János Szabadics

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 GABAA receptor-mediated transmission. Inhibitory postsynaptic currents (IPSCs) generated by GABAA receptors are typically extremely rapid synaptic events that do not last beyond a few milliseconds. Although unusually slow GABAA 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 GABAA,slow IPSCs originate from a specialized interneuron called neurogliaform cells. Compared with classical GABAA,fast IPSCs evoked by basket cells, single spikes in neurogliaform cells evoke extraordinarily prolonged GABAA responses that display tight regulation by transporters, low peak GABA concentration, unusual benzodiazepine modulation, and spillover. These results reveal a form of GABAA receptor mediated communication by a dedicated cell type that produces slow ionotropic responses with properties intermediate between phasic and tonic inhibition.

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 GABAA 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 GABAA and GABAB 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 GABAA and GABAB 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 GABAA receptors at various regions of target cells.

Complex Events Initiated by Individual Spikes in the Human Cerebral Cortex

Synaptic interactions between neurons of the human cerebral cortex were not directly studied to date. We recorded the first dataset, to our knowledge, on the synaptic effect of identified human pyramidal cells on various types of postsynaptic neurons and reveal complex events triggered by individual action potentials in the human neocortical network. Brain slices were prepared from nonpathological samples of cortex that had to be removed for the surgical treatment of brain areas beneath association cortices of 58 patients aged 18 to 73 y. Simultaneous triple and quadruple whole-cell patch clamp recordings were performed testing mono- and polysynaptic potentials in target neurons following a single action potential fired by layer 2/3 pyramidal cells, and the temporal structure of events and underlying mechanisms were analyzed. In addition to monosynaptic postsynaptic potentials, individual action potentials in presynaptic pyramidal cells initiated long-lasting (37 ± 17 ms) sequences of events in the network lasting an order of magnitude longer than detected previously in other species. These event series were composed of specifically alternating glutamatergic and GABAergic postsynaptic potentials and required selective spike-to-spike coupling from pyramidal cells to GABAergic interneurons producing concomitant inhibitory as well as excitatory feed-forward action of GABA. Single action potentials of human neurons are sufficient to recruit Hebbian-like neuronal assemblies that are proposed to participate in cognitive processes.

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.

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.

Functional Specificity of Mossy Fiber Innervation of GABAergic Cells in the Hippocampus

One million mossy fibers in the rat provide individually sparse but functionally important synaptic connections between the dentate gyrus and hippocampus. Although the majority of mossy fiber targets are GABAergic cells, the functional organization of the feedforward GABAergic machinery modulating the interactions of granule cells and CA3 pyramidal cells are not yet understood. We used mossy fiber bouton to GABA neuron paired recordings in the CA3 to demonstrate that mossy fibers provide cell type-specific innervation to distinct GABAergic neurons with specialized intra- and extrahippocampal outputs. Our results show that mossy fibers contact the perisomatically projecting fast-spiking and regular-spiking basket cells, in addition to the dendritically projecting ivy cells, and the septum-projecting spiny stratum lucidum cells. Monosynaptic mossy fiber inputs to fast-spiking basket cells and spiny stratum lucidum cells were found to be numerous, but they were small in amplitude and displayed low transmission probabilities. In contrast, regular-spiking basket cells and ivy cells were less likely to be innervated by mossy fibers, but the amplitudes of mossy fiber EPSCs were large and the transmission probabilities were high. The dependence of the numbers and strengths of the mossy fiber inputs to CA3 GABAergic cells on the postsynaptic cell type was correlated with the frequency of the background synaptic events, so that cells with weak but numerous mossy fiber inputs received high rates of spontaneous synaptic events. Together, these results reveal the diverse components and high degree of functional specificity of the GABAergic cellular machinery underlying the dentate gyrus–CA3 interface.

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 γ‐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.

Granule Cells in the CA3 Area

A fundamental property of neuronal networks in Ammons horn is that each area comprises a single glutamatergic cell population and various types of GABAergic neurons. Here we describe an exception to this rule, in the form of granule cells that reside within the CA3 area and function as glutamatergic nonprincipal cells with distinct properties. CA3 granule cells in normal, healthy rats, similarly to dentate gyrus granule cells, coexpressed calbindin and the homeobox protein Prox1. However, CA3 granule cells were located outside of the dentate gyrus, often hundreds of micrometers from the hilar border, in the lucidum and radiatum layers. CA3 granule cells were present in numbers that were comparable to the rarer GABAergic neuronal subtypes, and their somato-dendritic morphology, intrinsic properties, and perforant path inputs were similar to those of dentate gyrus granule cells. CA3 granule cell axons displayed giant mossy fiber terminals with filopodial extensions, demonstrating that not all mossy fibers originate from the dentate gyrus. Somatic paired recordings revealed that CA3 granule cells innervated CA3 pyramidal and GABAergic cells similarly to conventional mossy fiber synapses. However, CA3 granule cells were distinct in the specific organization of their GABAergic inputs. They received GABAergic synapses from cholecystokinin-expressing mossy fiber-associated cells that did not innervate the dentate granule cell layer, and these synapses demonstrated unusually strong activity-dependent endocannabinoid-mediated inhibition of GABA release. These results indicate that granule cells in the CA3 constitute a glutamatergic, nonprincipal neuronal subtype that is integrated into the CA3 synaptic network.

Cell-Type-Specific CCK2 Receptor Signaling Underlies the Cholecystokinin-Mediated Selective Excitation of Hippocampal Parvalbumin-Positive Fast-Spiking Basket Cells

Parvalbumin-positive (PV+) fast-spiking basket cells are thought to play key roles in network functions related to precise time keeping during behaviorally relevant hippocampal synchronous oscillations. Although they express relatively few receptors for neuromodulators, the highly abundant and functionally important neuropeptide cholecystokinin (CCK) is able to selectively depolarize PV+ basket cells, making these cells sensitive biosensors for CCK. However, the molecular mechanisms underlying the CCK-induced selective and powerful excitation of PV+ basket cells are not understood. We used single and paired patch-clamp recordings in acute rat hippocampal slices, in combination with post hoc identification of the recorded interneurons, to demonstrate that CCK acts via G-protein-coupled CCK2 receptors to engage sharply divergent intracellular pathways to exert its cell-type-selective effects. In contrast to CCK2 receptors on pyramidal cells that signal through the canonical Gq–PLC pathway to trigger endocannabinoid-mediated signaling events, CCK2 receptors on neighboring PV+ basket cells couple to an unusual, pertussis-toxin-sensitive pathway. The latter pathway involves ryanodine receptors on intracellular calcium stores that ultimately activate a nonselective cationic conductance to depolarize PV+ basket cells. CCK has highly cell-type-selective effects even within the PV+ cell population, as the PV+ dendrite-targeting bistratified cells do not respond to CCK. Together, these results demonstrate that an abundant ligand such as CCK can signal through the same receptor in different neurons to use cell-type-selective signaling pathways to provide divergence and specificity to its effects.

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