NMDA-receptor inhibition increases spine stability of denervated mouse dentate granule cells and accelerates spine density recovery following entorhinal denervation in vitro

Neuronal networks are reorganized following brain injury. At the structural level this is in part reflected by changes in the spine turnover of the denervated neurons. Using the entorhinal cortex lesion in vitro model, we recently showed that mouse dentate granule cells respond to entorhinal denervation with coordinated functional and structural changes: During the early phase after denervation spine density decreases, while excitatory synaptic strength increases in a homeostatic manner. At later stages spine density increases again, and synaptic strength decreases back to baseline. In the present study, we have addressed the question of whether the denervation-induced homeostatic strengthening of excitatory synapses could not only be a result of the deafferentation, but could, in turn, affect the dynamics of the spine reorganization process following entorhinal denervation in vitro. Using a computational approach, time-lapse imaging of neurons in organotypic slice cultures prepared from Thy1-GFP mice, and patch-clamp recordings we provide experimental evidence which suggests that the strengthening of surviving synapses can lead to the destabilization of spines formed after denervation. This activity-dependent pruning of newly formed spines requires the activation of N-methyl-d-aspartate receptors (NMDA-Rs), since pharmacological inhibition of NMDA-Rs resulted in a stabilization of spines and in an accelerated spine density recovery after denervation. Thus, NMDA-R inhibitors may restore the ability of neurons to form new stable synaptic contacts under conditions of denervation-induced homeostatic synaptic up-scaling, which may contribute to their beneficial effect seen in the context of some neurological diseases.

[The features of postsynaptic currents in primary culture of rat cortical neurons]

The generation features of postsynaptic currents were studied in primary culture of cortical neurons at 7-20 days in vitro (DIV). The use of specific blockers of postsynaptic ion channels after 10 DIV revealed all types of electrical activity found in adult cortex including miniature inhibitory (mIPSCs), excitatory (mEPSCs) and spontaneous giant excitatory currents and spikes. The frequency of mEPSCs increased exponentially from 7 to 20 DIV doubling every 2.2 days in parallel with changes in action potentials generation. The mEPSCs generated by NMDA and AMPA or by only AMPA receptor activation were found. The inhibition of NMDA receptors by magnesium ions or AP5 were shown to modulate the frequency and amplitude of mEPSCs, which differ primary culture from brain slices possibly because of the lack of glial control of synaptic transmission.

Investigations into the involvement of NMDA mechanisms in recognition memory

This review will focus on evidence showing that NMDA receptor neurotransmission is critical for synaptic plasticity processes within brain regions known to be necessary for the formation of object recognition memories. The aim will be to provide evidence concerning NMDA mechanisms related to recognition memory processes and show that recognition memory for objects, places or associations between objects and places depends on NMDA neurotransmission within the perirhinal cortex, temporal association cortex medial prefrontal cortex and hippocampus. Administration of the NMDA antagonist AP5, selectively into each of these brain regions has revealed that the extent of the involvement NMDA receptors appears dependent on the type of information required to solve the recognition memory task; thus NMDA receptors in the perirhinal cortex are crucial for the encoding of long-term recognition memory for objects, and object-in-place associations, but not for short-term recognition memory or for retrieval. In contrast the hippocampus and medial prefrontal cortex are required for both long-term and short-term recognition memory for places or associations between objects and places, or for recognition memory tasks that have a temporal component. Such studies have therefore confirmed that the multiple brain regions make distinct contributions to recognition memory but in addition that more than one synaptic plasticity process must be involved.

This article is part of the Special Issue entitled ‘Glutamate Receptor-Dependent Synaptic Plasticity’.

•

NMDAR blockade in PRH, mPFC and HPC produces different patterns of memory deficits.

•

NMDARs within these brain regions make distinct contributions to recognition memory.

•

NMDARs are also critical for synaptic plasticity in the same brain regions.

•

More than one synaptic plasticity process must be involved in recognition memory.

Extinction of conditioned opiate withdrawal in rats is blocked by intracerebroventricular infusion of an NMDA receptor antagonist

Maladaptive conditioned responses (CRs) contribute to psychiatric disorders including anxiety disorders and addiction. Methods of reducing these CRs have been considered as possible therapeutic approaches. One such method is extinction, which involves exposure to CR-eliciting cues in the absence of the event they once predicted. In animal models, extinction reduces both fear and addiction-related CRs, and in humans, extinction-based cue exposure therapy (CET) reduces fear CRs. However, CET is less effective in drug addicts, for reasons that are not clear. Increased understanding of the neurobiology of extinction of drug-related CRs as compared to fear CRs may help illuminate this issue. Here, we examine the N-methyl-d-aspartate (NMDA) receptor-dependence of extinction of conditioned opiate withdrawal in rats. Using a place conditioning paradigm, we trained morphine-dependent rats to associate an environment with naloxone-precipitated withdrawal. We then extinguished that association by returning the rats repeatedly to the environment in the absence of acute withdrawal. In some rats we administered the NMDA receptor antagonist d,l-2-amino-5-phosphovaleric acid (AP5) intracerebroventricularly immediately prior to extinction training. In a subsequent test session, these rats avoided the formerly naloxone-paired environment, similar to rats that had not undergone extinction training. By contrast, rats that received vehicle prior to extinction training did not avoid the formerly naloxone-paired environment. This finding indicates that extinction of a drug-related CR (conditioned opiate withdrawal) is dependent on NMDA receptors, similar to extinction of conditioned fear. The locus of the critical NMDA receptors is unclear but may include basolateral amygdala and/or medial prefrontal cortex.

Synaptic and extrasynaptic plasticity in glutamatergic circuits involving dentate granule cells following chronic N-methyl-D-aspartate receptor inhibition

Chronic global N-methyl-d-aspartate receptor (NMDAR) blockade leads to changes in glutamatergic transmission. The impact of more subunit-selective NMDAR inhibition on glutamatergic circuits remains incomplete. To this end, organotypic hippocampal slice cultures were treated for 17-21 days with the high-affinity competitive antagonist d-aminophosphonovaleric acid (d-APV), the allosteric GluN2B-selective antagonist Ro25-6981, or the newer competitive GluN2A-preferring antagonist NVP-AAM077. Electrophysiological recordings from dentate granule cells revealed that chronic d-APV treatment increased, whereas chronic Ro25-6981 reduced, epileptiform event-associated large-amplitude spontaneous excitatory postsynaptic currents (sEPSC) compared with all other treatment groups, consistent with opposite effects on glutamatergic networks. Presynaptically, chronic d-APV or Ro25-6981 increased small-amplitude sEPSCs and AMPA/kainate receptor-mediated miniature EPSCs (mEPSCAMPAR) frequency. Chronic d-APV or NVP-AAM077, but not Ro25-6981, increased putative vGlut1-positive glutamatergic synapses. Postsynaptically, chronic d-APV dramatically increased mEPSCAMPAR and profoundly decreased NMDAR-mediated mEPSC (mEPSCNMDAR) measures, suggesting increased AMPAR/NMDAR ratio. Ro25-6981 decreased mEPSCAMPAR charge transfer and modestly decreased mEPSCNMDAR frequency and decay, suggesting downward scaling of AMPAR and NMDAR function without dramatically altering AMPAR/NMDAR ratio. Extrasynaptically, threo-β-benzyloxyaspartate-enhanced "tonic" NMDAR current amplitude and activated channel number estimates were significantly increased only by chronic Ro25-6981. For intrinsic excitability, action potential threshold was slightly more negative following chronic d-APV or NVP-AAM077. The predominant pro-excitatory effects of chronic d-APV are consistent with increased glutamatergic transmission and network excitability. The minor effects of chronic NVP-AAM077 on action potential threshold and synapse number are consistent with minimal effects on circuit function. The chronic Ro25-6981-induced downward scaling of synaptic AMPAR and NMDAR function is consistent with decreased postsynaptic glutamate receptors and reduced network excitability.

Effect of barodenervation on cardiovascular responses elicited from the hypothalamic arcuate nucleus of the rat

We have previously reported that chemical stimulation of the hypothalamic arcuate nucleus (ARCN) in the rat elicited increases as well as decreases in blood pressure (BP) and sympathetic nerve activity (SNA). The type of response elicited from the ARCN (i.e., increase or decrease in BP and SNA) depended on the level of baroreceptor activity which, in turn, was determined by baseline BP in rats with intact baroreceptors. Based on this information, it was hypothesized that baroreceptor unloading may play a role in the type of response elicited from the ARCN. Therefore, the effect of barodenervation on the ARCN-induced cardiovascular and sympathetic responses and the neurotransmitters in the hypothalamic paraventricular nucleus (PVN) mediating the excitatory responses elicited from the ARCN were investigated in urethane-anesthetized adult male Wistar rats. Bilateral barodenervation converted decreases in mean arterial pressure (MAP) and greater splanchnic nerve activity (GSNA) elicited by chemical stimulation of the ARCN with microinjections of N-methyl-D-aspartic acid to increases in MAP and GSNA and exaggerated the increases in heart rate (HR). Combined microinjections of NBQX and D-AP7 (ionotropic glutamate receptor antagonists) into the PVN in barodenervated rats converted increases in MAP and GSNA elicited by the ARCN stimulation to decreases in MAP and GSNA and attenuated increases in HR. Microinjections of SHU9119 (a melanocortin 3/4 receptor antagonist) into the PVN in barodenervated rats attenuated increases in MAP, GSNA and HR elicited by the ARCN stimulation. ARCN neurons projecting to the PVN were immunoreactive for proopiomelanocortin, alpha-melanocyte stimulating hormone (alpha-MSH) and adrenocorticotropic hormone (ACTH). It was concluded that increases in MAP and GSNA and exaggeration of tachycardia elicited by the ARCN stimulation in barodenervated rats may be mediated via release of alpha-MSH and/or ACTH and glutamate from the ARCN neurons projecting to the PVN.

Kainate receptor-mediated synaptic transmissions in the adult rodent insular cortex

Kainate (KA) receptors are expressed widely in the central nervous system and regulate both excitatory and inhibitory synaptic transmission. KA receptors play important roles in fear memory, anxiety, and pain. However, little is known about their function in synaptic transmission in the insular cortex (IC), a critical region for taste, memory, and pain. Using whole cell patch-clamp recordings, we have shown that KA receptors contribute to fast synaptic transmission in neurons in all layers of the IC. In the presence of the GABA(A) receptor antagonist picrotoxin, the NMDA receptor antagonist AP-5, and the selective AMPA receptor antagonist GYKI 53655, KA receptor-mediated excitatory postsynaptic currents (KA EPSCs) were revealed. We found that KA EPSCs are ~5-10% of AMPA/KA EPSCs in all layers of the adult mouse IC. Similar results were found in adult rat IC. KA EPSCs had a significantly slower rise time course and decay time constant compared with AMPA receptor-mediated EPSCs. High-frequency repetitive stimulations at 200 Hz significantly facilitated the summation of KA EPSCs. In addition, genetic deletion of GluK1 or GluK2 subunit partially reduced postsynaptic KA EPSCs, and exposure of GluK2 knockout mice to the selective GluK1 antagonist UBP 302 could significantly reduce the KA EPSCs. These data suggest that both GluK1 and GluK2 play functional roles in the IC. Our study may provide the synaptic basis for the physiology and pathology of KA receptors in the IC-related functions.

The role of central 5-HT(2C) and NMDA receptors on LPS-induced feeding behavior in chickens

In mammals, LPS regulate feeding primarily through the 5-HT(1A) and 5-HT(2c) receptors within the brain. However, the central effect of 5-HT(1A) and 5-HT(2c) on LPS-induced feeding behavior has not been studied in non-mammalian species. Also, the role of glutamatergic system in LPS-induced anorexia has never been examined in either mammalian or non-mammalian species. Therefore, in this study, we examined the role of serotonergic and glutamatergic systems on LPS-induced anorexia in chickens. Food intake was measured in chickens after centrally administered lipopolysaccharide (LPS) (20 ng) (0 h), followed by intracerebroventricular (ICV) injection of the 5-HT(1A) autoreceptor agonist (8-OH-DPAT, 61 nmol), 5-HT(2c) receptor antagonist (SB 242084, 30 nm), and NMDA receptor antagonist (DL-AP5, 5 nm) at the onset of anorexia (4 h). In the following experiments, we used DL-AP5 before 5-HT (10 μg) and SB242084 before glutamate (300 nm) for evaluation of the interaction between 5-HTergic and glutamatergic systems on food intake. The results of this study showed that SB 242084 and DL-AP5 significantly attenuated food intake reduction caused by LPS (P < 0.05) but 8-OH-DPAT had no effect. In addition, 5-HT-induced anorexia was significantly attenuated by DL-AP5 pretreatment (P < 0.05), while SB 242084 had no effect on glutamate-induced hypophagia. These results indicated that 5-HT and glutamate (via 5-HT(2c) and NMDA receptor, respectively) dependently regulate LPS-induced hypophagia in chickens.

Salsolinol facilitates glutamatergic transmission to dopamine neurons in the posterior ventral tegmental area of rats

Although in vivo evidence indicates that salsolinol, the condensation product of acetaldehyde and dopamine, has properties that may contribute to alcohol abuse, the underlying mechanisms have not been fully elucidated. We have reported previously that salsolinol stimulates dopamine neurons in the posterior ventral tegmental area (p-VTA) partly by reducing inhibitory GABAergic transmission, and that ethanol increases glutamatergic transmission to VTA-dopamine neurons via the activation of dopamine D(1) receptors (D(1)Rs). In this study, we tested the hypothesis that salsolinol stimulates dopamine neurons involving activation of D(1)Rs. By using whole-cell recordings on p-VTA-dopamine neurons in acute brain slices of rats, we found that salsolinol-induced increase in spike frequency of dopamine neurons was substantially attenuated by DL-2-amino-5-phosphono-valeric acid and 6, 7-dinitroquinoxaline-2, 3-dione, the antagonists of glutamatergic N-Methyl-D-aspartic acid and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors. Moreover, salsolinol increased the amplitude of evoked excitatory postsynaptic currents (EPSCs) and the frequency but not the amplitude of spontaneous EPSCs. Additionally, SKF83566, a D(1)R antagonist attenuated the salsolinol-induced facilitation of EPSCs and of spontaneous firing of dopamine neurons. Our data reveal that salsolinol enhances glutamatergic transmission onto dopamine neurons via activation of D(1)Rs at the glutamatergic afferents in dopamine neurons, which contributes to salsolinol's stimulating effect on p-VTA dopamine neurons. This appears to be a novel mechanism which contributes toward rewarding properties of salsolinol.

Contractile properties and movement behaviour in neonatal rats with axotomy, treated with the NMDA antagonist DAP5

BACKGROUND

It is well known that axotomy in the neonatal period causes massive loss of motoneurons, which is reflected in the reduction of the number of motor units and the alteration in muscle properties. This type of neuronal death is attributed to the excessive activation of the ionotropic glutamate receptors (glutamate excitotoxicity). In the present study we investigated the effect of the NMDA antagonist DAP5 [D-2-amino-5-phosphonopentanoic acid] in systemic administration, on muscle properties and on behavioural aspects following peripheral nerve injury.

METHODS

Wistar rats were subjected to sciatic nerve crush on the second postnatal day. Four experimental groups were included in this study: a) controls (injection of 0.9% NaCl solution) b) crush c) DAP5 treated and d) crush and DAP5 treated. Animals were examined with isometric tension recordings of the fast extensor digitorum longus and the slow soleus muscles, as well as with locomotor tests at four time points, at P14, P21, P28 and adulthood (2 months).

RESULTS

1. Administration of DAP5 alone provoked no apparent adverse effects. 2. In all age groups, animals with crush developed significantly less tension than the controls in both muscles and had a worse performance in locomotor tests (p < 0.01). Crush animals injected with DAP5 were definitely improved as their tension recordings and their locomotor behaviour were significantly improved compared to axotomized ones (p < 0.01). 3. The time course of soleus contraction was not altered by axotomy and the muscle remained slow-contracting in all developmental stages in all experimental groups. EDL, on the other hand, became slower after the crush (p < 0.05). DAP5 administration restored the contraction velocity, even up to the level of control animals 4. Following crush, EDL becomes fatigue resistant after P21 (p < 0.01). Soleus, on the other hand, becomes less fatigue resistant. DAP5 restored the profile in both muscles.

CONCLUSIONS

Our results confirm that contractile properties and locomotor behaviour of animals are severely affected by axotomy, with a differential impact on fast contracting muscles. Administration of DAP5 reverses these devastating effects, without any observable side-effects. This agent could possibly show a therapeutic potential in other models of excitotoxic injury as well.

Microinjection of NMDA-type glutamate receptor agonist NMDA and antagonist D-AP-5 into the central nucleus of the amygdale alters water intake rather than food intake

OBJECTIVE

To investigate the role of N-Methyl-D-aspartic acid (NMDA)-type glutamate receptors in the central nucleus of the amygdale (CeA) in food and water intake.

METHODS

Male Sprague-Dawley rats with stainless steel cannulae implanted unilaterally into the CeA were used. The prototypic NMDA receptor agonist NMDA, or the selective NMDA receptor antagonist D(-)-2-amino-5-phosphonopentanoic acid (D-AP-5) was microinjected into the CeA of satiated and euhydrated rats.

RESULTS

Intra-CeA injection of 8.50, 17.00, or 34.00 nmol NMDA did not alter food intake but significantly increased water intake 0-1 h after the injection (F(3,32)=3.191, P=0.037) independent of food intake. Without affecting the food intake, injection of 6.34, 12.70, or 25.40 nmol D-AP-5 into the CeA significantly decreased water intake 0-1 h after the injection (F(3,28)=3.118, P=0.042) independent of food intake.

CONCLUSION

NMDA receptors in the CeA may participate in the control of water intake rather than food intake.

Repeated stress causes cognitive impairment by suppressing glutamate receptor expression and function in prefrontal cortex

Chronic stress could trigger maladaptive changes associated with stress-related mental disorders; however, the underlying mechanisms remain elusive. In this study, we found that exposing juvenile male rats to repeated stress significantly impaired the temporal order recognition memory, a cognitive process controlled by the prefrontal cortex (PFC). Concomitantly, significantly reduced AMPAR- and NMDAR-mediated synaptic transmission and glutamate receptor expression were found in PFC pyramidal neurons from repeatedly stressed animals. All these effects relied on activation of glucocorticoid receptors and the subsequent enhancement of ubiquitin/proteasome-mediated degradation of GluR1 and NR1 subunits, which was controlled by the E3 ubiquitin ligase Nedd4-1 and Fbx2, respectively. Inhibition of proteasomes or knockdown of Nedd4-1 and Fbx2 in PFC prevented the loss of glutamatergic responses and recognition memory in stressed animals. Our results suggest that repeated stress dampens PFC glutamatergic transmission by facilitating glutamate receptor turnover, which causes the detrimental effect on PFC-dependent cognitive processes.

Median preoptic nucleus and subfornical organ drive renal sympathetic nerve activity via a glutamatergic mechanism within the paraventricular nucleus

The paraventricular nucleus (PVN) of the hypothalamus is involved in the neural control of sympathetic drive, but the precise mechanism(s) that influences the PVN is not known. The activation of the PVN may be influenced by input from higher forebrain areas, such as the median preoptic nucleus (MnPO) and the subfornical organ (SFO). We hypothesized that activation of the MnPO or SFO would drive the PVN through a glutamatergic pathway. Neuroanatomical connections were confirmed by the recovery of a retrograde tracer in the MnPO and SFO that was injected bilaterally into the PVN in rats. Microinjection of 200 pmol of N-methyl-d-aspartate (NMDA) or bicuculline-induced activation of the MnPO and increased renal sympathetic activity (RSNA), mean arterial pressure, and heart rate in anesthetized rats. These responses were attenuated by prior microinjection of a glutamate receptor blocker AP5 (4 nmol) into the PVN (NMDA - ΔRSNA 72 ± 8% vs. 5 ± 1%; P < 0.05). Using single-unit extracellular recording, we examined the effect of NMDA microinjection (200 pmol) into the MnPO on the firing activity of PVN neurons. Of the 11 active neurons in the PVN, 6 neurons were excited by 95 ± 17% (P < 0.05), 1 was inhibited by 57%, and 4 did not respond. The increased RSNA after activation of the SFO by ANG II (1 nmol) or bicuculline (200 pmol) was also reduced by AP5 in the PVN (for ANG II - ΔRSNA 46 ± 7% vs. 17 ± 4%; P < 0.05). Prior microinjection of ANG II type 1 receptor blocker losartan (4 nmol) into the PVN did not change the response to ANG II or bicuculline microinjection into the SFO. The results from this study demonstrate that the sympathoexcitation mediated by a glutamatergic mechanism in the PVN is partially driven by the activation of the MnPO or SFO.

The afterhyperpolarizing potential following a train of action potentials is suppressed in an acute epilepsy model in the rat Cornu Ammonis 1 area

In hippocampal Cornu Ammonis 1 (CA1) neurons, a prolonged depolarization evokes a train of action potentials followed by a prominent afterhyperpolarizing potential (AHP), which critically dampens neuronal excitability. Because it is not known whether epileptiform activity alters the AHP and whether any alteration of the AHP is independent of inhibition, we acutely induced epileptiform activity by bath application of the GABA(A) receptor blocker gabazine (5 μM) in the rat hippocampal slice preparation and studied its impact on the AHP using intracellular recordings. Following 10 min of gabazine wash-in, slices started to develop spontaneous epileptiform discharges. This disinhibition was accompanied by a significant shift of the resting membrane potential of CA1 neurons to more depolarized values. Prolonged depolarizations (600 ms) elicited a train of action potentials, the number of which was not different between baseline and gabazine treatment. However, the AHP following the train of action potentials was significantly reduced after 20 min of gabazine treatment. When the induction of epileptiform activity was prevented by co-application of 6-cyano-7-nitroquinoxaline-2,3-dione disodium (CNQX, 10 μM) and D-(-)-2-amino-5-phosphonopentanoic acid (D-AP5, 50 μM) to block α-amino-3-hydroxy-5-methylisoxazolepropionate (AMPA) and N-methyl-d-aspartate (NMDA) receptors, respectively, the AHP was preserved despite of GABA(A) receptor inhibition suggesting that the epileptiform activity was required to suppress the AHP. Moreover, the AHP was also preserved when the slices were treated with the protein kinase blockers H-9 (100 μM) and H-89 (1 μM). These results demonstrate that the AHP following a train of action potentials is rapidly suppressed by acutely induced epileptiform activity due to a phosphorylation process-presumably involving protein kinase A.

Involvement of glutamatergic receptors in the nucleus cuneiformis in modulating morphine-induced antinociception in rats

The nucleus cuneiformis (CnF), located just ventrolateral to the periaqueductal gray, is part of the descending pain modulatory system. Neurons in the CnF project to medullary nucleus raphe magnus (NRM), which plays an important role on pain modulation. In this study, we investigated the effect of microinjection of the non-competitive NMDA receptor antagonist MK-801, the competitive NMDA receptor antagonist AP-7, and the kainate/AMPA receptor antagonist DNQX, alone or in combination with morphine into the nucleus cuneiformis on morphine-induced analgesia to understand the role of glutamatergic receptors in the modulating activity of morphine. Antinociception was assessed with the tail-flick test. Morphine (10, 20, 40 microg in 0.5 microl saline) had an antinociceptive effect, increasing tail-flick latency in a dose-dependent manner. Microinjection of MK-801 (10 microg/0.5 microl saline) and AP7 (3 microg/0.5 microl saline) prior to morphine microinjection (10 microg/0.5 microl saline) attenuated the antinociceptive effects of morphine, whereas DNQX (0.5 microg/0.5 microl saline) showed a partial antinociceptive effect and potentiated the analgesic effect of morphine. These results indicated that the NMDA receptor partially potentiates the antinociceptive effect of morphine. Our results suggest that NMDA but not non-NMDA receptors are involved in the antinociception produced by morphine in the CnF. The non-NMDA receptors in this area may have a facilitatory effect on nociceptive transmission. The fact that morphine's effect was potentiated by NMDA receptor suggests that projection neurons within the CnF are under tonic, glutamatergic input and when the influence of this input is blocked, the descending inhibitory system is inactivated.

Positive shifts of the GABAA receptor reversal potential due to altered chloride homeostasis is widespread after status epilepticus

PURPOSE

γ-Aminobutyric acid (GABA)ergic transmission plays an important role in the initiation of epileptic activity and the generation of ictal discharges. The functional alterations in the epileptiform hippocampus critically depend on GABAergic mechanisms and cation-chloride cotransporters.

METHODS

To understand the cellular basis of specific functional alterations in the epileptic hippocampus, we studied physiologic characteristics and pharmacologically isolated evoked GABA(A) receptor-mediated inhibitory postsynaptic currents (IPSCs) recorded from principal neurons in hippocampal slices from status epilepticus (SE) and control rats using whole-cell and gramicidin perforated patch-clamp recordings.

KEY FINDINGS

Whereas the resting membrane potential and input resistance were not significantly different between control and epileptic tissue, the reversal potential (E(GABA) ) of IPSCs was significantly shifted to more positive values in SE rats with regard to the resting membrane potential. Pharmacologic experiments and quantitative reverse transcriptase polymerase chain reaction (RT-PCR) showed that the observed changes in the epileptic tissue were due to a decreased ratio of the main Cl(-) extrusion transporter (K(+) -Cl(-) cotransporter, KCC2) to the main Cl(-) uptake transporter (Na(+) -K(+) -2Cl(-) cotransporter, NKCC1).

SIGNIFICANCE

Our results suggest that alterations of cation-chloride cotransporter functions, comprising a higher NKCC1 action, contribute to hyperexcitability within the hippocampus following SE.

Identification of transmitter systems and learning tag molecules involved in behavioral tagging during memory formation

Long-term memory (LTM) consolidation requires the synthesis of plasticity-related proteins (PRPs). In addition, we have shown recently that LTM formation also requires the setting of a "learning tag" able to capture those PRPs. Weak training, which results only in short-term memory, can set a tag to use PRPs derived from a temporal-spatial closely related event to promote LTM formation. Here, we studied the involvement of glutamatergic, dopaminergic, and noradrenergic inputs on the setting of an inhibitory avoidance (IA) learning tag and the synthesis of PRPs. Rats explored an open field (PRP donor) followed by weak (tag inducer) or strong (tag inducer plus PRP donor) IA training. Throughout pharmacological interventions around open-field and/or IA sessions, we found that hippocampal dopamine D1/D5- and β-adrenergic receptors are specifically required to induce PRP synthesis. Moreover, activation of the glutamatergic NMDA receptors is required for setting the learning tags, and this machinery further required α-Ca(2+)/calmodulin-dependent protein kinase II and PKA but not ERK1/2 activity. Together, the present findings emphasize an essential role of the induction of PRPs and learning tags for LTM formation. The existence of only the PRP or the tag was insufficient for stabilization of the mnemonic trace.

Allopregnanolone induces LHRH and glutamate release through NMDA receptor modulation

LHRH release from hypothalamus is influenced by the neurotransmitter glutamate that acts, among others, on NMDA receptors present in LHRH neurons. On the other hand, the neurosteroid allopregnanolone can modulate the activity of specific neurotransmitter receptors and affect neurotransmitter release. We examined the role of allopregnanolone on in vitro LHRH and glutamate release from mediobasal hypothalamus and anterior preoptic area of ovariectomized rats with estrogen and progesterone replacement. Moreover, we evaluated whether the neurosteroid might act through modulation of NMDA receptors. Allopregnanolone induced an increase in LHRH release. This effect was reversed when the NMDA receptors were blocked by the NMDA antagonist 2-amino-7-phosphonoheptanoic acid (AP-7) indicating that this neurosteroid would interact with NMDA receptors. Moreover allopregnanolone induced an augment in K(+) evoked [(3)H]-glutamate release from mediobasal hypothalamus-anterior preoptic area explants and this effect was also reversed when NMDA receptors were blocked with AP-7. These results suggest an important physiologic function of allopregnanolone on the regulation of neuroendocrine function in female adult rats. Not only appears to be involved in enhancing LHRH release through modulation of NMDA receptors but also in the release of glutamate which is critical in the control of LHRH release.

Glutamatergic neurons in rodent models respond to nanoscale particulate urban air pollutants in vivo and in vitro

BACKGROUND

Inhalation of airborne particulate matter (PM) derived from urban traffic is associated with pathology in the arteries, heart, and lung; effects on brain are also indicated but are less documented.

OBJECTIVE

We evaluated rodent brain responses to urban nanoscale (< 200 nm) PM (nPM).

METHODS

Ambient nPM collected near an urban freeway was transferred to aqueous suspension and reaerosolized for 10-week inhalation exposure of mice or directly applied to rat brain cell cultures.

RESULTS

Free radicals were detected by electron paramagnetic resonance in the nPM 30 days after initial collection. Chronic inhalation of reaerosolized nPM altered selected neuronal and glial activities in mice. The neuronal glutamate receptor subunit (GluA1) was decreased in hippocampus, whereas glia were activated and inflammatory cytokines were induced [interleukin-1α (IL-1α), tumor necrosis factor-α (TNFα)] in cerebral cortex. Two in vitro models showed effects of nPM suspensions within 24-48 hr of exposure that involved glutamatergic functions. In hippocampal slice cultures, nPM increased the neurotoxicity of NMDA (N-methyl-d-aspartic acid), a glutamatergic agonist, which was in turn blocked by the NMDA antagonist AP5 [(2R)-amino-5-phosphonopentanoate]. In embryonic neuron cultures, nPM impaired neurite outgrowth, also blocked by AP5. Induction of IL-1α and TNFα in mixed glia cultures required higher nPM concentrations than did neuronal effects. Because conditioned media from nPM-exposed glia also impaired outgrowth of embryonic neurites, nPM can act indirectly, as well as directly, on neurons in vitro.

CONCLUSIONS

nPM can affect embryonic and adult neurons through glutamatergic mechanisms. The interactions of nPM with glutamatergic neuronal functions suggest that cerebral ischemia, which involves glutamatergic excitotoxicity, could be exacerbated by nPM.

Glutamatergic regulation of ghrelin-induced activation of the mesolimbic dopamine system

Recently, we demonstrated that the central ghrelin signalling system, involving the ghrelin receptor (GHS-R1A), is important for alcohol reinforcement. Ghrelin targets a key mesolimbic circuit involved in natural as well as drug-induced reinforcement, that includes a dopamine projection from the ventral tegmental area (VTA) to the nucleus accumbens. The aim of the present study was to determine whether it is possible to suppress ghrelin's effects on this mesolimbic dopaminergic pathway can be suppressed, by interrupting afferent inputs to the VTA dopaminergic cells, as shown previously for cholinergic afferents. Thus, the effects of pharmacological suppression of glutamatergic, orexin A and opioid neurotransmitter systems on ghrelin-induced activation of the mesolimbic dopamine system were investigated. We found in the present study that ghrelin-induced locomotor stimulation was attenuated by VTA administration of the N-methyl-D-aspartic acid receptor antagonist (AP5) but not by VTA administration of an orexin A receptor antagonist (SB334867) or by peripheral administration of an opioid receptor antagonist (naltrexone). Intra-VTA administration of AP5 also suppressed the ghrelin-induced dopamine release in the nucleus accumbens. Finally the effects of peripheral ghrelin on locomotor stimulation and accumbal dopamine release were blocked by intra-VTA administration of a GHS-R1A antagonist (BIM28163), indicating that GHS-R1A signalling within the VTA is required for the ghrelin-induced activation of the mesolimbic dopamine system. Given the clinical knowledge that hyperghrelinemia is associated with addictive behaviours (such as compulsive overeating and alcohol use disorder) our finding highlights a potential therapeutic strategy involving glutamatergic control of ghrelin action at the level of the mesolimbic dopamine system.

[Edema in nervous tissue studied on model of hemorrhagic stroke in vitro]

The development of edema in the survival olfactory cortex slices under the long-term action of autoblood has been studied by monitoring the bioelectric activity of nervous cells. The level of disorder in electrogenesis of cells was revealed by comparing the focal potentials with their control values; the degree of the nervous tissue swelling in various periods of autoblood action was determined by weighing. In the model of hemorrhagic stroke, the dependence of edema growth on the level of activity of ionotropic glutamate receptors has been determined using the pharmacological blockade technique.

An examination of NMDA receptor contribution to conditioned responding evoked by the conditional stimulus effects of nicotine

RATIONALE

Research using a drug discriminated goal-tracking (DGT) task showed that the N-methyl-D: -aspartate (NMDA) channel blocker MK-801 (dizocilpine) reduced the nicotine-evoked conditioned response (CR).

OBJECTIVES

Given the unknown mechanism of the effect, Experiment 1 replicated the MK-801 results and included tests with NMDA receptor ligands. Experiments 2a and 2b tested whether MK-801 pretreatment blocked DGT via a state-dependency effect.

METHODS

In Experiment 1, adult male Sprague-Dawley rats received intermittent access to liquid sucrose following nicotine (0.4 mg base/kg); no sucrose was delivered on intermixed saline sessions. Conditioning was indicated by increased anticipatory dipper entries (goal-tracking) on nicotine compared to saline sessions. Antagonism and/or substitution tests were conducted with MK-801, phencyclidine, CGP 39551, d-CPPene (SDZ EAA 494), Ro 25,6981, L-701,324, ACPC, and NMDA. In Experiment 2a, rats received nicotine and sucrose on every session-no intermixed saline sessions without sucrose. Tests combined MK-801 or the non-competitive nicotinic acetylcholine receptor antagonist, mecamylamine with either nicotine or saline. Experiment 2b had sucrose delivered on saline sessions and no sucrose on intermixed nicotine sessions followed by MK-801 antagonism tests of the saline CS.

RESULTS

MK-801 and phencyclidine dose-dependently attenuated the CR in Experiment 1. Ro-25,6981 enhanced the CR, but did not substitute for nicotine. Other ligands showed inconsistent effects. In Experiment 2a, MK-801 pretreatment reduced goal-tracking when given before nicotine and saline test sessions; mecamylamine pretreatment had no effect. In Experiment 2b, MK-801 dose-dependently attenuated the saline-evoked CR.

CONCLUSIONS

Combined, the results suggest that MK-801 blocks discriminated goal-tracking by virtue of state-changing properties.

Role of the spinal cord NO/cGMP pathway in the control of arterial pressure and heart rate

The modulatory effect of nitric oxide/cyclic guanosine monophosphate (NO/cGMP) pathway on sympathetic preganglionic neurons still deserves further investigation. The present study was designed to examine the role of the spinal cord NO/cGMP pathway in controlling mean arterial pressure and heart rate. We observed that intrathecal administration of the NO synthase inhibitor Nω-Nitro-L-arginine methyl ester hydrochloride (L-NAME) causes an increase in mean arterial pressure but does not affect heart rate. Intrathecal administration of the soluble guanylyl cyclase inhibitor 1H-[1,2,4]Oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) does not change mean arterial pressure and heart rate. The precursor for NO synthesis, L-arginine, reduces both mean arterial pressure and heart rate while administration of ODQ before L-arginine impaired decreases in mean arterial pressure and heart rate. Administration of the N-methyl-D-aspartate (NMDA) receptor antagonist DL-2-amino-5-phosphonopentanoic acid (AP5) after L-NAME does not affect increases in mean arterial pressure promoted by NO synthase inhibition. Although the hypotensive and bradycardic responses induced by intrathecal administration of L-arginine depend on cGMP, our results indicate that NO acts to tonically inhibit SPNs, independent of either cGMP or NMDA receptors.

Intralaminar and interlaminar activity within the rodent superior colliculus visualized with voltage imaging

The superior colliculus (SC) is a midbrain structure that plays a role in converting sensation into action. Most SC research focuses on either in vivo extracellular recordings from behaving monkeys or patch-clamp recordings from smaller mammals in vitro. However, the activity of neuronal circuits is necessary to generate behavior, and neither of these approaches measures the simultaneous activity of large populations of neurons that make up circuits. Here, we describe experiments in which we measured changes in membrane potential across the SC map using voltage imaging of the rat SC in vitro. Our results provide the first high temporal and spatial resolution images of activity within the SC. Electrical stimulation of the SC evoked a characteristic two-component optical response containing a short latency initial-spike and a longer latency after-depolarization. Single-pulse stimulation in the superficial SC evoked a pattern of intralaminar and interlaminar spread that was distinct from the spread evoked by the same stimulus applied to the intermediate SC. Intermediate layer stimulation produced a more extensive and more ventrally located activation of the superficial layers than did stimulation in the superficial SC. Together, these results indicate the recruitment of dissimilar subpopulations of circuitry depending on the layer stimulated. Field potential recordings, pharmacological manipulations, and timing analyses indicate that the patterns of activity were physiologically relevant and largely synaptically driven. Therefore, voltage imaging is a powerful technique for the study of spatiotemporal dynamics of electrical signaling across neuronal populations, providing insight into neural circuits that underlie behavior.

In the adult hippocampus, chronic nerve growth factor deprivation shifts GABAergic signaling from the hyperpolarizing to the depolarizing direction

GABA, the main inhibitory transmitter in adulthood, early in postnatal development exerts a depolarizing and excitatory action. This effect, which results from a high intracellular chloride concentration ([Cl(-)](i)), promotes neuronal growth and synaptogenesis. During the second postnatal week, the developmental regulated expression of the cation-chloride cotransporter KCC2 accounts for the shift of GABA from the depolarizing to the hyperpolarizing direction. Changes in chloride homeostasis associated with high [Cl(-)](i) have been found in several neurological disorders, including temporal lobe epilepsy. Here, we report that, in adult transgenic mice engineered to express recombinant neutralizing anti-nerve growth factor antibodies (AD11 mice), GABA became depolarizing and excitatory. AD11 mice exhibit a severe deficit of the cholinergic function associated with an age-dependent progressive neurodegenerative pathology resembling that observed in Alzheimer patients. Thus, in hippocampal slices obtained from 6-month-old AD11 (but not wild-type) mice, the GABA(A) agonist isoguvacine significantly increased the firing of CA1 principal cells and, at the network level, the frequency of multiunit activity recorded with extracellular electrodes. In addition, in AD11 mice, the reversal of GABA(A)-mediated postsynaptic currents and of GABA-evoked single-channel currents were positive with respect to the resting membrane potential as estimated in perforated patch and cell attached recordings, respectively. Real-time quantitative reverse transcription-PCR and immunocytochemical experiments revealed a reduced expression of mRNA encoding for Kcc2 and of the respective protein. This novel mechanism may represent a homeostatic response that counterbalances within the hippocampal network the Alzheimer-like neurodegenerative pathology found in AD11 mice.