Distribution of mapacalcine receptors in the central nervous system of rat using the [125I]-labeled mapacalcine derivative.

Mapacalcine is a dimeric protein of Mr 19041 extracted from the marine sponge Cliona vastifica. Electrophysiological and pharmacological approaches have demonstrated that mapacalcine was blocking a calcium channel different from N-, L-, P-, T- or Q-type calcium channels on mouse intestinal smooth muscle. Recently a [125I]-labeled derivative of mapacalcine has been synthesized and characterized as a tool usable as a probe to investigate mapacalcine receptors. On rat brain membranes, it binds to its receptor with a K(d)=0.35 nM and a maximal binding capacity of 706 fmol/mg protein. We use here [125I]-mapacalcine to study the mapping of its receptors in the rat brain. Data obtained show a practically homogeneous labeling of the brain. Our experiments suggest that mapacalcine receptors are present on neuronal and glial cells. Interestingly, choroid plexus demonstrates a high density of mapacalcine receptors. These data would suggest that mapacalcine sensitive calcium channels could be involved in the control of calcium homeostasis of the cerebrospinal fluid.