NA+ CHANNELS AND EXCITABILITY IN HYPOXIA

In the past several years, one main focus of our laboratory has been to examine the response of central neurons in a number of brain regions to acute and chronic O2 deprivation. We are particularly interested in the mechanisms that can led to neuronal injury and those that, when activated, can prevent or delay injury. Recently, we have made interesting observations regarding membrane ionic events that link metabolism to excitability. One of these observations pertains to the voltage-sensitive Na+ channels in central neurons. An early event during anoxia in mature (adult) neocortical neurons seems to be a profound inhibition of the steady-state availability of these channels with a major reduction in Na+ current (iNa) and a decrease in neuronal excitability. This observation is important because this may be an adaptive strategy in the adult in that the decrease in excitability will lessen O2 consumption and minimize the mismatch between demand and supply. We are not sure how the immature neocortical cells will respond in terms of INa. We therefore focus in this proposal on the study of the Na+ current in neocortical neurons during graded hypoxia and examine 4 separate hypotheses: 1+ Neocortical neurons decrease their excitability during graded hypoxia by inhibiting INa in a graded manner in the mature but not in the immature; 2) the alterations in INa kinetics during O2 deprivation in neocortical neurons are due to changes in specific cytosolic factors and these are more pronounced in the mature than in the immature; 3) O2 deprivation alters INa via mechanisms that are either membrane-delimited or dependent on phosphorylation and 4) long term hypoxia modulates the expression of Na+ channels. These experiments will involve the use of the in-vitro slice and microelectrode technique as well as patch clamp with whole-cell and single channel recordings in freshly dissociated neurons. Optical measurements of Ca++ i and H+ i using confocal microscopy will also be performed. All techniques are available and routinely performed in the PI's laboratory. Our long term view and efforts are focussed on understanding the events that occur during anoxia in central neurons so that we can manipulate cell behavior and possibly render mammalian neurons more tolerant to lack of O2. This will have major implications on a vast number of diseases or conditions that span the age spectrum from the fetus to old age.