Immunohistochemistry for Ca2+/calmodulin-dependent protein kinase II (CaMKII) was performed on the rat cranial sensory ganglia. More than one half of neurons was immunoreactive for the enzyme in the trigeminal (60%), jugular (70%), petrosal (55%) and nodose ganglia (63%). These neurons were mainly small to medium-sized. The co-expression study demonstrated that one half of CaMKII-immunoreactive (it) neurons was also immunoreactive for calcitonin gene-related peptide (CGRP) or the vanilloid receptor subtype 1 (VR1) in the trigeminal, jugular and petrosal ganglia. In the nodose ganglion, CaMKII-it neurons were mostly devoid of CGRP-immunoreactivity (it) (8.2%) whereas the co-expression with VR1-ir was common among such neurons (72%). In the facial skin, nasal mucosa and palate, the epithelium and taste bud were innervated by CaMKII-it nerve fibers. In addition, the retrograde tracing study demonstrated that 39.6% and 44.8% of trigeminal neurons which were retrogradely traced with fluorogold from the facial skin and nasal mucosa exhibited CaMKII-ir. Forty-six percent of petrosal neurons which innervated the soft palate were immunoreactive for the enzyme. (C) 2004 Elsevier B.V. All rights reserved.