Odorant-evoked electrical responses in Grueneberg ganglion neurons rely on cGMP-associated signaling proteins

Abstract

The Grueneberg ganglion (GG) in the anterior nasal region of mice is considered as an olfactory compartment since its neurons were recently observed to be activated by chemical stimuli, in particular by the odorant 2,3-dimethylpyrazine (2,3-DMP). However, it is unclear whether the GG indeed serves an olfactory function since these findings are solely based on the expression of the activity-dependent gene c-Fos. Consequently, it is yet uncertain whether chemical compounds, such as given odorants, elicit electrical responses in GG neurons which are required to convey the chemosensory information to the brain. Therefore, in the present study, electrical recording experiments on tissue sections through the anterior nasal region of mice were conducted which revealed that 2,3-DMP induces electrical signals in the GG. These responses were restricted to sites harboring GG neurons, indicating that 2,3-DMP elicits an electrical signal only in these but not in other cells of the anterior nasal region.

2,3-DMP-sensitive GG neurons express signaling proteins associated with the second messenger substance cyclic guanosine monophosphate (cGMP); most notably the cyclic nucleotide-gated ion channel CNGA3 and the transmembrane guanylyl cyclase GC-G. Using mice deficient for CNGA3 or GC-G demonstrated that the 2,3-DMP-evoked electrical signals in the GG of these knockout mice were substantially lower than in the GG of wildtype conspecifics, indicating that cGMP signaling plays a crucial role for odorant-induced electrical responses in the GG.

Introduction

In the mammalian nose, odorous molecules elicit electrical responses in olfactory sensory neurons (OSNs) from distinct chemosensory compartments, including the main olfactory epithelium (MOE), the vomeronasal organ (VNO) and the septal organ [3], [14], [19]. In addition to the three above mentioned nasal organs harboring OSNs, a cluster of neuronal cells designated as Grueneberg ganglion (GG) exists in the anterior region of the nose in several mammalian species [10], [4]. Similar to OSNs, GG neurons express the olfactory marker protein (OMP) [5], [9], [11], [21], [22] and olfactory receptors [7], [8]. Furthermore, axonal processes of these cells project to the olfactory bulb of the brain, suggesting that they also function as olfactory neurons [5], [9], [11], [21], [22]. In line with this notion, recent studies have demonstrated that GG neurons are indeed activated by chemical compounds; however, these findings are solely based on experimental approaches using either Ca²⁺ imaging or the expression of the activity-dependent gene c-Fos, respectively [2], [15]. Thus, it is yet unclear whether GG neurons generate electrical signals upon exposure to appropriate odorous or pheromonal substances. Since electrical responses are crucial for conveying information from nasal sensory cells to the olfactory bulb for further processing, in the present study, we set out to identify electrical signals in the GG generated following odor exposure.

Based on recent c-Fos experiments, GG responsiveness to odorants has been found to depend on given signaling proteins associated with the second messenger substance cyclic guanosine monophosphate (cGMP), most notably the transmembrane guanylyl cyclase subtype GC-G and the cyclic nucleotide-gated (CNG) ion channel CNGA3 [16]. To further assess the relevance of these signaling elements for GG stimulation by odorants, electrical field potentials were recorded from the GG of both mice endowed with GC-G and CNGA3 and mice lacking these signaling proteins.