Autosomal recessive spastic ataxia of Charlevoix-Saguenay (ARSACS) is an early-onset neurodegenerative disease that includes a pronounced and progressive cerebellar dysfunction. ARSACS is caused by an autosomal recessive loss-of-function mutation in the Sacs gene that encodes the protein sacsin. To better understand the cerebellar pathophysiology in ARSACS, we studied synaptic and firing properties of Purkinje cells from a mouse model of ARSACS, Sacs mice. We found that excitatory synaptic drive was reduced onto Sacs Purkinje cells, and that Purkinje cell firing rate, but not regularity, was reduced at postnatal day (P)40, an age when ataxia symptoms were first reported. Firing rate deficits were limited to anterior lobules that later display Purkinje cell death, and were not observed in posterior lobules where Purkinje cells are not lost. Mild firing deficits were observed as early as P20, prior to the manifestation of motor deficits, suggesting that a critical level of cerebellar dysfunction is required for motor coordination to emerge. Finally, we observed a reduction in Purkinje cell innervation onto target neurons in the deep cerebellar nuclei (DCN) in Sacs mice. Together, these findings suggest that multiple alterations in the cerebellar circuit including Purkinje cell input and output contribute to cerebellar-related disease onset in ARSACS.

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A pertinent mechanism by which stress impacts learning and memory is through stress-induced plastic changes in glutamatergic transmission in the hippocampus. For instance, acute stress has been shown to alter the expression, binding, and function of the ionotropic glutamate N-methyl-D-aspartate receptor (NMDAR). However, the consequences of chronic stress, which could lead to various stress-related brain disorders, on NMDAR function remain unclear. While most studies on NMDARs focused on these receptors in synapses (synaptic NMDARs or sNMDARs), emerging findings have revealed functional roles of NMDARs outside synapses (extrasynaptic NMDARs or exNMDARs) that are distinct from those of sNMDARs. Using a restraint stress paradigm in adult rats, the objective of the current study is to examine whether sNMDARs and exNMDARs in the hippocampus are differentially regulated by acute and chronic stress. We examined sNMDAR and exNMDAR function in dorsal CA1 hippocampal neurons from brain slices of adult rats that were acutely (1 episode) or chronically (21 daily episodes) stressed by restraint (30 min). We found that acute stress increases sNMDAR but suppresses exNMDAR function. Surprisingly, we only observed a reduction in exNMDAR function after chronic stress. Taken together, our findings suggest that sNMDARs and exNMDARs may be differentially regulated by acute and chronic stress. Most importantly, the observed suppression in exNMDAR function by both acute and chronic stress implies crucial but overlooked roles of hippocampal exNMDARs in stress-related disorders.