K+-evoked [H-3]D-aspartate release in rat spinal cord synaptosomes: Modulation by neuropeptide Y and calcium channel antagonists

This study was conducted to investigate mechanisms regulating the release o f [H-3]D-aspattate (or endogenous glutamate) in the rat spinal cord. Presyn aptic modulation of glutamate release was studied in superfused synaptosome s depolarized with 20 mM KCl. Calcium-channel antagonists, omega -conotoxin GVIA (omega -CgTx GVIA; N-type), nifedipine (L-type), and omega -conotoxin MVIIC (omega -CmTx MVIIC; P/Q type), were used to characterize the voltage -operated Ca2+ channels (VOCCs) involved in this release. Nifedipine had no significant effect on the K+-evoked release of [H-3]D-aspartate, but the o mega -conotoxins GVIA and MVIIC produced dose-dependent inhibitory effects that were additive. The most substantial reduction (54.30% +/- 4.40%) was s een with omega -CgTx GVIA, indicating that N-type channels play a major rol e in the release of glutamate in this tissue. We investigated the effects o f neuropeptide Y (NPY), NPY13-36, and [Leu(31)][Pro(34)]Npy on Ca2+-depende nt, K+-evoked [H-3]D-aspartate release. NPY and NPY13-36 equipotently inhib ited the release of glutamate in a concentration-dependent manner. The half -maximal response was observed at about 12 nM; maximal inhibition of 44.22% +/- 4.60% was achieved with 0.3 muM. The selective GABA(B) agonist (-)bacl ofen inhibited K+-evoked [H-3]D-aspartate release from superfused spinal co rd synaptosomes by 50.00% +/- 4.80% at 10 muM. When NPY13-36 and (-)baclofe n were used together at maximal doses, their release-inhibiting effects wer e not additive. In addition, neither of the agonists was able to enhance th e inhibition produced by pretreating the synaptosomes with the selective in hibitor of N-type VOCCs omega -CgTx GVIA. These results are consistent with the hypothesis that presynaptic Y-2-like and GABA(B) receptors regulate gl utamate release by blocking Ca2+ currents through N-type VOCCs. Characteriz ation of the receptors that can inhibit the release of glutamate may provid e useful information for treatment of conditions characterized by excessive glutamatergic transmission in the spinal cord. J. Neurosci. Res. 62:722-72 9,2000. (C) 2000 Wiley-Liss, Inc.