PERIPHERAL VERSUS CENTRAL POTENCIES OF N-TYPE VOLTAGE-SENSITIVE CALCIUM-CHANNEL BLOCKERS

The ability of a series of synthetic analogues of omega-conopeptides M VIIA (SNX-111) and TVIA (SNX-185) to prevent electrically-evoked norep inephrine release from rat tail artery and hippocampal slice preparati ons was determined in an effort to identify voltage-sensitive calcium channel (VSCC) blockers that selectively target N-type VSCCs in centra l nervous system tissue. Electrical field stimulation (3 Hz, 1 ms in d uration, 80 V for 1 min) caused a high and consistent tritium outflow from rat tail artery and hippocampal slice preparations preloaded with [H-3]-norepinephrine. All conopeptides, chosen for their selective af finities for high-affinity SNX-111 binding sites (i.e., N-type VSCCs) over high-affinity omega-conopeptides MVIIC (SNX-230) binding sites (i .e., P/Q-type VSCCs), produced a concentration-dependent inhibition of calcium dependent electrically-evoked tritium outflow from both tail arteries and hippocampal slices; IC(50)s ranged from 1.2 nM to 1.2 mu M. Blocking potencies (IC(50)s) in the tail artery assay were signific antly correlated with those measured in the hippocampal slice preparat ion (r = 0.91, P = 0.00000012). There was a significant correlation be tween IC(50)s for blockade of hippocampal norepinephrine release and t he inhibition of high-affinity [I-125]-SNX-111 binding in rat brain sy naptosomes (r = 0.76, P = 0.00028). Blockade of hippocampal norepineph rine release was not significantly correlated with the inhibition of h igh-affinity SNX-230 binding (r = 0.46, P = 0.056). Maximum inhibition of tritium outflow in the tail artery assay was 22 +/- 1.4% of contro l, approximating the value (20.9 +/- 16.0% of control) obtained in the absence of extracellular Ca2+. In contrast, the maximum inhibition of tritium release from hippocampal slices was 36.8 +/- 2.5% of control (P < 0.05, compared to that of the tail artery assay). These re suits suggest that (1) N-type VSCCs alone mediate low frequency electrical s timulation-evoked neurotransmitter release from peripheral sympathetic efferents (tail artery) while both N-type and non-N type(s) mediate n eurotransmitter release from CNS neurons (hippocampus); and (2) analog ues of omega-conopeptides MVIIA and TVIA do not differentiate between N-type VSCCs mediating norepinephrine release from central and periphe ral neural tissues.