Inhibition from the brain stem of inhibitory interneurones of the cat's dorsal lateral geniculate nucleus.

Abstract

Brain-stem control of inhibitory circuits in the dorsal lateral geniculate nucleus (DLGN) of the cat was studied with extracellular recordings from functionally identified interneurons and with intracellular recordings from principal cells. Perigeniculate neurons, the recurrent inhibitory interneurons of the DLGN, were inhibited by low-threshold stimulation within a wide bilateral field of the brain-stem reticular formation extending from the rostral mesencephalon to the caudal medulla oblongata. The inhibition had a latency of 10-12 ms for stimulation sites in the mesencephalon and a duration of .apprx. 100 ms. The brain-stem stimulation evoked large hyperpolarizing potentials in intracellularly recorded perigeniculate neurons, indicating that the effect was due to post-synaptic inhibition. Intrageniculate interneurons, the feed-forward inhibitory interneurons of the DLGN, were inhibited with a similar time course from the same region of the brain stem. Both feed-forward and recurrent inhibitory post-synaptic potentials (LPSP) in principal cells were depressed by a preceding stimulation of brain-stem sites effective for the interneurons. The depression had about the same time course as the inhibition of the interneurons and it occurred without a concomitant change in the membrane potential of the recorded principal cells. A small depolarizing potential, with a latency of 10-20 ms, was observed in some principal cells after brain-stem stimulation. The potential reversed polarity when IPSP were reversed by current injection into the recorded cell indicating that it was due to disinhibition of the principal cells. The possible neuronal pathway for the inhibition of the DLGN interneurons is considered; the effect may be mediated by a group of neurons located in the caudal mesencephalon and the rostral pons close to the fibers of the brachium conjunctivum.