Networks of GABAergic interneurons are implicated in synchronizing cortical activity at gamma frequencies (30-70 Hz). Here we demonstrate that the combined electrical and GABAergic synaptic coupling of basket cells instantaneously entrained gamma-frequency postsynaptic firing in layers 2/3 of rat somatosensory cortex. This entrainment was mediated by rapid curtailment of gap junctional coupling potentials by GABAA receptor-mediated IPSPs. Electron microscopy revealed spatial proximity of gap junctions and GABAergic synapses on somata and dendrites. Electrical coupling alone entrained postsynaptic firing with a phase lag, whereas unitary GABAergic connections were ineffective in gamma-frequency phasing. These observations demonstrate precise spatiotemporal mechanisms underlying action potential timing in oscillating interneuronal networks.

Cortical synapses display remarkable structural, molecular and functional heterogeneity. Our knowledge regarding the relationship between the ultrastructural and functional parameters is still fragmented. Here we asked how the release probability and presynaptic [Ca2+] transients relate to the ultrastructure of rat hippocampal glutamatergic axon terminals. Two-photon Ca2+ imaging-derived optical quantal analysis and correlated electron microscopic reconstructions revealed a tight correlation between the release probability and the active zone area. The peak amplitude of [Ca2+] transients in single boutons also positively correlated with the active zone area. Freeze-fracture immunogold labeling revealed that the voltage-gated Ca2+ channel subunit Cav2.1 and the presynaptic protein Rim1/2 are confined to the active zone and their numbers scale linearly with the active zone area. Gold particles for Cav2.1 showed a nonrandom distribution within the active zones. Our results demonstrate that the number of several active zone proteins, including presynaptic Ca2+ channels, docked vesicles and the release probability scales linearly with the active zone area.

An ion channel's function depends largely on its location and density on neurons. Here we used high-resolution immunolocalization to determine the subcellular distribution of the hyperpolarization-activated and cyclic-nucleotide-gated channel subunit 1 (HCN1) in rat brain. Light microscopy revealed graded HCN1 immunoreactivity in apical dendrites of hippocampal, subicular and neocortical layer-5 pyramidal cells. Quantitative comparison of immunogold densities showed a 60-fold increase from somatic to distal apical dendritic membranes. Distal dendritic shafts had 16 times more HCN1 labeling than proximal dendrites of similar diameters. At the same distance from the soma, the density of HCN1 was significantly higher in dendritic shafts than in spines. Our results reveal the complex cell surface distribution of voltage-gated ion-channels, and predict its role in increasing the computational power of single neurons via subcellular domain and input-specific mechanisms.

The exact site of initiation and shape of action potentials vary among different neuronal types. The reason for this variability is largely unknown, but the subunit composition, density and distribution of voltage-gated sodium (Nav) and potassium (Kv) channels within the axon initial segment (AIS) are likely to play a key role. Here, we asked how heterogeneous are the density and distribution of Nav and Kv channels within the AISs of a variety of excitatory and inhibitory neurons. Most of the studied cell types expressed a high density of Nav1.6, Kv1.1, and Kv1.2 subunits in their AIS, but the Nav1.1 subunit could only be detected in GABAergic interneurons. A proximodistal gradient in the density of these subunits was observed within the AIS of certain nerve cells but not in others. For example, a gradual increase of the Nav1.6 subunit was observed in cortical layer 2/3 and hippocampal CA1 pyramidal cell (PC) AISs, whereas its density was rather uniform in layer 5 PC AISs. The Nav1.1 subunit was distributed evenly along the AIS of short-axon cells of the main olfactory bulb but was restricted to the proximal part of the AIS in cortical and cerebellar interneurons. Our results reveal a cell type-dependent expression of sodium and potassium channel subunits with varying densities along the proximo-distal axis of the AISs. This precise arrangement is likely to contribute to the diversity of firing properties observed among central neurons.

There are two types of inhibitory postsynaptic potentials in the cerebral cortex. Fast inhibition is mediated by ionotropic gamma-aminobutyric acid type A (GABA(A)) receptors, and slow inhibition is due to metabotropic GABA(B) receptors. Several neuron classes elicit inhibitory postsynaptic potentials through GABA(A) receptors, but possible distinct sources of slow inhibition remain unknown. We identified a class of GABAergic interneurons, the neurogliaform cells, that, in contrast to other GABA-releasing cells, elicited combined GABA(A) and GABA(B) receptor-mediated responses with single action potentials and that predominantly targeted the dendritic spines of pyramidal neurons. Slow inhibition evoked by a distinct interneuron in spatially restricted postsynaptic compartments could locally and selectively modulate cortical excitability.

SummaryElectrical synapses between interneurons contribute to synchronized firing and network oscillations in the brain. However, little is known about how such networks respond to excitatory synaptic input. To investigate this, we studied electrically coupled Golgi cells (GoC) in the cerebellar input layer. We show with immunohistochemistry, electron microscopy, and electrophysiology that Connexin-36 is necessary for functional gap junctions (GJs) between GoC dendrites. In the absence of coincident synaptic input, GoCs synchronize their firing. In contrast, sparse, coincident mossy fiber input triggered a mixture of excitation and inhibition of GoC firing and spike desynchronization. Inhibition is caused by propagation of the spike afterhyperpolarization through GJs. This triggers network desynchronization because heterogeneous coupling to surrounding cells causes spike-phase dispersion. Detailed network models predict that desynchronization is robust, local, and dependent on synaptic input properties. Our results show that GJ coupling can be inhibitory and either promote network synchronization or trigger rapid network desynchronization depending on the synaptic input.

Active invasion of the dendritic tree by action potentials (APs) generated in the axon is essential for associative synaptic plasticity and neuronal ensemble formation. In cortical pyramidal cells (PCs) this AP backpropagation is supported by dendritic voltage-gated Na + channels (Nav), whose molecular identity is unknown. Using a highly sensitive electron microscopic immunogold technique, we reveal the presence of the Nav1.6 subunit in hippocampal CA1 PC proximal and distal dendrites. Here the subunit density is lower by a factor of 35 to 80 than that found in axon initial segments. A gradual decrease in Nav1.6 density along the proximodistal axis of the dendritic tree was also detected without any labeling in dendritic spines. Our results reveal the characteristic subcellular distribution of the Nav1.6 subunit, identifying this molecule as a key substrate enabling dendritic excitability.Associative synaptic plasticity in cortical pyramidal cells (PC) is the most widely accepted cellular model of learning. An essential prerequisite of the model is that input synapses, which are distributed over an enormously large dendritic tree, must be capable of sensing the precise timing of the output signal. The most likely mechanism for this is the active invasion of the dendritic tree by fast sodium action potentials (APs) initiated in the axon initial segment (AIS). Modification of backpropagating APs therefore holds tremendous potential for altering synaptic plasticity and the formation of neuronal representations. Voltage-gated Na + (Nav) currents have been detected in hippocampal and neocortical PC dendrites, where they not only support AP backpropagation, but also underlie nonlinear synaptic integration and dendritic sodium spike generation (1-6; reviewed by 7-10). Patch-clamp experiments demonstrate that axonal and somato-dendritic Nav currents differ in their activation and inactivation properties (11,12), indicating either different Nav subunit compositions or distinct posttranslational modifications of identical subunits (12)(13)(14). However, the lack of subunit-specific drugs precludes the unequivocal identification of the subunit composition of the axo-somato-dendritic Nav channels using functional approaches. Immunohistochemistry with subunit-specific antibodies offers an alternative experimental approach to address this issue. Nav1.1, Nav1.2 and Nav1.6 are the Nav subunits expressed in adult brains (15, 16). They have been detected in nodes of Ranvier, AISs and preterminal axons using immunofluorescence localization (17)(18)(19)(20)(21)(22). Unfortunately this method has so far failed to visualize any of these Nav subunits in somato-dendritic compartments, most likely due to its insufficient sensitivity. We modified the immunofluorescence technique to increase its sensitivity and employed a highly sensitive, quantitative electron microscopic (EM) immunogold method (SDS-digested freeze-fracture replica-labeling: SDS-FRL, 23-25) to identify the Nav subunit content of the somato-dendritic plasma membra...

SummaryThe synaptic connectivity within neuronal networks is thought to determine the information processing they perform, yet network structure-function relationships remain poorly understood. By combining quantitative anatomy of the cerebellar input layer and information theoretic analysis of network models, we investigated how synaptic connectivity affects information transmission and processing. Simplified binary models revealed that the synaptic connectivity within feedforward networks determines the trade-off between information transmission and sparse encoding. Networks with few synaptic connections per neuron and network-activity-dependent threshold were optimal for lossless sparse encoding over the widest range of input activities. Biologically detailed spiking network models with experimentally constrained synaptic conductances and inhibition confirmed our analytical predictions. Our results establish that the synaptic connectivity within the cerebellar input layer enables efficient lossless sparse encoding. Moreover, they provide a functional explanation for why granule cells have approximately four dendrites, a feature that has been evolutionarily conserved since the appearance of fish.