Synaptology of the rat suprachiasmatic nucleus

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Abstract
Summary Within the suprachiasmatic nucleus (SCN) of the rat the fine structure of the synapses and some features of their topological arrangement were studied. Five types of synapses could be distinguished with certainty: A. Two types of Gray-type-I (GTI) or asymmetrical synapses (∼33%). The presynaptic elements contain strikingly different types of mitochondria. Size of clear vesicles: ∼ 450 Å. Synapses with subjunctional bodies often occur, among these also “crest synapses”. Localization: dendritic shafts and spines, rarely somata. B. Three types of Gray-type-2 (GTII) or symmetrical synapses (∼66%)∶:1) Axo-dendritic and -somatic (=AD) synapses. Size of clear vesicles: ∼500 Å. 2) Invaginated axo-dendritic and -somatic (=IAD) synapses with club-like postsynaptic protrusions within the presynaptic elements (PreEl). Size of clear vesicles is very variable: ∼ 400–1,000 Å. 3) Dendro-dendritic, -somatic and somato-dendritic (=DD) synapses occurring at least partly in reciprocal arrangements. They represent an intrinsic system. Shape of clear vesicles: often oval; sucrose treatment partly produces flattening. Dense core-vesicles (dcv) are found in all GTII- and most of the GTI-synapses after three-dimensional reconstruction. All types of synapses (mostly GTII-synapses) can be enclosed by multilamellar astroglial formations. The synapses often occur in complex synaptic arrangements. Dendrites and somata of females show significantly more multivesiculated bodies than those of males. Further pecularities of presynaptic (PreEls) and postsynaptic elements (PostEls) within the SCN are described and discussed.