The N-methyl-D-aspartate (NMDA) receptor contributes to synaptic plasticity in the central nervous system and is both serine-threonine and tyrosine phosphorylated. In CA1 pyramidal neurons of the hippocampus, activators of protein kinase C (PKC) as well as the G-protein-coupled receptor ligands muscarine and lysophosphatidic acid enhanced NMDA-evoked currents. Unexpectedly, this effect was blocked by inhibitors of tyrosine kinases, including a Src required sequence and an antibody selective for Src itself. In neurons from mice lacking c-Src, PKC-dependent upregulation was absent. Thus, G-protein-coupled receptors can regulate NMDA receptor function indirectly through a PKC-dependent activation of the non-receptor tyrosine kinase (Src) signaling cascade.

Dentate gyrus granule cells innervate inhibitory interneurons via a continuum of synapses comprised of either Ca(2+)-impermeable (CI) or Ca(2+)-permeable (CP) AMPA receptors. Synapses at the extreme ends of this continuum engage distinct postsynaptic responses, with activity at CI synapses being strongly influenced by NMDA receptor activation. NMDARs at CI synapses have a lower NR2B subunit composition and a higher open probability, which generate larger amplitude and more rapid EPSCs than their CP counterparts. A novel form of NMDAR-dependent long-term depression (iLTD) is associated with CI-mossy fiber synapses, whereas iLTD at CP synapses is dependent on Ca(2+)-permeable AMPA receptor activation. Induction of both forms of iLTD required elevation of postsynaptic calcium. Thus mossy fibers engage CA3 interneurons via multiple synapse types that will act to expand the computational repertoire of the mossy fiber-CA3 network.

Journal of Physiologyrelease at either excitatory or inhibitory synapses onto CA3 interneurons has not been elucidated.In the present paper, we examine the effects of GABA B receptor activation on excitatory and inhibitory synaptic transmission onto interneurons located in the CA3 st. radiatum. Our results indicate that activation of GABA B receptors inhibits both excitatory and inhibitory synaptic transmission at CA3 interneuron synapses via presynaptic mechanisms. METHODS Hippocampal slice preparationTransverse hippocampal slices (300 mm) were obtained from 16-18-day-old Sprague-Dawley rats, unless otherwise stated in the text, as described previously (Toth & McBain, 1998;Lei & McBain, 2002). Rats were deeply anaesthetized with isoflurane, rapidly decapitated, and the brain dissected out in ice-cold saline solution that contained (mM): 130 NaCl, 24 NaHCO 3 , 3.5 KCl, 1.25 NaH 2 PO 4 , 1.0 CaCl 2 , 5.0 MgCl 2 , and 10 glucose, saturated with 95 % O 2 and 5 % CO 2 , pH 7.4. All animal procedures conformed to the National Institutes of Health animal welfare guidelines. ElectrophysiologyWhole-cell patch-clamp recordings were made from visually identified interneurons located in the st. radiatum of CA3 by the use of an Axopatch 1D amplifier (Axon Instruments, Foster City, CA, USA) in voltage-clamp mode. Unless otherwise stated, recording electrodes were filled with the following (mM): 100 caesium gluconate, 0.6 EGTA, 5 MgCl 2 , 8 NaCl, 2 ATP 2 Na, 0.3 GTPNa, 40 Hepes, and 1 QX-314, pH 7.2-7.3. Biocytin (0.2 %) was routinely added to the recording electrode solution to allow post hoc morphological processing of recorded cells (Toth & McBain, 1998). In experiments where postsynaptic GABA B responses were investigated, caesium gluconate was replaced by equimolar potassium gluconate in the internal solution. The extracellular solution comprised the following (mM): 130 NaCl, 24 NaHCO 3 , 3.5 KCl, 1.25 NaHPO 4 , 1.5 MgCl 2 , 2.5 CaCl 2 and 10 glucose, saturated with 95 % O 2 and 5 % CO 2 , pH 7.4.

Sound features are blended together en route to the central nervous system before being discriminated for further processing by the cortical synaptic network. The mechanisms underlying this synaptic processing, however, are largely unexplored. Intracortical processing of the auditory signal was investigated by simultaneously recording from pairs of connected principal neurons in layer II/III in slices from A1 auditory cortex. Physiological patterns of stimulation in the presynaptic cell revealed two populations of postsynaptic events that differed in mean amplitude, failure rate, kinetics and short-term plasticity. In contrast, transmission between layer II/III pyramidal neurons in barrel cortex were uniformly of large amplitude and high success (release) probability (Pr). These unique features of auditory cortical transmission may provide two distinct mechanisms for discerning and separating transient from stationary features of the auditory signal at an early stage of cortical processing.

Summary The entorhinal cortex (EC) is regarded as the gateway to the hippocampus and thus is essential for learning and memory. Whereas the EC expresses a high density of GABAB receptors, the functions of these receptors in this region remain unexplored. Here we examined the effects of GABAB receptor activation on neuronal excitability in the EC and spatial learning. Application of baclofen, a specific GABAB receptor agonist, inhibited significantly neuronal excitability in the EC. GABAB receptor-mediated inhibition in the EC was mediated via activating TREK-2, a type of two-pore domain K+ channels and required the functions of inhibitory G proteins and protein kinase A pathway. Depression of neuronal excitability in the EC underlies GABAB receptor-mediated inhibition of spatial learning as assessed by Morris water maze. Our study indicates that GABAB receptors exert a tight control over spatial learning by modulating neuronal excitability in the EC.

Two distinct forms of long-term depression (LTD) exist at mossy fiber synapses between dentate gyrus granule cells and hippocampal CA3 stratum lucidum interneurons. Although induction of each form of LTD requires an elevation of postsynaptic intracellular Ca2+, at Ca2+-impermeable AMPA receptor (CI-AMPAR) synapses, induction is NMDA receptor (NMDAR) dependent, whereas LTD at Ca2+-permeable AMPA receptor (CP-AMPAR) synapses is NMDAR independent. However, the expression locus of either form of LTD is not known. Using a number of criteria, including the coefficient of variation, paired-pulse ratio, AMPA-NMDA receptor activity, and the low-affinity AMPAR antagonist γ-d-glutamyl-glycine, we demonstrate that LTD expression at CP-AMPAR synapses is presynaptic and results from reduced transmitter release, whereas LTD expression at CI-AMPAR synapses is postsynaptic. TheN-ethylmaleimide-sensitive fusion protein-AP2-clathrin adaptor protein 2 inhibitory peptide pep2m occluded LTD expression at CI-AMPAR synapses but not at CP-AMPAR synapses, confirming that CI-AMPAR LTD involves postsynaptic AMPAR trafficking. Thus, mossy fiber innervation of CA3 stratum lucidum interneurons occurs via two parallel systems targeted to either Ca2+-permeable or Ca2+-impermeable AMPA receptors, each with a distinct expression locus for long-term synaptic plasticity.

The postsynaptic density (PSD) at excitatory dendritic synapses comprises a protein complex of glutamate receptors, scaffolding elements, and signaling enzymes. For example, NMDA receptors (NMDARs) are linked to several proteins in the PSD, such as PSD-95, and are also tethered via binding proteins such as alpha-actinin directly to filamentous actin of the cytoskeleton. Depolymerization of the cytoskeleton modulates the activity of NMDARs, and, in turn, strong activation of NMDARs can trigger depolymerization of actin. Myosin, the motor protein of muscular contraction and nonmuscle motility, is also associated with NMDARs and the PSD. We show here that constitutively active myosin light chain kinase (MLCK) enhances NMDAR-mediated whole-cell and synaptic currents in acutely isolated CA1 pyramidal and cultured hippocampal neurons, whereas inhibitors of MLCK depress these currents. This MLCK-dependent regulation was observed in cell-attached patches but was lost after excision to inside-out patches. Furthermore, the enhancement induced by constitutively active MLCK and the depression of MLCK inhibitors were eliminated after depolymerization of the cytoskeleton. NMDARs and MLCK did not colocalize in clusters on the dendrites of cultured hippocampal neurons, further indicating that the effects of MLCK are mediated indirectly via actomyosin. Our results suggest that MLCK enhances actomyosin contractility to either increase the membrane tension on NMDARs or to alter physical relationships between the actin cytoskeleton and the linker proteins of NMDARs.

Whereas the entorhinal cortex (EC) receives noradrenergic innervations from the locus coeruleus of the pons and expresses adrenergic receptors, the function of norepinephrine (NE) in the EC is still elusive. We examined the effects of NE on GABAA receptor–mediated synaptic transmission in the superficial layers of the EC. Application of NE dose-dependently increased the frequency and amplitude of spontaneous inhibitory postsynaptic currents (IPSCs) recorded from the principal neurons in layer II/III through activation of α1 adrenergic receptors. NE increased the frequency and not the amplitude of miniature IPSCs (mIPSCs) recorded in the presence of TTX, suggesting that NE increases presynaptic GABA release with no effects on postsynaptic GABAA receptors. Application of Ca2+ channel blockers (Cd2+ and Ni2+), omission of Ca2+ in the extracellular solution, or replacement of extracellular Na+ with N-methyl-d-glucamine (NMDG) failed to alter NE-induced increase in mIPSC frequency, suggesting that Ca2+ influx through voltage-gated Ca2+ or other cationic channels is not required. Application of BAPTA-AM, thapsigargin, and ryanodine did not change NE-induced increase in mIPSC frequency, suggesting that Ca2+ release from intracellular stores is not necessary for NE-induced increase in GABA release. Whereas α1 receptors are coupled to Gq/11 resulting in activation of the phospholipase C (PLC) pathway, NE-mediated facilitation of GABAergic transmission was independent of PLC, protein kinase C, and tyrosine kinase activities. Our results suggest that NE-mediated facilitation of GABAergic function contributes to its antiepileptic effects in the EC.