Ion effects and protein synthesis in synaptosomal fraction
Authors: Morgan I. G. , Austin L. 
Published: January 2005

The capacity of the synaptosomal fraction to synthesize protein in vitro is affected by many ionic environments which also exert effects upon neuronal properties in vivo. 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 vivo 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 vivo (Austin and Morgan, 1967). This conclusion has been supported by an analysis of the pattern of labelling of brain subcellular fractions in vivo (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 in accord with the previously demonstrated ability of isolated nerve segments to incorporate labelled amino acid into axonal proteins (Edström, 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‐chondrial protein synthesis. A recent report (Gordon and Deanin, 1968) has suggested that brain mitochondria have similar properties to synaptosomes with regard to protein synthesis but these effects are possibly due to contamination by other subcellular particles (Yellin, personal communication).

The functional activity of the nervous system in conducting nerve impulses is extremely sensitive to extracellular ion concentrations (Eccles, 1964) and these effects have been studied in isolated neurons (Edwards et al., 1963). Ion effects upon a biochemical parameter, respiration, have been observed in isolated neurons (Giacobini, 1967). Many of these effects are believed to be mediated by changes in properties of the neuronal membrane.

Morphologically the synaptosome appears to be a membrane‐bound sample of axoplasm which typically contains mitochondria and synaptic vesicles (Whittaker, 1965). There is evidence that the synaptosomal membrane may retain some of the in uiuo functions of the neuronal membrane, namely that synaptosomal preparations take up noradrenaline and serotonin (Bogdansky et al., 1968), GABA (Varon et al., 1967), and choline (Potter, 1967). Active transport has not been demonstrated for sodium and potassium ions (Marchbanks, 1967), nor for acetylcholine (Marchbanks, 1968). But these studies were carried out at 0–5° where little enzymic activity would be expected. In view of the possible active functions of the synaptosomal membrane we set out to determine the effects of variations in the incubation medium upon protein synthesis in isolated synaptosomal fractions.