IN-VITRO LONG-TERM POTENTIATION OF ELECTROTONIC RESPONSES OF GOLDFISHMAUTHNER CELLS IS ACCOMPANIED BY ULTRASTRUCTURAL-CHANGES AT AFFERENT MIXED SYNAPSES

The potentiated afferent mixed synapses of the Mauthner cells of fry a nd adult goldfish in stumps of the medulla oblongata incubated long-te rm in vitro were studied by electrophysiological and electron microsco pic methods. It was shown that brief high-frequency stimulation of pos terior branches of the eighth nerve induced a long-term potentiation o f electrotonic transmission at large and small mixed club endings. It was about 135% upon subthreshold stimulation and about 200% upon supra threshold stimulation. The ultrastructural analysis of ultrathin secti ons of potentiated mixed synaptic endings revealed an increase in the dimensions of desmosome-like contacts which was proportional to the de gree of potentiation, about 135% or 200%, depending on the type of sti mulation. The dimensions of gap junctions remained unchanged. The dime nsions of active zones at potentiated synapses were reduced two-fold a s compared with their unpotentiated counterparts, irrespective of the type of stimulation. Considering that desmosome-like contacts consist predominantly of F-actin, a molecule which possesses electroconductivi ty, it can be assumed that this cytoskeletal protein is involved in th e process of potentiation. The increase in the synapse electrical cond uctivity can be mediated either directly, by shunting the synaptic jun ction with polymer actin filaments in the region of desmosome-like con tacts, or indirectly, via the interaction of actin with gap junction c onnections situated nearby. (C) 1998 IBRO. Published by Elsevier Scien ce Ltd.