Synaptically-evoked Prolonged Depolarizations in the Developing Auditory
System.
Vibhakar C. Kotak, Vibhakar C., and Dan H. Sanes.
Center for Neural Science and Department of Biology, 6 Washington Place,
New York University, New York, NY 10003.
APStracts 2:0180N, 1995.
SUMMARY AND CONCLUSIONS
1) Although synaptic transmission is known to influence many aspects of
neuronal development, activity rates are quite low at early ages. The present
study describes a long-lasting postsynaptic response to brief periods of
synaptic stimulation that may underlie such an influence. Whole-cell patch
clamp recordings were made from the lateral superior olive (LSO) in a brain
slice preparation from early postnatal gerbils. 2) Stimulation of the
excitatory afferent pathway from the cochlear nucleus elicited a prolonged
depolarization (PD) in about 60% of the LSO neurons tested. Low frequency
stimulation (1 Hz) was as effective as tetanic stimulation in producing PDs.
These synaptically-evoked depolarizations ranged in amplitude from 3 to 32 mV,
and recovered spontaneously after 0.5 to 35 mins. 3) The LSO neuron input
resistance declined during every PD episode, and remained significantly lower
even after the membrane potential had recovered. These prolonged
depolarizations were partially reversed by 2 mM Ni +2 , but 1 NM TTX and 10 NM
CNQX were ineffective. The metabotropic glutamate receptor agonist, t-ACPD (40
NM ), produced depolarizations that outlasted the exposure period by an
average of 20 mins, and were also partially repolarized by 2 mM Ni +2 . In
contrast, the depolarizations produced by AMPA or NMDA decayed within a much
shorter period of time. 4) To test whether in vivo discharge rates are, in
fact, very low during development, spontaneous activity was recorded from
neurons of the auditory midbrain in gerbils prior to and during the onset
of sound-evoked responses. The average discharge rate of auditory neurons was
quite low (X=0.4 spikes/sec), although many cells displayed brief periods of
rapid discharge rate (X=37 spikes/sec). Together, these results demonstrate a
novel form of developmental plasticity elicited by low rates of glutamatergic
transmission, and which may involve a metabotropic pathway and prolonged
calcium influx.
Received 23 March 1994; accepted in final form 4 June 1995.
APS Manuscript Number J195-5.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 6 July 1995.