Afferent influences on brain stem auditory nuclei of the chicken: Neuron number and size following cochlea removal

Abstract

_Studies of the avian auditory sytem 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, 1965). We also examined NM for neuron loss and soma shrinkage after blocking eighth nerve action potentials.TTX injected every 12 hr for 46 hr caused a 20% neuron loss and an 6% shrinkage of the remaining neurons. Similar reductions were found following cochlea removal (Born and Rubel, 1965). It is concluded that neuronal activity plays a major role in the maintenance of normal NM neurons. Furthermore, these results

Rapid regulation of microtubule-associated protein 2 in dendrites of nucleus laminaris of the chick following deprivation of afferent activity

“…Both manipulations cause an immediate cessation of action potentials of excitatory NM afferents to affected NL dendrites (Born et al, 1991) and both appear to produce similar changes in MAP2 immunoreactivity in NL dendrites, as shown in the current study. Cochlea removal does not directly damage the NM axons that innervate NL dendrites and deafferentation-induced cell death in NM does not occur for 2 days (Born and Rubel, 1985). Therefore, the changes in MAP2 immunoreactivity in NL dendrites are likely to be a result of the elimination of presynaptic action potentials rather than frank degenerative changes or the release of degeneration-related cytokines from the presynaptic endings.…”

“…The procedure described in Born and Rubel (1985) was used. Animals were anesthetized as described above.…”

Lithium increases bcl-2 expression in chick cochlear nucleus and protects against deafferentation-induced cell death

“…For example, mitral and tufted cells of the olfactory system die following unilateral naris occlusion (Frazier and Brunjes, 1988;Meisami and Safari, 1981), and in the visual system, the survival of tectal neurons depends on activity-dependent release of trophic factors from retinotectal axon terminals (Catsicas et al, 1992). In the auditory system, cell death can occur in the cochlear nucleus following deafness (Born and Rubel, 1985;Hashisaki and Rubel, 1989;Tierney et al, 1997). Although cell death following sensory deprivation has been widely studied in a variety of systems, little work has gone into efforts *Corresponding author.…”

“…Consequently, a common approach has been to remove one cochlea, thereby removing all excitatory afferent input to NM. Previous studies have shown that deafferentation by cochlea removal results in the death of 20-30% of the neurons in the ipsilateral NM and the remaining neurons show a decrease in soma size (Born and Rubel, 1985;Edmonds et al, 1999;Hyde and Durham, 1994). …”

Trophic Support of Cultured Spiral Ganglion Neurons by Depolarization Exceeds and Is Additive with that by Neurotrophins or cAMP and Requires Elevation of [Ca2+]iwithin a Set Range

“…Blockade of afferent input increases death in vivo in central (Catsicas et al, 1992;Galli-Resta et al, 1993) and peripheral neurons (Wright, 1981;Furber et al, 1987;Meriney et al, 1987;Maderdrut et al, 1988). In the avian auditory system, removal of the cochlea causes rapid atrophic changes, culminating in a 25-30% loss of neurons in the target, nucleus magnocellularis (Born and Rubel, 1985;Steward and Rubel, 1985;Sie and Rubel, 1992); blockade of electrical activity results in rapid atrophic changes and loss of magnocellularis neurons comparable with those after complete cochlear ablation (Born and Rubel, 1988;Pasic and Rubel, 1989;Rubel et al, 1990;Sie and Rubel, 1992).…”