Plasticity in an Electrosensory system II: Postsynaptic Events Associated With a Dynamic Sensory Filter. Bastian, Joseph. Department of Zoology, University of Oklahoma, Norman, OK 73019, USA.
APStracts 3:0104N, 1996.
SUMMARY AND CONCLUSIONS
1. This report summarizes studies of the changes in postsynaptic potentials that occur as pyramidal cells within the primary electrosensory processing nucleus learn to reject repetitive patterns of afferent input. The rejection mechanism employs "negative image inputs" which oppose or cancel electroreceptor afferent inputs or patterns of pyramidal hyperpolarization or depolarization caused by intracellular current injection. Feedback pathways carrying descending electrosensory, as well as other types of information provide the negative image inputs. This study focuses on the role of a directly descending projection from a second-order electrosensory nucleus, the n. praeeminentialis (nP), which provides excitatory and inhibitory inputs to the apical dendrites of electrosensory lateral line lobe (ELL) pyramidal cells. 2. Electrical stimulation of the pathway linking the nP to the ELL was used to activate descending inputs to the pyramidal cells. Pyramidal cell activity was typically increased due to stimulation of this pathway. Tetanic stimulation of the descending pathway paired with either electrosensory stimuli which inhibited pyramidal cells, or hyperpolarizing current injection, increased the excitation provided by subsequent stimulation of this pathway. Pairing tetanic stimulation with excitatory electrosensory stimuli, or depolarizing current injection, had the opposite effect. Subsequent activation of the descending pathway inhibited pyramidal cells. 3. Intracellular recordings showed that the increased firing of pyramidal cells evoked by stimulation of the descending pathway following tetanic stimulation paired with postsynaptic hyperpolarization resulted from larger amplitude and longer duration EPSPs. The shift in the effect of activity in this descending pathway to providing net inhibitory input to the pyramidal cells following paired presynaptic activity and postsynaptic depolarization probably results from the potentiation of IPSPs. The EPSP and IPSPs evoked by activity in this descending pathway can be continuously adjusted in amplitude thereby counterbalancing patterns of pyramidal cell excitation and inhibition received from the periphery with the result that repetitive patterns of afferent activity are strongly attenuated. 

Received 25 March 1995; accepted in final form 21 May 1996.
APS Manuscript Number J244-6.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 5 June 96