Synaptic basis of weak signal detection in the weakly electric fish, Apteronotus leptorhynchus.
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1
University of Ottawa, Cellular and Molecular Medicine, Canada
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2
University of Ottawa, Physics, Canada
The brown ghost knifefish, Apteronotus leptorhynchus, uses an electric organ discharge (EOD) to generate an electric field used for navigation, communication, as well as prey detection. Prey in surrounding water create distortions in the electric field, and these distortions are encoded by cutaneous eletroreceptors. Electroreceptor afferents project to pyramidal cells in the electrosensory lateral line lobe (ELL), which ultimately projects to higher brain centers. The electrosensory system of this weakly electric fish has been the focus of many studies on adaptive sensory processing and neural coding. To date, however, no one has found a mechanism that provides the sensitivity needed to account for detection of weak signals at the behavioral threshold. Computational studies of this sensory system revealed that a possible mechanism involved in enhancing weak signal detectability could involve short-term synaptic dynamics. We therefore used stimulation of electroreceptor afferents with natural response to weak prey signals in order to examine the contribution of both excitatory and inhibitory receptor dynamics in implementing weak signal detection at the eletroreceptor-pyramidal cell synapse. In vivo, the membrane potential of pyramidal cells are able to effectively track the weak signal encoded by electroreceptors. We find that in the absence of either NMDA or GABA-A mediated currents, pyramidal cells are not able to effectively encode these same signals. Blockade of NMDA or GABA-A receptors results in very fast excitatory post-synaptic potentials (EPSPs) showing little variability in amplitude. In contrast, in the presence of NMDA and GABA-A mediated currents we observe facilitation of EPSPs following fast successive stimuli, and inhibition following longer interspike intervals (ISIs). We show that NMDA summation along with GABA-A inhibition allows pyramidal cells to effectively reconstruct the weak prey signal. These results indicate that interplay between temporal summation via NMDA receptors and GABAergic inhibition are involved in optimizing weak signal detection at the P-unit-pyramidal cell synapse. We propose that NMDA receptors are necessary to amplify weak sensory signals while GABA-A receptors ensure that the NMDA response does not saturate. It is known that electroreceptor afferent spiking shows correlations of ISIs and a theoretical study has shown that removal of these correlations may underlie efficient weak signal detection. In line with this study, we also show that synaptic dynamics of the electroreceptor-pyramidal cell synapses are able to effectively decorrelate the correlated ISI sequence, allowing for efficient encoding of weak stimuli.
Keywords:
neural correlations,
Signal detection
Conference:
Tenth International Congress of Neuroethology, College Park. Maryland USA, United States, 5 Aug - 10 Aug, 2012.
Presentation Type:
Poster (but consider for student poster award)
Topic:
Sensory: Electrosensory
Citation:
Marcoux
C,
Maler
L and
Longtin
A
(2012). Synaptic basis of weak signal detection in the weakly electric fish, Apteronotus leptorhynchus..
Conference Abstract:
Tenth International Congress of Neuroethology.
doi: 10.3389/conf.fnbeh.2012.27.00363
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Received:
01 May 2012;
Published Online:
07 Jul 2012.
*
Correspondence:
Mr. Curtis Marcoux, University of Ottawa, Cellular and Molecular Medicine, Ottawa, Canada, cmarc070@uottawa.ca