MIXED SYNAPSES DISCOVERED AND MAPPED THROUGHOUT MAMMALIAN SPINAL-CORD

Previously, synaptic activity in the spinal cord of adult mammals was attributed exclusively to chemical neurotransmission. In this study, e vidence was obtained for the existence, relative abundance, and widesp read distribution of ''mixed'' (chemical and electrical) synapses on n eurons throughout the spinal cords of adult mammals. Using combined co nfocal microscopy and ''grid-mapped freeze fracture,'' 36 mixed synaps es containing 88 ''micro'' gap junctions (median = 45 connexons) were found and mapped to 33 interneurons and motor neurons in Rexed laminae III-IX in cervical, thoracic, and lumbosacral spinal cords of adult m ale and female rats. Gap junctions were adjacent to presumptive active zones, where even small gap junctions would be expected to increase s ynaptic efficacy. Two morphological types of mixed synapse were discer ned. One type contained distinctive active zones consisting of ''neste d'' concentric toroidal deformations of pre- and postsynaptic membrane s, which, because of their unusual topology, were designated as ''syna ptic sombreros.'' A second type had gap junctions adjacent to active z ones consisting of broad, flat, shallow indentations of the plasma mem brane. Morphometric analysis indicates that mixed synapses correspond to 3-5% of all synapses on the somata and proximal dendrites, but, bec ause of their subcellular location and morphology, they could represen t 30-100% of excitatory synapses. The relative abundance of mixed syna pses on several classes of neurons in spinal cords of adult rats sugge sts that mixed synapses provide important but previously unrecognized pathways for bidirectional communication between neurons in the mammal ian central nervous system.