The α2β2 isoform combination dominates the astrocytic Na+ /K+ -ATPase activity and is rendered nonfunctional by the α2.G301R familial hemiplegic migraine type 2-associated mutation

Synaptic activity results in transient elevations in extracellular K+ , clearance of which is critical for sustained function of the nervous system. The K+ clearance is, in part, accomplished by the neighboring astrocytes by mechanisms involving the Na+ /K+ -ATPase. The Na+ /K+ -ATPase consists of an α and a β subunit, each with several isoforms present in the central nervous system, of which the α2β2 and α2β1 isoform combinations are kinetically geared for astrocytic K+ clearance. While transcript analysis data designate α2β2 as predominantly astrocytic, the relative quantitative protein distribution and isoform pairing remain unknown. As cultured astrocytes altered their isoform expression in vitro, we isolated a pure astrocytic fraction from rat brain by a novel immunomagnetic separation approach in order to determine the expression levels of α and β isoforms by immunoblotting. In order to compare the abundance of isoforms in astrocytic samples, semi-quantification was carried out with polyhistidine-tagged Na+ /K+ -ATPase subunit isoforms expressed in Xenopus laevis oocytes as standards to obtain an efficiency factor for each antibody. Proximity ligation assay illustrated that α2 paired efficiently with both β1 and β2 and the semi-quantification of the astrocytic fraction indicated that the astrocytic Na+ /K+ -ATPase is dominated by α2, paired with β1 or β2 (in a 1:9 ratio). We demonstrate that while the familial hemiplegic migraine-associated α2.G301R mutant was not functionally expressed at the plasma membrane in a heterologous expression system, α2+/G301R mice displayed normal protein levels of α2 and glutamate transporters and that the one functional allele suffices to manage the general K+ dynamics.

Short-Term Synaptic Plasticity at Interneuronal Synapses Could Sculpt Rhythmic Motor Patterns

The output of a neuronal network depends on the organization and functional properties of its component cells and synapses. While the characterization of synaptic properties has lagged cellular analyses, a potentially important aspect in rhythmically active networks is how network synapses affect, and are in turn affected by, network activity. This could lead to a potential circular interaction where short-term activity-dependent synaptic plasticity is both influenced by and influences the network output. The analysis of synaptic plasticity in the lamprey locomotor network was extended here to characterize the short-term plasticity of connections between network interneurons and to try and address its potential network role. Paired recordings from identified interneurons in quiescent networks showed synapse-specific synaptic properties and plasticity that supported the presence of two hemisegmental groups that could influence bursting: depression in an excitatory interneuron group, and facilitation in an inhibitory feedback circuit. The influence of activity-dependent synaptic plasticity on network activity was investigated experimentally by changing Ringer Ca(2+) levels, and in a simple computer model. A potential caveat of the experimental analyses was that changes in Ringer Ca(2+) (and compensatory adjustments in Mg(2+) in some cases) could alter several other cellular and synaptic properties. Several of these properties were tested, and while there was some variability, these were not usually significantly affected by the Ringer changes. The experimental analyses suggested that depression of excitatory inputs had the strongest influence on the patterning of network activity. The simulation supported a role for this effect, and also suggested that the inhibitory facilitating group could modulate the influence of the excitatory synaptic depression. Short-term activity-dependent synaptic plasticity has not generally been considered in spinal cord models. These results provide further evidence for short-term plasticity between locomotor network interneurons. As this plasticity could influence the patterning of the network output it should be considered as a potential functional component of spinal cord networks.

Growth of rat cortical neurons on DuraGen, a collagen-based dural graft matrix

OBJECTIVES

DuraGen, a collagen-based dural graft matrix, is frequently used in clinical neurosurgery. In the present study we examined whether DuraGen influenced neuron survival of or process growth from cerebral cortex neurons in culture.

METHODS

Dissociated E19 rat cerebral cortical neurons were cultured at low density on poly-L-lysine or on cryostat-sectioned DuraGen. Neuron survival was assessed using morphological criteria, fluorescein diacetate (FDA) and propidium iodide (PI), nuclear staining and TUNEL labeling. Process growth was analysed using specific antibodies against MAP2 and the 200 kDa neurofilament subunit (NF-H) to identify dendrites and axons, respectively.

RESULTS

In immature cultures (3 days in vitro, DIV), nearly 70% of the neurons remained viable in control and DuraGen-exposed cells. In mature cultures (10 DIV), approximately 45% of the neurons were viable. Survival was similar in DuraGen cultures and controls. Cell viability also was similar when DuraGen conditioned the medium, but was not in contact with the neurons. When 10-day-old cultures were treated with glutamate (100 mumol/l for 24 hours) to elicit excitotoxic injury, a 40% decrease in neuron survival was observed. DuraGen's presence neither exacerbated nor attenuated glutamate-induced excitotoxic neuron death. The amount of necrotic or apoptotic cells also was similar in control and DuraGen cultures. Finally, DuraGen had an equal ability to support both axon and dendrite growth as poly-L-lysine.

CONCLUSION

Our findings demonstrate that DuraGen has no adverse effect on survival of or process growth from cerebral cortical neurons in vitro. These data support DuraGen's biosafety as a dural substitute in clinical neurosurgery.

[Inhibitory regulation of glutamate receptors in the frog motoneuron]

The interaction of exogenously applied excitatory (glutamate and their agonists NMDA, AMPA, kainate) and inhibitory (glycine and GABA) amino acid effects was studied intracellularly in the motoneurones of the isolated frog spinal cord. During simultaneous glycine or GABA bath applications GLU-, AMPA-, KA- and NMDA-evoked responses were, respectively, decreased up to 45.8 +/- 2.9% (n = 12) and 67.8 +/- 3.9% (n = 16), 13.9 +/- 4.3% (n = 9) and 32.1 +/- 8.3% (n = 12), 36.8 +/- 8.2% (n = 7) and 48.0 +/- 11.8% (n = 6), 7.7 +/- 3.5% (n = 9) and 18.1 +/- 3.8% (n = 14) from the control. Sequential applications of EAA after glycine or GABA as well as the applications of EAA-agonist and glycine (GABA) mixture demonstrated similar results. The decrease of EAA-responses by glycine and GABA was abolished by selective GlyR antagonist strychnine (1 microM) and the selective GABAR antagonist SR95531 (gabazine, 20 MM), respectively. The data revealed differences in inhibitory effect of glycine and GABA on the excitation responses mediated by different types of glutamate receptors in the frog motoneurones: the predominant inhibitory effect of glycine and GABA on NMDA-responses and weak inhibitory effect on KA- and GLU-responses. Inhibitory effect of glycine was twice as much as that of GABA at the same concentration.

[Spontaneous activity level and spike responses of cortical neurons to local supply of amino acids for dendrites and soma]

In guinea-pig parietal cortex slices, it was shown that neuronal spontaneous activity depended on dendro-somatic propagation of excitations evoked in dendrites. The functional dendritic properties are essentially non-uniform in the population of cortical neurons. Spike responses to direct soma activations are quite stable among neurons with various levels of spontaneous activity.

Identification of different types of respiratory neurones in the dorsal brainstem nucleus tractus solitarius of the rat

In Nembutal anaesthetised, spontaneously breathing rats, stereotaxic mapping of the nucleus tractus solitarius (NTS) for respiratory neuronal activity was undertaken. Eight different types of respiratory cells were found between 0.25 and 1.5 mm lateral to midline, extending 0.5 mm caudal to 1.5 mm rostral to obex, and 0.4-1.5 mm below the dorsal surface. A study of the respiratory motor (diaphragm EMG) and neuronal responses to excitatory amino acid (EAA) stimulation of the NTS areas was undertaken. Electrical stimulation of the vagus nerve was employed to study the NTS cellular responses to activation of pulmonary afferents. The effects of chemical activation of the midbrain periaqueductal grey (PAG) on NTS respiratory neuronal activity were investigated. EAA microinjections into the ventrolateral NTS rostral to the obex resulted in an increase in respiratory motor frequency along with increases to inspiratory cell discharge, whilst microinjections into the medial NTS caudal to the obex caused respiratory depression. EAA stimulation of calamus scriptorius produced apnea. NTS inspiratory neurones were inhibited following stimulation of ipsilateral vagus nerve, suggesting their involvement in the Hering-Breuer reflex pathway. PAG stimulation caused excitation of the NTS inspiratory cells indicating the presence of an excitatory respiratory pathway between the two nuclei. Following beta-adrenergic antagonist pre-treatment of ventrolateral NTS, EAA microinjections into PAG did not evoke a cardiorespiratory effect. Based on the various findings the role of NTS in organising respiration in the rat is discussed.

Inhibition by the chromaffin cell-derived peptide serine-histogranin in the rat's dorsal horn

The heptadecapeptide histogranin, synthesized by adrenal chromaffin cells, is implicated in the analgesia produced by transplanting chromaffin cells into the spinal cord, including block of hyperalgesia mediated by NMDA-subtype glutamate receptors. To examine the neurophysiological basis for this analgesia, we applied the stable analog [Ser(1)]-histogranin (SHG) by iontophoresis near extracellularly recorded wide-dynamic range (WDR) neurons in anesthetized rats. When SHG was applied during peripheral electrical stimulation of A and C fibers at 0.1Hz, the C-fiber response was significantly inhibited but the A-fiber response was unaffected. SHG also opposed the NMDA-receptor-dependent post-tetanic facilitation (wind-up) of C-fiber responses produced by increasing the rate of peripheral afferent stimulation to 1Hz for 20s. To test whether block of NMDA-subtype receptors could be wholly or partially responsible for this suppression, SHG was applied during sequential pulsed iontophoresis of three agonists targeting distinct excitatory synaptic receptors: NMDA, kainate and substance P. All three excitatory effects were reversed by SHG; this reversal outlasted the 10-30min observation period when higher SHG doses were applied (>60nA). Histogranin therefore probably produces prolonged spinal analgesia by opposing the basal and potentiating synaptic effects of C-fibers on dorsal horn neurons. Actions besides or in addition to NMDA-receptor antagonism (e.g., agonism at inhibitory postsynaptic receptors or block of voltage-gated cation channels on C-fibers) are implied by the diversity of excitatory transmitters opposed by SHG.

Variability and corresponding amplitude-velocity relation of activity propagating in one-dimensional neural cultures

We investigate the propagation of neural activity along one-dimensional rat hippocampal cultures patterned in lines over multielectrode arrays. Activity occurs spontaneously or is evoked by local electrical or chemical stimuli, with different resulting propagation velocities and firing rate amplitudes. A variability of an order of magnitude in velocity and amplitude is observed in spontaneous activity. A linear relation between velocity and amplitude is identified. We define a measure for neuron activation synchrony and find that it correlates with front velocity and is higher for electrically evoked fronts. We present a model that explains the linear relation between amplitude and velocity, which highlights the role of synchrony. The relation to current models for signal propagation in neural media is discussed.

Effects of galanin-like peptide on luteinizing hormone secretion in the rat: sexually dimorphic responses and enhanced sensitivity at male puberty

Reproductive function is exquisitely sensitive to adequacy of nutrition and fuel reserves, through mechanisms that are yet to be completely elucidated. Galanin-like peptide (GALP) has recently emerged as another neuropeptide link that couples reproduction and metabolism. However, although the effects of GALP on luteinizing hormone (LH) secretion have been studied, no systematic investigation on how these responses might differ along sexual maturation and between sexes has been reported. Moreover, the influence of metabolic status and potential interplay with other relevant neurotransmitters controlling LH secretion remain ill defined. These facets of GALP physiology were addressed herein. Intracerebral injection of GALP to male rats induced a dose-dependent increase in serum LH levels, the magnitude of which was significantly greater in pubertal than in adult males. In contrast, negligible LH responses to GALP were detected in pubertal or adult female rats at diestrus. Neonatal androgen treatment to females failed to "masculinize" the pattern of LH response to GALP. In addition, metabolic stress by short-term fasting did not prevent but rather amplified LH responses to GALP in pubertal males, whereas these responses were abrogated by pharmacological inhibition of nitric oxide synthesis. We conclude that the ability of GALP to evoke LH secretion is sexually differentiated, with maximal responses at male puberty, a phenomenon which was not reverted by manipulation of sex steroid milieu during the critical neonatal period and was sensitive to metabolic stress. This state of LH hyperresponsiveness may prove relevant for the mechanisms relaying metabolic status to the reproductive axis in male puberty.

Down-regulation of glutaredoxin by estrogen receptor antagonist renders female mice susceptible to excitatory amino acid mediated complex I inhibition in CNS

beta-N-oxalyl-amino-L-alanine, (L-BOAA), an excitatory amino acid, acts as an agonist of the AMPA subtype of glutamate receptors. It inhibits mitochondrial complex I in motor cortex and lumbosacral cord of male mice through oxidation of critical thiol groups, and glutaredoxin, a thiol disulfide oxido-reductase, helps maintain integrity of complex I. Since incidence of neurolathyrism is less common in women, we examined the mechanisms underlying the gender-related effects. Inhibition of complex I activity by L-BOAA was seen in male but not female mice. Pretreatment of female mice with estrogen receptor antagonist ICI 182,780 or tamoxifen sensitizes them to L-BOAA toxicity, indicating that the neuroprotection is mediated by estrogen receptors. L-BOAA triggers glutathione (GSH) loss in male mice but not in female mice, and only a small but significant increase in oxidized glutathione (GSSG) was seen in females. As a consequence, up-regulation of gamma-glutamyl cysteinyl synthase (the rate-limiting enzyme in glutathione synthesis) was seen only in male mouse CNS but not in females. Both glutathione reductase and glutaredoxin that reduce oxidized glutathione and protein glutathione mixed disulfides, respectively, were constitutively expressed at higher levels in females. Furthermore, glutaredoxin activity in female mice was down-regulated by estrogen antagonist indicating its regulation by estrogen receptor. The higher constitutive expression of glutathione reductase and glutaredoxin could potentially confer neuroprotection to female mice.

Roles of Ca2+and protein kinase C in the excitatory response to serotonin in embryonic molluscan ciliary cells

We examined the roles of Ca2+and protein kinase C (PKC) in the cilio-excitatory response to serotonin in pedal ciliary cells from Helisoma trivolvis embryos. Serotonin (5-hydroxytryptamine; 5-HT; 100 µmol/L) induced an increase in ciliary beat frequency (CBF) was abolished by microinjected BAPTA (50 mmol/L), but was only partially inhibited by the phospholipase C inhibitor U-73122 (10 µmol/L). The diacylglycerol analogs 1-oleoyl-2-acetyl-sn-glycerol (100 µmol/L) and 1,2-dioctanoyl-sn-glycerol (100 µmol/L) caused increases in [Ca2+]ithat were smaller than those induced by serotonin. In the absence of extracellular Ca2+, 1,2-dioctanoyl-sn-glycerol (100 µmol/L) failed to elicit an increase in both CBF and [Ca2+]i. In contrast, the serotonin-induced increase in CBF persisted in the absence of extracellular Ca2+, although the increase in [Ca2+]iwas abolished. PKC inhibitors bisindolylmaleimide (10 and 100 nmol/L) and calphostin C (10 nmol/L) partially inhibited the serotonin-induced increase in CBF, but didn’t affect the serotonin-induced change in [Ca2+]i. These findings suggest that an intracellular store-dependent increase in [Ca2+]imediates the cilio-excitatory response to serotonin. Furthermore, although PKC is able to cause an increase in [Ca2+]ithrough calcium influx, it contributes to the cilio-excitatory response to 5-HT through a different mechanism.

Ion permeation through a voltage- sensitive gating pore in brain sodium channels having voltage sensor mutations

Voltage-gated sodium channels activate in response to depolarization, but it is unknown whether the voltage-sensing arginines in their S4 segments pivot across the lipid bilayer as voltage sensor paddles or move through the protein in a gating pore. Here we report that mutation of pairs of arginine gating charges to glutamine induces cation permeation through a gating pore in domain II of the Na(V)1.2a channel. Mutation of R850 and R853 induces a K(+)-selective inward cationic current in the resting state that is blocked by activation. Remarkably, mutation of R853 and R856 causes an outward cationic current with the opposite gating polarity. These results support a model in which the IIS4 gating charges move through a narrow constriction in a gating pore in the sodium channel protein during gating. Paired substitutions of glutamine allow cation movement through the constriction when appropriately positioned by the gating movements of the S4 segment.

Glutamatergic stimulation of the medial amygdala induces steroid dependent c-fos expression within forebrain nuclei responsive to mating stimulation

Neurons within the posterodorsal medial amygdala of female rats are known to process vaginocervical stimulation received during mating through N-methyl-D-aspartate channel activation, conveying information to downstream hypothalamic cell groups that modulate neuroendocrine function. Stimulation of these neurons with an excitatory amino acid cocktail of glutamate, aspartate and glycine initiates 10-12 days of prolactin surge secretion that normally are observed only after the receipt of vaginocervical stimulation. Posterodorsal medial amygdala neurons responsive to vaginocervical stimulation also contain estrogen and progesterone receptors. The present experiment examined which downstream sites involved in prolactin secretion show c-fos expression following glutamate receptor activation within the posterodorsal medial amygdala and whether ovarian steroids influence cellular activation in these areas. Ovariectomized female rats implanted with unilateral cannulas directed at the posterodorsal medial amygdala received injections of estradiol benzoate and progesterone or oil before infusion treatment with either excitatory amino acid or control PBS. An additional group of estradiol benzoate+progesterone-treated females was infused with 1.0 microM glycine alone in PBS. Infusions were administered three times at 30 min intervals. FOS induction 90 min after infusion was determined immunohistochemically on the sides ipsilateral and contralateral to the infusion. Of the examined regions, excitatory amino acid treatment and hormone treatment induced three patterns of c-fos expression: 1) responses to both excitatory amino acid and hormone treatment [posterodorsal medial amygdala, medial preoptic area, ventrolateral ventromedial hypothalamic nucleus, bed nucleus of the stria terminalis]; 2) responses to estradiol benzoate+progesterone treatment only [anteroventral periventricular nucleus and dorsomedial nucleus]; and 3) responses to excitatory amino acid only [arcuate nucleus, suprachiasmatic nucleus, and paraventricular nucleus]. These data identify possible circuits by which vaginocervical stimulation, via activation of posterodorsal medial amygdala glutamate-type receptors, initiates and coordinates a series of events within a larger neuroendocrine circuit important for pregnancy.

Neuroprotective and abstinence-promoting effects of acamprosate: elucidating the mechanism of action

Acamprosate is an abstinence-promoting drug widely used in the treatment of alcohol dependence but which has a mechanism of action that has remained obscure for many years. Recently, evidence has emerged that this drug may interact with excitatory glutamatergic neurotransmission in general and as an antagonist of the metabotropic glutamate receptor subtype 5 (mGluR5) in particular. These findings provide, for the first time, a satisfactory, unifying hypothesis that can bring together and explain the diverse neurochemical effects of acamprosate. Glutamic acid is involved in several aspects of alcohol dependence and withdrawal, many of which can be modified by acamprosate. For example, during chronic exposure to alcohol, the glutamatergic system becomes upregulated, leaving the brain exposed to excessive glutamatergic activity when alcohol is abruptly withdrawn. The surge in glutamic acid release that occurs following alcohol withdrawal can be attenuated by acamprosate. The elevated extracellular levels of glutamic acid observed in withdrawal, together with supersensitivity of NMDA receptors, may expose vulnerable neurons to excitotoxicity, possibly contributing to the neuronal loss sometimes observed in chronic alcohol dependence. In vitro studies suggest that the excitotoxicity produced by ethanol can effectively be blocked by acamprosate. Moreover, glutamatergic neurotransmission plays an important role in the acquisition of cue-elicited drinking behaviours, which again can be modulated by acamprosate. In conclusion, the glutamatergic hypothesis of the mechanism of action of acamprosate helps explain many of its effects in human alcohol dependence and points the way to potential new activities, such as neuroprotection, that merit exploration in the clinic.

Divergent Pharmacological Activity of Novel Marine-Derived Excitatory Amino Acids on Glutamate Receptors

Kainate receptors show a particular affinity for a variety of natural source compounds, including dysiherbaine (DH), a potent agonist derived from the marine sponge Dysidea herbacea. In this study, we characterized the pharmacological activity and structural basis for subunit selectivity of neodysiherbaine (neoDH) and MSVIII-19, which are natural and synthetic analogs of DH, respectively. NeoDH and MSVIII-19 differ from DH in the composition of two functional groups that confer specificity and selectivity for ionotropic glutamate receptors. In radioligand binding assays, neoDH displayed a 15- to 25-fold lower affinity relative to that of DH for glutamate receptor (GluR)5 and GluR6 kainate receptor subunits but a 7-fold higher affinity for kainate (KA)2 subunits, whereas MSVIII-19 displaced [(3)H]kainate only from GluR5 subunits but not GluR6 or KA2 subunits. NeoDH was an agonist for kainate and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors in patch-clamp recordings; in contrast, MSVIII-19 acted as a potent antagonist for homomeric GluR5 receptor currents with weaker activity on other kainate and AMPA receptors. Neither neoDH nor MSVIII-19 activated group I metabotropic GluRs. Homology modeling suggests that two critical amino acids confer the high degree of selectivity between the dysiherbaine analogs and the GluR5 and KA2 subunits. In summary, these data describe the pharmacological activity of two new compounds, one of which is a selective GluR5 receptor antagonist that will be of use for understanding native receptor function and designing more selective ligands for kainate receptors.

Ocular inflammation alters swelling and membrane characteristics of rat Müller glial cells

Ocular inflammation is a common cause of retinal edema that may involve swelling of Müller glial cells. In order to investigate whether endotoxin-induced ocular inflammation in rats alters the swelling and membrane characteristics of Müller cells, lipopolysaccharide (LPS; 0.5%) was intravitreally injected. At 3 and 7 days after treatment, hypotonic challenge induced swelling of Müller cell somata that was not observed in non-treated control eyes. Müller cells of LPS-treated eyes displayed a downregulation of inward K(+) currents and upregulation of A-type K(+) currents that was associated with a decreased expression of Kir4.1 protein in retinal slices. The data suggest that ocular inflammation induces alterations of both the swelling characteristics and the K(+) channel expression of Müller cells.

Effects of KiSS-1 peptide, the natural ligand of GPR54, on follicle-stimulating hormone secretion in the rat

KiSS-1 was originally identified as a metastasis suppressor gene encoding an array of structurally related peptides, namely kisspeptins, which acting through the G protein-coupled receptor GPR54 are able to inhibit tumor progression. Unexpectedly, a reproductive facet of this newly discovered system has recently arisen, and characterization of the role of the KiSS-1/GPR54 system in the neuroendocrine control of gonadotropin secretion has been initiated. However, such studies have been so far mostly restricted to LH, and very little is known about the actual contribution of this system in the regulation of FSH release. To address this issue, the effects of KiSS-1 peptide on FSH secretion were monitored in vivo and in vitro under different experimental conditions. Intracerebroventricular administration of KiSS-1 peptide significantly stimulated FSH secretion in prepubertal and adult rats. Yet, dose-response analyses in vivo demonstrated an ED(50) value for the FSH-releasing effects of KiSS-1 of 400 pmol, i.e. approximately 100-fold higher than that of LH. In addition, systemic (ip and iv) injection of KiSS-1 significantly stimulated FSH secretion in vivo. However, KiSS-1 failed to elicit basal FSH release directly at the pituitary level, although it moderately enhanced GnRH-stimulated FSH secretion in vitro. Finally, mechanistic studies revealed that the ability of KiSS-1 to elicit FSH secretion was abolished by the blockade of endogenous GnRH actions, but it was persistently observed in different models of leptin insufficiency and after blockade of endogenous excitatory amino acid and nitric oxide pathways, i.e. relevant signals in the neuroendocrine control of gonadotropin secretion. In summary, our results extend previous recent observations on the role of KiSS-1 in the control of LH secretion and provide solid evidence for a stimulatory effect of KiSS-1 on FSH release, acting at central level. Overall, it is proposed that the KiSS-1/GPR54 system is a novel, pivotal downstream element in the neuroendocrine network governing gonadotropin secretion.

Mapping of chemical trigger zones for reward

Addictive drugs are thought to activate brain circuitry that normally mediates more natural rewards such as food or water. Drugs activate this circuitry at synaptic junctions within the brain; identifying the junctions at which this occurs provides clues to the neurochemical and anatomical characteristics of the circuitry. One approach to identifying the junctions at which drugs interact with this circuitry is to determine if animals will lever-press for site-specific microinjections of addictive drugs. This approach has identified GABAergic, dopaminergic, glutamatergic, and cholinergic trigger zones within meso-corticolimbic circuitry important for natural reward function.

Recent understanding in the mechanisms of addiction

The rewarding effects of drugs of abuse have been linked to increases in dopamine transmission. However, changes in brain chemistry and morphology that are produced in addiction underlie the long-lasting vulnerability to relapse and are more closely linked with the adaptations in excitatory transmission. The drug-induced changes in excitatory transmission seem to be pathologic exacerbations of normal forms of brain plasticity, and they occur in the brain areas linked by neuroimaging studies in addicted patients to craving and relapse. This review describes the brain adaptations produced in excitatory transmission by addictive drugs and identifies new potential sites of pharmacotherapeutic intervention to ameliorate addiction.

Excitatory amino acid induced oligodendrocyte cell death in vitro: receptor-dependent and -independent mechanisms

Oligodendroglia play an important role in axonal conduction in the CNS and are sensitive to oxidative toxicity induced by glutamate in the absence of ionotropic glutamate receptors. In this study, oligodendrocyte signalling cascades were examined, in response to glutamate-induced oxidative injury and to excitotoxicity. Rat cortical oligodendrocytes, differentiated in culture, were highly vulnerable to glutamate-induced cell death. Competitive inhibition of cystine uptake and increased oxidative stress appeared responsible for this death, and caused an accumulation of intracellular peroxides as well as chromatin fragmentation and condensation. Glutamate receptor subtype agonists (quisqualate, ibotenate) known to inhibit cystine uptake were cytotoxic, but not NMDA itself; moreover, glutamate receptor antagonists were not protective. Oligodendrocytes were also vulnerable to overactivation of glutamate receptors, as kainic acid and AMPA proved to be toxic. AMPA toxicity required the presence of cyclothiazide, suggesting rapid desensitization of AMPA receptors. Glutamate-induced oxidative stress and kainate/AMPA receptor stimulation activated the mitogen-activated protein kinase (MAP kinase) pathway, as well as the transcription factor ELK. However, MAP kinase kinase inhibitors only protected against injury from glutamate-induced oxidative stress. Oligodendrocytes were sensitive to oxygen-glucose deprivation injury as well, in a MAP kinase dependent fashion. Glutamate toxicity may conceivably be operative in neuropathological conditions that disrupt neuronal/oligodendrocyte interactions in axons, e.g. multiple sclerosis and ischaemia-reperfusion injury.

Novel metabotropic glutamate receptor negatively coupled to adenylyl cyclase in cultured rat cerebellar astrocytes

Several excitatory amino acid ligands were found potently to inhibit forskolin-stimulated cAMP accumulation in rat cultured cerebellar astrocytes: L-cysteine sulfinic acid (L-CSA) = L-aspartate > L-glutamate >/= the glutamate uptake inhibitor, L-PDC. This property did not reflect activation of conventional glutamate receptors, since the selective ionotropic glutamate receptor agonists NMDA, AMPA, and kainate, as well as several mGlu receptor agonists [(1S,3R)-ACPD, (S)-DHPG, DCG-IV, L-AP4, L-quisqualate, and L-CCG-I], were without activity. In addition, the mGlu receptor antagonists, L-AP3, (S)-4CPG, Eglu, LY341495, (RS)-CPPG, and (S)-MCPG failed to reverse 30 microM glutamate-mediated inhibitory responses. L-PDC-mediated inhibition was abolished by the addition of the enzyme glutamate-pyruvate transaminase. This finding suggests that the effect of L-PDC is indirect and that it is mediated through endogenously released L-glutamate. Interestingly, L-glutamate-mediated inhibitory responses were resistant to pertussis toxin, suggesting that G(i)/G(o) type G proteins were not involved. However, inhibition of protein kinase C (PKC, either via the selective PKC inhibitor GF109203X or chronic PMA treatment) augmented glutamate-mediated inhibitory responses. Although mGlu3 receptors (which are negatively coupled to adenylyl cyclase) are expressed in astrocyte populations, in our study Western blot analysis indicated that this receptor type was not expressed in cerebellar astrocytes. We therefore suggest that cerebellar astrocytes express a novel mGlu receptor, which is negatively coupled to adenylyl cyclase, and possesses an atypical pharmacological profile.

Estrogen-induced neurochemical and electrophysiological changes in the parabrachial nucleus of the male rat

Estrogen has previously been shown to significantly change sympathetic and parasympathetic system output via an action within the central nuclei responsible for regulating autonomic tone. These estrogen-induced changes were observed within 30 min of systemic administration and could be blocked by the direct microinjection of the estrogen receptor antagonist, ICI 182780, into the parabrachial nucleus (PBN) of the pons. In the present investigation, we sought to determine the possible mechanism(s) by which estrogen produced these rapid changes in autonomic tone by determining if estrogen modulates neuronal excitability within the PBN. Male Sprague-Dawley rats were anaesthetized with Inactin (sodium thiobutabarbitol, 100 mg/kg) and instrumented for the intravenous injection of estrogen and placed in a stereotaxic frame for the insertion of a microdialysis probe or glass recording electrode into the PBN. In the first experiment, we sought to determine the local concentration of estrogen in the cerebrospinal fluid in the PBN following systemic injection of estrogen. In the second experiment, we sought to determine the functional significance of systemic estrogen injection on neuronal activity and amino acid neurotransmitter levels in the PBN. Systemic estrogen injection resulted in a significant increase in local estrogen concentration in the PBN which corresponded to a decrease in neuronal excitability and extracellular glutamate levels while increasing GABA levels in the PBN. These results suggest that estrogen decreases neuronal excitability in the PBN by modulating synaptic transmission via an increased release of GABA and a decreased release of glutamate.

Dynamics of Ca2+ and Na+ in the dendrites of mouse cerebellar Purkinje cells evoked by parallel fibre stimulation

Ca2+ and Na+ play important roles in neurons, such as in synaptic plasticity. Their concentrations in neurons change dynamically in response to synaptic inputs, but their kinetics have not been compared directly. Here, we show the mechanisms and dynamics of Ca2+ and Na+ transients by simultaneous monitoring in Purkinje cell dendrites in mouse cerebellar slices. High frequency parallel fibre stimulation (50 Hz, 3-50-times) depolarized Purkinje cells, and Ca2+ transients were observed at the anatomically expected sites. The magnitude of the Ca2+ transients increased linearly with increasing numbers of parallel fibre inputs. With 50 stimuli, Ca2+ transients lasted for seconds, and the peak [Ca2+] reached approximately 100 microm, which was much higher than that reported previously, although it was still confined to a part of the dendrite. In contrast, Na+ transients were sustained for tens of seconds and diffused away from the stimulated site. Pharmacological interventions revealed that Na+ influx through alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors and Ca2+ influx through P-type Ca channels were essential players, that AMPA receptors did not operate as a Ca2+ influx pathway and that Ca2+ release from intracellular stores through inositol trisphosphate receptors or ryanodine receptors did not contribute greatly to the large Ca2+ transients.

Local differences in GABA release induced by excitatory amino acids during retina development: selective activation of NMDA receptors by aspartate in the inner retina

Glutamate and GABA are the major excitatory and inhibitory neurotransmitters in the CNS. In the retina, it has been shown that glutamate and aspartate and their agonists kainate and NMDA promote the release of GABA. In the chick retina, at embryonic day 14 (E14), glutamate and kainate were able to induce the release of GABA from amacrine and horizontal cells as detected by GABA-immunoreactivity. NMDA also induced GABA release restricted to amacrine cell population and its projections to the inner plexiform layer (E14 and E18). Although aspartate reduced GABA immunoreactivity, specifically in amacrine cells of E18 retinas, it was not efficient to promote GABA release from retinas at E14. As observed in differentiated retinas, dopamine inhibited the GABA release promoted by NMDA and aspartate but not by kainate. Our data show that different retinal sites respond to distinct EAAs via different receptor systems.

Antinociceptive profiles of crude extract from roots of Angelica gigas NAKAI in various pain models

To characterize the antinociceptive profiles of Angelica gigas NAKAI (ANG; Korean angelica), methanol extract from the dried roots of ANG was made and mice were administered orally at the various doses (from 0.25 to 3 g/kg). ANG produced the increased latencies of the tail-flick and hot-plate paw-licking responses in a dose-dependent manner. In acetic acid-induced writhing test, ANG dose-dependently decreased writhing numbers. Moreover, the cumulative response time of nociceptive behaviors induced by intraplantar formalin injection was reduced during both the 1st and the 2nd phases in a dose-dependent manner in ANG-treated mice. Furthermore, oral administration of ANG did not cause licking, scratching and biting responses induced by TNF-alpha (100 pg), IFN-gamma (100 pg) or IL-1beta (100 pg) injected intrathecally (i.t.), especially at higher dose (3 g/kg). Additionally, in ANG treated mice, the cumulative nociceptive response time for i.t. administration of substance P or capsaicin was dose-dependently diminished. Finally, nociceptive responses elicited by i.t. injection of glutamate (20 microg), N-methyl-D-aspartic acid (60 ng), alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (13 ng) or kainic acid (12 ng) were decreased by oral administration of ANG. Our results suggest that ANG produces antinociception via acting on the central nervous system and shows antinociceptive profiles in various pain models, especially inflammatory pain.

"In vivo" monitoring of neuronal network activity in zebrafish by two-photon Ca(2+) imaging

The zebrafish larva is a powerful model for the analysis of behaviour and the underlying neuronal network activity during early stages of development. Here we employ a new approach of "in vivo" Ca(2+) imaging in this preparation. We demonstrate that bolus injection of membrane-permeable Ca(2+) indicator dyes into the spinal cord of zebrafish larvae results in rapid staining of essentially the entire spinal cord. Using two-photon imaging, we could monitor Ca(2+) signals simultaneously from a large population of spinal neurons with single-cell resolution. To test the method, Ca(2+) transients were produced by iontophoretic application of glutamate and, as observed for the first time in a living preparation, of GABA or glycine. Glycine-evoked Ca(2+) transients were blocked by the application of strychnine. Sensory stimuli that trigger escape reflexes in mobile zebrafish evoked Ca(2+) transients in distinct neurons of the spinal network. Moreover, long-term recordings revealed spontaneous Ca(2+) transients in individual spinal neurons. Frequently, this activity occurred synchronously among many neurons in the network. In conclusion, the new approach permits a reliable analysis with single-cell resolution of the functional organisation of developing neuronal networks.

Lamotrigine and remacemide protect striatal neurons against in vitro ischemia: an electrophysiological study

In the present study, we investigated the cellular and synaptic mechanisms underlying the neuroprotective action of lamotrigine and remacemide. Both drugs, in fact, have been reported to exert a neuroprotective action in in vivo animal models of ischemia. To address this issue, electrophysiological recordings and cell swelling measurements were performed from striatal neurons in control condition and during combined oxygen and glucose deprivation (in vitro ischemia) in a brain slice preparation. Lamotrigine, remacemide, and the active desglycinyl metabolite of remacemide, D-REMA, induced a concentration-dependent reduction of both repetitive firing discharge and excitatory postsynaptic potentials. However, while remacemide and D-REMA exerted their inhibitory action on glutamatergic transmission by blocking NMDA receptors, lamotrigine exerted a preferential presynaptic action, as indicated by its ability to increase paired-pulse facilitation. Both remacemide and lamotrigine were found to be neuroprotective against the irreversible field potential loss and cell swelling induced by in vitro ischemia, and coadministration of low concentrations of these drugs exerted an additive neuroprotective action. A combined use of lamotrigine and remacemide could be employed in clinical trials to enhance neuroprotection in neurological disorders involving an abnormal striatal glutamatergic transmission.

pH modulates the vestibular afferent discharge and its response to excitatory amino acids

In the isolated inner ear of the axolotl (Ambystoma tigrinum) acid pH decreased and basic pH increased the resting and mechanically evoked spike discharge of semicircular canal afferent neurons. Variations in pH also modified the afferent neuron response to N-methyl-D-aspartic acid (NMDA) acid and to (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA). Responses to both excitatory amino acid agonists increased at pH 7.8 (41% and 22%, respectively) and decreased by perfusion of the preparation with a saline solution, of pH 7.0 (28% in both cases). These results indicate that vestibular endorgans have a significant sensitivity to pH that could play a significant role in various pathological states, and may also contribute to the post-transductional processing of sensory information.

17beta-estradiol inhibits excitatory amino acid-induced activity of neurons of the nucleus tractus solitarius

The effects of 17beta-estradiol (17betaE2) on spontaneous and excitatory amino acid (EAA) induced nucleus tractus solitarius (NTS) neuronal activity were investigated by electrophysiological and immunohistochemical experiments in ovariectomized female Sprague-Dawley rats. Out of 62 NTS neurons tested, 42 were inhibited (68%) following iontophoretic application of 17betaE2 in a current-dependent manner. The averaged firing rate decreased from 3.06+/-0.40 to 0.78+/-0.17 Hz. The inhibitory responses were rapid in onset (within 1 min) and variable in duration (2-4 min). The inhibitory effects of 17betaE2 were blocked by simultaneously applied 17betaE2 antagonist ICI182,780, but not by GABA antagonist, bicuculline and phaclofen. L-Glutamate, AMPA or NMDA enhanced the activity of 71, 73 or 69% of NTS cells tested, respectively. The excitatory effects of EAA were significantly inhibited in the presence of 17betaE2. Fluorescent immunohistochemistry revealed that all subnuclei of the NTS contained high levels of estrogen receptors (ERs) immunoreactivity. These results suggest that 17betaE2 inhibits spontaneous and EAA-induced NTS neuronal activity through 17betaE2 activation of ERs.

Role of heme oxygenase-1 protein in the neuroprotective effects of cyclopentenone prostaglandin derivatives under oxidative stress

Previously we found that some cyclopenteone prostaglandin derivatives (PGs), referred to as neurite outgrowth-promoting PGs (NEPPs), have dual biological activities of promoting neurite outgrowth and preventing neuronal death [Satoh et al. (2000) J. Neurochem., 75, 1092-1102; Satoh et al. (2001) J. Neurochem., 77, 50-62; Satoh et al. (2002) In Kikuchi, II. (ed.), Strategenic Medical Science Against Brain Attack. Springer-Verlag, Tokyo, pp. 78-93]. To investigate possible cellular mechanisms of the neuroprotective effects, we performed oligo hybridization-based DNA array analysis with mRNA isolated from HT22, a cell line that originated from a mouse hippocampal neuron. Several transcripts up-regulated by NEPP11 were identified. Because heme oxygenase 1 (HO-1) mRNA was the most prominently induced and was earlier reported to protect neuronal and non-neuronal cells against oxidative stress, we focused on it as a possible candidate responsible for the neuroprotective effects. We found NEPP11 to induce HO-1 protein (32 kDa) in HT22 cells in both the presence and the absence of glutamate, whereas non-neuroprotective prostaglandins (PGs) Delta12-PGJ2 or PGA2 did not. Overexpression of HO-1-green fluorescence protein (GFP) fusion protein significantly protected HT22 cells against oxidative glutamate toxicity, whereas that of GFP alone did not. Furthermore, biliverdin and bilirubin, products of HO-1 enzymatic activity on heme, protected HT22 cells from oxidative glutamate toxicity. These results, together with our previous results, suggest that NEPP11 activates the expression of HO-1 and that HO-1 produces biliverdin and bilirubin, which result in the inhibition of neuronal death induced by oxidative stress. NEPP11 is the first molecular probe reported to have a neuroprotective action through induction of HO-1 in neuronal cells.

How AMPA receptor desensitization depends on receptor occupancy

AMPA-type glutamate receptors mediate fast excitatory transmission at many central synapses, and rapid desensitization of these receptors can shape the decay of synaptic currents and limit the fidelity of high-frequency synaptic transmission. Here we use a combination of fast glutamate application protocols and kinetic simulations to determine how AMPA receptor desensitization depends on the number of subunits occupied by glutamate. We show that occupancy of a single subunit is sufficient to desensitize AMPA-type channels and that receptors with one to four glutamates bound enter desensitization at similar rates. We find that recovery from desensitization follows a similar sigmoid time course for channels with two to four glutamates bound but is faster and exponential for singly occupied channels. The results suggest that desensitization, at intermediate and high glutamate concentrations, is accompanied by two conformational changes that slow glutamate dissociation. We propose a kinetic scheme that accurately predicts several types of experimental results and differs significantly from previous models in the assignment of affinities for binding to closed and desensitized states. We conclude that desensitization involves a rearrangement that stabilizes the binding domains of one subunit in each dimer in a partially closed conformation. This stabilization likely results from an interaction at the dimer-dimer interface between the binding domains of adjacent subunits.

Neural progenitor cells resist excitatory amino acid-induced neurotoxicity

The applications of neural progenitor cells in clinical therapy for neural degeneration, such as Parkinson's disease, Huntington's disease, and cerebral infarction, have long been explored widely. It had been suggested that these cells may block the apoptosis of ischemia-induced neuronal damage and may themselves resist neurotoxic factors. In the present study, neural progenitor cells derived from the cortex of rodent embryos were cultured with the mitogenic agent epidermal growth factor. It was observed that these progenitor cells could self-renew and differentiate into a number of types of neurons and glial cells. By using sodium nitroprusside, glutamate, and N-methyl-D-aspartate, these neural progenitor cells were shown to have a higher resistance to neurotoxicity induced by these drugs compared with primary neuronal cells. However, the release of nitric oxide in response to glutamate by these neural progenitor cells was similar to the released by primary neuronal cells. Also, when the glutamate-stimulated increase in intracellular free Ca(2+) concentration was measured, stimulation of the glutamate receptors could not induce a significant influx of Ca(2+) into these progenitor cells until they differentiated. Our results suggest that the resistance of neural progenitor cells to neurotoxicity may be partially due to a lack of response to glutamate. In addition, some progenitor-generated neurotrophic factors may contribute to the resistance of these cells to nitric oxide-induced neurotoxicity.

Effect of nitric oxide on excitatory amino acid-evoked discharge of neurons in NTS

N-methyl-d-aspartate (NMDA) and non-NMDA excitatory amino acid (EAA) receptor subtypes are involved in the integration of visceral afferent inputs within the nucleus of the solitary tract (NTS). Microinjection studies indicate interactions between nitric oxide (NO) and EAA receptors within the NTS. To examine these interactions at the single cell level, this study characterized the effects of the NO synthase inhibitor N(G)-nitro-l-arginine methyl ester (l-NAME) and the NO donor 3-[2-hydroxy-2-nitroso-1-propylhydrazino]-1-propanamine (PAPA-NONOate) on the excitatory responses of vagus nerve (VN)-evoked NTS neurons to the activation of (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and NMDA receptors in rats. Iontophoresis of l-NAME did not alter spontaneous or VN-evoked discharges, but significantly decreased the number of action potentials (APs) evoked by iontophoretic application of AMPA. The effects of l-NAME on NMDA-evoked discharge were variable; for the population, l-NAME did not change the number of APs evoked by NMDA. PAPA-NONOate enhanced the spontaneous discharge and the number of APs elicited by AMPA but not NMDA. Iontophoresis of the inactive enantiomers N(G)-nitro-d-arginine methyl ester and hydroxydiazenesulfonic acid 1-oxide disodium salt had no effect on AMPA-evoked discharge. Our data suggest that NO facilitates AMPA-mediated neuronal transmission within the NTS.

Haemorrhage-evoked compensation and decompensation are mediated by distinct caudal midline medullary regions in the urethane-anaesthetised rat

Previous research using microinjections of excitatory amino acids suggested that the caudal midline medulla (including nucleus raphe obscurus and nucleus raphe pallidus) contained a mixed population of sympathoexcitatory and sympathoinhibitory neurones. The results of this study indicate that different anaesthetic regimes (urethane versus halothane) determine whether sympathoexcitatory (urethane only) or sympathoinhibitory (halothane only) responses are evoked by stimulation within distinct caudal midline medullary regions. In addition, anaesthetic regimes also affect the caudal midline medullary-mediated response to haemorrhage. Specifically, under conditions of urethane anaesthesia, inactivation (lignocaine) of the midline medullary region immediately caudal to the obex, prematurely triggered and dramatically potentiated the hypotension and bradycardia evoked by 15% haemorrhage; whereas under halothane anaesthesia, inactivation of the same region had no effect. In contrast, under urethane anaesthesia, inactivation of the midline medullary region immediately rostral to the obex, delayed the onset of the hypotension and bradycardia to 15% haemorrhage; inactivation of the same region under halothane anaesthesia blocked haemorrhage-evoked hypotension and bradycardia. Our findings indicate that topographically distinct parts of the caudal midline medulla contain neurones (i) that differentially regulate the timing and magnitude of the compensatory (normotensive) versus decompensatory (hypotensive) phases of the response to haemorrhage; and (ii) whose activity is altered by urethane versus halothane anaesthesia.

Neuronal death and tumor necrosis factor-alpha response to glutamate-induced excitotoxicity in the cerebral cortex of neonatal rats

Neuronal death and lactate dehydrogenase (LDH) activity were evaluated in the cerebral cortices of neonatal rats after exposure to monosodium L-glutamate (MSG) to induce neuroexcitotoxicity. A time-response profile for tumor necrosis factor-alpha (TNF-alpha) expression was drawn, with measurements taken every 6 h after the first dose of MSG during the first 8 postnatal days, and at days 10 and 14 after birth. An increase in neuronal loss accompanied by high LDH activity and high TNF-alpha levels was observed at 8 and 10 days. These results indicate that neuronal loss may occur via an apoptosis-like mechanism directed selectively against neurons that express glutamate receptors, mainly the N-methyl-D-aspartate, which it may be strengthen by high TNF-alpha levels through a feedback mechanism to induce cell death via apoptosis.

The effects of glutamate on spontaneous acetylcholine secretion processes in the rat neuromuscular synapse

Experiments on rat diaphragm muscles showed that glutamate (10 microM-1 mM) had no effect on the mean frequency, interspike intervals, and amplitude-time characteristics of miniature endplate potentials, but had a suppressive action on non-quantum secretion (the intensity of which was assessed in terms of the H effect). The effect of glutamate was markedly concentration-dependent and was completely overcome by blockade of NMDA receptors, inhibition of NO synthase, and by binding of NO molecules in the extracellular space by hemoglobin. It is suggested that glutamate can modulate the non-quantum release of acetylcholine, initiating the synthesis of NO molecules in muscle fibers via activation of NMDA receptors followed by the retrograde action of NO on nerve terminals.

Glutamatergic and serotonergic mechanisms in the dorsal facial area for common carotid artery blood flow control in the cat

This study explored which subtyes of glutamate receptors in the dorsal facial area are involved in the interaction between glutamatergic and serotonergic actions in controlling common carotid arterial blood flow. Microinjection of glutamate (25-100 nmol), N-methyl-D-aspartate (NMDA; 1-4 nmol), or alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA; 0.5-2 nmol) into the dorsal facial area dose-dependently increased common carotid arterial blood flow. The potency order was AMPA>NMDA>glutamate. The glutamate-induced increase in common carotid arterial blood flow was reduced by pretreatment with either D-2-amino-5-phosphonopentanoate (D-AP5; 2.5-5.0 nmol), or glutamate diethylester (25-50 nmol). The common carotid arterial blood flow was increased by ketanserin (1.0 nmol) and decreased by (+/-)-1-(2, 5-dimethoxy-4-iodophenyl)-2-aminopropane (1.0 nmol). Both effects were attenuated by pretreatment with either D-AP5 or glutamate diethylester. We conclude that activation of both NMDA and AMPA receptors in dorsal facial area is responsible for the increase in common carotid arterial blood flow, and AMPA receptor may play a greater role. Such response may be suppressed by 5-HT2 action.

Modulation of tianeptine on ion currents induced by inhibitory neurotransmitters in acutely dissociated dorsal raphe neurons of Sprague-Dawley rats

Classical antidepressant therapy has mainly used selective serotonin reuptake inhibitors (SSRIs) and tricyclic antidepressants (TCAs), but tianeptine appears to possess effects on 5-hydroxytryptamine (5-HT, serotonin) levels in the synaptic cleft contradictory to those of SSRIs. The mechanism behind its antidepressant effects is still unclear. The effect of tianeptine on the electrophysiological characteristics of neurons in the dorsal raphe, where serotonergic neurons mainly originate from, were investigated by the patch-clamp method in the present study. Tianeptine was shown to inhibit the ion currents induced by the inhibitory neurotransmitters GABA and glycine as well as the inwardly rectifying K(+) current induced by 5-HT and 8-hydroxy-2 (di-n-propylamino) tetraline hydrobromide (8-OH-DPAT), a 5-HT(1A) agonist. Tianeptine, however, exerted no effect on the ion current induced by the excitatory neurotransmitter glutamate. These results indicate that tianeptine has an inhibitory effect on ion currents evoked by GABA, glycine, and inwardly rectifying K(+) current induced by 5-HT in the dorsal raphe. This action may increase the excitability of serotonergic neurons of the dorsal raphe.

Evidence for the involvement of glutamatergic receptors in the antinociception caused in mice by the sesquiterpene drimanial

Drimanial, a new sesquiterpene isolated from the barks of the plant Drimys winteri (Winteraceae), given systemically, intraplantarly, or by spinal or supraspinal routes, produced pronounced antinociception against both phases of formalin-induced licking. The systemic injection of drimanial also inhibited, in a graded manner, the pain-related behaviours induced by intraplantar or intrathecal (i.t.) administration of glutamate. Moreover, drimanial also caused marked inhibition of the nociception induced by i.t. administration of a metabotropic glutamate agonist (1S,3R)-ACPD, without affecting nociceptive responses induced by ionotropic agonists (NMDA, kainate, AMPA) or by substance P. The antinociception caused by drimanial was not influenced by naloxone, nor did it interfere with the motor coordination of animals in the rota-rod test. Furthermore, drimanial caused graded inhibition of [(3)H]glutamate binding in cerebral cortical membranes from mice, with an IC(50) value of 4.39 micro M. Together, these results provide strong evidence indicating that the sesquiterpene drimanial produces antinociception in mice at peripheral, spinal and supraspinal sites. An interaction with metabotropic glutamate receptors seems to contribute to the mechanisms underlying its antinociceptive action.

Direct and indirect inhibition by nociceptin/orphanin FQ on noradrenaline release from rodent cerebral cortex in vitro

1 The modulation exerted by nociceptin/orphanin FQ (NC) on noradrenaline (NE) release in rodent cerebral cortex slices and synaptosomes was studied. 2 Rat, mouse and guinea-pig cortical slices and synaptosomes were preincubated with 0.1 micro M [(3)H]-NE and superfused. NE release was evoked by 2 min of electrical (3 Hz) stimulation in slices and by 1 min pulse of 10 mM KCl in synaptosomes. 3 In rat cortical slices, 0.01-3 micro M NC reduced the evoked [(3)H]-NE efflux (E(max)-54%), with a bell-shaped concentration-response curve, which regained its monotonic nature in the presence of either 0.1 micro M naloxone (NX) or 30 micro M bicuculline. In synaptosomes, the NC effect curve was sygmoidal in shape and reached a plateau at 1 micro M concentration. 4 In the rat, both 1 micro M [Phe(1)psi(CH(2)-NH)Gly(2)]NC(1-13)NH(2) and 10 micro M [Nphe(1)]NC(1-13)NH(2) (NPhe) antagonised NC-induced inhibition, without per se modifying [(3)H]-NE efflux. The effects of 0.3-1 micro M NC concentrations were partially prevented by 1 micro M NX; 1 micro M D-Phe-Cys-Thr-D-Trp-Orn-Thr-Pen-Thr-NH(2) (CTOP) was also an effective antagonist, but 0.1 micro M norbinaltorphimine was not. 5 In the mouse cerebral cortex, NC-induced inhibition of NE release (pEC(50) 6.87, E(max)-61%, in the slices) was prevented by Nphe but was NX-insensitive. In guinea-pig cortical slices, NC effect (pEC(50) 6.22, E(max)-38%) was prevented by Nphe, but was NX-insensitive. 6 These findings demonstrate that NC inhibits NE release from rodent cerebral cortex via presynaptically located ORL(1) receptors. In the rat, micro opioid and GABA(A) receptors are involved as well.

PSD-95 eliminates Src-induced potentiation of NR1/NR2A-subtype NMDA receptor channels and reduces high-affinity zinc inhibition

The channel activity of NMDA receptors is regulated by phosphorylation by protein kinases and by interaction with other proteins. Recombinant NR1/NR2A subtype NMDA receptor channels are potentiated by the protein tyrosine kinase Src, an effect which is mediated by a reduction in the high-affinity, voltage-independent Zn(2+) inhibition. However, it has been reported that Src-induced potentiation of NMDA receptor currents in hippocampus neurons is not mediated by a reduction in Zn(2+) inhibition. The post-synaptic density protein PSD-95 interacts with the C-terminus of NR2 subunits of the NMDA receptor. Here we demonstrate that PSD-95 eliminates the Src-induced potentiation of NR1/NR2A channels expressed in oocytes and reduces the sensitivity of the channels to Zn(2+). Our results reveal that the absence of Src-induced potentiation of PSD-95-coupled NR1/NR2A channels is not to due to the reduced sensitivity of these channels to Zn(2+). These results indicate that PSD-95 functionally modulates NR1/NR2A channels and explain why Src-induced potentiation of NMDA receptor currents in hippocampus neurons is not mediated by a reduction in Zn(2+) inhibition.

Alterations in responses of ventral pallidal neurons to excitatory amino acids after long-term dopamine depletion

The present study explored the possibility that excitatory amino acid (EAA) sensitivity within the ventral pallidum (VP) is altered by long-term removal of dopamine (DA). Electrophysiological experiments were conducted in chloral hydrate-anesthetized rats 21 to 28 days after they received unilateral substantia nigra injections of the dopaminergic toxin 6-hydroxydopamine (6-OHDA). VP neurons increased firing at low microiontophoretic ejection currents of the EAA agonists N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA); however, high currents decreased action potential amplitude and rapidly caused cessation of neuronal firing. These responses likely reflected the induction of depolarization block for they were reversed by coiontophoresis of the hyperpolarizing transmitter gamma-aminobutyric acid (GABA) at ejection current levels that normally suppressed firing. The ability of NMDA and AMPA to induce such inactivation was greater in the VP of 6-OHDA-lesioned hemispheres, but unchanged in reserpinized rats, verifying that the alterations in responding to NMDA were the result of chronic, rather than acute, DA removal. The adaptations do not appear to be the consequence of a diminished GABAergic tone for the ability of bicuculline to increase firing (due to blocking a tonic GABAergic input) was not changed. However, low ejection currents of GABA that were insufficient to alter firing rate greatly attenuated the ability of NMDA to induce an apparent depolarization inactivation when coiontophoresed with NMDA onto VP neurons of the lesioned, but not the unlesioned, hemisphere. These studies show that chronic DA removal altered the EAA-induced amplitude-decreasing (i.e., the apparent depolarization inactivation) effects in VP neurons in the absence of a decrease in GABAergic tone.

Episodic bursting activity and response to excitatory amino acids in acutely dissociated gonadotropin-releasing hormone neurons genetically targeted with green fluorescent protein

The gonadotropin-releasing hormone (GnRH) system, considered to be the final common pathway for the control of reproduction, has been difficult to study because of a lack of distinguishing characteristics and the scattered distribution of neurons. The development of a transgenic mouse in which the GnRH promoter drives expression of enhanced green fluorescent protein (EGFP) has provided the opportunity to perform electrophysiological studies of GnRH neurons. In this study, neurons were dissociated from brain slices prepared from prepubertal female GnRH-EGFP mice. Both current- and voltage-clamp recordings were obtained from acutely dissociated GnRH neurons identified on the basis of EGFP expression. Most isolated GnRH-EGFP neurons fired spontaneous action potentials (recorded in cell-attached or whole-cell mode) that typically consisted of brief bursts (2-20 Hz) separated by 1-10 sec. At more negative resting potentials, GnRH-EGFP neurons exhibited oscillations in membrane potential, which could lead to bursting episodes lasting from seconds to minutes. These bursting episodes were often separated by minutes of inactivity. Rapid application of glutamate or NMDA increased firing activity in all neurons and usually generated small inward currents (<15 pA), although larger currents were evoked in the remaining neurons. Both AMPA and NMDA receptors mediated the glutamate-evoked inward currents. These results suggest that isolated GnRH-EGFP neurons from juvenile mice can generate episodes of repetitive burst discharges that may underlie the pulsatile secretion of GnRH, and glutamatergic inputs may contribute to the activation of endogenous bursts.

Serotonin modulates oscillations of the membrane potential in isolated spinal neurons from lampreys

Studies were performed on spinal neurons from lampreys isolated by an enzymatic/mechanical method using pronase. The effects of 100 microM serotonin (5-HT) on membrane potential oscillations induced by a variety of excitatory amino acids were studied. 5-HT was found to depolarize branched cells (presumptive motoneurons and interneurons) by 2-6 mV without inducing membrane potential oscillations. However, when oscillations were already present because of an excitatory amino acid, 5-HT changed the parameters of these oscillations, increasing the amplitudes of all types of oscillations, increasing the frequency of irregular oscillations, and increasing the duration of the depolarization plateaus accompanied by action potentials. Serotonin modulation of the effects of excitatory amino acids and the electrical activity of cells in the neural locomotor network facilitates motor activity and leads to increases in the contraction of truncal muscles and more intense movements by the animal. The possible mechanisms of receptor coactivation are discussed, along with increases in action potential frequency and changes in the parameters of the locomotor rhythm.

Modulation of growth hormone, luteinizing hormone, and testosterone secretion by excitatory amino acids in boars

Ketamine hydrochloride, an n-methyl-d-aspartate (NMDA) receptor antagonist was used in an experiment that tested the hypothesis that fasting-induced increases in growth hormone (GH) secretion is mediated by excitatory amino acid (EAA) neurotransmission in boars. The effects of the drug on circulating concentrations of luteinizing hormone (LH) and testosterone were also evaluated. Blood was sampled at 15-min intervals for 8 h from 12 boars fitted with jugular vein catheters. At Hours 4 and 6, fasted boars (feed was withdrawn 48 h before the start of blood sampling) received i.m. injections of ketamine (19.9 mg/kg body weight; n=4) or .9% saline (n=4). Boars allowed feed on an ad libitum basis (n=4) received i.v. injections of n-methyl-d,l-aspartate (NMA; 2.5 mg/kg body weight), an NMDA receptor agonist, at Hours 4 and 6. Secretion of GH increased after NMA injections but was unaffected by treatment with ketamine or saline. Circulating concentrations of LH and testosterone were increased by injections of ketamine but were unaffected by injections of NMA or saline. Our results suggest that NMA is a potent GH secretagogue, but do not support the hypothesis that EAA neurotransmission drives the increased GH secretion displayed in fasted boars. Our finding that ketamine increased LH and testosterone release supports the notion that EAA have inhibitory effects on gonadotropin secretion in acutely fasted swine.

Changes in the control of gonadotrophin secretion by neurotransmitters during sexual development in rats

Gonadotrophin hormone releasing hormone (GnRH) is the primary messenger involved in sexual maturation and the onset of puberty. The activity of these neurons are controlled by several neurotransmitters systems. The onset of puberty implies changes from a prepubertal type of gonadotrophin secretion, characterized by a low activity of GnRH neurons, to an adult pattern of secretion with phasic and synchronous activation of GnRH neurons resulting in an increase in the amplitute and frequency of GnRH pulses. Neurotransmitter systems are involved in these changes of GnRH secretion during the onset of puberty by quantitative and qualitative modifications in the effect on GnRH secretion. Serotonin (5-HT), GABA and catecholamines (CA) have qualitative differences in the effects on GnRH and LH secretion in early prepubertal than in late prepubertal and adult female rats. The administration of 5-hydroxytryptophan a precursor of serotonin (5-HT) which increases 5-HT hypothalamic levels induces GnRH and LH release in early prepubertal female rats, these effects dissapear in late prepubertal stage having an inhibitory action in adult female rats. GABAergic system also stimulates GnRH and LH secretion in early prepubertal female rats and has an inhibitory action on this axis in late prepubertal period and in adult female rats. On the contrary the inhibition of catecholamines synthesis by alpha-methyl-p-tyrosine induced an increase of LH secretion in early prepubertal female rats and inhibitory effect in late prepubertal and adult stage. These effects indicate tha CA has an inhibitory effects on GnRH-LH secretion in early prepubertal female rats changing to an stimulatory action in the late puberty and adult rats. These qualitative modifications were observed only in female rats and are probably connected with the hypothalamic differentiation into a female type of gonadotrophin control. Opiadergic and excitatory amino acid systems have quantitative differences on GnRH-LH secretion during prepubertal and peripubertal and adult stages. Opiates has an high inhibitory tone in early prepubertal rats that is decreasing during sexual maturation to reach puberty. On the contrary EAA increases its stimulatory activity on GnRH-LH secretion during sexual maturation by increasing the hypothalamic release of aspartate and glutamate, the excitatory amino acids involved in GnRH release, and the sensibility of NMDA receptors to these amino acids. In conclusion sexual maturation and the onset of puberty in the female rats involve qualitative and quantitative modifications in the effects of neurotrasmitters system on GnRH secretion.

Serotonin (1A) receptor ligands act on norepinephrine neuron firing through excitatory amino acid and GABA(A) receptors: a microiontophoretic study in the rat locus coeruleus

It was previously shown that the excitatory effect of the 5-HT(1A) agonist 8-OH-DPAT on firing activity of locus coeruleus (LC) norepinephrine (NE) neurons and the inhibitory action of the 5-HT(1A) antagonist WAY 100,635 are dependent on the presence of 5-HT neurons, whereas the inhibitory action of the 5-HT(2) agonist DOI is not. Using in vivo extracellular unitary recordings performed in anesthetized rats, iontophoretic applications of the excitatory amino acid antagonist kynurenate attenuated the enhancement in firing produced by glutamate and kainate. In contrast, GABA applications decreased the firing activity of NE neurons which was attenuated by the enhancement produced by glutamate and kainate. In contrast, GABA applications decreased the firing activity of NE neurons which was attenuated by the GABA(A) receptor antagonist bicuculline. 8-OH-DPAT (10-60 microg kg(-1), i.v.) produced a dose-dependent enhancement in the firing activity of NE neurons that was abolished in the presence of kynurenate application. The selective 5-HT(1A) receptor antagonist WAY 100,635 (100 microg kg(-1), i.v.) suppressed NE firing which was reversed by the selective 5-HT(2A) antagonist MDL 100,907 (200 microg kg(-1), i.v.). In the presence of bicuculline, the inhibitory effect of WAY 100,635 was blunted. These results suggest that WAY 100,635 mainly attenuates NE neuron firing by blocking inhibitory 5-HT(1A) receptors on glutamatergic neurons, thereby enhancing glutamate release and activating excitatory amino acid receptors, possibly of the kainate subtype, on 5-HT terminals. The ensuing increased 5-HT release would then act on excitatory 5-HT(2A) receptors on GABA neurons that would ultimately mediate the inhibition of NE neurons. The prevention of the excitatory action of 8-OH-DPAT on NE neuron firing by kynurenate is also consistent with this neurocircuitry.

Effects of excitatory amino acids and nitric oxide on flight behavior elicited from the dorsolateral periaqueductal gray

Microinjection of excitatory amino acids (EAA) into the dorsolateral periaqueductal gray (dlPAG) induces flight reactions while EAA antagonists show anxiolytic effects. Part of the effects mediated by NMDA receptors may involve an increase in nitric oxide (NO) production. We showed that nitric oxide synthase (NOS) inhibitors injected into the dlPAG induced anxiolytic effects. Conversely, SIN-1, a NO donor, produced orientated flight reactions that resemble stimulation of the medial hypothalamus. This compound also produced extensive Fos-like immunoreactivity in this region and in other areas related to defensive reactions such as the medial amygdala and cingulate cortex. Since part of the effects of NO involves increases in guanylate cyclase levels, we found that intra-dlPAG injection of 8-Br-cGMP induced a brief flight reaction followed by increased locomotion. In another experiment, we showed that single or repeated restraint stress produced an increased expression of neuronal NOS in the dlPAG and other areas related to defense, as measured by in situ hybridization, diaphorase histochemistry and immunocytochemistry. Together, these data suggest that NO may participate in the modulation of defensive responses in the dlPAG.

Cytochrome P450 2B (CYP2B)-mediated activation of methyl-parathion in rat brain extracts

The role of cytochrome P450 (CYP) and the CYP isoform involved in the activation of the widely used pesticide methyl-parathion (MePA) were investigated in rat brain extracts by measuring the effect of different CYP inhibitors on acetylcholinesterase (AChE) inhibition by MePA. Brain extracts provide a useful tool to study the activation mechanisms of organophosphorus compounds (OP) since they contain both the activating enzyme(s) and the molecular target for OP toxicity. As expected, in incubations of rat brain extract supplemented with NADPH, AChE activity was non-competitively inhibited by the presence of MePA, indicating that MePA was activated to its reactive metabolite methyl-paraoxon (MePO). Indeed, Vmax(app) decreased from 13.4 to 8.7 micromol thionitrobenzoic acid (TNB)/min per mg protein. MePA activation by rat brain extracts, as measured by the AChE inhibition produced by the presence of the pesticide in the incubation, was fully prevented by previously bubbling the incubation mix with CO, by the presence of monoclonal anti-rat CYP2B1/2B2 antibodies and by the addition of phenobarbital (PB), a CYP2B substrate. Interestingly, MePA showed a greater affinity for CYP2B than PB. CYP1A1 antibodies showed no effect on MePA activation. The presence of cytochrome P450 2B (CYP2B) in the rat brain extracts was confirmed by immunoblotting. These results demonstrate indisputably the responsibility of CYP2B in MePA activation in the rat brain in vitro, suggesting that metabolic activation of OP compounds in situ might be crucial for their organ specific toxicity to the central nervous system also in vivo.