Location of interneurones in the recurrent inhibitory circuit of the rabbit lateral geniculate nucleus.

Injection of horseradish peroxidase (HRP) into the dorsal lateral geniculate nucleus (LGN) of the rabbit gave rise to retrograde labeling of neurones in the caudal part of the thalamic reticular nucleus. Electrophysiological observations demonstrated that these neurones met all criteria for interneurones in the recurrent inhibitory circuit of the geniculo-cortical pathway. They responded to stimulation of the visual cortex (Cx) or the optic chiasm (OX) with a burst of repetitive discharges, in agreement with the long-lasting IPSP from Cx or OX in relay cells of LGN. Results of collision test showed that the reticular neurones received excitatory input via axonal collaterals of relay cells. The latency of their response to stimulation of Cx or OX is about 1.8 ms shorter than that of the corresponding IPSP in the relay cells. Stimulation of LGN evoked an antidromic spike in reticular neurones with a latency of about 1.1 ms, indicating a monosynaptic projection from the latter to the relay cells. All evidence indicates that interneurones in the recurrent inhibitory circuit are most likely located in the caudal part of the thalamic reticular nucleus of the rabbit.