Previous studies from our laboratory have investigated field potentials evoked in the cerebellar cortex as a result of transmission within spino-olivocerebellar pathways in awake cats. Such studies have demonstrated that climbing fibre pathways are subject to powerful modulatory influences of central origin that limit their capacity to forward information to the cerebellum during active movements such as walking and reaching (for a review, see Apps, 1999). If this movement-related gating of transmission is to be considered an important and general phenomenon it is necessary that studies are carried out in species other than cats. The demonstration will show example results in which cerebellar cortical evoked field potentials have now been recorded in awake, behaving rats.

The experiments were carried out in accordance with UK Home Office guidelines, and at the end the animals were humanely killed. In brief, the experimental arrangements are as follows: Male Wistar rats (300-600 g) are trained for about 1-2 weeks to walk on a moving exercise belt (ca 0.15 m s^-1) after which an aseptic operation is performed under pentobarbitone (40 mg kg^-1 I.P.) general anaesthesia. A small craniotomy is performed to expose the surface of the cerebellar paravermis on the left side (e.g. in the region of the paramedian lobule) and from six to ten microwires (35 µm outer diameter; 90 % Platinum 10 % Iridium; Teflon insulated except at the tip; California Fine Wire) implanted into the cortex to a depth of about 1-2 mm. Bipolar stimulating electrodes are also implanted subcutaneously into the left forearm, together with bipolar recording electrodes into the belly of the extensor muscle triceps brachii in the same forelimb.

Following recovery from the operation a brief (0.1 ms) low-intensity, constant current, electrical stimulus is delivered via the subcutaneous stimulating electrodes to the forelimb once every 1.5 s to evoke field potentials at different cerebellar microwire recording sites. As in our previous studies in cats, variations in the size of these fields are used as a measure of changes in pathway transmission during quiet rest and during bouts of locomotion on the moving belt. Filter settings are 30 Hz-5 kHz for recording both the cerebellar fields and the forelimb EMG and all signals are digitised on-line at a sampling rate of at least 10 kHz, using Spike2 software in combination with a 1401Plus computer interface (both supplied by Cambridge Electronic Design, UK).

The EMG activity of the forelimb extensor muscle is used to identify the timing of the beginning of the stance phase in the step cycle of the ipsilateral forelimb and histograms are constructed, displaying variations in mean size of cerebellar response for different tenths of the normalised step cycle (cf. Apps et al. 1990).

The demonstration will include a video and examples of data obtained so far in the awake, behaving rat.

This work was supported by the MRC.

Apps, R. (1999). Prog. Neurobiol. 57, 537-562.

Apps, R., Lidierth, M. & Armstrong, D.M. (1990). J. Physiol. 424, 487-512. abstract