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)     

Effects of a selective 5-HT2 agonist, DOI, on 5-HT neuronal firing in the dorsal raphe nucleus and 5-HT release and metabolism in the frontal cortex.

Systemic administration of the 5-HT2 agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) (50 and 100 micrograms kg-1, i.v.) inhibited dorsal raphe neuronal firing. DOI (100 micrograms kg-1, i.v.) also produced a decrease in extracellular 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) in the frontal cortex measured by microdialysis. However, local administration of DOI into the frontal cortex produced no change in extracellular 5-HT and 5-HIAA at any dose given (1, 10 and 100ng). The results demonstrate that DOI is a potent inhibitor of 5-HT neuronal firing and terminal release and that the effects on release are not mediated by an action within the terminal region. The site of action and the receptor involved in the inhibition remains to be determined.     

5-HT4 receptor-mediated modulation of 5-HT release in the rat hippocampus in vivo.

1. In the present study, the ability of the 5-hydroxytryptamine, receptor (5-HT4 receptor) to modulate the release of 5-HT in the hippocampus of freely-moving rats was investigated by the in vivo microdialysis technique. 2. The 5-HT4 receptor agonist, renzapride (1.0-100 microM, administered via the microdialysis probe) increased extracellular hippocampal levels of 5-HT in concentration-dependent manner (approximately 200% maximal increase). The ability of renzapride (100 microM, administered via the microdialysis probe) to elevate extracellular levels of 5-HT remained in the presence of the selective 5-HT reuptake blocker, paroxetine (1.0 microM, administered via the microdialysis probe). Furthermore, another 5-HT4 receptor agonist 5-methoxytryptamine (5-MeOT; 10 microM, administered via the microdialysis probe, in the presence of the non-5-HT4 5-HT receptor antagonists pindolol (10 microM) and methysergide (10 microM)) maximally elevated extracellular levels of 5-HT by approximately 450% in the rat hippocampus. The elevation of extracellular 5-HT levels induced by either renzapride (100 microM) or 5-MeOT (10 microM) was completely prevented by combined administration of the selective 5-HT4 receptor antagonist, GR113808 (100 nM, administered via the microdialysis probe). GR113808 (100 nM, administered via the microdialysis probe) administered alone, however, reduced extracellular hippocampal 5-HT levels by some 60%. 3. Systemic administration of the 5-HT1A receptor agonist, 8-OH-DPAT (0.1 mg kg-1, s.c.) reduced extracellular levels of 5-HT in the rat hippocampus by approximately 40%. Prior administration of 8-OH-DPAT (0.1 mg kg-1, s.c.), with an associated reduction of extracellular hippocampal 5-HT levels by approximately 40-50%, however, failed to prevent a subsequent elevation of extracellular levels of 5-HT induced by renzapride (100 microM, administered via the microdialysis probe). 4. Systemic administration of the 5-HT4 receptor agonist, renzapride (0.25 and 1.0 mg kg-1, i.p.) increased extracellular levels of 5-HT in the hippocampus in a dose-dependent manner. The higher dose of renzapride increasing extracellular 5-HT levels by some 200%. The selective 5-HT4 receptor antagonist, GR125487D (1.0-100 micrograms kg-1, i.p.) caused a dose-dependent reduction in extracellular levels of 5-HT in the hippocampus (maximally approximately 80% reduction). Prior administration of GR125487D (10 micrograms kg-1, i.p.) prevented the elevation of extracellular levels of 5-HT induced by renzapride (1.0 mg kg-1, i.p.). 5. In conclusion, the present study provides evidence that activation of the 5-HT4 receptor facilitates 5-HT release in the rat hippocampus in vivo.     

The enantiomers of zacopride: an intra-species comparison of their potencies in functional and anxiolytic models.

1. The 5-HT3 receptor antagonist, zacopride, and its enantiomers, R(+)-zacopride and S(-)-zacopride, were examined in three pharmacological models: (i) 5-HT-induced depolarization of the mouse isolated vagus nerve preparation, (ii) the 5-HT-evoked von Bezold-Jarisch reflex in the mouse, and (iii) the mouse light:dark box model of anxiety. Other standard 5-HT3 receptor antagonists were also included for comparison in these studies. 2. Racemic zacopride, and both of the enantiomers, displayed potent 5-HT3 receptor antagonist activity in the isolated vagus nerve and in the von Bezold-Jarisch model. No 5-HT3 receptor agonist or partial agonist effects of these compounds were detected. 3. In the isolated vagus nerve, R(+)-zacopride and ondansetron were surmountable 5-HT3 receptor antagonists (pA2 values of 9.3 and 8.3, respectively), whereas racemic zacopride, S(-)-zacopride and tropisetron were insurmountable antagonists, markedly suppressing the maximum response to 5-HT. 4. In vivo, racemic zacopride, R(+)-zacopride, S(-)-zacopride and WAY100289 were potent antagonists of the 5-HT-evoked von Bezold-Jarisch reflex, with minimum effective doses (lowest dose required to reduce the reflex by > or = 85%; MED85) of 1.0, 3.0, 0.3 and 3.0 micrograms kg-1, s.c., respectively. 5. Racemic zacopride, R(+)-zacopride and S(-)-zacopride were active in the mouse light:dark box model of anxiety, with similar potencies (minimum effective dose 1 microgram kg-1, s.c.) and similar active dose-ranges (1-1000 micrograms kg-1, s.c.).(ABSTRACT TRUNCATED AT 250 WORDS)     

5-HT3 receptor agonism may be responsible for the emetic effects of zacopride in the ferret.

The racemic 5-HT3 receptor antagonist, zacopride (10-100 micrograms kg-1, i.m.) evoked an emetic response in ferrets. This property appeared to reside totally in the S-enantiomer which also produced emesis over the same dose range. This emesis could be prevented by pretreatment with ondansetron (1 mg kg-1, i.m.) or by R-zacopride (100 micrograms kg-1, i.m.). In urethane-anaesthetized ferrets, S-zacopride (0.3 micrograms kg-1, i.v.) evoked a profound Bezold-Jarisch reflex which was blocked by both ondansetron (30 micrograms kg-1, i.v.) and by R-zacopride (100 micrograms kg-1, i.v.). These results suggest that, in the ferret, S-zacopride possesses 5-HT3 receptor agonist properties which may be responsible for the emetic effect. In contrast R-zacopride does not appear to possess 5-HT3 receptor agonist properties in this species.     

The interaction of R(+)- and S(-)-zacopride with PCPA to modify rodent aversive behaviour.

The interaction of R(+)- and S(-)zacopride (0.00001-10 mg/kg i.p.) with parachlorophenylalanine (PCPA, 3 day treatment 100 mg/kg i.p.) to modify behaviour in an aversive situation was investigated in the mouse black and white test box and rat social interaction test. R(+)-Zacopride (but not S(-)zacopride) and PCPA had an anxiolytic profile of action to reduce aversive responding in both species. Their established anxiolytic profiles were abolished by a subsequent treatment with S(-)zacopride. In contrast, S(-)-zacopride was less or ineffective if administered simultaneously with R(+)-zacopride. A co-treatment of PCPA with R(+)-zacopride also inhibited the anxiolytic profiles observed to the individual treatments. It is concluded that there is a complex interaction between the isomers of zacopride to modify responding to an aversive situation that is dependent on the temporal sequence of drug administration, and which may be modified by changes in basal 5-hydroxytryptamine (5-HT) tone and agonist, partial agonist and antagonist effects at the 5-HT3 receptor.     

In vivo effects of sumatriptan (GR 43175) on extracellular levels of 5-HT in the guinea pig

The effect of sumatriptan, a selective 5-HT1D receptor agent, on extracellular levels of 5-HT in the frontal cortex of the guinea pig was measured by intracerebral dialysis. A constant infusion of sumatriptan (10(-8)-10(-7) M) dose-dependently reduced extracellular levels of 5-HT (e.g. 80 +/- 2% decrease from control levels of 5-HT at 10(-7) M). Peripheral administration of sumatriptan (50 micrograms/kg i.p. and 500 micrograms/kg i.p.) had no effect on extracellular levels of 5-HT in the frontal cortex. These data suggest that sumatriptan, when infused into the brain, inhibits the release of 5-HT by stimulating 5-HT autoreceptors and that the putative acute anti-migraine effects of sumatriptan are likely to be mediated by peripheral mechanisms.     

The 5-HT1 receptor agonist RU-24969 decreases 5-hydroxytryptamine (5-HT) release and metabolism in the rat frontal cortex in vitro and in vivo.

K+-stimulated release of [3H]-5-hydroxytryptamine ( [3H]-5-HT) from rat frontal cortex slices was decreased by the 5-HT receptor agonists 5-methoxy-n1N-dimethyltryptamine and 5-methoxy-3(1,2,3,6,-tetrahydro-4-pyrindinyl)-1H-indole (RU-24969) (1 X 10(-5)M). RU-24969 (10 mg kg-1, i.p.) decreased extracellular 5-HT and its metabolite 5-hydroxyindoleacetic acid measured in vivo by use of intracerebral dialysis combined with high performance liquid chromatography and electrochemical detection. The decrease in extracellular 5-hydroxyindoleacetic acid in vivo after RU-24969 (10 mg kg-1, i.p.) was also observed by in vivo voltammetry. The non-selective 5-HT antagonist metergoline prevented the RU-24969-induced decrease in 5-HT release and metabolism in vivo while the 5-HT2 receptor antagonist R-55669 (ritanserin) did not. The results support the view that RU-24969 stimulates a 5-HT1 receptor that is involved in the autoregulation of 5-HT release and metabolism.     

Desensitization of 5-HT(1A) autoreceptors by a low chronic fluoxetine dose effect of the concurrent administration of WAY-100635.

Using microdialysis, receptor autoradiography and in situ hybridization, we examined the effects of fluoxetine alone or with WAY-100635 on: (a) extracellular 5-HT in frontal cortex; and (b) density and sensitivity of 5-HT(1A) autoreceptors in rat brain. WAY-100635 (0.3 mg/kg, s.c.) doubled the increase in extracellular 5-HT produced by fluoxetine (3 mg/kg, i.p.) in frontal cortex. Two-week minipump treatments with these daily doses significantly raised extracellular 5-HT to 275 +/- 33% (fluoxetine) and 245 +/- 10% (fluoxetine plus WAY-100635) of controls. Fluoxetine 3 mg/kg.day desensitized dorsal raphe 5-HT(1A) autoreceptors, an effect prevented by the concurrent WAY-100635 administration. However, WAY-100635 (alone or with fluoxetine) did not change 5-HT(1A) autoreceptor sensitivity. The density of 5-HT(1A) receptors and its encoding mRNA, was unaffected by these treatments. These results suggest that prolonged blockade of 5-HT(1A) receptors in vivo prevents the autoreceptor desensitization induced by fluoxetine but does not result in receptor sensitization.     

Profiles of interaction of R(+)/S(-)-zacopride and anxiolytic agents in a mouse model.

The mouse black and white test box was used to measure changes in behaviour in an aversive situation where the administration of R(+)-zacopride (but not S(-)-zacopride) alone decreased aversive responding to the white area. A similar anxiolytic profile of action was observed using parachlorophenylalanine (PCPA), whose effects were antagonised by a co-treatment with R(+)-zacopride and reversed by S(-)-zacopride to an exacerbation of the aversive response. An anxiolytic profile of action was also observed using ondansetron, granisetron, chlordiazepoxide, diazepam, ritanserin, 8-OH-DPAT (8-hydroxy-2-(di-n-propylamino)tetralin), E4424 (2-[4-[4-(4-chloro-l-pyrazoyl)butyl]-l-piperazinyl]-pyrimidine), umepsirone, DuP753 (2-n-butyl-4-chloro-5-hydroxy-methyl-1-[2(1H-tetrazol-5-yl) biphenyl-4-yl)methyl)]-imidazole), SQ29,852 ((S)-1-[6-amino-2[hydroxy)(4-phenyl-butyl)phosphinyl]-oxy)-1- nexy]-2-proline), devazepide and guanfacine, and this was retained following co-treatment with PCPA. The anxiolytic profile of action of PCPA was also retained following co-treatment with renzapride which when administered alone failed to modify behaviour. However, the ability of chlordiazepoxide, diazepam, ondansetron and E4424 (but not devazepide, DuP753 or SQ29,852) to reduce aversive responding was inhibited by co-treatment with R(+) and/or S(-)-zacopride. It is concluded that the reduction in aversive responding caused by pharmacological manipulation at the benzodiazepine, 5-HT receptor subtypes 5-HT1A, 5-HT1C/5-HT2 and 5-HT3 (but not at the cholecystokin CCKA or angiotensin receptors or inhibition of angiotensin converting enzyme) can be inhibited by R(+) and S(-)-zacopride. The data is discussed in terms of zacopride having an agonist or partial agonist effect at the 5-HT3 receptor.     

Neurochemical evaluation of the novel 5-HT1A receptor partial agonist/serotonin reuptake inhibitor, vilazodone

Vilazodone has been reported to be an inhibitor of 5-hydoxytryptamine (5-HT) reuptake and a partial agonist at 5-HT1A receptors. Using [35S]GTPgammaS binding in rat hippocampal tissue, vilazodone was demonstrated to have an intrinsic activity comparable to the 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT). Vilazodone (1-10 mg/kg p.o.) dose-dependently displaced in vivo [3H]DASB (N,N-dimethyl-2-(2-amino-4-cyanophenylthio)benzylamine) binding from rat cortex and hippocampus, indicating that vilazodone occupies 5-HT transporters in vivo. Using in vivo microdialysis, vilazodone (10 mg/kg p.o.) was demonstrated to cause a 2-fold increase in extracellular 5-HT but no change in noradrenaline or dopamine levels in frontal cortex of freely moving rats. In contrast, administration of 8-OH-DPAT (0.3 mg/kg s.c.), either alone or in combination with a serotonin specific reuptake inhibitor (SSRI; paroxetine, 3 mg/kg p.o.), produced no increase in cortical 5-HT whilst increasing noradrenaline and dopamine 2 and 4 fold, respectively. A 2-fold increase in extracellular 5-HT levels (but no change in noradrenaline or dopamine levels) was observed after combination of the 5-HT(1A) receptor antagonist, N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(pyridinyl)cyclohexanecarboxamide) (WAY-100635; 0.3 mg/kg s.c.) and paroxetine (3 mg/kg p.o.). In summary, vilazodone behaved as a high efficacy partial agonist at the rat hippocampal 5-HT1A receptors in vitro and occupied 5-HT transporters in vivo. In vivo vilazodone induced a selective increase in extracellular levels of 5-HT in the rat frontal cortex. This profile was similar to that seen with a 5-HT1A receptor antagonist plus an SSRI but in contrast to 8-OH-DPAT either alone or in combination with paroxetine.     

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.     

Pharmacological characterization of 5-hydroxytryptamine-induced excitation of afferent cervical vagus nerve in anaesthetized rats.

1. An excitatory response to 5-hydroxytryptamine (5-HT) was measured from the afferent vagus nerve of anaesthetized rats. Measurements were determined by an extracellular recording from the whole nerve. 2. Intravenous bolus injection of 5-HT (1.56-100 micrograms kg-1) evoked a dose-dependent excitation of afferent vagus nerve activity. This response was blocked not only by a selective 5-HT3 receptor antagonist, GR38032F (10 and 100 micrograms kg-1), but also by a 5-HT2 receptor antagonist, ketanserin (10 and 100 micrograms kg-1). 3. Both a 5-HT3 receptor agonist, 2-methyl-5-HT (3.12-100 micrograms kg-1), and a 5-HT2 receptor agonist, alpha-methyl-5-HT (3.12-50 micrograms kg-1), produced a dose-dependent excitation of afferent vagus nerve activity. These excitatory effects were antagonized by GR38032F (10 micrograms kg-1) and ketanserin (10 micrograms kg-1), respectively. 4. A 5-HT1 like receptor agonist, 5-carboxamidotryptamine (50 micrograms kg-1), and a putative 5-HT4 receptor agonist, 5-methoxytryptamine (100 micrograms kg-1), failed to produce excitatory effects on the afferent vagus nerve. 5. These results suggest that the 5-HT-induced excitatory response of the afferent vagus nerve might be mediated not only via 5-HT3 receptors but also via 5-HT2 receptors in anaesthetized rats. It is unlikely, however, that either 5-HT1-like or putative 5-HT4 receptors are involved in the excitatory response of the afferent vagus nerve to 5-HT.     

Nitric oxide-dependent release of vasodilator quantities of calcitonin gene-related peptide from capsaicin-sensitive nerves in rabbit skin.

1. The involvement of presynaptic 5-hydroxytryptamine1A (5-HT1A) autoreceptors in the anxiolytic-like properties of lesopitron (E-4424) (2-(4-[4-(4-chloro-1-pyrazolyl)butyl]-1- piperazinyl)pyrimidine) was studied. Brain microdialysis was used to examine the effect of the drug on the release of 5-hydroxytryptamine (5-HT) and its metabolite 5-hydroxyindoleacetic acid (5-HIAA) in the frontal cortex of awake, freely moving rats. Moreover, extracellular cortical 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) were also studied to assess the possible participation of dopaminergic systems. 2. Lesopitron administered at a dose which induces anxiolytic behavior in rats (30 micrograms kg-1, i.p.) markedly reduced 5-HT levels (to 45% of the basal value) in cortical perfusates, having no effect on 5-HIAA, DOPAC and HVA. The effects of lesopitron were compared with those produced by the anxiolytic, and structurally related compound, buspirone. 3. Buspirone administered at a dose inducing anxiolytic-like effects in rats (5 mg kg-1, i.p.) produced a marked decrease in cortical 5-HT levels (to 20% of the basal value), but in contrast to lesopitron, buspirone produced a pronounced increase in cortical DOPAC (to 300% of the basal value) and HVA (to 400% of the basal value) levels. Buspirone administered at a low dose (30 micrograms kg-1, i.p.) was unable to affect cortical 5-HT levels. 4. To test the hypothesis that the 5-HT decreasing effect of lesopitron could be due to 5-HT1A autoreceptor (somatodendritic)-mediated inhibition of 5-HT neurotransmission, lesopitron was administered locally into the raphe nuclei.(ABSTRACT TRUNCATED AT 250 WORDS)     

Evidence that RU 24969-induced locomotor activity in C57/B1/6 mice is specifically mediated by the 5-HT1B receptor.

1. The behavioural effects of the 5-HT1B receptor agonists, RU 24969 and CGS 12066B, have been investigated in C57/B1/6 mice. 2. RU 24969 (1-30 mg kg-1) produced intense and prolonged hyperlocomotion and other behavioural changes. 3. CGS 12066B caused similar effects, but they were much less pronounced, inconsistent and transient irrespective of whether this drug was given i.p. (1-15 mg kg-1) or i.c.v. (0.2-40 micrograms). However, CGS 12066B (7.5 and 15 mg kg-1) caused a dose-related inhibition of RU 24969 (7.5 mg kg-1)-induced hyperlocomotion indicating that the former is a 5-HT1B partial agonist. 4. RU 24969 (7.5 mg kg-1 i.p.)-induced hyperlocomotion was inhibited by the (-)-, but not (+)-isomers of pindolol (4 mg kg-1) and propranolol (20 mg kg-1) but not by metoprolol (10 mg kg-1) or ICI 118,551 (5 mg kg-1), consistent with an involvement of 5-HT1A or 5-HT1B receptors. 5. The response was not altered by the selective 5-HT1A receptor antagonist, WAY 100135 (5 mg kg-1, s.c.), the 5-HT2A/5-HT2C receptor antagonist, ritanserin (0.1 mg kg-1), the selective 5-HT3 receptor antagonist, ondansetron (1 mg kg-1) or the non-selective 5-HT receptor antagonists methysergide (3 mg kg-1) and metergoline (3 mg kg-1). 6. Although spiroxatrine (0.1 mg kg-1) and ketanserin (1 mg kg-1) inhibited RU 24969-induced hyperlocomotion, these effects were probably due to antagonism of dopamine D2 receptors and alpha 1-adrenoceptors respectively.(ABSTRACT TRUNCATED AT 250 WORDS)     

Enhancement of 5-HT1B and 5-HT1D receptor antagonist effects on extracellular 5-HT levels in the guinea-pig brain following concurrent 5-HT1A or 5-HT re-uptake site blockade.

The effects of selective serotonin re-uptake inhibitor (SSRI), paroxetine, and 5-HT1A, 5-HT1B and 5-HT1B/1D receptor antagonists on in vivo extracellular 5-HT levels in the guinea-pig frontal cortex and dorsal hippocampus were investigated using the technique of microdialysis. The aim of the study was to further investigate the autoreceptor roles of the 5-HT1A, 5-HT1B and 5-HT1D receptors in the median vs dorsal raphe nuclei. In the frontal cortex, 5-HT1A (WAY 100635, 1 mg/kg i.p.) or 5-HT1B (SB-224289, 4 mg/kg i.p.) receptor antagonists had no effect on extracellular levels of 5-HT, whilst the mixed 5-HT1B/1D receptor antagonist (GR 127935, 0.3 mg/kg i.p) produced a significant decrease in extracellular 5-HT levels. Paroxetine (10 microM) significantly increased extracellular 5-HT levels when perfused locally into the cortex. Administration of SB-224289, followed 120 min later by WAY 100635, had no effect on extracellular 5-HT levels. In contrast, sequential administration of either WAY 100635 and GR 127935, or SB-224289 and paroxetine significantly increased extracellular 5-HT levels. In the dorsal hippocampus, whilst 5-HT1A receptor antagonism elicited by administration of WAY 100635 had no effect, both 5-HT1B and mixed 5-HT1B/1D receptor blockade significantly increased extracellular 5-HT levels. Administration of SB-224289 followed 120 min later with WAY 100635, or WAY 100635 followed 30 min later with GR 127935, potentiated the effect of the three compounds alone, significantly increasing extracellular 5-HT levels. These data demonstrate that to simultaneously increase extracellular 5-HT in both frontal cortex and dorsal hippocampus of the guinea-pig brain concurrent 5-HTA1A, 5-HT1B and 5-HT1D receptor blockade is required. Whereas in the dorsal hippocampus, 5-HT1B receptor blockade is sufficient to elicit an increase in extracellular 5-HT levels.     

Ability of 5-HT4 receptor ligands to modulate rat striatal dopamine release in vitro and in vivo.

1. The ability of 5-HT4 (5-hydroxytryptamine4) receptor ligands to modify dopamine release from rat striatal slices in vitro and in the striatum of freely moving rats was assessed by the microdialysis technique. 2. The release of dopamine from slices of rat striatum continually perfused with Krebs buffer was enhanced by 5-HT4 receptor agonists; 5-HT (10 microM), 5-methoxytryptamine (5-MeOT; 10 microM), renzapride (10 microM) and (S)-zacopride (10 microM) maximally increased dopamine release by 133 +/- 5, 214 +/- 25, 232 +/- 29 and 264 +/- 69%, respectively (mean +/- s.e.mean, n = 3-8). The drug-induced responses were maximal within the first 2 min of drug application, and subsequently declined. The non-selective 5-HT3/5-HT4 receptor antagonist, SDZ205-557 (10 microM), failed to modify basal dopamine release from striatal slices but completely antagonized the (S)-zacopride (10 microM)-induced increase in dopamine release. 3. To allow faster drug application, the modulation of dopamine release from rat striatal slices in a static release preparation was also investigated. The 5-HT4 receptor agonist, renzapride (10 microM) also enhanced dopamine release in this preparation (maximal increase = 214 +/- 35%, mean +/- s.e.mean, n = 14), whilst a lower concentration of renzapride (3 microM) was less effective. The renzapride-induced response was maximal within the first 2 min of drug application, before declining. In this preparation, the stimulation of dopamine release by renzapride (10 microM), was completely antagonized by the selective 5-HT4 receptor antagonist, GR113808 (100 nM). In addition, both the Na+ channel blocker, tetrodotoxin (100 nM) and the non-selective protein kinase A inhibitor, H7 (100 nM) completely prevented the stimulation of dopamine release induced by renzapride (10 microM). 4. In vivo microdialysis studies demonstrated that the 5-HT4 receptor agonists, 5-MeOT (10 microM), renzapride (100 microM) and (S)-zacopride (100 microM) maximally elevated extracellular levels of dopamine in the striatum by 220 +/- 20, 161 +/- 10 and 189 +/- 53%, respectively (mean +/- s.e.mean, n = 5-9). A lower concentration of renzapride (10 microM) was less effective. The elevation of extracellular striatal dopamine levels induced by either renzapride (100 microM) or (S)-zacopride (100 microM) were completely antagonized by the non-selective 5-HT4 receptor antagonist, SDZ205-557 (100 microM). In addition, the elevation of extracellular levels of dopamine induced by either 5-MeOT (10 microM) or renzapride (100 microM) was completely prevented by the selective 5-HT4 receptor antagonist, GR113808 (1 microM) and the renzapride (100 microM)-induced response was also completely prevented by the non-selective protein kinase A inhibitor, H7 (1 microM). In this in vivo preparation, both GR113808 (1 microM) and H7 (1 microM), when perfused alone, reduced extracellular levels of dopamine. 5. In conclusion, the present study provides evidence that the 5-HT4 receptor facilitates rat striatal dopamine release in vitro and in vivo.     

Pharmacological characterization of RS 25259-197, a novel and selective 5-HT3 receptor antagonist, in vivo.

1. The pharmacological effects in vivo, of RS 25259-197, a selective 5-HT3 receptor antagonist, have been investigated. 2. In anaesthetized rats, RS 25259-197, administered by the intravenous, intraduodenal or transdermal route, dose-dependently inhibited the von Bezold-Jarisch reflex induced by 2-methyl 5-HT (ID50 = 0.04 micrograms kg-1, i.v., 3.2 micrograms kg-1, i.d. and 32.8 micrograms per chamber, respectively). In this regard, when administered intraduodenally, RS 25259-197 was more potent and exhibited a longer duration of action than either ondansetron or granisetron. 3. In conscious ferrets, RS 25259-197, administered intravenously or orally, dose-dependently inhibited emesis induced by cisplatin. The ID50 estimates of RS 25259-197 were 1.1 micrograms kg-1, i.v. and 3.2 micrograms kg-1, p.o. In this respect, RS 25259-197 was more potent than ondansetron and equipotent with granisetron. 4. In conscious dogs, RS 25259-197, administered intravenously or orally, dose-dependently inhibited emesis induced by cisplatin (ID50 = 1.9 micrograms kg-1, i.v. and 8.5 micrograms kg-1, p.o.), dacarbazine (ID50 = 4.1 micrograms kg-1, i.v. and 9.7 micrograms kg-1, p.o.), actinomycin D (ID50 = 4.9 micrograms kg-1, i.v. and 2.5 micrograms kg-1, p.o.) and mechlorethamine (ID50 = 4.4 micrograms kg-1, i.v. and 3.0 micrograms kg-1, p.o.). Against each of the emetogenic agents, RS 25259-197 was very much more potent than ondansetron. When tested at equi-effective intravenous doses against cisplatin-induced emesis in dogs, RS 25259-197 had a longer duration of anti-emetic activity (7 h) than ondansetron (4 h).(ABSTRACT TRUNCATED AT 250 WORDS)     

Characterization of 5-HT3 and ''atypical'' 5-HT receptors mediating guinea-pig ileal contractions in vitro.

1. Neuronal 5-hydroxytryptamine (5-HT) receptors mediating contraction of guinea-pig ileal segments have been characterized in vitro by the use of methysergide to block 5-HT1-like and 5-HT2 receptors. Concentration-response curves to 5-HT were biphasic (first phase, defined as those responses occurring between 1 nM and 0.32 microM 5-HT, -log EC50 = 7.15 +/- 0.08; second phase, defined as these responses occurring between 0.32 microM and 32 microM 5-HT, -log EC50 = 5.32 +/- 0.03) but monophasic to 5-methoxytryptamine (-log EC50 = 7.0 +/- 0.08) and 2 methyl 5-HT (-log EC50 = 5.2 +/- 0.13). The maximal response of the first phase to 5-HT and the maximal response to 5-methoxytryptamine were 30 +/- 4% and 35 +/- 5% respectively of the maximum response to the second phase of the 5-HT concentration-effect curve (set at 100%). In contrast, the maximal response to 2-methyl-5-HT equalled that obtained with 5-HT (second phase). 2. The responses comprising the second phase of the concentration-effect curve to 5-HT were antagonized by 1 microM ICS 205-930, ondansetron, granisetron, quipazine, N-methyl-quipazine and (R,S)-zacopride and the following pKB values, with 5-HT as the agonist, were obtained at the 5-HT3 receptor: ICS 205-930 7.61 +/- 0.05, ondansetron 6.90 +/- 0.04, granisetron 7.90 +/- 0.04, (S)-zacopride 8.11 +/- 0.06, (R,S)-zacopride 7.64 +/- 0.11, and (R)-zacopride 7.27 +/- 0.06.(ABSTRACT TRUNCATED AT 250 WORDS)     

Role of uptake inhibition and autoreceptor activation in the control of 5-HT release in the frontal cortex and dorsal hippocampus of the rat

1. Using brain microdialysis, we compared the relative role of 5-hydroxytryptamine (5-HT; serotonin) blockade and somatodendritic 5-HT(1A) and/or terminal 5-HT(1B) autoreceptor activation in the control of 5-HT output. 2. Fluoxetine (10 mg kg(-1) i.p.) doubled the 5-HT output in frontal cortex and dorsal hippocampus. The 5-HT(1A) receptor antagonist WAY 100635, (0.3 mg kg(-1) s.c.) potentiated the effect of fluoxetine only in frontal cortex (to approximately 500 % of baseline). 3. Methiothepin (10 mg kg(-1) s.c.) further enhanced the 5-HT rise induced by fluoxetine+WAY 100635, to 835+/-179% in frontal cortex and 456+/-24% in dorsal hippocampus. Locally applied, methiothepin potentiated the fluoxetine-induced 5-HT rise more in the former area. 4. The selective 5-HT(1B) receptor antagonist SB-224289 (4 mg kg(-1) i.p.) enhanced the effect of fluoxetine (10 mg kg(-1) i.p.) in both areas. As with methiothepin, SB-224289 (4 mg kg(-1) i.p.) further enhanced the 5-HT increase produced by fluoxetine+WAY 100635 more in frontal cortex (613+/-134%) than in dorsal hippocampus (353+/-59%). 5. Locally applied, fluoxetine (10 - 300 microM; EC(50)=28 - 29 microM) and citalopram (1 - 30 microM; EC(50)=1.0 - 1.4 microM) increased the 5-HT output two to three times more in frontal cortex than in dorsal hippocampus. These data suggest that the comparable 5-HT increase produced by systemic fluoxetine in frontal cortex and dorsal hippocampus results from a greater effect of reuptake blockade in frontal cortex that is offset by a greater autoreceptor-mediated inhibition of 5-HT release. As a result, 5-HT autoreceptor antagonists preferentially potentiate the effect of fluoxetine in frontal cortex.