Similar Presynaptic Action Potential-Calcium Influx Coupling in Two Types of Large Mossy Fiber Terminals Innervating CA3 Pyramidal Cells and Hilar Mossy Cells

Morphologically similar axon boutons form synaptic contacts with diverse types of postsynaptic cells. However, it is less known to what extent the local axonal excitability, presynaptic action potentials (APs), and AP-evoked calcium influx contribute to the functional diversity of synapses and neuronal activity. This is particularly interesting in synapses that contact cell types that show only subtle cellular differences but fulfill completely different physiological functions. Here, we tested these questions in two synapses that are formed by rat hippocampal granule cells (GCs) onto hilar mossy cells (MCs) and CA3 pyramidal cells, which albeit share several morphologic and synaptic properties but contribute to distinct physiological functions. We were interested in the deterministic steps of the action potential-calcium ion influx coupling as these complex modules may underlie the functional segregation between and within the two cell types. Our systematic comparison using direct axonal recordings showed that AP shapes, Ca²⁺ currents and their plasticity are indistinguishable in synapses onto these two cell types. These suggest that the complete module that couples granule cell activity to synaptic release is shared by hilar mossy cells and CA3 pyramidal cells. Thus, our findings present an outstanding example for the modular composition of distinct cell types, by which cells employ different components only for those functions that are deterministic for their specialized functions, while many of their main properties are shared.

Functional specification of CCK+ interneurons by alternative isoforms of Kv4.3 auxiliary subunits

CCK-expressing interneurons (CCK+INs) are crucial for controlling hippocampal activity. We found two firing phenotypes of CCK+INs in rat hippocampal CA3 area; either possessing a previously undetected membrane potential-dependent firing or regular firing phenotype, due to different low-voltage-activated potassium currents. These different excitability properties destine the two types for distinct functions, because the former is essentially silenced during realistic 8–15 Hz oscillations. By contrast, the general intrinsic excitability, morphology and gene-profiles of the two types were surprisingly similar. Even the expression of Kv4.3 channels were comparable, despite evidences showing that Kv4.3-mediated currents underlie the distinct firing properties. Instead, the firing phenotypes were correlated with the presence of distinct isoforms of Kv4 auxiliary subunits (KChIP1 vs. KChIP4e and DPP6S). Our results reveal the underlying mechanisms of two previously unknown types of CCK+INs and demonstrate that alternative splicing of few genes, which may be viewed as a minor change in the cells’ whole transcriptome, can determine cell-type identity.

Feedforward inhibition is randomly wired from individual granule cells onto CA3 pyramidal cells

Feedforward inhibition (FFI) between the dentate gyrus (DG) and CA3 sparsifies and shapes memory‐ and spatial navigation‐related activities. However, our understanding of this prototypical FFI circuit lacks essential details, as the wiring of FFI is not yet mapped between individual DG granule cells (GCs) and CA3 pyramidal cells (PCs). Importantly, theoretically opposite network contributions are possible depending on whether the directly excited PCs are differently inhibited than the non‐excited PCs. Therefore, to better understand FFI wiring schemes, we compared the prevalence of disynaptic inhibitory postsynaptic events (diIPSCs) between pairs of individually recorded GC axons or somas and PCs, some of which were connected by monosynaptic excitation, while others were not. If FFI wiring is specific, diIPSCs are expected only in connected PCs; whereas diIPSCs should not be present in these PCs if FFI is laterally wired from individual GCs. However, we found single GC‐elicited diIPSCs with similar probabilities irrespective of the presence of monosynaptic excitation. This observation suggests that the wiring of FFI between individual GCs and PCs is independent of the direct excitation. Therefore, the randomly distributed FFI contributes to the hippocampal signal sparsification by setting the general excitability of the CA3 depending on the overall activity of GCs.

Adult-born granule cells mature through two functionally distinct states

Adult-born granule cells (ABGCs) are involved in certain forms of hippocampus-dependent learning and memory. It has been proposed that young but functionally integrated ABGCs (4-weeks-old) specifically contribute to pattern separation functions of the dentate gyrus due to their heightened excitability, whereas old ABGCs (>8 weeks old) lose these capabilities. Measuring multiple cellular and integrative characteristics of 3- 10-week-old individual ABGCs, we show that ABGCs consist of two functionally distinguishable populations showing highly distinct input integration properties (one group being highly sensitive to narrow input intensity ranges while the other group linearly reports input strength) that are largely independent of the cellular age and maturation stage, suggesting that ‘classmate’ cells (born during the same period) can contribute to the network with fundamentally different functions. Thus, ABGCs provide two temporally overlapping but functionally distinct neuronal cell populations, adding a novel level of complexity to our understanding of how life-long neurogenesis contributes to adult brain function.

DOI:http://dx.doi.org/10.7554/eLife.03104.001

Remembering what happened on different occasions involves a process in the brain called pattern separation, which allows us to separate and distinguish our memories. One part of the brain where pattern separation occurs is called the dentate gyrus, which sits in the hippocampus—the brain region that is in charge of certain forms of learning and memory.

Neurons called granule cells are thought to play a central role in hippocampal pattern separation. These cells, unlike the majority of nerve cells, can form at any time, and those that form in the mature brain are called adult born granule cells (ABGCs). Although it usually takes 10 weeks for these cells to fully mature, they are capable of communicating with each other about 3–4 weeks after being generated. Previously, it had been reported that while young, 4-week-old ABGCs are required for pattern separation, slightly older (8 week old) ABGCs are not.

What intrinsic properties make ABGCs capable of contributing to pattern separation? Is this property defined by the fate (i.e. a predetermined program) of the cell, or by the cell's experiences and activities?

To investigate these questions, Brunner et al. labeled ABGCs with a fluorescent tag when these neurons were born in adult male rats. Then, when the tagged cells were aged between 3 and 10 weeks old, the electrical properties of the labeled cells were measured from thin brain slices.

Brunner et al. found that ABGCs respond to input signals with two different levels of sensitivity. The youngest cells (3–5 weeks old) are exceptionally sensitive to a narrow range of input signal strengths, which is useful for pattern separation. The oldest investigated cells (10 weeks old), on the other hand, respond incrementally to a wide range of different input signal strengths. Under these experimental conditions, the cells changed how they respond to input signals some time between 5 and 9 weeks after being born. However, they either behaved like the youngest or like the oldest cells: no intermediate behavior was seen.

Unexpectedly, the switch is not directly related to the age of the cells: cells born at the same time don't necessarily change behavior at the same time, and cells born at different times may behave similarly. Thus, Brunner et al. suggest that it is the experience of the cells, and not their fate, that determines how they help the dentate gyrus function during the investigated period.

DOI:http://dx.doi.org/10.7554/eLife.03104.002

Neurogliaform cells in the molecular layer of the dentate gyrus as feed-forward γ-aminobutyric acidergic modulators of entorhinal-hippocampal interplay

Feed-forward inhibition from molecular layer interneurons onto granule cells (GCs) in the dentate gyrus is thought to have major effects regulating entorhinal-hippocampal interactions, but the precise identity, properties, and functional connectivity of the GABAergic cells in the molecular layer are not well understood. We used single and paired intracellular patch clamp recordings from post-hoc-identified cells in acute rat hippocampal slices and identified a subpopulation of molecular layer interneurons that expressed immunocytochemical markers present in members of the neurogliaform cell (NGFC) class. Single NGFCs displayed small dendritic trees, and their characteristically dense axonal arborizations covered significant portions of the outer and middle one-thirds of the molecular layer, with frequent axonal projections across the fissure into the CA1 and subicular regions. Typical NGFCs exhibited a late firing pattern with a ramp in membrane potential prior to firing action potentials, and single spikes in NGFCs evoked biphasic, prolonged GABA(A) and GABA(B) postsynaptic responses in GCs. In addition to providing dendritic GABAergic inputs to GCs, NGFCs also formed chemical synapses and gap junctions with various molecular layer interneurons, including other NGFCs. NGFCs received low-frequency spontaneous synaptic events, and stimulation of perforant path fibers revealed direct, facilitating synaptic inputs from the entorhinal cortex. Taken together, these results indicate that NGFCs form an integral part of the local molecular layer microcircuitry generating feed-forward inhibition and provide a direct GABAergic pathway linking the dentate gyrus to the CA1 and subicular regions through the hippocampal fissure.

Output of neurogliaform cells to various neuron types in the human and rat cerebral cortex

Neurogliaform cells in the rat elicit combined GABAA and GABAB receptor-mediated postsynaptic responses on cortical pyramidal cells and establish electrical synapses with various interneuron types. However, the involvement of GABAB receptors in postsynaptic effects of neurogliaform cells on other GABAergic interneurons is not clear. We measured the postsynaptic effects of neurogliaform cells in vitro applying simultaneous whole-cell recordings in human and rat cortex. Single action potentials of human neurogliaform cells evoked unitary IPSPs composed of GABAA and GABAB receptor-mediated components in various types of inteneuron and in pyramidal cells. Slow IPSPs were combined with homologous and heterologous electrical coupling between neurogliaform cells and several human interneuron types. In the rat, single action potentials in neurogliaform cells elicited GABAB receptor-mediated component in responses of neurogliaform, regular spiking, and fast spiking interneurons following the GABAA receptor-mediated component in postsynaptic responses. In conclusion, human and rat neurogliaform cells elicit slow IPSPs and reach GABAA and GABAB receptors on several interneuron types with a connection-specific involvement of GABAB receptors. The electrical synapses recorded between human neurogliaform cells and various interneuron types represent the first electrical synapses recorded in the human cortex.

Different transmitter transients underlie presynaptic cell type specificity of GABAA,slow and GABAA,fast

Phasic (synaptic) and tonic (extrasynaptic) inhibition represent the two most fundamental forms of GABA(A) receptor-mediated transmission. Inhibitory postsynaptic currents (IPSCs) generated by GABA(A) receptors are typically extremely rapid synaptic events that do not last beyond a few milliseconds. Although unusually slow GABA(A) IPSCs, lasting for tens of milliseconds, have been observed in recordings of spontaneous events, their origin and mechanisms are not known. We show that neocortical GABA(A,slow) IPSCs originate from a specialized interneuron called neurogliaform cells. Compared with classical GABA(A,fast) IPSCs evoked by basket cells, single spikes in neurogliaform cells evoke extraordinarily prolonged GABA(A) responses that display tight regulation by transporters, low peak GABA concentration, unusual benzodiazepine modulation, and spillover. These results reveal a form of GABA(A) receptor mediated communication by a dedicated cell type that produces slow ionotropic responses with properties intermediate between phasic and tonic inhibition.

Cell type-specific gating of perisomatic inhibition by cholecystokinin

Parvalbumin- and cholecystokinin (CCK)-expressing basket cells provide two parallel, functionally distinct sources of perisomatic inhibition to postsynaptic cells. We show that exogenously applied CCK enhances the output from rat parvalbumin-expressing basket cells, while concurrently suppressing GABA release from CCK-expressing neurons through retrograde endocannabinoid action. These results indicate that CCK may act as a molecular switch that determines the source of perisomatic inhibition for hippocampal principal cells.

Gap-junctional coupling between neurogliaform cells and various interneuron types in the neocortex

Electrical synapses contribute to the generation of synchronous activity in neuronal networks. Several types of cortical GABAergic neurons acting via postsynaptic GABA(A) receptors also form electrical synapses with interneurons of the same class, suggesting that synchronization through gap junctions could be limited to homogenous interneuron populations. Neurogliaform cells elicit combined GABA(A) and GABA(B) receptor-mediated postsynaptic responses in cortical pyramidal cells, but it is not clear whether neurogliaform cells are involved in networks linked by electrical coupling. We recorded from pairs, triplets, and quadruplets of cortical neurons in layers 2 and 3 of rat somatosensory cortex (postnatal day 20-35). Neurogliaform cells eliciting slow IPSPs on pyramidal cells also triggered divergent electrical coupling potentials on interneurons. Neurogliaform cells were electrically coupled to other neurogliaform cells, basket cells, regular-spiking nonpyramidal cells, to an axoaxonic cell, and to various unclassified interneurons showing diverse firing patterns and morphology. Electrical interactions were mediated by one or two electron microscopically verified gap junctions linking the somatodendritic domain of the coupled cells. Our results suggest that neurogliaform cells have a unique position in the cortical circuit. Apart from eliciting combined GABA(A) and GABA(B) receptor-mediated inhibition on pyramidal cells, neurogliaform cells establish electrical synapses and link multiple networks formed by gap junctions restricted to a particular class of interneuron. Widespread electrical connections might enable neurogliaform cells to monitor the activity of different interneurons acting on GABA(A) receptors at various regions of target cells.

Excitatory effect of GABAergic axo-axonic cells in cortical microcircuits

Axons in the cerebral cortex receive synaptic input at the axon initial segment almost exclusively from gamma-aminobutyric acid-releasing (GABAergic) axo-axonic cells (AACs). The axon has the lowest threshold for action potential generation in neurons; thus, AACs are considered to be strategically placed inhibitory neurons controlling neuronal output. However, we found that AACs can depolarize pyramidal cells and can initiate stereotyped series of synaptic events in rat and human cortical networks because of a depolarized reversal potential for axonal relative to perisomatic GABAergic inputs. Excitation and signal propagation initiated by AACs is supported by the absence of the potassium chloride cotransporter 2 in the axon.

Input and frequency-specific entrainment of postsynaptic firing by IPSPs of perisomatic or dendritic origin

Correlated activity of cortical neurons underlies cognitive processes. Networks of several distinct classes of gamma-aminobutyric acid (GABA)ergic interneurons are capable of synchronizing cortical neurons at behaviourally relevant frequencies. Here we show that perisomatic and dendritic GABAergic inputs provided by two classes of GABAergic cells, fast spiking and bitufted interneurons, respectively, entrain the timing of postsynaptic spikes differentially in both pyramidal cells and interneurons at beta and gamma frequencies. Entrainment of pyramidal as well as regular spiking non-pyramidal cells was input site and inhibitory postsynaptic potential frequency dependent. Gamma frequency input from fast spiking cells entrained pyramidal cells on the positive phase of an intrinsic cellular theta oscillation, whereas input from bitufted cells was most effective in gamma frequency entrainment on the negative phase of the theta oscillation. The discharge of regular spiking interneurons was phased at gamma frequency by dendritic input from bitufted cells, but not by perisomatic input from fast spiking cells. Action potentials in fast spiking GABAergic neurons were phased at gamma frequency by both other fast spiking and bitufted cells, regardless of whether the presynaptic GABAergic input was at gamma or beta frequency. The interaction of cell type-specific intrinsic properties and location-selective GABAergic inputs could result in a spatio-temporally regulated synchronization and gating of cortical spike propagation in the network.

Summation of Unitary IPSPs Elicited by Identified Axo-axonic Interneurons

We provided recent experimental evidence that coincident unitary events sum slightly sublinearly when targeting closely located postsynaptic sites. Simultaneous activation of many co-aligned inputs might lead to more significant nonlinear interactions especially in compartments of relatively small diameter. The axon initial segment of pyramidal cells has a limited volume and it receives inputs only from a moderate number of axo-axonic interneurons. We recorded the interaction of unitary axo-axonic inputs targeting a layer 4 pyramidal cell and determined the exact number and position of synapses mediating the effects. Both axo-axonic cells established three synaptic release sites on the axon initial segment of the postsynaptic cell which received a total of 19 synapses. The summation of identified inhibitory postsynaptic potentials (IPSPs) was slightly sublinear (9.4%) and the time course of sublinearity was slower than that of the IPSPs. Repeating the experiment while holding the postsynaptic cell in voltage clamp mode showed linear summation of inhibitory postsynaptic currents (IPSCs), suggesting that a local decrease in driving force could contribute to the sublinear summation measured in voltage recordings. The results indicate that moderate sublinearity during the interaction of neighboring inputs might be preserved in cellular compartments of relatively small volume, even if a considerable portion of all afferents converging to the same domain is simultaneously active.

Identified sources and targets of slow inhibition in the neocortex

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.

Mechanism of 4-aminopyridine block of the transient outward K-current in identified Helix neuron

The block of the transient outward K-current, I(K(A)) by 4-aminopyridine (4-AP) and blood-depressing substances (BDS) was investigated in identified Helix pomatia neurons (LPa3) using the two microelectrode voltage-clamp technique. The present study shows that 4-AP inhibits I(K(A)) in snail neurons in a voltage- and concentration-dependent manner. The 4-AP block of I(K(A)) involves the block of both open and closed states of the channel, however binding to open channels is preferred. It is suggested that 4-AP have two binding sites on the identified Helix neuron. One site causes an open channel block, which affects the N-type inactivation, and binding to the second site induces closed channel block, which affects C-type inactivation. In control solution the inactivating phase of the current is biexponential, suggesting simultaneous presence of two types of inactivation. The counterplay of these mechanisms results in the crossover of the current traces recorded from control and 4-AP blocked channels. It is assumed that use-dependence does not occur through blocker 'trapping', but rather by a different mechanism. BDS had no effect on Helix I(K(A)), suggesting that transient potassium channels in LPa3 neuron are not Kv3.4 type channels.

Cell type- and subcellular position-dependent summation of unitary postsynaptic potentials in neocortical neurons

Theoretical studies predict that the modes of integration of coincident inputs depend on their location and timing. To test these models experimentally, we simultaneously recorded from three neocortical neurons in vitro and investigated the effect of the subcellular position of two convergent inputs on the response summation in the common postsynaptic cell. When scattered over the somatodendritic surface, combination of two coincident excitatory or inhibitory synaptic potentials summed linearly in layer 2/3 pyramidal cells, as well as in GABAergic interneurons. Slightly sublinear summation with connection specific kinetics was observed when convergent inputs targeted closely placed sites on the postsynaptic cell. The degree of linearity of summation also depended on the type of connection, the relative timing of inputs, and the activation state of I(h). The results suggest that, when few inputs are active, the majority of afferent permutations undergo linear integration, maintaining the importance of individual inputs. However, compartment- and connection-specific nonlinear interactions between synapses located close to each other could increase the computational power of individual neurons in a cell type-specific manner.

Beta and gamma frequency synchronization by dendritic gabaergic synapses and gap junctions in a network of cortical interneurons

Distinct interneuron populations innervate perisomatic and dendritic regions of cortical cells. Perisomatically terminating GABAergic inputs are effective in timing postsynaptic action potentials, and basket cells synchronize each other via gap junctions combined with neighboring GABAergic synapses. The function of dendritic GABAergic synapses in cortical rhythmicity, and their interaction with electrical synapses is not understood. Using multiple whole-cell recordings in layers 2-3 of rat somatosensory cortex combined with light and electron microscopic determination of sites of interaction, we studied the interactions between regular spiking nonpyramidal cells (RSNPCs). Random samples of unlabeled postsynaptic targets showed that RSNPCs placed GABAergic synapses onto dendritic spines (53 +/- 12%) and shafts (45 +/- 10%) and occasionally somata (2 +/- 4%). GABAergic interactions between RSNPCs were mediated by 4 +/- 2 axodendritic synapses and phased postsynaptic activity at beta frequency but were ineffective in phasing at gamma rhythm. Electrical interactions of RSNPCs were transmitted via two to eight gap junctions between dendritic shafts and/or spines. Elicited at beta and gamma frequencies, gap junctional potentials timed postsynaptic spikes with a phase lag, however strong electrical coupling could synchronize presynaptic and postsynaptic activity. Combined unitary GABAergic and gap junctional connections of moderate strength produced beta and gamma frequency synchronization of the coupled RSNPCs. Our results provide evidence that dendritic GABAergic and/or gap junctional mechanisms effectively transmit suprathreshold information in a population of interneurons at behaviorally relevant frequencies. A coherent network of GABAergic cells targeting the dendrites could provide a pathway for rhythmic activity spatially segregated from perisomatic mechanisms of synchronization.

Pre- and postsynaptic effects of eugenol and related compounds on Helix pomatia L. neurons

We showed how eugenol blocks the synaptic transmission and gave a possible interpretation how it inhibits the excitation-contraction coupling that several authors described previously. Eugenol acts both in the pre- and postsynaptic side of the neurons. It blocks the Ca2+-currents, decreases the membrane potential of the neurons, increases the inward resistance and decreases the GABA, ACh and glutamate evoked excitatory responses in submillimolar concentration.