Extracellularly applied horseradish peroxidase increases the number of dense core vesicles in leech sensory neurons

The uptake of horseradish peroxidase (HRP), applied as an extracellular tracer, is a classical method for studying endo/exocytosis of synaptic vesicles at the ultrastructural level. It is generally not considered that HRP may affect neuronal function. Reported here is the finding that extracellularly applied HRP (0.1%) perturbs dense core vesicles in the synaptic processes of leech neurons. The strength of the effect varies with neuronal class. In sensory afferents, the number of dense core vesicles increases 5-fold, while there is only a 2-fold increase in central neurons. [Copyright &y& Elsevier]