Kainate receptors act as conditional amplifiers of spike transmission at hippocampal mossy fiber synapses.

Hippocampal mossy fiber (Mf) synapses are viewed as conditional detonators, assisting CA3 cells in complex network functions. By analyzing mice deficient for GluK2 (GluR6), GluK3 (GluR7) and GluK5 (KA2) genes we show that kainate receptors (KARs) play a crucial role in the control of synaptic integration and spike transmission efficacy at Mf synapses. We dissected out the role of the different KAR functions at Mf synapses and we show that presynaptic and postsynaptic KARs concur to amplify unitary Mf synaptic inputs to trigger spike discharge within a wide range of frequencies (from 1 to 50 Hz). Moreover, KARs strongly favor spike transmission in response to patterns of presynaptic activity mimicking in vivo dentate granule cell activity. By amplifying spike transmission, KARs also facilitate the induction of associative long-term potentiation in CA3. Hence the actions of KARs as amplifiers of spike transmission contribute largely to the "conditional detonator" function of Mf synapses and are likely important for spatial information processing.

Co-assembly of two GluR6 kainate receptor splice variants within a functional protein complex.

Kainate receptors (KAR) are composed of several distinct subunits and splice variants, but the functional relevance of this diversity remains largely unclear. Here we show that two splice variants of the GluR6 subunit, GluR6a and GluR6b, which differ in their C-terminal domains, do not show distinct functional properties, but coassemble as heteromers in vitro and in vivo. Using a proteomic approach combining affinity purification and MALDI-TOF mass spectrometry, we found that GluR6a and GluR6b interact with two distinct subsets of cytosolic proteins mainly involved in Ca(2+) regulation of channel function and intracellular trafficking. Guided by these results, we provide evidence that the regulation of native KAR function by NMDA receptors depends on the heteromerization of GluR6a and GluR6b and interaction of calcineurin with GluR6b. Thus, GluR6a and GluR6b bring in close proximity two separate subsets of interacting proteins that contribute to the fine regulation of KAR trafficking and function.

GluR6/KA2 kainate receptors mediate slow-deactivating currents.

Kainate receptors (KARs) are ionotropic glutamate receptors contributing to EPSCs with a slow-decaying component that is likely essential for synaptic integration. The slow kinetics of KAR-EPSCs markedly contrasts with the fast kinetics reported for recombinant KARs expressed in heterologous systems, for reasons that remain unexplained. Here we have studied the properties of recombinant heteromeric GluR6/KA2 receptors, which compose synaptic KARs. We report that, in response to brief glutamate applications, currents mediated by recombinant GluR6/KA2 receptors, but not GluR6 receptors, decay with a time course similar to KAR-EPSCs. Model simulations suggest that, after brief agonist exposures, GluR6/KA2 currents undergo slow deactivation caused by the stabilization of partially bound open states. We propose, therefore, that the GluR6/KA2 gating features could contribute to the slow KAR-EPSC decay kinetics.

Short-term plasticity of kainate receptor-mediated EPSCs induced by NMDA receptors at hippocampal mossy fiber synapses.

Kainate receptors (KARs) are heteromeric ionotropic glutamate receptors that play a variety of functions in the regulation of the activity of synaptic networks. Little is known about the regulation of the function of synaptic KARs in the brain. In the present study, we found that a conditioning activation of synaptic NMDA receptors (NMDARs) induces short-term depression of KAR-EPSCs but not of AMPA receptor-EPSCs at synapses between mossy fibers and CA3 pyramidal cells. Short-term depression of KAR-EPSCs by synaptic NMDARs peaked at 1 s and reversed within 20 s, was likely induced and expressed postsynaptically, and was homosynaptic. It depended on a rise of Ca2+ in the postsynaptic cell and on the activation of the phosphatase calcineurin that likely binds to the GluR6b (glutamate receptor subunit 6b) subunit splice variant allowing the dephosphorylation of KARs and inhibition of activity. Finally, we show in the current-clamp mode that short-term depression of KAR-EPSPs is induced by the coincident discharge of action potentials in the postsynaptic cell together with synaptic stimulation. Hence, this study describes a form of short-term synaptic plasticity that is postsynaptic, depends on the temporal order of presynaptic and postsynaptic spiking, and likely affects the summation properties of mossy fiber EPSPs.

Sensory representation of an auditory cued tactile stimulus in the posterior parietal cortex of the mouse.

Sensory association cortices receive diverse inputs with their role in representing and integrating multi-sensory content remaining unclear. Here we examined the neuronal correlates of an auditory-tactile stimulus sequence in the posterior parietal cortex (PPC) using 2-photon calcium imaging in awake mice. We find that neuronal subpopulations in layer 2/3 of PPC reliably represent texture-touch events, in addition to auditory cues that presage the incoming tactile stimulus. Notably, altering the flow of sensory events through omission of the cued texture touch elicited large responses in a subset of neurons hardly responsive to or even inhibited by the tactile stimuli. Hence, PPC neurons were able to discriminate not only tactile stimulus features (i.e., texture graininess) but also between the presence and omission of the texture stimulus. Whereas some of the neurons responsive to texture omission were driven by looming-like auditory sounds others became recruited only with tactile sensory experience. These findings indicate that layer 2/3 neuronal populations in PPC potentially encode correlates of expectancy in addition to auditory and tactile stimuli.

Distinct subunits in heteromeric kainate receptors mediate ionotropic and metabotropic function at hippocampal mossy fiber synapses.

Heteromeric kainate receptors (KARs) containing both glutamate receptor 6 (GluR6) and KA2 subunits are involved in KAR-mediated EPSCs at mossy fiber synapses in CA3 pyramidal cells. We report that endogenous glutamate, by activating KARs, reversibly inhibits the slow Ca2+-activated K+ current I(sAHP) and increases neuronal excitability through a G-protein-coupled mechanism. Using KAR knockout mice, we show that KA2 is essential for the inhibition of I(sAHP) in CA3 pyramidal cells by low nanomolar concentrations of kainate, in addition to GluR6. In GluR6(-/-) mice, both ionotropic synaptic transmission and inhibition of I(sAHP) by endogenous glutamate released from mossy fibers was lost. In contrast, inhibition of I(sAHP) was absent in KA2(-/-) mice despite the preservation of KAR-mediated EPSCs. These data indicate that the metabotropic action of KARs did not rely on the activation of a KAR-mediated inward current. Biochemical analysis of knock-out mice revealed that KA2 was required for the interaction of KARs with Galpha(q/11)-proteins known to be involved in I(sAHP) modulation. Finally, the ionotropic and metabotropic actions of KARs at mossy fiber synapses were differentially sensitive to the competitive glutamate receptor ligands kainate (5 nM) and kynurenate (1 mM). We propose a model in which KARs could operate in two modes at mossy fiber synapses: through a direct ionotropic action of GluR6, and through an indirect G-protein-coupled mechanism requiring the binding of glutamate to KA2.

Parvalbumin-Expressing GABAergic Neurons in Mouse Barrel Cortex Contribute to Gating a Goal-Directed Sensorimotor Transformation.

Sensory processing in neocortex is primarily driven by glutamatergic excitation, which is counterbalanced by GABAergic inhibition, mediated by a diversity of largely local inhibitory interneurons. Here, we trained mice to lick a reward spout in response to whisker deflection, and we recorded from genetically defined GABAergic inhibitory neurons in layer 2/3 of the primary somatosensory barrel cortex. Parvalbumin-expressing (PV), vasoactive intestinal peptide-expressing (VIP), and somatostatin-expressing (SST) neurons displayed distinct action potential firing dynamics during task performance. Whereas SST neurons fired at low rates, both PV and VIP neurons fired at high rates both spontaneously and in response to whisker stimulation. After an initial outcome-invariant early sensory response, PV neurons had lower firing rates in hit trials compared to miss trials. Optogenetic inhibition of PV neurons during this time period enhanced behavioral performance. Hence, PV neuron activity might contribute causally to gating the sensorimotor transformation of a whisker sensory stimulus into licking motor output.

Naltrexone attenuates plasma nitric oxide release following acute heat stress.

Previous studies have shown that naltrexone attenuates morbidity and mortality in heat stress by inhibiting endogenous opioids. In this study, we hypothesized that naltrexone can decrease heat stress by attenuating nitric oxide release. Male Sprague-Dawley rats were pretreated with naltrexone or normal saline, and exposed to 45 degrees C for 25 min; controls were exposed to 25 degrees C. Colonic temperatures were recorded and plasma samples from an in-dwelling i.v. cannula were analyzed for nitrate/nitrite levels. Following heat stress, peak colonic temperature was significantly diminished (P < 0.05) in naltrexone-treated rats compared to saline-treated rats. Plasma nitrate/nitrite levels were significantly lower (P < 0.05) in naltrexone-treated rats compared to saline-treated rats. These findings suggest that naltrexone is able to attenuate the rise in plasma nitric oxide levels commonly observed after heat stress.

Herbimycin A attenuates apoptosis during heat stress in rats.

Expression of heat shock proteins (HSPs) as a heat stress response is associated with acquisition of thermotolerance. Herbimycin A is a tyrosine kinase inhibitor that has been shown to induce HSPs. The present study aims to investigate the effects of herbimycin A on thermotolerance in rats subjected to heat stress exposure. Herbimycin A induced hsp70 to peak levels 12 h post-injection in rats without heat stress. No change in hsp70 levels was observed in the vehicle- and saline-treated rats. In rats exposed to heat stress at 45 degrees C for 25 min, 12 h post-treatment, lower peak temperatures were attained in herbimycin A-treated group as compared to the vehicle- and saline-treated groups. Terminal transferase-mediated d-UTP nick end labeling (TUNEL) showed that a significant decrease in apoptosis of hepatocytes in herbimycin A-treated rats as compared to the vehicle- and saline-treated rats. Caspase-3 activation was also lower in herbimycin A-treated rats, compared to the vehicle- and saline-treated rats. The present study has demonstrated that herbimycin A is effective for development of thermotolerance and therefore protects rats from heat stress.

Membrane potential correlates of sensory perception in mouse barrel cortex.

Neocortical activity can evoke sensory percepts, but the cellular mechanisms remain poorly understood. We trained mice to detect single brief whisker stimuli and report perceived stimuli by licking to obtain a reward. Pharmacological inactivation and optogenetic stimulation demonstrated a causal role for the primary somatosensory barrel cortex. Whole-cell recordings from barrel cortex neurons revealed membrane potential correlates of sensory perception. Sensory responses depended strongly on prestimulus cortical state, but both slow-wave and desynchronized cortical states were compatible with task performance. Whisker deflection evoked an early (<50 ms) reliable sensory response that was encoded through cell-specific reversal potentials. A secondary late (50-400 ms) depolarization was enhanced on hit trials compared to misses. Optogenetic inactivation revealed a causal role for late excitation. Our data reveal dynamic processing in the sensory cortex during task performance, with an early sensory response reliably encoding the stimulus and later secondary activity contributing to driving the subjective percept.