The molecular mechanism(s) of N-methyl-D-aspartate (NMDA) neuroprotective properties were investigated in primary cultures of cerebellar granule cell neurons. Granule cells express the neurotrophin receptor TrkB but not TrkA or TrkC. In these cells, the TrkB ligand brain-derived neurotrophic factor (BDNF) prevents glutamate toxicity. Therefore, we have tested the hypothesis that NMDA activates synthesis and release of BDNF, which may prevent glutamate toxicity by an autocrine loop. Exposure of granule cells for 2 and 5 min to a subtoxic concentration of NMDA (100 M) evoked an accumulation of BDNF in the medium without concomitant changes in the intracellular levels of BDNF protein or mRNA. The increase in BDNF in the medium is followed by enhanced TrkB tyrosine phosphorylation, suggesting that NMDA increases the release of BDNF and therefore the activity of TrkB receptors. To examine whether BDNF and TrkB signaling play a role in the NMDA-mediated neuroprotective properties, neurons were exposed to soluble trkB receptor-IgG fusion protein, which is known to inhibit the activity of extracellular BDNF, and to K252a, a tyrosine kinase inhibitor. Both compounds blocked the NMDA-mediated TrkB tyrosine phosphorylation and subsequently its neuroprotective properties. We suggest that NMDA activates the TrkB receptor via a BDNF autocrine loop, resulting in neuronal survival.Glutamate, the endogenous neurotransmitter required for normal physiological excitation, is also involved in the pathophysiology of hypoxic/ischemic neuronal injury (1-3). Although this neuropathological process can be mediated by any of the excitatory amino acid receptors, the N-methyl-D-aspartate (NMDA) 1 glutamate receptor subtype plays a major role. In addition, NMDA receptors mediate adaptive responses important for synaptic plasticity during development (4, 5). The molecular mechanism(s) by which NMDA receptors mediate such opposing effects are not clear.The role of NMDA receptors in glutamate-mediated excitotoxicity has been studied in rat cerebellar granule cells in vitro, in which overactivation of NMDA receptors results in neuronal cell death (6 -8). Paradoxically, in these cells subtoxic concentrations of NMDA have been shown to protect vulnerable neurons against an excitotoxic concentration of glutamate (9). Moreover, recent studies have shown that the neuroprotective effect of NMDA is blocked by coincubation with either the RNA synthesis inhibitor actinomycin D or the protein synthesis inhibitor cycloheximide (9). Thus, the neuroprotective effect of NMDA may involve a polypeptide(s) with neuroprotective properties.Cerebellar granule cells are responsive to various neurotrophic factors and in particular to the neurotrophins (10, 11), a family of trophic factors related by primary amino acid sequence homology, whose members include brain-derived neurotrophic factor (BDNF), nerve growth factor (NGF), neurotrophin-3 (NT-3), and NT-4/5 (12-17). The biological activity of neurotrophins depends upon the activation of high-affinity receptors (Trk...

Neurotrophins are critical to the development and maintenance of the mammalian central nervous system. Among them is brain-derived neurotrophic factor (BDNF), whose synthesis and release is targeted by activation of glutamate receptors. Perturbation of this process probably underlies neurodegenerative and psychiatric disorders. A naturally occurring variation in humans, in the form of a common single-nucleotide polymorphism in the pro region of the polypeptide at codon 66 (Val66-->Met), affects processing of the pro-BDNF polypeptide and its activation-dependent release. This variant is associated with differences in the volume of the hippocampal formation and with anxiety and depression-related phenotypes. Convergent findings supporting a role for BDNF in alterations to hippocampal structure and behavior are found in a "humanized" BDNF transgenic mouse. Also, recent human genetic studies have supported a role of BDNF signaling in addictive behaviors by allele-, genotype-, and haplotype-based association of the TrkB gene, which encodes the cognate receptor for BDNF, with alcohol dependence. A better understanding of the influence of BDNF-mediated pathways in cell survival and plasticity will aid in developing new approaches to restoring normal function in disease states.

The brain-derived neurotrophic factor (BDNF) gene is critical for neuronal function and survival, and is likely to be important in psychiatric disorders. In this study, we used single-nucleotide polymorphism (SNP) discovery, functional analyses, and genetic association studies to better understand the potential role of BDNF sequence variation in behavior. Screening 480 unrelated individuals for SNPs and genotyping was performed in US Caucasian, American Indian, and African American populations. Lifetime DSM-III-R psychiatric diagnoses were assigned and the Tridimensional Personality Questionnaire (TPQ) was administered to measure anxious temperament (harm avoidance (HA)) and novelty seeking (NS). A novel SNP (À281 C4A) in promoter 1 was discovered that had decreased DNA binding in vitro and decreased basal reporter gene activity in transfected rat hippocampal neurons. The frequency of the À281 A allele was 0.03 in a Caucasian sample, but was virtually absent in other populations. Association analyses in a community-based sample showed that individuals with the À281 A allele (13 heterozygotes) had lower TPQ HA (F ¼ 4.8, po0.05). In contrast, the Met 66 allele was associated with increased HA (F ¼ 4.1, p ¼ 0.02) and was most abundant in individuals with both anxiety disorders and major depression (po0.05). Among the Val66Val homozygotes, individuals who were -281 CA heterozygotes had significantly lower HA than the -281 CC homozygotes (po0.01). Our results suggest that in this population, the low activity -281 A allele may be protective against anxiety and psychiatric morbidity, whereas Met 66 may be a risk allele.