Repetitive optogenetic stimulation of glutamatergic neurons: An alternative to NMDA treatment for generating locomotor activity in spinalized zebrafish larvae

N-methyl-d-aspartate (NMDA) application has conventionally been used to activate spinal networks to induce locomotion in spinalized animals. We recently described an alternative approach in which application of continuous blue light activates channelrhodopsin-2 in vesicular glutamate transporter 2a (vglut2a)-expressing spinal neurons to produce organized, rhythmic locomotor activity in spinally-transected larval zebrafish. This technique arguably enhances research validity, because endogenous glutamate is released into existing synapses instead of activating only a subset of glutamatergic (NMDA) receptors with an exogenous compound. Here, we explored the viability of this approach in the context of using it for longer-term experiments. Fictive swimming was induced through repetitive application of 10-s blue light stimuli to spinalized preparations for up to 60 min at intervals of 1, 3, or 15 min. Locomotor activity was maintained throughout the experimental timecourse, demonstrating the robustness of the system. Although locomotor bursts remained organized into episodes of activity, the number of bursts elicited during each successive stimulus decreased. This was in contrast to NMDA bath application, in which bursts became less episodically organized while the overall number of bursts remained unchanged. The efficacy of the repetitive optogenetic stimulation paradigm was demonstrated through application of exogenous dopamine, which reversibly decreased the number of bursts produced per stimulus compared with untreated preparations. Finally, increasing the stimulus interval to 15 min lessened, but did not eliminate locomotor fatigue from repetitive activation. Altogether, we established repetitive optogenetic stimulation of vglut2a-expressing neurons as a viable alternative to NMDA application for activation of the zebrafish spinal locomotor network.

Lack of pulse and surge modes and glutamatergic stimulation of luteinising hormone release in Kiss1 knockout rats

Kisspeptin, encoded by the Kiss1 gene, has attracted attention as a key candidate neuropeptide in controlling puberty and reproduction via regulation of gonadotrophin-releasing hormone (GnRH) secretion in mammals. Pioneer studies with Kiss1 or its cognate receptor Gpr54 knockout (KO) mice showed the indispensable role of kisspeptin-GPR54 signalling in the control of animal reproduction, although detailed analyses of gonadotrophin secretion, especially pulsatile and surge-mode of luteinising hormone (LH) secretion, were limited. Thus, in the present study, we have generated Kiss1 KO rats aiming to evaluate a key role of kisspeptin in governing reproduction via pulse and surge modes of GnRH/LH secretion. Kiss1 KO male and female rats showed a complete suppression of pulsatile LH secretion, which is responsible for folliculogenesis and spermatogenesis, and an absence of puberty and atrophic gonads. Kiss1 KO female rats showed no spontaneous LH/follicle-stimulating hormone surge and an oestrogen-induced LH surge, suggesting that the GnRH surge generation system, which is responsible for ovulation, does not function without kisspeptin. Furthermore, challenge of major stimulatory neurotransmitters, such as monosodium glutamate, NMDA and norepinephrine, failed to stimulate LH secretion in Kiss1 KO rats, albeit they stimulated LH release in wild-type controls. Taken together, the results of the present study confirm that kisspeptin plays an indispensable role in generating two modes (pulse and surge) of GnRH/gonadotrophin secretion to regulate puberty onset and normal reproductive performance. In addition, the present study suggests that kisspeptin neurones play a critical role as a hub integrating major stimulatory neural inputs to GnRH neurones, using newly established Kiss1 KO rats, which serve as a useful model for detailed analysis of hormonal profiles.

From lamprey to salamander: an exploratory modeling study on the architecture of the spinal locomotor networks in the salamander

The evolutionary transition from water to land required new locomotor modes and corresponding adjustments of the spinal "central pattern generators" for locomotion. Salamanders resemble the first terrestrial tetrapods and represent a key animal for the study of these changes. Based on recent physiological data from salamanders, and previous work on the swimming, limbless lamprey, we present a model of the basic oscillatory network in the salamander spinal cord, the spinal segment. Model neurons are of the Hodgkin-Huxley type. Spinal hemisegments contain sparsely connected excitatory and inhibitory neuron populations, and are coupled to a contralateral hemisegment. The model yields a large range of experimental findings, especially the NMDA-induced oscillations observed in isolated axial hemisegments and segments of the salamander Pleurodeles waltlii. The model reproduces most of the effects of the blockade of AMPA synapses, glycinergic synapses, calcium-activated potassium current, persistent sodium current, and [Formula: see text]-current. Driving segments with a population of brainstem neurons yields fast oscillations in the in vivo swimming frequency range. A minimal modification to the conductances involved in burst-termination yields the slower stepping frequency range. Slow oscillators can impose their frequency on fast oscillators, as is likely the case during gait transitions from swimming to stepping. Our study shows that a lamprey-like network can potentially serve as a building block of axial and limb oscillators for swimming and stepping in salamanders.

Development of antipsychotic medications with novel mechanisms of action based on computational modeling of hippocampal neuropathology

A large number of cellular level abnormalities have been identified in the hippocampus of schizophrenic subjects. Nonetheless, it remains uncertain how these pathologies interact at a system level to create clinical symptoms, and this has hindered the development of more effective antipsychotic medications. Using a 72-processor supercomputer, we created a tissue level hippocampal simulation, featuring multicompartmental neuron models with multiple ion channel subtypes and synaptic channels with realistic temporal dynamics. As an index of the schizophrenic phenotype, we used the specific inability of the model to attune to 40 Hz (gamma band) stimulation, a well-characterized abnormality in schizophrenia. We examined several possible combinations of putatively schizophrenogenic cellular lesions by systematically varying model parameters representing NMDA channel function, dendritic spine density, and GABA system integrity, conducting 910 trials in total. Two discrete “clusters” of neuropathological changes were identified. The most robust was characterized by co-occurring modest reductions in NMDA system function (-30%) and dendritic spine density (-30%). Another set of lesions had greater NMDA hypofunction along with low level GABA system dysregulation. To the schizophrenic model, we applied the effects of 1,500 virtual medications, which were implemented by varying five model parameters, independently, in a graded manner; the effects of known drugs were also applied. The simulation accurately distinguished agents that are known to lack clinical efficacy, and identified novel mechanisms (e.g., decrease in AMPA conductance decay time constant, increase in projection strength of calretinin-positive interneurons) and combinations of mechanisms that could re-equilibrate model behavior. These findings shed light on the mechanistic links between schizophrenic neuropathology and the gamma band oscillatory abnormalities observed in the illness. As such, they generate specific falsifiable hypotheses, which can guide postmortem and other laboratory research. Significantly, this work also suggests specific non-obvious targets for potential pharmacologic agents.

Physiological role of β-catenin/TCF signaling in neurons of the adult brain

Wnt/β-catenin pathway, the effectors of which are transcription factors of the LEF1/TCF family, is primarily associated with development. Strikingly, however, some of the genes of the pathway are schizophrenia susceptibility genes, and the proteins that are often mutated in neurodegenerative diseases have the ability to regulate β-catenin levels. If impairment of this pathway indeed leads to these pathologies, then it likely plays a physiological role in the adult brain. This review provides an overview of the current knowledge on this subject. The involvement of β-catenin and LEF1/TCF factors in adult neurogenesis, synaptic plasticity, and the function of thalamic neurons are discussed. The data are still very preliminary and often based on circumstantial or indirect evidence. Further research might help to understand the etiology of the aforementioned pathologies.

Divide and conquer: functional segregation of synaptic inputs by astrocytic microdomains could alleviate paroxysmal activity following brain trauma

Traumatic brain injury often leads to epileptic seizures. Among other factors, homeostatic synaptic plasticity (HSP) mediates posttraumatic epileptogenesis through unbalanced synaptic scaling, partially compensating for the trauma-incurred loss of neural excitability. HSP is mediated in part by tumor necrosis factor alpha (TNFα), which is released locally from reactive astrocytes early after trauma in response to chronic neuronal inactivity. During this early period, TNFα is likely to be constrained to its glial sources; however, the contribution of glia-mediated spatially localized HSP to post-traumatic epileptogenesis remains poorly understood. We used computational model to investigate the reorganization of collective neural activity early after trauma. Trauma and synaptic scaling transformed asynchronous spiking into paroxysmal discharges. The rate of paroxysms could be reduced by functional segregation of synaptic input into astrocytic microdomains. Thus, we propose that trauma-triggered reactive gliosis could exert both beneficial and deleterious effects on neural activity.

ATPA induced GluR5-containing kainite receptor S-nitrosylation via activation of GluR5-Gq-PLC-IP(3)R pathway and signalling module GluR5·PSD-95·nNOS

GluR5-containing kainite receptor (GluR5-KAR) plays an important role in the pathophysiology of nervous system diseases, while S-nitrosylation exerts a variety of effects on biological systems. However, the mechanism of GluR5-KAR S-nitrosylation is still unclear up to now. Here our researches found that GluR5-KAR selective agonist ATPA stimulation activated the nonclassical GluR5-KAR-Gq-PLC-IP(3)R pathway and the signalling module GluR5·PSD-95·nNOS (the former is more important), led to Ca(2+) release from intracellular calcium stores endoplasmic reticulum (ER) to cytoplasm and extracellular calcium indrawal, respectively, which further resulted in nNOS activation and GluR5-KAR S-nitrosylation, and then inhibited GluR5-mediated whole-cell current attenuation and induced apoptosis in primary cultured hippocampal neurons. Clarification of the primary mechanisms of GluR5-KAR S-nitrosylation induced by ATPA and identification of critical cysteine for GluR5-2a S-nitrosylation (Cys231 and Cys804) open up a brand-new field for revealing downstream signalling pathway of GluR5-KAR and its molecular characteristics, exploring the pathogenesis of neurological diseases and searching for promising therapies.

Location-dependent effects of inhibition on local spiking in pyramidal neuron dendrites

Cortical computations are critically dependent on interactions between pyramidal neurons (PNs) and a menagerie of inhibitory interneuron types. A key feature distinguishing interneuron types is the spatial distribution of their synaptic contacts onto PNs, but the location-dependent effects of inhibition are mostly unknown, especially under conditions involving active dendritic responses. We studied the effect of somatic vs. dendritic inhibition on local spike generation in basal dendrites of layer 5 PNs both in neocortical slices and in simple and detailed compartmental models, with equivalent results: somatic inhibition divisively suppressed the amplitude of dendritic spikes recorded at the soma while minimally affecting dendritic spike thresholds. In contrast, distal dendritic inhibition raised dendritic spike thresholds while minimally affecting their amplitudes. On-the-path dendritic inhibition modulated both the gain and threshold of dendritic spikes depending on its distance from the spike initiation zone. Our findings suggest that cortical circuits could assign different mixtures of gain vs. threshold inhibition to different neural pathways, and thus tailor their local computations, by managing their relative activation of soma- vs. dendrite-targeting interneurons.

Does conventional anti-bipolar and antidepressant drug therapy reduce NMDA-mediated neuronal excitation by downregulating astrocytic GluK2 function?

Chronic treatment with anti-bipolar drugs (lithium, carbamazepine, and valproic acid) down-regulates mRNA and protein expression of kainate receptor GluK2 in mouse brain and cultured astrocytes. It also abolishes glutamate-mediated, Ca(2+)-dependent ERK(1/2) phosphorylation in the astrocytes. Chronic treatment with the SSRI fluoxetine enhances astrocytic GluK2 expression, but increases mRNA editing, abolishing glutamate-mediated ERK(1/2) phosphorylation and [Ca(2+)](i) increase, which are shown to be GluK2-mediated. Neither drug group affects Glu4/Glu5 expression necessary for GluK2's ionotropic effect. Consistent with a metabotropic effect, the PKC inhibitor GF 109203X and the IP(3) inhibitor xestospongin C abolish glutamate stimulation in cultured astrocytes. In CA1/CA3 pyramidal cells in hippocampal slices, activation of extrasynaptic GluK2 receptors, presumably including astrocytic, metabotropic GluK2 receptors, causes long-lasting inhibition of slow neuronal afterhyperpolarization mediated by Ca(2+)-dependent K(+) flux. This may be secondary to the induced astrocytic [Ca(2+)](i) increase, causing release of 'gliotransmitter' glutamate. Neuronal NMDA receptors respond to astrocytic glutamate release with enhancement of excitatory glutamatergic activity. Since reduction of NMDA receptor activity is known to have antidepressant effect in bipolar depression and major depression, these observations suggest that the inactivation of astrocytic GluK2 activity by antidepressant/anti-bipolar therapy ameliorates depression by inhibiting astrocytic glutamate release. A resultant strengthening of neuronal afterhyperpolarization may cause reduced NMDA-mediated activity.

The effect of nutrition on the neural mechanisms potentially involved in melatonin-stimulated LH secretion in female Mediterranean goats

This research examines which neural mechanisms among the endogenous opioid, dopaminergic, serotonergic and excitatory amino acid systems are involved in the stimulation of LH secretion by melatonin implantation and their modulation by nutritional level. Female goats were distributed to two experimental groups that received either 1.1 (group H; n=24) or 0.7 (group L; n=24) times their nutritional maintenance requirements. Half of each group was implanted with melatonin after a long-day period. Plasma LH concentrations were measured twice per week. The effects of i.v. injections of naloxone, pimozide, cyproheptadine and N-methyl-d,l-aspartate (NMDA) on LH secretion were assessed the day before melatonin implantation and again on days 30 and 45. The functioning of all but the dopaminergic systems was clearly modified by the level of nutrition, melatonin implantation and time elapsed since implantation. Thirty days after implantation, naloxone increased LH concentrations irrespective of the level of nutrition (P<0.05), similar to NMDA in the melatonin-implanted H goats (HM; P<0.01). On day 45, naloxone increased LH concentrations in the HM animals (P<0.05), similar to cyproheptadine in both the non-implanted H (HC) and the HM animals (P<0.01). Finally, at 45 days, NMDA increased the LH concentration in all subgroups (P<0.01). These results provide evidence that the effects of different neural systems on LH secretion are modified by nutritional level and melatonin implantation. Endogenous opioids seem to be most strongly involved in the inhibition of LH secretion on days 30 and 45 after melatonin implantation. However, the serotonergic mechanism appears to be most influenced by nutritional level.

Hypothalamic nutrient sensing activates a forebrain-hindbrain neuronal circuit to regulate glucose production in vivo

OBJECTIVE

Hypothalamic nutrient sensing regulates glucose production, but the neuronal circuits involved remain largely unknown. Recent studies underscore the importance of N-methyl-d-aspartate (NMDA) receptors in the dorsal vagal complex in glucose regulation. These studies raise the possibility that hypothalamic nutrient sensing activates a forebrain-hindbrain NMDA-dependent circuit to regulate glucose production.

RESEARCH DESIGN AND METHODS

We implanted bilateral catheters targeting the mediobasal hypothalamus (MBH) (forebrain) and dorsal vagal complex (DVC) (hindbrain) and performed intravenous catheterizations to the same rat for infusion and sampling purposes. This model enabled concurrent selective activation of MBH nutrient sensing by either MBH delivery of lactate or an adenovirus expressing the dominant negative form of AMPK (Ad-DN AMPK α2 [D¹⁵⁷A]) and inhibition of DVC NMDA receptors by either DVC delivery of NMDA receptor blocker MK-801 or an adenovirus expressing the shRNA of NR1 subunit of NMDA receptors (Ad-shRNA NR1). Tracer-dilution methodology and the pancreatic euglycemic clamp technique were performed to assess changes in glucose kinetics in the same conscious, unrestrained rat in vivo.

RESULTS

MBH lactate or Ad-DN AMPK with DVC saline increased glucose infusion required to maintain euglycemia due to an inhibition of glucose production during the clamps. However, DVC MK-801 negated the ability of MBH lactate or Ad-DN AMPK to increase glucose infusion or lower glucose production. Molecular knockdown of DVC NR1 of NMDA receptor via Ad-shRNA NR1 injection also negated MBH Ad-DN AMPK to lower glucose production.

CONCLUSIONS

Molecular and pharmacological inhibition of DVC NMDA receptors negated hypothalamic nutrient sensing mechanisms activated by lactate metabolism or AMPK inhibition to lower glucose production. Thus, DVC NMDA receptor is required for hypothalamic nutrient sensing to lower glucose production and that hypothalamic nutrient sensing activates a forebrain-hindbrain circuit to lower glucose production.

Dynamic and interacting profiles of *NO and O2 in rat hippocampal slices

Nitric oxide (*NO) is a ubiquitous signaling molecule that participates in the neuromolecular phenomena associated with memory formation. In the hippocampus, neuronal *NO production is coupled to the activation of the NMDA-type of glutamate receptor. Although *NO-mediated signaling has been associated with soluble guanylate cyclase activation, cytochrome oxidase is also a target for this gaseous free radical, for which *NO competes with O(2). Here we show, for the first time in a model preserving tissue cytoarchitecture (rat hippocampal slices) and at a physiological O(2) concentration, that endogenous NMDA-evoked *NO production inhibits tissue O(2) consumption for submicromolar concentrations. The simultaneous real-time recordings reveal a direct correlation between the profiles of *NO and O(2) in the CA1 subregion of the hippocampal slice. These results, obtained in a system close to in vivo models, strongly support the current paradigm for O(2) and *NO interplay in the regulation of cellular respiration.

Nitric oxide and pain: 'Something old, something new'

Challenges have emerged following the revival of nitric oxide (NO) from 'something old', a simple gas derived from nitrogen and oxygen with a role in the early stages of evolution, into 'something new', an endogenously formed biological mediator regulating a wide variety of physiological functions. Although pain is a common sensation, it encompasses multiple neurobiologic components, of which NO is only one. In pain research, the study of NO is complicated by convoluted problems related mostly to the effects of NO, which are pro- or anti-nociceptive depending on the circumstances. This dual function reflects the multi-faceted roles of the NO molecule described in physiology. This review covers current information about NO and its implications in pain mechanisms. In addition, it follows the pain pathways, demonstrating the role of NO in peripheral nociceptive transmission as well in central sensitization. This knowledge may provide the scientific basis for developing new drugs that are indicated for different types of pain, drugs that may be related to the chemical links of NO. A comprehensive approach to understanding the effects of NO will help clinicians identify novel agents that combine the pharmacological profile of native drugs with a controllable manner of NO release. Inhibitors of NO synthesis may have analgesic effects and would be of interest for treating inflammatory and neuropathic pain. Unfortunately, only a few of these compounds have reached the stage of clinical pain trials.

Prenatal development of NMDA receptor composition and function in trigeminal neurons

The prenatal development of neural circuits for rhythmical oral-motor behaviors used for feeding is essential for the survival of the newborn mammal. The N-methyl-D-aspartate (NMDA) receptor plays a critical role in brainstem circuits underlying postnatal oral-motor behaviors. To understand a role for the NMDA receptor in the emergence of sucking behavior we conducted physiological and immunohistochemical experiments using fetal rats. Physiology experiments examined the development of the NMDA dose response of the brainstem circuit responsible for generating rhythmical trigeminal activity by recording trigeminal motor outputs using an in vitro preparation. The high dose of NMDA agonist bath application affected the mean cycle duration of rhythmical trigeminal activity (RTA) at both embryonic day (E) 18-19 and E20-21 in comparison with standard concentration of NMDA agonist. NMDA receptor immunohistochemistry studies, using antibodies directed against subunits NR1, NR2A, NR2B, NR3A and NR3B were performed to determine the prenatal regulation of NMDA subunits in trigeminal motoneurons (Mo5), and mesencephalic trigeminal neurons (Me5) between E17 to E20. In Mo5, NR1, NR2A, NR2B and NR3A immunoreactivity was observed throughout the time frame sampled. NR3B immunoreactivity was not observed in Mo5 or Me5. In Mo5, there was a significant decrease in the percentage of NR2B immunoreactive neurons between E17 and E20, and a concurrent increase in the NR2A/NR2B ratio between E17 and E20. In Me5, NR1, NR2A and NR3A immunoreactivity was observed throughout the time frame sampled; a significant decrease in the percentage of NR2A immunoreactive neurons between E17 and E20, and NR3A immunoreactive neurons between E17 and E18 occurred. The timing of subunit changes between E17 and E18 is coincident with the prenatal emergence of rhythmical jaw movements, and in vitro rhythmical trigeminal activity, shown in earlier studies. Our data suggest that NMDA receptor plays an important role in the development and function of prenatal oral-motor circuits.

Estrous cycle variation of TRPV1-mediated cross-organ sensitization between uterus and NMDA-dependent pelvic-urethra reflex activity

Cross-organ sensitization between the uterus and the lower urinary tract (LUT) underlies the high concurrence of pelvic pain syndrome and LUT dysfunctions, and yet the role of gonadal steroids is still unknown. We tested the hypothesis that cross-organ sensitization on pelvic-urethra reflex activity caused by uterine capsaicin instillation is estrous cycle dependent. When compared with the baseline reflex activity (1.00 +/- 0.00 spikes/stimulation), uterine capsaicin instillation significantly increased reflex activity (45.42 +/- 9.13 spikes/stimulation, P < 0.01, n = 7) that was corroborated by an increase in phosphorylated NMDA NR2B (P < 0.05, n = 4) but not NR2A subunit (P > 0.05, n = 4) expression. Both intrauterine pretreatment with capsazepine (5.02 +/- 2.11 spikes/stimulation, P < 0.01, n = 7) and an intrathecal injection of AP5 (3.21 +/- 0.83 spikes/stimulation, P < 0.01, n = 7) abolished the capsaicin-induced cross-organ sensitization and the increment in the phosphorylated NR2B level (P < 0.05, n = 4). The degrees of the cross-organ sensitization increased in a dose-dependent manner with the concentration of instilled capsaicin from 100 to 300 microM in both the proestrus and metestrus stages, whereas they weakened when the concentrations were higher than 1,000 microM. Moreover, the cross-organ sensitization caused by the uterine capsaicin instillation increased significantly in the rats during the proestrus stage when compared with the metestrus stage (P < 0.01, n = 7). These results suggest that estrogen levels might modulate the cross-organ sensitization between the uterus and the urethra and underlie the high concurrence of pelvic pain syndrome and LUT dysfunctions.

Time course and mechanism of hippocampal neuronal death in an in vitro model of status epilepticus: role of NMDA receptor activation and NMDA dependent calcium entry

The hippocampus is especially vulnerable to seizure-induced damage and excitotoxic neuronal injury. This study examined the time course of neuronal death in relationship to seizure duration and the pharmacological mechanisms underlying seizure-induced cell death using low magnesium (Mg2+) induced continuous high frequency epileptiform discharges (in vitro status epilepticus) in hippocampal neuronal cultures. Neuronal death was assessed using cell morphology and fluorescein diacetate-propidium iodide staining. Effects of low Mg2+ and various receptor antagonists on spike frequency were assessed using patch clamp electrophysiology. We observed a linear and time-dependent increase in neuronal death with increasing durations of status epilepticus. This cell death was dependent upon extracellular calcium (Ca2+) that entered primarily through the N-methyl-d-aspartate (NMDA) glutamate receptor channel subtype. Neuronal death was significantly decreased by co-incubation with the NMDA receptor antagonists and was also inhibited by reduction of extracellular (Ca2+) during status epilepticus. In contrast, neuronal death from in vitro status epilepticus was not significantly prevented by inhibition of other glutamate receptor subtypes or voltage-gated Ca2+ channels. Interestingly this NMDA-Ca2+ dependent neuronal death was much more gradual in onset compared to cell death from excitotoxic glutamate exposure. The results provide evidence that in vitro status epilepticus results in increased activation of the NMDA-Ca2+ transduction pathway leading to neuronal death in a time-dependent fashion. The results also indicate that there is a significant window of opportunity during the initial time of continuous seizure activity to be able to intervene, protect neurons and decrease the high morbidity and mortality associated with status epilepticus.

Effect of exogenous glutamate and N-Methyl-D-aspartic acid on spontaneous activity of isolated human ureter

OBJECTIVES

While the neurotransmitter role of glutamate in the gastrointestinal tract has been shown, its effects on smooth muscle of the human ureter have not previously been investigated. In our study we have investigated the effects of exogenous glutamate on the spontaneous activity of isolated human ureter, taken from 14 adult patients after nephrectomy.

METHODS

The segment of ureter, excised 3 cm distal from the pyeloureteral junction, was isolated in an organ bath. Both longitudinal tension and intraluminal pressure of the segment were recorded simultaneously.

RESULTS

Glutamate administered in the lumen of the isolated ureteral segments (7.8 x 10(-7) M/L-3.5 x 10(-2) M/L) was ineffective. When added to the isolated organ bath from the serous side of the ureteral segment, glutamate (7.9 x 10(-6) M/L-10.6 x 10(-3) M/L) and N-Methyl-D-aspartic acid (NMDA) (9.1 x 10(-8) M/L-3.1 x 10(-5) M/L) produced a concentration-dependent increase in spontaneous activity of the isolated preparations, while kainic acid (6.3 x 10(-8) M/L-10.5 x 10(-5) M/L) and (+/-)-trans-1-Aminocyclopentane-trans-1,3-dicarboxylic acid (ACPD) (7.7 x 10(-8) M/L -6.5 x 10(-5) M/L) were ineffective.

CONCLUSIONS

The results of our study suggest that an excitatory neurotransmitter glutamate stimulates spontaneous activity of the human ureter through activation of NMDA ionotropic receptors, located on smooth muscle cells or intramural nerve fibers.

Asynchronous inputs and NMDA conductances predict excitatory responses in the cortical-cA1 pathway of the hippocampus

In the hippocampus, CA1 place cells are driven by a substantial input from CA3. There is a second pathway to CA1 from the entorhinal cortex. The mode of action of cortex on CA1 through this pathway is not known. The pathway supports CA1 place field activity after CA3 has been lesioned, yet stimulation of the pathway in rat slices results in strong feedforward inhibition that prevents pyramidal cell action potentials. We use a detailed conductance-based model of this pathway to simulate the response to cortical stimulation in slice experiments and in vivo spatial exploration. We find that the presence of NMDA conductances enable CA1 pyramidal cells to integrate cortical inputs over a time scale longer than that which is effective in recruiting the inhibitory response that can suppress action potentials. We then show that this asynchronous response mode supports place field formation in response to experimentally constrained spatially modulated cortical activity. Within this model, the inclusion of GABAB conductances and the hyperpolarisation activated current I(h) reduces the strength of the GABAA inputs required to balance the excitatory inputs, and this facilitates place field formation by reducing variability in the inhibitory inputs.

Increases in expression of 14-3-3 eta and 14-3-3 zeta transcripts during neuroprotection induced by delta9-tetrahydrocannabinol in AF5 cells

The molecular mechanisms involved in N-methyl-D-aspartate (NMDA)-induced cell death and Delta9-tetrahydrocannabinol (THC)-induced neuroprotection were investigated in vitro with an AF5 neural progenitor cell line model. By microarray analysis, Ywhah, CK1, Hsp60, Pdcd 4, and Pdcd 7 were identified as being strongly regulated by both NMDA toxicity and THC neuroprotection. The 14-3-3 eta (14-3-3eta; gene symbol Ywhah) and 14-3-3 zeta (14-3-3zeta; gene symbol Ywhaz) transcripts were deceased by NMDA treatment and increased by THC treatment prior to NMDA, as measured by cDNA microarray analysis and quantitative real-time RT-PCR. Other 14-3-3 isoforms were unchanged. Whereas up-regulation of 14-3-3zeta expression was observed 30 min after treatment with THC plus NMDA, down-regulation by NMDA alone was not seen until 16 hr after treatment. By Western blotting, THC increased 14-3-3 protein only in cells that were also treated with NMDA. Overexpression of 14-3-3eta or 14-3-3zeta by transient plasmid transfection increased 14-3-3 protein levels and decreased NMDA-induced cell death. These data suggest that increases in 14-3-3 proteins mediate THC-induced neuroprotection under conditions of NMDA-induced cellular stress.

Descending facilitation of spinal NMDA-dependent reflex potentiation from pontine tegmentum in rats

This study was conducted to investigate whether dorsolateral pontine tegmentum stimulation modulates spinal reflex potentiation (SRP) and whether serotonergic neurotransmission is involved in such a modulation. Reflex activities of the external urethra sphincter (EUS) electromyogram in response to a test stimulation (TS; 1/30 Hz) or repetitive stimulation (RS; 1 Hz) on the pelvic afferent nerve in 35 anesthetized rats were recorded with/without synchronized train pontine stimulation (PS; 300 Hz, 30 ms) and/or intrathecal administrations of 10 μl of 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo (F) quinoxaline (NBQX; 100 μM), d-2-amino-5-phosphonovalerate (APV; 100 μM), N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]- N-(2-pyridinyl) cyclohexanecarboxamide trihydrochloride (WAY 100635; 100 μM), and 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT; 100 μM). The TS evoked a single action potential (1.00 ± 0.00 spikes/stimulation), while the RS produced a long-lasting SRP (16.12 ± 1.59 spikes/stimulation) that was abolished by APV (1.57 ± 0.29 spikes/stimulation) and was attenuated by NBQX (7.42 ± 0.57 spikes/stimulation). Synchronized train PS with RS (PS+RS) produced facilitation in RS-induced SRP (25.17 ± 2.21 spikes/stimulation). Intrathecal WAY 100635 abolished the facilitation in SRP as a result of the synchronized PS (14.66 ± 1.58 spikes/stimulation). On the other hand, intrathecal 8-OH-DPAT elicited facilitation in the RS-induced SRP (25.16 ± 1.05 spikes/stimulation) without synchronized PS. Our findings suggest that dorsolateral pontine tegmentum may modulate N-methyl-d-aspartic acid-dependent SRP via descending serotonergic neurotransmission. This descending modulation may have physiological/pharmacological relevance in the neural controls of urethral closure.

Differential regulation of the Cdk5-dependent phosphorylation sites of inhibitor-1 and DARPP-32 by depolarization

While cyclin-dependent kinase 5 (Cdk5) is of growing importance to neuronal signaling, its regulation remains relatively unexplored. Examination of the mechanism by which NMDA modulates the phosphorylation of protein phosphatase inhibitor-1 at Ser6 and Ser67 and dopamine- and cAMP-regulated phosphoprotein M(r) 32 000 at Thr75 revealed that generalized depolarization, rather than specific activation of NMDA receptors, was sufficient to induce decreases in these Cdk5 sites. Although no evidence for the involvement of the Cdk5 cofactors p35 or p39, or for L- and T-type voltage-gated Ca(2+) channels, was found, evaluation of the role of phosphatases and extracellular cations revealed differential regulation of the three sites. NMDA-induced decreases in the phosphorylation of Thr75 of dopamine- and cAMP-regulated phosphoprotein M(r) 32 000 required protein phosphatase 1/2A activity and extracellular Ca(2+). In contrast, the effects on Ser6 and Ser67 of inhibitor-1 were not cation specific; either Na(+) or Ca(2+) sufficed. Furthermore, while the decrease in phosphorylation of Ser6 was partially dependent on protein phosphatase 2B, that of Ser67 was independent of the major protein serine/threonine phosphatases, likely indicating the presence of a pathway by which NMDA inhibits Cdk5 activity. Thus, in the striatum the regulation of phosphorylation of Cdk5-dependent sites by NMDA occurs through multiple distinct pathways.

Alternating and postictal psychoses: review and a unifying hypothesis

A comparison of the clinical and pathophysiological features of postictal psychosis and brief interictal or alternating psychosis was undertaken to examine if the underlying mechanisms are distinct in these 2 conditions. A selective review of the published literature in English on epilepsy and brief psychosis was carried out. The literature indicates that even though brief postictal and alternating psychoses are considered to be separate syndromes, they have a number of similarities. It can be argued that the underlying pathomechanisms are common, with the brain's inhibitory processes in response to seizures playing a key role in the development of the psychosis. These homeostatic mechanisms manifest as electrophysiological, cerebral blood flow, and neurotransmitter and receptor changes. Both syndromes are likely to be associated with prolonged inhibition in limbic circuits, with further seizures modifying the psychosis depending upon whether it is associated with disinhibition or hypersynchrony involving enhanced inhibition. The neurotransmitter with a key role is GABA, although ionic currents, catecholamines, opiates, adenosine, glutamate, and nitric oxide play a role. Brief postictal and alternating psychoses provide an opportunity to understand the complex relationships between epilepsy and schizophrenia-like brief psychotic episodes, and this understanding can assist in their management.

Application of electroencephalography to the study of cognitive and brain functions in schizophrenia

The electroencephalogram (EEG) recorded from the human scalp is widely used to study cognitive and brain functions in schizophrenia. Current research efforts are primarily devoted to the assessment of event-related potentials (ERPs) and event-related oscillations (EROs), extracted from the ongoing EEG, in patients with schizophrenia and in clinically unaffected individuals who, due to their family history and current mental status, are at high risk for developing schizophrenia. In this article, we discuss the potential usefulness of ERPs and EROs as genetic vulnerability markers, as pathophysiological markers, and as markers of possible ongoing progressive cognitive and cortical deterioration in schizophrenia. Our main purpose is to illustrate that these neurophysiological measures can offer valuable quantitative biological markers of basic pathophysiological mechanisms and cognitive dysfunctions in schizophrenia, yet they may not be specific to current psychiatry's diagnosis and classification. These biological markers can provide unique information on the nature and extent of cognitive and brain dysfunction in schizophrenia. Moreover, they can be utilized to gain deeper theoretical insights into illness etiology and pathophysiology and may lead to improvements in early detection and more effective and targeted treatment of schizophrenia. We conclude by addressing several key methodological, conceptual, and interpretative issues involved in this research field and by suggesting future research directions.

Are anticorrelated networks in the brain relevant to schizophrenia?

Most models of schizophrenia are based on basal ganglia-thalamocortical (BGTC) neuronal circuits or brain structures that project to them. Two new neuronal networks have been described which include many of the brain regions associated with BGTC neuronal circuits. These networks have been characterized with a new brain-imaging technique based on low-frequency fluctuations of the blood oxygen level-dependent (BOLD) signal. The new network associated with attention-demanding tasks is referred to as the task-related network and the network associated with stimulus-independent thought during the resting state is referred to as the default network. The 2 networks have been proposed to be negatively correlated or anticorrelated. This article critically reviews the rationale for these anticorrelated networks, the technique with which they are characterized, and preliminary findings in schizophrenia and other neuropsychiatric disorders. Regions associated with the default network overlap with regions important in motivation and are activated by memory retrieval, auditory hallucinations, and ketamine. Task-related networks are necessary for performance of neurocognitive tasks on which schizophrenic patients often perform poorly. It is concluded that anticorrelated networks can be viewed as complementary ways of understanding self-monitoring and task performance which extend present models of schizophrenia based on BGTC circuits. However, there are some limitations with regard the present understanding of brain structures involved in self-monitoring and the lack of asymmetry in the network which may mediate stimulus-independent thought. Further investigations of the default network assessed by low-frequency fluctuations in the BOLD signal seem warranted.

Effect of physiological activity on an NMDA-dependent form of cortical plasticity in human

Retention of motor learning can be enhanced or degraded by subsequent performance of a different task. Neurophysiologically this may reflect interference in synaptic plasticity by ongoing neural activity in the brain. Here we demonstrate that N-methyl-D-aspartate (NMDA) dependent aftereffects of repetitive transcranial magnetic stimulation (rTMS) also are subject to interference effects, suggesting that it may be possible to investigate these basic mechanisms in the intact human brain. We measured the motor-evoked potential (MEP) amplitude and short-interval intracortical inhibition (SICI) in the first dorsal interosseous (FDI) muscle after continuous or intermittent theta burst (cTBS/iTBS) forms of rTMS. In resting subjects, cTBS depressed MEPs and reduced SICI for about 20 min, whereas iTBS had the opposite effect. However, if subjects contracted the FDI during TBS, then effects on the MEP were abolished, although effects of cTBS on SICI remained. Contraction immediately after TBS enhanced the facilitatory effect of iTBS and reversed the usual inhibitory effect of cTBS into facilitation. Contraction 10 min after cTBS (iTBS not tested) had only a transient (3-4 min) effect on MEPs. These interactions with behavior may relate to mechanisms of interference between learning paradigms in human and be similar to effects on synaptic long-term potentiation/depression described in animal experiments.

Is metabolic syndrome X a disorder of the brain with the initiation of low-grade systemic inflammatory events during the perinatal period?

An imbalance between pro- and anti-inflammatory molecules occurs in metabolic syndrome X. High-energy diet, saturated fats and trans-fats during perinatal period could suppress Delta(6) and Delta(5) desaturases both in the maternal and fetal tissues, resulting in a decrease in the concentrations of long-chain polyunsaturated fatty acids (LCPUFAs): arachidonic acid (AA), eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) that have a negative feedback control on inflammation. EPA, DHA and AA augment endothelial nitric oxide synthesis, potentiate insulin action both in the peripheral tissues and brain and alter leptin production. LCPUFAs are essential for brain growth and development and synaptogenesis and modulate the action of several neurotransmitters and hypothalamic peptides. This suggests that metabolic syndrome X could be a disorder of the brain due to suboptimal LCPUFAs during perinatal period that triggers low-grade systemic inflammation, implying that perinatal strategies are needed to prevent its development.

Sex differences in the potency of kappa opioids and mixed-action opioids administered systemically and at the site of inflammation against capsaicin-induced hyperalgesia in rats

RATIONALE

Sex differences in the potency of the antinociceptive effects of kappa opioids have been reported in various acute pain models with evidence suggesting that these sex differences are mediated by activity in the N-methyl-D: -aspartate (NMDA) system.

OBJECTIVES

The purpose of the present study was to evaluate sex differences in the antihyperalgesic actions of selected kappa and mixed-action opioids in a persistent pain model and determine if the NMDA system modulates these effects in a sexually dimorphic manner.

METHODS

Using gonadally intact male and female F344 rats, hyperalgesia was induced by local administration of capsaicin in the tail, after which the tail was immersed in a mildly noxious thermal stimulus (45 degrees C water), and tail-withdrawal latency measured. Opioids were then administered systemically (s.c.) and locally (in the tail) alone, and in selected combinations with the noncompetitive NMDA antagonist dextromethorphan.

RESULTS

When administered systemically and locally, the kappa opioids spiradoline, U69,593 and U50,488, and the mixed-action opioids butorphanol and nalbuphine, produced dose-dependent antihyperalgesic effects. Whereas the kappa opioids were generally more potent in males, sex differences were not observed with the mixed-action opioids. Peripheral receptor activity was confirmed for local administration of kappa opioids by the antagonism observed after local, but not intracerebroventricular (i.c.v.), administration of the kappa antagonist nor-binaltorphamine (nor-BNI). Dextromethorphan was equally potent in attenuating the antihyperalgesia induced by kappa opioids in both males and females.

CONCLUSIONS

These findings demonstrate sex differences in kappa opioid activity in a persistent pain model. Although an NMDA antagonist blocked the effects of kappa opioids in this model, these effects were not sexually dimorphic as reported in most acute pain models.

NMDA neurotransmission as a critical mediator of borderline personality disorder

Studies of the neurobehavioural components of borderline personality disorder (BPD) have shown that symptoms and behaviours of BPD are partly associated with disruptions in basic neurocognitive processes, in particular, in the executive neurocognition and memory systems. A growing body of data indicates that the glutamatergic system, in particular, the N-methyl-D-aspartate (NMDA) subtype receptor, plays a major role in neuronal plasticity, cognition and memory and may underlie the pathophysiology of multiple psychiatric disorders. In this paper, we review the literature regarding BPD and its cognitive deficits and the current data on glutamatergic and NMDA neurotransmission. We propose that multiple cognitive dysfunctions and symptoms presented by BPD patients, like dissociation, psychosis and impaired nociception, may result from the dysregulation of the NMDA neurotransmission. This impairment may be the result of a combination of biological vulnerability and environmental influences mediated by the NMDA neurotransmission.

d-Aspartic acid: An endogenous amino acid with an important neuroendocrine role

D-Aspartic acid (d-Asp), an endogenous amino acid present in vertebrates and invertebrates, plays an important role in the neuroendocrine system, as well as in the development of the nervous system. During the embryonic stage of birds and the early postnatal life of mammals, a transient high concentration of d-Asp takes place in the brain and in the retina. d-Asp also acts as a neurotransmitter/neuromodulator. Indeed, this amino acid has been detected in synaptosomes and in synaptic vesicles, where it is released after chemical (K(+) ion, ionomycin) or electric stimuli. Furthermore, d-Asp increases cAMP in neuronal cells and is transported from the synaptic clefts to presynaptic nerve cells through a specific transporter. In the endocrine system, instead, d-Asp is involved in the regulation of hormone synthesis and release. For example, in the rat hypothalamus, it enhances gonadotropin-releasing hormone (GnRH) release and induces oxytocin and vasopressin mRNA synthesis. In the pituitary gland, it stimulates the secretion of the following hormones: prolactin (PRL), luteinizing hormone (LH), and growth hormone (GH) In the testes, it is present in Leydig cells and is involved in testosterone and progesterone release. Thus, a hypothalamus-pituitary-gonads pathway, in which d-Asp is involved, has been formulated. In conclusion, the present work is a summary of previous and current research done on the role of d-Asp in the nervous and endocrine systems of invertebrates and vertebrates, including mammals.

Instrumental learning within the spinal cord: underlying mechanisms and implications for recovery after injury

Using spinally transected rats, research has shown that neurons within the L4-S2 spinal cord are sensitive to response-outcome (instrumental) relations. This learning depends on a form of N-methyl-D-aspartate (NMDA)-mediated plasticity. Instrumental training enables subsequent learning, and this effect has been linked to the expression of brain-derived neurotrophic factor. Rats given uncontrollable stimulation later exhibit impaired instrumental learning, and this deficit lasts up to 48 hr. The induction of the deficit can be blocked by prior training with controllable shock, the concurrent presentation of a tonic stimulus that induces antinociception, or pretreatment with an NMDA or gamma-aminobutyric acid-A antagonist. The expression of the deficit depends on a kappa opioid. Uncontrollable stimulation enhances mechanical reactivity (allodynia), and treatments that induce allodynia (e.g., inflammation) inhibit learning. In intact animals, descending serotonergic neurons exert a protective effect that blocks the adverse consequences of uncontrollable stimulation. Uncontrollable, but not controllable, stimulation impairs the recovery of function after a contusion injury.

The G72/G30 gene complex and cognitive abnormalities in schizophrenia

A recently discovered gene complex, G72/G30 (hereafter G72, but now termed DAOA), was found to be associated with schizophrenia and with bipolar disorder, possibly because of an indirect effect on NMDA neurotransmission. In principle, if G72 increases risk for psychosis by this mechanism, it might impact with greater penetrance those cortically based cognitive and neurophysiological functions associated with NMDA signaling. We performed two independent family-based association studies (one sample contained more than 200 families and the other more than 65) of multiple SNPs in the G72 region and of multiple SNPs in the gene for D-amino acid oxidase (DAAO), which may be modulated by G72. We examined the relationship between select cognitive measures in attention, working memory, and episodic memory and a restricted set of G72 SNPs in over 600 normal controls, schizophrenic patients, and their nonpsychotic siblings using mixed model ANOVAs. We also determined genotype effects on neurophysiology measures in normal controls using the fMRI BOLD response obtained during activation procedures involving either episodic memory or working memory. There were no significant single G72 SNP associations and clinical diagnosis in either sample, though one approached significance (p=0.06). Diagnosis by genotype interaction effects for G72 SNP 10 were significant for cognitive variables assessing working memory and attention (p=0.05), and at the trend level for episodic memory, such that in the schizophrenia group an exaggerated allele load effect in the predicted directions was observed. In the fMRI paradigms, a strong effect of G72 SNP 10 genotype was observed on BOLD activation in the hippocampus during the episodic memory paradigm. Tests of association with DAAO were consistently nonsignificant. We present evidence that SNP variations in the G72 gene region increase risk of cognitive impairment in schizophrenia. SNP variations were not strongly associated with clinical diagnosis in family-based analyses.

Maturation-dependent reduced responsiveness of intracellular free Ca2+ ions to repeated stimulation by N-methyl-d-aspartate in cultured rat cortical neurons

In contrast to other ionotropic glutamate receptors, N-methyl-d-aspartate (NMDA) receptor channels are rather stable after the simulation. Brief exposure to NMDA at 50 microM rapidly increased the fluorescence intensity for increased intracellular free Ca(2+) levels in a reversible- and concentration-dependent manner in rat cortical neurons cultured for 3-15 days in vitro (DIV), while EC(50) values were significantly decreased in proportion to cellular maturation from 3 to 15 DIV. Although a constant increase was persistently seen in the fluorescence throughout the sustained exposure to NMDA for 60 min irrespective of the cell maturation from 3 to 15 DIV, the second brief exposure for 5 min resulted in a less efficient increase in the fluorescence than that found after the first brief exposure for 5 min in a manner dependent on intervals between the two repetitive brief exposures. In vitro maturation significantly shortened the interval required for the reduced responsiveness to the second brief exposure, while in immature neurons prolonged intervals were required for the reduced responsiveness to the second brief exposure to NMDA. Moreover, brief exposure to NMDA led to a marked decrease in immunoreactivity to extracellular loop of NR1 subunit in cultured neurons not permeabilized in proportion to the time after washing. These results suggest that cellular maturation would facilitate the desensitization process to repeated stimulation by NMDA, without markedly affecting that to sustained stimulation, through a mechanism related to the decreased number of NMDA receptors expressed at cell surfaces in cultured rat cortical neurons.

Selective Inhibition of Spontaneous But Not Ca2+-Dependent Release Machinery by Presynaptic Group II mGluRs in Rat Cerebellar Slices

Two main forms of neurotransmitter release are known: action potential-evoked and spontaneous release. Action potential-evoked release depends on Ca2+entry through voltage-gated Ca2+channels, whereas spontaneous release is thought to be Ca2+-independent. Generally, spontaneous and action potential-evoked release are believed to use the same release machinery to release neurotransmitter. This study shows, using the whole cell patch-clamp technique in rat cerebellar slices, that at the interneuron- Purkinje cell synapse activation of presynaptic group II metabotropic glutamate receptors suppresses spontaneous GABA release through a mechanism independent of voltage-gated Ca2+channels, store-operated Ca2+channels, and Ca2+release from intracellular Ca2+stores, suggesting that the metabotropic receptors target the release machinery directly. Voltage gated Ca2+channel-independent release following increased presynaptic cAMP production is similarly inhibited by these metabotropic receptors. In contrast, both voltage-gated Ca2+channel-dependent and presynaptic N-methyl-d-aspartate receptor-dependent GABA release were unaffected by activation of group II metabotropic glutamate receptors. Hence, the mechanisms underlying spontaneous and Ca2+-dependent GABA release are distinct in that only the former is blocked by group II metabotropic glutamate receptors. Thus the same neurotransmitter, glutamate, can activate or inhibit neurotransmitter release by selecting different receptors that target different release machineries.

A physiological mechanism to regulate d-aspartic acid and NMDA levels in mammals revealed by d-aspartate oxidase deficient mice

Free D-aspartic acid and NMDA are present in the mammalian central nervous system and endocrine glands at significant concentrations, but their physiological role is still matter of debate. The only enzyme known to metabolize in vitro selectively these D-amino acids is D-aspartate oxidase (DDO). To clarify the role in vivo of the enzyme, we generated mice with targeted deletion of Ddo gene by homologous recombination. Mutated animals showed increased amounts of both D-aspartic acid and NMDA in all tissues examined demonstrating a physiological role of DDO in the regulation of their endogenous levels.

Synaptic plasticity and dysconnection in schizophrenia

Current pathophysiological theories of schizophrenia highlight the role of altered brain connectivity. This dysconnectivity could manifest 1) anatomically, through structural changes of association fibers at the cellular level, and/or 2) functionally, through aberrant control of synaptic plasticity at the synaptic level. In this article, we review the evidence for these theories, focusing on the modulation of synaptic plasticity. In particular, we discuss how dysconnectivity, observed between brain regions in schizophrenic patients, could result from abnormal modulation of N-methyl-D-aspartate (NMDA)-dependent plasticity by other neurotransmitter systems. We focus on the implication of the dysconnection hypothesis for functional imaging at the systems level. In particular, we review recent advances in measuring plasticity in the human brain using functional magnetic resonance imaging (fMRI) and electroencephalography (EEG) that can be used to address dysconnectivity in schizophrenia. Promising experimental paradigms include perceptual and reinforcement learning. We describe how theoretical and causal models of brain responses might contribute to a mechanistic understanding of synaptic plasticity in schizophrenia.

Pitfalls And Hopes in Down Syndrome Therapeutic Approaches: In the Search for Evidence-Based Treatments

Trisomy 21 or Down syndrome (DS) is a complex syndrome, of genetic origin with multiple and variable neurobiological and neuropsychological manifestations. DS patients have consistent signs of brain damage along their lives, but understanding the biology of DS is complicated due to the extraordinary heterogeneity of the phenotypic signs. Thus, treatment of DS mental retardation poses significant challenges for clinicians and scientists. The review addresses the classical pharmacological and environmental treatments and also critically reviews the new possibilities that are emerging from the exciting advances in gene or cell therapy. We describe some of the most recent developments in the field and give a sense of the prospects for future prevention and therapy.

Bradykinin produces pain hypersensitivity by potentiating spinal cord glutamatergic synaptic transmission

Bradykinin, an inflammatory mediator, sensitizes nociceptor peripheral terminals reducing pain threshold. We now show that the B2 kinin receptor is expressed in rat dorsal horn neurons and that bradykinin, a B2-specific agonist, augments AMPA- and NMDA-induced, and primary afferent-evoked EPSCs, and increases the frequency and amplitude of miniature EPSCs in superficial dorsal horn neurons in vitro. Administration of bradykinin to the spinal cord in vivo produces, moreover, an NMDA-dependent hyperalgesia. We also demonstrate that nociceptive inputs result in the production of bradykinin in the spinal cord and that an intrathecal B2-selective antagonist suppresses behavioral manifestations of central sensitization, an activity-dependent increase in glutamatergic synaptic efficacy. Primary afferent-evoked central sensitization is, in addition, reduced in B2 receptor knock-out mice. We conclude that bradykinin is released in the spinal cord in response to nociceptor inputs and acts as a synaptic neuromodulator, potentiating glutamatergic synaptic transmission to produce pain hypersensitivity.

What have we learned from proton magnetic resonance spectroscopy about schizophrenia? A critical update

PURPOSE OF REVIEW

This review discusses recent studies investigating schizophrenia with proton magnetic resonance spectroscopy including the first meta-analysis [Steen RG, Hamer RM, Lieberman JA. Measurement of brain metabolites by 1H magnetic resonance spectroscopy in patients with schizophrenia: a systematic review and meta-analysis. Neuropsychology 2005; 30:1949-1962]. We also highlight methodological issues and suggest a modality for future research to further explore glutamatergic dysfunction in schizophrenia.

RECENT FINDINGS

Despite methodological differences, spectroscopy studies with schizophrenia show reductions in N-acetylaspartate in the medial temporal and prefrontal regions. Other areas such as the anterior cingulate, parietal cortex thalamus, and cerebellum may also have N-acetylaspartate reductions. The proton magnetic resonance spectroscopy studies at higher fields and with shorter echo time have revealed abnormalities in glutamate and glutamine. Animal studies have shown that the discrepancies in metabolites between patients and controls are not due to antipsychotic medication exposure, and that chronic exposure to N-methyl-D-aspartate antagonists has produced decreased N-acetylaspartate in the temporal cortex. The human and animal studies both support an excitoxic glutamatergically mediated process that may explain decreased N-acetylaspartate, volume loss, and the poor outcomes of schizophrenia.

SUMMARY

Use of higher field strengths and longitudinal studies may reveal a progressive excitoxic glutamatergic process that leads to N-acetylaspartate and volume reductions. This may lead to the development of neuroprotective agents that change the course of schizophrenia.

N-methyl-D-aspartate and group I metabotropic glutamate receptors are involved in the expression of ethanol-induced sensitization in mice

Effects of acamprosate and ionotropic uncompetitive N-methyl-D-aspartate receptor antagonists and group I metabotropic glutamatergic receptor antagonists on the expression of ethanol-induced sensitization were investigated in mice. The results indicated that acamprosate (200 and 400 mg/kg) and N-methyl-D-aspartate receptor antagonists, neramexane (10 and 20 mg/kg) and MK-801 (0.1 and 0.2 mg/kg), inhibited the expression of ethanol-induced sensitization. Acamprosate, but not the other compounds tested, also blocked the stimulant effect of acute injections of ethanol. Among the group I metabotropic glutamatergic receptor antagonists, only the metabotropic glutamatergic receptor 5 antagonist, MTEP (5, 10, and 20 mg/kg) showed an effect similar to the N-methyl-D-aspartate receptor antagonists. The metabotropic glutamatergic receptor 1 antagonist, EMQMCM (5, 10, and 20 mg/kg), however, potentiated the inhibitory effect of MK-801 on the expression of ethanol-induced sensitization. The findings indicate that glutamatergic neurotransmission is important in the ethanol-induced sensitization process, and suggest that co-administration of metabotropic glutamatergic receptor 1 antagonists and N-methyl-D-aspartate receptor antagonists may be useful in therapy for alcoholism.

N-methyl-D-aspartate blocks activation of JNK and mitochondrial apoptotic pathway induced by potassium deprivation in cerebellar granule cells

During the postnatal development of cerebellum, lack of excitatory innervation from the mossy fibers results in cerebellar granule cell (CGC) apoptosis during the migration of the cells toward the internal granule cell layer. Accordingly, CGCs die by apoptosis when cultured in physiological KCl concentrations (5 mm; K5), and they survive in the presence of depolarizing conditions such as high KCl concentration (25 mm; K25) or N-methyl-D-aspartate (NMDA). We have recently shown that NMDA is able to exert a long lasting neuroprotective effect when added to immature (2 days in vitro) CGC cultures by inhibition of caspase-3 activity. Here we show that NMDA- and K25-mediated neuroprotection is associated with an increase in the levels of Bcl-2, an inhibition of K5-mediated increase in Bax, and the inhibition of the release of apoptogenic factors from mitochondria such as Smac/DIABLO and cytochrome c. Moreover, we have shown that similar effects are observed when c-Jun N-terminal kinases (JNKs) are inhibited and that treatment of CGC cultures with NMDA blocks K5-mediated JNK activation. These results allow us to postulate that the inhibition of JNK-mediated release of apoptogenic factors from mitochondria is involved in the NMDA protection from K5-mediated apoptosis of CGCs.

AT4 receptor activation increases intracellular calcium influx and induces a non-N-methyl-D-aspartate dependent form of long-term potentiation

The angiotensin 4 receptor (AT4) subtype is heavily distributed in the dentate gyrus and CA1-CA3 subfields of the hippocampus. Neuronal pathways connecting these subfields are believed to be activated during learning and memory processing. ur laboratory previously demonstrated that application of the AT4 agonist, Norleucine1-angiotensin IV, enhanced baseline synaptic transmission and long-term potentiation, whereas perfusion with the AT4 antagonist, Norleucine1-Leu3-psi(CH2-NH2)3-4-angiotensin IV disrupted long-term potentiation stabilization in area CA1. The objective of the present study was to identify the mechanism(s) responsible for Norleucine1-angiotensin IV-induced increase in hippocampal long-term potentiation. Hippocampal slices perfused with Norleucine1-angiotensin IV for 20 min revealed a notable increase in baseline responses in a non-reversible manner and were blocked by the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptor antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione disodium salt. Infusions of Norleucine1-angiotensin IV prior to, but not after theta burst stimulation, significantly enhanced long-term potentiation compared with control slices. Further, N-methyl-D-aspartate receptor-independent long-term potentiation could be induced by tetanization during the perfusion of Norleucine1-angiotensin IV in the presence of the N-methyl-D-aspartate antagonist, D,L-2-amino-5-phosphonovaleric acid. Blockade of select voltage dependent calcium channels significantly reduced Norleucine1-angiotensin IV-induced increase in baseline responses and subsequent long-term potentiation suggesting that AT4 receptor activation increases intracellular calcium levels via altering voltage dependent calcium channels and triggers an N-methyl-D-aspartate-independent form of long-term potentiation. In support of this notion the application of Nle1-angiotensin IV to cultured rat hippocampal neurons resulted in increased intracellular calcium derived exclusively from extracellular sources. Consistent with these observations Nle1-angiotensin IV was capable of augmenting the uptake of 45Ca2+ into rat hippocampal slices. Taken together, these data indicate that increased calcium influx through postsynaptic calcium channels contribute to Norleucine1-angiotensin IV-induced enhancement of long-term potentiation.

Chronic hypoxia modulates NMDA-mediated regulation of the hypoxic ventilatory response in an amphibian, Bufo marinus

This study examined whether a hypoxia-tolerant amphibian, the Cane toad, undergoes mammalian-like ventilatory acclimatisation to hypoxia (VAH) and whether chronic hypoxia (CH) alters NMDA-mediated regulation of the acute hypoxic ventilatory response (HVR). Toads were exposed to 10 days of CH (10% O2) followed by acute hypoxic breathing trials or an intra-arterial injection of NaCN. Trials were conducted before and after i.p. treatment with an NMDA-receptor channel blocker (MK801). CH blunted the acute HVR but did not alter resting breathing. MK801 did not alter resting ventilation. In control animals, MK801 augmented breathing frequency (fR) during acute hypoxia by increasing the number of breaths per episode. This effect was attenuated following CH although MK801 did enhance the number of episodes per minute during acute hypoxia. MK801 enhanced the fR response to NaCN in both groups. The results indicate that CH did not produce mammalian-like VAH (i.e. increased resting ventilation and an augmented acute HVR) but did alter MK801-sensitive regulation of breathing pattern and the acute HVR.

The Arg473Cys-neuroligin-1 mutation modulates NMDA mediated synaptic transmission and receptor distribution in hippocampal neurons

Synapses mediate communication between neurons, thus playing a fundamental role in information processing in the CNS. Neuroligins form a family of heterophilic synaptic cell adhesion molecules, and neuroligin 1 (NL1) has been shown to be involved in the formation of excitatory synapses and have been suggested to associate indirectly with NMDA receptors by common binding to PSD95. A mutation in neuroligin 3 (Arg451Cys-NL3, human sequence numbering) identified in autistic patients is associated with altered spine density and has reduced binding capacity for its presynaptic partner beta-neurexin. Here, we investigated the role of NL1 and the homologous NL1 mutation Arg473Cys-NL1 (R473C-NL1) in excitatory synaptic transmission and NMDA receptor distribution. We demonstrate that R473C-NL1, when expressed in cultured hippocampal neurons, can induce a dramatic increase in NMDA current amplitude and that this change is accompanied by NMDA receptor clustering in the postsynaptic cell.

Respiratory and autonomic responses to microinjection of NMDA and AMPA into the commissural subnucleus of the NTS of awake rats

The changes in mean arterial pressure (MAP) and respiratory frequency (RF) in response to microinjection of NMDA or AMPA into the commissural subnucleus of the NTS (comNTS) at the calamus scriptorius level of awake rats were evaluated. Under tribromoethanol anesthesia, the rats received guide-cannulae in direction of the NTS and a catheter was inserted into the femoral artery for measurement of arterial pressure. Changes in RF were evaluated with the rats inside a plethysmographic chamber. Randomly microinjections of 5 doses of NMDA (0.001, 0.01, 0.1, 1.0 and 2 nmol/50 nL; n = 10) or AMPA (1, 5, 10, 25 and 50 pmol/50 nL; n = 8) into the comNTS were performed at 15 min intervals and produced a dose-dependent increase in MAP [NMDA (3 +/- 2, 4 +/- 3, 25 +/- 4, 41 +/- 4 and 51 +/- 4 mm Hg) and AMPA (0 +/- 1, 14 +/- 4, 17 +/- 3, 27 +/- 5 and 34 +/- 3 mm Hg)]. Microinjection of NMDA (1 nmol/50 nL; n = 7) or AMPA (50 pmol/50 nL; n = 4) into the comNTS produced a long lasting apnea. The pressor responses to microinjection of NMDA or AMPA into the comNTS were blocked by prazosin, a alpha(1)-adrenoceptor antagonist, indicating that the increase in arterial pressure in both cases was sympathetically mediated. The data show that microinjection of NMDA and AMPA into the comNTS produced pressor response and apnea, indicating that both ionotropic l-glutamate receptors may play a role in the neurotransmission/neuromodulation of the autonomic and respiratory components of the cardiovascular reflexes at this level.

Leptin: a potential cognitive enhancer?

It is well documented that the hormone leptin signals information regarding the status of fat stores to hypothalamic nuclei, which in turn control feeding behaviour and body weight. However, leptin and its receptor are widely expressed in many extra-hypothalamic brain regions, including hippocampus, brain stem and cerebellum. Moreover, evidence is accumulating that leptin has other neuronal functions that are unrelated to its effects on energy homeostasis. Indeed a role for leptin in neuronal development has been suggested as leptin-deficient rodents display abnormal brain development and leptin actively participates in the development of the hypothalamus. In the hippocampus, leptin is a potential cognitive enhancer as genetically obese rodents with dysfunctional leptin receptors display impairments in hippocampal synaptic plasticity. Moreover, direct administration of leptin into the hippocampus can facilitate hippocampal LTP (long-term potentiation) in vivo and improve memory processing in mice. At the cellular level, we have also shown that leptin has the capacity to convert short-term potentiation into LTP. Here, we review the data that leptin influences hippocampal synaptic plasticity via enhancing NMDA (N-methyl-D-aspartate) receptor function. We also provide evidence that rapid trafficking of NMDA receptors to the plasma membrane may underlie the effects of leptin on excitatory synaptic strength.

On the mechanism of anticonvulsant effect of tramadol in mice

The present study was conducted to examine the effects of tramadol, an atypical opioid on convulsive behaviour in maximal electroshock (MES) seizure test on mice. Moreover, an attempt was also made to investigate the role of possible receptor mechanisms involved. MES seizures were induced via transauricular electrodes (60 mA, 0.2 sec). Seizure severity was determined by (1) the duration of tonic hindlimb extensor (THE) phase and by (2) mortality due to electroconvulsions. Intraperitoneal (i.p.) administration of tramadol dose-dependently (10-50 mg/kg) decreased the duration of THE phase of MES. The anticonvulsant effect of tramadol was antagonized by the opioid antagonists, naloxone in high dose, and MR2266, a selective kappa antagonist but not by naltrindole, a delta opioid antagonist. Coadministration of either gamma-aminobutyric acid (GABA)-ergic drugs (diazepam, GABA, muscimol and baclofen) or N-methyl-D-aspartate (NMDA) receptor antagonist, MK801 with tramadol augmented the anticonvulsant effect of the latter drug. By contrast, flumazenil, a central benzodiazepine (BZD) receptor antagonist, counteracted the diazepam-induced facilitation of anti-MES effect of tramadol. Similarly, delta-aminovaleric acid (DAVA), a GABAB receptor antagonist, abolished the facilitatory effect of baclofen, a GABAB agonist on anti-MES action of tramadol. These BZD-GABAergic antagonists, flumazenil or DAVA, on their own also antagonized the anti-MES effect of tramadol administered alone. No significant effect on mortality was observed in any of the studied groups. Taken together, the current results have demonstrated a possible role for multitude of important neurotransmitter systems, i.e., opioid (kappa), GABAA-BZD receptors system, GABAB receptors and NMDA channel involvement in the antielectroshock effect of tramadol in mice.