Presence of Ras guanyl nucleotide-releasing protein in striosomes of the mature and developing rat

Ras signal transduction pathways have been implicated as key regulators in neuroplasticity and synaptic transmission in the brain. These pathways can be modulated by Ras guanyl nucleotide exchange factors, (GEF) which activate Ras proteins by catalysing the exchange of GDP for GTP. Ras guanyl nucleotide-releasing protein (RasGRP), a recently discovered Ras GEF, that links diacylglycerol and probably calcium to Ras signaling pathways, is expressed in brain as well as in T-cells. Here, we have used a highly selective monoclonal antibody against RasGRP to localize this protein within the striatum and related forebrain structures of developing and adult rats. RasGRP immunolabeling was found to be widespread in the mature and developing rat forebrain. Most notably, it presented a prominent patchy distribution throughout the striatum at birth and at all postnatal ages examined. These patches were found to correspond with the striosomal compartment of the striatum, as identified by micro-opioid receptor labeling in the adult. RasGRP-immunoreactivity was also observed in the matrix-like compartment surrounding these patches/striosomes but appeared later in development and was always weaker than in the patches. In both striatal compartments, RasGRP was exclusively expressed by medium-sized spiny neurons and showed no preference for neurons that project either directly or indirectly to the substantia nigra. At the ultrastructural level, immunogold labeling of RasGRP was confined to the cell bodies and dendritic shafts of these output neurons. We conclude that the prominent expression of RasGRP in striosomes may be of significance for diacylglycerol signaling in the striatum, and could be of importance for the processing of limbic-related activity within the basal ganglia.