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

Are the pharmacology and physiology of α₂ adrenoceptors determined by α₂-heteroreceptors and autoreceptors respectively?

α(2)-Adrenoceptors are important mediators of physiological responses to the endogenous catecholamines noradrenaline and adrenaline. In addition, α(2)-adrenoceptors are pharmacological targets for the treatment of hypertension, sympathetic overactivity and glaucoma. α(2)-Adrenoceptors are also targeted to induce sedation and analgesia in anaesthesia and intensive care. α(2)-Adrenoceptors were first described as presynaptic receptors inhibiting the release of various transmitters from neurons in the central and peripheral nervous systems. In addition to these presynaptic neuronal receptors, α(2)-adrenoceptors were also identified in many non-neuronal cell types of the body. Gene-targeting in mice provided a comprehensive assignment of the physiological and pharmacological functions of these receptors to specific α(2A)-, α(2B) - and α(2C)-adrenoceptor subtypes. However, the specific cell types and signalling pathways involved in these subtype-specific α(2)-adrenoceptor functions were largely unexplored until recently. This review summarizes recent findings from transgenic mouse models, which were generated to define the role of α(2)-adrenoceptors in adrenergic neurons, that is, α(2)-autoreceptors, versus α(2)-adrenoceptors in non-adrenergic neurons, termed α(2)-heteroreceptors. α(2)-Autoreceptors are primarily required to limit release of noradrenaline from sympathetic nerves and adrenaline from adrenal chromaffin cells at rest. These receptors are desensitized upon chronic activation as it may for instance occur due to enhanced sympathetic activity during chronic heart failure. In contrast, pharmacological effects of acutely administered α(2)-adrenoceptor agonist drugs essentially require α(2)-heteroreceptors in non-adrenergic neurons, including analgesia, sedation, hypothermia and anaesthetic-sparing as well as bradycardia and hypotension. Thus a clear picture has emerged of the significance of auto- versus heteroreceptors in mediating the physiological functions of α(2)-adrenoceptors and the pharmacological functions of α(2)-adrenoceptor agonist drugs respectively.

Morphine and clonidine synergize to ameliorate low back pain in mice

Chronic low back pain (LBP) is a debilitating condition associated with signs of axial and radiating pain. In humans with chronic LBP, opioids are often prescribed with varying outcomes and a multitude of side effects. Combination therapies, in which multiple pharmacological agents synergize to ameliorate pain without similar potentiation of adverse reactions, may be useful in improving therapeutic outcome in these patients. The SPARC-null mouse model of low back pain due to disc degeneration was used to assess the effects of opioid (morphine) and α₂-adrenergic agonist (clonidine) coadministration on measures of axial and radiating pain. The results indicate that systemic morphine and clonidine, coadministered at a fixed dose of 100 : 1 (morphine : clonidine), show a synergistic interaction in reversing signs of axial LBP, in addition to improving the therapeutic window for radiating LBP. Furthermore, these improvements were observed in the absence of synergy in assays of motor function which are indicative of side effects such as sedation and motor incoordination. These data show that the addition of low-dose systemic clonidine improves therapeutic outcome in measures of both axial and radiating pain. Combination therapy could be of enormous benefit to patients suffering from chronic LBP.

Effects of alpha 2-adrenoceptor agonists dexmedetomidine and guanfacine on morphine analgesia and tolerance in rats

BACKGROUND

Alpha 2 (α(2))-adrenoceptor agonists may be useful for their potential to increase or prolong opioid analgesia while attenuating the development of opioid tolerance. The purpose of this study was to investigate the effects of dexmedetomidine and guanfacine (α(2)-adrenoceptor agonists) on morphine analgesia and tolerance in rats.

METHODS

Adult male Wistar albino rats weighing 195-205 g were used. To constitute morphine tolerance, animals received morphine (50 mg/kg) once daily for 3 days. After the last dose of morphine had been injected on day 4, morphine tolerance was evaluated by analgesia tests. The analgesic effects of dexmedetomidine (20 ug/kg), guanfacine (0.5 mg/kg), MK-467 (0.25 mg/kg), and morphine were estimated at 30-min intervals (0, 30, 60, 90, and 120 min) by tail-flick and hot-plate analgesia tests.

RESULTS

Our findings indicate that dexmedetomidine and guanfacine attenuated the expression of morphine tolerance. In addition, administration of dexmedetomidine with morphine increased morphine analgesia. On the contrary, data suggested that MK-467 (an α(2)-adrenoceptor antagonist) decreased morphine analgesia and increased morphine tolerance in analgesia tests.

CONCLUSION

In conclusion, we observed that co-injection of dexmedetomidine or guanfacine with morphine attenuated the expression of tolerance to the analgesic effect of morphine and that dexmedetomidine enhanced the morphine analgesia.

A peripheral adrenoceptor-mediated sympathetic mechanism can transform stress-induced analgesia into hyperalgesia

BACKGROUND

Stress has paradoxical effects on pain, causing stress-induced analgesia but also exacerbating pain via poorly understood mechanisms. Adrenergic neurotransmission is integral in pathways that regulate the response to both pain and stress. Hyperalgesia is often associated with enhanced adrenergic sensitivity of primary afferents, but sympathetic nervous system outflow has not been demonstrated to exacerbate pain perception after stress.

METHODS

Rats or C57/BL6 wild-type mice treated with α-2 receptor antagonists or α-2A receptor knockout mice were exposed to ultrasonic noise stress or footshock stress and subsequently tested for hotplate paw withdrawal latencies. The sensory sensitivity of α-2A knockout mice to electrical and chemical stimuli was tested neurophysiologically and behaviorally. The effects of sympatholytic treatments were investigated.

RESULTS

Noise and footshock stressors induced thermal hyperalgesia in rats pretreated systemically with α-2 antagonists. Wild-type mice pretreated with α-2 antagonists and α-2A knockout mice also exhibited thermal hyperalgesia induced by noise stress. Local spinal or intraplantar injection of an α-2 antagonist counteracted stress-induced analgesia without causing hyperalgesia. The α-2A knockout mice had decreased thresholds for peripheral sensitization with sulprostone and for windup of the dorsal horn neuronal response to repetitive electrical stimuli. Stress-induced hyperalgesia was abolished and the sensitization was attenuated by sympathectomy or systemic administration of an α-1-adrenergic antagonist.

CONCLUSIONS

Sympathetic postganglionic nerves can enhance pain sensation via a peripheral α-1-adrenoceptor mechanism when sympathetic outflow is disinhibited. The net effect of stress on pain sensation reflects a balance between descending spinal inhibition and sympathetic outflow that can shift toward pain facilitation when central and peripheral α-2-adrenoceptor inhibitory mechanisms are attenuated.

Both α2B- and α2C-adrenoceptor subtypes are involved in the mediation of centrally induced gastroprotection in mice

α(2)-adrenoceptors are known to mediate gastroprotective effect in both acid-dependent and acid-independent ulcer models. The aim of the present study was to determine, which of the three α(2)-adrenoceptor subtypes (α(2A), α(2B) or α(2C)) is responsible for this protection. Various α(2)-adrenoceptor agonists and antagonists were administered intracerebroventricularly (i.c.v.) to C57BL/6 mice with deletion of genes encoding the different subtypes. The gastric mucosal damage was induced by orally injected acidified ethanol. Both the non-selective α(2)-adrenoceptor agonist clonidine (0.3-2.8 nmol) and the α(2B/C)-adrenoceptor subtype preferring agonist ST-91 (0.5-11.5 nmol) induced dose-dependent gastroprotective effect in wild type, α(2A)-, α(2B)- and α(2C)-KO mice. In contrast, the α(2A)-adrenoceptor subtype agonist oxymetazoline (0.07-84 nmol i.c.v.) reduced only slightly the development of ethanol-induced ulcers. The effect of clonidine was antagonized by the non-selective antagonist yohimbine (25 nmol) and the α(2B/C)-adrenoceptor antagonist ARC 239 (10.4 nmol), but not by the α(2A)-adrenoceptor antagonist BRL 44408 (7.5 nmol). ARC 239 also reversed the effect of clonidine in α(2A)-, α(2B)- and α(2C)-KO mice, while the selective α(2C)-adrenoceptor antagonist JP 1302 (52 nmol) antagonized that only in α(2B)-KO, but not in α(2A)- and α(2C)-KO mice. These results suggest that α(2B)- and α(2C)-adrenoceptor subtypes can equally contribute to the mediation of gastroprotective effect induced by α(2)-adrenoceptor agonists in mice.

α2C-adrenoceptor modulators: a patent review

INTRODUCTION

α2-Adrenoceptors (α2-ARs) are membrane proteins belonging to the superfamily of GPCRs. Detailed studies have shown three different subtypes, namely α2A, α2B and α2C. Although numerous α2-AR ligands exist, only a small set of compounds have shown even a degree of selectivity among the three α2-AR subtypes. Moreover, these compounds suffer from binding to receptor sites outside the α2-AR subfamily. Efforts made to understand the biological significance of each α2-AR subtype have greatly been assisted by genetically engineered mice. The main results obtained suggest that α2C-AR stimulation may represent a therapeutic strategy to get an analgesic response with reduced sedative effects and undesirable changes in blood pressure due to α2A-AR activation.

AREAS COVERED

This review summarizes the patent literature about the development of α2C-AR modulators from 2000 to early 2010 and their therapeutic effects evoked by the interaction with this receptor subtype.

EXPERT OPINION

Over 90 patents have been deposited in the last 10 years regarding different methods of α2C-AR modulation (use of agonists or antagonists, nucleic acids and polypeptides) for diagnosis, prognosis and treatment of disorders involving this receptor. Nevertheless, despite the numerous published patents, ligands highly selective for the α2C-AR subtype, which continues to be enigmatic, are lacking.

Increased presynaptic and postsynaptic α2-adrenoceptor activity in the spinal dorsal horn in painful diabetic neuropathy

Diabetic neuropathy is a common cause of chronic pain that is not adequately relieved by conventional analgesics. The α(2)-adrenoceptors are involved in the regulation of glutamatergic input and nociceptive transmission in the spinal dorsal horn, but their functional changes in diabetic neuropathy are not clear. The purpose of the present study was to determine the plasticity of presynaptic and postsynaptic α(2)-adrenoceptors in the control of spinal glutamatergic synaptic transmission in painful diabetic neuropathy. Whole-cell voltage-clamp recordings of lamina II neurons were performed in spinal cord slices from streptozotocin-induced diabetic rats. The amplitude of glutamatergic excitatory postsynaptic currents (EPSCs) evoked from the dorsal root and the frequency of spontaneous EPSCs (sEPSCs) were significantly higher in diabetic than vehicle-control rats. The specific α(2)-adrenoceptor agonist 5-bromo-6-(2-imidazolin-2-ylamino)quinoxaline (UK-14304) (0.1-2 μM) inhibited the frequency of sEPSCs more in diabetic than vehicle-treated rats. UK-14304 also inhibited the amplitude of evoked monosynaptic and polysynaptic EPSCs more in diabetic than control rats. Furthermore, the amplitude of postsynaptic G protein-coupled inwardly rectifying K(+) channel (GIRK) currents elicited by UK-14304 was significantly larger in the diabetic group than in the control group. In addition, intrathecal administration of UK-14304 increased the nociceptive threshold more in diabetic than vehicle-control rats. Our findings suggest that diabetic neuropathy increases the activity of presynaptic and postsynaptic α(2)-adrenoceptors to attenuate glutamatergic transmission in the spinal dorsal horn, which accounts for the potentiated antinociceptive effect of α(2)-adrenoceptor activation in diabetic neuropathic pain.

Advances in perioperative pain management: use of medications with dual analgesic mechanisms, tramadol & tapentadol

Recovery from ambulatory surgical procedures can be limited by postoperative pain. Inadequate analgesia may delay or prevent patient discharge and can result in readmission. More frequently, postoperative pain produces discomfort and interrupts sleep, contributing to postoperative fatigue. The development of effective analgesic regimens for the management of postoperative pain is a priority especially in patients with impaired cardiorespiratory, hepatic, or renal function. Tramadol and tapentadol hydrochloride are novel in that their analgesic actions occur at multiple sites. Both agents are reported to be mu-opioid receptor agonists and monoamine-reuptake inhibitors. In contrast to pure opioid agonists, both drugs are believed to have lower risks of respiratory depression, tolerance, and dependence. The Food and Drug Administration has approved both drugs for the treatment of moderate-to-severe acute pain in adults. This article provides an evidence-based account of the role of tramadol and tapentadol in modern clinical practice.

Determination of α(2)-adrenoceptor and imidazoline receptor involvement in augmentation of morphine and oxycodone analgesia by agmatine and BMS182874

Studies have demonstrated that clonidine (α(2)-adrenoceptor and imidazoline receptor agonist) and BMS182874 (endothelin ET(A) receptor antagonist) potentiate morphine and oxycodone analgesia. Agmatine, an endogenous clonidine-like substance, enhances morphine analgesia. However, its effect on oxycodone analgesia and its interaction with endothelin ET(A) receptor antagonists are not known. The present study was performed to determine the effect of agmatine on morphine and oxycodone analgesia and the involvement of α(2)-adrenoceptors, imidazoline receptors, opioid receptors, and endothelin receptors. Antinociception at various time intervals was determined by the tail-flick latency method in mice. Agmatine produced dose-dependent increase in tail-flick latency, while BMS182874 did not produce any change over the 360-min observation period. Agmatine significantly potentiated morphine as well as oxycodone analgesia which was not altered by BMS182874. BMS182874 pretreatment did not increase the analgesic effect produced by agmatine alone. Agmatine-induced potentiation of morphine and oxycodone analgesia was blocked by idazoxan (imidazoline receptor/α(2)-adrenoceptor antagonist) and yohimbine (α(2)-adrenoceptor antagonist). BMS182874-induced potentiation of morphine or oxycodone analgesia was not affected by yohimbine. However, idazoxan blocked BMS182874-induced potentiation of oxycodone but not morphine analgesia. This is the first report demonstrating that agmatine potentiates not only morphine but also oxycodone analgesia in mice. Potentiation of morphine and oxycodone analgesia by agmatine appears to involve α(2)-adrenoceptors, imidazoline receptors, and opioid receptors. In addition, imidazoline receptors may be involved in BMS182874-induced potentiation of oxycodone but not morphine analgesia. It is concluded that agmatine may be used as an adjuvant in opiate analgesia.

Systemic daily morphine enhances the analgesic effect of intrathecal dexmedetomidine via up-regulation of alpha 2 adrenergic receptor subtypes A, B and C in dorsal root ganglion and dorsal horn

OBJECTIVES

 It has been reported that the effect of intrathecally administered α2 adrenergic receptor (α2 AR) agonists is enhanced in mice that are chronically tolerant to systemic morphine. However, contributory factors have not been identified. Here we examined whether repeated systemic morphine affected the analgesic potency of intrathecal dexmedetomidine and the expression of subtype A, B and C α2 AR (α2A, α2B and α2C AR) in the dorsal root ganglion and dorsal horn in mice.

METHODS

After subcutaneous injection of morphine or saline for two weeks, dexmedetomidine was administered intrathecally to evaluate its antinociceptive effect. Also, the α2 AR subtypes and µ-opioid receptor mRNA expression in lumbar dorsal root ganglion was quantified using PCR, and α2A and α2C AR in lumbar dorsal root ganglion and dorsal horn were examined by immunohistochemistry.

KEY FINDINGS

Daily morphine enhanced the antinociceptive effect of intrathecal dexmedetomidine, increased all the α2 AR subtypes but decreased the µ-opioid receptor mRNA expression in dorsal root ganglion and increased immunoreactivity of α2A and α2C AR in dorsal root ganglion and dorsal horn.

CONCLUSIONS

These results suggest that systemic daily morphine enhances the analgesic effect of intrathecal dexmedetomidine via up-regulation of the α2A, α2B and α2C AR in lumbar dorsal root ganglion and dorsal horn.

The determination and application of fixed-dose analgesic combinations for treating multimodal pain

When the pathophysiology of a medical condition is multimodal, ie, related to multiple physiological causes or mediated by multiple pathways, the optimal strategy can be to use a drug or a combination of drugs that contribute multiple mechanisms to the therapeutic endpoint. In such situations, a rational multimodal approach can also result in the fewest adverse effects. We discuss the quantitative analysis of multimodal action using the treatment of pain as a practical example and give examples of its application to some widely used analgesic drugs.

PERSPECTIVE

This article reviews the medical relevance of the quantitative evaluation of drug combinations, using pain and combinations of analgesics as specific examples. Such measure can help clinicians who seek to maximize therapeutic effect while simultaneously minimizing adverse effects.

Interaction of morphine but not fentanyl with cerebral α2-adrenoceptors in α2-adrenoceptor knockout mice

Objectives

α2-Adrenergic and μ-opioid receptors belong to the rhodopsin family of G-protein coupled receptors and mediate antinociceptive effects via similar signal transduction pathways. Previous studies have revealed direct functional interactions between both receptor systems including synergistic and additive effects. To evaluate underlying mechanisms, we have studied whether morphine and fentanyl interacted with α2-adrenoceptor-subtypes in mice lacking one individual α2-adrenoceptor-subtype (α2-adrenoceptor knockout).

Methods

Opioid interaction with α2-adrenoceptors was investigated by quantitative receptor autoradiography in brain slices of α2A-, α2B- or α2C-adrenoceptor deficient mice. Displacement of the radiolabelled α2-adrenoceptor agonist [125I]paraiodoclonidine from α2-adrenoceptors in different brain regions by increasing concentrations of morphine, fentanyl and naloxone was analysed. The binding affinity of both opioids to α2-adrenoceptor subtypes in different brain regions was quantified.

Key findings

Morphine but not fentanyl or naloxone provoked dose-dependent displacement of [125I]paraiodoclonidine from all α2-adrenoceptor subtypes in the brain regions analysed. Binding affinity was highest in cortex, medulla oblongata and pons of α2A-adrenoceptor knockout mice.

Conclusions

Our results indicated that morphine interacted with α2-adrenoceptors showing higher affinity for the α2B and α2C than for the α2A subtype. In contrast, fentanyl and naloxone did not show any relevant affinity to α2-adrenoceptors. This effect may have an impact on the pharmacological actions of morphine.

Further studies on arylpiperazinyl alkyl pyridazinones: discovery of an exceptionally potent, orally active, antinociceptive agent in thermally induced pain

A number of pyridazinone derivatives bearing an arylpiperazinylalkyl chain were synthesized and tested icv in a model of acute nociception induced by thermal stimuli in mice (tail flick). The most interesting and potent compound in this series was 6a, which showed an ED(50) = 3.5 microg, a value about 3-fold higher with respect to morphine by the same route of administration. When administered per os, 6a was 4-fold more potent than morphine in the same test, suggesting a significant bioavailability. The same compound also showed high potency in the hot plate test. The antinociceptive effect of 6a was completely reversed by pretreatment with yohimbine both in the hot plate test and in the tail flick test. This demonstrated the involvement of the adrenergic system, which was confirmed by in vitro radioligand binding studies.

Protein kinase C mediates the synergistic interaction between agonists acting at alpha2-adrenergic and delta-opioid receptors in spinal cord

Coactivation of spinal alpha(2)-adrenergic receptors (ARs) and opioid receptors produces antinociceptive synergy. Antinociceptive synergy between intrathecally administered alpha(2)AR and opioid agonists is well documented, but the mechanism underlying this synergy remains unclear. The delta-opioid receptor (DOP) and the alpha(2A)ARs are coexpressed on the terminals of primary afferent fibers in the spinal cord where they may mediate this phenomenon. We evaluated the ability of the DOP-selective agonist deltorphin II (DELT), the alpha(2)AR agonist clonidine (CLON) or their combination to inhibit calcitonin gene-related peptide (CGRP) release from spinal cord slices. We then examined the possible underlying signaling mechanisms involved through coadministration of inhibitors of phospholipase C (PLC), protein kinase C (PKC) or protein kinase A (PKA). Potassium-evoked depolarization of spinal cord slices caused concentration-dependent release of CGRP. Coadministration of DELT and CLON inhibited the release of CGRP in a synergistic manner as confirmed statistically by isobolograpic analysis. Synergy was dependent on the activation of PLC and PKC, but not PKA, whereas the effect of agonist administration alone was only dependent on PLC. The importance of these findings was confirmed in vivo, using a thermal nociceptive test, demonstrating the PKC dependence of CLON-DELT antinociceptive synergy in mice. That inhibition of CGRP release by the combination was maintained in the presence of tetrodotoxin in spinal cord slices suggests that synergy does not rely on interneuronal signaling and may occur within single subcellular compartments. The present study reveals a novel signaling pathway underlying the synergistic analgesic interaction between DOP and alpha(2)AR agonists in the spinal cord.

Interaction of morphine with a new alpha2-adrenoceptor agonist in mice

Finding new chemicals or adjuvants with analgesic effects in the central nervous system is clinically relevant due to the limited number of drugs with these properties. Here, we present PT-31, which is chemically related to 3-benzyl-imidazolidine, with an analgesic profile that results from alpha(2)-adrenoceptor activation. Intraperitoneal administration of PT-31 dose-dependently produced antinociception in the hot plate test, and interacted synergistically with morphine. This effect was completely reversed by yohimbine, a non-selective antagonist of alpha(2)-adrenoceptors, and by BRL 44408, a selective alpha(2A)-adrenoceptor antagonist. The combination of morphine and PT-31 produced greater antinociceptive activity than either alone, and isobolographic analysis revealed a synergistic interaction between these compounds. Docking results confirm the high affinity of the PT-31 ligand at the alpha(2A)-adrenoceptor.

PERSPECTIVE

This study introduces a new analgesic compound (PT-31) that acts via alpha(2A)-adrenoceptor activation. A significant increase in analgesia was observed when co-administered with morphine. PT-31 is an interesting new substance for pain therapy.

Clinical evaluation of the efficacy and safety of a constant rate infusion of dexmedetomidine for postoperative pain management in dogs

OBJECTIVE

To compare postoperative analgesia provided by a constant rate infusion (CRI) of dexmedetomidine (DMED) to that of a well-established positive control [morphine (MOR)] in critically ill dogs. The sedative, cardiorespiratory effects and clinical safety of a 24-hour DMED CRI were also evaluated.

STUDY DESIGN

Prospective, randomised, blinded, positive-controlled parallel-group clinical study.

ANIMALS

Forty hospitalised, client-owned dogs requiring post-operative pain management after invasive surgery.

METHODS

After surgery, a loading dose of either DMED (25 microg m(-2)) or MOR (2500 microg m(-2)) followed by a 24-hour CRI of DMED (25 microg m(-2) hour(-1)) or MOR (2500 microg m(-2) hour(-1)) was administered. Pain was measured using the Short Form of the Glasgow Composite Measure Pain Scale, sedation and physiological variables were scored at regular intervals. Animals considered to be painful received rescue analgesia and were allocated to a post-rescue protocol; animals which were unresponsive to rescue analgesia were removed from the study. Data were analysed with anova, two-sample t-tests or Chi-square tests. Time to intervention was analysed with Kaplan-Meier methodology.

RESULTS

Forty dogs were enrolled. Twenty dogs (9 DMED and 11 MOR) did not require rescue analgesia. Eleven DMED and eight MOR dogs were allocated to the post-rescue protocol and seven of these removed from the study. Significant differences in pain scores between groups were not observed during the first 12 hours, however, DMED dogs were less (p = 0.009) painful during the last 12 hours. Sedation score over the entire 24-hour study was not significantly different between groups. CONCLUSION / CLINICAL RELEVANCE: Dexmedetomidine CRI was equally effective as MOR CRI at providing postoperative analgesia and no clinically significant adverse reactions were noted. This study shows the potential of DMED to contribute to a balanced postoperative analgesia regimen in dogs.

Effects of variation in the human alpha2A- and alpha2C-adrenoceptor genes on cognitive tasks and pain perception

BACKGROUND

The mechanisms underlying interindividual variability in pain perception and cognitive responses are undefined but highly heritable. alpha(2C)- and alpha(2A)-adrenergic receptors regulate noradrenergic activity and are important mediators of pain perception and analgesia. We hypothesized that common genetic variants in these genes, particularly the ADRA2C 322-325 deletion variant, affect pain perception or cognitive responses.

METHODS

We studied 73 healthy subjects (37 Caucasians and 36 African-Americans) aged 25.4+/-4.6years. Pain response to a cold pressor test was measured using a 10cm visual analog scale and again on the next day, after three infusions of the selective alpha(2)-agonist dexmedetomidine. Standardized cognitive tests were administered at baseline and after each infusion. The contribution of ADRA2C deletion genotype, dexmedetomidine concentration, and other covariates to pain perception and cognitive responses was determined using multiple linear regression models. Secondary analysis examined the effects of ADRA2A and other ADRA2C variants on pain perception.

RESULTS

ADRA2C Del homozygotes had higher pain scores in response to cold at baseline (6.3+/-1.8cm) and after dexmedetomidine (5.6+/-2.2cm) than insertion allele carriers (4.6+/-2.1cm [baseline] and 3.8+/-1.9cm [after dexmedetomidine]; adjusted P-values=0.019 and 0.004, respectively). Cognitive responses were unrelated to ADRA2C Ins/Del genotype. None of the other ADRA2A and ADRA2C variants was significantly related to cold pain sensitivity before dexmedetomidine; after dexmedetomidine, ADRA2A rs1800038 was marginally associated (P=0.03).

CONCLUSION

The common ADRA2C del322-325 variant affected pain perception before and after dexmedetomidine but did not affect other cognitive responses, suggesting that it contributes to interindividual variability in pain perception.

Coexpression of alpha 2A-adrenergic and delta-opioid receptors in substance P-containing terminals in rat dorsal horn

Agonists acting at alpha(2)-adrenergic and opioid receptors (alpha(2)ARs and ORs, respectively) inhibit pain transmission in the spinal cord. When coadministered, agonists activating these receptors interact in a synergistic manner. Although the existence of alpha(2)AR/OR synergy has been well characterized, its mechanism remains poorly understood. The formation of heterooligomers has been proposed as a molecular basis for interactions between neuronal G-protein-coupled receptors. The relevance of heterooligomer formation to spinal analgesic synergy requires demonstration of the expression of both receptors within the same neuron as well as the localization of both receptors in the same neuronal compartment. We used immunohistochemistry to investigate the spatial relationship between alpha(2)ARs and ORs in the rat spinal cord to determine whether coexpression could be demonstrated between these receptors. We observed extensive colocalization between alpha(2A)-adrenergic and delta-opioid receptors (DOP) on substance P (SP)-immunoreactive (-ir) varicosities in the superficial dorsal horn of the spinal cord and in peripheral nerve terminals in the skin. alpha(2A)AR- and DOP-ir elements were colocalized in subcellular structures of 0.5 mum or less in diameter in isolated nerve terminals. Furthermore, coincubation of isolated synaptosomes with alpha(2)AR and DOP agonists resulted in a greater-than-additive increase in the inhibition of K(+)-stimulated neuropeptide release. These findings suggest that coexpression of the synergistic receptor pair alpha(2A)AR-DOP on primary afferent nociceptive fibers may represent an anatomical substrate for analgesic synergy, perhaps as a result of protein-protein interactions such as heterooligomerization.

Pharmacological profiles of alpha 2 adrenergic receptor agonists identified using genetically altered mice and isobolographic analysis

Endogenous, descending noradrenergic fibers impose analgesic control over spinal afferent circuitry mediating the rostrad transmission of pain signals. These fibers target alpha 2 adrenergic receptors (alpha(2)ARs) on both primary afferent terminals and secondary neurons, and their activation mediates substantial inhibitory control over this transmission, rivaling that of opioid receptors which share a similar pattern of distribution. The terminals of primary afferent nociceptive neurons and secondary spinal dorsal horn neurons express alpha(2A)AR and alpha(2C)AR subtypes, respectively. Spinal delivery of these agents serves to reduce their side effects, which are mediated largely at supraspinal sites, by concentrating the drugs at the spinal level. Targeting these spinal alpha(2)ARs with one of five selective therapeutic agonists, clonidine, dexmedetomidine, brimonidine, ST91 and moxonidine, produces significant antinociception that can work in concert with opioid agonists to yield synergistic antinociception. Application of several genetically altered mouse lines had facilitated identification of the primary receptor subtypes that likely mediate the antinociceptive effects of these agents. This review provides first an anatomical description of the localization of the three subtypes in the central nervous system, second a detailed account of the pharmacological history of each of the six primary agonists, and finally a comprehensive report of the specific interactions of other GPCR agonists with each of the six principal alpha(2)AR agonists featured.

Clonidine and dexmedetomidine produce antinociceptive synergy in mouse spinal cord

BACKGROUND

Synergy between drugs manifests with increased potency and/or efficacy of the combination relative to either agonist given alone. Synergy is typically observed between drugs of different classes, as is the case with the alpha-adrenergic-opioid receptor synergy often observed in preclinical studies. However, rare studies report synergy between agonists of the same class. The current study examined the analgesic interaction between two intrathecally injected alpha2-adrenergic receptor (AR) agonists previously thought to act at the same receptor subtype when given spinally.

METHODS

Mice were given clonidine, dexmedetomidine, or the combination spinally to evaluate the interaction between these two agonists. The ED50 values were calculated, and the interactions were tested by isobolographic analysis. The rotarod test was performed in the same mice after the completion of analgesic assessment to assess motor or sedative effects. These experiments were performed in outbred mice as well as in mice with mutant alpha2A ARs, alpha2C AR knockout mice, or wild-type controls. Finally, analgesic cross-tolerance between clonidine and dexmedetomidine was evaluated.

RESULTS

Clonidine and dexmedetomidine interacted synergistically in all lines except the alpha2C AR knockout line, implicating alpha2C ARs in the interaction. In addition, clonidine and dexmedetomidine did not show analgesic cross-tolerance in the outbred strain, suggesting that the two drugs have distinct mechanisms of action.

CONCLUSIONS

The current study introduces a new synergistic agonist pair, clonidine-dexmedetomidine. These two drugs seem to require the alpha2A AR for spinal analgesia when given separately; when delivered as a combination, the resultant synergistic interaction requires the alpha2C AR as well.

Effect of combining dexmedetomidine and morphine for intravenous patient-controlled analgesia

BACKGROUND

Perioperative use of dexmedetomidine is associated with reduction in postoperative analgesic requirements. This study examined whether dexmedetomidine added to i.v. patient-controlled analgesia (PCA) morphine could improve analgesia while reducing opioid-related side-effects.

METHODS

In this double-blinded, randomized, controlled study, 100 women undergoing abdominal total hysterectomy were allocated to receive either morphine 1 mg ml(-1) alone (Group M) or morphine 1 mg ml(-1) plus dexmedetomidine 5 microg ml(-1) (Group D) for postoperative i.v. PCA, which was programmed to deliver 1 ml per demand with a 5 min lockout interval and no background infusion. Cumulative PCA requirements, pain intensities, cardiovascular and respiratory variables, and PCA-related adverse events were recorded for 24 h after operation.

RESULTS

Compared with Group M, patients in Group D required 29% less morphine during the 0-24 h postoperative period and reported significantly lower pain levels from the second postoperative hour onwards and throughout the study. Whereas levels of sedation were similar between the groups at each observational time point, decreases in heart rate and mean blood pressure from presurgery baseline at 1, 2, and 4 h after operation were significantly greater in Group D (by a range of 5-7 beats min(-1) and 10-13%, respectively). The 4-24 h incidence of nausea was significantly lower in Group D (34% vs 56.3%, P<0.05). There was no bradycardia, hypotension, oversedation, or respiratory depression.

CONCLUSIONS

The addition of dexmedetomidine to i.v. PCA morphine resulted in superior analgesia, significant morphine sparing, less morphine-induced nausea, and was devoid of additional sedation and untoward haemodynamic changes.

Peripheral mechanisms of pain and analgesia

This review summarizes recent findings on peripheral mechanisms underlying the generation and inhibition of pain. The focus is on events occurring in peripheral injured tissues that lead to the sensitization and excitation of primary afferent neurons, and on the modulation of such mechanisms. Primary afferent neurons are of particular interest from a therapeutic perspective because they are the initial generator of noxious impulses traveling towards relay stations in the spinal cord and the brain. Thus, if one finds ways to inhibit the sensitization and/or excitation of peripheral sensory neurons, subsequent central events such as wind-up, sensitization and plasticity may be prevented. Most importantly, if agents are found that selectively modulate primary afferent function and do not cross the blood-brain-barrier, centrally mediated untoward side effects of conventional analgesics (e.g. opioids, anticonvulsants) may be avoided. This article begins with the peripheral actions of opioids, turns to a discussion of the effects of adrenergic co-adjuvants, and then moves on to a discussion of pro-inflammatory mechanisms focusing on TRP channels and nerve growth factor, their signaling pathways and arising therapeutic perspectives.

Low doses of alpha 2-adrenoceptor antagonists augment spinal morphine analgesia and inhibit development of acute and chronic tolerance

BACKGROUND AND PURPOSE

Ultra-low doses of opioid receptor antagonists augment spinal morphine antinociception and block the induction of tolerance. Considering the evidence demonstrating functional and physical interactions between the opioid and alpha(2)-adrenoceptors, this study investigated whether ultra-low doses of alpha(2)-adrenoceptor antagonists also influence spinal morphine analgesia and tolerance.

EXPERIMENTAL APPROACH

Effects of low doses of the competitive alpha(2)-adrenoceptor antagonists-atipamezole (0.08, 0.8 ng), yohimbine (0.02, 2 ng), mirtazapine (0.02 ng) and idazoxan (0.08 ng) were investigated on intrathecal morphine analgesia, as well as acute and chronic morphine antinociceptive tolerance using the rat tail flick and paw pressure tests.

KEY RESULTS

At doses markedly lower than those producing alpha(2)-adrenoceptor blockade, atipamezole, yohimbine, mirtazapine and idazoxan, prolonged the antinociceptive effects of morphine. When co-administered with repeated acute spinal injections of morphine, all four agents blocked the induction of acute tolerance. Co-injection of atipamezole with morphine for 5 days inhibited the development of tolerance in a chronic treatment paradigm. Spinal administration of atipamezole also reversed established antinociceptive tolerance to morphine as indicated by the restoration of morphine antinociceptive potency. The effects of atipamezole on spinal morphine tolerance were not influenced by treatment with 6-hydroxydopamine.

CONCLUSIONS AND IMPLICATIONS

Low doses of competitive alpha(2)-adrenoceptor antagonists can augment acute morphine analgesia and block or reverse tolerance to spinal administration of morphine. These actions are interpreted in terms of their interaction with an opioid-alpha(2)-adrenoceptor complex, whose activity may have a function in the genesis of analgesic tolerance.

Action of dexmedetomidine on the substantia gelatinosa neurons of the rat spinal cord

Dexmedetomidine is a highly specific, potent and selective alpha(2)-adrenoceptor agonist. Although intrathecal and epidural administration of dexmedetomidine has been found to produce analgesia, whether this analgesia results from an effect on spinal cord substantia gelatinosa (SG) neurons remains unclear. Here, we investigated the effects of dexmedetomidine on postsynaptic transmission in SG neurons of rat spinal cord slices using the whole-cell patch-clamp technique. In 92% of the SG neurons examined (n = 84), bath-applied dexmedetomidine induced outward currents at -70 mV in a concentration-dependent manner, with the value of effective concentration producing a half-maximal response (0.62 microM). The outward currents induced by dexmedetomidine were suppressed by the alpha(2)-adrenoceptor antagonist yohimbine, but not by prazosin, an alpha(1)-, alpha(2B)- and alpha(2C)-adrenoceptor antagonist. Moreover, the dexmedetomidine-induced currents were partially suppressed by the alpha(2C)-adrenoceptor antagonist JP-1302, while simultaneous application of JP-1302 and the alpha(2A)-adrenoceptor antagonist BRL44408 abolished the current completely. The action of dexmedetomidine was mimicked by the alpha(2A)-adrenoceptor agonist oxymetazoline. Plots of the current-voltage relationship revealed a reversal potential at around -86 mV. Dexmedetomidine-induced currents were blocked by the addition of GDP-beta-S [guanosine-5'-O-(2-thiodiphosphate)] or Cs+ to the pipette solution. These findings suggest that dexmedetomidine hyperpolarizes the membrane potentials of SG neurons by G-protein-mediated activation of K+ channels through alpha(2A)- and alpha(2C)-adrenoceptors. This action of dexmedetomidine might contribute, at least in part, to its antinociceptive action in the spinal cord.

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.

Modulation of pain transmission by G-protein-coupled receptors

The heterotrimeric G-protein-coupled receptors (GPCR) represent the largest and most diverse family of cell surface receptors and proteins. GPCR are widely distributed in the peripheral and central nervous systems and are one of the most important therapeutic targets in pain medicine. GPCR are present on the plasma membrane of neurons and their terminals along the nociceptive pathways and are closely associated with the modulation of pain transmission. GPCR that can produce analgesia upon activation include opioid, cannabinoid, alpha2-adrenergic, muscarinic acetylcholine, gamma-aminobutyric acidB (GABAB), groups II and III metabotropic glutamate, and somatostatin receptors. Recent studies have led to a better understanding of the role of these GPCR in the regulation of pain transmission. Here, we review the current knowledge about the cellular and molecular mechanisms that underlie the analgesic actions of GPCR agonists, with a focus on their effects on ion channels expressed on nociceptive sensory neurons and on synaptic transmission at the spinal cord level.

ST91 [2-(2,6-diethylphenylamino)-2-imidazoline hydrochloride]-mediated spinal antinociception and synergy with opioids persists in the absence of functional alpha-2A- or alpha-2C-adrenergic receptors

Agonists acting at alpha2-adrenergic receptors (alpha2ARs) produce antinociception and synergize with opioids. The alpha2ARs are divided into three subtypes, alpha(2A)AR, alpha(2B)AR, and alpha(2C)AR. Most alpha2AR agonists require alpha(2A)AR activation to produce antinociception and opioid synergy. The same subtype also mediates the side effect of sedation, which limits the clinical utility of these compounds. Identification of a non-alpha(2A)AR-mediated antinociceptive agent would enhance the therapeutic utility of alpha2AR agonists in pain management. Previous studies have suggested that the alpha2AR agonist ST91 [2-(2,6-diethylphenylamino)-2-imidazoline hydrochloride] has a nonsedating, non-alpha(2A)AR mechanism of action. We examined the pharmacology of spinal ST91 and its interaction with the delta-opioid agonist deltorphin II (Tyr-D-Ala-Phe-Glu-Val-Val-Gly amide) in mice lacking either functional alpha(2A)ARs or alpha(2C)ARs. All drugs were administered by direct lumbar puncture, and drug interactions were evaluated using isobolographic analysis. In contrast to the majority of alpha2AR agonists, ST91 potency was only moderately reduced (3-fold) in the absence of the alpha(2A)AR. Studies with the alpha2AR subtype-preferring antagonists BRL-44408 (2-[2H-(1-methyl-1,3-dihydroisoindole)methyl]-4,5-dihydroimidazole maleate) and prazosin [[4-(4-amino-6,7-dimethoxy-quinazolin-2-yl) piperazin-1-yl]-(2-furyl)methanone] and the pan-alpha2AR antagonist SKF-86466 (6-chloro-2,3,4,5-tetrahydro-3-methyl-1-H-3-benzazepine) suggest a shift from alpha(2A)AR to the other alpha2AR subtype(s) in the absence of alpha(2A)AR. Antinociceptive synergy with deltorphin II was preserved in the absence of either alpha(2A)AR or alpha(2C)AR. In conclusion, ST91 activates both alpha(2A)AR and non-alpha(2A)AR subtypes to produce spinal antinociception and opioid synergy. This study confirms that the spinal pharmacology of ST91 differs from that of other alpha2AR agonists and extends those data to include spinal synergy with opioid agonists. The unique profile of ST91 may be advantageous in pain management.

Naloxone acts as a potent analgesic in transgenic mouse models of sickle cell anemia

Sickle cell anemia is a common genetic disorder in African Americans. Opioid analgesics are traditionally the treatment for the severe pain associated with this disease. Here we reveal that the opioid antagonist naloxone possesses potent analgesic activity in two transgenic mouse models of sickle cell anemia (NY1DD and hBERK1) and not in their respective controls (ICR-CD1 and C57BL/6J) when administered by three parenteral routes [intracerebroventricular (i.c.v.), intrathecal, and subcutaneous]. In the NY1DD mice, naloxone (i.c.v.) possessed approximately 300-fold greater potency than morphine (i.c.v.). Other opioid antagonists (naltrexone, norbinaltorphimine, and naltrindole) were substantially less effective in producing analgesia. Naloxone and morphine were synergistic in NY1DD mice, suggesting different receptor systems. Microarray analysis suggested naloxone-induced down-regulation of the CC chemokine receptor (CCR)5 in NY1DD mice but not in control mice. Pretreatment of control mice with CC chemokine ligand 5 [CCL5 (RANTES)] enabled naloxone to produce analgesia similar to that observed in NY1DD mice. Mu opioid receptor knockout mice treated similarly also displayed analgesia. That the effect of CCL5 was specifically related to CCR5 and/or CCR1 activation was demonstrated by antagonism of analgesia with the chemokine antagonist methionylated RANTES. Similar antagonism of naloxone-induced analgesia also was observed when NY1DD mice were pretreated with methionylated RANTES. These results indicate that CCR5/CCR1 receptors are directly or indirectly involved in analgesia produced by naloxone. The present study suggests that naloxone may be clinically useful in the treatment of pain associated with sickle cell disease and other disorders involving inflammation.

Potentiation of spinal alpha(2)-adrenoceptor analgesia in rats deficient in TRPV1-expressing afferent neurons

The alpha(2)-adrenoceptors (alpha(2)-ARs) are located on primary afferent terminals and on neurons in the spinal cord dorsal horn. However, their relative contribution to the analgesic effect of the alpha(2)-AR agonists is not known. In this study, we determined the role of certain presynaptic alpha(2)-ARs in the antinociceptive effect produced by intrathecal administration of the alpha(2)-AR agonist clonidine. TRPV1-expressing sensory neurons were removed by resiniferatoxin (RTX). The effect of intrathecal injection of clonidine was measured by testing the paw withdrawal response to noxious mechanical or heat stimuli. In RTX-treated rats, the alpha(2A)-AR-immunoreactivity co-expressed with TRPV1-expressing terminals in the spinal cord was eliminated. However, the alpha(2C)-AR-immunoreactivity in the spinal cord was little changed. Surprisingly, intrathecal administration of clonidine produced a much greater increase in the mechanical withdrawal threshold in RTX- than in vehicle-treated rats. The duration of the clonidine effect was also significantly increased in RTX-treated rats. Furthermore, in the vehicle-treated group, although intrathecal injection of clonidine produced a large increase in the thermal withdrawal latency, it only had a small and short-lasting effect on the mechanical withdrawal threshold. This study provides new information that the antinociceptive effect of spinally administered alpha(2)-AR agonists is largely modality-specific. Loss of TRPV1-expressing sensory neurons leads to a reduction in presynaptic alpha(2A)-ARs but paradoxically potentiates the effect of clonidine on mechano-nociception.

Noradrenergic pain modulation

Norepinephrine is involved in intrinsic control of pain. Main sources of norepinephrine are sympathetic nerves peripherally and noradrenergic brainstem nuclei A1-A7 centrally. Peripheral norepinephrine has little influence on pain in healthy tissues, whereas in injured tissues it has variable effects, including aggravation of pain. Its peripheral pronociceptive effect has been associated with injury-induced expression of novel noradrenergic receptors, sprouting of sympathetic nerve fibers, and pronociceptive changes in the ionic channel properties of primary afferent nociceptors, while an interaction with the immune system may contribute in part to peripheral antinociception induced by norepinephrine. In the spinal cord, norepinephrine released from descending pathways suppresses pain by inhibitory action on alpha-2A-adrenoceptors on central terminals of primary afferent nociceptors (presynaptic inhibition), by direct alpha-2-adrenergic action on pain-relay neurons (postsynaptic inhibition), and by alpha-1-adrenoceptor-mediated activation of inhibitory interneurons. Additionally, alpha-2C-adrenoceptors on axon terminals of excitatory interneurons of the spinal dorsal horn possibly contribute to spinal control of pain. At supraspinal levels, the pain modulatory effect by norepinephrine and noradrenergic receptors has varied depending on many factors such as the supraspinal site, the type of the adrenoceptor, the duration of the pain and pathophysiological condition. While in baseline conditions the noradrenergic system may have little effect, sustained pain induces noradrenergic feedback inhibition of pain. Noradrenergic systems may also contribute to top-down control of pain, such as induced by a change in the behavioral state. Following injury or inflammation, the central as well as peripheral noradrenergic system is subject to various plastic changes that influence its antinociceptive efficacy.

Adrenoceptors and signal transduction in neurons

The adrenergic system is an essential regulator of neuronal, endocrine, cardiovascular, vegetative, and metabolic functions. The endogenous catecholamines epinephrine and norepinephrine activate G-protein-coupled receptors to transmit their signal across the plasma membrane. These adrenoceptors can be divided into three different groups: the alpha(1)-receptors (alpha(1A), alpha(1B), alpha(1D)), alpha(2)-receptors (alpha(2A), alpha(2B), alpha(2C)), and beta-receptors (beta(1), beta(2), beta(3)). This review summarizes recent findings in the field of adrenoceptor signaling in neurons and includes a discussion of receptor-associated proteins, receptor dimerization, subcellular trafficking, and fluorescence optical methods for studying the kinetics of adrenergic signaling. Spatio-temporal imaging may become an important future tool for identifying the physiological significance of these complex signaling mechanisms in vivo. Gene-targeted mouse models carrying deletions in alpha(2)-adrenoceptor have provided detailed insights into specific neuronal functions of the three alpha(2)-receptor subtypes.

Tethered yohimbine analogs as selective human alpha2C-adrenergic receptor ligands

Yohimbine is a potent and relatively nonselective alpha(2)-adrenergic receptor (AR) antagonist. In an earlier report, we demonstrated that dimeric yohimbine analogs containing methylene and methylene-diglycine tethers were highly selective human alpha(2C)-AR ligands. Little work has been done to examine the role of the tether group or the absence of the second yohimbine pharmacophore on selectivity for human alpha(2)-AR subtypes. The goal of our study was to determine the binding affinities and functional subtype selectivities of a series of tethered yohimbine ligands in the absence of the second pharmacophore. The profiles of pharmacological activity for the yohimbine analogs on the three human alpha(2)-AR subtypes expressed in Chinese hamster ovary cells were examined using receptor binding and cAMP inhibition assays. All of the tethered yohimbine analogs exhibited higher binding affinities at the alpha(2C)- versus alpha(2A)- and alpha(2B)-AR subtypes. Notably, the benzyl carboxy alkyl amine and the carboxy alkyl amine analogs exhibited 43- and 1995-fold and 295- and 54-fold selectivities in binding to the alpha(2C)- versus alpha(2A)- and alpha(2B)-ARs, respectively. Data from luciferase reporter gene assays confirmed the functional antagonist activities and selectivity profiles of selected compounds from the tethered series. The data demonstrate that the second pharmacophore may not be essential to obtain alpha(2C)-AR subtype selectivity, previously observed with the dimers. Further changes in the nature of the tether will help in optimization of the structure-activity relationship to obtain potent and selective alpha(2C)-AR ligands. These compounds may be used as pharmacological probes and in the treatment of human disorders.

Alpha2C-adrenergic receptors exhibit enhanced surface expression and signaling upon association with beta2-adrenergic receptors

The alpha(2C)-adrenergic receptor (alpha(2C)AR) is known to be poorly trafficked to the cell surface when expressed in a variety of cell types. We tested the hypothesis that the surface expression and signaling of alpha(2C)AR might be enhanced by heterodimerization with other G protein-coupled receptors (GPCRs). Cotransfection of alpha(2C)AR with more than 25 related GPCRs revealed that only coexpression with the beta(2)-adrenergic receptor (beta(2)AR) increased the surface localization of alpha(2C)AR in human embryonic kidney-293 cells. Coimmunoprecipitation of alpha(2C)AR with beta(2)AR confirmed a physical interaction between the two receptors. Confocal microscopy studies demonstrated that alpha(2C)AR expressed alone was mainly intracellular, whereas alpha(2C)AR coexpressed with beta(2)AR was predominantly localized to the plasma membrane. Ligand binding studies revealed a significant increase in alpha(2C)AR binding sites upon coexpression with beta(2)AR, with no apparent change in affinity for alpha(2)AR ligands. Functional assays with the alpha(2)AR-specific agonist brimonidine (UK 14,304) revealed that coexpression of beta(2)AR with alpha(2C)AR enhanced alpha(2C)AR-mediated activation of extracellular signal-regulated kinase 1/2. Furthermore, analyses of agonist-promoted receptor endocytosis demonstrated enhanced alpha(2C)AR internalization in response to alpha(2)AR agonists when alpha(2C)AR and beta(2)AR were coexpressed. In addition, substantial cointernalization of alpha(2C)AR in response to betaAR agonists was observed when alpha(2C)AR was coexpressed with beta(2)AR. These data reveal that alpha(2C)AR can interact with beta(2)AR in cells in a manner that regulates alpha(2C)AR surface expression, internalization, and functionality.

The Effects of Dexmedetomidine on Perinatal Excitotoxic Brain Injury are Mediated by the ??2A-Adrenoceptor Subtype

We performed the current study in mice lacking individual alpha2-adrenoceptor subtypes to elucidate the contribution of alpha(2)-adrenoceptor subtypes to the neuroprotective properties of dexmedetomidine in a model of perinatal excitotoxic brain injury. On postnatal Day 5, wild-type mice and mice lacking alpha2A-adrenoceptor (alpha2A-KO) or alpha2C-adrenoceptor subtypes (alpha2C-KO) were randomly assigned to receive dexmedetomidine (3 microg/kg) or phosphate-buffered saline intraperitoneally. Thirty minutes after the intraperitoneal injection, the glutamatergic agonist ibotenate (10 microg) was intracerebrally injected, producing transcortical necrosis and white matter lesions that mimic perinatal human hypoxic-like lesions. Quantification of the lesions was performed on postnatal Day 10 by histopathologic examination. Dexmedetomidine reduced mean lesion size in the cortex of wild-type mice and alpha2C-KO mice by 44% and 49%, respectively. Ibotenate-induced white matter lesions were reduced by 71% (wild-type mice) and 75% (alpha2C-KO mice) after pretreatment with dexmedetomidine. In contrast, in alpha2A-KO mice, dexmedetomidine did not protect against the cortical excitotoxic insult, and white matter lesions were even more pronounced (82% increase of mean lesion size). Dexmedetomidine provides potent neuroprotection in a model of perinatal excitotoxic brain damage. This effect was completely abolished in alpha2A-KO mice, suggesting that the neuroprotective effect is mediated via the alpha2A-adrenoceptor subtype.