Characterization of CGRP protein expression in “satellite‐like” cells and dendritic arbours of the mouse olfactory bulb

Abstract

The main olfactory bulb (OB) is made up of several concentric layers, forming circuitries often involving dendro-dendritic synapses. Important interactions between OB neurons occur in the external plexiform layer (EPL), where dendrites of tufted and Van Gehuchten cells form synapses with dendrites of deeper lying mitral, tufted, and granule cells. OB neurons display a variety of neurotransmitters. Here, the focus is on calcitonin gene-related peptide (CGRP), a 37-amino acid neuropeptide transmitter that is widely distributed in the central and peripheral nervous system. In the OB, CGRP-immunoreactive (ir) cell bodies were mostly observed in the mitral cell layer (MCL) of normal mice, and their number increased following colchicine treatment. Sparsely distributed CGRP-ir cell bodies were also found in the EPL and granular cell layer. Double-immunofluorescence experiments revealed a lack of co-localization between CGRP-like immunoreactivity (LI) and corticotropin-releasing factor- or galanin-LI, two markers for mitral cells, and no CGRP-LI was found in cholecystokinin-, parvalbumin-, or vasoactive intestinal polypeptide-ir tufted/Van Gehuchten cells. CGRP-ir cell bodies were not found to co-localize glutamic acid decarboxylase 67 (GAD67)-green fluorescence protein, gamma-aminobutyric acid (GABA)-, or calretinin-LI, although the possibility remains that CGRP-ir cells may contain low levels of GABA and/or GAD67 not detected by our methodology. Dendrites of CGRP-ir cells extensively ramified deep in the EPL and double-immunofluorescence revealed them to be adjacent with, often apparently contacting, dendrites of granule, mitral, tufted, and Van Gehuchten cells. We propose that these CGRP-ir cell bodies in the mouse OB are "satellite-like" cells within and, occasionally, close to the MCL.