A specific 'axo-axonal' interneuron in the visual cortex of the rat.

Synaptic contacts located on the axon hillock and the initial axon segment of pyramidal neurons have been observed already by Palay et al. 9 and have been described in more detail by Peters et al.lO. Nothing, so far, is known about the origin and possible significance of these axo-axonic contacts. More recently we have come across a specific type of interneuron in the visual cortex of the rat that, due to the abundance in which it appears to occur in materials stained with our Golgi procedure, and due to its peculiar axon terminals, was deemed to be an interesting subject for a combined Golgi and EM study. The technique used will be reported in detail in a paper dealing with another major problem of cortical synaptology that can be solved with such a combined approach . It might suffice, therefore, to mention only very briefly that adult albino rats of our local breed were perfused through the heart with a solution of 1.25 % glutar-aldeh yde and 2 % paraformaldehyde in 0.1 M sodium phosphate buffer (pH 7.2-7.4) at room temperature for 30 min. The entire primary visual cortex was removed according to the stereotaxic coordinates given by Schober and Winkelmann l l. The tissue blocks were further immersed in the same fixation fluid and postosmicated in 2 % osmium tetroxide solution after 2 x I h washing in the buffer. The blocks were then immersed in 4 % potassium dichromate solution for one to two days and were transferred into a 0.75 % solution of silver nitrate. Sections of 100,um thickness were cut with the aid of a Sorvall TC-2 tissue chopper and were mounted after dehydration in Durcupan. Suitably stained areas were selected and re-embedded for electron microscopy. Serial thin sections were mounted on formvar coated single-slot grids. The contrast was enhanced by 'en-bloc' staining with uranyl acetate and of the sections with lead citrate. A continuous light microscopic control enabled us to follow the serial sectioning, so that not only Golgi-stained cells and their dendrites but also various axon branches and individual terminal boutons could be easily identified at the EM level. The specific interneuron that caught our interest is a cell that resembles slightly the so-called 'chandelier cell' of Szentagothai 13 ,14 but even more the interneuron type 4 of Jones 6 in the monkey somatosensory cortex. Such cells have been found in our material mainly …