Recent research indicates that the novel multimodular adaptor protein EHSH1 plays an important role in a partnership between clathrin-mediated endocytosis and intracellular signaling pathways, including the MAPK pathway, receptor-tyrosine kinase/ras-mediated pathway, and the rho family of the GTPase-dependent pathway. We report the detailed expression pattern of EHSH1 in the rat CNS, using separate cultures of neurons, astrocytes, and microglia, and biochemical and immunohistochemical analyses. Cultured neurons from the cortex express primarily the long isoform EHSH1 -l, as well as a small amount of the short isoform EHSH1-s. Cultured astrocytes express EHSH1-s, at a level similar to neurons, and a trace of EHSH1-l. Cultured microglia express only EHSH1-s. Double immunofluorescent staining of cortical sections showed that EHSH1 is expressed predominantly in neurons. These results suggest that EHSH1 -l is a primary isoform and that EHSH1 -l is highly enriched in neurons in the rat adult CNS. Immunohistochemistry of a series of brain sections revealed widespread distribution of EHSH1 throughout the brain. Particularly intense immunoreactivity was observed in the somatodendritic region of neurons in Layer III of the neocortex, hippocampus, globus pallidus, subthalamic nucleus, and substantia nigra. Interestingly, all pyramidal neurons in Layer III of the neocortex and hippocampus did not necessarily exhibit equal levels of immunostaining. In contrast, little EHSH1 immunoreactivity was detected in septofimbrial nucleus and subfornical organ of the septal region, and solitary tract and external cuneate nuclei of the medulla. Variety in the expression of EHSH1 in neurons of different regions may reflect different conditions in clathrin-mediated endocytosis and the following signal transduction. (C) 2002 Wiley-Liss, Inc.