The mechanism coupling exocytosis and endocytosis remains to be elucidated at central synapses. Here, we show that the mechanism linking these two processes is dependent on microdomain-[Ca2+](i) similar to that which triggers exocytosis, as well as the exocytotic protein synaptobrevin/VAMP. Furthermore, block of endocytosis has a limited, retrograde action on exocytosis, delaying recruitment of release-ready vesicles and enhancing short-term depression. This effect sets in so rapidly that it cannot be explained by the nonavailability of recycled vesicles. Rather, we postulate that perturbation of a step linking exocytosis and endocytosis temporarily prevents new vesicles from docking at specialized sites for exocytosis.

Recruitment of release-ready vesicles at synapses is one of the important factors, which determine dynamic properties of signaling] i . We could also demonstrate an approximate sixfold increase in release probability (facilitation) during the initial 5-15 AP-like stimuli of such trains in our experimental condition, regardless of EPSC depression.

Properties of glutamatergic synaptic transmission were investigated by simultaneously voltage-clamping a pair of connected bipolar cells and cells in the ganglion cell layer (GLCs) in the newt retinal slice preparation. Activation of the Ca2+ current in a single bipolar cell was essential for evoking the glutamatergic postsynaptic current in the GLC. Depolarization for as short as 15 msec activated both NMDA and non-NMDA receptors. On the other hand, analysis of the spontaneous glutamatergic synaptic currents of GLCs revealed that these currents consisted of mainly non-NMDA receptor activation with little contribution from NMDA receptors. This suggests that non-NMDA receptors of GLCs are clustered in postsynaptic membrane regions immediately beneath the release sites of bipolar cells and that NMDA receptors have lower accessibility to the released transmitter than non-NMDA receptors. Glutamate that is spilled over from the release sites may activate the NMDA receptors. When a prolonged depolarizing pulse was applied to a bipolar cell, the response induced by non-NMDA receptors was limited greatly by their fast desensitization, whereas NMDA receptors were able to produce a maintained response. The relationship between the pulse duration applied to the bipolar cell and the integrated charge of the response evoked in the GLC was almost linear. Therefore, we propose that both non-NMDA and NMDA receptors cooperate to transfer the graded photoresponses of bipolar cells proportionally to GLCs.

Key pointsr Spinocerebellar ataxia type 1 (SCA1) is a progressive neurodegenerative disease caused by a gene defect, leading to movement disorder such as cerebellar ataxia.r It remains largely unknown which functional defect contributes to the cerebellar ataxic phenotype in SCA1.r In this study, we report progressive dysfunction of metabotropic glutamate receptor (mGluR) signalling, which leads to smaller slow synaptic responses, reduced dendritic Ca 2+ signals and impaired synaptic plasticity at cerebellar synapses, in the early disease stage of SCA1 model mice.r We also show that enhancement of mGluR signalling by a clinically available drug, baclofen, leads to improvement of motor performance in SCA1 mice. r SCA1 is an incurable disease with no effective treatment, and our results may provide mechanistic grounds for targeting mGluRs and a novel drug therapy with baclofen to treat SCA1 patients in the future.Abstract Spinocerebellar ataxia type 1 (SCA1) is a progressive neurodegenerative disease that presents with cerebellar ataxia and motor learning defects. Previous studies have indicated that the pathology of SCA1, as well as other ataxic diseases, is related to signalling pathways mediated by the metabotropic glutamate receptor type 1 (mGluR1), which is indispensable for proper motor coordination and learning. However, the functional contribution of mGluR signalling to SCA1 pathology is unclear. In the present study, we show that SCA1 model mice develop a functional impairment of mGluR signalling which mediates slow synaptic responses, dendritic Ca 2+ signals, and short-and long-term synaptic plasticity at parallel fibre (PF)-Purkinje cell (PC) synapses in a progressive manner from the early disease stage (5 postnatal weeks) prior to PC death. Notably, impairment of mGluR-mediated dendritic Ca 2+ signals linearly correlated with a reduction of PC capacitance (cell surface area) in disease progression. Enhancement of mGluR signalling by baclofen, a clinically available GABA B receptor agonist, led to an improvement of motor performance in SCA1 mice and the improvement lasted ß1 week after a single application of baclofen. Moreover, the restoration of motor performance in baclofen-treated SCA1 mice matched the functional recovery of mGluR-mediated slow synaptic currents and mGluR-dependent short-and long-term synaptic plasticity. These results suggest that impairment of synaptic mGluR cascades is one of the important contributing factors to cerebellar ataxia in early and middle stages of SCA1 pathology, and that modulation of mGluR signalling by baclofen or other clinical interventions may be therapeutic targets to treat SCA1. , human Ataxin-1 with 76 uninterrupted glutamine repeats; BDNF, brainderived neurotrophic factor; Calbindin, calbindin D-28k; C57BL/6 mice, C57 black 6 mice; CF, climbing fibre; CPCCOEt, 7-hydroxyiminocyclopropan[b]chromen-1a-carboxylate ethyl ester; ER, endoplasmic reticulum; D-AP5, D-(-)-2-amino-5-phosphonopentanoic acid; GFP, green fluorescent protein; GPCR, G-protein-coupled rec...