5-HT3 receptors induce rises in cytosolic and nuclear calcium in NG108-15 cells via calcium-induced calcium release

Effect of ramosetron, a 5-HT3 receptor antagonist on the severity of seizures and memory impairment in electrical amygdala kindled rats

The entorhinal cortex (EC) plays a pivotal role in epileptogenesis and seizures. EC expresses high density of serotonergic receptors, especially 5-HT3 receptors. Cognitive impairment is common among people with epilepsy. The present study investigated the role of 5-HT3 receptor on the severity of seizures and learning and memory impairment by electrical kindling of amygdala in rats. The amygdala kindling was conducted in a chronic kindling manner in male Wistar rats. In fully kindled animals, ramosetron (as a potent and selective 5-HT3 receptor antagonist) was microinjected unilaterally (ad doses of 1, 10 or 100 µg/0.5 µl) into the EC 5 min before the novel object recognition (NOR) and Y-maze tests or kindling stimulations. Applying ramosetron at the concentration of 100 μg/0.5 µl (but not at 1 and 10 µg/0.5 µl) reduced afterdischarge (AD) duration and increased stage 4 latency in the kindled rats. Moreover, the obtained data from the NOR test showed that treatment by ramosetron (10 and 100 µg/0.5 µl) increased the discrimination index in the fully kindled animals. Microinjection of ramosetron (10 and 100 µg/0.5 µl) in fully kindled animals reversed the kindling induced changes in the percentage of spontaneous alternation in Y-maze task. The findings demonstrated an anticonvulsant role for a selective 5-HT3 receptor antagonist microinjected into the EC, therefore, suggesting an excitatory role for the EC 5-HT3 receptors in the amygdala kindling model of epilepsy. This anticonvulsive effect was accompanied with a restoring effect on cognitive behavior in NOR and Y-maze tests.

Mechanisms of Nausea and Vomiting: Current Knowledge and Recent Advances in Intracellular Emetic Signaling Systems

Nausea and vomiting are common gastrointestinal complaints that can be triggered by diverse emetic stimuli through central and/or peripheral nervous systems. Both nausea and vomiting are considered as defense mechanisms when threatening toxins/drugs/bacteria/viruses/fungi enter the body either via the enteral (e.g., the gastrointestinal tract) or parenteral routes, including the blood, skin, and respiratory systems. While vomiting is the act of forceful removal of gastrointestinal contents, nausea is believed to be a subjective sensation that is more difficult to study in nonhuman species. In this review, the authors discuss the anatomical structures, neurotransmitters/mediators, and corresponding receptors, as well as intracellular emetic signaling pathways involved in the processes of nausea and vomiting in diverse animal models as well as humans. While blockade of emetic receptors in the prevention of vomiting is fairly well understood, the potential of new classes of antiemetics altering postreceptor signal transduction mechanisms is currently evolving, which is also reviewed. Finally, future directions within the field will be discussed in terms of important questions that remain to be resolved and advances in technology that may help provide potential answers.

Intracellular Calcium Responses Encode Action Potential Firing in Spinal Cord Lamina I Neurons

Maladaptive plasticity of neurons in lamina I of the spinal cord is a lynchpin for the development of chronic pain, and is critically dependent on intracellular calcium signaling. However, the relationship between neuronal activity and intracellular calcium in these neurons is unknown. Here we combined two-photon calcium imaging with whole-cell electrophysiology to determine how action potential firing drives calcium responses within subcellular compartments of male rat spinal cord lamina I neurons. We found that single action potentials generated at the soma increase calcium concentration in the somatic cytosol and nucleus, and these calcium responses invade dendrites and dendritic spines by active backpropagation. Calcium responses in each compartment were dependent on voltage-gated calcium channels, and somatic and nuclear calcium responses were amplified by release of calcium from ryanodine-sensitive intracellular stores. Grouping single action potential-evoked calcium responses by neuron type demonstrated their presence in all defined types, as well as a high degree of similarity in calcium responses between neuron types. With bursts of action potentials, we found that calcium responses have the capacity to encode action potential frequency and number in all compartments, with action potential number being preferentially encoded. Together, these findings indicate that intracellular calcium serves as a readout of neuronal activity within lamina I neurons, providing a unifying mechanism through which activity may regulate plasticity, including that seen in chronic pain.SIGNIFICANCE STATEMENT Despite their critical role in both acute pain sensation and chronic pain, little is known of the fundamental physiology of spinal cord lamina I neurons. This is especially the case with respect to calcium dynamics within these neurons, which could regulate maladaptive plasticity observed in chronic pain. By combining two-photon calcium imaging and patch-clamp electrophysiological recordings from lamina I neurons, we found that action potential firing induces calcium responses within the somatic cytosol, nucleus, dendrites, and dendritic spines of lamina I neurons. Our findings demonstrate the presence of actively backpropagating action potentials, shifting our understanding of how these neurons process information, such that calcium provides a mechanism for lamina I neurons to track their own activity.

Granisetron Alleviates Alzheimer's Disease Pathology in TgSwDI Mice Through Calmodulin-Dependent Protein Kinase II/cAMP-Response Element Binding Protein Pathway

Alzheimer's disease (AD) is characterized by a compromised blood-brain barrier (BBB) and disrupted intracellular calcium homeostasis in the brain. Therefore, rectifying the BBB integrity and restoring calcium homeostasis could provide an effective strategy to treat AD. Recently, we developed a high throughput-screening assay to screen for compounds that enhance a cell-based BBB model integrity, which identified multiple hits among which is granisetron, a Food and Drug Administration approved drug. Here, we evaluated the therapeutic potential of granisetron against AD. Granisetron was tested in C57Bl/6J young and aged wild-type mice, and in a transgenic mouse model of AD namely TgSwDI for its effect on BBB intactness and amyloid-β (Aβ)-related pathology. Our study findings showed that granisetron enhanced BBB integrity in both aged and TgSwDI mice. This effect was associated with an overall reduction in Aβ load and neuroinflammation in TgSwDI mice brains. In addition, and supported by proteomics analysis, granisetron significantly reduced Aβ induced calcium influx in vitro, and rectified calcium dyshomeostasis in TgSwDI mice brains by restoring calmodulin-dependent protein kinase II/cAMP-response element binding protein pathway, which was associated with cognitive improvement. These results support granisetron repurposing as a potential drug to hold, slow, and/or treat AD.

Role of 5-HT3 receptor on food intake in fed and fasted mice

Background

Many studies have shown that 5-hydroxytryptamine (5-HT) receptor subtypes are involved in the regulation of feeding behavior. However, the relative contribution of 5-HT₃ receptor remains unclear. The present study was aimed to investigate the role of 5-HT₃ receptor in control of feeding behavior in fed and fasted mice.

Methodology/Principal Findings

Food intake and expression of c-Fos, tyrosine hydroxylase (TH), proopiomelanocortin (POMC) and 5-HT in the brain were examined after acute treatment with 5-HT₃ receptor agonist SR-57227 alone or in combination with 5-HT₃ receptor antagonist ondansetron. Food intake was significantly inhibited within 3 h after acute treatment with SR 57227 in fasted mice but not fed mice, and this inhibition was blocked by ondansetron. Immunohistochemical study revealed that fasting-induced c-Fos expression was further enhanced by SR 57227 in the brainstem and the hypothalamus, and this enhancement was also blocked by ondansetron. Furthermore, the fasting-induced downregulation of POMC expression in the hypothalamus and the TH expression in the brain stem was blocked by SR 57227 in the fasted mice, and this effect of SR 57227 was also antagonized by ondansetron.

Conclusion/Significance

Taken together, our findings suggest that the effect of SR 57227 on the control of feeding behavior in fasted mice may be, at least partially, related to the c-Fos expression in hypothalamus and brain stem, as well as POMC system in the hypothalamus and the TH system in the brain stem.

L-type calcium channels contribute to 5-HT3-receptor-evoked CaMKIIα and ERK activation and induction of emesis in the least shrew (Cryptotis parva)

Activation of serotonergic 5-HT3 receptors by its selective agonist 2-methyl serotonin (2-Me-5-HT) induces vomiting, which is sensitive to selective antagonists of both 5-HT3 receptors (palonosetron) and L-type calcium channels (LTCC) (amlodipine or nifedipine). Previously we demonstrated that 5-HT3 receptor activation also causes increases in a palonosetron-sensitive manner in: i) intracellular Ca(2+) concentration, ii) attachment of calmodulin (CaM) to 5-HT3 receptor, and iii) phosphorylation of Ca(2+)/calmodulin-dependent protein kinase IIα (CaMKIIα) and extracellular-signal-regulated kinase 1/2 (ERK1/2). Here, we investigate the role of the short-acting LTCC blocker nifedipine on 2-Me-5-HT-evoked intracellular Ca(2+) increase and on downstream intracellular emetic signaling, which have been shown to be coupled with 2-Me-5-HT׳s emetic effects in the least shrew. Using the cell-permeant Ca(2+) indicator fluo-4 AM, here we present evidence for the contribution of Ca(2+) influx through LTCCs (sensitive to nifedipine) in 2-Me-5-HT (1µM) -evoked rise in cytosolic Ca(2+) levels in least shrew brainstem slices. Nifedipine pretreatment (10mg/kg, s.c.) also suppressed 2-Me-5-HT-evoked interaction of 5-HT3 receptors with CaM as well as phosphorylation of CaMKIIα and ERK1/2 in the least shrew brainstem, and 5-HT3 receptors -CaM colocalization in jejunum of the small intestine. In vitro exposure of isolated enterochromaffin cells of the small intestine to 2-Me-5-HT (1µM) caused CaMKIIα phosphorylation, which was also abrogated by nifedipine pretreatment (0.1µM). In addition, pretreatment with the CaMKII inhibitor KN62 (10mg/kg, i.p.) suppressed emesis and also the activation of CaMKIIα, and ERK in brainstem caused by 2-Me-5-HT (5mg/kg, i.p.). This study provides further mechanistic explanation for our published findings that nifedipine can dose-dependently protect shrews from 2-Me-5-HT-induced vomiting.

Broad-spectrum antiemetic potential of the L-type calcium channel antagonist nifedipine and evidence for its additive antiemetic interaction with the 5-HT(3) receptor antagonist palonosetron in the least shrew (Cryptotis parva)

Cisplatin-like chemotherapeutics cause vomiting via release of multiple neurotransmitters (dopamine, serotonin (5-HT), or substance P (SP)) from the gastrointestinal enterochromaffin cells and/or the brainstem via a calcium dependent process. Diverse channels in the plasma membrane allow extracellular Ca(2+) entry into cells for the transmitter release process. Agonists of 5-HT3 receptors increase calcium influx through both 5-HT3 receptors and L-type Ca(2+) channels. We envisaged that L-type calcium agonists such as FPL 64176 should cause vomiting and corresponding antagonists such as nifedipine would behave as broad-spectrum antiemetics. Administration of FPL 64176 did cause vomiting in the least shrew in a dose-dependent fashion. Nifedipine and the 5-HT3 receptor antagonist palonosetron, potently suppressed FPL 64176-induced vomiting, while a combination of ineffective doses of these antagonists was more efficacious. Subsequently, we investigated the broad-spectrum antiemetic potential of nifedipine against diverse emetogens including agonists of serotonergic 5-HT3- (e.g. 5-HT or 2-Me-5-HT), SP tachykinin NK1- (GR73632), dopamine D2- (apomorphine or quinpirole), and cholinergic M1- (McN-A-343) receptors, as well as the non-specific emetogen, cisplatin. Nifedipine by itself suppressed vomiting in a potent and dose-dependent manner caused by the above emetogens except cisplatin. Moreover, low doses of nifedipine potentiated the antiemetic efficacy of non-effective or semi-effective doses of palonosetron against vomiting caused by either 2-Me-5-HT or cisplatin. Thus, our findings demonstrate that activation of L-type calcium channels causes vomiting, whereas blockade of these ion channels by nifedipine-like antagonists not only provides broad-spectrum antiemetic activity but can also potentiate the antiemetic efficacy of well-established antiemetics such as palonosetron. L-type calcium channel antagonists should also provide antiemetic activity against drug-induced vomiting as well as other emetogens including bacterial and viral proteins.

Serotonin 5-HT3 receptor-mediated vomiting occurs via the activation of Ca2+/CaMKII-dependent ERK1/2 signaling in the least shrew (Cryptotis parva)

Stimulation of 5-HT₃ receptors (5-HT₃Rs) by 2-methylserotonin (2-Me-5-HT), a selective 5-HT₃ receptor agonist, can induce vomiting. However, downstream signaling pathways for the induced emesis remain unknown. The 5-HT₃R channel has high permeability to extracellular calcium (Ca²⁺) and upon stimulation allows increased Ca²⁺ influx. We examined the contribution of Ca²⁺/calmodulin-dependent protein kinase IIα (Ca²⁺/CaMKIIα), interaction of 5-HT₃R with calmodulin, and extracellular signal-regulated kinase 1/2 (ERK1/2) signaling to 2-Me-5-HT-induced emesis in the least shrew. Using fluo-4 AM dye, we found that 2-Me-5-HT augments intracellular Ca²⁺ levels in brainstem slices and that the selective 5-HT₃R antagonist palonosetron, can abolish the induced Ca²⁺ signaling. Pre-treatment of shrews with either: i) amlodipine, an antagonist of L-type Ca²⁺ channels present on the cell membrane; ii) dantrolene, an inhibitor of ryanodine receptors (RyRs) Ca²⁺-release channels located on the endoplasmic reticulum (ER); iii) a combination of their less-effective doses; or iv) inhibitors of CaMKII (KN93) and ERK1/2 (PD98059); dose-dependently suppressed emesis caused by 2-Me-5-HT. Administration of 2-Me-5-HT also significantly: i) enhanced the interaction of 5-HT₃R with calmodulin in the brainstem as revealed by immunoprecipitation, as well as their colocalization in the area postrema (brainstem) and small intestine by immunohistochemistry; and ii) activated CaMKIIα in brainstem and in isolated enterochromaffin cells of the small intestine as shown by Western blot and immunocytochemistry. These effects were suppressed by palonosetron. 2-Me-5-HT also activated ERK1/2 in brainstem, which was abrogated by palonosetron, KN93, PD98059, amlodipine, dantrolene, or a combination of amlodipine plus dantrolene. However, blockade of ER inositol-1, 4, 5-triphosphate receptors by 2-APB, had no significant effect on the discussed behavioral and biochemical parameters. This study demonstrates that Ca²⁺ mobilization via extracellular Ca²⁺ influx through 5-HT₃Rs/L-type Ca²⁺ channels, and intracellular Ca²⁺ release via RyRs on ER, initiate Ca²⁺-dependent sequential activation of CaMKIIα and ERK1/2, which contribute to the 5-HT₃R-mediated, 2-Me-5-HT-evoked emesis.

Seizure susceptibility alteration through 5-HT(3) receptor: modulation by nitric oxide

There is some evidence that epileptic seizures could be induced or increased by 5-hydroxytryptamine (5-HT) attenuation, while augmentation of serotonin functions within the brain (e.g. by SSRIs) has been reported to be anticonvulsant. This study was performed to determine the effect of selective 5-HT(3) channel/receptor antagonist granisetron and agonist SR57227 hydrochloride on the pentylenetetrazole (PTZ)-induced seizure threshold in mice. The possible interaction of this effect with nitrergic system was also examined using the nitric oxide (NO) synthase inhibitor N(G)-nitro-l-arginine methyl ester (l-NAME) and the NO precursor l-arginine. SR57227 (10mg/kg, i.p.) significantly increased the seizure threshold compared to control group, while high dose granisetron (10mg/kg, i.p.) proved proconvulsant. Co-administration of sub-effective doses of the 5-HT(3) agonist with l-NAME (5 and 60mg/kg, i.p., respectively) exerted a significant anticonvulsive effect, while sub-effective doses of granisetron (3mg/kg) was observed to have a proconvulsive action with the addition of l-arginine (75mg/kg, i.p.). Our data demonstrate that enhancement of 5-HT(3) receptor function results in as anticonvulsant effect in the PTZ-induced seizure model, and that selective antagonism at the 5-HT(3) receptor yields proconvulsive effects. Furthermore, the NO system may play a role in 5-HT(3) receptor function.

Characteristics for enhanced response of serotonin-evoked ion dynamics in differentiated NG108-15 cells

Characteristics for the up-regulated response in the concentration of intracellular calcium ion ([Ca(2+)]( i )) and in the sodium ion (Na(+)) current by serotonin (5-HT) were investigated in differentiated neuroblastoma x glioma hybrid NG108-15 (NG) cells. The results for the changes in [Ca(2+)]( i ) by 5-HT were as follows, (1) The 5-HT-induced Ca(2+) response was inhibited by 3 x 10(-9) M tropisetron (a 5-HT(3) receptor blocker), but not by other types of 5-HT receptor blockers; (2) The 5-HT-induced Ca(2+) response was mainly inhibited by calciseptine (a L-type Ca(2+) blocker), but not by other types of Ca(2+) channel blockers or 10(-7) M TTX (a voltage-sensitive Na(+) channel blocker); (3) When the extracellular Na(+) was removed by exchange with choline chloride or N-methyl-D-glucamine, the 5-HT-induced Ca(2+) response was extremely inhibited. The results for the 5-HT-induced Na(+) current by the whole cell patch-clamp technique were as follows, (1) The 5-HT-induced Na(+) current in differentiated cells was significantly larger than that in undifferentiated cells; (2) The ED(50) value for 5-HT-induced Na(+) current in undifferentiated and differentiated cells was almost the same, about 4 x 10(-6) M each other; (3) The 5-HT-induced Na(+) current was completely blocked by 3 x 10(-9) M tropisetron, but not by other 5-HT receptor antagonists and 10(-7) M TTX. These results suggested that 5-HT-induced Ca(2+) response in differentiated NG cells was mainly due to L-type voltage-gated Ca(2+) channels allowing extracellular Na(+) to enter via 5-HT(3) receptors, but not through voltage-gated Na(+) channels.

Effects of calcium channel blockers on antidepressant action of Alprazolam and Imipramine

Alprazolam is effective as an anxiolytic and in the adjunct treatment of depression. In this study, the effects of calcium channel antagonists on the antidepressant action of alprazolam and imipramine were investigated. A forced swimming maze was used to study behavioral despair in albino mice. Mice were divided into nine groups (n = 7 per group). One group received a single dose of 1% Tween 80; two groups each received a single dose of the antidepressant alone (alprazolam or imipramine); two groups each received a single dose of the calcium channel blocker (nifedipine or verapamil); four groups each received a single dose of the calcium channel blocker followed by a single dose of the antidepressant (with same doses used for either in the previous four groups). Drug administration was performed concurrently on the nine groups. Our data confirmed the antidepressant action of alprazolam and imipramine. Both nifedipine and verapamil produced a significant antidepressant effect (delay the onset of immobility) when administered separately. Verapamil augmented the antidepressant effects of alprazolam and imipramine (additive antidepressant effect). This may be due to the possibility that verapamil might have antidepressant-like effect through different mechanism. Nifedipine and imipramine combined led to a delay in the onset of immobility greater than their single use but less than the sum of their independent administration. This may be due to the fact that nifedipine on its own might act as an antidepressant but blocks one imipramine mechanism that depends on L-type calcium channel activation. Combining nifedipine with alprazolam produced additional antidepressant effects, which indicates that they exert antidepressant effects through different mechanisms.

Characterization of cocaine-elicited cell vacuolation: the involvement of calcium/calmodulin in organelle deregulation

The sizes of organelles are tightly regulated in the cells. However, little is known on how cells maintain the homeostasis of these intracellular compartments. Using cocaine as a model compound, we have characterized the mechanism of deregulated vacuolation in cultured rat liver epithelial Clone 9 cells. The vacuoles were observed as early as 10 min following cocaine treatment. Removal of cocaine led to vacuole degeneration, indicating vacuolation is a reversible process. The vacuoles could devour intracellular materials and the vacuoles originated from late endosome/lysosome as indicated by immunofluorescence studies. Instant calcium influx and calmodulin were required for the initiation of vacuole formation. The unique properties of these late endosome/lysosome-derived vacuoles were further discussed. In summary, cocaine elicited a new type of deregulated vacuole and the involvement of calcium/calmodulin in vacuolation could shed light on prevention or treatment of cocaine-induced cytotoxicity.

Immunolabelling of the 5-HT3B receptor subunit in the central and peripheral nervous systems in rodents

The 5-HT(3) receptor is a member of the superfamily of neurotransmitter-gated ion channels involved in fast synaptic signalling and in modulation of neurotransmitter release. As for many other channel receptors, the electrophysiological properties and the functions of the 5-HT(3) receptor are determined by subunit composition of the pentameric channel. Because in situ hybridization did not allow the detection of mRNA encoding the 5-HT(3B) subunit in the rodent central nervous system, or in nearly half of the neurons expressing the 5-HT(3A) subunit in peripheral ganglia, it has been suggested that subunit composition could define at least two 5-HT(3) receptor-expressing neuronal populations. In order to challenge this hypothesis, we have developed polyclonal antibodies directed against a portion of the second intracytoplasmic loop of the mouse 5-HT(3B) subunit. Immunohistochemical analysis in the mouse and the rat revealed that immunolabelling was most prominent in peripheral ganglia, particularly in trigeminal ganglia (TG). In rats, transection or ligature of the infraorbital nerve resulted in a pronounced accumulation of immunoreactive material at the proximal side of the lesioned nerve, and an up-regulation of both subunits in 5-HT(3) receptor-expressing TG neurons. Surprisingly, nearly 100% of neurons expressing 5-HT(3A) subunits were also labelled by anti-5-HT(3B) antibodies. We also detected 5-HT(3B) immunoreactivity in the rat hippocampal CA1 layer and in scattered cortical neurons, indicating that detection of 5-HT(3) subunit mRNA by in situ hybridization might not provide really complete mapping of heteromeric 5-HT(3A/B) vs. homomeric 5-HT(3A) receptors in the peripheral and central nervous systems in rodents.

Characterization of the novel human serotonin receptor subunits 5-HT3C,5-HT3D, and 5-HT3E

Within the family of serotonin receptors, the 5-hydroxytryptamine-3 (5-HT(3)) receptor is the only ligand-gated ion channel. It is composed of five subunits, of which the 5-HT(3A) and 5-HT(3B) subunits are best characterized. Several studies, however, have reported on the functional diversity of native 5-HT(3) receptors, which cannot solely be explained on the basis of the 5-HT(3A) and 5-HT(3B) subunits. After our discovery of further putative 5-HT(3) serotonin receptor-encoding genes, HTR3C, HTR3D, and HTR3E, we investigated whether these novel candidates and the isoform 5-HT(3Ea) are able to form functional 5-HT(3) receptor complexes. Using immunofluorescence and immunoprecipitation studies of heterologously expressed proteins, we found that each of the respective candidates coassembles with 5-HT(3A). To investigate whether the novel subunits modulate 5-HT(3) receptor function, we performed radioligand-binding assays and calcium-influx studies in human embryonic kidney 293 cells. Our experiments revealed that the 5-HT(3C),5-HT(3D), 5-HT(3E), and 5-HT(3Ea) subunits alone cannot form functional receptors. Coexpression with 5-HT(3A), however, results in the formation of functional heteromeric complexes with different serotonin efficacies. Potencies of two agonists and antagonists were nearly identical with respect to homomeric 5-HT(3A) and heteromeric complexes. However, 5-HT showed increased efficacy with respect to 5-HT(3A/D) and 5-HT(3A/E) receptors, which is consistent with the increased surface expression compared with 5-HT(3A) receptors. In contrast, 5-HT(3A/C) and 5-HT(3A/Ea) receptors exhibited decreased 5-HT efficacy. These data show for the first time that the novel 5-HT(3) subunits are able to form heteromeric 5-HT(3) receptors, which exhibit quantitatively different functional properties compared with homomeric 5-HT(3A) receptors.

Ionotropic 5-HT3 Receptor Agonist-Induced Motor Responses in the Hindlimbs of Paraplegic Mice

Centrally expressed 5-HT3 receptors (5-HTR3) are well known for their role in wakefulness, cognition, and nociception. However, clear evidence of their participation in motor control is still lacking despite specific 5-HTR3 expression in hindlimb motor areas of the spinal cord (i.e., lumbar laminae VII-IX). Here, we studied the acute effects of 4-amino-(6-chloro-2-pyridyl)-1-piperidine hydrochloride (SR 57227A), a potent and selective 5-HTR3 agonist, on hindlimb movement generation in complete paraplegic mice. The induced movements were assessed in open-field, air-stepping, and treadmill conditions using a combination of qualitative and quantitative methods. The results revealed that SR 57227A (1–4 mg/kg ip) produced hindlimb movements corresponding to scores ranging from 1 to 5 on the motor scales of Basso, Beattie, and Bresnahan and of Antri, Orsal, and Barthe. Additional analyses revealed that one-third of the movements displayed on a treadmill were “locomotor-like” (i.e., bilateral alternation), whereas only nonlocomotor movements were observed in the other testing conditions suggesting a task-dependent contribution of peripheral afferent inputs to these effects. Locomotor-like movements could also be induced in open field and air stepping if SR 57227A was combined with subthreshold doses of 5-carboxytryptamine (5-HT1A/7 receptor agonist), suggesting synergistic actions of these drugs on central neurons. These results demonstrate that 5-HTR3 activation can induce motor activity and, under some conditions, rhythmic locomotor-like movements in the hindlimbs of paraplegic mice providing evidence of an unsuspected role for this receptor subtype in hindlimb motor control.

New aspects of nuclear calcium signalling

Nuclear calcium signalling has been a controversial battlefield for many years and the question of how permeable the nuclear pore complexes (NPCs) are to Ca2+ has been the subject of a particularly hot dispute. Recent data from isolated nuclei suggest that the NPCs are open even after depletion of the Ca2+ store in the nuclear envelope. Other research has suggested that a new Ca2+ -releasing messenger, nicotinic acid adenine dinucleotide phosphate (NAADP), can liberate Ca2+ only from acidic organelles, probably lysosomes, rather than from the traditional Ca2+ store in the endoplasmic reticulum (ER). Recent work indicates that NAADP can release Ca2+ from the nuclear envelope (NE), which has a thapsigargin-sensitive, ER-type Ca2+ store. NAADP acts in a manner similar to inositol (1,4,5)-trisphosphate [Ins(1,4,5)P3] or cyclic ADP-ribose (cADPR): all three messengers are equally able to reduce the Ca2+ concentration inside the NE and this is associated with a transient rise in the nucleoplasmic Ca2+ concentration. The NE contains ryanodine receptors (RyRs) and Ins(1,4,5)P3 receptors [Ins(1,4,5)P3Rs], and these can be activated separately and independently: the RyRs by either NAADP or cADPR, and the Ins(1,4,5)P3Rs by Ins(1,4,5)P3.

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.

5-HT3 receptors in the CNS: 3B or not 3B?

5-HT(3) receptors are widely distributed in the CNS and PNS where they participate in a variety of physiological processes. Native 5-HT(3) receptors in the CNS display functional and pharmacological heterogeneity, suggesting the existence of multiple receptor subunits. However, recent evidence suggests that of the two known subunits only the 5-HT(3A) receptor subunit (and not the 5-HT(3B) receptor subunit) is functionally present in the CNS. The molecular makeup of the 5-HT(3) receptor therefore remains an open question.

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.

L-Type calcium channels mediate calcium oscillations in early postnatal Purkinje neurons

Ca(2+) signaling is important in many fundamental neuronal processes including neurotransmission, synaptic plasticity, neuronal development, and gene expression. In cerebellar Purkinje neurons, Ca(2+) signaling has been studied primarily in the dendritic region where increases in local Ca(2+) have been shown to occur with both synaptic events and spontaneous electrical activity involving P-type voltage-gated Ca(2+) channels (VGCCs), the predominant VGCC expressed by Purkinje neurons. Here we show that Ca(2+) signaling is also a prominent feature of immature Purkinje neurons at developmental stages that precede expression of dendritic structure and involves L-type rather than P-type VGCCs. Immature Purkinje neurons acutely dissociated from postnatal day 4-7 rat pups exhibit spontaneous cytoplasmic Ca(2+) oscillations. The Ca(2+) oscillations require entry of extracellular Ca(2+), are blocked by tetrodotoxin, are communicated to the nucleus, and correlate closely with patterns of endogenously generated spontaneous and evoked electrical activity recorded in the neurons. Immunocytochemistry showed that L-, N-, and P/Q-types of VGCCs are present on the somata of the Purkinje neurons at this age. However, only the L-type VGCC antagonist nimodipine effectively antagonized the Ca(2+) oscillations; inhibitors of P/Q and N-type VGCCs were relatively ineffective. Release of Ca(2+) from intracellular Ca(2+) stores significantly amplified the Ca(2+) signals of external origin. These results show that a somatic signaling pathway that generates intracellular Ca(2+) oscillations and involves L-type VGCCs and intracellular Ca(2+) stores plays a prominent role in the Ca(2+) dynamics of early developing Purkinje neurons and may play an important role in communicating developmental cues to the nucleus.

Nicotinic receptors co-localize with 5-HT(3) serotonin receptors on striatal nerve terminals

Nicotinic acetylcholine receptors and 5-HT(3) serotonin receptors are present on presynaptic nerve terminals in the striatum, where they have been shown to be involved in the regulation of dopamine release. Here, we explored the possibility that both receptor systems function on the same individual nerve terminals in the striatum, as assessed by confocal imaging of synaptosomes. On performing sequential stimulation, nicotine (500 nM) induced changes in [Ca(2+)](i) in most of the synaptosomes ( approximately 80%) that had previously responded to stimulation with the 5-HT(3) receptor agonist m-chlorophenylbiguanide (mCPBG; 100 nM), whereas mCPBG induced [Ca(2+)](i) responses in approximately half of the synaptosomes that showed responses on nicotinic stimulation. The 5-HT(3) receptor-specific antagonist tropisetron blocked only the mCPBG-induced responses, but not the nicotinic responses on the same synaptosomes. Immunocytochemical staining revealed extensive co-localization of the 5-HT(3) receptor with the alpha4 nicotinic receptor subunit on the same synaptosomes, but not with the alpha3 and/or alpha5 subunits. Immunoprecipitation studies indicate that the 5-HT(3) receptor and the alpha4 nicotinic receptor subunit do not interact on the nerve terminals. The presence of nicotinic and 5-HT(3) receptors on the same presynaptic striatal nerve terminal indicates a convergence of cholinergic and serotonergic systems in the striatum.

Antibodies against the extracellular domain of the 5-HT3 receptor label both native and recombinant receptors

We have developed polyclonal antibodies (pAb120) against a peptide corresponding to a region within the extracellular domain of the 5-hydroxytryptamine3 (5-HT3) receptor subunit, thus permitting, for the first time, localization of 5-HT3 receptors at the cell surface in intact (non-permeabilized) systems. The antibodies are both specific and sensitive: pAb120 recognized as little as 63 ng of protein from HEK293 cells expressing recombinant 5-HT3 receptors, whilst Western blots of recombinant 5-HT3 receptors purified from Sf9 cells revealed two bands at 48 and 54 kDa, and native 5-HT3 receptors from N1E-115 cell membranes produced a broad band at 50-54 kDa with a smaller band at 35 kDa. These bands were also labelled by antibodies against the intracellular loop of the 5-HT3 receptor. Immunofluorescent labelling revealed a ring of intense fluorescence in the plasma membrane of non-permeabilized HEK293 cells expressing recombinant 5-HT3 receptors. Studies on native 5-HT3 receptors revealed that pAb120 could recognize 5-HT3 receptors on presynaptic terminals isolated from rat striatum, and immunohistochemical studies in rat brain sections revealed labelling of cell bodies, dendrites and varicose axons in hippocampus, cortex and lateral hypothalamus; all of these areas have been reported to express 5-HT3 receptors. We conclude that pAb120 is a highly specific and sensitive antiserum that will assist in clarifying fundamental questions about 5-HT3 receptor neurobiology.

The nicotinic alpha4 receptor subunit contributes to the lining of the ion channel pore when expressed with the 5-HT3 receptor subunit

To understand the wide variation of calcium permeability seen in native and recombinant 5-HT3 receptor (5-HT3R) channels, we reported previously the novel hypothesis that the serotonin 5-HT3R subunit can co-assemble with the alpha4 subunit of the nicotinic acetylcholine receptor (van Hooft, J. A., Spier, A. D., Yakel, J. L., Lummis, S. C. R. & Vijverberg, H. P. M. (1998) Proc. Natl. Acad. Sci. U. S. A. 95, 11456-11461). To test the hypothesis that the alpha4 subunit contributes to the lining of the pore of the resulting 5-HT3R channel, a mutant nicotinic alpha4 subunit with a reactive cysteine residue engineered into the putative pore region was constructed by substituting the leucine at position 285 (alpha4-L285C). The sulfhydryl-modifying reagent [2-(trimethylammonium) ethyl]methanethiosulfonate (MTSET) reduced the acetylcholine-induced current in oocytes expressing this mutant nicotinic alpha4-L285C subunit along with the nicotinic beta2 subunit by approximately 60%. When the alpha4-L285C subunit was co-expressed with the 5-HT3R subunit, both MTSET and silver nitrate (AgNO3), another cysteine-modifying reagent, significantly reduced the serotonin-induced current. No reduction was seen when the 5-HT3R was expressed alone or with the wild-type alpha4 subunit. These data provide direct molecular evidence that the nicotinic alpha4 subunit co-assembles with the 5-HT3R subunit and forms an integral part of the ion channel pore.

The induction of pain: an integrative review

The highly disagreeable sensation of pain results from an extraordinarily complex and interactive series of mechanisms integrated at all levels of the neuroaxis, from the periphery, via the dorsal horn to higher cerebral structures. Pain is usually elicited by the activation of specific nociceptors ('nociceptive pain'). However, it may also result from injury to sensory fibres, or from damage to the CNS itself ('neuropathic pain'). Although acute and subchronic, nociceptive pain fulfils a warning role, chronic and/or severe nociceptive and neuropathic pain is maladaptive. Recent years have seen a progressive unravelling of the neuroanatomical circuits and cellular mechanisms underlying the induction of pain. In addition to familiar inflammatory mediators, such as prostaglandins and bradykinin, potentially-important, pronociceptive roles have been proposed for a variety of 'exotic' species, including protons, ATP, cytokines, neurotrophins (growth factors) and nitric oxide. Further, both in the periphery and in the CNS, non-neuronal glial and immunecompetent cells have been shown to play a modulatory role in the response to inflammation and injury, and in processes modifying nociception. In the dorsal horn of the spinal cord, wherein the primary processing of nociceptive information occurs, N-methyl-D-aspartate receptors are activated by glutamate released from nocisponsive afferent fibres. Their activation plays a key role in the induction of neuronal sensitization, a process underlying prolonged painful states. In addition, upon peripheral nerve injury, a reduction of inhibitory interneurone tone in the dorsal horn exacerbates sensitized states and further enhance nociception. As concerns the transfer of nociceptive information to the brain, several pathways other than the classical spinothalamic tract are of importance: for example, the postsynaptic dorsal column pathway. In discussing the roles of supraspinal structures in pain sensation, differences between its 'discriminative-sensory' and 'affective-cognitive' dimensions should be emphasized. The purpose of the present article is to provide a global account of mechanisms involved in the induction of pain. Particular attention is focused on cellular aspects and on the consequences of peripheral nerve injury. In the first part of the review, neuronal pathways for the transmission of nociceptive information from peripheral nerve terminals to the dorsal horn, and therefrom to higher centres, are outlined. This neuronal framework is then exploited for a consideration of peripheral, spinal and supraspinal mechanisms involved in the induction of pain by stimulation of peripheral nociceptors, by peripheral nerve injury and by damage to the CNS itself. Finally, a hypothesis is forwarded that neurotrophins may play an important role in central, adaptive mechanisms modulating nociception. An improved understanding of the origins of pain should facilitate the development of novel strategies for its more effective treatment.

Ca2+ influx through voltage-gated Ca2+ channels regulates 5-HT3 receptor channel desensitization in rat glioma x mouse neuroblastoma hybrid NG108-15 cells

1. The kinetics of desensitization of the 5-HT3 receptor (5-HT3R)-gated ion channel were investigated using whole-cell and perforated-patch recording techniques in NG108-15 cells. 2. Rapid application of 5-HT (50 microM) elicited a 5-HT3R-mediated inward current response that desensitized completely in the continued presence of agonist. In the whole-cell recording configuration (holding potential of -70 mV) while buffering internal calcium (Cai2+) with 5 mM EGTA (0.5 mM added Ca2+; with an estimated free [Ca2+] of 30 nM), the rate of desensitization was initially rapid (with a half-time of approximately 230 ms), but dramatically slowed with time by 1120 +/- 160%. 3. This slowing in the rate of desensitization was reduced by stronger Ca2+ buffering (20 mM BAPTA, without added Ca2+), or by the bath application of cadmium (100 microM) to block voltage-gated Ca2+ channels. The rate of desensitization was also dependent on membrane potential. 4. In perforated-patch recordings, the rate of desensitization remained constant. However, a slowing in the desensitization rate could be induced by depolarizing cells immediately prior to the application of 5-HT. 5. The depolarization-induced slowing was blocked by incubating cells with BAPTA-AM (a membrane-permeant analogue of BAPTA) or by the bath application of cadmium. 6. These data suggest that Ca2+ influx through a cadmium-sensitive voltage-gated Ca2+ channel increases the cytoplasmic Ca2+ concentration ([Ca2+]i) and induces a dramatic slowing in the kinetics of desensitization of the 5-HT3R channel. These data provide evidence for cross-talk between voltage-gated Ca2+ channels and 5-HT3Rs in NG108-15 cells.