Influence of a GABA agonist, diazepam, on the vestibuloocular reflexes of the rabbit

Systemic pharmacological suppression of neural activity reverses learning impairment in a mouse model of Fragile X syndrome

The enhancement of associative synaptic plasticity often results in impaired rather than enhanced learning. Previously, we proposed that such learning impairments can result from saturation of the plasticity mechanism (Nguyen-Vu et al., 2017), or, more generally, from a history-dependent change in the threshold for plasticity. This hypothesis was based on experimental results from mice lacking two class I major histocompatibility molecules, MHCI H2-Kb and H2Db (MH-CI KbDb-/-), which have enhanced associative long-term depression at the parallel fiber-Purkinje cell synapses in the cerebellum (PF-Purkinje cell LTD). Here, we extend this work by testing predictions of the threshold metaplasticity hypothesis in a second mouse line with enhanced PF-Purkinje cell LTD, the Fmr1 knockout mouse model of Fragile X syndrome (FXS). Mice lacking Fmr1 gene expression in cerebellar Purkinje cells (L7-Fmr1 KO) were selectively impaired on two oculomotor learning tasks in which PF-Purkinje cell LTD has been implicated, with no impairment on LTD-independent oculomotor learning tasks. Consistent with the threshold metaplasticity hypothesis, behavioral pre-training designed to reverse LTD at the PF-Purkinje cell synapses eliminated the oculomotor learning deficit in the L7-Fmr1 KO mice, as previously reported in MHCI KbDb-/-mice. In addition, diazepam treatment to suppress neural activity and thereby limit the induction of associative LTD during the pre-training period also eliminated the learning deficits in L7-Fmr1 KO mice. These results support the hypothesis that cerebellar LTD-dependent learning is governed by an experience-dependent sliding threshold for plasticity. An increased threshold for LTD in response to elevated neural activity would tend to oppose firing rate stability, but could serve to stabilize synaptic weights and recently acquired memories. The metaplasticity perspective could inform the development of new clinical approaches for addressing learning impairments in autism and other disorders of the nervous system.

High thresholds for movement perception in schizophrenia may indicate abnormal extraneous noise levels of central vestibular activity

A theoretical argument proposes that thresholds for visual perception of movement should be abnormally high in schizophrenia. This may reflect a central vestibular dysfunction, consisting of abnormally high levels of extraneous noise within the neural activity of the central vestibulo-cerebellar complex. Two experiments are reported with results that support the hypothesis. To some extent, the disorder may explain the smooth pursuit eye movement dysfunction in schizophrenia. Relations to the dopamine hypothesis in schizophrenia are discussed.

Diazepam enhances cerebellar inhibition on vestibular neurons

The spontaneous neuronal activity of the lateral (LVN) and the superior (SVN) vestibular nuclei was analysed before and after the intravenous (i.v.) injection of diazepam in encéphale isolé', decerebrate and cerebellectomized rabbits. The inhibition of vestibular neurons was dependent on the integrity of cerebellar connections with LVN, while these links were partially responsible for the diazepam inhibition on SVN. A role of spinal and telediemesencephalic structures was not recognized. Considering that diazepam does not increase the activity of Purkinje cells, the drug effect ought to be exerted at the level of the Purkinje cell junctions with the cerebellar nuclei and with the vestibular neurons. GABA being the neurotransmitter released by Purkinje cells evidence is provided for a diazepam potentiation of the GABAergic mechanism at the level of vestibular system.