[SYNAPTIC AND ELECTROTONIC CONTACTS ON THE PRIMARY AFFERENT AXONS OF THE SPINAL CORD IN THE LAMPREY LAMPETRA...

Distribution of GABA and glycine immunoreactivity was studied in synapses on primary afferent axons of the spinal cord in the lamprey Lampetrafluviatilis by double labeling technique. Approximately 25 % of synapses on afferent axons revealed immunoreactivity to GABA and more than 70 % were im- munoreactive to both neurotransmitters. As in other vertebrates, axo-axonal contacts represented three-component synaptic complexes, the so-called triades, where the immunoreactive terminal was in sy- naptic contact both with the afferent axon and the dendrite contacting with this afferent. Contact zones with membrane specializations of gap junction type were found between adjacent afferents that suggests the presence of electrotonic interaction between afferents which serves, evidently, for synchronization of afferent flow and presents a structural base for a mechanism of fast interneuronal communication of func- tionally uniform neurons which is an important element in organization of coordinated locomotor acts. Besides, our studies provide evidence that afferent-afferent interaction may be mediated not only electro- tonically but also with the aid of chemical synapses. This fact gives grounds to suppose that depolariza- tion of primary afferents, produced by glutamate, occurs not only through autoreception mechanism but also by its direct action on the membrane of an afferent axon.