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

Activation of α 2A and α 2B -adrenergic receptors inhibits tactile stimulation-evoked parallel fiber-Purkinje cell synaptic transmission in mouse cerebellar cortex

The mechanism by which α2-adrenergic receptors (ARs) modulate the cerebellar parallel fiber-Purkinje cell (PF-PC) synaptic transmission is unclear. We investigated this issue using electrophysiological and neuropharmacological methods. Six- to eight-week-old ICR mice were used in the study. Under in vivo conditions, PF-PC synaptic transmission was evoked by facial stimulation of ipsilateral whisker pad, and recorded using cell-attached patch from PCs. Under in-vitro conditions, PF-PC synaptic transmission was evoked by electrical stimulation of the molecular layer in cerebellar slices, and was recorded using whole-cell recording from PCs. SR95531 (20 µM) was added to the ACSF during all recordings to prevent GABAA receptor-mediated inhibition. Air-puff stimulation of the ipsilateral whisker pad in-vivo evoked simple spike (eSS) firing of cerebellar PCs. Microapplication of noradrenaline (15 µM) to the molecular layer significantly decreased the numbers and frequency of eSS, an effect abolished by the α2-AR antagonist. Microapplication of an α2-AR agonist, UK14304 (1 µM), significantly decreased the numbers of eSS in PCs, which was abolished by either α2A- or α2B-AR antagonist, but not by α2C-AR antagonist. Under in-vitro conditions, application of UK 14304 significantly decreased the amplitude of PF-PC EPSCs and increased the paired-pulse ratio, which were abolished by either α2A- or α2B-AR antagonist. The present results indicate that activation of presynaptic α2A- and α2B-AR downregulates PF-PC synaptic transmission in mouse cerebellar cortex.

Adrenoceptors: A Focus on Psychiatric Disorders and Their Treatments

Research into the involvement of adrenoceptor subtypes in the cause(s) of psychiatric disorders is particularly challenging. This is partly because of difficulties in developing animal models that recapitulate the human condition but also because no evidence for any causal links has emerged from studies of patients. These, and other obstacles, are outlined in this chapter. Nevertheless, many drugs that are used to treat psychiatric disorders bind to adrenoceptors to some extent. Direct or indirect modulation of the function of specific adrenoceptor subtypes mediates all or part of the therapeutic actions of drugs in various psychiatric disorders. On the other hand, interactions with central or peripheral adrenoceptors can also explain their side effects. This chapter discusses both aspects of the field, focusing on disorders that are prevalent: depression, schizophrenia, anxiety, attention-deficit hyperactivity disorder, binge-eating disorder, and substance use disorder. In so doing, we highlight some unanswered questions that need to be resolved before it will be feasible to explain how changes in the function of any adrenoceptor subtype affect mood and behavior in humans and other animals.

Anesthesia and the neurobiology of fear and posttraumatic stress disorder

PURPOSE OF REVIEW

Dysfunction of fear memory systems underlie a cluster of clinically important and highly prevalent psychological morbidities seen in perioperative and critical care patients, most archetypally posttraumatic stress disorder (PTSD). Several sedative-hypnotics and analgesics are known to modulate fear systems, and it is theoretically plausible that clinical decisions of the anesthesiologist could impact psychological outcomes. This review aims to provide a focused synthesis of relevant literature from multiple fields of research.

RECENT FINDINGS

There is evidence in some contexts that unconscious fear memory systems are less sensitive to anesthetics than are conscious memory systems. Opiates may suppress the activation of fear systems and have benefit in the prevention of PTSD following trauma. There is inconsistent evidence that the use of propofol and benzodiazepines for sedation following trauma may potentiate the development of PTSD relative to other drugs. The benefits of ketamine seen in the treatment of major depression are not clearly replicated in PTSD-cluster psychopathologies, and its effects on fear processes are complex.

SUMMARY

There are multiple theoretical mechanisms by which anesthetic drugs can modulate fear systems and clinically important fear-based psychopathologies. The current state of research provides some evidence to support further hypothesis investigation. However, the absence of effectiveness studies and the inconsistent signals from smaller studies provide insufficient evidence to currently offer firm clinical guidance.

The Neurotransmitter Receptor Architecture of the Mouse Olfactory System

The uptake, transmission and processing of sensory olfactory information is modulated by inhibitory and excitatory receptors in the olfactory system. Previous studies have focused on the function of individual receptors in distinct brain areas, but the receptor architecture of the whole system remains unclear. Here, we analyzed the receptor profiles of the whole olfactory system of adult male mice. We examined the distribution patterns of glutamatergic (AMPA, kainate, mGlu_2/3, and NMDA), GABAergic (GABA_A, GABA_A(BZ), and GABA_B), dopaminergic (D_1/5) and noradrenergic (α₁ and α₂) neurotransmitter receptors by quantitative in vitro receptor autoradiography combined with an analysis of the cyto- and myelo-architecture. We observed that each subarea of the olfactory system is characterized by individual densities of distinct neurotransmitter receptor types, leading to a region- and layer-specific receptor profile. Thereby, the investigated receptors in the respective areas and strata showed a heterogeneous expression. Generally, we detected high densities of mGlu_2/3Rs, GABA_A(BZ)Rs and GABA_BRs. Noradrenergic receptors revealed a highly heterogenic distribution, while the dopaminergic receptor D_1/5 displayed low concentrations, except in the olfactory tubercle and the dorsal endopiriform nucleus. The similarities and dissimilarities of the area-specific multireceptor profiles were analyzed by a hierarchical cluster analysis. A three-cluster solution was found that divided the areas into the (1) olfactory relay stations (main and accessory olfactory bulb), (2) the olfactory cortex (anterior olfactory cortex, dorsal peduncular cortex, taenia tecta, piriform cortex, endopiriform nucleus, entorhinal cortex, orbitofrontal cortex) and the (3) olfactory tubercle, constituting its own cluster. The multimodal receptor-architectonic analysis of each component of the olfactory system provides new insights into its neurochemical organization and future possibilities for pharmaceutic targeting.

Dexmedetomidine attenuates sepsis-associated inflammation and encephalopathy via central α2A adrenoceptor

Sepsis-associated encephalopathy (SAE) is a significant clinical issue that is associated with increased mortality and cost of health care. Dexmedetomidine, an α2 adrenoceptor agonist that is used to provide sedation, has been shown to induce neuroprotection under various conditions. This study was designed to determine whether dexmedetomidine protects against SAE and whether α2 adrenoceptor plays a role in this protection. Six- to eight-week old CD-1 male mice were subjected to cecal ligation and puncture (CLP). They were treated with intraperitoneal injection of dexmedetomidine in the presence or absence of α2 adrenoceptor antagonists, atipamezole or yohimbine, or an α2A adrenoceptor antagonist, BRL-44408. Hippocampus and blood were harvested for measuring cytokines. Mice were subjected to Barnes maze and fear conditioning 14 days after CLP to evaluate their learning and memory. CLP significantly increased the proinflammatory cytokines including tumor necrosis factor α, interleukin (IL)-6 and IL-1β in the blood and hippocampus. CLP also increased the permeability of blood-brain barrier (BBB) and impaired learning and memory. These CLP detrimental effects were attenuated by dexmedetomidine. Intracerebroventricular application of atipamezole, yohimbine or BRL-44408 blocked the protection of dexmedetomidine on the brain but not on the systemic inflammation. Astrocytes but not microglia expressed α2A adrenoceptors. Microglial depletion did not abolish the protective effects of dexmedetomidine. These results suggest that dexmedetomidine reduces systemic inflammation, neuroinflammation, injury of BBB and cognitive dysfunction in septic mice. The protective effects of dexmedetomidine on the brain may be mediated by α2A adrenoceptors in the astrocytes.

Therapeutic Potential of Selectively Targeting the α2C-Adrenoceptor in Cognition, Depression, and Schizophrenia-New Developments and Future Perspective

α_2A- and α_2C-adrenoceptors (ARs) are the primary α₂-AR subtypes involved in central nervous system (CNS) function. These receptors are implicated in the pathophysiology of psychiatric illness, particularly those associated with affective, psychotic, and cognitive symptoms. Indeed, non-selective α₂-AR blockade is proposed to contribute toward antidepressant (e.g., mirtazapine) and atypical antipsychotic (e.g., clozapine) drug action. Both α_2C- and α_2A-AR share autoreceptor functions to exert negative feedback control on noradrenaline (NA) release, with α_2C-AR heteroreceptors regulating non-noradrenergic transmission (e.g., serotonin, dopamine). While the α_2A-AR is widely distributed throughout the CNS, α_2C-AR expression is more restricted, suggesting the possibility of significant differences in how these two receptor subtypes modulate regional neurotransmission. However, the α_2C-AR plays a more prominent role during states of low endogenous NA activity, while the α_2A-AR is relatively more engaged during states of high noradrenergic tone. Although augmentation of conventional antidepressant and antipsychotic therapy with non-selective α₂-AR antagonists may improve therapeutic outcome, animal studies report distinct yet often opposing roles for the α_2A- and α_2C-ARs on behavioral markers of mood and cognition, implying that non-selective α₂-AR antagonism may compromise therapeutic utility both in terms of efficacy and side-effect liability. Recently, several highly selective α_2C-AR antagonists have been identified that have allowed deeper investigation into the function and utility of the α_2C-AR. ORM-13070 is a useful positron emission tomography ligand, ORM-10921 has demonstrated antipsychotic, antidepressant, and pro-cognitive actions in animals, while ORM-12741 is in clinical development for the treatment of cognitive dysfunction and neuropsychiatric symptoms in Alzheimer's disease. This review will emphasize the importance and relevance of the α_2C-AR as a neuropsychiatric drug target in major depression, schizophrenia, and associated cognitive deficits. In addition, we will present new prospects and future directions of investigation.

Effects of abstinence from chronic cocaine self-administration on nonhuman primate dorsal and ventral noradrenergic bundle terminal field structures

Repeated exposure to cocaine is known to dysregulate the norepinephrine system, and norepinephrine has also been implicated as having a role in abstinence and withdrawal. The goal of this study was to determine the effects of exposure to cocaine self-administration and subsequent abstinence on regulatory elements of the norepinephrine system in the nonhuman primate brain. Rhesus monkeys self-administered cocaine (0.3 mg/kg/injection, 30 reinforcers/session) under a fixed-interval 3-min schedule of reinforcement for 100 sessions. Animals in the abstinence group then underwent a 30-day period during which no operant responding was conducted, followed by a final session of operant responding. Control animals underwent identical schedules of food reinforcement and abstinence. This duration of cocaine self-administration has been shown previously to increase levels of norepinephrine transporters (NET) in the ventral noradrenergic bundle terminal fields. In contrast, in the current study, abstinence from chronic cocaine self-administration resulted in elevated levels of [(3)H]nisoxetine binding to the NET primarily in dorsal noradrenergic bundle terminal field structures. As compared to food reinforcement, chronic cocaine self-administration resulted in decreased binding of [(3)H]RX821002 to α2-adrenoceptors primarily in limbic-related structures innervated by both dorsal and ventral bundles, as well as elevated binding in the striatum. However, following abstinence from responding for cocaine binding to α2-adrenoceptors was not different than in control animals. These data demonstrate the dynamic nature of the regulation of norepinephrine during cocaine use and abstinence, and provide further evidence that the norepinephrine system should not be overlooked in the search for effective pharmacotherapies for cocaine dependence.

Amphetamine decreases α2C-adrenoceptor binding of [11C]ORM-13070: a PET study in the primate brain

BACKGROUND

The neurotransmitter norepinephrine has been implicated in psychiatric and neurodegenerative disorders. Examination of synaptic norepinephrine concentrations in the living brain may be possible with positron emission tomography (PET), but has been hampered by the lack of suitable radioligands.

METHODS

We explored the use of the novel α2C-adrenoceptor antagonist PET tracer [(11)C]ORM-13070 for measurement of amphetamine-induced changes in synaptic norepinephrine. The effect of amphetamine on [(11)C]ORM-13070 binding was evaluated ex vivo in rat brain sections and in vivo with PET imaging in monkeys.

RESULTS

Microdialysis experiments confirmed amphetamine-induced elevations in rat striatal norepinephrine and dopamine concentrations. Regional [(11)C]ORM-13070 receptor binding was high in the striatum and low in the cerebellum. After injection of [(11)C]ORM-13070 in rats, mean striatal specific binding ratios, determined using cerebellum as a reference region, were 1.4±0.3 after vehicle pretreatment and 1.2±0.2 after amphetamine administration (0.3mg/kg, subcutaneous). Injection of [(11)C]ORM-13070 in non-human primates resulted in mean striatal binding potential (BP ND) estimates of 0.65±0.12 at baseline. Intravenous administration of amphetamine (0.5 and 1.0mg/kg, i.v.) reduced BP ND values by 31-50%. Amphetamine (0.3mg/kg, subcutaneous) increased extracellular norepinephrine (by 400%) and dopamine (by 270%) in rat striata.

CONCLUSIONS

Together, these results indicate that [(11)C]ORM-13070 may be a useful tool for evaluation of synaptic norepinephrine concentrations in vivo. Future studies are required to further understand a potential contribution of dopamine to the amphetamine-induced effect.

¹¹C-ORM-13070, a novel PET ligand for brain α₂C-adrenoceptors: radiometabolism, plasma pharmacokinetics, whole-body distribution and radiation dosimetry in healthy men

PURPOSE

(11)C-labelled 1-[(S)-1-(2,3-dihydrobenzo[1,2]dioxin-2-yl)methyl]-4-(3-methoxy-methylpyridin-2-yl)-piperazine ((11)C-ORM-13070) is a novel PET tracer for imaging of α2C-adrenoceptors in the human brain. Brain α2C-adrenoceptors may be therapeutic targets in several neuropsychiatric disorders, including depression, schizophrenia and Alzheimer's disease. To validate the use of (11)C-ORM-13070 in humans, we investigated its radiometabolism, pharmacokinetics, whole-body distribution and radiation dose.

METHODS

Radiometabolism was studied in a test-retest setting in six healthy men. After intravenous injection of (11)C-ORM-13070, blood samples were drawn over 60 min. Plasma samples were analysed by radio-HPLC for intact tracer and its radioactive metabolites. Metabolite-corrected plasma time-activity curves were used for calculation of pharmacokinetics. In a separate group of 12 healthy men, the whole-body distribution of (11)C-ORM-13070 and radiation exposure were investigated by dynamic PET/CT imaging without blood sampling.

RESULTS

Two radioactive metabolites of (11)C-ORM-13070 were detected in human arterial plasma. The proportion of unchanged (11)C-ORM-13070 decreased from 81 ± 4 % of total radioactivity at 4 min after tracer injection to 23 ± 4 % at 60 min. At least one of the radioactive metabolites penetrated into red blood cells, while the parent tracer remained in plasma. The apparent elimination rate constant and corresponding half-life of unchanged (11)C-ORM-13070 in arterial plasma were 0.0117 ± 0.0056 min(-1) and 73.6 ± 35.8 min, respectively. The organs with the highest absorbed doses were the liver (12 μSv/MBq), gallbladder wall (12 μSv/MBq) and pancreas (9.1 μSv/MBq). The mean effective dose was 3.9 μSv/MBq, with a range of 3.6 - 4.2 μSv/MBq.

CONCLUSION

(11)C-ORM-13070 was rapidly metabolized in human subjects after intravenous injection. The effective radiation dose of (11)C-ORM-13070 was in the same range as that of other (11)C-labelled brain receptor tracers. An injection of 500 MBq of (11)C-ORM-13070 would expose a subject to 2.0 mSv of radiation. This supports the use of (11)C-ORM-13070 in repeated PET scans, for example, in receptor occupancy trials with novel drug candidates.

A PET Tracer for Brain α2C Adrenoceptors, (11)C-ORM-13070: Radiosynthesis and Preclinical Evaluation in Rats and Knockout Mice

We report the development of a PET tracer for α2C adrenoceptor imaging and its preliminary preclinical evaluation. α2C adrenoceptors in the human brain may be involved in various neuropsychiatric disorders, such as depression, schizophrenia, and neurodegenerative diseases. PET tracers are needed for imaging of this receptor system in vivo.

METHODS

High-specific-activity (11)C-ORM-13070 (1-[(S)-1-(2,3-dihydrobenzo[1,4]dioxin-2-yl)methyl]-4-(3-(11)C-methoxymethylpyridin-2-yl)-piperazine) was synthesized by (11)C-methylation of O-desmethyl-ORM-13070 with (11)C-methyl triflate, which was prepared from cyclotron-produced (11)C-methane via (11)C-methyl iodide. Rats and mice were investigated in vivo with PET and ex vivo with autoradiography. The specificity of (11)C-ORM-13070 binding to α2 adrenoceptors was demonstrated in rats pretreated with atipamezole, an α2 adrenoceptor antagonist. The α2C adrenoceptor selectivity of the tracer was determined by comparing tracer binding in wild-type and α2A- and α2AC adrenoceptor knockout (KO) mice. (11)C-ORM-13070 and its radioactive metabolites in rat plasma and brain tissue were analyzed with radio-high-performance liquid chromatography and mass spectroscopy. Human radiation dose estimates were extrapolated from rat biodistribution data.

RESULTS

The radiochemical yield, calculated from initial cyclotron-produced (11)C-methane, was 9.6% ± 2.7% (decay-corrected to end of bombardment). The specific activity of the product was 640 ± 390 GBq/μmol (decay-corrected to end of synthesis). The radiochemical purity exceeded 99% in all syntheses. The highest levels of tracer binding were observed in the striatum and olfactory tubercle of rats and control and α2A KO mice-that is, in the brain regions known to contain the highest densities of α2C adrenoceptors. In rats pretreated with atipamezole and in α2AC KO mice, (11)C tracer binding in the striatum and olfactory tubercle was low, similar to that of the frontal cortex and thalamus, regions with low densities of α2C adrenoceptors. Two radioactive metabolites were found in rat plasma, but only one of them was found in the brain; their identity was not revealed. The estimated effective radiation dose was comparable with the average exposure level in PET studies with (11)C tracers.

CONCLUSION

An efficient method for the radiosynthesis of (11)C-ORM-13070 was developed. (11)C-ORM-13070 emerged as a potential novel radiotracer for in vivo imaging of brain α2C adrenoceptors.

α2-Adrenoceptors are targets for antipsychotic drugs

RATIONALE

Almost all antipsychotic drugs (APDs), irrespective of whether they belong to the first-generation (e.g. haloperidol) or second-generation (e.g. clozapine), are dopamine D2 receptor antagonists. Second-generation APDs, which differ from first-generation APDs in possessing a lower propensity to induce extrapyramidal side effects, target a variety of monoamine receptors such as serotonin (5-hydroxytryptamine) receptors (e.g. 5-HT1A, 5-HT2A, 5-HT2C, 5-HT6, 5-HT7) and α1- and α2-adrenoceptors in addition to their antagonist effects at D2 receptors.

OBJECTIVE

This short review is focussed on the potential role of α2-adrenoceptors in the antipsychotic therapy.

RESULTS

Schizophrenia is characterised by three categories of symptoms: positive symptoms, negative symptoms and cognitive deficits. α2-Adrenoceptors are classified into three distinct subtypes in mammals, α2A, α2B and α2C. Whereas the α2B-adrenoceptor seems to play only a minor role in the brain, activation of postsynaptic α2A-adrenoceptors in the prefrontal cortex improves cognitive functions. Preclinical models such as D-amphetamine-induced locomotion, the conditioned avoidance response and the pharmacological N-methyl-D-aspartate receptor hypofunction model have shown that α2C-adrenoceptor blockade or the combination of D2 receptor antagonists with idazoxan (α2A/2C-adrenoceptor antagonist) could be useful in schizophrenia. A potential benefit of a treatment combination of first-generation APDs with the α2A/2C-adrenoceptor antagonists idazoxan or mirtazapine was also demonstrated in patients with schizophrenia.

CONCLUSIONS

It is concluded that α2-adrenoceptors may be promising targets in the antipsychotic therapy.

Alteration of olfactory perceptual learning and its cellular basis in aged mice

Olfactory perceptual learning reflects an ongoing process by which animals learn to discriminate odorants thanks to repeated stimulations by these odorants. Adult neurogenesis is required for this learning to occur in young adults. The experiments reported here showed that olfactory perceptual learning is impaired with aging and that this impairment is associated with a reduction of neurogenesis and a decrease in granule cell responsiveness to the learned odorant in the olfactory bulb. Interestingly, we showed that the pharmacological stimulation of the noradrenergic system using dexefaroxan mimics olfactory perceptual learning in old mice, which is accompanied by an increase of granule cell responsiveness in response to the learned odorant without any improvement in neurogenesis. We provide the first published evidence that, in contrast to young adult mice, the improvement of olfactory performances in old mice is independent of the overall level of neurogenesis. In addition, restoring behavioral performances in old mice by stimulation of the noradrenergic system underlies the importance of this neuromodulatory system in regulating bulbar network plasticity.

The role of α₂ adrenoceptor in mediating noradrenaline action in the ventrolateral orbital cortex on allodynia following spared nerve injury

The present study examined the role of α₂ adrenoceptor in mediating noradrenaline action in the ventrolateral orbital cortex (VLO) on allodynia induced by spared nerve injury (SNI) in the rat. The mechanical paw withdrawal threshold (PWT) was measured using von-Frey filaments. Microinjection of noradrenaline (1, 2, 4 μg in 0.5 μl) into the VLO, contralateral to the site of nerve injury, reduced allodynia; PWT increased in a dose-dependent manner. Similar to noradrenaline, microinjection of selective α₂ adrenoceptor agonist clonidine into the same VLO site also reduced allodynia, and was blocked by selective α₂ adrenoceptor antagonist yohimbine. Furthermore, administration of γ-aminobutyric acid A (GABAA) receptor antagonist bicuculline or picrotoxin to the VLO significantly enhanced clonidine-induced inhibition of allodynia, while GABAA receptor agonist muscimol or THIP (2,5,6,7-retrahydroisoxazolo(5,4-c)pyridine-3-ol hydrochloride) attenuated clonidine-induced inhibition. These results suggest that noradrenaline acting in the VLO can potentially reduce allodynia induced by SNI, and this effect is mediated by α₂ adrenoceptor. Moreover, GABAergic disinhibition may participate in α₂ receptor mediating effects in neuropathic pain in the central nervous system.

α2C-Adrenoceptors modulate L-DOPA uptake in opossum kidney cells and in the mouse kidney

Targeted deletion or selective pharmacological inhibition of α(2C)-adrenoceptors in mice results in increased brain tissue levels of dopamine and its precursor l-3,4-dihydroxyphenylalanine (l-DOPA), without significant changes in l-DOPA synthesis. l-DOPA uptake is considered the rate-limiting step in dopamine synthesis in the kidney. Since α(2C)-adrenoceptors may influence the transport of l-DOPA, we investigated the effect of α(2C)-adrenoceptor activation on l-DOPA uptake in a kidney cell line (opossum kidney cells). l-DOPA and dopamine kidney tissue levels in α(2C)-adrenoceptor knockout (α(2C)KO) mice and in mice treated with the selective α(2C)-adrenoceptor antagonist JP-1302 were also evaluated. The α(2)-adrenoceptor agonist medetomidine (0.1-1,000 nM) produced a concentration-dependent decrease in l-DOPA uptake in opossum kidney cells (IC(50): 2.5 ± 0.5 nM and maximal effect: 28 ± 5% of inhibition). This effect was abolished by a preincubation with JP-1302 (300 nM). Furthermore, the effect of medetomidine (100 nM) was abolished by a preincubation with U-0126 (10 μM), a MEK1/2 inhibitor. Kidney tissue levels of l-DOPA were significantly higher in α(2C)KO mice compared with wild-type mice (wild-type mice: 58 ± 2 pmol/g tissue and α(2C)KO mice: 81 ± 15 pmol/g tissue, P < 0.05) and in mice treated with JP-1302 (3 μmol/kg body wt) compared with control mice (control mice: 62 ± 2 pmol/g tissue and JP-1302-treated mice: 75 ± 1 pmol/g tissue, P < 0.05), both without significant changes in dopamine kidney tissue levels. However, mice treated with JP-1302 on a high-salt diet presented significantly higher dopamine levels in the kidney and urine compared with control animals on a high-salt diet. In conclusion, in a kidney cell line, α(2C)-adrenoceptor activation inhibits l-DOPA uptake, and in mice, deletion or blockade of α(2C)-adrenoceptors increases l-DOPA kidney tissue levels.

The function of alpha-2-adrenoceptors in the rat locus coeruleus is preserved in the chronic constriction injury model of neuropathic pain

RATIONALE

Peripheral neuropathic pain is a chronic condition that may produce plastic changes in several brain regions. The noradrenergic locus coeruleus (LC) is a crucial component of ascending and descending pain pathways, both of which are frequently compromised after nerve injury.

OBJECTIVES

The objective of the study was to examine whether chronic constriction injury (CCI), a model of neuropathic pain, alters noradrenergic activity in the rat LC.

METHODS

Activity in the LC was assessed by electrophysiology and microdialysis, while protein expression was monitored in western blots and by immunohistochemistry.

RESULTS

The pain threshold had dropped in injured rats 7 days after inducing neuropathy. While alpha-2-adrenoceptors mediate activity in the LC and in its terminal areas, no alterations in either spontaneous neuronal activity or extracellular noradrenaline levels were observed following CCI. Moreover, alpha-2-adrenoceptor activity in the LC of CCI rats remained unchanged after systemic administration of UK14,304, RX821002 or desipramine. Accordingly, extracellular noradrenaline levels in the LC were similar in CCI and control animals following local administration of clonidine or RX821002. In addition, there were no changes in the expression of the alpha-2-adrenoceptors, Gαi/z subunits or the regulators of G-protein signaling. However, pERK1/2 (phosphorylated extracellular signal-regulated kinases 1/2) expression augmented in the spinal cord, paragigantocellularis nucleus (PGi) and dorsal raphe nucleus (DRN) following CCI.

CONCLUSIONS

Neuropathic pain is not accompanied by modifications in tonic LC activity after the onset of pain. This may indicate that the signals from the PGi and DRN, the excitatory and inhibitory afferents of the LC, cancel one another out.

Noradrenaline release in the locus coeruleus modulates memory formation and consolidation; roles for α- and β-adrenergic receptors

Noradrenaline, essential for the modulation of memory, is released in various parts of the brain from nerve terminals controlled by the locus coeruleus (LoC). Noradrenaline release consequent upon input from higher brain areas also occurs within the LoC itself. We examined the effect of noradrenaline on adrenergic receptors in the LoC on memory processing, using colored bead discrimination learning in the young domestic chick. We have shown previously that the release of noradrenaline in the hippocampus and cortex (mesopallium) is essential for acquisition and consolidation of short-term to intermediate and to long-term memory. Noradrenaline release within the LoC is triggered by the glutamatergic input from the forebrain. Inhibition by LoC injection of NMDA or AMPA receptor antagonists is rescued by injection of β2-and β3-adrenoceptor (AR) agonists in the hippocampus. We show that inhibition of α2A-ARs by BRL44408 in the LoC up to 30 min post-training consolidates weakly-reinforced learning. Conversely activation of α2A-ARs in the LoC at the times of consolidation between short-term and intermediate and long-term memory caused memory loss, which is likely to be due to a decreased release of noradrenaline within these two time windows. The α2A-AR antagonist will block presynaptic inhibitory receptors leading to an increase in extracellular noradrenaline. This interpretation is supported by the actions of noradrenaline uptake blockers that produce the same memory outcome. BRL44408 in the mesopallium also caused memory enhancement. β2-ARs are important in the first time window, whereas α1-, α2C-and β3-ARs are important in the second time window. The results reveal that for successful memory formation noradrenaline release is necessary within the LoC as well as in other brain regions, at the time of consolidation of memory from short-term to intermediate and from intermediate to long-term memory.

In vivo evaluation of limiting brain penetration of probes for α(2C)-adrenoceptor using small-animal positron emission tomography

To evaluate in vivo brain penetration of α(2C)-adrenoceptor (α(2C)-AR) antagonists as a therapeutic agent, we synthesized two new (11)C-labeled selective α(2C)-AR antagonists 4-(6,7-dimethoxy-1,2,3,4-tetrahydroisoquinolin-2-yl)methyl-2-aryl-7-methoxybenzofuran ([(11)C]MBF) and acridin-9-yl-[4-(4-methylpiperazin-1-yl)phenyl]amine ([(11)C]JP-1302) as α(2C)-AR-selective positron emission tomography (PET) probes. The radiochemical yield, specific activity, and radiochemical purity of these probes was appropriate for injection. To evaluate whether the brain penetration of these probes is related to the function of two major drug efflux transporters, P-glycoprotein (P-gp) and breast cancer resistance protein (BCRP), we performed PET studies using wild-type and P-gp/Bcrp knockout mice. In wild-type mice, the radioactivity level after injection with [(11)C]MBF initially increased and effluxed immediately from the brain, whereas that with [(11)C]JP-1302 was distributed throughout the brain. However, the regional distribution of radioactivity after injection with [(11)C]JP-1302 in the brain was different from that of α(2C)-ARs. In P-gp/Bcrp knockout mice, uptake of [(11)C]MBF was approximately 3.7-fold higher and that of [(11)C]JP-1302 was approximately 1.6-fold higher than those in wild-type mice. These results indicate that brain penetration of the two PET probes was affected by modulation of P-gp and Bcrp functions.

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.

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.

Noradrenergic regulation of GABAergic inhibition of main olfactory bulb mitral cells varies as a function of concentration and receptor subtype

The main olfactory bulb (MOB) receives a rich noradrenergic innervation from the pontine nucleus locus coeruleus (LC). Previous studies indicate that norepinephrine (NE) modulates the strength of GABAergic inhibition in MOB. However, the nature of this modulation and the NE receptors involved remain controversial. The goal of this study was to investigate the role of NE receptor subtypes in modulating the GABAergic inhibition of mitral cells using patch-clamp electrophysiology in rat MOB slices. NE concentration dependently and bi-directionally modulated GABA(A) receptor-mediated spontaneous and miniature inhibitory postsynaptic currents (sIPSCs/mIPSCs) recorded in mitral cells. Low doses of NE suppressed sIPSCs and mIPSCs because of activation of alpha2 receptors. Intermediate concentrations of NE increased sIPSCs and mIPSCs primarily because of activation of alpha1 receptors. In contrast, activation of beta receptors increased sIPSCs but not mIPSCs. These results indicate that NE release regulates the strength of GABAergic inhibition of mitral cells depending on the NE receptor subtype activated. Functionally, the differing affinity of noradrenergic receptor subtypes seems to allow for dynamic modulation of GABAergic inhibition in MOB as function of the extracellular NE concentration, which in turn, is regulated by behavioral state.

Developmental enhancement of alpha2-adrenoceptor-mediated suppression of inhibitory synaptic transmission onto mouse cerebellar Purkinje cells

Noradrenaline (NA) modulates glutamatergic and GABAergic transmission in various areas of the brain. It is reported that some alpha2-adrenoceptor subtypes are expressed in the cerebellar cortex and alpha2-adrenoceptors may play a role in motor coordination. Our previous study demonstrated that the selective alpha2-adrenoceptor agonist clonidine partially depresses spontaneous inhibitory postsynaptic currents (sIPSCs) in mouse cerebellar Purkinje cells (PCs). Here we found that the inhibitory effect of clonidine on sIPSCs was enhanced during postnatal development. The activation of alpha2-adrenoceptors by clonidine did not affect sIPSCs in PCs at postnatal days (P) 8-10, when PCs showed a few sIPSCs and interneurons in the molecular layer (MLIs) did not cause action potential (AP). In the second postnatal week, the frequency of sIPSCs increased temporarily and reached a plateau at P14. By contrast, MLIs began to fire at P11 with the firing rate gradually increasing thereafter and reaching a plateau at P21. In parallel with this rise in the rate of firing, the magnitude of the clonidine-mediated inhibition of sIPSCs increased during postnatal development. Furthermore, the magnitude of the clonidine-mediated firing suppression in MLIs, which seemed to be mediated by a reduction in amplitude of the hyperpolarization-activated nonselective cation current, I(h), was constant across development. Both alpha2A- and alpha2B-, but not alpha2C-, adrenoceptors were strongly expressed in MLIs at P13, and P31. Therefore, the developmental enhancement of the clonidine-mediated inhibition of sIPSCs is attributed to an age-dependent increase in AP-derived sIPSCs, which can be blocked by clonidine. Thus, presynaptic activation of alpha2-adrenoceptors inhibits cerebellar inhibitory synaptic transmission after the second postnatal week, leading to a restriction of NA signaling, which is mainly mediated by alpha1- and beta2-adrenoceptors in the adult cerebellar neuronal circuit.

Receptor occupancy of mirtazapine determined by PET in healthy volunteers

RATIONALE

Molecular tools are needed for assessing anti-depressant actions by positron emission tomography (PET) in the living human brain.

OBJECTIVES

This study determined whether [(11)C]mirtazapine is an appropriate molecular tool for use with PET to estimate the magnitude of neuroreceptor occupancy produced by daily intake of mirtazapine.

METHODS

This study used a randomised, double-blind, placebo-controlled, parallel-group, within-subject design. Eighteen healthy volunteers were PET-scanned twice with [(11)C]mirtazapine; once under baseline condition and again after receiving either placebo or mirtazapine (7.5 or 15 mg) for 5 days. We determined kinetic parameters of [(11)C]mirtazapine in brain regions by the simplified reference region method and used binding potential values to calculate receptor occupancy produced by mirtazapine.

RESULTS

Serum concentrations of mirtazapine ranged from 33 to 56 nmol/l after five daily doses of 7.5 mg mirtazapine and were between 41 and 74 nmol/l after 15 mg mirtazapine. Placebo treatment failed to alter the binding potential of [(11)C]mirtazapine from baseline values, whereas daily intake of mirtazapine markedly decreased the binding potential in cortex, amygdala and hippocampus. Receptor occupancy ranged from 74 to 96% in high-binding regions of the brain after five daily doses of 7.5 mg or 15 mg mirtazapine, whereas 17-48% occupancy occurred in low-binding regions.

CONCLUSIONS

[(11)C]Mirtazapine together with PET can determine the degree of receptor occupancy produced by daily doses of mirtazapine in regions of the living human brain.

The anatomy of co-morbid neuropsychiatric disorders based on cortico-limbic synaptic interactions

Many brain disorders appear to involve dysfunctions of aminergic systems. Alterations in dopamine activity may underpin both schizophrenia and the establishment and maintenance of drug dependence while disruption of serotonergic signalling may be crucial in both depression and schizophrenia. The co-existence of nicotine and alcohol abuse with depression and schizophrenia is well-documented as is the particular vulnerability of adolescents. At the same time, a common group of brain structures is increasingly implicated in neuropathological studies. For example, depression may involve a lack of serotonin signalling, particularly in the prefrontal cortex, while in schizophrenia there is evidence for reduced dopamine signalling in the same brain region, co-existing with hyperactivity in the mesolimbic dopamine pathway. Increased dopamine release from the mesolimbic dopamine pathway is also a common factor of drugs of abuse. Furthermore, the control of motivational behaviour and dopamine release is apparently modified by hippocampal and amygdala activity, both brain regions showing pathological changes in schizophrenia and depression. Our work has focused on the intricate synaptic interactions of aminergic terminals and cortical and subcortical neurons in order to unravel the anatomical basis for these disorders and their treatments. We show convergence of dopamine and cortical inputs onto single neurons in the nucleus accumbens, and between different cortical inputs to individual neurons, providing a basis for the gating mechanisms attributed to these interactions. We have also examined local and extrinsic connections in the prefrontal cortex and the basis for regulation of both cortical neurons and midbrain dopamine neurons by serotonin from the raph é nucleus. Together with data concerning subcellular receptor distributions, this information provides a detailed synaptic framework for interpreting behavioural, pharmacological and physiological data and enhances our understanding of possible circuitry underlying comorbidity of disorders such as schizophrenia and depression with drug abuse, information invaluable in the introduction of enhanced therapies.

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.

The alpha(2C)-adrenergic receptor mediates hyperactivity of coloboma mice, a model of attention deficit hyperactivity disorder

Drugs that modify noradrenergic transmission such as atomoxetine and clonidine are increasingly prescribed for the treatment of attention deficit hyperactivity disorder (ADHD). However, the therapeutic targets of these compounds are unknown. Norepinephrine is also implicated in the hyperactivity exhibited by coloboma mice. To identify the receptor subtypes that regulate the hyperactivity, coloboma mice were systematically challenged with adrenergic drugs. The beta-adrenergic receptor antagonist propranolol and the alpha(1)-adrenergic receptor antagonist prazosin each had little effect on the hyperactivity. Conversely, the alpha(2)-adrenergic receptor antagonist yohimbine reduced the activity of coloboma mice but not control mice. Subtype-selective blockade of alpha(2C)-, but not alpha(2A)- or alpha(2B)-adrenergic receptors, ameliorated hyperactivity of coloboma mice without affecting activity of control mice, suggesting that alpha(2C)-adrenergic receptors mediate the hyperactivity. Localized in the basal ganglia, alpha(2C)-adrenergic receptors are in a prime position to impact locomotor activity and are, therefore, potential targets of pharmacotherapy for ADHD.

Mild prenatal protein malnutrition increases α2C-adrenoceptor expression in the rat cerebral cortex during postnatal life

Mild reduction in the protein content in the diet of pregnant rats from 25 to 8% casein, calorically compensated by carbohydrates, does not alter body and brain weights of rat pups at birth, but results in significant changes of the concentration and release of cortical noradrenaline during postnatal life, together with impaired long-term potentiation and memory formation. Since some central noradrenergic receptors are critically involved in neuroplasticity, the present study evaluated, by utilizing immunohistochemical methods, the effect of mild prenatal protein malnutrition on the alpha 2C-adrenoceptor expression in the frontal and occipital cortices of 8- and 60-day-old rats. At day 8 of postnatal age, prenatally malnourished rats exhibited a three-fold increase of alpha 2C-adrenoceptor expression in both the frontal and the occipital cortices, as compared to well-nourished controls. At 60 days of age, prenatally malnourished rats showed normal expression levels scores of alpha 2C-adrenoceptor in the neocortex. Results suggest that overexpression of neocortical alpha 2C-adrenoceptors during early postnatal life, subsequent to mild prenatal protein malnutrition, could in part be responsible for neural and behavioral disturbances showing prenatally malnourished animals during the postnatal life.

Neuron specific alpha-adrenergic receptor expression in human cerebellum: implications for emerging cerebellar roles in neurologic disease

Recent data suggest novel functional roles for cerebellar involvement in a number of neurologic diseases. Function of cerebellar neurons is known to be modulated by norepinephrine and adrenergic receptors. The distribution of adrenergic receptor subtypes has been described in experimental animals, but corroboration of such studies in the human cerebellum, necessary for drug treatment, is still lacking. In the present work we studied cell-specific localizations of alpha1 adrenergic receptor subtype mRNA (alpha 1a, alpha 1b, alpha 1d), and alpha2 adrenergic receptor subtype mRNA (alpha 2a, alpha 2b, alpha 2c) by in situ hybridization on cryostat sections of human cerebellum (cortical layers and dentate nucleus). We observed unique neuron-specific alpha1 adrenergic receptor and alpha2 adrenergic receptor subtype distribution in human cerebellum. The cerebellar cortex expresses mRNA encoding all six alpha adrenergic receptor subtypes, whereas dentate nucleus neurons express all subtype mRNAs, except alpha 2a adrenergic receptor mRNA. All Purkinje cells label strongly for alpha 2a and alpha 2b adrenergic receptor mRNA. Additionally, Purkinje cells of the anterior lobe vermis (lobules I to V) and uvula/tonsil (lobules IX/HIX) express alpha 1a and alpha 2c subtypes, and Purkinje cells in the ansiform lobule (lobule HVII) and uvula/tonsil express alpha 1b and alpha 2c adrenergic receptor subtypes. Basket cells show a strong signal for alpha 1a, moderate signal for alpha 2a and light label for alpha 2b adrenergic receptor mRNA. In stellate cells, besides a strong label of alpha 2a adrenergic receptor mRNA in all and moderate label of alpha 2b message in select stellate cells, the inner stellate cells are also moderately positive for alpha 1b adrenergic receptor mRNA. Granule and Golgi cells express high levels of alpha 2a and alpha 2b adrenergic receptor mRNAs. These data contribute new information regarding specific location of adrenergic receptor subtypes in human cerebellar neurons. We discuss our observations in terms of possible modulatory roles of adrenergic receptor subtypes in cerebellar neurons responding to sensory and autonomic input signals, and review species differences in cerebellar adrenergic receptor expression.

Norepinephrine transporter knockout-induced up-regulation of brain alpha2A/C-adrenergic receptors

The norepinephrine transporter (NET) is responsible for the rapid removal of norepinephrine released from sympathetic neurons; this release is controlled by inhibitory alpha(2)-adrenergic receptors (alpha(2)ARs). Long-term inhibition of the NET by antidepressants has been reported to change the density and function of pre- and postsynaptic ARs, which may contribute to the antidepressant effects of NET inhibitors such as desipramine. NET-deficient (NET-KO) mice have been described to behave like antidepressant-treated mice. By means of quantitative real-time PCR we show that mRNAs encoding the alpha(2A)-adrenergic receptor (alpha(2A)AR) and the alpha(2C)-adrenergic receptor (alpha(2C)AR) are up-regulated in the brainstem, and that alpha(2C)AR mRNA is also elevated in the hippocampus and striatum of NET-KO mice. These results were confirmed at the protein level by quantitative autoradiography. The NET-KO mice showed enhanced binding of the selective alpha(2)AR antagonist [(3)H]RX821002 in several brain regions. Most robust increases (20-25%) in alpha(2)AR expression were observed in the hippocampus and in the striatum. Significant increases (16%) were also seen in the extended amygdala and thalamic structures. In an 'in vivo' test, the alpha(2)AR agonist clonidine (0.1 mg/kg) caused a significantly greater reduction of locomotor activity in NET-KO mice than in wild-type mice, showing the relevance of our findings at the functional level.

Inhibition of [11C]mirtazapine binding by α2-adrenoceptor antagonists studied by positron emission tomography in living porcine brain

We have developed [(11)C]mirtazapine as a ligand for PET studies of antidepressant binding in living brain. However, previous studies have determined neither optimal methods for quantification of [(11)C]mirtazapine binding nor the pharmacological identity of this binding. To obtain that information, we have now mapped the distribution volume (V(d)) of [(11)C]mirtazapine relative to the arterial input in the brain of three pigs, in a baseline condition and after pretreatment with excess cold mirtazapine (3 mg/kg). Baseline V(d) ranged from 6 ml/ml in cerebellum to 18 ml/ml in frontal cortex, with some evidence for a small self-displaceable binding component in the cerebellum. Regional binding potentials (pBs) obtained by a constrained two-compartment model, using the V(d) observation in cerebellum, were consistently higher than pBs obtained by other arterial input or reference tissue methods. We found that adequate quantification of pB was obtained using the simplified reference tissue method. Concomitant PET studies with [(15)O]-water indicated that mirtazapine challenge increased CBF uniformly in cerebellum and other brain regions, supporting the use of this reference tissue for calculation of [(11)C]mirtazapine pB. Displacement by mirtazapine was complete in the cerebral cortex, but only 50% in diencephalon, suggesting the presence of multiple binding sites of differing affinities in that tissue. Competition studies with yohimbine and RX 821002 showed decreases in [(11)C]mirtazapine pB throughout the forebrain; use of the multireceptor version of the Michaelis-Menten equation indicated that 42% of [(11)C]mirtazapine binding in cortical regions is displaceable by yohimbine. Thus, PET studies confirm that [(11)C]mirtazapine affects alpha(2)-adrenoceptor binding sites in living brain.

The analgesic efficacy of partial opioid agonists is increased in mice with targeted inactivation of the alpha2A-adrenoceptor gene

Alpha(2A)-Adrenoceptors mediate the antinociceptive effects of alpha(2)-adrenoceptor agonists in mice, and analgesic synergism between noradrenergic and opioidergic mechanisms has been reported to be lacking in mice devoid of functional alpha(2A)-adrenoceptors. We investigated whether the antinociceptive actions of opioid agonists with different efficacy would be altered in mice with targeted inactivation of the alpha(2A)-adrenoceptor gene (alpha(2A)-KO mice). The antinociceptive effects of fentanyl, morphine, buprenorphine and tramadol were assessed using conventional tail-flick and hot-plate assays. Antinociceptive responses to fentanyl were unaltered in the alpha(2A)-KO animals. Morphine analgesia was slightly accentuated in the tail-flick test. The naloxone-sensitive antinociceptive responses to both tested weak partial agonists were very markedly accentuated in both tests. For example, after 40 mg/kg tramadol administration, the tramadol-induced prolongation of tail-flick latency was 86+/-6% of the maximal possible effect (MPE) in alpha(2A)-KO and 22+/-2% of MPE in control mice; prolongation of hot-plate latency was 93+/-5% of MPE in alpha(2A)-KO mice and 8+/-2% of MPE in the controls (P<0.001 for both). The effects of alpha(2A)-KO were mimicked by pretreatment of wild-type control mice with the alpha(2)-adrenoceptor antagonists, atipamezole and yohimbine; the apparent efficacy of tramadol and buprenorphine now approached that of morphine and fentanyl. Other behavioural effects of the tested opioid agonists were not similarly influenced by alpha(2A)-KO. Antagonists of alpha(2A)-adrenoceptors may offer a novel mechanism to augment the antinociceptive actions of partial opioid agonists.

Activation of noradrenergic transmission by alpha2-adrenoceptor antagonists counteracts deafferentation-induced neuronal death and cell proliferation in the adult mouse olfactory bulb

The olfactory bulb is the target of neural progenitor cells that are generated in the subventricular zone of the lateral ventricle in the adult brain. This permanent neurogenesis is likely influenced by olfactory input to the bulb since previous studies have shown that cell proliferation and/or apoptotic death are stimulated by naris closure or surgical transection of the olfactory nerve. Since the olfactory bulb is densely innervated by noradrenergic afferents originating in the locus coeruleus, we have studied the impact of pharmacologically activating this noradrenergic system on cell death and proliferation following unilateral olfactory axotomy in the adult mouse olfactory bulb. We found that noradrenaline release in the olfactory bulb was significantly increased by intraperitoneal injections of the selective alpha(2)-adrenoceptor antagonists, dexefaroxan (0.63 mg/kg) and 5-fluoro-methoxyidazoxan (F 14413; 0.16 mg/kg). A chronic treatment with either compound for 7 days following olfactory axotomy significantly reduced neuronal death, glial activation and cell proliferation in the deafferented olfactory bulb. These data (1) confirm that alpha(2)-adrenoceptor antagonists, presumably by facilitating central noradrenergic transmission, afford neuroprotection in vivo, as previously shown in models of cerebral ischemia, excitotoxicity and devascularization-induced neurodegeneration, and (2) support a role of the locus coeruleus noradrenergic system in promoting survival of neurons in areas of the brain where neurogenesis persists in the adult.

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.

α2A-Adrenoceptors regulate d-amphetamine-induced hyperactivity and behavioural sensitization in mice

Stimulants, such as d-amphetamine, enhance the release of dopamine in the central nervous system (CNS) and induce locomotor activation in mice. When amphetamine is administered repeatedly, the locomotor activation is progressively increased. This behavioural sensitization may be associated with the development of drug craving, addiction and dependence. Also noradrenergic mechanisms participate in the mediation of the effects of psychostimulants. In this study we show that mice lacking the alpha(2)-adrenoceptor subtype A (alpha(2A)-AR knock-out (KO) on C57Bl/6J background) are supersensitive to the acute locomotor effects of d-amphetamine (5 mg/kg) in a novel environment compared to wild-type (WT) control mice. When both genotypes were treated repeatedly with d-amphetamine (2 mg/kg) they developed locomotor hyperactivation (sensitization), but its amplitude was lower in alpha(2A)-AR KO mice. Development of hyperactivation was reduced in both genotypes by pretreatment with the selective alpha(2)-adrenoceptor antagonist, atipamezole (1 mg/kg). Acute atipamezole also attenuated the expression of d-amphetamine-induced behavioural sensitization especially in WT mice. Interestingly, alpha(2A)-AR KO mice failed to exhibit persistent sensitization after 2 weeks of abstinence from repeated d-amphetamine. Rewarding properties of d-amphetamine, measured by conditioned place preference, were similar in both genotypes. These findings indicate that d-amphetamine-induced acute and sensitized locomotor effects are controlled by alpha(2)-adrenoceptors. Drugs antagonizing the alpha(2A)-adrenoceptor subtype may provide a novel approach for reducing drug sensitization and motor complications caused by dopaminergic agents.

Inhibition of perforant path input to the CA1 region by serotonin and noradrenaline

Bath-applied monoamines-dopamine (DA), serotonin (5-HT), and noradrenaline (NE)-strongly suppress the perforant path (PP) input to CA1 hippocampal region with very little effect on the Schaffer collaterals (SC) input. The effect of DA action on PP field excitatory postsynaptic potential (fEPSP) has been characterized in detail, but relatively little is known about the NE and 5-HT effects. Here we show that the maximal inhibition of the PP fEPSP by NE is approximately 55%, whereas 5-HT inhibition is weaker ( approximately 35%). The half-maximal inhibitory concentration of both 5-HT and NE is approximately 1 muM. Neither NE nor 5-HT affected paired-pulse facilitation, suggesting that the effect is not presynaptic. This is in contrast to DA, which does have a presynaptic effect. The NE effect was blocked by alpha2 antagonists, whereas the alpha1 antagonist corynanthine and beta-antagonist propranolol were ineffective. The effect of 5-HT was mimicked by the agonist, 5-carboxamidotryptamine maleate (5-CT), and not affected by adrenergic and dopaminergic antagonists. To determine the 5-HT receptors involved, we tested a number of 5-HT antagonists, but none produced a complete suppression of the 5-HT effect. Of these, only the 5-HT7 and 5-HT2 antagonists produced weak but significant inhibition of 5-HT effect. We conclude that NE inhibits the PP fEPSP through postsynaptic action on alpha2-adrenoceptors and that 5-HT7, 5-HT2, and some other receptor may be involved in 5-HT action in PP.

Novel pharmacological strategies for motor complications in Parkinson’s disease

In advanced Parkinson's disease, the combination of disease progression and levodopa therapy leads to the development of motor problems complicating the therapeutic response, known as motor response complications. The nonphysiological, pulsatile stimulation produced by most currently available dopaminergic therapies triggers a complicated series of responses resulting in the dysregulation of glutamate receptors and many other neurotransmitter systems on striatal neurons. Although a number of novel compounds that provide a more continuous dopaminergic stimulation are becoming available, no practical way to accomplish this in a truly physiological manner currently exists. Novel strategies for pharmacological intervention with the use of nondopaminergic treatments, with drugs targeting selected transmitter receptors expressed on striatal neurons appear more promising. These include NMDA or AMPA antagonists, or drugs acting on 5-hydroxytryptamine subtype 2A, alpha2-adrenergic, adenosine A2A and cannabinoid CB1 receptors. Future strategies may also target pre- and postsynaptic components that regulate firing pattern, like synaptic vesicle proteins, or nonsynaptic gap junction communication mechanisms, or drugs with actions at the signal transduction systems that modulate the phosphorylation state of NMDA receptors. These new therapeutic strategies, alone or in combination, hold the promise of providing effective control or reversal of motor response complications.

Mild prenatal protein malnutrition increases α2C-adrenoceptor density in the cerebral cortex during postnatal life and impairs neocortical long-term potentiation and visuo-spatial performance in rats

Mild reduction in the protein content of the mother's diet from 25 to 8% casein, calorically compensated by carbohydrates, does not alter body and brain weights of rat pups at birth, but leads to significant enhancements in the concentration and release of cortical noradrenaline during early postnatal life. Since central noradrenaline and some of its receptors are critically involved in long-term potentiation (LTP) and memory formation, this study evaluated the effect of mild prenatal protein malnutrition on the alpha2C-adrenoceptor density in the frontal and occipital cortices, induction of LTP in the same cortical regions and the visuo-spatial memory. Pups born from rats fed a 25% casein diet throughout pregnancy served as controls. At day 8 of postnatal age, prenatally malnourished rats showed a threefold increase in neocortical alpha2C-adrenoceptor density. At 60 days-of-age, alpha2C-adrenoceptor density was still elevated in the neocortex, and the animals were unable to maintain neocortical LTP and presented lower visuo-spatial memory performance. Results suggest that overexpression of neocortical alpha2C-adrenoceptors during postnatal life, subsequent to mild prenatal protein malnutrition, could functionally affect the synaptic networks subserving neocortical LTP and visuo-spatial memory formation.

In vivo regulation of dopamine and noradrenaline release by alpha2A-adrenoceptors in the mouse nucleus accumbens

The present study investigated the role of alpha2A-adrenoceptor (alpha2A-AR) subtype in the regulation of noradrenaline (NA) and dopamine (DA) release in the nucleus accumbens (NAc). The effect of locally infused and systemically injected alpha2-AR agonist, dexmedetomidine (DMT), and alpha2-AR antagonist, atipamezole, on NA and DA release was investigated in alpha2A-AR knockout and control mice by using in vivo microdialysis. In addition, we compared the drug effects on DA and NA release in the NAc to their effect on locomotor activity. Baseline NA and DA concentrations did not differ between genotypes. Local infusion of DMT decreased, in a concentration-dependent manner, NA, but not DA, levels in the control mice. However, systemic injection of DMT decreased both NA and DA levels in the control mice. In both cases DMT had no effects on transmitter release in alpha2A-AR knockout mice. Our results suggest that alpha2-ARs regulate the release of NA, but not DA, at the terminal level in the NAc. However, alpha2-ARs regulate DA release in the NAc indirectly by their effect on DA neurones in the ventral tegmental area via an unknown mechanism. In both cases the regulation is mediated by alpha2A-adrenoceptor subtype. Also the modulation of locomotor activity by alpha2-AR agonist and antagonist seems to be mediated via alpha2A-adrenoceptors.

[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.

Effects of tramadol on α2-adrenergic receptors in the rat brain

In recent years, it has been postulated that tramadol, used mainly for the treatment of moderate to severe pain, might display a potential as an antidepressant drug. The present study investigated the effects of acute and repeated tramadol administration on the binding of [3H]RX 821002, a selective alpha2-adrenergic receptor ligand, in the rat brain. Male Wistar rats were used. Tramadol (20 mg/kg, i.p.) administered acutely (single dose), at 24 h after dosing, induced a significant decrease in the alpha2-adrenergic receptors in all brain regions studied. The most pronounced effects were observed in all subregions of the olfactory system, nucleus accumbens and septum, thalamus, hypothalamus, amygdala, and cerebral cortex. Repeated treatment with tramadol (20 mg/kg, i.p., once daily for 21 days) also induced statistically significant downregulation of [3H]RX 821002 binding sites in the rat brain. However, the effect--although statistically significant--was less pronounced than in the group treated acutely with the drug. Since drugs such as mianserin and mirtazapine are potent antagonists of central alpha2-adrenergic receptors and are effective antidepressants, it is tempting to suggest that, in addition to other alterations induced by tramadol, downregulation of these receptors may represent a potential antidepressant efficacy. On the other hand, one should be careful to avoid the treatment of chronic pain with tramadol in patients already receiving antidepressant drugs. Tramadol-induced downregulation of alpha2-adrenergic receptors--when combined with ongoing antidepressant therapy with drugs, which themselves inhibit serotonin reuptake or are antagonists of alpha2-adrenergic receptors--might cause threatening complications.

Adrenergic receptor knockout mice: distinct functions of 9 receptor subtypes

The biological effects of epinephrine and norepinephrine are mediated via 9 different adrenergic receptor subtypes, which all belong to the superfamily of G protein-coupled receptors. Although pharmacological ligands for adrenergic receptors have an important place in medical therapy, the full therapeutic potential of the 9 adrenergic receptor subtypes has not been explored yet. To dissect the physiological relevance of adrenergic receptor subtype diversity, gene-targeted mouse models carrying deletions in these receptor genes ("knockout mice") have been generated. This review gives an overview of the phenotypes observed in mice deficient in adrenergic receptors and discusses the therapeutic relevance of subtype-specific drug therapy.

Noradrenergic Drugs for Levodopa-Induced Dyskinesia

Dyskinesia frequently mars the long-term therapeutic response to levodopa (LD) in Parkinson's disease (PD). New treatment strategies for levodopa-induced dyskinesia (LID) currently being investigated include some that target the nondopaminergic pathways. Indeed, LID in parkinsonism can be modulated by drugs acting on different neurotransmitters including glutamate, gamma-aminobutyric acid, noradrenaline, acetylcholine, serotonin, adenosine, and cholecystokinin. In many cases, the possibility of using specific compounds to counteract LID was raised by the previously shown efficacy of such compounds in the treatment of other types of dyskinesia. More data are now available on drugs that act on the noradrenergic system. Two studies have recently shown how the alpha-2 adrenoreceptor antagonist idazoxan can significantly reduce LID in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned primate model of parkinsonism and in patients with advanced PD. The experimental paper, which studied the antagonistic action of idazoxan on dyskinesia induced by both LD and apomorphine in marmosets with MPTP-induced parkinsonism, showed that the pharmacologic mechanisms underlying LID and apomorphine-induced dyskinesia in PD are probably distinct. LD, although not apomorphine-induced, dyskinesia was found to be influenced by adrenoreceptor antagonists. Indeed, the action of alpha-2 adrenoreceptor antagonists may involve the blockade of the action of noradrenaline synthesized from LD. The hypothesis is that because dopamine agonists are not metabolized to noradrenaline, alpha-2 adrenoreceptor antagonists do not reduce dyskinesia produced by such agents. This finding is particularly relevant in planning clinical studies in which LD or dopamine agonist challenges are used to assess the potential antidyskinetic properties of new drugs. The clinical study assessed the effects of idazoxan on LID in 18 patients with advanced PD: An improvement in LID, without the reappearance of parkinsonian symptoms, was observed. The practical outcome of this research is that, although the mechanisms underlying the manifestations and the priming process for dyskinesia have yet to be fully elucidated, a nondopaminergic approach to therapy may provide an effective way of preventing, or at least limiting, the expression of involuntary movements in PD.