Distribution of neurons immunoreactive for calcium-binding proteins varies across areas of cat primary somatosensory cortex

Characterization of extracellular spike waveforms recorded in wallaby primary visual cortex

Extracellular recordings were made from 642 units in the primary visual cortex (V1) of a highly visual marsupial, the Tammar wallaby. The receptive field (RF) characteristics of the cells were objectively estimated using the non-linear input model (NIM), and these were correlated with spike shapes. We found that wallaby cortical units had 68% regular spiking (RS), 12% fast spiking (FS), 4% triphasic spiking (TS), 5% compound spiking (CS) and 11% positive spiking (PS). RS waveforms are most often associated with recordings from pyramidal or spiny stellate cell bodies, suggesting that recordings from these cell types dominate in the wallaby cortex. In wallaby, 70-80% of FS and RS cells had orientation selective RFs and had evenly distributed linear and nonlinear RFs. We found that 47% of wallaby PS units were non-orientation selective and they were dominated by linear RFs. Previous studies suggest that the PS units represent recordings from the axon terminals of non-orientation selective cells originating in the lateral geniculate nucleus (LGN). If this is also true in wallaby, as strongly suggested by their low response latencies and bursty spiking properties, the results suggest that significantly more neurons in wallaby LGN are already orientation selective. In wallaby, less than 10% of recorded spikes had triphasic (TS) or sluggish compound spiking (CS) waveforms. These units had a mixture of orientation selective and non-oriented properties, and their cellular origins remain difficult to classify.

Laminar analysis of slow wave activity in humans

Brain electrical activity is largely composed of oscillations at characteristic frequencies. These rhythms are hierarchically organized and are thought to perform important pathological and physiological functions. The slow wave is a fundamental cortical rhythm that emerges in deep non-rapid eye movement sleep. In animals, the slow wave modulates delta, theta, spindle, alpha, beta, gamma and ripple oscillations, thus orchestrating brain electrical rhythms in sleep. While slow wave activity can enhance epileptic manifestations, it is also thought to underlie essential restorative processes and facilitate the consolidation of declarative memories. Animal studies show that slow wave activity is composed of rhythmically recurring phases of widespread, increased cortical cellular and synaptic activity, referred to as active- or up-state, followed by cellular and synaptic inactivation, referred to as silent- or down-state. However, its neural mechanisms in humans are poorly understood, since the traditional intracellular techniques used in animals are inappropriate for investigating the cellular and synaptic/transmembrane events in humans. To elucidate the intracortical neuronal mechanisms of slow wave activity in humans, novel, laminar multichannel microelectrodes were chronically implanted into the cortex of patients with drug-resistant focal epilepsy undergoing cortical mapping for seizure focus localization. Intracortical laminar local field potential gradient, multiple-unit and single-unit activities were recorded during slow wave sleep, related to simultaneous electrocorticography, and analysed with current source density and spectral methods. We found that slow wave activity in humans reflects a rhythmic oscillation between widespread cortical activation and silence. Cortical activation was demonstrated as increased wideband (0.3-200 Hz) spectral power including virtually all bands of cortical oscillations, increased multiple- and single-unit activity and powerful inward transmembrane currents, mainly localized to the supragranular layers. Neuronal firing in the up-state was sparse and the average discharge rate of single cells was less than expected from animal studies. Action potentials at up-state onset were synchronized within +/-10 ms across all cortical layers, suggesting that any layer could initiate firing at up-state onset. These findings provide strong direct experimental evidence that slow wave activity in humans is characterized by hyperpolarizing currents associated with suppressed cell firing, alternating with high levels of oscillatory synaptic/transmembrane activity associated with increased cell firing. Our results emphasize the major involvement of supragranular layers in the genesis of slow wave activity.

Neocortical neuron types in Xenarthra and Afrotheria: implications for brain evolution in mammals

Interpreting the evolution of neuronal types in the cerebral cortex of mammals requires information from a diversity of species. However, there is currently a paucity of data from the Xenarthra and Afrotheria, two major phylogenetic groups that diverged close to the base of the eutherian mammal adaptive radiation. In this study, we used immunohistochemistry to examine the distribution and morphology of neocortical neurons stained for nonphosphorylated neurofilament protein, calbindin, calretinin, parvalbumin, and neuropeptide Y in three xenarthran species-the giant anteater (Myrmecophaga tridactyla), the lesser anteater (Tamandua tetradactyla), and the two-toed sloth (Choloepus didactylus)-and two afrotherian species-the rock hyrax (Procavia capensis) and the black and rufous giant elephant shrew (Rhynchocyon petersi). We also studied the distribution and morphology of astrocytes using glial fibrillary acidic protein as a marker. In all of these species, nonphosphorylated neurofilament protein-immunoreactive neurons predominated in layer V. These neurons exhibited diverse morphologies with regional variation. Specifically, high proportions of atypical neurofilament-enriched neuron classes were observed, including extraverted neurons, inverted pyramidal neurons, fusiform neurons, and other multipolar types. In addition, many projection neurons in layers II-III were found to contain calbindin. Among interneurons, parvalbumin- and calbindin-expressing cells were generally denser compared to calretinin-immunoreactive cells. We traced the evolution of certain cortical architectural traits using phylogenetic analysis. Based on our reconstruction of character evolution, we found that the living xenarthrans and afrotherians show many similarities to the stem eutherian mammal, whereas other eutherian lineages display a greater number of derived traits.

Cyto- and chemoarchitecture of the cerebral cortex of an echidna (Tachyglossus aculeatus). II. Laminar organization and synaptic density

We have examined the distribution and morphology of neurons immunoreactive for nonphosphorylated neurofilament protein (SMI-32 antibody), calcium-binding proteins (parvalbumin, calbindin, calretinin), and neuropeptide Y as well as neurons reactive for NADPH diaphorase in the cerebral cortex of the Australian short-beaked echidna (Tachyglossus aculeatus). We have also studied synaptic morphology and density in S1 somatosensory cortex and assessed parameters associated with metabolic activity of the cerebral cortex (vessel volume density, mitochondrial volume density, and mitochondrial numerical density) in semi- and ultrathin sections. SMI-32 immunoreactivity was found mostly in layer V pyramidal neurons in selected cortical regions (S1, PV, V1, A). These neurons often showed atypical morphology compared with therian cortex. Neurons immunoreactive for calcium-binding proteins were broadly similar in both morphology and distribution to those seen in therian cortex, although calretinin-immunoreactive neurons were rare. Both Gray type I and Gray type II synapses could be identified in echidna S1 cortex and were similar to those seen in therian cortex. Peak synaptic density was in upper layer IV, followed by layer I, lower layer II, and upper layer III. Most synapses were of type I (72%), although types I and II were encountered with similar frequency in lower layer II and upper layer III. The capillary volume fraction values obtained for the echidna (from 1.18% in V1 to 1.34% in S1 cortex) fall within the values for rodent cortex. Similarly, values for mitochondrial volume fraction in echidna somatosensory cortex (4.68% +/- 1.76%) were comparable to those in eutherian cortex.

Distribution of calcium binding proteins in visual and auditory cortices of hamsters

The morphology and distribution of neurons immunoreactive (ir) to parvalbumin (PV), calretinin (CR) and calbindin (CB) were studied in the primary visual (V1) and auditory (A1) cortices of hamsters. Cortical cell populations were labelled immunohistochemically using a glucose oxidase-diaminobenzidine-nickel combined revelation method. Quantitative analysis revealed significant differences between V1 and A1 in the density and distribution of their neuronal population. CBir cells exhibited several typologies in both cortical regions. Most cells were multipolar even though many of them had bitufted or bipolar morphologies. These cells were distributed in layers II/III and in layer V of both A1 and V1, but were more numerous in layer V of V1. CRir cells were of the fusiform type with long bipolar dendritic arbours. These were similarly distributed in both cortices with a peak in superficial layers II/III. PVir cells were also found in both cortices and had round or oval-shaped somata with multipolar processes. They were mostly located in layer V for V1 and in layers III/IV for A1. Visual and auditory primary cortices can thus be differentiated on the basis of their immunoreactivity to specific calcium binding proteins.

Cortical Orofacial Motor Representation in Old World Monkeys, Great Apes, and Humans

This study presents a comparative stereologic investigation of neurofilament protein- and calcium-binding protein-immunoreactive neurons within the region of orofacial representation of primary motor cortex (Brodmann's area 4) in several catarrhine primate species (Macaca fascicularis, Papio anubis, Pongo pygmaeus, Gorilla gorilla, Pan troglodytes, and Homo sapiens). Results showed that the density of interneurons involved in vertical interlaminar processing (i.e., calbindin- and calretinin-immunoreactive neurons) as well pyramidal neurons that supply heavily-myelinated projections (i.e., neurofilament protein-immunoreactive neurons) are correlated with overall neuronal density, whereas interneurons making transcolumnar connections (i.e., parvalbumin-immunoreactive neurons) do not exhibit such a relationship. These results suggest that differential scaling rules apply to different neuronal subtypes depending on their functional role in cortical circuitry. For example, cortical columns across catarrhine species appear to involve a similar conserved network of intracolumnar inhibitory interconnections, as represented by the distribution of calbindin- and calretinin-immunoreactive neurons. The subpopulation of horizontally-oriented wide-arbor interneurons, on the other hand, increases in density relative to other interneuron subpopulations in large brains. Due to these scaling trends, the region of orofacial representation of primary motor cortex in great apes and humans is characterized by a greater proportion of neurons enriched in neurofilament protein and parvalbumin compared to the Old World monkeys examined. These modifications might contribute to the voluntary dexterous control of orofacial muscles in great ape and human communication.

A comparison of the distribution of GABA-ergic neurons in cortices representing different sensory modalities

It is well known that sensory receptive field properties are shaped by inhibitory processes. Given the physiological and perceptual distinctions among the different sensory modalities, it might be expected that the contribution of GABA-ergic inhibition to the process would vary from area to area, depending on the sensory modality represented. Furthermore, as receptive field properties become progressively more complex at higher cortical levels, differences in the inhibitory contributions to these computations would be reflected in differences in GABA-ergic neuronal distribution. These possibilities were examined in the cortices surrounding the cat Anterior Ectosylvian Sulcus (AES) which contains higher order visual (AEV), somatosensory (SIV) and auditory (Field AES) representations, and is located between the lower-level primary (AI) and secondary auditory (AII) and somatosensory (SII) areas. Using standard immunocytochemical and light-microscopic techniques, the distribution of GABA-ergic neurons (and their co-localized calcium-binding proteins: calbindin (CB), calretinin (CR) and parvalbumin (PV)) was determined for each area. When normalized for differences in cortical thickness, the depth distribution of each of the immunopositive types was plotted. These data confirmed that there were striking differences in the distribution of GABA-, CB-, CR- and PV-positive neurons. However, the laminar organization for a given marker was remarkably similar for the different subregions, irrespective of modality or hierarchical level. These data indicate that, instead of underlying processing differences among different sensory and hierarchical representations, the distribution of GABA-ergic inhibitory neurons reveals common organizational features across sensory cortex.

Electrophysiological classes of cat primary visual cortical neurons in vivo as revealed by quantitative analyses

To facilitate the characterization of cortical neuronal function, the responses of cells in cat area 17 to intracellular injection of current pulses were quantitatively analyzed. A variety of response variables were used to separate the cells into subtypes using cluster analysis. Four main classes of neurons could be clearly distinguished: regular spiking (RS), fast spiking (FS), intrinsic bursting (IB), and chattering (CH). Each of these contained significant subclasses. RS neurons were characterized by trains of action potentials that exhibited spike frequency adaptation. Morphologically, these cells were spiny stellate cells in layer 4 and pyramidal cells in layers 2, 3, 5, and 6. FS neurons had short-duration action potentials (<0.5 ms at half height), little or no spike frequency adaptation, and a steep relationship between injected current intensity and spike discharge frequency. Morphologically, these cells were sparsely spiny or aspiny nonpyramidal cells. IB neurons typically generated a low frequency (<425 Hz) burst of spikes at the beginning of a depolarizing current pulse followed by a tonic train of action potentials for the remainder of the pulse. These cells were observed in all cortical layers, but were most abundant in layer 5. Finally, CH neurons generated repetitive, high-frequency (350-700 Hz) bursts of short-duration (<0.55 ms) action potentials. Morphologically, these cells were layer 2-4 (mainly layer 3) pyramidal or spiny stellate neurons. These results indicate that firing properties do not form a continuum and that cortical neurons are members of distinct electrophysiological classes and subclasses.