Changes in local cerebral blood flow and neuronal activity during sensory stimulation in normal and sympathectomized cats

Neuronal regulation of the blood-brain barrier and neurovascular coupling

To continuously process neural activity underlying sensation, movement and cognition, the CNS requires a homeostatic microenvironment that is not only enriched in nutrients to meet its high metabolic demands but that is also devoid of toxins that might harm the sensitive neural tissues. This highly regulated microenvironment is made possible by two unique features of CNS vasculature absent in the peripheral organs. First, the blood-blood barrier, which partitions the circulating blood from the CNS, acts as a gatekeeper to facilitate the selective trafficking of substances between the blood and the parenchyma. Second, neurovascular coupling ensures that, following local neural activation, regional blood flow is increased to quickly supply more nutrients and remove metabolic waste. Here, we review how neural and vascular activity act on one another with regard to these two properties.

A Spatiotemporal Profile of In Vivo Cerebral Blood Flow Changes Following Intranasal Oxytocin in Humans

BACKGROUND

Animal and human studies highlight the role of oxytocin in social cognition and behavior and the potential of intranasal oxytocin (IN-OT) to treat social impairment in individuals with neuropsychiatric disorders such as autism. However, extensive efforts to evaluate the central actions and therapeutic efficacy of IN-OT may be marred by the absence of data regarding its temporal dynamics and sites of action in the living human brain.

METHODS

In a placebo-controlled study, we used arterial spin labeling to measure IN-OT-induced changes in resting regional cerebral blood flow (rCBF) in 32 healthy men. Volunteers were blinded regarding the nature of the compound they received. The rCBF data were acquired 15 min before and up to 78 min after onset of treatment onset (40 IU of IN-OT or placebo). The data were analyzed using mass univariate and multivariate pattern recognition techniques.

RESULTS

We obtained robust evidence delineating an oxytocinergic network comprising regions expected to express oxytocin receptors, based on histologic evidence, and including core regions of the brain circuitry underpinning social cognition and emotion processing. Pattern recognition on rCBF maps indicated that IN-OT-induced changes were sustained over the entire posttreatment observation interval (25-78 min) and consistent with a pharmacodynamic profile showing a peak response at 39-51 min.

CONCLUSIONS

Our study provides the first visualization and quantification of IN-OT-induced changes in rCBF in the living human brain unaffected by cognitive, affective, or social manipulations. Our findings can inform theoretical and mechanistic models regarding IN-OT effects on typical and atypical social behavior and guide future experiments (e.g., regarding the timing of experimental manipulations).

Movement and afferent representations in human motor areas: a simultaneous neuroimaging and transcranial magnetic/peripheral nerve-stimulation study

Neuroimaging combined with transcranial magnetic stimulation (TMS) to primary motor cortex (M1) is an emerging technique that can examine motor-system functionality through evoked activity. However, because sensory afferents from twitching muscles are widely represented in motor areas the amount of evoked activity directly resulting from TMS remains unclear. We delivered suprathreshold TMS to left M1 or gave electrical right median nerve stimulation (MNS) in 18 healthy volunteers while simultaneously conducting functional magnetic resonance imaging and monitoring with electromyography (EMG). We examined in detail the localization of TMS-, muscle afferent- and superficial afferent-induced activity in M1 subdivisions. Muscle afferent- and TMS-evoked activity occurred mainly in rostral M1, while superficial afferents generated a slightly different activation distribution. In 12 participants who yielded quantifiable EMG, differences in brain activity ascribed to differences in movement-size were adjusted using integrated information from the EMGs. Sensory components only explained 10–20% of the suprathreshold TMS-induced activity, indicating that locally and remotely evoked activity in motor areas mostly resulted from the recruitment of neural and synaptic activity. The present study appears to justify the use of fMRI combined with suprathreshold TMS to M1 for evoked motor network imaging.

Hemodynamic and electrophysiological spontaneous low-frequency oscillations in the cortex: directional influences revealed by Granger causality

We used a combined electrophysiological/hemodynamic system to examine low-frequency oscillations (LFOs) in spontaneous neuronal activities (spike trains and local field potentials) and hemodynamic signals (cerebral blood flow) recorded from the anesthetized rat somatosensory and visual cortices. The laser Doppler flowmetry (LDF) probe was tilted slightly to approach the area in which a microelectrode array (MEA) was implanted for simultaneous recordings. Spike trains (STs) were converted into continuous-time rate functions (CRFs) using the ST instantaneous firing rates. LFOs were detected for all three of the components using the multi-taper method (MTM). The frequencies of these LFOs ranged from 0.052 to 0.167 Hz (mean±SD, 0.10±0.026 Hz) for cerebral blood flow (CBF), from 0.027 to 0.26 Hz (mean±SD, 0.12±0.041 Hz) for the CRFs of the STs and from 0.04 to 0.19 Hz (mean±SD, 0.11±0.035 Hz) for local field potentials (LFPs). We evaluated the Granger causal relationships of spontaneous LFOs among CBF, LFPs and CRFs using Granger causality (GC) analysis. Significant Granger causal relationships were observed from LFPs to CBF, from STs to CBF and from LFPs to STs at approximately 0.1 Hz. The present results indicate that spontaneous LFOs exist not only in hemodynamic components but also in neuronal activities of the rat cortex. To the best of our knowledge, the present study is the first to identify Granger causal influences among CBF, LFPs and STs and show that spontaneous LFOs carry important Granger causal influences from neural activities to hemodynamic signals.

Visceral influences on brain and behavior

Mental processes and their neural substrates are intimately linked to the homeostatic control of internal bodily state. There are a set of distinct interoceptive pathways that directly and indirectly influence brain functions. The anatomical organization of these pathways and the psychological/behavioral expressions of their influence appear along discrete, evolutionarily conserved dimensions that are tractable to a mechanistic understanding. Here, we review the role of these pathways as sources of biases to perception, cognition, emotion, and behavior and arguably the dynamic basis to the concept of self.

Basal forebrain cholinergic lesions reduce heat shock protein 72 response but not pathology induced by the NMDA antagonist MK-801 in the rat cingulate cortex

Non-competitive N-methyl-D-aspartate (NMDA) antagonists, in addition to their neuroprotective potential, possess neurotoxic properties and induce seizures and psychosis. MK-801 induces cytoplasmic vacuoles and heat shock protein in pyramidal neurones in the rodent posterior cingulate and retrosplenial cortex. The mechanism of this neurotoxicity is unclear, involving many neurotransmitter systems. The aim of this study was to investigate the role of cholinergic pathways from the nucleus basalis of Meynert in mediating MK-801-induced neurotoxicity. Cholinergic projections from the nucleus basalis of Meynert were lesioned by focal injection of 192-IgG-saporin (80 ng), which after 7 days reduced the number of cholinergic cell bodies by 70% in the lesioned nucleus compared to the uninjected nucleus. Following a unilateral cholinergic lesion, MK-801 (5 mg/kg s.c.) induced expression of hsp72 mRNA (6 h) and HSP72 protein immunoreactivity (24 h) was reduced by 42 and 60%, respectively in the ipsilateral compared to the contralateral posterior cingulate. Despite this apparent protective effect, the unilateral cholinergic lesion did not affect the degree of neuronal vacuolation (6 h), necrosis (24 h) or the large and prolonged increase in cerebral blood flow which occurred over the first 9h following MK-801 administration. These results demonstrate that cholinergic neurones in the nucleus basalis of Meynert play an important role in the heat shock response to NMDA antagonist-induced neurotoxicity but also reveal an unexpected divergence between the heat shock response and the pathophysiological response. This suggests that other cholinergic pathways or non-cholinergic mechanisms are responsible for the pathological changes induced by MK-801.

Reading vascular changes in brain imaging: is dendritic calcium the key?

A key goal in functional neuroimaging is to use signals that are related to local changes in metabolism and blood flow to track the neuronal correlates of mental activity. Recent findings indicate that the dendritic processing of excitatory synaptic inputs correlates more closely than the generation of spikes with brain imaging signals. The correlation is often nonlinear and context-sensitive, and cannot be generalized for every condition or brain region. The vascular signals are mainly produced by increases in intracellular calcium in neurons and possibly astrocytes, which activate important enzymes that produce vasodilators to generate increments in flow and the positive blood oxygen level dependent signal. Our understanding of the cellular mechanisms of functional imaging signals places constraints on the interpretation of the data.

Functional Imaging with Intraoperative Ultrasound: Detection of Somatosensory Cortex in Dogs with Color-duplex Sonography

OBJECTIVE

To evaluate the capability of intraoperative color-duplex sonography to detect eloquent flow-activated areas and their anatomic relationship in dogs.

METHODS

After craniotomy, the sensory cortex of eight dogs was identified by recording the highest amplitude detected with a grid electrode evoked with somatosensory evoked potential stimulation of the nervus ischiadicus. A 7.5-MHz linear array transducer was placed on the dura, and eight images were taken in color-coded capture mode during baseline and somatosensory evoked potential stimulation of the ipsilateral (nonevoked) and contralateral (evoked) sensory cortex. The differences in flow velocity intensities were statistically compared (Wilcoxon test) in three arbitrary velocity ranges and across all colored pixels in a region of interest between baseline and stimulation in both hemispheres.

RESULTS

Comparing both hemispheres during stimulation, the evoked sensory cortex demonstrated an increase of 10% in the number of counted colored pixels during stimulation, whereas the number of counted colored pixels in the ipsilateral sensory cortex decreased by 2% (P < 0.05), indicating an overall increase in measured flow during stimulation. Comparing differences during nonstimulation and stimulation in single hemispheres, the lowest of the three velocity ranges (approximately 10-20 mm/s) demonstrated a statistically significant (P = 0.01) increase during stimulation, whereas no change was observed during stimulation in the ipsilateral hemisphere. This increase has been confirmed by regional cerebral blood flow measurement with colored microspheres.

CONCLUSION

This study indicates, for the first time, the capability of intraoperative ultrasound to detect functionally important areas during evoked stimulation.

Effect of stimulus frequency on human cerebral hemodynamic responses to electric median nerve stimulation: a near-infrared spectroscopic study

We examined the effect of stimulus frequency on optically recorded hemodynamic responses to electric median nerve stimulation. Electric stimuli were delivered to the right median nerve with an intensity of 90% of motor threshold. Four different stimulus frequencies (2, 5, 10, and 20 Hz) were administered in each subject. By means of a multi-channel near-infrared spectroscopic instrument, changes in concentration of oxygenated hemoglobin were continuously measured over the left scalp. After 20 Hz stimulation, we found two spatially and temporally distinct hemodynamic responses. One lasted beyond 60 s, and the center of this response was located over the secondary somatosensory area. The other had a transient duration starting immediately after the stimulus onset and was located in the primary somatosensory hand area. Both responses were linearly augmented as a function of the stimulus frequency. Since temporal activation patterns are different in two somatosensory areas, real-time optical monitoring is necessary in evaluation of hemodynamic responses to electric nerve stimulation.

How well do we understand the neural origins of the fMRI BOLD signal?

The successful use of functional magnetic resonance imaging (fMRI) as a way of visualizing cortical function depends largely on the important relationships between the signal observed and the underlying neuronal activity that it is believed to represent. Currently, a relatively direct correlation seems to be favoured between fMRI signals and population synaptic activity (including inhibitory and excitatory activity), with a secondary and potentially more variable correlation with cellular action potentials.

Relationship of spikes, synaptic activity, and local changes of cerebral blood flow

The coupling of electrical activity in the brain to changes in cerebral blood flow (CBF) is of interest because hemodynamic changes are used to track brain function. Recent studies, especially those investigating the cerebellar cortex, have shown that the spike rate in the principal target cell of a brain region (i.e. the efferent cell) does not affect vascular response amplitude. Subthreshold integrative synaptic processes trigger changes in the local microcirculation and local glucose consumption. The spatial specificity of the vascular response on the brain surface is limited because of the functional anatomy of the pial vessels. Within the cortex there is a characteristic laminar flow distribution, the largest changes of which are observed at the depth of maximal synaptic activity (i.e. layer IV) for an afferent input system. Under most conditions, increases in CBF are explained by activity in postsynaptic neurons, but presynaptic elements can contribute. Neurotransmitters do not mediate increases in CBF that are triggered by the concerted action of several second messenger molecules. It is important to distinguish between effective synaptic inhibition and deactivation that increase and decrease CBF and glucose consumption, respectively. In summary, hemodynamic changes evoked by neuronal activity depend on the afferent input function (i.e. all aspects of presynaptic and postsynaptic processing), but are totally independent of the efferent function (i.e., the spike rate of the same region). Thus, it is not possible to conclude whether the output level of activity of a region is increased based on brain maps that use blood-flow changes as markers.

Decoupling of the short-term hemodynamic response and the blood oxygen concentration

Neuronal activation leads to an increase in the local cortical perfusion. The exact regulatory mechanisms leading to these changes are still unknown. To elucidate the role of oxygen in the initial hemodynamic regulation, a disactivation paradigm was performed with fMRI. A stimulus was applied following a previous extended activation in order to evoke physiological variations of the local oxygen concentration. The results demonstrate that the initial hemodynamic response time is independent of the local blood oxygen concentration.

Frequency-dependent changes in cerebral blood flow and evoked potentials during somatosensory stimulation in the rat

Contrary to the concept of neuronal-vascular coupling, cortical evoked potentials do not always correlate with blood flow responses during somatosensory stimulation at changing stimulus rates. The goal of this study is to clarify the effects of stimulus frequency on the relationship between somatosensory evoked potentials (SEPs) and cerebral blood flow. In rats anesthetized with alpha-chloralose, we measured SEPs by signal-averaging field potentials recorded with an electrode placed on dura overlying the hindlimb somatosensory cortex. Regional blood flow was simultaneously assessed in the same region with a laser-Doppler flow (LDF) probe. The contralateral sciatic nerve was stimulated with 0.1 A pulses at the frequencies of 1, 2, 5, 10 and 20 Hz. SEPs (both P1 and N1 components) declined with increasing frequency regardless whether stimulus duration (20 s) or number (100) were kept constant, suggesting that frequency is an important determinant of neuronal activity. In contrast, LDF responses increased to a maximum at 5 Hz, and do not correlate with SEPs. Because CBF should reflect integrated neuronal activity, we computed the sum of SEPS (summation operatorSEP = SEP x stimulus frequency) as an index of total neuronal activity at each frequency. Summation operatorSEP indeed correlates positively (P<0.001) with LDF responses. Thus, during somatosensory stimulation at various frequencies, cerebral blood flow is coupled to integrated neuronal activity but not to averaged evoked potentials.

Visual processing in infants and children studied using functional MRI

We studied the development of visual processing in 58 children, ranging from 1 d to 12 y of age (median age 29 mo), using functional magnetic resonance imaging. All but nine children had either been sedated using chloral hydrate (n = 12) or pentobarbital (n = 28). Nine children were studied under a full halothane/ N2O:O2 anesthesia. In the first postnatal month, 30% of the neonates showed a positive blood oxygenation level-dependent (BOLD) contrast signal, whereas, for infants between the ages of 1 mo and 1 y, 27% did so. Thirty-one percent of children between 1 and 6 y of age and 71% of children aged 6 y and above showed a positive BOLD contrast signal change to our visual stimulation paradigm. Besides the usual positive BOLD contrast signal change, we also noted that a large portion of the children measured displayed a negative BOLD contrast signal change. This negative BOLD contrast signal change was observed in 30% of children up to 1 mo of age, in 27% between 1 mo and 1 y of age, in 47% between 1 and 6 y of age, and in 14% of children 6 y and older. In the children in which we observed a negative correlating BOLD contrast signal change, the locus was more anterior and more lateral than the positive BOLD contrast signal, placing it in the secondary visual cortical area. The results indicate that when using functional magnetic resonance imaging on children, the primary visual cortical area does not respond functionally in the same manner as that of the adult until 1.5 y of age. This supports earlier clinical and electrophysiologic findings that different cortical mechanisms seem to contribute to visual perception at different times postnatally.

Blood flow increases linearly in rat somatosensory cortex with increased whisker movement frequency

It has long been known that the level of neuronal activity is correlated to the level of localized blood flow. Despite the importance of functional hyperemia in the brain, the relationship between blood flow and electrical activity has not been clearly demonstrated parametrically in a single region of cerebral cortex. We investigated both the magnitude and temporal characteristics of the blood flow response in somatosensory cortex while varying the frequencies of whisker movement. The full whisker pad on one side of the rat's face was repeatedly moved for 13 s at frequencies of 1.5, 2, 3, 4, 6, 8, and 10.5 Hz, and the resulting changes in blood flow were quantified using Laser-Doppler flowmetry (LDF). The magnitude of the blood flow response increased linearly with increasing frequency while the temporal parameters of time to half maximal value and time to return halfway to baseline after stimulus termination did not vary. Baseline blood flow levels were elevated by breathing rats on a 5% CO2 mixture. No significant alteration in the LDF plateau response to whisker movement was observed compared to normal air, suggesting sustained vasodilation reserve capacity remained after CO2-induced vasodilation. These data demonstrate linear blood flow responses to presumptive linear increases in neuronal activity with sufficient vascular reserve capacity to overcome moderate CO2-induced dilation, and support the use of blood flow changes in neuroimaging studies. They provide a framework to study the neurobiological signal transduction mechanisms coupling neuronal electrical activity with regional alterations in blood flow.

Scopolamine abolishes cerebral blood flow response to somatosensory stimulation in anesthetized cats: PET study

The effect of the cholinergic blocker, scopolamine on the cerebral blood flow (CBF) response to vibrotactile stimulation of a fore paw was studied using high-resolution positron emission tomography and H2 15O in 5 pentobarbital-anesthetized cats. Before scopolamine injection, the CBF response to the stimulation was found in the contralateral somatosensory cortex (mean ratio (contralateral/ipsilateral) control: stimulated 1.02 +/- 0.02: 1.17 +/- 0.05; P < 0.01). After intravenous injection of scopolamine (0.35 mg/kg), the CBF response was abolished. However, the cerebral metabolic rate of glucose (CMRGlu) response to the same stimulation was unchanged after scopolamine injection in the same cats. We concluded that scopolamine abolishes the CBF response but not neuronal response to stimulation. We suggest that cholinergic mechanisms may play an important role for mediating CBF coupling to neuronal activity during physiological stimulation.

Persistent pain inhibits contralateral somatosensory cortical activity in humans

To assess cortical activity during pain perception, regional cerebral blood flow (rCBF) studies were done in humans using single photon emission computed tomography (SPECT) with the radiotracer Tc99m-HMPAO and magnetic resonance imaging localization. Normalized SPECT data were analyzed by region of interest and change distribution. Contralateral somatosensory rCBF was decreased when the digits of the hand were immersed in a hot water bath for 3 min which was rated as moderately painful (persistent pain). No decrease was observed when the hand was immersed in tepid water (control). In contrast, cortical rCBF was increased during vibratory and sensorimotor tasks, in the contralateral somatosensory and sensorimotor areas, respectively. These results indicate that pain perception in man is associated with somatosensory cortical inhibition.

Effects of physiological manipulations on locus coeruleus neuronal activity in freely moving cats. III. Glucoregulatory challenge

Insulin-induced hypoglycemia and the subsequent administration of glucose were examined for their effects on single unit activity of locus coeruleus noradrenergic (LC-NE) neurons in unanesthetized, unrestrained cats. LC-NE neuronal activity showed an inverse relationship to blood glucose levels. The activity of most cells increased during sustained hypoglycemia, and then decreased following glucose administration. Some neurons were unaffected by hypoglycemia, but were inhibited following glucose. The activation of LC-NE neurons in response to insulin administration generally paralleled the increase in plasma epinephrine, although the adrenal response was more sensitive. These data, together with those reported in the preceding papers, suggest the following general conclusions: (1) physiological stimuli can influence the activity of LC-NE neurons in unanesthetized subjects (although they do so less strongly than environmental stimuli); (2) these effects of physiological stimuli upon LC-NE neurons can be exerted independent of changes in behavioral state; (3) LC-NE neurons do not appear to play a specific role in the regulation of any of the systems examined, but may instead play a more global role in the response to physiological challenges in general; (4) LC-NE neurons are generally co-activated with both the neural and hormonal components of the sympatho-adrenal system, although sympathetic activation can occur in the absence of increased LC-NE activity. A previously hypothesized role for LC-NE neurons in facilitating the behavioral response to environmental stressors may thus be extended to include the response to physiological challenges, and perhaps facilitation of the physiological as well as the behavioral components of the stress response.

Right and left cortical lesions asymmetrically alter cerebrovascular permeability in the rat

Vascular permeability and intravascular space in the rat cerebral cortex and hypothalamus were assessed 5 and 14 days after the production of either right or left hemisphere frontal cortical suction lesions. Rats were intravenously injected with large (dextran, 50,000 kdalton) and small (inulin, 5000 kdalton) molecular tracers and tissue samples were removed for assay after 7 min. Five days following lesions of the right but not left hemisphere, inulin levels in the cortical region posterior and contralateral to the lesion site were significantly higher than control values. Dextran levels were not increased in this region by lesions of either hemisphere. However, both right and left hemisphere lesions led to a significant elevation of inulin and dextran levels (inulin higher than dextran) in the cortex surrounding the lesion site. Fourteen days after lesion of either hemisphere, tracer levels were not elevated over control in any region examined. These results suggest that both right and left hemisphere cortical lesions transiently increase local vasopermeability (and perhaps intravascular space), but that only right hemisphere lesions selectively increase inulin in cortex distant from the lesion site, probably from lesion-induced alterations in cerebrovascular permeability.

Vascular headaches and cerebral circulation: an overview

A complex network of neurotransmission systems underlies the control of the cerebral circulation. Classical neurotransmitters, vasoactive peptides and receptors have been found in cerebral arteries. Central and peripheral structures are also probably involved in the neurogenic control of the cerebral circulation. Vascular and neurotransmission changes reported in vascular headaches suggest that an alteration of the neurogenic control of the brain circulation may be implicated in vascular headaches. In particular, locus coeruleus, which may control the intracerebral adrenergic pathway, can induce vascular changes similar to those of migraine. Moreover, the trigeminal ganglion, which may induce the release of substance P, can change the extracranial and intracranial vasodilator activity. The vascular theory of migraine, proposed by Wolff, is re-evaluated on the grounds of a possible mediation of the vascular responses by neurotransmitters. It is hypothesized that a deficient modulation by enkephalins may cause alterations of locus coeruleus and/or trigeminal ganglion. The problem of pain in vascular headaches is also considered: whether it is of vascular origin or whether it is due to a dysfunction of the central nociceptive pathway. Knowledge of the neurogenic control of the cerebral circulation may be useful in understanding some pathogenetic mechanisms of vascular headaches.

Significance of the cerebrovascular effects of immobilization stress in the rabbit

The question of the significance of the cerebrovascular effects of stressful situations in animals is still controversial. In the present article, an experimental model of immobilization stress in the rabbit is described, and its specificity in relation to arterial blood pressure and PaCO2 is investigated. CBF was measured with the multiregional tissue sampling technique using [14C]-ethanol as tracer. After dissipation of althesin anesthesia, the stress reaction was elicited by tactile abdominal stimuli. The response was evidenced by an instantaneous acute hypertension (+33.8% during the CBF measurement period). Within the first minute of the reaction, the CBF was significantly increased in all nine structures studied by 39% (caudate nucleus) to 82% (parieto-temporal cortex). The study of the influence of arterial blood pressure and the PaCO2 on CBF showed that cerebrovascular autoregulation and CO2 sensitivity were differently affected in the various structures during the stress reaction. However, the stress response of the brain circulation could not be entirely ascribed to one or both of these two systemic factors, thus suggesting the contribution of a local intrinsic activation. The model presented here could be useful for long-term studies of cerebrovascular repercussions of repeated acute hypertensions of a stressful nature.

Evidence for involvement of the frontal cortex in pain-related cerebral events in cats: increase in local cerebral blood flow by noxious stimuli

Noxious stimuli were shown to induce a remarkable increase in local cerebral blood flow restricted to the forepart of the cerebral hemispheres bilaterally anterior to the posterior sigmoid gyrus in cats. This increase in local cerebral blood flow was averted by lesions in the bilateral ventromedial thalamus and attenuated by pretreatment with an intraventricular injection of 6-hydroxydopamine.

Long lasting suppression of firing of cortical neurons and decrease in cortical blood flow following train pulse stimulation of the locus coeruleus in the cat

Single neuron activity and local cerebral blood flow were recorded simultaneously in the same spot of the gyrus proreus in cats. Train pulse stimulation (10-20 Hz, 30 sec) of the ipsilateral locus coeruleus induced long lasting suppression of firing in up to 78% of neurons and decrease in local flow, which lasted 1.9-5.6 min and 3.8-6.5 min, respectively. Single pulse stimulation evoked inhibition of firing in 55% of the neurons investigated.