Complex synaptic arrangements in the rat suprachiasmatic nucleus: a possible basis for the "Zeitgeber" and non-synaptic synchronization of neuronal activity

A special type of complex synaptic arrangement occurs in the ventro-lateral portion of the rat suprachiasmatic nucleus. These arrangements are polycentric, with about equal numbers of pre- and postsynaptic elements. Because of an incomplete astroglial covering, these synaptic complexes are connected with each other and form a continuous reticulum or sponge-like system throughout the ventro-lateral region of the nucleus. In two partially reconstructed complex synaptic arrangements, boutons from retinal afferents could be seen to make up the majority of presynaptic elements. They form asymmetric and symmetric synaptic appositions with dendritic elements. Non-optic axo-dendritic synapses of unknown origin with asymmetric and symmetric appositions and dendro-dendritic synapses with symmetric appositions are also seen in complex synaptic arrangements. Within complex synaptic arrangements, dendrites often run in bundles, with some dendrites spiralling around others. Membranes of neighbouring dendrites are closely apposed. These interdendritic appositions are possibly ephapses and may, together with intersomatic contacts, mediate non-synaptic synchronization of neuronal activity in the suprachiasmatic nucleus, as described by other authors. The activity of optic and non-optic synapses in complex synaptic arrangements over a 24 h period may also produce an integrated response that influences the circadian rhythm of neuronal activity in this nucleus.