C-Terminal truncation of NR2A subunits impairs synaptic but not extrasynaptic localization of NMDA receptors

NMDA receptors interact via the extended intracellular C-terminal domain of the NR2 subunits with constituents of the postsynaptic density for purposes of retention, clustering, and functional regulation at central excitatory synapses. To examine the role of the C-terminal domain of NR2A in the synaptic localization and function of NR2A-containing NMDA receptors in hippocampal Schaffer collateral–CA1 pyramidal cell synapses, we analyzed mice which express NR2A only in its C-terminally truncated form. In CA1 cell somata, the levels, activation, and deactivation kinetics of extrasynaptic NMDA receptor channels were comparable in wild-type and mutant NR2A^(ΔC/ΔC) mice. At CA1 cell synapses, however, the truncated receptors were less concentrated than their full-length counterparts, as indicated by immunodetection in cultured neurons, synaptosomes, and postsynaptic densities. In the mutant, the NMDA component of evoked EPSCs was reduced in a developmentally progressing manner and was even more reduced in miniature EPSCs (mEPSCs) elicited by spontaneous glutamate release. Moreover, pharmacologically isolated NMDA currents evoked by synaptic stimulation had longer latencies and displayed slower rise and decay times, even in the presence of an NR2B-specific antagonist. These data strongly suggest that the C-terminal domain of NR2A subunits is important for the precise synaptic arrangement of NMDA receptors

Coordinated prefrontal state transition leads extinction of reward-seeking behaviors

Extinction learning suppresses conditioned reward responses and is thus fundamental to adapt to changing environmental demands and to control excessive reward seeking. The medial prefrontal cortex (mPFC) monitors and controls conditioned reward responses. Abrupt transitions in mPFC activity anticipate changes in conditioned responses to altered contingencies. It remains, however, unknown whether such transitions are driven by the extinction of old behavioral strategies or by the acquisition of new competing ones. Using in vivo multiple single-unit recordings of mPFC in male rats, we studied the relationship between single-unit and population dynamics during extinction learning, using alcohol as a positive reinforcer in an operant conditioning paradigm. To examine the fine temporal relation between neural activity and behavior, we developed a novel behavioral model that allowed us to identify the number, onset, and duration of extinction-learning episodes in the behavior of each animal. We found that single-unit responses to conditioned stimuli changed even under stable experimental conditions and behavior. However, when behavioral responses to task contingencies had to be updated, unit-specific modulations became coordinated across the whole population, pushing the network into a new stable attractor state. Thus, extinction learning is not associated with suppressed mPFC responses to conditioned stimuli, but is anticipated by single-unit coordination into population-wide transitions of the internal state of the animal

Different Forms of AMPA Receptor Mediated LTP and Their Correlation to the Spatial Working Memory Formation

Spatial working memory (SWM) and the classical, tetanus-induced long-term potentiation (LTP) at hippocampal CA3/CA1 synapses are dependent on L-α-amino-3-hydroxy-5-methylisoxazole-4-propionate receptors (AMPARs) containing GluA1 subunits as demonstrated by knockout mice lacking GluA1. In GluA1 knockout mice LTP and SWM deficits could be partially recovered by transgenic re-installation of full-length GluA1 in principle forebrain neurons. Here we partially restored hippocampal LTP in GluA1-deficient mice by forebrain-specific depletion of the GluA2 gene, by the activation of a hypomorphic GluA2(Q) allele and by transgenic expression of PDZ-site truncated GFP-GluA1(TG). In none of these three mouse lines, the partial LTP recovery improved the SWM performance of GluA1-deficient mice suggesting a specific function of intact GluA1/2 receptors and the GluA1 intracellular carboxyl-terminus in SWM and its associated behavior

A common molecular mechanism for cognitive deficits and craving in alcoholism

Alcohol-dependent patients commonly show impairments in executive functions that facilitate craving and can lead to relapse. The medial prefrontal cortex, a key brain region for executive control, is prone to alcohol-induced neuroadaptations. However, the molecular mechanisms leading to executive dysfunction in alcoholism are poorly understood. Here using a bi-directional neuromodulation approach we demonstrate a causal link for reduced prefrontal mGluR2 function and both impaired executive control and alcohol craving. By neuron-specific prefrontal knockdown of mGluR2 in rats, we generated a phenotype of reduced cognitive flexibility and excessive alcohol-seeking. Conversely, restoring prefrontal mGluR2 levels in alcohol-dependent rats rescued these pathological behaviors. Also targeting mGluR2 pharmacologically reduced relapse behavior. Finally, we developed a FDG-PET biomarker to identify those individuals that respond to mGluR2-based interventions. In conclusion, we identified a common molecular pathological mechanism for both executive dysfunction and alcohol craving, and provide a personalized mGluR2-mechanism-based intervention strategy for medication development of alcoholism

Sustained granule cell activity disinhibits juvenile mouse cerebellar stellate cells through presynaptic mechanisms

GABA release from cerebellar molecular layer interneurons can be modulated by presynaptic glutamate and/or GABAB receptors upon perfusing the respective agonists. However, it is unclear how release and potential spillover of endogenous transmitter lead to activation of presynaptic receptors. High frequency firing of granule cells, as observed in vivo upon sensory stimulation, could lead to glutamate and/or GABA spillover. Here, we established sustained glutamatergic activity in the granule cell layer of acute mouse cerebellar slices and performed 190 paired recordings from connected stellate cells. Train stimulation at 50 Hz reduced by about 30% the peak amplitude of IPSCs evoked by brief depolarization of the presynaptic cell in two week−old mice. A presynaptic mechanism was indicated by changes in failure rate, paired−pulse ratio and coefficient of variation of evoked IPSCs. Furthermore, Two−Photon Ca2+ imaging in identified Ca2+ hot spots of stellate cell axons confirmed reduced presynaptic Ca2+ influx after train stimulation within the granular layer. Pharmacological experiments indicated that glutamate released from parallel fibers activated AMPARs in stellate cells, evoking GABA release from surrounding cells. Consequential GABA spillover activated presynaptic GABABRs, which reduced the amplitude of eIPSCs. Two thirds of the total disinhibitory effect were mediated by GABABRs, one third being attributable to presynaptic AMPARs. This estimation was confirmed by the observation that bath applied baclofen induced a more pronounced reduction of evoked IPSCs than Kainate. Granule cell−mediated disinhibition persisted at near−physiological temperature but was strongly diminished in three week−old mice. At this age, GABA release probability was not reduced and presynaptic GABABRs were still detectable, but GABA uptake appeared to be advanced, attenuating GABA spillover. Thus, sustained granule cell activity modulates stellate cell−to−stellate cell synapses, involving transmitter spillover during a developmentally restricted period

Kindling increases N-methyl-D-aspartate potency at single N-methyl-D-aspartate channels in dentate gyrus granule cells

Dose-response studies of N-methyl-D-aspartate channel openings were carried out using cell-attached patches in dentate gyrus granule cells acutely isolated from control and kindled rats. The tips of the patch electrodes were first filled with regular extracellular solution, followed by backfilling through the shank with the agonist containing solution. As the two solutions joined, the agonist (N-methyl-D-aspartate, 25 microM) steadily diffused to the cell membrane, and the concentration gradually built up resulting in the progressive increase in the opening probability of N-methyl-D-aspartate channels. The reliability of this cell-attached diffusional drug delivery method was tested by determining the concentration dependence of competitive antagonism of N-methyl-D-aspartate induced channel activity by D(-)-2-amino-5-phosphonopentanoic acid. The Ki for D(-)-2-amino-5-phosphonopentanoic acid in the presence of 25 microM N-methyl-D-aspartate was found to be 6.8 microM. Twenty-four hours following the last seizure, N-methyl-D-aspartate channels on kindled neurons were consistently activated by lower N-methyl-D-aspartate concentrations than channels on control granule cells, indicating a higher potency of agonist at epileptic N-methyl-D-aspartate channels. The higher potency of the agonist is most likely a reflection of the long-term alterations in the modulation of N-methyl-D-aspartate receptor function in epileptic neurons

Triheteromeric NR1/NR2A/NR2B Receptors Constitute the Major N-Methyl-d-aspartate Receptor Population in Adult Hippocampal Synapses

NMDA receptors (NMDARs), fundamental to learning and memory and implicated in certain neurological disorders, are heterotetrameric complexes composed of two NR1 and two NR2 subunits. The function of synaptic NMDARs in postnatal principal forebrain neurons is typically attributed to diheteromeric NR1/NR2A and NR1/NR2B receptors, despite compelling evidence for triheteromeric NR1/NR2A/NR2B receptors. In synapses, the properties of triheteromeric NMDARs could thus far not be distinguished from those of mixtures of diheteromeric NMDARs. To find a signature of NR1/NR2A/NR2B receptors, we have employed two gene-targeted mouse lines, expressing either NR1/NR2A or NR1/NR2B receptors without NR1/NR2A/NR2B receptors, and compared their synaptic properties with those of wild type. In acute hippocampal slices of mutants older than 4 weeks we found a distinct voltage dependence of NMDA R-mediated excitatory postsynaptic current (NMDA EPSC) decay time for the two diheteromeric NMDARs. In wild-type mice, NMDA EPSCs unveiled the NR1/NR2A characteristic for this voltage-dependent deactivation exclusively, indicating that the contribution of NR1/NR2B receptors to evoked NMDA EPSCs is negligible in adult CA3-to-CA1 synapses. The presence of NR1/NR2A/NR2B receptors was obvious from properties that could not be explained by a mixture of diheteromeric NR1/NR2A and NR1/NR2B receptors or by the presence of NR1/NR2A receptors alone. The decay time for NMDA EPSCs in wild type was slower than that for NR1/NR2A receptors, and the sensitivity of NMDA EPSCs to NR2B-directed NMDAR antagonists was 50%. Thus, NR2B is prominent in adult hippocampal synapses as an integral part of NR1/NR2A/NR2B receptors

GABA release from cerebellar stellate cells is developmentally regulated by presynaptic GABAB receptors in a target−cell−specific manner

Transmitter release from boutons along a common axon is often regulated depending on the postsynaptic target. Here, GABA release from cerebellar stellate cells onto Purkinje cells and other stellate cells was examined in acute cerebellar slices of 2− and 4− week−old mice. Consistent with previous findings on action potential−dependent GABA release, we found a developmental decrease in inhibitory inputs onto Purkinje cells but not onto stellate cells when recording miniature inhibitory postsynaptic currents (mIPSCs). Although amplitudes of mIPSCs were developmentally reduced in both cell types, mIPSC frequencies were decreased in Purkinje cells but were increased in stellate cells. Similarly, modulation of GABA release by presynaptic GABAB receptors changed during development in Purkinje cells but not in stellate cells, as demonstrated by the baclofen−mediated depression of mIPSC frequency and evoked IPSC (eIPSC) amplitudes. The selectively diminished baclofen effect in 4−week−old Purkinje cells correlated with a selective downregulation of presynaptic GABAB receptors at stellate cell−to−Purkinje cell synapses observed by immunoelectron microscopy analysis. Thus, expression of GABAB receptors in stellate cell axons and presynaptic modulation of GABA release appear to change during development in a target−cell−specific manner

Properties of NMDA receptor channels in neurons acutely isolated from epileptic (kindled) rats

The hyperexcitability accompanying chronic epileptiform activity may result from long-term alterations of ligand- and voltage-gated channels. Previous studies have indicated that NMDA responses and other electrophysiological characteristics of dentate gyrus granule cells are profoundly altered following chronic epilepsy (kindling). We have now investigated channels activated by NMDA using whole-cell patch-clamp and cell-attached single-channel recordings in granule cells acutely isolated from control and epileptic (kindled) rats. In control neurons, the amplitude of whole-cell NMDA currents was not sensitive to the presence of an intracellular ATP regeneration system, whereas NMDA currents in kindled cells showed a great variability, with larger amplitudes consistently recorded in the presence of intracellular high- energy phosphates. The ratio of peak to steady-state NMDA current (desensitization) was comparable (approximately 51%) in control and kindled neurons. Single-channel conductance determined from fluctuation analysis of whole-cell NMDA currents ranged between 21 and 35 pS in control and between 17 and 37 pS in kindled cells. Whole-cell NMDA channel noise power spectra yielded a single normal distribution of long channel lifetimes (mean, 4.3 msec) in control neurons, and the sum of two normal distributions (means, 4.6 and 7.1 msec) in kindled cells. The voltage-dependent Mg2+ block of NMDA channels was altered following kindling. From curves fitted to voltage-ramp-evoked currents in the presence of NMDA, the calculated affinity for Mg2+ of kindled channels at 0 mV was lower (12 mM) than that of controls (1.7 mM). Cell-attached recordings in the absence of Mg2+ have substantiated the lack of effect of kindling on single-channel conductance (approximately 50 pS), and have demonstrated large increases in mean open times (from 1.26 msec in control to 2.05 msec in kindled), burst lengths (from 1.91 msec to 4.18 msec), and cluster lengths (from 9.11 msec to 20.86 msec) of NMDA channels in kindled neurons. In summary, kindling, an NMDA receptor- dependent form of activity-dependent neuronal plasticity induced in vivo, results in lasting modifications in the function of single NMDA receptor channels that can be studied in acutely dissociated neurons. Kindling-induced epilepsy predominantly affects the mean open time, burst, and cluster duration of NMDA channels, their sensitivity to intracellular high-energy phosphates, and their block by Mg2+, but not the desensitization or single-channel conductance. Such alterations may reflect a change in the molecular structure of NMDA channels and may underlie the maintenance of the epileptic state