In vitro and in vivo release of neurokinin A-like immunoreactivity from rat substantia nigra.

In vivo and in vitro perfusion techniques have been used to study the release of neurokinin A-like immunoreactivity from the rat substantia nigra. Potassium depolarization and electrical field stimulation evoked calcium-dependent release from nigral slices. Potassium depolarization was also effective in vivo. Tetrodotoxin (1 microM) completely blocked electrically stimulated release but only diminished release in response to depolarizing potassium. Neurokinin A-like immunoreactivity release showed frequency dependence and a clear facilitation phenomenon between 5 and 25 Hz. High-performance liquid chromatography analysis of the immunoreactivity released in vitro revealed the presence of neurokinin A, neuropeptide K and neurokinin B, along with their sulphoxide forms. A marked depletion of neuropeptide K and neurokinin B content was observed when the tachykinin content of the nigral slices was examined before and after stimulation. However, the neurokinin A content of the slices was unchanged or even increased, suggesting an accelerated processing of neurokinin A precursors during the stimulation. The tachykinin peptides were degraded at different rates by substantia nigra homogenates; degradation was fastest for neuropeptide K and slowest for neurokinin A. The addition of a mixture of peptidases inhibitors (thiorphan, phosphoramidon, bestatin and captopril) substantially reduced the degradation of all three tachykinins, but did not completely block degradation. GABA-A receptor antagonists such as bicuculline and, particularly, picrotoxin potentiated the stimulated neurokinin A-like immunoreactivity release in vitro, but the GABA-agonist muscimol had no effect. Picrotoxin was even more potent in vivo. The results presented in this study demonstrate that neurokinin A, neuropeptide K and neurokinin B can be released by depolarizing stimuli from rat substantia nigra. Furthermore, the features exhibited by this release suggest that these peptides may have a neurotransmitter/neuromodulator role in the rat substantia nigra.