Functional synaptic plasticity in the electrosensory system of the little skate, Raja erinacea

All sensory systems face the challenge of extracting behaviorally-relevant stimuli from a background of noise. The electrosensory system of an elasmobranch fish, Raja erinacea, separates important signals from noise at the very first synapse in the brain. Electrosensory information is carried by primary afferents which synapse onto the basilar dendrites of principal neurons (AENs) in the dorsal nucleus of the medulla, a cerebellum-like structure. In addition, AENs receive input on apical dendrites from thousands of parallel fibers (PFs) carrying a multitude of sensory feedback and correlates of motor commands. The nucleus acts as an adaptive filter; predictable electrosensory input that is coincident with PF activity is removed while novel electrosensory inputs are maintained.; Bodznick and Montgomery (1994) hypothesized that changes in synaptic strength at PF-AEN synapses could be the basis for the adaptive filter. Modeling studies (Nelson and Paulin, 1995) suggest that the PF-AEN synapses, if modifiable, could constitute the major component of the mechanism responsible for the adaptive filter phenomenon. The results of the studies described herein show that changes in PF-AEN synapses underlie the adaptive filter in the little skate. During a coupling period consisting of coincident activity of the PFs and AEN, the strength of PF-AEN synapses declines in a population of plastic AENs. When coupling ceases and PF stimulation is presented alone, the strength of PF-AEN synapses increases. The results show that there may be an increase in the apparent IPSP strength during the coupling period, indicating that GABA-ergic stellate cell-AEN synapses may be changing as a result of coincident PF-AEN activity.; Physiology of AENs and PF-AEN synapses as measured using the blind in vivo whole-cell patch method are described. Comparisons between recordings using sharp intracellular electrodes and whole-cell patch electrodes are made, responses of AENs to electrosensory stimuli are compared to responses elicited by direct intracellular current injection, and the physiological characteristics of plastic AENs, those that exhibit changes in synaptic strength as a result of coincident PF-AEN activity, are compared with non-plastic AENs. The two AEN classes have similar physiological characteristics and are therefore likely to have the same cell morphology.