Widespread release of peptides in the central nervous system: quantitation of tannic acid-captured exocytoses.

Tannic acid methods have been applied to capture the exocytosis of peptide-containing granules from peptidergic neurons. The captured exocytoses have been quantitated to assess the proportion and amount of peptide released at different parts of the neuronal membrane. Examination of hypothalamic synaptic boutons shows that only about one-half of the peptidergic vesicles is exocytosed into the synaptic cleft and also that exocytosis also occurs from undilated peptidergic axons. Study of the magnocellular neurosecretory system reveals that all parts of their extensive terminal arborization appear to be equally capable to exocytose peptide. Only about one-half of their peptide is released from their nerve endings, which about the capillaries. The remainder is released much deeper in the lobules of secretory tissue where its principal target(s) could be the pituicytes and/or neurosecretory axons. Dendrites of magnocellular neurons are also capable of releasing peptide by exocytosis and dendrites could release sufficient oxytocin and vasopressin to account for the peptide known to be released into the hypothalamus. We conclude that peptidergic neurons release substantial amounts of peptides from all of their processes and that this must be taken into account when considering what functions those peptides might serve.