COMPLEX SYNAPTIC ARRANGEMENTS IN THE RAT SUPRACHIASMATIC NUCLEUS - A POSSIBLE BASIS FOR THE ZEITGEBER AND NONSYNAPTIC SYNCHRONIZATION OF NEURONAL-ACTIVITY

A special type of complex synaptic arrangement occurs in the ventro-la teral portion of the rat suprachiasmatic nucleus. These arrangements a re polycentric, with about equal numbers of pre- and postsynaptic elem ents. Because of an incomplete astroglial covering, these synaptic com plexes are connected with each other and form a continuous reticulum o r sponge-like system throughout the ventro-lateral region of the nucle us. In two partially reconstructed complex synaptic arrangements, bout ons from retinal afferents could be seen to make up the majority of pr esynaptic elements. They form asymmetric and symmetric synaptic apposi tions with dendritic elements. Non-optic axo-dendritic synapses of unk nown origin with asymmetric and symmetric appositions and dendro-dendr itic synapses with symmetric appositions are also seen in complex syna ptic arrangements. Within complex synaptic arrangements, dendrites oft en run in bundles, with some dendrites spiralling around others. Membr anes of neighbouring dendrites are closely apposed. These interdendrit ic appositions are possibly ephapses and may, together with intersomat ic contacts, mediate non-synaptic synchronization of neuronal activity in the suprachiasmatic nucleus, as described by other authors. The ac tivity of optic and non-optic synapses in complex synaptic arrangement s over a 24 h period may also produce an integrated response that infl uences the circadian rhythm of neuronal activity in this nucleus.