Synaptic and electotonic contacts on primary afferent axons in the lamprey Lampetra fluviatilis spinal cord

Abstract

Distribution of GABA and glycine immunoreactivity was studied in synapses on primary afferent axons of the lamprey Lampetra fluviatilis spinal cord using a double labelling technique. Approximately 25% of synapses exhibit GABA immunoreactivity, while more than 70% are immunoreactive to both neurotransmitters. As in other vertebrates, axo-axonal contacts represent three-component synaptic complexes, the so-called triads, where the immunoreactive terminal make synaptic contact simultaneously with the afferent axon and the dendrite contacting this afferent. Contact zones with gap junction-like cell membrane specializations were found between adjacent afferents suggesting the presence of electrotonic interaction between them. This interaction appears to serve for the synchronization of the afferent flow and represents a structural correlate of the mechanism of rapid interneuronal communication between functionally uniform neurons, which is an important element in the organization of coordinated locomotor acts. Besides, our studies provide evidence that afferent–afferent interaction may be mediated not only electrotonically but also with the aid of chemical synapses. This finding suggests that glutamate-induced depolarization of primary afferents results not only from autoreception but also from the direct effect of glutamate on the afferent’s cell membrane.