Antipsychotic drugs reverse the AMPA receptor-stimulated release of 5-HT in the medial prefrontal cortex

The prefrontal cortex (PFC) is involved in the pathophysiology of schizophrenia. PFC neuronal activity is modulated by monoaminergic receptors for which antipsychotic drugs display moderate-high affinity, such as 5-HT(2A) and alpha(1)-adrenoceptors. Conversely, PFC pyramidal neurons project to and modulate the activity of raphe serotonergic neurons and serotonin (5-HT) release. Under the working hypothesis that atypical antipsychotic drugs may partly exert their action in PFC, we assessed their action on the in vivo 5-HT release evoked by increasing glutamatergic transmission in rat medial PFC (mPFC). This was achieved by applying S-AMPA in mPFC (reverse dialysis) or by disinhibiting thalamic excitatory afferents to mPFC with bicuculline. The application of haloperidol, chlorpromazine, clozapine and olanzapine in mPFC by reverse dialysis (but not reboxetine or diazepam) reversed the S-AMPA-evoked local 5-HT release. Likewise, the local (in mPFC) or systemic administration of these antipsychotic drugs reversed the increased prefrontal 5-HT release produced by thalamic disinhibition. These effects were shared by the 5-HT(2A) receptor antagonist M100907 and the alpha(1)-adrenoceptor antagonist prazosin. However, raclopride (DA D2 antagonist) had very modest effects. These results suggest that, besides their action in limbic striatum, antipsychotic drugs may attenuate glutamatergic transmission in PFC, possibly by interacting with 5-HT(2A) and/or alpha(1)-adrenoceptors.     

Region-dependent regulation of mesoaccumbens dopamine neurons in vivo by the constitutive activity of central serotonin2C receptors

Central serotonin2C receptors (5-HT(2C)Rs) control the mesoaccumbens dopamine (DA) pathway. This control involves the constitutive activity (CA) of 5-HT(2C)Rs, and is thought to engage regionally distinct populations of 5-HT(2C)Rs, leading to opposite functional effects. Here, using in vivo microdialysis in halothane-anesthetized rats, we investigated the relative contribution of ventral tegmental area (VTA) and nucleus accumbens shell (NAc) 5-HT(2C)Rs in the phasic/tonic control of accumbal DA release, to specifically identify the nature (inhibition/excitation) of the control, and the role of the 5-HT(2C)R CA. Intra-VTA injections of the selective 5-HT(2C)R antagonists SB 242084 and/or SB 243213 (0.1-0.5 microg/0.2 microL) prevented the decrease in accumbal DA outflow induced by the 5-HT(2C)R agonist Ro 60-0175 (3 mg/kg, i.p), but did not affect the increase in DA outflow induced by the 5-HT(2C)R inverse agonist SB 206553 (5 mg/kg, i.p). Intra-NAc infusions of SB 242084 (0.1-1 microM) blocked Ro 60-0175- and SB 206553-induced changes of DA outflow. Intra-NAc, but not intra-VTA administration of SB 206553 increased basal DA outflow. These findings demonstrate that both VTA and NAc 5-HT(2C)Rs participate in the inhibitory control exerted by 5-HT(2C)Rs on accumbal DA release, and that the NAc shell may represent a primary action site for the CA of 5-HT(2C)Rs.     

Serotonin-2C antagonism augments the effect of citalopram on serotonin and dopamine levels in the ventral tegmental area and nucleus accumbens

Many patients with major depression do not respond to selective serotonin reuptake inhibitors (SSRIs). Lack of response could be due to inhibition of dopamine (DA) release by serotonin (5-HT) through 5-HT_2C receptors. Combining an SSRI with a 5-HT_2C antagonist may result in improved efficacy by causing simultaneous increases of 5-HT and DA. In order to test this augmentation strategy, male Wistar rats were treated (s.c.) with an acute dose of the SSRI citalopram (Cit, 5 mg/kg), the 5-HT_2C antagonist SB 242084 (SB, 2 mg/kg), or Cit + SB, and the effect on 5-HT and DA release in the nucleus accumbens (NAcc) was assessed by microdialysis. In a separate experiment, animals were treated with vehicle, Cit (20 mg/kg/d), SB (2 mg/kg/d) or Cit + SB for a period of 2 days (s.c.), and the impact on the release of 5-HT and DA in the ventral tegmental area (VTA) and NAcc was studied. On the day of microdialysis, 5-HT_2C receptor sensitivity was assessed with an SB challenge. Acutely administered Cit + SB increased 5-HT release in the NAcc more than Cit alone. SB alone increased DA release in the NAcc (not in the VTA), but when administered together with Cit, this effect was abolished. A 2-day treatment with Cit or Cit + SB increased 5-HT release in both VTA and NAcc. Combining Cit with SB augmented the effect of Cit in the VTA. DA release in VTA and NAcc was only significantly increased after 2-days of treatment with Cit + SB. In conclusion, Cit + SB had synergistic effects on 5-HT and DA release after 2-days of treatment, probably related to a decreased tonic inhibition of DA release via 5-HT_2C receptors. Regional differences occur and future studies should elucidate if this augmentation strategy is beneficial at the behavioral level.     

5-HT(2A) and D(2) receptor blockade increases cortical DA release via 5-HT(1A) receptor activation: a possible mechanism of atypical antipsychotic-induced cortical dopamine release

Atypical antipsychotic drugs (APDs), all of which are relatively more potent as serotonin (5-HT)(2A) than dopamine D(2) antagonists, may improve negative symptoms and cognitive dysfunction in schizophrenia, in part, via increasing cortical dopamine release. 5-HT(1A) agonism has been also suggested to contribute to the ability to increase cortical dopamine release. The present study tested the hypothesis that clozapine, olanzapine, risperidone, and perhaps other atypical APDs, increase dopamine release in rat medial prefrontal cortex (mPFC) via 5-HT(1A) receptor activation, as a result of the blockade of 5-HT(2A) and D(2) receptors. M100907 (0.1 mg/kg), a 5-HT(2A) antagonist, significantly increased the ability of both S:(-)-sulpiride (10 mg/kg), a D(2) antagonist devoid of 5-HT(1A) affinity, and R:(+)-8-OH-DPAT (0.05 mg/kg), a 5-HT(1A) agonist, to increase mPFC dopamine release. These effects of M100907 were abolished by WAY100635 (0.05 mg/kg), a 5-HT(1A) antagonist, which by itself has no effect on mPFC dopamine release. WAY100635 (0.2 mg/kg) also reversed the ability of clozapine (20 mg/kg), olanzapine (1 mg/kg), risperidone (1 mg/kg), and the R:(+)-8-OH-DPAT (0.2 mg/kg) to increase mPFC dopamine release. Clozapine is a direct acting 5-HT(1A) partial agonist, whereas olanzapine and risperidone are not. These results suggest that the atypical APDs via 5-HT(2A) and D(2) receptor blockade, regardless of intrinsic 5-HT(1A) affinity, may promote the ability of 5-HT(1A) receptor stimulation to increase mPFC DA release, and provide additional evidence that coadministration of 5-HT(2A) antagonists and typical APDs, which are D(2) antagonists, may facilitate 5-HT(1A) agonist activity.     

The mGlu2/3 receptor agonist, LY354740, blocks immobilization-induced increases in noradrenaline and dopamine release in the rat medial prefrontal cortex

The metabotropic glutamate (mGlu2/3) receptor agonist, LY354740, exhibits anxiolytic-like properties in a number of rodent models. The present study utilized in vivo microdialysis to examine the effects of LY354740 on extracellular monoamine levels in the medial prefrontal cortex (mPFC) of animals subjected to 30 min immobilization stress. Immobilization stress significantly elevated extracellular levels of noradrenaline (NA) and dopamine (DA) in the mPFC, while systemic administration of LY354740 (30 mg/kg, s.c.) significantly attenuated immobilization-induced increases in both NA and DA. Reverse-dialysis of LY354740 (30 microm) into the mPFC significantly attenuated immobilization-induced increases in NA, but not DA without affecting basal levels of either amine. In separate studies in the presence of citalopram (1 microm; reverse dialysis into the mPFC), systemic administration of LY354740 attenuated immobilization-induced increases in NA and DA, but had no effect on serotonin (5-HT) levels. Co-administration of the selective mGlu2/3 receptor antagonist, LY341495, partially or fully reversed the attenuation in NA and DA levels produced by LY354740, respectively. Taken together, these data suggest that LY354740 may produce anti-stress actions, in part, by blocking stress-related increases in catecholamines in the mPFC via mGlu2/3 receptor stimulation.     

Stimulation by neurotensin of dopamine and 5-hydroxytryptamine (5-HT) release from rat prefrontal cortex: possible role of NTR1 receptors in neuropsychiatric disorders

The modulation of cortical dopaminergic and serotonergic neurotransmissions by neurotensin (NT) was studied by measuring the release of dopamine (DA) and 5-hydroxytryptamine (5-HT) from the prefrontal cortex (PFC) of freely moving rats. The samples were collected via transversal microdialysis. Dopamine and 5-HT levels in the dialysate were measured using high-performance liquid chromatography (HPLC) with an electrochemical detector. Local administration of neurotensin (1microM or 0.1microM) in the PFC via the dialysis probe produced significant, long-lasting, and concentration-dependent increase in the extracellular release of DA and 5-HT. The increase produced by 1microM neurotensin reached a maximum of about 210% for DA and 340% for 5-HT. A high-affinity selective neurotensin receptor (NTR1) antagonist {2-[(1-(7-chloro-4-quinolinyl)-5-(2,6-dimethoxyphenyl)pyrazol-3yl)carbonylamino tricyclo (3.3.1.1.(3.7)) decan-2-carboxylic acid} (SR 48692), perfused locally at a concentration of 0.1microM and 0.5microM in the PFC antagonized the effects of 1microM neurotensin. Our in vivo neurochemical results indicate, for the first time, that neurotensin is able to regulate cortical dopaminergic and serotonergic neuronal activity in freely moving rats. These effects are possibly mediated by interactions of neurotensin with neurons releasing DA or 5-HT, projecting to the PFC from the ventrotegmental area (VTA) and from the dorsal raphe nuclei (DRN), respectively. The potentiating effects of neurotensin on DA and 5-HT release in the PFC are regulated by NTR1 receptors, probably located on dopaminergic and serotonergic nerve terminals or axons.     

Target-specific glutamatergic regulation of dopamine neurons in the ventral tegmental area

Dopamine (DA) neurons in the ventral tegmental area (VTA) are thought to play a critical role in affective, motivational, and cognitive functioning. There are fundamental target-specific differences in the functional characteristics of subsets of these neurons. For example, DA afferents to the prefrontal cortex (PFC) have a higher firing and transmitter turnover rate and are more responsive to some pharmacological and environmental stimuli than DA projections to the nucleus accumbens (NAc). These functional differences may be attributed in part to differences in tonic regulation by glutamate. The present study provides evidence for this mechanism: In freely moving animals, blockade of basal glutamatergic activity in the VTA by the selective alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)/kainate antagonist LY293558 produced an increase in DA release in the NAc while significantly decreasing DA release in the PFC. These data support an AMPA receptor-mediated tonic inhibitory regulation of mesoaccumbens neurons and a tonic excitatory regulation of mesoprefrontal DA neurons. This differential regulation may result in target-specific effects on the basal output of DA neurons and on the regulatory influence of voltage-gated NMDA receptors in response to phasic activation by behaviorally relevant stimuli.     

Role of striatal serotonin2A and serotonin2C receptor subtypes in the control of in vivo dopamine outflow in the rat striatum

This study investigated, using in vivo microdialysis in the striatum of freely moving rats, the role of striatal serotonin2A (5-HT2A) and 5-HT2C receptor subtypes in the modulation of dopamine (DA) and 3, 4-dihydroxyphenylacetic acid (DOPAC) outflow, both in basal conditions and under activation induced by subcutaneous administration of 0.01 mg/kg haloperidol. The different 5-HT2 agents used were applied intrastriatally at a 1 microM concentration through the microdialysis probe. Basal DA efflux was enhanced (27%) by the 5-HT2A/2B/2C agonist 1-(4-iodo-2,5-dimethoxyphenyl)-2-aminopropane (DOI) and reduced (-30%) by the 5-HT2B/2C antagonist SB 206553. It was unaffected by infusion of the 5-HT2A antagonist SR 46349B. The effect of DOI was abolished by SB 206553 but not modified by SR 46349B. Haloperidol-stimulated DA efflux (65-70%) was reduced by both SR 46349B (-32%) and the 5-HT2A/2B/2C antagonist ritanserin (-30%) but not affected by SB 206553. Conversely, the effect of haloperidol was potentiated (22%) when DOI was coperfused with SB 206553. Also, haloperidol-stimulated DOPAC outflow (40-45%) was reduced (-20%) by SR 46349B and potentiated (25%) by the combination of SB 206553 with DOI. These results indicate that striatal 5-HT2A receptors, probably through activation of DA synthesis, positively modulate DA outflow only under activated conditions. In contrast, striatal 5-HT2C receptors exert a facilitatory control on basal DA efflux, which appears to be both tonic and phasic.     

Modulation of medical prefrontal cortical D1 receptors on the excitatory firing activity of nucleus accumbens neurons elicited by (-)-Stepholidine

(-)-Stepholidine (SPD), with D1 agonistic action, elicited an excitatory firing activity of nucleus accumbens (NAc) neurons by intravenous administration, but this effect was hardly observed by iontophoresis of SPD into the NAc. The present study intends to determine whether D1 receptors in the medial prefrontal cortex (mPFC) are involved in the action of SPD on the firing activity of NAc neurons in the chloral hydrate-anesthetized male rats. The results showed that the intra-mPFC microinjected SCH-23390 (D1 antagonist, 30 mM), but not the D2 antagonist spiperone (30 mM), significantly attenuated the enhanced firing activity induced by intravenous injection of SPD (2 mg/kg). Similarly, the excitatory firing of NAc neurons was also exhibited by the microinjection of either SPD or D1 agonist SKF-38393 into the mPFC. The SPD-induced excitatory effect was in a dose-dependent way from 277.8 +/- 51.3% (10 mM) to 1105.4 +/- 283.5% (30 mM) of NAc basal firing, which was completely reversed by SCH-23390 (i.v.). Furthermore, the direct D1 agonistic action of SPD on the mPFC neuron was observed with microiontophoresis. These results indicate that SPD possesses a direct agonistic action on the mPFC D1 receptors, by which it modulates the firing activity of NAc neurons.     

Peri-Pubertal Emergence of UNC-5 Homologue Expression by Dopamine Neurons in Rodents

Puberty is a critical period in mesocorticolimbic dopamine (DA) system development, particularly for the medial prefrontal cortex (mPFC) projection which achieves maturity in early adulthood. The guidance cue netrin-1 organizes neuronal networks by attracting or repelling cellular processes through DCC (deleted in colorectal cancer) and UNC-5 homologue (UNC5H) receptors, respectively. We have shown that variations in netrin-1 receptor levels lead to selective reorganization of mPFC DA circuitry, and changes in DA-related behaviors, in transgenic mice and in rats. Significantly, these effects are only observed after puberty, suggesting that netrin-1 mediated effects on DA systems vary across development. Here we report on the normal expression of DCC and UNC5H in the ventral tegmental area (VTA) by DA neurons from embryonic life to adulthood, in both mice and rats. We show a dramatic and enduring pubertal change in the ratio of DCC:UNC5H receptors, reflecting a shift toward predominant UNC5H function. This shift in DCC:UNC5H ratio coincides with the pubertal emergence of UNC5H expression by VTA DA neurons. Although the distribution of DCC and UNC5H by VTA DA neurons changes during puberty, the pattern of netrin-1 immunoreactivity in these cells does not. Together, our findings suggest that DCC:UNC5H ratios in DA neurons at critical periods may have important consequences for the organization and function of mesocorticolimbic DA systems.     

Serotonin-GABA interactions modulate MDMA-induced mesolimbic dopamine release

3,4,-Methylenedioxymethamphetamine (MDMA; 'ecstasy') acts at monoamine nerve terminals to alter the release and re-uptake of dopamine and 5-HT. The present study used microdialysis in awake rats to measure MDMA-induced changes in extracellular GABA in the ventral tegmental area (VTA), simultaneous with measures of extracellular dopamine (DA) in the nucleus accumbens (NAC) shell. (+)-MDMA (0, 2.5, 5 and 10 mg/kg, i.p.) increased GABA efflux in the VTA with a bell-shaped dose-response. This increase was blocked by application of TTX through the VTA probe. MDMA (5 mg/kg) increased 5-HT efflux in VTA by 1037% (p < 0.05). The local perfusion of the 5-HT(2B/2C) antagonist SB 206553 into the VTA reduced VTA GABA efflux after MDMA from a maximum of 229% to a maximum of 126% of basal values (p < 0.05), while having no effect on basal extracellular GABA concentrations. DA concentrations measured simultaneously in the NAC shell were increased from a maximum of 486% to 1320% (p < 0.05). The selective DA releaser d-amphetamine (AMPH) (4 mg/kg) also increased VTA GABA efflux (180%), did not alter 5-HT and increased NAC DA (875%) (p < 0.05), but the perfusion of SB 206553 into the VTA failed to alter these effects. These results suggest that MDMA-mediated increases in DA within the NAC shell are dampened by increases in VTA GABA subsequent to activation of 5-HT(2B/2C) receptors in the VTA.     

The 5-HT(1A) receptor agonist 8-OH-DPAT prevents prefrontocortical glutamate and serotonin release in response to blockade of cortical NMDA receptors

We studied the role of 5-HT(1A) receptors in controlling the release of glutamate (GLU) in the medial prefrontal cortex (mPFC) of conscious rats with the in vivo microdialysis technique. The effect of the 5-HT(1A) receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin infused in the prefrontal cortex was examined under basal conditions and on the rise of extracellular GLU (+106%) induced by co-infusion of the competitive N-methyl-d-aspartate receptor antagonist 3-[(R)-2-carboxypiperazin-4yl]-propyl-1-phosphonic acid (CPP). 8-OH-DPAT (0.3 and 3 microm) had no effect on basal extracellular GLU, but the higher concentration completely abolished the rise of extracellular GLU induced by CPP. CPP also increased extracellular serotonin (5-HT) in the mPFC (+50%) and this effect was antagonized by 3 microm 8-OH-DPAT which, by itself, had no effect on basal 5-HT release. The effects of 8-OH-DPAT on extracellular GLU and 5-HT were reversed by the 5-HT(1A) receptor antagonist WAY100 635 (100 microm), indicating a selective involvement of 5-HT(1A) receptors. WAY100 635 had no effect by itself. These results show that the stimulation of cortical 5-HT(1A) receptors prevents the CPP-evoked rise of extracellular GLU and 5-HT and suggest that these effects may contribute to the ability of intracortical 8-OH-DPAT to counteract cognitive deficits caused by the blockade of NMDA receptors.     

Monoamine Interactions Measured by Microdialysis in the Ventral Tegmental Area of Rats Treated Systemically with (±)-8-Hydroxy-2-(Di-n-Propylamino)tetralin

Abstract: The effect of (±)-8-hydroxy-2-(di- n -propylamino)tetralin (8-OH-DPAT), a selective serotonin 5-HT_1A agonist, on levels of extracellular norepinephrine (NE), dopamine (DA), and 5-HT (measured simultaneously) was investigated by microdialysis in the ventral tegmental area (VTA) of freely moving rats, and their behavioral activity was monitored. At 50 µg/kg s.c., 8-OH-DPAT reduced 5-HT levels but enhanced NE and DA levels in VTA dialysate. These effects were not altered by pretreatment with systemic idazoxan (5 mg/kg i.p.), a selective α₂ antagonist, or local sulpiride (10 µ M ), a selective D₂/D₃ antagonist. At 500 µg/kg s.c., 8-OH-DPAT further enhanced or more persistently reduced dialysate NE or 5-HT content but had little effect on dialysate DA content. Its DA level-increasing effect could be seen dramatically with local infusion of cocaine (30 µ M ) and, to a lesser extent, sulpiride (10 µ M ). Depletion of endogenous 5-HT with p -chlorophenylalanine attenuated both the 5-HT level-reducing and DA level-enhancing effects of 8-OH-DPAT without affecting its maximal NE effect and the locomotor-stimulatory effect. Partial depletion of endogenous NE with N -(2-chloroethyl)- N -ethyl-2-bromobenzylamine failed to change the monoamine response but diminished the locomotion induced by 8-OH-DPAT. These results suggested that (a) the low dose of 8-OH-DPAT may act at presynaptic 5-HT_1A receptors to modulate 5-HT and DA release, while acting at postsynaptic 5-HT_1A receptors to modulate NE release; (b) the high dose of 8-OH-DPAT may activate D₂ receptors to offset its DA level-increasing effect; and (c) the locomotor-stimulatory effect of 8-OH-DPAT might be mediated primarily by postsynaptic 5-HT_1A receptors and the NE system.     

Unilateral lesion of the nigrostriatal pathway decreases the response of GABA interneurons in the dorsal raphe nucleus to 5-HT1A receptor stimulation in the rat

This study examined the firing rate and pattern of electrophysiologically and chemically identified GABA interneurons in the dorsal raphe nucleus (DRN), and role of 5-HT_1A receptor agonist 8-OH-DPAT and the medial prefrontal cortex (mPFC) in the firing activity in rats with 6-hydroxydopamine lesions of the substantia nigra pars compacta (SNc). The interneurons in rats with lesions of the SNc showed a more burst-firing, while having no change in the firing rate; the mPFC and combined mPFC and SNc lesions in rats decreased the firing rate of the interneurons and firing pattern shifted towards a more burst-firing compared to rats with sham lesions of the SNc, respectively. In rats with sham lesions of the SNc, administration of 8-OH-DPAT (1–243 μg/kg, i.v.) produced excitatory–inhibitory, excitatory and inhibitory effects in the firing rate of individual interneurons. However, when these effects were averaged over the group, 8-OH-DPAT had no significant effect on firing rate. In rats with lesions of the SNc, mPFC and the paired lesions, 8-OH-DPAT, at the same doses, inhibited all interneurons tested, respectively. Cumulative doses producing inhibition in rats with the paired lesions were higher than that of rats with lesions of the mPFC. In contrast to rats with sham lesions of the SNc, SNc lesion reduced expression of 5-HT_1A receptor on parvalbumin positive neurons in the DRN, a subpopulation of GABA interneurons. Our results indicate that the SNc and mPFC regulate the firing activity of GABA interneurons in the DRN. Furthermore, response of likely GABA interneurons to systemic administration of 8-OH-DPAT is altered by lesion of the SNc and mPFC.     

Evidence for a complex influence of nicotinic acetylcholine receptors on hippocampal serotonin release

The effects of nicotine on 5-hydroxytryptamine (5-HT) release from serotonergic nerve endings in rat dorsal hippocampal slices were studied. Nicotine (50-500 microM:) caused a concentration-dependent increase in 5-HT release. This effect was antagonised by mecamylamine (0.5 microM:), indicating an action at nicotinic receptors. Nicotine-evoked 5-HT release was not affected by tetrodotoxin (3 microM:), cadmium chloride (0.1 mM:), or the absence of Ca(2+) or Na(+) in the superfusion medium. Unexpectedly, higher concentrations of mecamylamine alone (1-50 microM:) increased 5-HT release. This suggested the presence of inhibitory input to 5-HT neurones and that these inhibitory neurones possess tonically active nicotinic receptors. The effect of mecamylamine (50 microM:) on 5-HT release was reduced by the muscarinic M(1) receptor agonist, McN-A-343 (100 microM:), but pirenzepine (0.005-1 microM:), which blocks M(1) receptors, alone increased 5-HT release. Hippocampal serotonergic neurones are known to possess both excitatory nicotinic receptors and inhibitory M(1) receptors. Although there may be several explanations for our results, one possible explanation is that nicotine stimulates 5-HT release by activating nicotinic heteroreceptors on 5-HT terminals. Mecamylamine (0.5 microM:) antagonises this effect, but higher concentrations increase 5-HT release indirectly by blocking the action of endogenous acetylcholine on nicotinic receptors situated on cholinergic neurones that provide muscarinic inhibitory input to 5-HT neurones.     

Role of excitatory amino acids in the ventral tegmental area for central actions of non-competitive NMDA-receptor antagonists and nicotine

Summary The putative role of non-NMDA excitatory amino acid (EAA) receptors in the ventral tegmental area (VTA) for the increase in dopamine (DA) release in the nucleus acumbens (NAC) and the behavioural stimulation induced by systemically administered dizocilpine (MK-801) was investigated. Microdialysis was utilized in rats with probes in the VTA and NAC. The VTA was perfused with the AMPA and kainate receptor antagonist CNQX (0.3 or 1.0 mM) or vehicle and dialysates from the NAC were analyzed with high-performance liquid chromatography for DA. Forty min after onset of CNQX or vehicle perfusion of the VTA MK-801 (0.1 mg/kg) was injected subcutaneously (sc). Subsequently, typical MK-801 induced behaviours were assessed. The MK-801 induced hyperlocomotion was associated with a 50% increase of DA levels in NAC dialysates. Both the MK-801 evoked hyperlocomotion and DA release in the NAC were effectively antagonized by CNQX perfusion of the VTA. However, by itself the CNQX or vehicle perusion of the VTA did not affect DA levels in NAC or the rated behaviours. The results indicate that MK-801 induced hyperlocomotion and increased DA release in the NAC are largely elicited within the VTA via activation of non-NMDA EAA receptors, tentatively caused by locally increased EAA release. In contrast, the enhanced DA output in the NAC induced by systemic nicotine (0.5 mg/kg sc) was not antagonized by intra VTA infusion of CNQX (0.3 or 1.0 mM), but instead by infusion of the NMDA receptor antagonist AP-5 (0.3 or 1.0 mM) into the VTA, which by itself did not alter DA levels in the NAC. Thus, the probably indirect, EAA mediated activation of the mesolimbic DA neurons in the VTA by MK-801 and nicotine, respectively, seems to be mediated via different glutamate receptor subtypes.     

Serotonin 5-HT(2) receptor stimulation of dopamine release in the posterior but not anterior nucleus accumbens of the rat

The objective of the present study was to examine the involvement of serotonin 5-HT(2) receptors within the rat nucleus accumbens (Acc) in the regulation of dopamine (DA) release using in vivo microdialysis. Perfusion with the 5-HT(2) agonist (+)-1-(2, 5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI), at concentrations of 25-250 microM, through microdialysis probes located in the posterior Acc increased extracellular DA levels to a maximum of 200% of baseline. DOI-induced increases in the extracellular levels of DA were Ca(2+) dependent and were inhibited by co-perfusion with the 5-HT(2) antagonist LY-53,857. DOI enhancement of the extracellular concentrations of DA was observed when probes were implanted in the Acc core and shell regions posterior to anteroposterior +1.2 mm from bregma, whereas a small reduction in the extracellular levels of DA was observed in the anterior Acc. There were no differences between core and shell subdivisions within either the anterior or the posterior Acc. These results suggest that activation of 5-HT(2) receptors within the posterior, but not anterior, Acc stimulates DA release, indicating rostral-caudal differences in the interactions of 5-HT with DA systems in the Acc.     

Stimulation by neurotensin of dopamine and 5-hydroxytryptamine (5-HT) release from rat prefrontal cortex: Possible role of NTR1 receptors in neuropsychiatric disorders

The modulation of cortical dopaminergic and serotonergic neurotransmissions by neurotensin (NT) was studied by measuring the release of dopamine (DA) and 5-hydroxytryptamine (5-HT) from the prefrontal cortex (PFC) of freely moving rats. The samples were collected via transversal microdialysis. Dopamine and 5-HT levels in the dialysate were measured using high-performance liquid chromatography (HPLC) with an electrochemical detector. Local administration of neurotensin (1 μM or 0.1 μM) in the PFC via the dialysis probe produced significant, long-lasting, and concentration-dependent increase in the extracellular release of DA and 5-HT. The increase produced by 1 μM neurotensin reached a maximum of about 210% for DA and 340% for 5-HT. A high-affinity selective neurotensin receptor (NTR1) antagonist {2-[(1-(7-chloro-4-quinolinyl)-5-(2,6-dimethoxyphenyl)pyrazol-3yl)carbonylamino tricyclo (3.3.1.1.^3.7) decan-2-carboxylic acid} (SR 48692), perfused locally at a concentration of 0.1 μM and 0.5 μM in the PFC antagonized the effects of 1 μM neurotensin. Our in vivo neurochemical results indicate, for the first time, that neurotensin is able to regulate cortical dopaminergic and serotonergic neuronal activity in freely moving rats. These effects are possibly mediated by interactions of neurotensin with neurons releasing DA or 5-HT, projecting to the PFC from the ventrotegmental area (VTA) and from the dorsal raphe nuclei (DRN), respectively. The potentiating effects of neurotensin on DA and 5-HT release in the PFC are regulated by NTR1 receptors, probably located on dopaminergic and serotonergic nerve terminals or axons.     

Modulation of the Mesolimbic Dopamine System by Glutamate

Glutamate has been shown to modulate motor behavior, probably via N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors that are involved in the control of the mesolimbic dopamine (DA) system, that is, the ventral tegmental area (VTA)-nucleus accumbens (NAC). In the present study, we investigated the effects of uncompetitive (MK-801) and competitive [DL-2-amino-5-phosphonopentanoic acid (AP-5), CGP 40116] NMDA receptor antagonists and NMDA and AMPA on DA release in the mesolimbic system and on motor behavior. Systemic injection and intrategmental infusion of MK-801 increased DA levels in the VTA, but the systemic administration enhanced DA exclusively in the NAC and increased motor behavior. In contrast, intrategmental infusion of AP-5, but not the systemic administration of its lipophilic analogue CGP 40116, decreased the DA release in the two regions without affecting motor behavior. NMDA and AMPA infusion into the VTA increased DA levels in both areas. This increase was accompanied by a strong motor behavioral stimulation after NMDA but only a moderate increase after AMPA infusion. The present results indicate that mesolimbic DA neurons are controlled by the glutamatergic system and that the effects of uncompetitive and competitive NMDA receptor antagonists on DA release are mediated by an interaction with different brain areas. These findings may account for the different effects of NMDA receptor ligands on motor behavior.     

Opposite effects of presynaptic 5-HT3 receptor activation on spontaneous and action potential-evoked GABA release at hippocampal synapses

5-HT(3) (serotonin type 3) receptors are targets of antiemetics, antipsychotics, and antidepressants and are believed to play a role in cognition. Nevertheless, contrasting results have been obtained with respect to their functions in the CNS and in the control of transmitter release. We used rat hippocampal neurons in single-neuron microcultures to identify the roles of presynaptic 5-HT(3) receptors at central synapses. 5-HT (10 microm) caused a transient > 10-fold increase in the frequency of miniature inhibitory postsynaptic currents without affecting amplitudes or kinetics. This effect was abolished by tropisetron (30 nm) and when Ca(2+) channels were blocked by 100 microm Cd(2+) it was mimicked and occluded when neurons were depolarized by 20 mm, but not 10 mm, K(+). Thus, activation of presynaptic 5-HT(3) receptors increased spontaneous GABA release by causing depolarization and opening of voltage-gated Ca(2+) channels. In microculture neurons, 5-HT transiently reduced action potential-evoked inhibitory autaptic currents by > 50%; this effect was blocked by tropisetron and mimicked by 20 mm, but not 10 mm, K(+). Miniature excitatory postsynaptic currents were not altered by 5-HT. Excitatory autaptic currents were tonically reduced, an effect attenuated by 5-HT(1A) antagonists. Thus, presynaptic 5-HT(3) receptors control GABA, but not glutamate, release and mediate opposite effects on spontaneous and action potential-dependent release.