Calretinin mRNA and immunoreactivity in the medullary reticular formation of the rat: colocalization with glutamate receptors

Abstract Calretinin-positive cells were identified in the medullary reticular formation of the rat by both immunohistochemistry and in situ hybridization histochemistry. In addition, double immunocytochemical labeling was used to examine the degree of colocalization of calretinin with GluR 2 /R 3 , GluR 4 and GluR 5−7 glutamate receptor subtypes. Results indicated regional variation in calretinin expression across reticular formation regions with the exception of the largest cells which were mostly calretinin-positive. Calretinin mRNA was particularly abundant in the parvocellular reticular nucleus. Most calretinin-immunoreactive cells also expressed at least one of the glutamate receptor subtypes examined with the exception of the smallest calretinin-positive cells of the parvocellular reticular formation which were generally not immunoreactive for any of the glutamate receptors examined. Calretinin immunoreactivity was colocalized with immunoreactivity for all three glutamate receptor subtypes examined in most of the large cells of the reticular formation. Immunoreactivity for the GluR 4 antibody was least abundant in the reticular formation and GluR 4 immunoreactive cells were least likely to co-express calretinin. These results suggest that calretinin and glutamate receptor antibodies may be used to identify specific subsets of reticular formation neurons.