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 cerebe llar slices was assessed following stimulation of the parallel fibre i nput, which, in this preparation, preferentially activates the inhibit ory interneurones innervating Purkinje cells. Peristimulus-time histog ram 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 BZ(1) recep tors associated with postsynaptic GABA(A) receptors on Purkinje cells. 1 mu M Ro 15-4513 induced a similar response but 100 nM Ro 15-4513 in duced 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 predomin ated, the minimal effective concentration being 10 mu(M). 1 mu M fluma zenil blocked both components of this response to 100 nM Ro 15-4513, b ut at 100 nM flumazenil only blocked the decrease in inhibition. The a bility of Ro 15-4513 but not Ro 19-4603 to enhance inhibition and its relative insensitivity to 100 nM flumazenil, parallel the affinities o f these compounds for diazepam-insensitive (DI) binding sites in the c erebellum. These data suggest that the enhancement of inhibition induc ed by Ro 15-4513 results from its inverse agonist activity on DI recep tors causing disinhibition of both granule cells and their parallel fi bres and increased sensitivity to the electrical stimuli inducing acti vation of the inhibitory interneurones innervating Purkinje cells.