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.     

Antagonism of fenfluramine-induced hyperthermia in rats by some, but not all, selective inhibitors of 5-hydroxytryptamine uptake

The injection of fenfluramine (7.5 mg kg-1,i.p.) to rats housed at 27-28 degrees C was associated with an elevation of core body temperature which peaked at approximately 1 h post-injection. One h pretreatment with citalopram (20 mg kg-1, i.p.), chlorimipramine (10 mg kg-1, i.p.), femoxetine (10 mg kg-1, i.p.) and fluoxetine (20 mg kg-1, i.p.) resulted in an attenuated response to fenfluramine. In contrast, Org 6582 (20 mg kg-1) and zimelidine (20 mg kg-1) were devoid of an effect on fenfluramine-induced hyperthermia. The response to fenfluramine was was also blocked by i.p. injections of metergoline (0.2 mg kg-1), methysergide (5 mg kg-1) and mianserin (0.5 mg kg-1). Rectal temperature was unaltered by both the 5-hydroxytryptamine (5-HT) uptake inhibitors and the 5-HT receptor antagonists. The IC50 values (nM) for in vitro inhibition of [3H]-5-HT uptake into rat hypothalamic synaptosomes were for citalopram 2.4, chlorimipramine 8.8, femoxetine 14, fluoxetine 16, Org 6582 75 and zimelidine 250. The injection of all six compounds (20 mg kg-1, i.p.) 1 h before death was associated with an inhibition of [3H]-5-HT uptake into rat hypothalamic synaptosomes which ranged from 47.2% for chlorimipramine to 83.3% for citalopram. Rat hypothalamic 5-HT levels were decreased by approximately 50% 3 h after the injection of fenfluramine (15 mg kg-1, i.p.). This effect was blocked by a 1 h pretreatment with fluoxetine, Org 6582 and zimelidine (all 20 mg kg-1, i.p.). Ki values for displacement of specifically bound [3H]-5-HT (1 nM) to rat hypothalamic membranes were for metergoline 26 nM, methysergide 1.1 microM, mianserin 3.6 microM, chlorimipramine 9.2 microM and fluoxetine 32.7 microM. Values for citalopram, femoxetine, Org 6582 and zimelidine were in excess of 65.4 microM.(ABSTRACT TRUNCATED AT 250 WORDS)     

The effects of single and repeated anorectic doses of 5-hydroxytryptamine uptake inhibitors on indole levels in rat brain.

1. The effects of acute and repeated equiactive anorectic doses (ED50) of recently marketed 5-hydroxytryptamine (5-HT) uptake inhibitors on the content of brain indoles were compared in rats in relation to the brain regional concentrations of unchanged drug and its known active metabolite. 2. Single intraperitoneal (i.p.) doses of the anorectic ED50 of fluoxetine (35 mumol kg-1), fluvoxamine (60 mumol kg-1), paroxetine (20 mumol kg-1) and sertraline (49 mumol kg-1) slightly reduced brain 5-hydroxyindoleacetic acid (5-HIAA), with regional differences, this being compatible with 5-HT uptake blockade. Only fluvoxamine and sertraline significantly enhanced the content of 5-HT in the cortex. 3. The regional sensitivity to the acute effect of a given drug was not related to any preferential drug distribution, as these compounds distributed almost uniformly in the brain areas considered (cortex, striatum and hippocampus). 4. Repeating the same doses twice daily, i.p. for 14 days, however gave a different picture, fluvoxamine having little or no effect on the content of indoles and fluoxetine, paroxetine and sertraline lowering both 5-HT and 5-HIAA in all the brain regions compared to pair-fed control animals, 1 h after the last dose. 5. One week later only fluoxetine-treated animals still had reduced brain 5-HT, this probably being related to the accumulation of its main metabolite norfluoxetine in rat brain after chronic dosing.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of chronic administration of selective 5-hydroxytryptamine and noradrenaline uptake inhibitors on a putative index of 5-HT2C/2B receptor function

Chronic treatment with selective 5-HT reuptake inhibitors (SSRI) are therapeutic in obsessive compulsive disorder, depression, anxiety, bulimia nervosa and migraine. In the present study the possibility that SSRI's act by desensitizing 5-HT_2C/5-HT_2B receptors was assessed using a putative in vivo model of 5-HT_2C/5-HT_2B receptor function, mCPP-induced hypolocomotion. mCPP (2,4 and 6 mg/kg i.p. 20 min pretest) reduced locomotion and rears in rats treated acutely or chronically with saline. Acute oral administration of the SSRI's fluoxetine (10 mg/kg), paroxetine (10 mg/kg), or clomipramine (70 mg/kg) or the noradrenaline reuptake inhibitor, desipramine (10 mg/kg), all 1 hr pretest, did not prevent mCPP-induced hypolocomotion. In contrast, chronic treatment with the SSRI's paroxetine and fluoxetine (both 10 mg/kg p.o. daily × 21 days), significantly attenuated the effect of mCPP (4 and 6 mg/kg i.p.) on locomotion and rears 24 hr after the last pretreatment dose. Chronic clomipramine (70 mg/kg p.o. daily × 21 days) also significantly attenuated the effect of mCPP (4 mg/kg i.p.) on rears and tended to reduce the hypolocomotor response. However, chronic treatment with desipramine, (10 mg/kg p.o.daily × 21) had no effect on any of the parameters measured. As chronic fluoxetine and paroxetine did not reduce brain mCPP levels (determined by HPLC 30 min after 4 mg/kg i.p.) the results suggest that chronic SSRI's, but not desipramine, reduce 5-HT_2C/5-HT/_2B receptor responsivity. If this occurs in man, it may mediate or contribute to their reported therapeutic efficacy in depression, anxiety, bulimia, migraine and alcoholism. It may also be of particular relevance to their unique efficacy in OCD.     

High affinity binding of [3H]6-nitroquipazine to cortical membranes in the rat: inhibition by 5-hydroxytryptamine and 5-hydroxytryptamine uptake inhibitors

[3H]6-Nitroquipazine is a new, suitable radioligand for studying the uptake system for 5-hydroxytryptamine (5-HT; serotonin). In the present study, inhibition by drugs of the binding of [3H]6-nitroquipazine to uptake sites for 5-HT in the cerebral cortex of the rat was investigated. The inhibition of 5-HT and several inhibitors of the uptake of 5-HT (paroxetine, clomipramine, citalopram, Z-norzimelidine, fluoxetine, imipramine, desipramine and 5-methoxytryptoline) against the binding of [3H]6-nitroquipazine to membranes from the cortex of the rat were the same and competition curves indicated a single population of binding sites. The addition of 5-HT and the tricyclic inhibitors of the uptake of 5-HT, imipramine, clomipramine and desipramine, all produced changes in the apparent dissociation constant (Kd), without changes in the number of binding sites (Bmax). Also, the non-tricyclic inhibitors of the uptake of 5-HT, paroxetine, citalopram, fluoxetine and Z-norzimelidine, and 5-methoxytryptoline, all produced changes in Kd values without changes in the Bmax. These results suggest that all the drugs used in this experiment exhibited competitive interactions with the binding of [3H]6-nitroquipazine to uptake sites for 5-HT in the brain of the rat. These drugs may bind to common binding sites, which are likely to represent the substrate recognition sites for the uptake of 5-HT.     

Effects of inhibitors of neuronal uptake of 5-HT on sympathetic cardiovascular regulation.

The effect on sympathetic cardiovascular regulation of inhibition of the high-affinity neuronal uptake of 5-hydroxytryptamine (5-HT) by fluvoxamine, citalopram, and fluoxetine was studied in anesthetized rabbits. The mean arterial pressure, postganglionic renal sympathetic nerve activity, heart rate, clearance of [3H]norepinephrine from plasma and the plasma norepinephrine concentration were measured, and from the latter two parameters the spillover of norepinephrine into the blood was calculated. The effect of fluvoxamine and fluoxetine on the uptake of [3H]5-HT into platelets was also examined. Of four increasing doses of fluvoxamine (0.6, 1.7, 5, and 15 mg/kg i.v.) and citalopram (0.3, 1, 3, and 9 mg/kg i.v.), only the two highest ones decreased the renal sympathetic nerve activity, heart rate, and clearance of [3H]norepinephrine from plasma. Only the highest of four increasing doses of fluoxetine (0.2, 0.6, 1.7, and 5 mg/kg i.v.) inhibited sympathetic nerve activity. However, lower doses of fluvoxamine (0.6 and 1.7 mg/kg i.v.) and fluoxetine (0.6 and 1.7 mg/kg i.v.) already markedly inhibited the uptake of [3H]5-HT into platelets. The data indicate that selective inhibition of neuronal uptake of 5-HT has no effect on central sympathetic regulation in anesthetized rabbits. The sympathoinhibition observed at high doses of fluvoxamine and citalopram is probably due to inhibition of the neuronal uptake of norepinephrine.     

Reversal of learned helplessness by selective serotonin reuptake inhibitors in rats is not dependent on 5-HT availability

Serotonin (5-HT) and 5-HT(1A) receptors have been suggested to play a pivotal role in the mechanism of action of antidepressant drugs, particularly in the case of selective serotonin reuptake inhibitors (SSRIs). In the rat learned helplessness (LH) paradigm, a valid animal model of human depression, repeated treatment with the 5-HT(1A) receptor agonist 8-OH-DPAT (0.125 and 0.5mg/kg) and several classes of antidepressants such as the tricyclic agent desipramine (30 and 60mg/kg), the monoamine oxidase inhibitor (MAOI) pargyline (60mg/kg) and the SSRIs fluoxetine (15 and 30mg/kg), paroxetine (15 and 30mg/kg) and sertraline (30mg/kg) improved behavioural deficit in helpless rats. The involvement of serotonergic mechanisms in the antidepressant-like effect of these agents was investigated using the selective 5-HT(1A) receptor antagonist WAY 100,635 and the 5-HT synthesis inhibitor p-chlorophenylalanine (PCPA). Pretreatment with WAY 100,635 blocked the 8-OH-DPAT-induced reduction in escape failures, but did not counteract the antidepressant effect of fluoxetine and paroxetine. PCPA given alone did not modify helpless behaviour nor did it affect the behavioural effect of 8-OH-DPAT, fluoxetine and paroxetine. Adaptive changes in 5-HT(1A) receptor function were studied by measuring 8-OH-DPAT-mediated hypothermia and lower lip retraction (LLR) in the animals 24h after LH test session. Fluoxetine and paroxetine treatments caused a marked reduction in agonist-induced responses, an effect completely prevented by WAY 100,635 and PCPA. In conclusion, whereas direct agonist activity at postsynaptic 5-HT(1A) receptors attenuated helpless behaviour, the antidepressant-like effect of SSRIs was found to be independent of their actions on either 5-HT(1A) receptor function or extracellular 5-HT.     

Effects of acute paroxetine administration on tryptophan metabolism and disposition in the rat.

1 The effects of acute oral administration of paroxetine on tryptophan metabolism and disposition were examined in the rat. 2 Basal liver tryptophan pyrrolase activity was inhibited by paroxetine in vitro and after oral administration. Maximum inhibition was caused by a 1 mg kg-1 dose. 3 Paroxetine administration also inhibited pyrrolase activity that had previously been enhanced by hormonal induction by cortisol or cofactor activation by haematin. The cortisol induction of the enzyme was, however, not inhibited by pretreatment of rats with paroxetine. 4 Paroxetine increased tryptophan availability to the brain, because of the above pyrrolase-inhibitory mechanism. Cerebral 5-hydroxytryptamine (5-HT) synthesis was accordingly enhanced, though this was apparent only with doses of the drug of up to 1 mg kg-1. With larger doses, decreased 5-HT turnover, probably as a result of 5-HT uptake inhibition, was the more dominant feature. 5 Paroxetine lowered circulating corticosterone concentration, but did not influence those of albumin, non-esterified fatty acids or glucose. 6 It is concluded that, in addition to inhibiting brain 5-HT turnover, paroxetine also, in common with 20 other antidepressants, enhances 5-HT synthesis by increasing brain tryptophan concentration secondarily to inhibition of liver tryptophan pyrrolase activity.     

Measurements of tacrine and monoamines in brain by in vivo microdialysis argue against release of monoamines by tacrine at therapeutic doses.

1. The concentration of tacrine (tetrahydroaminoacridine or THA) in plasma, regions of brain and cerebral extracellular fluid has been studied in the rat at various times following injection of a dose of 5 mg kg-1, i.p. 2. The peak plasma THA concentration was 2.46 nmol ml-1, and occurred 30 min post injection and clearance was first order (t1/2 = 90 min). The concentration in the brain peaked between 30-60 min, and was around 30 times plasma concentration (striatum peak concentration = 65 +/- 3 nmol g-1). Extracellular cerebral concentration measured by in vivo microdialysis was similar to plasma concentration with the peak occurring 100 min post-injection. 3. No evidence was obtained by in vivo dialysis for THA inducing dopamine release from striatum or 5-hydroxytryptamine (5-HT) release from the frontal cortex. Enhanced release of dopamine did occur after (+)-amphetamine (5 mg kg-1, i.p.) injection, while KCl (100 mM) in the probe released both dopamine and 5-HT. 4. Since the minimum plasma THA concentration achieved in this study was at least twice that found in the plasma of patients given THA for the treatment of dementia, these results suggest that monoamine release in the brain does not occur during therapy.     

The profiles of interaction of yohimbine with anxiolytic and putative anxiolytic agents to modify 5-HT release in the frontal cortex of freely-moving rats.

1. The interaction of yohimbine with anxiolytic and putative anxiolytic agents to modify 5-hydroxytryptamine (5-HT) release in the frontal cortex of the freely-moving rat was assessed using the microdialysis technique. 2. The alpha 2-adrenoceptor antagonist, yohimbine (5.0 mg kg-1, i.p.) increased maximally the extracellular levels of 5-HT in the rat frontal cortex by approximately 230% of the basal levels. 3. The alpha 2-adrenoceptor agonist, clonidine (30-100 micrograms kg-1, i.p.) decreased dose-dependently the extracellular levels of 5-HT in the rat frontal cortex by approximately 0-60% of the basal levels. A 5 min pretreatment with clonidine (50 micrograms kg-1, i.p.) prevented the yohimbine-induced increase in the extracellular 5-HT levels. 4. The benzodiazepine receptor agonist, diazepam (2.5 mg kg-1, i.p.) and the 5-HT3 receptor antagonist, ondansetron (100 micrograms kg-1, i.p.) (5 min pretreatment) completely prevented the yohimbine (5.0 mg kg-1, i.p.)-induced increases in the extracellular levels of 5-HT. The 5-HT1A receptor agonist, 8-OH-DPAT (0.32 mg kg-1, s.c.) partially antagonized the yohimbine response. 5. A 5 min pretreatment with the 5-HT3/5-HT4 receptor ligand R(+)-zacopride (10 micrograms kg-1, i.p.) reversed the yohimbine (5.0 mg kg-1, i.p.)-induced increase in the extracellular levels of 5-HT to approximately 30% below the basal levels. A 5 min pretreatment with S(-)-zacopride (100 micrograms kg-1, i.p.) failed to modify the response to yohimbine.(ABSTRACT TRUNCATED AT 250 WORDS)     

Chlormethiazole, dizocilpine and haloperidol prevent the degeneration of serotonergic nerve terminals induced by administration of MDMA (‘Ecstasy’) to rats

An investigation has been made into the effect of 3,4-methylenedioxymethamphetamine (MDMA or ‘Ecstasy’) administration on the concentration of 5-hydroxytryptamine (5-HT), uptake of [³H]5-HT and [³H]paroxetine binding in rat cerebral cortex tissue. Four days after 2 injections of MDMA (20 mg/kg i.p., 6 hr apart) the concentrations of 5-HT and its metabolite 5-HIAA were reduced by 60%. The binding of [³H]paroxetine to the presynaptic 5-HT transporter was decreased and high affinity uptake of [³H]5-HT was reduced by a similar amount, indicating neurodegeneration of 5-HT terminals. Pretreatment with chlormethiazole (100 mg/kg i.p.), 10 min before each MDMA injection prevented the decrease in both [³H]parotextine binding and uptake of [³H]5-HT. The loss in 5-HT and 5-HIAA content was also attenuated. Pretreatment with dizocilpine (1 mg/kg i.p.) or haloperidol (2 mg/kg i.p.) also prevented the MDMA-induced loss of [³H]paroxetine binding and attenuated the loss of 5-HT and 5-HIAA content. All three compounds also decreased the degree of hyperthermia that follows MDMA administration, although previous studies suggest that the long term neurodegeneration is not associated with the acute hyperthermic response. These data support the findings of others that MDMA injection produces degeneration of 5-HT nerve terminals in the cortex, confirm that chlormethiazole, dizocilpine and haloperidol attenuate MDMA-induced neurotoxic loss of 5-HT and demonstrate for the first time that these compounds prevent the neurodegeneration of 5-HT nerve terminals that follows MDMA administration.     

Comparison of the effects of antidepressants on norepinephrine and serotonin concentrations in the rat frontal cortex: an in-vivo microdialysis study

The present study employed in-vivo microdialysis techniques in the freely moving rat to systematically compare the neurochemical effects of various antidepressant agents on extracellular concentrations of norepinephrine (NE) and serotonin (5-HT) in the frontal cortex. We found that acute administration of the tricyclic antidepressant, desipramine (3-30 mg/kg, s.c.) and the dual serotonin/norepinephrine reuptake inhibitor, venlafaxine (3-30 mg/kg, s.c.), produced dose-dependent and robust increases in cortical NE concentrations (498% and 403%, respectively). Conversely, acute injection of the selective serotonin reuptake inhibitors, fluoxetine (30 mg/kg, s.c.) and paroxetine (1-10 mg/kg, s.c.), did not alter forebrain NE concentrations. However, paroxetine did produce a significant increase in cortical NE concentrations (164%) when administered at 30 mg/kg. These changes in NE were not paralleled by 5-HT, which showed no increase following administration of desipramine, venlafaxine, paroxetine or fluoxetine. Combination treatment with the 5-HT1A receptor antagonist, WAY-100635 (0.3 mg/kg, s.c.), significantly enhanced extracellular 5-HT concentrations following venlafaxine (10 and 30 mg/kg), fluoxetine (30 mg/kg) and paroxetine (3-30 mg/kg). Alternatively, WAY-100635 produced no augmentation of the antidepressant-induced changes in extracellular NE. Collectively, these studies show that paroxetine, at low to intermediate doses, and fluoxetine are selective for 5-HT versus NE systems, whereas venlafaxine produces similar effects on both 5-HT and NE levels at the effective doses tested.     

5-HT loss in rat brain following 3,4-methylenedioxymethamphetamine (MDMA), p-chloroamphetamine and fenfluramine administration and effects of chlormethiazole and dizocilpine.

1. The present study has investigated whether the neurotoxic effects of the relatively selective 5-hydroxytryptamine (5-HT) neurotoxins, 3,4-methylenedioxymethamphetamine (MDMA or 'Ecstasy'), p-chloroamphetamine (PCA) and fenfluramine on hippocampal and cortical 5-HT terminals in rat brain could be prevented by administration of either chlormethiazole or dizocilpine. 2. Administration of MDMA (20 mg kg-1, i.p.) resulted in an approximate 30% loss of cortical and hippocampal 5-HT and 5-hydroxyindoleacetic acid (5-HIAA) content 4 days later. Injection of chlormethiazole (50 mg kg-1) 5 min before and 55 min after the MDMA provided complete protection in both regions, while dizocilpine (1 mg kg-1, i.p.) protected only the hippocampus. 3. Administration of a single dose of chlormethiazole (100 mg kg-1) 20 min after the MDMA also provided complete protection to the hippocampus but not the cortex. This regime also attenuated the sustained hyperthermia (approx +2.5 degrees C) induced by the MDMA injection. 4. Injection of PCA (5 mg kg-1, i.p.) resulted in a 70% loss of 5-HT and 5-HIAA content in hippocampus and cortex 4 days later. Injection of chlormethiazole (100 mg kg-1, i.p.) or dizocilpine (1 mg kg-1, i.p.) 5 min before and 55 min after the PCA failed to protect against the neurotoxicity, nor was protection afforded by chlormethiazole when a lower dose of PCA (2.5 mg kg-1, i.p.) was given which produced only a 30% loss of 5-HT content.(ABSTRACT TRUNCATED AT 250 WORDS)     

In vivo properties of SB 200646A, a 5-HT2C/2B receptor antagonist.

1. SB 200646A, N-(1-methyl-5-indolyl)-N'-(3-pyridyl) urea hydrochloride, the first reported selective 5-HT2C/2B over 5-HT2A receptor antagonist, (pK1 rat 5-HT2C receptor 6.9, pA2 rat 5-HT2B receptor 7.5, pK1 rat 5-HT2A receptor 5.2) dose-dependently blocked a putative rat model of 5-HT2C receptor activation; 1-(3-chlorophenyl)piperazine (mCPP, 5 mg kg-1, i.p. 20 min pretest)-induced hypolocomotion (estimated ID50 19.2 mg kg-1, p.o.). 2. SB 200646A also blocked another putative in vivo model of 5-HT2C receptor function; mCPP (5 mg kg-1, i.p. 20 min pretest)-induced hypophagia in 23 h food-deprived rats (estimated ID50 18.3 mg kg-1, p.o.). 3. SB 200646A did not antagonize 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI)-induced head shakes in rats at doses up to 200 mg kg-1, p.o., an effect thought to be mediated by 5-HT2A receptors for which SB 200646A has its next highest affinity (50 fold less) after the 5-HT2C and 5-HT2B sites. 4. SB 200646A (20, 40 mg kg-1, p.o., 1 h pretest) also reversed mCPP (0.5 mg kg-1, i.p., 30 min pretest)-induced anxiety in the social interaction test, under low light familiar conditions. 5. When given alone, under high light unfamiliar conditions, SB 200646A (2-40 mg kg-1, p.o.) increased active social interaction without affecting locomotor activity in the rat social interaction test. This is consistent with an anxiolytic action of SB 200646A.(ABSTRACT TRUNCATED AT 250 WORDS)     

Interaction between a selective 5-HT1A receptor antagonist and an SSRI in vivo: effects on 5-HT cell firing and extracellular 5-HT.

1. The acute inhibitory effect of selective 5-hydroxytryptamine (serotonin) reuptake inhibitors (SSRIs) on 5-HT neuronal activity may offset their ability to increase synaptic 5-HT in the forebrain. 2. Here, we determined the effects of the SSRI, paroxetine, and a novel selective 5-HT1A receptor antagonist, WAY 100635, on 5-HT cell firing in the dorsal raphé nucleus (DRN), and on extracellular 5-HT in both the DRN and the frontal cortex (FCx). Extracellular electrophysiological recording and brain microdialysis were used in parallel experiments, in anaesthetized rats. 3. Paroxetine dose-dependently inhibited the firing of 5-HT neurones in the DRN, with a maximally effective dose of approximately 0.8 mg kg-1, i.v. WAY 100635 (0.1 mg kg-1, i.v.) both reversed the inhibitory effect of paroxetine and, when used as a pretreatment, caused a pronounced shift to the right of the paroxetine dose-response curve. 4. Paroxetine (0.8 mg kg-1, i.v.), doubled extracellular 5-HT in the DRN, but did not alter extracellular 5-HT in the FCx. A higher dose of paroxetine (2.4 mg kg-1, i.v.) did increase extracellular 5-HT in the FCx, but to a lesser extent than in the DRN. Whereas 0.8 mg kg-1, i.v. paroxetine alone had no effect on extracellular 5-HT in the FCx, in rats pretreated with WAY 100635 (0.1 mg kg-1), paroxetine (0.8 mg kg-1, i.v.) markedly increased extracellular 5-HT in the FCx.(ABSTRACT TRUNCATED AT 250 WORDS)     

Allosteric modulation of the effect of escitalopram, paroxetine and fluoxetine: in-vitro and in-vivo studies

Clinical and preclinical studies have shown that the effect of citalopram on serotonin (5-HT) reuptake inhibition and its antidepressant activity resides in the S-enantiomer. In addition, using a variety of in-vivo and in-vitro paradigms, it was shown that R-citalopram counteracts the effect of escitalopram. This effect was suggested to occur via an allosteric modulation at the level of the 5-HT transporter. Using in-vitro binding assays at membranes from COS-1 cells expressing the human 5-HT transporter (hSERT) and in-vivo electrophysiological and microdialysis techniques in rats, the present study was directed at determining whether R-citalopram modifies the action of selective serotonin reuptake inhibitors (SSRIs) known to act on allosteric sites namely escitalopram, and to a lesser extent paroxetine, compared to fluoxetine, which has no affinity for these sites. In-vitro binding studies showed that R-citalopram attenuated the association rates of escitalopram and paroxetine to the 5-HT transporter, but had no effect on the association rates of fluoxetine, venlafaxine or sertraline. In the rat dorsal raphe nucleus, R-citalopram (250 microg/kg i.v.) blocked the suppressant effect on neuronal firing activity of both escitalopram (100 microg/kg i.v.) and paroxetine (500 microg/kg i.v.), but not fluoxetine (10 mg/kg i.v.). Interestingly, administration of R-citalopram (8 mg/kg i.p.) attenuated the increase of extracellular levels of 5-HT ([5-HT]ext) in the ventral hippocampus induced by both escitalopram (0.28 microM) and paroxetine (0.75 microM), but not fluoxetine (10 microM). In conclusion, the present in-vitro and in-vivo studies show that R-citalopram counteracts the activity of escitalopram and paroxetine, but not fluoxetine, by acting at the allosteric binding site of the 5-HT transporter, either located in the dorsal raphe nucleus or post-synaptically in the ventral hippocampus. This conclusion is strengthened by the observation that the inhibitory effect of fluoxetine, which has no stabilizing effect on the radioligand/hSERT complex, was not blocked by co-administration of R-citalopram.     

Differential modulation of extracellular levels of 5-hydroxytryptamine in the rat frontal cortex by (R)- and (S)-zacopride.

1. The ability of various anxiolytic and potential anxiolytic agents to modify 5-hydroxytryptamine (5-HT) release in the frontal cortex of the rat was assessed by the microdialysis technique. 2. The benzodiazepine receptor agonist, diazepam (2.5 mg kg-1, i.p.), the 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT, 0.32 mg kg-1, s.c.) and the 5-HT1A receptor partial agonist buspirone (4.0 mg kg-1, i.p.) maximally reduced extracellular levels of 5-HT in the rat frontal cortex by approximately 50-60%, 70-80% and 30-40%, respectively. 3. (R)-zacopride (1.0-100 micrograms kg-1, i.p.) dose-dependently reduced extracellular levels of 5-HT in the rat frontal cortex (approximately 80% maximal reduction) whereas the other 5-HT3 receptor antagonists ondansetron (10 micrograms kg-1, i.p.) and (S)-zacopride (10-100 micrograms kg-1, i.p.) were ineffective. 4. In contrast to (S)-zacopride (100 nM; administered via the microdialysis probe), (R)-zacopride (1.0-100 nM; administered via the microdialysis probe) induced a concentration-dependent reduction in extracellular levels of 5-HT in the rat frontal cortex (approximately 70% maximal reduction). 5. In contrast to ondansetron (100 micrograms kg-1, i.p.), (S)-zacopride (10-100 micrograms kg-1, i.p.) dose-dependently reversed the (R)-zacopride (10 micrograms kg-1, i.p.) induced reduction in extracellular levels of 5-HT in the rat frontal cortex. The highest dose of (S)-zacopride (100 micrograms kg-1, i.p.) completely prevented the (R)-zacopride response.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of chronic and acute administration of fluoxetine and its additive effect with morphine on the behavioural response in the formalin test in rats

Serotonergic systems are involved in the central regulation of nociceptive sensitivity. Fluoxetine, a selective inhibitor of the reuptake of serotonin (5-hydroxytryptamine, 5-HT), was administered orally (0.16, 0.32, 0.8 mg kg(-1) daily for 7 days), intraperitoneally (0.04, 0.08, 0.16 mg kg(-1) day(-1) for 7 days and a single dose of 0.32 mg kg(-1)) and intracerebroventricularly (10 microg/rat) to rats and nociceptive sensitivity was evaluated using the formalin test (50 microL of 2.5% formalin injected subcutaneously). The effect of fluoxetine was also studied in the presence of 5,7-dihydroxytryptamine creatinine sulfate (5,7-DHT) and after co-administration with morphine. Oral (0.8 mg kg(-1)), intraperitoneal (0.16 and 0.32 mg kg(-1)) and intracerebroventricular (10 microg/rat) fluoxetine induced antinociception in the late phase of the formalin test. Furthermore, intrathecal administration of 5-HT (100 microg/rat) induced an analgesic effect. The analgesic effect of fluoxetine (0.16 and 0.32 mg kg(-1), i.p.) and 5-HT (100 microg/rat, i.t.) was abolished by pre-treatment with 5,7-DHT (100 microg/rat, i.t.). In addition, the analgesic effect of 5-HT (100 microg/rat, i.t.) was decreased by pre-treatment with naloxone (2 mg kg(-1), i.p.). Morphine (5 mg kg(-1), i.p.) induced analgesia that was increased by fluoxetine (0.32 mg kg(-1), i.p.). These results suggest that fluoxetine has an antinociceptive effect in tonic inflammatory pain through functional alteration of the serotonergic system and also potentiates the analgesic effect of morphine.     

Inhibitors of serotonin and noradrenaline uptake in human plasma after withdrawal of zimelidine and clomipramine treatment

Ten patients with endogenous depression, who had not taken any antidepressive drugs for 3 months, were treated with 100 mg b.i.d. zimelidine (five patients) or 75 mg b.i.d. clomipramine (five patients) for 5-10 weeks. Blood samples taken before and at various times after stopping the treatment were analysed for plasma concentrations of the drugs and their desmethylated metabolites, the uptake of 14C-serotonin in platelets incubated in platelet rich plasma, the concentration of serotonin in whole blood, and the inhibitory effect of the plasma on the accumulation of 14C-serotonin and 3H-noradrenaline in rat hypothalamic synaptosomes. It was found that these uptake measures were normalized within 1 to 2 weeks after zimelidine withdrawal, whereas the effects after clomipramine persisted for 3 to 4 weeks. Norzimelidine, the desmethylated metabolite of zimelidine, caused the effects after zimelidine treatment. Both clomipramine and its metabolite desmethylclomipramine were involved in uptake inhibition after clomipramine treatment. The mean pretreatment values of 14C-serotonin uptake in the platelets did not differ significant from an age and sex matched control group.l     

Studies on the interaction between cerebral 5-hydroxytryptamine and gamma-aminobutyric acid in the mode of action of diazepam in the rat

The effect of the benzodiazepine, diazepam, administered for 7 days in doses between 1.25 and 5 mg kg-1 was studied on the turnover of 5-hydroxytryptamine (5-HT) in rat cerebral cortex. 5-HT turnover was assessed by calculating the ratio of the concentration of the major metabolite 5-hydroxyindoleacetic acid (5-HIAA) to that of 5-HT (i.e., 5-HIAA:5-HT). Diazepam (2.5 and 5 mg kg-1 i.p. daily for 7 days) significantly reduced cerebral cortical 5-HT turnover. The effect of manipulating cerebral gamma-aminobutyric acid (GABA) mechanisms on this action of diazepam was studied. Treatment of animals with a subconvulsive dose of picrotoxin (3 mg kg-1 i.p.) reversed the fall in cortical 5-HT turnover seen following diazepam. In contrast, however, treatment with the GABA transaminase inhibitors, amino-oxyacetic acid (25 mg kg-1) or ethanolamine-O-sulphate (250 mg kg-1, 7 days) which elevated cerebral GABA concentrations, enhanced the reduction in cortical 5-HT turnover following diazepam. Focal injection of picrotoxin (0.1 micrograms) into the region of the dorsal raphé nucleus reversed the decrease in cortical 5-HT turnover caused by diazepam. The hypothesis that doses of diazepam which result in total plasma concentrations comparable to those observed in man produce a reduction in 5-HT turnover mediated via GABA neurones is discussed.