BDNF down-regulates the caspase 3 pathway in injured geniculo-cortical neurones.

Visual cortex ablation in newborn rats causes a rapid and almost complete degeneration of neurones in the dorsal lateral geniculate nucleus (dLGN), as a consequence of the axotomy of geniculo-cortical fibres. Death of dLGN neurones occurs by apoptosis and is partially prevented (approximately 50%) by intraocular delivery of brain-derived neurotrophic factor (BDNF). Here we investigated the molecular mechanisms of BDNF-mediated neuroprotection. We found that exogenous administration of BDNF partially decreases (approximately 50%) the up-regulation of apoptotic proteins (phosphorylated c-Jun, cytochrome C and cleaved caspase 3), that occurs in dLGN neurones following visual cortex ablation at postnatal day 7. These results demonstrate that the neuroprotective action of BDNF on axotomised dLGN neurones involves the partial blockade of well-characterised apoptotic pathways.

Acute physiological response of mammalian central neurons to axotomy: ionic regulation and electrical activity.

The transection of the axon of central neurons has dramatic consequences on the damaged cells and nerves. Injury activates molecular programs leading to a complex repertoire of responses that, depending on the cellular context, include activation of sprouting, axonal degeneration, and cell death. Although the cellular mechanisms started at the time of lesion are likely to shape the changes affecting injured cells, the acute physiological reaction to trauma of mammalian central neurons is not completely understood yet. To characterize the physiology of the acute response to axonal transection, we have developed a model of in vitro axotomy of neurons cultured from the rodent cortex. Imaging showed that axotomy caused an increase of calcium in the soma and axon. Propagation of the response to the soma required the activation of voltage-dependent sodium channels, since it was blocked by tetrodotoxin. The electrophysiological response to axotomy was recorded in patched neurons kept in the current clamp configuration: injury was followed by vigorous spiking activity that caused a sodium load and the activation of transient calcium currents that were opened by each action potential. The decrease of the electrochemical gradient of sodium caused inversion of the Na-Ca exchanger that provided an additional mean of entry for calcium. Finally, we determined that inhibition of the physiological response to axotomy hindered the regeneration of a new neurite. These data provide elements of the framework required to link the axotomy itself to the downstream molecular machinery that contributes to the determination of the long-term fate of injured neurons and axons.

Intraocular delivery of BDNF following visual cortex lesion upregulates cytosolic branched chain aminotransferase (BCATc) in the rat dorsal lateral geniculate nucleus.

Visual cortex ablation in newborn rats determines the almost complete degeneration of neurons in the dorsal lateral geniculate nucleus (dLGN), as a consequence of the axotomy of the geniculo-cortical fibres. Death of dLGN neurons is massive and rapid, and occurs by apoptosis. We recently showed that exogenous administration of the neurotrophin brain-derived neurotrophic factor (BDNF) in the eye prevents the degeneration of dLGN neurons occurring after visual cortex lesion in newborn rats. To elucidate the molecular mechanisms of BDNF-mediated neuroprotection, we sought to identify novel genes regulated by BDNF in the rat dLGN after visual cortex lesion. By using mRNA fingerprinting, we isolated a cDNA fragment upregulated in the dLGN of lesioned rats treated with BDNF. This cDNA fragment shared 100% homology with the rat cytosolic branched chain aminotransferase (BCATc), a key enzyme of glutamate metabolism. Quantitative reverse transcription-polymerase chain reaction and in situ hybridization confirmed that BCATc mRNA is markedly overexpressed by exogenous supply of BDNF to axotomized dLGNs. Immunohistochemical analysis showed that upregulation of BCATc in the dLGN of lesioned rats treated with BDNF takes place in astrocytes. These results suggest that modulation of glutamate metabolism by astrocytes might play an important role in BDNF-mediated survival of axotomized dLGN neurons.