Capillary flow in the brain cortex during changes in oxygen supply and state of activation

Chronic central nervous system expression of HIV-1 Tat leads to accelerated rarefaction of neocortical capillaries and loss of red blood cell velocity heterogeneity

OBJECTIVES

HIV-1 infection of the CNS is associated with impairment of CBF and neurocognitive function, and accelerated signs of aging. As normal aging is associated with rarefaction of the cerebral vasculature, we set out to examine chronic viral effects on the cerebral vasculature.

METHODS

DOX-inducible HIV-1 Tat-tg and WT control mice were used. Animals were treated with DOX for three weeks or five to seven months. Cerebral vessel density and capillary segment length were determined from quantitative image analyses of sectioned cortical tissue. In addition, movement of red blood cells in individual capillaries was imaged in vivo using multiphoton microscopy, to determine RBCV and flux.

RESULTS

Mean RBCV was not different between Tat-tg mice and age-matched WT controls. However, cortical capillaries from Tat-tg mice showed a significant loss of RBCV heterogeneity and increased RBCF that was attributed to a marked decrease in total cortical capillary length (35-40%) compared to WT mice.

CONCLUSIONS

Cerebrovascular rarefaction is accelerated in HIV-1 Tat-transgenic mice, and this is associated with alterations in red cell blood velocity. These changes may have relevance to the pathogenesis of HIV-associated neurocognitive disorders in an aging HIV-positive population.

Nitric oxide modulates the CBF response to increased extracellular potassium

The response of the regional cerebral blood flow (rCBF) to brain topical superfusion of 20 mM K+ was characterized in a closed cranial window preparation in barbiturate anesthetized and ventilated rats: Increasing K+ in the artificial cerebrospinal fluid (ACSF) induced a rCBF elevation (measured by laser-Doppler flowmetry) of +85 +/- 37% above baseline (n = 19). This elevation was stable for > 3 h with continuous superfusion of increased K+ (n = 5) and partially reversible to a level of +18 +/- 19% above baseline when returning to a physiological K+ concentration. Nitric oxide synthase (NOS) inhibition by brain topical superfusion with N omega-nitro-L-arginine (L-NA) revealed (a) Addition of L-NA to high-potassium ACSF reduced the rCBF increase from +94 +/- 36% to +21 +/- 18% (p < or = 0.01, n = 7). (b) When L-NA was superfused for 60 min before increasing K+, rCBF decreased to -17 +/- 7% below baseline. Subsequent coapplication of L-NA and increased K+ induced only an elevation of +7 +/- 4% above baseline (n = 4). (c) When the NO donor S-nitroso-N-acetylpenicillamine (SNAP) was added during NOS inhibition to restore basal tissue NO levels, the resultant level of rCBF was +28 +/- 54% above baseline. Subsequent increase of K+ in the presence of NOS inhibition and SNAP elevated rCBF to +137 +/- 89% above baseline (n = 4).(ABSTRACT TRUNCATED AT 250 WORDS)

Cerebral autoregulation dynamics in humans

We studied the response of cerebral blood flow to acute step decreases in arterial blood pressure noninvasively and nonpharmacologically in 10 normal volunteers during normocapnia, hypocapnia, and hypercapnia. The step (approximately 20 mm Hg) was induced by rapidly deflating thigh blood pressure cuffs following a 2-minute inflation above systolic blood pressure. Instantaneous arterial blood pressure was measured by a new servo-cuff method, and cerebral blood flow changes were assessed by transcranial Doppler recording of middle cerebral artery blood flow velocity. In hypocapnia, full restoration of blood flow to the pretest level was seen as early as 4.1 seconds after the step decrease in blood pressure, while the response was slower in normocapnia and hypercapnia. The time course of cerebrovascular resistance was calculated from blood pressure and blood flow recordings, and rate of regulation was determined as the normalized change in cerebrovascular resistance per second during 2.5 seconds just after the step decrease in blood pressure. The reference for normalization was the calculated change in cerebrovascular resistance that would have nullified the effects of the step decrease in arterial blood pressure on cerebral blood flow. The rate of regulation was 0.38, 0.20, and 0.11/sec in hypocapnia, normocapnia, and hypercapnia, respectively. There was a highly significant inverse relation between rate of regulation and PaCO2 (p less than 0.001), indicating that the response rate of cerebral autoregulation in awake normal humans is profoundly dependent on vascular tone.

Cortical vasodilatation produced by vasoactive intestinal polypeptide (VIP) and by physiological stimuli in the cat

In chloralose-urethanized cats, vasoactive intestinal peptide (VIP), applied by superfusion in steady-state concentration (10(-10)-10(-6) M) onto cortical vessels in situ resulted in a rapid concentration-dependent vasodilatation in vessels that were mildly constricted by prostaglandin F2 alpha (PGF2 alpha) (5 X 10(-5) M) or hypocarbia (PaCO2 = 26). The maximum dilatation produced by VIP (10(-6) M) was about 60% over baseline in pial arteries and 40% in pial veins. Blockade of local neuronal activity with tetrodotoxin (TTX) (10(-5) M) had no effect on the VIP-evoked dilation of pial vessels. Activation of the cortex by either direct electrical stimulation or indirectly by stimulation of the mesencephalic reticular formation (MRF) resulted in a rapid dilatation of pial arterioles and venules. The vasodilatory effects of VIP and of cortical activation via direct cortical stimulation were not blocked by phentolamine (10(-4) M), propranolol (10(-4) M), atropine (10(-4) M), or naloxone (10(-4) M), indicating that the stimulated vasodilatation was not mediated by adrenergic, cholinergic, or opiate receptors. The dilatory effects of MRF, but not direct cortical stimulation, were not blocked by TTX. VIP antiserum (1:25) preincubated in cortical cups had no effect on resting vessel diameter, but resulted in a significant, though subtotal, reduction in the vasodilatation elicited by direct cortical and MRF stimulation. Normal rabbit sera or VIP antiserum preincubated with saturating amounts of VIP were ineffective. In similar experiments, pial arteriolar and venular dilation evoked by hypercarbia was not attenuated by cortically applied VIP antisera. These observations suggest that pial dilation evoked by local increases in neuronal activity may be mediated in part by the local release of VIP from intrinsic neurons. Such a substrate would define a close obligatory coupling between local neuronal activation and local perfusion, such that nutritive flow could be enhanced prior to the onset of any metabolic deficit.

Focal physiological uncoupling of cerebral blood flow and oxidative metabolism during somatosensory stimulation in human subjects

Coupling between cerebral blood flow (CBF) and cerebral metabolic rate of oxygen (CMRO2) was studied using multiple sequential administrations of 15O-labeled radiotracers (half-life, 123 sec) and positron emission tomography. In the resting state an excellent correlation (mean r, 0.87) between CBF and CMRO2 was found when paired measurements of CBF and CMRO2 from multiple (30-48) brain regions were tested in each of 33 normal subjects. Regional uncoupling of CBF and CMRO2 was found, however, during neuronal activation induced by somatosensory stimulation. Stimulus-induced focal augmentation of cerebral blood flow (29% mean) far exceeded the concomitant local increase in tissue metabolic rate (mean, 5%), when resting-state and stimulated-state measurements were obtained in each of 9 subjects. Stimulus duration had no significant effect on response magnitude or on the degree of CBF-CMRO2 uncoupling observed. Dynamic, physiological regulation of CBF by a mechanism (neuronal or biochemical) dependent on neuronal firing per se, but independent of the cerebral metabolic rate of oxygen, is hypothesized.