Local Cerebral Blood Flow Responses Research Articles

Cortical Hemodynamic Response to Multi-afferent Stimulation: an optical imaging study.

Multisensory stimulation plays an important role in the recovery of ischemic stroke. However, little is known about the interactions between neuronal activities with multi-afferent stimulations and their effects on hemodynamic responses. Optogenetics has been a useful tool in neuroscience research to unravel the mechanisms of neurovascular coupling at cell-specific level. In this study, we applied laser speckle contrast imaging (LSCI) to map the cortical hemodynamic response with high spatiotemporal resolution. The results showed that optogenetic inhibition of pyramidal neurons in sensorimotor cortex induced both local and distant increases of cerebral blood flow (CBF) with dual peaks, and the full width at half maximum (FWHM) was significantly larger than that of the CBF response to optogenetic excitation. Furthermore, optogenetic excitation of pyramidal neurons could significantly increase the local CBF response to sensory stimulation, whereas optogenetic inhibition of pyramidal neurons decreased the local CBF response at the early stage after sensory stimulation and increased the distant CBF response during the recovery period. Our work provided useful insights into the mechanisms of brain stimulation, which might help in clinical neurological applications.

Cerebrovascular Resistance: The Basis of Cerebrovascular Reactivity

The cerebral vascular network regulates blood flow distribution by adjusting vessel diameters, and consequently resistance to flow, in response to metabolic demands (neurovascular coupling) and changes in perfusion pressure (autoregulation). Deliberate changes in carbon dioxide (CO2) partial pressure may be used to challenge this regulation and assess its performance since CO2 also acts to change vessel diameter. Cerebrovascular reactivity (CVR), the ratio of cerebral blood flow (CBF) response to CO2 stimulus is currently used as a performance metric. However, the ability of CVR to reflect the responsiveness of a particular vascular region is confounded by that region’s inclusion in the cerebral vascular network, where all regions respond to the global CO2 stimulus. Consequently, local CBF responses reflect not only changes in the local vascular resistance but also the effect of changes in local perfusion pressure resulting from redistribution of flow within the network. As a result, the CBF responses to CO2 take on various non-linear patterns that are not well-described by straight lines. We propose a method using a simple model to convert these CBF response patterns to the pattern of resistance responses that underlie them. The model, which has been used previously to explain the steal phenomenon, consists of two vascular branches in parallel fed by a major artery with a fixed resistance unchanging with CO2. One branch has a reference resistance with a sigmoidal response to CO2, representative of a voxel with a robust response. The other branch has a CBF equal to the measured CBF response to CO2 of any voxel under examination. Using the model to calculate resistance response patterns of the examined branch showed sigmoidal patterns of resistance response, regardless of the measured CBF response patterns. The sigmoid parameters of the resistance response pattern of examined voxels may be mapped to their anatomical location. We show an example for a healthy subject and for a patient with steno-occlusive disease to illustrate. We suggest that these maps provide physiological insight into the regulation of CBF distribution.

Effect of cyclooxygenase-2 on the regulation of cerebral blood flow during neuronal activation in the rat

The present study was designed to clarify the precision of the main approach for investigating the regulation of local cerebral blood flow (CBF) response to neuronal activation in the brain (neurovascular coupling). In this study, we examined the effects of NS-398, a highly selective cyclooxygenase-2 inhibitor, on the physiological variables, baseline CBF, and local CBF response during rat somatosensory neuronal activation by laser-Doppler flowmetry. Blood pressure and heart rate were significantly decreased 3 h after i.v. infusion of NS-398. Baseline CBF and local CBF during somatosensory activation gradually decreased with an increase in time of NS-398 infusion up to 3 h, although neuronal activity in the somatosensory area was almost constant during the infusion. The results suggest that cyclooxygenase-2 participates in the regulation of local CBF during neuronal activation in rats. The present study also revealed the potential side-effects of dimethylsulfoxide, a solvent of NS-398, on neurovascular coupling.

Nonlinear correlation between field potential and local cerebral blood flow in rat somatosensory cortex evoked by changing the stimulus current

The relationship between local cerebral blood flow (LCBF) and field potential (FP) evoked by hindpaw stimulation in rat somatosensory cortex has been investigated while changing stimulus current. The change in LCBF was measured using laser-Doppler flowmetry and the field potential was acquired using a tungsten electrode inserted into the cortex of alpha-chloralose-anesthetized rats. The cortex was activated by electrical stimulation of the hind paw with 5 Hz pulses (0.1 ms) applied at currents of 1.0, 1.5, 2.0 and 2.5 mA for 5 s. It was found that the summed FP is nonlinear with respect to stimulus current, whereas the integrated LCBF response is linear across the range of currents used in the experiment. This means that the relationship between the summed FP and integrated LCBF is nonlinear as a function of stimulus current.

Blood flow activation in rat somatosensory cortex under sciatic nerve stimulation revealed by laser speckle imaging*

Abstract In many functional neuroimaging research the change of local cerebral blood flow (CBF) induced by sensory stimulation is regarded as an indicator of the change in cortical neuronal activity although a precise and full spatio-temporal description of local CBF response coupled to neural activity has still not been laid out. Using the laser speckle imaging technique a relatively large exposed area in somatosensory cortex of rat was imaged for the observation of the variations of CBF during sciatic nerve stimulation. The results showed that cerebral blood flow activation was spatially localized and discretely distributed in the targeted microvasculature. Individual arteries, veins and capillaries in different diameters were activated with the time going. The response pattern of CBF related to the function of brainactivity and energy metabolism is delineated exactly.

Frequency dependence of local cerebral blood flow induced by somatosensory hind paw stimulation in rat under normo- and hypercapnia.

We measured the field potential and the changes in local cerebral blood flow (LCBF) response during somatosensory activation (evoked LCBF) in alpha-chloralose--anesthetized rats by laser-Doppler flowmetry under normocapnia (PaCO(2)=34.3+/-3.8 mmHg) and hypercapnia (PaCO(2)=70.1+/-9.8 mmHg). Somatosensory activation was induced by electrical stimulation (0.2, 1, and 5 Hz with 1.5 mA for 5 s) of the hind paw. The neuronal activity of the somatosensory area of the hind paw was linear to the stimulus frequency, and there was no significant difference in the neuronal activity between hypercapnia and normocapnia. The baseline level of LCBF under hypercapnia was about 72.2% higher than that under normocapnia (p<0.01). The absolute response magnitude under hypercapnia was greater than that under normocapnia (p<0.05). The evoked LCBF under both conditions showed a frequency-dependent increase in the 0.2 to 5 Hz range, and the difference in the absolute response magnitude at the same stimulus frequency between normocapnia and hypercapnia became large with increasing stimulus frequency (p<0.05). On the other hand, after normalization to each baseline level there was no significant difference in the response magnitude of the normalized evoked LCBF between normocapnia and hypercapnia, indicating that the normalized evoked LCBF reflects neuronal activity even when the baseline LCBF was changed by the PaCO(2) level. The peak time and termination time of LCBF response curves with respect to the graded neuronal activity at 1 and 5 Hz stimulation increased significantly under hypercapnia, compared with those under normocapnia (p<0.05), although the rise time of 0.5 s was nearly constant. In conclusion, the results suggest a synergistic effect of the combined application of graded neuronal stimuli and hypercapnia on the LCBF response.

P-450 epoxygenase and NO synthase inhibitors reduce cerebral blood flow response toN-methyl-d-aspartate

Epoxyeicosatrienoic acids are cerebral vasodilators produced in astrocytes by cytochrome P-450 epoxygenase activity. The P-450 inhibitor miconazole attenuates the increase in cerebral blood flow (CBF) elicited by glutamate. We evaluated whether epoxygenase activity is involved in the CBF response to activation of the N-methyl-D-aspartate (NMDA) receptor subtype by using two structurally distinct inhibitors, miconazole and N-methylsulfonyl-6-(2-propargyloxyphenyl) hexanamide (MS-PPOH), a selective epoxygenase substrate inhibitor. Drugs were delivered locally through microdialysis probes in striata of anesthetized rats. Local CBF was measured by hydrogen clearance and compared with CBF in contralateral striatum receiving vehicle. Microdialysis perfusion of NMDA doubled CBF and increased nitric oxide (NO) production estimated by recovery of labeled citrulline in the dialysate during labeled arginine infusion. Perfusion of miconazole or MS-PPOH blocked the increase in CBF without decreasing citrulline recovery. Perfusion of N(omega)-nitro-L-arginine decreased baseline CBF and inhibited the CBF response to NMDA. Perfusion of MS-PPOH did not inhibit the CBF response to sodium nitroprusside. We conclude that both the P-450 epoxygenase and NO synthase pathways are involved in the local CBF response to NMDA receptor activation, and that the signaling pathway may be more complex than simply NO diffusion from neurons to vascular smooth muscle.

Nitric oxide and the cerebral-blood-flow response to somatosensory activation following deafferentation.

The single-vibrissa stimulation model in the rat was utilized to study the microvascular coupling between functional activation and local cerebral blood flow (LCBF) in both normal cortex and in cortex that had been peripherally deafferented. In addition, the role of chronic nitric oxide synthase (NOS) inhibition on the LCBF response to vibrissa stimulation was examined. One-day-old rats underwent deafferentation of all vibrissae on one side of the face, sparing C3, and received daily administration of either saline or N omega-nitro-L-arginine (L-NA). After seven weeks of treatment, LCBF was measured autoradiographically in conscious rats with [14C]N-isopropyl-p-iodoamphetamine while C3 was stimulated bilaterally. Stimulation produced a greater increase in LCBF in the deafferented cortex of both the saline (30.4%) and L-NA treated (25.7%) animals than in the intact cortex (19.9% and 16%, respectively). The area of activation of LCBF (0.176 mm2) was comparable to the area metabolically activated (0.149 mm2), and the increase in area of LCBF activation following deafferentation (169%) was smaller than the increase in area that was metabolically activated (287%). Chronic inhibition of NOS did not alter the spatial extent of the blood-flow response.

CBF change evoked by somatosensory activation measured by laser-Doppler flowmetry: independent evaluation of RBC velocity and RBC concentration.

The purpose of this study was to examine the timing and magnitude of cerebral blood flow (CBF) responses to neuronal activation. We measured the changes in local CBF (LCBF), red blood cell (RBC) velocity and RBC concentration by laser-Doppler flowmetry (LDF) as well as field potential recordings during activation of the somatosensory cortex of the rat in response to electrical stimulation of the hind paw. Electrical stimuli, 0.1 ms pulses of 1-1.5 mA for 5 s, were applied at 0.2, 0.5, 5, 10 and 50 Hz under alpha-chloralose anesthesia. LCBF showed the maximum increase at 5 Hz, and rose approximately 0.5 s after the onset of stimulation regardless of the frequency. The maximum frequency of the field potentials was also obtained at 5 Hz. During activation of the somatosensory cortex, the onset of rise in RBC concentration did not precede that of RBC velocity, and the peak RBC concentration was noted earlier than that of both LCBF and RBC velocity, suggesting that both arteriolar diameter and active changes in the capillary contributed to the LCBF response.

LCBF changes in rat somatosensory cortex during whisker stimulation monitored by dynamic H 2 clearance

We evaluated increases in local cerebral blood flow (LCBF) localized to single activated cortical columns by H 2 clearance methods. The rat whisker-barrel cortex is a model for cortical function and neural processing in active explorative behaviors. Up to four 30–40 μm Pt wire electrodes were inserted in or near the rat whisker-barrel cortex. Electrode positions were mapped by postmortem histology. H 2 was generated electrochemically by constant current from one electrode and detected by one or more other electrodes 300–500 μm away. Changes in LCBF produced inverse changes in PH 2. Shifts during steady H 2 generation were calibrated against standard H 2 inhalation clearance curves at rest and during inhalation of 7.5% CO 2 for 1 min for quantitative estimates of LCBF. Contralateral whisker stimulation at 3 Hz, 1 min duration and delivered every 2 min produced the largest increases in LCBF. LCBF responses were detected in ≈ 1 s. Stimulation of single whiskers produced the largest responses when an electrode was in the corresponding barrel. These results indicate that increased neural activity in a single cortical column produces blood flow responses primarily in that column.

Local cerebral blood flow responses in rats to hypercapnia and hypoxia in the rostral ventrolateral medulla and in the cortex

The effects of hypercapnia and hypoxia on two local cerebral blood flows in the parietal cortex (PC-BF) and rostral ventrolateral medulla (RVLM-BF) were examined using laser Doppler flowmetry in anesthetized rats. Hypercapnia for 45 s duration at the end-tidal CO 2 between 5% and 10%, induced by increasing inspiratory CO 2, increased both cerebral blood flows and systemic blood pressure in a degree-dependent manner. The response of RVLM-BF was significantly stronger than that of PC-BF. Both cerebral blood flow responses to hypercapnia were not influenced by cutting peripheral chemoreceptor afferent nerves. Hypoxia for 45 s duration at the end-tidal O 2 between 12% and 6%, induced by decreasing inspiratory O 2, produced an increase of similar magnitude in both RVLM and PC local blood flows in a degree-dependent manner and a decrease in systemic blood pressure. The responses of both PC-BF and RVLM-BF to hypoxia were significantly diminished after cutting peripheral chemoreceptor afferent nerves. It is concluded that: (1) the RVLM-BF is much more sensitive to hypercapnia than the PC-BF; and (2) activation of peripheral arterial chemoreceptors possibly contributes to hypoxia-induced increase in the RVLM-BF and PC-BF.

A possible involvement of central atrial natriuretic peptide in cerebral cortical microcirculation.

The possible involvement of atrial natriuretic peptide (ANP) in cerebral cortical microcirculation was investigated in rats by means of laser-Doppler flowmetry and immunohistochemistry. In the laser-Doppler study, local cerebral blood flow (LCBF) changes after the administration of 10(-6) to 10(-8) mol/LANP solution or vehicle (saline solution) as an intracortical injection for 5 minutes were continuously monitored throughout the 30 minutes of the study and were expressed as percentages of preinjection values represented as 0%. The administration of 10(-6) to 10(-8) mol/LANP caused a significant decrease in LCBF; the onset of LCBF responses occurred within a few minutes after the start of the injection and the decrease in LCBF reached the maximum level within 7 to 10 minutes after the completion of the administration, after which LCBF gradually recovered. In the immunohistochemical study, no specific ANP immunoreactivity was found associated with the intraparenchymal blood vessels; however, ANP-immunoreactive neurons were observed primarily in the hypothalamus and septum, in which high concentrations of ANP-containing neurons have been identified. The data from the laser-Doppler study suggest that central ANP may produce a vasoconstriction of the intraparenchymal blood vessels, regardless of whether through direct action on these vessels or through the mediation by some system in the central nervous system. Because there is no evidence for ANP-containing nerves around these vessels, the role of central ANP in the cerebral circulation must await identification of the source of perivascular ANP.

Variation in local cerebral blood flow response to high-dose pentobarbital sodium in the rat

Microvascular bed structure and functions are known to vary throughout the brain. Microvascular responses to high doses of pentobarbital sodium might therefore differ among brain areas. This possibility was examined by measuring local cerebral blood flow (LCBF) with [14C]iodoantipyrine in 52 brain areas at 5, 10, 25, and 60 min after intraperitoneal administration of pentobarbital (50 mg/kg). From 5 to 60 min, LCBF was significantly lowered in 17 of 25 forebrain gray matter areas but in only 1 of 18 hindbrain gray matter structures, the pontine nuclei. Smaller, shorter duration lowering of LCBF was also observed in ten other brain areas. In both control and treated rats, LCBF was found to vary within individual brain structures. The pattern of these LCBF variations was columnar in the cerebral cortex and the hippocampus but was patchy in the caudate-putamen, thalamus, and inferior colliculus. These results indicate that pentobarbital anesthesia more strongly alters LCBF in the forebrain than in the hindbrain and produces different patterns of changes in LCBF than in local cerebral glucose utilization, which was measured with 2-deoxyglucose in a companion study.

Local cerebral blood flow response to locally infused 2-chloroadenosine during hypotension in piglets

Brain interstitial adenosine increases during hypotension in piglets. If adenosine is to participate in the regulation of neonatal cerebral blood flow (CBF) during hypotension, it must retain its vasodilatory action under that condition. To examine this issue, we studied the effects of locally infused 2-chloroadenosine (2-CADO), a stable adenosine analog, on local CBF in the piglet frontal cortex during normotension and graded hemorrhagic hypotension. We used the modified brain microdialysis/hydrogen clearance technique to simultaneously infuse 2-CADO into the frontal cortex and measure local CBF from the same area. When 2-CADO from 10 −8 M to 10 −3 M was infused under control conditions ( n = 7), CBF increased 61% at 10 −5 M, 167% at 10 −4 M, and 210% at 10 −3 M. In hypotension experiments, local infusion of 10 −5 M 2-CADO ( n = 8) caused significant increases in CBF ( P < 0.05) under control conditions (MABP = 65 mmHg) and at hypotensive blood pressures of 55 mmHg and 44 mmHg, respectively. At a blood pressure of 33 mmHg, however, infusion of the analog failed to increase CBF. Local infusion of 10 −3 M 2-CADO also produced a similar change in CBF during graded hypotension. These results indicate that 2-CADO dilates intracerebral vessels during normotension, and mild and moderate hypotension, and support the hypothesis that endogenous adenosine mediates autoregulatory adjustments of CBF during hypotension in newborn piglets.