Climbing Fibre Responses in Olivo‐cerebellar Slice Cultures. I. Microelectrode Recordings from Purkinje Cells

Cerebellar slices prepared from newborn rats were co‐cultured with slices derived from the inferior olive of 4‐day‐old rats. After several weeks in vitro olivary fibres projecting into the cerebellar tissue could be assessed by anterograde labelling with the fluorescent dye 1,1‐dioctadecyl‐3,3,3,3′‐tetramethylindocarbocyanine perchlorate (Dil). Following electrical field stimulation of the olivary tissue, all‐or‐nothing complex spikes were generated in Purkinje cells, which closely resembled climbing fibre responses as seen in situ. These responses were completely and reversibly abolished by 6‐cyano‐7‐nitroquinoxaline‐2‐3‐dione (CNQX, 5 μM), an antagonist of non‐N‐methyl‐D‐aspartate excitatory amino acid receptors. Wash in of smaller concentrations of CNQX (0.5–2 μM) resulted in a graded dose‐dependent depression of the climbing fibre‐induced postsynaptic potentials and in a consecutive failure of distinct active components of the complex spikes. With climbing fibre synaptic transmission blocked by CNQX, complex spike‐like potentials could, however, still be evoked by intrasomatic injection of depolarizing current pulses. Increasing the concentration of Mg²⁺ in the bathing solution from 0.5 to up to 8 mM depressed regenerative complex‐spike components. Olivary stimulation elicited only monophasic postsynaptic potentials in Purkinje cells under these conditions. These observations indicate that voltage‐gated conductances which are substantially involved in the generation of the complex spike, are gated by the climbing fibre synaptic depolarization rather than directly by the climbing fibre transmitter.