[Leão's spreading depression in the study of the relationship of central structures]

The use of Leão's spreading depression for studying the action of connections between central structures is examined. Extracellular recordings of cortical and striatal spreading depressions with single microelectrodes are presented using both a DC channel and spike recordings systems. Striatal spreading depression was produced by peristaltic perfusion of a KC1 solution via a push-pull cannula system. The characteristics of cortical and striatal spreading depressions were studied and their effects on antidromically provoked cellular spikes. Using double microelectrode recordings modifications of spontaneous activity provoked by cortical spreading depression were examined in n. ventralis posterior (VP) and centralis lateralis (CL) of the thalamus. In both nuclei a silence corresponding to the block of spontaneous activity of a localised cortical area was observed. The controls originated from different cortical regions for each nucleus. The discharge preceeding the onset of the spreading depression slow wave is reflected by a similar discharge in VP but not in CL. The differences in the frequency following of CL and VP was examined in order to explain this discrepancy. The results just summarized can be explained if we accept that a tonic facilitatory control exists between localised cortical areas and the two thalamic nuclei studied. Similar experiments were performed to study the controls exerted by cortical areas on dorsal column nuclei. Only phasic transitory effects were observed which were either an increase or a decrease in activity. These facilitatory or depressive effects were attributed to the initial excitation signalling the propagation of a cortical spreading depression. No tonic effects were observed. Striatal spreading depression was not accompanied by a cortical effect but on the contrary the activity of cells in substantia nigra was significantly altered. In pars reticulata two populations could be distinguished. The first presented a phasic increase followed by a long period of decreased activity, they are proposed to be under the control of striatum via an excitatory pathway. The second was initially depressed then presented a long period of increased activity. This probably reflects the block of an inhibitory striato-nigral pathway. Both striato nigral effects were tonic in nature. The possibility of separating the effects due to cortical excitation or cortical block are discussed as well as the problems of interpretation of long distance changes in cellular activity.