Effects of glutamate-induced excitotoxicity on calretinin-expressing neuron populations in the area postrema of the rat.

We mapped the distribution of calretinin-immunoreactive neuron populations in a circumventricular organ of the rat, the area postrema, and investigated their sensitivity to excitotoxic stimuli mediated by subcutaneously administered monosodium glutamate. We were specifically interested to ascertain whether the presence of calretinin can, per se, confer an in vivo intrinsic resistance for area postrema neurons to glutamate excitotoxicity. We found that dense populations of calretinin-positive neurons displayed a subregional compartmentation in coronal sections of the area postrema along its rostrocaudal axis. We demonstrated that calretinin-positive neurons differ in their sensitivities to monosodium glutamate depending on their position within the area postrema. Neurons in the caudal area postrema were the most sensitive ones, while those in the rostral area postrema were spared of degeneration. We conclude that calretinin-positive neurons in the area postrema are not uniformly protected against glutamate excitotoxicity. It is possible that differences in the local concentrations of monosodium glutamate due to regional heterogeneities in density and permeability of the capillary bed rather than neuronal expression of calretinin account for the observed effects.