Synaptic contacts between cholinergic afferents and suprachiasmatic neurones of the rat

The relationship between cholinergic fibres in the suprachiasmatic nucleus (SCN) and neurones of the cell group was investigated at the light and electron microscopic level using choline acetyltransferase (ChAT) immunocytochemistry for the identification of cholinergic elements. Axosomatic and axodendritic synaptic contacts were found between ChAT-immunopositive axon terminals and SCN neurones. These synapses were asymmetrical. The observations provide the final morphological basis for the view already suggested by neuroanatomical, electrophysiological and pharmacological findings that the cholinergic elements in the SCN may act directly on the neurones of the nucleus.