be due to differences at the G-protein level or further down-stream in the effect cascade.

Expression of alpha(2)-adrenergic receptor subtypes in prenatal rat spinal cord

The results of molecular cloning have revealed three subtypes of the alpha(2)-adrenergic receptors (alpha(2) AR) that have been defined alpha(2)C10 (alpha(2A)), alpha(2)C2 (alpha(2B)) and alpha(2)C4 (alpha(2C)). The differential expression of alpha(2) AR subtypes is affected by developmental factors in rat submandibular gland, lung and brain. In the spinal cord of postnatal and adult rats, alpha(2A) and alpha(2C) AR subtypes are expressed and appear to mediate pain perception. However, the relative expression of alpha(2) AR subtypes in the prenatal spinal cord is unknown. In the present study subtype-specific antibodies and reverse transcription-polymerase chain reaction (RT-PCR) were used to determine the expression and localization of the alpha(2) AR subtypes in sections of embryonic day 14 rat spinal cords and primary cultures of cells isolated from these cords. Spinal cords were removed from day 14 embryos, and were sectioned or used for the preparation of cell cultures. After 9 days in culture, neurons were examined by immunofluorescence microscopy or used for preparation of total RNA. In both intact spinal cords and isolated cells, positive immunoreactivity was detected with antibodies against alpha(2A) and alpha(2B) subtypes, but not with antibodies against the alpha(2C) subtype. Using a dual-labeling approach, anti-alpha(2A) and anti-alpha(2B) immunoreactivity was present on the same population of neurons. RT-PCR results were consistent with immunofluorescence studies, and showed that mRNA encoding the alpha(2A) and alpha(2B) subtypes was present in total RNA prepared from primary cultures of rat spinal cord neurons. In contrast to spinal cords of postnatal or adult rats that express alpha(2A) and alpha(2C) AR subtypes on different neurons, prenatal spinal cords contain alpha(2A) and alpha(2B) AR subtypes, and these two subtypes appear to be co-expressed in the same cells.

Noradrenergic alpha-2 receptor agonists reverse working memory deficits induced by the anxiogenic drug, FG7142, in rats

Performance on working memory tasks, a measure of prefrontal cortical function, is impaired by exposure to mild stress as well as the anxiogenic drug, FG7142. Previous studies have shown that like stress, FG7142 increases catecholamine release in the prefrontal cortex (PFC) and that high levels of dopamine (DA) D(1) and norepinephrine (NE) alpha-1 receptor stimulation underlie the FG7142-induced cognitive impairment. Both the FG7142-induced DA turnover and working memory deficit can be blocked by pretreatment with the nonselective NE alpha-2/imidazoline I1 receptor agonist, clonidine. The present study examined the alpha-2 adrenoceptor subtype underlying this reversal in FG7142-induced working memory deficits by comparing the efficacy of clonidine with the more selective alpha-2A adrenoceptor agonist, guanfacine. The anxiogenic drug, FG7142 (0, 10, 20, or 30 mg/kg), dose-dependently impaired delayed alternation performance. Clonidine pretreatment (0.1 mg/kg, 30 min prior to FG7142) partially reversed the FG7142-induced impairment while guanfacine pretreatment (0.11 mg/kg) completely blocked the FG7142-induced impairment. Neither clonidine nor guanfacine had any effect on performance when administered alone. This study suggests that stimulation of the NE alpha-2A receptor subtype is sufficient to ameliorate the cognitive deficit induced by FG7142. Clonidine's sedative and hypotensive side effects limit its therapeutic usefulness; however, selective alpha-2A receptor agonists may be effective in treating prefrontal cognitive deficits in stress-related neuropsychiatric disorders with fewer side effects.

Alpha(2)-adrenoceptor-stimulated GTP gamma S binding in rat brain: an autoradiographic study

Agonist-stimulated [35S]GTP gamma S binding by alpha(2)-adrenoceptors was examined in rat brain by autoradiography. Epinephrine, norepinephrine, dexmedetomidine and brimonidine stimulated [35S]GTP gamma S binding in a dose-dependent manner. Agonist-stimulated binding was blocked by the specific alpha(2)-adrenoceptor antagonist (1, 4-benzodioxan-2-methoxy-2-yl)-2-imidazoline hydrochloride (RX821002). Each alpha(2)-adrenoceptor agonist stimulated [35S]GTP gamma S binding in the same brain regions, corresponding to alpha(2)-adrenoceptor distribution determined by [125I]para-iodoclonidine autoradiography. The order of antagonist potency (RX821002>idazoxan>rauwolscine>phentolamine>prazosin), and weak inhibition by propranolol and selective serotonin antagonists, indicate that epinephrine-stimulated [35S]GTP gamma S binding is mediated primarily by alpha(2)-adrenoceptors. Several antagonists increased [35S]GTP gamma S binding at very high concentrations, and this effect had anatomic and pharmacologic characteristics of binding mediated by 5-HT(1A) receptors. These studies demonstrate functional linkage of alpha(2)-adrenoceptors to G proteins in tissue sections, thus providing data on neuroanatomic localization and a means to examine drug specificity at alpha(2)-adrenoceptors in different brain regions.

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.

Effects of the I(1) imidazoline/alpha(2)-adrenergic receptor agonist moxonidine in comparison with clonidine in the formalin test in rats

Moxonidine is a mixed I(1) imidazoline/alpha(2)moxonidine=morphine. The I(1) imidazoline preferring antagonist efaroxan produced a dose-dependent antagonism of both moxonidine (5.0 mg/kg) and clonidine (0.5 mg/kg). In addition, the alpha(2)-adrenergic receptor antagonist yohimbine produced a dose-related antagonism of moxonidine, but only partially antagonized clonidine. Prazosin failed to block the effects of either moxonidine or clonidine, indicating a lack of involvement of alpha(1) as well as alpha(2B) and alpha(2C) receptors. The present results suggest that alpha(2)-adrenergic receptors play an important role in mediating the effects of moxonidine in producing antinociception in the formalin test. Further, the present results demonstrate that the mechanism of action of moxonidine and clonidine differ in that clonidine, but not moxonidine, produces an antinociceptive effect through a yohimbine-insensitive mechanism in the formalin test.

Antinociception induced by amitriptyline and imipramine is mediated by alpha2A-adrenoceptors

The involvement of alpha2-adrenoceptors in the antinociception induced by the tricyclic antidepressants amitriptyline and imipramine was investigated in mice by using the hot-plate and abdominal constriction tests. The antinociception produced by amitriptyline (15 mg/kg, i.p.) and imipramine (15 mg/kg, i.p.) was prevented by reserpine (2 mg/kg, i.p.) and yohimbine (3-10 mg/kg, i.p.) but not by naloxone (1 mg/kg, i.p.), atropine (5 mg/kg, i.p.), CGP 35348 (100 mg/kg, i.p.) and prazosin (1 mg/kg, i.p.). On the basis of the above data, it can be postulated that amitriptyline and imipramine exerted their antinociceptive effect by activation of alpha2-adrenoceptors. Administration of the alpha2A-adrenoceptor antagonist BRL 44408 (1 mg/kg, i.p.) prevented amitriptyline and imipramine antinociception, whereas the alpha2B/C-adrenoceptor antagonist ARC 239 (10 mg/kg, i.p.) was ineffective. These data indicate that the enhancement of the pain threshold produced by amitriptyline and imipramine is mediated by activation of alpha2A-adrenoceptors. Neither tricyclic antidepressants nor the antagonists used impaired mouse performance evaluated by the rota-rod and hole-board tests.

Characterization of a new radioiodinated probe for the alpha2C adrenoceptor in the mouse brain

[125I]17alpha-hydroxy-20alpha-yohimban-16beta-(N-4-p6 hydroxyphenethyl)carboxamide or [125I]rauwolscine-OHPC, a new radioiodinated probe derived from rauwolscine was synthesized and its binding characteristics investigated on sections of the mouse caudate putamen. [125I]rauwolscine-OHPC binding was saturable and revealed interaction with a single class of binding sites (KD= 0.171 nM, Bmax = 3082 pCi/mg of tissue). The kinetically derived affinity was in close agreement with the affinity evaluated by saturation experiments: k(-1)/k(+1)(0.0403 min(-1)/114 10(6) M(-1) min(-1))=0.35 nM. Competition studies revealed interaction with one single class of binding sites for each of the twelve compounds tested. The rank of potency suggested an interaction with alpha2 adrenoceptors (atipamezole > or = RX 821002 > yohimbine > (-)epinephrine). Moreover, the good affinity of [125I] rauwolscine-OHPC binding sites for spiroxatrine, yohimbine, WB 4101, the relatively good affinity for prazosin (Ki =37.4 nM) and the affinity ratio prazosin/oxymetazoline (37.4/43.4=0.86) were consistent with an alpha2C selective labelling of [125I]rauwolscine-OHPC. The distribution of [125I]rauwolscine-OHPC binding sites in mouse brain was characterized by autoradiography. The density of binding sites was high in the islands of Calleja, accumbens nucleus, caudate putamen and olfactory tubercles, moderate in the hippocampus, amygdala and anterodorsal nucleus of the thalamus. These findings demonstrated that [125I]rauwolscine-OHPC is a useful radioiodinated probe to label alpha2C adrenoceptors in mouse brain.

Noradrenaline release from cultured mouse postganglionic sympathetic neurons: autoreceptor-mediated modulation

The possible existence of alpha2-autoreceptors, P2-autoreceptors, and adenosine A1- or A2A-receptors was studied in cultured thoracolumbar postganglionic sympathetic neurons from mice. The cells were preincubated with [3H]noradrenaline and then superfused. The selective alpha2-adrenoceptor agonist UK 14,304 reduced the electrically evoked overflow of tritium. When the cultures were stimulated by trains of increasing pulse number, ranging from a single pulse to 72 pulses at 3 Hz, the concentration-inhibition curve of UK 14,304 was shifted progressively to the right and the maximal inhibition obtainable became progressively smaller. Six alpha-adrenoceptor antagonists shifted the concentration-inhibition curve of UK 14,304 in a parallel manner to the right. Neither ATP (3-300 microM), adenosine (0.01-100 microM), the selective A1-receptor agonist cyclopentyladenosine (1-1,000 nM), nor the selective A2A-receptor agonist CGS-21680 (1-10,000 nM) changed the basal or the electrically evoked overflow of tritium. It is concluded that the cultured neurons possess presynaptic, release-inhibiting alpha2-autoreceptors. As in intact tissues, the effectiveness of presynaptic alpha2-adrenergic inhibition depends on the "strength" of the releasing stimulus. The pK(D) values of the six antagonists against UK 14,304 indicate that the autoreceptors belong to the pharmacological alpha2D and hence the genetic alpha(2A/D) subtype of alpha2-adrenoceptor. Neither P2-autoreceptors nor receptors for adenosine, the degradation product of ATP, were detected.

Subtypes of functional alpha1- and alpha2-adrenoceptors

In this review, subtypes of functional alpha1- and alpha2-adrenoceptors are discussed. These are cell membrane receptors, belonging to the seven transmembrane spanning G-protein-linked family of receptors, which respond to the physiological agonists noradrenaline and adrenaline. Alpha1-adrenoceptors can be divided into alpha1A-, alpha1B- and alpha1D-adrenoceptors, all of which mediate contractile responses involving Gq/11 and inositol phosphate turnover. A 4th alpha1-adrenoceptor, the alpha1L-, has been postulated to mediate contractions in some tissues, but its relationship to cloned receptors remains to be established. Alpha2-adrenoceptors can be divided into alpha2A-, alpha2B- and alpha2C-adrenoceptors, all of which mediate contractile responses. Prejunctional inhibitory alpha2-adrenoceptors are predominantly of the alpha2A-adrenoceptor subtype (the alpha2D-adrenoceptor is a species orthologue), although alpha2C-adrenoceptors may also occur prejunctionally. Although alpha2-adrenoceptors are linked to inhibition of adenylate cyclase, this may not be the primary signal in causing smooth muscle contraction; likewise, prejunctional inhibitory actions probably involve restriction of Ca2+ entry or opening of K+ channels. Receptor knock-out mice are beginning to refine our knowledge of the functions of alpha-adrenoceptor subtypes.

Characterization of the enzymatic activity for biphasic competition by guanoxabenz (1-(2,6-dichlorobenzylidene-amino)-3-hydroxyguanidine) at alpha2-adrenoceptors. I. Description of an enzymatic activity in spleen membranes

The mechanism for formation of high-affinity binding of 1-(2,6-dichlorobenzylidene-amino)-3-hydroxyguanidine (guanoxabenz) to alpha2-adrenoceptors was studied in particulate fractions from the rat spleen. The proportion of apparent high versus low-affinity alpha2-adrenoceptor binding sites increased with increasing incubation time and was also augmented by Mg2+ ions. The formation of high-affinity guanoxabenz binding seemed to be inhibited by a series of N-hydroxyguanidine analogs to guanoxabenz, as well as by a series of metabolic inhibitors that included allopurinol, 1-chloro-2,4-dinitrobenzene, 5,5'-dithiobis-(2-nitrobenzoic acid), cibacron blue, phenyl-p-benzoquinone, didox, and trimidox. The formation of guanoxabenz high-affinity binding was also inhibited in a time- and concentration-dependent fashion by preincubating the membranes with the LW03 N-hydroxyguanidine analogue of guanoxabenz. Moreover, when the spleen membranes were extensively washed for 30 min with buffers at 25 degrees, the guanoxabenz high-affinity binding disappeared. However, when these washed membranes were supplemented with xanthine, the apparent affinity of guanoxabenz increased four to five-fold. Taken together, all data were compatible with the theory that the formation of high-affinity binding was dependent on the generation of a guanoxabenz metabolite that showed an approximate 100-fold greater affinity for the alpha2-adrenoceptors than guanoxabenz itself. Because the most potent blocker of the formation of high-affinity binding was allopurinol (apart from some N-hydroxyguanidine analogs to guanoxabenz) and since the activity could be restored with xanthine, a likely candidate responsible for the metabolic activation is xanthine oxidase.

Characterization of the enzymatic activity for biphasic competition by guanoxabenz (1-(2,6-dichlorobenzylidene-amino)-3-hydroxyguanidine) at alpha2-adrenoceptors. II. Description of a xanthine-dependent enzymatic activity in spleen cytosol

The mechanism for formation of high affinity binding of guanoxabenz (1-(2,6-dichlorobenzylidene-amino)-3-hydroxyguanidine) to alpha2-adrenoceptors by the rat spleen cytosol was studied. We report here that the spleen cytosolic fraction mediated the reduction of guanoxabenz to guanabenz (1-(2,6-dichlorobenzylidene-amino)-3-guanidine), the latter having an almost 100-fold higher affinity for rat alpha2A-adrenoceptors than guanoxabenz itself. The reaction product could be separated by high-performance liquid chromatography and its identity as guanabenz confirmed by nuclear magnetic resonance. The spleen cytosolic activity could be separated into high and low molecular weight components, the high molecular weight component requiring low molecular weight factors for maximal activity. Xanthine oxidase seems to be the most likely candidate responsible for the activity, as the guanoxabenz-reducing activity of the high molecular weight component could be sustained by exogenously applied xanthine, while it was potently blocked by allopurinol. The conversion of guanoxabenz by the cytosolic activity was also quite potently blocked by DWO1, 1-(3,4-dimethoxybenzylideneamino)3-hydroxyguanidine, a hydroxyguanidine analogue to guanoxabenz.

Effects of the alkylating agent EEDQ on regulatory G proteins and recovery of agonist and antagonist alpha2-adrenoceptor binding sites in rat brain

The aim of this study was to assess the effect of N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ)-induced alpha2-adrenoceptor inactivation on regulatory G proteins and the recovery of agonist and antagonist binding sites. EEDQ induced a rapid increase in the abundance of rat brain cortical Galphai1/2 proteins (30% at 6 h) which reached a maximum at 4 days (45%) and which then slowly returned (7-30 days) to control values. EEDQ did not alter significantly the levels of Galphai3 and Galphao proteins. By using the standard monoexponential model, the analysis of the recovery of alpha2-adrenoceptor density (6 h-30 days) with [3H]UK 14304 (bromoxidine) and [3H]RX 821002 (2-metoxy idazoxan) in the cerebral cortex did not reveal differences in receptor turnover parameters. However, the recovery of [3H]UK 14304 binding fitted best to a new biphasic recovery model, suggesting the existence of two distinct phases of recovery of agonist sites (r1 and r2 = 15.7 and 7.4 fmol mg protein(-1) day(-1); k1 and k2 = 0.51 and 0.25 day(-1); (t1/2)1 and (t1/2)2 = 1.4 and 2.7 days). In contrast, the recovery of [3H]RX 821002 antagonist sites did not fit to the biphasic model (r = 8.1, k = 0.14, t1/2 = 4.9). Because agonist binding requires coupling to G proteins, the present results suggest that the rapid over-expression of Galphai1/2 proteins induced by EEDQ is related to the biphasic recovery of [3H]UK 14304 binding. The possible implication of the faster recovery of alpha2-adrenoceptor function after EEDQ inactivation is discussed.

Characterization of alpha1-adrenoceptor subtypes in the pig

The identities of the alpha1-adrenoceptor subtypes present in various tissues of the pig were studied using [3H]prazosin radioligand binding. The subtypes were characterized by performing competition experiments for various subtype selective drugs. In the cerebral cortex, spleen and heart, both alpha1A- and alpha1B-adrenoceptors were detected. In the liver was found only the alpha1A-subtype, while in the aorta was found only the alpha1B-subtype. An alpha1-adrenoceptor subtype was present in the adrenal gland with a high affinity for prazosin, the pKd value being 9.6, but with relatively low affinities for other alpha1-adrenoceptor binding drugs. The adrenal gland alpha1-adrenoceptor did not seem to represent the classical alpha1D-subtype, since drugs selective for the alpha1D-subtype in other species, including BMY7378 and SKF104856, showed low affinities for the pig adrenal gland alpha1-adrenoceptor.

[3H]RS79948-197 binding to human, rat, guinea pig and pig alpha2A-, alpha2B- and alpha2C-adrenoceptors. Comparison with MK912, RX821002, rauwolscine and yohimbine

The Kd values of the recently introduced radioligand [3H]RS79948-197 ((8a R,12aS,13a-S)-5,8,8a,9,10,11,12,12a,13,13a-decahydro-3-metho xy-12-(ethylsulphonyl)-6H-isoquino[2,1-g][1,6]naphthyridine) were determined for the recombinant human and rat alpha2A-, alpha2B- and alpha2C- as well as guinea pig alpha2B- and alpha2c-adrenoceptors expressed in COS (CV-1 Origin, SV40) cells. In addition, the Kd values were also determined for [3H]RS79948-197 for the guinea pig spleen alpha2A-adrenoceptor and for pig alpha2A-, alpha2B- and alpha2C-adrenoceptors in membranes obtained from kidney and striatum. Available radioligands for alpha2-adrenoceptors, besides [3H]RS79948-197 are the tritiated forms of MK912 ((2S,12bS)1',3'-dimethylspiro(1,3,4,5',6,6',7,12b-octa hydro-2H-benzo[b]furo[2,3-a]quinazoline)-2,4'-pyrimidin-2'-one), RX821002 (2-methoxy-idazoxan), rauwolscine and yohimbine. In the present article the binding constants of all these substances for the alpha2A-, alpha2B- and alpha2C-adrenoceptor subtypes in human, pig, rat and guinea pig are reviewed. In all species tested MK912 was alpha2C-selective, RX821002 showed a minor alpha2A-selectivity, whereas [3H]RS79948-197 was non-selective among the alpha2-adrenoceptor subtypes, showing high affinity for all three subtypes. Rauwolscine and yohimbine showed relatively low affinities for nmost of the alpha2-adrenoceptor subtypes investigated, the exception being rauwolscine having high affinity for the human and porcine alpha2C-adrenoceptors.

Autoradiographic studies of central alpha 2A- and alpha 2C-adrenoceptors in the rat using [3H]MK912 and subtype-selective drugs

In the present study we examined the distribution of alpha 2A- and alpha 2C-adrenoceptors in tissue slices from the rat cervical spinal cord and from brain slices collected at the level of the striatum. To differentiate between alpha 2A- and alpha 2C-adrenoceptors, the slices were incubated with [3H]MK912 in the presence of graded concentrations of the alpha 2A-selective drug, BRL44408, or the alpha 2C-selective drug, spiroxatrine. Computer analysis of the autoradiograms indicated that 0.4 nM [3H]MK912 plus 185 nM BRL44408 selectively labeled alpha 2C-adrenoceptors, while 0.4 nM [3H]MK912 plus 220 nM spiroxatrine selectively labeled alpha 2A-adrenoceptors. Using this approach, alpha 2C-adrenoceptors were detected in the striatum, while alpha 2A-adrenoceptors predominated in the cortical layers 1-4, the spinal cord distal dorsal horn, the septum and the endopiriform nucleus.

The effects of alpha-2 agonist, medetomidine and its antagonist, atipamezole on reaction and movement times in a visual choice reaction time task in monkeys

Alpha-2 adrenoceptor agonists have been shown to improve the working memory task performance of aged monkeys. Suggestions offered to explain this finding include improved short-term memory processing, slight sedation, and decreased distractiveness. Although sedation is evident at high doses, it may also contribute to the working memory task performance at low doses. The aim of the present work was to find out whether the positive effects of an alpha-2 agonist, medetomidine, on working memory performance could be explained by its sedative effects. This was accomplished by measuring the reaction and movement times of monkeys performing a visual choice reaction time task under the influence of medetomidine or its antagonist atipamezole. In the task a trial began with the monkey holding a central pad. After a short period one of two lateral light emitting diodes was turned on for 300 ms and the monkeys were trained to release the central bar and touch either of the bars, situated below the diodes, depending on the location of the stimulus. The reaction and movement times were significantly longer than on saline control only at the highest dose of medetomidine (10.0 micrograms/kg). At the lowest dose of atipamezole (0.01 mg/kg), the reaction times were significantly shorter than on saline control. The results of this study demonstrate that low doses of medetomidine, which have earlier been shown to improve working memory performance, do not induce sedation as measured by reaction and movement times.

Reduced expression of alpha2C-adrenoceptors in rat striatum following antisense oligodeoxynucleotide infusion

The predominate subtypes of alpha2-adrenoceptors in the brain are alpha2A and alpha2C. The lack of selective ligands for these receptors hampers their functional characterization. We exploited an antisense strategy as an alternative pharmacological tool to study alpha2C-adrenoceptors. In rat striatum (caudate-putamen), alpha2-adrenoceptors were characterized using the subtype-non-selective antagonist [3H]2-(2-methoxy-1,4-benzodioxan-2-yl)-2-imidazoline ([3H]RX821002). Specific [3H]RX821002 binding was saturable and to a single class of high-affinity sites. Curves for the inhibition of [3H]RX821002 binding by the alpha2C-selective compound, prazosin, were fit best by a model assuming binding to two sites, presumably reflecting binding to alpha2A- and alpha2C-adrenoceptors. A 15-mer phosphorothioate oligodeoxynucleotide (alpha2C AS) complementary to the alpha2C-adrenoceptor mRNA, or a random sequence (RS) was administered to rats continuously for 4.5 days directly into the striatum. Compared to RS infusions, alpha2C AS infusions induced a 35% reduction in the Bmax of [3H]RX821002 in striatal homogenates (P < 0.05). Curves for the inhibition of [3H]RX821002 binding by prazosin were fit best by a model assuming a single interaction in alpha2C AS-infused rats and to a model assuming two sites in RS-infused rats. These results are consistent with the conjecture that both alpha2A- and alpha2C-adrenoceptors occur in the rat striatum and also demonstrate the feasibility of an antisense approach to examine the functional role of subtypes of alpha2C-adrenoceptors in the brain.

Mechanism of resistance to alpha-adrenergic receptor antagonists of renal nerve stimulation-induced vasoconstriction at low frequencies

To determine why renal vasoconstriction elicited by periarterial nerve stimulation (RNS) at lower frequencies (< 4 Hz) is resistant to alpha-adrenergic receptor blockade in the rat kidney, we reevaluated the effect of alpha-receptor antagonists on the vasoconstrictor response to norepinephrine (NE) and to RNS and on the release of adrenergic transmitter. The alpha-receptor antagonist prazosin (PZ) at 0.2 and 7 nM reduced the vasoconstrictor response to NE, and 2.4 microM PZ abolished it. PZ (0.2 or 7 nM) reduced RNS-induced vasoconstriction without altering the fractional tritium overflow. PZ (2.4 microM) enhanced fractional tritium overflow and reduced the vasoconstrictor response to RNS at 2-10 Hz, but not at 0.5 or 1 Hz. The effect of 0.2 nM PZ to reduce RNS-induced vasoconstriction was reversed by increasing the concentration to 2.4 microM. Corynanthine (COR; 2.6 microM), a preferential alpha-receptor blocker, or phenoxybenzamine (PBZ; 30 nM) abolished the vasoconstrictor response to NE but only partially reduced response to RNS and enhanced the fractional tritium overflow. Rauwolscine (RW; 2.5 nM), a preferential alpha 2-receptor antagonist, did not alter the vasoconstrictor response to NE but potentiated RNS-induced vasoconstriction and fractional tritium overflow. RW (7.7 microM) inhibited NE-induced vasoconstriction but potentiated the vasoconstrictor response to RNS and fractional tritium overflow. PZ (7 nM) abolished the potentiation by RW and reduced the vasoconstrictor response to RNS. These data suggest that a component of RNS-induced vasoconstriction in the rat kidney is attributable to co-release of a nonadrenergic transmitter with NE. The diminished effect of alpha-receptor antagonists at higher concentrations (e.g., PZ 2.4 microM) to reduce RNS-induced vasoconstriction is caused by their prejunctional action to enhance co-release of the nonadrenergic transmitter.

Effects of clonidine on a C-fibre reflex in the rat

A C-fibre reflex elicited by electrical stimulation within the territory of the sural nerve, was recorded from the ipsilateral biceps femoris muscle in anaesthetized rats. The temporal evolution of the response was studied using a constant stimulus intensity (3 x threshold) and recruitment curves were built by varying stimulus intensity from 0 to 7 x threshold. The intravenous administration of 0.02-0.2 mg/kg clonidine resulted in a dose-dependent depression of the C-fibre reflex. The alpha 2-adrenoceptor antagonist idazoxan completely prevented this depressive effect of clonidine. The effects of clonidine on the C-fibre reflex elicited by a wide range of stimulus intensities were investigated using recruitment curves: following 0.16 mg/kg clonidine, a dramatic shift of the recruitment curve to the right was seen with both an increase in the threshold and a decrease in the slope. Clonidine also produced a dose-dependent increase in blood pressure, but this was not correlated with the depression of the nociceptive reflex.

Adrenoceptor- and cholinoceptor-mediated mechanisms in the regulation of 5-hydroxytryptamine release from isolated tracheae of newborn rabbits

1. Isolated tracheae of newborn rabbits were incubated in vitro and the outflow of 5-hydroxytryptamine (5-HT) was determined by h.p.l.c. with electrochemical detection. Evidence has previously been provided that this 5-HT outflow derives from neuroendocrine epithelial (NEE) cells of the airway mucosa. 2. Phenylephrine (1, 10 and 30 microM) enhanced the outflow of 5-HT by 80, 290 and 205%, respectively. 5-HT outflow evoked by 10 microM phenylephrine was not affected by the presence of the neurotoxin tetrodotoxin (1 microM). 3. Rauwolscine, ARC 239 (an alpha(2B)-adrenoceptor preferring antagonist), yohimbine and prazosin antagonized the effect of 10 microM phenylephrine in a concentration-dependent manner with IC50 values of 150, 295, 300 and 1,700 nM, respectively. Comparison of the ratios (between all antagonists) of the present IC50 values with the corresponding ratios of Ki values obtained in binding studies for the alpha(2A)-, alpha(2B)-, alpha(2C)- and alpha(2D)-adrenoceptor subtypes strongly suggests the involvement of an alpha(2B)-receptor. 4. 5-HT outflow evoked by 10 microM phenylephrine was inhibited by 65% in the presence of 1 microM forskolin and abolished in the presence of 10 microM forskolin. 5. 5-HT outflow evoked by 10 microM phenylephrine was inhibited by about 45 and 70% in the presence of 0.1 and 1 microM isoprenaline, respectively. The inhibitory effect of 1 microM isoprenaline was only marginally antagonized by 1 microM, but blocked by 10 microM propranolol. 6. 5-HT outflow was not affected by the muscarine receptor agonist oxotremorine (10 microM), but was enhanced by 175% by 100 microM nicotine. The effect of nicotine was blocked by 100 microM hexamethonium and prevented by 1 microM tetrodotoxin or 1 microM yohimbine. 7. In conclusion, 5-HT release from NEE cells of the rabbit trachea is stimulated via alpha-adrenoceptors most likely of the alpha(2B)-subtype localized directly at the NEE cells. Activation of beta-adrenoceptors as well as direct activation of adenylyl cyclase by forskolin exert inhibitory effects on 5-HT release. Activation of nicotinic, but not of muscarinic receptors, also evokes the release of 5-HT. However, the effect of nicotine appears to be mediated indirectly via the release of noradrenaline.

Presynaptic alpha 2-autoreceptors in mouse heart atria: evidence for the alpha 2D subtype

Presynaptic alpha 2-autoreceptors in mouse atria were characterized in terms of the alpha 2A, alpha 2B, alpha 2C and alpha 2D subtypes. Segments of the atria were preincubated with 3H-noradrenaline and then superfused and stimulated electrically. The affinity of up to 16 antagonists for the autoreceptors was assessed as (1) pEC30% values. i.e. concentrations that increased previously autoinhibited release of 3H-noradrenaline (120 pulses, 3 Hz) by 30%, and (2) pKd values against the release-inhibiting effect of 5-bromo-6-(2-imidazolin-2-ylamino)-quinoxaline (UK 14,304) under conditions of no or little autoinhibition (2 trains of 20 pulses, 50 Hz, train interval 120 s). The pKd values correlated well with the pEC30% values (r = 0.98; P < 0.001; slope of regression line 0.93), indicating that UK 14,304 and released noradrenaline modulated the release of noradrenaline through pharmacologically identical receptors. Comparison with antagonist affinities for (1) prototypic native alpha 2 radioligand binding sites, (2) radioligand binding sites in COS cells transfected with alpha 2 subtype genes, and (3) previously classified presynaptic alpha 2-autoreceptors-all taken from the literature-indicated that the mouse atrial autoreceptors corresponded to the alpha 2D subtype. For example, the pKd values at mouse atrial auto-receptors correlated closely with pKd values at native alpha 2D binding sites in the bovine pineal gland (r = 0.96; P < 0.001); with pKd values at alpha 2D binding sites in COS cells transfected with the rat alpha 2D gene (r = 0.85; P < 0.01); and with pKd values at guinea-pig cerebral and atrial and mouse cerebral alpha 2D-autoreceptors (r = 0.96-0.98; P < 0.001). The antagonist pKd values at mouse atrial autoreceptors correlated less with pKd values at alpha 2A, alpha 2B and alpha 2C sites. It is concluded that the presynaptic alpha 2-autoreceptors in mouse atria are alpha 2D. This identification supports the hypothesis that at least the majority of alpha 2-autoreceptors belong to the alpha 2A/D pair of orthologous alpha 2-adrenoceptors.

alpha 2A/D-Adrenoceptor subtype predominates also in the neonatal rat spinal cord

alpha 2-Adrenoceptors are remarkably regulated by developmental factors. In this study alpha 2-adrenoceptor subtypes have been characterised in neonatal and adult rat spinal cords. In saturation experiments, a 5% proportion of [3H]rauwolscine binding has a high affinity component, representing the alpha 2C-subtype in both tissues. Competition studies with [3H]RX821002 indicate that in both tissues the alpha 2A/D subtype is expressed similarly.

Antagonists that differentiate between alpha 2A-and alpha 2D-adrenoceptors

Four antagonists were examined for their ability to differentiate alpha 2A-from the orthologous alpha 2D-adrenoceptors. The antagonists were (2S,12bS)1',3'-dimethylspiro(1,3,4,5',6,6',7,12b-octah ydro-2H- benzo[b]furo[2,3-a]quinolizine)-2,4'-pyrimidin-2'-one (MK912), 2-[2-(methoxy-1,4-benzodioxanyl)imidazoline (RX 821002), efaroxan and benoxathian. The alpha 2-autoreceptors in rabbit brain cortex were chosen as alpha 2A-and the alpha 2-autoreceptors in guinea-pig brain cortex as alpha 2D-adrenoceptors. Slices of the brain cortex were preincubated with 3H-noradrenaline and then superfused and stimulated electrically by brief pulse trains (4 pulses, 100 Hz) that led to little, if any, alpha 2-autoinhibition. 5-Bromo-6-(2-imidazolin-2-ylamino)-quinoxaline (UK 14,304) was used as an alpha 2-adrenoceptor agonist. UK 14, 304 decreased the stimulation-evoked overflow of tritium. The antagonists shifted the concentration-inhibition curve of UK 14, 304 to the right in an apparently competitive manner. Dissociation constants of the antagonists were calculated from the shifts. MK 912, RX 821002 and efaroxan had markedly higher affinity for (guinea-pig) alpha 2D-adrenoceptors (pKd values 10.0, 9.7 and 9.1, respectively) than for (rabbit) alpha 2A-adrenoceptors (pKd 8.9, 8.2 and 7.6, respectively). Benoxathian had higher affinity for alpha 2A-(pKd 7.4) than for alpha 2D-adrenoceptors (pKd 6.9). Ratios calculated from the Kd values of the four compounds differentiated between alpha 2A and alpha 2D up to 100 fold. It is concluded that MK 912, RX 821002, efaroxan and benoxathian are antagonists with high power to differentiate alpha 2A-from alpha 2D-adrenoceptors.

A comparative study of the reversal by different alpha 2-adrenoceptor antagonists of the central sympatho-inhibitory effect of clonidine

1. The recovery of the clonidine-induced hypotension, bradycardia and sympatho-inhibition produced by several putative alpha 2-adrenoceptor antagonists was investigated in pentobarbitone anaesthetized rats. The activity of four substances containing an imidazoline structure: idazoxan, methoxy-idazoxan, BRL44408 and atipamezole was compared with the effect of fluparoxan, yohimbine and L-657,743; in addition the effect of the alpha 1-adrenoceptor antagonist, prazosin, was also studied. 2. Prazosin (0.03-1 mg kg-1, i.v.) failed to alter the sympatho-inhibitory and hypotensive effects of clonidine (10 micrograms kg-1, i.v.). L-657,743 (0.01-1 mg kg-1, i.v.) induced a recovery of blood pressure, heart rate and renal sympathetic nerve activity. Yohimbine (0.03-3 mg kg-1, i.v.) completely reversed the sympatho-inhibitory effect of clonidine but did not alter its hypotensive effect. 3. The four imidazoline drugs: idazoxan (10-300 micrograms kg-1, i.v.), methoxy-idazoxan (1-100 micrograms kg-1, i.v.), BRL44408 (0.1-3 mg kg-1, i.v.) and atipamezole (0.03-1 mg kg-1, i.v.) and fluparoxan (10-300 micrograms kg-1, i.v.) reversed the clonidine-induced hypotension but produced only a partial recovery of the renal sympathetic nerve activity and of the heart rate. After pretreatment with prazosin (0.1 mg kg-1, i.v.), the recovery of the sympathetic nerve activity elicited by these compounds was significantly higher. In hexamethonium (10 mg kg-1, i.v.) pretreated rats, these five drugs induced dose-related hypertension which was reduced by pretreatment with prazosin (0.1 mg kg-1, i.v.). 4. Our results indicate that the putative alpha 2-adrenoceptor antagonists idazoxan, methoxy-idazoxan, BRL44408, atipamezole and fluparoxan also have a peripheral hypertensive effect which is mediated through activation of vascular alpha 1-adrenoceptors; this property of the compounds may be partly responsible for the reversal of the hypotensive action of clonidine. Considering the structure and the affinities of the drugs tested, our data indirectly suggest that alpha 2A-adrenoceptors may be implicated in the central sympatho-inhibitory effects of clonidine.

alpha 2-Adrenergic receptors in human spinal cord: specific localized expression of mRNA encoding alpha 2-adrenergic receptor subtypes at four distinct levels

alpha 2-Adrenergic receptor (AR) subtype mRNA (alpha 2a, alpha 2b, alpha 2c) neuronal localization in human spinal cord has not been described. We therefore performed in situ hybridization to identify cell bodies at four levels of human spinal cord (cervical, thoracic, lumbar, sacral) containing alpha 2AR subtype specific mRNA. alpha 2AR mRNA is present in gray matter only (ventral > dorsal; sacral > cervical > thoracic = lumbar). In addition to alpha 2AR mRNA in cell bodies in thoracic and lumbar intermediolateral (sympathetic) and sacral intermediate (parasympathetic) cell columns (lamina VII), all levels in dorsal horn laminae I, II, V, and ventral horn lamina IX, we demonstrate alpha 2AR mRNA in dorsal horn laminae III and IV, and dorsal nucleus of Clarke, where alpha 2ARs have not been described. Previously unreported heterogeneity in alpha 2AR subtype distribution (alpha 2a and alpha 2bAR mRNA present, alpha 2cAR mRNA virtually absent) is found at all sites of alpha 2AR mRNA expression in human spinal cord, including locations known to mediate effects of alpha 2AR agonist drugs on nociception, autonomic function and motor tone. Cervical spinal cord demonstrates a predominance of alpha 2a mRNA signal, while thoracic, lumbar, and sacral spinal cord demonstrate an increasing predominance of alpha 2bAR mRNA. If confirmed at a protein level, these findings have profound implications for therapeutic strategies in managing human pain.

Human adipocytes express alpha 2-adrenergic receptor of the alpha 2A-subtype only: pharmacological and genetic evidence

In the present study we have reinvestigated the subtype of alpha 2-adrenoceptors expressed in human adipocytes (from subcutaneous and internal fat deposits) by means of radioligand binding using subtype-selective antagonists, and RNase mapping using a set of specific probes prepared from human alpha 2-adrenoceptors genes (alpha 2C2, alpha 2C4 and alpha 2C10). Comparison of the pharmacological properties of the human adipocyte alpha 2-adrenoceptors with those of the different human adrenoceptors expressed in COS-7 cells demonstrated that: i) human adipocyte alpha 2-adrenoceptors displays a KD for [3H]RX821002 and [3H]MK912 identical to that found in COS-7 cells transfected with the alpha 2C10 gene; ii) yohimbine and oxymetazoline is 1,000-fold more potent than prazosin to inhibit [3H]antagonist binding. RNase protection assays on cellular RNA prepared from the three fat deposits showed the presence of substantial amounts of alpha 2C10 transcripts: in contrast, mRNAs from alpha 2C2 and alpha 2C4 genes were undetectable. Altogether these results definitively establish that human adipocytes express only one alpha 2-adrenoceptor which is of the alpha 2A-subtype and encoded by the alpha 2C10 gene.

Alpha 2C-adrenoceptor-modulated release of noradrenaline in human right atrium

1. The aim of the present study was to characterize the presynaptic alpha 2-autoreceptors in human right atrium in terms of the alpha 2A-D system. Segments of atrial appendages were preincubated with [3H]-noradrenaline and then superfused in the presence of cocaine and stimulated electrically. pEC30% values of eight alpha-adrenoceptor antagonists with discriminatory power were determined. pEC30% is the negative logarithm of the antagonist concentration that increased the stimulation-induced overflow of tritium by 30%. For four antagonists, the dissociation constant KD was determined, in addition to pEC30%, against the overflow-inhibiting effect of 5-bromo-6-(2-imidazolin-2-ylamino)-quinoxaline (UK 14,304) under autoinhibition-free conditions. 2. pEC30% and KD values yielded identical rank orders of antagonist affinity (rauwolscine > WB 4101 > phentolamine > prazosin) suggesting that both released noradrenaline and the exogenous agonist UK 14,304 activated the same receptor to inhibit release. 3. The eight antagonist pEC30% values obtained in right atrium correlated significantly with their pEC30% values, reported in the literature, at the presynaptic alpha 2C-autoreceptors in human kidney (r = 0.817; slope of the regression line 1.03). No significant correlation was obtained between pEC30% values at atrial autoreceptors and pKD values at previously characterized alpha 2A-autoreceptors in rabbit and alpha 2D-autoreceptors in rat, mouse and guinea-pig tissues. 4. Comparison of antagonist pEC30% values with their pKD values at native alpha 2 binding sites in cells or tissues that express a single subtype only, and with pKD values at alpha 2 binding sites in membranes of COS cells transfected with human alpha 2 subtype genes confirms the alpha 2C character of the atrial autoreceptors: significant correlations were obtained exclusively with the alpha 2C binding sites. 5. Ratios of KD values were computed for alpha 2-autoreceptors in human right atrium and for binding sites in COS cells transfected with human alpha 2 subtype genes. The autoreceptor ratios corresponded well with the respective ratios for the alpha 2C binding sites (maximal three fold deviation) but were, in part, markedly different from ratios calculated for alpha 2A and alpha 2B binding sites (up to 166 fold deviation). This outcome supports the alpha 2C designation of the autoreceptors. 6. In conclusion, the presynaptic alpha 2-autoreceptors in human right atrium are alpha 2C. In this they agree with the previously characterized alpha 2-autoreceptors in human kidney. The alpha 2C classification possibly separates, in general, human alpha 2-autoreceptors from those in lagomorph (rabbit) and rodent (rat, mouse, guinea pig) species that have been proposed to be predominantly alpha 2A or alpha 2D.