Kathleen S. Rockland

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.

Divergent feedback connections from areas V4 and TEO in the macaque

Extrastriate areas TEO and V4 have been associated with form and color vision. Area V4 has also been suggested to participate in processes concerned with attention, stimulus salience, and perceptual learning. In a continuing effort to elucidate the connectional interactions and microcircuitry of these areas, we describe in this report the pattern of feedback connections from TEO and V4. Connections were demonstrated by injections of the high-resolution anterograde tracers PHA-L or biocytin and further analyzed by reconstruction of 25 individual axons through serial sections. This analysis yielded several new results: (1) Both areas TEO and V4 have widespread feedback connections (defined by their preferential termination in layer 1 and avoidance of layer 4). From TEO, there are dense projections to area V4 and moderate ones to V2 and V1. From V4, there are dense projections to V2 and moderate ones to V3 and V1. (2) Terminal fields span large territories in area V1, up to 6.0 mm in the case of axons originating from TEO; up to 5.0 mm in the case of axons originating from V4. In V2, fields tend to be smaller, between 3.0-5.0 mm. (3) Many axons from TEO and some from V4 have terminations in both areas V1 and V2. (4) Because individual terminal clusters and segments are often larger than cytochrome oxidase compartments, especially in V1, we suggest they may not be correlated with this compartmental organization. These results are consistent with the hypothesis that feedback connections may contribute to processes other than perceptual discrimination.

Two types of corticopulvinar terminations: Round (type 2) and elongate (type 1)

Corticopulvinar axons were anterogradely labeled by Phaseolus vulgaris‐leucoagglutinin injections in the occipitotemporal cortex of the macaque to determine quantitative parameters of divergence and convergence, arbor size and shape, and distribution of terminal specializations. Forty individual axons were analyzed by serial section reconstruction and divided into two major groups. The majority of axons have numerous (typically 500–1,000) small, spinous endings (boutons terminaux). These axons have terminal fields that are beam‐like or elongated (E, corresponding to classical type 1) and highly divergent (1.0–3.0 mm). These frequently innervate several of the traditionally designated pulvinar subdivisions; namely inferior pulvinar (PI) and the ventral part of interal pulvinar (PL); medial pulvinar (PM) and dorsal PL, and (one axon) PM, dorsal PL, and PI. Some axons, however (R or round, corresponding to classical type 2), have a small number (typically 70–160) of primarily large, beaded endings (boutons en passant), which concentrate in sharply delimited, round arbors (diameters 100–125 μm). R axons appear to be larger caliber than E axons (1.0–1.5 μm vs. 0.5–1.0 μm, respectively). These differences in phenotype are probably associated with distinct types of projection neurons. In visual areas, corticopulvinar terminations are reported to originate from pyramidal cell subpopulations in layer 5. Indirect evidence, presented here, suggests that the more numerous medium‐sized neurons give rise to E axons, and the sparser giant pyramids give rise to R corticopulvinar axons. If this is correct, corticopulvinar connectivity may be involved in multiple transformations. Spatially, axons of giant neurons (with basal dendrites that collect intracortically from a disc‐like area, about 1.0 mm in diameter) converge onto a small number of pulvinar neurons. Axons of medium neurons (with basal dendrites that occupy a small intracortical disc, about 0.3 mm in diameter) diverge over 1.0–3.0 mm in the pulvinar and may form many contacts. Giant neurons, although numerically few in relation to medium pyramids (1 or 2: 50?), are likely to have distinctive membrane properties (functionally equivalent to bursting neurons?). Their larger boutons and axon caliber may be associated with a faster transmission that compensates for their small numbers. In primates, the E and R duality does not characterize cortical projections to the caudate, lateral geniculate nucleus, pons, or superior colliculus and thus may be essentially linked to pulvinar‐specific processes.

Some thoughts on cortical minicolumns

Although a columnar geometry is one of the defining features of cortical organization, major issues regarding its basic nature, key features, and functional significance remain unclear and often controversial. This review is intended to survey some of the basic anatomical features of columnar organization, and in particular the smaller scale dendritic minicolumns. One motive was simply to clarify what seem to be differences in terminology, where “minicolumn” can be used to refer to vertical rows of cells, pyramidal cell modules, or apical dendritic bundles. A second aim was to review anatomical details which over the years have tended increasingly to be overlooked. A third aim was to expand on recent results concerning the border of layers 1 and 2 as a specialized zone with its own micromodular organization. Views on columnar organization have arguably been heavily influenced by a desire for general principles; but re-examination of the complex underlying features may be both timely and worthwhile. We point out that what are defined as dendritic bundles do not extend through the full cortical thickness and are not strictly repetitive, but rather display significant inter- and intra-areal variation.

Organization of individual cortical axons projecting from area V1 (area 17) to V2 (area 18) in the macaque monkey.

The present study uses the anterograde tracer, Phaseolus vulgaris-leucoagglutinin (PHA-L), to investigate the detailed morphology of individual axons projecting from area V1 to prestriate area V2. Observations are derived from serial reconstructions of 45 axons. Axons are found to differ both in laminar distribution and in arbor size. The majority (25/45; 56%) terminate in the upper half of layer 4 and the lower part of layer 3. Terminal clusters typically measure about 200 microns in diameter (dimensions are uncorrected for shrinkage), and are either in one, two, or occasionally three patches. Patches are separated by 200-500 microns. Of these 25 axons, four also have minor collaterals to layer 5. Of the remaining 20 axons in our sample, eight have one or two terminal arbors (about 200 microns in diameter) mainly in layer 3; another eight have terminations, organized as a single field (about 350 microns in diameter), within layer 4; and four axons have much larger terminal fields (1.0-1.2 mm x 0.3 mm), in layers 3 and 4. These morphological differences might constitute a gradient or, alternately, indicate distinct subgroups within the striate efferent population. Large terminal fields are asymmetrical, with their long axis oriented in an anterior-posterior fashion toward the depth of the lunate sulcus. Axons with two terminal arbors have a similar bias. As this arrangement is approximately perpendicular to the border of V1, we suggest that striate axons may be extended preferentially along the length of the stripelike compartments in V2. These compartments are also arrayed perpendicular to the border between areas V1 and V2. Reconstruction of small groups of 2-4 convergent axons demonstrates that axons with different morphology (i.e. large or small terminal fields) can occur within the same projection focus. Terminal arbors belonging to different axons can overlap, but tend not to be superimposed exactly.

Neuron-specific distribution of P2X7 purinergic receptors in the monkey retina.

Extracellular ATP is a signaling molecule, working through P2X purinoceptors in the nervous system. P2X7 is a major subtype of the purinoceptors in the brain, where it is expressed mostly in glia cells and considered to work as a trigger of cytolysis. In the rodent retina, however, P2X7 is expressed in several classes of neurons including ganglion cells. In the present study we identified cells immunopositive for P2X7 by double immunolabeling. Immunoreactivity for P2X7 was observed in the inner nuclear layer (INL), the inner plexiform layer (IPL), and the ganglion cell layer (GCL). In the INL, strongly immunopositive cells corresponded to the subpopulation of horizontal cells. In the IPL, fine processes were immunopositive. In the GCL, most of the ganglion cells showed P2X7 immunoreactivity. At the ultrastructural level, immunoreactivity was confirmed in the cytoplasm of ganglion cells. No P2X7 immunoreactivity was found in non‐neural cells, i.e., Müller cells or microglia. The immunohistochemical distribution of other purinoceptor subtypes (P2X1, P2X2, and P2X4) was also examined in the monkey retina. Immunoreactivity for P2X1 was strongly detected in a band, in sublamina a of the IPL. The band existed at almost the same level as tyrosine hydroxylase immunoreactivity, but did not seem to actually overlap. P2X2 was not expressed in the retina, and P2X4 was only faintly expressed at the scleral margin of the INL. Because P2X7 in the primate retina is expressed exclusively in neurons, it may in this location be involved in neural transmission rather than in cytolysis, as found for glia cells. J. Comp. Neurol. 459:267–277, 2003.

Long-distance corticocortical GABAergic neurons in the adult monkey white and gray matter.

A subgroup of GABAergic neurons has been reported to project over long distances in several species. Here we demonstrate that long‐distance cortically projecting nonpyramidal neurons occur in monkeys in both white and gray matter. Nonpyramidal neurons were first identified morphologically. Visualization of Golgi‐like details was achieved by retrograde infection from injections of an adenovirus vector, producing enhanced green fluorescent protein (EGFP) under control of a neuron‐specific promoter. Injections in areas V1, V4, TEO, and posterior TE resulted in EGFP‐expressing nonpyramidal neurons up to 1.5 cm distant from the injections, mainly in the white matter. Some neurons occurred in the gray matter, mainly in layer 3, but also in layers 5 and 6, and, very occasionally, layer 1. As control, we injected cholera toxin subunit B, a standard retrograde tracer, in V4, and observed a similarly wide distribution of neurons in the white matter. Second, the GABAergic identity of EGFP‐expressing nonpyramidal neurons was established by colabeling for EGFP and GAD67 in selected tissue sections. Most neurons positive for EGFP and GAD67 were positive for somatostatin (SS; 90%). Of those neurons positive for EGFP and SS, almost all were also positive for neuronal nitric oxide synthase or m2 muscarinic receptor, but only 23% were also positive for calretinin. None were positive for parvalbumin. We conclude that long‐distance projecting GABAergic neurons 1) are phylogenetically conserved, although in monkeys most gray matter neurons are in the upper layers, and 2) are heterogeneous in terms of their neurochemistry, location, and potentially function. J. Comp. Neurol. 505:526–538, 2007.

Single axon analysis of pulvinocortical connections to several visual areas in the Macaque

The pulvinar nucleus is a major source of input to visual cortical areas, but many important facts are still unknown concerning the organization of pulvinocortical (PC) connections and their possible interactions with other connectional systems. In order to address some of these questions, we labeled PC connections by extracellular injections of biotinylated dextran amine into the lateral pulvinar of two monkeys, and analyzed 25 individual axons in several extrastriate areas by serial section reconstruction. This approach yielded four results: (1) in all extrastriate areas examined (V2, V3, V4, and middle temporal area [MT]/V5), PC axons consistently have 2–6 multiple, spatially distributed arbors; (2) in each area, there is a small number of larger caliber axons, possibly originating from a subpopulation of calbindin‐positive giant projection neurons in the pulvinar; (3) as previously reported by others, most terminations in extrastriate areas are concentrated in layer 3, but they can occur in other layers (layers 4,5,6, and, occasionally, layer 1) as collaterals of a single axon; in addition, (4) the size of individual arbors and of the terminal field as a whole varies with cortical area. In areas V2 and V3, there is typically a single principal arbor (0.25–0.50 mm in diameter) and several smaller arbors. In area V4, the principal arbor is larger (2.0‐ to 2.5‐mm‐wide), but in area MT/V5, the arbors tend to be smaller (0.15 mm in diameter). Size differences might result from specializations of the target areas, or may be more related to the particular injection site and how this projects to individual cortical areas.

Inferior parietal lobule projections to the presubiculum and neighboring ventromedial temporal cortical areas

The entorhinal and perirhinal cortices have long been accorded a special role in the communications between neocortical areas and the hippocampal formation. Less attention has been paid to the presubiculum, which, however, is also a component of the parahippocampal gyrus, receives dense inputs from several cortical areas, and itself is a major source of connections to the entorhinal cortex (EC). In part of a closer investigation of corticohippocampal systems, the authors applied single‐axon analysis to the connections from the inferior parietal lobule (IPL) to the presubiculum. One major result from this approach was the finding that many of these axons (at least 10 of 14) branch beyond the presubiculum. For 4 axons, branches were followed to area TF and to the border between the perirhinal and entorhinal cortices, raising the suggestion that these areas, which sometimes are viewed as serial stages, are tightly interconnected. In addition, the current data identify several features of presubicular organization that may be relevant to its functional role in visuospatial or memory processes: 1) Terminations from the IPL, as previously reported for prefrontal connections (Goldman‐Rakic et al. [1984] Neuroscience 12:719–743), form two to four patches in the superficial layers. These align in stripes, but only for short distances (≈1.5 mm). This pattern suggests a strong compartmentalization in layers I and II that is also indicated by cytochrome oxidase and other markers. 2) Connections tend to be bistratified, terminating in layers I–II and deeper in layer III. 3) Single axons terminate in layer I alone or in different combinations of layers. This may imply some heterogeneity of subtypes. 4) Individual axons, both ipsilateral projecting (n = 14 axons) and contralateral projecting (n = 6 axons), tend to have large arbors (0.3–0.8 mm across). Finally, the authors observe that projections from the IPL, except for its anteriormost portion, converge at the perirhinal‐entorhinal border around the posterior tip of the rhinal sulcus. These projections partially overlap with projections from ventromedial areas TE and TF, and this convergence may contribute to the severe deficits in visual recognition memory resulting from ablations of rhinal cortex. J. Comp. Neurol. 425:510–530, 2000.

Expression of COUP-TFII Nuclear Receptor in Restricted GABAergic Neuronal Populations in the Adult Rat Hippocampus

The COUP-TFII nuclear receptor, also known as NR2F2, is expressed in the developing ventral telencephalon and modulates the tangential migration of a set of subpallial neuronal progenitors during forebrain development. Little information is available about its expression patterns in the adult brain. We have identified the cell populations expressing COUP-TFII and the contribution of some of them to network activity in vivo. Expression of COUP-TFII by hippocampal pyramidal and dentate granule cells, as well as neurons in the neocortex, formed a gradient increasing from undetectable in the dorsal to very strong in the ventral sectors. In the dorsal hippocampal CA1 area, COUP-TFII was restricted to GABAergic interneurons and expressed in several, largely nonoverlapping neuronal populations. Immunoreactivity was present in calretinin-, neuronal nitric oxide synthase-, and reelin-expressing cells, as well as in subsets of cholecystokinin- or calbindin-expressing or radiatum-retrohippocampally projecting GABAergic cells, but not in parvalbumin- and/or somatostatin-expressing interneurons. In vivo recording and juxtacellular labeling of COUP-TFII-expressing cells revealed neurogliaform cells, basket cells in stratum radiatum and tachykinin-expressing radiatum dentate innervating interneurons, identified by their axodendritic distributions. They showed cell type-selective phase-locked firing to the theta rhythm but no activation during sharp wave/ripple oscillations. These basket cells in stratum radiatum and neurogliaform cells fired at the peak of theta oscillations detected extracellularly in stratum pyramidale, unlike previously reported ivy cells, which fired at the trough. The characterization of COUP-TFII-expressing neurons suggests that this developmentally important transcription factor plays cell type-specific role(s) in the adult hippocampus.