Inhibitory mechanisms in lateral geniculate nucleus of rat

1. An examination was made of the mechanisms responsible for the inhibition of synaptic transmission through the lateral geniculate nucleus (LGN) of the rat following a single shock to the optic nerve. 2. In the rat anaesthetized with paraldehyde we found no evidence that optic nerve stimulation produced any presynaptic inhibition in LGN. In agreement with other workers it was found that repetitive stimulation of visual cortex produced effects attributable to presynaptic inhibition. However, this was of small magnitude in the conditions of our experiments. 3. Stimulation of the optic nerve elicited an action potential in a P cell (principal cell) which was followed by a wave of hyperpolarization lasting about 150 msec (inhibitory post-synaptic potential, IPSP, wave). 4. The IPSP wave was chloride-dependent and was associated with inhibition of the P cell discharge. Occasional rippling on the IPSP wave suggests that it was produced by the repetitive discharge of I cells (interneurones). 5. These observations support the model proposed previously wherein P cells are inhibited by I cells which in turn are excited by axon collaterals of P cells. There is evidence for diffuse interconnexions between P cells and I cells. 6. The observation that the extracellularly recorded wave of hyperpolarization (P-wave) is usually negative suggests that most of the inhibitory synapses are not on the soma of the P cell.