Presynaptic localization of the PACAP-typeI-receptor in hippocampal and cerebellar mossy fibres

The distribution of PACAP-typeI-receptor (PACAP-I-R) mRNA and protein was s tudied in mouse using probes and a newly developed antiserum recognizing al l known splice variants. RNase protection assays revealed highest expressio n levels of PACAP-I-R mRNA in brain, in particular the hypothalamus and hip pocampus. At the cellular level, in situ hybridization analysis demonstrate d widespread distribution of PACAP-I-R mRNA in neurons throughout the brain , while glial cells did not express the gene. Highest expression levels of PACAP-I-R mRNA were observed in three regions: the limbic system, the hypot halamus, and the brainstem. In accordance with data obtained from in situ h ybridization analysis, immunohistochemistry showed widespread distribution of PACAP-I-R like immunoreactivity in the neuropil. Rather strong immunorea ctivity was found in cerebellar and hippocampal mossy fibres where double i mmunolabelling revealed the presynaptic localization of the receptor protei n. At the ultrastructural level, PACAP-I-R like immunoreactivity was observ ed around synaptic vesicles and close to the presynaptic grid in hippocampa l mossy fibre terminals. This finding is in contradiction to the described postsynaptic localization of the PACAP-I-R in dendritic processes of hippoc ampal granule cells in rat. Due to their presynaptic induction, mossy fibre LTPs are distinctly different from LTPs in all other hippocampal regions. Therefore, the presynaptic localization of the PACAP-I-R in mossy fibre ter minals may implicate this gene in influencing the synaptic strength of the mossy fibre pathway and hence memory consolidation. (C) 1999 Elsevier Scien ce B.V. All rights reserved.