Roles of hippocampal N-methyl-D-aspartate receptors and calcium/calmodulin-dependent protein kinase II in amphetamine-produced conditioned place preference in rats

This study investigates the roles of hippocampal N-methyl-D-aspartate (NMDA) glutamate receptors and CaMKII (calcium/calmodulin-dependent protein kinase II) in amphetamine-produced conditioned place preference (AMPH-CPP) in rats. An earlier report showed that AMPH-CPP resulted in the enhancement of hippocampal CaMKII activity. In this study, AMPH-CPP significantly increased hippocampal GluR1 receptors, though AMPH-CPP was impaired by either blockade of NMDA receptors (AP5) or inhibition of CaMKII (KN-93) during conditioning. These treatments also impaired CPP if administered before testing, but CPP recovered during the next testing session. Therefore, these treatments had no effect on the extinction of CPP. If the conditioned rats were, however, reexposed to AMPH-CPP after a hippocampal-infusion of AP5 or KN-93, the extinction of the original CPP was greater than that seen in the controls. The hippocampal-infusion of D-cycloserine before CPP testing enhanced the extinction of CPP. These results, taken together, indicate that NMDA receptor activation and CaMKII activity are essential for the AMPH-CPP. AMPH-CPP reexposure is similar to the memory reconsolidation process, being disrupted by either a blockade of the NMDA receptor or an inhibition of CaMKII. Furthermore, the extinction of CPP resembles new learning, which is an active process and is facilitated by a partial NMDA agonist.

Medial prefrontal cortex infusions of bupivacaine or AP-5 block extinction of amphetamine conditioned place preference

The present experiments used reversible lesion techniques and intra-mPFC infusions of the n-methyl D-aspartate (NMDA) receptor antagonist D,L-2-amino-5-phosphonovaleric acid (AP-5) to examine the role of the mPFC in extinction of an amphetamine conditioned place preference (CPP). Following initial training and testing for an amphetamine (2 mg/kg) CPP, adult male Long-Evans rats were given extinction trials that were identical to training, except in the absence of peripheral amphetamine injections. Immediately prior to each extinction trial, rats received intra-mPFC infusions of the anesthetic drug bupivacaine (0.75% solution/0.5 microl), AP-5 (1.25, 2.5, 5.0 microg/0.5 microl), or saline. Following extinction training, rats were given a second CPP test session. Rats receiving intra-mPFC infusions of saline displayed extinction of CPP behavior. In contrast, intra-mPFC infusions of bupivacaine or AP-5 (2.5, 5.0 microg) blocked CPP extinction. The findings indicate (1) the mPFC mediates extinction of approach behavior to drug-associated environmental contexts, and (2) NMDA receptor blockade within the mPFC is sufficient to block extinction of amphetamine CPP behavior.

GABAergic neurosteroids mediate the effects of ethanol on long-term potentiation in rat hippocampal slices

We previously found that ethanol has complex effects on hippocampal synaptic plasticity, inhibiting long-term potentiation (LTP) and long-term depression by different mechanisms. The block of long-term depression appears to be mediated by effects on N-methyl-d-aspartate receptors, whereas the block of LTP involves augmented inhibition via gamma-aminobutyric acid-A receptors (GABA(A)Rs). To pursue factors contributing to effects on LTP, we examined the ability of various concentrations of ethanol to block LTP in the CA1 region of rat hippocampal slices. Complete LTP block required 60 mm ethanol. LTP block was enhanced at lower ethanol concentrations in the presence of (3alpha5alpha)-3-hydroxypregnan-20-one, a GABA(A)R-potentiating neurosteroid, suggesting that neurosteroids may be important contributors to the effects of ethanol on LTP. Consistent with this, we found that block of LTP by 60 mm ethanol was overcome by coadministration of a cyclodextrin that binds and removes lipophilic neurosteroids. More specifically, treatment of slices with finasteride, an agent that inhibits the synthesis of 5alpha-reduced neurosteroids, or with an agent that inhibits the effects of 5alpha-reduced neurosteroids on GABA(A)Rs overcame the effects of 60 mm ethanol on LTP. Taken together, these results indicate that acute production of GABA(A)R-enhancing neurosteroids plays a key role in mediating the effects of ethanol on LTP.

Protection from inorganic mercury effects on the in vivo dopamine release by ionotropic glutamate receptor antagonists and nitric oxide synthase inhibitors

The possible role of ionotropics glutamate receptors on the HgCl(2)-induced dopamine (DA) release from rat striatum was investigated by using in vivo brain microdialysis technique after administration of selective NMDA and AMPA/Kainate receptors antagonists dizocilpine (MK-801), D (-)-2-amino-5-phoshonopentanoic acid (AP5), and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). Moreover, we have also studied the effects of nitric oxide synthase (NOS) inhibitors L-nitro-arginine methyl ester (L-NAME) and 7-nitro-indazol (7-NI) on HgCl(2)-induced DA release. Intraestriatal infusion of 1mM HgCl(2) increased striatal DA to 1717.2+/-375.4% respect to basal levels. Infusion of 1mM HgCl(2) in 400 microM MK-801 pre-treated animals produced an increase on striatal DA levels 61% smaller than that induced in non-pre-treated animals. In the case of AP5, this treatment reduced 92% the increase produced by HgCl(2) as compared to non-pre-treated rats. Nevertheless, the administration of CNQX did not produce any effect on HgCl(2)-induced dopamine release. Intrastriatal infusion of 1mM HgCl(2) in 100 microM L-NAME pre-treated animals produced an increase on extracellular DA levels 82% smaller than produced by HgCl(2) alone. In addition, the pre-treatment with 7-NI reduced 90% the increase produced by infusion of HgCl(2) alone in rats. Thus, HgCl(2)-induced DA release could be produced at last in part, by overstimulation of NMDA receptors with NO production, since administration of NMDA receptor antagonists and NOS inhibitors protected against HgCl(2) effects on DA release.

Orexin-induced feeding requires NMDA receptor activation in the perifornical region of the lateral hypothalamus

Food intake is stimulated following administration of orexin-A into the perifornical region of the lateral hypothalamus (LH/PFA). Orexin neurons originating in the LH/PFA interact with a number of hypothalamic systems known to influence food intake, including glutamatergic neurons. Glutamatergic systems in the LH/PFA were demonstrated to initiate feeding through N-methyl-d-aspartic acid (NMDA) receptors. Male Sprague-Dawley rats fitted with brain guide cannulas to the LH/PFA were used in two experiments. In the first experiment, a combination microdialysis/microinjection probe was used to deliver artificial cerebrospinal fluid (aCSF) or 500 pmol of orexin-A into the LH/PFA. Orexin-A increased interstitial glutamate to 143 ± 12% of baseline ( P < 0.05), which remained elevated over the 120-min collection period. In the second experiment, the NMDA receptor antagonist d-2-amino-5-phosphonopentanoic acid (d-AP5; 10 nmol) was administered before orexin-A. The orexin-induced increase in food intake (from 1.1 ± 0.4 to 3.2 ± 0.5 g, P < 0.05) during the first hour was absent in rats receiving d-AP5 + orexin-A (1.2 ± 0.5 g). There was no effect of d-AP5 alone on food intake. These data support glutamatergic systems in the LH/PFA mediating the feeding response to orexin-A through NMDA receptors.

Activation of the NMDA/glutamate receptor complex antagonizes the NMDA antagonist-induced antidepressant-like effects in the forced swim test

The antidepressant activity of NMDA receptor antagonists has been demonstrated, and their mechanism of action was based on the assumption of their selectivity for the NMDA receptor only. However, no direct evidence for the NMDA receptor role in this activity was demonstrated. Now, in order to prove the NMDA pathway of antidepressant-like action of the NMDA antagonists in the mouse forced swim test (FST) we examined if antidepressant activity of NMDA receptor antagonists is mediated by NMDA receptors and whether the activation of different modulatory sites of the NMDA receptor complex influence the action of the antagonists of different sites of NMDA receptor. In our study, we used two NMDA ligands: competitive NMDA glutamate site antagonist CGP 37849, and glycineB antagonist L-701,324; both at doses found to be effective in the FST. The antidepressant-like activity of the compounds was abolished by the N-methyl-D-aspartic acid (NMDA) or by D-serine co-treatment. Ligands at the doses active in the FST did not alter locomotor activity. The present study indicates the major role of the NMDA/glutamate pathway in the antidepressant-like activity of NMDA antagonists in the mouse FST.

NMDA receptor trafficking at recurrent synapses stabilizes the state of the CA3 network

Metaplasticity describes the stabilization of synaptic strength such that strong synapses are likely to remain strong while weak synapses are likely to remain weak. A potential mechanism for metaplasticity is a correlated change in both N-methyl-D-aspartate (NMDA) receptor-mediated postsynaptic conductance and synaptic strength. Synchronous activation of CA3-CA3 synapses during spontaneous bursts of population activity caused long-term potentiation (LTP) of recurrent CA3-CA3 glutamatergic synapses under control conditions and depotentiation when NMDA receptors were partially blocked by competitive antagonists. LTP was associated with a significant increase in membrane-bound NMDA receptors, whereas depotentiation was associated with a significant decrease in membrane-bound NMDA receptors. During burst activity, further depotentiation could be induced by sequential reductions in antagonist concentration, consistent with a depotentiation-associated reduction in membrane-bound NMDA receptors. The decrease in number of membrane-bound NMDA receptors associated with depotentiation reduced the probability of subsequent potentiation of weakened synapses in the face of ongoing synchronous network activity. This molecular mechanism stabilizes synaptic strength, which in turn stabilizes the state of the CA3 neuronal network, reflected in the frequency of spontaneous population bursts.

Offsetting of aberrations associated with seizure proneness in rat hippocampus area CA1 by theta pulse stimulation-induced activity pattern

Epileptiform activity induces long term aberrations in hippocampal network functions. This study was conducted in pentylenetetrazol (PTZ) -kindled rats to examine offsetting of aberrations associated with seizure proneness in hippocampus area CA1 by theta pulse stimulation (TPS: 5 Hz trains for 3 min) -induced activity pattern. In hippocampal slices from both control and kindled rats, the field excitatory postsynaptic potentials (fEPSP) and population spikes (PS) were simultaneously recorded through electrodes in the apical dendrites and stratum pyramidale, respectively. The following changes in kindled vs. control slices were observed. The fEPSP needed to be greater to produce the PS recorded in the cell body layer. The fEPSP was reduced by paired stimuli whereas the PS amplitude was increased. TPS selectively depressed the PS in a lasting fashion, and shifted the fEPSP slope and the PS amplitude relation toward what was observed in controls. Both the fEPSP and PS were increased by paired stimuli at 60 min after TPS application. The lasting depressive effect of TPS on the PS amplitude was converted into facilitation by adenosine A1 receptor antagonist 8-cyclopentyl-1, 3-dipropylxanthine (CPX). Potentiation of the PS amplitude by TPS in the presence of CPX was blocked by an N-methyl-d-aspartate receptor antagonist AP5. We hypothesize that the extracellular adenosine spillover, acting through adenosine A1 receptors, during TPS-induced activity pattern could trigger a homeostatic process for correcting network imbalances caused by epileptiform activity.

Distribution and electrophysiological effects of motilin in Purkinje cells

Evidence exists that motilin immunoreactivity is highly expressed in Purkinje cells. In this study, immunohistochemistry and whole-cell patch-clamp recording were performed to investigate the spatial distribution and electrophysiological effects of motilin receptors in the cerebellum. We show here that motilin receptors are strongly expressed in Purkinje cells of the human and rat cerebellum. Motilin at 10 nM depolarized Purkinje cells of the rat cerebellum, and this was mimicked by the motilin receptor agonist erythromycin. The depolarization evoked by motilin persisted in the presence of tetrodotoxin, glutamate and gamma-amino-n-butyric acid receptor antagonists, indicating that motilin excited the Purkinje cells by activating the receptor expressed on the neurons recorded. We suggest that motilin may serve specific neural functions in the cerebellum.

The impact of chronic network hyperexcitability on developing glutamatergic synapses

The effects recurring seizures have on the developing brain are an important area of debate because many forms of human epilepsy arise in early life when the central nervous system is undergoing dramatic developmental changes. To examine effects on glutamatergic synaptogenesis, epileptiform activity was induced by chronic treatment with GABAa receptor antagonists in slice cultures made from infant rat hippocampus. Experiments in control cultures showed that molecular markers for glutamatergic and GABAergic synapses recapitulated developmental milestones reported previously in vivo. Following a 1-week treatment with bicuculline, the intensity of epileptiform activity that could be induced in cultures was greatly diminished, suggesting induction of an adaptive response. In keeping with this notion, immunoblotting revealed the expression of NMDA and AMPA receptor subunits was dramatically reduced along with the scaffolding proteins, PSD95 and Homer. These effects could not be attributed to neuronal cell death, were reversible, and were not observed in slices taken from older animals. Co-treating slices with APV or TTX abolished the effects of bicuculline suggesting that effects were dependent on NMDA receptors and neuronal activity. Neurophysiological recordings supported the biochemical findings and demonstrated decreases in both the amplitude and frequency of NMDA and AMPA receptor-mediated miniature EPSCs (mEPSCs). Taken together these results suggest that neuronal network hyperexcitability interferes with the normal maturation of glutamatergic synapses, which could have implications for cognitive deficits commonly associated with the severe epilepsies of early childhood.

Inhibitory plasticity facilitates recovery of stimulus velocity tuning in the superior colliculus after chronic NMDA receptor blockade

The developing nervous system is shaped in important ways by spontaneous and stimulus-driven neural activity. Perturbation of normal activity patterns can profoundly affect the development of some neural response properties, whereas others are preserved through mechanisms that either compensate for or are unaffected by the perturbation. Most studies have examined the role of excitation in activity-dependent plasticity of response properties. Here, we examine the role of inhibition within the context of response selectivity for moving stimuli. The spatial extent of retinal input to the developing hamster superior colliculus (SC) can be experimentally increased by chronic NMDA receptor (NMDAR) blockade. Remarkably, stimulus velocity tuning is intact despite the increase in excitatory inputs. The goal of this study was to investigate whether plasticity in surround inhibition might provide the mechanism underlying this preservation of velocity tuning. Surround inhibition shapes velocity tuning in the majority of superficial layer SC neurons in normal hamsters. We show that despite the NMDAR blockade-induced increase in feedforward excitatory convergence from the retina, stimulus velocity tuning in the SC is maintained via compensatory plasticity in surround inhibition. The inhibitory surround increased in strength and spatial extent, and surround inhibition made a larger contribution to velocity tuning in the SC after chronic NMDAR blockade. These results show that inhibitory plasticity can preserve the balance between excitation and inhibition that is necessary to preserve response properties after developmental manipulations of neural activity. Understanding these compensatory mechanisms may permit their use to facilitate recovery from trauma or sensory deprivation.

The role of dorsal hippocampus and basolateral amygdala NMDA receptors in the acquisition and retrieval of context and contextual fear memories

The authors used 3-phase context preexposure facilitation methodology to study the contribution of N-methyl-D-aspartate (NMDA) receptors in dorsal hippocampus (DH) and the basal lateral region of the amygdala (BLA) to (a) acquisition of the context memory, (b) retrieval of the context memory, (c) acquisition of context-shock association, and (d) retrieval of the context-shock association. The NMDA receptor antagonist D-2-amino-5 phosphonopentanoic acid (D-AP5) was injected into either the DH or BLA prior to (a) the context preexposure phase, (b) the immediate shock phase, or (c) the test for contextual fear. Antagonizing NMDA receptors in the DH impaired the acquisition of the context memory but did not affect its retrieval or retrieval of the fear memory. Antagonizing NMDA receptors with D-AP5 in the BLA impaired acquisition of the context-shock association but had no effect on the expression of fear. However, both DL-AP5 and L-AP5 reduced the expression of fear when they were injected into the amygdala prior to testing for contextual fear.

Dorsal hippocampal N-methyl-D-aspartate receptors underlie spatial working memory performance during non-matching to place testing on the T-maze

Previous lesion studies have suggested a functional dissociation along the septotemporal axis of the hippocampus. Whereas the dorsal hippocampus has been implicated in spatial memory processes, the ventral hippocampus may play a role in anxiety. However, these lesion studies are potentially confounded by demyelination of fibres passing through the lesion site, and the possibility of secondary, downstream changes in associated brain structures as a consequence of their chronic denervation following the lesion. In the present study, we have used the microinfusion of muscimol to temporarily inactivate either the dorsal or ventral hippocampus in order to re-examine the contribution of the hippocampal sub-regions to spatial memory. Microinfusion studies spare fibres of passage and offer fewer opportunities for compensatory changes because the effects are transient and short-lasting. Rats were infused prior to spatial working memory testing on a non-matching to place T-maze alternation task. Spatial working memory was impaired by dorsal but not ventral hippocampal inactivation. In a second experiment, infusion of the NMDAR antagonist, D-AP5, into dorsal hippocampus also impaired spatial working memory performance, suggesting that NMDAR function within the dorsal hippocampus makes an essential contribution to this aspect of hippocampal information processing.

The relationship between blood flow and neuronal activity in the rodent olfactory bulb

In the brain, neuronal activation triggers an increase in cerebral blood flow (CBF). Here, we use two animal models and several techniques (two-photon imaging of CBF and neuronal calcium dynamics, intracellular and extracellular recordings, local pharmacology) to analyze the relationship between neuronal activity and local CBF during odor stimulation in the rodent olfactory bulb. Application of glutamate receptor antagonists or tetrodotoxin directly into single rat olfactory glomeruli blocked postsynaptic responses but did not affect the local odor-evoked CBF increases. This suggests that in our experimental conditions, odor always activates more than one glomerulus and that silencing one of a few clustered glomeruli does not affect the vascular response. To block synaptic transmission more widely, we then superfused glutamate antagonists over the surface of the olfactory bulb in transgenic G-CaMP2 mice. This was for two reasons: (1) mice have a thin olfactory nerve layer compared to rats and this will favor drug access to the glomerular layer, and (2) transgenic G-CaMP2 mice express the fluorescent calcium sensor protein G-CaMP2 in mitral cells. In G-CaMP2 mice, odor-evoked, odor-specific, and concentration-dependent calcium increases in glomeruli. Superfusion of glutamate receptor antagonists blocked odor-evoked postsynaptic calcium signals and CBF responses. We conclude that activation of postsynaptic glutamate receptors and rises in dendritic calcium are major steps for neurovascular coupling in olfactory bulb glomeruli.

A critical period for enhanced synaptic plasticity in newly generated neurons of the adult brain

Active adult neurogenesis occurs in discrete brain regions of all mammals and is widely regarded as a neuronal replacement mechanism. Whether adult-born neurons make unique contributions to brain functions is largely unknown. Here we systematically characterized synaptic plasticity of retrovirally labeled adult-born dentate granule cells at different stages during their neuronal maturation. We identified a critical period between 1 and 1.5 months of the cell age when adult-born neurons exhibit enhanced long-term potentiation with increased potentiation amplitude and decreased induction threshold. Furthermore, such enhanced plasticity in adult-born neurons depends on developmentally regulated synaptic expression of NR2B-containing NMDA receptors. Our study demonstrates that adult-born neurons exhibit the same classic critical period plasticity as neurons in the developing nervous system. The transient nature of such enhanced plasticity may provide a fundamental mechanism allowing adult-born neurons within the critical period to serve as major mediators of experience-induced plasticity while maintaining stability of the mature circuitry.

Two-pore-domain potassium channels contribute to neuronal potassium release and glial potassium buffering in the rat hippocampus

Two-pore-domain potassium (K2P) channels have been suggested to be involved in neuronal K+ release and glial K+ uptake. We studied effects of the K2P channel blockers quinine (200 or 500 microM), quinidine (500 microM), and bupivacaine (200 microM) on stimulus-induced and iontophoretically induced transient increases of the extracellular potassium concentration ([K+]o) in area CA1 of rat hippocampal slices, always in presence of AMPA/kainate and NMDA receptor antagonists. Increases in [K+]o evoked by repetitive alvear stimulation (20 Hz) were blocked by quinine and quinidine but amplitudes of population spikes were only modestly reduced. Bupivacaine suppressed both rises in [K+]o and population spikes. In contrast, iontophoretically induced rises in [K+]o were moderately augmented by quinine and quinidine while bupivacaine had no effect. Barium at concentrations of 2 mM which should block both potassium inward rectifier (Kir) and some K2P channels doubled iontophoretically induced rises in [K+]o also in presence of quinine, quinidine, and bupivacaine. The data suggest that quinine/quinidine-sensitive K2P channels mediate K+ release from neurons and possibly contribute to glial K+ buffering.

Glutamatergic ionotropic blockade within accumbens disrupts working memory and might alter the endocytic machinery in rat accumbens and prefrontal cortex

Effects of blocking N-methyl-D-aspartic acid (NMDA) and non-NMDA glutamatergic receptors on performance in the hole board test was studied in male rats bilaterally cannulated into the nucleus accumbens (Acc). Rats, divided into 5 groups, received either 1 microl injections of saline, (+/-) 2-amino-7-phosphonoheptanoic acid (AP-7) (0.5 or 1 microg) or 2,3-dioxo-6-nitro-1,2,3,4,tetrahydrobenzo-(f)quinoxaline-7-sulphonamide disodium (NBQX, 0.5 or 1 microg) 10 min before testing. An increase by AP-7 was observed in ambulatory movements (0.5 microg; p < 0.05), non-ambulatory movements and number of movements (1 microg; p < 0.05); sniffing and total exploration (1 microg; p < 0.01). When holes were considered in order from the first to the fifth by the number of explorations, the most visited holes (first and second) of the AP-7 group were significantly higher than the corresponding holes of saline group (p < 0.05 for 0.5 microg and p < 0.001 for 1 microg). When the second hole was compared with the first of his group, a difference was only observed in the AP-7 1 microg group (p < 0.001). Increasing differences between the other holes and the first were observed by drug treatment. At molecular level, it was observed that AP-7 induced an increase of the coat protein AP-2 expression in Acc, but not AP-180 neither the synaptic protein synaptophysin. The increase of AP-2 was also observed in the medial prefrontal cortex by the action of AP-7 but not NBQX. We conclude that NMDA glutamatergic blockade might induce an activation of the endocytic machinery into the Acc, leading to stereotypies and perseverations, lacking cortical intentional direction.

Enhanced glutamatergic transmission reduces the anticonvulsant potential of lamotrigine but not of felbamate against tonic-clonic seizures

The efficacy of lamotrigine and felbamate against maximal electroshock (MES)-induced seizures was assessed under conditions mimicking the pharmacoresistance associated with an increased excitatory neurotransmission. N-methyl-D-aspartate (NMDA), but not kainate applied at subconvulsive dose, reduced the activity of lamotrigine against MES-induced seizures increasing its ED50 value from 4.3 (3.2-5.6) to 6.1 (5.2-7.2) mg/kg (p < 0.001). This effect was reversed by co-application of an NMDAreceptor antagonist D-(E)-2-amino-4-methyl-5-phosphono-3-pentenoic acid (CGP 40116) at 0.1 mg/kg [4.5 (3.7-5.6) vs. 6.1 (5.2-7.2) mg/kg; p < 0.001]. The anticonvulsive action of felbamate was altered by neither NMDAnor kainate. In conclusion, the data presented here indicate that felbamate, but not lamotrigine, effectively prevents generalized tonic-clonic seizures, also when NMDA-mediated neurotransmission is enhanced. The impaired antiepileptic potential of lamotrigine might be restored in such scenario by the co-administration of a very low dose of NMDA receptor antagonist.

Involvement of AMPA/kainate, NMDA, and mGlu5 receptors in the nucleus accumbens core in cue-induced reinstatement of cocaine seeking in rats

RATIONALE

Nucleus accumbens glutamate transmission has been implicated in drug-seeking behavior, but the involvement of glutamate receptor subtypes in drug seeking maintained by drug-associated cues has not been fully investigated.

OBJECTIVE

This study examined the effects of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate, N-methyl-D-aspartate (NMDA) and mGlu5 receptor blockade in the nucleus accumbens core on cue-induced reinstatement of cocaine seeking.

METHOD

Wistar rats were trained to self-administer cocaine and associate a compound stimulus (light and tone) with the drug under an FR4(FR5:S) second-order schedule of reinforcement. After extinction, during which neither cocaine nor the compound stimulus was available, responding was reinstated by contingent presentations of the compound stimulus. The effects of the intra-accumbal AMPA/kainate receptor antagonist 6-cyano-7-nitro-quinoxaline-2, 3-dione (CNQX; 0, 0.01, and 0.03 microg/side), the NMDA antagonist D-2-amino-5-phosphonopentanoate (D-AP5; 0, 1, and 2 microg/side), and the mGluR5 antagonist 2-methyl-6-(phenylethynyl)-pyridine (MPEP; 0, 0.5, and 1 microg/side) on reinstatement were examined in a within-subjects design.

RESULTS

CNQX and D-AP5 attenuated cue-induced reinstatement of cocaine seeking dose-dependently. MPEP, however, decreased cocaine seeking only relative to baseline because also the saline vehicle included in the within-subjects series of injections decreased responding, possibly reflecting conditioned anhedonic effects of MPEP. In additional experiments, none of the antagonists attenuated locomotor activity or responding for sucrose pellets.

CONCLUSIONS

The results suggest that cue-induced reinstatement of cocaine seeking after a period of withdrawal from cocaine is sensitive to AMPA/kainate and NMDA receptor antagonism in the nucleus accumbens core and give further evidence for the role of the accumbal glutamate transmission in modulation of drug-seeking behavior.

Activity regulates the expression of AMPA receptor subunit GluR4 in developing visual cortex

In the developing visual cortex, the expression of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) subunit GluR4 precedes that of the other AMPAR subunits GluR1-3, and then declines to become almost absent in adults. The current study shows that the neuronal activity regulates the expression of GluR4 by a culture system in vitro and a dark-rearing (DR) system in vivo. Membrane depolarization by treatment of cultured neurons of the visual cortex with a high concentration of KCl (35 mm; HK) promoted a decline in the expression of GluR4. This effect of HK on the expression of GluR4 was significantly blocked by the addition of an N-methyl-d-aspartate receptor (NMDAR) antagonist, (D)-2-amino-5-phosphonovaleric acid (APV), but not by the voltage-sensitive calcium channel antagonist nifedipine. Moreover, the Ca(2+)-calmodulin-dependent kinase (CaMKII) inhibitor KN62 and the cAMP-dependent protein kinase A (PKA) inhibitor H-89 blocked this effect, which suggests the involvement of Ca(2+) influx via NMDAR and the subsequent activation of CaMKII and PKA. Conversely, the MAP kinase inhibitor PD98059 promoted the effect of HK on the expression of GluR4. Significantly, APV, KN62, H-89 and PD98059 either promoted or inhibited the expression of GluR4 even in normal KCl (5 mm) conditions. The developmental change in the expression of GluR4 was significantly attenuated in DR in vivo, and the results suggest that neuronal activity such as visual experience may be involved in the mechanism of the expression of GluR4, which is mediated by NMDAR and tuned by certain protein kinases at an early developmental stage in the visual cortex.

When administered into the nucleus accumbens core or shell, the NMDA receptor antagonist AP-5 reinstates cocaine-seeking behavior in the rat

Nucleus accumbens glutamate transmission plays a critical role in cocaine priming-induced reinstatement of drug seeking. Previous studies have demonstrated that systemic or intra-accumbens shell administration of an NMDA receptor antagonist reinstates cocaine-seeking behavior. However, it is unclear if antagonizing NMDA receptors in the nucleus accumbens core or shell subregions will differentially affect cocaine reinstatement. To investigate this possibility, we microinjected the competitive NMDA receptor antagonist AP-5 (0, 3 or 30 microg) into either the nucleus accumbens core or shell and assessed the reinstatement of cocaine-seeking behavior. When microinjected into the shell, both doses of AP-5 produced reinstatement of cocaine seeking. In contrast, when administered into the core, only the highest dose of AP-5 reinstated cocaine-seeking behavior; moreover, the magnitude of this effect was substantially less than when AP-5 was administered into the shell. This study provides evidence that pharmacological antagonism of NMDA receptors in the nucleus accumbens core or shell promotes the reinstatement of cocaine seeking.

Synergistic requirements for the induction of dopaminergic D1/D5-receptor-mediated LTP in hippocampal slices of rat CA1 in vitro

Dopaminergic D1/D5-receptor-mediated processes are important for certain forms of memory and its cellular model, i.e. hippocampal long-term potentiation (LTP) in CA1. D1/D5-receptor function is required for the induction of the protein synthesis-dependent maintenance of CA1-LTP (late-LTP) by activating the cAMP/PKA-pathway. In earlier studies we had reported a synergistic interaction of D1/D5-receptor function and N-methyl-D-aspartate (NMDA)-receptors (Frey, 2001, Long-lasting hippocampal plasticity: cellular model for memory consolidation? In: Richter, D. (Ed.), Cell Polarity and Subcellular RNA Localization. Springer-Verlag, Berlin-Heidelberg, pp. 27-40). Interestingly, the short-term application of D1/D5-receptor agonists (SKF38393 or 6-bromo-APB, 50 microM) can induce a slow-onset potentiation. This D1/D5-agonist-induced delayed-onset potentiation (D1/D5-LTP) resembles late-LTP, i.e. it is dependent on protein synthesis in the CA1 of rat hippocampal slices in vitro. The question arises as to whether D1/D5-LTP also requires glutamatergic stimulation, i.e. NMDA-receptor activation. We provide first evidence that a synergistic role of D1/D5- as well as NMDA-receptor-function is required in mediating processes relevant for the maintenance of this protein synthesis-dependent potentiation.

Glycine-induced long-term synaptic potentiation is mediated by the glycine transporter GLYT1

The negative symptoms of schizophrenia are reverted by treatment with glycine or other agonists of the glycine-B site which facilitate NMDA receptor function. On the other hand, there are experimental observations showing that exogenous application of glycine (0.5-10mM) results in a long-lasting potentiation of glutamatergic synaptic transmission (LTP-GLY). The characterization of the mechanisms underlying LTP-GLY could be useful to develop new therapies for schizophrenia. The main goal of this work is to deepen the understanding of this potentiation phenomenon. The present study demonstrates in rat hippocampal slices that superfusion of glycine 1mM during 30 min produces a potentiation of excitatory postsynaptic potentials in CA3-CA1 pathway lasting at least 1h. Glycine application does not modify neither presynaptic fiber volley nor paired-pulse facilitation of synaptic potentials. This LTP-GLY is independent of both strychnine-sensitive glycine receptors and nifedipine-sensitive calcium channels. Interestingly, LTP-GLY is not inhibited but strengthened by NMDA receptors antagonists such as AP-5 or MK-801. In contrast, LTP-GLY is partially or totally blocked with the antagonists of glycine transporter GLYT1, sarcosine or ALX-5407, respectively. These results indicate that LTP-GLY requires the activation of GLYT1, a glycine transporter co-localized and associated to NMDA receptors. In addition, the fact that NMDA receptor inhibition increases LTP-GLY magnitude, opens the possibility that these receptors could have a negative control on GLYT1 activity.

Endogenous D-serine contributes to NMDA-receptor-mediated light-evoked responses in the vertebrate retina

We have combined electrophysiology and chemical separation and measurement techniques with capillary electrophoresis (CE) to evaluate the role of endogenous d-serine as an NMDA receptor (NMDAR) coagonist in the salamander retina. Electrophysiological experiments were carried out using whole cell recordings from retinal ganglion cells and extracellular recordings of the proximal negative response (PNR), while bath applying two D-serine degrading enzymes, including d-amino acid oxidase (DAAO) and D-serine deaminase (DsdA). The addition of either enzyme resulted in a significant and rapid decline in the light-evoked responses observed in ganglion cell and PNR recordings. The addition of exogenous D-serine in the presence of the enzymes restored the light-evoked responses to the control or supracontrol amplitudes. Heat-inactivated enzymes had no effect on the light responses and blocking NMDARs with AP7 eliminated the suppressive influence of the enzymes as well as the response enhancement normally associated with exogenous d-serine application. CE was used to separate amino acid racemates and to study the selectivity of DAAO and DsdA against D-serine and glycine. Both enzymes showed high selectivity for D-serine without significant effects on glycine. Our results strongly support the concept that endogenous D-serine plays an essential role as a coagonist for NMDARs, allowing them to contribute to the light-evoked responses of retinal ganglion cells. Furthermore under our experimental conditions, these coagonist sites are not saturated so that modulation of NMDAR sensitivity can be achieved with further modulaton of d-serine.