[Effects of juxtafastigial stimulation on the rhythmic activity of Purkinje cells in harmaline-treated rats].

In the anaesthetized rat, harmaline induces an olivary activation which results in rhythmic complex-spike (CS) discharges of Purkinje cells (4--8/sec) in the vermian cortex. The temporal organization of the rhythmic CS activity was studied. While some Purkinje cells present long periods of continuous rhythmic activity, the rhythmic CS discharge of other cells is modulated by periodical suppression of activity, with a total cycle length of about 10 sec. This organization can be modified by electrical stimulation of the juxtafastigial region (JF). During periods without spontaneous rhythmical CS firing, single JF shocks produce a late reflex response (200--300 msec) of the Purkinje cells, which appears as a repetitive sequence of CS (up to 30) at the harmaline-like induced frequency. The response obtained is the same whether the electrical stimulus is single or given in succession at regular intervals; however, in order to obtain such an effect, the frequency of the JF stimulation has to be inferior to the rhythmic CS frequency produced by the drug. At a higher stimulation rate (10 c/sec, 3 sec) the JF-induced response of Purkinje cells is abolished and we observe the suppression--4 to 8 sec following stimulation - of any rhythmic CS activity. This experimental modulation of rhythmic activity of the Purkinje cells (inducement or suppression of the CS firing), controlled indirectly by the olivary system, reproduces the spontaneous fluctuations of the thythmicity under harmaline. A neurophysiological model is presented and the functional significance of the results is discussed.