Cannabinoids exert their psychomotor actions through the CB₁cannabinoid receptor in the brain. Genetic deletion of CB₁in mice causes various symptoms, including changes in locomotor activity, increased ring catalepsy, supraspinal hypoalgesia, and impaired memory extinction. Although the cerebellar cortex contains the highest level of CB₁, severe cerebellum-related functional deficits have not been reported in CB₁knock-out mice. To clarify the roles of CB₁in cerebellar function, we subjected CB₁knock-out mice to a delay version of classical eyeblink conditioning. This paradigm is a test for cerebellum-dependent discrete motor learning, in which conditioned stimulus (CS) (352 ms tone) and unconditioned stimulus (US) (100 ms periorbital electrical shock) are coterminated. We found that delay eyeblink conditioning performance was severely impaired in CB₁knock-out mice. In contrast, they exhibited normal performance in a trace version of eyeblink conditioning with 500 ms stimulus-free interval intervened between the CS offset and the US onset. This paradigm is a test for hippocampus-dependent associative learning. Sensitivity of CB₁knock-out mice to CS or US was normal, suggesting that impaired delay eyeblink conditioning is attributable to defects in association of responses to CS and US. We also found that intraperitoneal injection of the CB₁antagonist SR141716A [N-piperidino-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-3-pyrazole carboxamide] to wild-type mice caused severe impairment in acquisition but not extinction of delay eyeblink conditioning. SR141716A treatment had no effect on trace eyeblink conditioning with a 500 or 750 ms trace interval. These results indicate that endogenous cannabinoid signaling through CB₁is essential for cerebellum-dependent discrete motor learning, especially for its acquisition.

The metabotropic glutamate receptor subtype 1 (mGluR1, Grm1) in cerebellar Purkinje cells (PCs) is essential for motor coordination and motor learning. At the synaptic level, mGluR1 has a critical role in long-term synaptic depression (LTD) at parallel fiber (PF)-PC synapses, and in developmental elimination of climbing fiber (CF)-PC synapses. mGluR1a, a predominant splice variant in PCs, has a long carboxyl (C)-terminal domain that interacts with Homer scaffolding proteins. Cerebellar roles of the C-terminal domain at both synaptic and behavior levels remain poorly understood. To address this question, we introduced a short variant, mGluR1b, which lacks this domain into PCs of mGluR1-knock-out (KO) mice (mGluR1b-rescue mice). In mGluR1b-rescue mice, mGluR1b showed dispersed perisynaptic distribution in PC spines. Importantly, mGluR1b-rescue mice exhibited impairments in inositol 1,4,5-trisphosphate receptor (IP₃R)-mediated Ca²⁺release, CF synapse elimination, LTD induction, and delay eyeblink conditioning: they showed normal transient receptor potential canonical (TRPC) currents and normal motor coordination. In contrast, PC-specific rescue of mGluR1a restored all cerebellar defects of mGluR1-KO mice. We conclude that the long C-terminal domain of mGluR1a is required for the proper perisynaptic targeting of mGluR1, IP₃R-mediated Ca²⁺release, CF synapse elimination, LTD, and motor learning, but not for TRPC currents and motor coordination.

To elucidate the functional role of phospholipase Cβ4 (PLCβ4), which is highly expressed in the Purkinje cells of the rostral cerebellum, cerebellar long-term depression (LTD) and delay and trace eyeblink conditioning were investigated in PLCβ4-deficient mice. Rostral cerebellar LTD and delay eyeblink conditioning were severely impaired, whereas trace eyeblink conditioning was not. These results indicate that PLCβ4 is essential for LTD in the rostral cerebellum and delay conditioning, but not trace conditioning. Rostral cerebellar LTD may be required as a neural substrate for delay conditioning, but is not required for trace conditioning.