Visual experience prevents dysregulation of GABA_B receptor-dependent short-term depression in adult superior colliculus

Progressive loss of plasticity during development prevents refined circuits from regressing to an immature state and is thought to depend on maturation of GABAergic inhibition. For example, a gradual reduction in size of visual receptive fields (RFs) occurs in the superior colliculus (SC) during development. Maintenance of the refined state throughout adulthood requires early light exposure. Here we investigate the potential role of changes in long- or short-term plasticity in experiencedependent maintenance of refined RFs. Using an acute SC slice preparation, we found that long-term plasticity was not affected by visual deprivation, indicating that it does not underlie deprivationinduced RF enlargement. In contrast, visual deprivation altered short-term plasticity in an unexpected way. Specifically, GABA_B receptor (GABA_BR)-mediated paired pulse depression was increased in slices from dark-reared animals. This increase was mimicked by GABA_AR blockade in slices from normally reared animals, suggesting that experience-dependent maintenance of GABA_AR function prevents an increase in probability of neurotransmitter release. GABA_BR-mediated short-term depression in response to strong stimulation (such as occurs during vision) was reduced in slices from dark-reared animals. This change was mimicked in slices from normal animals by reducing GABA release. These results are consistent with the hypothesis that early visual experience maintains GABAergic inhibition and prevents later deprivation-induced alterations of short-term depression in SC. Identifying how plasticity is restricted in mature circuits could guide therapies to enhance recovery of function in adults.