THE ASTROCYTIC RESPONSE TO AFFERENT ACTIVITY BLOCKADE IN CHICK NUCLEUS MAGNOCELLULARIS IS INDEPENDENT OF SYNAPTIC ACTIVATION, AGE, AND NEURONAL SURVIVAL

Astrocytes in nucleus magnocellularis (NM) of the chick respond to aff erent activity blockade with increased immunoreactivity for glial fibr illary acidic protein (GFAP). NM neurons respond to the same manipulat ions with reduced protein synthesis, ribosomal dissociation, and subse quent death of a subset of these neurons. In the present study, we sou ght to evaluate the relationship between these neuronal and glial resp onses and to determine if similar activity-dependent mechanisms mediat e them. We first examined the anatomical relationship between NM neuro ns and astrocytic processes by electron microscopy and GFAP immunostai ning. Both methods showed that NM neurons deprived of activity for 6 h r were apposed by more glial processes than active NM neurons. However , we found no preferential positioning of GFAP-immunoreactive processe s near neurons of the dying or surviving populations, and there were n o differences in glial process apposition to dying versus surviving ne urons at the EM level. To determine whether the astrocytic response is similar to the neuronal response in age dependence, GFAP immunoreacti vity was analyzed in adult chickens following unilateral afferent acti vity blockade. Unlike the neuronal response to activity blockade, the astrocytic response is equally strong in adult animals. These results imply an independence of the neuronal and astrocytic responses to acti vity blockade, raising the possibility that these two cell types may b e responding to different activity-related signals. This possibility w as tested using an in vitro slice preparation. Unilateral stimulation of NM was provided in three ways: orthodromically, antidromically, and orthodromically in a low-calcium medium. The regulation of astrocytic GFAP immunoreactivity by these manipulations of activity was then ana lyzed. The results of these experiments show that, unlike neuronal pro tein synthesis, astrocytic GFAP immunoreactivity can be suppressed by either presynaptic or postsynaptic neuronal activity. Therefore, the a strocytes and neurons are regulated by different activity-dependent si gnals and, by the present measures, their responses to activity blocka de appear independent of one another.