Effect of subchronic lithium treatment on citalopram-induced increases in extracellular concentrations of serotonin in the medial prefrontal cortex

We investigated the effect of citalopram [a selective serotonin (5-HT) reuptake inhibitor; SSRI] and MKC-242 (a selective 5-HT1A agonist), following treatment with subchronic lithium (p.o., 1 week) on extracellular 5-HT concentrations in the medial prefrontal cortex (mPFC). Acute treatment with citalopram (3 and 30 mg/kg) led to significant increases in extracellular 5-HT concentrations. The subchronic lithium group showed significantly higher basal levels of extracellular 5-HT than normal diet controls. Acute citalopram (3 and 30 mg/kg) treatment together with subchronic lithium treatment showed significant increases in the extracellular 5-HT concentrations, compared with citalopram treatment alone. Acute MKC-242 (1 mg/kg) treatment showed significant decreases in extracellular 5-HT concentrations, in both the normal diet and lithium diet groups to the same extent. The addition of lithium did not change the effect of the 5-HT1A agonist on extracellular 5-HT concentrations. This study suggests that lithium augmentation of the antidepressant effect of SSRI is mediated by the additional increases in extracellular 5-HT concentrations following the co-administrations of lithium and SSRI.     

Local Activation of Metabotropic Glutamate Receptors Inhibits the Handling-Induced Increased Release of Dopamine in the Nucleus Accumbens but Not that of Dopamine or Noradrenaline in the Prefrontal Cortex: Comparison with Inhibition of Ionotropic Receptors

Abstract: On-line in vivo microdialysis was used to determine the effects of a 16-min handling period on release of dopamine (DA) in the nucleus accumbens and of DA and noradrenaline (NA) in the medial prefrontal cortex of awake, freely moving rats. DA and NA were determined in one HPLC run. Handling resulted in an immediate and strong increase of both catecholamines in the prefrontal cortex. Maximal values for DA were 295%, and for NA 225%, of controls. DA in the nucleus accumbens was also increased (to 135% of controls) but only after a short delay. Local inhibition of ionotropic glutamate receptors by continuous reversed dialysis of the drugs 6-cyano-7-nitroquinoxaline, d -2-amino-5-phosphonopentanoic acid, or dizocilpine did not significantly affect handling-induced increases in cortical DA and NA release. Neither did the agonist of metabotropic glutamate receptors, trans -(1 S ,3 R )-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD), or the GABA-B agonist baclofen. Reversed dialysis of dizocilpine in the nucleus accumbens was equally ineffective, but ACPD inhibited the increase in DA release in this area. Stimulation of metabotropic glutamate receptors in the nucleus accumbens was previously reported to inhibit activation of DA release in that area after stimulation of glutamatergic or dopaminergic afferents. It is concluded that metabotropic receptors in the nucleus accumbens are important for the control of activation of DA release in the accumbens by physiological stimuli but that a similar mechanism is lacking in the prefrontal cortex.     

Regulation of Serotonin Release in the Frontal Cortex and Ventral Hippocampus of Homozygous Mice Lacking 5-HT1B Receptors: In Vivo Microdialysis Studies

Abstract: To assess the involvement of the serotonin receptor subtype 5-HT_1B as terminal autoreceptor regulating 5-HT release in mice, we compared basal values and potassium-evoked changes of extracellular 5-HT levels obtained by in vivo microdialysis in two serotoninergic terminal projection areas of conscious wild-type mice with those measured in homozygous mutant mice lacking the gene encoding the 5-HT_1B receptor. In the frontal cortex and ventral hippocampus, basal and K⁺-evoked 5-HT release did not differ between the two strains of mice studied. The infusion via reverse microdialysis of the selective 5-HT_1B receptor agonist CP-93,129 (500 n M ) decreased significantly K⁺-evoked 5-HT release in the frontal cortex (by −44%) and ventral hippocampus (by −32%) of wild-type mice but had no effect in mutants. In a similar manner, the mixed 5-HT_1B-5-HT_1D receptor agonist sumatriptan (800 n M ) decreased significantly K⁺-evoked 5-HT release in the frontal cortex (by −46%) of wild-type mice but had no effect in mutants. These results demonstrated that 5-HT_1B knockout mice are not as sensitive to full (CP-93,129) and mixed (sumatriptan) 5-HT_1B receptor agonists as are wild-type mice. These data provide in vivo evidence that, in mice, 5-HT_1B, but not 5-HT_1D, autoreceptors inhibit 5-HT release at nerve terminals located in the frontal cortex and ventral hippocampus.     

Different receptor subtypes are involved in the serotonin-induced modulation of epileptiform activity in rat frontal cortex in vitro.

The frontal cortex is innervated by serotonergic terminals from the raphe nuclei and it expresses diverse 5-HT receptor subtypes. We investigated the effects of 5-HT and different 5-HT receptor subtype-selective agonists on spontaneous discharges which had developed in rat cortical slices perfused with a Mg2+-free medium and the GABA(A) receptor antagonist picrotoxin. The frequency of synchronous discharges, recorded extracellularly in superficial layers (II/III) of the frontal cortex, was dose-dependently enhanced by 5-HT (2.5-40 microM). That excitatory effect was blocked by the 5-HT2 receptor selective antagonist ketanserin. The 5-HT2A/2C receptor-selective agonist DOI and the 5-HT4 receptor agonist zacopride also increased the frequency of spontaneous discharges. In the presence of ketanserin, 5-HT decreased the discharge rate; a similar effect was observed when the 5-HT1A receptor agonist 8-OH-DPAT or the 5-HT1B receptor agonist CGS-12066B was applied. The 5-HT3 receptor agonist m-CPBG was ineffective. In conclusion, 5-HT produces multiple effects on epileptiform activity in the frontal cortex via activation of various 5-HT receptor subtypes. The excitatory action of 5-HT, which predominates, is mediated mainly by 5-HT2 receptors. The inhibitory effects can be attributed to activation of 5-HT1A and 5-HT1B receptors.     

Selective serotonin reuptake blockade increases extracellular dopamine in noradrenaline-rich isocortical but not prefrontal areas: dependence on serotonin-1A receptors and independence from noradrenergic innervation

The present study investigated the effects of two serotonin (5-HT) uptake inhibitors, citalopram and paroxetine, and of a non-selective noradrenaline (NA) and 5-HT uptake blocker, imipramine, on extracellular NA and dopamine (DA) in the prefrontal cortex (PfCX), parietal cortex (ParCX) and occipital cortex (OccCX). Citalopram, the most selective 5-HT uptake blocker, increased dialysate DA in the OccCX and ParCX but not in the PfCX and this effect was prevented in the OccCX by WAY-100635, an antagonist of serotonin-1A (5-HT(1A)) receptors, but not by dorsal noradrenergic bundle (DNAB) lesions that reduced to unmeasurable levels basal dialysate NA but did not affect dialysate DA. Paroxetine, a less selective 5-HT uptake inhibitor than citalopram, at the dose of 5 mg/kg, increased DA in the OccCX but not in the PfCX; however, at doses of 10 mg/kg, which increase PfCX NA, paroxetine increased DA also in this area. Imipramine increased dialysate DA and NA both in the PfCX and in the OccCX and this effect was abolished by DNAB lesions and was reduced but not abolished by WAY-100635. Administration of doses of reboxetine and citalopram that do not increase DA release in the OccCX if given separately, markedly increased DA when combined. These results indicate that endogenous 5-HT, raised by selective blockade of the 5-HT carrier, can increase extracellular DA in the OccCX and in the ParCX by stimulating 5-HT(1A) receptors independently from the presence of NA terminals, although blockade of 5-HT and NA carrier can strongly interact to raise extracellular DA in this area. These observations are consistent with the existence of DA neurons separate from the NA ones contributing to extracellular DA even in NA-rich/DA poor isocortical areas.     

Berberine produces antidepressant-like effects in the forced swim test and in the tail suspension test in mice

This study investigated the effect of berberine (BER) in the mouse forced swim test (FST) and in the tail suspension test (TST), two models predictive of antidepressant activity. We also investigated the antidepressant-like mechanism of BER by the combination of the desipramine [DES, an inhibitor of reuptake of noradrenaline (NA) and serotonin (5-HT)], maprotiline (MAP, selective NA reuptake inhibitor), fluoxetine (FLU, selective 5-HT reuptake inhibitor) and moclobemide [MOC, monoamine oxidase (MAO) A inhibitor). Then we further measured the levels of monoamines [NA, dopamine (DA) and 5-HT) in mice striatum, hippocampus and frontal cortex. The results show that BER (10, 20 mg/kg, p.o.), significantly reduced the immobility time during the FST and the TST. The immobility time after treatment with BER (20 mg/kg, p.o.) in FST was augmented by DES, FLU and MOC, and not affected by MAP. Furthermore, BER (20 mg/kg, p.o.) increased NA and 5-HT levels in the hippocampus and frontal cortex. Our findings support the view that BER exerts antidepressant-like effect. The antidepressant-like mechanism of BER may be related to the increase in NA and 5-HT levels in the hippocampus and frontal cortex.     

The activation of 5-HT receptors in prefrontal cortex enhances dopaminergic activity

Atypical antipsychotics show preferential 5-HT 2A versus dopamine (DA) D2 receptor affinity. At clinical doses, they fully occupy cortical 5-HT2 receptors, which suggests a strong relationship with their therapeutic action. Half of the pyramidal neurones in the medial prefrontal cortex (mPFC) express 5-HT 2A receptors. Also, neurones excited through 5-HT 2A receptors project to the ventral tegmental area (VTA). We therefore hypothesized that prefrontal 5-HT 2A receptors can modulate DA transmission through excitatory mPFC-VTA inputs. In this study we used single unit recordings to examine the responses of DA neurones to local (in the mPFC) and systemic administration of the 5-HT 2A/2C agonist 1-[2,5-dimethoxy-4-iodophenyl-2-aminopropane] (DOI). Likewise, using microdialysis, we examined DA release in the mPFC and VTA (single/dual probe) in response to prefrontal and systemic drug administration. The local (in the mPFC) and systemic administration of DOI increased the firing rate and burst firing of DA neurones and DA release in the VTA and mPFC. The increase in VTA DA release was mimicked by the electrical stimulation of the mPFC. The effects of DOI were reversed by M100907 and ritanserin. These results indicate that the activity of VTA DA neurones is under the excitatory control of 5-HT 2A receptors in the mPFC. These observations may help in the understanding of the therapeutic action of atypical antipsychotics.     

Brain amines and neocortical EEG in young and aged rats

1. Frontal and parieto-occipital electroencephalography (EEG) of young (4 months-old) and aged (17 and 22 months-old) Wistar rats were analyzed, both during movement and during waking immobility. 2. The levels of monoamines, serotonin and their metabolites were measured from the frontal cortex, parieto-occipital cortex, hippocampus, brainstem and midbrain. 3. In aged rats, as compared to young rats, the most apparent changes of the quantitative EEG spectrum were the decreased amplitude of alpha (5-10 Hz) and beta (10-20 Hz) frequency bands in the frontal and parieto-occipital cortices during both movement and waking immobility behavior (p less than 0.05). 4. The levels of dopamine (DA), homovanillinic acid (HVA), serotonin (5-HT), 5-hydroxyindoleacetic acid (5-HIAA) or the ratios of 5-HT/5-HIAA and DA/HVA did not differ between young and aged rats in any brain region studied, with the exceptions of brainstem DA and parieto-occipital 5-HIAA, which were elevated in aged rats (p less than 0.05). 5. In the frontal cortex, hippocampus and midbrain, noradrenaline (NA) levels of aged rats were slightly increased as compared to young rats (p less than 0.05). 6. NA levels of the parieto-occipital cortex and brainstem did not change during aging. 7. Furthermore, there were no clear correlations between the decreased amplitude of the quantitative EEG spectrum and monoamine or serotonin concentrations, or the ratios of 5-HT/5-HIAA and DA/HVA in the cerebral cortex of aging Wistar rat.(ABSTRACT TRUNCATED AT 250 WORDS)     

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.     

Rat brain monoamine and serotonin S2 receptor changes during pregnancy

The concentrations of noradrenaline (NA), dopamine (DA), serotonin (5-HT), and their metabolites were determined in 5 brain areas of non-pregnant, 15 and 20 day pregnant and 4 day post-partum rats. Striatal 5-HT content was significantly lower in 15 and 20 day pregnant rats than in estrous controls. A significant decrease in striatal and frontal cortex 5-hydroxyindole-3-acetic acid (5-HIAA) concentration was observed in 15 day pregnant rats. Significant increases in hypothalamic and hippocampal NA levels were observed at 4 days post-partum. Frontal cortex serotonin S2 receptorKd was reduced in 4 day post-partum rats. There was no significant change in S2 receptorB _max during pregnancy. Levels of progesterone were negatively correlated with striatal DA, homovanillic acid (HVA), 5-HT, and 5-HIAA levels, hypothalamic DA, hippocampal 5-HT, and frontal cortex 5-HIAA values as well as striatal HVA to DA, and HVA to 3,4-dihydroxyphenylacetic acid (DOPAC) ratios and amygdaloid HVA to DOPAC ratios. The limbic neurotransmitter changes might possibly contribute to mood changes which occur during pregnancy and post-partum.     

5-HT(1A) receptor mutant mice exhibit enhanced tonic, stress-induced and fluoxetine-induced serotonergic neurotransmission

Mutant mice that lack serotonin(1A) receptors exhibit enhanced anxiety-related behaviors, a phenotype that is hypothesized to result from impaired autoinhibitory control of midbrain serotonergic neuronal firing. Here we examined the impact of serotonin(1A) receptor deletion on forebrain serotonin neurotransmission using in vivo microdialysis in the frontal cortex and ventral hippocampus of serotonin(1A) receptor mutant and wild-type mice. Baseline dialysate serotonin levels were significantly elevated in mutant animals as compared with wild-types both in frontal cortex (mutant = 0.44 +/- 0.05 n M; wild-type = 0.28 +/- 0.03 n M) and hippocampus (mutant = 0.46 +/- 0.07 n M; wild-type = 0.27 +/- 0.04 n M). A stressor known to elicit enhanced anxiety-like behaviors in serotonin(1A) receptor mutants increased dialysate 5-HT levels in the frontal cortex of mutant mice by 144% while producing no alteration in cortical 5-HT in wild-type mice. There was no phenotypic difference in the effect of this stressor on serotonin levels in the hippocampus. Fluoxetine produced significantly greater increases in dialysate 5-HT content in serotonin(1A) receptor mutants as compared with wild-types, with two- and three-fold greater responses being observed in the hippocampus and frontal cortex, respectively. This phenotypic effect was mimicked in wild-types by pretreatment with the serotonin(1A) antagonist 4-iodo-N-[2-[4-(methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinyl-benzamide (p-MPPI). These results indicate that deletion of central serotonin(1A) receptors results in a tonic disinhibition of central serotonin neurotransmission, with a greater dysregulation of serotonin release in the frontal cortex than ventral hippocampus under conditions of stress or increased interstitial serotonin levels.     

Blockade of substance P (neurokinin 1) receptors enhances extracellular serotonin when combined with a selective serotonin reuptake inhibitor: an in vivo microdialysis study in mice

Abstract Substance P antagonists of the neurokinin-1 receptor type (NK1) are gaining growing interest as new antidepressant therapies. It has been postulated that these drugs exert this putative therapeutic effect without direct interactions with serotonin (5-HT) neurones. Our recent microdialysis experiment performed in NK1 receptor knockout mice suggested evidence of changes in 5-HT neuronal function (Froger et al. 2001). The aim of the present study was to evaluate the effects of coadministration of the selective 5-HT reuptake inhibitor (SSRI) paroxetine with a NK1 receptor antagonist (GR205171 or L733060), given either intraperitoneally (i.p.) or locally into the dorsal raphe nucleus, on extracellular levels of 5-HT ([5-HT]ext) in the frontal cortex and the dorsal raphe nucleus using in vivo microdialysis in awake, freely moving mice. The systemic or intraraphe administration of a NK1 receptor antagonist did not change basal cortical [5-HT]ext in mice. A single systemic dose of paroxetine (4 mg/kg; i.p.) resulted in a statistically significant increase in [5-HT]ext with a larger extent in the dorsal raphe nucleus (+ 138% over basal AUC values), than in the frontal cortex (+ 52% over basal AUC values). Co-administration of paroxetine (4 mg/kg; i.p.) with the NK1 receptor antagonists, GR205171 (30 mg/kg; i.p.) or L733060 (40 mg/kg; i.p.), potentiated the effects of paroxetine on cortical [5-HT]ext in wild-type mice, whereas GR205171 (30 mg/kg; i.p.) had no effect on paroxetine-induced increase in cortical [5-HT]ext in NK1 receptor knock-out mice. When GR205171 (300 micro mol/L) was perfused by 'reverse microdialysis' into the dorsal raphe nucleus, it potentiated the effects of paroxetine on cortical [5-HT]ext, and inhibited paroxetine-induced increase in [5-HT]ext in the dorsal raphe nucleus. Finally, in mice whose 5-HT transporters were first blocked by a local perfusion of 1 micro mol/L of citalopram into the frontal cortex, a single dose of paroxetine (4 mg/kg i.p.) decreased cortical 5-HT release, and GR205171 (30 mg/kg i.p.) reversed this effect. The present findings suggest that NK1 receptor antagonists, when combined with a SSRI, augment 5-HT release by modulating substance P/5-HT interactions in the dorsal raphe nucleus.     

The effect of active immunization with a conjugate of dopamine and bovine serum albumin on the development of an experimental MPTP-induced depressive syndrome and on the monoamine metabolism in the brain of rats

Active immunization with dopamine conjugated with bovine serum albumin (DA-BSA) or BSA with complete Freund's adjuvant (CFA) partly suppressed the development of the MPTP-induced depressive syndrome in rats preventing the appearance of "behavioral despair" symptoms: increase in immobility time and higher index of depression in forced-swim test. In DA-BSA-immunized rats the content of DOPA, DA, HVA, NA, and 5-HN in caudate putamen and that of NA in the frontal cortex was increased, while in BSA-immunized rats the content of 5-HT in both brain areas and that of DOPAC in the frontal cortex was decreased both in rats with reduced depressive syndrome and in saline control as compared with intact animals a day after the last drug injection. In DA-BSA-immunized rats with reduced depressive syndrome the increase in DA and 5-HT content in caudate putamen was less expressed and DOPAC content was lower than in saline control. In BSA-immunized depressive rats DA content in the frontal cortex was also reduced as compared to control.     

Alternating extremely low frequency magnetic field increases turnover of dopamine and serotonin in rat frontal cortex

The aim of this study was to evaluate the influence of an extremely low frequency sinusoidal magnetic field (ELF MF) with frequency of 10 Hz and intensity of 1.8-3.8 mT on the levels of the biogenic amines dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), 3-methoxytyramine (3-MT), 5-hydroxytryptamine (5-HT), 5-hydroxyindolacetic acid (5-HIAA), and noradrenaline (NA), as well as on DA and 5-HT turnover in corpus striatum and frontal cortex of adult male Wistar rats. We found that ELF MF exposure for 14 days, 1 h daily, did not influence the level of the examined biogenic amines and metabolites, but increased the rate of synthesis (turnover) of DA and 5-HT in rat frontal cortex as compared to control, sham exposed rats. On the basis of the present results and our previous findings, extremely low frequency magnetic field (ELF MF) exposure has been found to alter both turnover and receptor reactivity of monoaminergic systems, as well as some behaviors induced by these systems or their agonists and antagonists.     

Tandospirone potentiates the fluoxetine-induced increases in extracellular dopamine via 5-HT(1A) receptors in the rat medial frontal cortex.

Recent clinical studies suggest that 5-HT(1A) receptor agonists, including buspirone, may have an antidepressant effect and potentiate the efficacy of selective serotonin reuptake inhibitors (SSRI) in major depressive disorders. In the present study, we investigated the effect of tandospirone, a highly potent and selective 5-HT(1A) receptor agonist, on dopamine release and potentiation of fluoxetine-induced dopamine outflow in the medial frontal cortex using microdialysis in freely moving rats. Intraperitoneal injection of tandospirone (5 mg/kg) increased dopamine release to about 190% of basal levels. Pretreatment with the selective 5-HT(1A) receptor antagonist, WAY 100635 (1mg/kg), blocked the effect of tandospirone. Local application of WAY 100635 (10 microM) via microdialysis probe antagonized the increase in dopamine release in the medial frontal cortex induced by systemic injection of tandospirone. Fluoxetine (10 mg/kg) also increased dopamine release in the medial frontal cortex, to 200% of basal levels, and the simultaneous administration of tandospirone and fluoxetine increased the release to 380%. These results indicate that tandospirone potentiates the fluoxetine-induced increase in dopamine release via 5-HT(1A) receptors in the rat medial frontal cortex, and suggest that tandospirone may have therapeutic potential for the treatment of depression.     

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.     

Histochemical demonstration of transmitter release from noradrenaline,dopamine and 5-hydroxytryptamine nerve terminals in field stimulated rat brain slices

Summary The effect of electrical field stimulation on noradrenaline (NA), dopamine (DA) and 5-hydroxytryptamine (5-HT) nerve terminals in rat brain slicesin vitro was investigated. Slices prepared from the cerebral cortex or the neostriatum were incubated in physiologic buffer for 30 min and then superfused by buffer and stimulated by an electrical field (biphasic pulses, 10 Hz, 12 mA, 2 ms) for various time periods. The effect of the stimulation was studied at the cellular level with the histochemical fluorescence technique of Falck and Hillarp. The transmitter overflow into the superfusing buffer caused by the stimulation was studied with isotope technique. Cerebral Cortex NA Nerve Terminals. Stimulation caused release of NA from cortical NA nerve terminals. Already after 2 min stimulation a slight decrease of the fluorescence intensity of the nerve terminals could be found. Stimulation for 15 to 30 min greatly reduced the fluorescence intensity. In slices preincubated with³H-NA the stimulation-induced overflow of tritium during 2 min stimulation was about 15% (i.e. 15% of the tissue tritium content was overflowing into the superfusing buffer in response to stimulation for 2 min). During prolonged stimulation there was a considerable decline of the tritium efflux. Cerebral Cortex 5-HT Nerve Terminals. The 5-HT-analogue 6-hydroxytryptamine (6-HT) which is readily taken up into 5-HT nerve terminals was used to permit good visualization of these nerve terminals. Uptake of 6-HT into cortical NA nerve terminals was prevented by preincubation with 6-hydroxydopamine (6-OH-DA) or protriptyline. Stimulation for 15 to 30 min reduced the fluorescence intensity of the 5-HT nerve terminals. In slices preincubated with³H-5-HT the stimulation-induced overflow of tritium during 2 min stimulation was about 5%. The tritium efflux slowly decreased during continuous stimulation. Neostriatal DA Nerve Terminals. In slices frozen directly after preparation an intense diffuse fluorescence could be seen. After incubation in drug-free buffer at 37° C the fluorescence was localized in the varicosities of the DA nerve terminals. The central parts of the slices almost completely lacked specific fluorescence, while the outer zones were brightly fluorescent. No clear reduction of the fluorescence intensity of the DA nerve terminals in the outer zone could be observed after stimulation for 30 min. In slices preincubated with³H-DA the stimulation-induced overflow of tritium during 2 min stimulation was about 2%. The tritium efflux slowly decreased during continuous stimulation.It is suggested that the differences in release between the various nerve terminal systems foundin vitro reflect differences in transmitter release occurringin vivo. The comparably high release of NA per impulse from the cortical NA nerve terminals implicate that the discharge rate of these neuronsin vivo is very low.This investigation has been supported by grants from the Swedish Medical Research Council (B72-14X-2330-05A) and Magnus Bergwall's Foundation.The author is greatly indebted to Mrs. Annika Hamberger for her skillful technical assistance. For generous supplies of drugs thanks are due to Hässle, Göteborg, Sweden, through Dr. H. Corrodi (6-HT, 6-OH-DA and H44/68).     

Microiontophoresis of biogenic amines on cortical neurons: amounts of NA, DA and 5-HT ejected, compared with tissue content

The present series of studies were carried out to quantify the amounts of dopamine (DA), noradrenaline (NA) and serotonin (5-HT) ejected from iontophoresis micropipettes and that produce inhibitory and modulatory effects on cortical neurons, in the frontoparietal cortex of the rat and in the occipital cortex of the cat. Using radioactive isotopes of the biogenic amines the effective transport number (n) was found to be 0.08 for DA; 0.13 for NA, and 0.22 for 5-HT. In addition, similar determinations were made, for comparison purposes, of the transport numbers of the neurotransmitters acetylcholine (ACh; n = 0.44), gamma-amino-n-butyrate (GABA, n = 13), and glutamate (GLU; n = 0.27). The quantities ejected under in vivo conditions were then estimated using Faraday's formula and fell in the pmol range (10(-12) mol). The effects of DA, NA and 5-HT on cortical units were studied; the amounts ejected were compared with the endogenous tissue content of these amines, determined by means of specific and sensitive radioenzymatic assays in the regions where the microiontophoretic experiments were performed. These results are discussed in the light of the anatomical, biochemical and electrophysiological data suggesting a modulatory role for the biogenic amines in the cerebral neocortex.     

Stimulation of 5-hydroxytryptamine (5-HT(2C) ) receptors in the ventrotegmental area inhibits stress-induced but not basal dopamine release in the rat prefrontal cortex

The present study investigated whether 5-HT(2C) receptors in the ventrotegmental area and prefrontal cortex regulate basal and stimulus-evoked dopamine release in the prefrontal cortex. Using the in vivo microdialysis technique in conscious rats, we studied the effect of a selective 5-HT(2C) receptor agonist, Ro60-0175, on basal and immobilization stress-induced dopamine release in the prefrontal cortex. Ro60-0175 intraperitoneally (2.5 mg/kg) and into the ventrotegmental area (10 microg/0.5 microL) completely antagonized the effect of stress on extracellular dopamine without altering basal levels. Infusion of 10 microm Ro60-0175 through the cortical probe had no significant effect on basal and stress-induced dopamine release. SB242084 (10 mg/kg), a selective antagonist of 5-HT(2C) receptors, significantly increased basal extracellular dopamine and completely prevented the effect of intraperitoneal and intraventrotegmental Ro60-0175 on the stress-induced rise of extracellular dopamine, but had no effect itself in stressed rats. The results show that Ro60-0175 suppresses cortical dopamine release induced by immobilization stress through the stimulation of 5-HT(2C) receptors in the ventrotegmental area. While confirming that endogenous 5-HT acting on 5-HT(2C) receptors tonically inhibit basal dopamine release in the prefrontal cortex, the present findings suggest that the stimulation of 5-HT(2C) receptors with an exogenous agonist preferentially inhibit stimulated release.     

Venlafaxine enhances the effect of bupropion on extracellular dopamine in rat frontal cortex

Venlafaxine is recognised as an effective treatment for depression and is known to inhibit the reuptake of serotonin (5-HT) and noradrenaline (NA). Another antidepressant, bupropion, acts to inhibit dopamine (DA) and NA reuptake and is commonly co-administered with other antidepressants to improve the efficacy of the antidepressant effect. The present study was designed to investigate the acute effect of combining the 2 drugs on extracellular levels of 5-HT, DA, and NA in rat frontal cortex using brain microdialysis, with the drugs being administered by intraperitoneal injection (i.p). Bupropion (10 mg/kg body mass, i.p.) alone had no effect on extracellular 5-HT levels, whereas venlafaxine (10 mg/kg, i.p.) alone significantly elevated extracellular 5-HT over basal values. As expected, bupropion alone elevated extracellular dopamine above basal values at 40 min post-drug administration, and this effect lasted for a further 2 h. Venlafaxine alone did not statistically elevate extracellular dopamine. The co-administration of venlafaxine with bupropion resulted in a dramatic increase in extracellular dopamine, and this effect was significantly greater than that seen with bupropion alone. In the frontal cortex, NA was elevated by bupropion alone and venlafaxine alone, relative to the control animals. The combination of bupropion and venlafaxine resulted in a marked elevation of NA.