Extracellular Serotonin Levels Research Articles

Fatigue of cholestasis and the serotoninergic neurotransmitter system in the rat†

Fatigue associated with cholestasis may impair health-related quality of life. The pathogenesis of this symptom is largely unknown, but it has been suggested that central serotoninergic neurotransmission may be implicated and that serotonin 1A receptor agonists may yield improvement. The aim of this study was to study the central serotoninergic system, specifically the serotonin (5-HT)(1A) receptor-mediated pathway of serotoninergic neurotransmission, in a bile duct resection rat model of cholestasis. Fatigue was assessed in the forced swim test in sham and bile duct-resected rats. The serotonin behavioral syndrome, which includes hyperlocomotion, was assessed in both groups of rats after escalating doses of the 5-HT(1A) receptor agonist 8-hydroxy(di-n-propylamine)tetralin (8-OH DPAT). 5-HT(1A) and 5-HT(2) receptor densities were explored in four brain regions using a receptor-binding assay. Extracellular 5-HT and 5-hydroxyindoleacetic acid were measured via in vivo brain dialysis. Bile duct-resected rats spent more time floating in the forced swim test, and 8-OH DPAT decreased floating time in cholestatic rats (P < .01). Dose-response curves created with 8-OH DPAT for the serotonin behavioral syndrome were similar in bile duct-resected and sham-operated rats. 5-HT(1A) and 5-HT(2) receptor densities in most brain regions and extracellular serotonin levels were similar in both groups of rats. In conclusion, 5-HT(1A) receptor agonist-induced amelioration of fatigue in cholestatic rats may be nonspecific and not linked to reversal of the pathophysiology of fatigue associated with cholestasis; however, these data do not exclude a potential role of the central serotoninergic system in the evolution of fatigue.

SHORT COMMUNICATIONS: Increases in extracellular levels of 5‐HT and dopamine in the basolateral, but not in the central, nucleus of amygdala induced by aversive stimulation of the inferior colliculus

Consistent evidence has shown that dopamine release in the prefrontal cortex is increased by electrical stimulation of the inferior colliculus (IC) as unconditioned stimulus. Recent reports have also demonstrated that inactivation of the basolateral nucleus of the amygdala (BLA) with muscimol enhances the behavioural consequences of the aversive stimulation of the IC and reduces the dopamine release in the prefrontal cortex. Moreover, neurotoxic lesions of the BLA enhance whereas those of the central nucleus of the amygdala (CeA) reduce the aversiveness of the electrical stimulation of the IC. Based on these findings the present study examined the effects of the electrical stimulation of the IC on the extracellular levels of serotonin and dopamine in the BLA and CeA. To this end, rats implanted with a stimulation electrode in the IC also bore a microdialysis probe in the BLA or CeA for determination of the release of dopamine and serotonin. IC electrical stimulation at the freezing and escape thresholds increased the levels of serotonin ( approximately 70%) and dopamine ( approximately 60%) in the BLA related to the basal values. Similarly, the metabolites DOPAC and 5-HIAA increased in a parallel fashion in BLA. No significant changes could be detected in these biogenic amines and metabolites in CeA following IC aversive stimulation. These findings point to a differential role of serotonergic and dopaminergic mechanisms of the BLA and CeA in the setting up of adaptive responses to fear states generated at the inferior colliculus level.

Modafinil enhances the increase of extracellular serotonin levels induced by the antidepressant drugs fluoxetine and imipramine: A dual probe microdialysis study in awake rat

In view of a postulated role of the vigilance-promoting drug modafinil in depression, the interaction of modafinil and two classical antidepressant drugs, fluoxetine and imipramine, were studied in 5-HT levels in the dorsal raphe-cortical system using dual-probe microdialysis. Fluoxetine (1-10 mg/kg) dose-dependently increased dorsal raphe-cortical 5-HT levels. Modafinil at a very low dose (3 mg/kg), by itself ineffective, enhanced the fluoxetine (5 mg/kg)-induced increases of 5-HT levels in both brain areas. A synergistic interaction was observed in the prefrontal cortex with fluoxetine (1 mg/kg) in terms of 5-HT release, but not in the dorsal raphe. Imipramine (1.3 mg/kg) increased 5-HT levels in the dorsal raphe, but not in the prefrontal cortex, while the higher doses (10.9-21.8 mg/kg) caused substantial increases in both brain areas. Modafinil (3 mg/kg), injected before imipramine (1.3 mg/kg), which by itself was ineffective on cortical 5-HT levels, increased cortical 5-HT levels. On other hand, modafinil failed to affect the high-dose imipramine (10.9 mg/kg)-induced increase of 5-HT levels in the prefrontal cortex and the imipramine (1.3; 10.9 mg/kg)-induced increase of 5-HT levels in the dorsal raphe nucleus. These results demonstrate that modafinil in low doses enhances the acute effects of fluoxetine and imipramine on 5-HT levels in the dorsal raphe nucleus (fluoxetine only) and especially in the prefrontal cortex of the awake rat. These findings suggest a therapeutic potential of low doses of modafinil in the treatment of depression when combined with low doses of classical antidepressants, especially by increasing 5-HT transmission in cortical regions.

Amygdalic levels of dopamine and serotonin rise upon exposure to conditioned fear stress without elevation of glutamate

Conditioned fear is an artificial stress, induced by a stimulus, such as a tone, that does not elicit fear in nature. This fear response is acquired by experimental animals when tone is combined with an unconditioned stimulus, such as electrical foot shock. The amygdala is considered to be the area involved in acquisition, consolidation and recall of fear. A series of previous pharmacological studies showed antagonists of dopamine D 1 and D 2, glutamate N-methyl- d-asparatate and (±)-α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptors to prevent the acquisition of conditioned fear. However, little is known about the types of neurotransmitters released when conditioned fear is acquired and recalled. The present study was designed to continuously monitor changes in extracellular levels of glutamate, dopamine and serotonin in the amygdala, at the acquisition of conditioned fear on Day 1 and at fear recall in response to a tone as a conditioned stimulus on Day 2, using the in vivo microdialysis method. Glutamate was elevated only on Day 1, while dopamine and serotonin rose on both days. The periods of elevated dopamine and serotonin were longer on Day 1 than on Day 2. These results suggest that greater amounts of glutamate, dopamine and serotonin are necessary for acquisition than for recall of conditioned fear.

Neuropharmacological profiles of antagonists of group II metabotropic glutamate receptors

Glutamatergic abnormalities play roles in several psychiatric disorders. Glutamate acts at two classes of receptors, ionotropic and metabotropic glutamate receptors (mGluR), the latter is classified into three group, based on receptor homology and signaling mechanisms. Among them, recent pharmacological and histochemical studies suggest that the group II mGluR (mGluR2 and mGluR3) plays crucial roles in the control of emotional states. We previously reported that MGS0039, a selective group II mGluR antagonist, exhibited dose-dependent antidepressant-like effects in some animal models. However, the mechanism by which group II mGluR antagonists exhibit such effects is still unclear. In the present two studies, we examined neuropharmacological effects of group II mGluR antagonists on monoaminergic neurons. In an electrophysiological study, MGS0039 dose-dependently and significantly increased the firing rate of dorsal raphe nucleus (DRN) serotonergic neurons. LY341495, another group II mGluR antagonist, also increased DRN serotonergic neural activity significantly. Consistent with the findings of this electrophysiological study, MGS0039 significantly increased extracellular level of serotonin in rat medial prefrontal cortex in a microdialysis study. In contrast, MGS0039 had no effect on the activity of locus coeruleus noradrenergic neurons. These findings suggest that modulation of serotonergic neuron might be, at least in part, responsible for the antidepressant-like effects of group II mGluR antagonists.

Serotonergic 5-HT2C Receptors as a Potential Therapeutic Target for the Design Antiepileptic Drugs

A variety of clinical observations suggest that certain forms of epilepsy are due to long-term, progressive changes in neural networks that eventually provoke spontaneous and recurring seizures. Recently, there has been growing evidence that serotonergic neurotransmission modulates experimentally induced seizures and is involved in the enhanced seizure susceptibility observed in some genetically epilepsy-prone animals. Generally, agents that elevate extracellular serotonin (5-Hydroxytryptamine, 5-HT) levels, such as 5-hydroxytryptophan, and 5-HT reuptake blockers inhibit both limbic and generalized seizures. Conversely, depletion of brain 5-HT lowers the threshold to audiogenically, chemically and electrically-evoked convulsions. More specifically, the recent finding that the 5-HT2B/2C receptor agonist, 1-(m-chlorophenyl)-piperazine (mCPP) is anticonvulsant has kindled an interest into the investigation of the serotonergic 5-HT2C receptor subtype as a potential target for the treatment of epilepsy. Further pharmacological evaluation of selective activation or inactivation of the 5-HT2C receptor subtype with selective agonist/positive modulators and antagonists will provide important information about the therapeutic contribution of this receptor to the epileptic circuitry in the brain. Future development of serotonergic antiepileptic drugs will be a significant addition to the therapeutic armamentarium against epilepsy.

The 5-HT 1A receptor modulates the effects of cocaine on extracellular serotonin and dopamine levels in the nucleus accumbens

The regulation of extracellular levels of serotonin (5-HT) and dopamine in response to cocaine by 5-HT 1A receptors was examined using in vivo microdialysis and the 5-HT 1A receptor antagonist 4-(2′-methoxy-)-phenyl-1-[2′-( N-2″-pyridinyl)- p-fluorobenzamido-]ethyl-piperazine ( p-MPPF). Pretreatment with p-MPPF significantly augmented the increase in extracellular levels of both 5-HT and dopamine in the nucleus accumbens produced by systemic administration of cocaine. Levels of 5-HT or dopamine were unaffected by p-MPPF given alone. Extracellular levels of 5-HT and dopamine were increased dramatically by cocaine infused locally into the nucleus accumbens. Systemic injection of cocaine given during the cocaine infusion reduced 5-HT and dopamine levels, presumably by activating inhibitory 5-HT and dopamine autoreceptors outside of the locus of infusion. The reduction of 5-HT and dopamine levels by systemic cocaine during accumbal infusion was blocked by pretreatment with the 5-HT 1A receptor antagonist p-MPPF. Taken together, these findings suggest that the 5-HT 1A autoreceptor acts to modulate the effects of cocaine on both 5-HT and dopamine levels in the nucleus accumbens.

Brain Serotonin Transporter Binding Potential Measured With Carbon11–Labeled DASB Positron Emission Tomography

Although brain serotonin transporter (5-HTT) density has been investigated in subjects with a history of major depressive episodes (MDE), there has never been an investigation of brain 5-HTT during a current MDE. Brain 5-HTT binding potential (BP) may have an important role during MDE due to major depressive disorder, because the 5-HTT regulates extracellular 5-HT. The BP is an index of receptor density. Carbon 11-labeled 3-amino-4-(2-dimethylaminomethyl-phenylsulfanyl)-benzonitrile (DASB) positron emission tomography (PET) is the first brain imaging technique that can measure the 5-HTT BP in cortical and subcortical brain regions in vivo. The purposes of this study were to investigate 5-HTT BP during MDE and to determine the relationship between 5-HTT BP and negativistic dysfunctional attitudes during MDE. Dysfunctional attitudes are negatively biased assumptions and beliefs regarding oneself, the world, and the future. Our recent publication of increased serotonin2 BP in MDE with severely negativistic dysfunctional attitudes suggests that this subgroup of MDE subjects has very low levels of extracellular serotonin. Regional 5-HTT BP was measured in 20 nonsmoking medication-free (> or =3 months) depressed subjects and 20 age-matched nonsmoking, medication-free, healthy subjects using [11C]DASB PET. Dysfunctional attitudes were measured using the Dysfunctional Attitudes Scale. No difference in regional 5-HTT BP was found between MDE and healthy subjects; however, the subgroup of MDE subjects with highly negativistic dysfunctional attitudes had significantly higher 5-HTT BP compared with healthy subjects in brain regions mainly sampling serotonergic nerve terminals (prefrontal cortex, anterior cingulate, thalamus, bilateral caudate, and bilateral putamen; average, 21% greater; F(1,26), 5.6-12.2 [P values, .03-.002]). In the MDE subjects, increased 5-HTT BP was strongly associated with more negativistic dysfunctional attitudes in brain regions primarily sampling serotonergic nerve terminals (prefrontal cortex, anterior cingulate, thalamus, caudate, and putamen; r = 0.64-0.74 [P values, .003 to <.001]). Serotonin transporters play an important role during depression. The magnitude of regional 5-HTT BP can provide a vulnerability to low levels of extracellular serotonin and symptoms of extremely negativistic dysfunctional attitudes.

Fluoxetine increases extracellular levels of 3‐methoxy‐4‐hydroxyphenylglycol in cultured COLO320 DM cells

Fluoxetine (Prozac) is a serotonin reuptake inhibitor. It increases extracellular levels of serotonin and is used in relieving the depressive symptoms of cancer patients. It has been reported that the drug may enhance the growth of certain cancer cells. This study investigates whether fluoxetine enhances the growth of a human colon cancer cell line (COLO320 DM) and if it affects the extracellular levels of serotonin or its metabolite, 5-hydroxyindole-3-acetic acid (5-HIAA) and other monoamines and metabolites at two cell densities. The extracellular levels of serotonin, 5-HIAA and other monoamines and metabolites were measured simultaneously by high performance liquid chromatography from cell-culture media after incubation of cells both with and without fluoxetine for 3 days. The viability of COLO320 DM cells was evaluated using 3-(4,5-cimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT). At low cell densities (1.25x10(5) cells ml-1), fluoxetine at 1-10 microM significantly increased the extracellular levels of serotonin (p<0.005), 5-HIAA (p<0.005), and 3-methoxy-4-hydroxyphenylglycol (MHPG; p<0.001) as compared to the controls. Fluoxetine at 10-100 microM significantly inhibited the growth of COLO320 DM (p<0.005). At high cell densities (2x10(6) cells ml-1), fluoxetine at 1-10 microM significantly increased the extracellular levels of MHPG (p<0.01), and at 10 microM it significantly increased the extracellular levels of 5-HIAA (p<0.05). Fluoxetine at 100 microM significantly inhibited the growth of the cells (p<0.0001). These results suggest that fluoxetine at 1 microM of effective concentration may increase the extracellular levels MHPG, in addition to serotonin and 5-HIAA levels, yet not inhibit the growth of COLO320 DM.

Antinociceptive effects of the antidepressants amitriptyline, duloxetine, mirtazapine and citalopram in animal models of acute, persistent and neuropathic pain

The effects of acute, systemic administration of amitriptyline, duloxetine and mirtazapine (antidepressant drugs that variously affect extracellular noradrenaline and serotonin levels) and the selective serotonin reuptake inhibitor (SSRI) citalopram were compared in rat models of experimental pain. None of the drugs (all 3–30 mg/kg, i.p.) affected acute nociceptive responses as measured in the tail flick test. In the hot plate test, duloxetine and mirtazapine significantly increased ( P<0.05) the nociceptive response latency, whereas amitriptyline and citalopram were ineffective. In the formalin test, duloxetine and citalopram significantly attenuated, whereas amitriptyline and mirtazapine increased, second phase flinching behaviour (all P<0.05). However, amitriptyline and mirtazapine reduced second phase licking behaviour. In the chronic constriction injury model of neuropathic pain, thermal hyperalgesia of the injured hindpaw was significantly attenuated by all four drugs ( P<0.05); only amitriptyline and duloxetine fully reversed thermal hypersensitivity. None of the drugs tested attenuated mechanical allodynia. In contrast amitriptyline, duloxetine and mirtazapine significantly reduced mechanical hyperalgesia ( P<0.05); citalopram was ineffective. No drug-related effects on motor performance in the rotarod test were observed. These results (a) highlight the difficulty in correlating antinociceptive effects of drugs from different antidepressant classes across a range of animal pain models and (b) suggest that antidepressants that variously affect both noradrenaline and serotonin levels have more potent and efficacious antinociceptive effects than SSRIs (as exemplified by citalopram), against a range of pain-like behaviours in an animal model of neuropathic pain.

Infant rats with chronic neonatal isolation experience show decreased extracellular serotonin levels in ventral striatum at baseline and in response to cocaine

Previously, we demonstrated that the early life stress of neonatal isolation enhances extracellular dopamine (DA) levels in ventral striatum in response to psychostimulants in infant rats. Yet, neonatal isolation does not alter baseline DA levels. DA levels are affected by serotonin (5-HT) and striatal levels of this transmitter are also enhanced by cocaine. Other early life stresses are reported to alter various 5-HT neural systems. Thus, the purpose of this study is to test whether neonatal isolation alters ventral striatal 5-HT levels at baseline or in response to cocaine. Litters were subjected to neonatal isolation (1-h individual isolation/day on postnatal days 2–9) or to non-handled conditions and pups assigned to one of three cocaine doses (0, 2.5, or 5.0 mg/kg) groups. On postnatal day 10, probes were implanted in the ventral striatum. Dialysate samples obtained over a 60-min baseline period and for 120 min post cocaine injections were assessed for levels of 5-HT and its metabolite, 5-HIAA. ISO decreased ventral striatal 5-HT levels at baseline and after cocaine administration but did not alter 5-HIAA levels. These data add to the literature on the immediate effects of early life stress on 5-HT systems by showing alterations in the ventral striatal system. Because serotonergic effects in this neural area are associated with reward and with emotion and affect regulation, the results of this study suggest that early life stress may be a risk factor for addiction and other psychiatric disorders.

Hypericum perforatum L (St John's wort) preferentially increases extracellular dopamine levels in the rat prefrontal cortex.

The effects of hydro-alcoholic extracts of Hypericum perforatum L on extracellular serotonin (5-HT), noradrenaline (NA) and dopamine (DA) levels and the acidic metabolites (3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA) and 5-hydroxy-3-indoleacetic acid (5-HIAA)) were examined by in vivo microdialysis in the prefrontal cortex of awake rats. Thus, a single dose (60 mg kg(-1) i.p. or 300 mg kg(-1) p.o.) of H. perforatum increased DA concentrations to 165 and 140% of control values, respectively, and increased locomotor activity in nonhabituated rats. DOPAC and HVA levels were markedly reduced. 5-HT concentrations were elevated only moderately, while the NA levels were not affected by any treatment. The whole-tissue analysis revealed that hypericum increased, whereas the monoamine oxidase (MAO) A/B inhibitor phenelzine decreased DA and 5-HT turnover. The present data indicate that the mechanism of action of hypericum extract in vivo is more complex than the inhibition of monoamine reuptake or metabolism observed in vitro. The finding of preferential enhancement of DA transmission is in agreement with human studies measuring DA-mediated neuroendocrine responses.

Inactivation of 5-HT2C Receptors Potentiates Consequences of Serotonin Reuptake Blockade

The enhancement of central serotonin system function underlies the therapeutic effects of selective serotonin reuptake inhibitors (SSRIs), which have become the most commonly used class of antidepressant agents. However, many individuals experience depressive episodes that are resistant to SSRI treatment. Homeostatic mechanisms that limit the extent to which SSRIs enhance serotonergic neurotransmission have been implicated in this phenomenon. Here, we report a novel strategy for enhancing the efficacy of SSRIs. Using in vivo microdialysis methods in rats, the nonselective 5-HT2 receptor antagonist ketanserin was observed to produce a robust augmentation of citalopram-, fluoxetine-, and sertraline-induced elevations of hippocampal extracellular serotonin levels. Similar effects were also observed in cortex. The potentiation of SSRI-induced increases in hippocampal serotonin levels was reproduced by the 5-HT(2C) receptor-selective antagonists SB 242084 and RS 102221, but not by the 5-HT(2A) receptor-selective antagonist MDL 100 907. Although 5-HT(2C) receptor antagonists augmented the actions of SSRIs, they had no effect on extracellular serotonin levels or tail suspension responses when administered alone. These results were in strong accord with independent findings using a line of 5-HT(2C) receptor-null mutant mice. Although this mutation did not affect baseline extracellular serotonin levels or tail suspension test (TST) behavior, it enhanced fluoxetine effects on serotonin levels and immobility in the TST. These findings reveal an unanticipated pharmacological action of 5-HT(2C) receptors that warrants consideration in the development of novel strategies for the treatment of depression.

Dissociable roles for the dorsal and median raphé in the facilitatory effect of 5-HT1A receptor stimulation upon cocaine-induced locomotion and sensitization.

A distinct role for serotonin transmission from the dorsal and median raphé nuclei (DRN and MRN, respectively) was identified in regulating the behavioral and neurochemical effects of acute and repeated cocaine administration. Serotonin 1A (5-hydroxytryptophan (5-HT)1A) receptors were stimulated by intraraphé microinjection of 8-hydroxy-2-(di-n-propylamino)tetralin (DPAT; 5 or 10 microg) and behavior, as well as extracellular neurotransmitter content in the nucleus accumbens was measured. Pretreatment of the DRN with DPAT caused a sensitization-like potentiation of acute cocaine-induced motor activity and an elevation in extracellular dopamine and glutamate. In contrast, DPAT microinjection into the MRN did not alter acute cocaine-induced motor activity or extracellular levels of dopamine or glutamate. Acutely, DPAT microinjection into either raphé nucleus reduced the basal and acute cocaine-stimulated levels of extracellular serotonin. Pretreatment with DPAT before systemic cocaine administration was continued for 5 days, and 3 weeks after the last injection, all rats were administered a cocaine challenge injection. The sensitized behavioral and neurochemical response produced by repeated cocaine in control subjects was unaffected by the intra-DRN administration of DPAT. However, in animals administered DPAT into the MRN, both the sensitized motor response and the increase in glutamate were augmented, while the sensitized serotonin response was blocked, without altering dopamine sensitization. These data show a differential role for 5-HT1A receptors in the DRN and MRN in the acute and sensitized effects of cocaine. While the DRN is involved in the acute effects of cocaine, neuroadaptations in the MRN may regulate the long-term consequences of repeated cocaine exposure.

Extracellular serotonin, dopamine and glutamate levels are elevated in the hypothalamus in a serotonin syndrome animal model induced by tranylcypromine and fluoxetine

Serotonin (5-HT) syndrome is a potentially fatal condition associated with various combinations of serotonergic drugs. The present study was undertaken to demonstrate that nervous systems other than the 5-HT system also participate in the pathophysiology of 5-HT syndrome. Concentrations of 5-HT, dopamine (DA) and glutamate in the hypothalamus were measured in two different 5-HT syndrome animal models using a microdialysis technique. The first model was induced by tranylcypromine, a nonselective monoamine oxidase (MAO) inhibitor (3.5 mg/kg) and fluoxetine, a selective serotonin reuptake inhibitor (SSRI) (10 mg/kg). The second model was induced by clorgyline, an MAO-A inhibitor (1.2 mg/kg) and 5-hydroxy- l-tryptophan, a precursor of 5-HT (5-HTP) (80 mg/kg). In the first model, the levels of 5-HT and DA increased by 40-fold and 44-fold, respectively, compared with the preadministration levels. In the second model, the concentrations of 5-HT increased by up to 140-fold, whereas DA levels increased by only 10-fold, of the preadministration levels. Although the level of glutamate in the second model barely changed, a delayed increase in the glutamate level was observed in the first model. These findings suggest that not only hyperactivity of the 5-HT system, but also hyperactivity of the DA system, are present in 5-HT syndrome, and that the glutamatergic system is influenced in some 5-HT syndrome cases in which the DA concentration markedly increases.

Animal pain models: Neurochemical changes in the RVM associated with a peripheral inflammatory pain stimulus

The purpose of the current study was to investigate neurochemical changes in the rostral ventromedial medulla (RVM) following a peripheral inflammatory pain stimulus in rats. In vivo microdialysis probes were surgically implanted into the RVM. Baseline neurochemical samples were collected every 20-min, after which baseline thermal sensitivity was assessed. After neurochemical and behavioral baselines were established, a dose of 0.1 mg of prostaglandin E2 (PGE2 ) was then injected (0.05 ml) into the left hind footpad and thermal hypersensitivity was assessed every 20-min starting 10-min after PGE2 injection; microdialysate was drawn every 20-min starting 20-min after PGE2 injection. PGE2 produced maximal thermal hypersensitivity 10-min after administration, which correlated with a 230\% increase of extracellular serotonin levels in the RVM. Thermal hypersensitivity persisted even though serotonin levels decreased. In contrast, norepinephrine, dopamine, GABA and glutamate levels were not significantly changed from baseline over the course of the study. A dose of morphine (5.6 mg/kg; IP) that attenuated PGE2-induced thermal hypersensitivity also attenuated the increase of serotonin in the RVM. In contrast, morphine did not modify other neurochemicals. The results of the present study show that increases in serotonin correlate with the initiation of thermal hypersensitive but not the maintenance of peripheral inflammatory pain. Additionally, the current study demonstrates that the combination of behavioral endpoints with microdialysis could lead to important insights related to pain circuitry and novel target identification.

5‐HT6 receptor antagonist SB‐271046 enhances extracellular levels of monoamines in the rat medial prefrontal cortex

The present study investigated the neurochemical effects of the selective 5-HT(6) receptor antagonist SB-271046 in the rat medial prefrontal cortex (mPFC). The effect of SB-271046 on extracellular levels of dopamine (DA), norepinephrine (NE), and serotonin (5-HT) in the mPFC was examined using in vivo microdialysis in the freely moving rat. SB-271046 (10 mg/kg, p.o.) produced a significant increase in extracellular levels of both DA and NE without altering 5-HT neurotransmission. These results further support the rationale for the use of 5-HT(6) receptor antagonists in the treatment of cognitive dysfunction associated with psychiatric diseases.

Total sleep deprivation increases extracellular serotonin in the rat hippocampus

Sleep deprivation exerts antidepressant effects after only one night of deprivation, demonstrating that a rapid antidepressant response is possible. In this report we tested the hypothesis that total sleep deprivation induces an increase in extracellular serotonin (5-HT) levels in the hippocampus, a structure that has been proposed repeatedly to play a role in the pathophysiology of depression. Sleep deprivation was performed using the disk-over-water method. Extracellular levels of 5-HT were determined in 3 h periods with microdialysis and measured by high performance liquid chromatography coupled with electrochemical detection. Sleep deprivation induced an increase in 5-HT levels during the sleep deprivation day. During an additional sleep recovery day, 5-HT remained elevated even though rats displayed normal amounts of sleep. Stimulus control rats, which had been allowed to sleep, did not experience a significant increased in 5-HT levels, though they were exposed to a stressful situation similar to slee-deprived rats. These results are consistent with a role of 5-HT in the antidepressant effects of sleep deprivation.

Regulation of the release of serotonin in the dorsal raphe nucleus by alpha1 and alpha2 adrenoceptors.

To investigate the modulation of serotonin release in the dorsal raphe nucleus (DRN) by alpha(1) and alpha(2) adrenoceptors, dual-probe microdialysis was performed in conscious rats. The specific alpha(1) and alpha(2) adrenoceptor agonists and antagonists were locally infused into the DRN via retrograde microdialysis. The release of serotonin was simultaneously sampled from the DRN and prefrontal cortex (PFC). Infusion of the alpha(1) adrenoceptor agonist cirazoline into the DRN (100 microM) produced an increase in the release of serotonin in the DRN to 200% of the basal levels, but no effect was seen in the PFC. After infusion of the alpha(1) adrenoceptor antagonist prazosin into the DRN (100 microM) the release of serotonin decreased in the DRN and PFC to about 40% and 65% of the basal levels, respectively. Infusion of the alpha(2) adrenoceptor agonist clonidine into the DRN (100 microM) slightly but significantly decreased the level of serotonin in the DRN as well as in the PFC to about 70% of the basal levels. Infusion of the alpha(2A) adrenoceptor antagonist BRL 44408 into the DRN (100 microM) caused an increase of serotonin release in the DRN to 270% of the basal levels, but at the same time no changes were seen in the extracellular levels of serotonin in the PFC. The present study demonstrates that alpha(1) as well as alpha(2) adrenoceptors in the DRN modulate the release of serotonin in the DRN, and that alpha(1) adrenoceptors in the DRN are maximally stimulated during resting conditions.

Dopamine depletion abolishes apomorphine- and amphetamine-induced increases in extracellular serotonin levels in the striatum of conscious rats: a microdialysis study

We investigated how serotonergic neurotransmission was affected by 6-hydroxydopamine (6-OHDA) lesioning of the adult rat brain dopamine (DA) system. In this animal model for Parkinson's disease (PD), the effect of destroying ascending DA pathways on extracellular levels of serotonin (5-HT) and 5-HT innervation in rat striatum were examined. Profound unilateral lesions of the nigrostriatal DA pathways were made by infusing 6-OHDA unilaterally into either the right medial forebrain bundle or the right substantia nigra. At 5 weeks after lesioning extracellular levels of DA and 5-HT were determined with microdialysis and high-pressure liquid chromatography under basal conditions and after systemic injections of apomorphine or amphetamine. DA nerve-terminal destruction and 5-HT innervation were determined with quantitative autoradiography. 6-OHDA lesioning reduced extracellular levels of DA below detection limits and led to statistically significant increases in extracellular 5-HT. Apomorphine, and amphetamine, respectively increased extracellular 5-HT to 8.2- and 2.2-fold above baseline levels in intact animals; these effects were absent in 6-OHDA-lesioned animals. Basal levels of [ 3H]paroxetine binding to 5-HT transporters in caudate-putamen increased by 41% in 6-OHDA-lesioned animals. These results suggest that 6-OHDA lesioning led to hyperinnervation of 5-HT nerve terminals and increases in basal extracellular 5-HT levels, but also to an unexplained loss of apomorphine and amphetamine-induced release of 5-HT. Addressing whether this impairment has significance in the onset of PD might lead to development of new strategies to manage parkinsonian symptoms.