Effects of 5-HT1B receptor ligands microinjected into the accumbal shell or core on the cocaine-induced locomotor hyperactivity in rats.

The present study was designed to examine the effect of 5-HT1B receptor ligands microinjected into the subregions of the nucleus accumbens (the shell and the core) on the locomotor hyperactivity induced by cocaine in rats. Male Wistar rats were implanted bilaterally with cannulae into the accumbens shell or core, and then were locally injected with GR 55562 (an antagonist of 5-HT1B receptors) or CP 93129 (an agonist of 5-HT1B receptors). Given alone to any accumbal subregion, GR 55562 (0.1-10 microg/side) or CP 93129 (0.1-10 microg/side) did not change basal locomotor activity. Systemic cocaine (10 mg/kg) significantly increased the locomotor activity of rats. GR 55562 (0.1-10 microg/side), administered intra-accumbens shell prior to cocaine, dose-dependently attenuated the psychostimulant-induced locomotor hyperactivity. Such attenuation was not found in animals which had been injected with GR 55562 into the accumbens core. When injected into the accumbens shell (but not the core) before cocaine, CP 93129 (0.1-10 microg/side) enhanced the locomotor response to cocaine; the maximum effect being observed after 10 microg/side of the agonist. The later enhancement was attenuated after intra-accumbens shell treatment with GR 55562 (1 microg/side). Our findings indicate that cocaine induced hyperlocomotion is modified by 5-HT1B receptor ligands microinjected into the accumbens shell, but not core, this modification consisting in inhibitory and facilitatory effects of the 5-HT1B receptor antagonist (GR 55562) and agonist (CP 93129), respectively. In other words, the present results suggest that the accumbal shell 5-HT1B receptors play a permissive role in the behavioural response to the psychostimulant.     

5-HT1B serotonin receptors and antidepressant effects of selective serotonin reuptake inhibitors ]

We used knockout mice and receptor antagonist strategies to investigate the contribution of the serotonin (5-hydroxytryptamine, 5-HT) 5-HT1B receptor subtype in mediating the effects of selective serotonin reuptake inhibitors (SSRIs). Using in vivo intracerebral microdialysis in awake mice, we show that a single systemic administration of paroxetine (1 or 5 mg/kg, i.p.) increased extracellular serotonin levels [5-HT]ext in the ventral hippocampus and frontal cortex of wild-type and mutant mice. However, in the ventral hippocampus, paroxetine at the two doses studied induced a larger increase in [5-HT]ext in knockout than in wild-type mice. In the frontal cortex, the effect of paroxetine was larger in mutants than in wild-type mice at the 1 mg/kg dose but not at 5 mg/kg. In addition, either the absence of the 5-HT1B receptor or its blockade with the mixed 5-HT1B/1D receptor antagonist, GR 127935, potentiates the effect of a single administration of paroxetine on [5-HT]ext more in the ventral hippocampus than in the frontal cortex. Furthermore, we demonstrate that SSRIs decrease immobility in the forced swimming test; this effect is absent in 5-HT1B knockout mice and blocked by GR 127935 in wild-type suggesting therefore that activation of 5-HT1B receptors mediate the antidepressant-like effects of SSRIs. Taken together these data demonstrate that 5-HT1B autoreceptors appear to limit the effects of SSRI on dialysate 5-HT levels particularly in the hippocampus while presynaptic 5-HT1B heteroreceptors are likely to be required for the antidepressant activity of SSRIs.     

Combined Administration of a 5-Hydroxytryptamine (5-HT)1D Antagonist and a 5-HT Reuptake Inhibitor Synergistically Increases 5-HT Release in Guinea Pig Hypothalamus In Vivo

Abstract: In vivo microdialysis in guinea pig hypothalamus was used to study the effect of serotonin [5-hydroxytryptamine (5-HT)] subtype 1D autoreceptor blockade on the increase in extracellular 5-HT levels produced by a selective 5-HT reuptake inhibitor (SSRI). Administration of the selective 5-HT_1D antagonist GR127935 at 0.3 mg/kg had no effect, but 5 mg/kg significantly increased extracellular levels of 5-HT and 5-hydroxyindoleacetic acid to 135% of basal values. Moreover, at these doses GR127935 significantly attenuated the decrease in extracellular 5-HT levels following local perfusion with the selective 5-HT_1D agonist CP-135,807. The SSRI sertraline at 2 mg/kg increased 5-HT levels to 130% of basal levels. The combination of this low dose of sertraline with either dose of GR127935 resulted in a pronounced, long-lasting increase in 5-HT levels to 230% of basal values. These results indicate that the effects of an SSRI on terminal 5-HT are significantly enhanced by coadministration of a 5-HT_1D antagonist and confirm that in addition to somatodendritic 5-HT_1A autoreceptors, terminal 5-HT_1D autoreceptors mitigate the effect of SSRIs on terminal 5-HT. As such, antagonists of the 5-HT_1D autoreceptor could be useful as rapidly acting antidepressants and may shorten the onset of antidepressant action when combined with SSRIs.     

5-HT1B Autoreceptors limit the effects of selective serotonin re-uptake inhibitors in mouse hippocampus and frontal cortex

We used knockout mice and receptor antagonist strategies to investigate the contribution of the serotonin (5-hydroxytryptamine, 5-HT) 1B receptor subtype in mediating the effects of selective serotonin re-uptake inhibitors (SSRIs). Using in vivo intracerebral microdialysis in awake mice, we show that a single systemic administration of paroxetine (1 or 5 mg/kg, i.p.) increased extracellular serotonin levels [5-HT]ext in the ventral hippocampus and frontal cortex of wild-type and mutant mice. However, in the ventral hippocampus, paroxetine at the two doses studied induced a larger increase in [5-HT]ext in knockout than in wild-type mice. In the frontal cortex, the effect of paroxetine was larger in mutants than in wild-type mice at the 1 mg/kg, but not at 5 mg/kg. In addition, either the absence of the 5-HT1B receptor or its blockade with the mixed 5-HT1B/1D receptor antagonist, GR 127935, potentiated the effect of a single administration of paroxetine on extracellular 5-HT levels more in the ventral hippocampus than in the frontal cortex. These data suggest that 5-HT1B autoreceptors limit the effects of SSRIs on dialysate 5-HT levels at serotonergic nerve terminals.     

Blockade of substance P (neurokinin 1) receptors enhances extracellular serotonin when combined with a selective serotonin reuptake inhibitor: an in vivo microdialysis study in mice

Abstract Substance P antagonists of the neurokinin-1 receptor type (NK1) are gaining growing interest as new antidepressant therapies. It has been postulated that these drugs exert this putative therapeutic effect without direct interactions with serotonin (5-HT) neurones. Our recent microdialysis experiment performed in NK1 receptor knockout mice suggested evidence of changes in 5-HT neuronal function (Froger et al. 2001). The aim of the present study was to evaluate the effects of coadministration of the selective 5-HT reuptake inhibitor (SSRI) paroxetine with a NK1 receptor antagonist (GR205171 or L733060), given either intraperitoneally (i.p.) or locally into the dorsal raphe nucleus, on extracellular levels of 5-HT ([5-HT]ext) in the frontal cortex and the dorsal raphe nucleus using in vivo microdialysis in awake, freely moving mice. The systemic or intraraphe administration of a NK1 receptor antagonist did not change basal cortical [5-HT]ext in mice. A single systemic dose of paroxetine (4 mg/kg; i.p.) resulted in a statistically significant increase in [5-HT]ext with a larger extent in the dorsal raphe nucleus (+ 138% over basal AUC values), than in the frontal cortex (+ 52% over basal AUC values). Co-administration of paroxetine (4 mg/kg; i.p.) with the NK1 receptor antagonists, GR205171 (30 mg/kg; i.p.) or L733060 (40 mg/kg; i.p.), potentiated the effects of paroxetine on cortical [5-HT]ext in wild-type mice, whereas GR205171 (30 mg/kg; i.p.) had no effect on paroxetine-induced increase in cortical [5-HT]ext in NK1 receptor knock-out mice. When GR205171 (300 micro mol/L) was perfused by 'reverse microdialysis' into the dorsal raphe nucleus, it potentiated the effects of paroxetine on cortical [5-HT]ext, and inhibited paroxetine-induced increase in [5-HT]ext in the dorsal raphe nucleus. Finally, in mice whose 5-HT transporters were first blocked by a local perfusion of 1 micro mol/L of citalopram into the frontal cortex, a single dose of paroxetine (4 mg/kg i.p.) decreased cortical 5-HT release, and GR205171 (30 mg/kg i.p.) reversed this effect. The present findings suggest that NK1 receptor antagonists, when combined with a SSRI, augment 5-HT release by modulating substance P/5-HT interactions in the dorsal raphe nucleus.     

Electrophysiological assessment of putative antagonists of 5-hydroxytryptamine receptors: a single-cell study in the rat dorsal raphe nucleus

The intravenous administration of low doses of lysergic acid diethylamide (LSD) or of the selective 5-hydroxytryptamine1A (5-HT1A) receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) depresses the firing activity of dorsal raphe 5-HT-containing neurons, presumably via the activation of 5-HT1A receptors. The present studies were undertaken to determine the effect of different types of 5-HT receptor antagonists on this effect of LSD and 8-OH-DPAT. (-)-Propranolol (2 mg/kg i.v.), methiothepin (2 mg/kg i.p., twice daily for 4 days followed by an additional dose of 2 mg/kg i.p., prior to the experiment), pelanserine (0.5 mg/kg i.v.), and indorenate (125 micrograms/kg i.v.) failed to block the effects of either LSD or 8-OH-DPAT on the firing activity of 5-HT neurons of the dorsal raphe nucleus. However, spiperone (1 mg/kg i.v.) significantly reduced the effect of both LSD and 8-OH-DPAT. These results indicate that, among the five putative 5-HT receptor antagonists tested, only spiperone can antagonize the suppressant effect of 5-HT receptor agonists on the firing of dorsal raphe 5-HT neurons.     

Role of 5-HT1B/D receptors in canine gastric accommodation: effect of sumatriptan and 5-HT1B/D receptor antagonists

The 5-HT1B/D receptor agonist sumatriptan has been proposed to treat dyspeptic symptoms, because it facilitates gastric accommodation. It is unknown whether stimulation of 5-HT1B/D receptors is involved. Thus, in four conscious dogs, we compared the effects of sumatriptan alone or combined with N-[4-methoxy-3-(4-methyl-1-piperazinyl) phenyl]-2'-methyl-4'-(5-methyl-1,2,4-oxadiazol-3-yl)-[1,1-biphenyl]-4-carboxamide hydrocloride (GR-127935), N-[3-[3 (dimethylamino)-ethoxy]-4-methoxyphenyl]-2'-[methyl-4'-(5-methyl-1,2,4-oxadiazol-3-yl)]-[1,1-biphenyl]-4-carboxamide hydrocloride (SB-216641 hydrochloride), or 3-[4-(4-chloro-phenyl)piperazin-1-yl]-1,1-diphenyl-2-propanol hydrochloride (BRL-15572 hydrochloride) (respectively, nonselective 5-HT1B/D, selective 5-HT1B, and selective 5-HT1D receptor antagonists) on gastric accommodation to isobaric distensions performed with a barostat. An exponential and a linear model were used to fit the pressure-volume relationship. An exponential equation fitted the data better than a linear equation. Sumatriptan (800 nmol/kg iv) induced an immediate gastric relaxation (Deltavolume: 112 +/- 44 ml, P < 0.05). After sumatriptan, the pressure-volume curve was shifted toward significantly higher volumes. This effect was fully reversed by GR-127935 or SB-216641 but not by BRL-15572. In conclusion, 5-HT1B receptors seem to play an important role in modulating gastric accommodation to a distending stimulus. An exponential model for pressure-volume curves fits well with the concept of gastric adaptive relaxation.     

Potentiation of the Fluoxetine-Induced Increase in Dialysate Levels of Serotonin (5-HT) in the Frontal Cortex of Freely Moving Rats by Combined Blockade of 5-HT1A and 5-HT1B Receptors with WAY 100,635 and GR 127,935

Abstract: In this study, we examined the influence of blockade of serotonin (5-HT)_1A and/or 5-HT_1B autoreceptors on the fluoxetine-induced increase in dialysate levels of 5-HT as compared with dopamine (DA) and noradrenaline (NAD) in single samples of the frontal cortex (FCx) of freely moving rats. Fluoxetine (10.0 mg/kg, s.c.) elicited a twofold increase in dialysate levels of 5-HT relative to baseline values. The selective 5-HT_1A antagonist WAY 100,635 (0.16 mg/kg, s.c.) did not influence 5-HT release alone but doubled the influence of fluoxetine on basal levels. Similarly, the selective 5-HT_1B/1D antagonist GR 127,935 (2.5 mg/kg, s.c.) did not alter basal 5-HT levels alone and doubled the fluoxetine-induced increase in 5-HT levels. Combined administration of WAY 100,635 and GR 127,935 elicited an (at least) additive rise in the fluoxetine-induced increase in 5-HT levels to eightfold basal values, without modifying resting 5-HT levels. These changes were selective for 5-HT inasmuch as the parallel (twofold) increase in DA and NAD levels provoked by fluoxetine was not potentiated. The present data demonstrate that combined blockade of 5-HT_1A and 5-HT_1B autoreceptors markedly and selectively potentiates the fluoxetine-induced increase in dialysate levels of 5-HT versus DA and NAD in the FCx of freely moving rats. These observations suggest that 5-HT_1A/1B antagonism may represent a novel strategy for the improvement in the therapeutic profile of 5-HT reuptake inhibitor antidepressant agents and that 5-HT may be primarily involved in such interactions.     

Effects of chronic paroxetine treatment on dialysate serotonin in 5-HT1B receptor knockout mice

The role of serotonin (5-HT)1B receptors in the mechanism of action of selective serotonin re-uptake inhibitors (SSRI) was studied by using intracerebral in vivo microdialysis in conscious, freely moving wild-type and 5-HT1B receptor knockout (KO 5-HT1B) mice in order to compare the effects of chronic administration of paroxetine via osmotic minipumps (1 mg per kg per day for 14 days) on extracellular 5-HT levels ([5-HT]ext) in the medial prefrontal cortex and ventral hippocampus. Basal [5-HT]ext values in the medial prefrontal cortex and ventral hippocampus, approximately 20 h after removing the minipump, were not altered by chronic paroxetine treatment in both genotypes. On day 15, in the ventral hippocampus, an acute paroxetine challenge (1 mg/kg i.p.) induced a larger increase in [5-HT]ext in saline-pretreated mutant than in wild-type mice. This difference between the two genotypes in the effect of the paroxetine challenge persisted following chronic paroxetine treatment. Conversely, in the medial prefrontal cortex, the paroxetine challenge increased [5-HT]ext similarly in saline-pretreated mice of both genotypes. Such a challenge produced a further increase in cortical [5-HT]ext compared with that in saline-pretreated groups of both genotypes, but no differences were found between genotypes following chronic treatment. To avoid the interaction with raphe 5-HT1A autoreceptors, 1 micro m paroxetine was perfused locally through the dialysis probe implanted in the ventral hippocampus; similar increases in hippocampal [5-HT]ext were found in acutely or chronically treated wild-type mice. Systemic administration of the mixed 5-HT1B/1D receptor antagonist GR 127935 (4 mg/kg) in chronically treated wild-type mice potentiated the effect of a paroxetine challenge dose on [5-HT]ext in the ventral hippocampus, whereas systemic administration of the selective 5-HT1A receptor antagonist WAY 100635 did not. By using the zero net flux method of quantitative microdialysis in the medial prefrontal cortex and ventral hippocampus of wild-type and KO 5-HT1B mice, we found that basal [5-HT]ext and the extraction fraction of 5-HT were similar in the medial prefrontal cortex and ventral hippocampus of both genotypes, suggesting that no compensatory response to the constitutive deletion of the 5-HT1B receptor involving changes in 5-HT uptake capacity occurred in vivo. As steady-state brain concentrations of paroxetine at day 14 were similar in both genotypes, it is unlikely that differences in the effects of a paroxetine challenge on hippocampal [5-HT]ext are due to alterations of the drug's pharmacokinetic properties in mutants. These data suggest that there are differences between the ventral hippocampus and medial prefrontal cortex in activation of terminal 5-HT1B autoreceptors and their role in regulating dialysate 5-HT levels. These presynaptic receptors retain their capacity to limit 5-HT release mainly in the ventral hippocampus following chronic paroxetine treatment in mice.     

Regionally and functionally distinct serotonin3 receptors control in vivo dopamine outflow in the rat nucleus accumbens

Central serotonin(3) (5-HT(3)) receptors control the mesoaccumbens dopamine (DA) pathway. This control is thought to be conditional and might involve regionally distinct subpopulations of 5-HT(3) receptors. Here, using in vivo microdialysis in rats, we assessed the relative contribution of nucleus accumbens (Nacc) 5-HT(3) receptors to the overall influence exerted by 5-HT(3) receptors on accumbal DA release induced by different drugs or treatments. In freely moving rats, pre-treatment with 5-HT(3) antagonists (0.1 mg/kg ondansetron and/or 0.03 mg/kg MDL 72222, s.c.) reduced DA efflux enhanced by morphine (1-10 mg/kg, s.c.) and haloperidol (0.01 mg/kg, s.c.), but not amphetamine (1-2.5 mg/kg, i.p.) or cocaine (10-20 mg/kg, i.p.), the latter two drugs do not trigger depolarization-stimulated DA exocytosis. Intra-Nacc administration of ondansetron (1 microm) in freely moving rats reduced the DA effects elicited by 10 mg/kg morphine, but not 1 mg/kg morphine or haloperidol. The 5-HT(1A) agonist 8-OH-DPAT (0.1 mg/kg, s.c.), known to decrease central 5-HT tone, reduced 10 but not 1 mg/kg morphine-stimulated DA outflow in freely moving rats. In halothane-anaesthetized rats, intra-Nacc ondansetron (1 microm) application reduced dorsal raphe nucleus electrical stimulation (20Hz)-induced DA outflow. Our results show that regionally distinct populations of 5-HT(3) receptors control the depolarization-dependent exocytosis of DA and suggest that the involvement of Nacc 5-HT(3) receptors occurs only when central DA and 5-HT tones are concomitantly increased.     

Possible involvement of 5-HT and 5-HT2 receptors in acceleration of gastrointestinal transit by escin Ib in mice

We have reported previously that escin Ib accelerated gastrointestinal transit (GIT) in mice, and that its effect may be mediated by the release of endogenous prostaglandins (PGs) and nitric oxide (NO). In this study, the possible involvement of 5-HT and 5-HT receptors in the GIT acceleration of escin Ib was investigated in mice. The acceleration of GIT by escin Ib (25 or 50 mg/kg, p.o.) was attenuated by pretreatment with ritanserin (0.5-5 mg/kg, s.c., a 5-HT(2A/2C/2B) receptor antagonist), but not with MDL 72222 (1 and 5 mg/kg, s.c.) and metoclopramide (10 mg/kg, s.c.) (5-HT3 receptor antagonists) or tropisetron (1 and 10 mg/kg, s.c., a 5-HT(3/4) receptor antagonist). Furthermore, pretreatment with ketanserin (0.05-5 mg/kg, s.c.), haloperidol (1-5 mg/kg, s.c.) and spiperone (0.5-5 mg/kg, s.c.) (5-HT2A receptor antagonists), as well as a bolus of dl-p-chlorophenylalanine methyl ester (PCPA, 1000 mg/kg, p.o., 1, 6 or 24 h before administration of the sample) (an inhibitor of 5-HT synthesizing enzyme tryptophan hydroxylase) and reserpine (5 mg/kg, p.o.) (a 5-HT depletor), but not 6-hydroxydopamine (80 mg/kg, i.p., a dopamine depletor) or repeated PCPA (300 mg/kg x2, p.o., 72 and 48 h before administration of the sample), also attenuated the effects of escin Ib. It is postulated that escin Ib accelerates GIT, at least in part, by stimulating the synthesis of 5-HT to act through 5-HT2, possibly 5-HT2A receptors, which in turn causes the release of NO and PGs.     

Precursor utilization of 5-hydroxytryptophan for 5-hydroxytryptamine biosynthesis in isolated and perfused rabbit and rat lungs

The present study was designed to investigate whether lungs can utilize 5-hydroxytryptophan (5-HTP), formed elsewhere and transported, for the synthesis of 5-hydroxytryptamine (5-HT). [14C]5-HTP uptake was 7.7 +/- 1.1 and 3.9 +/- 0.2% by rabbit and rat lungs, respectively, after 1 h of perfusion with 10 microM [14C]5-HTP. There was an increase in the lung uptake of [14C]5-HTP when the lungs were preperfused with 0.5 mM chlorphentermine (CP) and the uptake was low when the lungs were preperfused with 0.1 mM hydroxybenzylhydrazine dihydrochloride (HBH). The perfusate concentration of 5-hydroxyindole acetic acid (5-HIAA) increased significantly (3-4 micrograms/100 mL) during rabbit lung perfusion with 10 microM [14C]5-HTP and this did not change significantly when the lungs were preperfused with 0.5 mM CP. However, 5-HT increased with time in the perfusate. 5-HT, but not 5-HIAA, was detected in the perfusate and increased with time of perfusion when the rat lungs were perfused either with 10 microM 5-HTP or with 0.5 mM CP and 10 microM 5-HTP. However, no metabolites were detected in either the rabbit lung or rat lung perfusates when they were preperfused with 0.1 mM HBH. Lung contents of 5-HT and 5-HIAA were significantly higher in the rat lungs and only 5-HIAA increased in rabbit lungs after 1 h of perfusion with 10 microM 5-HTP. Preperfusion with 0.5 mM CP resulted in a greater increase in the 5-HT content of both rabbit and rat lungs.(ABSTRACT TRUNCATED AT 250 WORDS)     

Selective blockade of neurokinin (NK)(1) receptors facilitates the activity of adrenergic pathways projecting to frontal cortex and dorsal hippocampus in rats

The selective NK(1) receptor antagonist, GR205,171 (2.5-40.0 mg/kg, i.p.), dose-dependently elevated dialysate levels of noradrenaline (NA), but not serotonin (5-HT), in the frontal cortex of freely moving rats. This action was exerted stereospecifically inasmuch as its less active isomer, GR226,206, was ineffective. In the dorsal hippocampus, GR205,171 (but not GR226,206) also significantly increased dialysate levels of NA, whereas levels of 5-HT were unaffected. Further, in anaesthetized rats, GR205,171 dose-dependently (1.0-4.0 mg/kg, i.v.) increased the firing rate of adrenergic perikarya in the locus coeruleus. In contrast, their activity was not modified by GR226,206. These findings indicate that selective blockade of NK(1) receptors enhances the activity of ascending adrenergic pathways in rats. Adrenergic mechanisms may, thus, be involved in the potential antidepressant and other functional properties of NK(1) receptor antagonists.     

Synthesis and 5-hydroxytryptamine (5-HT) activity of 2,3,4,4a-tetrahydro-1H-pyrazino[1,2-a]quinoxalin-5-(6H)ones and 2,3,4,4a,5,6-hexahydro-1H-pyrazino[1,2-a]quinoxalines

A series of 2,3,4,4a-tetrahydro-1H-pyrazino[1,2-a]quinoxalin-5-(6H)ones and 2,3,4,4a,5,6-hexahydro-1H-pyrazino[1,2-a]quinoxalines was shown to exhibit 5-HT2C agonist binding and functional activity. Compound 21R inhibited food intake over 2 h in fasted, male Sprague Dawley rats with ED50 values of 2 mg/kg (i.p.) and 10 mg/kg (p.o.).     

The role of 5-hydroxytryptamine3 and 5-hydroxytryptamine4 receptors in the regulation of gut motility in the ferret

The role of 5-hydroxytryptamine (5-HT)3 and 5-HT4 receptors in the regulation of gut motility in the ferret was investigated. The selective 5-HT3 receptor antagonist ramosetron (1 - 10 microg/kg s.c.) prolonged the interval of gastric antral migrating motor complex, but had only slight effect on small intestinal and colonic motility in unfed animals. The selective 5-HT4 receptor antagonist SB 204070 did not affect motility throughout gut in unfed animals. Neither ramosetron nor SB 204070 affected the motility throughout gut in fed animals. In conclusion, neither 5-HT3 nor 5-HT4 receptors tonically regulate ferret gut motility except that 5-HT3 receptors have a key role in the occurrence of migrating motor complex specifically in the stomach. The role of 5-HT3 and 5-HT4 receptor system in the regulation of gut motility in ferrets is similar to that in other mammalian species studied, including humans. This similarity suggests that the ferret is a suitable model animal to study gut motor functions in humans.     

Complex role of 5-HT in the regulation of TSH secretion in the male rat

The role of 5-hydroxytryptamine (5-HT) in the regulations of TSH secretion was studied in male rats using both peripheral and central administration of the drugs. Basal TSH levels were not modified by moderate doses of 5-HT (subcutaneously) or its precursors or antagonists (intraperitoneally) given 1 h before decapitation. The cold-stimulated TSH secretion was decreased by L-tryptophan (L-TRP, 400 mg/kg i.p.), quipazine (10 mg/kg i.p.) and 5-HT (1 or 5 mg/kg s.c. or i.v.) as well as by p-chlorophenylalanine (pCPA, 20 or more mg/kg i.p.) when the drugs were given 1 h before sampling. pCPA (100-400 mg/kg i.p.) was active 24-48 h after the injection but repetitive administration did not affect TSH levels. 5-HT (5 mg/kg s.c.) was effective also in pinealectomized animals. L-TRP and 5-hydroxytryptophan potentiated the TRH-stimulated TSH secretion when given 1 h before killing. 5-HT (10 microgram/rat) infused into the third ventricle enhanced the cold-stimulated TSH secretion when given 30-45 min before sampling. When injected into the medial basal hypothalamus, 50-HT (1-10 microgram/rat) had no effect on basal or stimulated TSH levels. The results suggest: (1) 5-HT does not play any role in the regulation of basal TSH secretion; (2) in the cold-stimulated TSH secretion 5-HT has a stimulatory action evidently inside the blood-brain barrier and also an inhibitory effect obviously outside this barrier.     

Sustained pharmacological blockade of NK1 substance P receptors causes functional desensitization of dorsal raphe 5-HT 1A autoreceptors in mice

Antagonists at NK1 substance P receptors have demonstrated similar antidepressant properties in both animal paradigms and in human as selective serotonin reuptake inhibitors (SSRIs) that induce desensitization of 5-HT 1A autoreceptors within the dorsal raphe nucleus (DRN). We investigated whether this receptor adaptation also occurs upon NK1 receptor blockade. C57B/L6J mice were treated for 21 days with the selective NK1 receptor antagonist GR 205171 (10 mg/kg daily) through subcutaneously implanted osmotic mini pumps, and DRN 5-HT 1A autoreceptor functioning was assessed using various approaches. Recording of DRN serotonergic neurons in brainstem slices showed that GR 205171 treatment reduced (by approximately 1.5 fold) the potency of the 5-HT 1A receptor agonist, ipsapirone, to inhibit cell firing. In parallel, the 5-HT 1A autoreceptor-mediated [35S]GTP-gamma-S binding induced by 5-carboxamidotryptamine onto the DRN in brainstem sections was significantly decreased in GR 205171-treated mice. In vivo microdialysis showed that the cortical 5-HT overflow caused by acute injection of the SSRI paroxetine (1 mg/kg) was twice as high in GR 205171-treated as in vehicle-treated controls. In the DRN, basal 5-HT outflow was significantly enhanced by GR 205171 treatment. These data supported the hypothesis that chronic NK1 receptor blockade induces a functional desensitization of 5-HT 1A autoreceptors similar to that observed with SSRIs.     

In vivo efflux of serotonin in the dorsal raphe nucleus of 5-HT1A receptor knockout mice

In the dorsal raphe nucleus (DR), extracellular serotonin (5-HT) regulates serotonergic transmission through 5-HT1A autoreceptors. In this work we used in vivo microdialysis to examine the effects of stressful and pharmacological challenges on DR 5-HT efflux in 5-HT1A receptor knockout (5-HT1A-/-) mice and their wild-type counterparts (5-HT1A+/+). Baseline 5-HT concentrations did not differ between both lines of mice, which is consistent with a lack of tonic control of 5-HT1A autoreceptors on DR 5-HT release. (R)-(+)-8-Hydroxy-2-(di-n-propylamino)tetralin hydrobromide (8-OH-DPAT, 0.5 mg/kg) reduced 5-HT levels to 30% of basal values in 5-HT1A+/+ mice, but not in 5-HT1A-/- mice. The selective 5-HT1B receptor agonist 1,4-dihydro-3-(1,2,3,6-tetrahydro-4-pyridinyl)-5H-pyrrolo[3,2-b]pyridin-5-one dihydrochloride (CP 93129, 300 micro m) reduced dialysate 5-HT to the same extent (30-40% of baseline) in the two genotypes, which suggests a lack of compensatory changes in 5-HT1B receptors in the DR of such mutant mice. Both a saline injection and handling for 3 min increased DR dialysate 5-HT in mutants, but not in 5-HT1A+/+ mice. Fluoxetine (5 and 20 mg/kg) elevated 5-HT in a dose-dependent manner in both genotypes. However, this effect was markedly more pronounced in the 5-HT1A-/- mice. The increased responsiveness of the extracellular 5-HT in the DR of 5-HT1A receptor knockout mice reflects a lack of the autoinhibitory control exerted by 5-HT1A autoreceptors.     

Effects of novel 5-HT1A receptor antagonists on measures of postsynaptic 5-HT1A receptor activation in vivo

The effects of two putative 5-HT_1A antagonists, 4-(2′-methoxyphenyl)-1-[2′-[N-(2″-pyridinyl)-p-iodobenzamido]ethyl]piperazine (p-MPPI) and 4-(2′-methoxyphenyl)-1-[2′-[N-(2″-pyridinyl)-p-flourobenzamido]ethyl]piperazine (p-MPPF), were examined in vivo in two tests of postsynaptic 5-HT_1A receptor activation, hypothermia and reciprocal forepaw treading, in the rat. Both p-MPPI (10 mg/Kg, I.p.) and p-MPPF (10 mg/Kg, I.p.) antagonized the hypothermia induced by the 5-HT_1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) (0.5 mg/Kg, S.c.). Neither p-MPPI nor p-MPPF administered alone at a dose of 10 mg/kg (i.p.) induced hypothermia. Similarly, both p-MPPI (10 mg/Kg, I.p.) and p-MPPF (2.5 mg/Kg, I.p.) completely antagonized 8-OH-DPAT (2 mg/Kg, S.c.)-induced forepaw treading in rats pretreated with reserpine (1 mg/Kg, S.c., 18–24 hours prior to the experiment). p-MPPI and p-MPPF, at doses of 10 mg/kg (i.p.) did not induce forepaw treading in reserpine pretreated animals. The results of the present study demonstrate that p-MPPI and p-MPPF act as 5-HT_1A receptor antagonists in these measures of postsynaptic 5-HT_1A a receptor activation.     

5-Hydroxytryptamine affects rat migrating myoelectric complexes through different receptor subtypes: evidence from 5-hydroxytryptophan administration

The effect of the serotonin precursor 5-hydroxytryptophan (5-HTP) on jejunal migrating myoelectric complexes (MMCs) was investigated in conscious rats. Subcutaneous administration of low doses of 5-HTP (1-2 mg/kg) shortened the period between migrating complexes, whereas high doses of the compound (4-8 mg/kg) disrupted the MMC pattern. The serotonin (5-HT2) antagonist methysergide (8 mg/kg s.c.) did not alter basal MMC, neither did it prevent the effect of a low dose of 5-HTP; conversely, it antagonized the disruption due to the high dose. The 5-HT3 antagonist ICS 205-930 (30 micrograms/kg s.c.) decreased MMC frequency; administration of 2 mg/kg 5-HTP following ICS 205-930 brought the frequency of myoelectric complexes back to basal values. Both effects of 5-HTP were prevented by the decarboxylase inhibitor benserazide (85 mg/kg i.p.), which per se caused a transient inhibition of spiking activity. The results suggest that rat MMCs can be influenced in a composite fashion by progressively increasing concentrations of 5-HT, which in turn activate different receptor subtypes. A peripheral neuronal receptor, probably belonging to the 5-HT3 subclass, mediates the increase in MMC frequency observed after low doses of 5-HTP; higher levels of serotonin activate 5-HT2 receptors, causing disruption of cycling activity. Additionally, 5-HT3 receptors, but not 5-HT2, appear to be relevant for the regulation of the MMC pattern by the endogenous amine.