The role of feedforward inhibition onto Purkinje cells during learning is still not well understood. Here, the authors report that selective genetic removal of GABAA receptor–mediated inhibition onto Purkinje cells modulates fine-scale patterns of Purkinje cell activity. These patterns may mediate the induction of downstream plasticity and, ultimately, the consolidation of cerebellar motor learning. Although feedforward inhibition onto Purkinje cells was first documented 40 years ago, we understand little of how inhibitory interneurons contribute to cerebellar function in behaving animals. Using a mouse line (PC-Δγ2) in which GABAA receptor–mediated synaptic inhibition is selectively removed from Purkinje cells, we examined how feedforward inhibition from molecular layer interneurons regulates adaptation of the vestibulo-ocular reflex. Although impairment of baseline motor performance was relatively mild, the ability to adapt the phase of the vestibulo-ocular reflex and to consolidate gain adaptations was strongly compromised. Purkinje cells showed abnormal patterns of simple spikes, both during and in the absence of evoked compensatory eye movements. On the basis of modeling our experimental data, we propose that feedforward inhibition, by controlling the fine-scale patterns of Purkinje cell activity, enables the induction of plasticity in neurons of the cerebellar and vestibular nuclei.

TARPs, which are associated with AMPA-type glutamate receptors and which regulate their properties and trafficking, have not been shown to specifically regulate calcium-permeable AMPARs. In this study, the authors report that the stargazin-related protein g-5, which is expressed in Bergmann glia, preferentially regulates calcium-permeable AMPARs. Although the properties and trafficking of AMPA-type glutamate receptors (AMPARs) depend critically on associated transmembrane AMPAR regulatory proteins (TARPs) such as stargazin (γ-2), no TARP has been described that can specifically regulate the important class of calcium-permeable (CP-) AMPARs. We examined the stargazin-related protein γ-5, which is highly expressed in Bergmann glia, a cell type possessing only CP-AMPARs. γ-5 was previously thought not to be a TARP, and it has been widely used as a negative control. Here we find that, contrary to expectation, γ-5 acts as a TARP and serves this role in Bergmann glia. Whereas γ-5 interacts with all AMPAR subunits, and modifies their behavior to varying extents, its main effect is to regulate the function of AMPAR subunit combinations that lack short-form subunits, which constitute predominantly CP-AMPARs. Our results suggest an important role for γ-5 in regulating the functional contribution of CP-AMPARs.

The authors studied the dynamic regulation of Ca2+-permeable AMPARs (CP-AMPARs) in oligodendrocyte precursor cells. Group 1 mGluRs regulate CP-AMPARs via a pathway that requires intracellular Ca2+, PI3 kinase, PICK-1 and JNK. Purinergic receptor activation decreases CP-AMPAR expression. Oligodendrocyte precursor cells (OPCs), a major glial cell type that gives rise to myelinating oligodendrocytes in the CNS, express calcium-permeable AMPA receptors (CP-AMPARs). Although CP-AMPARs are important for OPC proliferation and neuron-glia signaling, they render OPCs susceptible to ischemic damage in early development. We identified factors controlling the dynamic regulation of AMPAR subtypes in OPCs from rat optic nerve and mouse cerebellar cortex. We found that activation of group 1 mGluRs drove an increase in the proportion of CP-AMPARs, reflected by an increase in single-channel conductance and inward rectification. This plasticity required the elevation of intracellular calcium and used PI3K, PICK-1 and the JNK pathway. In white matter, neurons and astrocytes release both ATP and glutamate. Unexpectedly, activation of purinergic receptors in OPCs decreased CP-AMPAR expression, suggesting a capacity for homeostatic regulation. Finally, we found that stargazin-related transmembrane AMPAR regulatory proteins, which are critical for AMPAR surface expression in neurons, regulate CP-AMPAR plasticity in OPCs.

The kinetics of IPSCs influence many neuronal processes, such as the frequencies of oscillations and the duration of shunting inhibition. The subunit composition of recombinant GABA_Areceptors (GABA_ARs) strongly affects the deactivation kinetics of GABA-evoked currents. However, for GABAergic synapses, the relationship between subunit composition and IPSC decay is less clear. Here we addressed this by combining whole-cell recordings of miniature IPSCs (mIPSCs) and quantitative immunolocalization of synaptic GABA_AR subunits. In cerebellar stellate, thalamic relay, and main olfactory bulb (MOB) deep short-axon cells of Wistar rats, the only synaptic α subunit was α1, and zolpidem-sensitive mIPSCs had weighted decay time constants (τ_w) of 4–6 ms. Nucleus reticularis thalami neurons expressed only α3 as the synaptic α subunit and exhibited slow (τ_w= 28 ms), zolpidem-insensitive mIPSCs. By contrast, MOB external tufted cells contained two α subunit types (α1 and α3) at their synapses. Quantitative analysis of multiple immunolabeled images revealed small within-cell, but large between-cell, variability in synaptic α1/α3 ratios. This corresponded to large cell-to-cell variability in the decay (τ_w= 3–30 ms) and zolpidem sensitivity of mIPSCs. Currents evoked by rapid application of GABA to patches excised from HEK cells expressing different mixtures of α1 and α3 subunits displayed highly variable deactivation times that correlated with the α1/α3 cDNA ratio. Our results demonstrate that diversity in the decay of IPSCs can be generated by varying the expression of different GABA_AR subunits that alone confer different decay kinetics, allowing the time course of inhibition to be tuned to individual cellular requirements.

Ionotropic glutamate receptors, which underlie a majority of excitatory synaptic transmission in the CNS, associate with transmembrane proteins that modify their intracellular trafficking and channel gating. Significant advances have been made in our understanding of AMPA-type glutamate receptor (AMPAR) regulation by transmembrane AMPAR regulatory proteins. Less is known about the functional influence of cornichons—unrelated AMPAR-interacting proteins, identified by proteomic analysis. Here we confirm that cornichon homologs 2 and 3 (CNIH-2 and CNIH-3), but not CNIH-1, slow the deactivation and desensitization of both GluA2-containing calcium-impermeable and GluA2-lacking calcium-permeable (CP) AMPARs expressed in tsA201 cells. CNIH-2 and -3 also enhanced the glutamate sensitivity, single-channel conductance, and calcium permeability of CP-AMPARs while decreasing their block by intracellular polyamines. We examined the potential effects of CNIHs on native AMPARs by recording from rat optic nerve oligodendrocyte precursor cells (OPCs), known to express a significant population of CP-AMPARs. These glial cells exhibited surface labeling with an anti-CNIH-2/3 antibody. Two features of their AMPAR-mediated currents—the relative efficacy of the partial agonist kainate (I_KA/I_Glu ratio 0.4) and a greater than fivefold potentiation of kainate responses by cyclothiazide—suggest AMPAR association with CNIHs. Additionally, overexpression of CNIH-3 in OPCs markedly slowed AMPAR desensitization. Together, our experiments support the view that CNIHs are capable of altering key properties of AMPARs and suggest that they may do so in glia.