Cannabinoids modulate associative cerebellar learning via alterations in behavioral state

Abstract

Attempts to identify cellular mechanisms underlying learning often include knocking out genes involved in candidate forms of synaptic plasticity and assessing subsequent effects on behavior. Within the cerebellum, multiple plasticity mechanisms have been proposed as cellular substrates of learning. For example, type 1-cannabinoid receptors (CB1Rs) mediate several forms of synaptic plasticity in the cerebellar cortex and have been implicated in cerebellum-dependent delay eyeblink conditioning in knockout experiments. However, recent work has shown that eyeblink conditioning is modulated by behavioral state, and global CB1KO mice are known to be hypoactive. We therefore asked to what extent altered locomotor activity vs. impaired CB1-dependent plasticity within the cerebellar cortex contribute to learning impairments in these mice. We find that eyeblink conditioning deficits in global CB1KOs can be fully accounted for by their hypoactivity. Impairments disappear when the level of locomotor activity is taken into account, and externally controlling running speed rescues learning. Moreover, both global and cerebellar granule cell-specific CB1KOs exhibit normal cerebellum-dependent locomotor adaptation. Our results suggest that the apparent effects of CB1R deletion on cerebellar learning are not due to direct effects on CB1-dependent plasticity, but rather, arise as a secondary consequence of hypoactivity. These findings highlight the importance of considering general changes in behavioral state as a powerful means through which individual genes contribute to complex behaviors, particularly in transgenic models.

Neural instructive signals for associative cerebellar learning

“…In general, the properties of learning to an optogenetic CF-US matched those of normal sensory CS+US conditioning in wildtype mice 51,52 . Learning to an optogenetic-US emerged over several days, with the amplitudes of learned eyelid closures, measured on CS-only trials, increasing gradually across sessions (Fig.…”

“…We investigated neural instructive signals for delay eyeblink conditioning in head-fixed mice walking on a motorized treadmill 51,52 . In classical eyeblink conditioning experiments (Fig.…”

“…Table 1). Cre lines were: For Purkinje cells, L7-Cre strain #004146 from The Jackson Laboratory 51,92 ; For granule cells, Gabra6-Cre (MMRRC 000196-UCD; 51,52,93,94 ); For glutamatergic neurons (within the IO), vGlut2-Cre (strain #016963 from The Jackson Laboratory; 57,95,96 ). To target inputs to the IO, we used the Thy1-ChR2/EYFP mouse line obtained from The Jackson Laboratory 65,66 (strain #007612).…”

“…The experimental setup for eyeblink conditioning was based on previous work 51,52 . For all behavioral experiments, mice head-fixed and walking on a Fast-Trac Activity Wheel (Bio-Serv).…”

“…We therefore tested whether learning to an optogenetic-US could also be adapted for varying CS+US interstimulus intervals. Indeed, when the interval between CS and CF-ChR2-US onset was shifted from 300ms to 500ms, mice adapted the timing of their learned responses 52,59 (Fig. 1m).…”