Ultrastructural Changes in the Mixed Synapses of Mauthner Neurons Related to Long-Term Potentiation and Natural Modification of the Motor Function

We examined changes in the ultrastructure of afferent mixed synapses on the membrane of Mauthner neurons (M cells) of the goldfish, which were related to two functional states, long-term potentiation (LTP) of the electrotonic response (a model form of the memory trace) and adaptation (resistivity to fatigue resulting from long-lasting motor training and considered a natural form of the memory trace manifested on the neuronal level). LTP was induced in medullary slices using high-frequency electrical stimulation of the afferent input. Adaptation was produced using natural vestibular stimulation (everyday motor training, which modified motor behavior of the fish and function of the M cell). It was supposed that if the LTP phenomenon is involved in the formation of natural memory, both the adaptation and the LTP states should be accompanied by similar specific structural modifications. Indeed, it was found that in both cases the number of fibrillar bridges in the gaps of desmosome-like contacts (DLC) in the mixed synapses on the M cell surface demonstrated an about twofold increase. These bridges are known to include actin filaments, which function as conductors of cationic signals; thus, the LTP-related increase in the density of bridges corresponds to increased efficacy of electrotonic coupling via mixed synapses. Such a structural correlate of LTP, which probably has the same functional significance in mixed synapses of the adapted M cells, allows us to suppose that LTP is a natural property of the nervous system. The LTP-type intensification of the relay function of mixed synapses, which corresponds to adaptation, is probably a compensatory rearrangement allowing M cells to maintain some balance of the synaptic influences and, at the same time, to remain in a stable and plastic state; this is necessary for stable functioning under changing environmental conditions.