Excitatory amino acid (EAA) neurotransmitters have been shown to play important roles as molecular signals influencing the structure of neurons during development. However, the signal transduction and effector mechanisms responsible for these effects have yet to be fully elucidated. We have previously provided evidence that EAA agonists induce the synthesis and release of proteoglycans (PGs) with neurite-promoting activity from fetal hippocampal neurons (Sugiura M., Dow K.E. Dev. Biol. 164:102, 1994). In the present studies, we examined expression of the neuron-specific growth associated gene Tα1 α-tubulin (Tα1) and genes that encode for proteins targetted to axons (growth-associated protein-43 (GAP-43)) and dendrites (microtubule-associated protein-2 (MAP-2)) following exposure to EAAs.

Hippocampal neuronal cultures were established from the dissociated hippocampus of 19 day fetal rats. Total RNA was isolated from cultured neurons. Competitive reverse transcription-polymerase chain reaction (RT-PCR) was performed to quantitate mRNA levels. Exposure of neurons to glutamate (100μM) for 5 minutes resulted in increases in Tα1, GAP-43 and MAP-2 mRNA levels which peaked at between 8 and 12 hours following exposure to glutamate(p < 0.001 vs control). Heparin, which inhibits the neurite growth-promoting effects of PGs in vitro (Riopelle R.J., Dow K.E. Brain Res. 525:92, 1990), also inhibited the glutamate-induced induction of Tα1, GAP-43 and MAP-2 mRNA expression when added to culture medium at 25μg/ml following glutamate exposure.

Emerging data have suggested a role for PGs in activity-dependent neuronal growth. The present studies provide the first evidence for an autocrine influence of PGs as effectors of EAA stimulated growth responses.