Ultrastructure of a histaminergic synapses in Aplysia.

The ultrastructure of histaminergic synaptic terminals was studied by the means of intrasomatic injection of horseradish peroxidase into the identified histaminergic neuron C2 of Aplysia. The axonal tree of C2 was found to consist, in part, of varicosities that display putative release sites similar in morphology to those described in other neurons in Aplysia. The varicosities contain at least two populations of vesicles: a conspicuous class of of large vesicles with an electron-dense core that almost fills the entire vesicle and a heterogeneous class of large and small electron-lucent vesicles. The small lucent vesicles preferentially cluster near active zones.