CHARACTERIZATION OF SYNAPTIC CONNECTIONS BETWEEN CORTEX AND DEEP NUCLEI OF THE RAT CEREBELLUM IN-VITRO

Intracellular recordings were used to characterize the inhibitory syna pses formed by Purkinje cells on neurons in the deep cerebellar nuclei of the rat. This work was performed on organotypic cerebellar culture s where functional connections between Purkinje cells and deep cerebel lar neurons are formed de novo. After blocking ionotropic excitatory a mino acid, and GABA(A) receptors with 6-cyano-7-nitro-quinoxaline-2,3- dione, D-2-amino-5-phosphonovalerate and bicuculline, respectively, th e majority of deep cerebellar neurons fired spontaneously without acco mmodation. This tonic firing was linearly dependent on membrane potent ial and was abolished with hyperpolarization. Bath application of musc imol and baclofen reversibly hyperpolarized deep cerebellar nuclei cel ls. In the presence of excitatory amino acid receptor antagonists, hel d stimulation within the Purkinje cell layer induced monosynaptic inhi bitory potentials in deep cerebellar neurons that were graded and comp letely blocked by bicuculline. Inhibitory potential amplitudes were no t markedly reduced during fast repetitive stimulation of Purkinje cell s, and the resulting hyperpolarization was not affected by the competi tive GABA(B) receptor antagonist CGP 35348. A single inhibitory potent ial temporarily interrupted trains of action potentials induced in dee p cerebellar cells by short depolarizing pulses. Trains of five inhibi tory postsynaptic potentials, evoked at 20 Hz, induced a hyperpolariza tion which transiently blocked the spontaneous firing of deep cerebell ar cells. The efficiency to block action potential discharges depended on the frequency of evoked inhibitory potentials. Bath application of bicuculline induced burst discharges in the control solution. When th e excitatory amino acid receptors were pharmacologically blocked, bicu culline depolarized deep cerebellar neurons inducing sustained action potential discharges. In the presence of tetrodotoxin, bicuculline abo lished miniature inhibitory postsynaptic potentials and resulted in a membrane depolarization of deep cerebellar cells. We conclude that dee p cerebellar neurons isolated from synaptic inputs display a pacemaker -like activity. Although these neurons possess GABA(A) and GABA(B) rec eptors, we confirm that only GABA(A) receptors were involved in the ge neration of inhibitory postsynaptic potentials, even with high frequen cy stimulation. The amplitude of evoked inhibitory potentials was weak ly frequency-dependent, thus allowing a powerful inhibition of the pac emaker-like activity by trains of evoked inhibitory postsynaptic poten tials. Additionally, spontaneous and miniature inhibitory potentials c ontrol the excitability of deep cerebellar neurons by exerting a conti nuous hyperpolarizing tone.