CHANGES IN NEUROTROPHIC FACTOR EXPRESSION AND RECEPTOR ACTIVATION FOLLOWING EXPOSURE OF HIPPOCAMPAL NEURON ASTROCYTE COCULTURES TO KAINIC ACID

Neurotrophic factor expression in the adult mammalian CNS is largely n euronal. However, upon traumatic injury reactive astrocytes express a number of neurotrophic factors including ciliary neurotrophic factor ( CNTF), fibroblast growth factor (FGF), and NGF. In this study, we exam ined whether the upregulation of neurotrophic factors in reactive astr ocytes and cultured astrocytes is a consequence of separation from the ir neuronal counterparts, and whether neurotrophic factor levels can b e regulated by placing astrocytes into coculture with neurons. We show that reintroduction of rat hippocampal neurons to rat hippocampal ast rocytes in vitro leads to a time dependent downregulation in astrocyte s of the neurotrophic factors CNTF, NGF, and neurotrophin 3 (NT-3). In contrast, brain-derived neurotrophic factor (BDNF) mRNA, which is onl y expressed in neurons in these cultures is slightly increased. Once n eurotrophic factor levels in cocultures had reached a steady state in the neuron/glia cocultures, we initiated a traumatic event with the ex citotoxin kainic acid. BDNF protein was rapidly upregulated within 24 hr after lesion, whereas CNTF protein upregulation was delayed reachin g maximal levels by 3 d. Despite the endogenous upregulation of both o f these trophic factors, no activation of their respective receptors, as measured by tyrosine phosphorylation, was detectable following kain ate administration. However, following addition of exogenous CNTF at a ny time point up to 24 hr after kainate administration, the beta compo nents of the CNTF receptor (LIFR beta and gp130) could be phosphorylat ed. Furthermore, although activation of neuronal LIFR beta and gp130 b y exogenous CNTF declined during the period of neuronal death, these r eceptors reappeared on astrocytes and could be activated by CNTF. in c ontrast, phosphorylation of TrkB by exogenous BDNF was undetectable by 24 hr and could not be reactivated after this point. These data sugge st that the intimate association of astrocytes and neurons in the CNS serves to suppress astrocyte-derived neurotrophic factor expression an d that neuronal loss leads to a derepression of neurotrophic factor sy nthesis in astrocytes. However, the upregulation of endogenous BDNF an d CNTF observed after excitotoxic lesion in this culture model are ins ufficient to activate signal transduction and protect against neuronal loss.