The Journal of Neuroscience, April 15, 1998, 18(8):3073-3087
Pharmacological Specialization of Learned Auditory Responses in
the Inferior Colliculus of the Barn Owl
Daniel E.
Feldman and
Eric I.
Knudsen
Department of Neurobiology, Stanford University School of Medicine,
Stanford, California 94305
Neural tuning for interaural time difference (ITD) in the optic
tectum of the owl is calibrated by experience-dependent plasticity occurring in the external nucleus of the inferior colliculus (ICX). When juvenile owls are subjected to a sustained lateral displacement of
the visual field by wearing prismatic spectacles, the ITD tuning of ICX
neurons becomes systematically altered; ICX neurons acquire novel
auditory responses, termed "learned responses," to ITD values outside their normal, pre-existing tuning range. In this study, we
compared the glutamatergic pharmacology of learned responses with that
of normal responses expressed by the same ICX neurons. Measurements
were made in the ICX using iontophoretic application of glutamate
receptor antagonists. We found that in early stages of ITD tuning
adjustment, soon after learned responses had been induced by
experience-dependent processes, the NMDA receptor antagonist D,L-2-amino-5-phosphonopentanoic acid (AP-5) preferentially
blocked the expression of learned responses of many ICX neurons
compared with that of normal responses of the same neurons. In
contrast, the non-NMDA receptor antagonist
6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) blocked learned and normal
responses equally. After long periods of prism experience, preferential
blockade of learned responses by AP-5 was no longer observed. These
results indicate that NMDA receptors play a preferential role in the
expression of learned responses soon after these responses have been
induced by experience-dependent processes, whereas later in development or with additional prism experience (we cannot distinguish which), the
differential NMDA receptor-mediated component of these responses disappears. This pharmacological progression resembles the changes that
occur during maturation of glutamatergic synaptic currents during early
development.
Key words:
sound localization; experience-dependent plasticity; inferior colliculus; silent synapses; NMDA receptors; development
Copyright © 1998 Society for Neuroscience 0270-6474/98/1883073-15$05.00/0
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