Alpha 5 subunit-containing GABAA receptors affect the dynamic range of mouse hippocampal kainate-induced gamma frequency oscillations in vitro.

Though all in vitro models of gamma frequency network oscillations are critically dependent on GABAA receptor-mediated synaptic transmission little is known about the specific role played by different subtypes of GABAA receptor. Strong expression of the alpha5 subunit of the GABAA receptor is restricted to few brain regions, amongst them the hippocampal dendritic layers. Receptors containing this subunit may be expressed on the extrasynaptic membrane of principal cells and can mediate a tonic GABAA conductance. Using hippocampal slices of wild-type (WT) and alpha5-/- mice we investigated the role of alpha5 subunits in the generation of kainate-induced gamma frequency oscillations (20-80 Hz). The change in power of the oscillations evoked in CA3 by increasing network drive (kainate, 50-400 nm) was significantly greater in alpha5-/- than in WT slices. However, the change in frequency of gamma oscillations with increasing network drive seen in WT slices was absent in alpha5-/- slices. Raising the concentration of extracellular GABA by bathing slices in the GABA transaminase inhibitor vigabatrin and blocking uptake with tiagabine reduced the power of gamma oscillations more in WT slices than alpha5-/- slices (43%versus 15%). The data suggest that loss of this GABAA receptor subunit alters the dynamic profile of gamma oscillations to changes in network drive, possibly via actions of GABA at extrasynaptic receptors.

GABA-enhanced collective behavior in neuronal axons underlies persistent gamma-frequency oscillations.

Gamma (30-80 Hz) oscillations occur in mammalian electroencephalogram in a manner that indicates cognitive relevance. In vitro models of gamma oscillations demonstrate two forms of oscillation: one occurring transiently and driven by discrete afferent input and the second occurring persistently in response to activation of excitatory metabotropic receptors. The mechanism underlying persistent gamma oscillations has been suggested to involve gap-junctional communication between axons of principal neurons, but the precise relationship between this neuronal activity and the gamma oscillation has remained elusive. Here we demonstrate that gamma oscillations coexist with high-frequency oscillations (>90 Hz). High-frequency oscillations can be generated in the axonal plexus even when it is physically isolated from pyramidal cell bodies. They were enhanced in networks by nonsomatic gamma-aminobutyric acid type A (GABA(A)) receptor activation, were modulated by perisomatic GABAA receptor-mediated synaptic input to principal cells, and provided the phasic input to interneurons required to generate persistent gamma-frequency oscillations. The data suggest that high-frequency oscillations occurred as a consequence of random activity within the axonal plexus. Interneurons provide a mechanism by which this random activity is both amplified and organized into a coherent network rhythm.

A model of atropine-resistant theta oscillations in rat hippocampal area CA1.

Theta frequency oscillations are a predominant feature of rhythmic activity in the hippocampus. We demonstrate that hippocampal area CA1 generates atropine-resistant theta population oscillations in response to metabotropic glutamate receptor activation under conditions of reduced AMPA receptor activation. This activity occurred in the absence of inputs from area CA3 and extra-ammonic areas. Field theta oscillations were co-expressed with pyramidal distal apical dendritic burst spiking and were temporally related to trains of IPSPs with slow kinetics. Pyramidal somatic responses showed theta oscillations consisted of compound inhibitory synaptic potentials with initial IPSPs with slow kinetics followed by trains of smaller, faster IPSPs. Pharmacological modulation of IPSPs altered the theta oscillation suggesting an inhibitory network origin. Somatic IPSPs, dendritic burst firing and stratum pyramidale interneuron activity were all temporally correlated with spiking in stratum oriens interneurons demonstrating intrinsic theta-frequency oscillations. Disruption of spiking in these interneurons was accompanied by a loss of both field theta and theta frequency IPSP trains. We suggest that population theta oscillations can be generated as a consequence of intrinsic theta frequency spiking activity in a subset of stratum oriens interneurons controlling electrogenesis in pyramidal cell apical dendrites.

Gap junctions between interneuron dendrites can enhance synchrony of gamma oscillations in distributed networks.

Gamma-frequency (30-70 Hz) oscillations in populations of interneurons may be of functional relevance in the brain by virtue of their ability to induce synchronous firing in principal neurons. Such a role would require that neurons, 1 mm or more apart, be able to synchronize their activity, despite the presence of axonal conduction delays and of the limited axonal spread of many interneurons. We showed previously that interneuron doublet firing can help to synchronize gamma oscillations, provided that sufficiently many pyramidal neurons are active; we also suggested that gap junctions, between the axons of principal neurons, could contribute to the long-range synchrony of gamma oscillations induced in the hippocampus by carbachol in vitro. Here we consider interneuron network gamma: that is, gamma oscillations in pharmacologically isolated networks of tonically excited interneurons, with frequency gated by mutual GABA(A) receptor-mediated IPSPs. We provide simulation and electrophysiological evidence that interneuronal gap junctions (presumably dendritic) can enhance the synchrony of such gamma oscillations, in spatially extended interneuron networks. There appears to be a sharp threshold conductance, below which the interneuron dendritic gap junctions do not exert a synchronizing role.

Axo-axonal coupling. a novel mechanism for ultrafast neuronal communication.

We provide physiological, pharmacological, and structural evidence that axons of hippocampal principal cells are electrically coupled, with prepotentials or spikelets forming the physiological substrate of electrical coupling as observed in cell somata. Antidromic activation of neighboring axons induced somatic spikelet potentials in neurons of CA3, CA1, and dentate gyrus areas of rat hippocampal slices. Somatic invasion by these spikelets was dependent on the activation of fast Na(+) channels in the postjunctional neuron. Antidromically elicited spikelets were suppressed by gap junction blockers and low intracellular pH. Paired axo-somatic and somato-dendritic recordings revealed that the coupling potentials appeared in the axon before invading the soma and the dendrite. Using confocal laser scanning microscopy we found that putative axons of principal cells were dye coupled. Our data thus suggest that hippocampal neurons are coupled by axo-axonal junctions, providing a novel mechanism for very fast electrical communication.

Impaired electrical signaling disrupts gamma frequency oscillations in connexin 36-deficient mice.

Neural processing occurs in parallel in distant cortical areas even for simple perceptual tasks. Associated cognitive binding is believed to occur through the interareal synchronization of rhythmic activity in the gamma (30-80 Hz) range. Such oscillations arise as an emergent property of the neuronal network and require conventional chemical neurotransmission. To test the potential role of gap junction-mediated electrical signaling in this network property, we generated mice lacking connexin 36, the major neuronal connexin. Here we show that the loss of this protein disrupts gamma frequency network oscillations in vitro but leaves high frequency (150 Hz) rhythms, which may involve gap junctions between principal cells (Schmitz et al., 2001), unaffected. Thus, specific connexins differentially deployed throughout cortical networks are likely to regulate different functional aspects of neuronal information processing in the mature brain.

A possible role for gap junctions in generation of very fast EEG oscillations preceding the onset of, and perhaps initiating, seizures.

PURPOSE: We propose an experimentally and clinically testable hypothesis, concerning the origin of very fast (> approximately 70 Hz) EEG oscillations that sometimes precede the onset of focal seizures. These oscillations are important, as they may play a causal role in the initiation of seizures. METHODS: Subdural EEG recordings were obtained from children with focal cortical dysplasias and intractable seizures. Intra- and extracellular recordings were performed in rat hippocampal slices, with induction of population activity, as follows: (a) bath-applied tetramethylamine (an intracellular alkalinizing agent, that opens gap junctions); (b) bath-applied carbachol, a cholinergic agonist; and (c) focal pressure ejection of hypertonic K+ solution. Detailed network simulations were performed, the better to understand the cellular mechanisms underlying oscillations. A major feature of the simulations was inclusion of axon-axon gap junctions between principal neurons, as supported by recent experimental data. RESULTS: Very fast oscillations were found in children before seizure onset, but also superimposed on bursts during the seizure, and on interictal bursts. In slice experiments, very fast oscillations had previously been seen on interictal-like bursts; we now show such oscillations before, between, and after epileptiform bursts. Very fast oscillations were also seen superimposed on gamma (30-70 Hz) oscillations induced by carbachol or hypertonic K+, and in the latter case, very fast oscillations became continuous when chemical synapses were blocked. Simulations replicate these data, when axonal gap junctions are included. CONCLUSIONS: Electrical coupling between principal neurons, perhaps via axonal gap junctions, could underlie very fast population oscillations, in seizure-prone brain, but possibly also in normal brain. The anticonvulsant potential of gap-junction blockers such as carbenoxolone, now in clinical use for treatment of ulcer disease, should be considered.

Differential expression of synaptic and nonsynaptic mechanisms underlying stimulus-induced gamma oscillations in vitro.

Gamma frequency oscillations occur in hippocampus in vitro after brief tetani delivered to afferent pathways. Previous reports have characterized these oscillations as either (1) trains of GABA(A) inhibitory synaptic events mediated by depolarization of both pyramidal cells and interneurons at least in part mediated by metabotropic glutamate and acetylcholine receptors, or (2) field potential oscillations occurring in the near absence of an inhibitory synaptic oscillation when cells are driven by depolarizing GABA responses and local synchrony is produced by field effects. The aim of this study was to investigate factors involved in the differential expression of these synaptically and nonsynaptically gated oscillations. Field effects were undetectable in control recordings but manifested when slices were perfused with hypo-osmotic solutions or a reduced level of normal perfusate. These manipulations also reduced the amplitude of the train of inhibitory synaptic events associated with an oscillation and enhanced the depolarizing GABA component underlying the post-tetanic depolarization. The resulting field oscillation was still dependent, at least in part, on inhibitory synaptic transmission, but spatiotemporal aspects of the oscillation were severely disrupted. These changes were also accompanied by an increase in estimated [K(+)](o) compared with control. We suggest that nonsynaptic oscillations occur under conditions also associated with epileptiform activity and constitute a phenomenon that is distinct from synaptically gated oscillations. The latter remain a viable model for in vivo oscillations of cognitive relevance.

Inhibition-based rhythms: experimental and mathematical observations on network dynamics.

An increasingly large body of data exists which demonstrates that oscillations of frequency 12-80 Hz are a consequence of, or are inextricably linked to, the behaviour of inhibitory interneurons in the central nervous system. This frequency range covers the EEG bands beta 1 (12-20 Hz), beta 2 (20-30 Hz) and gamma (30-80 Hz). The pharmacological profile of both spontaneous and sensory-evoked EEG potentials reveals a very strong influence on these rhythms by drugs which have direct effects on GABA(A) receptor-mediated synaptic transmission (general anaesthetics, sedative/hypnotics) or indirect effects on inhibitory neuronal function (opiates, ketamine). In addition, a number of experimental models of, in particular, gamma-frequency oscillations, have revealed both common denominators for oscillation generation and function, and subtle differences in network dynamics between the different frequency ranges. Powerful computer and mathematical modelling techniques based around both clinical and experimental observations have recently provided invaluable insight into the behaviour of large networks of interconnected neurons. In particular, the mechanistic profile of oscillations generated as an emergent property of such networks, and the mathematical derivation of this complex phenomenon have much to contribute to our understanding of how and why neurons oscillate. This review will provide the reader with a brief outline of the basic properties of inhibition-based oscillations in the CNS by combining research from laboratory models, large-scale neuronal network simulations, and mathematical analysis.

A model of gamma-frequency network oscillations induced in the rat CA3 region by carbachol in vitro.

Carbachol (> 20 microM) and kainate (100 nM) induce, in the in vitro CA3 region, synchronized neuronal population oscillations at approximately 40 Hz having distinctive features: (i) the oscillations persist for hours; (ii) interneurons in kainate fire at 5-20 Hz and their firing is tightly locked to field potential maxima (recorded in s. radiatum); (iii) in contrast, pyramidal cells, in both carbachol and kainate, fire at frequencies as low as 2 Hz, and their firing is less tightly locked to field potentials; (iv) the oscillations require GABAA receptors, AMPA receptors and gap junctions. Using a network of 3072 pyramidal cells and 384 interneurons (each multicompartmental and containing a segment of unmyelinated axon), we employed computer simulations to examine conditions under which network oscillations might occur with the experimentally determined properties. We found that such network oscillations could be generated, robustly, when gap junctions were located between pyramidal cell axons, as suggested to occur based on studies of spontaneous high-frequency (> 100 Hz) network oscillations in the in vitro hippocampus. In the model, pyramidal cell somatic firing was not essential for the oscillations. Critical components of the model are (i) the plexus of pyramidal cell axons, randomly and sparsely interconnected by gap junctions; (ii) glutamate synapses onto interneurons; (iii) synaptic inhibition between interneurons and onto pyramidal cell axons and somata; (iv) a sufficiently high rate of spontaneous action potentials generated in pyramidal cell axons. This model explains the dependence of network oscillations on GABA(A) and AMPA receptors, as well as on gap junctions. Besides the existence of axon-axon gap junctions, the model predicts that many of the pyramidal cell action potentials, during sustained gamma oscillations, are initiated in axons.

Proximally targeted GABAergic synapses and gap junctions synchronize cortical interneurons.

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.

On the mechanism of the gamma --> beta frequency shift in neuronal oscillations induced in rat hippocampal slices by tetanic stimulation.

Tetanic stimulation of the CA1 region of rat hippocampal slices can induce gamma frequency population oscillations (30-100 Hz) after a latency of 50-150 msec that are synchronized to within 1-2 msec when simultaneous stimuli are delivered to two sites 2 mm or more apart. When tetanic stimuli, twice-threshold for eliciting gamma oscillations, are used, new phenomena occur. (1) After a period of gamma, there is a switch to beta frequencies (10-25 Hz); (2) during the switch, pyramidal cell spike afterhyperpolarizations (AHPs) increase and rhythmic EPSPs occur in pyramidal cells; and (3) after an episode of single-site, twice-threshold-induced gamma/beta oscillations, simultaneous two-site threshold stimuli induce gamma oscillations that are locally synchronized, but no longer are capable of long-range synchrony. We studied the cellular mechanisms of the gamma/beta switch with electrophysiological techniques and computer simulations. Our model predicts that the observed increases in both pyramidal cell AHPs and in pyramidal/pyramidal cell EPSPs are necessary and sufficient for the beta switch to occur. Firing patterns generated by the model, both for pyramidal cells and for interneurons, resemble experimental records. A one-site twice-threshold stimulus might lead to an inability of the two sites to synchronize at gamma frequencies, after subsequent two-site stimulation, via this mechanism. If depression is induced at synapses coupling pyramidal cells at one site to interneurons at the other site, then two-site stimulation cannot produce interneuron doublets; hence, as shown previously, the two sites will be unable to synchronize. This mechanism works in simulations, and we provide experimental evidence that synaptic depression and loss of doublets occur after a sufficiently strong local tetanus to one site. We suggest that long-range excitatory connections onto interneurons determine whether different pyramidal cell "assemblies" can synchronize at gamma frequencies, whereas excitatory connections onto pyramidal cells determine whether such assemblies can synchronize at beta frequencies.

Cholinergic activation and tonic excitation induce persistent gamma oscillations in mouse somatosensory cortex in vitro.

1. Concomitant application of the cholinergic agonist carbachol and nanomolar doses of kainate can elicit persistent gamma frequency oscillations in all layers of the mouse somatosensory cortex in vitro. Receptor pharmacology with bath-applied antagonists indicated that oscillatory network activity depended crucially on the participation of cholinergic muscarinic, (S)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate and GABAA receptors. 2. The timing of action potentials and the occurrence of excitatory as well as inhibitory postsynaptic events was highly correlated with the phasic change of extracellularly recorded population activity. Firing probability was lowest during the peak negativity of IPSPs and gradually increased during their ensuing decay. In conjunction with the effect of a barbiturate to decrease the frequency of gamma oscillations, this suggests a crucial role of IPSPs in phasing the suprathreshold activity of principal neurons. 3. At nearby (< 1 mm) sites contained within any given cortical layer, oscillatory extra- and intracellular activity was highly synchronous with no apparent phase lag. However, interlaminar mapping experiments demonstrated a phase reversal of both extra- and intracellularly recorded activity near the lower border of thalamo-recipient layer 4, thus corroborating findings that have been obtained in vivo. 4. In conclusion, a modest increase of tonic excitatory drive in conjunction with the activation of cholinergic muscarinic receptors can elicit persistent gamma frequency network oscillations in the rodent somatosensory cortex. These findings (re)emphasize the role of the cholinergic ascending system in the cortical processing of sensory information.

The dynamics of synchronized neurotransmitter release determined from compound spontaneous IPSCs in rat dentate granule neurones in vitro.

1. The properties of GABAA receptor-mediated spontaneous IPSCs generated in hippocampal dentate granule neurones were analysed using whole-cell voltage-clamp techniques in order to explore the functional consequences of the low number (6-12) and close proximity of synaptic contacts made by single GABAergic interneurones. 2. Spontaneous IPSCs (sIPSCs) occurred with a frequency of 14.0 +/- 9.1 Hz (n = 31) and revealed a multi-modal positively skewed amplitude distribution (39.0 +/- 19.8 pA, median values). 3. The variance of 10-90% rise times and decay kinetics between IPSCs decreased with increasing peak amplitude. Larger amplitude events had significantly faster rise times, consistent with their site of generation being proximal to the soma. The decay kinetics of sIPSCs did not significantly change with amplitude. 4. Large amplitude sIPSCs occurred singularly or in discrete bursts, repeated regularly at low frequency. The rising phase of such sIPSCs were multi-phasic, composed of clear step-like inflections that were not a product of noise. The variability between the rising phase of individual sIPSCs was quantified by calculating their standard deviation, which produced fast rising (0.22 +/- 0.05 ms time to peak, n = 16) functions with half-widths of 0.38 +/- 0.10 ms, which declined to plateaux. 5. Computer simulations demonstrated that IPSCs with properties similar to those recorded experimentally could be generated by the linear summation of groups of temporally dispersed component events. Standard deviation functions of the rising phase of simulated IPSCs accurately described distributions of the temporal dispersion of unitary components. 6. The GABA uptake inhibitor (R)-N[4,4-bis(3-methyl-2-thienyl)but-3-enl-yl] nipecotic acid (tiagabine) (10 microM, n = 12) significantly prolonged the decay of mIPSCs (6.5 +/- 0.8 to 8.7 +/- 1.0 ms, median values) and sIPSCs (6.2 +/- 0.4 to 7.3 +/- 1.2 ms, median values), but failed to alter the frequency of occurrence, 10-90% rise times or peak amplitude of events. The application of flurazepam (30 microM, n = 7; 50 microM, n = 4) prolonged the decay of sIPSCs regardless of their amplitude. 7. These data indicate that sIPSCs are formed by the summation of unitary components that occur asynchronously and that GABA released from multiple sites has independent post-synaptic actions.

Salient features of synaptic organisation in the cerebral cortex.

The neuronal and synaptic organisation of the cerebral cortex appears exceedingly complex, and the definition of a basic cortical circuit in terms of defined classes of cells and connections is necessary to facilitate progress of its analysis. During the last two decades quantitative studies of the synaptic connectivity of identified cortical neurones and their molecular dissection revealed a number of general rules that apply to all areas of cortex. In this review, first the precise location of postsynaptic GABA and glutamate receptors is examined at cortical synapses, in order to define the site of synaptic interactions. It is argued that, due to the exclusion of G protein-coupled receptors from the postsynaptic density, the presence of extrasynaptic receptors and the molecular compartmentalisation of the postsynaptic membrane, the synapse should include membrane areas beyond the membrane specialisation. Subsequently, the following organisational principles are examined: 1. The cerebral cortex consists of: (i) a large population of principal neurones reciprocally connected to the thalamus and to each other via axon collaterals releasing excitatory amino acids, and, (ii) a smaller population of mainly local circuit GABAergic neurones. 2. Differential reciprocal connections are also formed amongst GABAergic neurones. 3. All extrinsic and intracortical glutamatergic pathways terminate on both the principal and the GABAergic neurones, differentially weighted according to the pathway. 4. Synapses of multiple sets of glutamatergic and GABAergic afferents subdivide the surface of cortical neurones and are often co-aligned on the dendritic domain. 5. A unique feature of the cortex is the GABAergic axo-axonic cell, influencing principal cells through GABAA receptors at synapses located exclusively on the axon initial segment. The analysis of these salient features of connectivity has revealed a remarkably selective array of connections, yet a highly adaptable design of the basic circuit emerges when comparisons are made between cortical areas or layers. The basic circuit is most obvious in the hippocampus where a relatively homogeneous set of spatially aligned principal cells allows an easy visualization of the organisational rules. Those principles which have been examined in the isocortex proved to be identical or very similar. In the isocortex, the basic circuit, scaled to specific requirements, is repeated in each layer. As multiple sets of output neurones evolved, requiring subtly different needs for their inputs, the basic circuit may be superimposed several times in the same layer. Tangential intralaminar connections in both the hippocampus and isocortex also connect output neurones with similar properties, as best seen in the patchy connections in the isocortex. The additional radial superposition of several laminae of distinct sets of output neurones, each representing and supported by its basic circuit, requires a co-ordination of their activity that is mediated by highly selective interlaminar connections, involving both the GABAergic and the excitatory amino acid releasing neurones. The remarkable specificity in the geometry of cells and the selectivity in placement of neurotransmitter receptors and synapses on their surface, strongly suggest a predominant role for time in the coding of information, but this does not exclude an important role also for the rate of action potential discharge in cortical representation of information.

Cholinergic induction of network oscillations at 40 Hz in the hippocampus in vitro.

Acetylcholine is vital for cognitive functions of the brain. Although its actions in the individual cell are known in some detail, its effects at the network level are poorly understood. The hippocampus, which receives a major cholinergic input from the medial septum/diagonal band, is important in memory and exhibits network activity at 40 Hz during relevant behaviours. Here we show that cholinergic activation is sufficient to induce 40-Hz network oscillations in the hippocampus in vitro. Oscillatory activity is generated spontaneously in the CA3 subfield and can persist for hours. During the oscillatory state, principal neurons fire action potentials that are phase-related to the extracellular oscillation, but each neuron fires in only a small proportion of the cycles. Both excitatory and inhibitory synaptic events participate during the network oscillation in a precise temporal pattern. These results indicate that subcortical cholinergic input can control hippocampal memory processing by inducing fast network oscillations.

Morphological features of the entorhinal-hippocampal connection.

The goal of this review in an overview of the structural elements of the entorhinal-hippocampal connection. The development of the dendrites of hippocampal neurons will be outlined in relation to afferent pathway specificity and the mature dendritic structure compared. Interneurons will be contrasted to pyramidal cells in terms of processing of physiological signals and convergence and divergence in control of hippocampal circuits. Mechanisms of axonal guidance and target recognition, target structures, the involvement of receptor distribution on hippocampal dendrites and the involvement of non-neuronal cellular elements in the establishment of specific connections will be presented. Mechanisms relevant for the maintenance of shape and morphological specializations of hippocampal dendrites will be reviewed. One of the significant contexts in which to view these structural elements is the degree of plasticity in which they participate, during development and origination of dendrites, mature synaptic plasticity and after lesions, when the cells must continue to maintain and reconstitute function, to remain part of the circuitry in the hippocampus. This review will be presented in four main sections: (1) interneurons-development, role in synchronizing influence and hippocampal network functioning; (2) principal cells in CA1, CA3 and dentate gyrus regions-their development, function in terms of synaptic integration, differentiating structure and alterations with lesions; (3) glia and glia/neuronal interactions-response to lesions and developmental guidance mechanisms; and (4) network and circuit aspects of hippocampal morphology and functioning. Finally, the interwoven role of these various elements participating in hippocampal network function will be discussed.

Differentially interconnected networks of GABAergic interneurons in the visual cortex of the cat.

Networks of GABAergic neurons have been implicated in neuronal population synchronization. To define the extent of cellular interconnections, we determined the effect, number, and subcellular distribution of synapses between putative GABAergic neurons in layers II-IV of the cat visual cortex using paired intracellular recordings in vitro followed by correlated light and electron microscopy. All neurons having interneuronal electrophysiological properties were classified by their postsynaptic target profile and were identified as basket (BC; n = 6), dendrite-targeting (DTC; n = 1), and double bouquet (DBC; n = 2) cells. In four out of five anatomically fully recovered and reconstructed cell pairs, synaptic connections were found to be reciprocal. Generally BCs established synaptic junctions closer (21 +/- 20 micron) to postsynaptic somata than did DBCs (43 +/- 19 micron; p < 0.01). The unitary number of synapses (n values, 10, 7, and 20) in each of three BC-to-BC pairs was higher than that in three BC-to-DBC (n values, 1, 2, and 2) and three DBC-to-BC (n values, 1, 4, and 4) connections (p < 0.05). A BC innervated a DTC through two synaptic junctions. Unitary postsynaptic effects mediated by five BCs could be recorded in two BCs, two DBCs, and a DTC. The BCs elicited short-duration fast IPSPs, similar to those mediated by GABAA receptors. At a membrane potential of -55.0 +/- 6.4 mV, unitary IPSPs (n = 5) had a mean amplitude of 919 +/- 863 microV. Postsynaptic response failures were absent when an IPSP was mediated by several release sites. Thus, distinct GABAergic interneurons form reciprocally interconnected networks. The strength of innervation and the proximal placement of synapses suggest a prominent role for BCs in governing the activity of intracortical GABAergic networks in layers II-IV.

Unitary IPSPs evoked by interneurons at the stratum radiatum-stratum lacunosum-moleculare border in the CA1 area of the rat hippocampus in vitro.

1. Hippocampal non-principal neurons at the stratum radiatum-stratum lacunosum-moleculare border (R-LM interneurons) of the CA1 area may constitute several cell classes and have been implicated in the generation of GABAergic unitary IPSPs. Using biocytin-filled electrodes we recorded R-LM interneurons intracellularly in vitro and determined their postsynaptic effects in concomitantly recorded pyramidal cells. 2. Light microscopic analysis revealed four populations of R-LM interneurons with distinct axons: (1) basket cells (n = 4) with axons predominantly ramifying in the pyramidal cell layer; (2) Schaffer collateral/commissural pathway-associated interneurons (n = 10) stratifying in stratum radiatum and, to a lesser extent, stratum oriens; (3) perforant pathway-associated interneurons (n = 6) innervating the perforant path termination zone in stratum lacunosum-moleculare of the CA1 area as well as equivalent portions of the dentate gyrus and subiculum; and (4) neurogliaform interneurons (n = 2) characterized by their dense, compact axonal and dendritic arbour. 3. Random electron microscopic sampling of synaptic targets revealed a preponderance of pyramidal neurons as postsynaptic elements. Basket cells had a synaptic target preference for somata and proximal dendrites, whereas the remainder of R-LM interneurons innervated dendritic shafts and spines. The axon of dendrite-targeting cells formed up to six putative contacts with individual postsynatpic pyramidal cells. 4. Anatomically recovered R-LM interneurons (n = 22) had a mean resting membrane potential of -56.7 +/- 3.6 mV, a membrane time constant of 12.9 +/- 7.7 ms and an input resistance of 86.4 +/- 29.2 M omega. Depolarizing current pulses generally elicited overshooting action potentials (70.8 +/- 6.9 mV) which had a mean duration, when measured at half-amplitude, of 0.7 +/- 0.1 ms. In response to prolonged (> 200 ms) depolarizing current pulses all R-LM interneurons displayed (a varying degree of) spike frequency adaptation. 5. Basket cells, Schaffer-associated and neurogliaform interneurons elicited small-amplitude (< 2 mV), short-latency IPSPs in postsynaptic pyramids (n = 5, 13 and 1, respectively). Those interactions in which an effect was elicited with the repetitive activation of the presynaptic neuron (n = 13) showed a substantial degree of postsynaptic response summation. Unitary IPSPs had fast kinetics and, whenever tested (n = 5; 1 basket cell and 4 Schaffer-associated interneurons), were abolished by the GABAA receptor antagonist bicuculline. 6. Thus, R-LM interneurons comprise several distinct populations which evoke fast GABAA receptor mediated IPSPs. The domain-specific innervation of postsynaptic pyramidal cells suggests functionally diverse effects on the integration of afferent information in functionally non-equivalent compartments of pyramidal cells.