Impact of CB1 Receptor Deletion on Visual Responses and Organization of Primary Visual Cortex in Adult Mice.

PURPOSE: The endocannabinoids (eCBs) and their receptors are expressed in the cortex of developing animals where they act as a neuromodulating system during critical stages of brain development such as cell proliferation and migration, and axon guidance. Little is known on the impact of the cannabinoid system on cortical map formation and receptive field properties of cortical sensory neurons. The present study evaluates in vivo the functional organization of the primary visual cortex (V1) of mice lacking cannabinoid CB1R receptor (cnr1-/-).

METHODS: Using optical imaging of intrinsic signals, azimuth, and elevation maps of cnr1-/- mice were compared with their wild-type littermates (cnr1+/+).

RESULTS: Topographic maps were affected in mutant mice as they exhibited narrower visual field and changes in the shape of V1. CB1R exerted its action in an axis dependent manner as all changes were observed in the azimuth axis. Spatial frequency and contrast sensitivity were also compared between the two groups. Both properties were affected by the chronic lacking of CB1R as mutant mice exhibited a significantly lower contrast sensitivity as well as lower spatial frequency selectivity.

CONCLUSIONS: Taken together, these results suggest an important role for CB1R in cortical map formation. Our results also clearly demonstrate the impact of CB1R in the development of visual properties of primary visual cortex neurons. Because psychoactive effects of cannabis consumption on visual experience are mediated mainly through CB1R, our results could possibly explain neuronal mechanisms involved in those perceptual changes.