Different functional effect of Ro 15-4513 and Ro 19-4603 on synaptic responses of Purkinje cells in the rat cerebellar slice.

Modulation of GABA-mediated neurotransmissions by Ro 15-4513 in cerebellar slices was assessed following stimulation of the parallel fibre input, which, in this preparation, preferentially activates the inhibitory interneurones innervating Purkinje cells. Peristimulus-time histogram analysis of inhibitory responses of spontaneously-active Purkinje cells showed only a decrease in the duration of inhibition induced by Ro 19-4603. This is consistent with inverse agonism on the BZ1 receptors associated with postsynaptic GABAA receptors on Purkinje cells. 1 microM Ro 15-4513 induced a similar response but 100 nM Ro 15-4513 induced a biphasic response, with an increase in duration of inhibition preceding the decrease during continued perfusion of the compound. At lower concentrations of Ro 15-4513 the increase in inhibition predominated, the minimal effective concentration being 10 pM. 1 microM flumazenil blocked both components of this response to 100 nM Ro 15-4513, but at 100 nM flumazenil only blocked the decrease in inhibition. The ability of Ro 15-4513 but not Ro 19-4603 to enhance inhibition and its relative insensitivity to 100 nM flumazenil, parallel the affinities of these compounds for diazepam-insensitive (DI) binding sites in the cerebellum. These data suggest that the enhancement of inhibition induced by Ro 15-4513 results from its inverse agonist activity on DI receptors causing disinhibition of both granule cells and their parallel fibres and increased sensitivity to the electrical stimuli inducing activation of the inhibitory interneurones innervating Purkinje cells.