Neural activity increases local blood flow in the central nervous system (CNS), which is the basis of BOLD (blood oxygen level dependent) and PET (positron emission tomography) functional imaging techniques^{1, 2, 3}. Blood flow is assumed to be regulated by precapillary arterioles, because capillaries lack smooth muscle. However, most (65%) noradrenergic innervation of CNS blood vessels terminates near capillaries rather than arterioles⁴, and in muscle and brain a dilatory signal propagates from vessels near metabolically active cells to precapillary arterioles^{5, 6}, suggesting that blood flow control is initiated in capillaries. Pericytes, which are apposed to CNS capillaries and contain contractile proteins⁷, could initiate such signalling. Here we show that pericytes can control capillary diameter in whole retina and cerebellar slices. Electrical stimulation of retinal pericytes evoked a localized capillary constriction, which propagated at 2 m s^-1 to constrict distant pericytes. Superfused ATP in retina or noradrenaline in cerebellum resulted in constriction of capillaries by pericytes, and glutamate reversed the constriction produced by noradrenaline. Electrical stimulation or puffing GABA (-amino butyric acid) receptor blockers in the inner retina also evoked pericyte constriction. In simulated ischaemia, some pericytes constricted capillaries. Pericytes are probably modulators of blood flow in response to changes in neural activity, which may contribute to functional imaging signals and to CNS vascular disease.

1. Department of Physiology, University College London, Gower Street, London WC1E 6BT, UK
2. *These authors contributed equally to this work

Correspondence to: David Attwell¹ Correspondence and requests for materials should be addressed to D.A. (Email: D.Attwell@ucl.ac.uk).

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