Sensory perception depends not only on the physical signals captured by peripheral organs, but also on the internal dynamics of central neuronal networks. In the sensory cortex, so-called desynchronized, or "activated," states are associated with more reliable stimulus-evoked responses and enhanced perception, whereas strongly synchronized states tend to dampen sensitivity to external inputs. The anterior piriform cortex, a sensory paleocortex involved in olfactory perception, is notable for not receiving direct thalamic inputs, unlike the sensory neocortex. It does, however, receive massive projections from the main olfactory bulb and contains a dense network of associative fibers linking principal excitatory neurons over long distances, making it an ideal substrate for studying the interplay between external signals and the internal dynamics of circuits. Type 1 cannabinoid receptors (CB1R), known to influence network coordination in the hippocampus, are present there, but their role in the activity of this region and in odor perception remained unknown.
To address this question, the authors combined pharmacological manipulations with multi-electrode recordings or fiber-optic photometry, carried out in the anterior piriform cortex of freely moving male mice. CB1R blockade was achieved either by systemic administration or by local infusion of the compound directly into the region of interest. Analyses focused on the waking state, with brain states distinguished on the basis of accelerometry signals and hippocampal and cortical spectrograms. Oscillatory activity, the synchronization of neuronal populations, and behavioral thresholds for olfactory detection could thus be characterized.
The results show that CB1R blockade, whether systemic or local, increases the amplitude of gamma oscillations in the anterior piriform cortex, while reducing the frequency of synchronized population events involving excitatory neurons. In animals exposed to odor sources, this same blockade decreases the correlation between excitatory units and lowers the behavioral thresholds for odor detection, reflecting enhanced olfactory sensitivity.
The authors conclude that endogenous endocannabinoid signaling promotes synchronized population events and dampens gamma oscillations in the anterior piriform cortex, resulting in reduced sensitivity to external sensory inputs. The endocannabinoid system thus emerges as a modulator of the balance between internal dynamics and peripheral signals, capable of finely tuning olfactory perception according to the state of the cortical network.