Architecture and synaptic relationships in the intermediolateral nucleus of the thoracic spinal cord of the rat: HRP labelling, catecholamine histochemistry and electron microscopic studies.

The organization of the intermediolateral nucleus (IML) of the thoracic spinal cord was examined using glyoxylic acid-induced fluorescence histochemistry, retrograde horseradish peroxidase (HRP) labelling and electron microscopy. In serial sections of T2, it was found that the distribution of catecholamine nerve terminals was intimately related to the neuronal perikarya of IML. Potassium permanganate fixation and 5-hydroxydopamine treatment revealed small dense-cored vesicles in axon varicosities with or without synaptic specializations. A gelatinous region, composed of small diameter dendrites and unmyelinated axons, formed a narrow longitudinal bundle in the centre of the nucleus. The population of the axon varicosities in the IML was 0.17 +/- 0.02/micrometer2 in 75 nm sections. The average size of the axon varicosities with flat synaptic vesicles was 1.44 +/- 0.05 micrometers2 and that of varicosities with spherical vesicles was 0.97 +/- 0.02 micrometers2. After HRP injection into the superior cervical ganglion, ipsilateral IML neurons were labelled in T1-T3 segments of the spinal cord. Axon varicosities with flat and others with spherical synaptic vesicles synapsing on the dendrites labelled by HRP. Among axon varicosities synapsing on the preganglionic sympathetic neurons, 74.8 +/- 7.1% at axo-somatic synapses and 46.0 +/- 6.7% at synapses on proximal dendrites contained flat synaptic vesicles.