RECIPROCAL INTERACTIONS BETWEEN INTRALAMINAR AND LATERAL THALAMIC NUCLEI IN RATS

Reciprocal interactions between intralaminar thalamic nuclei (ncl. cen tralis lateralis, CL, and ncl. parafascicularis, Pf), the pretectal ar ea (Pt) and lateral thalamic nuclei (ventrobasal complex, VB, ncl. ant erior ventralis, AV, and ncl. ventralis anterior, VA) have been observ ed in ketamine-anaesthetized rats. Extracellular single unit activity has been recorded after single electrical stimuli. Electrical stimulat ion of the VB evoked a short latency orthodromic response followed by a pause in spontaneous activity in neurones of medial thalamic nuclei. Lateral thalamic neurones responded to electrical stimulation of the intralaminar nuclei or the pretectal area with the same pattern of res ponse. Striatal, sensorimotor cortical or peripheral electrical stimul ation also evoked similar responses. The pauses in spontaneous activit y were shown to be the result of inhibition since the responsiveness o f the intralaminar nuclei or the lateral thalamic neurones to all inpu ts was abolished or reduced after a conditioning electrical single-sho ck stimulation in the VB or in the intralaminar nuclei, respectively. The two components of the response were of a different origin, since m ost of the short latency responses disappeared after medullary, upper cervical sections or large decortications, while the inhibitions persi sted. These inhibitions were shown to be of thalamic origin since thei r duration was decreased after extensive decortications increased afte r medullary section. It is concluded that the neuroneal properties stu died in this report are probably broadly represented throughout the th alamus and that thalamic neurones are under inhibitory control elicite d by afferent volleys. This inhibitory control includes a relay in the nucleus reticularis thalami (nRT). The mechanisms of sensory interact ion can be purely thalamic, but they can be modulated by suprathalamic and/or mesencephalic loops.