Responses of intracranial pressure (ICP) produced by stimulating the pressor area in the brainstem at various levels of blood pressure and ICP in cats.

An increase in intracranial pressure (ICP) was produced by stimulating brainstem pressor sites in cats anesthetized with alpha-chloralose. The ICP responses were augmented by lowering prestimulus BP and reduced by elevating prestimulus BP. In contrast, stimulus-induced pressor response of BP showed no consistent correlation to prestimulus BP. When the mean amplitude of stimulus-induced ICP responses at the control prestimulus ICP (within 18 mmHg) was plotted against the mean of the prestimulus BP levels for each site examined, the sites were classified into 2 groups by the regression line; sites generating a marked ICP response above the line and those generating a small ICP response on and under the line. The former sites were located in the paramedian region of the reticular formation including nuclei parvocellularis and gigantocellularis. The latter sites scattered throughout the brainstem pressor area. The ICP response at the former sites was markedly increased at an elevated prestimulus ICP. The peak ICP response at 30-50 mmHg of prestimulus ICP was 70-100 mmHg, similar to plateau waves. The ratio of ICP response size to BP response size was negatively correlated to prestimulus BP and the regression line was 2-5 times steeper at an elevated prestimulus ICP (18-60 mmHg) than at the control ICP. On the other hand, the negative relation between the response ratio and the BP for the latter sites produced no such change at the increased prestimulus ICP. These findings suggest that the ICP response is produced primarily by neurogenic intracranial vasodilation, which works most effectively at moderately decreased cerebral perfusion pressure. This mechanism may be involved in a series of events that results in plateau waves.