Fine structure of the synaptic discs separated from the goldfish medulla oblongata

Abstract

Synaptic discs are structures localized in the club ending synapses on the Mauthner cell lateral dendrite of the goldfish medulla oblongata. The synaptic discs present a hexagonal array of particles approximately 8.5 nm center-to-center when observed in en face view. This lattice covers the entire surface Divalent cations are important in the stabilization of this particular hexagonal array of particles When a synaptic disc-rich fraction is treated with chelating agents (EDTA or EGTA), definite changes occur in the hexagonal lattice. First, the synaptic membranes show zones without particles interspersed with zones covered with the hexagonal array of particles Second, the synaptic discs break down and a new structure characterized by two parallel dense bands (7 nm each), separated by a 4 nm gap, is observed. The negative stain fills the gap region showing striations spaced approximately 10 nm center-to-center crossing the gap, but it does not penetrate the dense bands This "double band" structure is interpreted as an edge on view of a fragment of the synaptic membrane complex. Further treatment of this fraction with a chelating agent plus 0.3% deoxycholate produces an increase in the number of double band structures. However, EDTA plus Triton X-100 (a treatment known to produce solubilization of membrane proteins) never shows such double band structure An ordered material was observed associated with the cytoplasmic leaflets of the double bands This material consists of rows of beads approximately 4 nm in diameter and spaced at intervals of approximately 7 nm. Each of these beads is joined to the band by a thin stalk.