Noradrenaline Efflux Research Articles

P2X receptor activation elicits transporter-mediated noradrenaline release from rat hippocampal slices.

This study was designed to test the hypothesis of whether activation of presynaptic P2X receptor-gated ion channels elicits noradrenaline release from central catecholaminergic terminals. ATP, alpha,beta-methylene-adenosine 5'-triphosphate (alpha,beta-methyleneATP), and ADP elicited concentration-dependent [3H]noradrenaline outflow from superfused rat hippocampal slices with the following rank order of agonist potency: alpha,beta-methyleneATP > ATP > ADP. Among P2 receptor antagonists, pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid (30 microM), 4,4',4",4"'-[carbonylbis(imino-5,1,3-benzenetriyl-bis(carbonylimino))]tetrakis-1,3-benzenedisulfonic acid (100 nM), and 8,8'-[carbonybis(imino-3,1-phenylenecarbonylimino)]bis1,3,5-naphthalenetrisulphonic acid (10 microM) significantly inhibited the outflow of [3H]noradrenaline, evoked by ATP, whereas Brilliant Blue G (100 nM), 2'-deoxy-N6-methyladenosine 3',5'-bisphosphate tetraammonium (10 microM), the A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (250 nM), and the A2A receptor antagonist 3,7-dimethyl-1-propargylxanthine (250 nM) were ineffective. Pretreatment with the Gi protein inhibitor pertussis toxin (2.5 microg/ml) did not change the effect of ATP on [3H]noradrenaline outflow. In contrast, a decrease in extracellular pH from 7.4 to 6.6 significantly attenuated the response by ATP. When extracellular Na+ was replaced by choline chloride and in the presence of the noradrenaline uptake inhibitor desipramine (10 microM), the ATP-evoked [3H]noradrenaline outflow was almost completely abolished, indicating that its underlying mechanism is the sodium-dependent reversal of the noradrenaline transporter. Reverse transcription-polymerase chain reaction analysis revealed that mRNA encoding P2X1, P2X2, P2X3, P2X4, P2X6, P2X7 and P2Y1 receptor subunits were expressed in the brainstem containing catecholaminergic nuclei projecting to the hippocampus, whereas mRNA encoding P2X5, P2Y2, P2Y4, and P2Y6 receptors were absent. Taken together, these results indicate that noradrenergic terminals of the rat hippocampus are equipped with presynaptic facilitatory P2X receptors, displaying a pharmacological profile similar to homomeric P2X1 and P2X3 receptors.

Noradrenaline and dopamine efflux in the prefrontal cortex in relation to appetitive classical conditioning.

We trained rats to learn that an auditory stimulus predicted delivery of reward pellets in the Skinner box. After 2 d of training, we measured changes in efflux of noradrenaline (NA) and dopamine (DA) in the medial prefrontal cortex using microdialysis on the third day. Animals were subjected to a normal rewarded session and an extinction session, in which the auditory stimulus was presented alone. In the rewarded session, both NA and DA efflux were increased, but in extinction, only NA was activated. The data suggest that NA has a role in the reaction to reward-predicting stimuli, which complements that of DA.

5-HT has contrasting effects in the frontal cortex, but not the hypothalamus, on changes in noradrenaline efflux induced by the monoamine releasing-agent, d-amphetamine, and the reuptake inhibitor, BTS 54 354

There is extensive evidence for functional interactions between central noradrenergic and serotonergic neurones. Here, dual-probe microdialysis was used in freely-moving rats to compare the effects of 5-HT on noradrenergic transmission in the rat frontal cortex and hypothalamus. We studied the effects of the 5-HT synthesis inhibitor, para-chlorophenylalanine ( pCPA; which depleted 5-HT stores in both the frontal cortex and the hypothalamus), on spontaneous efflux of noradrenaline and on the noradrenergic responses to d-amphetamine, and the monoamine reuptake inhibitor, BTS 54 354. pCPA pretreatment alone did not affect spontaneous noradrenaline efflux in either brain region, whether or not α 2-autoreceptors were inactivated by administration of the α 2-antagonist, atipamezole (1 mg/kg i.p). However, in the frontal cortex, pCPA pretreatment augmented the amplitude of, and prolonged, the noradrenergic response to local infusion of d-amphetamine (10 μM). In contrast, pCPA abolished the increase in cortical noradrenaline efflux induced by local infusion of BTS 54 354 (50 μM). In the hypothalamus, pCPA did not affect the amplitude of the response to either of these agents but did prolong the effects of d-amphetamine on noradrenaline efflux. These findings suggest that serotonergic transmission has complex effects on the noradrenergic response to drugs that increase noradrenergic transmission in the frontal cortex, but has less influence in the hypothalamus.

Ischemia-induced norepinephrine efflux from cardiac sympathetic nerve endings

Ischemia-induced norepinephrine efflux from cardiac sympathetic nerve endings

Noradrenaline clearance differs in the frontal cortex and hypothalamus: a dual-probe microdialysis study in freely-moving rats

The noradrenergic innervation of the frontal cortex and the hypothalamus arises from different brainstem nuclei and neurones projecting to these two brain regions differ topographically and morphologically. In the present study, we investigated whether there is a difference in noradrenaline clearance in these two brain areas using dual-probe microdialysis in freely-moving rats. Noradrenaline clearance was determined simultaneously in each brain area by measuring the noradrenaline extraction fraction using the ‘No Net Flux’ method, with both brain regions perfused with the same solutions. The noradrenaline extraction fraction was greater in the frontal cortex than in the hypothalamus (72 and 54%, respectively). This finding suggests that the processes involved in noradrenaline clearance, especially uptake, are more efficient in the former brain area, which could explain the differences in the increase in noradrenaline efflux when these brain areas are challenged with drugs that alter the function of the central nervous system.

Differences in the mechanisms that increase noradrenaline efflux after administration of d-amphetamine: a dual-probe microdialysis study in rat frontal cortex and hypothalamus.

1. The extent to which impulse-independent release of noradrenaline and/or inhibition of its reuptake contribute to the response to d-amphetamine in vivo is unclear. Here, dual-probe microdialysis was used to investigate this question in the rat frontal cortex and hypothalamus. 2. After systemic administration of d-amphetamine (10 mg kg(-1)), or its local infusion (10 micro M), the increase in noradrenaline efflux in the hypothalamus was greater than in the frontal cortex. 3. In contrast, during local infusion of the noradrenaline reuptake inhibitor, BTS 54 354 (50 micro M), the noradrenaline response was similar in the frontal cortex and hypothalamus, even after systemic administration of the alpha(2)-antagonist, atipamezole, to block presynaptic inhibition of transmitter release and neuronal firing. 4. In the frontal cortex, but not the hypothalamus, the noradrenaline response to 10 micro M d-amphetamine was constrained by activation of alpha(2)-adrenoceptors. This suggests that, at this concentration, inhibition of reuptake of noradrenaline, following its impulse-dependent release, is evident in the frontal cortex, but that the noradrenaline response in the hypothalamus derives mostly from impulse-independent release (retrotransport). 5. Atipamezole did not affect the noradrenaline response to 100 micro M d-amphetamine in either brain region possibly because, at this higher concentration, retrotransport of noradrenaline masks any compensatory reduction in impulse-evoked release. 6. It is concluded that inhibition of reuptake and retrotransport make different contributions to the noradrenaline response to d-amphetamine in the frontal cortex and hypothalamus and that retrotransport increases with the concentration of d-amphetamine.

Reboxetine modulates norepinephrine efflux in the frontal cortex of the freely moving rat: the involvement of α 2 and 5-HT 1A receptors

The effect of reboxetine on norepinephrine (NE) efflux in the frontal cortex of freely moving rats has been studied using in vivo microdialysis. Reboxetine was administered either by injection (10 and 30 mg/kg i.p.) or directly to the frontal cortex via the dialysis probe (10 and 100 μM). To further elucidate the mechanism of action of reboxetine in this region yohimbine (10 μM) was co-infused with reboxetine via the frontal cortex probe. Both routes of administration of reboxetine resulted in a drug-induced, dose-dependent decrease in frontal cortex NE efflux. This effect was reversed following co-infusion of yohimbine, which has been shown to possess both α 2-adrenoceptor antagonist and 5-HT 1A agonist properties.

Stereoselective effects of ketamine on dopamine, serotonin and noradrenaline release and uptake in rat brain slices

Ketamine (2-(2-chlorophenyl)-(1-methylamino)-cyclohexanone) is a rapid-acting dissociative general anaesthetic whose hallucinogenic properties have made it a popular drug of abuse. Ketamine comprises two optical isomers, with differing pharmacology. In the present study, the effects of (+)- and (−)-ketamine on stimulated efflux and reuptake of dopamine (DA), noradrenaline (NA) and serotonin (5-HT) were compared in isolated superfused slices of the rat caudatoputamen (CPu), ventral bed nucleus of the stria terminalis (BSTV) or dorsal raphe nucleus (DRN), respectively. Monoamine efflux was elicited by local electrical stimulation (20 pulses, 100 Hz trains) at tungsten microelectrodes and measured at adjacent carbon fibre microelectrodes using fast cyclic voltammetry (FCV). In CPu (+)-ketamine increased stimulated DA efflux and slowed DA reuptake in a concentration-dependent manner (25–200 μM). At 100 μM (+)-ketamine increased DA efflux by 109±20% (mean±S.E.M., n=13) of control values after 30 min ( P<0.001 versus control) and prolonged uptake half-time ( t 1/2) by 76±38% ( n=9, P<0.001) of control. In contrast (−)-ketamine (100 μM) had no effect on DA efflux or uptake. In DRN, both isomers (100 μM) increased stimulated 5-HT efflux. (−)-Ketamine had a larger effect ( P<0.001), an 88±15% increase in 5-HT efflux ( n=9) versus 46±10% ( n=8) for the (+)-isomer. The isomers had similar effects on 5-HT uptake, increasing t 1/2 by approximately 200%. No evidence of stereospecificity was seen in BSTV: both isomers had small effects (+)- and (−)-ketamine (100 μM) increasing NA efflux by 43±10% ( n=7, P<0.001) and 29±8% ( n=7, P<0.001), respectively. The isomers also had identical effects on NA uptake, each increasing uptake t 1/2 by approximately 100%. In summary, our data show that the optical isomers of ketamine have strikingly different stereospecificity for the monoamine systems and one might predict, therefore, a different psychotomimetic potential.

Implication of ionotropic glutamate receptors in the release of noradrenaline in hippocampal CA1 and CA3 subregions under oxygen and glucose deprivation

A strong linkage between adrenergic and glutamatergic systems exists in the CNS but it is still unclear whether the excessive release of noradrenaline under ischemic conditions is modulated by excitatory amino acids. We studied the effect of selective glutamate receptor antagonists on the release of [ 3 H ]noradrenaline evoked by glucose and oxygen deprivation in hippocampal CA1, CA3 and dentate gyrus subregions. The release of glutamate, aspartate and GABA was measured by HPLC. Omission of oxygen and glucose increased the release of [ 3 H ]noradrenaline as well as the release of amino acids. Maximum effect on noradrenaline release was observed in CA1 region. The relative increase of the release after 30 min energy deprivation ( R 2) versus the basal release under normal conditions ( R 1), i.e. the R 2/ R 1 ratio was 7.1±1.0, 3.87±0.4 and 3.26±0.27 for CA1, CA3 and dentate gyrus, respectively. The [ 3 H ]noradrenaline outflow in response to glucose and oxygen deprivation was abolished at low temperature, but not by Ca 2+ removal, suggesting a cytoplasmic release process. In CA1 and CA3 [ 3 H ]noradrenaline release was significantly attenuated by MK-801, an NMDA receptor antagonist. The AMPA receptor antagonist GYKI-53784 had no effect in CA3, but partly reduced noradrenaline release in CA1. Our results suggest that ionotropic glutamate receptors seem to be implicated in the massive cytoplasmic release of noradrenaline in CA1 what may contribute to its selective vulnerability.

Enhancer substances: Selegiline and R-(–)-1-(benzofuran-2-yl)-2-propylaminopentane [(–)-BPAP] enhance the neurotrophic factor synthesis on cultured mouse astrocytes

R-(–)-1-(Benzofuran-2-yl)-2-propylaminopentane [(–)-BPAP] is a potent “catecholaminergic and serotonergic activity enhancer (CAE/SAE)”, which enhances the impulse-evoked catecholamines and serotonin release, e.g. (–)-BPAP enhances in vitro norepinephrine efflux from the slices of locus coeruleus in a bipolar manner with the two effective ranges of low (fM–pM level) and high (nM–μM level) concentrations. Here, the effects of (–)-BPAP and selegiline on the cultured mouse astrocytes were studied. The protein levels of the neurotrophic factors (NGF, BDNF and GDNF) in the conditioned medium of cultured astrocytes were determined by using ELISA. In the cultured astrocytes incubated for 24 h with selegiline, the synthesis of NGF and BDNF was significantly enhanced in the concentration dependent manner, with minimum effective concentrations of 4 × 10 −4 and 5 × 10 −4 M, respectively. (–)-BPAP also enhanced the NGF, BDNF and GDNF synthesis, with minimum effective concentrations of 5 × 10 −5, 1 × 10 −5, and 1 × 10 −6 M, respectively. Although the effects of (–)-BPAP on the NGF synthesis was tested in the range of 1 × 10 −15–5 × 10 −4 M, the concentration response curve of (–)-BPAP was a single bell shape with the peak effect at 1 × 10 −4 M, and did not show any effects in low concentrations such as fM–pM level. Each concentration response curve of (–)-BPAP on BDNF and GDNF synthesis was a single bell shape with peak effects at 1 × 10 −3 M and 1 × 10 −4 M, respectively.

The CB1 receptor antagonist SR141716A selectively increases monoaminergic neurotransmission in the medial prefrontal cortex: implications for therapeutic actions.

1. In order to explore potential therapeutic implications of cannabinoid antagonists, the effects of the prototypical cannabinoid antagonist SR141716A on monoamine efflux from the medial prefrontal cortex and the nucleus accumbens of the rat were investigated by in vivo microdialysis. 2. SR141716A moderately increased serotonin efflux and concentrations of its metabolite 5-HIAA, both in the medial prefrontal cortex and the nucleus accumbens, and increased norepinephrine, dopamine and their metabolites in the medial prefrontal cortex. In contrast, it had no effect on norepinephrine, dopamine and their metabolites in the nucleus accumbens. 3. At the same doses, SR141716A increased acetylcholine efflux in the medial prefrontal cortex, in agreement with previous studies; contrary to the effects in cortex, SR141716A had no effect on acetylcholine efflux in the nucleus accumbens. 4. The efficacy of SR141716A in the psychostimulant-induced hyperlocomotion and the forced swimming paradigms was also explored in mice. SR141716A attenuated phenylcyclidine- and d-amphetamine-induced hyperlocomotion, without affecting locomotor activity when administered alone, and decreased immobility in the forced swimming test. 5. These results suggest that the cortical selectivity in the release of catecholamines, dopamine in particular, induced by the cannabinoid antagonist SR141716A, its procholinergic properties, together with its mild stimulatory effects on serotonin and norepinephrine efflux make similar compounds unique candidates for the treatment of psychosis, affective and cognitive disorders.

Neuroanatomical substrates involved in the anxiogenic-like effect of acute fluoxetine treatment

An initial exacerbation of anxiety can be observed in animals and humans treated with selective serotonin reuptake inhibitors (SSRIs). The neurobiological substrates and mechanism(s) underlying this effect are not clear. We used Fos expression as a marker of neuronal activation to investigate effects of acute fluoxetine treatment in rats submitted to two different models of emotional stress, airjet and immobilization. Exposure to both stressors induced Fos expression in various brain regions implicated in fear/anxiety mechanisms. Acute treatment with 5 mg/kg fluoxetine facilitated airjet-induced escape responses and enhanced the airjet-, as well as immobilization-induced Fos expression exclusively in the locus coeruleus (LC), but not in other areas including the amygdala, hypothalamus or septum. Fluoxetine also facilitated airjet-induced noradrenaline efflux in the medial prefrontal cortex, a projection area of LC noradrenergic neurons. A higher dose of fluoxetine (10 mg/kg) did not change escape responses and had no effect on stress-induced Fos expression in the LC, but decreased airjet-induced Fos expression in the medial amygdala. The results indicate that anxiogenic effects of acute fluoxetine treatment occur in a specific dose range and can be mimicked by exacerbation of escape responses in the airjet model. Furthermore, facilitation of escape responses by fluoxetine is linked to enhanced activity in the LC/noradrenaline system.

Facilitatory role of NO in neural norepinephrine release in the rat kidney

We examined modulation by nitric oxide (NO) of sympathetic neurotransmitter release and vasoconstriction in the isolated pump-perfused rat kidney. Electrical renal nerve stimulation (RNS; 1 and 2 Hz) increased renal perfusion pressure and renal norepinephrine (NE) efflux. Nonselective NO synthase (NOS) inhibitors [N(omega)-nitro-L-arginine methyl ester (L-NAME) or N(omega)-nitro-L-arginine], but not a selective neuronal NO synthase inhibitor (7-nitroindazole sodium salt), suppressed the NE efflux response and enhanced the perfusion pressure response. Pretreatment with L-arginine prevented the effects of L-NAME on the RNS-induced responses. 2-(4-Carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (carboxy-PTIO), which eliminates NO by oxidizing it to NO(2), suppressed the NE efflux response, whereas the perfusion pressure response was less susceptible to carboxy-PTIO. 8-Bromoguanosine cGMP suppressed and a guanylate cyclase inhibitor [4H-8-bromo-1,2,4-oxadiazolo(3,4-d)benz(b)(1,4)oxazin-1-one] enhanced the RNS-induced perfusion pressure response, but neither of these drugs affected the NE efflux response. These results suggest that endogenous NO facilitates the NE release through cGMP-independent mechanisms, NO metabolites formed after NO(2) rather than NO itself counteract the vasoconstriction, and neuronal NOS does not contribute to these modulatory mechanisms in the sympathetic nervous system of the rat kidney.

Norepinephrine Content And Adrenoceptor Function In The Bladder Of Cats With Feline Interstitial Cystitis

Interstitial cystitis is a chronic syndrome affecting humans and domestic animals, including cats (feline interstitial cystitis). The aggravation of interstitial cystitis symptoms by stress suggests involvement of the sympathetic nervous system. Studies have identified increased sympathetic nervous system activity in patients with interstitial cystitis but to our knowledge effects on bladder function have not been reported. To address this question we measured bladder norepinephrine (NE) content, the electrical field stimulation flux of NE and acetylcholine (ACh), and the effects of feline interstitial cystitis on adrenoceptor (AR) mediated bladder strip contractility. Bladders were obtained from healthy cats and cats with feline interstitial cystitis. In experiment 1 bladder tissue NE content was determined and the simultaneous release of 3H-NE and 14C-ACh in perfusion bath effluent after electrical field stimulation was measured. NE and ACh release was calculated from the area under the efflux curve. In experiment 2 electrical field stimulation induced contractility of bladder body strips was measured in the presence of 100 nM. to 25 microM. NE only or combined with atipamezole (an alpha2-AR antagonist), propranolol (a beta-AR antagonist) or phentolamine (an alpha-AR antagonist). Antagonists were added to the bath at least 15 minutes before stimulation, after which NE was added in cumulative doses and dose response curves were constructed. Significant increases in NE content and efflux in the absence of alterations in ACh efflux were identified. In the bladder strip studies decreased alpha2 and beta1-AR function was found in strips from cats with feline interstitial cystitis, whereas beta3 or atypical beta-ARs were tentatively identified. These results support and extend previous studies by identifying an effect of increased sympathetic activity on bladder function in cats with feline interstitial cystitis.

Modulatory effects of ketamine on catecholamine efflux from in vivo cardiac sympathetic nerve endings in cats

With the use of the microdialysis technique, we examined the modulatory effect of ketamine on catecholamine efflux from in vivo cardiac sympathetic nerve endings. A dialysis probe was implanted in the left ventricular myocardium, and dialysate norepinephrine (NE) levels in anesthetized cats were measured with liquid chromatogram–electrical detection. A 60-min occlusion of the left anterior descending coronary artery caused increases in dialysate NE levels. Through the dialysis probe, locally applied ketamine (10 mM) augmented the dialysate NE responses to coronary occlusion in the presence and absence of desipramine (membrane NE transport blocker). Thus, the ketamine-induced NE increment is not mediated through the neuronal NE transporter. The sympathomimetic action of ketamine may augment the NE efflux evoked by myocardial ischemia.

Role of bradykinin in preconditioning and protection of the ischaemic myocardium.

Since the first description of bradykinin more than 50 years ago (Rocha e Silva et al., 1949), actions of the peptide in a variety of physiological and pathological responses have been extensively researched (reviewed in Bhoola et al., 1992; Wirth et al., 1997; Calixto et al., 2000). In the cardiovascular system, the classical action of bradykinin is vasodilatation, mediated in several vascular beds by the release of nitric oxide (NO) and prostacyclin (Hatta et al., 1997; Wirth et al., 1997). In the heart, exogenously-administered bradykinin is a potent coronary artery vasodilator substance, although the contribution of endogenous bradykinin to the regulation of coronary vascular tone is unclear. Several actions of bradykinin in the heart are of particular interest as they are independent of the vasodilator actions of the peptide. Such actions include the modulation of cell growth and division in the heart and the modulation of myocardial responses to ischaemia-reperfusion. The ability of bradykinin to act as an endogenous cytoprotective mediator in the ischaemic myocardium has received a great deal of attention in recent years. Much of this research on bradykinin has been fuelled by a growing appreciation of ischaemic preconditioning, an adaptive mechanism in which bradykinin plays an important role. This review focuses on the cytoprotective actions of bradykinin in the ischaemic and reperfused myocardium, discusses its role in the ischaemic preconditioning response, and examines the potential for manipulating endogenous bradykinin for therapeutic benefit in myocardial ischaemia.

Lack of prejunctional modulation of noradrenaline release by endogenous nitric oxide in guinea pig pulmonary artery

The regulation of noradrenaline (NA) release by endogenous endothelium-derived compounds was investigated in the isolated guinea pig pulmonary artery preloaded with [ 3 H ]NA. The radioactivity uptake, the basal and electrical field stimulation (10 Hz, 2 ms, 360 shocks) evoked release of [ 3 H ]NA was similar in arteries with intact endothelium and after removal of the endothelium. The wide selectivity nitric oxide (NO) synthase inhibitor N-ω-nitro- l-arginine (100 μM) did not affect significantly the basal and stimulation-evoked release of [ 3 H ]NA in control and endothelium-denuded preparations. These results indicate that neither endogenous NO nor other compounds derived from the endothelium have substantial influence on the NA outflow from sympathetic nerves innervating the pulmonary artery.

Characterisation of a modified approach to the study of sympathetic neurotransmission and its presynaptic modulation in the isolated rabbit thoracic aorta

Introduction: The quantification of [ 3H]noradrenaline spillover from electrically stimulated, sympathetic nerves is a widely used method to study presynaptic effects of hormones, transmitters and drugs. Although a straightforward approach, the execution of the experiments is not trivial. This holds true mainly for a reliable control of the experimental conditions, a major pitfall of the commonly used superfusion setup, and problems concerning the sampling of the tritium containing probes. Methods: These difficulties prompted us to develop a variant of this method, which is based on a classical organ bath setup. Rabbit thoracic aortic rings were incubated with [ 3H]-labeled noradrenaline. Instead of being constantly washed away by superfusion, the [ 3H]noradrenaline is allowed to accumulate in the medium. Results: Electrical field stimulation (EFS) (2 Hz, 3 ms, 150 mA) caused a significant increase of [ 3H]noradrenaline outflow by approximately a factor 4.2 ( P<.05). The fractional release of noradrenaline during consecutive periods of stimulation remained unaltered (FR 2/FR 1 0.99±0.03). The EFS-evoked release could be nearly abolished by the selective sodium channel blocker tetrodotoxin (1 μM) (FR 2/FR 1 0.06±0.03, P<.05). The N-type calcium antagonist ω-conotoxin GVIA (0.3 μM) abolished the stimulation-evoked outflow (FR 2/FR 1 0.01±0.06, P<.05), whereas the antisympathotonic agent guanethidine (10 μM) attenuated the EFS-evoked noradrenaline outflow by approximately a factor 2 (FR 2/FR 1 0.46±0.07, P<.05). Angiotensin II (0.1 and 1 nM) enhanced the EFS-evoked [ 3H]noradrenaline outflow by nearly a factor 1.5 and 2, respectively (FR 2/FR 1 of 1.43±0.11 (0.1 nM) and 2.03±0.11 (1 nM); n=6–8, P<.05). All agents failed to influence basal outflow. Discussion: Our modified experimental approach appears to be suitable to study presynaptic influences on sympathetic transmission in the rabbit thoracic aorta. In addition to optimal control of the experimental conditions, the method offers the advantage of a safe and reliable sampling.

Effects of ketamine on in vivo cardiac sympathetic nerve endings.

Using the dialysis technique, we examined the effect of ketamine on dialysate norepinephrine (NE) levels in the myocardial interstitial space in anesthetized cats. Dialysis probes were implanted in the left ventricular myocardium, and we measured the dialysate NE levels serving as an indicator of NE output at the cardiac sympathetic nerve endings. During local administration of ketamine (10 mM), we examined the time-course of the change in dialysate NE levels and the dialysate NE response to coronary occlusion. Dialysate NE levels significantly increased from 39+/-7 pg/ml at control to 133+/-22 pg/ml 30 min after beginning the ketamine administration. Addition of either omega-conotoxin GVIA (N-type calcium channel blocker) at 10 microg/kg intravenously or desipramine (neuronal NE transport blocker) at 100 microM did not inhibit the increment in dialysate NE evoked by ketamine. These findings suggest that the increase in dialysate NE evoked by ketamine is dependent neither on the activity of NE exocytosis nor on the neuronal NE transport. Left descending coronary artery occlusion evoked increments in dialysate NE. The addition of ketamine augmented the dialysate NE response to coronary occlusion. A ketamine-induced increment in dialysate NE might occur as a consequence of NE exocytosis independent or membrane NE transport insensitive efflux of NE.

The cannabinoid CB 1 receptor antagonist SR141716A increases norepinephrine outflow in the rat anterior hypothalamus

The effects of the selective cannabinoid CB 1 receptor antagonist N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1 H-pyrazole-3-carboxamide-hydrochloride (SR141716A) on extracellular concentrations of norepinephrine and 5-hydroxytryptamine (5-HT) were assessed by in vivo microdialysis in the anterior hypothalamus of freely moving rats. SR14716A (0.3, 1, 3 mg/kg, i.p.) dose-dependently increased norepinephrine efflux to about 300% of baseline, without affecting 5-HT levels. This increase in norepinephrine outflow could play an important role in the pharmacological and potentially therapeutic actions of SR141716A.