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

Effects of low-doses of methamphetamine on d-fenfluramine-induced head-twitch response (HTR) in mice during ageing and c-fos expression in the prefrontal cortex

BACKGROUND

The head-twitch response (HTR) in mice is considered a behavioral model for hallucinogens and serotonin 5-HT_2A receptor function, as well as Tourette syndrome in humans. It is mediated by 5-HT_2A receptor agonists such as ( ±)- 2,5-dimethoxy-4-iodoamphetamine (DOI) in the prefrontal cortex (PFC). The 5-HT_2A antagonist EMD 281014, can prevent both DOI-induced HTR during ageing and c-fos expression in different regions of PFC. Moreover, the nonselective monoamine releaser methamphetamine (MA) suppressed DOI-induced HTR through ageing via concomitant activation of inhibitory 5-HT_1A receptors, but enhanced DOI-evoked c-fos expression. d-Fenfluramine is a selective 5-HT releaser and induces HTR in mice, whereas MA does not. Currently, we investigated whether EMD 281014 or MA would alter: (1) d-fenfluramine-induced HTR frequency in 20-, 30- and 60-day old mice, (2) d-fenfluramine-evoked c-fos expression in PFC, and (3) whether blockade of inhibitory serotonergic 5-HT_1A- or adrenergic ɑ₂-receptors would prevent suppressive effect of MA on d-fenfluramine-induced HTR.

RESULTS

EMD 281014 (0.001-0.05 mg/kg) or MA (0.1-5 mg/kg) blocked d-fenfluramine-induced HTR dose-dependently during ageing. The 5-HT_1A antagonist WAY 100635 countered the inhibitory effect of MA on d-fenfluramine-induced HTR in 30-day old mice, whereas the adrenergic ɑ₂ antagonist RS 79948 reversed MA's inhibitory effect in both 20- and 30- day old mice. d-Fenfluramine significantly increased c-fos expressions in PFC regions. MA (1 mg/kg) pretreatment significantly increased d-fenfluramine-evoked c-fos expression in different regions of PFC. EMD 281014 (0.05 mg/kg) failed to prevent d-fenfluramine-induced c-fos expression, but significantly increased it in one PFC region (PrL at - 2.68 mm).

CONCLUSION

EMD 281014 suppressed d-fenfluramine-induced HTR but failed to prevent d-fenfluramine-evoked c-fos expression which suggest involvement of additional serotonergic receptors in the mediation of evoked c-fos. The suppressive effect of MA on d-fenfluramine-evoked HTR is due to well-recognized functional interactions between stimulatory 5-HT_2A- and the inhibitory 5-HT_1A- and ɑ₂-receptors. MA-evoked increases in c-fos expression in PFC regions are due to the activation of diverse monoaminergic receptors through increased synaptic concentrations of 5-HT, NE and/or DA, which may also account for the additive effect of MA on d-fenfluramine-evoked changes in c-fos expression. Our findings suggest potential drug receptor functional interaction during development when used in combination.

Expression of 22 serotonin-related genes in rat brain after sub-acute serotonin depletion or reuptake inhibition

OBJECTIVE

Although the assessment of expression of serotonin-related genes in experimental animals has become a common strategy to shed light on variations in brain serotonergic function, it remains largely unknown to what extent the manipulation of serotonin levels causes detectable changes in gene expression. We therefore chose to investigate how sub-acute depletion or elevation of brain serotonin influences the expression of a number of serotonin-related genes in six brain areas.

METHODS

Male Wistar rats were administered a serotonin synthesis inhibitor, para-chlorophenylalanine (p-CPA), or a serotonin reuptake inhibitor, paroxetine, for 3 days and then sacrificed. The expression of a number of serotonin-related genes in the raphe nuclei, hypothalamus, amygdala, striatum, hippocampus and prefrontal cortex was investigated using real-time quantitative PCR (rt-qPCR).

RESULTS

While most of the studied genes were uninfluenced by paroxetine treatment, we could observe a robust downregulation of tryptophan hydroxylase-2 in the brain region where the serotonergic cell bodies reside, that is, the raphe nuclei. p-CPA induced a significant increase in the expression of Htr1b and Htr2a in amygdala and of Htr2c in the striatum and a marked reduction in the expression of Htr6 in prefrontal cortex; it also enhanced the expression of the brain-derived neurotrophic factor (Bdnf) in raphe and hippocampus.

CONCLUSION

With some notable exceptions, the expression of most of the studied genes is left unchanged by short-term modulation of extracellular levels of serotonin.

Chronic deep brain stimulation of the rat ventral medial prefrontal cortex disrupts hippocampal-prefrontal coherence

Deep brain stimulation (DBS) of the subgenual cingulate gyrus (SCG) has been used to treat patients with treatment-resistant depression. As in humans, DBS applied to the ventromedial prefrontal cortex of rats induces antidepressant-like responses. Physiological interactions between structures that play a role in depression and antidepressant treatment are still unknown. The present study examined the effect of DBS on inter-region communication by measuring the coherence of local field potentials in the rat infralimbic cortex (IL; homologue of the SCG) and one of its major afferents, the ventral hippocampus (VH). Rats received daily IL DBS treatment (100 μA, 90 μs, 130 Hz; 8h/day). Recordings were conducted in unrestrained, behaving animals on the day before treatment, after 1 and 10 days of treatment, and 10 days stimulation offset. VH-IL coherence in the 2-4 Hz range was reduced in DBS-treated animals compared with shams after 10 days, but not after only 1 day of treatment. No effect of DBS was observed in the 6-10 Hz (theta) range, where coherence was generally high and could be further evoked with a loud auditory stimulus. Finally, coherence was not affected by fluoxetine (10mg/kg), suggesting that the effects of DBS were not likely mediated by increased serotonin levels. While these data support the hypothesis that DBS disrupts communication between regions important for expectation-based control of emotion, they also suggest that lasting physiological effects require many days of treatment and, furthermore, may be specific to lower-frequency patterns, the nature and scope of which await further investigation.

Evidence for a role of transporter-mediated currents in the depletion of brain serotonin induced by serotonin transporter substrates

Serotonin (5-HT) transporter (SERT) substrates like fenfluramine and 3,4-methylenedioxymethamphetamine cause long-term depletion of brain 5-HT, while certain other substrates do not. The 5-HT deficits produced by SERT substrates are dependent upon transporter proteins, but the exact mechanisms responsible are unclear. Here, we compared the pharmacology of several SERT substrates: fenfluramine, d-fenfluramine, 1-(m-chlorophenyl)piperazine (mCPP) and 1-(m-trifluoromethylphenyl)piperainze (TFMPP), to establish relationships between acute drug mechanisms and the propensity for long-term 5-HT depletions. In vivo microdialysis was carried out in rat nucleus accumbens to examine acute 5-HT release and long-term depletion in the same subjects. In vitro assays were performed to measure efflux of [(3)H]5-HT in rat brain synaptosomes and transporter-mediated ionic currents in SERT-expressing Xenopus oocytes. When administered repeatedly to rats (6 mg/kg, i.p., four doses), all drugs produce large sustained elevations in extracellular 5-HT (>5-fold) with minimal effects on dopamine. Importantly, 2 weeks after dosing, only rats exposed to fenfluramine and d-fenfluramine display depletion of brain 5-HT. All test drugs evoke fluoxetine-sensitive efflux of [(3)H]5-HT from synaptosomes, but d-fenfluramine and its bioactive metabolite d-norfenfluramine induce significantly greater SERT-mediated currents than phenylpiperazines. Our data confirm that drug-induced 5-HT release probably does not mediate 5-HT depletion. However, the magnitude of transporter-mediated inward current may be a critical factor in the cascade of events leading to 5-HT deficits. This hypothesis warrants further study, especially given the growing popularity of designer drugs that target SERT.

The role of serotonin, vasopressin, and serotonin/vasopressin interactions in aggressive behavior

Aggression control has been investigated across species and is centrally mediated within various brain regions by several neural systems that interact at different levels. The debate over the degree to which any one system or region affects aggressive responding, or any behavior for that matter, in some senses is arbitrary considering the plastic and adaptive properties of the central nervous system. Nevertheless, from the reductionist point of view, the compartmentalization of evolutionarily maladaptive behaviors to specific regions and systems of the brain is necessary for the advancement of clinical treatments (e.g., pharmaceutical) and novel therapeutic methods (e.g., deep brain stimulation). The general purpose of this chapter is to examine the confluence of two such systems, and how their functional interaction affects aggressive behavior. Specifically, the influence of the serotonin (5HT) and arginine vasopressin (AVP) neural systems on the control of aggressive behavior will be examined individually and together to provide a context by which the understanding of aggression modulation can be expanded from seemingly parallel neuromodulatory mechanisms, to a single and highly interactive system of aggression control.

Fenfluramine-induced hypothermia is associated with cutaneous dilation in conscious rats

The antiobesity agent, fenfluramine, produces hypothermia in rodents by an, as yet, uncharacterized mechanism. The present study was conducted in conscious rats to determine if fenfluramine-induced hypothermia was associated with cutaneous dilation. In animals maintained at 16 degrees C, core body temperature (T(CORE)) was measured telemetrically, and tail surface temperature was monitored with thermocouples fixed to the tail (T(TAIL)). D-Fenfluramine (10 mg/kg ip) produced a rapid increase in T(TAIL) of 7.7+/-0.4 degrees C (P<.001) and a decline in T(CORE) of 4+/-0.3 degrees C (P<.001). Two findings indicate that the increase in T(TAIL) was due to the withdrawal of a sympathetic vasoconstrictor tone. First, pretreatment with the ganglionic blocker, pentolinium, prevented fenfluramine-induced changes in T(TAIL). Second, when sympathetic tone to the tail was physiologically withdrawn by increasing the environmental temperature to 28 degrees C, fenfluramine treatment produced no increase in T(TAIL). Moreover, the effects of fenfluramine on T(TAIL) and T(CORE) depended on the uptake of fenfluramine into serotonergic neurons because these effects were markedly attenuated by pretreatment with the selective serotonin re-uptake inhibitor, fluoxetine. The hypothermic effect of fenfluramine occurred despite the fact that total body oxygen consumption increased by 20%. The results suggest that heat loss due to the dilation of the cutaneous circulation contributes to fenfluramine-induced hypothermia.

Anticonvulsant action of hippocampal dopamine and serotonin is independently mediated by D2 and 5-HT1A receptors

The present microdialysis study evaluated the anticonvulsant activity of extracellular hippocampal dopamine (DA) and serotonin (5-HT) with concomitant assessment of the possible mutual interactions between these monoamines. The anticonvulsant effects of intrahippocampally applied DA and 5-HT concentrations were evaluated against pilocarpine-induced seizures in conscious rats. DA or 5-HT perfusions protected the rats from limbic seizures as long as extracellular DA or 5-HT concentrations ranged, respectively, between 70-400% and 80-350% increases compared with the baseline levels. Co-perfusion with the selective D(2) blocker remoxipride or the selective 5-HT(1A) blocker WAY-100635 clearly abolished all anticonvulsant effects. These anticonvulsant effects were mediated independently since no mutual 5-HT and DA interactions were observed as long as extracellular DA and 5-HT levels remained within these protective ranges. Simultaneous D(2) and 5-HT(1A) receptor blockade significantly aggravated pilocarpine-induced seizures. High extracellular DA (> 1000% increases) or 5-HT (> 900% increases) concentrations also worsened seizure outcome. The latter proconvulsive effects were associated with significant increases in extracellular glutamate (Glu) and mutual increases in extracellular monoamines. Our results suggest that, within a certain concentration range, DA and 5-HT contribute independently to the prevention of hippocampal epileptogenesis via, respectively, D(2) and 5-HT(1A) receptor activation.

Effects of fenfluramine and antidepressants on protein kinase C activity in rat cortical synaptoneurosomes

Fenfluramine releases serotonin (5-HT) via the 5-HT transporter (SERT). Previous work has shown that amphetamine increases particulate protein kinase C (PKC) activity in striatal synaptoneurosomes. The increased PKC activity is linked to the outward transport of dopamine, and when release is diminished, the inward transport of amphetamine inhibits PKC instead. Since there is homology among monoamine transporters, this study was undertaken to determine if D-fenfluramine has similar effects on PKC. The role of 5-HT receptors and endogenous 5-HT were also examined. Naive rats and rats pretreated with p-chlorophenylalanine (PCPA), a 5-HT synthesis inhibitor, were sacrificed. Cortical synaptoneurosomes were prepared and incubated with fenfluramine. PKC activity was determined by thiophosphorylation of endogenous substrates. It was found that 5-HT, D/L-fenfluramine, and D-fenfluramine increased PKC activity in a time- and dose-dependent manner. The 5-HT-mediated increase in PKC activity was attenuated by pretreatment with the 5-HT(2) antagonist ketanserin, but not with the SERT inhibitor fluoxetine. The D-fenfluramine-induced increase in PKC activity was completely prevented, however, by pretreatment with SERT inhibitors and partially with ketanserin. It was also attenuated by pretreatment with PCPA, resulting in a dose-dependent inhibition of PKC instead. Thus, when 5-HT release was diminished the uptake of D-fenfluramine inhibited PKC. Similar effects have been observed with amphetamine. Unlike D-fenfluramine, the D/L-fenfluramine-induced increase in PKC activity was partially resistant to PCPA pretreatment but was attenuated with bupropion, a dopamine transporter (DAT) inhibitor. SERT inhibitors (sertraline, paroxetine, citalopram, and fluoxetine) also increased PKC activity. Nefazodone and bupropion increased PKC activity, but mirtazapine was relatively inactive. The SERT inhibitor-induced increase in PKC was unaffected by pretreatment with PCPA but was inhibited by calcium. Similar effects on PKC activity have been observed with DAT inhibitors. These results, showing that D-fenfluramine altered PKC activity similar to D-amphetamine, suggest that the topographic homology between DAT and SERT may extend to their effects on PKC activity.

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 [(3)H]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.

Effect of 5-HT on binding of [(11)C] WAY 100635 to 5-HT(IA) receptors in rat brain, assessed using in vivo microdialysis nd PET after fenfluramine

By using a combination of positron emission tomography (PET) and postmortem tissue dissection, the effect of increased endogenous serotonin on specific binding of [(11)C]WAY 100635 to the 5-HT(1A) receptor was investigated in rat brain in vivo. The binding studies were complemented by in vivo microdialysis to monitor 5-HT levels in similarly treated isoflurane-anaesthetised rats, with the dialysis probe locations corresponding to two of the tissues sampled for specific binding of the radioligand. Fenfluramine treatment (10 mg/kg i.p.) resulted in a approximately 5-fold increase in extracellular 5-HT in medial prefrontal cortex and a approximately 15-fold increase in lateral hippocampus, maximal at approximately 40 min after injection. PET scan duration was either 60 or 90 min, beginning 30 min after fenfluramine injection. The specific binding of [(11)C]WAY 100635 was reduced by 10-20% in hippocampus, which showed highest binding in control animals. Specific binding, however, was unaffected in both prefrontal cortex and midbrain raphe, each additional high binding regions. The minimal effects are consistent with a low baseline occupancy of the 5-HT(1A) receptor by 5-HT in vivo, so that only a large change in endogenous agonist concentration will affect radioligand binding. This implies that utilisation of [(11)C]WAY 100635 in human PET to quantify 5-HT(1A) receptor expression can be extended to pathology where synaptic 5-HT levels are altered as a consequence of the disease state.

Serotonin transporters, serotonin release, and the mechanism of fenfluramine neurotoxicity

Administration of d,l-fenfluramine (FEN), or the more active isomer d-fenfluramine (dFEN), causes long-term depletion of forebrain serotonin (5-HT) in animals. The mechanism underlying FEN-induced 5-HT depletion is not known, but appears to involve 5-HT transporters (SERTs) in the brain. Some investigators have postulated that 5-HT release evoked by FEN is responsible for the deleterious effects of the drug. In the present work, we sought to examine the relationship between drug-induced 5-HT release and long-term 5-HT depletion. The acute 5-HT-releasing effects of dFEN and the non-amphetamine 5-HT agonist 1-(m-chlorophenyl)piperazine (mCPP) were evaluated using in vivo microdialysis in rat nucleus accumbens. The ability of dFEN and mCPP to interact with SERTs was assessed using in vitro assays for [3H]-transmitter uptake and release in rat forebrain synaptosomes. Drugs were subsequently tested for potential long-lasting effects on brain tissue 5-HT after repeated dosing (2.7 or 8.1 mg/kg, ip x 4). dFEN and mCPP were essentially equipotent in their ability to stimulate acute 5-HT release in vivo and in vitro. Both drugs produced very selective effects on 5-HT with minimal effects on dopamine. Interestingly, when dFEN or mCPP was administered repeatedly, only dFEN caused long-term 5-HT depletion in the forebrain at 2 weeks later. These data suggest that acute 5-HT release per se does not mediate the long-term 5-HT depletion associated with dFEN. We hypothesize that dFEN and other amphetamine-type releasers gain entrance into 5-HT neurons via interaction with SERTs. Once internalized in nerve terminals, drugs accumulate to high concentrations, causing damage to cells. The relevance of this hypothesis for explaining clinical side effects of FEN and dFEN, such as cardiac valvulopathy and primary pulmonary hypertension, warrants further study.

Sibutramine does not decrease the number of 5-HT re-uptake sites in rat brain and, like fluoxetine, protects against the deficits produced by dexfenfluramine

The effect of sibutramine and dexfenfluramine on 5-HT re-uptake sites, labelled with [(3)H]paroxetine, have been determined in various rat brain regions. In addition, the ability of fluoxetine and sibutramine to protect against the changes in [(3)H]paroxetine binding produced by dexfenfluramine was examined. Sibutramine (9 mg/kg, p.o.) and dexfenfluramine (1, 3 and 10 mg/kg, p.o.) were administered twice daily (before 09.00 h and after 16.00 h) for four days, followed by a 14 day drug-free period. In the protection studies, fluoxetine (10 mg/kg, i.p.) and sibutramine (9 mg/kg, p.o.) were given 1 h prior to dexfenfluramine (10 mg/kg, p.o.) using the same dosing regimen as described above. Sibutramine (9 mg/kg, p.o.; three times its ED(50) to inhibit food intake at 2 h) had no significant effect on the number or affinity of 5-HT re-uptake sites the brain regions studied. In contrast, dexfenfluramine at an equivalent dose (3 mg/kg, p.o.) significantly decreased the number of 5-HT re-uptake sites in frontal cortex (by 35%), hippocampus (by 47%) and hypothalamus (by 27%). This effect was dose-dependent with marked decreases (by 58-84%) in the number of sites following 10 mg/kg, p.o. These effects were not associated with changes in binding affinity. Fluoxetine (10 mg/kg, i.p.) completely blocked the effect of dexfenfluramine (10 mg/kg, p.o.) without having any significant effect alone. Sibutramine (9 mg/kg, p.o.) also blocked the effects of dexfenfluramine, although the reversal was only partial in frontal cortex, hippocampus and hypothalamus. Thus sibutramine, unlike dexfenfluramine, does not alter brain 5-HT re-uptake sites. Furthermore, sibutramine and fluoxetine protect against the deficits in 5-HT re-uptake sites produced by dexfenfluramine. These data provide further evidence that sibutramine is a 5-HT re-uptake inhibitor and it does not have neurotoxic potential.

Homeostatic regulation of serotonergic function by the serotonin transporter as revealed by nonviral gene transfer

With the aim of exploring the relationship between the serotonin transporter (5-HTT or SERT) and the activity level of serotonin (5-HT) neurotransmission, in vivo expression of this protein was specifically altered using a nonviral DNA transfer method. Plasmids containing the entire coding sequence or a partial antisense sequence of the 5-HTT gene were complexed with the cationic polymer polyethylenimine and injected into the dorsal raphe nucleus of adult male rats. Significant increase or decrease in both [(3)H]citalopram binding and [(3)H]5-HT synaptosomal uptake were observed in various brain areas up to 2 weeks after a single administration of the sense plasmid or 7 d after injection of the short antisense plasmid, respectively. Such changes in 5-HTT expression were associated with functional alterations in 5-HT neurotransmission, as shown by the increased capacity of 5-HT(1A) receptor stimulation to enhance [(35)S]GTP-gamma-S binding onto the dorsal raphe nucleus in sections from rats injected with the sense plasmid. Conversely, both a decrease in 5-HT(1A)-mediated [(35)S]GTP-gamma-S binding and a reduced potency of the 5-HT(1A) receptor agonist ipsapirone to inhibit neuronal firing were observed in the dorsal raphe nucleus of antisense plasmid-injected rats. Furthermore, changes in brain 5-HT and/or 5-HIAA levels, and sleep wakefulness circadian rhythm in the latter animals demonstrated that altered expression of 5-HTT by recombinant plasmids has important functional consequences on central 5-HT neurotransmission in adult rats.

Comparative effects of continuous infusion of mCPP, Ro 60-0175 and d-fenfluramine on food intake, water intake, body weight and locomotor activity in rats

1. The aim of the study was to compare the effects of 14 day subcutaneous infusion of the 5-HT(2C) receptor agonists, m-chlorophenylpiperazine (mCPP, 12 mg kg(-1) day(-1)) and Ro 60-0175 (36 mg kg(-1) day(-1)) and the 5-HT releasing agent and re-uptake inhibitor, d-fenfluramine (6 mg kg(-1) day(-1)), on food and water intake, body weight gain and locomotion in lean male Lister hooded rats. 2. Chronic infusion of all three drugs significantly reduced food intake and attenuated body weight gain. In contrast, drug infusion did not lead to significant reductions in locomotor activity in animals assessed 2 and 13 days after pump implantation. 3. In a subsequent 14 day study that was designed to identify possible tolerance during days 7 - 14, animals were given a subcutaneous infusion of mCPP (12 mg kg(-1) day(-1)) or d-fenfluramine (6 mg kg(-1) day(-1)) for either 7 or 14 days. During the first 7 days both drugs significantly reduced body weight gain compared to saline-infused controls; however, from day 7 onwards animals withdrawn from drug treatment exhibited an increase in body weight such that by day 14 they were significantly heavier than their 14-day drug-treated counterparts. 4. Both mCPP and d-fenfluramine reduced daily food intake throughout the infusion periods. For 14-day treated animals this hypophagia was marked during the initial week of the study but only minor during the second week. In light of the sustained drug effect on body weight, the data suggest that weight loss by 5-HT(2C) receptor stimulation may be only partly dependent on changes in food consumption and that 5-HT(2C) receptor agonists may have effects on thermogenesis. 5. These data suggest tolerance does not develop to the effects of d-fenfluramine, mCPP and Ro 60-0175 on rat body weight gain.

Effect of acute treatment with YM992 on extracellular serotonin levels in the rat frontal cortex

(S)-2-[[(7-fluoroindan-4-yl)oxy]methyl]morpholine monohydrochloride (YM992) is a novel putative antidepressant exhibiting both selective serotonin (5-hydroxytryptamine, 5-HT) reuptake inhibition and 5-HT(2A) receptor antagonism. In vivo microdialysis revealed that a single treatment with YM992 (3, 10, 30 mg/kg i.p.) dose-dependently increased extracellular 5-HT levels in the rat frontal cortex. Fluoxetine, citalopram and venlafaxine also produced significant increases in 5-HT levels at doses of 10-30 mg/kg. However, the increase in 5-HT levels induced by YM992 was significantly larger than increases elicited by these three compounds at 30 mg/kg. The combined administration of R-(+)-alpha-(2, 3-dimethoxyphenyl)-1-[2-(4-fluorophenyl)ethyl]-4-piperidine-methanol (MDL100,907) (a selective 5-HT(2A) receptor antagonist) and citalopram produced no additional increase in 5-HT levels compared with citalopram treatment alone. YM992 moderately enhanced [3H]5-HT release from rat cerebral cortex synaptosomes using different mechanisms than p-chloroamphetamine. In comparison, 10-microM fluoxetine markedly induced 5-HT release in vitro, while citalopram and venlafaxine had no noticeable effect on release. YM992 produces a more robust increase of 5-HT levels acutely than other antidepressants in vivo and the effect may be due to 5-HT releasing properties of the drug.

Effects of antihistamines on fenfluramine-induced depletion of indoles in the brain of rats

Various effects of chlorpheniramine (CPA), diphenhydramine (DIPH), tripelennamine (TRIP), and pyrilamine (PYRI) on fenfluramine (FEN)-induced depletion of serotonin in the brain of rats were observed to be dependent on body temperature. Levels of 5-HT and 5-HIAA in the frontal cortex, hippocampus, and striatum of rats treated with FEN (10 mg/kg, once or twice daily x 4 days) decreased to approximately 30% (P < 0.01) that of controls with no significant changes after CPA, DIPH, TRIP, and PYRI. Treatment with FEN plus CPA (5, 10, 20 mg/kg) and FEN plus DIPH (20 mg/kg), but not FEN plus TRIP (20 mg/kg) and FEN plus PYRI (20 mg/kg), increased brain serotonin levels 2- to 3-fold more than those treated with FEN plus saline. Treatment with FEN plus CPA and FEN plus DIPH, but not FEN plus TRIP and FEN plus PYRI, decreased rectal temperature with no significant change after FEN. The antihistamines alone decreased temperature at a 1-hour period and enhanced FEN-induced reduction in body weight. Possible mechanisms of the different effects of antihistamines on FEN-induced depletion of serotonin are discussed.

Functional consequences of central serotonin depletion produced by repeated fenfluramine administration in rats

Repeated administration of D,L-fenfluramine (FEN) is known to cause prolonged depletion of forebrain serotonin (5-HT) in animals. Ironically, few studies have evaluated functional consequences of such FEN-induced 5-HT loss. In the present work, we examined neuroendocrine and behavioral responses evoked by acute FEN injection in rats that had previously received a 4 d FEN-dosing regimen known to deplete forebrain 5-HT (D,L-FEN, 20 mg/kg, s.c., b. i.d.). Rats were fitted with indwelling jugular catheters before the study to allow for repeated intravenous challenge injections and stress-free blood sampling. At 1 and 2 weeks after the 4 d dosing regimen, acute FEN (1.5 or 3.0 mg/kg, i.v.) produced dose-related elevations in plasma corticosterone and prolactin; these hormonal responses were markedly attenuated in FEN-pretreated rats. Behavioral effects of acute FEN, namely flat body posture and forepaw treading, were also blunted in FEN-pretreated rats. Interestingly, rats exposed to repeated FEN did not display overt abnormalities in hormonal or behavioral parameters under basal (i.e., unprovoked) conditions, despite dramatic decreases in postmortem tissue levels of 5-HT in numerous brain areas. Our results suggest that FEN-induced 5-HT depletion is accompanied by multiple impairments in 5-HT function. Although the clinical relevance of our data are debatable, the findings clearly show the utility of the FEN challenge test for uncovering in vivo functional deficits that might otherwise go undetected. FEN should remain an important pharmacological tool for determining the role of 5-HT neurons in mediating diverse physiological and behavioral processes.

dl-Fenfluramine inhibits ethanol-induced ascorbic acid release in rat striatum studied by microdialysis

The effects of dl -fenfluramine, dl -5-hydroxytryptophan(5-HTP) and fluoxetine on ethanol-induced striatal ascorbic acid (AA) release in rat were studied by microdialysis coupled to high performance liquid chromatography with electrochemical detection. Ethanol (3.0 g/kg, i.p.) stimulated striatal AA release to more than 200% above the baseline. dl -Fenfluramine (20 mg/kg, i.p. or 40 mug/rat, i.c.v.), 10 min before ethanol administration, markedly inhibited ethanol-induced AA release. A similar result was also observed following dl -5-HTP (100 mg/kg, i.p.) administration. However, fluoxetine (10, 30 mg/kg, i.p.) showed no antagonistic effect on ethanol-induced AA release. The suppressing effect of dl -fenfluramine and dl -5-HTP on ethanolinduced AA release could be reversed by the 5-HT receptor antagonist cyproheptadine (10 mg/kg, s.c.). All these drugs had no effect on basal AA release. The results give a first evidence for the involvement of central serotonergic system, and suggest that differential activities may exist between dl -fenfluramine, dl -5-HTP and fluoxetine in regulating ethanol-induced AA release in rat striatum.

Cocaine and selective monoamine uptake blockers (sertraline, nisoxetine, and GBR 12935) prevent the d-fenfluramine-induced head-twitch response in mice

Serotonin release subsequent to 5-HT precursor loading mainly occurs via exocytosis. Acute cocaine or sertraline administration promote the ability of 5-HT precursors (e.g. L-tryptophan) to induce the 5-HT2A receptor-mediated head-twitch response (HTR) in rodents. The 5-HT releaser, d-fenfluramine, at behaviorally active doses, can induce the head-twitch response in rodents by releasing cytoplasmic 5-HT via the serotonin uptake carrier working in reverse. The purpose of the present study was to utilize the d-fenfluramine-induced HTR to determine the serotonergic and nonserotonergic components of cocaine's actions on the d-fenfluramine-sensitive pool of cytoplasmic 5-HT. Because a dramatic differential potentiation in HTR frequency is obtained when cocaine is administered prior relative to after L-tryptophan injection, the effects of varying doses of cocaine and the selective serotonin (sertraline), dopamine (DA) (GBR 12935), and norepinephrine (NE) (nisoxetine) uptake blockers were investigated on the d-fenfluramine-induced behavior in two experimental protocols. Thus, each uptake inhibitor was administered either 10 min following (protocol 1) or 10 min prior to (protocol 2) d-fenfluramine injection. All the tested uptake inhibitors attenuated the d-fenfluramine-induced HTR in a dose-dependent manner in both experimental protocols. However, their order of potency in either protocol 1 (nisoxetine > GBR 12935 > cocaine > sertraline) or protocol 2 (cocaine > GBR 12935 > nisoxetine = sertraline) does not agree with in vitro affinity of these drugs for the 5-HT transporter. In addition, the potency order for cocaine and nisoxetine in protocol 1 was significantly reversed in protocol 2. The inhibitory effects of the cited drugs on the d-fenfluramine-induced HTR are discussed in terms of: 1) high doses of selective monoamine uptake blockers may not exhibit as much selectivity for their target uptake sites as indicated by in vitro tests; and 2) possible pharmacokinetic interactions between d-fenfluramine and the monoamine uptake blockers.

Deficits in D-fenfluramine-sensitive pool of brain 5-HT following withdrawal from chronic cocaine exposure

Recent head-twitch response (HTR) studies in mice have indicated that withdrawal from chronic cocaine exposure produces deficits in CNS conversion of L-tryptophan to 5-HT. In the present study, the ability of 5-HT releaser, d-fenfluramine, was utilized to induce the HTR in mice following abstinence from chronic cocaine exposure. d-Fenfluramine-induced HTR, is a 5-HT2A receptor-mediated phenomenon and its induction frequency can be regarded as an indirect but in vivo measure of basal brain 5-HT concentration. Thus, different groups of mice were injected with cocaine twice daily (0, 0.1, 0.5, 2.5, 5 or 10 mg/kg, i.p.) for either 7 or 13 days. At 24 h after last cocaine injection, the treated mice received d-fenfluramine (5 mg/kg, i.p.) and the induced HTR (mean+/-SEM) was recorded for the next 30 min. Cocaine attenuated the d-fenfluramine-induced HTR frequency by 30-37% in the 13-day regimen and significant effects were observed from 0.5 mg/kg dose. At 24 h withdrawal in the 7-day cocaine exposure group, the mean HTR frequencies were attenuated, however, they did not achieve statistical significance. Extended abstinence studies (i.e. 24, 48, 72 and 96 h postwithdrawal) from chronic cocaine exposure (0, 0.5 and 5 mg/kg/day for either 7 or 13 days) indicated that in the 7-day exposure group, significant reductions (26, 39 and 22%) in HTR frequency occurred at 48, 72 and 96 h following withdrawal from 0.5 mg/kg cocaine, whereas its 5 mg/kg dose failed to induce a significant effect. In the 13-day exposure group significant reductions in HTR frequency were observed at 24 h abstinence (27%) for the 0.5 mg/kg cocaine dose and at 24 and 48 h for the 5 mg/kg. Overall, these results indicate that abstinence from chronic exposure to cocaine produces enduring deficits in basal 5-HT concentration. Lastly, serotonergic function appears to be uniquely sensitive to chronic administration of low doses of cocaine.

Possible in vivo 5-HT reuptake blocking properties of 8-OH-DPAT assessed by measuring hippocampal extracellular 5-HT using microdialysis in rats

1. The 5-hydroxytryptamine (5-HT)1A receptor agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), has been shown to label 5-HT reuptake sites. 2. To study the functional consequences of this property, the effects of 8-OH-DPAT were compared with those of the 5-HT reuptake inhibitors, paroxetine and clomipramine, and of the 5-HT1A receptor agonist flesinoxan, in vitro on 5-HT reuptake, and in vivo on the extracellular concentration of 5-HT by use of microdialysis, in rat hippocampus. Because 5-HT reuptake inhibitors reportedly attenuate the ability of (+)-fenfluramine to increase the extracellular concentration of 5-HT, the possible reversal of these effects of 8-OH-DPAT and by paroxetine were examined. 3. 8-OH-DPAT, paroxetine and clomipramine inhibited [3H]-5-HT reuptake in rat hippocampal synaptosomes (pIC50: 6.00, 8.41 and 7.00, respectively). In contrast, flesinoxan did not alter 5-HT reuptake (pIC50 < 5). 4. 8-OH-DPAT (10 and 100 microM), paroxetine (0.1 microM) and clomipramine (1 microM), administered through the dialysis probe, significantly increased the hippocampal extracellular concentration of 5-HT. In contrast, flesinoxan (100 microM) did not alter extracellular 5-HT. Moreover, the effects of 100 microM 8-OH-DPAT were not blocked by the 5-HT1A receptor antagonist, WAY-100635 (0.16 mg kg-1, s.c.). 5. The increase in extracellular 5-HT induced by 10 mg kg-1, i.p., (+)-fenfluramine was prevented not only by 0.1 microM paroxetine, but also by 100 microM 8-OH-DPAT. In addition, systemic administration of 10 mg kg-1, but not 2.5 mg kg-1, i.p. 8-OH-DPAT attenuated the increase in extracellular 5-HT induced by 2.5 mg kg-1, i.p., (+)-fenfluramine. 6. These findings suggest that the increase in extracellular 5-HT produced by local administration of 8-OH-DPAT does not involve its 5-HT1A receptor agonist properties, but may result, at least in part, from its 5-HT reuptake blocking properties.

Fluoxetine effects on serotonin function and aggressive behavior

Fluoxetine inhibits serotonin uptake selectively and increases extracellular concentrations of serotonin in brain regions. The enhanced serotonergic neurotransmission resulting from increased action of that extracellular serotonin on postsynaptic receptors on target neurons results in various functional changes, reflecting the wide distribution of serotonin nerve terminals in brain regions that regulate numerous physiological functions. One consequence of fluoxetine administration in animals is a reduction of aggressive behavior, consistent with a larger body of data implicating serotonin as an important neurotransmitter modulator of aggression. In humans, preliminary data suggest that fluoxetine may also decrease aggressive behavior and feelings of anger or hostility. Further investigation of the potential usefulness of fluoxetine and other drugs that increase serotonergic function as a means of reducing anger, hostility, and aggressive behavior seems warranted.

The mechanism by which the selective 5-HT1A receptor antagonist S-(-) UH 301 produces head-twitches in mice

Electrophysiological studies indicate that certain 5-HT1A receptor antagonists increase the basal firing rate of some but not all raphe neurons by antagonizing the inhibitory endogenous serotonin tone operating on the somatodendritic pulse-modulating presynaptic 5-HT1A autoreceptors. This effect should enhance the synaptic concentration of 5-HT (5-hydroxytryptamine) in serotonergic terminal fields, which may then activate postsynaptic 5-HT receptors. However, in vivo microdialysis studies show that generally such 5-HT1A antagonists by themselves do not increase the basal 5-HT release but potentiate the ability of serotonin reuptake blockers to increase the neuronal serotonin terminal output in the rat brain via the above mechanism. The purpose of the present study was to determine whether antagonism of the proposed endogenous serotonin tone on the 5-HT1A autoreceptors can potentiate the activity of other postsynaptic serotonin receptors. To this end, we utilized the head-twitch response (HTR) in mice as an in vivo model of postsynaptic 5-HT2A receptor function. The selective and silent 5-HT1A receptor antagonist, S-(-)UH 301, by itself, in a dose-dependent manner, produced the HTR in normal but not in reserpinized animals. The 5-HT2A antagonist, SR 46349B, completely prevented S-(-)UH 301-induced HTR. Pretreatment with S-(-)UH 301 also potentiated 5-hydroxytryptophan (5-HTP)-induced HTR both in normal and in the reserpinized mice. At low doses (0.06-0.25 mg/kg), the 5-HT2A selective agonist, 8-OH DPAT, significantly but partially inhibited 5-HTP-induced HTR. However, further attenuation was not observed following the administration of larger doses of 8-OH DPAT. Depending upon the dose used, S-(-)UH 301 pretreatment not only antagonized but also broke through the inhibitory effect of 8-OH DPAT on 5-HTP-induced HTR. The selective (sertraline) and nonselective (cocaine) serotonin reuptake blockers potentiated the ability of 5-HTP to induce the head-twitch behavior in mice. Pretreatment with S-(-)UH 301 enhanced the potentiating effect of serotonin reuptake blockers on the 5-HTP induced HTR. These results suggest that an endogenous 5-HT tone via the discussed mechanism controls the terminal field synapticactivity of serotonergic neurons in mice. In addition, disinhibition of pulse-modulating 5-HT1A autoreceptors by S-(-)UH 301 can potentiate the synaptic effects of serotonin reuptake blockers as well as the serotonin precursor 5-HTP. However, a more firm general conclusion regarding antagonism of presynaptic 5-HT1A receptors leading to indirect functional enhancement of other postsynaptic serotonergic receptors can only be made when the above hypothesis is further tested with other selective 5-HT1A receptor antagonists (such as WAY 100 635), which we were unable to obtain. The present study is the first report to show that a selective 5-HT1A antagonist by itself can produce a serotonin-mediated function via indirect stimulation of another serotonin receptor subtype in mice.

On the in-vivo modulation of neostriatal dopamine release by fluoxetine and 5-hydroxy-L-tryptophan in conscious rats

To help determine the nature of serotonergic regulation of dopamine activity in the brain an in-vivo microdialysis study has been performed in conscious rats to investigate the modulation of dopamine release in the neostriatum by 5-hydroxytryptamine (5-HT). The 5-HT uptake inhibitor, fluoxetine, and the 5-HT precursor, 5-hydroxy-L-tryptophan (5-HTP), were used to produce an increase in extracellular 5-HT concentration Systemic administration of fluoxetine (10 mg kg-1, s.c.) produced a 2- to 3-fold increase in extracellular 5-HT concentration but did not change extracellular dopamine concentration in the neostriatum. Co-administration of fluoxetine and 5-HTP (40 mg kg-1, s.c.; 60-90 min after fluoxetine) caused a highly significant tenfold increase in extracellular 5-HT concentration in the neostriatum with a slight but non-significant decrease in extracellular dopamine concentration. Pergolide, a dopamine D2 agonist, given systemically caused a dramatic decrease in extracellular dopamine concentration demonstrating the responsiveness of the neurons. These results demonstrate that high concentrations of extracellular 5-HT do not modulate dopamine release in the neostriatum. The possibility that different 5-HT receptor subtypes may mediate different regulation of dopamine release remains to be explored.

Simultaneous brain and blood microdialysis study with a new removable venous probe. Serotonin and 5-hydroxyindolacetic acid changes after D-norfenfluramine or fluoxetine

A removable intravenous microdialysis probe was developed and simultaneously used with a removable microdialysis probe placed in the lateral hypothalamus (LH). Serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) changes in blood and brain dialysates were measured by HPLC-EC after an i.p. injection of 5 mg/kg d-norfenfluramine (dNF) or 10 mg/kg fluoxetine (FLU) in freely moving rats. 5-HT in the LH significantly increased after both drugs, but the rise was larger and faster with dNF [F(7,28)=4.0 p<0.05] than with FLU [F(5,20)=5.0 p<0.01]. By contrast, in venous blood 5-HT increased after FLU [F(5,20)=2.96 p<0.05] but not after dNF. 5-HIAA after both drugs continued decreasing significantly in the LH [dNF F(7,28)=11.4 p<0.01; FLU F(5,20)=22.8 p<0.01], but it did not change in blood. Simultaneous dialysis in brain and blood allowed evaluation of the differential effects of dNF and FLU on 5-HT and 5-HIAA in the two places. Removable venous probes prevented the inflammatory reaction that may occur around permanently implanted probes, and the dialysis could be more efficient and with less risk of clogging.

Role of 5-HT1A autoreceptors in the mechanism of action of serotoninergic antidepressant drugs: recent findings from in vivo microdialysis studies

Although a new generation of selective serotonin reuptake inhibitors (SSRIs) has been introduced in therapeutics as antidepressant drugs, a two to four week lag period still occurs between starting treatment with SSRIs and the onset of therapeutic effects in man. In vivo cerebral microdialysis can be used to measure extracellular concentrations of serotonin (5-hydroxytryptamine, 5-HT), which reflect intrasynaptic events. With the coupling of this new experimental method to very sensitive analytical assays such as liquid chromatography with electrochemical detection, it has recently been possible to obtain two major arguments supporting the hypothesis that somatodendritic 5-HT1A autoreceptors situated in the raphe nuclei play an important role in the mechanism of action of SSRIs. First, in the rat, single administration of SSRIs at low doses comparable to those used therapeutically increases extracellular 5-HT concentrations in the vicinity of the cell body and the dendrites of serotoninergic neurones of the raphe nuclei. This effect is more marked than that observed in regions rich in nerve endings (frontal cortex). The magnitude of the activation of the serotoninergic neurotransmission depends on the brain area studied and the dose of the SSRIs administered to rats. This could be explained by simultaneous activation of somatodendritic 5-HT1A autoreceptors by endogenous 5-HT in the raphe nuclei, thereby limiting the corticofrontal effects of the antidepressant. Second, SSRIs cause a larger increase in extracellular 5-HT concentrations in the nerve endings when administered chronically: 5-HT autoreceptors may have gradually desensitized during the 2-4 weeks of treatment with SSRIs. Preliminary studies of patients with depression appear to confirm these experimental results, as co-administration of a 5-HT1A autoreceptor antagonist and a SSRI accelerated the onset of the antidepressant effect (< 1 week).

Serum corticosterone increases reflect enhanced uptake inhibitor-induced elevation of extracellular 5-hydroxytryptamine in rat hypothalamus

The increase in extracellular 5-hydroxytryptamine (5-HT) in rat hypothalamus following administration of fluoxetine, a 5-HT-uptake inhibitor, was enhanced by the injection of LY206130(1-[1-H-indol-4-yloxy]-3-[cyclohexylamino]-2-prop ano l maleate), a 5HT1A receptor antagonist, or by L-5-hydroxytryptophan (L-5-HTP), the 5-HT precursor. Elevation of serum corticosterone, measured as a functional output of hypothalamic 5-HT pathways, was greater in rats treated with fluoxetine plus LY206130 or with fluoxetine plus L-5-HTP than in rats treated with the agents alone. Synergism between effects of fluoxetine and L-5HTP has often been reported, but this is the first report of an increased functional effect when a 5-HT1A receptor antagonist is combined with a 5-HT uptake inhibitor to augment the increase in extracellular 5-HT.

Biological actions of drugs affecting serotonin and eating

The present status of knowledge on drugs affecting food intake and presumably acting via a serotoninergic mechanism is reviewed. The mechanism of action of these drugs is analyzed at the neurochemical level. All the drugs, to various extents, inhibit the uptake of serotonin (5HT), increase the release of 5HT and decrease brain levels of 5HT and 5HIAA. However, the underlying mechanisms are not identical as exemplified by comparisons made with d-fenfluramine, d-norfenfluramine, fluoxetine, sertraline and paroxetine. An analysis of the role of 5HT in the inhibition of food intake reveals that only d-fenfluramine is inhibited by antiserotonin agents. The role of the different 5HT receptor-subtypes in this antagonism is discussed. More selective 5HT antagonists are needed to establish which 5HT receptor(s) controls food intake.

Release of serotonin induced by 3,4-methylenedioxymethamphetamine (MDMA) and other substituted amphetamines in cultured fetal raphe neurons: further evidence for calcium-independent mechanisms of release

The substituted amphetamines 3,4-methylenedioxymethamphetamine (MDMA), 3,4-methylenedioxyamphetamine (MDA), p-chloro-amphetamine (PCA) and fenfluramine (FEN) all exert their effects by releasing serotonin (5-HT) from presynaptic nerve terminals. In the current study, we examined the ability of these agents to induce the release of 5-HT in culture fetal raphe neurons. The data indicate that the rank order of release potencies for these agents was (+/-)PCA>(+)MDMA=(+)MDA=(+/-)FEN. Studies examining the role fo calcium in 5-HT release demonstrate that preventing calcium influx with L- and N-type calcium channel blockers inhibits potassium-stimulated release of -3H-5-HT but has no effect on release induced by the substituted amphetamines. Furthermore, omitting calcium from the extracellular media or depleting the vesicular pool of neurotransmitter with continual potassium stimulation did not affect the release of -3H-5-HT induced by these compounds. Administration of fluoxetine prior to the substituted amphetamines significantly attenuated the releasing effects of these agents, while producing no effect on potassium-stimulated release. These results are consistent with the notion that the amphetamines induce release of cytoplasmic 5-HT via the plasma membrane transporter.

Effects of fluoxetine on basal and K(+)-induced tritium release from synaptosomes preloaded with [3H]serotonin

Synaptosomes from rat brain cortex and spinal cord were preloaded with [3H]serotonin ([3H]5-HT), superfused and exposed to fluoxetine and/or 15 mM K+. In both regions 10 microM, but not 1 microM fluoxetine evoked a marked tritium overflow, about 2 min later than the immediate [3H]5-HT release induced by K+, and mainly (73%) due to the efflux of a tritiated metabolite of 5-HT, possibly [3H]5-hydroxy-indoleacetic acid. These findings confirm previous data in the rat hippocampus and are probably due to fluoxetine interacting with the 5-HT storage vesicles. One microM fluoxetine significantly reduced the d-fenfluramine-induced [3H]5-HT overflow, in accordance with its action as 5-HT uptake blocker, but did not affect the K(+)-induced [3H]5-HT overflow. This latter finding does not confirm that fluoxetine inhibits the depolarization-induced Ca(2+)-influx, suggested to involve a drug interaction with the L-type Ca(2+)-channels. Thus, the overflow induced by 10 microM fluoxetine was additive with the depolarization-induced overflow, when the two stimuli were applied together. When 10 microM fluoxetine was added 7 min before 15 mM K+, there was no depolarization-induced overflow. Such inhibition might be only apparent and due either to the fluoxetine-induced loss of vesicular 5-HT or to a fluoxetine-induced alterations of synaptic vesicles. The in vivo relevance of the fluoxetine releasing effect remains to be assessed.

Serotonin uptake inhibitors: uses in clinical therapy and in laboratory research

Fluoxetine, zimelidine, sertraline, paroxetine, fluvoxamine, indalpine and citalopram are the selective inhibitors of serotonin uptake that have been most widely studied. Some of these compounds are or have been used clinically in the treatment of mental depression, obsessive-compulsive disorder and bulimia, and therapeutic benefit has been claimed in additional diseases as well. By blocking the membrane uptake carrier which transports serotonin from the extracellular space to inside the serotonin nerve terminals, these compounds increase extracellular concentrations of serotonin and amplify signals sent by serotonin neurons. Because serotonin neurons are widespread in the central nervous system, the functional consequences of blocking serotonin uptake are diverse, but are generally subtle. Animals treated with serotonin uptake inhibitors look normal in gross appearance, but effects such as reduced aggressive behavior, decreased food intake and altered food selection, analgesia, anticonvulsant activity, endocrine changes and neurochemical changes have been demonstrated and characterized. Serotonin uptake inhibitors have helped in revealing some dynamics of serotonin neurons; for example, when uptake is inhibited and extracellular serotonin concentration increases, presynaptic as well as postsynaptic receptors for serotonin are activated to a greater degree. A consequence of increased activation of autoreceptors on serotonin cell bodies and nerve terminals is a reduction in firing of serotonin neurons and a decrease in serotonin synthesis and release. The result is a limit on the degree to which extracellular serotonin and serotonergic neurotransmission are increased.(ABSTRACT TRUNCATED AT 250 WORDS)

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 microM 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-HT1A autoreceptor activation, and suggests that the 5-HT releasing effect of dexfenfluramine in vivo depends on nerve terminal depolarization.

Short-term fluoxetine treatment alters monoamine levels and turnover in discrete brain nuclei

The effects of short-term fluoxetine administration on monoamine levels and turnover were assessed in discrete brain nuclei. Adult male rats received fluoxetine HCl (10 mg/kg) or saline injections intraperitoneally for 4 days and monoamine levels determined by high performance liquid chromatography. The major metabolite of 5-HT, 5-hydroxyindoleacetic acid (5-HIAA), was decreased by fluoxetine treatment in the ventromedial hypothalamic nucleus (VMN), the lateral hypothalamic area and the CA1 region of the hippocampus. Fluoxetine treatment significantly increased serotonin (5-HT) levels in the VMN but did not change 5-HT levels in any other area examined. Norepinephrine (NE) levels were higher in fluoxetine-treated rats in the dorsomedial hypothalamic nucleus, dorsal raphe nucleus and parietal motor cortex (MCTX). 5-HT and NE turnover were also determined by the pargyline method. Fluoxetine treatment decreased 5-HT turnover in the VMN and increased 5-HT turnover in the median raphe. NE turnover was decreased in the preoptic area, the MCTX and parietal sensory cortex by fluoxetine administration. These results demonstrate that brain areas with similar 5-HT innervation respond differently to fluoxetine administration and fluoxetine, which selectively alters 5-HT uptake, also affects NE levels and turnover in several brain nuclei.

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.

Effect of fluoxetine on the spontaneous electrical activity of fronto-cortical neurons

The effect of fluoxetine on spontaneous extracellular activity of fronto-cortical neurons of chloral hydrate-anesthetized rats was investigated. Fluoxetine significantly increased the basal firing rate of cortical neurons in a dose-dependent manner (0.1-1000 micrograms kg-1 i.v.), with a maximum excitatory effect of 53% at 1000 micrograms kg-1. Selective destruction of ascending serotoninergic pathways induced by intracerebroventricular injections of 150 micrograms 5,7-dihydroxytryptamine, in desipramine-pretreated rats, antagonized the excitatory effect of fluoxetine. The present results suggest that fluoxetine significantly increases the electrical activity of the fronto-cortical neurons acting on serotoninergic uptake mechanisms localized at the level of raphe nuclei.

Psychological stress increases serotonin release in the rat amygdala and prefrontal cortex assessed by in vivo microdialysis

The effects of psychological stress on serotonin (5-HT) release were studied in the basolateral amygdaloid nucleus and the prefrontal cortex in conscious rats with in vivo microdialysis. Psychological stress, wherein emotional factors were predominantly involved, significantly increased extracellular 5-HT levels in these two areas. These findings suggest that activation of serotonergic neurons in these brain regions is involved in the emotional and/or cognitive states in animals.