Nociceptin is a peptide transmitter belonging to the opioid family. Nociceptin has recently attracted considerable interest since it appears to exhibit a number of differences to the other opioids. In the present study, we used a nociceptin antibody to map the distribution of nociceptin in the human trigeminal ganglion. In addition, we studied the nociceptin receptor at mRNA levels by RT-PCR and the vasomotor response to nociceptin in human cerebral vessels using a sensitive in vitro method. About 70% of all neuronal cells in trigeminal ganglia were nociceptin immunopositive. Nociceptin was predominantly (78%) expressed in medium-sized cells (30-60 microm). Nociceptin also distributed in small-sized cells (14% of positive cell bodies; <30 microm) and in large-sized cells (8% of positive cell bodies; >60 microm). Double immunostaining showed that in the human trigeminal ganglion nociceptin colocalized with calcitonin gene-related peptide (CGRP), substance P (SP), nitric oxide synthase (NOS) or pituitary adenylate cyclase activating peptide (PACAP). About 61% of nociceptin positive cells contained CGRP, 54% contained SP, 50% contained NOS and 68% contained PACAP. Immunoreactivity to nociceptin was not detected in human cerebral blood vessels. Reverse transcriptase-polymerase chain reaction detected the expression of nociceptin receptor mRNA in trigeminal ganglia but not in basilar arteries. To further examine whether there are functional nociceptin receptors in human cerebral arteries, a pharmacological study was done, where cerebral arteries revealed strong contractions to 60 mM K(+) and U466166 and strong relaxation to CGRP. Nociceptin failed to elicit contraction or relaxation. In conclusion, nociceptin is expressed in human trigeminal ganglia but not in cerebral blood vessels. Nociceptin is colocalized with CGRP, SP, NOS and PACAP. Nociceptin receptor mRNA is expressed in human trigeminal ganglia but not in basilar arteries. The functional role of nociceptin may be at the presynaptic level.