Presynaptic 5-HT3 receptors evoke an excitatory response in dorsal vagal preganglionic neurones in anaesthetized rats

5-HT3 Receptors in Rat Dorsal Root Ganglion Neurons: Ca2+ Entry and Modulation of Neurotransmitter Release

Rat dorsal root ganglion (DRG) neurons express 5-hydroxytryptamine receptors (5-HT3Rs). To elucidate their physiological role in the modulation of sensory signaling, we aimed to quantify their functional expression in newborn and adult rat DRG neurons, as well as their ability to modulate the Ca²⁺-dependent neurotransmitter release, by means of electrophysiological techniques combined with fluorescence-based Ca²⁺ imaging. The selective 5-HT3R agonist mCPBG (10 μM) elicited whole-cell currents in 92.5% of adult DRG neurons with a significantly higher density current than in responding newborn cells (52.2%), suggesting an increasing serotoninergic modulation on primary afferent cells during development. Briefly, 5-HT3Rs expressed by adult DRG neurons are permeable to Ca²⁺ ions, with a measured fractional Ca²⁺ current (i.e., the percentage of total current carried by Ca²⁺ ions, Pf) of 1.0%, similar to the value measured for the human heteromeric 5-HT3_A/B receptor (P_f = 1.1%), but lower than that of the human homomeric 5-HT3_A receptor (P_f = 3.5%). mCPBG applied to co-cultures of newborn DRG and spinal neurons significantly increased the miniature excitatory postsynaptic currents (mEPSCs) frequency in a subset of recorded spinal neurons, even in the presence of Cd²⁺, a voltage-activated Ca²⁺ channel blocker. Considered together, our findings indicate that the Ca²⁺ influx through heteromeric 5-HT3Rs is sufficient to increase the spontaneous neurotransmitter release from DRG to spinal neurons.

DMV extrasynaptic NMDA receptors regulate caloric intake in rats

Acute high-fat diet (aHFD) exposure induces a brief period of hyperphagia before caloric balance is restored. Previous studies have demonstrated that this period of regulation is associated with activation of synaptic N-methyl-D-aspartate (NMDA) receptors on dorsal motor nucleus of the vagus (DMV) neurons, which increases vagal control of gastric functions. Our aim was to test the hypothesis that activation of DMV synaptic NMDA receptors occurs subsequent to activation of extrasynaptic NMDA receptors. Sprague-Dawley rats were fed a control or high-fat diet for 3-5 days prior to experimentation. Whole-cell patch-clamp recordings from gastric-projecting DMV neurons; in vivo recordings of gastric motility, tone, compliance, and emptying; and food intake studies were used to assess the effects of NMDA receptor antagonism on caloric regulation. After aHFD exposure, inhibition of extrasynaptic NMDA receptors prevented the synaptic NMDA receptor-mediated increase in glutamatergic transmission to DMV neurons, as well as the increase in gastric tone and motility, while chronic extrasynaptic NMDA receptor inhibition attenuated the regulation of caloric intake. After aHFD exposure, the regulation of food intake involved synaptic NMDA receptor-mediated currents, which occurred in response to extrasynaptic NMDA receptor activation. Understanding these events may provide a mechanistic basis for hyperphagia and may identify novel therapeutic targets for the treatment of obesity.

Synthesis and Pharmacological Evaluation of [11C]Granisetron and [18F]Fluoropalonosetron as PET Probes for 5-HT3 Receptor Imaging

Serotonin-gated ionotropic 5-HT₃ receptors are the major pharmacological targets for antiemetic compounds. Furthermore, they have become a focus for the treatment of irritable bowel syndrome (IBS) and there is some evidence that pharmacological modulation of 5-HT₃ receptors might alleviate symptoms of other neurological disorders. Highly selective, high-affinity antagonists, such as granisetron (Kytril) and palonosetron (Aloxi), belong to a family of drugs (the "setrons") that are well established for clinical use. To enable us to better understand the actions of these drugs in vivo, we report the synthesis of 8-fluoropalonosetron (15) that has a binding affinity (K_i = 0.26 ± 0.05 nM) similar to the parent drug (K_i = 0.21 ± 0.03 nM). We radiolabeled 15 by nucleophilic ¹⁸F-fluorination of an unsymmetrical diaryliodonium palonosetron precursor and achieved the radiosynthesis of 1-(methyl-¹¹C)-N-granisetron ([¹¹C]2) through N-alkylation with [¹¹C]CH₃I, respectively. Both compounds [¹⁸F]15 (chemical and radiochemical purity >95%, specific activity 41 GBq/μmol) and [¹¹C]2 (chemical and radiochemical purity ≥99%, specific activity 170 GBq/μmol) were evaluated for their utility as positron emission tomography (PET) probes. Using mouse and rat brain slices, in vitro autoradiography with both [¹⁸F]15 and [¹¹C]2 revealed a heterogeneous and displaceable binding in cortical and hippocampal regions that are known to express 5-HT₃ receptors at significant levels. Subsequent PET experiments suggested that [¹⁸F]15 and [¹¹C]2 are of limited utility for the PET imaging of brain 5-HT₃ receptors in vivo.

The antidepressant bupropion is a negative allosteric modulator of serotonin type 3A receptors

The FDA-approved antidepressant and smoking cessation drug bupropion is known to inhibit dopamine and norepinephrine reuptake transporters, as well as nicotinic acetylcholine receptors (nAChRs) which are cation-conducting members of the Cys-loop superfamily of ion channels, and more broadly pentameric ligand-gated ion channels (pLGICs). In the present study, we examined the ability of bupropion and its primary metabolite hydroxybupropion to block the function of cation-selective serotonin type 3A receptors (5-HT_3ARs), and further characterized bupropion's pharmacological effects at these receptors. Mouse 5-HT_3ARs were heterologously expressed in HEK-293 cells or Xenopus laevis oocytes for equilibrium binding studies. In addition, the latter expression system was utilized for functional studies by employing two-electrode voltage-clamp recordings. Both bupropion and hydroxybupropion inhibited serotonin-gated currents from 5-HT_3ARs reversibly and dose-dependently with inhibitory potencies of 87 μM and 112 μM, respectively. Notably, the measured IC₅₀ value for hydroxybupropion is within its therapeutically-relevant concentrations. The blockade by bupropion was largely non-competitive and non-use-dependent. Unlike its modulation at cation-selective pLGICs, bupropion displayed no significant inhibition of the function of anion-selective pLGICs. In summary, our results demonstrate allosteric blockade by bupropion of the 5-HT_3AR. Importantly, given the possibility that bupropion's major active metabolite may achieve clinically relevant concentrations in the brain, our novel findings delineate a not yet identified pharmacological principle underlying its antidepressant effect.

QCM-4, a 5-HT₃ receptor antagonist ameliorates plasma HPA axis hyperactivity, leptin resistance and brain oxidative stress in depression and anxiety-like behavior in obese mice

Several preclinical studies have revealed antidepressant and anxiolytic-like effect of 5-HT3 receptor antagonists. In our earlier study, we have reported the antidepressive-like effect of 3-methoxy-N-p-tolylquinoxalin-2-carboxamide (QCM-4) in obese mice subjected to chronic stress. The present study deals with the biochemical mechanisms associated with depression co-morbid with obesity. Mice were fed with high fat diet (HFD) for 14 weeks, further subjected for treatment with QCM-4 (1 and 2mg/kg p.o.) and standard antidepressant escitalopram (ESC) (10mg/kg p.o.) for 28 days. Behavioral assays for depression such as sucrose preference test (SPT), forced swim test (FST) and for anxiety such as light and dark test (LDT) and hole board test (HBT) were performed in obese mice. Biochemical assessments including plasma leptin and corticosterone concentration followed by brain oxidative stress parameters malonaldehyde (MDA) and reduced glutathione (GSH) were performed. Results confirmed that QCM-4 exhibits antidepressive effect by increasing the sucrose consumption in SPT, reducing immobility time in FST and anxiolytic effect by increasing transitions and time in light chamber in LDT, increasing head dip and crossing score in HBT. Furthermore, QCM-4 attenuated the hypothalamic-pituitary-adrenal (HPA) axis hyperactivity by reducing the plasma corticosterone, reversing altered plasma leptin, restoring the imbalance of brain MDA and GSH concentration. In conclusion, QCM-4 showed antidepressive and anxiolytic effect by reversing the behavioral alterations that were supported by biochemical estimations in obese mice.

Cisplatin induces neuronal activation and increases central AMPA and NMDA receptor subunit gene expression in mice

Although rats and mice do not vomit, these species are widely studied as models of energy balance and sickness behavior. Previous work has shown that rats exhibit similar neuroanatomical activation of brain and visceral afferent pathways following cisplatin chemotherapy compared to vomiting species. However, the neural response to cisplatin in mice is understudied. Here, food intake, body weight, and central c-Fos immunofluorescence were analyzed in the hindbrains of male C57BL/6 mice following IP saline or cisplatin (5mg/kg, and 20mg/kg doses). As glutamate receptor signaling is classically linked to inhibitory feeding pathways in the rodent, gene expression of selected α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and N-methyl-d-aspartic acid (NMDA) receptor subunits were assessed in the dorsal vagal complex (DVC), parabrachial nucleus (PBN), amygdala, and bed nucleus of the stria terminalis (BNST). Our results show dose-dependent reductions in food intake and body weight following cisplatin treatment, as well as increases in cisplatin-induced c-Fos in the PBN and throughout the DVC. Quantitative PCR analysis shows cisplatin-induced increases in NMDA receptor subunit expression, particularly NR2B, in the DVC, PBN, BNST, and amygdala. In addition, upregulation of AMPA receptor subunits (GluA1 and/or GluA2) were observed in all regions examined except the amygdala. Taken together, these results suggest similar neural pathways mediating cisplatin effects in mice compared to other well-studied species, which are likely mediated by central upregulation of AMPA and NMDA receptors.

The role of central 5-HT(2C) and NMDA receptors on LPS-induced feeding behavior in chickens

In mammals, LPS regulate feeding primarily through the 5-HT(1A) and 5-HT(2c) receptors within the brain. However, the central effect of 5-HT(1A) and 5-HT(2c) on LPS-induced feeding behavior has not been studied in non-mammalian species. Also, the role of glutamatergic system in LPS-induced anorexia has never been examined in either mammalian or non-mammalian species. Therefore, in this study, we examined the role of serotonergic and glutamatergic systems on LPS-induced anorexia in chickens. Food intake was measured in chickens after centrally administered lipopolysaccharide (LPS) (20 ng) (0 h), followed by intracerebroventricular (ICV) injection of the 5-HT(1A) autoreceptor agonist (8-OH-DPAT, 61 nmol), 5-HT(2c) receptor antagonist (SB 242084, 30 nm), and NMDA receptor antagonist (DL-AP5, 5 nm) at the onset of anorexia (4 h). In the following experiments, we used DL-AP5 before 5-HT (10 μg) and SB242084 before glutamate (300 nm) for evaluation of the interaction between 5-HTergic and glutamatergic systems on food intake. The results of this study showed that SB 242084 and DL-AP5 significantly attenuated food intake reduction caused by LPS (P < 0.05) but 8-OH-DPAT had no effect. In addition, 5-HT-induced anorexia was significantly attenuated by DL-AP5 pretreatment (P < 0.05), while SB 242084 had no effect on glutamate-induced hypophagia. These results indicated that 5-HT and glutamate (via 5-HT(2c) and NMDA receptor, respectively) dependently regulate LPS-induced hypophagia in chickens.

The dorsal motor nucleus of the vagus and regulation of pancreatic secretory function

Recent investigation of the factors and pathways that are involved in regulation of pancreatic secretory function (PSF) has led to development of a pancreatic vagovagal reflex model. This model consists of three elements, including pancreatic vagal afferents, the dorsal motor nucleus of the vagus (DMV) and pancreatic vagal efferents. The DMV has been recognized as a major component of this model and so this review focuses on the role of this nucleus in regulation of PSF. Classically, the control of the PSF has been viewed as being dependent on gastrointestinal hormones and vagovagal reflex pathways. However, recent studies have suggested that these two mechanisms act synergistically to mediate pancreatic secretion. The DMV is the major source of vagal motor output to the pancreas, and this output is modulated by various neurotransmitters and synaptic inputs from other central autonomic regulatory circuits, including the nucleus of the solitary tract. Endogenously occurring excitatory (glutamate) and inhibitory amino acids (GABA) have a marked influence on DMV vagal output to the pancreas. In addition, a variety of neurotransmitters and receptors for gastrointestinal peptides and hormones have been localized in the DMV, emphasizing the direct and indirect involvement of this nucleus in control of PSF.

Peripheral 5-HT1A and 5-HT7 serotonergic receptors modulate parasympathetic neurotransmission in long-term diabetic rats

We analyzed the modulation of serotonin on the bradycardia induced in vivo by vagal electrical stimulation in alloxan-induced long-term diabetic rats. Bolus intravenous administration of serotonin had a dual effect on the bradycardia induced either by vagal stimulation or exogenous Ach, increasing it at low doses and decreasing it at high doses of 5-hydroxytryptamine (5-HT), effect reproduced by 5-carboxamidotryptamine maleate (5-CT), a 5-HT_1/7 agonist. The enhancement of the bradycardia at low doses of 5-CT was reproduced by 5-HT_1A agonist 8-hydroxy-2-dipropylaminotetralin hydrobromide (8-OH-DPAT) and abolished by WAY-100,635, 5-HT_1A antagonist. Pretreatment with 5-HT₁ antagonist methiothepin blocked the stimulatory and inhibitory effect of 5-CT, whereas pimozide, 5-HT₇ antagonist, only abolished 5-CT inhibitory action. In conclusion, long-term diabetes elicits changes in the subtype of the 5-HT receptor involved in modulation of vagally induced bradycardia. Activation of the 5-HT_1A receptors induces enhancement, whereas attenuation is due to 5-HT₇ receptor activation. This 5-HT dual effect occurs at pre- and postjunctional levels.

Both GABAA and GABAB receptors mediate vagal inhibition in nucleus tractus solitarii neurones in anaesthetized rats

GABA receptors in the nucleus tractus solitarius (NTS) are known to play an important role in the mediation/modulation of various cardiovascular/respiratory functions. Vagal afferent activation of these neurones usually evokes an initial excitation followed by a long lasting inhibition. The present study examines the role of GABA(B) as well as GABA(A) receptors in the mediation of this inhibition in anaesthetized rats, using CGP 35348 and bicuculline, respective antagonists at these receptors, applied topically or by ionophoresis. Bicuculline delayed the onset and reduced the duration of this inhibition. The duration of this inhibition was further and significantly decreased when CGP 35348 was administered along with bicuculline but the delay in onset was unaffected. CGP 35348 application alone, had no effect on the vagal afferent-evoked inhibition. From intracellular recordings the early and late components of this inhibition were found to have reversal potentials close to E(Cl) and E(K), respectively. Therefore it is concluded that this inhibition is mediated by both GABA(A) and GABA(B) receptors, although GABA(B) mediated inhibition can only be unmasked when GABA(A) receptors are blocked.

The role of 5-HT3 and other excitatory receptors in central cardiorespiratory responses to hypoxia: implications for sudden infant death syndrome

Although brainstem serotonergic (5-HT) systems are involved in the protective responses to hypoxia, abnormalities of 5-HT function are strongly implicated in SIDS, and the neurochemical mechanisms by which 5-HT receptors influence brainstem cardiorespiratory responses to hypoxia remains unclear. This study focuses on the role of excitatory neurotransmission, including 5-HT3 signaling, to cardiac vagal neurons (CVNs) that dominate the control of heart rate. Excitatory synaptic inputs to CVNs, located in the nucleus ambiguus (NA), were recorded simultaneously with respiratory activity in in vitro brainstem slices. During control conditions excitatory inputs to CVNs were blocked by application of NMDA and AMPA/kainate glutamatergic receptor antagonists, whereas the 5-HT3 and purinergic receptor antagonists ondansetron and pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS), respectively, had no effect. However, during hypoxia ondansetron inhibited excitatory neurotransmission to CVNs. In recovery from hypoxia, spontaneous and respiratory-related excitatory events were blocked by glutamatergic and purinergic receptor blockers, respectively, whereas ondancetron had no effect. These results demonstrate that hypoxia recruits a 5-HT pathway to CVNs that activates 5-HT3 receptors on CVNs to maintain parasympathetic cardiac activity during hypoxia. Exaggeration of this 5-HT neurotransmission could increase the incidence of bradycardia and risk of sudden infant death during hypoxia.

The 5-HT3 receptor--the relationship between structure and function

The 5-hydroxytryptamine type-3 (5-HT3) receptor is a cation-selective ion channel of the Cys-loop superfamily. 5-HT3 receptor activation in the central and peripheral nervous systems evokes neuronal excitation and neurotransmitter release. Here, we review the relationship between the structure and the function of the 5-HT3 receptor. 5-HT3A and 5-HT3B subunits are well established components of 5-HT3 receptors but additional HTR3C, HTR3D and HTR3E genes expand the potential for molecular diversity within the family. Studies upon the relationship between subunit structure and the ionic selectivity and single channel conductances of 5-HT3 receptors have identified a novel domain (the intracellular MA-stretch) that contributes to ion permeation and selectivity. Conventional and unnatural amino acid mutagenesis of the extracellular domain of the receptor has revealed residues, within the principle (A-C) and complementary (D-F) loops, which are crucial to ligand binding. An area requiring much further investigation is the subunit composition of 5-HT3 receptors that are endogenous to neurones, and their regional expression within the central nervous system. We conclude by describing recent studies that have identified numerous HTR3A and HTR3B gene polymorphisms that impact upon 5-HT3 receptor function, or expression, and consider their relevance to (patho)physiology.

Diabetes-induced changes in 5-hydroxytryptamine modulation of vagally-induced bradycardia in rat heart

1. In the present study, we investigated how alloxan-induced diabetes affects the ability of 5-hydroxytryptamine (5-HT) to modulate bradycardia induced in vivo by electrical stimulation of the vagus nerve in pithed rats. We also analysed the type and/or subtype of 5-HT receptors involved. 2. Diabetes was induced in male Wistar rats with a single injection of alloxan (150 mg/kg, s.c.). Four weeks later, rats were anaesthetized, pretreated with atenolol and pithed. Electrical stimulation (3, 6 and 9 Hz) of the vagus nerve resulted in frequency dependent decreases in heart rate (HR). 3. In diabetic rats, intravenous bolus administration of high doses of 5-HT (100 and 200 microg/kg) increased the bradycardia induced by vagal electrical stimulation. Similarly, low doses (10 microg/kg) of the 5-HT(1/7) receptor agonist 5-carboxamidotryptamine (5-CT), increased vagally induced bradycardia. However, at high doses (50, 100 and 150 microg/kg), 5-CT reduced the bradycardia. Attenuation of the vagally induced bradycardia evoked by the higher doses of 5-CT was reproduced by L-694,247 (50 microg/kg), a selective agonist for the non-rodent 5-HT(1B) and 5-HT(1D) receptors. Enhancement of the vagally induced bradycardia elicited by low doses of 5-CT was reproduced by the selective 5-HT(1A) receptor agonist 8-hydroxydipropylaminotretalin hydrobromide (8-OH-DPAT; 50 microg/kg). These stimulatory and inhibitory actions on vagal stimulation-induced bradycardia in diabetic rats were also observed after administration of exogenous acetylcholine. 4. Vagally induced bradycardia in diabetic rats was not affected by administration of the selective 5-HT(2) receptor agonist alpha-methyl-5-HT (150 microg/kg), the selective 5-HT(3) receptor agonist 1-phenylbiguanide (150 microg/kg) or the selective 5-HT(1B) receptor agonist CGS-12066B (50 microg/kg). 5. Enhancement of the electrical stimulation-induced bradycardia in diabetic rats caused by 5-CT (10 microg/kg) or 8-OH-DPAT (50 microg/kg) was abolished by the selective 5-HT(2/7) receptor antagonist mesulergine (1 mg/kg) and the selective 5-HT(1A) receptor antagonist WAY-100,635 (100 microg/kg), respectively. Similarly, pretreatment with the non-selective 5-HT(1) receptor antagonist methiothepin (0.1 mg/kg) blocked the inhibitory effect of 5-CT (50 microg/kg) on the bradycardia induced by vagal electrical stimulation in diabetic rats. BRL-15572 (2 microg/kg), a selective 5-HT(1D) receptor antagonist, inhibited the action of L-694,247 (50 microg/kg), a selective agonist for the non-rodent 5-HT(1B) and 5-HT(1D) receptors, on the vagally induced bradycardia. 6. In conclusion, in the present study, experimental diabetes evoked changes in both the nature and 5-HT receptor types/subtypes involved in vagally induced bradycardia.

Cardiac vagal preganglionic neurones in the intermediate zone of the brainstem in anaesthetized cats

Cardiac vagal preganglionic neurones (CVPNs) show respiratory modulation in the nucleus ambiguus but not in the dorsal vagal nucleus. Both types of neurones can be activated by pulmonary C fibre afferents. Another brainstem area that has been identified as containing CVPNs is the intermediate zone between the dorsal vagal nucleus and nucleus ambiguus. Experiments were carried out in alpha-chloralose-anaesthetized cats to determine the physiological properties of these CVPNs and their responses to pulmonary C fibre afferent activation. Seven CVPN axons in the right cardiac vagal branches were identified and found to be localized in the intermediate zone with a conduction velocity of between 1.2 and 1.6 m s(-1), in the C fibre range. These seven CVPNs [either showing spontaneous activity (n = 1) or having activity induced by dl-homocysteic acid applied ionophoretically (n = 3)] were neither respiratory modulated nor did they receive a baroreceptor input, thus being similar to those found in the dorsal vagal nucleus. Right atrial injections of phenylbiguanide excited all four CVPNs tested. In conclusion, CVPNs located in the intermediate zone have similar properties to those in the dorsal vagal nucleus but not the nucleus ambiguus.

5-HT2 and 5-HT3 receptors in the lateral parabrachial nucleus mediate opposite effects on sodium intake

The present study investigated the role of several 5-HT receptor subtypes in the lateral parabrachial nucleus (LPBN) in the control of sodium appetite (i.e. NaCl consumption). Male Holtzman rats had cannulas implanted bilaterally into the LPBN for the injection of 5-HT receptor agonists and antagonists in conjunction with either acute fluid depletion or 24-h sodium depletion. Following these treatments, access to 0.3 M NaCl was provided and the intakes of saline and water were measured for the next 2 h. Bilateral injections of the 5-HT2A receptor antagonist, ketanserin or the 5-HT2C receptor antagonist, mianserin into the LPBN increased 0.3 M NaCl intake without affecting water intake induced by acute fluid-depletion. Bilateral injections of the 5-HT2B receptor agonist, BW723C86 hydrochloride, had no effect on 0.3 M NaCl or water intake under these conditions. Treatment of the LPBN with the 5-HT2B/2C receptor agonist, 2-(2-methyl-4-clorophenoxy) propanoic acid (mCPP) caused dose-related reductions in 0.3 M NaCl intake after 24 h sodium depletion. The effects of mCPP were prevented by pretreating the LPBN with the 5-HT2B/2C receptor antagonist, SDZSER082. Activation of 5-HT3 receptors by the receptor agonist, 1-phenylbiguanide (PBG) caused dose-related increases in 0.3 M NaCl intake. Pretreatment of the LPBN with the 5-HT3 receptor antagonist, 1-methyl-N-[8-methyl-8-azabicyclo (3.2.1)-oct-3-yl]-1H-indazole-3-carboxamide (LY-278,584) abolished the effects of PBG, but LY-278,584 had no effects on sodium or water intake when injected by itself. PBG injected into the LPBN did not alter intake of palatable 0.06 M sucrose in fluid replete rats. The results suggest that activation of the 5-HT2A and 5-HT2C receptor subtypes inhibits sodium ingestion. In contrast, activation of the 5-HT3 receptor subtype increases sodium ingestion. Therefore, multiple serotonergic receptor subtypes in the LPBN are implicated in the control of sodium intake, sometimes by mediating opposite effects of 5-HT. The results provide new information concerning the control of sodium intake by LPBN mechanisms.

Isoflurane Decreases Extracellular Serotonin in the Mouse Hippocampus

The serotonergic system may play a role during general anesthesia. Furthermore, alterations in serotonergic neurotransmission in the hippocampus have been linked to depression and anxiety as well as to changes in arousal and cognition. Little is known about the effects of volatile anesthetics on hippocampal serotonin (5-HT) levels. In this study we examined the effects of isoflurane on hippocampal 5-HT levels in mice. Adult male 129/SvEv mice were exposed to either isoflurane 1 or 1.5 minimum alveolar concentration (MAC) both in 40% O2 in air or to 40% O2 in air alone (control) for a period of 80 min, and hippocampal 5-HT levels were measured by microdialysis coupled with high performance liquid chromatography. Within 20-40 min of administration, both doses of isoflurane similarly produced a significant decrease in hippocampal 5-HT to 41.5% +/- 11.0% and 36.4% +/- 13.9% of the baseline level in the isoflurane 1 MAC and 1.5 MAC groups, respectively. Furthermore, when additional dialysates were obtained on termination of anesthesia in the isoflurane 1.5 MAC group, the decrease in extracellular 5-HT levels persisted for several hours. To determine if isoflurane-induced changes in extracellular 5-HT involve the serotonin transporter (SERT), similar microdialysis studies were performed in C57BL/6 wild-type (SERT +/+) and homozygous SERT knockout (SERT -/-) mice exposed to either 1 MAC isoflurane in 40% O2 in air or to 40% O2 in air alone for a period of 80 min. Isoflurane produced a significant decrease in hippocampal 5-HT in SERT +/+ and SERT -/-, and this decrease was larger in SERT -/- compared with SERT +/+: to 22.4% +/- 8.5% versus 50.2% +/- 17.4% of the baseline 5-HT level, respectively. These data suggest that isoflurane produces a decrease in hippocampal 5-HT, independent of SERT function.

Serotonin as a modulator of glutamate- and GABA-mediated neurotransmission: implications in physiological functions and in pathology

The neurotransmitter serotonin (5-HT), widely distributed in the central nervous system (CNS), is involved in a large variety of physiological functions. In several brain regions 5-HT is diffusely released by volume transmission and behaves as a neuromodulator rather than as a "classical" neurotransmitter. In some cases 5-HT is co-localized in the same nerve terminal with other neurotransmitters and reciprocal interactions take place. This review will focus on the modulatory action of 5-HT on the effects of glutamate and gamma-amino-butyric acid (GABA), which are the principal neurotransmitters mediating respectively excitatory and inhibitory signals in the CNS. Examples of interaction at pre-and/or post-synaptic levels will be illustrated, as well as the receptors involved and their mechanisms of action. Finally, the physiological meaning of neuromodulatory effects of 5-HT will be briefly discussed with respect to pathologies deriving from malfunctioning of serotonin system.

The role of central 5-HT3 receptors in vagal reflex inputs to neurones in the nucleus tractus solitarius of anaesthetized rats

Brainstem 5-hydroxytryptamine (5-HT, serotonin)-containing neurones modulate cardiovascular reflex responses but the differing roles of the many 5-HT receptors have not been thoroughly investigated. The present experiments on anaesthetized rats investigated the role of 5-HT3 receptors in modulating vagal afferent evoked activity of nucleus tractus solitarius (NTS) neurones. Recordings were made from 301 NTS neurones receiving an input at long (> 20 ms) minimum onset latency from stimulation of the vagus nerve. These included 140 neurones excited by activating non-myelinated cardiopulmonary afferents by right atrial injection of phenylbiguanide (PBG). Ionophoretic application of PBG, a highly selective 5-HT3 receptor agonist, significantly increased activity (from 2.4 +/- 0.4 to 5.5 +/- 0.8 spikes s(-1)) in 96 of 106 neurones tested and in all 17 neurones tested the increase in activity (3.4 +/- 1.1 to 7.0 +/- 1.9 spikes s(-1)) was significantly attenuated (3.0 +/- 0.9 to 3.8 +/- 1.1 spikes s(-1)) by the selective 5-HT3 receptor antagonist granisetron. Ionophoretic application of PBG potentiated responses to vagus nerve and cardiopulmonary afferent stimulation, and granisetron significantly attenuated this cardiopulmonary input (20.2 +/- 5.7 to 10.6 +/- 4.1 spikes burst(-1)) in 9 of 10 neurones tested. Ionophoretic application of AMPA and NMDA also excited NTS neurones and these excitations could be selectively antagonized by the non-NMDA and NMDA receptor antagonists DNQX and AP-5, respectively. At these selective currents, DNQX and AP-5 also attenuated PBG- and cardiopulmonary input-evoked increases in NTS activity. These data are consistent with the hypothesis that vagal inputs, including non-myelinated cardiopulmonary inputs to the NTS, utilize a 5-HT-containing pathway which activates 5-HT3 receptors. This excitatory response to 5-HT3 receptor activation may be partly a direct postsynaptic action but part may also be due to facilitation of the release of glutamate which in turn acts on either non-NMDA or NMDA receptors to evoke excitation.

Modulation of human 5-hydroxytryptamine type 3AB receptors by volatile anesthetics and n-alcohols

Functional 5-hydroxytryptamine type 3 (5-HT3) receptors can be formed by 5-HT3A subunits alone or in combination with the 5-HT3B subunit, but only the 5-HT3A receptor has been previously studied with respect to the modulation by volatile anesthetics and n-alcohols. Using two-electrode voltage-clamp, we show for the first time the modulation of heteromeric human (h)5-HT3AB receptors, expressed in Xenopus oocytes, by a series of n-alcohols and halogenated volatile anesthetics. At twice their anesthetic concentration, compounds having a molecular volume of less than 110 A3 enhanced submaximal 5-HT-evoked current. Compounds larger than 110 A3 inhibited submaximal 5-HT-evoked current. In experiments examining 5-HT concentration-response relationships, chloroform and butanol caused a slight decrease in the 5-HT EC50. Sevoflurane and octanol inhibited 5-HT-evoked current at all 5-HT concentrations tested but had no effect upon the 5-HT EC50. Compared with previous data on homomeric h5-HT3A receptors, the presence of the h5-HT3B subunit reduces the enhancement of h5-HT3 receptors by smaller halogenated volatile anesthetics and n-alcohols. In summary, these results suggest that heteromeric h5-HT3AB receptors are modulated by halogenated volatile anesthetics at clinically relevant concentrations, in addition to n-alcohols, suggesting that these receptors may be another physiological target for these compounds. The modulation is dependent upon the molecular volume of the compound, further supporting the concept of an anesthetic binding pocket of limited volume common on other Cys-loop ligand-gated ion channels. Incorporation of the 5-HT3B subunit alters either the anesthetic binding site or the allosteric interactions between anesthetic binding and channel opening.

Vagal control of the heart: central serotonergic (5-HT) mechanisms

Cardiac vagal preganglionic neurones (CVPNs) are located within the dorsal vagal nucleus (DVN) and the nucleus ambiguus (nA). In mammals, CVPNs within the nA have small myelinated axons and mediate major chronotropic effects, those in the DVN have non-myelinated axons and mediate smaller chronotropic, dromotropic and inotropic effects. Numerous studies demonstrate important influences of serotonin (5-HT) at multiple sites controlling autonomic outflows including the nucleus tractus solitarius (NTS) where cardiorespiratory afferent fibres terminate, and the CVPNs and rostral ventrolateral medulla (RVLM), the location of sympathetic premotor neurones. We have demonstrated roles for some of the numerous 5-HT receptor subtypes (5-HT1, 5-HT2, 5-HT3, 5-HT4 and 5-HT7) in brainstem regions involved in cardiac control. Intracisternal application of selective ligands was used to study the effect of 5-HT receptors on heart rate and its reflex control. Further electrophysiological studies were also carried out to delineate their location and the mechanisms of action of these ligands. Blocking 5-HT1A receptors attenuated bradycardias evoked by stimulating baroreceptor and cardiopulmonary afferents but not arterial chemoreceptors, whereas antagonizing 5-HT7 receptors markedly attenuated all these reflex bradycardias. Within the DVN, nA and NTS, activation of 5-HT1A receptors could excite or inhibit neurones. In the NTS 5-HT2 receptors also had variable effects; 5-HT2B receptors excite and 5-HT2C receptors inhibit. Antagonism of 5-HT3 receptors attenuated upper airway and cardiopulmonary reflex bradycardias; this is compatible with data showing that 5-HT3 receptors excite DVN and NTS neurones by a glutamate-dependent mechanism. The origin of the glutamate (neuronal or glial) remains unresolved but glia are a possibility as baroreceptor-sensitive NTS neurones receive few direct 5-HT-containing synaptic contacts. Thus, 5-HT plays a critical role in the control of vagal outflow to the heart; however, why so many different receptors are involved, and their relative functional roles, remains unresolved.

5-HT3A receptor subunits in the rat medial nucleus of the solitary tract: subcellular distribution and relation to the serotonin transporter

The 5-hydroxytryptamine 3 (5HT3) receptor is a serotonin-gated ion channel implicated in reflex regulation of autonomic functions within the nucleus of the solitary tract (NTS). To determine the relevant sites for 5-HT3 receptor mediated transmission in this region, we used electron microscopic immunocytochemistry to examine the subcellular distribution of the 5HT3 receptor subunit A (5HT3A) in relation to the serotonin transporter (SERT) in the intermediate medial NTS (mNTS) of rat brain. The 5HT3A immunolabeling was detected in many axonal as well as somatodendritic and glial profiles. The axonal profiles included small axons and axon terminals in which the 5HT3A immunoreactivity was localized to membranes of synaptic vesicles and extrasynaptic plasma membranes. In dendrites and glia, the 5HT3A immunoreactivity was located on the plasma membranes or in association with membranous cytoplasmic organelles. The dendritic plasmalemmal 5HT3A labeling was prominent within and near excitatory-type synapses from terminals including those that resemble vagal afferents. The 5HT3A-labeled glial processes apposed 5HT3A-immunoreactive axonal and dendritic profiles, some of which also contained SERT. Terminals containing 5-HT3A and/or SERT were among those providing synaptic input to 5HT3A-labeled dendrites. Thus, 5HT3A has a subcellular distribution consistent with the involvement of 5-HT3 receptors in modulation of both presynaptic release and postsynaptic responses of mNTS neurons, some of which are serotonergic. The results further suggest that the neuronal as well as glial 5HT3 receptors can be activated by release of serotonin from presynaptic terminals or by diffusion facilitated by SERT distribution at a distant from the synapse.

Substance P potentiates 5-HT3 receptor-mediated current in rat trigeminal ganglion neurons

The present study aimed to investigate the interaction between the coexistent SP receptor and 5-HT3 receptor in trigeminal ganglion (TG) neurons using whole-cell patch clamp technique. The majority of the neurons examined responded to 5-HT with an inward current (I5-HT) (78.2%, 79/101) that could be blocked by 5-HT3 receptor antagonist, ICS-205,930. The I5-HT was potentiated by preapplication of SP (10(-10) to 10(-8) M) in most 5-HT-sensitive cells(78.5%, 62/79). Coapplication of SP and GR-82334, antagonist of NK1 receptor, had no enhancing effect on I5-HT. The concentration-response curves for 5-HT with and without SP preapplication show that: (1) the threshold 5-HT concentrations with and without SP preapplication are basically the same, while SP preapplication increased the maximal value of I5-HT by 38.0% of its control; (2) the EC50 values of the curves with and without SP pretreatment are very close, i.e. 1.89 x 10(-5) M and 2.08 x 10(-5) M (P > 0.1; n = 9), respectively. Intracellular dialysis of GDP-beta-S, a non-hydrolyzable GDP analog, and GF-109203X, a selective protein kinase C inhibitor, removed the SP potentiation of I5-HT. These results may offer a clue to understanding the mechanism underlying the generation and/or regulation of peripheral pain caused by tissue damage inflammation, etc.

Ionotropic NMDA receptor evokes an excitatory response in superior salivatory nucleus neurons in anaesthetized rats

Extracellular recordings were taken from preganglionic superior salivatory nucleus (SSN) neurons projecting to submandibular and intra-lingual ganglia, in order to study the action of SSN neurons resulting from ionophoretic application of ionotropic NMDA receptor agonist in urethane-chloralose anaesthetized rats. Single SSN neurons were identified by their antidromic spike responses following stimulation of the chorda-lingual nerve (CLN), chorda tympani branches (CTBs) and the lingual nerve (LN). About one-third (33%, 10/30) of the identified SSN neurons were induced to fire by ionophoretic application of the NMDA receptor agonists used, dl-homocysteic acid (DLH) and N-methyl-D-aspartic acid (NMDA). More than half exhibited firing at high frequencies, often exceeding 40 Hz. About one-fifth (20%; 6/30) of the identified SSN neurons exhibited orthodromic spike responses to the combination of NMDA receptor agonist application and sensory nerve (CLN or LN) stimulus. These excitatory responses evoked by application of NMDA receptor agonist were attenuated (n = 4) by ionophoretic application of DL-2-amino-5-phosphonovaleric acid (AP5; NMDA receptor antagonist). About half (47%) of the neurons did not respond to any combination of NMDA receptor agonist and sensory nerve stimuli. No differences were observed between SSN neurons with B fibre axons and those with C fibre axons in response to ionophoresis of the NMDA receptor agonists. The NMDA-sensitive neurons, which exhibited high frequency firing, were predominantly found in the rostral part of the SSN. In summary, activation of ionotropic NMDA receptors exerts an excitatory effect on about half of the SSN neurons. These data support the view that NMDA receptors are involved in information processing and transmission on SSN neurons.

Role of nucleus tractus solitarius 5-HT3 receptors in the defense reaction-induced inhibition of the aortic baroreflex in rats

Different stressful conditions elicit a typical behavior called the defense reaction. Our aim was to determine whether 5-HT3 receptors in the nucleus tractus solitarius (NTS) are involved in 1) the inhibition of the baroreflex bradycardia and 2) the rise in blood pressure, which are known to occur during the defense reaction. In urethane-anesthetized rats, the defense reaction was elicited by electrical stimulation of the dorsomedial nucleus of the hypothalamus (DMH) or the dorsal part of the periaqueductal gray (dPAG). Direct electrical stimulation of the aortic depressor nerve was used to trigger the typical baroreflex responses. Aortic stimulation at high (100-150 microA) and low (50-90 microA) intensity produced a decrease in heart rate of -39 to -44% (relative to baseline, Group 1 responses, n = 113) and -19 to -24% (Group 2 responses, n = 43), respectively. In spontaneously breathing rats, Group 1 and Group 2 bradycardiac responses were inhibited during DMH (-75 +/- 4% and -96 +/- 4%, n = 38 and n = 11, respectively), as well as dPAG (-81 +/- 3% and -95 +/- 4%, n = 36 and n = 10, respectively) stimulation. The aortic baroreflex bradycardia was hardly affected by DMH or dPAG stimulation when bicuculline (5 pmol), a specific GABAA receptor antagonist, had previously been microinjected into the NTS. Likewise, NTS microinjections of granisetron, a specific 5-HT3 receptor antagonist, prevented, in a dose-dependent manner, the baroreflex bradycardia inhibition. In addition, intra-NTS granisetron did not affect the rise in blood pressure induced by either site stimulation. These data show that 5-HT3 receptors in the NTS are involved in the GABAergic inhibition of the aortic baroreflex bradycardia, but not in the rise in blood pressure, occurring during the defense reaction elicited by DMH or dPAG stimulation.

Involvement of NMDA receptors in the pressor response to microinjection of 5-HT3 agonist into the NTS of awake rats

Previous studies have shown that the activation of 5-HT3 receptors in the nucleus tractus solitarii (NTS) increases the baseline mean arterial pressure (MAP). In the present study, we evaluated the possible involvement of NMDA receptors in this pressor response. Four days before the experiments, under tribromoethanol anesthesia, rats received two guide cannulas in the direction of the NTS, and 1 day before the experiments, under tribromoethanol anesthesia, the femoral artery was cannulated for pulsatile arterial pressure (PAP), MAP, and heart rate (HR) measurements. On the day of the experiments, 2-methyl-serotonin, a 5-HT3 agonist, was microinjected into the NTS after microinjection of saline or AP-5, a selective NMDA receptor antagonist. Microinjection of 2-methyl-serotonin (5 nmol/50 nl) into the NTS after the vehicle (saline) produced a significant increase in MAP (+ 20 +/- 5 mm Hg, n = 8) while microinjection of 2-methyl-serotonin after microinjection of AP-5 (10 nmol/50 nl) produced no change in baseline MAP (-1 +/- 3 mm Hg, n = 11). Microinjection of AP-5 into the NTS produced no significant changes in the baseline MAP and HR. The data show that the increase in MAP in response to microinjection of a 5-HT3 agonist into the NTS is dependent on NMDA receptors.

Central cardiovascular regulation and 5-hydroxytryptamine receptors

Evidence is provided to support the view that central 5-HT(1A) and 5-HT(2) receptors are the major receptor subtypes important in cardiovascular regulation. Data are also provided to implicate 5-HT(1B/1D/1F) receptors in central cardiovascular regulation. Activation of 5-HT(2) receptors generally causes sympathoexcitation and a rise in blood pressure and this is mainly mediated by 5-HT(2A) receptors. However, presympathetic vasomotor neurones located in the hindbrain (RVLM), controlling sympathetic outflow to the heart, are not activated in the same way as other presympathetic vasomotor neurones, although activation of 5-HT(2) receptors located in the midbrain can activate sympathetic outflow to the heart. Furthermore, at least in the rat, these midbrain 5-HT(2A) receptors are also responsible for the release of vasopressin by activation of a central angiotensinergic pathway. The ability of vasopressin directly and/or indirectly to modify renal sympathetic outflow involves the activation of central 5-HT(2B) receptors, which in turn, when activated via the i.c.v. route, can cause selective renal sympathoexcitation. Evidence is also provided which indicates that the reflex control of parasympathetic outflow to the heart and to other organs involves central 5-HT(1A) receptors located in the vicinity of these preganglionic vagal neurones. Finally, 5-HT(3) receptors are implicated in the afferent regulation of central sympathetic and parasympathetic tone.

The role of central 5-HT(1A) receptors in the control of B-fibre cardiac and bronchoconstrictor vagal preganglionic neurones in anaesthetized cats

1. Experiments were performed to determine whether 5-HT(1A) receptors (a) modulate the activity of cardiac and bronchoconstrictor vagal preganglionic neurones (CVPNs and BVPNs) in the nucleus ambiguus (NA) and (b) are involved in pulmonary C-fibre afferent-evoked excitation of CVPNs, by right-atrial injections of phenylbiguanide (PBG). These experiments were carried out on alpha-chloralose-anaesthetized, artificially ventilated and atenolol (1 mg kg(-1))-pretreated cats. 2. The ionophoretic application of 8-OH-DPAT (a selective 5-HT(1A) receptor agonist) influenced the activity of 16 of the 19 CVPNs tested. 8-OH-DPAT tended to cause inhibition at low currents (40 nA) and excitation at high currents (120 nA). The activity of 15 of these neurones increased in response to the application of 8-OH-DPAT. In six of the CVPNs tested, this excitatory action of 8-OH-DPAT was attenuated by co-application of the selective 5-HT(1A) receptor antagonist WAY-100635. 3. The pulmonary C-fibre afferent-evoked excitation of eight CVPNs was attenuated by ionophoretic application of WAY-100635. 4. In three out of four CVPNs, the ionophoretic application of PBG caused excitation. 5. In five out of the nine identified BVPNs that were tested with ionophoretic application of 8-OH-DPAT, excitation was observed that was attenuated by WAY-100635. 6. WAY-100635 (I.V. or intra-cisternally) also reversed bradycardia, hypotension and the decrease in phrenic nerve activity evoked by the I.V. application of 8-OH-DPAT (42 microg kg(-1)). 7. In conclusion, the data indicate that 5-HT(1A) receptors located in the NA play an important role in the reflex activation of CVPNs and BVPNs, and support the view that overall, these receptors play a fundamental role in the reflex regulation of parasympathetic outflow.

Postsynaptic target regulates functional responses induced by 5-HT3 serotonin receptors on axonal varicosities of NG108-15 hybrid neuroblastoma cells

Rat brain presynaptic 5-HT3 serotonin receptors, members of the ligand-gated ion channel superfamily, induce changes in nerve terminal [Ca2+]i in a manner distinct from that found for somatic 5-HT3 receptors. Here, we assessed the role of postsynaptic target in regulating the nature of presynaptic receptor-induced responses, using the hybrid neuroblastoma cell line NG108-15 as a model neuronal system that expresses 5-HT3 receptors. Using immunocytochemistry, 5-HT3 receptors were found to be present on the presynaptic-like varicosities of differentiated NG108-15 cells, indicating that these receptors possess an inherent ability to localize to potential presynaptic sites. In the absence of postsynaptic target, 5-HT3 receptors localized to the varicosities induce rapid but transient changes in [Ca2+]i that were initiated by voltage-gated Ca2+ channels, as assessed using Ca2+ channel blockers, these properties being typical of those found for somatic 5-HT3 receptors. In co-cultures containing rat myotubes, with which NG108-15 cells form functional cholinergic synapses, the 5-HT3 receptor-induced changes in [Ca2+]i in the axonal varicosities shifted over time (three to 10 days) to that found for brain nerve endings: sustained responses that were insensitive to blockade by antagonists of voltage-gated Ca2+ channels. The effect of co-culturing myotubes with the NG108-15 cells was mimicked by conditioned media from myotube cultures. These results indicate that regulatory molecules from the target postsynaptic cell dictate the functional responses elicited by presynaptic 5-HT3 receptors. Because the target-induced changes required several days before they were evident, we hypothesize that changes in protein expression, perhaps the consequence of altered gene regulation, underlie the changes in the responses to 5-HT3 receptor activation in the axonal varicosities of this neuronal cell line.

An excitatory role for 5-HT in spinal inflammatory nociceptive transmission; state-dependent actions via dorsal horn 5-HT(3) receptors in the anaesthetized rat

The involvement of 5-HT(3) receptor mediated modulation of formalin and carrageenan induced inflammatory transmission was investigated. The effects of the selective 5-HT(3) receptor antagonist ondansetron on the electrically evoked responses of dorsal horn neurones in normal animals were compared to those following carrageenan. The effect of pre-treatment on the formalin response was also studied. Ondansetron had no significant effect on the electrically evoked responses of dorsal horn neurones in normal animals or following carrageenan induced inflammation, but significantly inhibited both phases of the formalin response. Our results suggest that 5-HT(3) receptors in the spinal cord have no significant role under normal conditions. However, during formalin (but not carrageenan) induced inflammation this system is activated, maintaining the response of nociceptive spinal neurones to peripheral formalin.

Effect of pulmonary C-fibre afferent stimulation on cardiac vagal neurones in the nucleus ambiguus in anaesthetized cats

It has been demonstrated previously that the vagal bradycardia evoked by activation of pulmonary C-fibres is not respiratory modulated. Experiments were carried out in alpha-chloralose anaesthetized cats to determine if these cardiac vagal preganglionic neurones (CVPNs) in the nucleus ambiguus (NA), which have respiratory modulated activity, can be activated when pulmonary C-fibre afferents are stimulated by right atrial injections of phenylbiguanide (PBG). Eleven CVPNs with B-fibre axons in the right cardiac vagal branches were identified and found to be localized within or ventrolateral to the nucleus ambiguus. Ionophoretic application of a high current of dl-homocysteic acid (DLH) induced a vagally mediated bradycardia and hypotension in six of eight sites from which CVPNs were recorded. The activity of B-fibre CVPNs, whether spontaneous (n = 4) or induced by ionophoresis of DLH (n = 7) was respiratory modulated, firing perferentially during post-inspiration and stage 2 expiration. This activity also correlated with the rising phase of the arterial blood pressure wave consistent with these CVPNs receiving an arterial baroreceptor input. Right atrial injections of PBG excited nine of eleven CVPNs tested. In eight of these activated neurones the onset latency of the excitation was within the pulmonary circulation time, consistent with being activated only by pulmonary C-fibre afferents. In two neurones the PBG-evoked excitation still occurred when central inspiratory drive was inhibited, as indicated by the disappearance of phrenic nerve activity. In conclusion, B-fibre respiratory modulated CVPNs can be activated following stimulation of pulmonary C-fibre afferents.

Characterization of the in vitro effects of 5-hydroxytryptamine (5-HT) on identified neurones of the rat dorsal motor nucleus of the vagus (DMV)

1 Whole cell patch clamp techniques were used on thin brainstem slices to investigate the effects of 5-hydroxytryptamine (5-HT) on gastrointestinal-projecting dorsal motor nucleus of the vagus (DMV) neurones. Neurones were identified as projecting to the stomach (n=122) or intestine (n=84) if they contained the fluorescent tracer Dil after it had been applied to the gastric fundus, corpus or antrum/pylorus or to the duodenum or caecum. 2 A higher proportion of intestinal neurones (69%) than gastric neurones (47%) responded to 5-HT with a concentration-dependent inward current which was antagonized fully by the 5-HT2A receptor antagonist ketanserin (1 microM). 3 Stimulation of the nucleus tractus solitarius (NTS) induced inhibitory synaptic currents that were reduced in amplitude by application of the 5-HT1A receptor agonist 8-OHDPAT (1 microM) or the 5-HT1A/1B receptor agonist TFMPP (1 microM) in 61% and 52% of gastric- and intestinal-projecting neurones, respectively. 5-HT also significantly reduced the frequency but not the amplitude of spontaneous inhibitory currents. 4 These data show that 5-HT excites directly a larger proportion of intestinal projecting neurones than gastric-projecting neurones, as well as inhibiting synaptic transmission from the NTS to the DMV. These data imply that the response to DMV neurones to 5-HT may be determined and classified by their specific projections.

Electrophysiological and morphological heterogeneity of rat dorsal vagal neurones which project to specific areas of the gastrointestinal tract

1. The electrophysiological properties of rat dorsal motor nucleus of the vagus (DMV) neurones (n = 162) were examined using whole cell patch clamp recordings from brainstem slices. Recordings were made from DMV neurones whose projections to the gastrointestinal tract had been identified by previously applying fluorescent retrograde tracers to the gastric fundus, corpus or antrum/pylorus, or to the duodenum or caecum. 2. The neuronal groups were markedly heterogeneous with respect to several electrophysiological properties. For example, neurones which projected to the fundus had a higher input resistance (400 +/- 25 Momega), a smaller and shorter after-hyperpolarization (16.7 +/- 0.49 mV and 63.5 +/- 3.9 ms) and a higher frequency of action potential firing (19.3 +/- 1.4 action potentials s-1) following injection of depolarizing current (270 pA) when compared with caecum-projecting neurones (302 +/- 22 Momega; 23. 5 +/- 0.87 mV and 81.1 +/- 5.3 ms; 9.7 +/- 1.1 action potentials s-1; P < 0.05 for each parameter). Differences between neuronal groups were also apparent with respect to the distribution of several voltage-dependent potassium currents. Inward rectification was present only in caecum-projecting neurones, for example. 3. Neurones (n = 82) were filled with the intracellular stain Neurobiotin allowing post-fixation morphological reconstruction. Neurones projecting to the caecum had the largest cell volume (5238 +/- 535 microm3), soma area (489 +/- 46 microm2) and soma diameter (24.6 +/- 1.24 microm) as well as the largest number of dendritic branch segments (23 +/- 2). 4. In summary, these results suggest that DMV neurones are heterogeneous with respect to some electrophysiological as well as some morphological properties and can be divided into subgroups according to their gastrointestinal projections.