Effects of chronic tramadol on pre- and post-synaptic measures of monoamine function

The atypical analgesic tramadol has strong structural similarities to the antidepressant venlafaxine and is a mixed noradrenaline (NA) and serotonin (5-HT) uptake inhibitor. Because tramadol has been found active in the forced swim test, a common predictor of antidepressant efficacy, we therefore examined the effects of chronic tramadol on various pre- and post-synaptic monoamine measures. Male Wistar rats (150-200 g) received tramadol (20 mg/kg i.p.) or vehicle for 21 days and were sacrificed 24 h after the last dose. Quantitative autoradiography revealed that specific frontocortical [3H]dihydroalprenolol and [3H]ketanserin binding was lower in the chronic tramadol group than controls (beta: 37+/-8 and 217+/-56 fmol/mg; 5-HT2A: 23+/-3 and 44+/-7 fmol/mg, respectively, p < 0.05). Chronic tramadol had no effect on the magnitude of electrically stimulated noradrenaline (NA) efflux or uptake in locus coeruleus (LC) slices. Although dexmedetomidine (10 nM) decreased LC NA efflux equally (by approximately 60%) in chronic tramadol and vehicle groups, desipramine (50 nM) increased LC NA efflux more in vehicle (to 164+/-7%) than tramadol-treated rats (144+/-6%; p < 0.05). Chronic tramadol had no effect on dorsal raphé (DRN) or median raphé (MRN) 5-HT efflux. However, 5-HT uptake in tramadol-treated rats was slower (p < 0.05) in MRN and nearly so (p = 0.055) in DRN. The selective 5-HT1A agonist 8-OH-DPAT reduced 5-HT efflux in both DRN and MRN. Its effect in DRN was greater in rats given chronic tramadol than in vehicle controls (54+/-2 versus 32+/-6% reduction in 5-HT efflux, respectively). In conclusion, we suggest that tramadol has many of the pre- and postsynaptic neurochemical features of a conventional antidepressant, as might be predicted from its pharmacology.

Noradrenergic modulation of serotonin release in rat dorsal and median raphé nuclei via alpha(1) and alpha(2A) adrenoceptors

The rat rostral raphé nuclei receive catecholaminergic innervation from the locus coeruleus and other areas. In the present study, we investigated noradrenergic modulation of 5-HT release in rat dorsal and median raphé nuclei (DRN and MRN) slices (350 microm thick) superfused with artificial cerebrospinal fluid (aCSF). The raphé was locally stimulated (0.1 ms pulses, 10 mA) and 5-HT release was monitored at carbon fibre microelectrodes using fast cyclic voltammetry. The selective noradrenaline reuptake inhibitor desipramine (50 nM) did not increase stimulated (20 pulses, 100 Hz) 5-HT release but significantly slowed 5-HT reuptake in both DRN and MRN. On short stimulus trains (10 pulses, 200 Hz), the alpha(2)-selective agonist dexmedetomidine (10nM) decreased evoked 5-HT release in DRN and MRN (to 44+/-3 and 43+/-7% of pre-drug values, respectively, at minimum). In both nuclei, this response was antagonised by the selective alpha(2A)-antagonist BRL 44408 (1 microM: P<0.001 vs. dexmedetomidine) but not by the selective alpha(2B/C)-adrenoceptor antagonist ARC 239 (500 nM), the selective 5-HT(1A) antagonist WAY 100635 (100 nM) or the alpha(1)-selective antagonist prazosin (1 microM), suggesting that the effect of dexmedetomidine is wholly attributable to alpha(2A)-receptor activation. The alpha(1)-adrenoceptor agonist phenylephrine (5 microM) significantly decreased 5-HT release (to 49+/-7 and 41+/-4% of pre-drug values in DRN and MRN, respectively). The response was blocked by prazosin (P<0.001) and BRL 44408 (P<0.01) in DRN and by prazosin, BRL 44408 and WAY 100635 (all P<0.05) in MRN, suggesting that the effect of phenylephrine is, under these conditions, only partly mediated via alpha(1)-adrenoceptors. On long stimuli (30 pulses, 10 Hz), BRL 44408 (1 microM) increased evoked 5-HT efflux to 187+/-17 and 178+/-2% of pre-drug values in DRN and MRN, respectively (both P<0.001 vs. vehicle). Collectively, these data show that activation of both alpha(1) and alpha(2A)-adrenoceptors can decrease stimulated 5-HT release in the rostral raphé nuclei. Since the effect of dexmedetomidine was not antagonised by prazosin, we suggest that its effect was mediated directly, possibly through alpha(2A) receptors located on 5-HT cell elements, and not transduced indirectly through alpha(1)-adrenoceptor activation, as previously suggested by others.

Comparison of ketamine stereoisomers on tissue metabolic activity in an in vitro model of global cerebral ischaemia

Ketamine (2-o-chlorophenenyl-2-methylaminocyclohexanone hydrochloride) is a dissociative general anaesthetic with neuroprotective properties. Since ketamine is optically active, we compared the neuroprotective efficacy of the (+)- or (-)-enantiomers in global cerebral ischaemia. Rat corticostriatal slices superfused with, or incubated in, artificial CSF at 34 degrees C were subjected to a brief ischaemic insult. Dopamine efflux was measured using fast cyclic voltammetry. Tissue metabolism was determined with 2,3,5-triphenyltetrazolium chloride staining, a marker of mitochondrial enzyme activity. In control slices, ischaemia caused rapid striatal dopamine release (to 122 microM over 18 s) after an initial delay of 149s. Racemic ketamine (100 micromol/l) significantly delayed (by 24%, P<0.05), slowed (by 63%, P<0.01) and reduced (by 27%, P<0.05) ischaemia-induced dopamine release. Ischaemia (10 min) also caused significant decreases in striatal (25%, P<0.01) and cortical (31%, P<0.001) metabolic activity, manifested as a drop in mean TTC staining intensity. Racemic ketamine and its (+)- and (-)-enantiomers (each 100 microM) attenuated the loss of metabolic activity in the striatum. However, in the cortex, only (+)-ketamine (100 microM) was significantly neuroprotective. We conclude that neuroprotection by ketamine in cerebral ischaemia is both region- and isomer-dependent.

Effects of sevoflurane on dopamine, glutamate and aspartate release in an in vitro model of cerebral ischaemia

Release of excitatory amino acids and dopamine plays a central role in neuronal damage after cerebral ischaemia. In the present study, we used an in vitro model of ischaemia to investigate the effects of sevoflurane on dopamine, glutamate and aspartate efflux from rat corticostriatal slices. Slices were superfused with artificial cerebrospinal fluid at 34 degrees C and episodes of 'ischaemia' were mimicked by removal of oxygen and reduction in glucose concentration from 4 to 2 mmol litre(-1) for < or = 30 min. Dopamine efflux was monitored in situ by voltammetry while glutamate and aspartate concentrations in samples of the superfusate were measured by HPLC with fluorescence detection. Neurotransmitter outflow from slices was measured in the absence or presence of sevoflurane (4%). After induction of ischaemia in control slices, there was a mean (SEM) delay of 166 (7) s (n = 5) before sudden efflux of dopamine which reached a maximum extracellular concentration of 77.0 (15.2) micromol litre(-1). Sevoflurane (4%) reduced the rate of dopamine efflux during ischaemia (6.90 (1.5) and 4.73 (1.76) micromol litre(-1) s(-1) in controls and sevoflurane-treated slices, respectively; P<0.05), without affecting its onset or magnitude. Excitatory amino acid efflux was much slower. lschaemia-induced glutamate efflux had not reached maximum after 30 min of ischaemia. Basal (pre-ischaemic) glutamate and aspartate efflux per slice was 94.8 (24.8) and 69.3 (31.5) nmol litre(-1) superfusate (n = 4) and was not significantly reduced by 4% sevoflurane. lschaemia greatly increased glutamate and aspartate efflux (to a maximum of 919 (244)% and 974 (489)% of control, respectively). However, ischaemia-induced efflux of both glutamate and aspartate was significantly reduced by 4% sevoflurane (P < 0.001 for glutamate, P < 0.01 for aspartate). In summary, sevoflurane may owe part of its reported neuroprotective effect to a reduction of ischaemia-induced efflux of excitatory amino acids and, to a lesser extent, dopamine.

Multiple 5-HT(1) autoreceptor subtypes govern serotonin release in dorsal and median raphé nuclei

The present study investigated the possibility of multiple 5-HT(1) autoreceptor subtypes in the rostral raphé nuclei. Slices (350 microm) of rat dorsal or median raphé nucleus (DRN/MRN) were taken from male Wistar rats and superfused with artificial cerebrospinal fluid at 32 degrees C. Fast cyclic voltammetry at carbon fibre microelectrodes was used to monitor serotonin (5-HT) release following local electrical stimulation. In both DRN and MRN, 5-HT release on short trains was reduced by the selective 5-HT(1A) agonist 8-OH-DPAT (1 microM), an effect blocked by the selective 5-HT(1A) antagonist WAY 100635 (0.1 microM) but not by SB 216641 (0.05 and 0.2 microM) or BRL 15572 (0.5 microM), selective antagonists at the 5-HT(1B) and 5-HT(1D) receptors respectively. The selective 5-HT(1B) agonist CP 93129 (0.3 microM) also reduced 5-HT release in both nuclei. Its effect was blocked by SB 216641 but not by WAY 100635 or BRL 15572. The 5-HT(1D/1B) agonist sumatriptan (0.5 microM) decreased 5-HT release in both DRN and MRN. In DRN, the effect of sumatriptan was blocked by BRL 15572 but not by WAY 100635 or SB 216641. In MRN, the effect of sumatriptan was not blocked by any of the above antagonists. BRL 15572 increased 5-HT release on long stimulations in DRN and MRN while WAY 100635 had no effect. SB 216641 increased 5-HT release in MRN but not DRN. WAY 100635 potentiated the effect of SB 216641 in DRN but not MRN. The data suggest that 5-HT release in DRN is controlled by 5-HT(1A), 5-HT(1B) and 5-HT(1D) autoreceptors. 5-HT release in MRN is controlled by 5-HT(1A) and 5-HT(1B) autoreceptors and another, as yet unidentified mechanism.

LY393615, a novel neuronal Ca(2+) and Na(+) channel blocker with neuroprotective effects in models of in vitro and in vivo cerebral ischemia

In the present studies we have examined the effects of a new calcium channel blocker, LY393615 ((N-Butyl-[5,5-bis-(4-fluorophenyl)tetrahydrofuran-2-yl]methylamine hydrochloride, NCC1048) in a model of hypoxia-hypoglycaemia in vitro and in a gerbil model of global and in two rat models of focal cerebral ischaemia in vivo. Results indicated that LY393615 protected against hypoxia-hypoglycaemic insults in brain slices and also provided significant protection against ischaemia-induced hippocampal damage in gerbil global cerebral ischaemia when dosed at 10, 12.5 (P<0.05) or 15 mg/kg i.p. (P<0.01) 30 min before and 2 h 30 min after occlusion. The compound penetrated the brain well after a 15 mg/kg i.p. dose and had a half-life of 2.5 h. In further studies LY393615 was protective 1 h post-occlusion when administered at 15 mg/kg i.p. followed by 2 doses of 5 mg/kg i.p. 2 and 3 h later. LY393615 dosed at 15 mg/kg i.p. followed by 2 further doses of 5 mg/kg i.p. (2 and 3 h later) also produced a significant reduction in the infarct volume following Endothelin-1 (Et-1) middle cerebral artery occlusion in the rat when administration was initiated immediately (P<0.01) or 1 h (P<0.05) after occlusion. The compound was also evaluated in the intraluminal monofilament model of focal ischaemia. The animals had the middle cerebral artery occluded for 2 h, and 15 min after reperfusion LY393615 was administered at 15 mg/kg i.p. followed by 2 mg/kg/h i.v. infusion for 6 h. There was no reduction in infarct volume using this dosing protocol. In conclusion, in the present studies we have reported that a novel calcium channel blocker, LY393615, with good bioavailability protects against neuronal damage caused by hypoxia-hypoglycaemia in vitro and both global and focal cerebral ischaemia in vivo. The compound is neuroprotective when administered post-occlusion and may therefore be a useful anti-ischaemic agent.

Differential recruitment of N-, P- and Q-type voltage-operated calcium channels in striatal dopamine release evoked by 'regular' and 'burst' firing

This study used the peptides omega-conotoxin GVIA, omega-agatoxin IVA and omega-conotoxin MVIIC, singly and in combination, to investigate the relative involvement of N-, P- and Q-type voltage-operated calcium channels in the control of striatal dopamine release. Electrically stimulated dopamine release was measured by fast cyclic voltammetry at carbon fibre microelectrodes in rat striatal slices. The contribution of these channel subtypes was compared in dorsolateral and medial neostriatum for 'regular' (discrete) and 'burst' stimulation modalities. In dorsolateral neostriatum, a role for N-, P- and Q-type channels was demonstrated for discrete stimulations, whilst at least one other unidentified channel was also involved in dopamine release on 'burst' stimulations. Similarly, in the medial axis of the neostriatum, N-, P- and Q-type channels were involved in dopamine release for discrete stimulations, and N-, Q- and at least one other channel type for 'burst' stimulations. However, blockade of P-type channels had no effect on dopamine release for 'burst' stimulations in the medial axis. In both regions and stimulation paradigms, N-type channels played a greater role than P/Q-type channels. In the medial axis of the neostriatum there was a smaller contribution by N- and P-type channels and the unidentified component, but a greater Q-type contribution to DA release. 'Burst' stimulations induced a lesser involvement of N- and P-type channels than discrete stimulations, and a greater role of the unidentified component. In summary, this study suggests that there is heterogeneity in the distribution of functional voltage-operated calcium channel subtypes in the neostriatum, and differences in subtype recruitment for different firing patterns.

Rapid quantification of ischaemic injury and cerebroprotection in brain slices using densitometric assessment of 2,3,5-triphenyltetrazolium chloride staining

2,3,5-Triphenyltetrazolium chloride (TTC), a marker of mitochondrial enzyme activity, is widely used to assess the effects of cerebral ischaemia in vivo. In the present study, we characterised its utility as a simple rapid macrohistological measure of ischaemic damage in brain slices. Coronal rat corticostriatal slices were incubated in oxygenated artificial cerebrospinal fluid (aCSF) until subjected to 'ischaemia' (deoxygenated, hypoglycaemic aCSF) for up to 12 min. After a further 30 min to 16 h of reincubation in oxygenated aCSF, slices were stained with TTC, fixed with formalin and transferred to cover slips. The slices were scanned in 8-bit greyscale using a standard desktop scanner and the staining analysed by densitometry of the acquired images. Control slices stained a rich pink/red. Ischaemia (10 min) reduced both the area and intensity of staining. Both measures of striatal staining were negatively correlated with the duration of ischaemia (0-12 min). Furthermore, staining in the striatum correlated significantly with cortical TTC staining. The effects of TTC concentration (0.063-0.5% w/v) and post-ischaemic interval (30 min to 16 h) were examined upon the intensity of TTC staining. (+)-MK 801 prevented the ischaemia-induced reduction in TTC staining, consistent with cerebroprotection. These data suggest that TTC staining of brain slices may be used to quantify ischaemic injury and cerebroprotection.

Control of dorsal raphé 5-HT function by multiple 5-HT(1) autoreceptors: parallel purposes or pointless plurality?

The serotonergic cells of the dorsal raphé nucleus innervate much of the forebrain and are thought to be involved in the mechanism of action of antidepressants. Dysfunction of these cells might be involved in the neural mechanisms underlying depression and suicide. The traffic in pathways emanating from the dorsal raphé nucleus is controlled by 5-HT(1) autoreceptors. Until recently it was thought that the autoreceptors in the dorsal raphé nucleus were solely of the 5-HT(1A) subtype. In this article, we discuss evidence that the situation is more complex and that multiple 5-HT(1) subtypes govern different aspects of 5-HT function in the dorsal raphé nucleus presenting new therapeutic opportunities.

Spatiotemporal interaction of alpha(2) autoreceptors and noradrenaline transporters in the rat locus coeruleus: implications for volume transmission

We investigated the roles of alpha(2) autoreceptors and noradrenaline (NA) transporters on NA efflux and uptake in the rat locus coeruleus after electrical stimulation. NA efflux was evoked by various trains (50 pulses, 10-500 Hz) and measured by fast cyclic voltammetry. NA efflux and uptake half-time (t(1/2)) were stimulus-dependent, ranging from 43 +/- 3 nM and 2.45 +/- 0.21 s, respectively, with 500-Hz stimuli to 127 +/- 11 nM and 4.41 +/- 0.34 s, respectively, with 100-Hz trains. Based on these data, we calculate that each transporter removes 0.19 NA molecules from the extracellular space every second, a velocity compatible more with transporter-than channel-mode conduction. Dexmedetomidine (10 nM) decreased NA efflux by approximately 30% on stimulations of < or =1 s in duration. BRL 44408 (1 microM) increased NA efflux on stimuli of > or =2 s (by up to 92 +/- 16%). Desipramine (50 nM) increased NA efflux on stimuli of > or =1 s (by 113 +/- 24%) but slowed NA uptake on all stimuli. When given together, the effects of desipramine and BRL 44408 were additive at stimuli of >or =1 s but showed potentiation on shorter trains. There was a significant time delay for the elevation of NA efflux by blockade of uptake (0.79 s) or autoreceptors (1.14 s), suggesting that both are located extrasynaptically and that NA must diffuse through the extracellular space to these structures. We suggest that released NA may interact with alpha(2) autoreceptors and NA transporters as far as 10 microm from the release sites, an action compatible with a volume transmission role of NA in the locus coeruleus.

Propofol decreases stimulated dopamine release in the rat nucleus accumbens by a mechanism independent of dopamine D2, GABAA and NMDA receptors

Although propofol (2,6-di-isopropylphenol) is a popular i.v. general anaesthetic, it has been suggested to have abuse potential. As many drugs of abuse act preferentially via release of dopamine in the limbic system, we investigated the action of propofol on stimulated dopamine release in the rat nucleus accumbens. Nucleus accumbens slices were superfused (1.6 ml min-1) with artificial cerebrospinal fluid at 32 degrees C. Dopamine release was evoked by electrical stimulation (10 pulses, 0.1 ms, 10 mA, 10 Hz, every 10 min) and monitored by fast cyclic voltammetry. Propofol 100 mumol litre-1 reduced stimulated dopamine release over the 2 h after administration, relative to intralipid controls, to mean 30 (SEM 2)% (P < 0.01). The dopamine D2 receptor antagonist metoclopramide 0.3 mumol litre-1 increased dopamine release but did not block the effect of propofol (38 (3)%). The selective GABAA antagonist bicuculline 24 mumol litre-1 also failed to antagonize the action of propofol (45 (3)%). The NMDA receptor antagonist dextromethorphan 10 mumol litre-1 decreased dopamine release to 57 (6)% (P < 0.01) but failed to block the inhibitory effect of propofol (46 (6)%). Although propofol has been reported to bind to D2, GABAA and NMDA receptors, failure of metoclopramide and bicuculline to block its effects suggests that an agonist action at D2 or GABAA receptors does not mediate the effects of propofol on dopamine release in the rat nucleus accumbens. The lack of effect of dextromethorphan makes an NMDA receptor antagonist action unlikely.

Effect of chronic paroxetine treatment on 5-HT1B and 5-HT1D autoreceptors in rat dorsal raphe nucleus

This study reports the effect of chronic paroxetine (10 mg/kg p.o., 21 days) on 5-HT1B and 5-HT1D autoreceptors controlling stimulated 5-HT efflux in slices of rat dorsal raphe nucleus. Electrically evoked 5-HT (10 pulses, 200 Hz, 0.1 ms, 10 mA) was measured using fast cyclic voltammetry. 5-HT efflux was inhibited by CP 93129 (10 nM-10 microM) and by sumatriptan (1 nM-1 microM) agonists at 5-HT1B and 5-HT1D receptors, respectively. Chronic paroxetine did not, initially, appear to alter the sensitivity of the 5-HT1B autoreceptors to CP 93129. However, when constructed in the presence of WAY 100635 (10 nM) the selective and silent 5-HT1A antagonist, there was a significant (P < 0.001) rightward shift of the CP 93129 concentration-response curve in the paroxetine-treated rats but not in the controls, implying a desensitisation of the 5-HT1B autoreceptor by paroxetine. Chronic paroxetine did not affect the sumatriptan concentration-response curve, even with WAY 100635 present, implying that there was no (de)sensitisation of the 5-HT1D autoreceptor. These data suggest that chronic paroxetine treatment may desensitise 5-HT1B autoreceptors in the dorsal raphe nucleus but that this effect is unmasked only when the dominant 5-HT1A autoreceptor control is antagonised.

Effects of dizocilpine (MK 801) on noradrenaline, serotonin and dopamine release and uptake

In the present study, we examined the actions of the NMDA antagonist dizocilpine (MK801) on electrically evoked release and uptake of noradrenaline (NA) in the locus coeruleus (LC), serotonin (5-HT) in the dorsal raphe nucleus (DRN) and dopamine (DA) in the nucleus accumbens (NAc), measured by fast cyclic voltammetry (FCV) in rat brain slices. Dizocilpine (10 microM) significantly increased NA (to 248 +/- 15%) and 5-HT release (to 184 +/- 29%) and slowed monoamine uptake in the LC (t1/2 = 853 +/- 129%) and the DRN (t1/2 = 387 +/- 70%), respectively. However, dizocilpine had no effect on DA release or uptake in NAc. Actions on monoamines are thus likely and should be considered in the interpretation of data regarding dizocilpine.

Effects of tramadol stereoisomers on norepinephrine efflux and uptake in the rat locus coeruleus measured by real time voltammetry

Despite its structural similarity to codeine, tramadol is an unusual analgesic whose antinociceptive efficacy is not solely a result of opioid actions but also of its apparent capacity to block monoamine uptake. Tramadol is a mixture of stereoisomers. In this study, we have examined the actions of racemic, (+)- and (-)-tramadol, in addition to O-desmethyltramadol (the main human metabolite), on electrically evoked norepinephrine efflux and uptake in the locus coeruleus brain slice, measured by fast cyclic voltammetry. Racemic tramadol and its (+)- and (-)-enantiomers (all at 5 mumol litre-1) significantly increased stimulated norepinephrine efflux (P < 0.01) by mean 66 (SEM 10)%, 57 (7)% and 64 (13)%, respectively. However, only (-)-tramadol blocked norepinephrine reuptake (P < 0.01), increasing the reuptake half-time to 499 (63)% of pre-drug values. The metabolite O-desmethyl tramadol was inactive at the concentration tested (5 mumol litre-1). In the case of (-)-tramadol, the effect on norepinephrine efflux was directly proportional to, but significantly smaller than, the effect on norepinephrine uptake (P < 0.01). This appeared to be a result of compensatory alpha 2A autoreceptor tone as the selective alpha 2A autoreceptor antagonist BRL 44408 (1 mumol litre-1) eliminated this difference when its own effects on norepinephrine reuptake were taken into account. The efficacy of (-)-tramadol on norepinephrine uptake, at clinically relevant concentrations, may contribute to its antinociceptive efficacy.

Ascorbic acid is neuroprotective against global ischaemia in striatum but not hippocampus: histological and voltammetric data

Following reports that ascorbic acid (AA) blocks NMDA receptors, we examined its possible neuroprotective properties in vivo (gerbil bilateral carotid artery occlusion model: BCAO) and in vitro (ischaemia-induced dopamine (DA) release in brain slices). Five minutes of BCAO caused substantial cell loss of 90-95% and 40-50% in gerbil CA1 hippocampus and striatum, respectively, measured in haematoxylin and eosin-stained sections, 5 days post-insult. AA (500 mg kg(-1) day(-1) i.p. for 312 days, first dose 1 h before occlusion) significantly (P<0.05) reduced striatal cell loss (from 40 to 13%) while only reducing CA1 cell loss from 95 to 88%. A lower dose (250 mg kg(-1) day(-1) i.p. for 312 days) was ineffective in either region. AA (750 mg kg(-1) day(-1) i.p. for 312 days) caused significant striatal protection (cell loss reduced from 49 to 20%) if treatment was initiated 1 h before occlusion. Initiation of treatment immediately post occlusion did not cause significant protection. Neither treatment regime protected CA1 hippocampus. In separate experiments we examined the effect of AA on DA release, monitored by voltammetry, in an in vitro model of striatal ischaemia. Four DA release variables were measured: T(on)--time from initiation of ischaemia to the onset of DA release, T(pk)--the time from onset of DA release to maximum, deltaDA/deltat--the mean rate of DA release and [DA](max)-- the maximum extracellular DA concentration. Control values in drug-naive slices were: T(on)=193+/-8 s, T(pk) = 24 +/- 4 s, [DA](max) = 69 +/- 6 microM and deltaDA/deltat = 4.2 +/- 0.7 microM s(-1) (means+/-S.E.M., n=15). 212 h pretreatment with AA (0.4 to 10 mM) did not affect T(on) or [DA](max) but increased T(pk) and decreased deltaDA/deltat (P<0.05) with an EC50 of 1.66 mM. NMDA (100 microM) shortened T(on). N-ethylmaleimide (20 microM) had no effect on the response to AA but potentiated the action of NMDA on T(on). AA (2 or 10 mM) had no effect on the response to NMDA. We conclude that AA is neuroprotective against global ischaemia in the striatum and that some of this action may be due to attenuation of ischaemia-induced DA release. This action is mediated neither by blockade of the NMDA receptor nor modulation of its redox status.

Stereospecific effects of ketamine on dopamine efflux and uptake in the rat nucleus accumbens

In addition to being a general anaesthetic, ketamine is a recognized drug of abuse. Many, if not all, drugs of abuse have been shown to increase dopamine efflux in the nucleus accumbens (NAc). As ketamine is optically active, we examined if its actions on dopamine efflux in the NAc were stereoselective. Slices of rat NAc were superfused with artificial CSF at 32 degrees C. Dopamine efflux was evoked by electrical stimulation (1 or 20 pulses, 100 Hz) and measured using fast cyclic voltammetry. (+/-)-Ketamine 100 mumol litre-1 increased dopamine efflux (to mean 174 (SEM 17)% of control, P < 0.05) and slowed dopamine uptake half-time (T1/2) to 164 (17)% of control, as did (+)-ketamine 100 mumol litre-1 (efflux 236 (16)% (P < 0.001); uptake T1/2 177 (25)% (P < 0.05)). The (-)-isomer was inactive. The effect of (+)-ketamine on dopamine efflux did not correlate with its action on dopamine uptake. (+)-Ketamine increased dopamine efflux on single pulse stimulation but to a lesser extent than on 20 pulse trains (P < 0.05). (+)-Ketamine was unable to block the inhibitory effect of quinpirole on single pulse dopamine efflux. Neither MK 801 10 mumol litre-1 nor metoclopramide 1 mumol litre-1 had any effect on dopamine release after short train stimuli (20 pulses, 100 Hz). We conclude that the (+)-isomer is the active form of ketamine and increases NAc dopamine efflux not by block of dopamine uptake; autoreceptors or NMDA receptors, but by mobilization of the dopamine storage pool to releasable sites.

Effects of metabolic alterations on dopamine release in an in vitro model of neostriatal ischaemia

Release of neurotransmitters, including dopamine (DA), plays a central role in neuronal death during cerebral ischaemia. We investigated the effects of changes in energy demand and supply on DA release in cerebral ischaemia in vitro. Rat striatal slices were superfused (400 ml/h) with an artificial cerebrospinal fluid at 34 degrees C, unless otherwise stated. Ischaemia were mimicked by removal of O2 and reduction in glucose concentration from 4 to 2 mM. DA release was monitored by voltammetry. The profile of ischaemia-induced DA release was temperature-dependent. Hypothermia (to 24 degrees C) delayed, slowed, and reduced ischaemia-induced DA release relative to 34 degrees C. Pretreatment of the slices for 3 h with creatine (25 mM) delayed and slowed ischaemia-induced DA release. Conversely, blockade of Na+/K+ ATPase with ouabain induced an anoxic depolarisation and rapid DA release similar to ischaemia. In summary, the onset of DA release in this model is controlled by the balance between energy supply and utilisation. Strategies that increase availability of energy substrates for the membrane sodium pump (i.e., pre-incubation with creatine) or decrease their utilisation (hypothermia) slow and delay DA release. Hypothermia may owe part of its neuroprotective effect to a delay and slowing of ischaemia-induced release of DA and/or other neurotransmitters.

Alpha2A- but not alpha2B/C-adrenoceptors modulate noradrenaline release in rat locus coeruleus: voltammetric data

In this study, we used subtype-selective antagonists to determine the subtype of alpha2-adrenoceptor controlling noradrenaline release in rat locus coeruleus. Noradrenaline release was measured in locus coeruleus slices using fast cyclic voltammetry at carbon fibre microelectrodes. On long stimulation trains (40 pulses, 20 Hz), the alpha2A-adrenoceptor selective antagonist BRL 44408 (2-[2H-(1-methyl-1,3-dihydroisoindole) methyl]-4,5-dihydroimidazole) at 100 nM and 1 microM significantly increased stimulated noradrenaline release, whereas the alpha2B/C-selective antagonist ARC 239 (2-[2[4-(o-methoxyphenyl)piperazin-1-yl] ethyl]-4,4dimethyl-1,3-(2H,4H)-isoquinolinedione) at 50 and 500 nM had no effect. On short stimuli (20 pulses, 200 Hz), the non-specific alpha2-adrenoceptor agonist dexmedetomidine (10 nM) significantly decreased noradrenaline release, an effect reversed by BRL 44408 (1 microM) but not by ARC 239 (500 nM). These data demonstrate that autoreceptor control of noradrenaline release in the locus coeruleus is mediated by alpha2A but not alpha2B/C-adrenoceptors.

Contrasting effects of chronic paroxetine on 5-HT1A control of dorsal raphe cell firing and 5-HT release

To test the role of 5-HT1A receptors in the action of antidepressants, we investigated the effect of chronic paroxetine (10 mg/kg, p.o. for 21 days) on functional assays of 5-HT1A sensitivity. We constructed cumulative concentration response curves to the selective 5-HT1A agonist (+)-8-OH-DPAT on both extracellular recordings of 5-HT neurones and electrically stimulated 5-HT release in dorsal raphe brain slices. Chronic paroxetine desensitized the 5-HT1A receptors controlling firing, with an increase in EC50 from 10.7 nM to 46.2 nM 8-OH-DPAT. Chronic paroxetine did not, however, desensitize the 5-HT1A receptors controlling 5-HT release but increased the 8-OH-DPAT Emax from 54.9% to 79.2% inhibition of 5-HT release. These data suggest that there are either two distinct populations of 5-HT1A receptors or separate second messenger systems, one controlling 5-HT release and another influencing firing. Furthermore chronic paroxetine treatment can differentially modulate these different populations.

Control of pulsatile 5-HT/insulin secretion from single mouse pancreatic islets by intracellular calcium dynamics

1. Glucose-induced insulin release from single islets of Langerhans is pulsatile. We have investigated the correlation between changes in cytosolic free calcium concentration ([Ca2+]i) and oscillatory insulin secretion from single mouse islets, in particular examining the basis for differences in secretory responses to intermediate and high glucose concentrations. Insulin release was monitored in real time through the amperometric detection of the surrogate insulin marker 5-hydroxytryptamine (5-HT) via carbon fibre microelectrodes. The [Ca2+]i was simultaneously recorded by whole-islet fura-2 microfluorometry. 2. In 82 % of the experiments, exposure to 11 mM glucose evoked regular high-frequency (average, 3.4 min-1) synchronous oscillations in amperometric current and [Ca2+]i. In the remaining experiments (18 %), 11 mM glucose induced an oscillatory pattern consisting of high-frequency [Ca2+]i oscillations that were superimposed on low-frequency (average, 0.32 min-1) [Ca2+]i waves. Intermittent high-frequency [Ca2+]i oscillations gave rise to a similar pattern of pulsatile 5-HT release. 3. Raising the glucose concentration from 11 to 20 mM increased the duration of the steady-state [Ca2+]i oscillations without increasing their amplitude. In contrast, both the duration and amplitude of the associated 5-HT transients were increased by glucose stimulation. The amount of 5-HT released per secretion cycle was linearly related to the duration of the underlying [Ca2+]i oscillations in both 11 and 20 mM glucose. The slopes of the straight lines were identical, indicating that there is no significant difference between the ability of calcium oscillations to elicit 5-HT/insulin release in 11 and 20 mM glucose. 4. In situ 5-HT microamperometry has the potential to resolve the high-frequency oscillatory component of the second phase of glucose-induced insulin secretion. This component appears to reflect primarily the duration of the underlying [Ca2+]i oscillations, suggesting that glucose metabolism and/or access to glucose metabolites is not rate limiting to fast pulsatile insulin release.

Characteristics of the NMDA receptor modulating hypoxia/hypoglycaemia-induced rat striatal dopamine release in vitro

We investigated the functional characteristics of the NMDA receptor that modulates hypoxia/hypoglycaemia-induced striatal dopamine release. Dopamine release was detected by fast cyclic voltammetry in rat neostriatal slices. Four variables were measured: T(on) -- time from initiation of hypoxia/hypoglycaemia to the onset of dopamine release, Tpk -- time from onset to maximum, deltaDA/delta(t) -- rate of dopamine release and DAmax -- maximum extracellular dopamine concentration. In controls, T(on) = 164.9 +/- 1.7 s, Tpk = 20.9 +/- 0.9 s, deltaDA/delta(t) = 5.31 +/- 0.44 microM/s and DAmax = 79.1 +/- 2.5 microM (means +/- S.E.M., n = 203). Cis-4-(phosphonomethyl)piperidine-2-carboxylic acid (CGS 19755, 20 microM) lengthened, while N-methyl-D-aspartate (NMDA) (100 microM) shortened T(on). (5R,10S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,1 0-imine hydrogen maleate (MK 801, 1 and 10 microM) and dextromethorphan (10 and 100 microM) increased Tpk and decreased DAmax. Neither glycine (100 microM), 7-chlorokynurenic acid (50 microM) nor 5-nitro-6,7-dichloro-1,4-dihydroquinoxaline-2,3-dione (ACEA 1021, 100 microM) had any effect although 7-chlorokynurenic acid blocked the effect of NMDA. Increasing [Mg2+] from 1.3 to 3.7 mM, increased Tpk and decreased deltaDA/delta(t). Dithiothreitol (1 mM) accelerated T(on) while 5.5-dithio-bis-(2-nitrobenzoic acid) (1 mM) delayed T(on). Neither drug affected Tpk, DAmax or deltaDA/delta(t). Neither spermidine (100 microM) nor arcaine (100 microM) affected T(on), Tpk or deltaDA/delta(t) although arcaine decreased DAmax. In conclusion, hypoxia/hypoglycaemia-induced dopamine release was influenced by an NMDA receptor although modulation of the glycine recognition site of the receptor was ineffective, as were agents acting at polyamine modulatory zones. These findings highlight differences between recombinant and native NMDA receptors and suggest caution in extrapolating molecular biology to functional studies.

Sodium channel blockade unmasks two temporally distinct mechanisms of striatal dopamine release during hypoxia/hypoglycaemia in vitro

Massive striatal dopamine release during cerebral ischaemia has been implicated in the resulting neuronal damage. Sodium influx is an early event in the biochemical cascade during ischaemia and blockade of sodium channels may increase resistance to ischaemia by reducing energy demand involved in compensation for sodium and potassium fluxes. In this study, we have determined the effects of opening and blockade of voltage-gated sodium channels on hypoxia/hypoglycaemia-induced dopamine release. Slices of rat caudate nucleus were maintained in a slice chamber superfused by an oxygenated artificial cerebrospinal fluid containing 4 mM glucose. Ischaemia (hypoxia/hypoglycaemia) was mimicked by a switch to a deoxygenated artificial cerebrospinal fluid containing 2 mM glucose and dopamine release was measured using fast cyclic voltammetry. In drug-free (control) slices, there was a 2-3 min delay after the onset of hypoxia/hypoglycaemia followed by a rapid dopamine release event which was associated with anoxic depolarization. In slices treated with the Na+ channel opener, veratridine (1 microM), the time to onset of dopamine release was shortened (101 +/- 20 s, compared with 171 +/- 8 s in controls, P < 0.05). Conversely, phenytoin (100 microM), lignocaine (200 microM) and the highly selective sodium channel blocker, tetrodotoxin (1 microM) markedly delayed and slowed dopamine release vs paired controls. In the majority of cases, dopamine release was biphasic after sodium channel blockade: a slow phase preceded a more rapid dopamine release event. The latter was associated with anoxic depolarization. Neither the fast nor the slow release events were affected by pretreatment with the selective dopamine uptake blocker GBR 12935 (0.2 microM), suggesting that uptake carrier reversal did not contribute to these events. In conclusion, sodium channel antagonism delays and slows hypoxia/hypoglycaemia-induced dopamine release in vitro. Furthermore, sodium channel blockade delays anoxic depolarization and its associated neurotransmitter release, revealing an earlier dopamine release event that does not result from reversal of the uptake carrier.

Actions of tramadol, its enantiomers and principal metabolite, O-desmethyltramadol, on serotonin (5-HT) efflux and uptake in the rat dorsal raphe nucleus

Tramadol is an atypical centrally acting analgesic agent with relatively weak opioid receptor affinity in comparison with its antinociceptive efficacy. Evidence suggests that block of monoamine uptake may contribute to its analgesic actions. Therefore, we have examined the actions of (+/-)-tramadol, (+)-tramadol, (-)-tramadol and O-desmethyltramadol (M1 metabolite) on electrically evoked 5-HT efflux and uptake in the dorsal raphe nucleus (DRN) brain slice, measured by fast cyclic voltammetry. Racemic tramadol and its (+)-enantiomer (both 5 mumol litre-1) significantly blocked DRN 5-HT uptake (both P < 0.05) and increased stimulated 5-HT efflux (P < 0.01 (+/-)-tramadol; P < 0.05 (+)-tramadol). The (-)-enantiomer and metabolite, O-desmethyltramadol, were inactive at the concentration tested (5 mumol litre-1). For both (+/-)-tramadol and the (+)-enantiomer, the action on 5-HT efflux preceded an effect on 5-HT uptake, suggesting that uptake block was not the cause of the increased 5-HT efflux and that tramadol might therefore have a direct 5-HT releasing action. This activity, at clinically relevant concentrations, may help to explain the antinociceptive efficacy of tramadol despite weak mu opioid receptor affinity and adds to evidence that tramadol exerts actions on central monoaminergic systems that may contribute to its analgesic effect.

Chronic paroxetine desensitises 5-HT1D but not 5-HT1B autoreceptors in rat lateral geniculate nucleus

The present study examined the effect of chronic paroxetine (10 mg/kg p.o., 21 days) on the 5-HT1B and 5-HT1D autoreceptors controlling 5-HT efflux in slices of rat ventrolateral geniculate nucleus. Electrically stimulated 5-HT efflux (10 pulses, 200 Hz, 0.1 ms, 10 mA) was measured using fast cyclic voltammetry. Peak 5-HT efflux was greater (P < 0.01) after chronic paroxetine (22.2 +/- 1.4 nM, mean +/- S.E.M.) than water (15.8 +/- 1.4 nM). 5-HT efflux was inhibited by CP 93129 (1 nM-10 microM) and sumatriptan (1 nM-1 microM), agonists at 5-HT1B and 5-HT1D receptors, respectively. Chronic paroxetine did not affect the sensitivity of the 5-HT1B autoreceptor but shifted the sumatriptan concentration-response curve to the right (P < 0.05). These data suggest that chronic paroxetine increases evoked 5-HT efflux. This may be the result of desensitisation of 5-HT1D but not 5-HT1B autoreceptors.

(+)-WAY 100135, a partial agonist, at native and recombinant 5-HT1B/1D receptors

1. We have studied the effects of the purportedly selective 5-HT1A receptor antagonist (+)-WAY 100135 on electrically stimulated 5-hydroxytryptamine (5-HT) efflux in the ventrolateral geniculate nucleus (vLGN), and its affinity at human 5-HT1B and 5-HT1D receptors stably expressed in Chinese hamster ovary (CHO) cells. 2. On short 'pseudo single pulse' stimulations (20 pulses at 100 Hz, 190 ms train duration), (+)-WAY 100135 (1.0 microM) decreased 5-HT efflux in the vLGN to 68 +/- 8% of pre-drug values (P < 0.01). This decrease could be blocked by the 5-HT1D/1B receptor antagonist GR 127935 (50 nM). Conversely, when long stimulations (20 pulses at 20 Hz, 950 ms train) were used, (+)-WAY 100135 had no effect on 5-HT efflux (84 +/- 8% of pre-drug values) although both methiothepin (200 nM) and GR 127935 (50 nM) caused significant increases (to 175 +/- 18 and 130 +/- 10% of pre-drug values, respectively). 3. Paroxetine (100 nM), the selective 5-HT reuptake inhibitor, increased stimulated 5-HT efflux and reuptake half-life (to 145 +/- 18% and 649 +/- 121%, respectively) on pseudo single pulse stimulations. When (+)-WAY 100135 was added in combination with the uptake blocker, the effect of paroxetine on stimulated 5-HT efflux was potentiated to 282 +/- 48% (P < 0.01) without further effect on the 5-HT reuptake half-life. 4. The affinity and intrinsic activity of (+)-WAY 100135 were determined at recombinant human 5-HT1B and 5-HT1D receptors expressed in CHO cells, by use of radioligand binding and [35S]-GTP gamma S binding (+)-WAY 100135 was a partial agonist at human 5-HT1B and 5-HT1D receptors with moderately high affinity for 5-HT1D receptors (pEC50 = 7.61). 5. In conclusion, (+)-WAY 100135 was found to be not a selective 5-HT1A autoreceptor antagonist but may act as a partial agonist at the 5-HT1B/1D receptor, displaying agonist or antagonist properties depending on the stimulation protocol used and the resultant 5-HT 'tone' at the receptor.

Involvement of N- and P/Q- but not L- or T-type voltage-gated calcium channels in ischaemia-induced striatal dopamine release in vitro

Calcium influx and transmitter efflux are central events in the neuropathological cascade that occurs during and following cerebral ischaemia. This study explored the role of voltage-gated calcium channels (VGCCs) in ischaemia-induced striatal dopamine (DA) release in vitro. Slices (350 microm thickness) of rat neostriatum were superfused (400 ml/h) with an artificial cerebrospinal fluid (aCSF) at 34 degrees C and subjected to episodes of 'ischaemia' by reduction of the glucose concentration from 4 to 2 mM and gassing with 95% N2/5% CO2. DA release was monitored with fast cyclic voltammetry at implanted carbon fibre microelectrodes. The time to onset, time to peak, rate and magnitude of DA release were measured. Non-selective blockade of VGCCs with a high concentration of Ni2+ (2.5 mM), markedly delayed (P < 0.01) and slowed (P < 0.05) DA release but preferential blockade of T-type VGCCs with a lower concentration (200 microM) had no effect. DA release was also unaffected by selective antagonism of L-type VGCCs with nimodipine and nicardipine (10 microM each). Selective blockade of N-type VGCCs with omega-conotoxin GVIA (100 nM) delayed DA release (P < 0.05) but did not affect its rate or magnitude. Blockade of P- and possibly Q-type VGCCs with omega-agatoxin IVA (up to 200 nM) both delayed (P < 0.05) and slowed (P < 0.05) DA release. Preferential blockade of P- type VGCCs with neomycin (500 microM) also delayed (P < 0.05) and slowed (P < 0.05) DA release. These findings suggest that N-, P- and possibly Q- but not L- or T-type VGCCs mediate ischaemia-induced DA release. Although it is not possible to say, on the basis of these results, that the effects are directly upon the dopamine terminals, these calcium channels nevertheless constitute promising targets for therapeutic intervention.

Synergism of 5-HT 1B/D antagonists with paroxetine on serotonin efflux in rat ventral lateral geniculate nucleus slices

Serotonin (5-HT) efflux in rat ventral lateral geniculate nucleus (vLGN) slices was evoked by electrical stimulation (20 pulses at 100 Hz, 10 mA, 190 ms train) and measured, along with 5-HT uptake, by fast cyclic voltammetry at implanted carbon fibre microelectrodes. Paroxetine (100 nM), a selective serotonin reuptake inhibitor (SSRI), increased stimulated 5-HT efflux to 194 +/- 25% of pre-drug values at maximum (mean +/- SEM, n = 5) and the half-life of uptake to 684 +/- 135%. When given alone, neither the selective 5-HT 1B antagonist isamoltane (1 microM) nor the 5-HT 1D/B antagonist GR 127935 (50 nM), affected 5-HT efflux or uptake under this stimulation paradigm. When added in combination with paroxetine, both isamoltane and GR 127935 significantly potentiated the effect of paroxetine on stimulated 5-HT efflux: isamoltane to 302 +/- 48% at maximum (p < 0.05 vs. paroxetine alone), GR 127935 to 318 +/- 95% (p < 0.05 vs. paroxetine alone) of pre-drug values. Neither isamoltane nor GR 127935 had any effect on 5-HT uptake. The selective 5-HT 1A antagonist WAY 100635 (10 nM) had no effect on 5-HT efflux or uptake, alone or in combination with paroxetine. These data suggest that, under these experimental conditions, paroxetine gives rise to tonic activation of the vLGN terminal 5-HT autoreceptors. Furthermore, these data show that 5-HT 1B and possibly 5-HT 1D antagonists block this inhibitory autoreceptor tone and may thus be a useful addition to SSRI treatment in the clinic.

Real time electrochemical detection of 5-HT/insulin secretion from single pancreatic islets: effect of glucose and K+ depolarization

We report a highly sensitive electrochemical approach suitable for the real time measurement of insulin release from single islets of Langerhans, the functional endocrine units in the pancreas. The method is based on the detection of the insulin surrogate 5-hydroxytryptamine (5-HT) by carbon fibre microelectrodes implanted in the islets. Based on the combination of this novel approach with the simultaneous microfluorometric recording of cytosolic free Ca2+ concentration ([Ca2+]i), we demonstrate that glucose-stimulated islets secrete 5-HT/insulin in a pulsatile fashion under physiological conditions, and that this activity is encoded by synchronous [Ca2+]i oscillations. The sensitivity to detect variations in minute amounts of secreted materials is partially conferred by the fact that the tracer is released into a relatively confined space (the intraislet interstitial space).

Doctor in the lab: what is it like for a doctor to work with scientists?

As clinical academic medical departments strive to improve the quality of their research, clinicians and scientists are forced into closer liaison. In many cases, clinical departments now have research laboratories directed by "basic scientists" but often staffed, in part at least, by doctors. To someone who has not worked in one, these laboratories may seem uncompromising and forbidding work environments. This article presents a "case report" written from the viewpoints of the doctor, the scientist, and the professor.

'Real time' measurement of dopamine release in an in vitro model of neostriatal ischaemia

Dopamine (DA) is released in large quantities during ischaemia and may be neurotoxic. For instance, reduction of DA release is associated with a better histological outcome following experimental stroke. In the present study, we report the measurement of transmitter release in an in vitro model of cerebral ischaemia using brain slices. Striatal slices were subjected to 'ischaemia' by reducing the O2 and glucose content of the superfusate in a controlled fashion. The resultant monoamine release, measured in real time by fast cyclic voltammetry at carbon fibre microelectrodes, was shown to be DA by electrochemical criteria. Upon imposition of an ischaemic episode, there was a lag period (159 +/- 2 s, mean +/- S.E.M., n = 99) followed by a sudden release of DA, reaching a maximum extracellular concentration of 95 +/- 4 microM in 25 +/- 2 s. This overall profile of DA release was qualitatively similar to that measured in the striatum in vivo following cardiac arrest. The DA uptake inhibitor GBR 12935 (1.0 microM) had no effect on any DA release variable. We conclude that this model mimics many of the features of cerebral ischaemia in vivo and may be a suitable vehicle for the investigation of neuroprotective drugs.

Rauwolscine potentiates the effect of desipramine on limbic noradrenaline efflux

Desipramine, like most antidepressants, does not exert clinical benefit until 2-3 weeks after the onset of treatment. It has been suggested that this delay might be due to enhanced autoreceptor activation, counteracting the acute uptake blockade. We therefore tested whether autoreceptor blockade might enhance the response to acute uptake blockade, using voltammetry at carbon fibre microelectrodes to monitor stimulated noradrenaline (NA) efflux in rat bed nucleus of stria terminalis brain slices. Desipramine significantly increased NA efflux and slowed NA uptake. The combination of rauwolscine and desipramine increased NA efflux significantly more than desipramine alone. We suggest that alpha 2 autoreceptor blockade functionally mimics alpha 2 autoreceptor down-regulation and thus may allow the full therapeutic effect of desipramine to be manifested more rapidly.

Serotonin efflux in the rat ventral lateral geniculate nucleus assessed by fast cyclic voltammetry is modulated by 5-HT1B and 5-HT1D autoreceptors

Fast cyclic voltammetry (FCV) was used to measure electrically stimulated monoamine efflux in the rat ventral lateral geniculate nucleus (vLGN). The electrochemical characteristics of the released species resembled 5-HT but not dopamine or noradrenaline. Amine efflux was abolished by the sodium channel blocker tetrodotoxin (0.1 microM), Ro 4-1284 (1.0 microM), the fast-acting reserpine analogue, and removal of Ca2+ from the superfusate. Amine efflux was unaffected by the monoamine oxidase inhibitor clorgyline (0.1 microM). Of paroxetine (0.1 microM), desipramine (50 nM) and vanoxerine (0.5 microM), selective blockers of 5-HT, noradrenaline and dopamine uptake respectively, only paroxetine increased monoamine efflux (to 194 +/- 25%, mean +/- SEM) and prolonged the removal half-life (to 638 +/- 105%). The non-specific 5-HT1 antagonist methiothepin (0.2 microM) increased 5-HT efflux on long (20 pulses at 20 Hz) but not short trains (20 pulses at 100 Hz). When tested on pseudo-one-pulse stimulations (5 pulses, 100 Hz), the selective 5-HT1A agonist 8-OHDPAT (1.0 microM) had no effect. CP 93129 (0.3 microM), the selective 5-HT1B agonist, decreased 5-HT efflux to 37 +/- 4% of control and was antagonised by the 5-HT1B blocker isamoltane (0.5 microM) and by the 5-HT1D/B antagonist GR 127935 (50 nM). The preferential 5-HT1D agonist sumatriptan (0.5 microM) also decreased 5-HT efflux, to 55 +/- 6% and was antagonised by GR 127935 (50 nM) but not isamoltane (0.5 microM). These results suggest that 5-HT released in the vLGN can be measured by FCV. Furthermore, released 5-HT is taken up by the 5-HT transporter and may be under the influence of 5-HT1B and 5-HT1D autoreceptors.

Descending control of pain

Thanks largely to the study of the brainstem nuclei that mediate stimulation analgesia, the involvement of the monoamines in the descending control of pain is now well established. The periaqueductal grey, the raphe nuclei (NRM and DRN) and the locus coeruleus are all key brainstem sites for the control of nociceptive transmission in the spinal cord. Although the initial emphasis was on 5-HT as the transmitter mediating this control at spinal levels, it is clear from more recent work that NA has an equally important part to play. How (or even if) the two amines differ in their roles and actions in analgesia is, however, still an open question. The small size and complexity of the brainstem areas from which analgesia may be elicited by electrical stimulation complicates the interpretation of the data. Stimulating currents may spread to surrounding regions mediating opposite effects to that of the main region stimulated. Opiates and GABA are clearly involved in descending control at both brainstem and spinal levels, although the relative roles of the different types of amino-acid and opiate receptors is still hotly debated. Despite the fact that the first report on stimulation analgesia appeared more than a quarter of a century ago in 1969, the precise connections and cord synaptology are still the basis of ongoing research. It is perhaps ironic, in an issue dedicated to new molecules and mechanisms, that those transmitters most involved in descending inhibition should be such old and familiar friends.

The effect of paroxetine on 5-HT efflux in the rat dorsal raphe nucleus is potentiated by both 5-HT1A and 5-HT1B/D receptor antagonists

Serotonin (5-HT) efflux in slices of rat dorsal raphe nucleus (DRN) was evoked by pseudo one pulse electrical stimulation (20 pulses at 100 Hz, 190 ms train duration) and measured, along with 5-HT uptake, by fast cyclic voltammetry (FCV). The selective serotonin re-uptake inhibitor (SSRI) paroxetine (10(-7) M) increased 5-HT efflux to 147 +/- 6% of pre-drug values at maximum (mean +/- SEM, n = 5) and the half-life of uptake to 443 +/- 38%. The non-selective 5-HT1 antagonist methiothepin (2 x 10(-7) M) increased 5-HT efflux to 147 +/- 9% at maximum but had no effect on uptake half-life. In contrast, (+)-WAY 100135 (10(-6) M) and GR 127935 (5 x 10(-8) M), selective antagonists at 5-HT1A and 5-HT1B/D receptors, respectively, affected neither 5-HT efflux nor uptake. When given in combination with paroxetine, the antagonists significantly increased the effect of paroxetine on efflux: methiothepin to 228 +/- 24% (P < 0.001), (+)-WAY 100135 to 212 +/- 31% (P < 0.05) and GR 127935 to 203 +/- 23% (P < 0.01). These data suggest that, under these experimental conditions, DRN 5-HT autoreceptors are tonically activated in the presence of the uptake blocker and that the antagonists act by blocking this counteracting autoinhibitory tone. The data also strongly indicate that 5-HT efflux in the rat DRN is under the control not only of 5-HT1A but also of 5-HT1B/D receptors.

Evidence that 5-hydroxytryptamine release in rat dorsal raphé nucleus is controlled by 5-HT1A, 5-HT1B and 5-HT1D autoreceptors

Electrically stimulated 5-hydroxytryptamine (5-HT) release was monitored in slices of rat dorsal raphé nucleus (DRN) by fast cyclic voltammetry. Pseudo-single pulse stimulations (5 pulses at 100 Hz) were used to enable the effect of various receptor agonists to be seen without competition from endogenously released transmitter. The selective 5-HT1A receptor agonist, (+)-8-OH-DPAT (1.0 microM) decreased stimulated 5-HT release to 31 +/- 3% of controls. This decrease was inhibited by the 5-HT1A receptor antagonists, (+)-WAY-100135 (1.0 microM) and WAY-100635 (0.1 microM) but not by the 5-HT1D/B antagonist, GR127935 (0.05 microM). The selective 5-HT1B receptor agonist, CP-93129 (0.3 microM) decreased stimulated 5-HT release to 61 +/- 4% of control. This effect was antagonized by the 5-HT1B receptor antagonist, isamoltane (0.5 microM) but not by (+)-WAY-100135. The 5-HT1D agonist, sumatriptan (0.5 microM) decreased stimulated 5-HT release to 52 +/- 2% of controls. This decrease was blocked by GR-127935 but not by WAY-100635. These results suggest that 5-HT release in the rat DRN is under the control of 5-HT1A, 5-HT1B and 5-HT1D autoreceptors.

Actions of morphine on noradrenaline efflux in the rat locus coeruleus are mediated via both opioid and alpha 2 adrenoceptor mechanisms

A recent report showed that morphine inhibited [3H]clonidine binding to human platelet alpha 2 receptors. As the analgesic effects of morphine and clonidine are clinically additive, we investigated the possibility that morphine might stimulate alpha 2 receptors or alpha 2 mechanisms in rat locus coeruleus (LC) slices. Stimulated LC noradrenaline efflux was measured by fast cyclic voltammetry. Cumulatively applied morphine 10(-8)-10(-4) mol litre-1 had no effect on noradrenaline efflux evoked by pseudo-single-pulse stimulations (20 pulses at 100 Hz) while the alpha 2 agonist dexmedetomidine 2 x 10(-10)-10(-7) mol litre-1 decreased efflux of noradrenaline in a concentration-dependent manner. Administration of single concentrations of morphine 10(-6)-10(-4) mol litre-1 significantly decreased efflux of noradrenaline (by 22% and 17%, respectively) and attenuated the effect of dexmedetomidine in a concentration-dependent fashion. Morphine 10(-6)-10(-4) mol litre-1 also decreased efflux of noradrenaline on long stimulus trains (50 pulses at 50 Hz). These data suggest that the analgesic potentiation of alpha 2 and opioid agonists is not mediated via LC alpha 2 receptors.

Real-time monitoring of endogenous noradrenaline release in rat brain slices using fast cyclic voltammetry: 3. Selective detection of noradrenaline efflux in the locus coeruleus

Fast cyclic voltammetry (FCV) at carbon fibre microelectrodes was used to monitor 'real time' endogenous noradrenaline (NA) efflux in superfused slices of rat locus coeruleus (LC) following local electrical stimulation. When stimulated with a standard train (30 pulses, 100 Hz, 0.2 ms, 10 mA, every 5 min), efflux of monoamine was constant over the experimental period (2.5 h): Amine efflux declined by only 16 +/- 5% while uptake half-life lengthened by only 9 +/- 8%. When calibrated in solutions of NA, peak amine efflux corresponded to 0.31 +/- 0.04 microM (mean +/- S.E.M., n = 28) and was removed by uptake with a half-life of 2.93 +/- 0.28 s (n = 16). The released compound was confirmed as NA on the basis of pharmacological and electrochemical criteria. Stimulated monoamine efflux was reversibly reduced by 78% by omission of Ca2+ from the superfusate for 30 min (P < 0.05). Ro 4-1284 (1 microM), a fast-acting reserpine-like drug, decreased amine efflux by 86% (P < 0.05). The monoamine oxidase inhibitor pargyline (2 microM) increased efflux by 30% (P < 0.05). Desipramine (0.05 microM), a selective NA uptake blocker, significantly increased amine efflux (by 96%, P < 0.05) and uptake half-life (by 314%, P < 0.05). Fluvoxamine (0.5 microM), the selective serotonin (5HT) uptake blocker, increased efflux by 59% (P < 0.05) and the uptake half-life by 122% (P < 0.05). Vanoxerine (GBR 12909: 0.3 microM), the dopamine (DA) uptake blocker, had no effect on amine efflux or uptake half-life. The voltammogram of the released amine had single oxidation and reduction peaks.(ABSTRACT TRUNCATED AT 250 WORDS)

Simultaneous "real-time" electrochemical and electrophysiological recording in brain slices with a single carbon-fibre microelectrode

Many previous studies have demonstrated the value of carbon-fibre microelectrodes (CFMs) for single-unit activity recording and for fast cyclic voltammetry. In this report we show that these two independent methodologies can be combined at a single CFM and used to study simultaneous electrochemical and electrophysiological events in brain slices. In superfused slices of rat locus coeruleus, dorsal raphe and substantia nigra, we were able to record stable electrophysiological signals and stimulated monoamine efflux for periods of at least 2 h, thereby allowing quantitative pharmacological interventions. The simultaneous recording of amine efflux and unit activity at the same locus facilitates comparison of drug effects at pre- and post-synaptic sites. Furthermore, the system described here uses commercially available instrumentation. The circuitry is described and examples of its application are shown.

Actions of the hypnotic anaesthetic, dexmedetomidine, on noradrenaline release and cell firing in rat locus coeruleus slices

We have examined the effects of the highly selective alpha 2-adrenoceptor agonist, dexmedetomidine, on noradrenaline release and cell firing in isolated, superfused slices of rat locus coeruleus. Dexmedetomidine decreased both noradrenaline release and cell firing rate in a concentration-dependent fashion, with an EC50 of 3.97 (SEM 0.97) x 10(-9) mol litre-1 for noradrenaline release and 0.92 (0.53) x 10(-9) mol litre-1 for unit activity. Both effects were reversed completely by the selective alpha 2 antagonist, atipamezole 10(-6) mol litre-1. These results suggest that cell firing and noradrenaline release are under alpha 2 receptor control and that dexmedetomidine potently stimulates these receptors. We conclude that these effects are consistent with the locus coeruleus being a major site of action of the hypnotic anaesthetic alpha 2 agonists.

Early age-dependent changes in noradrenaline efflux in the bed nucleus of stria terminalis: voltammetric data in rat brain slices

Fast cyclic voltammetry (FCV) was applied to the detection of stimulated noradrenaline (NA) efflux in the bed nucleus of stria terminalis (BSTV) of the superfused rat brain slice from rats of different ages. Three age groups were compared: young (5 weeks; 130 +/- 8 g), adult (12 weeks; 308 +/- 12 g), and mature rats (36 weeks; 575 +/- 21 g). The effect of train duration (20-99 pulses at 100 Hz) and frequency of stimulation (50 pulses at 10-500 Hz) on NA efflux were examined. The effect of the alpha 2 antagonist yohimbine (1 microM) was also investigated. NA efflux was significantly (p < 0.05) greater in young than adult or mature rats on all train durations and stimulus frequencies tested: Maximal NA efflux on the longest train (99 pulses, 100 Hz, 10 mA, 0.2 ms) was 423 +/- 48 nM (young), 135 +/- 24 nM (adult), and 155 +/- 26 nM (mature). There were no significant differences between adult and mature rats. Yohimbine (1 microM) elevated NA efflux to a greater extent at lower (10 & 20 Hz) than higher frequencies in all age groups. Yohimbine also potentiated NA efflux more (p < 0.05) in young and adult rats than in mature animals. There were no significant differences between young and adult rats. The results indicate early maturational differences in both NA efflux and its control by alpha 2 adrenoceptors. Interestingly, the decreases in NA efflux and in yohimbine response were not temporally linked.

Neurochemical evidence of functional A10 dopamine terminals innervating the ventromedial axis of the neostriatum: in vitro voltammetric data in rat brain slices

The neostriatum (CPu) and nucleus accumbens (NAc) receive their primary dopamine (DA) afferents from the A9 and A10 cell groups, respectively. Anatomical evidence has, however, shown that some clusters of A10 afferents innervate the medial (periventricular) aspect of the CPu14. The present study used fast cyclic voltammetry (FCV) at carbon fibre microelectrodes to locate and measure the size of these clusters on the basis of diagnostic differences in DA efflux in A9 and A10 terminal regions. All experiments were conducted in CPu or NAc slices superfused with oxygenated artificial CSF at 32 degrees C. Carbon fibre microelectrodes were placed 80 microns below the slice surface and bipolar stimulating electrodes were located 200 microns away. Except in experiments where the stimulation frequency, pulse width or number of pulses were investigated, DA efflux was evoked using 0.1 ms, 10 mA pulses applied singly (1p) or in 20 pulse trains (20p) at 50 Hz and monitored using FCV. The CPu was first mapped on the basis of the ratio of 20p:1p DA efflux. The CPu consisted mainly (64%) of low ratio (< 3) sites while the NAc core comprised exclusively high ratio (> 6) loci. Population analysis revealed a small percentage (10%) of striatal sites with high (> 6) ratios. These high ratio sites matched the reported distribution of A10 afferent clusters, being found almost entirely along the ventromedial axis of the CPu. Individual clusters of high ratio sites (20p:1p ratios > 6), 'mapped' on the basis of evoked 20p DA efflux, were found to be irregular in outline and around 500 microns across. In order to characterise the clusters further, the influence of stimulation frequency, train duration (number of pulses) and pulse width on DA efflux were examined. Peak DA efflux in the clusters and NAc occurred at 50 Hz while the striatal matrix had a flat frequency response. Both clusters and NAc showed a similar dependence of DA efflux on the number of pulses in the stimulus train. In the CPu matrix, DA efflux was less dependent on the number of pulses. In the striatal matrix, increasing the stimulation pulse width enhanced DA efflux on trains more than on single pulses while, in both NAc and striatal clusters, there was no preferential effect on trains. The medial location of these clusters within the CPu more closely matches the anatomical distribution of the A10 afferents of Gerfen et al. than the more evenly dispersed striosomes.(ABSTRACT TRUNCATED AT 400 WORDS)

Real-time monitoring of endogenous noradrenaline release in rat brain slices using fast cyclic voltammetry. 2. Operational characteristics of the alpha 2 autoreceptor in the bed nucleus of stria terminalis, pars ventralis

Fast cyclic voltammetry (FCV) at carbon fibre microelectrodes was used to monitor stimulated noradrenaline (NA) efflux in slices of the ventral part of the rat bed nucleus of stria terminalis (BSTV) superfused with artificial cerebrospinal fluid at 32 degrees C. NA efflux was evoked by local electrical stimulation (trains of 10-50 pulses, 0.2 ms duration, 10 mA constant current at 10-500 Hz). The effects of four alpha 2 antagonists (yohimbine, rauwolscine, prazosin and WB 4101) and three alpha 2 agonists (clonidine, oxymetazoline and UK 14304) were examined. All drugs (1 microM) were added via the superfusate. Yohimbine and rauwolscine increased NA efflux on the lower but not the higher frequency trains: maximum increases (on 20 Hz, 50 pulse stimulation) were to 392 +/- 63% (yohimbine) and 243 +/- 7% (rauwolscine). There was a threshold train duration for demonstration of autoreceptor antagonism of 500-1000 ms. Prazosin and WB 4101 did not increase NA efflux but caused a modest decrease at the higher (100-500 Hz) frequencies. The effects of the alpha 2 agonists were also affected by stimulus train duration. Longer trains reduced agonist (clonidine) effects. When tested on pseudo-one pulse (POP) stimulations (less than 100 ms duration), the alpha 2 agonists decreased NA efflux. UK 14304 reduced NA efflux on 20 pulse/200 Hz stimulation to a greater degree (86 +/- 7%) than the partial agonists clonidine (39 +/- 3%) or oxymetazoline (40 +/- 8%). The present results demonstrate that alpha 2 autoreceptors are a major mechanism in the control of NA efflux in the BSTV.(ABSTRACT TRUNCATED AT 250 WORDS)

Activity of two primary human metabolites of nomifensine on stimulated efflux and uptake of dopamine in the striatum: in vitro voltammetric data in slices of rat brain

Several antidepressant drugs have active metabolites. This study sought to establish whether two of the main human metabolites of nomifensine (M2: 8-amino-2-methyl-4-(3-methoxy-4-hydroxyphenyl)-1,2,3,4- tetrahydroisoquinoline and M3: 8-amino-2-methyl-4-(3-hydroxy-4-methoxyphenyl)-1,2,3,4- tetrahydroisoquinoline) had actions on the release and uptake of dopamine (DA). Experiments were conducted in superfused striatal slices of the rat. The efflux of DA was evoked by single constant-current pulses (0.1 msec, 10 mA) and trains (20 pulses, 50 Hz), applied alternately every 10 min and monitored using fast cyclic voltammetry at carbon fibre microelectrodes. Nomifensine (5 x 10(-7) M) significantly increased the efflux of DA on both single pulse (302% of pre-drug) and train stimuli (529%) and increased the uptake half-life (178% of pre-drug). The M2 metabolite had similar potency on the efflux of DA (260%: pulse, 570%: train) but without any effect on uptake of DA. Nomifensine and M2 increased efflux of DA more on trains than on single pulses. The M3 metabolite (5 x 10(-7) M) increased efflux of DA only moderately. The selective blocker of the uptake of DA, GBR 12909 (3 x 10(-7) M), increased efflux of DA on single pulse (430%) and train stimuli (645%) and blocked uptake of DA (t1/2: 292%). Amfonelic acid, the psychomotor stimulant (10(-7) M) blocked uptake of DA (t1/2: 234%) and elevated efflux of DA to a greater extent on trains (1007%) than on single pulses (495%).(ABSTRACT TRUNCATED AT 250 WORDS)

Real-time monitoring of endogenous noradrenaline release in rat brain slices using fast cyclic voltammetry: 1. Characterisation of evoked noradrenaline efflux and uptake from nerve terminals in the bed nucleus of stria terminalis, pars ventralis

Fast cyclic voltammetry (FCV) at carbon fibre microelectrodes was used to monitor endogenous noradrenaline (NA) efflux in superfused slices of bed nucleus of stria terminalis pars ventralis (BSTV) in 'real time'. NA efflux was evoked by local electrical stimulation at bipolar tungsten stimulating electrodes. Confirmation of the identity of the released species as NA was made on the basis of anatomical, electrochemical and pharmacological proofs. Firstly, the signal matched the NA innervation density; efflux of monoamine was greater in BSTV than in the pars dorsalis of the nucleus. Secondly, the voltammogram of the released species was indistinguishable from those of the catecholamines NA and dopamine (DA) but dissimilar to that of the indoleamine serotonin (5-hydroxytryptamine, 5-HT). Thirdly, amine efflux was influenced in a predictable fashion by the drugs tested. Tetrodotoxin (10(-6) M) or omission of Ca2+ from the superfusate reversibly reduced amine efflux by 90.2 and 88.0% respectively. Ro 4-1284 (10(-6) M) decreased amine efflux by 75.8%. Desipramine (5 x 10(-8) M), the selective NA uptake blocker, significantly increased amine efflux and uptake half-life (to 214.3 and 389.5% of control respectively). Fluvoxamine (5 x 10(-7) M) and GBR 12909 (3 x 10(-7) M), blockers of 5-HT and DA uptake respectively, had no effect on amine efflux, although fluvoxamine caused a modest (91.0%) increase in the uptake half-life. Pargyline (2 x 10(-6) M) affected neither efflux nor uptake. The combined anatomical, electrochemical and pharmacological data confirm that the monoamine detected in BSTV by local electrical stimulation was NA. Stimulated NA efflux was stable and reproducible over at least 2.5 h (longest period tested). This study demonstrates the ability of FCV to selectively monitor endogenous NA efflux and uptake in 'real time' and with high spatial resolution.

Voltammetric evidence that subsensitivity to reward following chronic mild stress is associated with increased release of mesolimbic dopamine

Chronic exposure to mild unpredictable stress caused a decrease in rats' consumption of a palatable weak sucrose solution, which was reversed by chronic (5 weeks) administration of imipramine (5 mg/kg/day). Dopamine (DA) release in the nucleus accumbens (NAc) and caudate putamen (CPu) was measured in vivo using fast cyclic voltammetry, following electrical stimulation of the medial forebrain bundle. Experiments were performed under chloral hydrate anaesthesia 48 h after the termination of stress and the final imipramine injection. DA release was increased in the NAc of both stressed and imipramine-treated animals; imipramine did not normalize the increased DA release in stressed animals. In a further experiment, brain slices from stressed animals tended to be subsensitive to the inhibition of DA release in the NAc by quinpirole. No changes were observed in the CPu in any experiment. We discuss the relationship of these effects to stress-induced anhedonia.