Laser light scattering determination of size and dispersity of synaptosomes and synaptic vesicles isolated from squid (Loligo pealei) optic lobes

Quasi-elastic laser light scattering has been used to investigate the size and dispersity of synaptosomes and synaptic vesicles isolated from optic lobes of the squid Loligo pealei. Synaptosomal fractions were highly polydisperse ( \({{\mu _2 } \mathord{\left/ {\vphantom {{\mu _2 } {\bar \Gamma }}} \right. \kern-0em} {\bar \Gamma }}^2 = 0.5\) ) and the mean diameter ( \(\bar d\) ) ranged from 0.5–2.0 μm. Size distribution histograms yielded two major components — smaller particles ( \(\bar d \sim 300 - 700{\text{ nm}}\) ) and a larger group of particles ( \(\bar d \sim 1,500 - 5,000{\text{ nm}}\) ). The heterogeneity of the synaptosomal particles detected in solution is in agreement with published data obtained using electron microscopy. Purified synaptic vesicle fractions also yielded complex particle size distribution data. A component with a mean diameter in the range 150–250 nm was detected, though a smaller particle ( \(\bar d \sim 40 - 110{\text{ nm}}\) ) dominated the scattering signal. This smaller particle closely resembles in size the electron lucent vesicles seen in the majority of squid optic lobe nerve terminals when examined by electron microscopy. Osmotically-induced shirnkage and swelling of the synptosomes was detected. Depolarization by veratridine (1.0×10⁻⁴ M) did not result in a detectable change in the size of synaptosomal particles.