Elsevier

Brain Research

Volume 1120, Issue 1, 20 November 2006, Pages 13-22
Brain Research

Research Report
Distribution of pyramidal cells associated with perineuronal nets in the neocortex of rat

https://doi.org/10.1016/j.brainres.2006.08.069Get rights and content

Abstract

Perineuronal nets are lattice-like accumulations of extracellular matrix components around the cell body and perisomatic portion of certain neurons. Whereas interneurons associated to this specific neuron-associated sheath have been elaborately classified, less effort has been undertaken to describe the occurrence of perineuronal nets around pyramidal neurons. Our aim was to give a detailed and comparative description of the occurrence of net-associated pyramidal cells throughout the rat neocortex as well as to systematically and comparatively analyze the relation of main projection types of principal neurons to the presence of perineuronal nets. The present study revealed that perineuronal nets stained with WFA were associated rather rarely to pyramidal cells compared to interneurons in layers II/III and V/VI of rat neocortex. However, their frequency was considerably different between various cortical areas with a maximum in visual cortex and with a minimum in secondary motor cortices. Further analysis revealed that neuron-associated matrix sheaths around principal cells were more common in the primary than in the secondary fields of corresponding areas and they were more numerous in infra-than in supragranular layers in most regions. Subfields of cortical areas also differed regarding the occurrence of net-associated principal cells, and the subtlety of cortical representation seemed to correlate with the frequency of perineuronal nets around pyramidal neurons in the primary somatosensory cortex. It appears that net-associated pyramidal cells do not have a projection pattern restricted to distinct target regions. Rather a functional heterogeneity of the pyramidal cell population contributing to specific intra-or subcortical projections is suggested.

Introduction

More than hundred years after their discovery (Golgi, 1893, Ramón y Cajal, 1898), the morphological and functional characterization of perineuronal nets (PNs) of the extracellular matrix gained increasing interest among neuroscientists. Despite series of thorough and versatile investigations, the role of these neuron-associated sheaths is not clear (Celio and Blümcke, 1994, Celio et al., 1998). PNs function has been implicated in synaptic stabilization and in depression of synaptic plasticity (Hockfield et al., 1990, Lander et al., 1997, Fox and Caterson, 2002, Pizzorusso et al., 2002), in the support of ion homeostasis around highly active neurons (Brückner et al., 1993, Brückner et al., 1996a, Brückner et al., 1996b, Brückner et al., 2006, Härtig et al., 1999, Härtig et al., 2001, Hobohm et al., 1998) or in neuroprotection (Brückner et al., 1999, Schüppel et al., 2002, Morawski et al., 2004). The functional significance of this specialized neuronal environment has been suggested and supported by morphological findings describing the location, structure and composition of PNs. They ensheath the perikaryon, proximal parts of dendrites, the axon initial segment and the presynaptic boutons attached to these structures (Hendry et al., 1988, Brückner et al., 1993, Brückner et al., 1996b, Brückner et al., 2006). The main components of PNs are polyanionic chondroitin sulfate proteoglycans (CSPG) and associated molecules such as hyaluronic acid and tenascins (Brückner et al., 1993, Brückner et al., 2000, Köppe et al., 1997, Hagihara et al., 1999, Matsui et al., 1998, Matthews et al., 2002). Lectins, such as Wisteria floribunda agglutinin (WFA), are established markers for PNs for their strong affinity to N-acetylgalactosamine, which is a constituent amino sugar of glycosaminoglycan chains of CSPG (Härtig et al., 1992).

Further advances revealed that PNs in the cerebral cortex are primarily associated with GABAergic interneurons (Brückner et al., 1994, Mulligan et al., 1989, Naegele et al., 1988, Nakagawa et al., 1987) most of which express the calcium-binding protein parvalbumin (Kosaka and Heizmann, 1989, Härtig et al., 1992, Morino-Wannier et al., 1992). However, PNs labeled with WFA or the lectin Vicia villosa were detected around pyramidal neurons as well in different mammalian species, but these perineuronal sheaths were in most cases less established, hence, faintly stained (Ohyama and Ojima, 1997, Brückner et al., 1999, Härtig et al., 1999). Previous studies revealed regional differences in the occurrence and frequency of PNs: primary sensory or motor areas contain considerably more net-associated pyramidal cells (naPCs) than secondary or higher order association areas (Hendry et al., 1988, McGuire et al., 1989, Hausen et al., 1996, Brückner et al., 1999). However, further characterization of different pyramidal cell populations in relation to PNs has only begun (Hendry et al., 1988, Ohyama and Ojima, 1997, Wegner et al., 2003). Similarly, a complete and comprehensive description of the association of PNs to pyramidal cells as well as to their various projection types in rat neocortex has been lacking.

The present study quantitatively analyzes naPCs in the rat neocortex and investigates the association of pyramidal neurons with different projection patterns to the presence of perineuronal sheath in the neocortex of the rat.

Section snippets

Quantitative analysis of the density of naPCs in the various cortical areas

Following WFA histochemistry, naPCs were found in different number and density in the distinct cortical areas (Fig. 1, Fig. 2, Table 1, Table 2). The frequency of naPCs was characterized by the number of naPCs per 1 mm2 (Table 2). Brain regions could be classified into five groups according to their relative density of naPCs. Primary and secondary visual cortices (V1 and V2, respectively) had the highest density values, primary auditory (Au1) had high/medium, the parietal association cortex

Discussion

Previous studies have offered thorough descriptions of PNs around cortical GABAergic interneurons (Brückner et al., 1994, Mulligan et al., 1989, Naegele et al., 1988, Nakagawa et al., 1987) and suggested versatile implications in cell functions including synaptic plasticity, neuroprotection or ion homeostasis (Brückner et al., 1993, Brückner et al., 1996a, Brückner et al., 1996b, Brückner et al., 1999, Pizzorusso et al., 2002). Whereas subtle details of PNs of interneurons have been

Animals

Seventeen five-months-old Wistar rats of both sexes were used. Treatment of animals was in accordance with the Ethical guideline of SOTE and the European Communities Council Directive (86/609/EEC; November 24, 1996).

WFA histochemistry

Five rats were perfused transcardially in deep anesthesia, first with saline (0.9% NaCl) for 1–2 min and then with a fixative containing 4% paraformaldehyde and 0.1% glutaraldehyde in 0.1 M phosphate buffer (pH 7.4) for 30 min. The brains were postfixed in a fixative containing 4%

Acknowledgments

We are grateful for Dr. Gábor Gerber, Dr. Márk Kozsurek and Dr. József Takács for generously providing us the animals used in this study. We would like to thank Dr. Markus Morawski, Dr. László Négyessy and Dr. Zita Puskár for his useful advises. This study was supported by the Basic Research Foundation of Hungary (grant OTKA F 048350), the Hirnliga e.V. and by the Interdisziplinäres Zentrum für Klinische Forschung (IZKF) Leipzig at the Faculty of Medicine of the Universität Leipzig (C1).

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