Estimating the parameters of a vesicular storage and release system in rat hippocampus

During high frequency stimulation, synaptic efficacy decreases due to the depletion of the readily releasable pool (RRP), and its replenishment is vital in maintaining synaptic transmission. According to the 'mobilization by liberation' model, stimulation and subsequent [Ca++] i accumulation frees vesicles from their cytoskeletal constraints, leading to the replenishment of the RRP. Additionally, okadaic acid (a phosphotase inhibitor) and staurosporine (a protein kinase inhibitor) have been shown to affect vesicular mobility. In this thesis, the 'mobilization by liberation' model is tested by evaluating how the dynamics of the vesicular system change with stimulation, [Ca++]o, temperature, and compounds which affect vesicular mobility. The parameters of a vesicular storage and release model were estimated using the excitatory post-synaptic currents recorded from rat hippocampal CA1 neurons. The fractional release increases with [Ca ++]o, whereas the replenishment rate is [Ca++] o-independent. During long, high frequency stimulation, the fractional release remains constant and the replenishment rate decreases markedly and rapidly. Thus, stimulation induced changes of vesicular dynamics are not determined by [Ca++]i accumulation. Agents that affect vesicular mobility did not influence the replenishment rate, and these compounds were ineffective in influencing how the replenishment rate decreases with stimulation. Therefore, the replenishment of the RRP is unlikely to be associated with vesicular movement. The replenishment rate decreases during stimulation at low temperatures (< 22°C), and it increases at high temperatures (> 28°C). After prolonged stimulation, the replenishment rate recovers rapidly, to a steady-state level that is above its pre-stimulation estimate. The recovery of synaptic efficacy is thus an unreliable index of the replenishment rate. In conclusion, our results do not support the 'mobilization by liberation' model in excitatory synapses of the rat hippocampus, and future studies should clarify the underlying biophysical process of vesicular replenishment.