Reflex sensitization in the frog (Rana pipiens): Stimulation-independent influences

Abstract

Daily stimulation of sites on the dorsum of leopard frogs can result in an enhancement of a leg-wiping reflex. This enhancement, while long lasting, is not specific, i.e., the eliciting of one reflex sensitizes reflexes of other limbs. Animals which receive handling each day but which are not stimulated also demonstrate an increased frequency of reflex elicitation though animals which don't receive daily handling do not. This, coupled with seasonal variations in responsivity suggested that nonspecific factors are largely responsible for enhancement of the reflex. The-potential ot this preparation as a simple system model for the study of learning mechanisms is questioned, and the need for detailed behavioral characterizations of such preparations prior to the undertaking of neurophysiological studies is emphasized.Comparative investigations of learning mechanisms are making increasing use of "simple system" preparations in which naturally occurring or artificially induced restrictions in system complexity provide an enhanced possibility of elucidating fundamental mechanisms. Optimization of this strategy requires that systems be found which demonstrate modifiable behaviors as well as ease of electrophysiological and/or biochemical manipulation (Abraham, Palka, Peeke, & Willows, 1971). Additionally, care must be taken to fully characterize the plasticity at the behavioral level lest spurious neuronal correlations be hypothesized (Lahue & Corning, 1975). The present study helps to clarify an interesting phenomenon at the behavioral level and suggests that further studies of underlying mechanisms in the preparation need reconsideration.Tactile stimulation of the dorsal skin of intact or spinal frogs occasionally elicits one of several wiping reflexes. The reflex elicited is specifically related to the site of stimulation, i.e., either the toes of the ipsilateral front or back leg or the heel of the back leg performs the wiping, dependent upon whether the anterior, major, or caudal zones, respectively, are stimulated. Franzisket (I963) reported that repetitive stimulation of a specific point resulted in an increased probability of eliCiting the appropriate reflex. This enhancement was specific to the zone stimulated and persisted for over 20 days. Since the a c c essibility of the frog spinal cord to neurophysiological analysis has been well established, the possibility exists of exploring the mechanisms of this reflex sensitization. However, attempts at replication of the behavioral results with the readily available Rana pipiens (Franzisket used a European form, Rana esculenta have not been completely successful. Kimble and Ray (1964) found that slight variation in the site of Requests for reprints should be sent to R. Lahue, Department of Psychology, University of Waterloo, Waterloo, Ontario N2L 3Gl, Canada. stimulation was necessary in order to enhance the reflex above a 30% base rate. Repeated application of the stimulus to a very restricted locus resulted in a decrease in reflex elicitation. Corning and Lahu...