"Distribution of glutaminergic, gabaergic, cholinergic, serotonergic, and adrenergic receptors as a function of cortical layer in human V2.

"Distribution of glutaminergic, gabaergic, cholinergic, serotonergic, and adrenergic receptors as a function of cortical layer in human V2.

is found in layers II and III, with lower densities in layers I, IV, V, and VI. In contrast, the a2A receptors show a higher density in area V1 than in V2. This difference is largely due to the high density of a2A receptors in area V1. In V2, the highest density of a2A receptors is found in layers II, V, and VI, peaking in layer VI.

The distribution of serotonergic receptors can also be used to distinguish area V2 from area V1. 5-HT1 binding sites have a higher density in area V1 than in V2. 5-HT1A binding sites have a similar laminar distribution in both areas, peaking in layers I and II. 5-HT2 binding sites, determined through [3H]keran-serin autoradiography, have a high density in layer 4C of V1, and layers II and V have the higher density in area V2. 5-HT2A receptor mRNA has its highest density in layer IVC of area V1, lower densities in layers III and II, and lowest densities in layers IVB, V, and VI. In V2, 5-HT2A receptors show their highest densities in layers III and V, and lower densities were found in layers II and IV.

The distribution of nicotinamide adenine dinucleo-tide phosphate-diaphorase (NADPH-d) immunoreac-tivity has been used to distinguish area V2 from area V1. NADPH-d immunohistochemistry labels both the neurophil and several sizes of neurons in both the gray and white matter. In V1, NADPH-d activity in the neuropil is most dense in layer IV, and regions of high activity can be observed to extend into the supragranular layers. In V2, the NADPH-d-back-ground activity is more diffuse, extending from layer IV into the supragranular layers. NADPH-d-positive cells in areas V1 and V2 can be subdivided into large gray matter cells (16 x 16 mm soma size) with round, oval, or pyramidal cell bodies with sparely spinous dendrites, large white matter cells (12 x 19 mm soma size) with oval cell bodies that are horizontally oriented and contain spinous dendrites, and small gray matter cells (3.6 x 4 mm soma size) with stellate shape. The small cells are distributed in layers II-VI of areas V1 and V2 and they are most numerous in layer IV.

Area V2 also contains a rich complement of neurons and fibers immunoreactive for neuropeptide Y. In both areas V1 and V2, immunoreactive pyramidal and nonpyramidal cells are found in layers I-VI. The nonpyramidal cells are either bipolar, bitufted, or multipolar aspiny cells. In layers II and V, most immunoreactive cells are pyramidal in shape. In addition to these cells located in the cortical gray matter, the majority of neuropeptide Y immunoreac-tive cells are located in the underlying white matter. Many of these cells are located just below the border of layer VI, whereas other cells are located deeper within the white matter. Immunoreactive fibers are found in two plexuses: one in layers I and II and a second in layer V. This pattern is distinct from that seen in area V1, where a third plexus in layers IVB and IVC is observed. Many of these immunoreactive fibers are found to surround small blood vessels, suggesting that they play a role in the regulation of cerebral blood flow. The localization of neuropeptide Y in both aspinous and spinous pyramidal cells suggests that it plays a role in both inhibitory and excitatory neurotransmission. The localization in layer III and V pyramidal cells suggests that this peptide is involved in cotransmission in long-range corticocortical and corticofugal pathways.

Calcium-binding proteins parvalbumin (PV), cal-bindin (CB), and calretinin (CR) have been reported to be colocalized in GABAergic neurons in human visual cortex. In general, these GABAergic cells are inter-neurons of the chandelier and basket cell types located in layers II and IV (PV), double bouquet cells of layers II-III and V (CB), or bipolar and fusiform cells in layers II and III (CR). In addition, some pyramidal cells in layers II-III are immunoreactive for calcium-binding proteins. In some cases, more than one calcium-binding protein has been localized in individual cells in areas V1 and V2 of human visual cortex. In area V2(area 18 near the V1 border), 3.28% of cells are immunoreactive for both PV and CR, whereas 7.39%

of cells are immunoreactive for both CR and CB. The colocalization of PV and CB is rare in both areas V1 and V2. In addition to the single- and double-labeled cells located within the gray matter of V2, single- and double-labeled fibers were observed in the underlying white matter. These labeled fibers may belong to projection neurons from the lateral geniculate nucleus or pulvinar because these cells are often immunopositive for calcium-binding proteins.

Many parvalbumin immunoreactive neurons are surrounded by extracellular lattice coatings called perineuronal nets that are stainable with lectins such as Wisteria floribunda agglutinin (WFA). Areas V1 and V2 differ in the density, laminar distribution, and target cells of WFA immunoreactivity. In V1, WFA staining is largely distributed in two bands; one located in layer IVB and the second in layer VI. Lighter staining is observed in layers II and III. In V2 a different laminar pattern of labeling is observed. Dense labeling is observed in layers IIIB-IIIC, and less intense labeling is observed in layer V. In addition to these laminar differences, areas V1 and V2 differ in the size of neurons that were enveloped by WFA finding. In both areas, medium-sized cells (13-21 mm) are the most commonly enveloped cell type. In V1 a greater percentage of small cells (5-12 mm) are labeled in layers II-III, IVA, IVB, and IVC. In contrast, a larger percentage of large cells (22-30 mm) are labeled in layers IIIB, IIIC, and V of area V2. Finally, layer VI of V1 contains a larger proportion of labeled large cells.

The density and laminar distributions of neurons that are immunoreactive to SMI-32 that recognizes nonphosphorylated neurofilament proteins, also provide a basis for distinguishing area V1 from area V2. In area V1, densely stained SMI-32 immunoreactive neurons are found in layer 4B and the Meynert cells of layer VI. In addition to these large cells, smaller immunoreactive cells are found in layers III, V, and VI. In area V2, SMI-32 immunoreactivity is found in the large cells of layers III and V (see Fig. 11A). Area V2 also contains a population of SMI-32 immunoreactive cells that are affected in cases of Alzheimer's disease. Although area V2 contained significantly less neuritic plaques and neurofibrillary tangles than prefrontal and temporal cortex, a significant loss of SMI-32 immunoreactive neurons is found in layers III and VI of area V2 (average loss of 18%). These results reinforce the view that Alzheimer's disease preferentially affects neurons with long corticocortical axons such as those in layers III and VI of V2 that project to distant extrastriate cortical areas or to subcortical targets (Fig. 11B).

Figure 11 Photomontage of SMI-32 immunoreactivity in human area 18. (A) Normal cortex. Large SMI-32-ir neurons are present in layers Illd and V. (B) V2 in Alzheimer's disease is characterized by a major loss in the density of SMI-32-ir neurons in layers Illd and V. From Hof and Morrison (1990).
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