Deleterious effects of minocycline after in vivo target deprivation of thalamocortical neurons in the immature, metallothionein-deficient mouse brain.

Compared with adults, immature metallothionein I and II knockout (MT(-/-)) mice incur greater neuronal loss and a more rapid rate of microglia accumulation after target deprivation-induced injury. Because minocycline has been proposed to inhibit microglial activation and associated production of neuroinflammatory factors, we investigated its ability to promote neuronal survival in the immature, metallothionein-deficient brain. After ablation of the visual cortex, 10-day-old MT(-/-) mice were treated with minocycline or saline and killed 24 or 48 hr after injury. By means of stereological methods, the number of microglia and neurons were estimated in the ipsilateral dorsal lateral geniculate nucleus (dLGN) by an investigator blinded to the treatment. No effect on neuronal survival was observed at 24 hr, but 48 hr after injury, an unanticipated but significant minocycline-mediated increase in neuronal loss was detected. Further, while failing to inhibit microglial accumulation, minocycline treatment increased the proportion of amoeboid microglia in the ipsilateral dLGN. To understand the molecular mechanisms underlying this neurotoxic response, we identified minocycline-mediated changes in the expression of three potentially proapoptotic/inflammatory genes: growth arrest- and DNA damage-inducible gene 45gamma (GADD45gamma); interferon-inducible protein 1 (IFI1), and cytokine-induced growth factor. We also observed increased mitogen-activated protein kinase p38 phosphorylation with minocycline treatment. Although minocycline inhibited calpain activity at 12 hr after injury, this effect was not sustained at 24 hr. Together, these results help to explain how minocycline has a deleterious effect on neuronal survival in this injury model.

Basic science; metallothionein I and II attenuate the thalamic microglial response following traumatic axotomy in the immature brain.

The clinical manifestations of inflicted traumatic brain injury in infancy most commonly result from intracranial hemorrhage, axonal stretch and disruption, and cerebral edema. Often hypoxia ischemia is superimposed, leading to early forebrain and later thalamic neurodegeneration. Such acute and delayed cellular injury activates microglia in the CNS. Although activated microglia provide important benefits in response to injury, microglial release of reactive oxygen species can be harmful to axotomized neurons. We have previously shown that the antioxidants metallothionein I and II (MT I & II) promote geniculocortical neuronal survival after visual cortex lesioning. The purpose of this investigation was to determine the influence of MT I & II on the density and rate of thalamic microglial activation and accumulation following in vivo axotomy. We ablated the visual cortex of 10-day-old and adult MT I & II knock out (MT(-/-)) and wild-type mice and then determined the density of microglia in the dorsal lateral geniculate nucleus (dLGN) over time. Compared to the wild-type strain, microglial activation occurred earlier in both young and adult MT(-/-) mice. Similarly, microglial density was significantly greater in young MT(-/-) mice 30, 36, and 48 hours after injury, and 3, 4, and 5 days after injury in MT(-/-) adults. In both younger and older mice, time and MT I & II deficiency each contributed significantly to greater microglial density. Only in younger mice did MT I & II expression significantly slow the rate (density x time) of microglial accumulation. These results suggest that augmentation of MT I & II expression may provide therapeutic benefits to infants with inflicted brain injury.