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 primary afferent fibres were physiologically characterized and intracellularly labelled with horseradish peroxidase. The cells were prepared for combined light and electron microscopy, and synaptic arrangements formed by axon collaterals of interneurons and synapses formed with their dendrites and somata were examined with the electron microscope. Immunogold reactions for gamma-aminobutyric acid, glycine and glutamate were performed to determine if these synapses were excitatory or inhibitory. Axon collaterals in lamina VI formed synapses with somata and dendrites of other neurons, and collaterals of one cell also formed axoaxonic synapses. It was concluded that one cell from the sample was inhibitory, whereas the remainder were probably excitatory. Dendrites and cell bodies of interneurons were contacted by several types of synaptic bouton. The first type of bouton displayed immunoreactivity for glutamate, the second type contained both gamma-aminobutyric acid and glycine, the third type contained glycine alone, and the fourth type contained gamma-aminobutyric acid alone. Some large glutamatergic boutons were postsynaptic to other boutons. Presynaptic boutons at these axoaxonic synapses always contained gamma-aminobutyric acid but a minority also contained glycine. The results of this study demonstrate the heterogeneity of dorsal horn group II interneurons and provide evidence that they include inhibitory and probably also excitatory neurons. Boutons originating from several chemically different classes of neuron are responsible for postsynaptic inhibition of these interneurons, and the presence of axoaxonic synapses indicates that their excitatory input is also controlled presynaptically.

Subject headings

MEDICIN OCH HÄLSOVETENSKAP  -- Medicinska och farmaceutiska grundvetenskaper -- Fysiologi (hsv//swe)

MEDICAL AND HEALTH SCIENCES  -- Basic Medicine -- Physiology (hsv//eng)

Keyword

Animals

Axons

physiology

Cats

physiology

Dendrites

physiology

ultrastructure

Image Processing

Computer-Assisted

Immunohistochemistry

Interneurons

physiology

ultrastructure

Microscopy

Electron

Neurotransmitter Agents

analysis

Presynaptic Terminals

physiology

ultrastructure

Spinal Cord

cytology

physiology

Synapses

physiology

ultrastructure

Publication and Content Type

ref (subject category)

art (subject category)