Increase In Extracellular Serotonin Research Articles

The potential of the beta-Microprobe, an intracerebral radiosensitive probe, to monitor the [(18)F]MPPF binding in the rat dorsal raphe nucleus.

The aim of this study was to demonstrate the ability of a recently developed beta(+)-range sensitive intracerebral probe (beta-Microprobe) to measure the binding kinetics of [(18)F]MPPF, a well-documented 5-HT(1A) serotoninergic receptor ligand, in the dorsal raphe nucleus (DRN) of the anaesthetised rat. This midbrain nucleus presents a high concentration of 5-HT(1A) receptors known to be implicated in the effects of antidepressants. The difficulty confronting this study lay in the fact that the dimensions of the DRN are smaller than the detection volume of the beta-Microprobe. In the first part of the study, we studied the feasibility of this measurement from a theoretical point of view by autoradiography and a Monte Carlo simulation. We determined the optimal beta-Microprobe location close to the DRN and verified that this configuration allowed accurate determination of [(18)F]MPPF specific binding in the nucleus. In the second part of our study, we measured the in vivo time-concentration curves of [(18)F]MPPF binding in the DRN in comparison with the cerebellum. The specificity of [(18)F]MPPF binding in the DRN was confirmed by its displacement after non-labelled 5-HT(1A)antagonist injection (MPPF or WAY-100635). Moreover, we verified the feasibility of using beta-Microprobe monitoring and simultaneous validation by microdialysis to study the effect of an increase in extracellular serotonin, induced by fenfluramine injection, on [(18)F]MPPF binding in the DRN. Our theoretical simulations, confirmed by our experimental results, demonstrate the ability of this new device to monitor in vivo the binding of [(18)F]MPPF in the DRN of anaesthetised rodents.

Light-potentiation of acoustic startle response (ASR) and monoamine efflux related to fearfulness in Fyn-deficient mice

Fyn tyrosine kinase deficient mice are known to show increased fearfulness. We investigated the fear response of these mice using the light-potentiation of the acoustic startle response (ASR) and examined its neurochemical correlates using in vivo microdialysis. Female homozygous Fyn-deficient mice showed an enhancement of the startle amplitude under a bright light while heterozygotes and wild-types did not show such a change. Along with these behavioral findings, the homozygous Fyn-deficient mice showed an increase in extracellular serotonin (5-HT) and dopamine (DA) in the prefrontal cortex and 5-HT in the hippocampus when they were exposed to bright light, while heterozygous and wild-type mice did not show such changes. These results suggest that the increased fearfulness of Fyn-deficient mice is related to enhanced serotonergic and dopaminergic activity in the prefrontal cortex and limbic system.

Differential effects of coadministration of fluoxetine and WAY-100635 on serotonergic neurotransmission in vivo: sensitivity to sequence of injections.

Serotonin 5-HT(1A) receptor antagonists potentiate the effects of serotonin reuptake inhibitors on extracellular serotonin levels in a variety of brain regions. These effects are quite variable, however, with reports indicating potentiations of anywhere from 100-1900%. One factor that might impact the magnitude of such potentiations is the timing of administration of the two agents; reports in which the reuptake inhibitor is given prior to the serotonin receptor antagonist consistently report larger potentiations than reports in which the antagonist is given first. To test this relationship directly, microdialysis and electrophysiology studies were performed to assess the magnitude of increase in extracellular serotonin and changes in cellular activity produced by the serotonin reuptake inhibitor fluoxetine and the 5-HT(1A) receptor antagonist WAY-100635 under various dosing regimens. In microdialysis studies, when WAY-100635 (0.5 mg/kg s.c.) was administered 80 min after fluoxetine (10 mg/kg i.p.) the increase in serotonin was more than twice that observed when the compounds were coadministered. In electrophysiology studies in vivo, WAY-100635 reversed the depression of cell firing produced by fluoxetine when administered 30 min after fluoxetine, but when the two compounds were coadministered, a depression in firing rate was observed comparable to that produced by fluoxetine alone. In contrast, slice recording studies showed that WAY-100635 blocked the effects of fluoxetine regardless of the order of administration. These results indicate that fluoxetine and WAY-100635 can interact in a fashion not predicted by the currently accepted model. It is likely that neuronal circuitry outside of the raphe nuclei underlies this relationship.

Influence of serotoninergic drugs on in vivo dopamine extracellular output in rat striatum.

In vivo microdialysis was used to investigate the mechanism behind the increase in extracellular dopamine (DA) induced by increase in extracellular serotonin (5-HT) level and 5-HT1 and 5-HT2 receptor activation. The following serotoninergic drugs were perfused in the absence or presence of nomifensine (5 microM) or tetrodotoxin (TTX; 2 microM): clomipramine (10, 500 and 1,000 microM), a selective 5-HT reuptake inhibitor; 8-OH-DPAT (50 and 500 microM), a 5-HT1A receptor agonist; and alpha-methyl-5-HT (1, 5 and 50 microM), a 5-HT2 receptor agonist. All the serotoninergic drugs studied increased DA extracellular output in a dose-dependent manner. The presence of nomifensine attenuated the effect of perfusion of clomipramine (500 microM) and completely abolished the effect of perfusion of 8-OH-DPAT (500 microM) and alpha-methyl-5-HT (5 microM) on DA extracellular output. Clomipramine (100-1,000 microM) perfusion produced a dose dependent increase in DOPAC extracellular output, which was stronger when clomipramine (500 microM) was co-perfused with nomifensine. 8-OH-DPAT and alpha-methyl-5-HT perfusion decreased DOPAC overflow. Addition of TTX to the perfusion fluid one hour before serotoninergic drugs perfusion, did not completely abolish the effect on dopamine extracellular output produced by the serotoninergic drugs. These data seem to indicate that increase in extracellular 5-HT level and 5-HT1 and 5-HT2 receptor activation increase in vivo DA extracellular output in the striatum mainly by a nonexocytotic mechanism involving DA uptake sites and, secondarily, by activation of 5-HT receptors.

Effects of 5-HT1A receptor antagonists on fluoxetine-induced changes in extracellular serotonin concentrations in rat frontal cortex.

Clinical studies in which serotonin specific reuptake inhibitors have been co-administered with pindolol have demonstrated a shortened time to onset of antidepressant activity. This effect has been attributed to the antagonist effects of pindolol at the presynaptic 5-HT1A receptor which augments the action of the serotonin specific reuptake inhibitors. In the present study, we demonstrate that acute fluoxetine-induced increases in extracellular serotonin concentrations, as measured by microdialysis in the frontal cortex, can be potentiated by 5-HT1A receptor blockade using N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(pyridinyl)cyclohexa necarboxamide (WAY100635), the silent and selective 5-HT1A receptor antagonist. WAY100635 at doses as low as 0.03 mg/kg s.c. maintained this potentiation effect across a range of fluoxetine doses. In addition, using antagonists with different intrinsic agonist activities for the 5-HT1A receptor, we have determined that only compounds with very low intrinsic agonist activity can produce a potentiation of the acute fluoxetine-induced increases in extracellular serotonin.

Anticonvulsant doses of carbamazepine increase hippocampal extracellular serotonin in genetically epilepsy-prone rats: dose response relationships

The antiepileptic drug carbamazepine produces dose related anticonvulsant effects in genetically epilepsy-prone rats (GEPRs) and most other animal seizure models. Carbamazepine releases serotonin as part of the pharmacodynamic action by which it suppresses convulsions in GEPRs and it releases serotonin in non-epileptic Sprague–Dawley rats. The two strains which make up the GEPR seizure model (moderate seizure GEPR-3s and severe seizure GEPR-9s) experience anticonvulsant effects in response to different doses of carbamazepine (GEPR-3 ED 50=25 mg/kg; GEPR-9 ED 50=3 mg/kg). The present study determined that carbamazepine produces a dose related increase in extracellular serotonin in each of the two GEPR strains. The doses of carbamazepine required to increase extracellular serotonin are similar to the doses required for an anticonvulsant effect in each of the strains. This result provides further support for the hypothesis that release of serotonin by carbamazepine is an important part of the pharmacodynamic action by which this drug suppresses seizures.

Effect of 5-HT1A Receptor Antagonists on Citalopram-induced Increase in Extracellular Serotonin in the Frontal Cortex, Striatum and Dorsal Hippocampus

The aim of the present study was to compare the effects of citalopram, either alone or combined with 5-HT1A receptor antagonists, on extracellular serotonin levels in brain regions innervated by the dorsal or median raphe nuclei. Using intracerebral microdialysis in awake rats with separate probes in the frontal cortex or dorsal hippocampus, we studied the ability of 8 mg/kg s.c. (—)penbutolol, a β-adrenoceptor antagonist with antagonist action at 5-HT1A and 5-HT1B receptors, and 0.3 mg/kg s.c. WAY-100635, a selective 5-HT1A receptor blocker, to modify the effect of 1 and 10 mg/kg i.p. citalopram on extracellular serotonin. Both doses of citalopram had more effect on extracellular serotonin levels in the dorsal hippocampus than in the frontal cortex. The effect of 1 mg/kg citalopram was significantly potentiated by (—)penbutolol in the frontal cortex only, but a clear-cut potentiation of the effect of citalopram was seen in both regions at a dose of 10 mg/kg. The effect of 10 mg/kg citalopram was potentiated by WAY-100635 in the frontal cortex but not in the dorsal hippocampus.In a second set of experiments, the combined effect of WAY-100635 and citalopram was studied in the same rat implanted with vertical probes in the striatum and dorsal hippocampus. Citalopram (1 and 10 mg/kg i.p.) raised extracellular serotonin to a similar extent in both regions. However, 0.3 mg/kg s.c. WAY-100635 potentiated the effect of 10 mg/kg citalopram in the striatum but not in the dorsal hippocampus. The results suggest that only a combined blockade of 5-HT1A and 5-HT1B receptors potentiates the effect of citalopram on extracellular concentrations of serotonin in the dorsal hippocampus. The findings may be relevant in designing clinical trials aimed at enhancing the antidepressant action of selective serotonin re-uptake inhibitors by combining them with serotonin receptor antagonists. © 1997 Elsevier Science Ltd.

The 5-HT 1A antagonist WAY-100635 selectively potentiates the presynaptic effects of serotonergic antidepressants in rat brain

The increases in extracellular serotonin (5-hydroxytryptamine; 5-HT) produced by some antidepressant drugs in forebrain are attenuated by the activation of somatodendritic 5-HT 1A autoreceptors by the excess 5-HT induced by these agents in the midbrain raphe. Using microdialysis, we have examined the effects of the selective 5-HT 1A antagonist WAY-100635 in rats pretreated with the selective 5-HT reuptake inhibitors (SSRIs) citalopram, fluoxetine, fluvoxamine, the tricyclic antidepressants clomipramine and desipramine and the monoamine oxidase inhibitor phenelzine. WAY-100635 markedly potentiated the increases in 5-HT produced by the SSRIs, clomipramine and phenelzine but it did not alter that produced by desipramine. These results indicate that the effects of serotonergic antidepressant drugs (but not those of desipramine, which mainly blocks noradrenaline reuptake) can be potentiated by 5-HT 1A autoreceptor blockade.

Involvement of the dorsal raphe but not median raphe nucleus in morphine-induced increases in serotonin release in the rat forebrain.

In vivo microdialysis was used to determine if morphine produces increases in extracellular serotonin in specific brain sites. With citalopram included in the dialysis solution to block reuptake, serotonin was measured in 11 brain sites of unanesthetized rats. After systemic morphine (10 mg/kg, s.c.), increases in extracellular serotonin were observed in the nucleus accumbens, amygdala, frontal cortex, striatum, thalamus, hypothalamus and ventral hippocampus. These seven areas are innervated either by dorsal raphe nucleus projections alone, or by projections from both dorsal and median raphe nucleus. In contrast, serotonin was not significantly increased in the medial septal nucleus and dorsal hippocampus. These two areas are selectively innervated by projections from the median raphe nucleus. After systemic morphine, serotonin was increased in the dorsal raphe nucleus, but not in the medial raphe nucleus. Local infusion of morphine through a microdialysis probe in the dorsal raphe nucleus induced a dose-dependent increase of serotonin in the nucleus accumbens, but not in the medial septum. In contrast, infusion of morphine into the median raphe nucleus had little effect on serotonin in either the nucleus accumbens or septum. Infusion of morphine into either the dorsal or median raphe nucleus elicited increased behavioral activity and hyperthermia. These data provide evidence that morphine acts in the area of the dorsal raphe nucleus, but not the median raphe nucleus, to enhance serotonin release in specific forebrain sites, and that the increases in serotonin in the dorsal raphe nucleus projection sites are not an indirect effect of changes in behavioral state or body temperature.

8-OH-DPAT attenuates the dexfenfluramine-induced increase in extracellular serotonin: an in vivo dialysis study

Rats with frontocortical microdialysis probes were treated with dexfenfluramine or dexfenfluramine with 8-hydroxy-2-(di- n-propylamino)tetralin (8-OH-DPAT) pretreatment. Dexfenfluramine (10 mg/kg i.p.) increased extracellular serotonin (5-hydroxytryptamine, 5-HT) (calculated area under the curve (AUC) for the 0 to 105-min period after dexfenfluramine treatment = 8.22 ± 2.66 pmol 5-HT). Systemic (0.025 mg/kg i.p.) or local (0.01 μM into the dorsal raphe nucleus) 8-OH-DPAT pretreatement decreased the dexfenfluramine response (AUC: 1.03 ± 0.07 and 0.44 ± 0.04 pmol 5-HT, respectively). This result might be explained by the decrease in 5-HT neuronal discharge caused by somatodendritic 5-HT 1A autoreceptor activation, and suggests that the 5-HT releasing effect of dexfenfluramine in vivo depends on nerve terminal depolarization.

The anticonvulsant effect of the broad spectrum anticonvulsant loreclezole may be mediated in part by serotonin in rats: a microdialysis study

Loreclezole is an experimental anticonvulsant drug. We found previously that several established anticonvulsants increase extracellular serotonin as measured by microdialysis. We have concluded that the increase in extracellular serotonin and the anticonvulsant effect produced by these anticonvulsant drugs are related in a cause and effect manner. To determine if anticonvulsant doses of loreclezole increase extracellular serotonin, we determined anticonvulsant dose-response relationships in genetically epilepsy-prone rats (GEPRs). Then, we administered ED 99 doses of loreclezole to GEPRs and determined the effect on extracellular serotonin as measured by microdialysis in the striatum. We conclude that loreclezole produces a dose-related anticonvulsant effect in GEPRs and that anticonvulsant doses of loreclezole increase extracellular serotonin in these animals.

Increase in extracellular serotonin produced by uptake inhibitors is enhanced after chronic treatment with fluoxetine

The effect of prolonged uptake inhibition with fluoxetine (10 mg/kg/day i.p. × 14 days) on extracellular serotonin (5-HT) in the rat diencephalon was monitored using in vivo microdialysis. The increase in extracellular 5-HT after repeated administration of fluoxetine was significantly greater than the increase produced by a single injection of this uptake blocker. This difference may have been due to a decrease in somatodendritic autoreceptor sensitivity, since the response to a low dose of the 5-HT 1A agonist 8-OH-DPAT (25 μg/kg i.v.) was abolished in the chronic rats, while the response to a high dose (100 μg/kg i.v.) was attenuated as compared to animals injected once with fluoxetine.

Uptake inhibitors increase extracellular serotonin concentration measured by brain microdialysis

The physiological role of the serotonin transporter on serotonin neuronal membranes apparently is to inactivate serotonin that has been released into the synaptic cleft. Drugs that inhibit the uptake of serotonin increase the amount of serotonin in the synaptic cleft and enhance serotonergic neurotransmission. As an adaptive response to the increased amount of serotonin in the synaptic cleft, serotonin neurons decrease their firing and release of serotonin to limit the magnitude of the increase in extracellular serotonin concentration. The increase in extracellular serotonin in rat brain caused by inhibitors of the serotonin uptake carrier has been characterized by brain microdialysis coupled to liquid chromatography with electrochemical detection. These drugs cause rapid accumulation of extracellular serotonin in several brain regions, although the increase in frontal cortex may be smaller than in other nerve terminal regions or in the cell body-containing raphe region.

Morphine induces an increase in extracellular serotonin in the rat diencephalon

The effect of systemic morphine on extracellular serotonin (5-HT) in the diencephalon of unanesthetized, unrestrained rats was investigated by in vivo microdialysis coupled to high performance liquid chromatography with electrochemical detection. Administration of morphine resulted in a dose dependent increase in extracellular 5-HT. Significant increases were first seen at a dose of 5 mg/kg, and a maximal increase occurred at 10 mg/kg. This increase was blocked by pretreatment with naltrexone, indicating that the effect of morphine of 5-HT was mediated by opiate receptors. Morphine also had a significant effect on extracellular 5-hydroxy indoleacetic acid (5-HIAA), leading to a gradual increase across a range of doses. The effect of morphine on 5-HT was compared to effects of morphine on nociception and catalepsy. Increases in 5-HT and 5-HIAA were first detected at doses that were analgesic but too low to elicit catalepsy. Consistent with many reports that opioids cause an increase in synthesis and turnover of 5-HT in the rat forebrain, the present results provide more direct evidence that 5-HT release is increased after morphine administration.

Fluoxetine attenuates the dl-fenfluramine-induced increase in extracellular serotonin as measured by in vivo dialysis

Rats with hippocampal dialysis probes were treated with dl-fenfluramine (FEN), fluoxetine, or FEN with fluoxetine pre-treatment. FEN (12.5 mg/kg) increased extracellular serotonin (5-HT) from 0.4±0.04 to 25.2±4.16 pg/10 μl. Fluoxetine (10.0 mg/kg) increased extracellular 5-HT levels from 0.4±0.05 to 2.4±0.33 pg/10 μl. FEN-induced increases in extracellular 5-HT were attenuated by 66% with fluoxetine pre-treatment. This result supports the view that the 5-HT releasing properties of FEN are mediated by the 5-HT uptake transporter.

Neuroactive metabolises of L-tryptophan, serotonin and quinolinic acid, in striatal extracellular fluid effect of tryptophan loading

Extracellular fluid levels of the neurotoxin quinolinic acid in the corpus striatum of rats, measured by in vivo microdialysis, were increased in a dose-dependent manner following the intraperitoneal administration of tryptophan. The lowest dose of tryptophan (12.5 mg/kg), equivalent to about 5% of the normal daily intake, increased peak quinolinic acid levels nearly 3-fold. At higher doses of tryptophan (up to 250 mg/kg), concentrations of quinolinic acid increased over 200-fold and exceeded potentially neurotoxic levels (10 μM). In contrast, the increase in extracellular serotonin following even the highest tryptophan dose was small (less than 2-fold). These data indicate that quinolinic acid is present in them extracellular fluid where it may function as a neuromodulator and that it is very responsive to physiological changes in precursor availability.

Fenfluramine administered systemically or locally increases extracellular serotonin in the lateral hypothalamus as measured by microdialysis

Microdialysis was used to monitor serotonin, 5-hydroxyindoleacetic acid (5-HIAA) and the metabolites of dopamine, dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in the perifornical lateral hypothalamus (PFH) of freely moving rats. Systemically administered d-fenfluramine (d-FEN) increased extracellular serotonin, DOPAC and HVA, while decreasing serotonin's metabolite 5-HIAA. Local application of d-FEN directly to the hypothalamic terminal region caused large increases in extracellular serotonin and had a tendency to decrease all 3 metabolites. This effect was confirmed when d-FEN was infused locally by reverse dialysis. These results provide direct evidence that the anorectic drug d-FEN can increase extracellular serotonin in the hypothalamus in vivo and suggest a serotonergic action in the perifornical region. This finding is consistent with our report that a meal also increases serotonin in this region.

Feeding increases extracellular serotonin in the lateral hypothalamus of the rat as measured by microdialysis

Microdialysis probes inserted into chronically implanted guide shafts allowed the collection of serotonin and 5-hydroxyindoleacetic acid (5-HIAA) from the lateral hypothalamus of rats during feeding behavior. After the collection of baseline samples, animals were offered a palatable diet that they could only see and smell for 60-min, then they were allowed access to the food for an hour. An additional three samples were collected after food was removed. Extracellular serotonin increased during the first half hour of access when the animals actually ate the food, and then returned to baseline level throughout the remainder of the test. 5-HIAA decreased gradually with no increase during feeding. These data suggest that eating a meal of palatable food causes a short-term increase in extracellular serotonin in the lateral hypothalamus. This increased serotonin may play a role in the control of lateral hypothalamic feeding and reward.

Serotonergic function in diabetic rats: Psychotherapeutic implications

Extracellular serotonin in striatum was studied in untreated streptozotocin-induced diabetic rats and in untreated nondiabetic rats that served as age-, food-, and sex-matched controls. Extracellular serotonin was studied under anesthesia in vivo and dynamically with voltammetry. The results showed that an early and significant increase in extracellular serotonin occurred in striatum in the untreated acutely (3 days) diabetic rat. In untreated long-term (3–7 weeks) diabetic rats, however, the increase in serotonin in extracellular fluid in striatum decreased and returned to normal. The findings show a change in serotonergic function in acutely diabetic rats. The serotonergic alteration may have psychotherapeutic implications.

Simultaneous microdialysis and amphetamine infusion in the nucleus accumbens and striatum of freely moving rats: Increase in extracellular dopamine and serotonin

To test the effects of systemic and local amphetamine on dopamine and serotonin release in freely moving rats, guide cannulas were implanted in the nucleus accumbens and ventral striatum for removable 200 μ. microdialysis probes. Comparing 45 min samples before and after IP amphetamine (2 mg/kg), dopamine (DA) in dialysate from the accumbens increased from a baseline of 3 pg/20 μl to 11 pg/20 μl; whereas dopamine metabolites, DOPAC and HVA decreased. This was probably due to block of DA reuptake and inhibition of monoamine oxidase, MAO. Accumbens serotonin increased from a baseline of 8 to 11 pg/20 μl. Changes in the ventral striatum were similar. In the second experiment, microdialysis was performed before and after local injection of amphetamine (4 μg) to reveal effects of amphetamine in the terminal area only instead of the whole brain. DA in the accumbens increased from 11 to 147 pg per sample; serotonin, from 11 to 107. The effect was even larger in the ventral striatum. DA increased from 12 to 409 pg/20 μl: serotonin, 3 to 139. To avoid handling the rat or disturbing brain tissue in experiment three, amphetamine was infused via the microdialysis probe during ongoing dialysis sampling. Perfusate containing amphetamine was switched into the flow line which allowed roughly 10% or 4 μg to diffuse out into the extracellular space. During this 20 min, extracellular DA in the accumbens increased from a baseline of 10 pg/20 μl to 300 pg/20 μl sample. Serotonin increased from barely detectable to 60 pg/20 μl. DOPAC decreased significantly: HVA and 5HIAA drifted lower. The observed increase in extracellular serotonin may be important in understanding the behavioral effects of amphetamine as well as the better known effects of dopamine.