Regional differences in the in vivo regulation of the extracellular levels of noradrenaline and its metabolites in rat brain

Microdialysis Unveils the Role of the α2-Adrenergic System in the Basolateral Amygdala during Acquisition of Conditioned Odor Aversion in the Rat

Previous work has shown that β-adrenergic and GABAergic systems in the basolateral amygdala (BLA) are involved in the acquisition of conditioned odor aversion (COA) learning. The involvement of α₂-adrenoreceptors, however, is poorly documented. In a first experiment, male Long-Evans rats received infusions of 0.1 μg of the selective α₂-antagonist dexefaroxan (Dex) in the BLA before being exposed to COA learning. In a second experiment, levels of norepinephrine (NE) were analyzed following Dex retrodialysis into the BLA. While microdialysis data showed a significant enhancement of NE release in the BLA with Dex, behavioral results showed that pre-CS infusion of Dex impaired, rather than facilitated, the acquisition of COA. Our results show that the NE system in the BLA is involved in the acquisition of COA, including a strong α₂-receptor modulation until now unsuspected. Supported by the recent literature, the present data suggest moreover that the processes underlying this learning are probably mediated by the balanced effects of NE excitatory/inhibitory signaling in the BLA, in which interneurons are highly involved.

Monoaminergic Modulation of Motor Cortex Function

Elaboration of appropriate responses to behavioral situations rests on the ability of selecting appropriate motor outcomes in accordance to specific environmental inputs. To this end, the primary motor cortex (M1) is a key structure for the control of voluntary movements and motor skills learning. Subcortical loops regulate the activity of the motor cortex and thus contribute to the selection of appropriate motor plans. Monoamines are key mediators of arousal, attention and motivation. Their firing pattern enables a direct encoding of different states thus promoting or repressing the selection of actions adapted to the behavioral context. Monoaminergic modulation of motor systems has been extensively studied in subcortical circuits. Despite evidence of converging projections of multiple neurotransmitters systems in the motor cortex pointing to a direct modulation of local circuits, their contribution to the execution and learning of motor skills is still poorly understood. Monoaminergic dysregulation leads to impaired plasticity and motor function in several neurological and psychiatric conditions, thus it is critical to better understand how monoamines modulate neural activity in the motor cortex. This review aims to provide an update of our current understanding on the monoaminergic modulation of the motor cortex with an emphasis on motor skill learning and execution under physiological conditions.

Blockade of α2-adrenergic receptors in prelimbic cortex: impact on cocaine self-administration in adult spontaneously hypertensive rats following adolescent atomoxetine treatment

RATIONALE

Research with the spontaneously hypertensive rat (SHR) model of attention deficit/hyperactivity disorder demonstrated that chronic methylphenidate treatment during adolescence increased cocaine self-administration established during adulthood under a progressive ratio (PR) schedule. Compared to vehicle, chronic atomoxetine treatment during adolescence failed to increase cocaine self-administration under a PR schedule in adult SHR.

OBJECTIVES

We determined if enhanced noradrenergic transmission at α2-adrenergic receptors within prefrontal cortex contributes to this neutral effect of adolescent atomoxetine treatment in adult SHR.

METHODS

Following treatment from postnatal days 28-55 with atomoxetine (0.3 mg/kg) or vehicle, adult male SHR and control rats from Wistar-Kyoto (WKY) and Wistar (WIS) strains were trained to self-administer 0.3 mg/kg cocaine. Self-administration performance was evaluated under a PR schedule of cocaine delivery following infusion of the α2-adrenergic receptor antagonist idazoxan (0 and 10-56 μg/side) directly into prelimbic cortex.

RESULTS

Adult SHR attained higher PR break points and had greater numbers of active lever responses and infusions than WKY and WIS. Idazoxan dose-dependently increased PR break points and active lever responses in SHR following adolescent atomoxetine vs. vehicle treatment. Behavioral changes were negligible after idazoxan pretreatment in SHR following adolescent vehicle or in WKY and WIS following adolescent atomoxetine or vehicle.

CONCLUSIONS

α2-Adrenergic receptor blockade in prelimbic cortex of SHR masked the expected neutral effect of adolescent atomoxetine on adult cocaine self-administration behavior. Moreover, greater efficacy of acute idazoxan challenge in adult SHR after adolescent atomoxetine relative to vehicle is consistent with the idea that chronic atomoxetine may downregulate presynaptic α2A-adrenergic autoreceptors in SHR.

Layer- and area-specific actions of norepinephrine on cortical synaptic transmission

The cerebral cortex is a critical target of the central noradrenergic system. The importance of norepinephrine (NE) in the regulation of cortical activity is underscored by clinical findings that involve this catecholamine and its receptor subtypes in the regulation of a large number of emotional and cognitive functions and illnesses. In this review, we highlight diverse effects of the LC/NE system in the mammalian cortex. Indeed, electrophysiological, pharmacological, and behavioral studies in the last few decades reveal that NE elicits a mixed repertoire of excitatory, inhibitory, and biphasic effects on the firing activity and transmitter release of cortical neurons. At the intrinsic cellular level, NE can produce a series of effects similar to those elicited by other monoamines or acetylcholine, associated with systemic arousal. At the synaptic level, NE induces numerous acute changes in synaptic function, and ׳gates' the induction of long-term plasticity of glutamatergic synapses, consisting in an enhancement of engaged and relevant cortical synapses and/or depression of unengaged synapses. Equally important in shaping cortical function, in many cortical areas NE promotes a characteristic, most often reversible, increase in the gain of local inhibitory synapses, whose extent and temporal properties vary between different areas and sometimes even between cortical layers of the same area. While we are still a long way from a comprehensive theory of the function of the LC/NE system, its cellular, synaptic, and plastic effects are consistent with the hypothesis that noradrenergic modulation is critical in coordinating the activity of cortical and subcortical circuits for the integration of sensory activity and working memory. This article is part of a Special Issue entitled SI: Noradrenergic System.

Noradrenergic influences in the basolateral amygdala on inhibitory avoidance memory are mediated by an action on α2-adrenoceptors

The role of norepinephrine (NE) in the consolidation of inhibitory avoidance learning (IA) in rats is known to involve α1- and β-adrenoceptor systems in the basolateral nucleus of the amygdala (BLA). However, the amygdala also contains α2-adrenoceptor subtypes, and local microinfusions of the selective α2-adrenoceptor antagonist idazoxan and agonist UK 14,304 respectively into the BLA enhance and inhibit IA performances when administered before acquisition. The present study investigated whether the effects of idazoxan and UK 14,304 on IA were associated with changes in NE release within the BLA before and after one-trial inhibitory avoidance training. Male Sprague-Dawley rats were unilaterally implanted with a microdialysis probe in the BLA and were administered idazoxan (0.1mM) or UK 14,304 (10 μM) by retrodialysis infusion 15 min before the acquisition of IA. Dialysates were collected every 15 min for analysis of NE. Retrodialysis of idazoxan potentiated the release of NE induced by footshock application, whereas UK 14,304 decreased NE release to the extent that the footshock failed to induce any measurable effect on NE levels. Idazoxan infusion enhanced IA retention tested 24h later and this effect was directly related to the level of NE release in the BLA measured during IA acquisition. In contrast, the infusion of UK 14,304 did not modify IA performances in comparison to control animals, possibly due to compensatory activity of the contralateral BLA. These results are consistent with previous evidence that amygdala NE is involved in modulating memory consolidation, and provide evidence for an involvement of presynaptic α2-autoceptors in the BLA in this process.

The alpha2-adrenoceptor antagonist dexefaroxan enhances hippocampal neurogenesis by increasing the survival and differentiation of new granule cells

The generation of new neurons in the hippocampus is a dynamic process regulated by environmental, endocrine, and pharmacological factors. Since enhancement of hippocampal neurogenesis has been associated with learning and memory, and the locus coeruleus-noradrenergic system has been shown to modulate these cognitive functions, we hypothesized that activation of noradrenergic neurotransmission might enhance neurogenesis in the adult hippocampus. To test this hypothesis in vivo, we induced the release of noradrenaline in the hippocampus by blocking presynaptic inhibitory autoreceptors with the selective alpha2-adrenoceptor antagonist dexefaroxan. Confocal microscopy showed that noradrenergic afferents make contact with proliferating and differentiating cells, suggesting a direct noradrenergic influence on neurogenesis. Chronic systemic treatment of rats with dexefaroxan did not affect cell proliferation per se in the dentate gyrus (as monitored by bromodeoxyuridine-labeling), but promoted the long-term survival of newborn neurons by reducing apoptosis. Dexefaroxan treatment also enhanced the number and complexity of the dendritic arborizations of polysialated neural cell adhesion molecule-positive neurons. The trophic effects of dexefaroxan on newborn cells might involve an increase in brain-derived neurotrophic factor, which was upregulated in afferent noradrenergic fiber projection areas and in neurons in the granule cell layer. By promoting the survival of new endogenously formed neurons, dexefaroxan treatment represents a potential therapeutic strategy for maintaining adult neurogenesis in neurodegenerative conditions, such as Alzheimer's disease, that affect the hippocampus.

Serotonergic influence on the potentiation of D-amphetamine and apomorphine-induced rotational behavior by the α2-adrenoceptor antagonist 2-methoxy idazoxan in hemiparkinsonian rats

The alpha(2)-adrenoceptor antagonists potentiate both ipsilateral and contralateral rotations induced by amphetamine and apomorphine respectively in hemiparkinsonian rats. The present study investigated the role of serotonergic transmission in this potentiation in unilaterally 6-hydroxydopamine nigral lesioned rats. D-amphetamine (0.5 mg/kg, i.p.) produced ipsilateral rotations, which were decreased by the dopamine receptor antagonist haloperidol (0.2 mg/kg, i.p.) and the alpha(1)-receptor antagonist prazosin (1 mg/kg, i.p.). The selective alpha(2)-antagonist 2-methoxy idazoxan (0.2 mg/kg, i.p.) potentiated the amphetamine-induced ipsilateral rotations, that were attenuated by haloperidol and prazosin. The selective serotonin re-uptake inhibitor citalopram (10 mg/kg, i.p.) and selective serotonin synthesis inhibitor p-chlorophenylalanine (150 mg/kg, i.p., 3 days) decreased and increased the observed potentiation respectively. Apomorphine (0.2 mg/kg, s.c.) produced contralateral rotations, which were decreased by haloperidol but not by prazosin. 2-methoxy idazoxan potentiated these rotations which were attenuated by haloperidol but not by prazosin. Citalopram and p-chlorophenylalanine increased and decreased the observed potentiation respectively. Citalopram and p-chlorophenylalanine had no effect by per se on D-amphetamine and apomorphine-induced rotations. 2-methoxy idazoxan alone increased both ipsilateral and contralateral spontaneous rotations. Taken together, these findings indicate that an increase in noradrenergic tone by 2-methoxy idazoxan potentiates both D-amphetamine-induced ipsilateral and apomorphine induced contralateral rotations. alpha(1)-Antagonism attenuates D-amphetamine induced ipsilateral rotations and its potentiation by 2-methoxy idazoxan but not apomorphine rotations or its potentiation. Increasing and decreasing the serotonergic transmission decreases and increases D-amphetamine potentiation, whereas increases and decreases apomorphine potentiation respectively. The possible mechanisms for these findings are discussed.

Role of presynaptic alpha2-adrenoceptors in antidepressant action: recent findings from microdialysis studies

The therapeutic effect of an antidepressant drug takes at least 2 to 3 weeks to develop and a significant proportion of patients have no or only partial benefit regardless of the class of antidepressant used. Research into the neurobiological basis of antidepressant action has suggested new strategies to improve the antidepressant effect. Recent microdialysis studies show that hypofunction of the presynaptic autoreceptors enhances the increase of extracellular serotonin (5-HT) induced by selective serotonin reuptake inhibitors (SSRIs) so it has been suggested that the antidepressant effect may be speeded up by blockade of the autoreceptors. The similarity between the synaptic mechanisms controlling serotonergic and noradrenergic transmission has stimulated preclinical research into the role of presynaptic alpha(2)-adrenoceptors in the effect of noradrenaline (NA) reuptake inhibitors (NRIs) on NA availability at central synapses. The microdialysis studies reviewed here indicate that NRIs including desipramine, reboxetine and atomoxetine, the mixed 5-HT/NA reuptake inhibitors sibutramine, duloxetine, venlafaxine or the NA/DA reuptake inhibitor amineptine, increased extracellular NA in various regions of the rat brain. The effect was enhanced by chronic treatment and even more by the co-administration of alpha(2)-adrenoceptor antagonists. The results support the theory that desensitization of the alpha(2)-adrenoceptor contributes to enhancing the effect of NRIs seen after chronic administration and may account for the slow onset of the antidepressant effect. Finally, they suggest that co-administration of an alpha(2)-adrenoceptor antagonist may improve the therapeutic effect of NRI.

Estimation of endogenous noradrenaline release in rat brain in vivo using [3H]RX 821002

Noradrenaline plays an important role in many normal brain functions, e.g., attention, memory, and emotion. Dysfunction in the noradrenergic system is thought to lead to a number of abnormal brain conditions. The lack of suitable in vivo tracers to monitor noradrenaline release, levels, and regulation has hampered our fully understanding the roles that it plays in the brain. Presented here are data showing that the in vivo binding of the alpha2-adrenoceptor antagonist [3H]RX 821002 is sensitive to endogenous noradrenaline. Elevation of extracellular noradrenaline, using three different pharmacological challenges in rat, led to a reduction in the binding potential (BP) of [3H]RX 821002 when compared with vehicle controls. The challenges used were i.p. administration of D-amphetamine, the imidazoline2 binding site-selective ligand BU224, and L-deprenyl. Of the cortical regions measured, the reduction in BP reached significance in the anterior cingulate cortex for all of these pharmacological challenges. These initial observations in rat indicate that labelling of the alpha2-adrenoceptors with RX 821002 can be used to estimate changes in extracellular noradrenaline concentration in the cortex. This has the potential to enable the investigation of the role that noradrenaline plays both in the normal and abnormal brain and, if the ligand can be radiolabelled with a suitable positron-emitting isotope at high specific radioactivity, it could be an invaluable PET tracer.

Behavioral arousal and increased dopamine efflux after blockade of NMDA-receptors in the prefrontal cortex are dependent on activation of glutamatergic neurotransmission

Blockade of NMDA/glutamate receptors induces altered behavior in humans and experimental animals. At the same time a differential activation of dopaminergic (DA) systems has been reported. To study the involvement of the medial prefrontal cortex (mPFC) in these effects, we used bilateral perfusions of the rat mPFC with the competitive NMDA-antagonist D-AP-5 and simultaneous determination of spontaneous behavior and local DA efflux. D-AP-5 concentration-dependently induced arousal and motor activity and also increased DA efflux. These effects were shown to have a similar time-scale but no causal relationship: combined D1/D2 receptor blockade in the mPFC did not inhibit the behavioral activation. As bilateral perfusion of the nucleus accumbens with D-AP-5 resulted in similar behavioral effects, but no change in DA efflux, we conclude that DA is not involved in the behavioral activation induced by these local perfusions. However, local blockade of non-NMDA glutamate receptors or stimulation of GABA-B receptors completely blocked the effects on behavior and DA efflux, suggesting that the arousal and locomotor activity induced by NMDA receptor blockade in mPFC is primarily dependent on activation of glutamatergic mechanisms. The mPFC appears to be an important site of action for NMDA antagonists to induce behavioral alterations.

Effects of imidazoline antihypertensive drugs on sympathetic tone and noradrenaline release in the prefrontal cortex

1. The aim of the present study was to compare the effects of the centrally acting antihypertensive drugs rilmenidine, moxonidine, clonidine and guanabenz on sympathetic tone with their effects on noradrenaline release in the cerebral cortex. In particular, the hypothesis was tested that rilmenidine and moxonidine, due to their high affinity for sympatho-inhibitory imidazoline I(1) receptors and low affinity for alpha(2)-adrenoceptors, lower sympathetic tone without causing an alpha(2)-adrenoceptor-mediated inhibition of cerebrocortical noradrenaline release. 2. In rats anaesthetized with urethane, blood pressure and heart rate were measured and the concentration of noradrenaline in arterial blood plasma was determined. The release of noradrenaline in the medial prefrontal cortex was estimated by microdialysis. Intravenous administration of rilmenidine (30, 100, 300 and 1000 microg kg(-1)), moxonidine (10, 30, 100 and 300 microg kg(-1)), clonidine (1, 3, 10 and 30 microg kg(-1)) and guanabenz (1, 3, 10 and 30 microg kg(-1)) led to dose-dependent hypotension and bradycardia; the plasma noradrenaline concentration also decreased. After the two highest doses, all four drugs lowered noradrenaline release in the prefrontal cortex. At doses eliciting equal hypotensive and sympatho-inhibitory responses, rilmenidine and moxonidine inhibited cerebral cortical noradrenaline release at least as much as clonidine and guanabenz. 3. The results show that rilmenidine and moxonidine lower cerebrocortical noradrenaline release at doses similar to those which cause sympatho-inhibition. This effect was probably due to an alpha(2)-adrenoceptor-mediated inhibition of the firing of locus coeruleus neurons and, in addition, to presynaptic inhibition of noradrenaline release at the level of the axon terminals in the cortex. The results argue against the hypothesis that rilmenidine and moxonidine, due to their selectivity for sympatho-inhibitory I(1) imidazoline receptors, do not suppress noradrenergic neurons in the central nervous system.

Dialysate dihydroxyphenylglycol as a window for in situ axoplasmic norepinephrine disposition

To examine basal axoplasmic norepinephrine (NE) kinetics at the in situ cardiac sympathetic nerve ending, we applied a dialysis technique to the heart of anesthetized cats and performed the dialysate sampling with local administration of a pharmacological tool through a dialysis probe. The dialysis probe was implanted in the left ventricular wall, and dihydroxyphenylglycol (DHPG, an index of axoplasmic NE) levels were measured by liquid chromatogram-electrochemical detection. Control dialysate DHPG levels were 161+/-19 pg/ml. Pargyline (monoamine oxidase inhibitor, 1 mM) decreased the dialysate DHPG levels to 38+/-10 pg/ml. Further alpha-methyl-para-tyrosine, omega-conotoxin GVIA, desipramine (NE synthesis, release and uptake blockers) decreased the dialysate DHPG levels to 64+/-19, 106+/-15, 110+/-22 pg/ml, respectively. In contrast, reserpine (vesicle NE transport inhibitor, 10 microM) increased the dialysate DHPG levels to 690+/-42 pg/ml. Thus, NE synthesis, metabolism and recycling (release, uptake and vesicle transport) affected basal intraneuronal NE disposition at the nerve endings. Measurement of DHPG levels through a dialysis probe provides information about basal intraneuronal NE disposition at the cardiac sympathetic nerve endings. Yohimbine (alpha(2)-adrenoreceptor blocker, 10 microM) and U-521 (catechol-O-methyltransferase blocker, 100 microM) did not alter the dialysate DHPG levels. Furthermore, there were no significant differences in the reserpine induced DHPG increment between the presence and absence of desipramine (10 microM) or alpha-methyl-para-tyrosine (100 mg/kg i.p.). These results may be explained by the presence of two axoplasmic pools of NE, filled by NE taken up and synthesized, and by NE overflow from vesicle. The latter pool of NE may be closed to the monoamine oxidase system in the axoplasma.

Dopamine and noradrenaline efflux in the prefrontal cortex in the light and dark period: effects of novelty and handling and comparison to the nucleus accumbens

We used on-line microdialysis measurements of dopamine and noradrenaline extracellular concentrations in the medial prefrontal cortex of awake, freely moving rats during the dark and the light period of the day to study whether (i) basal efflux would be higher in the active, dark period than in the inactive, light period; (ii) the activation induced by environmental stimuli would be dependent on these conditions. When determined one day after cannula placement, noradrenaline and dopamine levels were higher during the dark. Maximal relative increases induced by novelty and handling were 150% and 175-200%, respectively, and were very similar in the light and the dark, but the net increases were higher in the dark. Separate groups were tested one week after cannula placement to ensure recovery of possibly disturbed circadian rhythms. While basal levels in the dark were now approximately twice those in the light, the maximal relative and net increases after both novelty and handling were very similar. Basal levels of dopamine in the nucleus accumbens (one day after cannula placement) were not different in the light or dark, but were increased by novelty and handling to about 130% only in the light period, not in the dark. Thus, in the prefrontal cortex, dopamine strongly resembles noradrenaline, in that basal efflux was state dependent, whereas activation by stimuli was not. In the nucleus accumbens, basal dopamine efflux was not state dependent, but activation by stimuli was. These results suggest that there are differential effects of circadian phase on basal activity and responsiveness of the mesolimbic vs the mesocortical dopamine system.

Effects of moxonidine on corticocerebral blood flow under normal and ischemic conditions in conscious rabbits

Hypertension associated with excessive liberation of circulating and tissue catecholamines is an independent risk factor for further cardiovascular complications and an important predictor of stroke. Moxonidine is a centrally acting anti-hypertensive drug with potent action on I1-imidazoline receptors. It inhibits catecholamine release and is therefore expected to exert an antiadrenergic effect at various levels in the regulation of the cardiovascular system. The aim of this study was to investigate the effect of moxonidine (0.025-0.1 mg/kg, i.v.) on the normal and unilateral carotid occlusion-induced impaired corticocerebral blood flow (cCBF) determined by hydrogen polarography, on mean arterial blood pressure (MABP) and heart rate (HR) in conscious rabbits. Moxonidine produced a reduction of MABP and HR. On the other hand, after administration of the drug, a significant increase in the normal and impaired cCBF was observed. Because the improvement in cCBF was conspicuous in both normal and ischemic conditions, moxonidine might be beneficial not only in the treatment of hypertension but also in the management of cerebral ischemia.

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

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

Studies on the acute and chronic effects of reboxetine on extracellular noradrenaline and other monoamines in the rat brain

1 The effect of reboxetine, a novel antidepressant drug that potently and selectively inhibits neuronal noradrenaline (NA) uptake, on brain extracellular monoamines was studied by microdialysis. 2 Fifteen mg kg-1 i.p. reboxetine raised extracellular NA in the frontal cortex (by 242%) and dorsal hippocampus (by 240%). 3 Idazoxan (1 mg kg-1 s.c.), given 60 min after 15 mg kg-1 reboxetine, markedly potentiated the effect on extracellular NA in the frontal cortex (by 1580%) and dorsal hippocampus (by 1360%), but had no effect by itself. 4 Twenty-four hours after the last injection of a chronic schedule (15 mg kg-1 i.p. once daily for 14 days) reboxetine had no effect on basal extracellular concentrations of NA in the dorsal hippocampus and a challenge dose of reboxetine (15 mg kg-1) raised extracellular NA similarly in rats treated chronically with reboxetine (by 353%) and saline (by 425%). 5 Ten and 20 microg kg-1 i.p. clonidine dose-dependently reduced hippocampal extracellular NA similarly in rats given chronic reboxetine (by 32% and 57%) and saline (by 42% and 56%). 6 Extracellular concentrations of dopamine and 5-HT in the striatum were similar in rats treated chronically with reboxetine and saline. A challenge dose of reboxetine (15 mg kg-1) had no effect on striatal extracellular dopamine and slightly increased striatal extracellular 5-HT to a similar extent in rats treated chronically with reboxetine (by 137%) and saline (by 142%). 7 The results suggest that combining reboxetine with an alpha2-adrenoceptor antagonist may facilitate its antidepressant activity. Repeated treatment confirmed that reboxetine is fairly selective for the noradrenergic system but provided no evidence of adaptive changes in that system that could facilitate its effect on extracellular NA.

Neuroprotective effects of the alpha2-adrenoceptor antagonists, (+)-efaroxan and (+/-)-idazoxan, against quinolinic acid-induced lesions of the rat striatum

A deficient control of neuronal repair mechanisms by noradrenergic projections originating from the locus coeruleus may be a critical factor in the progression of neurodegenerative diseases. Blockade of presynaptic inhibitory alpha2-adrenergic autoreceptors can disinhibit this system, facilitating noradrenaline release. In order to test the neuroprotective potential of this approach in a model involving excitotoxicity, the effects of treatments with the alpha2-adreneceptor antagonists, (+)-efaroxan (0.63 mg/kg i.p., thrice daily for 7 days) or (+/-)-idazoxan (2.5 mg/kg i.p., thrice daily for 7 days), were evaluated in rats which received a quinolinic acid-induced lesion of the left striatum. Both drug treatments resulted in a reduced ipsiversive circling response to apomorphine and a reduced choline acetyltransferase deficit in the lesioned striatum. The mechanisms underlying this effect are not known for certain, but may include noradrenergic receptor modulation of glial cell function, growth factor synthesis and release, activity of glutamatergic corticostriatal afferents, and/or events initiated by NMDA receptor activation. These results suggest a therapeutic potential of alpha2-adrenoceptor antagonists in neurodegenerative disorders where excitotoxicity has been implicated.

Elevation of extracellular cortical noradrenaline may contribute to the antidepressant activity of zotepine: an in vivo microdialysis study in freely moving rats

The antipsychotic, zotepine, as well as possessing affinity for dopamine D1- and D2-1ike receptors, has high affinity for the noradrenaline (NA) transporter and inhibits [3H]NA uptake by rat frontal cortex synaptosomes, in vitro. The present studies investigated the effects of zotepine on extracellular NA in the frontal cortex of freely moving rats using in vivo microdialysis. Removal of calcium from the perfusate reduced extracellular NA by 70.5% and prevented the 50 mM KCl-stimulated increase in NA levels. Zotepine (0.5-1.5 mg kg(-1) i.p.), evoked biphasic, dose-dependent rises in extracellular NA with maximal increases observed at 60 min (+ 171.0%) and 240 min (+ 211.5%) post-treatment. The increases in NA levels were sustained for up to 100 min post-dosing. Clozapine (10.0 mg kg(-1) i.p.), resulted in a smaller, transient increase in NA levels (+ 72.0%) which lasted for 20 min post-treatment. Neither ziprasidone (3.0 mg kg(-1) i.p.) nor olanzapine (1.0 mg kg(-1) i.p.) influenced extracellular NA. Systemic treatment with the antidepressant desipramine (0.3 mg kg(-1) i.p.) resulted in a prolonged elevation of NA levels over 240 min (maximal increase of + 354.3%), whilst local infusion of nisoxetine (1-100 microM) through the dialysis probe increased NA levels in a concentration-dependent manner (up to 587.8% of control values). These data suggest that the inhibition of NA uptake by zotepine and its subsequent prolonged elevation of extracellular cortical NA may underlie the reported antidepressant properties of zotepine in schizophrenic patients.

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.

Modulation of catecholamine release by alpha 2-adrenoceptors and I1-imidazoline receptors in rat brain

The physiological and pharmacological effects of imidazoli(di)ne derivatives, such as clonidine, have been related not only to the interaction with alpha 2-adrenoceptors but also to their activity on non-adrenoceptor sites termed imidazoline receptors. The modulation of catecholamine release by imidazoline drugs was studied by monitoring extracellular levels of norepinephrine (NE), dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) with microdialysis in cingulate cortex of rats, with or without irreversible alpha 2-adrenoceptor blockade. NE and DA levels were in the 1 nM range whereas DOPAC and HIVA levels were approximately equal to 100 nM. NE and DA levels were increased when the uptake blocker desipramine (1 microM) or KCl (100 mM) were added to the perfusion medium. Clonidine induced a dose-dependent (0.3-1.2 mg/kg i.p.) decrease in NE (max 61%) and DA (max 40+) levels that was reversed by the alpha 2-adrenoceptor antagonist RX821002. After alpha 2-adrenoceptor irreversible blockade with the alkylating agent N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ), [3H]clonidine binding to alpha 2-adrenoceptors was reduced by 94 +/- 1%. Under such conditions, clonidine elicited a paradoxical dose-dependent (0.6-2.4 mg/kg i.p.) increase of NE (max 56%) without modifications in DA, DOPAC and HVA levels. The stimulatory effect of clonidine was prevented by the imidazoline receptor antagonist idazoxan (10 mg/kg i.p.) but not by RX821002 (5 mg/kg i.p.). In rats pretreated with EEDQ, cirazoline (I1/I2-imidazoline receptor agonist), moxonidine (I1-imidazoline receptor agonist), but not guanabenz (I2-imidazoline receptor agonist) (1.2-2.4 mg/kg i.p.) elicited an increase of NE levels in a similar manner to clonidine (11-82%). Idazoxan also abolished these responses to cirazoline or moxonidine. In contrast to systemic administration, local perfusion of clonidine (10-100 microM) through the microdialysis probe under alpha 2-adrenoceptor alkylating conditions, did not modify extracellular levels of NE and DA suggesting an indirect mechanism. The results demonstrate that clonidine and related imidazoli(di)ne drugs are able not only to inhibit NE release in rat cerebral cortex involving an alpha 2-adrenoceptor mechanism, but also to induce a paradoxical NE release through an indirect extracortical mechanism. The findings evidence that the indirect modulation of NE levels by imidazoline drugs is mainly due to a functional activity on I1-imidazoline receptors.

Norepinephrine induced growth and expression of virulence associated factors in enterotoxigenic and enterohemorrhagic strains of Escherichia coli

The small intestine is richly innervated by the sympathetic nervous system. High concentrations of monoamines, most notably norepinephrine, are found throughout the various intestinal layers. In order to determine whether norepinephrine is capable of influencing bacterial pathogenesis, the growth and production of virulence factors in ETEC and EHEC were examined in a physiologically relevant medium utilizing very low initial bacterial inoculums to more closely mimie in vivo conditions. The growth of ETEC strain B44 and the production of the K99 pilus adhesin on a protein equivalent basis was greatly increased in the presence of norepinephrine. Growth of EHEC O157:H7 was also increased in norepinephrine containing medium as well as production of SLT-I and SLT-II. The ability of norepinephrine to increase both bacterial growth and expression of virulence factors was shown to be non-nutritional in nature. Given the abundant adrenergic innervation in the small intestine, these in vitro results suggest that the neurohumoral environment of the host may play a role in bacterial growth and expression of virulence factors.

Production of Shiga-like toxins by Escherichia coli O157:H7 can be influenced by the neuroendocrine hormone norepinephrine

To examine whether the neuroendocrine hormone norepinephrine may influence the production of the Shiga-like toxins (SLTs), several Escherichia coli O157:H7 clinical isolates were grown in the presence or absence of norepinephrine. An in vitro culture system consisting of low (<1500 colony-forming units/ml) initial concentrations of inocula into a serum-based medium was used to more closely approximate in vivo conditions. The growth of all isolates was increased several logs in the presence of norepinephrine, as compared with the growth in controls, during a 24-hour growth period. Controls included additional dextrose as well as the use of the norepinephrine metabolite normetanephrine, which contains one more methyl group than norepinephrine and hence would serve as a better energy source for growth if the effect were solely nutritionally mediated. During the 24 hours of growth, the production of cell-associated SLT-I on a protein-equivalent basis was shown to be increased over 100-fold in norepinephrine-cultured bacteria as compared with controls. SLT-II elaboration into culture supernatants was also greatly increased in norepinephrine-cultured bacteria as compared with controls. Maximal detection of cell-associated SLT-II occurred at least 12 hours before maximum levels were achieved in culture supernatants. Because norepinephrine represents one of the largest pools of monoamines present throughout the small intestine, these results suggest that the neuroendocrine environment of the small intestine may play a role in the growth of O157:H7 and the production of SLTs.

Effects of two volatile anesthetics (sevoflurane and halothane) on the hypothalamic noradrenaline release in rat brain

There are marked increases in noradrenaline (NA) release during emergence from general anesthesia induced with volatile anesthestics. These changes in NA in the posterior hypothalamus of the rat were assessed by intracranial microdialysis. Sevoflurane and halothane in equipotent concentrations were used to obtain the same depth of anesthesia. NA release increased similarly with the two agents during recovery. However, NA release remained elevated longer with halothane, from which recovery was also slower.

Novelty-induced increase in dopamine release in the rat prefrontal cortex in vivo: inhibition by diazepam

The effects of graded stressful conditions on extracellular concentrations of dopamine (DA) in the medial prefrontal cortex of rats were measured in vivo using microdialysis. Picking up the rat twice with a 20-min interval increased extracellular DA to 120%, exposure to a novel environment by placement in a clean cage for 20 min to 150% and holding the rat in the hands for 20 min to over 200%. Diazepam (5 mg/kg) decreased DA to about 75% and attenuated the novelty- and handling-induced increases. Exposure to novelty or handling are easy and simple methods to obtain graded increases of in vivo cortical DA release.

Effects of neonatal exposure to clonidine on basal and activated central noradrenaline metabolism and in vivo overflow

The occurrence of persistent effects of chronic neonatal exposure to the alpha 2-adrenoceptor agonist clonidine was investigated by determination of tissue concentrations of monoamines and metabolites and in vivo overflow of noradrenaline and its metabolites, in various rat brain regions during adulthood. Rat pups were treated with clonidine from postnatal day 10-20 and all measurements were carried out between postnatal day 40 and 58. Tissue concentrations of monoamines and metabolites of the early clonidine-treated rats did not differ significantly from the control group. A challenge with yohimbine did not reveal altered responses of monoaminergic systems, except for the failure of an increased serotonergic activity in the medulla pons. In vivo microdialysis measurements revealed an elevated basal extracellular noradrenaline level in amygdala, but not in frontal cortex and hippocampus. Pharmacological challenge in vivo with idazoxan did not reveal differences between clonidine- and saline-exposed rats. These results confirm previous findings that continuous activation of alpha 2-adrenoceptors during a particular period of rat brain development may result in long-lasting but small changes in monoaminergic activity. However, these alterations are not very consistent and may depend on the parameter chosen to reflect monoaminergic activity and are not revealed more clearly by activating (challenging) the noradrenaline system with alpha2-adrenoceptor antagonists.

Increased alpha 2-adrenoceptor mediated regulation of adult rat brain noradrenaline overflow after chronic neonatal exposure to propranolol; a microdialysis study

Direct and persistent effects of chronic neonatal administration of the beta-adrenoceptor antagonist propranolol on brain noradrenergic activity were investigated by measuring tissue concentrations of noradrenaline and its metabolites and in vivo overflow of noradrenaline during adulthood. Rat pups were chronically treated with propranolol from postnatal day 1 to day 10. Determination of monoamine metabolism after the last injection showed an increase in noradrenaline metabolism in frontal cortex, limbic system and hippocampus of propranolol-exposed rats, but 47 days after this last injection it was apparent that these effects were not long-lasting. Moreover, basal noradrenaline overflow in vivo in the hippocampus of 40-55 day-old propranolol-exposed rats did not differ from that in controls. However, the regulation of noradrenaline release seemed to have been altered, since a pharmacological challenge with the alpha 2-adrenoceptor antagonist idazoxan induced an enhanced increase in the in vivo noradrenaline overflow in propranolol-exposed rats compared to controls. It is suggested that the neonatal beta-blockade induced a supersensitivity of the presynaptic alpha 2-adrenoceptor. The precise mechanism underlying this effect has to be elucidated.