Continuous measurement of blood flow in the superior sagittal sinus of the lamb

Endotoxin-induced cerebral pathophysiology: differences between fetus and newborn

As the comparative pathophysiology of perinatal infection in the fetus and newborn is uncertain, this study contrasted the cerebral effects of endotoxemia in conscious fetal sheep and newborn lambs. Responses to intravenous bacterial endotoxin (lipopolysaccharide, LPS) or normal saline were studied on three consecutive days in fetal sheep (LPS 1 μg/kg, n = 5; normal saline n = 5) and newborn lambs (LPS 2 μg/kg, n = 10; normal saline n = 5). Cerebro-vascular function was assessed by monitoring cerebral blood flow (CBF) and cerebral vascular resistance (CVR) over 12 h each day, and inflammatory responses were assessed by plasma TNF alpha (TNF-α), nitrate and nitrite concentrations. Brain injury was quantified by counting both resting and active macrophages in the caudate nucleus and periventricular white matter (PVWM). An acute cerebral vasoconstriction (within 1 h of LPS injection) occurred in both the fetus (ΔCVR +53%) and newborn (ΔCVR +63%); subsequently prolonged cerebral vasodilatation occurred in the fetus (ΔCVR -33%) in association with double plasma nitrate/nitrite concentrations, but not in the newborn. Abundant infiltration of activated macrophages was observed in both CN and PVWM at each age, with the extent being 2-3 times greater in the fetus (P < 0.001). In conclusion, while the fetus and newborn experience a similar acute disruption of the cerebral circulation after LPS, the fetus suffers a more prolonged circulatory disruption, a greater infiltration of activated macrophages, and an exaggerated susceptibility to brain injury.

Acute and chronic effects of endotoxin on cerebral circulation in lambs

The impact of endotoxemia on cerebral endothelium and cerebral blood flow (CBF) regulation was studied in conscious newborn lambs. Bacterial endotoxin [LPS, 2 μg/kg iv] was infused on 3 consecutive days. Cerebrovascular function was assessed by monitoring CBF and cerebral vascular resistance (CVR) over 12 h each day and by the endothelium-dependent vasodilator bradykinin (BK) ( n = 10). Inflammatory responses were assessed by plasma tumor necrosis factor-α (TNF-α, n = 5). Acutely, LPS disrupted the cerebral circulation within 1 h, with peak cerebral vasoconstriction at 3 h (CBF −28 and CVR +118%, P < 0.05) followed by recovery to baseline by 12 h. TNF-α and body temperature peaked ∼1 h post-LPS. BK-induced vasodilatation (CVR −20%, P < 0.05) declined with each LPS infusion, was abolished after 3 days, and remained absent for at least the subsequent 5 days. Histological evidence of brain injury was found in four of five LPS-treated newborns. We conclude that endotoxin impairs cerebral perfusion in newborn lambs via two mechanisms: 1) acute vasoconstriction (over several hours); and 2) persistent endothelial dysfunction (over several days). Endotoxin-induced circulatory impairments may place the newborn brain at prolonged risk of CBF dysregulation and injury as a legacy of endotoxin exposure.

Tissue oxygenation index measured using spatially resolved spectroscopy correlates with changes in cerebral blood flow in newborn lambs

BACKGROUND/OBJECTIVE

Abnormal cerebral haemodynamics in very preterm infants undergoing neonatal intensive care have been associated with adverse outcome, but cerebral blood flow (CBF) is difficult to assess at the cotside. Spatially resolved spectroscopy (SRS) continuously measures cerebral tissue oxygen saturation expressed as tissue oxygenation index (TOI, %), and TOI would reflect changes in CBF with constant cerebral metabolic demand. We aimed to evaluate the relationship between simultaneous measurements of CBF (Transonic System) and TOI (Hamamatsu NIRO-200) in newborn lambs (n = 8). We hypothesised that alterations in CBF of different magnitudes and frequencies would be reflected as changes of TOI in both time domain and frequency domain analyses.

METHODS

A silicon cuff positioned around the common brachiocephalic artery was inflated to induce 10-30% reductions in cerebral perfusion pressure so as to produce mild-to-moderate variations of CBF. Relationships between changes in CBF and TOI were evaluated in the time domain (Pearson correlation) and frequency domain (Coherence).

RESULTS

In time domain analysis, there was significant correlation between DeltaTOI (%) and DeltaCBF (%) (R2 = 0.69, P < 0.001). In frequency domain analysis, CBF-TOI coherence was > or = 0.5 at frequencies below 0.1 Hz, but <0.5 at higher frequencies.

CONCLUSIONS

We conclude that overall TOI changes are concordant with CBF variations, when arterial oxygen saturation and cerebral oxygen consumption are constant. While TOI reflects CBF, it is more sensitive to variations of CBF of low frequency (<0.1 Hz) than to more rapid, higher frequency changes.

Cerebral sympathetic nerve activity has a major regulatory role in the cerebral circulation in REM sleep

Sympathetic nerve activity (SNA) in neurons projecting to skeletal muscle blood vessels increases during rapid-eye-movement (REM) sleep, substantially exceeding SNA of non-REM (NREM) sleep and quiet wakefulness (QW). Similar SNA increases to cerebral blood vessels may regulate the cerebral circulation in REM sleep, but this is unknown. We hypothesized that cerebral SNA increases during phasic REM sleep, constricting cerebral vessels as a protective mechanism against cerebral hyperperfusion during the large arterial pressure surges that characterize this sleep state. We tested this hypothesis using a newly developed model to continuously record SNA in the superior cervical ganglion (SCG) before, during, and after arterial pressure surges occurring during REM in spontaneously sleeping lambs. Arterial pressure (AP), intracranial pressure (ICP), cerebral blood flow (CBF), cerebral vascular resistance [CVR = (AP − ICP)/CBF], and SNA from the SCG were recorded in lambs ( n = 5) undergoing spontaneous sleep-wake cycles. In REM sleep, CBF was greatest (REM > QW = NREM, P < 0.05) and CVR was least (REM < QW = NREM, P < 0.05). SNA in the SCG did not change from QW to NREM sleep but increased during tonic REM sleep, with a further increase during phasic REM sleep (phasic REM > tonic REM > QW = NREM, P < 0.05). Coherent averaging revealed that SNA increases preceded AP surges in phasic REM sleep by 12 s ( P < 0.05). We report the first recordings of cerebral SNA during natural sleep-wake cycles. SNA increases markedly during tonic REM sleep, and further in phasic REM sleep. As SNA increases precede AP surges, they may serve to protect the brain against potentially damaging intravascular pressure changes or hyperperfusion in REM sleep.

Endotoxin has acute and chronic effects on the cerebral circulation of fetal sheep

We studied the impact of endotoxemia on cerebral blood flow (CBF), cerebral vascular resistance (CVR), and cerebral oxygen transport (O2 transport) in fetal sheep. We hypothesized that endotoxemia impairs CBF regulation and O2 transport, exposing the brain to hypoxic-ischemic injury. Responses to lipopolysaccharide (LPS; 1 μg/kg iv on 3 consecutive days, n = 9) or normal saline ( n = 5) were studied. Of LPS-treated fetuses, five survived and four died; in surviving fetuses, transient cerebral vasoconstriction at 0.5 h (ΔCVR approximately +50%) was followed by vasodilatation maximal at 5–6 h (ΔCVR approximately −50%) when CBF had increased (approximately +60%) despite reduced ABP (approximately −20%). Decreased CVR and increased CBF persisted 24 h post-LPS and the two subsequent LPS infusions. Cerebral O2 transport was sustained, although arterial O2 saturation was reduced ( P < 0.05). Histological evidence of neuronal injury was found in all surviving LPS-treated fetuses; one experienced grade IV intracranial hemorrhage. Bradykinin-induced cerebral vasodilatation (ΔCVR approximately −20%, P < 0.05) was abolished after LPS. Fetuses that died post-LPS ( n = 4) differed from survivors in three respects: CVR did not fall, CBF did not rise, and O2 transport fell progressively. In conclusion, endotoxin disrupts the cerebral circulation in two phases: 1) acute vasoconstriction (1 h) and 2) prolonged vasodilatation despite impaired endothelial dilatation (24 h). In surviving fetuses, LPS causes brain injury despite cerebral O2 transport being maintained by elevated cerebral perfusion; thus sustained O2 transport does not prevent brain injury in endotoxemia. In contrast, cerebral hypoperfusion and reduced O2 transport occur in fetuses destined to die, emphasizing the importance of sustaining O2 transport for survival.

Integration of cerebrovascular CO2 reactivity and chemoreflex control of breathing: mechanisms of regulation, measurement, and interpretation

Cerebral blood flow (CBF) and its distribution are highly sensitive to changes in the partial pressure of arterial CO(2) (Pa(CO(2))). This physiological response, termed cerebrovascular CO(2) reactivity, is a vital homeostatic function that helps regulate and maintain central pH and, therefore, affects the respiratory central chemoreceptor stimulus. CBF increases with hypercapnia to wash out CO(2) from brain tissue, thereby attenuating the rise in central Pco(2), whereas hypocapnia causes cerebral vasoconstriction, which reduces CBF and attenuates the fall of brain tissue Pco(2). Cerebrovascular reactivity and ventilatory response to Pa(CO(2)) are therefore tightly linked, so that the regulation of CBF has an important role in stabilizing breathing during fluctuating levels of chemical stimuli. Indeed, recent reports indicate that cerebrovascular responsiveness to CO(2), primarily via its effects at the level of the central chemoreceptors, is an important determinant of eupneic and hypercapnic ventilatory responsiveness in otherwise healthy humans during wakefulness, sleep, and exercise and at high altitude. In particular, reductions in cerebrovascular responsiveness to CO(2) that provoke an increase in the gain of the chemoreflex control of breathing may underpin breathing instability during central sleep apnea in patients with congestive heart failure and on ascent to high altitude. In this review, we summarize the major factors that regulate CBF to emphasize the integrated mechanisms, in addition to Pa(CO(2)), that control CBF. We discuss in detail the assessment and interpretation of cerebrovascular reactivity to CO(2). Next, we provide a detailed update on the integration of the role of cerebrovascular CO(2) reactivity and CBF in regulation of chemoreflex control of breathing in health and disease. Finally, we describe the use of a newly developed steady-state modeling approach to examine the effects of changes in CBF on the chemoreflex control of breathing and suggest avenues for future research.

Sympathetic withdrawal augments cerebral blood flow during acute hypercapnia in sleeping lambs

STUDY OBJECTIVES

Cerebral sympathetic activity constricts cerebral vessels and limits increases in cerebral blood flow (CBF), particularly in conditions such as hypercapnia which powerfully dilate cerebral vessels. As hypercapnia is common in sleep, especially in sleep disordered breathing, we tested the hypothesis that sympathetic innervation to the cerebral circulation attenuates the CBF increase that accompanies increases in PaCO2 in sleep, particularly in REM sleep when CBF is high.

DESIGN

Newborn lambs (n = 5) were instrumented to record CBF, arterial pressure (AP) intracranial pressure (ICP), and sleep-wake state (quiet wakefulness (QW), NREM, and REM sleep). Cerebral vascular resistance was calculated as CVR = [AP-ICP]/CBF. Lambs were subjected to 60-sec tests of hypercapnia (FICO2 = 0.08) during spontaneous sleep-wake states before (intact) and after sympathectomy (bilateral superior cervical ganglionectomy).

RESULTS

During hypercapnia in intact animals, CBF increased and CVR decreased in all sleep-wake states, with the greatest changes occurring in REM (CBF 39.3% +/- 6.1%, CVR -26.9% +/- 3.6%, P < 0.05). After sympathectomy, CBF increases (26.5% +/- 3.6%) and CVR decreases (-21.8% +/- 2.1%) during REM were less (P < 0.05). However the maximal CBF (27.8 +/- 4.2 mL/min) and minimum CVR (1.8 +/- 0.3 mm Hg/ min/mL) reached during hypercapnia were similar to intact values.

CONCLUSION

Hypercapnia increases CBF in sleep and wakefulness, with the increase being greatest in REM. Sympathectomy increases baseline CBF, but decreases the response to hypercapnia. These findings suggest that cerebral sympathetic nerve activity is normally withdrawn during hypercapnia in REM sleep, augmenting the CBF response.

Impaired autoregulation in preterm infants identified by using spatially resolved spectroscopy

OBJECTIVE

The absence of cerebral autoregulation in preterm infants has been associated with adverse outcome, but its bedside assessment in the immature brain is problematic. We used spatially resolved spectroscopy to continuously measure cerebral oxygen saturation (expressed as a tissue-oxygenation index) and used the correlation of tissue-oxygenation index with spontaneous fluctuations in mean arterial blood pressure to assess cerebral autoregulation.

PATIENTS AND METHODS

The tissue-oxygenation index and mean arterial blood pressure were continuously measured in very premature infants (n = 24) of mean (+/-SD) gestational age of 26 (+/-2.3) weeks at a mean postnatal age of 28 (+/-22) hours. The correlation between mean arterial blood pressure and tissue-oxygenation index in the frequency domain was assessed by using cross-spectral analysis techniques (coherence and transfer-function gain). Values of coherence reflect the strength of linear correlation, whereas transfer-function gain reflects the amplitude of tissue-oxygenation index changes relative to mean arterial blood pressure changes.

RESULTS

High coherence (coherence > or = 0.5) values were found in 9 infants who were of lower gestational age, lower birth weight, and lower mean arterial blood pressure than infants with coherence of < 0.5; high-coherence infants also had higher median Clinical Risk Index for Babies scores and a higher rate of neonatal deaths. Coherence of > or = 0.5 predicted mortality with a positive predictive value of 67% and negative predictive value of 100%. In multifactorial analysis, coherence alone was the best predictor of mortality and Clinical Risk Index for Babies score alone was the best predictor of coherence.

CONCLUSIONS

High coherence between mean arterial blood pressure and tissue-oxygenation index indicates impaired cerebral autoregulation in clinically sick preterm infants and is strongly associated with subsequent mortality. Cross-spectral analysis of mean arterial blood pressure and tissue-oxygenation index has the potential to provide continuous bedside assessment of cerebral autoregulation and to guide therapeutic interventions.

Sympathetic nerve activity in the superior cervical ganglia increases in response to imposed increases in arterial pressure

Sympathetic vasoconstriction of cerebral vessels has been proposed to be a protective mechanism for the brain, limiting cerebral perfusion and microcirculatory pressure during transient increases in arterial pressure. To furnish direct neural evidence for this proposition, we aimed to develop a method for recording cerebral sympathetic nerve activity (SNA) from the superior cervical ganglion (SCG). We hypothesized that SNA recorded from the SCG increases during imposed hypertension, but not during hypotension. Lambs (n = 11) were anesthetized (alpha-chloralose, 20 mg.kg(-1).h(-1)) and ventilated. SNA was measured using 25-microm tungsten microelectrodes inserted into the SCG. Arterial blood pressure (AP) was pharmacologically raised (adrenaline, phenylephrine, or ANG II, 1-50 microg/kg iv), mechanically raised (intravascular balloon in the thoracic aorta), or lowered (sodium nitroprusside, 1-50 microg/kg iv). In response to adrenaline (n = 10), mean AP increased 135 +/- 10% from baseline (mean +/- SE), and the RMS value of SNA (Square Root of the Mean of the Squares, SNA(RMS)) increased 255 +/- 120%. In response to mechanically induced hypertension, mean AP increased 43 +/- 3%, and SNA(RMS) increased 53 +/- 13%. Generally, (9 of 10 animals), SNA(RMS) did not increase, as AP was lowered with sodium nitroprusside. Using a new model for direct recording of cerebral SNA from the SCG, we have demonstrated that SNA increases in response to large induced rises, but not falls, in AP. These findings furnish direct support for the proposed protective role for sympathetic nerves in the cerebral circulation.

Cerebral blood flow and oxygenation in ovine fetus: responses to superimposed hypoxia at both low and high altitude

For the fetus, although the roles of arterial blood gases are recognized to be critical in the regulation of cerebral blood flow (CBF) and cerebral oxygenation, the relation of CBF, cortical tissue P(O2) (tP(O2)), sagittal sinus P(O2), and related indices of cerebral oxygenation to arterial blood gases are not well defined. This is particularly true for that fetus subjected to long-term hypoxia (LTH). In an effort to elucidate these interrelations, we tested the hypothesis that in the fetus acclimatized to high altitude, cerebral oxygenation is not compromised relative to that at low altitude. By use of a laser Doppler flowmeter with a fluorescent O2 probe, in near-term fetal sheep at low altitude (n = 8) and those acclimatized to high altitude hypoxia (3801 m for 90 +/- 5 days; n = 6), we measured laser Doppler CBF (LD-CBF), tP(O2), and related variables in response to 40 min superimposed hypoxia. At both altitudes, fetal LD-CBF, cerebral O2 delivery, tP(O2), and several other variables including sagittal sinus P(O2), correlated highly with arterial P(O2) (P(a,O2)). In response to superimposed hypoxia (P(a,O2) = 11 +/- 1 Torr), LD-CBF was significantly blunted at high altitude, as compared with that at low altitude. In the two altitude groups fetal cerebral oxygenation was similar under both control conditions and with superimposed hypoxia, cortical tP(O2) decreasing from 8 +/- 1 and 6 +/- 1 Torr, respectively, to 2 +/- 1 Torr. Also, for these conditions sagittal sinus P(O2) and [HbO2] values were similar. In response to superimposed hypoxia, cerebral metabolic rate for O(2) decreased approximately 50% in each group (P < 0.05). For both the fetus at low altitude and that acclimatized to high altitude LTH, we present the first dose-response data on the relation of LD-CBF, cortical tP(O2), and sagittal sinus blood gas values to P(a,O2). In addition, despite differences in several variables, the fetus at high altitude showed evidence of successful acclimatization, supporting the hypothesis that such fetuses demonstrate no compromise in cerebral oxygenation.

Sleep-dependent changes in cerebral oxygen consumption in newborn lambs

During rapid-eye-movement (REM) sleep in adult subjects, the cerebral metabolic rate of oxygen consumption (CMRO(2)) is as high as that during wakefulness. We investigated whether CMRO(2) during active sleep is already at the waking level in newborn life, to support the role of active sleep as a state of endogenous brain activation during early postnatal development. Newborn lambs, 2-5 days old (n = 6), were instrumented with electrodes for sleep-state scoring, catheters for blood sample withdrawal and pressure monitoring, and a transit-time ultrasonic blood-flow probe around the superior sagittal sinus. At the age of 19 +/- 3 days, blood samples were obtained simultaneously from the carotid artery and the superior sagittal sinus during uninterrupted epochs of wakefulness, quiet sleep, and active sleep. The arteriovenous difference in blood oxygen concentration was multiplied by cerebral blood flow to determine CMRO(2). CMRO(2) during active sleep (47 +/- 5 micromol min(-1)) was similar to the value in wakefulness (44 +/- 6 micromol min(-1)) and significantly higher than in quiet sleep (39 +/- 5 micromol min(-1), P < 0.05). These data show that active sleep provides newborn lambs with brain activity at a level similar to that in wakefulness in terms of cerebral oxygen metabolism. The high CMRO(2) during active sleep supports its functional role during early postnatal development, when time spent in active sleep is at a lifetime maximum, albeit constituting a metabolic challenge for newborns, because of the impairment of systemic and cerebral vascular regulation in this sleep state.

Autoregulation of the cerebral circulation during sleep in newborn lambs

Autoregulation is a vital protective mechanism that maintains stable cerebral blood flow as cerebral perfusion pressure changes. We contrasted cerebral autoregulation across sleep-wake states, as little is known about its effectiveness during sleep. Newborn lambs (n= 9) were instrumented to measure cerebral blood flow (flow probe on the superior sagittal sinus) and cerebral perfusion pressure, then studied during active sleep (AS), quiet sleep (QS) and quiet wakefulness (QW). We generated cerebral autoregulation curves by inflating an occluder cuff around the brachiocephalic artery thereby lowering cerebral perfusion pressure. Baseline cerebral blood flow was higher (P < 0.05) and cerebral vascular resistance lower (P < 0.05) in AS than in QW (76 +/- 8% and 133 +/- 15%, respectively, of the AS value, mean +/-s.d.) and in QS (66 +/- 11% and 158 +/- 30%). The autoregulation curve in AS differed from that in QS and QW in three key respects: firstly, the plateau was elevated relative to QS and QW (P < 0.05); secondly, the lower limit of the curve (breakpoint) was higher (P < 0.05) in AS (50 mmHg) than QS (45 mmHg); and thirdly, the slope of the descending limb below the breakpoint was greater (P < 0.05) in AS than QS (56% of AS) or QW (56% of AS). Although autoregulation functions in AS, the higher breakpoint and greater slope of the descending limb may place the brain at risk for vascular compromise should hypotension occur.

The haemodynamic and metabolic effects of epinephrine in experimental hyperdynamic septic shock

OBJECTIVE

To study the effect of epinephrine (EPI) infusion on vital organ blood flow and metabolic variables during sepsis.

DESIGN AND SETTING

Randomised placebo-controlled animal trial in an animal laboratory.

ANIMALS

Seven merino cross-ewes.

INTERVENTIONS

Chronic implantation of flow probes (aorta, renal, mesenteric and coronary artery and sagittal sinus). Induction of sepsis by intravenous injection of E. coli. Random allocation of sheep to EPI (0.4 microg kg(-1) min(-1)) or vehicle for 6 h.

MEASUREMENTS AND RESULTS

E. coli induced hypotension and hyperlactataemia and increased cardiac output, renal, mesenteric and coronary blood flows. Compared to vehicle, EPI restored mean arterial blood pressure (69 vs. 86 mmHg) and further increased cardiac output (6.4 vs. 7.1 l/min). EPI, however, decreased renal blood flow (330 vs. 247 ml/min) and renal conductance. EPI also reduced mesenteric and coronary conductance without changes in flows. Compared to vehicle, EPI increased urine output (293 vs. 544 ml/6 h) but not creatinine clearance. EPI increased lactate (1.8 vs. 15.7 mmol/l) with accompanying acidosis (serum bicarbonate: 25.2 vs. 15.7 mmol/l), hyperglycaemia (2.6 vs. 13.5 mmol/l) and hypokalaemia (4.3 vs. 3.0 mmol/l).

CONCLUSIONS

Hyperdynamic sepsis increased blood flow to heart, gut and kidney. Although EPI infusion further increased cardiac output, blood pressure and myocardial performance, it was also associated with potent metabolic effects, decreased mesenteric, coronary and renal conductance and a significant reduction in renal blood flow.

Norepinephrine and vital organ blood flow during experimental hyperdynamic sepsis

OBJECTIVE

To study the effect of norepinephrine (NE) infusion on cerebral, coronary, renal and mesenteric blood flow during sepsis.

DESIGN AND SETTING

Randomised placebo-controlled animal trial in the animal laboratory of university physiology institute.

ANIMALS

Seven merino cross-ewes.

INTERVENTIONS

Chronic implantation of flow probes (aorta, renal, mesenteric and coronary artery and sagittal sinus). Induction of sepsis by intravenous bolus of E. coli (3 x 10(9)). After the onset of hyperdynamic sepsis sheep were randomly allocated to either NE (0.4 microg kg(-1) min(-1)) or placebo for 6 h.

MEASUREMENTS AND RESULTS

E. coli induced hypotension, fever, oliguria, tachycardia and tachypnoea. It increased cardiac output and renal, mesenteric and coronary blood flows. Sagittal flow remained unchanged. Compared to placebo NE infusion restored mean arterial blood pressure and further increased cardiac output. The increases in renal, mesenteric and coronary blood flow were unaffected. Sagittal flow was also unaltered. Compared to placebo NE increased myocardial performance, mean urine output and creatinine clearance at 2 h.

CONCLUSIONS

We conclude that hyperdynamic sepsis increases blood flow to heart, gut and kidney and that NE further increases cardiac output, blood pressure, myocardial performance, and urine output and creatinine clearance while maintaining regional blood flow.

Cerebral leucine uptake and protein synthesis in the near-term ovine fetus: relation to fetal behavioral state

Behavioral/sleep state activity may impact on synthetic processes within the brain, thus accounting for the developmental change in such activity and suggesting a role in the brain's growth and development. We have therefore determined the cerebral uptake of leucine and [(14)C]leucine during continuous tracer infusion as measures of leucine metabolism in relation to behavioral state activity, as well as the regional flux of leucine into brain tissue in the ovine fetus near term. The cerebral fractional protein synthetic rate and the absolute protein synthetic rate averaged approximately 20%/day and approximately 1 g/day, respectively, as measured for the whole brain, which is considerably higher than anticipated protein accretion and indicates a high rate of protein turnover with protein synthesis closely linked to protein degradation. Measures of protein synthesis were significantly higher in the pituitary gland, which may be attributed to the active synthesis and export of peptide hormones from this region. Cerebral leucine and [(14)C]leucine uptakes averaged approximately 630 and approximately 1,000 nmol. 100 g(-1). min(-1), with the latter higher than leucine unidirectional flux and thus supporting a degree of leucine oxidation by the brain. Cerebral leucine metabolism as studied was affected by behavioral state activity, with uptake measurements for both leucine and [(14)C]leucine significantly increased during the high-voltage electrocortical/non-rapid eye movement state by 1.7-fold and 2.8-fold, respectively, indicating that protein synthesis and degradation must also be increased at this time, and supporting a role for behavioral state activity in the brain's growth and development.

Nitric oxide inhibition abolishes sleep-wake differences in cerebral circulation

Nitric oxide (NO), being produced by active neurones and also being a cerebral vasodilator, may couple brain activity and blood flow in sleep, particularly during active sleep (AS), which is characterized by widespread neural activation and markedly elevated cerebral blood flow (CBF) compared with quiet wakefulness (QW) and quiet sleep (QS). This study examined CBF and cerebral vascular resistance (CVR) in lambs ( n = 6) during spontaneous sleep-wake cycles before and after infusion of N ω-nitro-l-arginine (l-NNA), an inhibitor of NO synthase. l-NNA infusion produced increases in CVR and decreases in CBF during all sleep-wake stages, with the greatest changes occurring in AS (ΔCVR, 88 ± 19%; ΔCBF −24 ± 8%). The characteristic CVR and CBF differences among AS, QS, and QW disappeared within 1–3 h ofl-NNA infusion, but had reappeared by 24 h despite persisting cerebral vasoconstriction. These experiments show that NO promotes cerebral vasodilatation during sleep as well as wakefulness, particularly during AS. Additionally, NO is the major, although not sole, determinant of the CBF differences that exist between sleep-wake states.