ABSTRACT
Presynaptic kainate receptors play an important role in synaptic transmission and short-term plasticity to profoundly regulate network activity in many parts of the mammalian brain. In primary sensory neocortex, where short-term synaptic plasticity is important for receptive field structure and information processing, kainate receptors are highly expressed and regulate thalamocortical inputs, particularly during development. However, the mechanisms of the kainate receptor-dependent presynaptic regulation of thalamocortical transmission are unclear. We therefore investigated this issue using electrophysiology in neonatal thalamocortical slices of barrel cortex combined with pharmacology and biochemical analyses. We show that presynaptic kainate receptors can both facilitate or depress synaptic transmission depending on the extent of their activation. This bi-directional regulation is mediated in part by kainate receptors that directly influence thalamocortical axonal excitability, but also likely involves receptors acting at thalamocortical terminals to regulate transmitter release. The efficacy of kainate in regulating thalamocortical transmission is low compared to that reported for other inputs. Consistent with this low efficacy, our biochemical analyses indicate that the presynaptic kainate receptors regulating neonatal thalamocortical inputs likely lack the high kainate affinity GluK4 and 5 subunits. Thus thalamocortical transmission can be bi-directionally regulated by low affinity kainate receptors through two mechanisms. Such presynaptic regulation provides a potentially powerful mechanism to influence sensory processing during development of barrel cortex.
Subject(s)
Receptors, Kainic Acid/physiology , Receptors, Presynaptic/physiology , Somatosensory Cortex/physiology , Synaptic Transmission/physiology , Thalamus/physiology , Animals , Animals, Newborn , Dose-Response Relationship, Drug , In Vitro Techniques , Kainic Acid/pharmacology , Mice , Mice, 129 Strain , Mice, Inbred C57BL , Neural Pathways/drug effects , Neural Pathways/physiology , Patch-Clamp Techniques , Receptors, Kainic Acid/biosynthesis , Somatosensory Cortex/drug effects , Synaptic Transmission/drug effectsABSTRACT
Kainate receptors (KARs) modulate neuronal network activity. The molecular mechanisms that control the assembly and trafficking of KARs are unclear. Here, we examined the role of Q/R editing and subunit composition on KAR subunit assembly and subcellular distribution. The majority of GluK2 subunits undergo editing at the Q/R site in the channel pore loop. Cell surface biotinylation, cross-linking, Endoglycosidase-H analysis and gradient separation of KAR subunit assembly states revealed that Q/R editing reduces oligomerization, endoplasmic reticulum (ER) export, plasma membrane expression and stability of homomeric GluK2-containing KARs. These results indicate that Q/R editing of GluK2 may orchestrate channel subunit composition during KAR assembly in the ER. GluK2/GluK5 heteromers are the most abundant KAR subtype in the brain. While subcellular fractionation of brain tissue confirmed that both GluK2/3 and GluK5 are present in synaptosomes and tightly associated with post-synaptic density fractions, biochemical analysis revealed that endogenous GluK2/3 subunits show less complete assembly and trafficking compared with GluK5. In transgenic mice, the loss of the key assembly partner GluK2 leads to dramatic reduction in GluK5 expression. These results support the idea that the assembly and intracellular distribution of KARs is determined by RNA editing at the Q/R site and subunit composition.
Subject(s)
RNA Editing , Receptors, Kainic Acid/metabolism , Amino Acid Sequence , Animals , Brain/metabolism , Cell Line , Humans , Mice , Mice, Transgenic , Microsomes/metabolism , Molecular Sequence Data , Protein Multimerization , Protein Subunits/genetics , Protein Subunits/metabolism , Receptors, Kainic Acid/genetics , Synaptosomes/metabolism , GluK2 Kainate Receptor , GluK3 Kainate ReceptorABSTRACT
Metabotropic glutamate receptor 7 (mGluR7), a receptor with a distinct brain distribution and a putative role in anxiety, emotional responding, and spatial working memory, could be an interesting therapeutic target for fear and anxiety disorders. mGluR7-deficient (mGluR7-/-) mice showed essentially normal performance in tests for neuromotor and exploratory activity and passive avoidance learning but prominent anxiolytic behavior in two anxiety tests. They showed a delayed learning curve during the acquisition of the hidden-platform water maze, and three interspersed probe trials indicated that mGluR7-/- mice were slower to acquire spatial information. Working memory in the water maze task and the radial arm maze was impaired in mGluR7-/- mice compared with mGluR7+/+. mGluR7-/- mice also displayed a higher resistance to extinction of fear-elicited response suppression in a conditioned emotional response protocol. In a non-fear-based water maze protocol, mGluR7-/- mice displayed similar delayed extinction. These observed behavioral changes are probably not attributable to changes in AMPA or NMDA receptor function because expression levels of AMPA and NMDA receptors were unaltered. Extinction of conditioned fear is an active and context-dependent form of inhibitory learning and an experimental model for therapeutic fear reduction. It appears to depend on glutamatergic and higher-level brain functions similar to those involved in spatial working memory but functionally dissociated from those that mediate constitutional responses in anxiety tests.
Subject(s)
Anxiety/physiopathology , Extinction, Psychological/physiology , Fear/psychology , Memory, Short-Term/physiology , Receptors, Metabotropic Glutamate/deficiency , Analysis of Variance , Animals , Avoidance Learning/physiology , Behavior, Animal , Conditioning, Operant/physiology , Disease Models, Animal , Evoked Potentials, Visual/physiology , Exploratory Behavior/physiology , Female , Gene Expression/physiology , Immunohistochemistry , Interpersonal Relations , Male , Maze Learning/physiology , Mice , Mice, Inbred C57BL , Mice, Knockout , Motor Activity/genetics , Psychomotor Performance/physiology , Receptors, AMPA/metabolism , Receptors, N-Methyl-D-Aspartate/metabolismABSTRACT
Mechanisms of long-term potentiation and depression (LTP and LTD) change considerably during development, but the importance of these changes and the factors that control them is not clear. We found that visual experience triggered a switch in mechanisms of LTD in rat perirhinal cortex, an area critical for visual recognition memory. Thus, changes in synaptic plasticity mechanisms were correlated with the changing physiological demands on the CNS.
Subject(s)
Cerebral Cortex/growth & development , Long-Term Synaptic Depression/physiology , Neuronal Plasticity/physiology , Animals , Cerebral Cortex/metabolism , Light , Organ Culture Techniques , Rats , Receptors, Glutamate/biosynthesis , Receptors, Muscarinic/biosynthesis , Vision, Ocular/physiologyABSTRACT
Kainate receptors (KARs) modulate synaptic transmission at both pre-synaptic and post-synaptic sites. The overlap in the distribution of KA-2 and GluR6/7 subunits in several brain regions suggests the co-assembly of these subunits in native KARs. The molecular mechanisms that control the assembly and surface expression of KARs are unknown. Unlike GluR5-7, the KA-2 subunit is unable to form functional homomeric KAR channels. We expressed the KA-2 subunit alone or in combination with other KAR subunits in HEK-293 cells. The cell surface expression of the KAR subunit homo- and heteromers were analysed using biotinylation and agonist-stimulated cobalt uptake. While GluR6 or GluR7 homomers were expressed on the cell surface, KA-2 alone was retained within the endoplasmic reticulum. We found that the cell surface expression of KA-2 was dramatically increased by co-expression with either of the low-affinity KAR subunits GluR5-7. However, co-expression with other related ionotropic glutamate receptor subunits (GluR1 and NR1) does not facilitate the cell surface expression of KA-2. The analysis of subcellular fractions of neocortex revealed that synaptic KARs have a relatively high KA-2 content compared to microsomal ones. Thus, KA-2 is likely to contain an endoplasmic reticulum retention signal that is shielded on assembly with other KAR subunits.
