A reinforcing circuit action of extrasynaptic GABAA receptor modulators on cerebellar granule cell inhibition.

GABAA receptors (GABARs) are the targets of a wide variety of modulatory drugs which enhance chloride flux through GABAR ion channels. Certain GABAR modulators appear to acutely enhance the function of δ subunit-containing GABAR subtypes responsible for tonic forms of inhibition. Here we identify a reinforcing circuit mechanism by which these drugs, in addition to directly enhancing GABAR function, also increase GABA release. Electrophysiological recordings in cerebellar slices from rats homozygous for the ethanol-hypersensitive (α6100Q) allele show that modulators and agonists selective for δ-containing GABARs such as THDOC, ethanol and THIP (gaboxadol) increased the frequency of spontaneous inhibitory postsynaptic currents (sIPSCs) in granule cells. Ethanol fails to augment granule cell sIPSC frequency in the presence of glutamate receptor antagonists, indicating that circuit mechanisms involving granule cell output contribute to ethanol-enhancement of synaptic inhibition. Additionally, GABAR antagonists decrease ethanol-induced enhancement of Golgi cell firing. Consistent with a role for glutamatergic inputs, THIP-induced increases in Golgi cell firing are abolished by glutamate receptor antagonists. Moreover, THIP enhances the frequency of spontaneous excitatory postsynaptic currents in Golgi cells. Analyses of knockout mice indicate that δ subunit-containing GABARs are required for enhancing GABA release in the presence of ethanol and THIP. The limited expression of the GABAR δ subunit protein within the cerebellar cortex suggests that an indirect, circuit mechanism is responsible for stimulating Golgi cell GABA release by drugs selective for extrasynaptic isoforms of GABARs. Such circuit effects reinforce direct actions of these positive modulators on tonic GABAergic inhibition and are likely to contribute to the potent effect of these compounds as nervous system depressants.

Prolonging the postcomplex spike pause speeds eyeblink conditioning.

Climbing fiber input to the cerebellum is believed to serve as a teaching signal during associative, cerebellum-dependent forms of motor learning. However, it is not understood how this neural pathway coordinates changes in cerebellar circuitry during learning. Here, we use pharmacological manipulations to prolong the postcomplex spike pause, a component of the climbing fiber signal in Purkinje neurons, and show that these manipulations enhance the rate of learning in classical eyelid conditioning. Our findings elucidate an unappreciated aspect of the climbing fiber teaching signal, and are consistent with a model in which convergent postcomplex spike pauses drive learning-related plasticity in the deep cerebellar nucleus. They also suggest a physiological mechanism that could modulate motor learning rates.

Photochemical control of endogenous ion channels and cellular excitability.

Light-activated ion channels provide a precise and noninvasive optical means for controlling action potential firing, but the genes encoding these channels must first be delivered and expressed in target cells. Here we describe a method for bestowing light sensitivity onto endogenous ion channels that does not rely on exogenous gene expression. The method uses a synthetic photoisomerizable small molecule, or photoswitchable affinity label (PAL), that specifically targets K+ channels. PALs contain a reactive electrophile, enabling covalent attachment of the photoswitch to naturally occurring nucleophiles in K+ channels. Ion flow through PAL-modified channels is turned on or off by photoisomerizing PAL with different wavelengths of light. We showed that PAL treatment confers light sensitivity onto endogenous K+ channels in isolated rat neurons and in intact neural structures from rat and leech, allowing rapid optical regulation of excitability without genetic modification.

Conformational dynamics of hSGLT1 during Na+/glucose cotransport.

This study examines the conformations of the Na(+)/glucose cotransporter (SGLT1) during sugar transport using charge and fluorescence measurements on the human SGLT1 mutant G507C expressed in Xenopus oocytes. The mutant exhibited similar steady-state and presteady-state kinetics as wild-type SGLT1, and labeling of Cys507 by tetramethylrhodamine-6-maleimide had no effect on kinetics. Our strategy was to record changes in charge and fluorescence in response to rapid jumps in membrane potential in the presence and absence of sugar or the competitive inhibitor phlorizin. In Na(+) buffer, step jumps in membrane voltage elicited presteady-state currents (charge movements) that decay to the steady state with time constants tau(med) (3-20 ms, medium) and tau(slow) (15-70 ms, slow). Concurrently, SGLT1 rhodamine fluorescence intensity increased with depolarizing and decreased with hyperpolarizing voltages (DeltaF). The charge vs. voltage (Q-V) and fluorescence vs. voltage (DeltaF-V) relations (for medium and slow components) obeyed Boltzmann relations with similar parameters: zdelta (apparent valence of voltage sensor) approximately 1; and V(0.5) (midpoint voltage) between -15 and -40 mV. Sugar induced an inward current (Na(+)/glucose cotransport), and reduced maximal charge (Q(max)) and fluorescence (DeltaF(max)) with half-maximal concentrations (K(0.5)) of 1 mM. Increasing [alphaMDG](o) also shifted the V(0.5) for Q and DeltaF to more positive values, with K(0.5)'s approximately 1 mM. The major difference between Q and DeltaF was that at saturating [alphaMDG](o), the presteady-state current (and Q(max)) was totally abolished, whereas DeltaF(max) was only reduced 50%. Phlorizin reduced both Q(max) and DeltaF(max) (K(i) approximately 0.4 microM), with no changes in V(0.5)'s or relaxation time constants. Simulations using an eight-state kinetic model indicate that external sugar increases the occupancy probability of inward-facing conformations at the expense of outward-facing conformations. The simulations predict, and we have observed experimentally, that presteady-state currents are blocked by saturating sugar, but not the changes in fluorescence. Thus we have isolated an electroneutral conformational change that has not been previously described. This rate-limiting step at maximal inward Na(+)/sugar cotransport (saturating voltage and external Na(+) and sugar concentrations) is the slow release of Na(+) from the internal surface of SGLT1. The high affinity blocker phlorizin locks the cotransporter in an inactive conformation.

Expression of GFP-tagged neuronal glutamate transporters in cerebellar Purkinje neurons.

Of the five excitatory amino acid transporters (EAATs) identified, two genes are expressed by neurons (EAAT3 and EAAT4) and give rise to transporters confined to neuronal cell bodies and dendrites. At an ultrastructural level, EAAT3 and EAAT4 proteins are clustered at the edges of postsynaptic densities of excitatory synapses. This pattern of localization suggests that postsynaptic EAATs may help to limit spillover of glutamate from excitatory synapses. In an effort to study transporter localization in living neurons and ultimately to manipulate uptake at intact synapses, we have developed viral reagents encoding neuronal EAATs tagged with GFP. We demonstrate that these fusion proteins are capable of Na(+)-dependent glutamate uptake, that they generate ionic conductances indistinguishable from their wild-type counterparts, and that GFP does not alter their glutamate dose-dependence. Two-photon microscopy was used to examine fusion protein expression in Purkinje neurons in acute cerebellar slices. Both EAAT3-GFP and EAAT4-GFP were observed at high levels in the dendritic spines of transfected Purkinje neurons. These findings indicate that functional EAAT fusion proteins can be synthesized and appropriately trafficked to postsynaptic compartments. Furthermore, they validate a powerful system for looking at EAAT function in situ.

Excitation of cerebellar interneurons by group I metabotropic glutamate receptors.

Cerebellar basket and stellate neurons (BSNs) provide feed-forward inhibition to Purkinje neurons (PNs) and thereby play a principal role in determining the output of the cerebellar cortex. During low-frequency transmission, glutamate released at parallel fiber synapses excites BSNs by binding to AMPA receptors; high-frequency transmission also recruits N-methyl-d-aspartate (NMDA) receptors. We find that, in addition to these ligand-gated receptors, a G-protein-coupled glutamate receptor subtype participates in exciting BSNs. Stimulation of metabotropic glutamate receptor 1alpha (mGluR1alpha) with the mGluR agonist (RS)-3,5-dihydroxyphenylglycine (DHPG) leads to an increase in spontaneous firing of BSNs and indirectly to an increase in the frequency of spontaneous inhibitory postsynaptic currents (sIPSCs) recorded in PNs. Under conditions in which ligand-gated glutamate receptors are blocked, parallel fiber stimulation generates a slow excitatory postsynaptic current (EPSC) in BSNs that is inhibited by mGluR1alpha-selective antagonists. This slow EPSC is capable of increasing BSN spiking and indirectly increasing sIPSCs frequency in PNs. Our findings reinforce the idea that distinct subtypes of glutamate receptors are activated in response to different patterns of activity at excitatory synapses. The results also raise the possibility that mGluR1alpha-dependent forms of synaptic plasticity may occur at excitatory inputs to BSNs.