Studies on cholinergic transmission in the medial geniculate nucleus.

1. Studies were made on the effects of iontophoretically and intravenously administered cholinergic antagonists on the synaptic responses of medial geniculate (MG) neurones evoked by stimulation of the auditory cortex, inferior colliculus and mesencephalic reticular formation.2. Atropine specifically blocked a proportion of the excitatory responses evoked by stimulating the auditory cortex, inferior colliculus and reticular formation, although it was without effect on some of them.3. Neostigmine and eserine facilitated some excitatory synaptic responses evoked by inferior collicular stimulation.4. It is suggested that the feline MG nucleus receives excitatory cholinergic, as well as non-cholinergic, pathways from the auditory cortex, inferior colliculus and lower brain stem. The cholinergic pathways from the auditory cortex may be either corticofugal fibres or recurrent axon collaterals of afferent projections from the MG nucleus to the cortex. Those from the lower brain stem are possibly the cholinesterase-containing fibres described by Shute & Lewis (1967).