Modulatory action of serotonin on glutamate-induced excitation of cerebellar Purkinje cells

The Discovery of the Monoaminergic Innervation of the Cerebellum: Convergence of Divergent and Point-to-Point Systems

This Cerebellar Classic highlights the landmark discovery of the innervation of the cerebellar cortex and cerebellar nuclei by noradrenergic and serotoninergic axons emanating, respectively, from the locus coeruleus and the raphé nuclei. Since then, modulation of the activity of cerebellar neurons by the monoamine systems has been studied extensively, as well as their reorganization and modifications during development, plasticity, and disease. The discovery of noradrenergic and serotoninergic innervation of the cerebellum has been a crucial step in understanding the neurochemical relationships between brainstem nuclei and the cerebellum, and the attempts to treat cerebellar ataxias pharmacologically. The large neurochemical repertoire of the cerebellum represents one of the complexities and challenges in the modern appraisal of cerebellar disorders.

[Recent Findings on the Mechanism of Cough Hypersensitivity as a Cause of Chronic Cough]

An increasing number of patients complain to medical institutions about a cough that persists for more than 8 weeks, namely chronic cough. The cough observed in patients with chronic cough is not responsive to conventional antitussive agents such as dihydrocodeine and dextromethorphan, and this is a major clinical problem. The most common pathology of chronic cough in Japan is dry cough. Two causes of dry cough are increased sensitivity of cough receptors (cough hypersensitivity) and increased contraction of bronchial smooth muscle. Among these, the mechanisms of cough hypersensitivity are diverse, and understanding these mechanisms is important for the diagnosis and treatment of chronic cough. In this paper I will review the regulatory mechanisms of cough hypersensitivity, especially the regulation of Aδ fiber excitability by C fibers. Furthermore, the central mechanisms involved cough reflex are discussed in relation to central acting antitussives.

The 5-HT7 receptor triggers cerebellar long-term synaptic depression via PKC-MAPK

The 5-HT7 receptor (5-HT7R) mediates important physiological effects of serotonin, such as memory and emotion, and is emerging as a therapeutic target for the treatment of cognitive disorders and depression. Although previous studies have revealed an expression of 5-HT7R in cerebellum, particularly at Purkinje cells, its functional role and signaling mechanisms have never been described. Using patch-clamp recordings in cerebellar slices of adult mice, we investigated the effects of a selective 5-HT7R agonist, LP-211, on the main plastic site of the cerebellar cortex, the parallel fiber-Purkinje cell synapse. Here we show that 5-HT7R activation induces long-term depression of parallel fiber-Purkinje cell synapse via a postsynaptic mechanism that involves the PKC-MAPK signaling pathway. Moreover, a 5-HT7R antagonist abolished the expression of PF-LTD, produced by pairing parallel fiber stimulation with Purkinje cell depolarization; whereas, application of a 5-HT7R agonist impaired LTP induced by 1 Hz parallel fiber stimulation. Our results indicate for the first time that 5-HT7R exerts a fine regulation of cerebellar bidirectional synaptic plasticity that might be involved in cognitive processes and neuropsychiatric disorders involving the cerebellum.

Increase in telencephalic dopamine and cerebellar norepinephrine contents by hydrostatic pressure in goldfish: the possible involvement in hydrostatic pressure-related locomotion

Fish are faced with a wide range of hydrostatic pressure (HP) in their natural habitats. Additionally, freshwater fish are occasionally exposed to rapid changes in HP due to heavy rainfall, flood and/or dam release. Accordingly, variations in HP are one of the most important environmental cues for fish. However, little information is available on how HP information is perceived and transmitted in the central nervous system of fish. The present study examined the effect of HP (water depth of 1.3 m) on the quantities of monoamines and their metabolites in the telencephalon, optic tectum, diencephalon, cerebellum (including partial mesencephalon) and vagal lobe (including medulla oblongata) of the goldfish, Carassius auratus, using high-performance liquid chromatography. HP affected monoamine and metabolite contents in restricted brain regions, including the telencephalon, cerebellum and vagal lobe. In particular, HP significantly increased the levels of dopamine (DA) in the telencephalon at 15 min and that of norepinephrine (NE) in the cerebellum at 30 min. In addition, HP also significantly increased locomotor activity at 15 and 30 min after HP treatment. It is possible that HP indirectly induces locomotion in goldfish via telencephalic DA and cerebellar NE neuronal activity.

Monoamine receptors and immature cerebellum cytoarchitecture after cisplatin injury

The experimental model of cisplatin treatment provides the opportunity to identify the precise function of the neurotransmitters in some crucial events of brain development, and their interactions or modulatory roles. The serotonin and noradrenaline monoamines influence the formation of the cerebellar cortex circuitry. In this study we found changes in the expression of the serotonin and noradrenaline receptors after a single injection of cisplatin in 10-day-old rats. The growth of Pc dendrites was early altered in lobules VI-VIII of cerebellum vermis. In these lobules, at postnatal day (PD) 17, the cisplatin-induced increase of the serotoninergic receptor 5-HT2AR, a factor that inhibits Pc dendrite growth by acting post-synaptically, occurred in all cerebellar layers, suggesting also alteration of granule cell proliferation and migration. The decreased labelling of beta l adrenergic receptor (beta1AR) in the soma of some Pc at PD11 can be correlated with the altered expression of glutamate receptors and GAD65 (glutamic acid decarboxylase) of and on Pc we have previously described [Pisu, M.B., Guioli, S., Conforti, E., Bernocchi, G., 2003. Signal molecules and receptors in the differential development of cerebellum lobules. Acute effects of cisplatin on nitric oxide and glutamate system in Purkinje cell population. Dev. Brain Res. 145, 229-240; Pisu, M.B., Roda, E., Avella, D., Bernocchi, G., 2004. Developmental plasticity of rat cerebellar cortex after cisplatin injury: inhibitory synapses and differentiating Purkinje neurons. Neuroscience 129, 655-664]. Moreover, beta1AR seems to be the key factor in the cerebellar reorganization between PD17 and PD30. The expression of this receptor was maintained in the molecular layer (ML), in particular in the inhibitory interneurons, despite their different distributions. The labelling of 5-HT1AR in the ML areas lacking Pc dendrite branches could contribute to the recovery phase of the cerebellar cytoarchitecture in cisplatin-treated rats. In general these findings should be taken into consideration in therapeutic interventions for developmental CNS disorders with a morphological basis.

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.

Nitric oxide-evoked glutamate release and cGMP production in cerebellar slices: control by presynaptic 5-HT1D receptors

We previously reported that pre- and postsynaptic 5-hydroxytryptamine (5-HT) receptors effectively control glutamatergic transmission in adult rat cerebellum. To investigate where 5-HT acts in the glutamate ionotropic receptors/nitric oxide/guanosine 3',5'-cyclic monophosphate (cGMP) pathway, in the present study 5-HT modulation of the cGMP response to the nitric oxide donor S-nitroso-penicillamine (SNAP) was studied in adult rat cerebellar slices. While cGMP elevation produced by high-micromolar SNAP was insensitive to 5-HT, 1 microM SNAP, expected to release nitric oxide in the low-nanomolar concentration range, elicited cGMP production and endogenous glutamate release both of which could be prevented by activating presynaptic 5-HT1D receptors. Released nitric oxide appeared responsible for cGMP production and glutamate release evoked by 1 microM SNAP, as both the effects were mimicked by the structurally unrelated nitric oxide donor 2-(N,N-diethylamino)-diazenolate-2-oxide (0.1 microM). Dependency of the 1 microM SNAP-evoked release of glutamate on external Ca2+, sensitivity to presynaptic release-regulating receptors and dependency on ionotropic glutamate receptor functioning, suggest that nitric oxide stimulates exocytotic-like, activity-dependent glutamate release. Activation of ionotropic glutamate receptors/nitric oxide synthase/guanylyl cyclase pathway by endogenously released glutamate was involved in the cGMP response to 1 microM SNAP, as blockade of NMDA/non-NMDA receptors, nitric oxide synthase or guanylyl cyclase, abolished the cGMP response. To conclude, in adult rat cerebellar slices low-nanomolar exogenous nitric oxide could facilitate glutamate exocytotic-like release possibly from parallel fibers that subsequently activated the glutamate ionotropic receptors/nitric oxide/cGMP pathway. Presynaptic 5-HT1D receptors could regulate the nitric oxide-evoked release of glutamate and subsequent cGMP production.

Treatment of Ataxia in Cortical Cerebellar Atrophy with the GABAergic Drug Gabapentin

The aim of this work was to investigate the efficacy of the GABAergic drug gabapentin in the treatment of the cerebellar signs caused by cortical cerebellar atrophy (CCA). Ten patients with CCA received gabapentin in single doses of 400 mg in an open-label study; thereafter, daily administration of 900-1,600 mg of gabapentin was continued during at least 4 weeks. An ataxia scale based on clinical findings was used to evaluate the cerebellar signs at baseline and after administration of the drug. A statistically significant improvement of the ataxia scores was found after single doses of 400 mg of gabapentin and after the administration of 900-1,600 mg of this drug during 4 weeks, as compared to the results obtained at baseline. An important clinical amelioration was also evident. Gabapentin has been demonstrated to be capable of improving the cerebellar signs in cases of CCA, after single doses and after continued administration of the drug during 4 weeks. GABAergic enhancement or supplementation could play an important role in the treatment of diseases of the cerebellar cortex associated with a deficit of GABA.

Cerebellar aminergic neuromodulation: towards a functional understanding

Although a number of neuromodulators influence the cerebellar circuitry, their functions remain largely unknown. By reviewing and combining results from data-driven and theory-driven studies, we attempt to provide an integrated systems view of cerebellar neuromodulation. First, we review the short- and long-term effects of neuromodulators on the cerebellar circuitry. Second, we review recent theories of the cerebellum and show that a number of modulatory signals are needed for powerful cerebellar learning and control. Finally, we attempt to match each theoretically derived modulatory signal with a specific neuromodulator. In particular, we propose that serotonin controls the 'responsibility' of each cerebellar unit (or microcomplex) in cerebellar learning and control; norepinephrine gates unsupervised learning in the cerebellar cortex; dopamine enhances goal-oriented cerebellar learning; and, finally, acetylcholine controls the speed of supervised learning in Purkinje cells.

The effects of serotonin on functionally diverse isolated lamprey spinal cord neurons

The experiments reported here showed that application of serotonin (5-hydroxytryptamine, 5-HT) (100 microM) did not induce any significant current through the membranes of any of the spinal neurons studied (n = 62). At the same time, the membranes of most motoneurons and interneurons (15 of 18) underwent slight depolarization (2-6 mV) in the presence of 5-HT, which was not accompanied by any change in the input resistance of the cells. Depolarization to 10-20 mV occurred in some cells (3 of 18) of these functional groups, this being accompanied by 20-60% decreases in input resistance. The same concentration of 5-HT induced transient low-amplitude depolarization of most sensory spinal neurons (dorsal sensory cells), this changing smoothly to long-term hyperpolarization by 2-7 mV. The input resistance of the cell membranes in these cases showed no significant change (n = 8). Data were obtained which provided a better understanding of the mechanism by which 5-HT modulates the activity of spinal neurons. Thus, 5-HT facilitates chemoreceptive currents induced by application of NMDA to motoneurons and interneurons, while the NMDA responses of dorsal sensory cells were decreased by 5-HT. 5-HT affected the post-spike afterresponses of neurons. 5-HT significantly decreased the amplitude of afterhyperpolarization arising at the end of the descending phase of action potentials in motoneurons and interneurons and increased the amplitude of afterdepolarization in these types of cells. In sensory spinal neurons, 5-HT had no great effect on post-spike afterresponses. The results obtained here support the suggestion that 5-HT significantly modulates the activity of spinal neurons of different functional types. 5-HT facilitates excitation induced by subthreshold depolarization in motoneurons and some interneurons, facilitating the generation of rhythmic discharges by decreasing afterhyperpolarization. In sensory cells, 5-HT enhances inhibition due to hyperpolarization, suppressing NMDA currents. The differences in the effects of 5-HT on functionally diverse neurons are presumed to be associated with the combination of different types of 5-HT receptors on the membranes of these types of spinal neurons.

Influence of serotonin on the glutamate-induced excitations of secondary vestibular neurons in the rat

The excitatory responses evoked by glutamate and its agonists in secondary vestibular neurons of the rat were studied during microiontophoretic application of 5-hydroxytryptamine (5-HT). Ejection of 5-HT modified neuronal responsiveness to glutamate in 86% of the studied units, the effect being a depression of the excitatory responses in two-thirds of cases and an enhancement in the remaining third. 5-HT was also effective in modifying 94% of the responses evoked by N-methyl-d-aspartate (NMDA), inducing a depressive effect in 76% of cases and an enhancement in the remaining ones. Quisqualate-evoked effects were depressed and enhanced by 5-HT in about the same number of cases; in contrast, kainate-evoked responses were enhanced. The depressive action of 5-HT was mimicked by application of alpha-methyl-5-hydroxytryptamine (alpha-Me-5-HT), a 5-HT(2) receptor agonist, whereas the enhancing effect could be evoked by application of 8-hydroxy-2(di-n-propylamino)tetralin (8-OH-DPAT), a selective 5-HT(1A) receptor agonist. The 5-HT(2) receptor antagonist ketanserin was able to reduce, but not to block totally, the depressive action of 5-HT on glutamate- or NMDA-evoked responses. No significant difference was detected between neuronal responses in the lateral and the superior vestibular nucleus. These results indicate that 5-HT is able to modulate the responsiveness of secondary vestibular neurons to excitatory amino acids. Its action is mostly depressive, involves 5-HT(2) receptors, and is exerted on NMDA receptors. A minor involvement of other 5-HT receptors (at least 5-HT(1A)) and other glutamate receptors (for quisqualate and kainate) in the modulatory action of 5-HT is plausible.

Serotonergic neuromodulation in the cerebellar cortex: cellular, synaptic, and molecular basis

The cerebellum, like most sensorimotor areas of the brain, receives a serotonergic innervation from neurons of the reticular formation. It is well established that local application of serotonin modulates the firing rate of cerebellar Purkinje cells in vivo and in vitro, but the mechanisms by which serotonin affects the cerebellar function are still poorly understood. Whereas interactions between serotonin, glutamate, and GABA have been reported to increase or decrease the firing frequency of Purkinje cells, there is little evidence for a modulation of excitatory and inhibitory synapses by serotonin in the cerebellar cortex. Changes in the intrinsic electrical properties of Purkinje cells upon application of serotonin have also been reported, but their impact on Purkinje cell firing is unclear. The recent finding that serotonin specifically modulates the activity of Lugaro cells, a class of inhibitory interneurons of the cerebellar cortex, offers new insights on the action of this neuromodulator. The peculiar axonal projection and specific interneuronal targets of the Lugaro cells suggest that the action of serotonin might occur upstream of Purkinje cells through a resetting of the computational properties of the cerebellar cortex. Understanding the mechanisms of the serotonergic modulation of the cerebellar cortex is of clinical relevance, as abnormal serotonin metabolism has been observed in animal models and pathological cases of motor disorders involving the cerebellum, and as chronic intravenous administration of L-5-hydroxytryptophan (5-HTP), a precursor of serotonin, was the first treatment shown to improve significantly cerebellar symptoms.

Long term depletion of serotonin leads to selective changes in glutamate receptor subunits

The present study was carried out to clarify possible modulation mechanism of serotonin (5-HT) on glutamatergic neurotransmission in the rat cerebral cortex. 5-HT was depleted by a 5-HT metabolite blocker (para-chlorophenylalanine; pCPA) for a week. Receptor binding experiments using (S)-[(3)H]alpha-amino-3-hydroxy-5-methylisoxazol-4-propionic acid (AMPA) showed a considerable increase in B(max) value of the membrane samples prepared from the cerebral cortex of rats compared with that of control animals received saline. In contrast, B(max) value of the [(3)H]MK-801 binding experiments for NMDA receptor was not changed by pCPA-treatment. Changes in the density of each AMPA receptor subtype were examined in the cerebral cortex by immunoblot analyses using antibodies against AMPA receptor subunits. The density of immunoreactive bands with receptor subtype specific antibodies against GluR2/3 and GluR2 receptors was increased, whereas that of GluR1 receptors was decreased. Considering GluR2 receptor subtype inhibits Ca(2+) influx into neurons, the present study suggests that 5-HT appears to modulate synaptic plasticity by regulating the density of each AMPA receptor subtype.

Central serotonin system in Dystonia musculorum mutant mice: biochemical, autoradiographic and immunocytochemical data

The autosomal recessive mutation dystonia musculorum (dt(J)/dt(J)) causes degenerative alterations of peripheral and central sensory pathways that lead to ataxia. To investigate possible changes in the central serotonin system of these mice, HPLC measurements of 5-hydroxytryptophan, 5-hydroxy-tryptamine (serotonin; 5-HT), and 5-HT metabolites were obtained from 22 brain regions and the spinal cord of wild type and dt(J)/dt(J) mutant mice. Also, 5-HT transporters were quantified by [(3)H]citalopram autoradiography in 72 brain regions, subregions, and nuclei, and the 5-HT innervation visualized by immunocytochemistry throughout the brain and spinal cord. In all brain regions measured for indoleamine content, there were no significant differences between the two genotypes. In the spinal cord, an increased tissue concentration of 5-HT (+34%), 5-hydroxyindole-3-acetic acid (+33%), 5-hydroxytryptophol (+21%), and 5-hydroxytryptophan (+45%) in dt(J)/dt(J) actually corresponded to the same total amount of each of these indoleamines in the entire spinal cord, when taking into account its reduced size in the mutants. Quantification of the binding to 5-HT transporters showed increases in the medial geniculate nucleus (+14%), medial (+24%) and lateral (+18%) hypothalamus, interpeduncular (+13%), vestibular (+22%), and deep cerebellar nuclei (+37%) of dt(J)/dt mice, and decreases in the ventral tegmental area (-13%), median and linear raphe nuclei (-20%), as well as in the solitary complex (-35%). There were no apparent differences in the distribution of 5-HT-immunostained fibers in these and other regions of brain and in the spinal cord of dt(J)/dt(J) compared to wild type mice. The bulk of these results indicates a relative sparing of the central 5-HT system in the dt(J)/dt(J) mice, even though alterations in 5-HT transporters could justify attempts at improving the sensorimotor dysfunction by administration of serotoninergic agents in these mice.

Weak effect of neuromodulators on climbing fiber-activated [Ca(2+)](i) increases in rat cerebellar Purkinje neurons

The effect of several neuromodulators (carbachol (CCh), serotonin (5-HT), noradrenaline (NE), and dopamine (DA)) on the climbing fiber (CF)-induced [Ca(2+)](i) increase in the dendrites of cerebellar Purkinje cells was examined in slices from the rat cerebellum. Purkinje cells were filled with the Ca(2+) indicator bis-fura-2 with patch electrodes on the soma. [Ca(2+)](i) changes were measured from regions of interest in the dendrites with a high speed camera. Changes evoked by one or three responses were measured in control conditions and with neuromodulators added to the bath. None of these four classic modulators caused a significant change in the CF-induced [Ca(2+)](i) amplitude. Buspirone, a partial 5-HT(1A) agonist and a weak DA receptor antagonist caused a small (10-15%) reduction in the response.

The treatment of spinocerebellar ataxias: facts and hypotheses

Actual therapeutic assays in spinocerebellar ataxias, i.e. in Friedreich's ataxia (FA) and olivopontocerebellar atrophy (OPCA) are discussed in relation to (i) the serotoninergic theory; (ii) the excitotoxic action of glutamate; and (iii) cerebrospinal fluid thiamine deficiency in ataxic patients. Data from the literature show that neurochemical deficiencies arising from cerebellar damage in both FA and OPCA patients are multiple. Assays of replacement and neuroprotective therapeutics with a single drug have produced controversial data or mildly effective results. Consequently, it is hypothesized that a drug cocktail, i.e. L-5-hydroxytryptophan, thiamine and amantadine hydrochloride, would be more beneficial. This cocktail proved to be useful in open studies, improving respiratory disorders in FA patients. More powerful inhibitors of N-methyl-D aspartate receptor channels should be tried initially in animal experiments.

Regional distribution of 5-HT transporters in the brain of wild type and 'Purkinje cell degeneration' mutant mice: a quantitative autoradiographic study with [3H]citalopram

The neurological mutant 'Purkinje cell degeneration' (pcd) is characterized by a primary degeneration of Purkinje cells, as well as by retrograde and secondary partial degeneration of cerebellar granule cells and inferior olivary neurons, and can be considered as an animal model of human degenerative ataxias. The serotonin (5-HT) innervation was examined in wild type and pcd mice, by quantifying 5-HT uptake sites, or transporters, using [3H]citalopram binding autoradiography. In both wild type and pcd mutants, the highest densities of 5-HT transporters were in mesencephalic and rostral pontine regions, in limbic structures, in hypothalamus and in discrete thalamic divisions, while the lowest labelling was found in cerebellum and brainstem reticular formation. In pcd mice, although [3H]citalopram labelling was higher in cerebellar cortex and deep cerebellar nuclei, when binding densities were corrected for surface area, the up-regulation of 5-HT transporters was present only in deep cerebellar nuclei. Also, higher labelling was found in nuclei raphe dorsalis and medialis, in ventral divisions of rostral neostriatum, caudal neostriatum, rostral globus pallidus, posteromedial amygdaloid nucleus, septum, olfactory tubercles, vertical limb of Broca's diagonal band, periventricular, latero-ventral and medio-ventral thalamic nuclei, medial geniculate nucleus, anterior hypothalamus and entorhinal cortex. The results indicate a relative integrity of the 5-HT innervation, but with a reorganization of serotoninergic terminals in the cerebellum, in particular in the deep cerebellar nuclei. This suggests that in progressive cerebellar degeneration, as found in the pcd mutant, the modified 5-HT system may still participate in motor functions by exerting an overall modulation of excitatory amino acid neurotransmission, but the availability of 5-HT may be altered in defined brain targets, as is the case for other spontaneous cerebellar mutants, in particular for the 'Lurcher' mutant mouse, a model of human olivopontocerebellar atrophy.

Regional distribution of the 5-HT innervation in the brain of normal and lurcher mice as revealed by [3H]citalopram quantitative autoradiography

The neurological cerebellar mutant lurcher is characterized by a primary degeneration of Purkinje cells as well as retrograde secondary partial degeneration of cerebellar granule cells and inferior olivary neurons. Since serotonin (5-HT) has been implicated in the modulation of excitatory amino acid systems of the cerebellum, the 5-HT innervation of the normal and lurcher mice was examined by quantifying uptake sites using [3H]citalopram autoradiography, and by biochemical assays of the indoles 5-HT, 5-hydroxy-L-tryptophan and 5-hydroxyindole-3-acetic acid using high-performance liquid chromatography. Comparable results were found between [3H]citalopram binding and 5-HT tissue concentrations in different brain regions. The highest [3H]citaslopram labelling was observed in defined structures of the mesencephalic and upper pontine regions, in limbic strutures, in hypothalamus and in discrete thalamic divisions, while the lowest labelling of uptake sites was documented in cerebellum and brainstem reticular formation. In lurcher mutants, the histology confirmed cell degeneration and the reduction in width, leading to 65%, 45% and 25% atrophies of total cerebellum, deep nuclei and inferior olivary nucleus, respectively. The [3H]citalopram labelling corrected for surface loss was 45% and 20% higher to cerebellar deep nuclei and red nucleus, respectively, but remained unchanged in the cerebellar cortex and inferior olivary nucleus. Moreover, higher labelling was found in nucleus raphe dorsalis, ventral tegmental area, inferior colliculus, locus coeruleus, pontine central grey and anterior thalamic nuclei, areas known to be part of cerebellar afferent and efferent systems. The present results indicate that in such pathological conditions as described for the lurcher mutant, the 5-HT system may modulate motor function not only at the level of the cerebellum, but also in other forebrain structures functionally related to the motor system.

Low nanomolar serotonin inhibits the glutamate receptor/nitric oxide/cyclic GMP pathway in slices from adult rat cerebellum

The function of serotonin afferents to the cerebellum has been investigated by monitoring the effects of serotoninergic drugs on the production of cyclic GMP elicited in cerebellar slices by activation of ionotropic glutamate receptors. Exposure of adult rat cerebellar slices to N-methyl-D-aspartate (1 nM to 1 microM) or to (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA; 1 nM to 10 microM) elicited concentration-dependent and saturable rises in the levels of cyclic GMP. These responses were blocked by selective antagonists at the N-methyl-D-aspartate or AMPA receptors and by inhibiting nitric oxide synthase, but were insensitive to tetrodotoxin. When tested between 0.1 and 10 nM, serotonin, the serotonin1A receptor agonist (+/-)-8-hydroxy-2-(di-n-propylamino)tetralin and the serotonin2 receptor agonist (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane inhibited, concentration-dependently, the cyclic GMP responses evoked by near-maximal (0.1 microM) concentrations of N-methyl-D-aspartate or AMPA. The EC50 values (concentrations causing half-maximal effect) ranged between 0.7 and 2.1 nM. The actions of serotonin were totally abolished by methiothepin, a mixed-type serotonin receptor antagonist. Thus, the serotonergic cerebellar afferents may exert a potent inhibitory control on the excitatory transmission mediated by N-methyl-D-aspartate and AMPA receptors; the inhibition occurs through both serotonin1A and serotonin2 receptors. As the glutamate receptor-dependent cyclic GMP responses involve production of nitric oxide, a diffusible activator of guanylate cyclase, the above inhibitory serotonin receptors may have multiple localization.(ABSTRACT TRUNCATED AT 250 WORDS)

NMDA-induced glutamate and aspartate release from rat cortical pyramidal neurones: evidence for modulation by a 5-HT1A antagonist

1. We have investigated an aspect of the regulation of cortical pyramidal neurone activity. Microdialysis was used to assess whether topical application of drugs (in 10 microliter) to fill a burr hole over the frontal cortex, where part of the corticostriatal pathway originates, would change concentrations of the excitatory amino acids glutamate and aspartate in the striatum of the anaesthetized rat. 2. Topical application of N-methyl-D-aspartate (NMDA, 2 and 20 mM) dose-dependently increased glutamate and aspartate concentrations in the striatum. Coapplication of tetrodotoxin (10 microM) blocked the NMDA-evoked rise in these amino acids. A calcium-free medium, perfused through the probe also blocked the rise, indicating that it was due to an exocytotic mechanism in the striatum. 3. It was hypothesized that the rise observed was due to an increase in the activity of the corticostriatal pathway. As 5-hydroxytryptamine1A (5-HT1A) receptors are enriched on cell bodies of corticostriatal neurones, a selective 5-HT1A-antagonist (WAY 100135) was coapplied with the lower dose of NMDA. Compared to NMDA alone, coapplication of 50 microM WAY 100135 significantly increased glutamate release. This effect was sensitive to tetrodotoxin and calcium-dependent. Application of 50 microM WAY 100135 alone significantly enhanced glutamate release above baseline; this was also tested at 100 microM (not significant). 4. Compared to NMDA alone, coapplication of WAY 100135 (20 microM) significantly enhanced aspartate release; the mean value was also increased (not significantly) with 50 microM. This rise was calcium-dependent, but not tetrodotoxin-sensitive. WAY 100135 (100 microM) reduced NMDA-induced aspartate release. WAY 100135 (100 microM) reduced NMDA-induced aspartate release. Application of the drug alone had no effect on basal aspartate release.5. Coapplication of the 5-HT1A agonist, 8-OHDPAT (5 sanM) with NMDA did not affect the NMDA evoked increase in glutamate and aspartate.6. Topical application of high potassium (100 sanM) to the surface of the cortex did not result in a detectable rise in striatal glutamate or aspartate.7. Perfusion of WAY 100135 (tested at 50 microM) through the dialysis probe did not affect glutamate oraspartate concentrations.8. It was concluded that a selective 5-HT1A-antagonist can increase the activity of corticostriatal pyramidal neurones. As in Alzheimer's disease hypoactivity of pyramidal neurones almost certainly exists, a selective 5-HT1A-antagonist may be potentially useful in the treatment of the cognitive symptoms of this disease.

Calcitonin gene-related peptide modulates neuronal activity in the mammalian cerebellar cortex

Calcitonin gene related peptide (CGRP) has been localized within specific populations of mossy fibers in the cat's cerebellar cortex. The intent of the present study was to determine the physiological role of this peptide in cerebellar circuitry. CGRP was iontophoretically applied and its effects on spontaneous, amino acid-induced, and synaptically-mediated activity were recorded. In addition, interactions between CGRP and serotonin (5HT), another neuromodulator in cerebellar circuitry, also were analyzed. The findings of this study reveal that the primary effect of CGRP is to suppress spontaneous and excitatory amino acid-induced activity. However, CGRP has a more potent effect in suppressing aspartate- and quisqualate-induce activity as compared to that elicited by glutamate. CGRP slowed or completely blocked synaptic activity mediated by stimulation of the inferior cerebellar peduncle. Finally, the individual suppressive effects of 5HT and CGRP were potentiated when both were applied simultaneously. However, the potentiation was greater when the neuron was exposed to 5HT before CGRP was applied. In summary, the presence of CGRP in selected populations of mossy fibers, together with serotoninergic afferents, decreases the responsiveness of Purkinje cells to excitatory amino acids as well as synaptically-driven activity. Thus, activation of an afferent system to the cerebellum can elicit distinct effects on different populations of neurons that are dependent on the microenvironment of the cell at a particular point in time.

Different functions of spinal 5-HT1A and 5-HT2 receptor subtypes in modulating behaviour induced by excitatory amino acid receptor agonists in mice

The modulating effects of 5-HT1A and 5-HT2 receptor agonists on behaviour spinal excitatory amino acid (EAA) agonists were examined. Intrathecal (i.th.) administration of both N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) produce a behavioural syndrome of caudally directed biting and scratching. Serotonin (5-HT) agonists were coadministered with either NMDA or AMPA, and changes in EAA-induced behaviour were scored. All drugs were administered i.th. The 5-HT1A agonist 8-hydroxy-2-(di-n-propylamino)tetralin hydrobromide (8-OH-DPAT) (15-60 nmol) reduced both NMDA (0.25 nmol) and AMPA (0.06 nmol) induced behaviour in a dose-dependent manner, and preadministration of the 5-HT1A receptor antagonist, 1-(2-methoxyphenyl)-4-[4-(2-phthalimido)butyl]piperazine hydrobromide (NAN-190) (20 nmol) reversed this effect. The administration of the 5-HT2 agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) (0.7-28 nmol) produced a dose-dependent behavioural syndrome similar to the EAA agonists. This was reversed by preadministration of ritanserin (10 nmol), a 5-HT2 antagonist. When DOI was coadministered with NMDA (0.25 nmol) or MAPA (0.06 nmol) there was an increase in the behaviour recorded and this effect was antagonised by ritanserin. The results of this study implicate that in the spinal cord subtypes of 5-HT receptors have different effects on modulation of behaviour induced by activation of the NMDA or the AMPA receptors; the activated 5-HT1A receptors have an inhibitory effect whereas activation of the 5-HT2 receptors enhance the induced behaviour.

Sensory responsiveness of brain noradrenergic neurons is modulated by endogenous brain serotonin

Previous results have indicated that application of serotonin (5-HT) onto noradrenergic locus coeruleus (LC) neurons selectively attenuates the response of these cells to excitatory amino acids (EAAs). Other studies revealed that certain sensory responses of LC neurons are mediated by EAA inputs. We examined the role of endogenous 5-HT in modulating sensory responses of LC neurons that are EAA-mediated. LC neurons recorded in rats pretreated with the serotonin (5-HT) depletor, p-chlorophenylalanine (PCPA), exhibited increased responsiveness to electrical stimulation of a rear footpad. Conversely, injection of the 5-HT precursor, 5-hydroxytryptophan (5-HTP), reversed this effect of PCPA and attenuated this sensory response of LC neurons in drug-naive animals. Neither treatment altered the spontaneous discharge rate of LC neurons. These results are consistent with previous findings indicating that 5-HT has potent but selective effects on EAA-mediated responses of LC neurons, and in addition point to a possible functional role for endogenous 5-HT in controlling sensory-evoked LC activity.

Effects of serotonin and the 5-HT2/1C receptor agonist DOI on neurons of the cerebellar dentate/interpositus nuclei: possible involvement of a GABAergic interneuron

The present study was designed to examine the effects of iontophoretically applied serotonin (5-HT) on neurons of the cerebellar dentate/interpositus nuclei in an in vitro slice preparation and to determine if the 5-HT2/1C receptor subtype could be responsible for mediating any effects noted with 5-HT. 5-HT and the 5-HT2/1C-selective agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane hydrochloride (DOI) were iontophoretically applied alone and during superfusion of the 5-HT2/1C-selective antagonist, ritanserin. 5-HT and DOI elicited either inhibition or excitation of the spontaneous activity of dentate/interpositus neurons. An inhibitory response was induced by both compounds in the majority of cells responding. Ritanserin significantly attenuated the inhibitory response elicited by both 5-HT and DOI. In addition, the inhibitory response to DOI was significantly attenuated by the gamma-aminobutyric acid (GABA) antagonists, bicuculline and picrotoxin. Our results suggest that the 5-HT2/1C receptor subtype may be partially responsible for mediating 5-HT-induced inhibition of dentate/interpositus neurons, possibly via activation of GABAergic interneurons.

Serotonin depresses excitatory amino acid-induced excitation of cerebellar Purkinje cells in the adult rat in vivo

The modulatory effects of serotonin (5-HT) on excitatory amino acid (EAA)-induced excitations of Purkinje cells (PCs) were examined in urethane-anesthetized adult male rats using microiontophoresis and extracellular recordings. Application of 5-HT had minimal effects on the spontaneous firing rates of PCs but depressed excitations elicited by glutamate (Glu), aspartate (Asp), kainate (KA), and quisqualate (QA), and to a lesser extent those of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA). Excitations induced by the metabotropic EAA agonist, (+-)-1-aminocyclopentane-trans-1,3-dicarboxylate (t-ACPD), were unaffected by 5-HT. In summary, 5-HT depressed EAA-mediated excitations with the following rank order of effectiveness: Glu = Asp = KA = QA > AMPA >> t-ACPD. These findings suggest that 5-HT shows some selectivity in its modulation of EAA-mediated excitations of PCs and thus may serve an important neuromodulatory role in the cerebellum.

The physiological effects of serotonin are mediated by the 5HT1A receptor in the cat's cerebellar cortex

Serotonin is present in a fine beaded plexus in the cerebellar cortex of several mammalian species. In the cat, serotoninergic afferents arise from neurons located within the lateral, paramedian and peri-olivary reticular nuclei (Kerr and Bishop, J. Comp. Neurol., 304 (1991) 502-515). In addition to serotoninergic afferents, these same nuclei also contain a separate population of neurons that give rise to mossy fibers to the cerebellar cortex. Physiological studies have shown that mossy fibers are excitatory to their target neurons. The intent of the present study was to determine the physiological effects of serotonin in the cat's cerebellum in an in vivo preparation and to identify the receptor(s) that mediate the observed responses. Iontophoretic application of serotonin (5HT) onto Purkinje cells reduces the spontaneous firing rate of all cells tested (n = 12). Serotonin also blocks the excitatory effects elicited by the application of aspartate in 17 of 19 units tested and of glutamate (n = 62) in all cases. In addition, 5HT potentiated the inhibitory action of GABA (n = 12). Iontophoretic application of the 5HT1A agonists, 8-OH-DPAT and ipsapirone, mimic the suppressive action of serotonin in a dose-dependent manner. This response, as well as the 5HT mediated suppression are blocked by the application of spiperone, a 5HT1A antagonist. Compounds selective for the 5HT1C,2 and 3 receptors are physiologically ineffective. The present data are in partial agreement with previous studies in the rat's cerebellar cortex.(ABSTRACT TRUNCATED AT 250 WORDS)

Serotonin1A facilitation of frog motoneuron responses to afferent stimuli and to N-methyl-D-aspartate

The effects of serotonin and excitatory amino acids on motoneurons were examined by sucrose gap recordings from the ventral root of the isolated, hemisected frog spinal cord superfused with magnesium-free, carbonate-buffered Ringer solution. Low concentrations of serotonin (0.1 microM) and the serotonin1A agonist 8-hydroxy-2-(n-dipropylamino)tetralin (8-OH-DPAT; 0.01 microM) significantly increased the duration and amplitude of the polysynaptic components of ventral root potentials produced by dorsal root stimulation. The facilitations of the ventral root potentials were blocked by the serotonin1A antagonist spiroxatrine, but were unaffected by the serotonin2 antagonist ketanserin or the serotonin3 antagonist 1 alpha H,3 alpha,5 alpha H-tropan-3-yl-3,-dichlorobenzoate (MDL 72222). The actions of 0.1 microM serotonin on motoneuron depolarizations evoked by the putative excitatory amino acid transmitters L-glutamate and L-aspartate were quite variable, but in the presence of ketanserin (20 microM), small consistent increases in amino acid-induced motoneuron depolarizations were observed. 8-OH-DPAT significantly enhanced motoneuron depolarizations elicited by the selective excitatory amino acid agonist N-methyl-D-aspartate in both normal and tetrodotoxin-containing Ringer solution. Quisqualate-induced motoneuron depolarizations were also facilitated by 8-OH-DPAT in normal Ringer solution, but these increases were eliminated by addition of either tetrodotoxin or the N-methyl-D-aspartate antagonist D(-)-2-amino-5-phosphonovalerate to the Ringer superfusate. Kainate-depolarizations were not altered by low concentrations of serotonin or 8-OH-DPAT. Prior exposure of the cord to spiperone, but not ketanserin or MDL 72222 blocked the enhancement of N-methyl-D-aspartate-induced motoneuron depolarizations by 8-OH-DPAT.(ABSTRACT TRUNCATED AT 250 WORDS)

Serotonin fiber innervation of cerebellar cell suspensions intraparenchymally grafted to the cerebellum of pcd mutant mice

One aspect of integration of implanted neurons into the neuronal circuitry of a defective host brain is the re-establishment of a host-to-graft afferent innervation. We addressed this issue by using the adult cerebellum of 'Purkinje cell degeneration' (pcd) mutant mice, which lack virtually all Purkinje cells after postnatal day (P) 45. Purkinje cells constitute one of the cerebellar cell types being innervated by axons of raphé serotonin (5-HT) neurons. In normal mice, 5-HT-immunoreactive fibers are distributed to all cerebellar folia. Following Purkinje cell loss in pcd mice, cerebellar 5-HT-immunoreactive fibers persist. Cerebellar cell suspensions were prepared from embryonic day (E) 11-13 normal mouse embryos and were intraparenchymally grafted into the cerebellum of pcd mutants either directly or after pre-treatment with 5,7-dihydroxytryptamine (5,7-DHT) to selectively remove 5-HT cells of donor origin. The state of Purkinje cells and 5-HT axons was monitored in alternate sections by 28-kDa Ca(2+)-binding protein (CaBP) and 5-HT immunocytochemistry, respectively. Serotonin-immunoreactive axons were seen in the grafts from 5 to 32 days after transplantation. In some of the grafts which had not been pre-treated with 5,7-DHT, a small number of 5-HT-immunoreactive cell bodies was found, indicating that part of the 5-HT fiber innervation of the graft could actually derive from donor cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Changes in motor performance and rubral single unit activity in cats after microinjections of serotonin into the red nucleus area

The serotonergic control exerted on the red nucleus (RN) was studied in unrestrained cats during the performance of a simple reaction time task which consisted of releasing a lever in response to an auditory go-signal. The effects of microinjections of serotonin-oxalate salt into the rubral area on the motor activity and on the firing of neurons recorded concomitantly in the red nucleus were investigated. Injections of serotonin (5-HT) (200-400 ng) into the red nucleus or its dorsal border induced subtle alterations in the conditioned motor performances but had no major effects on the spontaneous motor behavior. The changes in the conditioned motor output (an increase in the static force exerted on the lever and a speeding up of the lever release) are reminiscent of the facilitatory influence of serotonin on various motor reflexes previously reported. Changes in the neuronal activity were observed concomitantly with the effects on the motor output: 5-HT either enhanced or reduced the firing rate of the rubral neurons. These effects were apparently dependent on the discharge pattern of the neurons during the static motor activity. The results suggest that the serotonergic input to the red nucleus may participate in motor control by exerting a dual modulatory action on the activity of rubral neurons.

Serotonin-immunoreactivity in the cerebellum of two neurological mutant mice and the corresponding wild-type genetic stocks

Using a serotonin (5-HT)-specific antibody, we examined the 5-HTergic innervation of the cerebellum in the normal mouse (+/+) and in two neurological mutants: weaver (wv/wv), which are characterized by a genetically determined loss of granule cells, and 'Purkinje cell degeneration' (pcd/pcd), which are characterized by a genetically determined loss of Purkinje cells. In normal cerebellum, serotonin-immunoreactive (5-HT-ir) fibers are discrete and ascend to all three layers of the cerebellar cortex. Serotonin-immunoreactive fibers have a much higher density in the atrophic cerebella of both weaver and pcd mutants, where they form multidirectional contours. These anatomical findings provide a profile of 5-HT axon innervation of mouse cerebellum and extend previous neurochemical observations on the metabolic state of cerebellar 5-HT in neurological mutants.

Effects of 8-hydroxy-2-(di-n-propylamino)tetralin, a selective agonist of 5-HT1A receptors, on the cough reflex in rats

The effects of the agonist of 5-HT1A receptors, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), on the capsaicin-induced cough reflex in rats were studied. I.p. injection of 8-OH-DPAT, at doses of 0.1 and 0.3 mg/kg, significantly decreased the number of coughs in a dose-dependent manner. The antitussive effect of 8-OH-DPAT (0.3 mg/kg) was blocked by prior injection of methysergide (3 mg/kg i.p.) and spiperone (0.3 mg/kg i.p.), whereas ketanserin (3 mg/kg i.p.) had no effect on the antitussive effect of 8-OH-DPAT. The antitussive effects of dihydrocodeine (1 mg/kg i.p.) and dextromethorphan (3 mg/kg i.p.) were also antagonized by methysergide and spiperone. However, these cough-depressant effects were not reduced by ketanserin. These results suggest that the antitussive action of 8-OH-DPAT may be related to the enhancement of the function of 5-HT1A receptors, and that antitussives interact with the 5-HT1A receptors.

Serotonin modulates muscimol- and baclofen-elicited inhibition of cerebellar Purkinje cells

The present study was designed to test whether serotonin (5-HT) preferentially affects the GABAA or GABAB receptor subtypes in the cerebellum using muscimol and baclofen as the selective agonists. Serotonin significantly augmented baclofen- and muscimol-elicited inhibitions with minimal effects of 5-HT on the spontaneous activity. The magnitudes of augmentation of baclofen and muscimol effects seen in the presence of 5-HT were comparable. In summary, this study further describes the neuromodulatory role of serotonin on GABAergic inhibition and based upon the present study, it can be concluded that both GABAA and GABAB receptor subtypes are modulated by 5-HT.

Modulation of [3H]-glutamate binding by serotonin in the rat hippocampus: an autoradiographic study

Serotonin (5-HT) added in vitro (10 microM) increased [3H]-glutamate specific binding in the rat hippocampus, reaching statistical significance in layers rich in N-Methyl-D-Aspartate sensitive glutamate receptors. This effect was explained by a significant increase in the apparent affinity of [3H]-glutamate when 5-HT is added in vitro. Two days after lesion of serotonergic afferents to the hippocampus with 5,7-Dihydroxytryptamine [3H]-glutamate binding was significantly decreased in the CA3 region and stratum lacunosum moleculare of the hippocampus, this reduction being reversed by in vitro addition of 10 microM 5-HT. The decrease observed is due to a significant reduction of quisqualate-insensitive (radiatum CA3) and kainate receptors (strata oriens, radiatum, pyramidal of CA3). Five days after lesion [3H]-glutamate binding increased significantly in the CA3 region of the hippocampus but was not different from sham animals in the other hippocampal layers. Two weeks after lesion [3H]-glutamate binding to quisqualate-insensitive receptors was increased in all the hippocampal layers, while kainate and quisqualate-sensitive receptors were not affected. These data are consistent with the possibility that 5-HT is a direct positive modulator of glutamate receptor subtypes.

Synaptic receptors and intracellular signal transduction in the cerebellum

Glutamate receptor subtypes mediating excitatory synaptic neurotransmission in the cerebellar cortex are briefly reviewed from molecular biological, electrophysiological and pharmacological points of view. In particular, molecular biological findings of a novel family of AMPA-selective glutamate receptors are introduced, and the pharmacological and electrophysiological properties and the identity of cerebellar N-methyl-D-aspartate-sensitive receptors probably existing on Purkinje cells are discussed in comparison with well-established cerebral NMDA receptors. As possible intracellular mechanisms of the long-term depression of parallel fiber-Purkinje cell neurotransmission, the perspective of the roles of novel messengers, nitric oxide and arachidonic acid, is particularly commented based on recent information about cerebral long-term events. The specificity and possible independence of cerebellar excitatory amino acid receptors and linked intracellular second messengers are also suggested, taking the highly active guanylate cyclase system in Purkinje cells and other cerebellum-specific proteins into consideration.

Effects of N-methyl-D-aspartate antagonists on the cough reflex

The effects of antagonists of N-methyl-D-aspartate (NMDA) on the capsaicin-induced cough reflex in rats were studied. Intracisternal (i. cist.) injection of MK-801, a non-competitive antagonist of NMDA, significantly decreased the number of coughs in a dose-dependent manner. The competitive antagonists of NMDA, 2-DL-amino-5-phosphonovalerate and 2-DL-amino-7-phosphonoheptanoate, also decreased the number of coughs after i. cist. injection. The antitussive potencies of both the competitive and non-competitive antagonists were similar to that of dextromethorphan. Intraperitoneal injection of MK-801 also decreased the number of coughs in a dose-dependent manner. These data suggest that excitatory amino acid neurotransmitters and NMDA receptors may be involved in the regulation of the cough reflex.

Selective effects of serotonin upon excitatory amino acid-induced depolarizations of Purkinje cells in cerebellar slices from young rats

The effects of serotonin on responses induced in Purkinje cells (PCs) by microiontophoretic administration of excitatory amino acids (EAAs) in their dendritic fields were tested in vitro by extracellular recording and by single electrode voltage clamp methods in cerebellar slices from rats aged 16-22 days. Serotonin diminished excitations produced by glutamate (Glu) and quisqualate (Quis) selectively, those caused by N-methyl-D-aspartate (NMDA) being affected much less. These suppressions of Glu- and Quis-induced responses generally occurred without there being any effect on intrinsic membrane properties of PCs, although on occasion serotonin increased membrane conductance slightly and/or induced an outward current in the recorded cells. All these effects of serotonin were maintained in the presence of tetrodotoxin and reversed upon removal of the amine. On the few occasions when serotonin enhanced Quis-induced responses, the effect was mimicked by ejection from a control solution of saline, made up at the same pH as the drug solution of serotonin.

Modulatory role of serotonin on GABA-elicited inhibition of cerebellar Purkinje cells

The present study was designed to directly examine the postsynaptic actions of serotonin on GABA-mediated inhibition of cerebellar Purkinje cells. The findings indicate that serotonin at currents that produced minimal effects on the spontaneous firing rates of Purkinje cells modified GABA effects in a biphasic manner. Serotonin initially decreased GABA-mediated inhibitions followed secondarily by either continued inhibition or, in the majority of cases, augmentation of GABA responses. When a comparison was made of the secondary effects of serotonin on GABA-mediated inhibition with the initial spontaneous firing rates of the Purkinje cells, the group in which serotonin augmented GABA actions had a significantly higher initial firing frequency than the group in which serotonin attenuated GABA-mediated inhibition. Furthermore, with increasing firing rates, the proportion of cells showing augmentation of GABA inhibition increased, and the proportion of cells displaying attenuation of GABA effects decreased. Serotonin affected beta-alanine-mediated inhibitions in a manner similar to that seen with GABA, whereas glycine was differentially altered. This study identifies another neuromodulatory role of serotonin on Purkinje cells in the cerebellum. Furthermore, the effects of serotonin on GABA inhibition seem to be governed by some intrinsic property of the Purkinje cell, which is apparently related to the firing rate of the cell.

Modulatory effects of serotonin on excitatory amino acid responses and sensory synaptic transmission in the ventrobasal thalamus

Excitatory amino acid receptors are thought to mediate sensory input to the ventrobasal thalamus. There is evidence for a brainstem serotonergic projection to the ventrobasal thalamus which may have a modulatory role. The possibility that serotonin may selectively modulate responses to excitatory amino acid receptor agonists, and its effects on sensory synaptic transmission has been examined in the rat ventrobasal thalamus in vivo. Iontophoretic ejection of serotonin at low currents produced a marked facilitation of responses to excitatory amino acids. In contrast, excitatory responses to cholinomimetic agonists were attenuated. Synaptic transmission was concomitantly enhanced or unchanged in these circumstances. Higher serotonin ejection currents reversed the facilitation, or inhibited excitatory amino acid responses and synaptic transmission. It is concluded that serotonin can modulate responses to excitatory amino acids relatively selectively and that synaptic transmission of somatosensory information through the ventrobasal thalamus may be susceptible to brainstem serotonergic modulation.

Histamine modulates local inhibition in the rat hippocampal slice

1. These experiments investigated the action of histamine on local inhibition in the CA1 region of the in vitro hippocampal slice preparation using a paired-pulse paradigm. 2. We observed that histamine produced a concentration-dependent and reversible attenuation of paired-pulse inhibition. This effect was reduced by the H2 receptor antagonist, cimetidine, and mimicked by the H2 receptor agonist, impromidine. 3. We also observed that histamine produced concentration-dependent effects on the amplitude of the population spike that could be correlated with alterations in the field excitatory postsynaptic potential (EPSP) amplitude and input fiber volley. High concentrations of histamine produced a reduction in the amplitude of the population spike which was always accompanied by a reduction in the EPSP and fiber volley amplitude. 4. These results suggest that histamine, through the occupancy of H2 receptors, acts to modulate the efficacy of the local synaptic circuitry which is involved in producing paired-pulse inhibition in the hippocampus.

The pattern of distribution of serotoninergic fibers in the anterior horn of the chick spinal cord

The pattern of distribution of serotonin positive fibers in the motor nuclei of the chick spinal cord was examined immunohistochemically by using an antiserum against serotonin. A dense aggregation of serotoninergic fibers was located around anterior horn cells in the cervical spinal cord. In the brachial spinal cord, serotoninergic fibers were densely aggregated in the medial motor column and in the parts of the lateral motor column. There were two regions of serotonin immunoreactivity in the lateral motor column of the brachial spinal cord; one located in the ventromedial regions where a dense aggregation of serotoninergic fibers was found, and the reminder of the lateral motor column where only a few serotoninergic fibers were observed. The region containing a dense cluster of serotoninergic fibres around profiles of motoneuron somata and proximal dendrites appears to correspond to motor neuron pools of flexor muscles. In the thoracic spinal cord a high density of serotoninergic fibers was found in the motor nucleus. In the lumbosacral spinal cord (segments LS1-LS8) serotoninergic fibers were not observed in the medial motor column. However, there were five regions in the lateral motor column, where a high density of serotoninergic fibers was found. These very likely correspond to motor neuron pools of muscles which extend the hip joint.

Pentobarbital augments serotonin-mediated inhibition of cerebellar Purkinje cells

The ability of pentobarbital to modify the direct effects of iontophoretically ejected serotonin on the firing rates of cerebellar Purkinje cells was examined. Serotonin elicited inhibition, excitation, or a biphasic effect on cerebellar Purkinje cells. With continuous application of iontophoretic pentobarbital at currents found to potentiate GABA-induced inhibition, serotonin-mediated inhibitions were also augmented consistently. When application of serotonin elicited excitation, including a late component of biphasic responses, iontophoretic pentobarbital converted the effect to, primarily, inhibition. Besides increasing the magnitude of serotonin-mediated inhibition, iontophoretic pentobarbital increased the duration of this effect. In another series of experiments using pentobarbital rather than urethan as the anesthetic, serotonin-mediated inhibition was significantly augmented for all ejection currents tested. The GABA antagonists bicuculline, pentylenetetrazole and picrotoxin attenuated pentobarbital augmentation of serotonin-elicited inhibition. We conclude that serotonin-mediated inhibition of Purkinje cells is modifiable by pentobarbital and this effect bears a strong semblance to the actions of barbiturates on GABAergic neurotransmission.

Serotonin concentration and turnover in cerebellum and other brain regions of pcd mutant mice

The Purkinje cell degeneration (pcd) mutant mouse is characterized by a loss of Purkinje cells in the cerebellum. Loss of granule cells occurs and is severe in pcd mutants after 9 months of age. Since Purkinje cells and granule cells represent two groups of target cells for serotonin neurons projecting from raphe nuclei and other brain areas, the content and turnover of serotonin in the cerebellum were determined in pcd mice aged 3-15 months. The content of serotonin was not decreased in pcd mouse cerebellum but tended to be slightly increased after 7 months. The ratio of 5-hydroxyindoleacetic acid (5-HIAA) to serotonin was significantly decreased in cerebellum at 7-15 months but not at 3 or 6 months. The decrease in this ratio is indicative of decreased serotonin turnover. Similar changes were not seen in brainstem or hypothalamus in mice up to 14 months old, but slight decreases were observed at 15 months. Another index of turnover, the accumulation of 5-HIAA after administration of probenecid to block its efflux from brain, was decreased by 46% in 7-month-old pcd mice in the cerebellum but not in the brainstem or hypothalamus. The decrease in serotonin turnover in pcd mouse cerebellum occurs subsequent to and perhaps due to the loss of the target Purkinje and granule cells.

The effects of serotonin on N-methyl-D-aspartate and synaptically evoked depolarizations in rat neocortical neurons

The effects of serotonin (5-HT) on neuronal responses to the excitatory amino acid agonist N-methyl-D-aspartate (NMDA) were examined in neocortical slices of the Fischer rat using current-clamp and single-electrode voltage-clamp techniques. Layer V neocortical neurons responded to application of NMDA by depolarization with no change or an apparent increase in input resistance. Following perfusion with 10(-5) M 5-HT, the response of these neurons to NMDA was significantly increased in both amplitude and duration, whereas neuronal responses to quisqualic acid and acetylcholine were not altered by 5-HT. Furthermore, the enhanced response to NMDA in 5-HT was long-lasting, and could not be reversed during the course of the experiment. Resting membrane potential and the postspike train afterhyperpolarization were not significantly altered by 5-HT, although the input resistance was decreased by 5-HT. Excitatory postsynaptic potentials (EPSPs) were usually not affected or reversibly decreased by 5-HT. However, in a few cells exhibiting a complex voltage-dependent EPSP, 5-HT produced a long-lasting enhancement in the amplitude of the EPSP. Under voltage-clamp conditions, with Na+- and K+-channels blocked, 5-HT enhanced the inward current stimulated by application of NMDA. It is suggested that 5-HT selectively enhances the voltage- and Ca2+-dependent NMDA response.

Developmental changes in density and distribution of serotoninergic fibers in the chick spinal cord

Developmental changes of serotoninergic innervation in the chick spinal cord (third lumbosacral segment) were examined with an immunohistochemical technique using an antiserum to serotonin. In the 1-day-old hatched chick, serotoninergic fibers were located in laminae I, II, VII, IX, and X. A large number of serotonin-positive fibers and terminals were found around somal profiles of large neurons and in the neuropil of the medial and lateral parts of the lateral motor column (LMC). In the 1-week-old chick, the density of serotoninergic fibers was greatly increased in the posterior columns, and serotoninergic fibers were most densely aggregated in the dorsolateral part of the LMC. In the 2-week-old chick, a considerable decrease in the density of serotoninergic fibers was observed in the lateral funiculus and the gray matter (laminae I, II, VII, IX, and X). In the LMC, serotonin-positive fibers and terminals were largely absent from the neuropil, but were found preferentially around the somal profiles of large neurons. Between 1 and 2 weeks after hatching the density of varicosities and terminals in the neuropil of the dorsolateral and medial parts of the LMC decreased by 33% and 56%, respectively. In the 3-month-old chick, the density of serotoninergic fibers in laminae I, II, V, VII, and X had increased compared to younger ages. Serotonin-positive fibers were not evenly distributed in the LMC of the adult chicken; rather, they were densely aggregated around the soma and proximal dendrites of motoneurons in the dorsolateral LMC. Many neuronal soma in the medial and intermediate regions of the LMC lacked serotoninergic fibers.

The modulation of excitatory amino acid responses by serotonin in the cat neocortex in vitro

1. The electrophysiological actions of excitatory amino acids and serotonin were investigated in slices from cat neocortex in vitro. Intracellular recordings were obtained from neurons (mainly in layer V) and the drugs applied extracellularly to the same neurons by microiontophoresis. 2. Serotonin, and to some extent noradrenaline, facilitated the excitatory actions of N-methyl-D-aspartate (NMDA), glutamate, and quisqualate but caused no changes in the passive neuronal membrane properties when presented alone. Serotonin had no effect on evoked excitatory postsynaptic potentials (EPSPs) or spike afterhyperpolarizations. 3. The facilitatory effect of serotonin on the responses to NMDA was observed with both somatic and dendritic applications. It persisted during Mg2+ depletion and in the presence of tetrodotoxin and tetraethylammonium. The effect was attenuated by the serotonin antagonist cinanserin but not by methysergide. A possible underlying receptor modulation is discussed.

Modulation of cerebellar granule cell activity by iontophoretic application of serotonergic agents

Serotonergic fibers have been identified within the granule cell layer of the cerebellar cortex; however, their functional significance has not been identified. In this study the effect of serotonin on granule cell spontaneous activity was determined in the rat cerebellum. Of the 136 granule cells tested, 44.8% displayed a decrease in firing rate, 21.3% increased firing rate and 33.8% were not affected. The serotonin-induced changes in activity were not blocked by bicuculline or methysergide. The serotonin agonist 1,3 (trifluoromethylphenyl) piperazine mimicked the serotonin-induced suppressive response. Iontophoretically applied serotonin was also found to modulate GABA-induced suppression of granule cell activity. The variable effects of serotonin on spontaneous activity suggests the presence of more than one type of serotonergic receptor in the cerebellar granule cell layer.