ACTIVE POLYSOMES ARE PRESENT IN THE LARGE PRESYNAPTIC ENDINGS OF THE SYNAPTOSOMAL FRACTION FROM SQUID BRAIN

Previous data have suggested that the large nerve terminals present in the synaptosomal fraction from squid optic lobe are capable of protei n synthesis (Crispino et al., 1993a,b). We have further examined this issue by comparing the translation products of synaptosomal and micros omal polysomes. Both preparations programmed an active process of tran slation, which was completely abolished by their previous treatment wi th EDTA. After immunoabsorption of the newly synthesized neurofilament (NF) proteins, the labeling ratio of the 60 and 70 kDa NF proteins wa s found to differ, in agreement with comparable differences obtained w ith intact synaptosomes. These observations indicate that the set of m RNAs translated by synaptosomes differs from that translated by nerve cell bodies. Hence, because NF proteins are neuron-specific, they supp ort the view that the active synaptosomal polysomes are mostly localiz ed in the large nerve terminals that represent the most abundant neuro nal component of the fraction. This hypothesis was confirmed (1) by el ectron spectroscopic data demonstrating the presence of ribosomes and polysomes within the large nerve endings of the synaptosomal fraction, as well as in the carrot-like nerve endings of the retinal photorecep tors that constitute the only large terminals in the optic lobe, and ( 2) by light and high resolution autoradiography of synaptosomal sample s incubated with [H-3]leucine, showing that most labeled proteins are associated with the large nerve endings. This response was abolished b y cycloheximide. Taken together, the data provide the first unequivoca l demonstration that presynaptic nerve terminals are capable of protei n synthesis.