Chemosensory cues from the environment are detected by the peripheral nervous system which then transmits sensory information to the central nervous system and activates discrete neuronal ensembles. Studies on patterns of neuronal activation provide useful insights into roles of different neuronal population and brain regions in regulating distinct animal behaviors. IEG expression is induced rapidly when neurons are activated by membrane depolarization, seizure or some sensory signals
] and the expression pattern of IEGs is a convenient tool for visualization of neuronal activities
]. Neuronal IEGs comprise of several categories including transcription factors (c-fos, Fosb, c-jun, Junb, Egr1, Egr2, Egr3, Npas4, Nr4a1, Nr4a2
, etc.) and postsynaptic proteins (Arc, Homer1a
, etc.). Previous studies which used long-term potentiation (LTP) or long-term enhancement paradigm indicated that different IEGs have different thresholds for transcriptional induction; c-fos,
for instance, has a high threshold compared to that of Egr1
]. Guzowski et al.
, (2001) found a higher responsiveness of Arc
than that of c-fos
in the rat hippocampus after spatial learning
Olfactory sensory neurons (OSNs) in the main olfactory epithelium (MOE) can detect a vast array of odorous molecules by the olfactory receptors (ORs)
]. OSNs make synapses directly to second-order neurons in the central nervous system. Each OSN projects to a single glomerulus and the OSNs which express a particular OR usually converge to a single glomerulus both in the medial and lateral halves of the OB and thus OB glomeruli form a topographical map of ORs
]. Consequently, afferent inputs through OSNs trigger activity in the OB which is often traced by specific induction of IEGs. However, it should be noted that in addition to peripheral stimulation, centrifugal inputs can significantly influence the pattern of activity in the OB, particularly in the granule cell layer
Interaction of odorants with ORs in vertebrate OSNs activates the olfaction-specific G protein (Golf
) which in turn stimulates other components of the signaling cascades including the adenylyl cyclase type III (ACIII) and the olfactory cyclic nucleotide-gated channel (CNGC)
]. Previous knockout mice studies have confirmed that the cAMP signaling pathway plays the key role for detection of odorants
]. Belluscio et al.
(1998) reported that most Golf
-deficient mice showed neonatal mortality
]. In addition, electro-olfactogram (EOG) recordings, which measure electrical activity detected by an electrode placed on the olfactory epithelium, indicated severe reduction in odor-evoked response in Golf
]. The odorant-induced EOG response was found to be completely ablated and the odorant-dependent avoidance learning was impaired in ACIII mutant mice
]. The mice which have mutation in the cyclic nucleotide-gated channel subunit A2 (Cnga2
) gene also show general anosmia
]. In 1 day-old Cnga2
-mutants there was no detectable EOG response even when the olfactory epithelium was exposed to complex olfactory stimuli such as mouse urine (conspecific odor cues) or coyote urine (predator odor cues)
]. Behavioral studies in adult Cnga2
-null male mice showed that they fail to mate or fight and it is suggested that the MOE has an essential role in regulating these social behaviors
]. Although the mutant male mice failed to show preference for female urine
], it remains unclear whether the female urine odor activated the OB glomeruli or not. Intriguingly, Lin et al.
(2004) showed that several odorants, including putative pheromones, were behaviorally detected by the Cnga2-
null mice and electrophysiological and immunohistochemical studies revealed that those odors indeed produced responses in the MOE, OB and piriform cortex (PC) in the mutants
For mapping neuronal activity using IEGs two important criteria should be the low basal expression and the high induction of the IEG being used. It is also advantageous to analyze several IEGs since the induction thresholds of IEGs vary depending on the IEG, the stimulus and the tissue. In this present study we used ISH to analyze expression patterns of ten IEGs in the mouse brain using different odor stimuli and compared inducibility and sensitivity of these IEGs for detection of sensory-evoked neuronal activities. We then asked how disruption of the cAMP signaling cascade in the olfactory pathway affects neuronal activation using Cnga2-null male mice which show general anosmia and sexual deficits. We exposed the mutants to different odorants including a predator odor and female odors to observe behavioral responses and the pattern of brain activities mediated by any CNGA2-independent olfactory signaling pathway(s) using ISH of IEGs.