Abstract
BackgroundSensitive detection of sensory-evoked neuronal activation is a key to mechanistic understanding of brain functions. Since immediate early genes (IEGs) are readily induced in the brain by environmental changes, tracing IEG expression provides a convenient tool to identify brain activity. In this study we used in situ hybridization to detect odor-evoked induction of ten IEGs in the mouse olfactory system. We then analyzed IEG induction in the cyclic nucleotide-gated channel subunit A2 (Cnga2)-null mice to visualize residual neuronal activity following odorant exposure since CNGA2 is a key component of the olfactory signal transduction pathway in the main olfactory system.ResultsWe observed rapid induction of as many as ten IEGs in the mouse olfactory bulb (OB) after olfactory stimulation by a non-biological odorant amyl acetate. A robust increase in expression of several IEGs like c-fos and Egr1 was evident in the glomerular layer, the mitral/tufted cell layer and the granule cell layer. Additionally, the neuronal IEG Npas4 showed steep induction from a very low basal expression level predominantly in the granule cell layer. In Cnga2-null mice, which are usually anosmic and sexually unresponsive, glomerular activation was insignificant in response to either ambient odorants or female stimuli. However, a subtle induction of c-fos took place in the OB of a few Cnga2-mutants which exhibited sexual arousal. Interestingly, very strong glomerular activation was observed in the OB of Cnga2-null male mice after stimulation with either the neutral odor amyl acetate or the predator odor 2, 3, 5-trimethyl-3-thiazoline (TMT).ConclusionsThis study shows for the first time that in vivo olfactory stimulation can robustly induce the neuronal IEG Npas4 in the mouse OB and confirms the odor-evoked induction of a number of IEGs. As shown in previous studies, our results indicate that a CNGA2-independent signaling pathway(s) may activate the olfactory circuit in Cnga2-null mice and that neuronal activation which correlates to behavioral difference in individual mice is detectable by in situ hybridization of IEGs. Thus, the in situ hybridization probe set we established for IEG tracing can be very useful to visualize neuronal activity at the cellular level.
Highlights
Sensitive detection of sensory-evoked neuronal activation is a key to mechanistic understanding of brain functions
We found that the exploration of odor cues only for a brief period significantly induced the expression of several Immediate early gene (IEG) in the mouse hippocampus (Figure 4E-F', data not shown)
Exposure to estrous odors increased Fos immunoreactivity in the medial amygdala [39] and the medial amygdala was found to regulate attraction to female odor cues [40]. We found that both in wild type mice and Cyclic nucleotide-gated channel subunit A2 (Cnga2) mutants which interacted with female mice, a conspicuous induction of IEGs occurred in the posterodorsal part of the medial amygdaloid nucleus (MePD)
Summary
Sensitive detection of sensory-evoked neuronal activation is a key to mechanistic understanding of brain functions. In this study we used in situ hybridization to detect odor-evoked induction of ten IEGs in the mouse olfactory system. Olfactory sensory neurons (OSNs) in the main olfactory epithelium (MOE) can detect a vast array of odorous molecules by the olfactory receptors (ORs) [8]. Afferent inputs through OSNs trigger activity in the OB which is often traced by specific induction of IEGs. it should be noted that in addition to peripheral stimulation, centrifugal inputs can significantly influence the pattern of activity in the OB, in the granule cell layer [11,12,13,14]
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