SYNAPTIC CONNECTIONS OF DORSAL HORN GROUP-II SPINAL INTERNEURONS - SYNAPSES FORMED WITH THE INTERNEURONS AND BY THEIR AXON COLLATERALS

Five dorsal horn interneurons with monosynaptic input from group II pr imary afferent fibres were physiologically characterized and intracell ularly labelled with horseradish peroxidase. The cells were prepared f or combined light and electron microscopy, and synaptic arrangements f ormed by axon collaterals of interneurons and synapses formed with the ir dendrites and somata were examined with the electron microscope. Im munogold reactions for gamma-aminobutyric acid, glycine and glutamate were performed to determine if these synapses were excitatory or inhib itory. Axon collaterals in lamina VI formed synapses with somata and d endrites of other neurons, and collaterals of one cell also formed axo axonic synapses. It was concluded that one cell from the sample was in hibitory, whereas the remainder were probably excitatory. Dendrites an d cell bodies of interneurons were contacted by several types of synap tic bouton. The first type of bouton displayed immunoreactivity for gl utamate, the second type contained both gamma-aminobutyric acid and gl ycine, the third type contained glycine alone, and the fourth type con tained gamma-aminobutyric acid alone. Some large glutamatergic boutons were postsynaptic to other boutons. Presynaptic boutons at these axoa xonic synapses always contained gamma-aminobutyric acid but a minority also contained glycine. The results of this study demonstrate the het erogeneity of dorsal horn group II interneurons and provide evidence t hat they include inhibitory and probably also excitatory neurons. Bout ons originating from several chemically different classes of neuron ar e responsible for postsynaptic inhibition of these interneurons, and t he presence of axoaxonic synapses indicates that their excitatory inpu t is also controlled presynaptically. (C) 1997 Wiley-Liss, Inc.