Synaptic structure of the monoamine and peptide nerve terminals in the intermediolateral nucleus of the guinea pig thoracic spinal cord

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Abstract
Synaptic organization of the intermediolateral nucleus of the guinea pig thoracic spinal cord was examined with particular focus on monoamine- and peptide-containing nerve terminals. Axon varicosities having flat synaptic vesicles constituted 17% of all axons in the nucleus and formed exclusively symmetric synapses. Enkephalin-, substance P-, somatostatin-, 5-hydroxytryptamine-, and catecholamine-immunoreactive nerve terminals were densely distributed, while neurotensin-, vasoactive intestinal polypeptide-, oxytocin-, and cholecystokinin-8-immunoreactive nerves were sparse in the nucleus. Coexistence of 5-hydroxytryptamine and enkephalin was demonstrated, and coexistence of somatostatin and enkephalin as well as somatostatin and 5-hydroxytryptamine in the same axons was also shown by serial semithin sections. Catecholamine axons labelled by 5-hydroxydopamine formed axodendritic and axosomatic synapses and made direct synaptic contacts on the preganglionic sympathetic neurons identified by retrograde transport of horseradish peroxidase. Direct synaptic contacts from enkephalin- and substance P-immunoreactive axons to preganglionic sympathetic neurons were also revealed. Enkephalin-, substance P-, and 5-hydroxytryptamine-imunoreactive axons formed axodendritic and axosomatic synapses. Catecholamine axon varicosities constituted 19% of all axon varicosities in the nucleus and 30% of them showed synaptic specializations in a sectional plane. Axon varicosities immunoreactive to enkephalin, 5-hydroxytryptamine, and substance P constituted approximately 35, 19, and 13% of all axon varicosities, respectively, while those with synaptic contacts made up 27, 30, and 26%, respectively, in a sectional plane. Enkephalin-, 5-hydroxytryptamine-, and noradrenaline-immunoreactive axons showed mainly symmetric synaptic contacts.

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