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Andris K. Tebcis

1. Studies involving the electrophoretic administration of antagonists of ACh (atropine, DHE) and cholinesterase inhibitors (neostigmine, physostigmine) to MGN neurones indicate that ACh is an excitatory transmitter in the feline MGN, most probably released from fibres which originate in or traverse the mesencephalon.

2. Auditory afferents to the MGN, cortico-geniculate fibres and the excitatory fibres which mediate `spontaneous' firing of MGN neurones are unlikely to be cholinergic.

3. Almost all geniculo-cortical relay cells are excited by ACh, this excitation being mediated by receptors which have both muscarinic and nicotinic properties. The excitation of relay cells by ACh is sometimes preceded or followed by a depression of firing which is resistant to atropine and DHE, but the significance of this depression is unknown.

4. The firing of many unidentified MGN neurones is depressed by ACh in the absence of any excitation, and this depression is blocked by both atropine and DHE, and potentiated by anticholinesterases. This type of depression by ACh may be related to cholinergic inhibition, but this possibility has yet to be investigated.