SYNAPTIC AND NONSYNAPTIC MONOAMINERGIC NEURON SYSTEMS IN THE LAMPREY SPINAL-CORD

In the lamprey spinal cord, dopamine- (DA) and 5-hydroxytryptamine (5- HT) containing cells appear to play an important role in controlling t he firing properties of motoneurons and interneurons and, thereby, in modulating the efferent motor pattern. To determine the detailed morph ology and synaptic connectivity of the intraspinal DA and 5-HT systems in Lampetra fluviatilis and Ichthyomyzon unicuspis, DA and 5-HT antis era were used in light and electron microscopic immunocytochemical exp eriments. Two main groups of labeled cells were distinguished: DA-cont aining Liquor-contacting (LC) cells distributed along the central cana l, and 5-HT+DA-containing multipolar cells located near the midline ve ntral to the central canal. Both types were synaptically connected wit h other neuronal elements. The DA-immunoreactive LC cells, which exten ded a ciliated process into the central canal, received symmetrical sy napses from unlabeled terminals containing small synaptic vesicles. Th e distal process of the LC cells could be traced to the lateral cell c olumn, to the ventral aspect of the dorsal column, or to the ventromed ial area. Ultrastructural analysis of DA fibers in these regions showe d the presence of labeled terminals containing numerous small synaptic vesicles and a few dense-core vesicles. These terminals formed symmet rical synapses with unlabeled cell bodies and dendrites, with GABA-imm unopositive LC cells, and with the multipolar DA+5-HT cells. The multi polar DA+5-HT cells also received input from unlabeled synapses. Intra cellular recording from these cells showed that they received excitato ry postsynaptic potentials in response to stimulation of fibers in the ventromedial tracts and dorsal roots. The terminals of the multipolar DA+5-HT neurons in the ventromedial spinal cord contained numerous de nse-core vesicles and small synaptic vesicles, but no synaptic special izations could be detected. In addition, a small number of larger DA-i mmunoreactive cells were observed in the lateral cell column at rostra l levels. The lamprey spinal cord thus contains distinct populations o f synaptically interconnected monoaminergic neurons. Dopamine-containi ng LC cells synapse onto DA+5-HT-containing multipolar cells, in addit ion to GABAergic LC cells and unidentified spinal neurons. In contrast , the multipolar cells appear to exert their influence by nonsynaptic mechanisms. (C) 1996 Wiley-Liss, Inc.