Characterization of Ca2+-channels responsible for K+-evoked [3H]noradrenaline release from rat brain codex synaptosomes and their response to amyotrophic lateral sclerosis IgGs

The contribution of the different Ca²⁺-channel subtypes to the K⁺- evoked [³H]noradrenaline release from rat cerebral cortex synaptosomes has been investigated. In the same experimental model, it was also verified whether the calcium-mediated neurotransmitter release is influenced by IgGs purified from sera of seven patients affected by sporadic amyotrophic lateral sclerosis. Synaptosome treatment with 3.0 μM nifedipine or 2.0 μM calciseptine, which block L-type channels, slightly decreased [³H]noradrenaline release, the reduction being 7 and 13% of the control values, respectively. The blockade of N-type Ca²⁺-channels with ω- conotoxin-GVIA (0.001-1.0 μM) induced a concentration-dependent reduction of the neurotransmitter release, with maximum effect of 34%. ω-Agatoxin-IVA failed to significantly affect the studied release, which was instead markedly reduced by ω-conotoxin-MVIIC. After the blockade of N-type channels with maximal concentrations of ω-conotoxin-GVIA, 3.0 μM ω-conotoxin-MVIIC reduced the release by 58%. Synaptosome treatment with amyotrophic lateral sclerosis IgGs enhanced the K⁺-evoked [³H]noradrenaline release, which was mostly mediated by P/Q- and N-type Ca²⁺-channels. The increase induced by pathologic IgGs (0.2 mg/ml) ranged from 11 to 62% for the different patients, and it was concentration-dependent. The basal release was instead unaffected by IgG treatment. The results of the present study suggest that the K⁺- evoked [³H]noradrenaline release from brain cortex synaptosomes is mainly mediated by activation of P/Q- and N-type Ca²⁺channels. Autoantibodies present in the sera of patients affected by sporadic amyotrophic lateral sclerosis may interact with these channels by producing an increased calcium influx, with consequent enhancement of the neurotransmitter release. Preliminary results of the present study have been published in abstract form (Martire et al., 1997, Pharmacol. Res. 35:9).