|
|
COLES Jonathan A. | |||
|
 Metabolic exchanges between neurons and glia The shape and position of astrocytes, with endfeet ensheathing blood capillaries and ramifications apposed to neurons, has encouraged speculation that they may take up glucose beyond their own energy requirements and supply metabolic substrate to neurons. For the mammalian brain itself, available evidence is indirect and conflicting. We investigate the question of metabolic interactions between neurons and glia mainly in two model preparations, the retina of the honeybee drone, and rat vagus nerve. The retina of the drone (male) honeybee
The drone has a big eye for detecting black dots against a blue sky that might be queen bees (Vallet et al,1992). The picture is a section through the retina parallel to the cornea. The rosettes are groups of 6 photoreceptor neurons and the other cells are glial cells. A long series of studies, mainly by Tsacopoulos and colleagues, have shown that glucose is taken up exclusively by the neurons (see Coles, 1989). The glial cells supply substrate, probably alanine, to the neurons. The glial cells are equipped with a NH4+,Cl- cotransporter (the first NH4+-selective transporter described on an animal cell) and it appears that in the neurons alanine is deaminated to pyruvate with release of ammonium that returns to the glial cells (see Marcaggi & Coles 2000, Marcaggi et al, 2004). Using homemade ion-selective microelectrodes of various types (selective for H+, NH4+, K+, etc) we are currently analysing the kinetics of this transfer with a view to refining our description of the sequence of metabolic events set in train by stimulation of the photoreceptors with a light flash (see Tsacopoulos et al, 1983). Rat vagus nerve
Compared to drone retina,rat vagus nerve is one step closer to mammalian brain. Most of the axons are unmyelinated, but they are still surrounded by Schwann glial cells. It would be interesting, better to understand the effects of diabetes on the nervous system, to know whether glucose is taken up by glial cells or neurons. In general, it is surprisingly difficult to show this. The glucose analog, 2-deoxyglucose (DG), is a useful tool; it enters cells on the glucose transporter, is phosphorylated to DG-6P, but is hardly metabolized further. If the DG is radioactive, one can make autoradiographs to localize it. In mammalian brain, the spatial resolution is not good enough to distinguish neurons from astrocytes; in mammalian retina, there are conflicting reports; in drone retina the result is clear (the DG-6P is all in the glia). In vagus nerve, we have used a trick to get the result. We applied *DG at one point and measured how far the resulting *DG-6P had diffused along the nerve at various times. Most of the diffusion was slowed by blockers of gap junctions. There are no gap junctions along axons, so this fraction of the *DG-6P must have been diffusing within the Schwann cells and from one Schwann cell to the next. Analysis of the diffusion profiles suggested that 78% of the DG was taken up by Schwann cells. Since it is mainly the axons that require energy, the Schwann cells must be supplying them with substrate (Véga et al, 2003). We are building apparatus to do further experiments using optical methods. Multiphoton microscopy In collaboration with the Laboratoire de Spectrométrie Physique of Grenoble University (UJF), we have set up a two-photon microscope configured for work on mouse cortex and isolated vagus nerve. NMR in vivo 1H spectroscopy for studying brain metabolism See the home pages of Peggy Provent and Anne Ziegler |