Reciprocal interactions between intralaminar and lateral thalamic nuclei in rats

Reciprocal interactions between intralaminar thalamic nuclei (ncl. centralis lateralis, CL, and ncl. parafascicularis, Pf), the pretectal area (Pt) and lateral thalamic nuclei (ventrobasal complex, VB, ncl. anterior ventralis, AV, and ncl. ventralis anterior, VA) have been observed in ketamine-anaesthetized rats. Extracellular single unit activity has been recorded after single electrical stimuli. Electrical stimulation 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 response. Striatal, sensorimotor cortical or peripheral electrical stimulation also evoked similar responses. The pauses in spontaneous activity were shown to be the result of inhibition since the responsiveness of the intralaminar nuclei or the lateral thalamic neurones to all inputs was abolished or reduced after a conditioning electrical single-shock stimulation in the VB or in the intralaminar nuclei, respectively. The two components of the response were of a different origin, since most of the short latency responses disappeared after medullary, upper cervical sections or large decortications, while the inhibitions persisted. These inhibitions were shown to be of thalamic origin since their duration was decreased after extensive decortications increased after medullary section. It is concluded that the neuroneal properties studied in this report are probably broadly represented throughout the thalamus and that thalamic neurones are under inhibitory control elicited by afferent volleys. This inhibitory control includes a relay in the nucleus reticularis thalami (nRT). The mechanisms of sensory interaction can be purely thalamic, but they can be modulated by suprathalamic and/or mesencephalic loops.