In vitro release of immunoreactive substance P from putative afferent nerve endings in bovine pia arachnoid

The release of substance P-like immunoreactivity was examined using bovine pia arachnoid and its attendant blood vessels in vitro. At concentrations of 20,51, and 100 mM, potassium ions evoked the release of substance P-like immunoreactivity in a dose-dependent manner. The drug capsaicin released substance P at concentrations greater than 10(-8) M. Both potassium- and capsaicin-induced release were abolished by omitting calcium ions from the superfusion buffer. When subjected to separation by reverse phase high performance liquid chromatography, the superfusate from capsaicin perfused tissues contained a peak of immunoreactivity which migrated at the retention time corresponding to substance P. During basal and stimulated states, the percent endogenous substance P released ranged between 0.4-6.5 X 10(-2) and 1.3-11.6 X 10(-2) per minute, rates comparable to those previously reported by others using slices of dorsal horn or spinal cord segments. The immunoreactivity measurable in the conditioned buffer probably reflected release from afferent nerve endings in as much as most of the substance P immunoreactivity in pia arachnoid arises from trigeminal ganglia. Release of substance P, a cerebrovasodilating peptide from perivascular nerve endings in pia arachnoid suggests a possible role for substance P in the pathophysiology of disorders associated with pain of cerebrovascular origin.