Perivascular spaces balance vascular dimensions without impairing brain clearance mechanisms

Perivascular spaces (PVS) contribute to brain clearance, as they connect the cerebrospinal fluid (CSF) in the subarachnoid space and ventricular system with the interstitial fluid. We tested the hypothesis that the size and exchange function of PVS depend on the degree of blood vessel dilatation. To induce vasodilatation, we subjected mice to hypercapnia and compared this to normocapnic conditions. The study comprised three experimental approaches. In the first series, cranial window surgery and two-photon microscopy were used to assess CO₂-induced changes in vascular and PVS morphology. The second series investigated tracer influx following cisterna magna tracer injection and 30-min exposure to either normocapnia or hypercapnia, with subsequent confocal imaging of brain slices. The third approach evaluated tracer efflux and parenchymal diffusion after intracortical injection via a cranial window port and widefield fluorescence imaging. Arterial blood sampling confirmed hypercapnia. Hypercapnia led to vasodilatation in cerebral arteries, but not cerebral veins. The periarterial spaces balanced vasodilatation by a decrease in size. No effect was found on perivenous spaces. Despite changes in size of the PVS, no significant differences were found in CSF tracer influx from the subarachnoid space into the brain parenchyma, or subregions of the brain. Additionally, hypercapnia did not affect the efflux of fluorescent tracer from the cortex over the course of one hour. Hypercapnia-induced vasodilatation was accommodated by shrinkage of the periarterial spaces. Veins and perivenous spaces were not affected. We propose that PVS might act primarily as a buffer for volume changes induced by vasomotor activity. Changes in PVS dimensions, induced by relatively mild vasomotor changes had no effect on brain clearance related parameters under the current experimental conditions.