Effects of inhalational anesthetics on biochemical events in growing neuronal tips

The influence of general anesthetics on developing organs has been a source of concern for many years. The central nervous system, which is developing rapidly at the time of birth, is of special interest in this regard. In this study, the biochemical characteristics of developing neural tips (growth cones) were examined after exposure to anesthetics to elucidate the molecular mechanism by which long-lasting alterations in the nervous system, including neuroteratogenicity, as previously described, were evoked.

Neonatal rats were exposed to an atmosphere containing inhalational anesthetics (1% halothane or 75% nitrous oxide) or a control atmosphere (25% O2 and 75% N2) for 6 h at postnatal day 1. After this exposure, growth cone particles were isolated from the forebrain using a recently devised cell fractionation method at postnatal days 2, 3, 4, and 5. Protein composition, phosphoprotein patterns, and protein kinase C (PKC) activities of the isolated growth cones were compared between each group exposed to anesthetics and the control group. The dose-response relationship of the action of anesthetics on PKC activity was also examined (at 0.5 and 0.75% halothane and 25 and 50% N2O).

The increase in body weight and brain wet weight were not significantly affected by exposure to either anesthetic. No apparent influence on protein composition was observed by sodiumdodecylsulfate polyacrylamide gel electrophoresis (SDS-PAGE). However, calcium-dependent protein phosphorylation of the 46 kDa protein and of the 80 kDa protein, which is reported to be mediated by PKC, were significantly reduced after exposure to the anesthetics. A direct assay of PKC activity in growth cone particles indicated that PKC activity in the growth cone was 70.6 +/- 9.6% of the control value at 24 h after exposure to 1% halothane, and 63.2 +/- 4.9% after exposure to 75% nitrous oxide. Exposure to 0.75% halothane or 50% nitrous oxide had a similar, but lesser, effect on this parameter. In contrast, exposure to 0.5% halothane or 25% nitrous oxide evoked no apparent effect. Thus the PKC activity in growth cone particles, which is thought to play an important role in signal transduction in the developing brain, was shown to be affected by exposure to inhalational anesthetics over a range of concentrations.

Considering the crucial role of growth cones in the establishment of the neuronal network, the interruption of signal transduction in the growth cone at a time that is critical in directing the neurite extension may evoke a long-lasting alteration in the neural network. Therefore, the effect of inhalational anesthetics on the growth cone enzyme observed in this study may have a major role in the mechanism that induces morphologic or behavioral neuroabnormalities in later life.