A functional correlate for the dihydropyridine binding site in rat brain.

Calcium channels, controlling the influx of extracellular Ca2+ and hence neurotransmitter release, exist in the brain. However, drugs classed as calcium antagonists and which inhibit Ca2+ entry through voltage-activated Ca2+ channels in heart and smooth muscle, seem not to affect any aspect of neuronal function in the brain at pharmacologically relevant concentrations. Yet the dihydropyridine calcium antagonists (for example, nitrendipine) bind stereospecifically with high affinity to a recognition site on brain-cell membranes thought to represent the Ca2+ channel and consequently, the physiological relevance of these sites has been questioned. However, activation of voltage-dependent Ca2+ channels can increase cytoplasmic Ca2+ and neurotransmitter release in neuronal tissue. We show here that Bay K8644, a dihydropyridine Ca2+-channel activator, can augment K+-stimulated release of serotonin from rat frontal cortex slices and that these effects can be antagonized by low concentrations of calcium antagonists. As 3H-dihydropyridine binding to cortical membrane preparations resembles the binding in heart and smooth muscle where there are good functional correlates we conclude that the dihydropyridine binding sites in the brain represent functional Ca2+ channels that can be unmasked under certain circumstances.