This study examines the axonal projections of so-called inverted pyramids and other neurons with their major dendritic shaft oriented in the direction of the white matter ('inverted cells') in the adult rabbit cortex. Single injections of horseradish peroxidase wheat germ agglutinin were made into cortical or subcortical sites. The resulting retrograde labelling in the cortex was analysed and the distribution across areas and layers of inverted cells contributing to each of these projections was estimated. In addition, the radial distribution of inverted cells was independently determined from rapid Golgi-impregnated and Nissl-stained material. All three procedures revealed that inverted cells lay overwhelmingly in infragranular layers, but congregated at the border between layers 5 and 6. Inverted cells, identified by retrograde labelling, seldom furnished non-telencephalic centres; in contrast, these cells constituted a major source for the projections to the ipsi- or the contralateral cortex, the claustrum or the nucleus caudatus. In general, each set of inverted cells (when defined by its specific destination as a group) was located below the typically oriented cells whose axons were aimed at the same target. Thus, the inverted cells of the rabbit cortex are characterized not only by their unique morphology and their corticocortical, corticoclaustral and corticostriatal projections, but also by their distinctive radial locations. These findings suggest that inverted cells, even though possibly composed of different cell types, are a specific class of projection neurons.

No abstract

To study the distribution of L-homocysteate in the rat retina, specific polyclonal and monoclonal anti-homocysteate antibodies have been used in combination with a highly sensitive postembedding method for light microscopic immunocytochemistry. In central and peripheral retina, the most strongly immunoreactive cell bodies lay in the inner nuclear layer. They represented about 17% of the total neuronal cell population of the layer and were identified as bipolar cells (19-20% of cells in the outer half of the inner nuclear layer) and amacrine cells (15% of cells in the inner half of the inner nuclear layer). A third cell type showing heavy homocysteate-like immunoreactivity was identified as Müller glial cells. Characteristically, their descending processes formed three immunoreactive bands in the inner plexiform layer. Furthermore, the outer and inner limiting membranes as well as glia around and between ganglion cell axons and in the vicinity of blood vessels were labelled intensely. Photoreceptors and their terminals, and ganglion cells, were not immunostained. These findings indicate the presence of homocysteate in some bipolar and amacrine cells of the inner nuclear layer and support a role for this sulphur-containing excitatory amino acid as a neurotransmitter candidate in the retina.