Differential sensitivity of [3H]nitrendipine binding to cations of toxicological interest in various rat brain areas

In vitro and in vivo effects of lead on specific 3H-PN200-110 binding to dihydropyridine receptors in the frontal cortex of the mouse brain

It is assumed that several neurotoxic substances interfere with neuronal calcium channels. Therefore, we studied the effects of the heavy metals, cadmium, copper, lead, manganese, and zinc on the L-type calcium channels in the mouse brain. Characterization of the calcium channels was carried out using binding studies on homogenates from the frontal cortex with the DHP (dihydropyridine)-derivative, 3H-PN200-110, which binds with high affinity to the DHP-receptor inside the L-type calcium channel. Furthermore, the in vivo effects of lead on the DHP-receptors were investigated in perinatally exposed mice. In these animals, the analysis of saturation experiments with 3H-PN200-110 showed no changes in receptor density or ligand affinity due to the lead exposure. In vitro, 3H-PN200-110 binding is absolutely dependent on the presence of calcium. Divalent cations, such as magnesium or manganese, which normally block the physiological effects of calcium, also enhance DHP-receptor binding. Interestingly, ions such as lead, cadmium and copper stimulate 3H-PN200-110 binding at low concentrations (0.1-10 microM), but inhibit binding at higher concentrations. In contrast, zinc blocked DHP-receptor binding at low concentrations (< 100 microM) without any stimulating effects. These results suggest that modulation of the L-type calcium channel by heavy metal cations is one possible mechanism by which the regulation of calcium homeostasis in neurons is altered.

Concomitant regulation of hippocampal calcium antagonist receptors and calcium uptake by substance P

The interaction of various neuropeptides with calcium antagonist binding was investigated in rat hippocampus. Among the peptides examined Substance P selectively increased the binding of phenylalkylamine and dihydropyridine calcium antagonists; this action was receptor mediated. No effect was observed with Substance P in other brain areas and with neurotensin and met-enkephalin in all the areas examined. The modification in calcium antagonist binding is functionally paralleled by an area specific increase in voltage-dependent calcium uptake. These data suggest that in hippocampus Substance P may be an endogenous regulator of voltage sensitive calcium channels.

Regional modification of brain calcium antagonist binding after in vivo chronic lead exposure

Lead toxicity in the central nervous system seems to be partially related to specific effects of the metal on calcium metabolism and in particular on calcium transport. On this line, the present study investigates the characteristics of [3H]nitrendipine binding to several rat brain regions after in vitro lead addition or after in vivo chronic exposure to this metal. In vivo a lead induced increase in [3H]nitrendipine binding, Bmax, is observed in cerebral cortex and striatum while the binding is unmodified in hippocampus. The in vitro studies are in agreement with in vivo data; lead addition stimulates the binding in synaptic membranes prepared from cortex and striatum but not from hippocampus where the binding is slightly inhibited. The data suggest that lead interferes with neuronal calcium channels in an area-selective manner.