The inulin space of the lamprey spinal cord

The distribution of [14C]inulin was measured in isolated spinal cords of larval and feeding stage adult forms of sea lamprey (Petromyzon marinus) and expressed in per cent of total cord wet weight. In larval cord the apparent inulin space reached a plateau value of 32--33% within 2.5. min. This correlates well with electrophysiological experiments in which 10(-7) M tetrodotoxin added to the perfusion fluid blocked the responses of giant interneurons to both intracellular and rostral cord stimulation in 1 to 2 min. Thus the plateau level of inulin space probably represents the extracellular space. [14C]Mannitol did not reach a steady distribution space even after 30 min of incubation. Therefore, mannitol is not an accurate extracellular space indicator in the isolated lamprey spinal cord. The inulin space increased with increasing temperature of incubation. Average inulin spaces for larval spinal cords incubated at 5, 10 and 22 degrees C were approximately 26%, 33% and 42% respectively. The inulin space of isolated adult lamprey spinal cords was about 18--19%. Since in larvae the inulin space did not vary consistently with the sizes (and therefore presumably the ages) of the animals, it is likely that the reduction in inulin space during maturation does not occur gradually during the larval phase, but probably occurs during transformation. The difference between the inulin spaces of isolated larval and adult spinal cords is reflected qualitatively in the electron microscopic appearance of the extracellular space. We conclude that the inulin space in the lamprey spinal cord behaves similarly to the picture of the mammalian brain extracellular space which has emerged in recent years. Because of the rapidity of inulin diffusion in the lamprey cord and the unambiguous time-dependent behavior of the inulin space of the isolated lamprey cord, the latter would seem to be useful model for the extracellular space of the vertebrate central nervous system.