Journal of Neuroscience, Vol 8, 901-919, Copyright © 1988 by Society for Neuroscience
Afferent influences on brain stem auditory nuclei of the chicken: presynaptic action potentials regulate protein synthesis in nucleus magnocellularis neurons
DE Born and EW Rubel
Department of Otolaryngology, University of Virginia School of Medicine, Charlottesville 22908.
Studies of the avian auditory system indicate that neurons in nucleus
magnocellularis (NM) and nucleus laminaris of young animals are
dramatically altered by changes in the auditory receptor. We examined the
role of presynaptic activity on these transneuronal regulatory events. TTX
was used to block action potentials in the auditory nerve. TTX injections
into the perilymph reliably blocked all neuronal activity in the cochlear
nerve and NM. Far-field recordings of sound- evoked potentials revealed
that responses returned within 6-12 hr after a single TTX injection.
Changes in protein synthesis by NM neurons were measured by determining the
incorporation of 3H-leucine using autoradiography. NM neurons on the side
of the brain ipsilateral to the TTX injection were compared to normally
active cells on the other side of the same tissue section. Grain counts
over individual neurons revealed that a single injection of TTX produced a
40% decrease in grain density in ipsilateral NM neurons within 1.5 hr after
the TTX injection. However, by 24 hr after a single TTX injection, grain
densities were not different on the 2 sides of the brain. Continuous
activity blockade for 6 hr caused the cessation of amino acid incorporation
in a portion of NM neurons and a 15-20% decrease in the remaining neurons.
These changes in amino acid incorporation are comparable to those following
complete removal of the cochlea (Steward and Rubel, 1985). We also examined
NM for neuron loss and soma shrinkage after blocking eighth nerve action
potentials. TTX injected every 12 hr for 48 hr caused a 20% neuron loss and
an 8% shrinkage of the remaining neurons. Similar reductions were found
following cochlea removal (Born and Rubel, 1985). It is concluded that
neuronal activity plays a major role in the maintenance of normal NM
neurons. Furthermore, these results suggest that transneuronal
morphological changes seen in neurons following deafferentation or
alterations of sensory experience are a result of changes in the level of
presynaptic activity.
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