Electrophysiological properties of parallel fiber synapses in vivo

In vitro studies have revealed intriguing properties of parallel fiber synaptic transmisson. Most notably, these synapses have been shown to induce highly variable synaptic responses, have a pronounced paired pulse facilitation and to induce later, variable synaptic responses in their target neurons. In the present study, whole cell recordings from stellate cells and basket cells were made to evaluate the properties of parallel fiber synaptic transmission under non-anesthetized in vivo conditions in decerebrated cats. Electrical parallel fiber stimulation at threshold intensity was used to study the variability as well as the delayed activation of transmission at single parallel fiber synapses. In addition, the synaptic responses to brief manual skin stimulation, which mimicked the natural synaptic activation pattern and evoked intense spike bursts in these neurons, were analyzed to gain clues about the physiological role of these parallel fiber-specific properties. The data suggest that the high variability pronounced paired-pulse facilitation and the delayed responses reported for parallel fiber synapses in vitro are virtually non-existent in vivo.