tropisetron and phenyl-biguanide

To determine the role of central serotonin 5-HT(3) receptors in respiratory motor control, respiratory motor bursts were recorded from hypoglossal (XII) nerve rootlets on isolated adult turtle brainstems during bath-application of 5-HT(3) receptor agonists and antagonists. mCPBG and PBG (5-HT(3) receptor agonists) acutely increased XII burst frequency and regularity, and decreased bursts/episode. Tropisetron and MDL72222 (5-HT(3) antagonists) increased bursts/episode, suggesting endogenous 5-HT(3) receptor activation modulates burst timing in vitro. Tropisetron blocked all mCPBG effects, and the PBG-induced reduction in bursts/episode. Tropisetron application following mCPBG application did not reverse the long-lasting (2h) mCPBG-induced decrease in bursts/episode. We conclude that endogenous 5-HT(3) receptor activation regulates respiratory frequency, regularity, and episodicity in turtles and may induce a form of respiratory plasticity with the long-lasting changes in respiratory regularity.

Topics: Action Potentials; Afferent Pathways; Animals; Biguanides; Dose-Response Relationship, Drug; Hypoglossal Nerve; In Vitro Techniques; Indoles; Physical Stimulation; Receptors, Serotonin, 5-HT3; Respiration; Respiratory Center; Serotonin 5-HT3 Receptor Agonists; Serotonin 5-HT3 Receptor Antagonists; Serotonin Agents; Time Factors; Tropanes; Tropisetron; Turtles

The aims of the present study were to investigate whether the activation of the 5-HT receptor subtypes (5-HT(4) and 5-HT(3)) acted significantly on the modification of the tetrodotoxin-resistant sodium current (I(NaR)) in small-sized rat trigeminal ganglion (TG) neurons and whether the inhibition of the transient K(+) current (I(A)) contributed to the excitability in those neurons. 5-HT applications in at concentrations ranging from 0.01-10 microM significantly increased the peak I(NaR). One micromolar 5-HT application caused the greatest increase in the peak I(NaR) amplitude accompanied by a hyperpolarizing shift in the activation curve. A similar modification of I(NaR) properties was also obtained via the application of the 5-HT(4) receptor agonist, RS 67333, in concentrations ranging from 0.001-1 microM. The largest effects of 5-HT (1 microM) and RS 67333 (0.1 microM) on the modification of I(NaR) were abolished by pretreatment with ICS 205-930 (a 5-HT(3/4) receptor antagonist, 10 microM), which showed no significant effect on the baseline I(NaR). However, ICS 205-930 application at 30 microM caused a significant decrease in the baseline I(NaR). Phenylbiguanide (a 5-HT(3) receptor agonist) did not significantly alter I(NaR) properties when applied in concentrations ranging from 1 to 100 microM. The application of 0.1 microM RS 67333 decreased the transient K(+) current (I(A)) by approximately 31%. The threshold for action potential generation was significantly lower after the application of 0.1 microM RS 67333. Furthermore, 0.1 microM RS 67333 application increased the number of action potentials and the resting membrane potential got more positive, but it decreased the duration of depolarization phase of action potential. In addition, neither the additional application of 1 microM 5-HT in the presence of 10 microM forskolin, a stimulator of adenylyl cyclase, nor the opposite applications of 5-HT and forskolin caused the enhancement of increased I(NaR), which indicates the presence of an 'occluding effect.' These results suggest that the 5-HT-induced modification of I(NaR) is mediated by the activation of 5-HT(4) receptors, involving a cAMP-dependent signaling pathway, and that the inhibition of I(A) following the application of a 5-HT(4) receptor agonist also contributes to the increased number of action potentials.

Topics: Aniline Compounds; Animals; Animals, Newborn; Antibodies, Heterophile; Biguanides; Biophysics; Cells, Cultured; Dose-Response Relationship, Drug; Electric Stimulation; Indoles; Membrane Potentials; Neurons; Patch-Clamp Techniques; Piperidines; Rats; Rats, Wistar; Serotonin; Serotonin Antagonists; Serotonin Receptor Agonists; Sodium Channel Blockers; Tetrodotoxin; Trigeminal Ganglion; Tropisetron

The effect of iontophoretic application of the 5-HT3 receptor agonist, phenylbiguanide (PBG), on the excitation of the trigeminal spinal nucleus oralis (TSNO) neurons to tooth-pulp (TP) stimulation was examined. The PBG application inhibited the TP-evoked TSNO neuronal excitation, and this inhibition was completely blocked by co-application of a GABAA receptor antagonist, bicuculline. The results suggest that the activation of 5-HT3 receptors elicits GABA release in the TSNO.

Topics: Action Potentials; Animals; Bicuculline; Biguanides; Dental Pulp; Dose-Response Relationship, Drug; Drug Interactions; GABA Antagonists; gamma-Aminobutyric Acid; Indoles; Interneurons; Iontophoresis; Male; Rats; Rats, Wistar; Receptors, Serotonin, 5-HT3; Serotonin Antagonists; Serotonin Receptor Agonists; Trigeminal Nucleus, Spinal; Tropisetron

Whole-cell voltage-clamp recordings were performed to investigate the serotonergic modulation of neurotransmitter release onto rat area postrema neurons in vitro. The bath application of serotonin (5-HT; 50 microM) or phenylbiguanide (PBA; 50 microM), a potent 5-HT3 receptor agonist, increased the frequency of spontaneous excitatory postsynaptic currents (sEPSCs) or miniature EPSCs (mEPSCs) in 35 of 83 neurons (42%). These increases occurred in all electrophysiological cell classes. No cells exhibited a decrease in EPSC frequency. The majority of responding cells showed no inward currents during the application of serotonergic agonists (n = 34/35). However, the amplitude of mEPSCs was increased in 11/11 cells with 5-HT or 3/11 cells with PBA. ICS-205,930, a potent 5-HT3 receptor antagonist, markedly suppressed the 5-HT-induced facilitation of sEPSCs (n = 5) or mEPSCs (n = 5). An increase in the frequency of mEPSCs after PBA exposure was found, even with media containing Cd2+ (50 microM) or zero Ca2+. mEPSCs and evoked EPSCs were completely blocked in media containing the non-NMDA ionotropic receptor antagonist, CNQX (10 microM), indicating that EPSCs were glutamate events. These results suggest that glutamate release is increased in the area postrema by presynaptic 5-HT3 receptor activation. Furthermore, we present evidence that 5-HT3 receptor activation may be able to directly release glutamate from terminals, bypassing a requirement for voltage-dependent calcium entry into terminals. Such a mechanism may contribute to the chemosensitive function of area postrema neurons.

Topics: 6-Cyano-7-nitroquinoxaline-2,3-dione; Animals; Antiemetics; Area Postrema; Biguanides; Excitatory Amino Acid Agonists; Excitatory Postsynaptic Potentials; Glutamic Acid; In Vitro Techniques; Indoles; Neurons; Patch-Clamp Techniques; Rats; Rats, Sprague-Dawley; Receptors, Presynaptic; Serotonin; Serotonin 5-HT3 Receptor Agonists; Serotonin 5-HT3 Receptor Antagonists; Serotonin Receptor Agonists; Synaptic Transmission; Tropisetron

In rats, the mechanosensitive cardiorenal baroreflex influencing renal excretory function might be impaired by serotonin occurring in coronary arteries, e.g., in hypertension. Because the afferent limb of this reflex could be affected, we investigated the responses of nodose ganglion cells (one neuron of reflex) to osmotic, mechanical stress in presence or absence of the serotonin 5-HT(3) receptor agonist phenylbiguanide (PBG). Current-voltage relationships (from -100 to +50 mV) were obtained using cell patch recordings while the cells were exposed to control or hypoosmotic solutions to induce mechanical stress. This protocol was repeated after low doses of PBG (10 microM), angiotensin II (10 nM), or the stretch-activated channel blocker gadolinium (20 microM) were added to the extracellular medium (EM). Hypoosmotic EM induced significant changes in cellular conductance. The full-range current-voltage relationship allowed for the calculation of a mean reversal potential of -13 +/- 1.2 mV with respect to this change in cellular conductance (n = 44). This increase in conductance was impaired after addition of either PBG or gadolinium to the EM,which was statistically evaluated at a voltage of -80 mV, where influences of voltage-gated channels are not likely to interfere with the responses recorded. The serotonin 5-HT(3) receptor antagonist tropisetron (10 nM) prevented the PBG effect on conductance responses. Angiotensin II had no influence. Hence, serotonin might decrease the mechanical sensitivity of afferent cardiac nerves controlling renal sympathetic nerve activity.

Topics: Afferent Pathways; Angiotensin II; Animals; Biguanides; Cells, Cultured; Electric Conductivity; Gadolinium; Heart; Hypotonic Solutions; Indoles; Male; Mechanoreceptors; Neurons; Nodose Ganglion; Osmolar Concentration; Patch-Clamp Techniques; Rats; Rats, Sprague-Dawley; Receptors, Serotonin; Receptors, Serotonin, 5-HT3; Serotonin Antagonists; Serotonin Receptor Agonists; Stress, Mechanical; Tropisetron

The modulation of acetylcholine (ACh) release by 5-HT3 receptor activation was studied using in vivo microdialysis. Spontaneous and K+-stimulated ACh release were measured in frontoparietal cortex and hippocampus of freely moving rats. Two consecutive exposures to high K+ produced ACh release of similar magnitude. In the cortex, serotonin (5-HT) failed to alter spontaneous ACh release, but caused a concentration-dependent decrease of K+-evoked ACh release. Phenylbiguanide (PBG) and m-chlorophenylbiguanide, two selective 5-HT3 agonists, mimicked the 5-HT responses, but 8-hydroxy-2-(di-n-propylamino)tetralin, a selective 5-HT1A agonist, was without effect. However, PBG failed to modify K+-evoked ACh release from the hippocampus. Systemic and local administration of a highly selective 5-HT3 antagonist, tropisetron ((3-alpha-tropanyl)1H-indole-carboxylic acid ester) blocked the effect of both 5-HT and PBG. The inhibition of ACh release by PBG was sensitive to tetrodotoxin. These observations provide direct evidence that, in rat cortex, 5-HT modulates in-vivo release of ACh through activation of 5-HT3 receptors.

Topics: Acetylcholine; Animals; Biguanides; Cerebral Cortex; Hippocampus; Indoles; Male; Potassium; Rats; Rats, Wistar; Serotonin Antagonists; Serotonin Receptor Agonists; Tetrodotoxin; Tropisetron

1. Extracellular recording were made from 141 vagal preganglionic neurones in the dorsal vagal nucleus (DVN). The effects of ionophoretic administration of 5-hydroxytryptamine (5-HT), the 5-HT3 receptor agonist, phenylbiguanide (PBG) and the antagonists, granisetron and tropisetron (ICS 205-930) on these vagal preganglionic neurones were studied in pentobarbitone sodium anaesthetized rats. 2. Ionophoretic application of 5-HT at low currents (< 10 nA) increased the activity in 46 (73%) of 63 neurones tested. Application of granisetron (5-20 nA) or tropisetron (5-20 nA) abolished or attenuated the 5-HT excitatory responses in 8 out of 11 and 5 out of 5 neurones respectively. At the currents used, neither antagonist had any effect on baseline firing rate. 3. Ionophoresis of the selective 5-HT3 receptor agonist, phenylbiguanide (0-40 nA) excited 54 (82%) of the 66 vagal neurones tested, whilst the remaining 12 neurones were unaffected. 4. Granisetron applied either ionophoretically (8/11) or intravenously (3/3),abolished or attenuated the excitations evoked by PBG. Similarly, tropisetron administered either ionophoretically (2/3) or intravenously (2/2), attenuated the PBG excitation. In contrast, the PBG excitations were unaffected by the 5-HT2 receptor antagonist, cinanserin (2/2), and the selective 5-HT1A receptor antagonist, WAY- 100802 (6/6). 5. In conclusion, excitation of vagal preganglionic neurones evoked by ionophoretic application of 5- HT is mediated in part by 5-HT3 receptors and activation of 5-HT3 receptors on and/or in the vicinity of vagal motoneurones causes excitation of these neurones.

Topics: Anesthesia, General; Animals; Biguanides; Extracellular Space; Granisetron; Indoles; Iontophoresis; Male; Motor Neurons; Nerve Fibers; Rats; Rats, Sprague-Dawley; Receptors, Serotonin; Serotonin; Serotonin Antagonists; Serotonin Receptor Agonists; Tropisetron; Vagus Nerve

The 5-HT3-receptor antagonist, ondansetron, has been shown to have positive effects in selected in-vivo models of memory impairment and anxiety. The exact mechanisms underlying such bioactivities are unknown. In the present work, an 86Rb efflux bioassay was used to show that ondansetron has a unique ability to block voltage-gated potassium channels in TE671 human neuroblastoma cells. This intrinsic potassium-channel-blocking (KCB) property is relatively weak (IC50 20 microM), but is not shared by other 5-HT3-receptor ligands including zatosetron, MDL 72222, LY 278, 584, zacopride, 1-phenylbiguanide, and ICS 205-930 (tropisetron). Pre-incubation of the target neuroblastoma cells with several 5-HT-receptor ligands including 5-hydroxytryptamine, 8-OH-DPAT, ketanserin, 2-methyl-5-HT, as well as a number of potent 5-HT3 agonists and antagonists and two selective neurotoxins, failed to abolish the KCB action of ondansetron. A preliminary structure-activity relationship analysis indicates that the KCB activity of ondansetron is almost entirely attributable to its structural nucleus, 2,3-dihyro-9-methyl-4(1H)-carbazolone. It is hypothesized that the KCB action of ondansetron is mediated through receptors other than 5-HT3 receptors. The KCB activity of ondansetron may be a significant factor in the in-vivo cognition-enhancing activities of this compound, conceivably due to depolarization of the hippocampal synaptic membranes and a consequent augmentation of neurotransmission.

Topics: Anti-Anxiety Agents; Benzamides; Benzofurans; Biguanides; Bridged Bicyclo Compounds, Heterocyclic; Humans; Hypoglycemic Agents; Indazoles; Indoles; Neuroblastoma; Neurotoxins; Ondansetron; Potassium Channels; Receptors, Serotonin; Receptors, Serotonin, 5-HT3; Rubidium Radioisotopes; Serotonin Antagonists; Serotonin Receptor Agonists; Structure-Activity Relationship; Tropanes; Tropisetron; Tumor Cells, Cultured

We have investigated the behavioral effect of the 5-hydroxytryptamine3 (5-HT3) receptor agonists 1-phenylbiguanide (PBG) and m-chlorophenylbiguanide (mCPBG) in rats after i.p. and i.c.v. injection. It was hoped that this approach may provide an alternative means of studying the role of 5-HT3 receptors on animal behavior, for the majority of related studies have used antagonists at this subtype. Both PBG (3-60 mg/kg, i.p.) and mCPBG (1-30 mg/kg i.p.) produced abdominal constrictions, writhing and salivation in some, but not all, rats. The most marked behaviors were seen after mCPBG (30 mg/kg, i.p.), where paw shakes and chin rubbing was also recorded. Almost certainly as a consequence of these behaviors, PBG (3-30 mg/kg, i.p.) and mCPBG (0.3-10 mg/kg, i.p.) produced a conditioned place aversion. Pretreatment with the 5-HT3 antagonists ondansetron (0.01-0.1 mg/kg, s.c.), ICS205-930 and quaternized ICS205-930 (both 0.1 mg/kg, i.p.) blocked the PBG (30 mg/kg, i.p.)-induced place aversion. PBG (30 mg/kg, i.p.) and mCPBG (10 mg/kg, i.p.) also produced a conditioned taste aversion. The central administration of PBG (1-30 micrograms, i.c.v.) and mCPBG (0.1-10 micrograms, i.c.v.) enhanced locomotor- and gnawing-related behavior, although the effects with PBG seemed more consistent. These PBG (10 micrograms, i.c.v.)-induced behaviors were completely blocked by haloperidol (0.01-0.1 mg/kg, s.c.). In contrast, ondansetron (0.0001-1 mg/kg, s.c.) and ICS205-930 (0.1 mg/kg, i.p.) produced only a mild and inconsistent attenuation of these responses. PBG (1-30 micrograms, i.c.v.) failed to produce any place conditioning (i.e., neither a preference nor aversion was found). It is concluded that activation of peripheral 5-HT3 receptors leads to aversive-type behaviors, which may be related to gastrointestinal discomfort or malaise. In contrast, central injection of PBG and mCPBG produced a range of dopamine-related behaviors; however, a 5-HT3 receptor involvement is unclear. Because both PBG and mCPBG have dopamine releasing properties, the use of 5-HT3 agonists lacking such effects and/or the use of more discrete microinjection studies are needed to more clearly elucidate the roles of 5-HT3 receptors in the central nervous system.

Topics: Animals; Behavior, Animal; Biguanides; Conditioning, Psychological; Dopamine; Haloperidol; Indoles; Injections, Intraventricular; Male; Rats; Rats, Wistar; Receptors, Serotonin; Serotonin Receptor Agonists; Tropisetron

The ability of 5-HT3 receptor agonists to modulate the resting efflux or K(+)-evoked release of [3H]5-HT from superfused synaptosomes from the spinal cord of the rat was investigated. Phenylbiguanide did not alter the resting efflux of [3H]5-HIAA or [3H]5-HT or modify the K(+)-evoked release of [3H]5-HT. 2-Methyl-5-HT (10 microM) caused an increase in resting efflux of [3H]5-HIAA, an effect that was blocked by the inhibitor of the uptake of 5-HT fluoxetine. No effect on K(+)-evoked release of tritium was observed. Bufotenine (100-1000 nM) increased the resting efflux of [3H]5-HT and [3H]5-HIAA. These effects were not antagonized by the 5-HT3 antagonist ICS 205-930 but were antagonized by fluoxetine. The drug ICS 205-930 (1 microM) did not alter resting efflux or block the ability of serotonin (30 and 100 nM) to decrease K(+)-evoked release of tritium. Quipazine, a potent antagonist of peripheral 5-HT3 receptors (subnanomolar concentrations), was also unable to alter resting or K(+)-evoked release of [3H]5-HT. It did, however, attenuate the inhibitory effect 5-HT on K(+)-evoked release. The concentrations required were in the micromolar range, consistent with the ability of the drug to antagonize the 5-HT1B autoreceptor. These results support the idea that 5-HT3 receptors do not act as nerve terminal autoreceptors in the spinal cord of the rat.

Topics: Animals; Biguanides; Bufotenin; Hydroxyindoleacetic Acid; In Vitro Techniques; Indoles; Male; Potassium; Quipazine; Rats; Rats, Inbred Strains; Receptors, Serotonin; Serotonin; Serotonin Antagonists; Spinal Cord; Synaptosomes; Tropisetron