Giant synaptosomes.

Investigations using synaptosomes, pinched-off nerve ending particles from brain, have greatly improved our knowledge of presynaptic function. However, these structures, like most nerve endings, are too small to be penetrated with microelectrodes. We have treated synaptosomal preparations from rat brain with a neutral protease and obtained fused structures large enough to be recorded from directly with microelectrodes; we report here that these particles (30-250 microM in diameter) contain mitochondria and structures resembling synaptic vesicles, morphological features characteristic of synaptosomes. These 'giant synaptosomes' have resting membrane potentials in the range -45 to -76 mV, are depolarized by increasing concentrations of K+, show responses to a variety of neuroactive substances and exhibit active membrane responses to depolarizing current pulses. These results suggest that this preparation will be of value in further studies of nerve terminal function.