RESUMEN
Using immunoelectron microscopic technique the SPergic autoregulatory synapses in the nucleus raphe magnus were studied. The results showed that there were SPlike perikarya and nerve fibers. The positive perikarya mainly were large polypolar cells. The positive fibers appeared as beaded-like and formed axodendritic synapses with SP-positive dendrites besides synapses formed with SP-negative structures. In the autoregulatory synapses the pre- and postsynaptic element were both labeled with immunoreactive products which precipitated at the periphery of small clear synaptic vesicles and the dendritic tubes, on the outer membrane of mitochondria, and in the cytoplasmic matrix. The pre- and postsynaptic membrane were symmetrical, and the synaptic vesicles aggregated near the presynaptic membrane. The synaptic cleft was about 20 nm in width and contained electron dense materials. The generality, structural characteristics and functional significance of the autoregulatory sysnapses were discussed.