A large variety of GABAergic interneurons control information processing in hippocampal circuits governing the formation of neuronal representations. Whether distinct hippocampal interneuron types contribute differentially to information-processing during behavior is not known. We employed a novel technique for recording and labeling interneurons and pyramidal cells in drug-free, freely-moving rats. Recorded parvalbumin-expressing basket interneurons innervate somata and proximal pyramidal cell dendrites, whereas nitric-oxide-synthase-and neuropeptide-Y-expressing ivy cells provide synaptic and extrasynaptic dendritic modulation. Basket and ivy cells showed distinct spike timing dynamics, firing at different rates and times during theta and ripple oscillations. Basket but not ivy cells changed their firing rates during movement, sleep and quiet wakefulness, suggesting that basket cells coordinate cell assemblies in a behavioral state-contingent manner, whereas persistently-firing ivy cells might control network excitability and homeostasis. Different interneuron types provide GABA to specific subcellular domains at defined times and rates, thus differentially controlling network activity during behavior.GABAergic interneurons control information processing in cortical circuits as percussionists set the rhythm for a melody, or traffic lights regulate the movement of cars through a city. Interneurons generate oscillatory activity 1, 2 , synchronize the activity of pyramidal cells 3 and set time windows for synaptic integration 4 . A large diversity of interneuronal types is a hallmark of cortical circuits. Different domains of pyramidal cells, such as the soma, axoninitial-segment, proximal or distal dendrites 5 are innervated by distinct types of GABAergic interneuron. They also have distinct inputs and membrane properties 6-10 and show different firing patterns during network oscillations induced in vitro [11][12][13][14] or recorded in anesthetized animals 15 , indicating distinct roles for specific interneuron types. However, research on interneurons in drug-free animals that can freely change their behavior, has so far been limited to recordings from unidentified interneurons because of technical limitations. In the barrel cortex of head-restrained mice, groups of interneurons with distinct membrane dynamics during different behavioral states have been described 16,17 and in the hippocampus unidentified interneurons or interneurons belonging to heterogeneous groups expressing parvalbumin and/or somatostatin have been reported [18][19][20][21] to fire with different firing patterns during network oscillations. But, how do specific types of identified interneurons control the activity of cortical circuits in freely-moving animals? CouldCorrespondence and requests for materials should be addressed to D.L. (damien.lapray@pharm.ox.ac.uk) Results Identification of neurons recorded in freely-moving ratsWe recorded the activity of parvalbumin (PV)-expressing basket, ivy and pyramidal cells in the dorsal CA...

Hippocampal sharp waves are population discharges initiated by an unknown mechanism in pyramidal cell networks of CA3. Axo-axonic cells (AACs) regulate action potential generation through GABAergic synapses on the axon initial segment. We found that CA3 AACs in anesthetized rats and AACs in freely moving rats stopped firing during sharp waves, when pyramidal cells fire most. AACs fired strongly and rhythmically around the peak of theta oscillations, when pyramidal cells fire at low probability. Distinguishing AACs from other parvalbumin-expressing interneurons by their lack of detectable SATB1 transcription factor immunoreactivity, we discovered a somatic GABAergic input originating from the medial septum that preferentially targets AACs. We recorded septo-hippocampal GABAergic cells that were activated during hippocampal sharp waves and projected to CA3. We hypothesize that inhibition of AACs, and the resulting subcellular redistribution of inhibition from the axon initial segment to other pyramidal cell domains, is a necessary condition for the emergence of sharp waves promoting memory consolidation.During slow wave sleep (SWS), quiet wakefulness and consummatory behavior, largeamplitude, 30-120-ms-duration 'sharp wave' voltage deflections have been observed in extracellular recordings throughout the mammalian hippocampal formation 1 , which occur simultaneously with 130-230 Hz 'ripples' most pronounced in stratum pyramidale (sPyr) of CA1 (refs. 2,3). These sharp wave-ripple complexes (SWRs) are required for memory © 2013 Nature America, Inc. All rights reserved.Reprints and permissions information is available online at http://www.nature.com/reprints/index.html.Correspondence should be addressed to T.J.V. (tim.viney@pharm.ox.ac.uk), B.L. (balint.lasztoczi@meduniwien.ac.at) or P.S. (peter.somogyi@pharm.ox.ac.uk). AUTHOR CONTRIBUTIONS T.J.V., B.L., L.K., M.G.C., J.J.T., T.K. and P.S. collected and analyzed data and wrote the paper. To expand on the equalcontributions footnote, each of the first four authors made important-though different-contributions, and hence they should be considered equal first authors.Note: Any Supplementary Information and Source Data files are available in the online version of the paper. COMPETING FINANCIAL INTERESTSThe authors declare no competing financial interests. Europe PMC Funders GroupAuthor Manuscript Nat Neurosci. Author manuscript; available in PMC 2015 June 18. Published in final edited form as:Nat Neurosci. 2013 December ; 16(12): 1802-1811. doi:10.1038/nn.3550. Europe PMC Funders Author ManuscriptsEurope PMC Funders Author Manuscripts consolidation 4 and have been postulated to originate from groups of pyramidal neurons in CA3 participating in a synchronous 'population burst', which is transmitted to the downstream CA1 region via Schaffer collaterals 5 . A proposed mechanism for SWR initiation is through disinhibition of pyramidal cells via dynamic state-dependent interactions between GABAergic neurons and glutamatergic inputs 1 . Indeed, in vivo recordings show ...

The temporary interaction of distinct gamma oscillators effects binding, association, and information routing. How independent gamma oscillations are generated and maintained by pyramidal cells and interneurons within a cortical circuit remains unknown. We recorded the spike timing of identified parvalbumin-expressing basket cells in the CA1 hippocampus of anesthetized rats and simultaneously detected layer-specific gamma oscillations using current-source-density analysis. Spike timing of basket cells tuned the phase and amplitude of gamma oscillations generated around stratum pyramidale, where basket cells selectively innervate pyramidal cells with GABAergic synapses. Basket cells did not contribute to gamma oscillations generated at the apical tuft of pyramidal cells. This gamma oscillation was selectively modulated by a subset of local GABAergic interneurons and by medial entorhinal cortex layer 3 neurons. The generation of independent and layer-specific gamma oscillations, implemented onto hippocampal pyramidal cells along their somato-dendritic axis, can be explained by selective axonal targeting and precisely controlled temporal firing of GABAergic interneurons.

The behaviour-contingent rhythmic synchronization of neuronal activity is reported by local field potential oscillations in the theta, gamma and sharp wave-related ripple (SWR) frequency ranges. In the hippocampus, pyramidal cell assemblies representing temporal sequences are coordinated by GABAergic interneurons selectively innervating specific postsynaptic domains, and discharging phase locked to network oscillations. We compare the cellular network dynamics in the CA1 and CA3 areas recorded with or without anaesthesia. All parts of pyramidal cells, except the axon initial segment, receive GABA from multiple interneuron types, each with distinct firing dynamics. The axon initial segment is exclusively innervated by axo-axonic cells, preferentially firing after the peak of the pyramidal layer theta cycle, when pyramidal cells are least active. Axo-axonic cells are inhibited during SWRs, when many pyramidal cells fire synchronously. This dual inverse correlation demonstrates the key inhibitory role of axo-axonic cells. Parvalbumin-expressing basket cells fire phase locked to field gamma activity in both CA1 and CA3, and also strongly increase firing during SWRs, together with dendrite-innervating bistratified cells, phasing pyramidal cell discharge. Subcellular domain-specific GABAergic innervation probably developed for the coordination of multiple glutamatergic inputs on different parts of pyramidal cells through the temporally distinct activity of GABAergic interneurons, which differentially change their firing during different network states.

Three distinct gamma oscillations, generated in different CA1 layers, occur at different phases of concurrent theta oscillation. In parallel, firing of place cells displays phase advancement over successive cycles of theta oscillations while an animal passes through the place field. Is the theta-phase-precessing output of place cells shaped by distinct gamma oscillations along different theta phases during place field traversal? We simultaneously recorded firing of place cells and three layer-specific gamma oscillations using current-source-density analysis of multi-site field potential measurements in mice. We show that spike timing of place cells can tune to all three gamma oscillations, but phase coupling to the mid-frequency gamma oscillation conveyed from the entorhinal cortex was restricted to leaving a place field. A subset of place cells coupled to two different gamma oscillations even during single-place field traversals. Thus, an individual CA1 place cell can combine and relay information from multiple gamma networks while the animal crosses the place field.

BackgroundGlutamate (Glu) and γ-aminobutyric acid (GABA) transporters play important roles in regulating neuronal activity. Glu is removed from the extracellular space dominantly by glial transporters. In contrast, GABA is mainly taken up by neurons. However, the glial GABA transporter subtypes share their localization with the Glu transporters and their expression is confined to the same subpopulation of astrocytes, raising the possibility of cooperation between Glu and GABA transport processes.Methodology/Principal FindingsHere we used diverse biological models both in vitro and in vivo to explore the interplay between these processes. We found that removal of Glu by astrocytic transporters triggers an elevation in the extracellular level of GABA. This coupling between excitatory and inhibitory signaling was found to be independent of Glu receptor-mediated depolarization, external presence of Ca2+ and glutamate decarboxylase activity. It was abolished in the presence of non-transportable blockers of glial Glu or GABA transporters, suggesting that the concerted action of these transporters underlies the process.Conclusions/SignificanceOur results suggest that activation of Glu transporters results in GABA release through reversal of glial GABA transporters. This transporter-mediated interplay represents a direct link between inhibitory and excitatory neurotransmission and may function as a negative feedback combating intense excitation in pathological conditions such as epilepsy or ischemia.

Hippocampal CA3 area generates temporally structured network activity such as sharp waves and gamma and theta oscillations. Parvalbumin-expressing basket cells, making GABAergic synapses onto cell bodies and proximal dendrites of pyramidal cells, control pyramidal cell activity and participate in network oscillations in slice preparations, but their roles in vivo remain to be tested. We have recorded the spike timing of parvalbumin-expressing basket cells in areas CA2/3 of anesthetized rats in relation to CA3 putative pyramidal cell firing and activity locally and in area CA1. During theta oscillations, CA2/3 basket cells fired on the same phase as putative pyramidal cells, but, surprisingly, significantly later than downstream CA1 basket cells. This indicates a distinct modulation of CA3 and CA1 pyramidal cells by basket cells, which receive different inputs. We observed unexpectedly large dendritic arborization of CA2/3 basket cells in stratum lacunosum moleculare (33% of length, 29% surface, and 24% synaptic input from a total of ϳ35,000), different from the dendritic arborizations of CA1 basket cells. Area CA2/3 basket cells fired phase locked to both CA2/3 and CA1 gamma oscillations, and increased firing during CA1 sharp waves, thus supporting the role of CA3 networks in the generation of gamma oscillations and sharp waves. However, during ripples associated with sharp waves, firing of CA2/3 basket cells was phase locked only to local but not CA1 ripples, suggesting the independent generation of fast oscillations by basket cells in CA1 and CA2/3. The distinct spike timing of basket cells during oscillations in CA1 and CA2/3 suggests differences in synaptic inputs paralleled by differences in dendritic arborizations.

Hippocampal oscillations reflect coordinated neuronal activity on many timescales. Distinct types of GABAergic interneuron participate in the coordination of pyramidal cells over different oscillatory cycle phases. In the CA3 area, which generates sharp waves and gamma oscillations, the contribution of identified GABAergic neurons remains to be defined. We have examined the firing of a family of cholecystokinin-expressing interneurons during network oscillations in urethane-anesthetized rats and compared them with firing of CA3 pyramidal cells. The position of the terminals of individual visualized interneurons was highly diverse, selective, and often spatially coaligned with either the entorhinal or the associationalinputstoareaCA3.Thespiketiminginrelationtothetaandgammaoscillationsandsharpwaveswascorrelatedwiththeinnervated pyramidal cell domain. Basket and dendritic-layer-innervating interneurons receive entorhinal and associational inputs and preferentially fire on the ascending theta phase, when pyramidal cell assemblies emerge. Perforant-path-associated cells, driven by recurrent collaterals of pyramidal cells fire on theta troughs, when established pyramidal cell assemblies are most active. In the CA3 area, slow and fast gamma oscillations occurred on opposite theta oscillation phases. Perforant-path-associated and some COUP-TFII-positive interneurons are strongly coupled to both fast and slow gamma oscillations, but basket and dendritic-layer-innervating cells are weakly coupled to fast gamma oscillations only. During sharp waves, different interneuron types are activated, inhibited, or remain unaffected. We suggest that specialization in pyramidal cell domain and glutamatergic input-specific operations, reflected in the position of GABAergic terminals, is the evolutionary drive underlying the diversity of cholecystokinin-expressing interneurons.