5-HT receptor subtypes involved in the stimulatory effect of 5-HT on the peristaltic reflex in vitro

The effect of serotonin (5-HT) and of more selective 5-HT agonists on the peristaltic reflex evoked in the isolated guinea-pig ileum was investigated. Using the Trendelenburg technique, peristaltic contractions were elicited by increasing intraluminal pressure, and rhythmic contractions of the longitudinal and circular muscle were measured after serosal administration of the drugs. 5-HT potently stimulated contractions of the longitudinal muscle. The effect of 5-HT was partly antagonized by the 5-HT₄ receptor antagonist SDZ 205–557. Of the potent 5-HT_1A receptor agonists, 8-OH-DPAT, 5-carboxamidotryptamine (5-CT) and dipropyl-5-CT (DP-5-CT), only 5-CT caused a substantial stimulation. Of the 5-HT_1C-/5-HT₂ receptor agonists DOI and 5-methoxytryptamine (5-MeOT), DOI was inactive, whereas 5-MeOT potently stimulated contractions. 5-HT_1D receptor agonists (5-CT₁ sumatriptan) had a stimulatory effect. The effect of sumatriptan was antagonized by the 5-HT_1D receptor antagonist metitepine but not by the 5-HT₄ receptor antagonist SDZ 205–557. The 5-HT₃ receptor agonist 2-methyl-5-HT and the antagonists ICS 205–930 and granisetron did not influence the peristaltic reflex. 6-OH-indalpine, a 5-HT_1P agonist, was inactive. This data suggest that 5-HT stimulates the peristaltic reflex in the isolated guinea-pig ileum by activation of 5-HT₄- and 5-HT_1D receptors; other 5-HT receptor subtypes appear not to play a significant role in the modulation of this reflex.     

Influence of serotoninergic drugs on in vivo dopamine extracellular output in rat striatum

In vivo microdialysis was used to investigate the mechanism behind the increase in extracellular dopamine (DA) induced by increase in extracellular serotonin (5-HT) level and 5-HT₁ and 5-HT₂ receptor activation. The following serotoninergic drugs were perfused in the absence or presence of nomifensine (5 μM)or tetrodotoxin (TTX; 2 μM): clomipramine (10, 500 and 1,000 μM), a selective 5-HT reuptake inhibitor; 8-OH-DPAT (50 and 500 μM), a 5-HT_1A receptor agonist; and α-methyl-5-HT (1, 5 and 50 μM), a 5-HT₂ receptor agonist. All the serotoninergic drugs studied increased DA extracellular output in a dose-dependent manner. The presence of nomifensine attenuated the effect of perfusion of clomipramine (500 μM) and completely abolished the effect of perfusion of 8-OH-DPAT (500 μM) and α-methyl-5-HT (5 μM) on DA extracellular output. Clomipramine (100–1,000 μM) perfusion produced a dose dependent increase in DOPAC extracellular output, which was stronger when clomipramine (500 μM) was co-perfused with nomifensine. 8-OH-DPAT and α-methyl-5-HT perfusion decreased DOPAC overflow. Addition of TTX to the perfusion fluid one hour before serotoninergic drugs perfusion, did not completely abolish the effect on dopamine extracellular output produced by the serotoninergic drugs. These data seem to indicate that increase in extracellular 5-HT level and 5-HT₁ and 5-HT₂ receptor activation increase in vivo DA extracellular output in the striatum mainly by a non-exocytotic mechanism involving DA uptake sites and, secondarily, by activation of 5-HT receptors. J. Neurosci. Res. 52:591–598, 1998. © 1998 Wiley-Liss, Inc.     

Heterogenous effects of serotonin in the dorsal horn of rat: the involvement of 5-HT1 receptor subtypes

The effect of ionophoretically applied serotonin (5-HT) was tested on cutaneous sensory responses of multireceptive dorsal horn neurones in the anaesthetized rat. Three types of 5-HT action were discerned: selective inhibition of nociceptive responses (10/18 cells), non-selective inhibition of responses to both noxious and innocous stimuli as well as to excitatory amino acids (4/18 cells) and non-selective excitation of evoked responses (1/18 cells). A few cells (3/18) were unaffected by 5-HT. The use of agonists, shown to discriminate between subtypes of 5-HT₁ receptor revealed that a 5-HT_1A receptor agonist mimicked the non-selective effects of 5-HT, whereas a 5-HT_1B receptor agonist mimicked the selective antinociptive effects of 5-HT. A 5-HT₂ receptor agonist, in contrast, was without effect. Both the selective and the non-selective effects were reversed by a 5-HT₁ receptor antagonist, but not a 5-HT₂ antagonist.     

Detection of the 5-HT1A receptor and 5-HT1A receptor mRNA in the rat bowel and pancreas: Comparison with 5-HT1Preceptors

We tested the hypothesis that the rat bowel and pancreas contain 5-HT_1A receptors. ₃H-8-hydroxy-2-(di-n-propylamino) tetralin (₃H-8-OH-DPAT) was used as a radioligand. Binding of ₃H-8-OH-DPAT to membranes derived from the myenteric plexus and the pancreas was investigated by rapid filtration. Alternatively, radioautography was employed to locate ₃H-8-OH-DPAT binding sites in frozen sections of unfixed bowel or pancreas. An excess of 5-HT (10 μM) was used to define nonspecific binding. Saturable, high affinity binding of ₃H-8-OH-DPAT to enteric (K_d= 2.8 ± 1.1 nM; B_max =83.8 ± 4.3 fmol/mgproteim) and pancreatic (K_d = 6.6 ± 1.3 nM; B_max = 44 ± 2.2 fmol/mg protein) membranes was found. The binding of ₃H-8-OH-DPAT to enteric and pancreatic membranes was inhibited by 8-OH-DPAT, NAN-190, and spiperone. In contrast, the binding of ₃H-8-OH-DPAT to enteric and pancreatic membranes was not inhibited by 5-carboxyamidotryptamine, or by avariety of compounds known to bind to other subtypes of 5-HT receptor. Digoxigenin-labeled oligonucleotides were found to detect mRNA encoding the 5-HT_1A receptor in a subset of neurons in myenteric and submucosal ganglia. In contrast, 5-HT_1A mRNA was not found in the pancreas. Radioautography revealed that the highest density of ₃H-8-OH-DPAT binding sites was found in the stomach. These sites were especially numerous in the lamina propria adjacent to gastric glands, and in myenteric ganglia. Pancreatic 5-HT_1Areceptors were located on nerves, lymphoid tissue (especially the capsule of nodes), and on cells scattered in the pancreatic parenchyma. The concentration of ₃H-8-OH-DPAT binding sites in the rat bowel and pancreas was less than that of ₃H-5-HT binding sites; however, the distribution of ₃H-8-OH-DPAT binding sites was similar to that of sites that bind ₃H-5-HT. It is concluded that the rat gut and its extension in the pancreas contains 5-HT_1A receptors. Many, if not all, of the nerve cells and processes that express 5-HT_1A receptors express 5-HT_1p receptors as well. The function of these receptors in the physiology of the entero-pancreatic innervation remains to be determined. © 1993 Wiley-Liss, Inc.     

Pre- and post-synaptic effects of the 5-HT3 agonist 2-Methyl-5-HT on the 5-HT system in the rat brain

Microiontophoretic applications of 5-HT and of the 5-HT₃ agonist 2-methyl-5-HT produced a current-dependent suppression of firing activity of both hippocampal (CA₁ and CA₃) and cortical neurons in anesthetized rats. Concomitant microiontophoretic applications of the 5-HT₃ antagonists BRL 46470A and S-zacopride, as well as their intravenous injection, did not antagonize the inhibitory effect of 5-HT and 2-methyl-5-HT. In contrast, the 5-HT_1A antagonist BMY 7378, applied by microiontophoresis or administered intravenously, significantly reduced the inhibitory action of 5-HT and 2-methyl-5-HT. The firing activity of dorsal raphe 5-HT neurons was also reduced by 5-HT, 2-methyl-5-HT and the 5-HT_1A agonist 8-OH-DPAT applied by microiontophoresis. While BRL 46470A (0.1 and 1 mg/kg, i.v.) did not antagonize the inhibitory effect of the three 5-HT agonists on 5-HT neuronal firing activity, only that of 8-OH-DPAT was attenuated by the 5-HT_1A antagonist (+) WAY 100135. R-zacopride significantly reduced the duration of suppression of firing activity of CA₃ pyramidal neurons induced by the electrical stimulation of the ascending 5-HT pathway, and this reducing effect was prevented by the three 5-HT₃/5-HT₄ antagonists renzapride, S-zacopride and tropisetron, but not by BRL 46470A. Finally, in in vitro superfusion experiments, both BRL 46470A and S-zacopride antagonized the enhancing action of 2-methyld-HT on the electrically-evoked release of [³H]-5-HT in both rat frontal cortex and hippocampus slices. These findings suggest that, in vivo, the suppressant effect of 2-methyl-5-HT on the firing activity of dorsal hippocampus pyramidal, somatosensory cortical, and dorsal raphe 5-HT neurons is not mediated by 5-HT₃ receptors, but rather by 5-HT_1A receptors. The attenuating effect of R-zacopride on the effectiveness of the stimulation of the ascending 5-HT pathway is not mediated by 5-HT₃ receptors. In contrast, in vitro, the enhancing action of 2-methyl-5-HT on the electrically-evoked release of [³H]5-HT in both frontal cortex and hippocampus slices is mediated by 5-HT₃ receptors. © 1995 Wiley-Liss, Inc.     

Evidence for 5-HT autoreceptor-mediated,nerve impulse-independent,control of 5-HT synthesis in the rat brain

To gain further insight into the operation of 5-HT autoreceptor-mediated feedback control of 5-HT biosynthesis in serotonergic nerve terminal areas, the effect of the 5-HT_1B and the 5-HT_1A receptor agonists, TFMPP and 8-OH-DPAT, respectively, were investigated in the rat central nervous system (CNS) using in vivo and in vitro neurochemical approaches. TFMPP suppressed 5-HT synthesis (5-HTP accumulation after decarboxylase inhibition) both in vivo and in vitro. In vivo, the 5-HT synthesissuppressing effect of the drug (3.0 mg/kg, s.c.) proved resistant to either acute hemitransection or reserpine (5 mg/kg, i.p.; 90 min before) pretreatment. In vitro, in cortical, hippocampal and striatal slice preparations, TFMPP (0.1–10 μM) decreased 5-HT synthesis under basal and stimulated (30 mM K⁺) conditions, an effect which was unaltered by prior in vivo reserpine-induced 5-HT depletion but was attenuated in the presence of 5-HT_1B receptor antagonists such as methiothepin, cyanopindolol or propranolol. The 8-OH-DPAT (0.1 mg/kg, s.c.)-induced decrease of 5-HT synthesis in vivo was abolished by hemitransection but resistant to acute reserpine pretreatment; 8-OH-DPAT (10 μM) did not decrease 5-HT synthesis in vitro. In conclusion, the present study confirms the importance of 5-HT autoreceptors in the feedback control of nerve terminal 5-HT biosynthesis. Specifically, our data indicate: (1) that the reduction of rat brain 5-HT synthesis after TFMPP is mediated by 5-HT_1B autoreceptors located on the serotonergic axon terminals, and (2) that the effect is directly mediated and occurs independently of 5-HT neuronal firing and intact monoamine stores. © 1995 Wiley-Liss, Inc.     

Increased binding at 5-HT1A, 5-HT1B, and 5-HT2A receptors and 5-HT transporters in diet-induced obese rats

5-Hydroxytryptamine (5-HT, serotonin), synthesized in midbrain raphe nuclei and released in various hypothalamic sites, decreases food intake but the specific 5-HT receptor subtypes involved are controversial. Here, we have studied changes in the regional density of binding to 5-HT receptors and transporters and the levels of tryptophan hydroxylase, in rats with obesity induced by feeding a palatable high-energy diet for 7 weeks. We mapped binding at 5-HT receptor subtypes and transporters using quantitative autoradiography and determined tryptophan hydroxylase protein levels by Western blotting. In diet-induced obese (DiO) rats, specific binding to 5-HT_1A receptors ([³H]8-OH-DPAT) was significantly increased in the dorsal and median raphe by 90% (P<0.01) and 132% (P<0.05), respectively, compared with chow-fed controls. 5-HT_1B receptor binding sites ([¹²⁵I]cyanopindolol) were significantly increased in the hypothalamic arcuate nucleus (ARC) of DiO rats (58%; P<0.05), as were 5-HT_2A receptor binding sites ([³H]ketanserin) in both the ARC (44%; P<0.05) and lateral hypothalamic area (LHA) (121%; P<0.05). However, binding to 5-HT_2C receptors ([³H]mesulgergine) in DiO rats was not significantly different from that in controls in any hypothalamic region. Binding to 5-HT transporters ([³H]paroxetine) was significantly increased (P<0.05) in both dorsal and median raphe, paraventricular nuclei (PVN), ventromedial nuclei (VMH), anterior hypothalamic area (AHA) and LHA of DiO rats, by 47%–165%. Tryptophan hydroxylase protein levels in the raphe nuclei were not significantly different between controls and DiO rats. In conclusion, we have demonstrated regionally specific changes in binding to certain 5-HT receptor subtypes in obesity induced by voluntary overeating of a palatable diet. Overall, these changes are consistent with reduced 5-HT release and decreased activity of the 5-HT neurons. Reduction in the hypophagic action of 5-HT, possibly acting at 5-HT_1A, 5-HT_1B and 5-HT_2A receptors, may contribute to increased appetite in rats presented with highly palatable diet.     

Study of mechanisms of calcitonin analgesia in mice: Involvement of 5-HT3 receptors

The analgesic effect of calcitonin when serotonin (5-HT) concentration is increased and the involvement of some 5-HT receptors were studied using the writhing test in mice. 5-hydroxytryptophan (5-HTP) administration increased both 5-HT levels in the central nervous system (CNS) and calcitonin analgesia. The 5-HT_1A agonist (±)-8-hydroxy-2-dipropylaminotetralin hydrobromide (8-OH-DPAT) diminished calcitonin analgesia, this effect being antagonised by the 5-HT_1A antagonist (WAY 100, 135). As the stimulation of 5-HT_1A autoreceptors reduces the turnover of 5-HT, the effect of 8-OH-DPAT on calcitonin analgesia may be attributed to this decrease. The 5-HT_2A–2C agonist (±)-1-(4-iodo-2,5-dimethoxyphenyl)-2-aminopropane hydrochloride (DOI) diminished calcitonin analgesia. A sub-analgesic dose of the 5-HT_2A antagonist ketanserin failed to prevent this effect. The 5-HT₃ agonist (±)-2-methyl-5-hydroxytryptamine maleate (2-methyl-5-HT) potentiated calcitonin analgesia, whereas it was significantly reduced by the 5-HT₃ antagonist tropisetron. The effect of 2-methyl-5-HT on calcitonin analgesia was also reversed by tropisetron, This result suggests that the 5-HT₃ receptor may play an important role in the relationship between calcitonin and the serotonergic system. Tropisetron also reversed the analgesia induced by calcitonin plus 5-HTP corroborating importance of the 5-HT₃ receptors.     

5-HT1A and 5-HT2 receptors differentially regulate the excitability of 5-HT-containing neurones of the guinea pig dorsal raphe nucleus in vitro

We have used intracellular recording techniques to examine the effects of 5-hydroxytryptamine (5-HT, serotonin) on 5-HT-containing neurones of the guinea pig dorsal raphe nucleus in vitro. Bath-applied 5-HT (30–300 μM) had two opposing effects on the membrane excitability of these cells, reflecting the activation of distinct 5-HT receptor subtypes. As demonstrated previously in the rat, 5-HT evoked a hyperpolarization and inhibition of 5-HT neurones, which appeared to involve the activation of an inwardly rectifying K⁺ conductance. This hyperpolarizing response was blocked by the 5-HT_1A receptor-selective antagonist WAY-100635 (30–100 nM). In the presence of WAY-100635, 5-HT induced a previously unreported depolarizing, excitatory response of these cells, which was often associated with an increase in the apparent input resistance of the neurone, likely due to the suppression of a K⁺ conductance. Like the hyperpolarizing response to 5-HT, this depolarization could be recorded in the presence of the Na⁺ channel blocker tetrodotoxin. In addition, the response was not significantly attenuated by the α₁-adrenoceptor antagonist prazosin (500 nM), indicating that it is not due to the release of noradrenaline, or to the direct activation of α₁-adrenoceptors by 5-HT. The 5-HT₃ receptor antagonist granisetron (1 μM) and the 5-HT₄ receptor antagonist SB 204070 (100 nM) failed to reduce the depolarizing response to 5-HT; however, ketanserin (100 nM), mesulergine (100 nM) and lysergic acid diethylamide (1 μM) significantly reduced or abolished the depolarization, indicating that this effect of 5-HT is mediated by 5-HT₂ receptors.     

Mosapride citrate (AS-4370), a new gastroproltinetic agent, is a partial 5-HT4 receptor agonist in the gut

Abstract We examined the possible involvement of 5-HT₄ receptors in the gastroprokinetic effects of mosapride citrate (AS-4370). In isolated longitudinal muscle myenteric plexus (LMMP) preparations from the guinea-pig ileum, mosapride, cisapride, zacopride and metoclopramide enhanced electrically evoked contractions with EC₅₀ values of 74.2, 32.2, 50.3 and 1047.4 nM, respectively. The maximal increases in the amplitude of the contractions caused by mosapride, cisapride, zacopride and metoclopramide were 58, 78, 100 and 92% of those caused by 5-HT, respectively. The enhancing effect of mosapride was non-surmountably antagonised by high concentrations of tropisetron (10⁻-^7–3 ± 10⁻-⁶ M), but not by methysergide (10⁻-⁶ M) or ondansetron (10⁻-⁶ M). In the ileal LMMP preparations, mosapride, cisapride, zacopride and tropisetron had an inhibitory effect related to concentration on the 5-HT (3 ± 10⁻-⁷ M) induced contractions with IC₅₀ values of 0.59, 0.71, 0.55 and 7.9 μM, respectively. Mosapride (10⁻-⁸-⁻10⁻-⁵ M) alone, unlike cisapride and zacopride, did not cause contraction in the ileal LMMP preparations. These results suggest that mosapride is a partial 5-HT₄ receptor agonist and possesses 5-HT₄ receptor antagonist activity at high concentrations. In conscious dogs with force transducers implanted, mosapride (I mg kg⁻¹, i.v.) enhanced the antral motor activity in the postprandial state, and this enhancement was clearly antagonised by a high dose of tropisetron (10 mg kg⁻¹ h⁻¹, i.v. infusion) that did not affect basal motor activity, indicating the involvement of 5-HT₄ receptors in the in vivo gastroprokinetic effect of mosapride.     

Ex vivo inhibitory effect of the 5-HT uptake blocker citalopram on 5-HT synthesis

Summary Serotonin (5-hydroxytryptamine, 5-HT) synthesis was determined in vivo by measuring the accumulation of 5-hydroxytryptophan (5-HTP) in rat frontal cortex after inhibition of aromatic amino acid decarboxylase by administration of m-hydroxybenzylhydrazine (NSD 1015) (100 mg/kg, i.p.). The selective 5-HT reuptake inhibitor, citalopram, the 5-HT_1a agonists, (±)8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT), ipsapirone, gepirone and the 5-HT_1a/b agonist, 7-trifluoromethyl-4(4-methyl-1-piperazinylpyrolo[1,2-a]-quinoxaline (CGS 12066B), the 5-HT_1a/b ligands and -adrenoceptor antagonists, (±)pindolol and (±)alprenolol, and the non-selective 5-HT ligands, m-chlorophenylpiperazine (mCPP) and metergoline, all inhibited the synthesis of 5-HT. The 5-HT_1a /5-HT₂ antagonist, spiperone, alone, had no effect on basal 5-HT synthesis, however it attenuated the effect of 8-OH-DPAT by 56% and CGS 12066B by 39% but only barely that of citalopram by 17%. The selective 5-HT_1a antagonist, WAY 100635, which did not modify by itself 5-HT synthesis, had no effect on citalopram-induced reduction of 5-HT synthesis. Neither the 5-HT₂ agonist, (±)1-(2,5-dimethoxy-4-indophenyl)-2-aminopropane (DOI) nor the 5-HT₂ antagonist, ritanserin, had any effect on the synthesis of 5-HT. In addition, ritanserin did not modify the inhibitory effect of citalopram. Methiothepin was the only compound to increase 5-HT synthesis. These results suggest that the effect of citalopram on the synthesis of 5-HT is not mediated by 5-HT_1a or 5-HT₂ receptors and that other receptors may be involved.     

LSD has high efficacy relative to serotonin in enhancing the cationic current Ih: Intracellular studies in rat facial motoneurons

The effects of LSD (d-lysergic acid diethylamide) on rat facial motoneurons were compared to those of 5-hydroxytryptamine (5-HT) in brain slices by means of current clamp and single-electrode voltage-clamp recordings. As previously reported, 5-HT, in part by decreasing a resting potassium conductance, produced a reversible depolarization (～5 mV), an increase in input resistance, and an enhancement in electrical excitability. LSD also produced an increase in electrical excitability, although with a much slower onset and longer duration. However, in contrast to 5-HT, LSD produced only a slight depolarization (1-2 mV). Moreover, in the presence of LSD the depolarizing effect of 5-HT was markedly attenuated. The 5-HT₂/5-HT_1c agonist 1-(2,5-dimethoxy-4-io-dophenyl)-2-aminopropane (DOI) produced effects intermediate between LSD and 5-HT. The LSD-induced increase in electrical excitability was completely reversed by spiperone, a 5-HT₂/5-HT_1A antagonist, and by ritanserin, a 5-HT₂/5-HT_1c antagonist; the effects of 5-HT were also reduced by these 2 antagonists, but complete blockade did not occur at the concentrations and durations tested. Surprisingly, LSD was found to enhance the hyperpolarization-activated nonspecific cation current I_h to a greater extent than did 5-HT; this enhancement was blocked by both spiperone and ritanserin. These results indicate that, despite having low efficacy relative to 5-HT in decreasing resting potassium conductance, LSD has high efficacy in enhancing the I_h current in rat facial motoneurons; possible mechanisms for this difference are discussed. © 1993 Wiley-Liss, Inc.     

Pharmacological characterization of 5-hydroxytryptamine receptor types in the guinea-pig proximal colon

It was investigated which 5-hydroxytryptamine (5-HT) receptors mediate the responses to 5-HT in the longitudinal muscle layer of the guinea-pig proximal colon, using selective 5-HT receptor antagonists and the 5-HT analogues α-methyl-5-HT (2-Me-5-HT), 2-methyl-5-HT (2-Me-5-HT), and 5-methoxytryptamine (5-MeOT). 5-HT as well as its analogues induced concentration-related contractions, at low concentrations preceded by relaxations. The 5-HT concentration-contractile response curve was biphasic whilst the curves to α-Me-5-HT, 2-Me-5-HT, and 5-MeOT were monophasic. Tetrodotoxin (TTX) abolished the relaxations, and it inhibited the contractions to all agonists. In the presence of TTX, blockade of either 5-HT₂ (ketanserin) or 5-HT₃ receptors (ondansetron, tropisetron) reduced the contractions to 5-HT, whereas blockade of both 5-HT receptor types at the same time abolished them. In the absence of TTX, the contractions to 5-HT were inhibited by antagonists at 5-HT₂ (ketanserin), 5-HT₃ (granisetron, nanomolar concentration of tropisetron) and also 5-HT₄ receptors (micromolar concentration of tropisetron). Contractions to α-Me-5-HT did not seem to be sensitive to 5-HT₂ receptor blockage with ketanserin, but in the presence of TTX the contractions were abolished by the 5-HT₂ receptor antagonist. The 5-HT₃ receptor antagonist granisetron abolished contractions to 2-Me-5-HT. In the presence of TTX, the 5-HT₂ receptor antagonist ketanserin abolished contractions to 5-MeOT, and in the absence of TTX the contractions to 5-MeOT were highly sensitive blockade of 5-HT₄ receptors with tropisetron. Blockage of either 5-HT₁ (methiothepin), 5-HT₂ (ketanserin), 5-HT₃ (ondansetron, granisetron, tropisetron) or 5-HT₄ (tropisetron) receptors did not abolish the relaxations to 5-HT or 5-MeOT. In conclusion, 5-HT induces contractions of the longitudinal muscle of the guinea-pig proximal colon, through the stimulation of 5-HT₂ receptors on the smooth muscle cells and 5-HT₃ receptors and putative neuronal 5-HT₄ receptors. 5-HT evokes relaxations via an unknown neuronal receptor.     

Involvement of 5-HT1B receptors in the anticonflict effect of m-CPP in rats

Summary The anticonflict activity of m-CPP, a non-selective agonist of 5-HT receptors, was studied in the drinking conflict test in rats. m-CPP administered in doses of 0.125–0. 5 mg/kg increased the number of punished licks, the maximum effect having been observed after a dose of 0.25 mg/kg. The anticonflict effect of m-CPP (0.25 mg/kg) was antagonized by the non-selective 5-HT antagonist metergoline (1–4 mg/kg) and by the -adrenoceptor blocker SDZ 21009 (2 and 4 mg/kg) with affinity for 5-HT_1A and 5-HT_1B receptors. On the other hand, the 5-HT_1A receptor antagonist NAN-190 (0.5 and 1 mg/kg), the 5-HT₂ receptor antagonist ritanserin (0.25 and 0.5 mg/kg), and the -blockers betaxolol (8 mg/kg) and ICI 118,551 (8 mg/kg) with no affinity for 5-HT receptors did not affect the effect of m-CPP. The effect of m-CPP was not modified, either, in animals with the 5-HT lesion produced by p-chloroamphetamine.These results suggest that the anticonflict effect of m-CPP described above results from stimulation of 5-HT_1B receptors — most probably these which are located postsynaptically.     

Differential effects of serotonin (5-HT) lesions and synthesis blockade on neuropeptide-Y immunoreactivity and 5-HT1A, 5-HT1B/1D and 5-HT2A/2C receptor binding sites in the rat cerebral cortex

The present study was aimed at comparing the effects of serotonin (5-HT) synthesis blockade using chronic administration of p-chlorophenylalanine (PCPA) and 5,7-dihydroxytryptamine injections of variable volume (3 vs. 6 μl) on the density of NPY immunoreactive (Ir) neurons and binding of [³H]8-OH-DPAT, S-CM-G[¹²⁵I]TNH₂ and [¹²⁵I]DOI to 5-HT_1A, 5-HT_1B/1D, and 5-HT_2A/2C receptors in rat cortical regions. Three weeks after large but partial (89% depletion in 5-HT tissue concentration) lesions of 5-HT neurons no changes in neither NPY immunoreactivity nor 5-HT receptor binding were detected. The complete 5,7-DHT lesions produced increases in the number of NPY-Ir neurons in the upper regions of the cingular (134%), frontal (140%) and parietal cortex (48%) and corresponding decreases in 5-HT_2A/2C binding (16–26%). No changes in 5-HT_1A and 5-HT_1B/1D binding were observed after lesions of this kind. After PCPA treatment, decreases in NPY-Ir neurons density (22–40%) and increases in 5-HT_1A and 5-HT_1B/1D receptor binding sites (20–50%) were distributed in both upper and deeper cortical regions. The lack of effect of the partial lesion suggests that spared 5-HT neurons may exert compensatory mechanisms up to a large extent. The changes in NPY immunoreactivity and 5-HT_2A/2C binding detected in the upper regions of the cortex after complete 5-HT lesions probably result from local cellular rearrangements, whereas blocking 5-HT synthesis has more widespread influence on NPY neurons and on 5-HT_1A and 5-HT_1B/1D receptor subtypes. Moreover, decreases in DOPAC concentrations detected only after complete lesions suggest that the involvement of catecholaminergic transmission may also differentiate 5,7-DHT and PCPA treatments. Altogether, these data suggest that different receptor subtypes might be involved in 5-HT–NPY relationships.     

Feedback Stimulation of Somatodendritic Serotonin Release: A 5-HT3 Receptor-Mediated Effect in the Raphe Nuclei of the Rat

Slices from rat midbrain containing the raphe nuclei and from hippocampus were prepared, loaded with [³H]5-HT and superfused and the resting and the electrically stimulated [³H]5-HT release was measured. The 5-HT₃ receptor agonist 2-methyl-5-HT (1 to 10 μmol/l) increased the resting tritium outflow in superfused raphe nuclei slices, EC₅₀ 5.3 μmol/l. The 2-methyl-5-HT-induced increase of tritium outflow was an external Ca²⁺-independent process and was not altered by reserpine pretreatment but it was reversed by addition of the 5-HT uptake inhibitor fluoxetine (1 μmol/l). The 5-HT₃ receptor antagonists ondansetron and GYKI-46 903 (1 μmol/l) did not antagonize the stimulatory effect of 2-methyl-5-HT on resting tritium outflow. 2-Methyl-5-HT in lower concentration increased the electrically induced tritium overflow from raphe nuclei slices (EC₅₀ 0.56 μmol/l) and also from hippocampal slices preloaded with [³H]5-HT. These effects were reversed by 1 μmol/l of ondansetron and GYKI-46903. The 5-HT₃ receptor antagonists (1 μmol/l) were without effects on depolarization-evoked [³H]5-HT release at 2 Hz stimulation, when 10 Hz stimulation was used, ondansetron and GYKI-46 903 reduced the tritium overflow from raphe nuclei slices. These data indicate that 5-HT₃ receptors positively alter depolarization-induced somatodendritic 5-HT release in the raphe nuclei. They also show that 2-methyl-5-HT is able to evoke 5-HT release not only from vesicles but also from cytoplasmic stores via a transporter-dependent exchange process.     

5-Hydroxytryptamine and functional bowel disorders

Abstract The possibility that 5-hydroxytryptamine (5-HT) acts as a key sensitising agent in the aetiology of irritable bowel syndrome (IBS) is reviewed. The strategic locations of 5-HT and its receptors are described, the most dominant being the 5-HT₃ and 5-HT₄ type. 5-HT, acting mostly at 5-HT₃ or 5-HT₃-like receptors, enhances the sensitivity of visceral neurones projecting between the gut and the central nervous systems. 5-HT, acting at 5-HT₄ receptors promotes the sensitivity of enteric neurones that react to luminal stimuli. 5-HT₄ and 5-HT₃ receptors also mediate, respectively, sensitzsing and physiological actions of 5-HT on gastrointestinal motor and secretory functions. This distribution implies that some 5-HT₃ receptor antagonists might reduce certain symptoms of IBS, such as pain, by reducing the reactivity of the visceral afferent neurones linking the gut with the brain and spinal cord. However, such antagonists are not likely to find widespread clinical acceptance because they can also affect normal lower bowel function and promote constipation. 5-HT₄ receptor antagonists, by contrast, reduce 5-HT-induced enteric nerve hypersensitivity without notably affecting the function of the normal bowel. Accordingly, these agents may reduce the symptoms of IBS directly, by reducing the incidence of defecation and diarrhoea and indirectly, by reducing both ‘rebound’ constipation and the post-prandial discomfort and pain associated with gastrointestinal hyper-reactivity.     

DOI, a 5-HT2A/2C receptor agonist, attenuates clozapine-induced cortical dopamine release

(±)-1-(2,5-Dimethoxy-4-iodophenyl)-2-aminopropane hydrochloride (DOI, 1.25, 2.5 and 5 mg/kg), a serotonin (5-HT)_2A/2C agonist, produced an inverted U-shaped increase in DA release in rat medial prefrontal cortex (mPFC) with a significant effect only at 2.5 mg/kg. This effect was completely abolished by M100907 (0.1 mg/kg), a 5-HT_2A antagonist, and WAY100635 (0.2 mg/kg), a 5-HT_1A antagonist, neither of which when given alone affected dopamine release. DOI (2.5 mg/kg), but not the 5-HT_2C agonist Ro 60-0175 (3 mg/kg), attenuated clozapine (20 mg/kg)-induced mPFC dopamine release. These results suggest that 5-HT_2A receptor stimulation increases basal cortical dopamine release via 5-HT_1A receptor stimulation, and inhibits clozapine-induced cortical dopamine release by diminishing 5-HT_2A receptor blockade.     

5-HT receptors on interstitial cells of Cajal,smooth muscle and enteric nerves

Abstract The majority of the body's serotonin (5-HT) is produced by the gastrointestinal tract. 5-HT has several functions in the gastrointestinal tract. 5-HT is a paracrine signalling molecule released from enterochromaffin cells, a survival and proliferating factor and a neurotransmitter. The actions of 5-HT are transduced by a large family of 5-HT receptors, several of which are expressed on different gastrointestinal cell types including enteric nerves, smooth muscle and interstitial cells of Cajal (ICC). This review will summarize recent advances in understanding the role of 5-HT in regulating function of ICC, and the expression and function of 5-HT receptors on muscle and enteric nerves in human tissue. Rodent ICC express several 5-HT receptors including 5-HT_2B receptors which regulate ICC survival and proliferation. Human smooth muscle and enteric neurons also express several 5-HT receptor subtypes. Expression and function of these receptors is significantly different from small laboratory animals. 5-HT₇ receptor activation causes relaxation of muscle, whereas 5-HT_2B receptors increase muscle activity. The 5-HT₄ receptor appears to mediate both inhibition and activation of smooth muscle involving myogenic as well as neural actions. Despite the abundant expression of 5-HT₃ receptors in the human enteric nervous system no functional correlate has been as yet demonstrated.     

Inhibition of NMDA-receptor mediated response in the rat medial prefrontal cortical pyramidal cells by the 5-HT3 receptor agonist SR 57227A and 5-HT: Intracellular studies

The techniques of intracellular recording and single-electrode voltage-clamp were used to study the effect of serotonin (5-HT) and the selective 5-HT₃ receptor agonist SR 57227A on N-methyl-D-aspartic acid (NMDA)-evoked responses in pyramidal cells of the rat medial prefrontal cortex (mPFC) in in vitro brain slice preparations. Bath application of 5-HT or SR 57227A produced a concentration-dependent inhibition of NMDA-induced membrane depolarization, action potentials, and inward current. The depressant action of 5-HT and SR 57227A had a slow onset and showed no signs of receptor desensitization. This action was markedly attenuated or completely blocked by the selective 5-HT₃ receptor antagonists granisetron and BRL 46470A, but not other receptor antagonists. In addition to inhibiting NMDA-evoked responses, SR 57227A also depressed significantly pharmacologically isolated, NMDA receptor-mediated, monosynaptic excitatory postsynaptic currents (EPSCs) elicited by electrical stimulation of the forceps minor; this inhibitory action was blocked by BRL 46470A but not other 5-HT receptor antagonists. Perfusion of Ca²⁺-free or low Ca²⁺ plus Cd²⁺ artificial cerebrospinal fluid prevented electrical stimulation-induced EPSCs, but did not affect the inhibitory action of 5-HT and SR 57227A. In conclusion, we demonstrate for the first time that 5-HT and SR 57227A interact with 5-HT₃-like receptors to produce a direct inhibitory action on NMDA receptor-mediated response in pyramidal cells of the mPFC. Synapse 29:257–268, 1998. © 1998 Wiley-Liss, Inc.