Local cerebral blood flow in the conscious rat as measured with 14C-antipyrine, 14C-iodoantipyrine and 3H-nicotine

Brain endothelial permeability, transport, and flow assessed over 10 orders of magnitude using the in situ brain perfusion technique

BACKGROUND

Cerebral blood flow normally places a limit on the magnitude of brain vascular permeability (P) that can be measured in vivo. At normal cerebral blood flow, this limit falls at the lower end of lipophilicity for most FDA-approved CNS drugs. In this study, we report on two methods that can be used to overcome this limitation and measure brain vascular permeability values that are up to ~1000 times higher using the in situ brain perfusion technique.

METHODS

Rat brain was perfused with physiological saline at increased flow rate and in the presence of various concentrations of plasma protein, serum albumin or alpha-acid glycoprotein. Plasma protein was added to the saline perfusion fluid to lower extraction into the measurable range using the Crone Renkin "diffusion-flow" equation to calculate brain PoS.

RESULTS

Cerebrovascular Po was determined for 125 solutes, of which 78 showed little or no evidence of active efflux transport. Fifty of the solutes were in the lipophilicity zone (Log Poct 1-5) of most FDA-approved CNS drugs. Care was taken to ensure the integrity of the brain vasculature during perfusion and to measure flow accurately using markers that had been verified for the flow rates. The results showed a linear relationship between Log Po and Log Poct over ~10 orders of magnitude with values for diazepam, estradiol, testosterone, and other agents that exceed prior published values by fivefold to 200-fold.

CONCLUSIONS

The results show that brain vascular permeability can be measured directly in vivo for highly lipophilic solutes and the PS values obtained match reasonably with that predicted by the Crone-Renkin flow diffusion equation with care taken to validate the accuracy for the component measurements and with no need to invoke "enhanced" or "induced" dissociation.

Impact of Nicotine Transport across the Blood-Brain Barrier: Carrier-Mediated Transport of Nicotine and Interaction with Central Nervous System Drugs

Nicotine, an addictive substance, is absorbed from the lungs following inhalation of tobacco smoke, and distributed to various tissues such as liver, brain, and retina. Recent in vivo and in vitro studies suggest the involvement of a carrier-mediated transport process in nicotine transport in the lung, liver, and inner blood-retinal barrier. In addition, in vivo studies of influx and efflux transport of nicotine across the blood-brain barrier (BBB) revealed that blood-to-brain influx transport of nicotine is more dominant than brain-to-blood efflux transport of nicotine. Uptake studies in TR-BBB13 cells, which are an in vitro model cell line of the BBB, suggest the involvement of H⁺/organic cation antiporter, which is distinct from typical organic cation transporters, in nicotine transport at the BBB. Moreover, inhibition studies in TR-BBB13 cells showed that nicotine uptake was significantly reduced by central nervous system (CNS) drugs, such as antidepressants, anti-Alzheimer's disease drugs, and anti-Parkinson's disease drugs, suggesting that the nicotine transport system can recognize these molecules. The cumulative evidence would be helpful to improve our understanding of smoking-CNS drug interaction for providing appropriate medication.

Scaling of cerebral blood perfusion in primates and marsupials

The evolution of primates involved increasing body size, brain size and presumably cognitive ability. Cognition is related to neural activity, metabolic rate and rate of blood flow to the cerebral cortex. These parameters are difficult to quantify in living animals. This study shows that it is possible to determine the rate of cortical brain perfusion from the size of the internal carotid artery foramina in skulls of certain mammals, including haplorrhine primates and diprotodont marsupials. We quantify combined blood flow rate in both internal carotid arteries as a proxy of brain metabolism in 34 species of haplorrhine primates (0.116-145 kg body mass) and compare it to the same analysis for 19 species of diprotodont marsupials (0.014-46 kg). Brain volume is related to body mass by essentially the same exponent of 0.70 in both groups. Flow rate increases with haplorrhine brain volume to the 0.95 power, which is significantly higher than the exponent (0.75) expected for most organs according to 'Kleiber's Law'. By comparison, the exponent is 0.73 in marsupials. Thus, the brain perfusion rate increases with body size and brain size much faster in primates than in marsupials. The trajectory of cerebral perfusion in primates is set by the phylogenetically older groups (New and Old World monkeys, lesser apes) and the phylogenetically younger groups (great apes, including humans) fall near the line, with the highest perfusion. This may be associated with disproportionate increases in cortical surface area and mental capacity in the highly social, larger primates.

Modeling cyclosporine A inhibition of the distribution of a P-glycoprotein PET ligand, 11C-verapamil, into the maternal brain and fetal liver of the pregnant nonhuman primate: impact of tissue blood flow and site of inhibition

Through PET imaging, our laboratory has studied the dynamic biodistribution of (11)C-verapamil, a P-gp substrate, in the nonhuman primate Macaca nemestrina. To gain detailed insight into the kinetics of verapamil transport across the blood-brain barrier (BBB) and the blood-placental barrier (BPB), we analyzed these dynamic biodistribution data by compartmental modeling.

METHODS

Thirteen pregnant macaques (gestational age, 71-159 d; term, ∼172 d) underwent PET imaging with (11)C-verapamil before and during infusion (6, 12, or 24 mg/kg/h) of cyclosporine A (CsA, a P-glycoprotein [P-gp] inhibitor). Dynamic (11)C-verapamil brain or fetal liver (reporter of placental P-gp function) activity was assessed by a 1- or 2-tissue-compartment model.

RESULTS

The 1-tissue-compartment model best explained the observed brain and fetal liver distribution of (11)C-radioactivity. When P-gp was completely inhibited, the brain and fetal liver distribution clearance (K1) approximated tissue blood flow (Q); that is, extraction ratio (K1/Q) was approximately 1, indicating that in the absence of P-gp function, the distribution of (11)C-verapamil radioactivity into these compartments is limited by blood flow. The potency of CsA to inhibit P-gp was tissue-independent (maternal BBB half-maximal inhibitory concentration [IC50], 5.67 ± 1.07 μM, vs. BPB IC50, 7.63 ± 3.16 μM).

CONCLUSION

We propose that on deliberate or inadvertent P-gp inhibition, the upper boundary of increase in human brain (or fetal) distribution of lipophilic drugs such as verapamil will be limited by tissue blood flow. This finding provides a means to predict the magnitude of P-gp-based drug interactions at the BBB and BPB when only the baseline distribution of the drug (i.e., in the absence of P-gp inhibition) across these barriers is available through PET. Our data suggest that P-gp-based drug interactions at the human BBB and BPB can be clinically significant, particularly for those P-gp substrate drugs for which P-gp plays a significant role in excluding the drug from these privileged compartments.

Pharmacokinetics and modeling of immune cell trafficking: quantifying differential influences of target tissues versus lymphocytes in SJL and lipopolysaccharide-treated mice

BACKGROUND

Immune cell trafficking into the CNS and other tissues plays important roles in health and disease. Rapid quantitative methods are not available that could be used to study many of the dynamic aspects of immune cell-tissue interactions.

METHODS

We used pharmacokinetics and modeling to quantify and characterize the trafficking of radioactively labeled lymphocytes into brain and peripheral tissues. We used variance from two-way ANOVAs with 2 × 2 experimental designs to model the relative influences of lymphocytes and target tissues in trafficking.

RESULTS

We found that in male CD-1 mice, about 1 in 5,000 intravenously injected lymphocytes entered each gram of brain. Uptake by brain was 2 to 3 times higher in naïve SJL females, but uptake by spleen and clearance from blood was lower, demonstrating a dichotomy in immune cell distribution. Treatment of CD-1 mice with lipopolysaccharide (LPS) increased immune cell uptake into brain but decreased uptake by spleen and axillary nodes.

CONCLUSIONS

Differences in brain uptake and in uptake by spleen between SJL and CD-1 mice were primarily determined by lymphocytes, whereas differences in uptake with LPS were primarily determined by lymphocytes for the brain but by the tissues for the spleen and the axillary lymph node. These results show that immune cells normally enter the CNS and that tissues and immune cells interact in ways that can be quantified by pharmacokinetic models.

Heterogeneous blood-tumor barrier permeability determines drug efficacy in experimental brain metastases of breast cancer

PURPOSE

Brain metastases of breast cancer appear to be increasing in incidence, confer significant morbidity, and threaten to compromise gains made in systemic chemotherapy. The blood-tumor barrier (BTB) is compromised in many brain metastases; however, the extent to which this influences chemotherapeutic delivery and efficacy is unknown. Herein, we answer this question by measuring BTB passive integrity, chemotherapeutic drug uptake, and anticancer efficacy in vivo in two breast cancer models that metastasize preferentially to brain.

EXPERIMENTAL DESIGN

Experimental brain metastasis drug uptake and BTB permeability were simultaneously measured using novel fluorescent and phosphorescent imaging techniques in immune-compromised mice. Drug-induced apoptosis and vascular characteristics were assessed using immunofluorescent microscopy.

RESULTS

Analysis of over 2,000 brain metastases from two models (human 231-BR-Her2 and murine 4T1-BR5) showed partial BTB permeability compromise in greater than 89% of lesions, varying in magnitude within and between metastases. Brain metastasis uptake of ¹⁴C-paclitaxel and ¹⁴C-doxorubicin was generally greater than normal brain but less than 15% of that of other tissues or peripheral metastases, and only reached cytotoxic concentrations in a small subset (∼10%) of the most permeable metastases. Neither drug significantly decreased the experimental brain metastatic ability of 231-BR-Her2 tumor cells. BTB permeability was associated with vascular remodeling and correlated with overexpression of the pericyte protein desmin.

CONCLUSIONS

This work shows that the BTB remains a significant impediment to standard chemotherapeutic delivery and efficacy in experimental brain metastases of breast cancer. New brain permeable drugs will be needed. Evidence is presented for vascular remodeling in BTB permeability alterations.

MR molecular imaging of tumor vasculature and vascular targets

Tumor angiogenesis and the ability of cancer cells to induce neovasculature continue to be a fascinating area of research. As the delivery network that provides substrates and nutrients, as well as chemotherapeutic agents to cancer cells, but allows cancer cells to disseminate, the tumor vasculature is richly primed with targets and mechanisms that can be exploited for cancer cure or control. The spatial and temporal heterogeneity of tumor vasculature, and the heterogeneity of response to targeting, make noninvasive imaging essential for understanding the mechanisms of tumor angiogenesis, tracking vascular targeting, and detecting the efficacy of antiangiogenic therapies. With its noninvasive characteristics, exquisite spatial resolution and range of applications, magnetic resonance imaging (MRI) techniques have provided a wealth of functional and molecular information on tumor vasculature in applications spanning from "bench to bedside". The integration of molecular biology and chemistry to design novel imaging probes ensures the continued evolution of the molecular capabilities of MRI. In this review, we have focused on developments in the characterization of tumor vasculature with functional and molecular MRI.

Regional differences in capillary density, perfusion rate, and P-glycoprotein activity: a quantitative analysis of regional drug exposure in the brain

The in situ brain perfusion technique was used to assess the impact of local capillary density, blood flow rate and P-gp-mediated efflux activity on regional drug exposure for the P-gp substrates colchicine, quinidine, verapamil, and loperamide, the perfusion flow rate marker diazepam, and the vascular volume marker inulin, in mdr1a(+/+) and mdr1a(-/-) mice. Regional perfusion flow rate varied 7.5-fold, and capillary density (based on vascular volume) varied 3.7-fold, across the 13 brain regions examined. The rate of regional flow, as well as P-gp-mediated colchicine efflux activity, was directly proportional to local capillary density. A decrease in perfusion rate attenuated verapamil brain uptake and had significant effect on P-gp-mediated efflux activity for this substrate in brain regions with lower capillary density. Regional brain uptake and calculated logD at pH 7.4 (clogD(7.4)) were well-related in P-gp-deficient mice, indicating that in the absence of P-gp-mediated efflux, physicochemical properties of the compound (i.e., lipophilicity) serve as the primary determinant of regional brain uptake. Loperamide regional brain uptake and P-gp effect during a 60-s brain perfusion or at 30min after subcutaneous administration were significantly correlated with local capillary density. The highest P-gp-mediated efflux activity was consistently observed in cerebral cortex and midbrain regions for loperamide following short-term brain perfusion and at all time points following subcutaneous administration. These results in intact animal emphasize that the regionality of substrate exposure in brain as measured by the in situ brain perfusion technique is actually biologically relevant.

Central nervous system drug disposition: the relationship between in situ brain permeability and brain free fraction

The dispositions of 50 marketed central nervous system (CNS) drugs into the brain have been examined in terms of their rat in situ (P) and in vitro apparent membrane permeability (P(app)) alongside lipophilicity and free fraction in rat brain tissue. The inter-relationship between these parameters highlights that both permeability and brain tissue binding influence the uptake of drugs into the CNS. Hydrophilic compounds characterized by low brain tissue binding display a strong correlation (R(2) = 0.82) between P and P(app), whereas the uptake of more lipophilic compounds seems to be influenced by both P(app) and brain free fraction. A nonlinear relationship is observed between logP(oct) and P over the 6 orders of magnitude range in lipophilicity studied. These findings corroborate recent reports in the literature that brain penetration is a function of both rate and extent of drug uptake into the CNS.

Brain uptake kinetics of nicotine and cotinine after chronic nicotine exposure

Blood-brain barrier (BBB) nicotine transfer has been well documented in view of the fact that this alkaloid is a cerebral blood flow marker. However, limited data are available that describe BBB penetration of the major tobacco alkaloids after chronic nicotine exposure. This question needs to be addressed, given long-term nicotine exposure alters both BBB function and morphology. In contrast to nicotine, it has been reported that cotinine (the major nicotine metabolite) does not penetrate the BBB, yet cotinine brain distribution has been well documented after nicotine exposure. Surprisingly, therefore, the literature indirectly suggests that central nervous system cotinine distribution occurs secondarily to nicotine brain metabolism. The aims of the current report are to define BBB transfer of nicotine and cotinine in naive and nicotine-exposed animals. Using an in situ brain perfusion model, we assessed the BBB uptake of [3H]nicotine and [3H]cotinine in naive animals and in animals exposed chronically to S-(-)nicotine (4.5 mg/kg/day) through osmotic minipump infusion. Our data demonstrate that 1) [3H]nicotine BBB uptake is not altered in the in situ perfusion model after chronic nicotine exposure, 2) [3H]cotinine penetrates the BBB, and 3) similar to [3H]nicotine, [3H]cotinine BBB transfer is not altered by chronic nicotine exposure. To our knowledge, this is the first report detailing the uptake of nicotine and cotinine after chronic nicotine exposure and quantifying the rate of BBB penetration by cotinine.

Optical imaging of the spatiotemporal dynamics of cerebral blood flow and oxidative metabolism in the rat barrel cortex

Oxidative metabolism and cerebral blood flow (CBF) are two of the most important measures in neuroimaging. However, results from concurrent imaging of the two with high spatial and temporal resolution have never been published. We used flavoprotein autofluorescence (AF) and laser speckle imaging (LSI) in the anaesthetized rat to map oxidative metabolism and CBF in response to single vibrissa stimulation. Autofluorescence responses reflecting oxidative metabolism demonstrated a fast increase with a delay of 0.1 s. The sign-reversed speckle contrast reflecting CBF started to rise with a delay of 0.6 s and reached its maximum 1.4 s after the stimulation offset. The fractional signal changes were 2.0% in AF and 9.7% in LSI. Pixelwise modelling revealed that CBF maps spread over an area up to 2.5-times larger than metabolic maps. The results provide evidence that the increase in cerebral oxidative metabolism in response to sensory stimulation is considerably faster and more localized than the CBF response. This suggests that future developments in functional imaging concentrating on the metabolic response promise an increased spatial resolution.

Quantitative cerebral blood flow measurements in the rat using a beta-probe and H2 15O

Beta-probes are a relatively new tool for tracer kinetic studies in animals. They are highly suited to evaluate new positron emission tomography tracers or measure physiologic parameters at rest and after some kind of stimulation or intervention. In many of these experiments, the knowledge of CBF is highly important. Thus, the purpose of this study was to evaluate the method of CBF measurements using a beta-probe and H2 15O. CBF was measured in the barrel cortex of eight rats at baseline and after acetazolamide challenge. Trigeminal nerve stimulation was additionally performed in five animals. In each category, three injections of 250 to 300 MBq H2 15O were performed at 10-minute intervals. Data were analyzed using a standard one-tissue compartment model (K1 = CBF, k2 = CBF/p, where p is the partition coefficient). Values for K1 were 0.35 +/- 0.09, 0.58 +/- 0.16, and 0.49 +/- 0.03 mL x min(-1) x mL(-1) at rest, after acetazolamide challenge, and during trigeminal nerve stimulation, respectively. The corresponding values for k2 were 0.55 +/- 0.12, 0.94 +/- 0.16, and 0.85 +/- 0.12 min(-7), and for p were 0.64 +/- 0.05, 0.61 +/- 0.07, and 0.59 +/- 0.06. The standard deviation of the difference between two successive experiments, a measure for the reproducibility of the method, was 10.1%, 13.0%, and 5.7% for K1, k2, and p, respectively. In summary, beta-probes in conjunction with H2 15O allow the reproducible quantitative measurement of CBF, although some systematic underestimation seems to occur, probably because of partial volume effects.

Effect of hypertension on the integrity of blood brain and blood CSF barriers, cerebral blood flow and CSF secretion in the rat

Hypertension has been related to the development of brain damage, dementia and other CNS dysfunctions. Disruption of the blood-brain barrier (BBB) is thought to contribute to these disorders. In this study, the integrity of both blood-brain and blood-CSF barriers during chronic hypertension was investigated. For this, the entry of [14C]sucrose and of lanthanum into brain tissue, choroid plexus, and CSF was studied. Also brain regional blood flow and brain [14C]sucrose volume of distribution were measured using indicator fractionation and ventriculo-cisternal perfusion methods, respectively. The above measurements were performed in normotensive (WKY) rats and in the spontaneously hypertensive rats (SHR). Choroid plexus and CSF uptakes of [14C]sucrose were found to be significantly greater in SHR compared to WKY rats (P<0.05). Intercellular entry of lanthanum was observed in choroidal tissue of SHR but not in that of WKY rats and at the BBB. Choroid plexus blood flow was significantly greater in SHR, 2.82+/-0.21 ml g(-1) min(-1), compared to 2.4+/-0.08 ml g(-1) min(-1) in WKY (P<0.05). There were no significant differences (P>0.05) in brain % water content and extracellular fluid [14C]sucrose volume of distribution between SHR and WKY rats. However, choroid plexus showed greater % water content in SHR (85.7+/-1.9%) compared to the WKY rats (81.5+/-1.7%). These results suggest that chronic hypertension in SHR may cause more pronounced defects in the integrity of the blood-CSF barrier than in the BBB.

Development of a new reporter gene system--dsRed/xanthine phosphoribosyltransferase-xanthine for molecular imaging of processes behind the intact blood-brain barrier

We report the development of a novel dual-modality fusion reporter gene system consisting of Escherichia coli xanthine phosphoribosyltransferase (XPRT) for nuclear imaging with radiolabeled xanthine and Discosoma red fluorescent protein for optical fluorescent imaging applications. The dsRed/XPRT fusion gene was successfully created and stably transduced into RG2 glioma cells, and both reporters were shown to be functional. The level of dsRed fluorescence directly correlated with XPRT enzymatic activity as measured by ribophosphorylation of [14C]-xanthine was in vitro (Ki = 0.124 +/- 0.008 vs. 0.00031 +/- 0.00005 mL/min/g in parental cell line), and [*]-xanthine octanol/water partition coefficient was 0.20 at pH = 7.4 (logP = -0.69), meeting requirements for the blood-brain barrier (BBB) penetrating tracer. In the in vivo experiment, the concentration of [14C]-xanthine in the normal brain varied from 0.20 to 0.16 + 0.05% dose/g under 0.87 + 0.24% dose/g plasma radiotracer concentration. The accumulation in vivo in the transfected flank tumor was to 2.4 +/- 0.3% dose/g, compared to 0.78 +/- 0.02% dose/g and 0.64 +/- 0.05% dose/g in the control flank tumors and intact muscle, respectively. [14C]-Xanthine appeared to be capable of specific accumulation in the transfected infiltrative brain tumor (RG2-dsRed/XPRT), which corresponded to the 585 nm fluorescent signal obtained from the adjacent cryosections. The images of endogenous gene expression with the "sensory system" have to be normalized for the transfection efficiency based on the "beacon system" image data. Such an approach requires two different "reporter genes" and two different "reporter substrates." Therefore, the novel dsRed/XPRT fusion gene can be used as a multimodality reporter system in the biological applications requiring two independent reporter genes, including the cells located behind the BBB.

β-Adrenergics enhance brain extraction of levodopa

We sought to determine whether beta-adrenergic agonists enhance the brain extraction of L-dopa and L-leucine. Systemic administration of beta-adrenergic agonists increase brain concentrations of L-dopa and other large neutral amino acids (LNAA) in rats and monkeys and may improve symptoms and reduce daily L-dopa requirement in patients with Parkinson's disease. Cerebral blood flow (CBF) using [3H]nicotine and the extraction fraction of 14C-labeled L-dopa or L-leucine were measured simultaneously in various brain regions of conscious rats using the dual-isotope indicator fractionation technique after intraperitoneal administration of isoproterenol (a peripheral nonselective beta-adrenergic agonist), or clenbuterol (a beta2-adrenergic agonist that crosses the blood-brain barrier), or beta-adrenergic agonist preceded by nadolol (a peripheral nonselective beta-adrenergic antagonist), or saline vehicle. Both beta-adrenergic agonists increased regional brain extraction fraction of L-dopa and L-leucine tracers by 35-45%, without altering regional CBF. These changes were accompanied by about a 30% decrease in plasma branched chain LNAA concentrations. Nadolol blocked all these effects. beta-Adrenergic agonists increase the brain extraction of L-dopa and leucine, mainly by peripheral mechanisms that reduce the levels of other competing plasma LNAAs for transport. Thus, beta-adrenergic agonists might be useful in the treatment of patients with Parkinson's disease by enhancing delivery of L-dopa to the brain.

Comparison of blood-brain barrier permeability in mice and rats using in situ brain perfusion technique

Here we present a method for measuring the permeability coefficient-surface area product (PS) values at the blood-brain barrier in mice, using the in situ brain perfusion technique originally developed for rats by Takasato et al. (Am J Physiol Heart Circ Physiol 247: H484-H493, 1984). Retrograde infusion into the right external carotid artery increased the carotid perfusion pressure in proportion to the perfusion rate. Intravascular volume and cerebral perfusion fluid flow at a perfusion rate of 1.0 ml/min in mice were similar to those in rats. In addition, the contribution of systemic blood to total flow in the hemisphere was small (only 3. 2%). These findings indicated that this perfusion rate is suitable for mice. The PS values of more than 20 different compounds were determined in mice by using the in situ brain perfusion technique, and comparisons were made with data from rats. There was a close relationship (1:1) between the PS values in mice and rats, indicating that brain capillary permeabilities are similar in mice and rats.

Hemodilution, cerebral O2 delivery, and cerebral blood flow: a study using hyperbaric oxygenation

Hemodilution reduces blood viscosity and O2 content (CaO2) and increases cerebral blood flow (CBF). Viscosity and CaO2 may contribute to increasing CBF after hemodilution. However, because hematocrit is the major contributor to blood viscosity and CaO2, it has been difficult to assess their relative importance. By varying blood viscosity without changing CaO2, prior investigation in hemodiluted animals has suggested that both factors play roughly equal roles. To further investigate the relationship of hemodilution, blood viscosity, CaO2, and CBF, we took the opposite approach in hemodiluted animals, i.e., we varied CaO2 without changing blood viscosity. Hyperbaric O2 was used to restore CaO2 to normal after hemodilution. Pentobarbital sodium-anesthetized rats underwent isovolumic hemodilution with 6% hetastarch, and forebrain CBF was measured with [3H]nicotine. One group of animals did not undergo hemodilution and served as controls (Con). In the three experimental groups, hematocrit was reduced from 44% to 17-19%. Con and hemodiluted (HDil) groups were ventilated with 40% O2 at 101 kPa (1 atmosphere absolute), which resulted in CaO2 values of 19.7 +/- 1.3 and 8.1 +/- 0.7 (SD) ml O2/dl, respectively. A second group of hemodiluted animals (HBar) was ventilated with 100% O2 at 506 kPa (5 atmospheres absolute) in a hyperbaric chamber, which restored CaO2 to an estimated 18.5 +/- 0.5 ml O2/dl by increasing dissolved O2. A fourth group of hemodiluted animals (HCon) served as hyperbaric controls and were ventilated with 10% O2 at 506 kPa, resulting in CaO2 of 9.1 +/- 0.6 ml O2/dl. CBF was 79 +/- 19 ml. 100 g-1. min-1 in the Con group and significantly increased to 123 +/- 9 ml. 100 g-1. min-1 in the HDil group. When CaO2 was restored to baseline with dissolved O2 in the HBar group, CBF decreased to 104 +/- 20 ml. 100 g-1. min-1. When normoxia was maintained during hyperbaric exposure in the HCon group, CBF was 125 +/- 18 ml. 100 g-1. min-1, a value indistinguishable from that in normobaric HDil animals. Our data demonstrate that the reduction in CaO2 after hemodilution is responsible for 40-60% of the increase in CBF.

Vesamicol receptor mapping of brain cholinergic neurons with radioiodine-labeled positional isomers of benzovesamicol

Alzheimer's disease is characterized by progressive cerebral cholinergic neuronal degeneration. Radiotracer analogs of benzovesamicol, which bind with high affinity to the vesamicol receptor located on the uptake transporter of acetylcholine storage vesicles, may provide an in vivo marker of cholinergic neuronal integrity. Five positional isomers of racemic iodobenzovesamicol (4'-, 5-, 6-, 7-, and 8-IBVM) were synthesized, exchange-labeled with iodine-125, and evaluated as possible in vivo markers for central cholinergic neurons. Only two isomers, 5-IBVM (5) and 6-IBVM (10), gave distribution patterns in mouse brain consistent with cholinergic innervation: striatum >> hippocampus > or = cortex > hypothalamus >> cerebellum. The 24-h tissue-to-cerebellum concentration ratios for 5-IBVM (5) were 3-4-fold higher for striatum, cortex, and hippocampus than the respective ratios for 6-IBVM (10). Neither 8-IBVM (16) nor 4'-IBVM (17) exhibited selective retention in any of the brain regions examined. In the heart, only 5-IBVM (5) exhibited an atria-to-ventricles concentration ratio consistent with high peripheral cholinergic neuronal selectivity. The 7-IBVM (14) isomer exhibited an anomalous brain distribution pattern, marked by high and prolonged retention in the five brain regions, most notably the cerebellum. This isomer was screened for binding in a series of 26 different biological assays; 7-IBVM (14) exhibited affinity only for the delta-receptor with an IC50 of approximately 30 nM. Drug-blocking studies suggested that brain retention of 7-IBVM (14) reflects high-affinity binding to both vesamicol and delta-receptors. Competitive binding studies using rat cortical homogenates gave IC50 values for binding to the vesamicol receptor of 2.5 nM for 5-IBVM (5), 4.8 nM for 6-IBVM (10), and 3.5 nM for 7-IBVM (14). Ex vivo autoradiography of rat brain after injection of (-)-5-[125I]IBVM ((-)-[125I]5) clearly delineated small cholinergic-rich areas such as basolateral amygdala, interpeduncular nucleus, and facial nuclei. Except for cortex, regional brain levels of (-)-5-[123I]IBVM ((-)-[123I]5) at 4 h exhibited a linear correlation (r2 = 0.99) with endogenous levels of choline acetyltransferase.

CONCLUSION

Vesamicol receptor mapping of cholinergic nerve terminals in murine brain can be achieved with 5-IBVM (5) and less robustly with 6-IBVM (10), whereas the brain localization of 7-IBVM (14) reflects high-affinity binding to both vesamicol and delta-receptors.

Animal models for studying transport across the blood-brain barrier

There are many reasons for wishing to determine the rate of uptake of a drug from blood into brain parenchyma. However, when faced with doing so for the first time, choosing a method can be a formidable task. There are at least 7 methods from which to choose: indicator dilution, brain uptake index, microdialysis, external registration, PET scanning, in situ perfusion, and compartmental modeling. Each method has advantages and disadvantages. Some methods require very little equipment while others require equipment that can cost millions of dollars. Some methods require very little technical experience whereas others require complex surgical manipulation. The mathematics alone for the various methods range from simple algebra to complex integral calculus and differential equations. Like most things in science, as the complexity of the technique increases, so does the quantity of information it provides. This review is meant to serve as a starting point for the researcher who wishes to study transport and uptake across the blood-brain barrier in animal models. An overview of the mathematical theory, as well as an introduction to the techniques, is presented.

Digital imaging of umbelliferone clearance: a method for repeated measurements of cerebral cortical blood flow with high temporal and spatial resolution

We have developed a procedure for digital imaging of the exposed cerebral cortex during elution of a fluorescent dye. This avoids disturbing the cortex and has provided a method for the repeated estimation of regional CBF (rCBF) with a high topographical resolution. Under varying conditions of MABP and arterial blood gases, grey-level images of the exposed cortex irradiated with ultraviolet light (340 or 370 nm) were digitised (8 bits) at 15-s intervals after the injection of 1-2 ml of saturated umbelliferone solution into the lingual or external carotid artery of anaesthetised cats and rabbits. Specifically designed software allowed (a) regions of interest (ROIs) in the exposed cortex to be defined that were automatically applied to the sequence of images in a selected clearance and (b) solution of the initial slope equation for rCBF from the decay in grey-level fluorescence by exponential regression. Separate software that solved the equation at the level of a single pixel allowed a pseudocolour map of cortical rCBF to be generated. The factors affecting the resolution of this technique have been identified and quantified. Thus consistent and reproducible results were obtained provided that the fluorescence enhancement exceeded 20 grey levels and the r2 coefficient for regression was 90% or above. Mean rCBF values of 99.5 [95% confidence interval (CI), 89.4-110] ml 100 g-1 min-1 were obtained for rabbits (N = 12; mean MABP = 75.2; mean PaCO2 = 32.9; PaO2 = 111.8; pH 7.38) and 65.1 (95% CI, 55.1-75.1) ml 100 g-1 min-1 for cats (N = 8; mean MABP = 92.8; PaCO2 = 31.5; PaO2 = 114.6, pH 7.40).(ABSTRACT TRUNCATED AT 250 WORDS)

PET studies of the uptake of (S)- and (R)-[11C]nicotine in the human brain: difficulties in visualizing specific receptor binding in vivo

(S)- and (R)-[11C]nicotine were synthesized by methylation of (S)- and (R)-nornicotine using [11C]methyl iodide. Following their intravenous injection in tracer doses to smoking and nonsmoking healthy males the radioactivity in arterial blood showed a sharp peak at about 1 min followed by a plateau level for the remaining 50 min of recording. Uptake in the brain, as measured by positron emission tomography (PET), was rapid with a peak at 5 min followed by a steady decline towards the end of the measurement. The regional distribution of radioactivity followed essentially the distribution of gray matter with high uptake in the cortex, the thalamus and the basal ganglia and low uptake in the pons, cerebellum and white matter. Levels of the labelled natural enantiomer, (S)-[11C]nicotine, were higher than those of the synthetic enantiomer, (R)-[11C]nicotine, particularly in the smokers. The time-activity curves of (S)-[11C]nicotine uptake were not changed by co-administration of 1.0 mg of unlabelled nicotine with the labelled nicotine. Similarly administration of unlabelled nicotine at the peak of radioactivity, 6 min following (S)-[11C]nicotine, had no effect on the time-activity curves. Thus essential criteria for visualizing receptor binding with the PET technique could not be fulfilled. Calculation of kinetic constants using a two-compartment model gave values indicating that the brain uptake of [11C]nicotine is mainly determined by the cerebral blood flow, extraction of the tracer over the blood-brain barrier and unspecific binding. Thus 11C-labelled nicotine does not seem to be a suitable tracer for PET studies of nicotinic cholinergic receptors in the human brain.

Microwave fixation for the determination of cerebral blood volume in rats

The cerebral blood volume (CBV) is sensitive to changing hydrostatic pressures. Thus, measurement methods that rely on removing tissue from unfixed brain may lead to underestimates of the CBV due to the loss of blood from the tissue. In situ fixation of tissue before removal may offer improved accuracy. We employed a triple-label method to measure simultaneously whole brain CBF and CBV in halothane-anesthetized Sprague-Dawley rats, which were then killed either by focused microwave irradiation (approximately 8 kW of incident power x 770 ms) or by decapitation. CBF was measured with [3H]nicotine while the CBV was determined as the sum of the cerebral red cell volume (CRCV--measured with 99mTc-labeled red cells) and the cerebral plasma volume (CPV--measured with [14C]dextran). Animals were studied during hypocarbic (PaCO2 approximately 25 mm Hg), normocarbic, or hypercarbic (PaCO2 approximately 70 mm Hg) conditions. Added studies were performed to verify that the microwave irradiation scheme used was capable of fixing previously administered tracers in place, and also halting the entry of tracer given after irradiation. Results indicate that the method of killing had no effects on CBF measurements, as assessed either by absolute values during normocarbia or responsiveness to changing PaCO2. However, all three volume measurements made using nondiffusible tracers (CRCV, CPV, and CBV) were significantly lower in animals killed by decapitation. Furthermore, CO2 responsiveness for all three variables (as assessed by the slope of the PaCO2/volume) was not evident in decapitated animals. We conclude that in situ fixation offers significant advantages when examining the cerebral distribution space of nondiffusible tracers.

Application of carbon-11 labelled nicotine in the measurement of human cerebral blood flow and other physiological parameters

Using positron emission tomography (PET), we measured the regional cerebral blood flow (rCBF) in five normal human subjects after intravenous injection of carbon-11 labeled (R)nicotine. The rCBF of the same subjects was measured by PET using the C15O2 inhalation steady-state method. The distribution of 11C activity in the brain after injection of 11C-(R)nicotine was almost equivalent to the CBF image obtained with the C15O2 inhalation stead-state method. The kinetics of 11C-(R)nicotine in the brain was analysed using a two-compartment model consisting of vascular and brain tissue compartments. The rCBF values obtained with 11C-(R)nicotine were higher than with C15O2 gas. The relatively long fixed distribution of 11C-(R)nicotine with a short uptake period allows stimulation studies by measurement of CBF to be performed with better photon flux and a longer imaging time than are possible with H215O.

A quantitative method for measuring regional in vivo fatty-acid incorporation into and turnover within brain phospholipids: review and critical analysis

An experimental method and its associated mathematical model are described to quantitate in vivo incorporation rates into and turnovers of fatty acids (FAs) within stable brain metabolic compartments, particularly phospholipids. A radiolabeled FA is injected i.v. in a rat, and arterial plasma unacylated FA radioactivities and unlabeled concentrations are sampled until the animal is killed after 15 min, when the brain is analyzed biochemically or with quantitative autoradiography. Unbound unacylated label in blood easily crosses the blood-brain barrier; rapidly equilibrates in the unacylated FA, acyl-CoA and phosphatidate-diacylglycerol brain pools; then is incorporated into phospholipids and other stable metabolic compartments. Uptake and incorporation of labeled FAs are independent of cerebral blood flow at constant brain blood volume. Different labeled FAs enter specific sn positions of different brain phospholipids, suggesting that a combination of probes can be used to investigate metabolism of these phospholipids. Thus, [9,10-3-H]palmitate preferentially labels the sn1 position of phosphatidylcholine; [1-14C]arachidonate the sn2 positions of phosphatidylinositol and phosphatidylcholine; and [1-14C]docosahexaenoate the sn2 positions of phosphatidylethanolamine and phosphatidylcholine. The FA model provides an operational equation for rates of incorporation of FAs into brain phospholipids, taking into account intracerebral recycling and de novo synthesis of the FA, as well as entry into brain of FA from acylated blood sources. The equation is essentially independent of specific details of the proposed model, and can be used to calculate turnovers and half-lives of FAs within different phospholipid classes. For the model to be most applicable, experiments should satisfy conditions for pulse-labeling of the phospholipids, with brain sampling times short enough to minimize exchange of label between stable metabolic compartments. A 15-20 min sampling time satisfies these criteria. The FA method has been used to elucidate the dynamics of brain phospholipids metabolism in relation to brain development, brain tumor, chronically reduced auditory input, transient ischemic insult, axotomy with and without nerve regeneration, and cholinergic stimulation in animals with or without a chronic unilateral lesion of the nucleus basalis magnocellularis.

Responses of regional cerebral blood flow to intravenous administration of thyrotropin releasing hormone in aged rats

The effects of i.v. administration of thyrotropin releasing hormone (TRH) on regional cerebral blood flow (rCBF) were examined in both healthy adult (3-5 months old) and healthy aged (24-25 months old) male Wistar rats under halothane anesthesia. The rCBFs in 9 different brain regions-cerebral cortex, caudate putamen, hippocampus, thalamus + hypothalamus, superior colliculus, inferior colliculus, cerebellum, pons, and medulla-were measured by [14C]iodoantipyrine method. In the adult rats, i.v. administration of TRH (300 micrograms/kg) produced significant increases in rCBFs in cerebral cortex, caudate putamen, hippocampus, thalamus + hypothalamus and superior colliculus. In the aged rats, the rCBFs in all brain regions measured did not change significantly by TRH administration. From these results, it is suggested that the system involved in TRH-induced vasodilatation of cerebral blood vessels was impaired with aging.

The role of electrode size on the incidence of spreading depression and on cortical cerebral blood flow as measured by H2 clearance

Cerebral blood flow was measured by the H2 clearance method 30 and 60 min after the implantation of 300, 250, 125, or 50 microns diameter platinum-iridium electrodes 2 mm deep into the right parietal cortex of normothermic, normocarbic halothane-anesthetized rats. Another group of animals had 50 microns electrodes inserted 1 mm. In all animals, the presence or absence of a wave of spreading depression (SD) was noted at the time of implantation, with recordings made with glass micropipettes. H2 flow values were compared with those measured in gray matter from the same anatomical region (but from different rats), using [3H]nicotine. The incidence of SD ranged from 60% following insertion of 300 microns electrodes to 0% with 50 microns electrodes. H2 clearance flows also varied with electrode size, from 77 +/- 21 ml 100 g-1 min-1 (mean +/- standard deviation) with 300 microns electrodes to 110 +/- 31 and 111 +/- 16 ml 100 g-1 min-1 with 125 and 50 microns electrodes, respectively (insertion depth of 2 mm). A CBF value of 155 +/- 60 ml 100 g-1 min-1 was obtained with 50 micron electrodes inserted only 1 mm. Cortical gray matter blood flow measured with [3H]nicotine was 154 +/- 35 ml 100 g-1 min-1. When the role of SD in subsequent flow measurements was examined, there was a gradual increase in CBF between 30 and 60 min after electrode insertion in those animals with SD, while no such change was seen in rats without SD.(ABSTRACT TRUNCATED AT 250 WORDS)

Vascular factors in the neurotoxic damage caused by 1,3-dinitrobenzene in the rat

Using a 3 x 10 mg/kg dose schedule of 1,3-dinitrobenzene (DNB) over two days in Fischer rats, we have found the following changes in vascular function and structure during the early phase of the symmetrical brain stem lesions. 1. Marked increase in cerebral blood flow generally but especially in the inferior colliculi, from 6 h after the final dose of DNB. 2. Increasing incidence of petechial haemorrhages in inferior colliculi, cerebellar roof, vestibular and superior olivary nuclei from 12 h. 3. Focal leakage of horseradish peroxidase and many sleeve-like arteriolar haemorrhages seen in vibratome sections and by scanning electron microscopy (SEM) in these regions from 12 h. 4. Periarteriolar oedema and protein leakage present in step-serial sections in these regions from 12 h, with astrocyte swelling and occasional small infarcts. These changes suggest that the vascular bed may play an important role in the pathogenesis of these lesions, perhaps in parallel with early astroglial damage. They are discussed in relation to (i) the known presence of xanthine oxidase in the vascular bed of the brain and the likelihood of "useless redox cycling' with free radical generation from this enzyme's interaction with nitroheterocyclic compounds, and (ii) the possible role of free radical damage to endothelial cells in this intoxication and in the analogous lesions of natural and experimental Wernicke's encephalopathy.

Cortical cerebral blood flow governed by the basal forebrain: age-related impairments

This study sought to compare resting and evoked increases in cortical microvascular perfusion elicited by electrical microstimulation of the basal forebrain (BF) in young (4-6 months) and aged (22-26 months) Sprague-Dawley rats. Regional cerebral blood flow (rCBF) was measured in chloralose-anesthetized rats for twelve bilateral regions using 14C-iodoantipyrine with regional brain dissection, while second-to-second changes in tissue perfusion were concurrently assessed using laser-doppler flowmetry (LDF). In young animals, BF stimulation elicited significant ipsilateral increases in CBF in parietal (+123%) and frontal (+107%) cortices, caudate nucleus (+63%) and thalamus (+59%) (p less than 0.05). The BF-elicited increases were preserved in frontal cortex and thalamus, but not in parietal cortex or caudate nucleus of aged animals. No frequency- or current-specific attenuations were observed in the spared frontal cortex of aged animals. However, there was a significant (+70%) age-related increase in the latency to reach maximal blood flow increases (p less than 0.05), without any change in the total time of increased blood flow. These findings support the hypothesis that cortical CBF is in part governed by BF neurons, and suggest that regionally selective, age-related impairments of cortical coupling of neuronal to dynamic vascular responses exist. It remains to be determined whether the mechanism of this impairment relates to an age-related impairment in coupling of blood flow and metabolism.

The energy metabolism of RIF-1 tumours following hydralazine

Phosphorus-31 Magnetic Resonance Spectroscopy (MRS) was used to observe the effect of two doses of the vasodilator hydralazine on the energy status of RIF-1 tumours. An intravenous dose of 5 mg/kg hydralazine reduced the high energy phosphate metabolites PCr and ATP, lowered pHMRS and raised the levels of inorganic phosphate of tumours within 20 min of administering the drug. The levels of high energy metabolites continued to decrease for at least 24 h. Normal muscle spectra obtained up to 1 h after drug administration remained unchanged. An intravenous dose of 0.5 mg/kg hydralazine also reduced NTP/Pi and PCr/Pi levels of tumours up to at least 5 h after drug administration, but the effect was smaller than for the higher dose. Blood flow measurements and measurements of systemic blood pressure demonstrated that 5 mg/kg of hydralazine produced a reduction in both systemic blood pressure and tumour blood flow relative to most normal tissues investigated. It is concluded that the changes in the P-31 MRS spectra of tumours were due to a reduction in tumour vascular perfusion following administration of hydralazine.

Responses of regional cerebral blood flow following focal electrical stimulation of the nucleus basalis of Meynert and the medial septum using the [14C]iodoantipyrine method in rats

The effects of focal electrical stimulation of the nucleus basalis of Meynert (NBM) and the medial septum (MS) on regional cerebral blood flow (rCBF) of the 14 brain regions were examined in halothane-anesthetized rats using the [14C]iodoantipyrine ([14C]IAP) method. The stimulation of the unilateral NBM (with parameters of 200 microA, 0.5 ms, 50 Hz for 60 s) produced significant increases in frontal, parietal and occipital cortical blood flows in the hemisphere ipsilateral to the stimulated NBM; no rCBFs in all other brain regions examined were influenced by the stimulation. The stimulation of the MS produced significant increases in bilateral hippocampal rCBFs, but rCBFs in other brain regions were not influenced by the stimulation. In summary, the response of increase in rCBF following focal electrical stimulation of the NBM or MS is restricted to regions that receive cholinergic nerve projections from the NBM or MS.

Dissociation by chloralose of the cardiovascular and cerebrovascular responses evoked from the cerebellar fastigial nucleus

We studied the effects of chloralose anesthesia on the elevation in arterial pressure (AP), heart rate (HR), and regional CBF (rCBF) elicited by stimulation of the cerebellar fastigial nucleus (FN). Rats were anesthetized with an initial dose of chloralose (40 mg/kg s.c.), paralyzed, and artificially ventilated. The FN was stimulated (50-100 microA, 50 Hz, 1 s on/1 s off) with microelectrodes stereotaxically implanted. During the stimulation AP was carefully maintained within cerebrovascular autoregulation. CBF was measured by the [14C]iodoantipyrine technique with regional dissection. In rats that received only the initial dose of chloralose, FN stimulation elevated rCBF in brain and spinal cord, up to 209 +/- 13% of control in frontal cortex (n = 5; p less than 0.01, analysis of variance). Administration of additional chloralose (10 mg/kg i.v., 30 min prior to measurement of CBF) did not affect resting rCBF (n = 5), the EEG, or the elevation in AP and HR elicited by FN stimulation (n = 4). However, the additional chloralose abolished the elevations in rCBF (n = 5; p greater than 0.05). Thus, the cerebrovasodilation elicited from the FN is more susceptible to the effects of additional anesthesia than the elevation in AP and HR. These results indicate that the cerebrovascular and cardiovascular responses elicited from the FN are functionally distinct and provide additional evidence for the notion that these responses are mediated by different neural pathways and transmitters.

Blood flow and blood volume in a transplanted rat fibrosarcoma: comparison with various normal tissues

Blood flow measurements following i.v. infusion of iodo-antipyrine labelled with 14C(14C-IAP) and blood volume measurements following i.v. injection of 125I human serum albumin and 51Cr-labelled red blood cells were made in a transplanted rat fibrosarcoma for comparison with various normal tissues. The tumour-blood partition co-efficient for 14C-IAP was found to be 0.79 +/- 0.07 which is similar to most of the normal tissues studied. The solubility of 14C-IAP in plasma was found to be higher than that in whole blood. Blood flow to tumours less than 1000 mm3 was found to be 17.9 +/- 4.0 ml blood 100 g tissue-1.min-1. These values were considered to be primarily measurements of nutritive flow. Blood in the tumours was found to occupy around 1% of the tissue space which was similar to that found for normal muscle and skin. There was no direct correlation between % blood volume and blood flow for the different tissues studied. The haematocrit of blood contained in tumour tissue was calculated to be significantly lower than that of blood contained in the normal tissues. It was suspected that permeability of tumour blood vessel walls to 125I-HSA could have accounted for this difference.

Lesions of rostral ventrolateral medulla abolish some cardio- and cerebrovascular components of the cerebellar fastigial pressor and depressor responses

We sought to establish whether the C1 area of the rostral ventrolateral reticular nucleus (RVL) of the medulla oblongata mediates: (1) the elevations in arterial pressure (AP), heart rate (HR) and regional cerebral blood flow (rCBF) elicited by electrical stimulation of the rostral cerebellar fastigial nucleus (FN), the fastigial pressor response (FPR); (2) the reductions in AP and HR elicited by chemical stimulation of intrinsic neurons of FN with excitatory amino acids, the fastigial depressor response (FDR). Studies were conducted on rats anesthetized (chloralose), paralyzed and artificially ventilated. The FN was stimulated electrically through microelectrodes and chemically by microinjection of D.L-homocysteic acid (100 nmol in 100 nl). rCBF was measured in homogenates of 11 brain regions by the 14C-iodoantipyrine technique. Bilateral electrolytic lesions restricted to the RVL abolished the elevations in AP, HR and rCBF elicited by electrical stimulation as well as the fall of AP and HR elicited by chemical stimulation of the FN. The disappearance of the responses was anatomically selective and could not be attributed to changes in resting AP, HR or rCBF, loss of reactivity of preganglionic sympathetic neurons, or variations in blood gases. Since the FN neither projects to nor receives afferents from the RVL the pathway to RVL is indirect. We conclude that: (1) the FPR results from excitation and the FDR inhibition of reticulospinal sympathoexcitatory axons of RVL; (2) the FPR is a consequence of excitation of axons arising from neurons in an as yet unidentified area of lower brainstem projecting to or through the FN and with collateral branches innervating RVL mono- or polysynaptically; (3) the FDR, in contrast, represents excitation of intrinsic FN neurons with a polysynaptic projection to RVL through unidentified regions of lower brainstem; (4) the RVL is a relay mediating the increase in rCBF associated with the FPR; and (5) the RVL plays a critical role in integrating actions on the systemic and cerebral circulation represented in cerebellum.

Sphenopalatine ganglion stimulation increases regional cerebral blood flow independent of glucose utilization in the cat

Regional cerebral blood flow was determined using the tracer [14C]iodoantipyrine and regional brain dissection, and regional cerebral glucose utilization determined using the 2-deoxyglucose method, in the alpha-chloralose-anesthetized cat to evaluate the effect of electrical stimulation of the sphenopalatine (pterygopalatine) ganglion. Unilateral stimulation for either a short period (7-10 min) or a longer period (45 min) resulted in increases in blood flow in the ipsilateral cerebral cortex of up to 45% (parietal cortex) with, in addition, increased flow in the white matter of the corpus callosum (42%). The flow changes for both brief and prolonged stimulation were not significantly different. Flow was not altered in either the brainstem or basal ganglia (caudate nucleus). In contrast to these changes in cerebral blood flow no changes in cerebral glucose utilization were seen in any of the brain areas studied and in particular there were no changes in the areas in which blood flow increased. These data provide clear evidence that the innervation of the cerebral vasculature from the main parasympathetic ganglion can alter cerebral blood flow independent of cerebral metabolism.

Global reduction in cerebral blood flow and metabolism elicited from intrinsic neurons of fastigial nucleus

We sought to determine whether the global increase in regional cerebral blood flow (rCBF) produced by electrical stimulation of the rostral cerebellar fastigial nucleus (FN) is a consequence of excitation of intrinsic neurons of the FN or of axons of fibers passing through or projecting into it. Studies were conducted in rats anesthetized with chloralose, paralyzed and ventilated. rCBF was measured with [14C]iodoantipyrine as tracer and regional cerebral glucose utilization (rCGU) by [14C]2-deoxyglucose in homogenates of 11 brain regions. Neuronal perikarya in FN were excited chemically by local microinjection of the glutamate analogue kainic acid (KA) (5 nmol in 100 nl). KA elicited a transient and significant fall of arterial pressure and heart rate, the fastigial depressor response (FDR). Associated was a significant and symmetrical reduction in rCBF, to 44% of control in all regions except medulla. The response was site- and agent-specific and unrelated to the hypotension. KA also significantly and proportionally reduced, to 52% of control, rCGU in the same 10 areas of brain. In all regions the magnitudes of the reductions in rCBF and rCGU elicited by KA were linearly related. Intrinsic neurons of FN were chronically destroyed by local microinjection of the excitotoxin ibotenic acid (IBO) (10 micrograms/microliters in 0.4 microliter). Destruction of intrinsic FN neurons had no effect on resting rCBF nor on the global cerebrovascular vasodilation elicited by electrical stimulation of the FN. We conclude that: (a) excitation of intrinsic neurons of FN elicits a widespread reduction of cerebral metabolism and, secondarily, blood flow; (b) FN neurons do not exert a long-term tonic influence on brain blood flow nor metabolism; (c) the global increase in rCBF elicited by electrical stimulation of the FN is a consequence of excitation of axons projecting into or through the nucleus.

A model of the pathophysiology of cerebral arteriovenous malformations by a carotid-jugular fistula in the rat

A model of a carotid-jugular fistula in the rat was created such that the arterial feeding vessel is derived from the intracranial arterial circulation and the venous drainage communicates with a major intracranial venous drainage system. This fistula was created in 28 rats on the right side with an additional 11 rats designed as controls with a right carotid ligation and 6 rats without previous surgery. After 12 weeks convalescence, 6 rats with a carotid-jugular fistula and 6 rats without previous surgery underwent cerebral angiography. All fistulae were patent and the model was verified. All of the 33 remaining rats underwent regional cerebral blood flow (rCBF) determination by [14C]iodoantipyrine autoradiography under barbiturate anesthesia. Of the rats with the fistula, 11 had this fistula obliterated 5 min prior to rCBF determination ('closed' group) while 11 had rCBFs determined with the fistula ('open' group). The rCBF was measured from each hemisphere from 7 anatomical regions. The rCBF in the control animals ranged from a median of 82 to 112 ml/100 g/min, in the 'open fistula' group 46 to 68 ml/100 g/min, and in the 'closed' group 118 to 187 ml/100 g/min. This experimental model stimulates the pathophysiologic perturbations in the parenchyma induced by cerebral arteriovenous malformations. It supports the findings that non-infarctional hypoperfusion can result from arteriovenous malformations and that following extirpation of arteriovenous malformations hyperemia may ensue.

Effect of acute electrode placement on regional CBF in the gerbil: a comparison of blood flow measured by hydrogen clearance, [3H]nicotine, and [14C]iodoantipyrine techniques

Regional cerebral blood flow (rCBF) was compared in the gerbil by means of [3H]nicotine, [14C]-iodoantipyrine, and hydrogen clearance techniques. In agreement with other studies, nicotine and iodoantipyrine methods gave virtually identical results. With these methods, it was observed that a reduction in blood flow occurred shortly after insertion of an electrode into the striatum for hydrogen clearance measurement, affecting rCBF throughout the impaled hemisphere. The reduction was moderate (30%) in the striatum and hippocampus, but much greater (70%) in cortical regions. Identical deficits were observed following brief penetrations involving only cortex. Following chronic electrode placement in the striatum, regional blood flow values obtained with [3H]nicotine returned to the control range within 6 h. Blood flow estimates obtained in the striatum with the implanted electrode increased with a similar time course, so that by 6-24 h, hydrogen clearance gave values indistinguishable from control values obtained with [3H]nicotine. These results clearly demonstrate that reduction of CBF subsequent to electrode placement can account for the low values frequently obtained with the hydrogen clearance method in small animals. The distribution of the deficit and the time course of its recovery are similar to blood flow changes associated with spreading depression. While mechanisms responsible for this effect remain to be fully identified, chronic implantation is a practical solution that allows the continued use of hydrogen clearance as a convenient method for repeated measurement of blood flow in the same animal.

Low frequency stimulation of the locus coeruleus reduces regional cerebral blood flow in the spinalized cat

Regional cerebral blood flow (RCBF) was studied during low frequency (15/s) and high frequency (50/s) electrical stimulation of the locus coeruleus (LC) in the alpha-chloralose-anesthetized cat using the freely diffusible tracer [14C]iodoantipyrine and regional brain dissection. The responses were determined in animals spinalized at the C1/C2 level to eliminate systemic effects of pontine stimulation such as alterations in blood pressure and heart rate. The spinalization, itself, did not alter resting RCBF or reactivity to hypercapnia. Low frequency stimulation reduced regional cerebral blood flow in the cortex, basal ganglia and white matter of the corpus callosum. The reductions in RCBF were maximal (35%) in the occipital cortex whereas no changes were seen in the colliculi. No changes were seen in any brain areas with high frequency stimulation. The relevance of this brainstem effect on cerebral blood flow to pathological states such as stroke and migraine is discussed.

In vivo specific binding of [3H]1-nicotine in the mouse brain

[3H] 1-Nicotine was used as a receptor ligand in the intact mouse. It was injected i.v., and radioactivity in brain regions was assayed. Nonspecific binding was estimated by pretreatment with unlabelled 1-nicotine. Radioactivity entered the brain rapidly, was heterogeneously distributed, and declined after 5 min. Estimated specific binding was highest in the medial and posterior cortex, midbrain, thalamus/hypothalamus and medulla/pons; intermediate in the cerebellum, caudate/putamen, frontal and frontoparietal cortex; and lowest in the hippocampus and olfactory bulb. Autoradiography showed similar patterns. Coinjection of unlabelled 1-nicotine reduced specific binding so that it approached estimated nonspecific binding. Nicotinic agonists reduced radioactivity in the thalamus/hypothalamus, but nicotinic antagonists were less active. Non-nicotinic drugs did not reduce brain radioactivity. The results suggest that radiolabelled nicotine may be used for in vivo receptor studies despite problems in estimating nonspecific binding.

Regional cerebral glucose metabolism and blood flow during the silent phase of methylmercury neurotoxicity in rats

Methylmercuric chloride was given to rats in a neurotoxic dose regimen (six daily doses of 8 mg kg-1 p.o.). During the silent (asymptomatic) phase of intoxication, the rates of cerebral glucose influx and cerebral glucose phosphorylation were measured simultaneously using 2-deoxyglucose. Regional cerebral blood flow was also measured using iodoantipyrine. The unidirectional flux of glucose into brain was not affected by methylmercury, and differences in the rates of glucose phosphorylation from region to region remained coupled to the regional cerebral blood flow. However, the blood flow was reduced throughout the brain, an observation suggesting that the operational level of metabolically regulated blood flow had been reset. Thus, in spite of a generalised reduction in blood flow, there was no indication of impaired cerebral glucose supply or utilization during the silent phase of methylmercury intoxication.

Effect of monoamine oxidase inhibition on the regional cerebral blood flow response to circulating noradrenaline

The effect of an acute i.v. infusion of noradrenaline (NA) on regional cerebral blood flow (rCBF) was investigated in the awake rat using [14C]iodoantipyrine as diffusible tracer. The contribution of vascular monoamine oxidase (MAO) to the efficiency of the enzymatic blood-brain barrier (BBB) to catecholamines was assessed by measuring the multiregional cerebrovascular response to circulating NA given alone or after i.v. administration of the monoamine oxidase inhibitor, clorgyline. Since i.v. infusion of NA elevates blood pressure, the influence of NA on the cerebrovascular bed was first studied by determining the relationship between rCBF and the mean arterial pressure (MAP). When the MAP was only slightly increased (to approximately 130 mm Hg), a trend to flow decrease under NA infusion was observed. Secondly, we compared the effects of NA on rCBF in animals treated or not treated with clorgyline. This was performed under moderate hypertension (within the 'autoregulated' range of MAP) to avoid any risk of mechanical damage to the BBB. Clorgyline administration alone did not significantly modify rCBF, but the subsequent i.v. infusion of NA induced an increase in rCBF (weighted mean 14%) in all structures investigated. The differences being statistically significant (P less than 0.05) in 5 out of 13 structures by up to 20%. Compared to studies involving disruption of the morphological BBB in which plasma NA elicits a widespread important increase in blood flow, the weak cerebrovascular effects we observed provide indirect evidence for the efficiency of the BBB to catecholamines in the conscious rat within the autoregulated range of arterial pressure.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparative effects of stimulation of the trigeminal ganglion and the superior sagittal sinus on cerebral blood flow and evoked potentials in the cat

The superior sagittal sinus (SSS) and the trigeminal ganglion (Vg) of anesthetized cats were stimulated electrically and field potentials in the upper cervical spinal cord and regional cerebral blood flow were recorded. Stimulation of the entire ganglion produced smaller field potential changes in two regions (medioventral area (MVA); dorsolateral area (DLA] of the upper spinal cord than did stimulation of the sagittal sinus (Vg/SSS response ratio = 17% for the MVA and 48% for the DLA). Stimulation of the trigeminal ganglion increased blood flow in only the frontal and parietal cortices (+93% and +33%), whereas stimulation of the sinus produced both larger changes in these areas (+137% and +139%) and also produced changes in regional cerebral blood flow in the thalamus (+122%).

Tracer 2-deoxyglucose kinetics in brain regions of rats given kainic acid

The initial distribution of tracer amounts of 2-deoxyglucose between plasma and brain tissue, relative to native glucose, and the rate of accumulation of 2-deoxyglucose-6-phosphate were determined in brain regions of rats given kainic acid intravenously. Regional plasma flow was measured in a comparable group of animals. A previously described compartmental model was used to obtain estimates of rates of glucose transport and of glucose phosphorylation. Both rates were significantly increased in entorhinal cortex, hippocampus, amygdala, and septal nucleus. From measured brain tissue and plasma glucose concentrations, glucose fluxes were also calculated in terms of either irreversible or reversible Michaelis-Menten kinetics. In all brain regions of control rats and in six of the ten regions studied in rats given kainic acid, rates of glucose transport calculated in terms of the Michaelis-Menten models were consistent with those estimated by the tracer 2-deoxyglucose procedure. However, in the four regions in which glucose metabolism was stimulated, rates of glucose transport calculated from the behaviour of tracer 2-deoxyglucose were considerably higher than rates calculated from measured concentrations of glucose in plasma and brain tissue using Michaelis-Menten models. The possibility is considered that in those regions that are metabolically stimulated by kainate, there is an increasing asymmetry between the luminal and abluminal membranes of the capillary endothelium in the permeability to glucose and its analogs. An alternative proposal is that in the model used to analyse the tracer 2-deoxyglucose data, the assumption of a rapid mixing of tracer throughout the endogenous pool of tissue glucose prior to phosphorylation becomes invalid. The discrepancies between tracer and native glucose in these particular regions of rats given kainate are consistent with an apparent metabolic compartmentation. The influence of kainate on plasma flow was found to differ regionally, with flow in entorhinal cortex, hippocampus, and amygdala being unchanged. There is some evidence for increased rates of glycolysis relative to oxidative metabolism in these regions.

Experimental model for repetitive ischemic attacks in the gerbil: the cumulative effect of repeated ischemic insults

An experimental model for repeated ischemic attacks, which allows easy induction of cerebral ischemia of any desired duration and frequency, has been developed in the gerbil. With this procedure, a pronounced cumulative effect on development of edema and tissue injury was observed using 3 separate, 5-min bilateral occlusions of the common carotid arteries spaced at various time intervals. This effect was most evident when the occlusions were carried out at 1-h intervals, i.e., during the period of marked postischemic hypoperfusion. Such animals, killed after 24 h of recirculation, showed significantly more severe edema and brain tissue injury in the areas exposed to ischemia than was observed in animals killed 24 h after single 5- or 15-min occlusions. The changes of regional CBF, assayed with a [3H]nicotine method, indicated a relatively rapid onset of hypoperfusion of similar degree after each release of arterial occlusion. The hypoperfusion recovered significantly within 6 h of recirculation following either single or multiple occlusions, and no residual hypoperfusion was observed in animals which, when killed at 24 h, showed severe edema and brain tissue injury. This model should prove useful in elucidating the pathophysiological mechanisms operative in repetitive cerebral ischemia.

Tracer disposition kinetics in the determination of local cerebral blood flow by a venous equilibrium model, tube model, and distributed model

Tracer distribution kinetics in the determination of local cerebral blood flow (LCBF) were examined by using three models, i.e., venous equilibrium, tube, and distributed models. The technique most commonly used for measuring LCBF is the tissue uptake method, which was first developed and applied by Kety (1951). The measurement of LCBF with the 14C-iodoantipyrine (IAP) method is calculated by using an equation derived by Kety based on the Fick's principle and a two-compartment model of blood-tissue exchange and tissue concentration at a single data point (Sakurada et al., 1978). The procedure, in which the tissue is to be in equilibrium with venous blood, will be referred to as the tissue equilibration model. In this article, effects of the concentration gradient of tracer along the length of the capillary (tube model) and the transverse heterogeneity in the capillary transit time (distributed model) on the determination of LCBF were theoretically analyzed for the tissue sampling method. Similarities and differences among these models are explored. The rank order of the LCBF calculated by using arterial blood concentration time courses and the tissue concentration of tracer based on each model were tube model (model II) less than distributed model (model III) less than venous equilibrium model (model I). Data on 14C-IAP kinetics reported by Ohno et al. (1979) were employed. The LCBFs calculated based on model I were 45-260% larger than those in models II or III. To discriminate among three models, we propose to examine the effect of altering the venous infusion time of tracer on the apparent tissue-to-blood concentration ratio (lambda app). A range of the ratio of the predicted lambda app in models II or III to that in model I was from 0.6 to 1.3. In the future, there may be a need to determine which model should be used to calculate the LCBF based on this discriminator and to develop another discriminator by using multiple data points based on positron emission tomography.

Radiolabeled hypoxic cell sensitizers: tracers for assessment of ischemia

Hypoxic, non-functional, but viable, tissue may exist in heart and brain following an arterial occlusion. Identification of such tissue in vivo is crucial to the development of effective treatment strategies. It has been suggested that certain compounds capable of sensitizing hypoxic tumor cells to killing by x-rays (i.e., misonidazole) might serve as in vivo markers of hypoxic tissue in ischemic myocardium or brain if properly radiolabeled. To this end we have radiolabeled two fluorinated analogs of nitroimidazole based hypoxic cell sensitizers with the 110 minute half-lived positron-emitting fluorine-18. The ability of these tracers to quantitate the presence of hypoxic tissue has been studied in a gerbil stroke model. The in vivo uptake of one of these tracers [F-18]-fluoronormethyoxymisonidazole is dependent on the extent of tissue hypoxia, and thus, appears to have potential as a diagnostic indicator of non-functional but viable tissue when the tracer is used in conjunction with positron emission tomography.