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.     

Somatodendritic localization of 5-HT1A and preterminal axonal localization of 5-HT1B serotonin receptors in adult rat brain

The 5-HT1A and 5-HT1B receptors of serotonin play important roles as auto- and heteroreceptors controlling the release of serotonin itself and of other neurotransmitters/modulators in the central nervous system (CNS). To determine the precise localization of these receptors, we examined their respective cellular and subcellular distributions in the nucleus raphe dorsalis and hippocampal formation (5-HT1A) and in the globus pallidus and substantia nigra (5-HT1B), using light and electron microscopic immunocytochemistry with specific antibodies. Both immunogold and immunoperoxidase preembedding labelings were achieved. In the nucleus raphe dorsalis, 5-HT1A immunoreactivity was found exclusively on neuronal cell bodies and dendrites, and mostly along extrasynaptic portions of their plasma membrane. After immunogold labeling, the density of membrane-associated 5-HT1A receptors could be estimated to be at least 30-40 times that in the cytoplasm. In the hippocampal formation, the somata as well as dendrites of pyramidal and granule cells displayed 5-HT1A immunoreactivity, which was also prominent on the dendritic spines of pyramidal cells. In both substantia nigra and globus pallidus, 5-HT1B receptors were preferentially associated with the membrane of fine, unmyelinated, preterminal axons, and were not found on axon terminals. A selective localization to the cytoplasm of endothelial cells of microvessels was also observed. Because the 5-HT1A receptors are somatodendritic, they are ideally situated to mediate serotonin effects on neuronal firing, both as auto- and as heteroreceptors. The localization of 5-HT1B receptors to the membrane of preterminal axons suggests that they control transmitter release from nonserotonin as well as serotonin neurons by mediating serotonin effects on axonal conduction. The fact that these two receptor subtypes predominate at extrasynaptic and nonsynaptic sites provides further evidence for diffuse serotonin transmission in the CNS.     

Excitatory action of 5-HT on deglutitive substrates in the rat solitary complex

The excitatory effect of serotonin (5-HT) on the pharyngeal stage of swallowing was investigated in urethane anaesthetised rats with respect to the involvement of neural substrates located in the central and intermediolateral regions of the nucleus tractus solitarii (NTS). Micropneumophoretic ejection of 5-HT 5-50 pmol either produced deglutitory responses or selectively facilitated the S-glutamate-evoked pharyngeal responses when applied in 1-10 pmol prepulses. The excitatory/facilitatory effect of 5-HT was enhanced by intravenous threshold doses of the 5-HT-mimetic, quipazine (0.3-1 mumol/kg) and reversibly blocked by the 5-HT2-receptor antagonists, methysergide, metergoline and ketanserin. 5-HT doses exceeding 10-60 pmol gave rise to a non-selective reversible inhibition of glutamate- and acetylcholine (ACh)-evoked pharyngeal or oesophageal responses which was not prevented or reversed by 5-HT2-receptor antagonists, but was readily overcome by increasing the amount of glutamate or ACh ejected. Non-selective deglutitive inhibition after high doses of 5-HT could, therefore, result from neuronal desensitization secondary to excessive stimulation or activation of a different type of 5-HT receptor. These results corroborate an excitatory role of 5-HT in both reflex and automatic swallowing and demonstrate that the NTS is a major site of serotoninergic facilitation of swallowing.     

Early glial activation, synaptic changes and axonal pathology in the thalamocortical system of Niemann-Pick type C1 mice

Niemann-Pick disease type C (NPC) is an inherited lysosomal storage disease characterised by accumulation of cholesterol and glycosphingolipids. NPC patients suffer a progressive neurodegenerative phenotype presenting with motor dysfunction, mental retardation and cognitive decline. To examine the onset and progression of neuropathological insults in NPC we have systematically examined the CNS of a mouse model of NPC1 (Npc1(-/-) mice) at different stages of the disease course. This revealed a specific spatial and temporal pattern of neuropathology in Npc1(-/-) mice, highlighting that sensory thalamic pathways are particularly vulnerable to loss of NPC1 resulting in neurodegeneration in Npc1(-/-) mice. Examination of markers of astrocytosis and microglial activation revealed a particularly pronounced reactive gliosis in the thalamus early in the disease, which subsequently also occurred in interconnected cortical laminae at later ages. Our examination of the precise staging of events demonstrate that the relationship between glia and neurons varies between brain regions in Npc1(-/-) mice, suggesting that the cues causing glial reactivity may differ between brain regions. In addition, aggregations of pre-synaptic markers are apparent in white matter tracts and the thalamus and are likely to be formed within axonal spheroids. Our data provide a new perspective, revealing a number of events that occur prior to and alongside neuron loss and highlighting that these occur in a pathway dependent manner.     

The 5-HT syndrome and the drug discrimination paradigm in rats: application in behavioral studies on the central 5-HT system

The use of two behavioral tests, the 5-HT motor syndrome in rats and the drug discrimination paradigm for behavioral analysis of the central 5-HT system are discussed. The motor syndrome induced by L-5-HTP in rats is a simple test, allowing the identification of inhibitors of the 5-HT reuptake system and MAO-A. 5-HT-antagonists and 5HT-agonists can also be detected. The degree of selectivity of 5-HT agonists for the 5-HT system can be determined by the use of specific 5-HT-antagonists, or by testing drugs in MAO-inhibited or 5,7-DHT-lesioned animals. The advantages of the drug discrimination paradigm are mainly the low doses of training drugs needed, the objectivity of the test (no observer involved) and the possibility to test for both agonistic and antagonistic activity in the same animals. The drug discrimination paradigm may also contribute to the behavioral differentiation between drugs interacting with 5-HT1, 5-HT2 and possibly tryptamine receptors.     

Insights into the three-dimensional structure of the oculomotor nuclear complex and fascicles.

The authors report the case of a patient with an ischemic lesion in the left midbrain. The patient presented with paresis of left inferior rectus, pupil, right superior rectus, convergence and transiently, of the left medial rectus. A lesion in the left dorsal midbrain close to the oculomotor nuclear complex, selectively involving the fascicles innervating the above muscles, is proposed. Fine magnetic resonance sections showed a consistent lesion in the left paramedian dorsal midbrain. A detailed, three-dimensional, schematic computer model of the oculomotor nucleus and fascicles was constructed. Using this model, the authors topographically validate the putative site of the lesion. The medial rectus subnucleus is divided into three subgroups, A, B, and C. Subgroup C is thought to be the site of the majority of neurons controlling convergence. In the above model, the putative lesion is closer to subgroup A than to C; this suggests that subgroup A, rather than subgroup C, may have a higher concentration of neurons involved in convergence.     

Voltammetric study of 8-OH-DPAT effect on the raphe-trigeminal 5-HT system.

The effect of i.p. administration of the selective 5-HT1A agonist 8-hydroxy-2-(di-N-propylamino) tetralin (8-OH-DPAT) (100 micrograms kg-1) has been investigated by in vivo 5-hydroxyindole electrochemical (peak 3) detection in the nucleus raphe magnus (NRM) and medullary dorsal horn (MDH) of acute anaesthetized and unanaesthetized freely moving rats. 8-OH-DPAT induced a small but significant decrease in peak 3 in the NRM and MDH of anaesthetized rats. In freely moving animals, a similar small effect was observed at both NRM and MDH levels. With reference to similar in vivo studies demonstrating differential responsiveness of ascending serotonergic systems to 8-OH-DPAT, it is concluded that the serotonergic NRM-dorsal horn system is slightly affected by this 5-HT1A agonist.     

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.     

Different distributions of the 5-HT reuptake complex and the postsynaptic 5-HT(2A) receptors in Brodmann areas and brain hemispheres

The aim of the present study was to determine the distribution of the presynaptic 5-HT reuptake complex and the 5-HT(2A) receptors through Brodmann areas from two control subjects, together with the possible existence of laterality between both brain hemispheres. A left laterality was observed in the postsynaptic 5-HT(2A) binding sites, with significantly higher B(max) values in the left frontal and cingulate cortex. In frontal cortex, [3H]imipramine and [3H]paroxetine binding showed the highest B(max) values in areas 25, 10 and 11. In cingulate cortex, the highest [3H]imipramine and [3H]paroxetine B(max) values were noted in Brodmann area 33 followed by area 24, while postsynaptic 5-HT(2A) receptors were mainly distributed through Brodmann areas 23 and 29. In temporal cortex, the highest [3H]imipramine and [3H]paroxetine B(max) was noted in Brodmann areas 28 and 34, followed by areas 35 and 38. All Brodmann areas from parietal cortex (1, 2, 3, 4, 5, 6, 7, 39, 40 and 43) showed similar presynaptic and postsynaptic binding values. In occipital cortex no differences were observed with regard to the brain hemisphere or to the Brodmann area (17, 18 and 19). These results suggest the need to carefully define the brain hemisphere and the Brodmann areas studied, as well to avoid comparisons between studies including different Brodmann areas or brain hemispheres.     

Postnatal development of 5-HT1 receptors: [3H]5-HT binding sites and 5-HT induced adenylate cyclase activations in rat brain cortex

The postnatal development of the 5-HT1 receptor system was studied in young rat brain cortex from birth to adulthood (14 successive ages). The high-affinity binding of [3H]5-HT was low at birth but developed markedly between the 8th and the 15th day postnatally. The basal adenylate cyclase activity produced 50 pmoles cAMP/mg protein/min at birth and increased from the 8th to the 15th day. 5-HT could stimulate the adenylate cyclase activity in adult rat brain cortex with two different affinity constants: Km = 1 nM and Km = 0.5 microM; these low- and high-affinity constants presumably correspond to 5-HT1A and 5-HT1non-A.non-B.non-C (5-HT1D) respectively. These two activities developed parallelly from the 14-15th to the 28th day. The 8-hydroxy-2-(di-n-propylamino-tetralin) (8-OH-DPAT)-induced activity described a curve similar to the one that corresponded to 10 microM 5-HT. These results establish that 5-HT1A and 5-HT1non-A.non-B.non-C receptors mainly develop during the synaptogenesis.     

Regulation of 5-HT(1A) and 5-HT(1B) receptor systems by phospholipid signaling cascades.

The 5-HT(1A) and 5-HT(1B) receptor systems play central roles in the control of serotonergic neurotransmission and feature prominently in many behaviors and physiological functions. In addition, the regulation of these receptors and their effector mechanisms has been the focus of intense interest because of their potential importance in the therapeutic actions of anxiolytic and antidepressant drugs. Here we describe the regulation of 5-HT(1A) and 5-HT(1B) receptor-mediated inhibition of adenylyl cyclase activity by receptors which activate phospholipid signaling cascades. Although it might be expected that these two highly homologous Gi-coupled receptors would be regulated similarly by activation of phospholipase C (PLC) and phospholipase A(2) (PLA(2)), we have found that the regulation differs markedly between these receptor systems. Further, our data suggest that the modulation of agonist efficacy at these receptor subtypes is dependent on the nature of receptor coupling to PLC and PLA(2) activation. Moreover, regulation at the level of the effector (e.g., adenylyl cyclase) appears to play a significant role in the regulation of both the 5-HT(1A) and 5-HT(1B) receptor systems by the PLA(2) signaling cascade. Such data illustrate multiple levels for control of biochemical signaling cascades within cells and demonstrate that although different receptors may couple to the same effector pathways, the ultimate cellular effects produced by these receptors may differ due to differential cross-talk regulation.     

The development of axonal connections in the central olfactory system of rats

The development of the cytoarchitecture and axonal connections of the central olfactory system were studied in fetal and neonatal rats from E16. In contrast to neocortical development, the olfactory cortex lacks a distinct cortical plate. In the piriform cortex and the olfactory tubercle the cellular laminae emerge simultaneously, while in the anterior olfactory nucleus, there are morphogenetic gradients from superficial to deep as well as from caudal to rostral which parallel the known cytogenetic gradients. Parallel morphogenetic and cytogenetic gradients are also present in the lateral to medial axis of the olfactory tubercle. The projection from the olfactory bulb and the associational projections from the piriform cortex begin to develop well before birth. At E17 fibers from the bulb are limited to the lateral olfactory tract (LOT) and the molecular layer just deep to it, and then spread out caudally, laterally, and medially away from the LOT. This sequence of innervation parallels and predicts the density of innervation in the adult: those areas which are innervated first (such as the piriform cortex deep to the LOT) ultimately receive the heaviest innervation; conversely, those areas which are innervated very late (such as the medial olfactory tubercle) receive the lightest projection. The intracortical projections from the anterior and posterior piriform cortex extend into layer I ipsilaterally by E20 and obtain their adult distribution by the middle of the first postnatal week. On the other hand, fibers from the anterior olfactory nucleus and the entorhinal area do not reach their full adult extent until the second postnatal week. Similarly, the crossed projection of the anterior piriform cortex to the contralateral posterior piriform cortex does not grow into layer I until this later time. The timing of fiber ingrowth showed no relation to the trajectory or eventual areal or laminar termination of fibers. As with the olfactory bulb projection, the timing may influence the density of termination. Centrifugal fibers to the bulb are demonstrable around the time of birth both by the retrograde transport of horseradish peroxidase (HRP) and by the anterograde transport of 3H-leucine. The arrival of additional fibers during the remainder of the first postnatal week parallels the known cytogenetic and morphogenetic gradients in the areas in which they arise. The projections of the olfactory cortex to the lateral hypothalamic area and the mediodorsal thalamic nucleus are evident before birth. This correlates with the early generation of the cells which give rise to these projections.     

Ephrin-A5 promotes the formation of terminal thalamocortical arbors

Ephrins-A5 are expressed in the cortical target layer of thalamic afferents at the time when these axons form terminal arbors. Previous in-vitro studies provided evidence that ephrin-A5 supports the branching of thalamic axons, but there is no direct in-vivo evidence for such a growth-promoting effect. Here we examined thalamocortical projections in ephrins-A5 deficient mice. Our results demonstrate that the laminar specificity of thalamic afferents in ephrin-A5 mutants remains preserved, but axonal arbor formation is greatly reduced. Thus, ephrin-A5 specifically regulates branch formation of thalamic axons, but does not affect target layer selection. Ephrin-A5-mutant mice are, therefore, a unique model to study the effects of reduced thalamic innervation on the assembly of cortical circuits and sensory processing.     

RGS7 complex formation and colocalization with the Gbeta5 subunit in the adult rat brain and influence on Gbeta5gamma2-mediated PLCbeta signaling

This study describes the colocalized distribution and dimeric complex formation between RGS7, a GTPase-activating protein for several heterotrimeric Galpha protein families, and the Gbeta5 subunit in the adult rat brain. Confocal dual immunofluorescence labeling studies indicated a broad regional specificity in the cellular coexpression between RGS7 and Gbeta5 within the cerebral cortical layers I and V-VI, hippocampal formation, caudate-putamen, medial habenula, most thalamic nuclei, and cerebellar molecular and granular layers. In all instances, Gbeta1-beta4 immunoreactivities exhibited no observable colocalization with RGS7, despite their widespread codistribution throughout similar neuronal networks. Coimmunoprecipitation studies confirmed the selective protein-protein interaction between RGS7 and Gbeta5 within brain regions that displayed immunohistochemical colocalization. The influence of RGS7 to modulate Gbeta5gamma2-mediated phosphatidyl inositol (PI) production was examined in COS-7-cotransfected cells. In the presence of Gbeta5gamma2 only, intracellular PI accumulation was increased by 25% above basal levels; addition of RGS7 produced no significant alteration in Gbeta5gamma2-mediated PI accumulation. A similar trend was exhibited when full-length RGS7 was substituted with an RGS7 construct lacking the Gbeta5-interacting region (G protein gamma-like domain; GGL domain) or with RGS4. In conclusion, RGS7/Gbeta5 dimers occurred within most brain regions in which both proteins were cellularly coexpressed. However, an influence of RGS7 on Gbeta5gamma2-mediated PLCbeta signaling activity was not apparent, athough this was in COS-7 cell transfection studies.     

The effects of manipulation of presynaptic 5-HT nerve terminals on postsynaptic 5-HT1 and 5-HT2 binding sites of the rat brain

Summary The effects of long-term treatment of rats with alaproclate and amiflamine on the number and kinetics of 5-HT₁ and 5-HT₂ binding sites were investigated usingin vitro receptor binding techniques. Some other studies have reported down-regulatory effects of alaproclate and amiflamine on 5-HT₂ binding sites in certain regions of the rat forebrain, but no such effects could be detected in the present study. Induction of a high-affinity binding site for³H-5-HT after long-term antidepressant treatment, as has been reported elsewhere, was not obtained in the present study. The results are compared to the effects obtained by treatment of rats with para-chloroarnphetamine (PCA), which depletes the presynaptic neurons of monoamines. These different types of treatment do not cause any change in the binding properties of the specific 5-HT binding sites. It is thus concluded that such manipulations of the presynaptic 5-HT neurons do not affect the postsynaptic 5-HT₁ and 5-HT₂ binding sites.     

Insights into brain function through the examination of art: the influence of neurodegenerative diseases

There has been intense interest in determining how the visual brain processes the outside world, and in identifying the neuroanatomical correlates of the ability to create art. Comparing art production before and after illness onset permits some speculation on the function of selective brain regions affected by the neurodegenerative process. In this review of cases of neurodegenerative illness in visual artists, the evolution of abstraction may be argued to either reflect an enhancement of previous skills or, that the emergence of abstraction, when it was previously absent, is a manifestation of cognitive and visuo-spatial decline. We present examples of each, illustrating both perspectives and suggest that the two opposing views may not be mutually exclusive.     

Variation within the serotonin (5-HT) 5-HT2C receptor system aligns with vulnerability to cocaine cue reactivity

Cocaine dependence remains a challenging public health problem with relapse cited as a major determinant in its chronicity and severity. Environmental contexts and stimuli become reliably associated with its use leading to durable conditioned responses (‘cue reactivity'') that can predict relapse as well as treatment success. Individual variation in the magnitude and influence of cue reactivity over behavior in humans and animals suggest that cue-reactive individuals may be at greater risk for the progression to addiction and/or relapse. In the present translational study, we investigated the contribution of variation in the serotonin (5-HT) 5-HT_2C receptor (5-HT_2CR) system in individual differences in cocaine cue reactivity in humans and rodents. We found that cocaine-dependent subjects carrying a single nucleotide polymorphism (SNP) in the HTR2C gene that encodes for the conversion of cysteine to serine at codon 23 (Ser23 variant) exhibited significantly higher attentional bias to cocaine cues in the cocaine-word Stroop task than those carrying the Cys23 variant. In a model of individual differences in cocaine cue reactivity in rats, we identified that high cocaine cue reactivity measured as appetitive approach behavior (lever presses reinforced by the discrete cue complex) correlated with lower 5-HT_2CR protein expression in the medial prefrontal cortex and blunted sensitivity to the suppressive effects of the selective 5-HT_2CR agonist WAY163909. Our translational findings suggest that the functional status of the 5-HT_2CR system is a mechanistic factor in the generation of vulnerability to cocaine-associated cues, an observation that opens new avenues for future development of biomarker and therapeutic approaches to suppress relapse in cocaine dependence.     

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.