Journal of Neuroscience, Vol 8, 463-471, Copyright © 1988 by Society for Neuroscience
Characterization of the electrically evoked release of substance P from dorsal root ganglion neurons: methods and dihydropyridine sensitivity
GG Holz 4th, K Dunlap and RM Kream
Department of Physiology, Tufts University School of Medicine, Boston, Massachusetts 02111.
The mechanism by which dihydropyridines (DHPs) modulate the electrically
evoked or KCI-induced release of substance P (SP) from embryonic chick
dorsal root ganglion (DRG) neurons was investigated in the present study.
The release of SP, as measured by radioimmunoassay (RIA), was characterized
in terms of its dependence on extracellular calcium ion, its
stimulus-response relationship, its sensitivity to the calcium-channel
blocker omega conus toxin (omega-CgTx), and its modulation by the DHPs Bay
K 8644 and nifedipine. Here it is reported that omega-CgTx (1 microM)
blocked the electrically evoked release of SP. In contrast, the
calcium-channel agonist Bay K 8644 (5 microM) facilitated the release of SP
(by 45%), whereas the calcium-channel antagonist nifedipine (5 microM) was
without effect. When the release of SP was triggered by depolarization of
cultures with 60 mM KCI, the actions of the DHPs became much more
pronounced. Under these conditions, Bay K 8644 facilitated (by 115%),
whereas nifedipine inhibited (by 58%), peptide secretion. Voltage-clamp
analysis of DRG cell calcium currents demonstrated that these actions of
omega-CgTx, Bay K 8644, and nifedipine are explicable in terms of their
effects on the slowly inactivating (L-type) calcium current. On the basis
of these findings, it is suggested that the SP release mechanism exhibits
DHP sensitivity due to the involvement of L-type calcium channels in the
neurosecretory process. This model predicts that the voltage and time-
dependent antagonist actions of nifedipine are sufficient to explain its
failure to inhibit the electrically evoked release of SP.