Presynaptic Calcium Signalling in Cerebellar Mossy Fibres

Whole-cell recordings were obtained from mossy fibre terminals in adult turtles in order to characterize the basic membrane properties. Calcium imaging of presynaptic calcium signals was carried out in order to analyse calcium dynamics and presynaptic GABA B inhibition. A tetrodotoxin (TTX)-sensitive fast Na+ spike faithfully followed repetitive depolarizing pulses with little change in spike duration or amplitude, while a strong outward rectification dominated responses to long-lasting depolarizations. High-threshold calcium spikes were uncovered following addition of potassium channel blockers. Calcium imaging using Calcium-Green dextran revealed a stimulus-evoked all-or-none TTX-sensitive calcium signal in simple and complex rosettes. All compartments of a complex rosette were activated during electrical activation of the mossy fibre, while individual simple and complex rosettes along an axon appeared to be isolated from one another in terms of calcium signalling. CGP55845 application showed that GABA B receptors mediated presynaptic inhibition of the calcium signal over the entire firing frequency range of mossy fibres. A paired-pulse depression of the calcium signal lasting more than 1 s affected burst firing in mossy fibres; this paired-pulse depression was reduced by GABA B antagonists. While our results indicated that a presynaptic rosette electrophysiologically functioned as a unit, topical GABA application showed that calcium signals in the branches of complex rosettes could be modulated locally, suggesting that cerebellar glomeruli may be dynamically sub-compartmentalized due to ongoing inhibition mediated by Golgi cells. This could provide a fine-grained control of mossy fibre-granule cell information transfer and synaptic plasticity within a mossy fibre rosette

Stellate cell inhibition of Purkinje cells in the turtle cerebellum in vitro

1. The stellate cell-mediated inhibition of Purkinje cells was studied by intracellular recordings in an in vitro slice preparation of the turtle cerebellar cortex. A graded inhibitory postsynaptic potential (IPSP) was recorded in Purkinje cells upon stimulation of the parallel fibre-stellate cell pathway. 2. The IPSP was abolished by bicuculline, and had a reversal potential around -75 mV, consistent with a GABAA receptor-operated Cl- conductance dominating the response investigated here. 3. Paired recordings from synaptically coupled stellate cells and Purkinje cells demonstrated that the inhibitory input from a single stellate cell is sufficient to reduce the firing in a Purkinje cell. 4. The extracellular-evoked IPSP interacted with the active postsynaptic membrane properties in the Purkinje cell. Interaction with both the Na+ plateau and the IA prolonged the responses to an IPSP, making the net effect of the inhibitory response dependent on the membrane potential in each postsynaptic neurone. 5. A precisely timed IPSP was particularly efficient in reducing dendritic Ca2+ influx. 6. The voltage-dependent Ca2+ component of a climbing fibre response (CFR) as well as of a parallel fibre (PF) input was reduced by the IPSP. 7. It is suggested that Ca2+ spike-mediated reduction in Purkinje cell excitability may be prevented by the stellate cell IPSP-mediated reduction in Ca2+ influx