Somatic recording of GABAergic autoreceptor current in cerebellar stellateand basket cells

Patch-clamp recordings were performed from stellate and basket cells in rat cerebellar slices. Under somatic voltage clamp, short depolarizing pulses were applied to elicit action potentials in the axon. After the action pote ntial, a bicuculline- and Cd2+- sensitive current transient was observed. A similar response was obtained when eliciting axonal firing by extracellula r stimulation. With an isotonic internal Cl- solution, the peak amplitude o f this current varied linearly with the holding potential, yielding an extr apolated reversal potential of -20 to 0 mV. Unlike synaptic or autaptic GAB Aergic currents obtained in the same preparation, the current transient had a slow rise-time and a low variability between trials. This current was bl ocked when 10 mM BAPTA was included in the recording solution. In some expe riments, the current transient elicited axonal action potentials. The curre nt transient was reliably observed in animals aged 12-15 d, with a mean amp litude of 82 pA at -70 mV, but was small and rare in the age group 29-49 d. Numerical simulations could account for all properties of the current tran sient by assuming that an action potential activates a distributed GABAergi c conductance in the axon. The actual conductance is probably restricted to release sites, with an estimated mean presynaptic current response of 10 p A per site (-70 mV, age 12-15 d). We conclude that in developing rats, stel late and basket cell axons have a high density of GABAergic autoreceptors a nd that a sizable fraction of the corresponding current can be measured fro m the soma.