Excitation and axonal flow: Autoradiographic study on motoneurons intracellularly injected with a 3H-amino acid

Abstract

Tritiated glycine has been injected intracellularly into cat motoneurons by means of cross barrel iontophoresis through microelectrodes. The amino acid is incorporated into proteins in the cell body and the synthesized radioactive proteins have been localized by autoradiography. The single-cell-injection technique has several advantages over the usual intraperitoneal, intravenous or intrathecal injection, e.g. distribution of the radiochemical is strictly confined to the injected neuron; the amount of injected substance can be estimated; it is easy to investigate simultaneously electrophysiological and morphological characteristics of a defined neuron. Since the injected cell body releases radioactive proteins into the axon, the transport of these substances can be studied under various conditions. In this study the influence of antidromic stimulation has been studied. Stimulated motoneurons demonstrate higher radioactivity in their cell bodies than unstimulated ones, reflecting an increased protein synthesis. The differences are even more pronounced in the axons of stimulated cells when compared with unstimulated ones. A significant, higher amount of radioactive material — up to 100% increase as demonstrated by silver grain counting — can be seen in the axons of stimulated neurons. Although considerable differences seem to exist for the quantity of the exported proteins no significant differences have been detected for the velocity of transportation. In axons of stimulated and unstimulated neurons proteins advance toward the periphery at the same rate.