Soybean lectin binding neurons in the visual cortex of the rat contain parvalbumin and are covered by glial nets.
We carried out qualitative and quantitative studies on the distribution of soybean agglutinin-labelled cells in the visual cortex of the rat. Lectin-positive nerve cells mostly showed the morphological characteristics of small and large multipolar basket cells. Only a few cells appeared to be bipolar with a vertical or horizontal orientation. By light microscopy, soybean agglutinin binding sites were seen as discontinuous, punctate perineuronal staining (or pericellular nets) on the surface of about 9% of cortical neurons. Lectin-positive cells were predominantly localized in layers IV and V (16.9 and 12.4% of all neurons), where the intensity of staining was also the strongest. Combining lectin- and immunohistochemistry on cryo semithin sections, lectin-positive cells were shown to contain parvalbumin. Nerve cells in the visual cortex containing the related calcium binding-proteins, calbindin or calretinin were never soybean agglutinin-positive. Some glial cells and their processes were also soybean agglutinin-positive. The structure of the soybean agglutinin-positive pericellular nets was similar to that of glial nets visualized with the Golgi-method. Electron microscopy revealed that lectin binding sites were localized on the membranes and cytoplasm of glial processes ensheathing the axon terminals that impinged upon neurons and their proximal dendrites. Synaptic clefts and axon terminals were never reactive, thus explaining the discontinuous punctate labelling of the neuronal surface. Lectin binding sites were also found on the trans-face of the Golgi-complex of some lectin positive neurons suggesting that N-acetylgalactosamine-containing glycoconjugates, which are selectively detected by the lectin, are synthesized, at least partly, within the labelled neurons. We therefore consider possible the existence of specific interactions between parvalbumin positive basket cells and the glial network surrounding them, by which the neuron may determine the conditions of its own microenvironment.