Rapid deafferentation-induced upregulation of bcl-2 mRNA in the chick cochlear nucleus

Neuronal survival in developing animals is often dependent on afferent activity. In the posthatch chick, approximately 30% of the neurons in the avian cochlear nucleus, nucleus magnocellularis (NM) die following elimination of VIIIth nerve activity. The factors that influence death or survival of an individual NM neuron are largely unknown. Previous studies indicate that both cell death and cell survival mechanisms compete to determine cell fate. One factor that has been shown to suppress cell death cascades in a variety of systems is bcl-2. If this gene product plays a role in regulating cell survival following deafferentation, then one might expect the expression of this gene to be influenced by removal of afferent input. In the present study, in situ hybridization revealed a rapid and transient increase in bcl-2 mRNA in NM neurons following deafferentation. Enhanced bcl-2 mRNA expression was observed at 6 and 12 h following deafferentation, but not at 3 or 24 h. Surprisingly, the upregulation of bcl-2 mRNA was limited to a subpopulation (20-30%) of deafferented neurons corresponding to the number of NM neurons that eventually die following cochlea removal. The robust and rapid upregulation of this gene suggests that cell death cascades regulated by bcl-2 may be initiated following deafferentation.