Effects of recurrent collateral inhibition on Purkinje cell activity in the immature rat cerebellar cortex - an in vivo electrophysiological study

Abstract

We present here a study on the effects of inhibitory recurrent collaterals of Purkinje cell (PC) axons on the activity of the immature rat cerebellar cortex. Simultaneous extracellular recordings of pairs of PCs were performed in rat pups aged 5–8 days postnatal. Bicuculline was applied to the surface of the cortex in order to functionally antagonize PC recurrent collaterals. At this early developmental stage these are the only inhibitory links in the network. Dye marks from the microelectrode tips and 3D serial-section reconstruction of the structure allowed the exact determination of the distance separating recorded cells and of their respective orientation in the cortex. Standard statistical tests and an informational entropy index were used to calculate levels of cooperativity. By comparing PC activity under control conditions and after bicuculline superfusion it is shown that recurrent collateral inhibition has a structurating effect on the PC activity and that it increases the informational content of the network. Inhibition decreases the activity of the cells by 35% and drastically changes the interspike interval histograms. This leads to a more constrained state of the system. Three types of coupling via recurrent collaterals are present: symmatrical, asymmetrical or non existent. The exact type of coupling follows a simple vicinity rule and strongly influences the cooperativity level between the recorded cells. This cooperativity was also found to be spatially compartmentalized. Several pairs were driven by common inputs via climbing fibers or parallel fibers. Using the predictive value of a theoretical model of this immature structure 16 we propose a complementary explanation of the role of the recurrent collaterals at this stage of development: that of a spatial and temporal filter, specific to each different microzone.