The capacity of the synaptosomal fraction to synthesize protein in uitro is affected by many ionic environments which also exert effects upon neuronal properties in uiuo. Protein synthesis in this fraction is at a maximum in high sodium ion, and low but not zero potassium ion medium. The effects of changes in the ionic composition of the incubation medium upon synaptic fraction protein synthesis resemble the effects observed in uiuo on the neuronal membrane from which it is derived. Evidence is obtained for the function of Na-ATPase in amino acid uptake into the particles.
Since the experiments of Weiss and Hiscoe, 1948, it has been generally accepted that pre-synaptic and axonal proteins of neurons are supplied from the cell-body to these areas by the process of somato-axonal flow. However we have argued from the time-course of incorporation of labelled amino acids into proteins of the synaptosomal fraction in chopped brain tissue, that synaptic protein synthesis occurs in uiuo (Austin and Morgan, 1967). This conclusion has been supported by an analysis of the pattern of labelling of brain subcellular fractions in uiuo (Von Hungen et al., 1968) which indicated that synaptosomal membrane fragments were capable of protein synthesis.
Recently we have shown that synaptosomal fractions isolated from immature rat brain, synthesize protein in a cell-free system (Morgan and Austin, 1968). This ability of isolated pre-synaptic material to synthesize protein has been supported by other studies on protein synthesis in synaptosomal preparations (Autilio, et al., 1968; Gordin, et al., 1968) and is in accord with the previously demonstrated ability of isolated nerve segments to incorporate labelled amino acid into axonal proteins (Edstrom, 1966).
Although a high rate of protein synthesis has been observed in synaptosomal preparations, the morphological entities with which it is associated have not yet been unequivocally identified. Previous studies (Morgan and Austin, 1968) demonstrated that the properties of the protein synthesis were indicative of the operation of a eukaryotic protein synthesizing system though a small component was attributed to mito-155