Multichannel tetrode array recording in awake behaving animals provides a powerful method to record the activity of large numbers of neurons. The power of this method could be extended if further information concerning the intracellular state of the neurons could be extracted from the extracellularly recorded signals. Toward this end, we have simultaneously recorded intracellular and extracellular signals from hippocampal CA1 pyramidal cells and interneurons in the anesthetized rat. We found that several intracellular parameters can be deduced from extracellular spike waveforms. The width of the intracellular action potential is defined precisely by distinct points on the extracellular spike. Amplitude changes of the intracellular action potential are reflected by changes in the amplitude of the initial negative phase of the extracellular spike, and these amplitude changes are dependent on the state of the network. In addition, intracellular recordings from dendrites with simultaneous extracellular recordings from the soma indicate that, on average, action potentials are initiated in the perisomatic region and propagate to the dendrites at 1.68 m/s. Finally we determined that a tetrode in hippocampal area CA1 theoretically should be able to record electrical signals from approximately 1, 000 neurons. Of these, 60-100 neurons should generate spikes of sufficient amplitude to be detectable from the noise and to allow for their separation using current spatial clustering methods. This theoretical maximum is in contrast to the approximately six units that are usually detected per tetrode. From this, we conclude that a large percentage of hippocampal CA1 pyramidal cells are silent in any given behavioral condition.

Septo-hippocampal GABAergic neurons immunoreactive for parvalbumin are thought to play a crucial role in the generation of hippocampal theta oscillations associated with a specific stage of memory formation. Here we use in vivo juxtacellular recording and filling in the medial septum followed by immunocytochemical identification of the recorded cells containing parvalbumin to determine their firing pattern, phase relationship with hippocampal theta, morphology, and to thereby reveal their involvement in the generation of hippocampal theta activity. We have demonstrated that GABAergic medial septal neurons form two distinct populations exhibiting highly regular bursting activity that is tightly coupled to either the trough (178°) or the peak (330°) of hippocampal theta waves. Additionally, different types of bursting as well as nonbursting activity patterns were also observed. The morphological reconstruction of theta-bursting neurons revealed extensive axon arbors of these cells with numerous local collaterals establishing symmetrical synapses; thus, synchrony among the septal pacemaker units may be brought about by their recurrent collateral interactions. Long projecting axons could also be found running dorsally toward the hippocampus and ventrally in the direction of basal forebrain regions. We conclude that GABAergic neurons in the medial septum, which are known to selectively innervate hippocampal interneurons, are in a position to induce rhythmic disinhibition in the hippocampus and other theta-related subcortical areas at two different phases of hippocampal theta.

Information processing in the hippocampus critically relies on its reciprocal interaction with the medial septum (MS). Synchronization of the septo-hippocampal system was demonstrated during both major hippocampal activity states, the regular theta rhythm and the large amplitude irregular activity. Previous experimental and modeling data suggest that the MS provides rhythmic drive to the hippocampus, and hippocampo-septal feedback synchronizes septal pacemaker units. However, this view has recently been questioned based on the possibility of intrahippocampal theta genesis. Previously, we identified putative pacemaker neurons expressing parvalbumin (PV) and/or the pacemaker hyperpolarization-activated and cyclic nucleotide-gated nonselective cation channel (HCN) in the MS. In this study, by analyzing the temporal relationship of activity between the PV/HCN-containing medial septal neurons and hippocampal local field potential, we aimed to uncover whether the sequence of events during theta formation supports the classic view of septal drive or the challenging theory of hippocampal pacing of theta. Importantly, by implementing a circular statistical method, a temporal lead of these septal neurons over the hippocampus was observed on the course of theta synchronization. Moreover, the activity of putative hippocampal interneurons also preceded hippocampal local field theta, but by a shorter time period compared with PV/HCN-containing septal neurons. Using the concept of mutual information, the action potential series of PV/HCN-containing neurons shared higher amount of information with hippocampal field oscillation than PV/HCN-immunonegative cells. Thus, a pacemaker neuron population of the MS leads hippocampal activity, presumably via the synchronization of hippocampal interneurons.

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...

Cortical information processing is under state-dependent control of subcortical neuromodulatory systems. Although this modulatory effect is thought to be mediated mainly by slow nonsynaptic metabotropic receptors, other mechanisms, such as direct synaptic transmission, are possible. Yet, it is currently unknown if any such form of subcortical control exists. Here, we present direct evidence of a strong, spatiotemporally precise excitatory input from an ascending neuromodulatory center. Selective stimulation of serotonergic median raphe neurons produced a rapid activation of hippocampal interneurons. At the network level, this subcortical drive was manifested as a pattern of effective disynaptic GABAergic inhibition that spread throughout the circuit. This form of subcortical network regulation should be incorporated into current concepts of normal and pathological cortical function.