Presynaptic alpha 2A-adrenoceptors inhibit the release of endogenous dopamine in rabbit caudate nucleus slices

Presynaptic Adrenoceptors

Presynaptic α2-adrenoceptors are localized on axon terminals of many noradrenergic and non-noradrenergic neurons in the peripheral and central nervous systems. Their activation by exogenous agonists leads to inhibition of the exocytotic release of noradrenaline and other transmitters from the neurons. Most often, the α2A-receptor subtype is involved in this inhibition. The chain of molecular events between receptor occupation and inhibition of the exocytotic release of transmitters has been determined. Physiologically released endogenous noradrenaline elicits retrograde autoinhibition of its own release. Some clonidine-like α2-receptor agonists have been used to treat hypertension. Dexmedetomidine is used for prolonged sedation in the intensive care; It also has a strong analgesic effect. The α2-receptor antagonist mirtazapine increases the noradrenaline concentration in the synaptic cleft by interrupting physiological autoinhibion of release. It belongs to the most effective antidepressive drugs. β2-Adrenoceptors are also localized on axon terminals in the peripheral and central nervous systems. Their activation leads to enhanced transmitter release, however, they are not activated by endogenous adrenaline.

Enduring dysregulation of nucleus accumbens catecholamine and glutamate transmission by developmental exposure to phenylpropanolamine

For over 50 years, the sympathomimetic phenylpropanolamine (PPA; ±-norephedrine) was a primary active ingredient in over-the-counter nasal decongestants for both children and adults and continues to be prevalent in the vast majority of countries today. Previously, we reported that juvenile PPA exposure alters the developmental trajectory of catecholamine and amino acid neurotransmitter systems in the nucleus accumbens (NAC), impacting the motivational valence of cocaine in later life. The present study employed a combination of in vivo microdialysis and immunoblotting approaches to better understand how juvenile PPA exposure impacts catecholamine and glutamate function within the NAC. For this, C57BL/6J mice were pretreated repeatedly with PPA (0 or 40 mg/kg) during postnatal days 21-33. Starting at 70 days of age, the function and expression of receptors and transporters regulating extracellular dopamine and glutamate were determined. Juvenile PPA pretreatment completely abolished the capacity of selective dopamine and epinephrine reuptake inhibitors to increase NAC levels of both catecholamines, without impacting D2 or α2 receptor regulation of catecholamine release. Juvenile PPA pretreatment facilitated the rise in NAC glutamate elicited by dopamine, norepinephrine and glutamate transporter inhibitors and blunted mGlu2/3 inhibition of glutamate release in this region. These data confirm that juvenile exposure to PPA produces protracted perturbations in the regulation of extracellular catecholamine and glutamate levels within the NAC and further the hypothesis that early exposure to sympathomimetic drugs found in cough, cold and allergy medicines, have long-lasting effects upon neurotransmission within brain regions gating motivation.

Dopaminergic Disturbances in Tourette Syndrome: An Integrative Account

Tourette syndrome (TS) is thought to involve dopaminergic disturbances, but the nature of those disturbances remains controversial. Existing hypotheses suggest that TS involves 1) supersensitive dopamine receptors, 2) overactive dopamine transporters that cause low tonic but high phasic dopamine, 3) presynaptic dysfunction in dopamine neurons, or 4) dopaminergic hyperinnervation. We review evidence that contradicts the first two hypotheses; we also note that the last two hypotheses have traditionally been considered too narrowly, explaining only small subsets of findings. We review all studies that have used positron emission tomography and single-photon emission computerized tomography to investigate the dopaminergic system in TS. The seemingly diverse findings from those studies have typically been interpreted as pointing to distinct mechanisms, as evidenced by the various hypotheses concerning the nature of dopaminergic disturbances in TS. We show, however, that the hyperinnervation hypothesis provides a simple, parsimonious explanation for all such seemingly diverse findings. Dopaminergic hyperinnervation likely causes increased tonic and phasic dopamine. We have previously shown, using a computational model of the role of dopamine in basal ganglia, that increased tonic dopamine and increased phasic dopamine likely increase the propensities to express and learn tics, respectively. There is therefore a plausible mechanistic link between dopaminergic hyperinnervation and TS via increased tonic and phasic dopamine. To further bolster this argument, we review evidence showing that all medications that are effective for TS reduce signaling by tonic dopamine, phasic dopamine, or both.

Dopamine, Noradrenaline and Serotonin Receptor Densities in the Striatum of Hemiparkinsonian Rats following Botulinum Neurotoxin-A Injection

Parkinson's disease (PD) is characterized by a degeneration of dopaminergic neurons in the substantia nigra pars compacta (SNpc) that causes a dopamine (DA) deficit in the caudate-putamen (CPu) accompanied by compensatory changes in other neurotransmitter systems. These changes result in severe motor and non-motor symptoms. To disclose the role of various receptor binding sites for DA, noradrenaline, and serotonin in the hemiparkinsonian (hemi-PD) rat model induced by unilateral 6-hydroxydopamine (6-OHDA) injection, the densities of D₁, D₂/D₃, α₁, α₂, and 5HT_2A receptors were longitudinally visualized and measured in the CPu of hemi-PD rats by quantitative in vitro receptor autoradiography. We found a moderate increase in D₁ receptor density 3 weeks post lesion that decreased during longer survival times, a significant increase of D₂/D₃ receptor density, and 50% reduction in 5HT_2A receptor density. α₁ receptor density remained unaltered in hemi-PD and α₂ receptors demonstrated a slight right-left difference increasing with post lesion survival. In a second step, the possible role of receptors on the known reduction of apomorphine-induced rotations in hemi-PD rats by intrastriatally injected Botulinum neurotoxin-A (BoNT-A) was analyzed by measuring the receptor densities after BoNT-A injection. The application of this neurotoxin reduced D₂/D₃ receptor density, whereas the other receptors mainly remained unaltered. Our results provide novel data for an understanding of the postlesional plasticity of dopaminergic, noradrenergic and serotonergic receptors in the hemi-PD rat model. The results further suggest a therapeutic effect of BoNT-A on the impaired motor behavior of hemi-PD rats by reducing the interhemispheric imbalance in D₂/D₃ receptor density.

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.

Modulation of L-DOPA's antiparkinsonian and dyskinetic effects by α2-noradrenergic receptors within the locus coeruleus

Long-term l-DOPA use for Parkinson's disease (PD) is frequently complicated by the emergence of a debilitating motor side effect known as l-DOPA-induced dyskinesia (LID). Accumulating evidence has implicated the norepinephrine (NE) system in the pathogenesis of LID. Here we used the unilateral 6-hydroxydopamine rat model of PD to determine the role of the α2-adrenoceptors (α2R) in l-DOPA's therapeutic and detrimental motor-inducing effects. First, we characterized the effects of systemic α2R stimulation with clonidine, or blockade with atipamezole, on LID using the rodent abnormal involuntary movements scale, and l-DOPA's therapeutic effects using the forepaw adjusting steps test and locomotor activity chambers. The anatomical locus of action of α2R in LID was investigated by directly infusing clonidine or atipamezole into the locus coeruleus prior to systemic l-DOPA administration. Results showed systemic clonidine treatment reduced LID and locomotor activity but did not interfere with l-DOPA's antiparkinsonian benefits. Conversely, systemic atipamezole pretreatment prolonged LID and locomotor activity but did not modulate l-DOPA's antiparkinsonian benefits. Intra-LC infusions of clonidine and atipamezole mirrored systemic effects where clonidine reduced, and atipamezole increased, LID. Collectively, these results demonstrate that α2R play an important modulatory role in l-DOPA-mediated behaviors and should be further investigated as a potential therapeutic target.

Targeting the noradrenergic system for gender-sensitive medication development for tobacco dependence

INTRODUCTION

Tobacco use remains the leading cause of morbidity and mortality for both women and men in the United States, and women often experience poorer smoking cessation outcomes than men. Preliminary evidence suggests there are sex differences in medication effectiveness for smoking cessation. However, current medications do not take into account gender-sensitive treatment development and efficacy, underscoring the importance of this underdeveloped area of research.

METHODS

We reviewed preclinical and clinical evidence for gender differences in the inability to quit smoking by examining (a) the effect of increased negative affect and stress reactivity on smoking outcomes in women and (b) smoking for nicotine reinforcement in men. We also reviewed the current literature targeting the noradrenergic system as a novel gender-sensitive treatment strategy for tobacco dependence.

RESULTS

We hypothesize that noradrenergic agents that normalize noradrenergic activity may differentially attenuate stress reactivity in women and nicotine-related reinforcement in men, indicating that targeting the noradrenergic system for smoking cessation may be effective for both genders, with benefits operating through sex-specific mechanisms.

CONCLUSIONS

Converging lines of preclinical and clinical evidence suggest that gender-sensitive approaches to medication development for smoking cessation are a critical next step for addressing low quit rates and exacerbated health risks among women. Evidence reviewed indicates that smoking activates different brain systems modulated by noradrenergic activity in women versus men, and noradrenergic compounds may preferentially target these gender-sensitive systems.

Beneficial effects of desipramine on cognitive function of chronically stressed rats are mediated by alpha1-adrenergic receptors in medial prefrontal cortex

Chronic stress is a risk factor for many psychopathological conditions, including depression and anxiety disorders. Cognitive impairments associated with prefrontal cortical dysfunction are a major component of such illnesses. Using an attentional set-shifting test (AST), we have previously shown that elevating noradrenergic activity in rat medial prefrontal cortex (mPFC) can facilitate cognitive set-shifting, and that chronic unpredictable stress (CUS) caused set-shifting deficits. It is not known, however, if noradrenergic modulatory function is compromised by chronic stress, perhaps contributing to the stress-induced cognitive deficit. Thus, the first study investigated whether acutely elevating noradrenergic activity in mPFC still enhances cognitive function after chronic stress. As previously demonstrated, CUS impaired cognitive set-shifting on the AST. This deficit was abolished by acute systemic administration of the alpha(2)-adrenergic autoreceptor antagonist, atipamezole. Microdialysis revealed no differences in extracellular norepinephrine (NE) levels in mPFC of CUS-exposed and unstressed control rats at baseline or during behavioral testing, and comparable increases after atipamezole. In the second experiment, rats were treated chronically with the selective NE reuptake blocker, desipramine, during the CUS treatment through behavioral testing. Again, CUS impaired cognitive set-shifting in vehicle-treated rats, and chronic desipramine treatment prevented such deficits. Acute blockade of post-synaptic alpha(1)-adrenergic receptors in mPFC prior to testing blocked the beneficial effect of desipramine on cognitive set-shifting. These results suggest that desipramine restores cognitive set-shifting capability that has been compromised by chronic stress by activating alpha(1)-adrenergic receptors in the mPFC. Thus, noradrenergic modulatory capability in mPFC remains intact after CUS, and this represents one possible substrate by which antidepressants may exert their beneficial effects in the treatment of depression.

Role of adrenoceptors in the regulation of dopamine/DARPP-32 signaling in neostriatal neurons

Studies in animal models of Parkinson's disease have revealed that degeneration of noradrenaline neurons is involved in the motor deficits. Several types of adrenoceptors are highly expressed in neostriatal neurons. However, the selective actions of these receptors on striatal signaling pathways have not been characterized. In this study, we investigated the role of adrenoceptors in the regulation of dopamine/dopamine- and cAMP-regulated phosphoprotein of M(r) 32 kDa (DARPP-32) signaling by analyzing DARPP-32 phosphorylation at Thr34 [protein kinase A (PKA)-site] in mouse neostriatal slices. Activation of beta(1)-adrenoceptors induced a rapid and transient increase in DARPP-32 phosphorylation. Activation of alpha(2)-adrenoceptors also induced a rapid and transient increase in DARPP-32 phosphorylation, which subsequently decreased below basal levels. In addition, activation of alpha(2)-adrenoceptors attenuated, and blockade of alpha(2)-adrenoceptors enhanced dopamine D(1) and adenosine A(2A) receptor/DARPP-32 signaling. Chemical lesioning of noradrenergic neurons mimicked the effects of alpha(2)-adrenoceptor blockade. Under conditions of alpha(2)-adrenoceptor blockade, the dopamine D(2) receptor-induced decrease in DARPP-32 phosphorylation was attenuated. Our data demonstrate that beta(1)- and alpha(2)-adrenoceptors regulate DARPP-32 phosphorylation in neostriatal neurons. G(i) activation by alpha(2)-adrenoceptors antagonizes G(s)/PKA signaling mediated by D(1) and A(2A) receptors in striatonigral and striatopallidal neurons, respectively, and thereby enhances D(2) receptor/G(i) signaling in striatopallidal neurons. alpha(2)-Adrenoceptors may therefore be a therapeutic target for the treatment of Parkinson's disease.

The pharmacology of anxiety

Understanding the neurochemistry of anxiety is of fundamental importance in the development and use of novel anxiolytics. Through measuring peripheral markers of brain biochemistry, direct pharmacological challenges and brain neuroimaging techniques our understanding of this field has increased substantially in the past few decades. We review the four most studied neurotransmitter systems with respect to in anxiety disorders: gamma amino-butyric acid, serotonin, noradrenaline and dopamine. We have focussed upon clinical studies to highlight the current techniques used to determine brain neurochemistry in vivo. Future research in this field will greatly benefit from recent advances in neuroimaging techniques and the discovery of novel ligands targeting specific receptors.

Mesolimbic alpha-, but not beta-adrenoceptors control the accumbal release of dopamine that is derived from reserpine-sensitive storage vesicles

Mesolimbic beta-, but not alpha-adrenoceptors control the accumbal release of dopamine that is derived from alpha-methyl-para-tyrosine-sensitive pools of newly synthesized neurotransmitter. The aim of this study was to investigate which of these adrenoceptors control the accumbal release of dopamine that is derived from reserpine-sensitive pools of previously stored neurotransmitter. Rats, that were divided in low-responders and high-responders to novelty, were pretreated with 1 mg/kg of reserpine before the alpha-adrenergic-agent phentolamine or the beta-adrenergic-agent isoproterenol was locally applied into the nucleus accumbens. The original finding that phentolamine and isoproterenol increased accumbal dopamine levels in low-responders and high-responders was replicated. Reserpine reduced the phentolamine-induced increase of accumbal dopamine in both types of rat. However, phentolamine could still increase accumbal dopamine levels in reserpine-treated high-responders, but not anymore in reserpine-treated low-responders. Reserpine did not reduce the isoproterenol-induced increase of accumbal dopamine in any type of rat. This study demonstrates that mesolimbic alpha-, but not beta-adrenoceptors control the accumbal release of dopamine that is derived from reserpine-sensitive storage vesicles. In addition, these data confirm our previous finding that dopamine can still be released from storage vesicles of reserpinized high-responders, but not of reserpinized low-responders. The collected data underline our notion that alpha- and beta-adrenergic drugs may have therapeutic effects in patients suffering from diseases in which accumbal dopamine is involved.

Chronic treatment and withdrawal of the cannabinoid agonist WIN 55,212-2 modulate the sensitivity of presynaptic receptors involved in the regulation of monoamine syntheses in rat brain

Brain monoamines are involved in many neurochemical and behavioral effects of cannabinoids, but little is known on the regulation of noradrenaline, dopamine, and serotonin (5-HT) synthesis in cannabinoid addiction. This study investigated in rat brain the chronic effects of the potent cannabinoid agonist WIN 55,212-2 and of rimonabant-precipitated withdrawal, as well as the sensitivity of synthesis-modulating inhibitory receptors, on the accumulation of L-3,4-dihydroxyphenylalanine (DOPA) and 5-HTP after decarboxylase inhibition. Acute WIN (8 mg/kg; 1 h) increased DOPA synthesis in cortex (52%), hippocampus (51%), and cerebellum (56%) and decreased DOPA accumulation in striatum (31%). Acute WIN also decreased the synthesis of 5-HTP in all brain regions (40-53%). Chronic WIN (2-8 mg/kg; 5 days) and/or antagonist-precipitated withdrawal induced tolerance to the acute effects of WIN on the accumulation of DOPA (cortex and striatum) and 5-HTP (all brain regions). The inhibitory effect of clonidine (alpha2-agonist; 1 mg/kg) on the accumulation of DOPA (15-41%) and 5-HTP (22-41%) was markedly decreased or abolished after chronic WIN and precipitated withdrawal, mainly in noradrenergic and serotonergic brain regions, which indicated desensitization of alpha2-autoreceptors and alpha2-heteroreceptors regulating the synthesis of noradrenaline and 5-HT. In WIN-dependent rats (chronic and withdrawal states), the effect of a low dose of (+/-)-8-hydroxy-2-(di-n-propylamino)-tetralin (5-HT1A agonist; 0.1 mg/kg) on the accumulation of precursor amino acids was markedly potentiated in cerebellum and striatum, indicating the induction of supersensitivity of 5-HT1A-autoreceptors and 5-HT1A-heteroreceptors that regulate the synthesis of 5-HT, noradrenaline, and dopamine in these brain regions. These chronic adaptations in presynaptic receptor function could play a relevant role in cannabinoid addiction.

Dual influence of the striatum on neuropathic hypersensitivity

We studied whether striatal alpha(2)-adrenoceptors or N-methyl-d-aspartate (NMDA) receptors influence descending regulation of neuropathic hypersensitivity in the rat by microinjecting an alpha(2)-adrenoceptor agonist or NMDA-receptor antagonist into the dorsal striatum in animals with a spinal nerve ligation-induced neuropathy. Hypersensitivity was assessed in the hind limb by monofilaments and paw pressure test. Various neurotransmitter receptor antagonists were administered into the striatum or intrathecally to determine striatal and spinal neurotransmitters mediating the modulatory influence. The results indicate that the striatum has a dual effect on neuropathic hypersensitivity via two distinct pathways descending to the spinal cord. First, hypersensitivity is reduced following activation of noradrenergic alpha(2)-adrenoceptors and downstream dopamine D2 receptors in the striatum. This antihypersensitivity effect is predominantly ipsilateral and it descends via parallel dopaminergic and serotoninergic pathways to act on spinal dopamine D2 and 5-HT(1A) receptors, respectively. Second, tonic activation of striatal NMDA receptors promotes hypersensitivity by suppressing spinal GABAergic inhibition. The antihypersensitivity actions induced by striatal drug administrations were not associated with motor effects as suggested by lack of effect on the threshold of the uninjured limb or amplitude of the innocuous H-reflex. Involvement of striatal dopamine D2 receptors in the noradrenergic pain inhibitory circuitry may explain why disorders causing hypofunction of the striatal dopaminergic system, such as in Parkinson's disease, have been associated with pain. Furthermore, our findings indicate that striatal NMDA receptors provide a tonic supramedullary drive for medullospinal facilitatory influence that is known to be of importance for neuropathic hypersensitivity.

TDIQ (5,6,7,8-tetrahydro-1,3-dioxolo [4,5-g]isoquinoline): discovery, pharmacological effects, and therapeutic potential

Chemically, TDIQ (5,6,7,8-tetrahydro-1,3-dioxolo[4,5-g]isoquinoline) can be viewed as a conformationally restricted phenylalkylamine that is related in structure to amphetamine but does not stimulate (or depress) locomotor activity in rodents. In radioligand binding studies TDIQ displays selective affinity for alpha(2)-adrenergic receptor subsites (i.e., alpha(2A)-, alpha(2B)-, and alpha(2C)-adrenergic receptors), and behavioral data suggest that it might exert an agonist (or partial agonist) effect at alpha(2)-adrenergic receptors or interact at alpha(2)-adrenergic heteroreceptors. Drug discrimination studies in rats indicate that TDIQ: (1) serves as a discriminative stimulus, (2) may be useful in the treatment of symptoms associated with the abuse of cocaine, and (3) exhibits a low potential for abuse. In addition, TDIQ exhibits a dose-dependent and wide dissociation between doses that produce an anxiolytic-like effect or an inhibition of "snack" consumption in mice and doses that produce minimal, if any, effects in tests that measure a potential for disruption of coordinated movement or motor activity. Also, TDIQ displays negligible effects on the heart rate (HR) and blood pressure (BP) of mice. Taken together, the preclinical data suggest that TDIQ exhibits a favorable ratio of therapeutic-like effects (anxiolytic, therapeutic adjunct in the treatment of cocaine abuse, and appetite suppression) to side effect-like activities (behavioral impairment, drug abuse, or adverse cardiovascular effect). As such, TDIQ could: (1) be a forerunner for a new type of chemical entity in the treatment of certain forms of anxiety and/or obesity and (2) serve as a structural template in the discovery and development of additional agents that might be selective for alpha(2)-adrenergic receptors.

Guanfacine produces differential effects in frontal cortex compared with striatum: assessed by phMRI BOLD contrast

RATIONALE

Guanfacine (an alpha-(2A) adrenoreceptor agonist) is a drug of benefit in the treatment of attention deficit hyperactivity disorder (ADHD) (Taylor FB, Russo J, J Clin Psychopharmacol 21:223-228, 2001). Assessment of this drug using neuroimaging will provide information about the brain regions involved in its effects.

OBJECTIVES

The pharmacological magnetic resonance imaging blood oxygenation level dependent (BOLD) response was determined in rat brain regions following administration of guanfacine.

METHODS

Male rats were individually placed into a 2.35 T Bruker magnet for 60 min to achieve basal recording of changes in signal intensity. Either saline (n = 9) or guanfacine (0.3 mg/kg, i.p.; n = 9) was then administered and recording was continued for a further 90 min. Data were analysed for BOLD effects using statistical parametric maps. Respiration rate, blood pressure and blood gases were monitored and remained constant throughout scanning.

RESULTS

The main changes observed were negative BOLD effects in the caudate putamen and nucleus accumbens with positive BOLD effects in frontal association, prelimbic and motor cortex areas.

CONCLUSIONS

These data suggest that guanfacine can decrease neuronal activity in the caudate while increasing frontal cortex activity. This ability to change neuronal activity in specific areas of rat brain that are known to be impaired in ADHD (Solanto MV, Behav Brain Res 130:65-71, 2002) may contribute to guanfacine's beneficial effects.

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.

Quantification and pharmacological characterization of dialysate levels of noradrenaline in the striatum of freely-moving rats: release from adrenergic terminals and modulation by alpha2-autoreceptors

Information concerning striatal levels of noradrenaline (NA) remains inconsistent. Here we have addressed this issue using a sensitive method of HPLC coupled to amperometric detection. The NA reuptake-inhibitor, reboxetine, selectively elevated levels of NA versus dopamine (DA), and NA levels were also selectively elevated by the alpha2-adrenoceptor (AR) antagonist, atipamezole. The actions of atipamezole were mimicked by the preferential alpha2A-AR antagonist, BRL44408, while JO-1 and prazosin, preferential antagonists at alpha2C-ARs, caused less marked elevations in NA levels. In contrast to antagonists, the alpha2-AR agonist, S18616, decreased NA levels and likewise suppressed those of DA. Unilateral lesions of the substantia nigra with 6-hydroxydopamine depleted DA levels without affecting those of NA. Further, the D3/D2 receptor agonist, quinelorane, decreased levels of DA without modifying those of NA. However, the D3/D2 receptor antagonists, haloperidol and raclopride, and the DA reuptake-inhibitor, GBR12935, elevated levels of both DA and NA. Levels of 5-HT (but not of NA or DA) were increased only by the 5-HT reuptake-inhibitor, citalopram. They were decreased by S18616 and prazosin, reflecting the inhibitory and excitatory influence of alpha2- and alpha1-ARs, respectively, upon serotonergic pathways. In conclusion, NA in the striatum is derived from adrenergic terminals. Its release is subject to tonic, inhibitory control by alpha2-ARs, possibly involving both alpha2A- and alpha2C-AR subtypes, though their respective contribution requires clarification. A role of dopaminergic terminals in the reuptake of NA likely explains the elevation in its levels elicited by DA reuptake-inhibitors and D3/D2 receptor antagonists.

Atipamezole, an alpha(2)-adrenoceptor antagonist, has disease modifying effects on epileptogenesis in rats

Stimulation of alpha(2)-adrenoceptors delays the development of kindling, a model of epileptogenesis in humans. Blocking alpha(2)-adrenoceptors is proconvulsant, but has beneficial effects on somatomotor recovery after experimental stroke. We investigated whether atipamezole, a selective alpha(2)-adrenoceptor antagonist, affects the recovery process from status epilepticus (SE)-induced brain damage, which affects the risk of epileptogenesis. Vehicle or atipamezole (100 microg/kg/h) treatment was started 1 week after the induction of SE and continued for 9 weeks using Alzet minipumps (n = 70). Development and severity of epilepsy, spatial and emotional learning, and histologic analysis were used as outcome measures. There were no differences in the percentage of animals with epilepsy in the different treatment groups. In the atipamezole group, however, daily seizure frequency was lower (P < 0.01), a higher percentage of epileptic animals had mild epilepsy (<1 seizure/day; P < 0.01), and seizure frequency did not increase over time compared with the vehicle group. The atipamezole group had milder hilar cell damage (P < 0.05) and less intense mossy fiber sprouting (P < 0.05). Behavioral impairments were similar between groups. Our data indicate that chronic treatment with atipamezole does not prevent epileptogenesis. There is, however, a disease-modifying effect; that is, the epilepsy that develops is milder and non-progressive. These data warrant further studies.

Properties of dopamine release and uptake in the songbird basal ganglia

Vocal learning in songbirds requires a basal ganglia circuit termed the anterior forebrain pathway (AFP). The AFP is not required for song production, and its role in song learning is not well understood. Like the mammalian striatum, the striatal component of the AFP, Area X, receives dense dopaminergic innervation from the midbrain. Since dopamine (DA) clearly plays a crucial role in basal ganglia-mediated motor control and learning in mammals, it seems likely that DA signaling contributes importantly to the functions of Area X as well. In this study, we used voltammetric methods to detect subsecond changes in extracellular DA concentration to gain better understanding of the properties and regulation of DA release and uptake in Area X. We electrically stimulated Ca(2+)- and action potential-dependent release of an electroactive substance in Area X brain slices and identified the substance as DA by the voltammetric waveform, electrode selectivity, and neurochemical and pharmacological evidence. As in the mammalian striatum, DA release in Area X is depressed by autoinhibition, and the lifetime of extracellular DA is strongly constrained by monoamine transporters. These results add to the known physiological similarities of the mammalian and songbird striatum and support further use of voltammetry in songbirds to investigate the role of basal ganglia DA in motor learning.

Atipamezole, an alpha2-adrenoceptor antagonist, augments the effects of L-DOPA on evoked dopamine release in rat striatum

The effects of atipamezole, an alpha(2)-adrenoceptor antagonist, L-3,4-dihydroxyphenylalanine (L-DOPA) and the combination of these drugs on dopamine overflow were studied in dopaminergic presynaptic terminals of rat caudate and nucleus accumbens. Dopamine overflow evoked by 100 pulses of electrical stimulation of the medial forebrain bundle at a low (20 Hz) and high (50 Hz) frequency was measured by in vivo voltammetry. L-DOPA (15 mg/kg) increased dopamine overflow in the caudate nucleus, but this dose had no effects in the nucleus accumbens. Atipamezole (300 microg/kg) had no effects on its own on dopamine overflow, but it did increase the size of the readily releasable storage pool and the effects of L-DOPA treatment in both structures. The combination of the drugs increased dopamine overflow to a larger extent at high compared to low stimulation frequencies. We conclude that the rat caudate nucleus is more sensitive than the nucleus accumbens to the effects of L-DOPA, and the effects of L-DOPA treatment might be effectively enhanced by antagonism of alpha(2)-adrenoceptors.

In vivo regulation of dopamine and noradrenaline release by alpha2A-adrenoceptors in the mouse prefrontal cortex

The present study investigated the role of alpha2A-adrenoceptor subtype in the regulation of noradrenaline and dopamine release in the medial prefrontal cortex. The effect of local introduction of the alpha2-adrenoceptor agonist dexmedetomidine (10-9-10-8 m) on noradrenaline and dopamine release was investigated in alpha2A-adrenoceptor knockout and control mice by using in vivo microdialysis. Furthermore, to reveal a possible distinction between regulation of baseline and peak release, we sampled the dialysate during both rest and handling-induced mild stress. Baseline noradrenaline and dopamine concentrations did not differ between alpha2A-adrenoceptor knockout and control mice. Dexmedetomidine decreased, in a concentration-dependent manner, noradrenaline and dopamine levels in both genotypes. However, the effect of dexmedetomidine on noradrenaline release was attenuated in the alpha2A-adrenoceptor knockout mice, whereas the effect on dopamine release did not differ between the genotypes. The first handling episode increased noradrenaline and dopamine levels to the same extent in both genotypes. However, in alpha2A-adrenoceptor knockout mice the noradrenaline and dopamine levels remained elevated in the samples following the first handling whilst, in the control mice, transmitter levels returned to baseline levels. In control mice the handling-induced peak noradrenaline and dopamine levels were lower after the administration of dexmedetomidine than during the first handling episode, but in alpha2A-adrenoceptor knockout mice no drug effect on handling-induced peak noradrenaline and dopamine levels was found. Our results suggest that the release of noradrenaline in the medial prefrontal cortex is mainly regulated via alpha2A-adrenoceptors, whilst other alpha-adrenoceptor subtypes play a significant role in the regulation of dopamine release.

Two alpha(2)-adrenergic receptor subtypes, alpha(2A) and alpha(2C), inhibit transmitter release in the brain of gene-targeted mice

alpha(2)-Adrenergic receptors play an essential role in regulating neurotransmitter release from sympathetic nerves and from adrenergic neurons in the CNS. However, the role of each of the three highly homologous alpha(2)-adrenergic receptor subtypes (alpha(2A), alpha(2B), alpha(2C)) in this process has not been determined unequivocally. To address this question, the regulation of norepinephrine and dopamine release was studied in mice carrying deletions in the genes encoding the three alpha(2)-adrenergic receptor subtypes. Autoradiography and radioligand binding studies showed that alpha(2)-receptor density in alpha(2A)-deficient brains was decreased to 9 +/- 1% of the respective wild-type value, whereas alpha(2)-receptor levels were reduced to 83 +/- 4% in alpha(2C)-deficient mice. These results indicate that approximately 90% of mouse brain alpha(2)-receptors belong to the alpha(2A) subtype and 10% are alpha(2C)-receptors. In isolated brain cortex slices from wild-type mice a non-subtype-selective alpha(2)-receptor agonist inhibited release of [(3)H]norepinephrine by maximally 96%. Similarly, release of [(3)H]dopamine from isolated basal ganglion slices was inhibited by 76% by an alpha(2)-receptor agonist. In alpha(2A)-receptor-deficient mice, the inhibitory effect of the alpha(2)-receptor agonist on norepinephrine and dopamine release was significantly reduced but not abolished. Only in tissues from mice lacking both alpha(2A)- and alpha(2C)-receptors was no alpha(2)-receptor agonist effect on transmitter release observed. The time course of onset of presynaptic inhibition of norepinephrine release was much faster for the alpha(2A)-receptor than for the alpha(2C)-subtype. After prolonged stimulation with norepinephrine, presynaptic alpha(2C)-adrenergic receptors were desensitized. From these data we suggest that two functionally distinct alpha(2)-adrenergic receptor subtypes, alpha(2A) and alpha(2C), operate as presynaptic inhibitory receptors regulating neurotransmitter release in the mouse CNS.

Expression of catecholaminergic mRNAs in the hyperactive mouse mutant coloboma

The SNAP-25 deficient mouse mutant coloboma (Cm/+) is an animal model for investigating the biochemical basis of locomotor hyperactivity. The spontaneous hyperactivity exhibited by coloboma is three times greater than control mice and is a direct result of the SNAP-25 deletion. SNAP-25 is a presynaptic protein that regulates exocytotic neurotransmitter release; coloboma mice express only 50% of normal protein concentrations. Previous research has determined that there is an increase in the concentration of norepinephrine but a decrease in dopamine utilization in the striatum and nucleus accumbens of coloboma mice. In situ hybridization analysis revealed that there were corresponding increases in tyrosine hydroxylase (TH) mRNA expression in noradrenergic cell bodies of the locus coeruleus of Cm/+ mice. In contrast, TH mRNA expression in substantia nigra appeared normal in the mutant mouse. alpha(2)-Adrenergic receptors are important modulators of central noradrenergic function and dopamine release. In situ hybridization data revealed that alpha(2A)-adrenergic receptor mRNA expression is upregulated in Cm/+ mice. These results suggest an underlying abnormality in noradrenergic regulation in this hyperactive mouse mutant.

Changes in serotonin, dopamine and noradrenaline levels in striatum and nucleus accumbens after repeated administration of the abused drug MDMA in rats

The selective neurotoxic action of the abused drug 3,4-methylenedioxymethamphetamine (MDMA) on the serotonergic axons ascending from the dorsal raphe nucleus (DRN) is well known. The present study examined the long-term effects of subchronic MDMA treatment on rat brain tissue contents of catecholaminergic neurotransmitters. Two and four weeks after cessation of repeated MDMA treatment (ten consecutive days, 20 mg/kg/day), the tissue neurotransmitter concentrations were measured by means of electrochemical detected HPLC in several forebrain areas and DRN. We found reduced serotonin levels in the whole forebrain at both instants of time. In nucleus accumbens (NAC), the noradrenaline levels were also decreased, whereas dopamine levels were increased 4 weeks after treatment. It is concluded that MDMA causes changes of monoamine transmitter levels outlasting cessation of drug intake for at least 4 weeks. Decreased noradrenaline and/or serotonin may subsequently cause the augmentation of dopamine in NAC, a structure crucially involved in motivation circuits. With exception of transmitter alterations in the NAC, the post drug effects are opposite to the acute effects of MDMA and may underlie the psychiatric changes after MDMA intake in humans.

Clonidine treatment increases tics in patients with Tourette syndrome: case report

Clonidine usually serves as a tic-suppressing agent in patients with Tourette syndrome. A case study is presented in which clonidine at subclinical concentrations led to an exacerbation of the tics and caused severe systemic heat sensations. Involvement of functional abnormalities of the hypothalamus in Tourette syndrome is hypothesized.

An alpha(2)-adrenergic antagonist, atipamezole, facilitates behavioral recovery after focal cerebral ischemia in rats

Previous studies suggest that enhanced noradrenergic neurotransmission promotes functional recovery following cerebral lesions. The present study investigated whether systemic administration of an alpha(2)-adrenergic antagonist, atipamezole, facilitates recovery following transient focal cerebral ischemia in rats. The effect of atipamezole therapy on recovery from ischemia was compared with the effect of enriched-environment housing in rats. Ischemia was induced by occlusion of the right middle cerebral artery (MCA) for 120 min using the intraluminal filament model. Daily atipamezole treatment (1 mg/kg, subcutaneously) was started on day 2 after ischemia induction and drug administration stopped after 10 days. Another group of rats was housed in an enriched environment from day 2 following ischemia induction until the end of the experiment. Several different behavioral tests were used to measure functional recovery during the 26 days following the induction of focal cerebral ischemia. There was improved performance in the limb-placing test from the beginning of atipamezole treatment to day 8, and in wheel-running in the foot-slip test on days 2 and 4. Enriched-environment housing facilitated recovery in the foot-slip test in a later phase of the test period (days 8 to 10). Discovery of a hidden platform in a water-maze task was also facilitated in rats housed in the enriched environment, but this was probably due to the increased swimming speed of these rats. The present data suggest that the alpha(2)-adrenergic antagonist, atipamezole, facilitates sensorimotor recovery after focal ischemia, but has no effect on subsequent water-maze tests assessing spatial learning and memory, when assessed 11 days after the cessation of drug administration.

Reciprocal autoreceptor and heteroreceptor control of serotonergic, dopaminergic and noradrenergic transmission in the frontal cortex: relevance to the actions of antidepressant agents

The frontal cortex (FCX) plays a key role in processes that control mood, cognition and motor behaviour, functions which are compromised in depression, schizophrenia and other psychiatric disorders. In this regard, there is considerable evidence that a perturbation of monoaminergic input to the FCX is involved in the pathogenesis of these states. Correspondingly, the modulation of monoaminergic transmission in the FCX and other corticolimbic structures plays an important role in the actions of antipsychotic and antidepressant agents. In order to further understand the significance of monoaminergic systems in psychiatric disorders and their treatment, it is essential to characterize mechanisms underlying their modulation. Within this framework, the present commentary focuses on our electrophysiological and dialysis analyses of the complex and reciprocal pattern of auto- and heteroreceptor mediated control of dopaminergic, noradrenergic and serotonergic transmission in the FCX. The delineation of such interactions provides a framework for an interpretation of the influence of diverse classes of antidepressant agent upon extracellular levels of dopamine, noradrenaline and serotonin in FCX. Moreover, it also generates important insights into strategies for the potential improvement in the therapeutic profiles of antidepressant agents.

P2 receptor-mediated inhibition of dopamine release in rat neostriatum

Axon terminal nucleotide P2 receptors mediating an inhibition of transmitter release have, so far, been detected in various sympathetically innervated tissues,(8,27) and on central noradrenergic,(14,26) glutamatergic(15) and serotonergic neurons. (28) We have now investigated the effect of ATP and related nucleotides on the release of endogenous dopamine from slices of rat neostriatum using fast cyclic voltammetry. Mutual interactions between the two neurotransmitters have been observed previously: ATP and related nucleotides induce a release of dopamine in PC12 pheochromocytoma cells, a frequently used model for sympathetic neurons;(10,22) they also increase the dopamine concentration in rat brain measured by in vivo microdialysis(16,32) and stimulate the uptake of dopamine by rat striatal synaptosomes.(3) Dopamine, in contrast, facilitates activation of ligand-gated cation channels (i. e. P2X(2) receptors) by ATP.(11,20) Here, we show that ATP and two of its analogues decrease the electrically evoked release of endogenous dopamine in rat neostriatum. The inhibitory effect of ATP is blocked by the P2 receptor antagonists suramin, reactive blue 2 and cibacron blue 3GA. Suramin, in addition, partly prevents the attenuation of dopamine release evoked by a single stimulus that follows a brief train of high-frequency pulses.These findings suggest the existence of release-inhibiting P2 receptors on dopaminergic nerve terminals and indicate that dopaminergic transmission in rat neostriatum might be modulated by an endogenous P2 receptor ligand, presumably ATP.

Mirtazapine enhances frontocortical dopaminergic and corticolimbic adrenergic, but not serotonergic, transmission by blockade of alpha2-adrenergic and serotonin2C receptors: a comparison with citalopram

Mirtazapine displayed marked affinity for cloned, human alpha2A-adrenergic (AR) receptors at which it blocked noradrenaline (NA)-induced stimulation of guanosine-5'-O-(3-[35S]thio)-triphosphate ([35S]-GTPgammaS) binding. Similarly, mirtazapine showed high affinity for cloned, human serotonin (5-HT)2C receptors at which it abolished 5-HT-induced phosphoinositide generation. Alpha2-AR antagonist properties were revealed in vivo by blockade of UK-14,304-induced antinociception, while antagonist actions at 5-HT2C receptors were demonstrated by blockade of Ro 60 0175-induced penile erections and discriminative stimulus properties. Mirtazapine showed negligible affinity for 5-HT reuptake sites, in contrast to the selective 5-HT reuptake inhibitor, citalopram. In freely moving rats, in the dorsal hippocampus, frontal cortex (FCX), nucleus accumbens and striatum, citalopram increased dialysate levels of 5-HT, but not dopamine (DA) and NA. On the contrary, mirtazapine markedly elevated dialysate levels of NA and, in FCX, DA, whereas 5-HT was not affected. Citalopram inhibited the firing rate of serotonergic neurons in dorsal raphe nucleus, but not of dopaminergic neurons in the ventral tegmental area, nor adrenergic neurons in the locus coeruleus. Mirtazapine, in contrast, enhanced the firing rate of dopaminergic and adrenergic, but not serotonergic, neurons. Following 2 weeks administration, the facilitatory influence of mirtazapine upon dialysate levels of DA and NA versus 5-HT in FCX was maintained, and the influence of citalopram upon FCX levels of 5-HT versus DA and NA was also unchanged. Moreover, citalopram still inhibited, and mirtazapine still failed to influence, dorsal raphe serotonergic neurons. In conclusion, in contrast to citalopram, mirtazapine reinforces frontocortical dopaminergic and corticolimbic adrenergic, but not serotonergic, transmission. These actions reflect antagonist properties at alpha2A-AR and 5-HT2C receptors.

The effects of a specific alpha(2)-adrenoceptor antagonist, atipamezole, on cognitive performance and brain neurochemistry in aged Fisher 344 rats

The present experiments investigated the effects of a specific and potent alpha(2)-adrenoceptor antagonist, atipamezole, on cognitive performance and neurochemistry in aged rats. Aged control Fisher 344 rats, which had lower activities of choline acetyltransferase in the frontal cortex, were impaired in the acquisition of the linear arm maze task both in terms of repetition errors and their behavioural activity (the speed of arm visits), and they needed longer time to complete this task as compared to adult control rats. Atipamezole treatment (0.3 mg/kg) facilitated the acquisition of this task in the aged rats as they committed fewer errors and completed the task more quickly than saline-treated aged control rats. A separate experiment indicated that atipamezole enhanced the turnover of noradrenaline both in the adult and aged rats, but this effect was more pronounced in the aged rats. Furthermore, atipamezole enhanced significantly the turnover of serotonin and dopamine only in the aged rats when analysed in the whole brain samples. As alpha(2)-adrenoceptor antagonists are known to alleviate akinesia in the experimental models of Parkinson's disease, the present results could be especially relevant for the development of palliative treatment for demented Parkinsonian patients.

Activation of muscarinic M3-like receptors and beta-adrenoceptors, but not M2-like muscarinic receptors or alpha-adrenoceptors, directly modulates corticostriatal neurotransmission in vitro

The aim of this study was to characterize the modulation of synaptic transmission in the glutamatergic corticostriatal pathway by cholinergic and adrenergic receptors. In coronal slices of mouse brain, negative-going field potentials were recorded in the dorsal striatum in response to stimulation of the overlying white matter, and their susceptibility to various pharmacological manipulations was studied. The responses were mediated by alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-type glutamate receptors, since they were augmented by aniracetam (0.5-1.5 mM), a positive modulator of AMPA-type glutamate receptors, and blocked by 6-cyano-7-nitroquinoxaline-2,3-dione (> or = 10 microM), a selective antagonist of AMPA receptors. Carbachol (10 microM), a muscarinic agonist, reduced the size of responses and abolished paired-pulse depression; these effects being consistent with previous studies indicating that muscarinic activation inhibits release of glutamate in the corticostriatal pathway. Muscarinic antagonists could block the effect of carbachol. Their rank order was: 10 microM scopolamine (a non-selective muscarinic antagonist) > or = 1 microM 4-diphenylacetoxy-N-methyl-piperidine (M3/M1 antagonist)>1 microM pirenzepine (M1 antagonist)>10 microM methoctramine (M2 antagonist). McN-A-343 (1-10 microM), an M1 muscarinic agonist, was ineffective in this preparation. In contrast, isoproterenol (10-30 microM), a beta-adrenergic agonist, slightly increased the synaptic responses, but it did not affect paired-pulse depression. None of alpha-adrenergic agents (30 nM-1.0 microM dexmedetomidine, an alpha2-adrenergic agonist, 0.3 microM atipamezole, an alpha2-adrenergic antagonist or 30 microM phenylephrine, an alpha1-adrenergic agonist) influenced the size of the responses; neither did these drugs alter paired-pulse depression. These results indicate that the activation of striatal M3-like muscarinic receptors and beta-adrenoceptors, but not M2-like muscarinic receptors and alpha-adrenoceptors, modulates directly corticostriatal glutamatergic neurotransmission.

The involvement of noradrenergic transmission in the morphine-induced locomotor hyperactivity in mice withdrawn from repeated morphine treatment

1. Our previous studies suggest that in addition to the cerebral dopaminergic systems the noradrenergic ones have a crucial role in the morphine-induced behavioural sensitization in mice. Therefore the effects of alpha2-adrenoceptor antagonist, idazoxan (1 and 3 mg kg(-1), i.p.) on morphine-induced locomotor hyperactivity as well as on morphine-induced changes in cerebral noradrenaline (NA) and striatal dopamine (DA) metabolism were studied in mice withdrawn for 3 days from 5 day repeated morphine treatment. The concentrations of NA, free 3-methoxy-4-hydroxyphenylethylene glycol (MOPEG), DA, 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), and 3-methoxytyramine (3-MT) were determined. 2. Acute morphine (10 mg kg(-1), s.c.) increased locomotor activity in control and in morphine-withdrawn mice; idazoxan alone did not alter the activity. Idazoxan pretreatment did not alter the locomotor hyperactivity induced by acute morphine in control mice but potentiated it in morphine-withdrawn mice. 3. Acute morphine elevated MOPEG less but increased DOPAC and HVA more clearly in morphine-withdrawn mice than in controls, and decreased 3-MT only in controls. Idazoxan alone did not alter the NA or DA metabolite concentrations in control mice, but elevated MOPEG as well as DOPAC in morphine-withdrawn mice. 4. In control mice idazoxan enhanced acute morphine's elevating effect on MOPEG. In withdrawn mice idazoxan counteracted the tolerance so that acute morphine elevated MOPEG in these mice to about similar level as in controls. 5. Idazoxan pretreatment abolished the HVA increasing effect of acute morphine both in control and withdrawn mice. In control mice idazoxan enhanced morphine's elevating effect on DOPAC and abolished morphine's decreasing effect on 3-MT. Idazoxan did not alter morphine's effects on DOPAC or 3-MT concentrations in withdrawn mice. 6. Our results show that in morphine-withdrawn mice idazoxan pretreatment reveals the morphine-induced locomotor sensitization. This most probably occurs by overcoming the tolerance towards the acute morphine-induced increase of cerebral NA turnover and release. It is suggested that in mice the cerebral noradrenergic in addition to the dopaminergic systems are major determinants of the behavioural sensitization to morphine.

Alpha 2 adrenergic agonists. Neurochemistry, efficacy, and clinical guidelines for use in children

The alpha 2 adrenergic agonists are used to treat a variety of psychiatric disorders and their usage has been increasing. This article presents the rationale and neurochemical basis for treatment of psychiatric disorders with alpha 2 agents, reviews studies examining clinical efficacy, and develops guidelines for usage. Case vignettes are presented to illustrate how the alpha 2 agents can successfully be used in practice.

Clonidine reduces dopamine and increases GABA in the nucleus accumbens: an in vivo microdialysis study

The effects of clonidine, an alpha2 adrenoceptor agonist, on extracellular concentrations of dopamine and gamma-aminobutyric acid (GABA) in the nucleus accumbens of rats were studied by using in vivo brain microdialysis. Clonidine (5 microg/kg i.v.) significantly decreased the brain microdialysate concentration of dopamine in the nucleus accumbens up to a maximum of 16% at its peak effect. This effect was inhibited by a dose of idazoxan (10 microg/kg i.v.), an alpha2-adrenoceptor antagonist. which itself did not affect the efflux of dopamine. A smaller dose of clonidine (1 microg/kg i.v.), which had no significant effect on dopamine efflux per se, decreased the dopamine efflux (21% reduction) when given together with an ineffective dose of midazolam (0.075 mg/kg i.v.), a benzodiazepine receptor agonist. The effect of clonidine (5 microg/kg i.v.) on mesolimbic dopamine efflux was abolished by bicuculline (1 mg/kg i.v.), a GABA(A) receptor antagonist, counteracted by beta-carboline-3-carboxylate ethyl ester (beta-CCE, 3 mg/kg i.p.), a benzodiazepine receptor inverse agonist, but not affected by flumazenil (6 microg/kg i.v.), a benzodiazepine receptor antagonist. Clonidine (5 microg/kg i.v.) increased the dialysate concentration of GABA in the nucleus accumbens up to a maximum of 250% at its peak effect, but not in the ventral tegmental area. It is hypothesized that GABA(A) binding sites in the nucleus accumbens form part of the sequence of events that is triggered by clonidine in an alpha2-adrenergic-specific manner and that ultimately results in a decreased release of dopamine in the nucleus accumbens.

Simultaneous quantification of serotonin, dopamine and noradrenaline levels in single frontal cortex dialysates of freely-moving rats reveals a complex pattern of reciprocal auto- and heteroreceptor-mediated control of release

In the present study, a novel and exceptionally sensitive method of high-performance liquid chromatography coupled to coulometric detection, together with concentric dialysis probes, was exploited for an examination of the role of autoreceptors and heteroceptors in the modulation of dopamine, noradrenaline and serotonin levels in single samples of the frontal cortex of freely-moving rats. The selective D3/D2 receptor agonist, CGS 15855A [(+/-)-trans-1,3,4,4a,5,10b-hexahydro-4-propyl-2H-[1]benzopyrano[3 ,4-b]-pyridin-9-ol], and antagonist, raclopride, respectively decreased (-50%) and increased (+60%) levels of dopamine without significantly modifying those of serotonin and noradrenaline. The selective alpha2-adrenergic receptor agonist, dexmedetomidine, markedly decreased noradrenaline levels (-100%) and likewise suppressed those of serotonin and dopamine by -55 and -45%, respectively. This effect was mimicked by the preferential alpha2-adrenergic receptor agonist, guanabenz (-100%, -60% and -50%). Furthermore, the alpha2-adrenergic receptor antagonist, RX 821,002 [2(2-methoxy-1,4-benzodioxan-2-yl)-2-imidazoline], and the preferential alpha2A-adrenergic receptor antagonist, BRL 44408 [2-(2H-(1-methyl-1,3-dihydroisoindole)methyl)-4,5-dihydroimidaz ole], both evoked a pronounced elevation in levels of noradrenaline (+212%, +109%) and dopamine (+73%, +85%). In contrast, the preferential alpha(2B/2C)-adrenergic receptor antagonist, prazosin, did not modify noradrenaline and dopamine levels. RX 821,002 and BRL 44408 did not significantly modify levels of serotonin, whereas prazosin decreased these levels markedly (-55%), likely due to its alpha1-adrenergic receptor antagonist properties. The selective serotonin-1A receptor agonist, 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT), reduced serotonin levels (-65%) and increased those of dopamine and noradrenaline by +100%), and +175%, respectively. The selective serotonin-1A antagonist, WAY 100,635 [N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl)cyclo- hexanecarboxamide], which had little affect on monoamine levels alone, abolished the influence of 8-OH-DPAT upon serotonin and dopamine levels and significantly attenuated its influence upon noradrenaline levels. Finally, the selective serotonin-1B agonist, GR 46611 [3-[3-(2-dimethylaminoethyl)-1H-indol-5-yl]-N-(4-methoxybenzyl)acrylamid e], decreased serotonin levels (-49%) and the serotonin-1B antagonist, GR 127,935 [N-[4-methoxy-3-(4-methylpiperazin-1-yl)phenyl]-2'-methyl-4'-(5-me thyl-1,2,4-oxadiazol-3-yl)-biphenyl-4-carboxamide], which did not significantly modify serotonin levels alone, abolished this action of GR 46611. Levels of dopamine and noradrenaline were not affected by GR 46611 or GR 127,935. In conclusion, there is a complex pattern of reciprocal autoreceptor and heteroceptor control of monoamine release in the frontal cortex. Most notably, activation of alpha2-adrenergic receptors inhibits the release of noradrenaline, dopamine and serotonin in each case, while stimulation of serotonin-1A receptors suppresses serotonin, yet facilitates noradrenaline and dopamine release. In addition, dopamine D2/D3 autoreceptors restrain dopamine release while (terminal-localized) serotonin-1B receptors reduce serotonin release. Control of serotonin release is expressed phasically and that of noradrenaline and dopamine release tonically.

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

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

Alpha2-adrenergic control of dopamine overflow and metabolism in mouse striatum

The effects of the alpha2-adrenoceptor drugs, medetomidine and atipamezole, on dopamine overflow evoked by low (6 Hz-10 s) and high (50 Hz-4 s) frequency electrical stimulation of the median forebrain bundle were studied in striatum of BALB/C mice anaesthetized with chloral hydrate with fast in vivo voltammetry techniques. The effects of these drugs on the basal concentrations of dopamine metabolites were also investigated by means of differential pulse voltammetry. Medetomidine dose dependently decreased dopamine overflow in nucleus accumbens in the dose range 5-100 microg/kg, s.c. This effect was seen only at low frequency stimulation and reached 85% at a dose of 100 microg/kg. Medetomidine also decreased the basal concentration of striatal homovanillic acid. This effect did not exceed 35%. Atipamezole antagonized the inhibitory effects of medetomidine on the dopamine overflow. but showed no effect itself. We suggest that alpha2-adrenoceptors in dopaminergic terminal fields in the mouse striatum are involved in the regulation of dopamine release at physiological stimulation frequencies.

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

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

Distribution of alpha 2A-adrenergic receptor-like immunoreactivity in the rat central nervous system

In this study, we analyzed immunohistochemically the distribution of the A subtype of alpha 2-adrenergic receptor (alpha 2A-AR) in the rat central nervous system using light level immunohistochemistry. By using affinity-purified antisera, we found perikaryal labeling was diffuse and/or punctate; immunoreactive puncta were heterogeneous in size and number in a region-specific manner. Dense deposits of immunoreaction product were found associated with neuropil also, particularly in the lateral parabrachial nucleus, locus coeruleus, lateral septum, diagonal band, stratum lacunosum-moleculare of CA1, and various nuclei of the amygdala and extended amygdala. Prominently immunoreactive olfactory structures include the anterior olfactory nucleus and the granular layer of the olfactory bulb. The cortex was generally light to moderately labeled with greater immunoreactivity in the cingulate and insular cortices. alpha 2A-AR-like immunoreactivity was intense in the basal forebrain and continuous from the nucleus accumbens through the substantia innominata and fundus of the striatum. Most immunoreactivity in the diencephalon was restricted to the hypothalamus with light to moderate labeling in the thalamus. Generally light immunoreactivity was observed in midbrain structures. In the pons and medulla, both perikaryal and neuropil labeling were observed. Together with the accompanying paper describing the neural distribution of alpha 2C-AR-like immunoreactivity, our results provide an extensive immunohistochemical cartography of alpha 2-ARs in the adult rat central nervous system.

Alpha 2-autoreceptors and alpha 2-heteroreceptors modulating tyrosine and tryptophan hydroxylase activity in the rat brain in vivo: an investigation into the alpha 2-adrenoceptor subtypes

The subtype determination of auto- and hetero-alpha 2-adrenoceptors modulating the synthesis of noradrenaline (NA) and serotonin (5-HT), respectively, was assessed using the accumulation of 3,4-dihydroxyphenylalanine (dopa) and 5-hydroxytryptophan (5-HTP) after decarboxylase inhibition as a measure of the rate of tyrosine and tryptophan hydroxylation in the rat brain in vivo. In the cerebral cortex and hippocampus, Org 3770 (non-selective alpha 2-adrenoceptor antagonist, 0.5-10 mg/kg, i.p.) increased (43%-58%) and clonidine (non-selective alpha 2-adrenoceptor agonist, 1 mg/kg) decreased (37%-49%) the synthesis of dopa. Also the antagonist ARC 239 (alpha 2B/C selective, 5-40 mg/kg) increased the synthesis of dopa in cortex (39%-46%) and hippocampus (17%-85%). In contrast, the antagonist BRL 44408 (alpha 2D selective, 1-10 mg/kg) did not increase the synthesis of dopa in cortex, and increased it modestly in hippocampus only. The agonist guanoxabenz (alpha 2B/C selective, 0.03-3 mg/kg) decreased the synthesis of dopa in both brain regions (20%-65%), whereas the agonist oxymetazoline (alpha 2D selective, 0.1-3 mg/kg) failed to do so. These results indicated that the alpha 2-autoreceptors that modulate the synthesis of dopa/NA are probably associated with the alpha 2B/C-subtypes. In cortex and hippocampus, clonidine decreased (35%-53%) the synthesis of 5-HTP but Org 3770 failed to induce the opposite effect (except the 2 mg/kg dose in cortex). BRL 44408 markedly increased the synthesis of 5-HTP in cortex (113%-148%) but not in hippocampus. Similarly, also ARC239 increased the formation of 5-HTP in cortex (36%-48%) but not in hippocampus, where it was decreased (30%-55%). Oxymetazoline decreased the synthesis of 5-HTP in hippocampus (28%-30%) but failed to do so in cortex. Guanoxabenz in the low dose range (0.03-0.3 mg/kg) did not decrease the synthesis of 5-HTP in any brain region. These results indicated that the alpha 2-heteroreceptors that modulate the synthesis of 5-HTP/5-HT may well be different from the proposed alpha 2B/C-autoreceptors modulating the synthesis of dopa/NA. These alpha 2-heteroreceptors appear to be associated with the alpha 2D-subtype.

Subclassification of presynaptic alpha 2-adrenoceptors: alpha 2A-autoreceptors in rabbit atria and kidney

The study was devised to classify, by means of antagonist affinities, the presynaptic alpha 2-autoreceptors of rabbit atria and kidney in terms of alpha 2A, alpha 2B, alpha 2C and alpha 2D. A set of antagonists was chosen that was able to discriminate between the four subtypes. Small pieces of the left atrium and slices of the kidney cortex were preincubated with 3H-noradrenaline and then superfused and stimulated electrically. In one series of experiments, tissue pieces were stimulated by relatively long pulse trains (1 or 2 min) leading to alpha 2-autoinhibition. All 11 (atria) or 10 (kidney) antagonists increased the evoked overflow of tritium. pEC30% values (concentrations causing 30% increase) were interpolated from concentration-response curves. In a second series of experiments, tissue pieces were stimulated by brief pulse trains (0.4 s) that did not lead to alpha 2-autoinhibition, and concentration-inhibition curves of the alpha 2-selective agonist 5-bromo-6-(2-imidazolin-2-ylamino)-quinoxaline (UK 14,304) were determined. Most of the 11 (atria) or 8 (kidney) antagonists shifted the concentration-inhibition curve of UK 14,304 to the right. pKd values of the antagonists were calculated from the shifts. pEC30% values correlated with pKd values, both in atria (r = 0.728) and in the kidney (r = 0.930). pEC30% values in atria correlated with pEC30% values in the kidney (r = 0.988) and pKd values in atria correlated with pKd values in kidney (r = 0.923). It is concluded that the alpha 2-autoreceptors in atria and the kidney are the same. Comparison with antagonist affinities for prototypic native alpha 2 binding sites, alpha 2 binding sites in cells transfected with alpha 2 subtype genes, and previously classified presynaptic alpha 2-adrenoceptors--all taken from the literature--indicates that both autoreceptors are alpha 2A. This conclusion is reached with either of the two independent estimates of autoreceptor affinity, pEC30% and pKd. The results are compatible with the hypothesis that at least the majority of alpha 2-autoreceptors belong to the alpha 2A/D branch of the alpha 2-adrenoceptor tree, across mammalian or at least rodent and lagomorph species.

Subclassification of presynaptic alpha 2-adrenoceptors: alpha 2D-autoreceptors and alpha 2D-adrenoceptors modulating release of acetylcholine in guinea-pig ileum

The study was designed to classify in terms of alpha 2A, alpha 2B, alpha 2C and alpha 2D the presynaptic alpha 2-autoreceptors, as well as the alpha 2-receptors modulating the release of acetylcholine, in the myenteric plexus-longitudinal muscle (MPLM) preparation of the guinea-pig ileum. A set of antagonists was chosen that was able to discriminate between the four subtypes. Small pieces of the MPLM preparation were preincubated with 3H-noradrenaline or 3H-choline and then superfused and stimulated electrically. The stimulation periods used (3H-noradrenaline: 3 trains of 20 pulses, 50 Hz, train interval 60 s; 3H-choline: single trains of 30 pulses, 0.2 Hz) did not lead to alpha 2-autoinhibition or inhibition of 3H-acetylcholine release by endogenous noradrenaline. The alpha 2-selective agonist 5-bromo-6-(2-imidazolin-2-ylamino)-quinoxaline (UK 14,304) reduced the evoked overflow of tritium in both 3H-noradrenaline and 3H-choline experiments. Most (3H-noradrenaline) or all (3H-choline) of the 10 antagonists shifted the concentration-inhibition curves of UK 14,304 to the right. pKd values of the antagonists were calculated from the shifts. pKd values from 3H-noradrenaline experiments correlated with pKd values from 3H-choline experiments (r = 0.981). It is concluded that alpha 2-autoreceptors and alpha 2-heteroreceptors modulating the release of acetylcholine in the MPLM preparation are of the same subtype. Comparison with antagonist affinities for prototypic native alpha 2 binding sites, binding sites in cells transfected with alpha 2 subtype genes, and previously classified presynaptic alpha 2-adrenoceptors--all taken from the literature--indicates that both are alpha 2D.(ABSTRACT TRUNCATED AT 250 WORDS)

Subclassification of presynaptic alpha 2-adrenoceptors: alpha 2D-autoreceptors in mouse brain

The study was devised to classify, by means of antagonist affinities, the presynaptic alpha 2-autoreceptors in mouse cerebral cortex in terms of alpha 2A, alpha 2B, alpha 2C and alpha 2D. A set of antagonists was chosen that was able to discriminate between the four subtypes. Slices of the cortex were preincubated with 3H-noradrenaline and then superfused and stimulated electrically. The stimulation periods used (4 pulses, 100 Hz) did not lead to alpha 2-autoinhibition as shown by the lack of an increase by rauwolscine of the evoked overflow of tritium. The alpha 2-selective agonists 5-bromo-6-(2-imidazolin-2-ylamino)-quinoxaline (UK 14,304) and alpha-methylnoradrenaline reduced the evoked overflow. All 10 antagonists shifted the concentration-inhibition curve of UK 14,304 to the right. Rauwolscine also shifted the concentration-inhibition curve of alpha-methylnoradrenaline to the right. pKd values of the antagonists were calculated from the shifts. The pKd values of rauwolscine against UK 14,304 and alpha-methylnoradrenaline were very similar (8.0 and 7.9, respectively). Comparison with antagonist affinities for prototypic native alpha 2 binding sites, alpha 2 binding sites in cells transfected with alpha 2 subtype genes, and previously classified presynaptic alpha 2-adrenoceptors--all taken from the literature--indicates that the alpha 2-autoreceptors in mouse brain cortex are alpha 2D. This is the first subtype determination of alpha 2-autoreceptors in the mouse. It supports the hypothesis that at least the majority of alpha 2-autoreceptors belong to the alpha 2A/D branch of the alpha 2-adrenoceptor tree.