Brain stem-thalamus reciprocal influences in the cat

Intracellular recordings were made from Mth neurons in acute 'encéphale isole' preparations during low- and high-frequency BP and Mes stimulation. Low-frequency BP stimulation induced short-latency (1-5 msec) EPSPs on Mth neurons, followed by IPSPs which could also appear in isolation. High-frequency BP activation produced mixed excitatory and inhibitory effects. Low-frequency Mes stimulation mainly induced long-latency and long-lasting IPSPs while high-frequency activation blocked spontaneous firing of Mth neurons with or without increase in membrane potential. Similar effect was seen during high-frequency Mth stimulation. DCN stimulation never affected any of the Mth studied neurons. Control experiments have excluded the co-stimulation of spinothalamic and cerebello-rubro-thalamic pathways. Intracellular injection of hyperpolarizing and depolarizing current showed that BP effects were postsynaptic whereas Mes inhibition was due to removal of excitation (i.e., disfacilitation) on Mth neurons. High-frequency BP stimulation induced an increase of cell firing and reduction of the inhibitory phase in the thalamically induced sequences, while Mes activation reduced the EPSPs without apparent changes of IPSPs. Low- and high-frequency Mth stimulation evoked short-latency (1-4 msec) EPSPs on BP and Mes neurons with an increase of firing which was more evident during high-frequency stimulation. Antidromic invasion in some BP and Mes neurons was also found. The results indicate the existence of a reciprocal brain stem-thalamic linkage which is excitatory in the descending, while it is excitatory and inhibitory in the ascending way from BP regions, and mainly disfacilitatory from the midbrain.