Nerve growth factor (
NGF) is known to intensify
pain in various ways, so perturbing pertinent effects without negating its essential influences on neuronal functions could help the search for much-needed
analgesics. Towards this goal, cultured neurons from neonatal rat trigeminal ganglia-a locus for craniofacial sensory nerves-were used to examine how
NGF affects the Ca2+-dependent release of a
pain mediator,
calcitonin gene-related peptide (CGRP), that is triggered by activating a key signal transducer, transient receptor potential vanilloid 1 (TRPV1) with
capsaicin (CAP). Measurements utilised neurons fed with or deprived of
NGF for 2 days. Acute re-introduction of
NGF induced Ca2+-dependent CGRP exocytosis that was inhibited by
botulinum neurotoxin type A (
BoNT/A) or a chimera of/E and/A (/EA), which truncated SNAP-25 (synaptosomal-associated
protein with Mr = 25 k) at distinct sites.
NGF additionally caused a Ca2+-independent enhancement of the
neuropeptide release evoked by low concentrations (<100 nM) of CAP, but only marginally increased the peak response to ≥100 nM. Notably,
BoNT/A inhibited CGRP exocytosis evoked by low but not high CAP concentrations, whereas/EA effectively reduced responses up to 1 µM CAP and inhibited to a greater extent its enhancement by
NGF. In addition to establishing that sensitisation of sensory neurons to CAP by
NGF is dependent on SNARE-mediated membrane fusion, insights were gleaned into the differential ability of two regions in the C-terminus of SNAP-25 (181-197 and 198-206) to support CAP-evoked Ca2+-dependent exocytosis at different intensities of stimulation.