HABITUATION OF A MONOSYNAPTIC RESPONSE IN FROG SPINAL-CORD - EVIDENCE FOR A PRESYNAPTIC MECHANISM

Using the isolated spinal cord of bullfrogs (Rana catesbiana), intracellular correlates of habituation-like depression of the monosynaptic response elicited in motoneurons by lateral column (LC) stimulation were investigated. No alteration was found in electrical properties of motoneurons as a result of habituation. To determine whether transmitter release mechanisms were changing over trials, the LC was stimulated with pairs of stimuli separated by 6 ms presented at 0.5/s. Facilitation ratios increased. Habituation apparently involves a process intrinsic to the LC-motoneuron synapse and is not totally mediated by receptor desensitization. Habituation is probably not mediated by a mechanism extrinsic to the LC-motoneuron synapse that depolarizes terminal endings, e.g., presynaptic inhibition or accumulation of extracellular K+ and is not produced by transmitter depletion. Any of these possibilities has as a necessary consequence that facilitation ratios remain unchanged. Possible mechanisms that could mediate habituation are alterations in mobilization and/or release of transmitter or probability of invasion of terminal branches of the presynaptic fiber by the action potential.