Alex M. Thomson

Petilla terminology: nomenclature of features of GABAergic interneurons of the cerebral cortex.

Neuroscience produces a vast amount of data from an enormous diversity of neurons. A neuronal classification system is essential to organize such data and the knowledge that is derived from them. Classification depends on the unequivocal identification of the features that distinguish one type of neuron from another. The problems inherent in this are particularly acute when studying cortical interneurons. To tackle this, we convened a representative group of researchers to agree on a set of terms to describe the anatomical, physiological and molecular features of GABAergic interneurons of the cerebral cortex. The resulting terminology might provide a stepping stone towards a future classification of these complex and heterogeneous cells. Consistent adoption will be important for the success of such an initiative, and we also encourage the active involvement of the broader scientific community in the dynamic evolution of this project.

Temporal and spatial properties of local circuits in neocortex

A large body of anatomical data has detailed many complexities of neocortical circuitry, and physiological studies have indicated some roles for this circuitry in the complex functions of the cortex. Until recently, however, we have little precise information about the spatio-temporal properties of synaptic connections between individual neocortical neurones. Studies of synaptic responses elicited in one neocortical neurone by action potentials in another, and parallel morphological studies that have identified these neurones and the synaptic connections between them, have now described these parameters for certain types of local circuit connection in the neocortex. Some of these studies confirmed previous observations and inferences, but others provided major surprises. Evidence indicates that the class of both the presynaptic and postsynaptic neurone together determine a wide range of synaptic properties, such as the type of postsynaptic receptors involved and the temporal pattern of transmitter release, so that each type of synapse displays unique properties. A role for retrograde diffusable messages in determining the temporal properties of these circuits is postulated.

Functional maps of neocortical local circuitry

This review aims to summarize data obtained with different techniques to provide a functional map of the local circuit connections made by neocortical neurones, a reference for those interested in cortical circuitry and the numerical information required by those wishing to model the circuit. A brief description of the main techniques used to study circuitry is followed by outline descriptions of the major classes of neocortical excitatory and inhibitory neurones and the connections that each layer makes with other cortical and subcortical regions. Maps summarizing the projection patterns of each class of neurone within the local circuit and tables of the properties of these local circuit connections are provided. This review relies primarily on anatomical studies that have identified the classes of neurones and their local and long distance connections and on paired intracellular and whole-cell recordings which have documented the properties of the connections between them. A large number of different types of synaptic connections have been described, but for some there are only a few published examples and for others the details that can only be obtained with paired recordings and dye-filling are lacking. A further complication is provided by the range of species, technical approaches and age groups used in these studies. Wherever possible the range of available data are summarised and compared. To fill some of the more obvious gaps for the less well-documented cases, data obtained with other methods are also summarized.

Single axon IPSPs elicited in pyramidal cells by three classes of interneurones in slices of rat neocortex.

1. Using dual intracellular recordings in slices of adult rat neocortex, twenty‐four IPSPs activated by single presynaptic interneurones were studied in simultaneously recorded pyramidal cells. Fast spiking interneurones inhibited one in four or five of their close pyramidal neighbours. No reciprocal connections were observed. After recordings neurones were filled with biocytin. 2. Interneurones that elicited IPSPs were classified as classical fast spiking (n = 10), as non‐classical fast spiking (n = 3, including one burst‐firing interneurone), as unclassified, or slow interneurones (n = 8), or as regular spiking interneurones (n = 3), i.e. interneurones whose electrophysiological characteristics were indistinguishable from those of pyramidal cells. 3. All of the seven classical fast spiking cells anatomically fully recovered had aspiny, beaded dendrites. Their partially myelinated axons ramified extensively, varying widely in shape and extent, but randomly selected labelled axon terminals typically innervated somata and large calibre dendrites on electron microscopic examination. One ‘autapse’ was demonstrated. One presumptive regular spiking interneurone axon made four somatic and five dendritic connections with unlabelled targets. 4. Full anatomical reconstructions of labelled classical fast spiking interneurones and their postsynaptic pyramids (n = 5) demonstrated one to five boutons per connection. The two recorded IPSPs that were fully reconstructed morphologically (3 and 5 terminals) were, however, amongst the smallest recorded (< 0.4 mV). Some connections may therefore involve larger numbers of contacts. 5. Single axon IPSPs were between 0.2 and 3.5 mV in average amplitude at ‐55 to ‐60 mV. Extrapolated reversal potentials were between ‐70 and ‐82 mV. IPSP time course correlated with the type of presynaptic interneurone, but not with IPSP latency, amplitude, reversal potential, or sensitivity to current injected at the soma. 6. Classical fast spiking interneurones elicited the fastest IPSPs (width at half‐amplitude 14.72 +/‐ 3.83 ms, n = 10) and unclassified, or slow interneurones the slowest (56.29 +/‐ 23.44 ms, n = 8). Regular spiking interneurone IPSPs had intermediate half‐widths (27.3 +/‐ 3.68 ms, n = 3). 7. Increasingly brief presynaptic interspike intervals increased the peak amplitude of, but not the area under, the summed IPSP. Only at interspike intervals between 10 and 20 ms did IPSP integrals exhibit paired pulse facilitation. Paired pulse depression was apparent at < 10 and 20‐60 ms. During longer spike trains, summing IPSPs decayed to a plateau potential that was relatively independent of firing rate (100‐250 Hz). Thereafter, the voltage response could increase again. 8. Summed IPSPs elicited by two to fifteen presynaptic spike trains decayed as, or more rapidly than, single‐spike IPSPs. Summed IPSPs elicited by > 20 spikes (> 150 Hz), however, resulted in an additional, more slowly decaying component (latency > 50 ms, duration > 200 ms). The possible involvement of GABAB receptors in this component is discussed. 9. It is suggested that three broad classes of interneurones may activate GABAA receptors on relatively proximal portions of neocortical pyramidal neurones. The different time courses of the IPSPs elicited by the three classes may reflect different types of postsynaptic receptor rather than dendritic location. An additional class, burst firing, spiny interneurones appear to activate GABAA receptors on more distal sites.

Activity‐dependent properties of synaptic transmission at two classes of connections made by rat neocortical pyramidal axons in vitro

1 To compare the dynamics of synaptic transmission at different types of connection, dual intracellular recordings were made from pairs of neurones in slices of adult rat neocortex. Excitatory postsynaptic potentials (EPSPs) were elicited by single spikes, spike pairs and brief spike trains in presynaptic pyramidal cells and responses recorded in postsynaptic pyramidal cells and in inter neurones. 2 Pyramid–pyramid EPSPs were strongly voltage dependent and this resulted in a range of paired pulse effects. At thirty‐two of sixty‐nine pyramid–pyramid connections, the 2nd EPSP was the same shape as the 1st, indicating minimal interaction between active synapses. In these thirty‐two connections, paired pulse depression (PPD) was apparent (2nd EPSP integral 46 ± 21% of the 1st, at 5–20 ms), which recovered within 60–70 ms. 3 In eleven additional pyramid–pyramid pairs, the 2nd EPSP was also the same shape as the 1st, but paired pulse facilitation (PPF, 149 ± 32%) decaying within 50–60 ms was apparent. Even these connections displayed frequency‐dependent depression, however, as 3rd EPSPs were smaller than 1st EPSPs at intervals < 100 ms. 4 At twenty‐five pyramid–pyramid connections, 2nd EPSPs were broader than 1st EPSPs and in sixteen of these, voltage‐ and NMDA receptor‐dependent enhancement was large enough to obscure the underlying PPD. PPD was revealed by postsynaptic hyperpolarization (4 pairs), N‐methyl‐D‐aspartate (NMDA) receptor blockade (3 pairs), or if Mg2+ was removed (in the one case studied). If synapse location allowed significant depolarization of one active site by another, voltage‐dependent enhancement could produce supralinear EPSP summation and overcome PPD. Third EPSPs were, however, consistently smaller than 1st EPSPs. 5 In striking contrast, profound frequency‐dependent facilitation, independent of voltage or NMDA receptors was seen at fifteen connections involving two classes of postsynaptic inter neurones. 6 At these pyramid–interneurone connections, facilitation of the 2nd EPSP (655 ± 380% at 5–20 ms) decayed rapidly, within 50–60 ms. Third and fourth EPSPs showed additional facilitation which decayed more slowly, within 90 ms and 2 s, respectively. Facilitation due to five to six spike trains was still apparent at 3 s. Therefore, once initiated by a brief high frequency spike train, facilitation was maintained at lower frequencies.

Physiological and morphological diversity of immunocytochemically defined parvalbumin- and cholecystokinin-positive interneurones in CA1 of the adult rat hippocampus

To investigate the electrophysiological properties, synaptic connections, and anatomy of individual parvalbumin‐immunoreactive (PV‐IR) and cholecystokinin‐immunoreactive (CCK‐IR) interneurones in CA1, dual intracellular recordings using biocytin‐filled microelectrodes in slices of adult rat hippocampus were combined with fluorescence labelling of PV‐ and CCK‐containing cells. Of 36 PV‐IR cells, 29 were basket cells, with most of their axonal arbours in the stratum pyramidale (SP). Six were bistratified cells with axons ramifying throughout stratum oriens (SO) and stratum radiatum (SR). One was a putative axo‐axonic cell with an axonal arbour confined to half of the SP and a narrow adjacent region of the SO. Of 27 CCK‐IR neurones, 13 were basket cells, with most of their axonal arbours in the SP, and included basket cells with somata in the SP (6), SO (3), and SR (2) and at the border between the stratum lacunosum‐moleculare (SLM) and the SR (2). In addition, several dendrite‐targeting cell classes expressed CCK‐IR: 4 of 9 bistratified cells with axons ramifying in the SO and SR; all five Schaffer‐associated cells whose axons ramified extensively in the SR; both cells classified as quadrilaminar because their axons ramified in the SO, SP, SR, and SLM; one SO‐SO cell whose dendritic and axonal arbours were contained within the SO; and one perforant path‐associated cell with axonal and dendritic arbours within the distal SR and SLM. The majority (31 of 36) of PV‐IR neurones recorded were fast‐spiking, and most fast‐spiking cells tested (25 of 29 basket, 1 axo‐axonic, and 5 of 6 bistratified cells) were PV‐IR. However, 1 of 6 regular‐spiking basket, 1 of 4 regular‐spiking bistratified, and 3 of 5 burst‐firing basket cells were also PV‐IR. In contrast, the majority (17 of 27) of the CCK‐IR neurones recorded were regular‐spiking, 3 were burst‐firing, and 7 were fast‐spiking. These data confirm that the majority of PV‐IR and CCK‐IR axon terminals innervate proximal portions of CA1 pyramidal cells. Most PV‐IR cells are fast‐spiking, whereas most CCK‐IR cells are regular‐spiking. In both neurochemical classes basket cells predominate, but both groups included subpopulations of dendrite‐targeting cells. Despite these similarities, the two populations exhibited very different somatic distributions, and each contained cellular morphologies not represented in the other. J. Comp. Neurol. 443:346–367, 2002.

CA1 pyramid-pyramid connections in rat hippocampus in vitro : dual intracellular recordings with biocytin filling

In adult rat hippocampus, simultaneous intracellular recordings from 989 pairs of CA1 pyramidal cells revealed nine monosynaptic, excitatory connections. Six of these pairs were sufficiently stable for electrophysiological analysis. Mean excitatory postsynaptic potential amplitude recorded at a postsynaptic membrane potential between -67 and -70 mV was 0.7 +/- 0.5 mV (0.17-1.5 mV), mean 10-90% rise time was 2.7 +/- 0.9 ms (1.5-3.8 ms) and mean width at half-amplitude was 16.8 +/- 4.1 ms (11.6-25 ms). Cells were labelled with biocytin and identified histologically. For one pair that was fully reconstructed morphologically, excitatory postsynaptic potential average amplitude was 1.5 mV, 10-90% rise time 2.8 ms and width at half-amplitude 11.6 ms (at -67 mV). In this pair, correlated light and electron microscopy revealed that the presynaptic axon formed two synaptic contacts with third-order basal dendrites of the postsynaptic pyramid, one with a dendritic spine, the other with a dendritic shaft. In the four pairs tested, postsynaptic depolarization increased excitatory postsynaptic potential amplitude and duration. In two, D-2-amino-5-phosphonovalerate (50 microM) reduced the amplitude and duration of the excitatory postsynaptic potential. The remainder of the excitatory postsynaptic potential now increased with postsynaptic hyperpolarization and was abolished by 20 microM 6-cyano-7-nitroquinoxaline-2,3-dione (n = 1). Paired-pulse depression was evident in the four excitatory postsynaptic potentials tested. This depression decreased with increasing inter-spike interval. These results provide the first combined electrophysiological and morphological illustration of synaptic contacts between pyramidal neurons in the hippocampus and confirm that connections between CA1 pyramidal neurons are mediated by both N-methyl-D-aspartate and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate/kainate receptors.

Single axon excitatory postsynaptic potentials in neocortical interneurons exhibit pronounced paired pulse facilitation

In slices of adult rat somatosensory/motor cortex, paired recordings were made from pyramidal and non-pyramidal neurons. Single axon excitatory postsynaptic potentials evoked in the non-pyramidal neuron by action potentials in the pyramidal neuron were large and fast and demonstrated large fluctuations in amplitude, with coefficients of variation between 0.1 and 1.25. Excitatory postsynaptic potential amplitude distributions included a large number of apparent failures of transmission as well as some extremely large events. This contrasted dramatically with the relatively narrow distribution of amplitudes for pyramid-pyramid connections in neocortex. Excitatory postsynaptic potentials increased in amplitude with postsynaptic membrane hyperpolarization. Very small changes in the coefficient of variation when mean amplitudes increased substantially were consistent with the increase being due to a change in quantal amplitude. These excitatory postsynaptic potentials displayed profound paired pulse facilitation. Moreover, third and fourth spikes in a presynaptic burst also evoked large responses. This facilitation was associated with a decrease in the proportion of apparent failures in transmission and a change in the shape of the excitatory postsynaptic potential amplitude distribution, both indicative of an increase in the probability of transmitter release. However a large change in the mean amplitude was not associated with a similar change in the inverse square of the coefficient of variation. The result of this third test, taken in isolation, might therefore suggest that quantal amplitude had increased with paired-pulse facilitation. However, of the three tests applied, this last is the most heavily model-dependent and produced a result inconsistent with the results of the other two tests. The possibility is therefore discussed that both the shape of the excitatory postsynaptic potential amplitude distribution and the failure of coefficient of variation analysis to detect an apparently presynaptic change might result from the release at these synapses being poorly fit by a simple model. Based on a more complex model of synaptic release proposed by Faber and Korn [Faber and Korn (1991) Biophys. J. 60, 1288-1294] and a hypothesis proposed by Scharfman et al. [Scharfman et al. (1990) Neuroscience 37, 693-707], two hypotheses arising from the present study are discussed: (i) that branch point failure contributes to the pattern of synaptic activation at these connections; and (ii) that both presynaptic pyramidal firing pattern and axonal geometry contribute to the selection of the type of postsynaptic neurone preferentially activated.(ABSTRACT TRUNCATED AT 400 WORDS)

FACILITATING PYRAMID TO HORIZONTAL ORIENS-ALVEUS INTERNEURONE INPUTS : DUAL INTRACELLULAR RECORDINGS IN SLICES OF RAT HIPPOCAMPUS

1 In adult rat hippocampal slices, simultaneous intracellular recordings from pyramidal cells in CA1 and interneurones near the stratum oriens‐alveus border revealed excitatory connections that displayed facilitation on repetitive activation in twelve of thirty‐six pairs tested. 2 Postsynaptic interneurones were classified as horizontal oriens‐alveus interneurones by the pronounced ‘sag’ in response to hyperpolarizing current injection, high levels of spontaneous synaptic activity and by the morphology of their somata and dendrites, which were confined to stratum oriens‐alveus and their axons which projected to stratum lacunosum‐moleculare where they ramified extensively, in the region of entorhinal cortex input to CA1. 3 Excitatory postsynaptic potentials (EPSPs) elicited by single pyramidal cells were 0 to 12 mV in amplitude. Mean EPSP amplitude (single spikes) was 0.93 ± 1.06 mV at −70 ± 2.3 mV (n= 10). The rise time was 1.2 ± 0.5 ms and the width at half‐amplitude was 7.5 ± 4.7 ms. 4 EPSPs fluctuated greatly in amplitude; the mean coefficient of variation was 0.84 ± 0.37 for the first EPSP and 0.47 ± 0.24 for the second. Apparent failures of transmission frequently occurred after first presynaptic spikes but less frequently after the second or subsequent spikes in brief trains. 5 EPSPs displayed facilitation at membrane potentials between −80 mV and spike threshold. Second EPSPs within 20 ms of the first were 253 ± 48% (range, 152–324%) of the mean first EPSP amplitude. Third EPSPs within 60 ms were 266 ± 70 % (range, 169–389%) and fourth EPSPs within 60–120 ms were 288 ± 71% (range, 188–393%). Both proportions of apparent failures of transmission and coefficient of variation analysis indicated a presynaptic locus for this facilitation.

Relationships between morphology and physiology of pyramid-pyramid single axon connections in rat neocortex in vitro.

1. Double intracellular recordings were made from 1163 pairs of pyramidal neurones in layer V‐VI of the rat somatomotor cortex in vitro using sharp electrodes filled with biocytin. Monosynaptically connected pairs of cells were identified when an action potential in one could elicit a constant latency excitatory postsynaptic potential (EPSP) in the other and the cells were filled with biocytin. Labelled cells were subsequently identified histologically with avidin‐horseradish peroxidase. 2. Thirty‐four pairs of cells were found to be monosynaptically connected. Fifteen of these pairs were sufficiently stable for electrophysiological recordings and three of these were recovered sufficiently to permit full morphological reconstruction. 3. The EPSP recorded between the first pair of pyramids varied in amplitude between 0 and 3 mV (mean 1.33 +/‐ 1.06 mV) and fluctuated considerably (coefficient of variation, 0.796). This was largely due to a high incidence of apparent failures of transmission. On reconstruction two boutons from the presynaptic pyramid axon were in close apposition to the proximal portions of basal dendrites of the postsynaptic cell. 4. In the second pair of pyramids the EPSP had a mean amplitude of 1.06 mV, and displayed a 10‐90% rise time of 2.8 ms and a width at half‐amplitude of 23 ms. This EPSP did not alter significantly with changes in membrane potential at the soma. The presynaptic axon closely apposed the distal apical dendrite of the postsynaptic cell in eight places. 5. In the third pair of pyramids, the EPSPs, recorded at a relatively depolarized membrane potential, were long lasting and could elicit slow dendritic spikes with long and variable latencies. These slow spikes suggested that the postsynaptic recording site was dendritic and on reconstruction a possible location was identified on the apical dendrite. A total of five presynaptic boutons closely apposed three separate, proximal branches of the postsynaptic apical dendrite. 6. These results provide the first illustration of a morphological basis for variations in functional properties of pyramid‐pyramid connections in the neocortex.