New insights into the classification and nomenclature of cortical GABAergic interneurons

A systematic classification and accepted nomenclature of neuron types is much needed but is currently lacking. This article describes a possible taxonomical solution for classifying GABAergic interneurons of the cerebral cortex based on a novel, web-based interactive system that allows experts to classify neurons with pre-determined criteria. Using Bayesian analysis and clustering algorithms on the resulting data, we investigated the suitability of several anatomical terms and neuron names for cortical GABAergic interneurons. Moreover, we show that supervised classification models could automatically categorize interneurons in agreement with experts' assignments. These results demonstrate a practical and objective approach to the naming, characterization and classification of neurons based on community consensus.

Neuroplasticity of neocortical circuits in schizophrenia

The core features of schizophrenia include deficits in cognitive processes mediated by the circuitry of the dorsolateral prefrontal cortex (DLPFC). These deficits are associated with a range of molecular and morphological alterations in the DLPFC, each of which could be a cause, consequence, or compensation in relation to other changes, and thus reflect the neuroplasticity of the brain in response to the underlying disease process. In this review, we consider disturbances in excitatory, inhibitory, and modulatory connections of DLPFC circuitry from the perspective of disease- and development-related neuroplasticity and discuss their implications for the identification of novel therapeutic targets.

Cluster Analysis–Based Physiological Classification and Morphological Properties of Inhibitory Neurons in Layers 2–3 of Monkey Dorsolateral Prefrontal Cortex

In primates, little is known about intrinsic electrophysiological properties of neocortical neurons and their morphological correlates. To classify inhibitory cells (interneurons) in layers 2–3 of monkey dorsolateral prefrontal cortex we used whole cell voltage recordings and intracellular labeling in slice preparation with subsequent morphological reconstructions. Regular spiking pyramidal cells have been also included in the sample. Neurons were successfully segregated into three physiological clusters: regular-, intermediate-, and fast-spiking cells using cluster analysis as a multivariate exploratory technique. When morphological types of neurons were mapped on the physiological clusters, the cluster of regular spiking cells contained all pyramidal cells, whereas the intermediate- and fast-spiking clusters consisted exclusively of interneurons. The cluster of fast-spiking cells contained all of the chandelier cells and the majority of local, medium, and wide arbor (basket) interneurons. The cluster of intermediate spiking cells predominantly consisted of cells with the morphology of neurogliaform or vertically oriented (double-bouquet) interneurons. Thus a quantitative approach enabled us to demonstrate that intrinsic electrophysiological properties of neurons in the monkey prefrontal cortex define distinct cell types, which also display distinct morphologies.

Functional Properties of Fast Spiking Interneurons and Their Synaptic Connections With Pyramidal Cells in Primate Dorsolateral Prefrontal Cortex

Recent studies suggest that fast-spiking (FS) interneurons of the monkey dorsolateral prefrontal cortex (DLPFC) exhibit task-related firing during working-memory tasks. To gain further understanding of the functional role of FS neurons in monkey DLPFC, we described the in vitro electrophysiological properties of FS interneurons and their synaptic connections with pyramidal cells in layers 2/3 of areas 9 and 46. Extracellular spike duration was found to distinguish FS cells from non-FS interneuron subtypes. However, a substantial overlap in extracellular spike duration between these populations would make classification of individual interneurons difficult. FS neurons could be divided into two main morphological groups, chandelier and basket neurons, with very similar electrophysiological properties but significantly different horizontal spread of the axonal arborization. In paired cell recordings, unitary inhibitory postsynaptic potentials (IPSPs) elicited by FS neurons in pyramidal cells had rapid time course, small amplitude at resting membrane potential, and were mediated by GABAAreceptors. Repetitive FS neuron stimulation, partially mimicking the sustained firing of interneurons in vivo, produced short-term depression of the unitary IPSPs, present at connections made by both basket and chandelier neurons and due at least in part to presynaptic mechanisms. These results suggest that FS neurons and their synaptic connections with pyramidal cells have homogeneous physiological properties. Thus different functional roles of basket and chandelier neurons in the DLPFC in vivo must arise from the distinct properties of the interneuronal axonal arborization or from a different functional pattern of excitatory and inhibitory connections with other components of the DLPFC neuronal network.

Synaptic Efficacy during Repetitive Activation of Excitatory Inputs in Primate Dorsolateral Prefrontal Cortex

Neurons in the monkey dorsolateral prefrontal cortex (DLPFC) fire persistently during the delay period of working memory tasks. To determine how repetitive firing affects the efficacy of synaptic inputs to DLPFC layer 3 neurons, we examined the effects of repetitive presynaptic stimulation on the amplitude and temporal summation of EPSPs. Recordings were obtained in monkey DLPFC brain slices from regular spiking (RS) pyramidal cells and two types of interneurons, fast spiking (FS) and adapting non-pyramidal (ANP) cells. Repetitive stimulation of presynaptic axons in layer 3 caused EPSP depression in RS and FS neurons, but EPSP facilitation in ANP cells. A shorter EPSP duration produced weaker temporal summation in FS neurons compared to the other cell classes. Thus, due to the combined effects of dynamic changes in EPSP amplitude and differences in temporal summation, the effect of a presynaptic spike train differed according to the postsynaptic cell class. Similar results were obtained when recording unitary EPSPs evoked in connected pairs of presynaptic RS pyramidal cells and postsynaptic RS, FS or ANP neurons. In addition, similar differences in the efficacy of sustained inputs among cell classes were observed when delay-related firing was reproduced in vitro by stimulating inputs with the timing of spike trains recorded from the DLPFC of monkeys performing a delayed-response task. We suggest that the transition from baseline firing rates to higher frequency delay-related firing may lead to the differential activation of distinct cell populations, with corresponding significant effects on the patterns of activity in local prefrontal circuits.

Dopamine modulation of neuronal function in the monkey prefrontal cortex

We developed a brain slice preparation that allowed us to apply whole-cell recordings to examine the electrophysiological properties of identified synapses, neurons, and local circuits in the dorsolateral prefrontal cortex (DLPFC) of macaque monkeys. In this article, we summarize the results from some of our recent and current in vitro studies in the DLPFC with special emphasis on the modulatory effects of dopamine (DA) receptor activation on pyramidal and nonpyramidal cell function in superficial layers in DLPFC areas 46 and 9.

Selective reduction by dopamine of excitatory synaptic inputs to pyramidal neurons in primate prefrontal cortex

We have employed in vitro physiological methods to investigate dopaminergic modulation of excitatory synaptic transmission in monkey prefrontal cortex (PFC) circuits. We show that combined activation of D1-like and D2-like dopamine receptors results in the reduction of extracellular stimulation-evoked isolated EPSCs in layer 3 pyramidal neurons. Using paired recordings from synaptically connected pyramidal neurons we have determined the basic properties of unitary synaptic connections between layer 3 pyramids in the primate PFC and, interestingly, we found that dopamine does not reduce synaptic transmission between nearby pairs of synaptically coupled PFC pyramidal neurons. This input specificity may be a critical aspect of the dopaminergic regulation of recurrent excitatory circuits in the PFC.

Voltage-gated sodium channels shape subthreshold EPSPs in layer 5 pyramidal neurons from rat prefrontal cortex

The role of voltage-dependent channels in shaping subthreshold excitatory postsynaptic potentials (EPSPs) in neocortical layer 5 pyramidal neurons from rat medial prefrontal cortex (PFC) was investigated using patch-clamp recordings from visually identified neurons in brain slices. Small-amplitude EPSPs evoked by stimulation of superficial layers were not affected by the N-methyl-D-aspartate receptor antagonist D-2-amino-5-phosphonopentanoic acid but were abolished by the AMPA receptor antagonist 6-cyano-7-nitroquinoxalene-2,3-dione, suggesting that they were primarily mediated by AMPA receptors. AMPA receptor-mediated EPSPs (AMPA-EPSPs) evoked in the apical dendrites were markedly enhanced, or increased in peak and duration, at depolarized holding potentials. Enhancement of AMPA-EPSPs was reduced by loading the cells with lidocaine N-ethylbromide (QX-314) and by local application of the Na(+) channel blocker tetrodotoxin (TTX) to the soma but not to the middle/proximal apical dendrite. In contrast, blockade of Ca(2+) channels by co-application of Cd(2+) and Ni(2+) to the soma or apical dendrite did not affect the AMPA-EPSPs. Like single EPSPs, EPSP trains were shaped by Na(+) but not Ca(2+) channels. EPSPs simulated by injecting synaptic-like current into proximal/middle apical dendrite (simEPSPs) were enhanced at depolarized holding potentials similarly to AMPA-EPSPs. Extensive blockade of Ca(2+) channels by bath application of the Cd(2+) and Ni(2+) mixture had no effects on simEPSPs, whereas bath-applied TTX removed the depolarization-dependent EPSP amplification. Inhibition of K(+) currents by 4-aminopyridine (4-AP) and TEA increased the TTX-sensitive EPSP amplification. Moreover, strong inhibition of K(+) currents by high concentrations of 4-AP and TEA revealed a contribution of Ca(2+) channels to EPSPs that, however, seemed to be dependent on Na(+) channel activation. Our results indicate that in layer 5 pyramidal neurons from PFC, Na(+), and K(+) voltage-gated channels shape EPSPs within the voltage range that is subthreshold for somatic action potentials.

Synaptic targets of the intrinsic axon collaterals of supragranular pyramidal neurons in monkey prefrontal cortex

The principal axons of supragranular pyramidal neurons in the cerebral cortex travel through the white matter and terminate in other cortical areas, whereas their intrinsic axon collaterals course through the gray matter and form both local and long-distance connections within a cortical region. In the monkey prefrontal cortex (PFC), horizontally oriented, intrinsic axon collaterals from supragranular pyramidal neurons form a series of stripe-like clusters of axon terminals (Levitt et al. [1993] J Comp Neurol 338:360-376; Pucak et al. [1996] J Comp Neurol 376:614-630). The present study examined the synaptic targets of the intrinsic axon collaterals arising from supragranular pyramidal neurons within the same stripe (local projections). Approximately 50% of the within-stripe axon terminals in monkey PFC area 9 targeted dendritic spines. In contrast, for both the intrinsic axon collaterals that travel between stripes (long-range projections), and the axon terminals that project to other PFC areas (associational projections), over 92% of the postsynaptic structures were dendritic spines (Melchitzky et al. [1998] J Comp Neurol 390:211-224). The other 50% of the within-stripe terminals synapsed with dendritic shafts. Dual-labeling studies confirmed that these within-stripe terminals contacted gamma-aminobutyric acid-immunoreactive dendritic shafts, including the subpopulation that contains the calcium-binding protein parvalbumin. The functional significance of the differences in synaptic targets between local and long-range intrinsic axon collaterals was supported by whole-cell, patch clamp recordings in an in vitro slice preparation of monkey PFC. Specifically, the small amplitude responses observed in layer 3 pyramidal neurons during long-range, low-intensity stimulation were exclusively excitatory, whereas local stimulation also evoked di/polysynaptic inhibitory responses. These anatomic and electrophysiological findings suggest that intrinsic connections of the PFC differ from other cortical regions and that within the PFC, feedback (within-stripe) inhibition plays a greater role in regulating the activity of supragranular pyramidal neurons than does feedforward inhibition either between stripes or across regions.

Horizontal synaptic connections in monkey prefrontal cortex: an in vitro electrophysiological study

In monkey dorsolateral prefrontal cortex (PFC), long-distance, horizontally oriented intrinsic axon collaterals interconnect clusters of pyramidal neurons in the supragranular layers. In order to study the electrophysiological responses mediated by these long-distance projections, an in vitro slice preparation of monkey PFC was used to obtain whole-cell patch clamp recordings from layer 3 pyramidal neurons. Using in vivo tracer injections, we found that long-distance projections were well preserved in PFC slices cut in the coronal plane. Postsynaptic currents were evoked by low-intensity electrical extracellular stimulation applied successively to 20-30 discrete sites located up to 2200 micron lateral to the recorded cell. Several criteria were applied to discriminate between mono- and polysynaptic responses. Long-distance monosynaptic connections were mediated by fibers with relatively slow conduction velocity (0.14 m/s). Excitatory postsynaptic currents (EPSCs) evoked by stimulation of short- or long-distance horizontal connections did not differ in kinetic properties. The majority (77%) of the 35 layer 3 PFC neurons studied were monosynaptic targets of long-distance connections. EPSCs mediated by long-distance connections had amplitudes that were similar or even larger than short-distance EPSCs, suggesting that excitatory input provided by the former was relatively robust. For most neurons (87.5%) in which a full complement of monosynaptic EPSCs was evoked by multisite stimulation, the EPSC amplitude as a function of stimulation distance from the recorded cells exhibited statistically significant peaks. The spacing between peaks was similar to the spacing between interconnected clusters of neurons observed in previous anatomical studies. The results show that long-distance excitatory connections constitute a significant intrinsic pathway of synaptic communication in layer 3 of monkey PFC.