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.     

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.     

Opposing roles for 5-HT1B and 5-HT3 receptors in the control of 5-HT release in rat hippocampus in vivo.

1. Intracerebral microdialysis was used to determine whether 5-hydroxytryptamine (5-HT) release in the ventral hippocampus of rats anaesthetized with chloral hydrate was modulated by 5-HT3 receptors. 2. It was confirmed that 5-methoxy-3-(1,2,3,6-tetrahydro-4-pyridinyl)-1H-indole (RU 24969), a selective 5-HT1B receptor agonist, decreased 5-HT release in a dose- and concentration-related manner when administered i.p. (1 and 5 mg kg-1) or via the dialysis probe (0.1 and 1 microM) respectively. The effect of RU 24969 infusion (1 microM) was attenuated by concurrent infusion of metitepine (10 microM) into the hippocampus. 3. When infused into the hippocampus for 15 min, the selective 5-HT3 receptor agonist, 2-methyl-5-hydroxytryptamine (2-methyl-5-HT; 0.1- 10 microM) increased dialysate 5-HT levels in a concentration-related manner; an effect which was abolished by concurrent infusion of 3-tropanyl-3,5-dichlorobenzoate (1 microM, MDL 72222), a selective 5-HT3 antagonist. 4. MDL 72222 had no effects on hippocampal 5-HT release when administered via the dialysis probe (1 or 10 microM). 5. The data show that 5-HT3 and 5-HT1B receptors have opposing roles in the control of 5-HT release in the hippocampus, with 5-HT3 receptors facilitating and 5-HT1B receptors inhibiting 5-HT efflux, respectively. They also indicate that the facilitatory 5-HT3 receptors are not tonically activated.     

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)     

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.     

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)     

Modulation of 5-HT4 receptor function in the rat isolated ileum by fluoxetine: the involvement of endogenous 5-hydroxytryptamine

The effect of the selective serotonin reuptake inhibitor fluoxetine was examined on the 5-HT4 receptor-mediated relaxation in the rat isolated ileum. Fluoxetine unsurmountably antagonized the relaxation to exogenous 5-HT with abolition of the response at 10 microM. Fluoxetine (10 microM) also caused a gradual loss of the resting tension. These effects of fluoxetine were prevented by a prior addition of the 5-HT4 receptor selective antagonist GR113808 (100 nM), which itself caused a contraction of the tissues when administered alone. Fluoxetine (10 microM) also failed to prevent the relaxation due to exogenous 5-HT and the 5-HT4 receptor agonist 5-methoxytryptamine in tissues taken from the rats treated with para-chlorophenylalanine (300 mg kg-1) for 3 and 6 days, which reduced the 5-HT level in the mucosa by 88 and 97.5% respectively. The contraction of the tissues with GR113808 indicates the presence of an endogenous 5-HT tone at the 5-HT4 receptor in the rat ileum. It is hypothesized that in the presence of fluoxetine, the concentration of endogenous 5-HT at the receptor was increased sufficiently to reduce or abolish the relaxation to 5-HT added exogenously. The inability of fluoxetine to prevent the relaxation to 5-HT in the presence of GR113808 or after the p-CPA treatment supports this hypothesis.     

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.     

The effects of selective serotonin reuptake inhibitors on extracellular 5-HT levels in the hippocampus of 5-HT(1B) receptor knockout mice

The effects of two selective serotonin reuptake inhibitors on 5-hydroxy-tryptamine (5-HT) in the hippocampus were studied in wildtype and in 5-HT(1B) receptor knockout mice using in vivo microdialysis. Basal 5-HT levels in the hippocampus were not different between the two genotypes. The functional absence of 5-HT(1B) receptors was examined in the knockout mice by local infusion of the 5-HT(1B) receptor agonist, 1,4-Dihydro-3-(1,2,3,6-tetrahydro-4-pyridinyl)-5H-pyrrolo[3,2-b]pyridin-5-one (CP93129) into the hippocampus. CP93129 (1 microM) decreased 5-HT levels in wildtype mice, but not in 5-HT(1B) knockout mice. Systemic administration of the selective 5-HT reuptake inhibitor paroxetine (5 mg/kg, i.p.) increased extracellular 5-HT levels. The increase of 5-HT in 5-HT(1B) knockout mice was almost twofold higher than in wildtype mice. Systemic administration of selective 5-HT reuptake inhibitors stimulates both terminal 5-HT(1B) autoreceptors and somatodendritic 5-HT(1A) autoreceptors. Therefore, the selective 5-HT reuptake inhibitor, fluvoxamine, was applied locally into the hippocampus to investigate the role of the terminal 5-HT(1B) autoreceptors. Local administration of 0.3 microM fluvoxamine resulted in comparable increases in extracellular 5-HT in both genotypes, whereas 1.0 microM fluvoxamine produced a twofold greater increase in 5-HT levels in 5-HT(1B) knockout as compared to wildtype mice. In conclusion, the differences in hippocampal 5-HT output between wildtype and 5-HT(1B) knockout mice after local or systemic administration of selective 5-HT reuptake inhibitors show that 5-HT(1B) autoreceptors play a significant role in the inhibition of 5-HT release at serotonergic nerve terminals. In addition, the different dose-response to fluvoxamine suggests that 5-HT(1B) knockout mice have possible adaptations of 5-HT transporters in order to compensate for the loss of the terminal 5-HT(1B) autoreceptor.     

Synergism between 5-HT1B/1D and 5-HT1A receptor antagonists on turnover and release of 5-HT in guinea-pig brain in vivo

The effects on 5-HT turnover (5-HIAA/5-HT ratio) and extracellular 5-HT and 5-HIAA levels (in vivo microdialysis in freely moving animals) were analysed in guinea-pig brains following the 5-HT_1B receptor antagonist, GR 127935 {N-[4-methoxy-3-(4-methyl-1-piper_{azinyl)phenyl]-2’-methyl-4’-(5-methyl-1,2,4-oxadiazol-3-}yl) [1,1-biphenyl]-4-carboxamide}, or the 5-HT_1A receptor antagonist, WAY-100635 (N-{2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl}-N-(2-pyridinyl) cyclohexanecarboxamide trihydrochloride), administered alone or in combination. GR 127935, injected alone, increased 5-HT turnover with maximal effects approximately 50% above the control levels in the four brain regions examined (hypothalamus, hippocampus, striatum and frontal cortex). GR 127935 significantly increased extracellular concentrations of 5-HT and 5-HIAA in frontal cortex (40%), whereas 5-HIAA, but not 5-HT, was elevated in striatum (20–30%). WAY-100635 did not significantly change 5-HT turnover but caused a small significant increase in the extracellular 5-HT and 5-HIAA concentrations in both striatum and frontal cortex. The combined treatment with GR 127935 and WAY-100635 resulted in an increased 5-HT turnover reaching maximal effects of 70–90% above the control values in all brain regions tested and produced a significant elevation of striatal and frontal cortex extracellular 5-HT (40% and 60%, respectively) and 5-HIAA (60% and 70%, respectively) concentrations. The synergistic effect of the two receptor antagonists on the 5-HT turnover and the terminal release of 5-HT indicate somatodendritic 5-HT release and stimulation of inhibitory 5-HT_1A receptors at this level. Extracellular 5-HIAA seems to be a better marker than 5-HT itself for the evoked 5-HT release when the reuptake mechanism is intact. Received: 2 September 1998 / Accepted: 19 November 1998     

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.     

Evidence that 5-hydroxytryptamine release in rat dorsal raphé nucleus is controlled by 5-HT1A, 5-HT1B and 5-HT1D autoreceptors.

Electrically stimulated 5-hydroxytryptamine (5-HT) release was monitored in slices of rat dorsal raphé nucleus (DRN) by fast cyclic voltammetry. Pseudo-single pulse stimulations (5 pulses at 100 Hz) were used to enable the effect of various receptor agonists to be seen without competition from endogenously released transmitter. The selective 5-HT1A receptor agonist, (+)-8-OH-DPAT (1.0 microM) decreased stimulated 5-HT release to 31 +/- 3% of controls. This decrease was inhibited by the 5-HT1A receptor antagonists, (+)-WAY-100135 (1.0 microM) and WAY-100635 (0.1 microM) but not by the 5-HT1D/B antagonist, GR127935 (0.05 microM). The selective 5-HT1B receptor agonist, CP-93129 (0.3 microM) decreased stimulated 5-HT release to 61 +/- 4% of control. This effect was antagonized by the 5-HT1B receptor antagonist, isamoltane (0.5 microM) but not by (+)-WAY-100135. The 5-HT1D agonist, sumatriptan (0.5 microM) decreased stimulated 5-HT release to 52 +/- 2% of controls. This decrease was blocked by GR-127935 but not by WAY-100635. These results suggest that 5-HT release in the rat DRN is under the control of 5-HT1A, 5-HT1B and 5-HT1D autoreceptors.     

The novel 5-HT1A receptor antagonist, SDZ 216-525, decreases 5-HT release in rat hippocampus in vivo.

1. Recent evidence suggests that the novel compound SDZ 216-525 is a selective and possibly silent 5-HT1A receptor antagonist. Here we have examined the action of SDZ 216-525 on central 5-HT1A autoreceptor function. The experiments involved measurement of drug effects on extracellular 5-HT in the ventral hippocampus of the chloral hydrate anaesthetized rat by use of microdialysis. 2. Acute injection of SDZ 216-525 (0.1, 0.3, 1.0 and 3 mg kg-1, s.c.) caused a dose-related decrease in 5-HT output with an estimated ED50 of at least 0.3 mg kg-1. This ED50 value is 20-30 times greater than ED50 values previously obtained for 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) and NAN-190. In comparison, SDZ 216-525 is reported to have slightly higher affinity for the 5-HT1A site than 8-OH-DPAT and NAN-190. 3. The inhibitory effect of SDZ 216-525 (1 mg kg-1, s.c.) on 5-HT was blocked by the 5-HT1/beta-adrenoceptor antagonist, (-)-pindolol (8 mg kg-1, s.c.) but not by a combination of the beta 1- and beta 2-selective adrenoceptor antagonists metoprolol and ICI 118,551 (4 mg kg-1, each). 4. Although in several experimental models SDZ 216-525 has high affinity, selectivity and lacks intrinsic activity at the 5-HT1A receptor, our experiments show that the drug decreases extracellular 5-HT in ventral hippocampus of the chloral hydrate anaesthetized rat via a pindolol-sensitive mechanism.(ABSTRACT TRUNCATED AT 250 WORDS)     

Activation of 5-HT(1B/1D) receptors in the mesolimbic dopamine system increases dopamine release from the nucleus accumbens: a microdialysis study

This study was designed to investigate the role of 5-hydroxytryptamine (5-HT)(1B) receptors located in the ventral tegmental area and nucleus accumbens in the modulation of accumbal dopaminergic transmission. The selective 5-HT(1B) receptor agonist CP 93129 [3-(1,2,5,6-tetrahydro-4-pyridyl)pyrrolo[3,2-b]pyrid-5-one] was administered into the ventral tegmental area or nucleus accumbens of freely moving Sprague-Dawley rats via retrograde microdialysis. The effects of intra-accumbal and intra-tegmental CP 93129 on extracellular dopamine levels in the nucleus accumbens were measured using one- and dual-probe microdialysis, respectively. For dual-probe microdialysis, one probe was in the ventral tegmental area for drug administration and the other in the ipsilateral nucleus accumbens for dopamine measurement. The results show that infusion of CP 93129 (2, 5 and 10 microM) into the nucleus accumbens increased local dopamine levels in a concentration-related manner. Infusion of CP 93129 (10 and 20 microM) into the ventral tegmental area also increased dopamine levels in the ipsilateral nucleus accumbens. The increased dopamine release in the nucleus accumbens produced by intra-accumbal or intra-tegmental CP 93129 was antagonized by co-infusion of cyanopindolol (5 microM), a 5-HT(1B/1A) receptor antagonist, but not by WAY-100635 [N-(2-(4-(2-methoxyphenyl)-1-piperazinyl)ethyl)-N-2-pyridinyl-cyclohexanecarboxamide] (5 microM), a highly selective 5-HT(1A) receptor antagonist. In addition, augmentations of dopamine release in the nucleus accumbens induced by intra-accumbal CP 93129 were sensitive to Na(+) channel blockade with tetrodotoxin. These results are not in opposition to the concept that 5-HT(1B) receptors within the ventral tegmental area and nucleus accumbens are all involved in the modulation of dopamine release in the terminal area of the mesolimbic dopamine system.     

Functional effects of corticosterone on 5-HT(1A) and 5-HT(1B) receptor activity in rat brain: in vivo microdialysis studies

Glucocorticoid hormones are known to be elevated in depression, and to interact with serotonin 5-HT(1A) receptors at both the presynaptic and postsynaptic levels. Since one of the presumed mechanisms of action of antidepressant drugs is induction of changes in sensitivity of 5-HT(1A) and also 5-HT(1B) receptors, the effects of repeated administration of corticosterone (50 mg/kg s.c. b.i.d. for 10 days) on activities of these receptors were determined using in vivo microdialysis in freely moving rats. Presynaptic 5-HT(1A) receptor activity, as measured by the effect of a challenge dose (0.2 mg/kg s.c.) of the 5-HT(1A) agonist 8-hydroxy-2 (di-n-propylamino) tetralin (8-OH-DPAT) to reduce 5-HT levels in the hypothalamus, was not affected by corticosterone administration. Presynaptic 5-HT(1B) receptor activity, as measured by the effect of the 5-HT(1B) receptor antagonist (N-[4-methoxy-3-(4-methyl-1-piperizinyl)phenyl]-2'-methyl-4'-(5-methyl-1,2,4-oxadiazole-3-yl)[1,1'-biphenyl]-carboxamide (GR 127935) (5 mg/kg s.c.) to increase 5-HT levels, was increased in hypothalamus but not hippocampus of corticosterone-treated rats. Postsynaptic 5-HT(1A) receptor activity, as measured by the effect of 8-OH-DPAT to increase cyclic AMP levels in the hippocampus, was not affected by corticosterone administration. The decrease in presynaptic 5-HT(1B) receptor activity after chronic administration of antidepressant drugs complements the increases in 5-HT(1B) receptor number observed in animal models of depression.     

An evaluation of the effect of NAS-181, a new selective 5-HT<Subscript>1B</Subscript> receptor antagonist,on extracellular 5-HT levels in rat frontal cortex

In the mammalian brain 5-HT(1B) receptors are present as autoreceptors regulating the release of serotonin (5-HT) by inhibitory feedback. The antagonistic properties of NAS-181 ((R)-(+)-2-[[[3-(Morpholinomethyl)-2H-chromen-8-yl]oxy]methyl] morpholine methane sulfonate), a new selective antagonist for the rodent 5-HT(1B) receptor, were determined by using an agonist-induced decrease of extracellular 5-HT. The 5-HT(1B) receptor agonist CP93129 (0.030.3 microM) applied by reversed microdialysis, dose-dependently reduced 5-HT levels in rat frontal cortex. The suppressant effect of CP93129 (0.1 microM) was smaller in the presence of fluvoxamine (3-10 microM), a 5-HT reuptake inhibitor. The effects of NAS-181 on CP93129 were compared with GR127935, a mixed 5-HT (1B/1D) receptor antagonist, and SB224289, a 5-HT(1B) receptor antagonist. Both in the presence and absence of fluvoxamine, the suppressant effect of CP93129 on extracellular 5-HT was attenuated by NAS-181 (1 microM) and GR127935 (10 microM), but not by SB224289 (1 microM). In the absence of fluvoxamine, GR127935, SB224289 and NAS-181 all reduced 5-HT levels, suggesting partial agonistic properties of these compounds. In conclusion, the results show that NAS-181 is a potent 5-HT(1B) receptor antagonist.     

Increase of noradrenaline release in the hypothalamus of freely moving rat by postsynaptic 5-hydroxytryptamine1A receptor activation.

1. 5-Hydroxytryptamine (5-HT) plays a role in the regulation of noradrenergic neurones in the brain, but the precise mechanism of regulation of noradrenaline (NA) release by 5-HT1A receptors has not been defined. The present study describes the effect of a highly potent and selective 5-HT1A receptor agonist, 5-(3-[[(2S)-1,4-benzodioxan-2-ylmethyl)]amino]propoxy)-1,3-b enzodioxole HC1 (MKC-242), on NA release in the hypothalamus using microdialysis in the freely moving rat. 2. Subcutaneous injection of MKC-242 (0.5 mg kg-1) increased extracellular levels of NA and its metabolite, 3-methoxy-4-hydroxyphenylglycol, in the hypothalamus and hippocampus. 3. The 5-HT1A receptor agonists, 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) (0.2 mg kg-1) and buspirone (3 mg kg-1) mimicked the effect of MKC-242 in increasing NA release in the hypothalamus. 4. The effects of MKC-242 and 8-OH-DPAT in the hypothalamus were antagonized by pretreatment with WAY100135 (10 mg kg-1), a silent 5-HT1A receptor antagonist. 5. Local administration of 8-OH-DPAT (10-100 microM), citalopram (1 microM), a 5-HT reuptake inhibitor, and MDL72222 (10 microM), a 5-HT3 receptor antagonist, into the hypothalamus, had no effect on NA release. 6. Intracerebroventricular injection with 5,7-dihydroxytryptamine caused a marked reduction in brain 5-HT content, but the treatment affected neither basal NA levels nor the MKC-242-induced increase in NA release. 7. The effect of MKC-242 in increasing NA release was not attenuated by repeated treatment with the drug (0.5 mg kg-1, once a day for 2 weeks).(ABSTRACT TRUNCATED AT 250 WORDS)     

In vivo/ex vivo and behavioural study on central effects of 5-HT1B/1D and 5-HT1A antagonists in guinea pigs

Serotonin 5-HT1A and 5-HT1B/1D receptors control serotonin (5-HT) release and are targets for the pharmacological treatment of psychiatric disorders. We investigated effects of the 5-HT1B/1D antagonist GR127935, the 5-HT1A antagonist WAY 100635 and a combination of both in guinea pigs on the behaviour in the forced swimming test and on extracellular 5-HT in the hippocampus and the prefrontal cortex using in vivo microdialysis. Tissue content of 5-HT, 5-HIAA and 5-HT turnover (ratio 5-HIAA/5-HT) were determined in a sample containing i) the median and dorsal raphe nuclei, ii) the frontal cortex, or iii) the ventral hippocampus ex vivo. BEHAVIOUR: Administration of WAY 100635 (0.3-3.0 mg/kg, i.p.) or GR127935 (1.0-10.0 mg/kg, i.p.) or the combination of both delayed immobility in the forced swim test. MICRODIALYSIS: Systemic administration of WAY 100635 (1 mg/kg i.p.), perfusion with GR127935 (10 microM perfused into the frontal cortex) in the medial prefrontal cortex or the combination of both treatments had no significant effect on extracellular 5-HT. 5-HT TISSUE CONTENT AND 5-HT TURNOVER IN THE TISSUE: Compared to controls, WAY 100635, GR127935 and the combination thereof, decreased cortical 5-HT (-30%), increased 5-HIAA and consequently 5-HT turnover in the cortex threefold and the raphe nuclei twofold. WAY 100635 decreased 5-HT in the hippocampus (-40%), too. WAY 100635 and GR127935 and their combination increased hippocampal 5-HIAA and 5-HT turnover twofold, compared to controls. The results suggest that both 5-HT1 antagonists have subtle effects on 5-HT function under resting conditions; combined treatment has no superior effects compared to solitary treatment.     

Investigation into the 5-hydroxytryptamine receptor mediating smooth muscle relaxation in the rat oesophagus.

1. An investigation has been made into the 5-hydroxytryptamine (5-HT) receptor mediating relaxation of rat oesophagus in preparations precontracted with carbachol. 2. In tissues treated with pargyline (100 microM) and in the presence of corticosterone (30 microM) and cocaine (30 microM) the potency of 5-HT and 5-methoxytyramine (5-MeOT) was not changed but the maximum response to these agonists was reduced. Thus there was no evidence of metabolism and/or uptake through an amine depleting mechanism. 3. The relaxant concentration-effect curves to 5-HT were shifted to the left in a concentration-related manner by isobutylmethylxanthine (1 and 10 microM), suggesting the involvement of adenosine 3':5'-cyclic monophosphate in these responses. 4. 5-HT produced concentration-related relaxations of rat oesophagus with an EC50 value of 0.24 microM. Several indole agonists were tested and the following rank order of potency of key agonists obtained: 5-HT greater than alpha-methyl-5-hydroxytryptamine = 5-carboxamidotryptamine (5-CT) greater than 5-MeOT. In contrast, 2-methyl-5-hydroxytryptamine, sumatriptan and 8-hydroxy-2-(di-n-propylamino) tetralin were weak or inactive. 5. The substituted benzamides, metoclopramide, cisapride, renzapride and R,S-zacopride acted as partial agonists, producing 60-70% of the 5-HT maximum. 6. The relaxation responses to 5-HT were neither inhibited by antagonists selective for 5-HT1 or 5-HT2 receptors nor by the 5-HT3 receptor antagonists, ondansetron, granisetron or MDL 72222. 7. The relaxation responses induced by 5-HT, 5-CT, 5-MeOT and renzapride were selectively inhibited by high concentrations of ICS 205-930 with pKB values of approximately 6.(ABSTRACT TRUNCATED AT 250 WORDS)     

5-HT1 agonists reduce 5-hydroxytryptamine release in rat hippocampus in vivo as determined by brain microdialysis.

1. An intracerebral perfusion method, brain microdialysis, was used to assess changes of 5-hydroxytryptamine (5-HT) release in the ventral hippocampus of the chloral hydrate-anaesthetized rat in response to systemic administration of a variety of 5-HT1 receptor agonists. 2. A stable output of reliably detectable endogenous 5-HT was measured in dialysates collected from ventral hippocampus with the 5-HT reuptake inhibitor, citalopram, present in the perfusion medium. 3. Under these conditions the putative 5-HT1A agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) caused a dose-dependent (5-250 micrograms kg-1, s.c.) reduction of 5-HT in hippocampal dialysates. 4. Similarly, the putative 5-HT1A agonists gepirone (5 mg kg-1, s.c.), ipsapirone (5 mg kg-1, s.c.) and buspirone (5 mg kg-1, s.c.) markedly reduced levels of 5-HT in hippocampal perfusates whereas their common metabolite 1-(2-pyrimidinyl) piperazine (5 mg kg-1, s.c.), which does not bind to central 5-HT1A recognition sites, had no effect. 5. 5-Methoxy-3-(1,2,3,6-tetrahydro-4-pyridinyl)-1H-indole (RU 24969), a drug with reported high affinity for brain 5-HT1B binding sites, also produced a dose-dependent (0.25-5 mg kg-1, s.c.) decrease of hippocampal 5-HT output. 6. These data are direct biochemical evidence that systemically administered putative 5-HT1A and 5-HT1B agonists markedly inhibit 5-HT release in rat ventral hippocampus in vivo.