Jörg R. P. Geiger

Relative abundance of subunit mRNAs determines gating and Ca2+ permeability of AMPA receptors in principal neurons and interneurons in rat CNS

Recording of glutamate-activated currents in membrane patches was combined with RT-PCR-mediated AMPA receptor (AMPAR) subunit mRNA analysis in single identified cells of rat brain slices. Analysis of AMPARs in principal neurons and interneurons of hippocampus and neocortex and in auditory relay neurons and Bergmann glial cells indicates that the GluR-B subunit in its flip version determines formation of receptors with relatively slow gating, whereas the GluR-D subunit promotes assembly of more rapidly gated receptors. The relation between Ca2+ permeability of AMPAR channels and the relative GluR-B mRNA abundance is consistent with the dominance of this subunit in determining the Ca2+ permeability of native receptors. The results suggest that differential expression of GluR-B and GluR-D subunit genes, as well as splicing and editing of their mRNAs, account for the differences in gating and Ca2+ permeability of native AMPAR channels.

Dynamic Control of Presynaptic Ca2+ Inflow by Fast-Inactivating K+ Channels in Hippocampal Mossy Fiber Boutons

Analysis of presynaptic determinants of synaptic strength has been difficult at cortical synapses, mainly due to the lack of direct access to presynaptic elements. Here we report patch-clamp recordings from mossy fiber boutons (MFBs) in rat hippocampal slices. The presynaptic action potential is very short during low-frequency stimulation but is prolonged up to 3-fold during high-frequency stimulation. Voltage-gated K(+) channels in MFBs inactivate rapidly but recover from inactivation very slowly, suggesting that cumulative K(+) channel inactivation mediates activity-dependent spike broadening. Prolongation of the presynaptic voltage waveform leads to an increase in the number of Ca(2+) ions entering the terminal per action potential and to a consecutive potentiation of evoked excitatory postsynaptic currents at MFB-CA3 pyramidal cell synapses. Thus, inactivation of presynaptic K(+) channels contributes to the control of efficacy of a glutamatergic synapse in the cortex.

Fast synaptic inhibition promotes synchronized gamma oscillations in hippocampal interneuron networks

Networks of GABAergic interneurons are of critical importance for the generation of gamma frequency oscillations in the brain. To examine the underlying synaptic mechanisms, we made paired recordings from “basket cells” (BCs) in different subfields of hippocampal slices, using transgenic mice that express enhanced green fluorescent protein (EGFP) under the control of the parvalbumin promoter. Unitary inhibitory postsynaptic currents (IPSCs) showed large amplitude and fast time course with mean amplitude-weighted decay time constants of 2.5, 1.2, and 1.8 ms in the dentate gyrus, and the cornu ammonis area 3 (CA3) and 1 (CA1), respectively (33–34°C). The decay of unitary IPSCs at BC–BC synapses was significantly faster than that at BC–principal cell synapses, indicating target cell-specific differences in IPSC kinetics. In addition, electrical coupling was found in a subset of BC–BC pairs. To examine whether an interneuron network with fast inhibitory synapses can act as a gamma frequency oscillator, we developed an interneuron network model based on experimentally determined properties. In comparison to previous interneuron network models, our model was able to generate oscillatory activity with higher coherence over a broad range of frequencies (20–110 Hz). In this model, high coherence and flexibility in frequency control emerge from the combination of synaptic properties, network structure, and electrical coupling.

Submillisecond AMPA Receptor-Mediated Signaling at a Principal Neuron–Interneuron Synapse

Glutamatergic transmission at a principal neuron-interneuron synapse was investigated by dual whole-cell patch-clamp recording in rat hippocampal slices combined with morphological analysis. Evoked EPSPs with rapid time course (half duration = 4 ms; 34 degrees C) were generated at multiple synaptic contacts established on the interneuron dendrites close to the soma. The underlying postsynaptic conductance change showed a submillisecond rise and decay, due to the precise timing of glutamate release and the rapid deactivation of the postsynaptic AMPA receptors. Simulations based on a compartmental model of the interneuron indicated that the rapid postsynaptic conductance change determines the shape and the somatodendritic integration of EPSPs, thus enabling interneurons to detect synchronous principal neuron activity.

Combined Analog and Action Potential Coding in Hippocampal Mossy Fibers

In the mammalian cortex, it is generally assumed that the output information of neurons is encoded in the number and the timing of action potentials. Here, we show, by using direct patchclamp recordings from presynaptic hippocampal mossy fiber boutons, that axons transmit analog signals in addition to action potentials. Excitatory presynaptic potentials result from subthreshold dendritic synaptic inputs, which propagate several hundreds of micrometers along the axon and modulate action potential–evoked transmitter release at the mossy fiber–CA3 synapse. This combined analog and action potential coding represents an additional mechanism for information transmission in a major hippocampal pathway.

Energy-Efficient Action Potentials in Hippocampal Mossy Fibers

Minimizing Brain Energy Consumption How much energy is actually required to generate neuronal activity and information processing? By combining direct recordings at physiological temperatures from mossy fiber axons in rat brain slices with modeling and simulation approaches, Alle et al. (p. 1405; see the Perspective by Magistretti) found that the regenerative action potentials in nonmyelinated axons of mammalian hippocampus are remarkably energy efficient. The data indicate a surprisingly minor contribution of action potentials to the entire energy expenditure of neural information processing. Mammalian neurons have developed highly efficient ways to limit energy consumption while propagating neuronal information. Action potentials in nonmyelinated axons are considered to contribute substantially to activity-dependent brain metabolism. Here we show that fast Na+ current decay and delayed K+ current onset during action potentials in nonmyelinated mossy fibers of the rat hippocampus minimize the overlap of their respective ion fluxes. This results in total Na+ influx and associated energy demand per action potential of only 1.3 times the theoretical minimum, in contrast to the factor of 4 used in previous energy budget calculations for neural activity. Analysis of ionic conductance parameters revealed that the properties of Na+ and K+ channels are matched to make axonal action potentials energy-efficient, minimizing their contribution to activity-dependent metabolism.

Ca(2+)-permeable AMPA and NMDA receptor channels in basket cells of rat hippocampal dentate gyrus

1. Glutamate receptor (GluR) channels were studied in basket cells in the dentate gyrus of rat hippocampal slices. Basket cells were identified by their location, dendritic morphology and high frequency of action potentials generated during sustained current injection. 2. Dual‐component currents were activated by fast application of glutamate to outside‐out membrane patches isolated from basket cell somata (10 microM glycine, no external Mg2+). The fast component was selectively blocked by 6‐cyano‐7‐nitroquinoxaline‐2,3‐dione (CNQX), the slow component by D‐2‐amino‐5‐phosphonopentanoic acid (D‐AP5). This suggests that the two components were mediated by alpha‐amino‐3‐ hydroxy‐5‐methyl‐4‐isoxazolepropionate receptor (AMPAR)/kainate receptor and N‐methyl‐D‐aspartate receptor (NMDAR) channels, respectively. The mean ratio of the peak current of the NMDAR component to that of the AMPAR/kainate receptor component was 0.22 (1 ms pulses of 10 mM glutamate). 3. The AMPAR/kainate receptor component, which was studied in isolation in the presence of D‐AP5, was identified as AMPAR mediated on the basis of the preferential activation by AMPA as compared with kainate, the weak desensitization of kainate‐activated currents, the cross‐desensitization between AMPA and kainate, and the reduction of desensitization by cyclothiazide. 4. Deactivation of basket cell AMPARs following 1 ms pulses of glutamate occurred with a time constant (tau) of 1.2 +/‐ 0.1 ms (mean +/‐ S.E.M.). During 100 ms glutamate pulses AMPARs desensitized with a tau of 3.7 +/‐ 0.2ms. 5. The peak current‐voltage (I‐V) relation of AMPAR‐mediated currents in Na(+)‐rich extracellular solution showed a reversal potential of ‐4.0 +/‐ 2.6 mV and was characterized by a a doubly rectifying shape. The conductance of single AMPAR channels was estimated as 22.6 +/‐ 1.6 pS using non‐stationary fluctuation analysis. AMPARs expressed in hippocampal basket cells were highly Ca2+ permeable (PCa/PK = 1.79). 6. NMDARs in hippocampal basket cells were studied in isolation in the presence of CNQX. Deactivation of NMDARs activated by glutamate pulses occurred bi‐exponentially with mean tau values of 266 +/‐ 23 ms (76%) and 2620 +/‐ 383 ms (24%). 7. The peak I‐V relation of the NMDAR‐mediated component in Na(+)‐rich extracellular solution showed a reversal potential of 1.5 +/‐ 0.6 mV and a region of negative slope at negative membrane potentials in the presence of external Mg2+, due to voltage‐dependent block by these ions. The conductance of single NMDAR channels in the main open state was 50.2 +/‐ 1.8 pS.(ABSTRACT TRUNCATED AT 400 WORDS)

PTP and LTP at a hippocampal mossy fiber-interneuron synapse

The mossy fiber-CA3 pyramidal neuron synapse is a main component of the hippocampal trisynaptic circuitry. Recent studies, however, suggested that inhibitory interneurons are the major targets of the mossy fiber system. To study the regulation of mossy fiber-interneuron excitation, we examined unitary and compound excitatory postsynaptic currents in dentate gyrus basket cells, evoked by paired recording between granule and basket cells or extracellular stimulation of mossy fiber collaterals. The application of an associative high-frequency stimulation paradigm induced posttetanic potentiation (PTP) followed by homosynaptic long-term potentiation (LTP). Analysis of numbers of failures, coefficient of variation, and paired-pulse modulation indicated that both PTP and LTP were expressed presynaptically. The Ca2+ chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N′,N′-tetraacetic acid (BAPTA) did not affect PTP or LTP at a concentration of 10 mM but attenuated LTP at a concentration of 30 mM. Both forskolin, an adenylyl cyclase activator, and phorbolester diacetate, a protein kinase C stimulator, lead to a long-lasting increase in excitatory postsynaptic current amplitude. H-89, a protein kinase A inhibitor, and bisindolylmaleimide, a protein kinase C antagonist, reduced PTP, whereas only bisindolylmaleimide reduced LTP. These results may suggest a differential contribution of protein kinase A and C pathways to mossy fiber-interneuron plasticity. Interneuron PTP and LTP may provide mechanisms to maintain the balance between synaptic excitation of interneurons and that of principal neurons in the dentate gyrus-CA3 network.

Impaired GABAergic transmission and altered hippocampal synaptic plasticity in collybistin-deficient mice

Collybistin (Cb) is a brain‐specific guanine nucleotide exchange factor that has been implicated in plasma membrane targeting of the postsynaptic scaffolding protein gephyrin found at glycinergic and GABAergic synapses. Here we show that Cb‐deficient mice display a region‐specific loss of postsynaptic gephyrin and GABAA receptor clusters in the hippocampus and the basolateral amygdala. Cb deficiency is accompanied by significant changes in hippocampal synaptic plasticity, due to reduced dendritic GABAergic inhibition. Long‐term potentiation is enhanced, and long‐term depression reduced, in Cb‐deficient hippocampal slices. Consistent with the anatomical and electrophysiological findings, the animals show increased levels of anxiety and impaired spatial learning. Together, our data indicate that Cb is essential for gephyrin‐dependent clustering of a specific set of GABAA receptors, but not required for glycine receptor postsynaptic localization.

GABAergic Spill-Over Transmission onto Hippocampal Mossy Fiber Boutons

Presynaptic ionotropic GABAA receptors have been suggested to contribute to the regulation of cortical glutamatergic synaptic transmission. Here, we analyzed presynaptic GABAA receptor-mediated currents (34°C) recorded from mossy fiber boutons (MFBs) in rat hippocampal slices. In MFBs from young and adult animals, GABA puff application activated currents that were blocked by GABAA receptor antagonists. The conductance density of 0.65 mS · cm2 was comparable to that of other presynaptic terminals. The single-channel conductance was 36 pS (symmetrical chloride), yielding an estimated GABAA receptor density of 20–200 receptors per MFB. Presynaptic GABAA receptors likely contain α2-subunits as indicated by their zolpidem sensitivity. In accordance with the low apparent GABA affinity (EC50 = 60 μm) of the receptors and a tight control of ambient GABA concentration by GABA transporters, no tonic background activation of presynaptic GABAA receptors was observed. Instead, extracellular high-frequency stimulation led to transient presynaptic currents, which were blocked by GABAA receptor antagonists but were enhanced by block of GAT 1 (GABA transporter 1), indicating that these currents were generated by GABA spill-over and subsequent presynaptic GABAA receptor activation. Presynaptic spill-over currents were depressed by pharmacological cannabinoid 1 (CB1) receptor activation, suggesting that GABA was released predominantly by a CB1 receptor-expressing interneuron subpopulation. Because GABAA receptors in axons are considered to act depolarizing, high activity of CB1 receptor-expressing interneurons will exert substantial impact on presynaptic membrane potential, thus modulating action potential-evoked transmitter release at the mossy fiber–CA3 synapse.