Regulation of cerebello-cortical transmission in the rat ventromedial thalamic nucleus

On the basis of antidromic stimulation we have identified two distinct neuronal populations in the rat ventromedial thalamic nucleus. The largest population (96%) are thalamo-cortical relay cells which project via the internal capsule to the cerebral cortex. The smaller population of cells (4%) project caudally to the reticular formation and superior colliculus. These two cell types could be distinguished further on the basis of their patterns of spontaneous discharge. Relay cells fluctuate between two activity patterns (i) a rhythmic pattern characterized by periods of high-frequency bursting, and (ii) a more tonic discharge pattern of single spikes. The caudally projecting cells had a characteristic fast, regular type of spontaneous firing. Brachium conjunctivum stimulation evokes two distinct responses in thalamic relay cells. (i) a short-latency single spike, (ii) a longer latency, rhythmic response of 2-3 spikes. Both excitatory responses are followed by a period of cell quiescence. The type of response is dependent upon the cell's firing pattern. The short-latency response occurs during tonic, single-spike activity whilst the longer latency response occurs during high-frequency bursting activity. The short-latency response can be altered to the long latency response by increasing the level of anaesthesia or by applying a conditioning shock to known inhibitory pathways. Conversely the long latency response can be altered to the short-latency response by decreasing anaesthesia or by stimulation of the reticular formation. It is argued that both response types are evoked monosynaptically by activation of the same cerebello-thalamic fibres but that different ionic conductances which are active at different levels of membrane polarization are responsible for the two response patterns. Efficient time-locked cerebello-thalamo-cortical transmission occurs only during tonic single-spike activity, when cerebellar stimulation evokes a short-latency response. Such transmission is allowed or disallowed by the fine balance between converging excitatory and inhibitory afferents. In addition to a monosynaptic excitatory input from the cerebellar nuclei, relay cells received converging synaptic inputs from the substantia nigra, cerebral cortex, reticular formation and superior colliculus. Due to the anatomical arrangement in the rat it proved impossible to assess the role of the pallidum. The population of caudally projecting cells also received several converging synaptic inputs, but unlike those influencing relay cells, these inputs were all excitatory.(ABSTRACT TRUNCATED AT 400 WORDS)