PACAP is a member of the vasoactive intestinal polypeptide (VIP), secretin, glucagon, family of peptides, which are known to act as neuroendocrine hormones and neurotransmitters by interacting with G protein-coupled receptors. PACAP has been described to enhance cAMP formation, phosphoinositide breakdown and elevate [Ca 2+ ] i by mobilizing intracellular calcium stores.
To characterize cell types carrying PACAP-type1-R we investigated the cellular and subcellular distribution of the PACAP-typeI-receptor in the brain of mice. In addition, we compared the localization of PACAP-type1-R in the hippocampus with the distribution of specific marker proteins using immunofluorescence and immunoperoxidase techniques in combination with confocal laserscanning microscopy and electron microscopy (Otto et al., 1999). In general, PACAP-type1-R immunoreactivity was found ubiquitously as punctate staining in the neuropil of the CNS. Most conspicuous was the punctate localization of the receptors in hilus and stratum lucidum of the hippocampal CA3 region, where PACAP-type1-R matched well with the distribution of the calbindin D 28k immunoreactive mossy fiber projection. PV-IR basket cells in the CA3 region carried numerous PACAP-type1-R stained dots on their proximal dendrites and perikarya. Immunocytochemical double stainings with presynaptic markers, as for instance, synaptophysin revealed an extensive overlap with PACAP-type1-R in the mossy fiber axon terminals of stratum lucidum in CA3 and the hilus. In contrast, antibodies against dendritic cytoskeleton proteins (MAP2) and postsynaptic markers (ProSAP1) did not show a considerable colocalization with PACAP-type1-R in CA3 pyramidal cells. Ultrastructural investigations corroborated the presynaptic localization of PACAP-type1-R at mossy fiber terminals, although, in other contacts, it was identified postsynaptically. The immunocytochemical and ultrastructural findings give evidence for a presynaptic modulatory role of PACAP-type1-R at specific synapses relevant for learning and memory processes.
P. Gass, C. Otto, G. Schütz (German Cancer Reseach Center, Heidelberg)