Neurotransmitter receptor binding in bovine cerebral microvessels

Histamine in the neocortex: Towards integrating multiscale effectors

Histamine is a modulatory neurotransmitter, which has received relatively less attention in the central nervous system than other neurotransmitters. The functional role of histamine in the neocortex, the brain region that controls higher-order cognitive functions such as attention, learning and memory, remains largely unknown. This article focuses on the emerging roles and mechanisms of histamine release in the neocortex. We describe gaps in current knowledge and propose the application of interdisciplinary tools to dissect the detailed multiscale functional logic of histaminergic action in the neocortex ranging from sub-cellular, cellular, dendritic and synaptic levels to microcircuits and mesoscale effects.

Monoamines: Dopamine, Norepinephrine, and Serotonin, Beyond Modulation, "Switches" That Alter the State of Target Networks

How do monoamines influence the perceptual and behavioral aspects of brain function? A library of information regarding the genetic, molecular, cellular, and function of monoamines in the nervous system and other organs has accumulated. We briefly review monoamines' anatomy and physiology and discuss their effects on the target neurons and circuits. Monoaminergic cells in the brain stem receive inputs from sensory, limbic, and prefrontal areas and project extensively to the forebrain and hindbrain. We review selected studies on molecular, cellular, and electrophysiological effects of monoamines on the brain's target areas. The idea is that monoamines, by reversibly modulating the "primary" information processing circuits, regulate and switch the functions of brain networks and can reversibly alter the "brain states," such as consciousness, emotions, and movements. Monoamines, as the drivers of normal motor and sensory brain operations, including housekeeping, play essential roles in pathogenesis of neuropsychiatric diseases.

Delta Opioid Receptors and Cardioprotection

The opioid receptor family, with associated endogenous ligands, has numerous roles throughout the body. Moreover, the delta opioid receptor (DORs) has various integrated roles within the physiological systems, including the cardiovascular system. While DORs are important modulators of cardiovascular autonomic balance, they are well-established contributors to cardioprotective mechanisms. Both endogenous and exogenous opioids acting upon DORs have roles in myocardial hibernation and protection against ischaemia-reperfusion (I-R) injury. Downstream signalling mechanisms governing protective responses alternate, depending on the timing and duration of DOR activation. The following review describes models and mechanisms of DOR-mediated cardioprotection, the impact of co-morbidities and challenges for clinical translation.

Vascular dysfunction in the pathogenesis of Alzheimer's disease--A review of endothelium-mediated mechanisms and ensuing vicious circles

Late-onset dementia is a major health concern in the ageing population. Alzheimer's disease (AD) accounts for the largest proportion (65-70%) of dementia cases in the older population. Despite considerable research effort, the pathogenesis of late-onset AD remains unclear. Substantial evidence suggests that the neurodegenerative process is initiated by chronic cerebral hypoperfusion (CCH) caused by ageing and cardiovascular conditions. CCH causes reduced oxygen, glucose and other nutrient supply to the brain, with direct damage not only to the parenchymal cells, but also to the blood-brain barrier (BBB), a key mediator of cerebral homeostasis. BBB dysfunction mediates the indirect neurotoxic effects of CCH by promoting oxidative stress, inflammation, paracellular permeability, and dysregulation of nitric oxide, a key regulator of regional blood flow. As such, BBB dysfunction mediates a vicious circle in which cerebral perfusion is reduced further and the neurodegenerative process is accelerated. Endothelial interaction with pericytes and astrocytes could also play a role in the process. Reciprocal interactions between vascular dysfunction and neurodegeneration could further contribute to the development of the disease. A comprehensive overview of the complex scenario of interacting endothelium-mediated processes is currently lacking, and could prospectively contribute to the identification of adequate therapeutic interventions. This study reviews the current literature of in vitro and ex vivo studies on endothelium-mediated mechanisms underlying vascular dysfunction in AD pathogenesis, with the aim of presenting a comprehensive overview of the complex network of causative relationships. Particular emphasis is given to vicious circles which can accelerate the process of neurovascular degeneration.

The effect of morphine on regional cerebral blood flow measured by 99mTc-ECD SPECT in dogs

To gain insights into the working mechanism of morphine, regional cerebral blood flow (rCBF) patterns after morphine administration were assessed in dogs. In a randomized cross-over experimental study, rCBF was estimated with ^99mTc-Ethylcysteinate Dimer single photon emission computed tomography in 8 dogs at baseline, at 30 minutes and at 120 minutes after a single bolus of morphine. Perfusion indices (PI) in the frontal, parietal, temporal and occipital cortex and in the subcortical and cerebellar region were calculated. PI was significantly decreased 30 min after morphine compared to baseline in the right frontal cortex. The left parietal cortex and subcortical region showed a significantly increased PI 30 min after morphine compared to baseline. No significant differences were noted for the other regions or at other time points. In conclusion, a single bolus of morphine generated a changing rCBF pattern at different time points.

The response to rapid infusion of fentanyl in the human brain measured using pulsed arterial spin labelling

OBJECTIVE

We evaluated the sensitivity of pulsed Arterial Spin Labelling (pASL) for the detection of changes in regional cerebral blood perfusion (CBP) during and after intra-venous (i.v.) infusion of an opioid agonist (fentanyl) and an opioid antagonist (naloxone).

MATERIALS AND METHODS

Twenty-three subjects were scanned four times, receiving i.v. infusion of fentanyl, naloxone, placebo and a second fentanyl administration, in four separate scanning sessions in randomised order. End-tidal CO(2), respiration rate and heart rate were recorded continuously throughout each scan. pASL time series were collected using single shot EPI for 15 min (including 5 min of baseline prior to infusion).

RESULTS

Significant increases in CBP were detected during and after administration of fentanyl, (when compared to placebo and naloxone), in most areas of high concentration of mu-opioid receptors (thalamus, lingual gyrus, para-hippocampal gyrus, and insula); near-significant increases were also observed in the insula. No increases in perfusion were observed during or after naloxone infusion. No correlation was found between regional rCBF changes and end-tidal CO(2), respiration rate or heart rate. Good reliability was found between the first and second fentanyl sessions but the regions of high reliability did not overlap completely with those of highest perfusion change.

CONCLUSION

pASL is a suitable method for examining rapid, dynamic effects of opioid administration on brain physiology.

The physiologic implications of isolated alpha(1) adrenergic stimulation

Phenylephrine and methoxamine are direct-acting, predominantly α(1) adrenergic receptor (AR) agonists. To better understand their physiologic effects, we screened 463 articles on the basis of PubMed searches of "methoxamine" and "phenylephrine" (limited to human, randomized studies published in English), as well as citations found therein. Relevant articles, as well as those discovered in the peer-review process, were incorporated into this review. Both methoxamine and phenylephrine increase cardiac afterload via several mechanisms, including increased vascular resistance, decreased vascular compliance, and disadvantageous alterations in the pressure waveforms produced by the pulsatile heart. Although pure α(1) agonists increase arterial blood pressure, neither animal nor human studies have ever shown pure α(1)-agonism to produce a favorable change in myocardial energetics because of the resultant increase in myocardial workload. Furthermore, the cost of increased blood pressure after pure α(1)-agonism is almost invariably decreased cardiac output, likely due to increases in venous resistance. The venous system contains α(1) ARs, and though stimulation of α(1) ARs decreases capacitance and may transiently increase venous return, this gain may be offset by changes in afterload, venous compliance, and venous resistance. Data on the effects of α(1) stimulation in the central nervous system show conflicting changes, while experimental animal data suggest that renal blood flow is reduced by α(1)-agonists, and both animal and human data suggest that gastrointestinal perfusion may be reduced by α(1) tone.

Expression levels of the alpha4 subunit of the GABA(A) receptor in differentiated neuroblastoma cells are correlated with GABA-gated current

The alpha4 subunit of the GABA(A) receptor (GABAR) is capable of rapid plasticity, increased by chronic exposure to positive GABA modulators, such as the neurosteroid 3alpha-OH-5alpha[beta]-pregnan-20-one (THP). Here, we show that 48 h exposure of differentiated neuroblastoma cells (IMR-32) to 100 nM THP increases alpha4 expression, without changing the current density or the concentration-response curve. Increased expression of alpha4-containing GABAR was verified by a relative insensitivity of GABA (EC(20))-gated current to modulation by the benzodiazepine (BZ) lorazepam (0.01-100 microM), and potentiation of current by flumazenil and RO15-4513, characteristic of alpha4betagamma2 pharmacology. In contrast to THP, compounds which decrease GABA-gated current, such as the BZ inverse agonist DMCM, the GABAR antagonist gabazine and the open channel blocker penicillin, decreased alpha4 expression after a 48 h exposure, without changing BZ responsiveness. However, pentobarbital, another positive GABA modulator, increased alpha4 expression, while the BZ antagonist flumazenil had no effect. In order to test whether changes in current were responsible for increased alpha4 expression, decreases in the Cl(-) driving force were produced by chronic exposure to the NKCC1 blocker bumetanide (10 microM). When applied under these conditions of reduced GABA-gated current, THP failed to increase alpha4 expression. The results of this study suggest that alpha4 expression is correlated with changes in GABA-gated current, rather than simply through ligand-receptor interactions. These findings have relevance for GABAR subunit plasticity produced by fluctuations in endogenous steroids across the menstrual cycle, when altered BZ sensitivity is reported.

[Is morphine still the analgesic of choice in acute myocardial infarction?]

Chest pain is the most common symptom of patients who present with ischemic heart disease. Morphine has traditionally been the drug of choice for managing chest pain in acute coronary syndrome (ACS) due to its high analgesic potency, though its physiological effects are poorly understood. Routinely used for managing chest pain, morphine is recommended in the 2002 guidelines of the American College of Cardiology/American Heart Association. This recommendation, however, is not based on a high level of scientific evidence but on expert opinion. Studies have found both for and against the use of morphine in ACS, suggesting that its benefits are perhaps not altogether clear. This review examines the pathophysiological effects of morphine and their cardiac implications, with special attention to a possible negative effect on ACS. We reviewed articles in the MEDLINE database from 1982 to 2006.

Dexmedetomidine for the treatment of postanesthesia shivering in children

BACKGROUND

Shivering is a common postanesthesia adverse event with multiple etiologies and multiple suggested prophylactic and abortive treatment regimens. Dexmedetomidine, a centrally acting alpha(2)-adrenergic agonist, has been used as a sedative agent and is known to reduce the shivering threshold. We hypothesized that children with postanesthesia shivering would reduce shivering behavior following a single bolus dose of dexmedetomidine.

METHODS

Dexmedetomidine was administered in a prospective, open-label fashion. The anesthesia management was uniform consisting of maintenance inhaled anesthesia (sevoflurane) and the intraoperative administration of fentanyl (1-2 microg.kg(-1)) plus a regional anesthetic technique (either a neuraxial or peripheral block) for postoperative analgesia. Criteria for treatment included: (i) shivering, (ii) successful extubation, and (iii) no other complaint/indication of pain. All children who met the criteria were treated with a single intravenous bolus dose of dexmedetomidine (0.5 microg.kg(-1)) over 3-5 min. Following the completion of drug administration, shivering activity was recorded every minute (up to 10 min) with any adverse effects or complaints. The efficacy of shivering reduction at 5 min in this cohort is compared with previous reports from the literature of the efficacy of clonidine and meperidine.

RESULTS

Twenty-four children ranging in age from 7 to 16 years (11.5 +/- 2.5 years) were treated. All children had a cessation of shivering behavior within 5 min following the completion of dexmedetomidine administration. The onset of effect was 3.5 +/- 0.9 min. No adverse effects were observed. No shivering behavior recurred.

CONCLUSIONS

This study demonstrates the efficacy of dexmedetomidine in the treatment of postanesthesia shivering.

Pharmacological FMRI: measuring opioid effects on the BOLD response to hypercapnia

Opioid binding to the cerebral blood vessels may affect vascular responsiveness and hence confound interpretation of blood oxygen level-dependent (BOLD) responses, which are usually interpreted as neuronal in origin. Opioid binding varies in different brain regions. It is unclear whether opioids alter neurovascular coupling, or whether their effects are purely neuronal. This study used BOLD functional magnetic resonance imaging (FMRI) to investigate the effect of a mu-opioid agonist remifentanil, on cerebrovascular CO(2) reactivity (being one component of neurovascular coupling). Hypercapnic challenges were delivered to human volunteers, while controlling potential opioid-induced respiratory depression. The BOLD signal increase to hypercapnia was compared before and during remifentanil administration. Remifentanil was shown not to have a generalised effect on CO(2) responsiveness in the cerebral vasculature. However, it caused a significant reduction in the positive BOLD response to hypercapnia in the bilateral primary sensorimotor cortices, bilateral extrastriate visual areas, left insula, left caudate nucleus, and left inferior temporal gyrus. We conclude that remifentanil does not modulate cerebrovascular CO(2) reactivity, as we saw no difference in BOLD response to hypercapnia in areas with high opioid receptor densities. We did however see a focal reduction in areas related to motor control and putative task activation, which we conclude to be related to changes in neuronal activity related to the sedative effects of remifentanil. Our method of controlling CO(2) levels effectively mitigated the potential confound of respiratory depression and allowed comparison over a similar range of CO(2) levels. We suggest that similar methodology should be used when investigating other potentially vasoactive compounds with FMRI.

Interaction of adenosine and naloxone on regional cerebral blood flow in morphine-dependent rats

The present research aimed at investigating the opioid-adenosine interaction on regional cerebral blood flow (rCBF). Therefore rCBF in the sensory cortex of morphine-naive and -dependent rats was measured using the laser-Doppler flowmetry technique. The results showed that adenosine (10(-5), 10(-4), 10(-3) M) significantly increased rCBF in morphine-dependent rats (MDR) (P < 0.01). This effect was inhibited by theophylline (5 x 10(-5) M). Also systemic naloxone (0.5, 1.5 and 3 mg/kg, s.c.) significantly increased rCBF in MDR and it was accompanied by elevated blood pressure and heart rate. Local adenosine (10(-4) M) significantly augmented naloxone (0.5 mg/kg)-induced increase in rCBF of MDR but had no significant effect on naloxone's (1.5 and 3 mg/kg) increasing effect on rCBF. Theophylline also has no effect on naloxone increasing effect on rCBF. These data suggest that adenosine receptors responsiveness increase in sensory cortex of MDR. Naloxone also highly increased rCBF of MDR that probably not interfere with adenosine receptors. Also, it seems that adenosine acts as a modulator in rCBF regulation of morphine-dependent and morphine withdrawal rats.

Cardioprotective action of fentanyl in a model of central sympathetic overactivity in rabbits: antiarrhythmic and anti-ischemic effects

BACKGROUND

Sympathetic overactivity resulting from perioperative noxious stimuli elicits hyperdynamic cardiovascular responses that may lead to myocardial ischemia and/or ventricular arrhythmia, especially in patients presenting with coronary artery disease. In the present study we investigated the cardioprotective effects of clinically relevant doses of fentanyl in an experimental model of sympathetic overactivity associated with myocardial ischemia in anesthetized rabbits.

METHODS

Central sympathetic stimulation was achieved through intracerebroventricular (i.c.v.) injection of L-glutamate (10 micro mol), with simultaneous inhibition of nitric oxide (NO) synthesis through i.v. administration of N(omega)-nitro-l-arginine methyl ester (L-NAME; 40 mg kg(-1)).

RESULTS

L-glutamate triggered ventricular arrhythmia and electrocardiographic alterations indicative of myocardial ischemia. The intravenous administration of fentanyl (5, 10 or 50 micro g kg(-1)) reduced the incidence of ST-segment shift (70, 20 and 10%, respectively, vs. 66.7% in controls) as well as of T-wave inversion from 58.3% to 30, 20 and 10%, respectively. The total number of ventricular premature complexes per minute fell from 65.2 +/- 16 in the control group to 6.8 +/- 3, 3.5 +/- 2 and 2.6 +/- 1.5, respectively. The occurrence of ventricular tachycardia and bigeminy was completely abolished by fentanyl. Finally, the i.v. administration of fentanyl did not induce significant hemodynamic effects (except for dP/dt(max) in the 50 micro g kg(-1)-dose).

CONCLUSION

Fentanyl elicits significant cardioprotective effects in a model of arrhythmia resulting from the association of central sympathetic overactivity with myocardial ischemia in rabbits, independently from its systemic hemodynamic actions.

Traumatic Cerebral Vascular Injury: The Effects of Concussive Brain Injury on the Cerebral Vasculature

In terms of human suffering, medical expenses, and lost productivity, head injury is one of the major health care problems in the United States, and inadequate cerebral blood flow is an important contributor to mortality and morbidity after traumatic brain injury. Despite the importance of cerebral vascular dysfunction in the pathophysiology of traumatic brain injury, the effects of trauma on the cerebral circulation have been less well studied than the effects of trauma on the brain. Recent research has led to a better understanding of the physiologic, cellular, and molecular components and causes of traumatic cerebral vascular injury. A more thorough understanding of the direct and indirect effects of trauma on the cerebral vasculature will lead to improvements in current treatments of brain trauma as well as to the development of novel and, hopefully, more effective therapeutic strategies.

Role of Nociceptin/Orphanin FQ in the physiologic and pathologic control of the cerebral circulation

Nociceptin/orphanin FQ is a newly described member of the opioid family. Previous minireviews in this series have described the contribution of important factors, including opioids, in the regulation of the cerebral circulation during physiologic and pathologic conditions. The present review extends these initial comments to an opioid whose vascular actions have only very recently been appreciated. In particular, this review discusses the contribution of nociceptin/orphanin FQ to impaired cerebral hemodynamics after cerebral hypoxia/ischemia and traumatic brain injury.

Interactions between the endothelium-derived relaxing factor/nitric oxide system and the endogenous opiate system in the modulation of cerebral and spinal vascular CO2 responsiveness

The role of the L-arginine-nitric oxide (NO) system, the role of the endogenous morphine-like substances (endorphins), and the possible interaction between these two systems in the modulation of regional cerebral and spinal CO2 responsiveness was investigated in anesthetized, ventilated, normotensive, normoxic cats. Regional cerebral blood flow was measured with radiolabeled microspheres in hypocapnic, normocapnic, and hypercapnic conditions in nine individual cerebral and spinal cord regions. General opiate receptor blockade by 1 mg/kg naloxone intravenously alone or NO synthase blockade by 3 mg/kg N(omega)-nitro-L-arginine-methyl ester (L-NAME) intravenously alone caused no changes in regional CO2 responsiveness. Combined administration of these two blocking agents in the very same doses, however, resulted in a strong potentiation, with a statistically significant reduction of the CO2 responsiveness observed. Separation of the blood flow response to hypercapnia and hypocapnia indicates that this reduction occurs only during hypercapnia. Specific mu and delta opiate receptors were blocked by 0.5 mg kg(-1) IV beta-funaltrexamine and 0.4 mg kg(-1) IV naltrindole, respectively. The role of specific mu and delta opiate receptors in the NO-opiate interaction was found to be negligible because neither mu nor delta receptor blockade along with simultaneous NO blockade were able to decrease CO2 responsiveness. The current findings suggest a previously unknown interaction between the endothelium-derived relaxing factor/nitric oxide (EDRF/NO) system and the endogenous opiate system in the cerebrovascular bed during hypercapnic stimulation, with the phenomenon not mediated by mu or delta opiate receptors.

Distribution of the histamine H(2) receptor in monkey brain and its mRNA localization in monkey and human brain

The distribution of histamine H(2) receptor mRNA was determined by in situ hybridization histochemistry in human and monkey brain. In the case of monkey brain, we combined this technique with receptor ligand autoradiography to compare the distribution of mRNA and receptor binding sites. [(125)I]Iodoaminopotentidine ([(125)I]-APT), a reversible, high specific activity antagonist with high affinity and selectivity for the H(2) receptor, was used for receptor autoradiography. Radiolabeled oligonucleotides derived from the human mRNA sequence encoding this receptor were used as hybridization probes. The highest density of the H(2) receptor mRNA in human and monkey brain was found in caudate and putamen nuclei and external layers of cerebral cortex. Moderate levels were seen in the hippocampal formation and lower densities in the dentate nucleus of cerebellum. Areas such as globus pallidus, amygdaloid complex, cerebellar cortex, and substantia nigra were devoid of hybridization signal. The distribution of H(2) receptor mRNA in monkey brain is generally in good agreement with that of the corresponding binding sites: prominent in caudate, putamen, accumbens nuclei, and cortical areas. The hippocampus showed lower densities of receptors and low levels were detected in the globus pallidus pars lateralis. No binding sites were seen in amygdaloid complex and substantia nigra. The distribution of histaminergic innervation is in good correlation with the areas of high density for H(2) receptors: caudate, putamen, and external layers of cerebral cortex in monkey and human brain. The presence of mRNA in caudate and putamen nuclei, together with its absence from substantia nigra, suggests that the H(2) receptors found in the striatum are synthesized by intrinsic cells and not by nigral dopaminergic cells. These striatal H(2) receptors may be located on short circuit striatal interneurons or somatodendritically on striatal projection neurons which project to the globus pallidus pars lateralis. In conclusion, the present results, which constitute, to our knowledge, the first report of the regional distribution of mRNA encoding H(2) receptors detected by in situ hybridization, define the sites of synthesis of H(2) receptors and are the basis for future, more detailed studies that should result in a better understanding of H(2) receptor function.

The histaminergic system in the brain: structural characteristics and changes in hibernation

Histaminergic neurons in adult vertebrate brain are confined to the posterior hypothalamic area, where they are comprised of scattered groups of neurons referred to as the tuberomammillary nucleus. Histamine regulates hormonal functions, sleep, food intake, thermoregulation and locomotor activity, for example. In the zebrafish, Danio rerio, histamine was detected only in the brain, where also the histamine synthesizing enzyme L-histidine decarboxylase (HDC) was expressed. It is possible that histamine has first evolved as a neurotransmitter in the central nervous system. We established sensitive quantitative in situ hybridization methods for histamine H(1) and H(2) receptors and HDC, to study the modulation of brain histaminergic system under pathophysiological conditions. A transient increase in H(1) receptor expression was seen in the dentate gyrus and striatum after a single injection of kainic acid, a glutamate analog. H(1) antagonists are known to increase duration of convulsions, and increased brain histamine is associated with reduced convulsions in animal models of epilepsy. No HDC mRNA was detected in brain vessels by in situ hybridization, which suggests lack of histamine synthesis by brain endothelial cells. This was verified by lack of HDC mRNA in a rat brain endothelial cell line, RBE4 cells. Both H(1) and H(2) receptor mRNA was found in this cell line, and the expression of both receptors was downregulated by dexamethasone. The findings are in agreement with the concept that histamine regulates blood-brain barrier permeability through H(1) and H(2) receptor mediated mechanisms. Hibernation is characterized by a drastic reduction of central functions. The activity of most transmitter systems is maintained at a very low level. Surprisingly, histamine levels and turnover were clearly elevated in hibernating ground squirrels, and the density of histamine-containing fibers was higher than in euthermic animals. It is possible that histamine actively maintains the low activity of other transmitters during the hibernation state.

Lack of histamine synthesis and down-regulation of H1 and H2 receptor mRNA levels by dexamethasone in cerebral endothelial cells

The purpose of this work was to determine whether cerebral endothelial cells have the capacity to synthesize histamine or to express mRNA of receptors that specifically respond to available free histamine. The histamine concentrations and the expression of L-histidine decarboxylase (HDC) and histamine H1 and H2 receptor mRNA, both in adult rat brain and in cultured immortalized RBE4 cerebral endothelial cells, were investigated. In this study endothelial cells were devoid of any kind of detectable histamine production, both in vivo and in the immortalized RBE4 cells in culture. Both the immunostainings for histamine and the in situ hybridizations for HDC were negative, as well as histamine determinations by HPLC, indicating that endothelial cells do not possess the capacity to produce histamine. Also, glucocorticoid (dexamethasone) treatment failed to induce histamine production in the cultured cells. Although the cerebral endothelial cells lack histamine production, a nonsaturable uptake in RBE4 cells is demonstrated. The internalized histamine is detected both in the cytoplasm and in the nucleus, which could indicate a role for histamine as an intracellular messenger. Histamine H1 and H2 receptor mRNA was expressed in RBE4 cells, and glucocorticoid treatment down-regulated the mRNA levels of both H1 and H2 receptors. This mechanism may be involved in glucocorticoid-mediated effects on cerebrovascular permeability and brain edema.

Effects on intracranial pressure of fentanyl in severe head injured patients

Despite opioids are routinely used for analgesia in head injured patients, the effects of such drugs on ICP and cerebral hemodynamics remain controversial. Cerebrovascular autoregulation (CAR) could be an important factor in the ICP increases reported after opioid administration. In order to describe the effects on intracranial pressure of fentanyl and correlated such effects with autoregulation status, we studied 30 consecutive severe head injury patients who received fentanyl (2 micrograms/kg) intravenously over one minute. Prior to study, CAR was assessed. Monitoring included MAP, HR, SaO2, ETCO2, SjO2 and ICP. Changes in cerebral blood flow (CBF) were estimated from relative changes in AVDO2. Patients mean GCS was 5.7 +/- 1.7 (mean +/- STD) and mean ICP on admission was 23.8 +/- 16.3 mmHg. Fentanyl caused significant increases in ICP and decreases in MAP and CPP, but CBF remained unchanged when estimated by AVDO2. In patients with preserved CAR (34.5%), opioid-induced ICP increase was greater (but not statistically significant) than in those with impaired CAR (65.5%). We conclude than fentanyl moderately increased ICP and decreased MAP and CPP. Our data suggests that in patients with preserved CAR, potent opioids could cause greater increases of ICP, probably due to activation of the vasodilatadory cascade.

Role of opioids in hypoxic pial artery dilation is stimulus duration dependent

Because methionine enkephalin contributes to and dynorphin opposes dilation during a 10-min hypoxic exposure, opioids modulate pial artery dilation to this stimulus. However, such modulation may be dependent on the duration of hypoxia. The present study was designed to characterize the modulation of hypoxic pial dilation by opioids as a function of stimulus duration in newborn pigs equipped with a closed cranial window. Hypoxic dilation was decremented in both moderate and severe groups (PO2 approximately 35 and 25 mmHg, respectively) during 20-min and 40-min exposure periods compared with the response during 5 or 10 min of stimulation (24 +/- 1, 25 +/- 1, 18 +/- 1, and 14 +/- 1% for 5, 10, 20, and 40 min of moderate hypoxia; means +/- SE). Moderate and severe hypoxia had no effect on cerebral spinal fluid (CSF) methionine enkephalin or dynorphin concentration during a 5-min exposure period. During a 10-min exposure, however, both opioids were increased in CSF. During 20- and 40-min exposure periods, CSF dynorphin continued to increase, whereas methionine enkephalin steadily decreased (962 +/- 18, 952 +/- 21, 2,821 +/- 15, 2,000 +/- 81, and 1,726 +/- 58 pg/ml methionine enkephalin for control, 5, 10, 20, and 40 min of moderate hypoxia, respectively). The mu-opioid (methionine enkephalin) antagonist beta-funaltrexamine had no influence on dilation during the 5-min exposure, decremented the 10- and 20-min exposures, but had no effect on 40-min exposure hypoxic dilation. Whereas the kappa-opioid (dynorphin) antagonist norbinaltorphimine similarly had no effect on a 5-min exposure dilation, it, in contrast, potentiated 10-, 20-, and 40-min exposure hypoxic dilations (23 +/- 1 vs. 23 +/- 1, 24 +/- 1 vs. 32 +/- 1, 16 +/- 1 vs. 24 +/- 2, and 13 +/- 1 vs. 23 +/- 3% for 5, 10, 20, and 40-min hypoxic dilation before and after norbinaltorphimine). These data show that opioids do not modulate hypoxic pial dilation during short but do so during longer exposure periods. Moreover, hypoxic pial dilation is diminished during longer exposure periods. Decremented hypoxic pial dilation during longer exposure periods results, at least in part, from decreased release of methionine enkephalin and accentuated release of dynorphin. These data suggest that the relative role of opioids in hypoxic pial dilation changes with the stimulus duration.

Postnatal expression of H1-receptor mRNA in the rat brain: correlation to L-histidine decarboxylase expression and local upregulation in limbic seizures

Histamine is implicated in the regulation of brain functions through three distinct receptors. Endogenous histamine in the brain is derived from mast cells and neurons, but the importance of these two pools during early postnatal development is still unknown. The expression of histamine H1-receptor in the rat brain was examined using in situ hybridization during postnatal development and in adults. For comparison, the expression of L-histidine decarboxylase (HDC) in the two pools was revealed. H1-receptor was evenly expressed throughout the brain on the first postnatal days, but resembled the adult, uneven pattern already on postnatal day 5 (P5). HDC was expressed in both mast cells and tuberomammillary neurons from birth until P5, after which the mast cell expression was no more detectable. In adult rat brain, high or moderate levels of H1-receptor expression were found in the hippocampus, zona incerta, medial amygdaloid nucleus and reticular thalamic nucleus. In most areas of the adult brain the expression of H1-receptor mRNA correlates well with binding data and histaminergic innervation. A notable exception is the hypothalamus, with high fibre density but moderate or low H1-receptor expression. Systemic kainic acid administration induced increased expression of H1-receptor mRNA in the caudate-putamen and dentate gyrus, whereas no change was seen in the hippocampal subfields CA1-CA3 or in the entorhinal cortex 6 h after kainic acid injections. This significant increase supports the concept that histaminergic transmission, through H1-receptor, is involved in the regulation of seizure activity in the brain.

P2 purinoceptors in cultured bovine middle cerebral artery endothelial cells

Extracellular adenosine triphosphate (ATP) plays an important role in the regulation of endothelial function. However, its receptors and their signal-transduction pathways in major cerebral arterial endothelial cells are largely unknown. This study was undertaken functionally to classify the P2 purinoceptors in cultured bovine middle cerebral artery endothelial cells by using [Ca2+]i microfluorimetry. The rank order of potency to increase [Ca2+]i was 2-methylthio-ATP approximately ATP approximately uridine triphosphate (UTP) > adenosine diphosphate (ADP) >> adenosine monophosphate (AMP) > alpha,beta-methylene-ATP > adenosine, suggesting that the effect was mediated by both P2y and P2u receptors. ATP, 2-methylthio-ATP, and UTP mobilized Ca2+ from intracellular stores and triggered Ca2+ entry. The effects of ATP, 2-methylthio-ATP, and UTP were reduced by phospholipase C inhibitor 2-nitro-4-carboxyphenyl-N,N-diphenylcarbamate (NCDC), but only the effects of ATP and UTP were attenuated by pertussis toxin, indicating that P2y and P2u receptors may activate the same effector mechanisms by coupling to different G proteins. The [Ca2+]i entry caused by UTP was significantly reduced by the receptor-regulated Ca2+ channel blocker SK&F 96365, by P-450 inhibitor econazole and by inorganic Ca2+ entry blocker lanthanum. P2-receptor antagonists suramin, pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS), and reactive blue 2 reduced the effects of ATP and 2-methylthio-ATP, but not those of UTP, in a concentration-dependent manner. These studies suggest a coexistence of P2y and P2u receptors in cultured bovine middle cerebral artery endothelial cells.

Cerebral vessels in ageing and Alzheimer's disease

The integrity of the cerebral vasculature is crucial to the maintenance of cognitive functions during ageing. Prevailing evidence suggests that cerebrovascular functions decline during normal ageing, with pronounced effects in Alzheimer's disease (AD). The causes of these changes largely remain unknown. While previous studies recorded ageing-related impairments, such as atherosclerosis and loss of innervation in basal surface arteries of the brain, it only recently has been realized that a number of subtle alterations in both the intracranial resistance vessels and the smaller capillaries is apparent in both ageing animals and humans. The dominant changes include alterations in composition of connective tissues and smooth muscle of large vessel walls, thickening of the vascular basement membrane, thinning of the endothelium in some species, loss of endothelial mitochondria and increased pericytes. Some of these attributes appear more affected in AD. Other abnormalities entail profound irregularities in the course of microvessels, unexplained inclusions in the basement membrane and changes in unique proteins and membrane lipids associated with the blood-brain barrier. Brain imaging and permeability studies show no clear functional evidence to support the structural and biochemical anomalies, but it is plausible that focal and transient breach of the blood-brain barrier in ageing, and more notably in AD, occurs. Thus, circumscribed neuronal populations in certain brain regions could become vulnerable. Furthermore, the characteristic deposition of amyloid in vessels in AD may exacerbate the decline in vascular function and promote chronic hypoperfusion. Although not explicit from current studies, it is likely that the brain vasculature is continually modified by growth and repair mechanisms in attempts to maintain perfusion during ageing and disease.

Role of impaired cAMP and calcium-sensitive K+ channel function in altered cerebral hemodynamics following brain injury

Previous studies have shown that pial arteries constricted and responses to dilator opioids were blunted after fluid percussion injury (FPI) in newborn pigs. Membrane potential of vascular muscle is a major determinant of vascular tone and activity of K+ channels is a major regulator of membrane potential. Recent data show that opioids elicit dilation via the sequential production of cAMP and subsequent activation of calcium-sensitive K+ (K(Ca2+)) channels by this second messenger. The present study was designed to investigate the effect of FPI on cAMP and K(Ca2+) channel function. Chloralose-anesthetized piglets equipped with a closed cranial window were connected to a percussion device consisting of a saline-filled cylindrical reservoir and a metal pendulum. Brain injury of moderate severity (1.9-2.1 atm) was produced by allowing the pendulum to strike a piston on the cylinder. FPI blunted dilation to the cAMP analogs 8-Bromo cAMP and Sp 8-Bromo cAMPs (10(-8), 10(-6) M), (9 +/- 1 and 16 +/- 1 vs. 2 +/- 1 and 3 +/- 1% dilations to 8-Bromo cAMP before and after FPI, respectively, n = 8). Similarly, FPI attenuated dilation to pituitary adenylate cyclase activating peptide (PACAP), an endogenous activator of adenylate cyclase, and NS 1619, a K(Ca2+) channel agonist (9 +/- 1 and 16 +/- 1 vs. 3 +/- 1 and 5 +/- 1% for NS 1619 10(-8), 10(-6) M before and after FPI, respectively, n = 8). Moreover, FPI attenuated PACAP, methionine enkephalin, leucine enkephalin, and dynorphin induced elevations in CSF cAMP concentration (940 +/- 2, 1457 +/- 50, and 2191 +/- 53 vs. 810 +/- 17, 1033 +/- 36, and 1218 +/- 49 fmol/ml for control, PACAP 10(-8), 10(-6) M before and after FPI, respectively, n = 8). These data show that cAMP and K(Ca2+) channel function is impaired after FPI. Further these data suggest that impaired cAMP and K(Ca2+) channel function contribute to altered cerebral hemodynamics following FPI.

Alpha2B adrenoceptor mRNA expression during rat brain development

The expression of alpha2B adrenoceptor mRNA in developing and adult rat brain was examined, using in situ hybridization with S-labeled riboprobes. In the adult we have detected more widespread expression than previously reported, with moderate to strong hybridization signals in the fundus striati, olfactory tubercles, septum, thalamus and Purkinje cells of the cerebellum. In addition, there was low expression in the endopiriform nucleus, claustrum, cortex, caudate-putamen and spinal trigeminal nucleus of the brain stem. During embryonic development, there was intense, transient mRNA expression in the developing vascular plexus and vasculature which disappeared by birth. In most brain areas which exhibit mRNA expression in the adult, expression started during late embryonic development; with the exception of the thalamus, where expression was differentially regulated in sensory and non-sensory thalamic nuclei starting at the end of the first postnatal week. In addition, there was transiently upregulated expression in the caudate-putamen and cerebellar Purkinje cells during late embryonic and early postnatal development, respectively. This transient expression correlates with the time of neuronal migration and differentiation in these structures and complements the developmental expression of alpha2A and alpha2C adrenoceptors. These results suggest that alpha2B adrenoceptors may play a role in angiogenesis and in mediating neurotrophic functions of norepinephrine in some brain areas.

Intraventricular dexmedetomidine decreases cerebral blood flow during normoxia and hypoxia in dogs

We tested the hypothesis that a centrally administered alpha 2-receptor agonist could alter the cerebrovascular response to hypoxia, without evidence of systemic absorption of the drug. Beagle dogs were anesthetized with 1.4% isoflurane and exposed to hypoxic hypoxia (Pao2 approximately 22 mm Hg) before and after ventricular-cisternal perfusion with mock cerebrospinal fluid (CSF group, n = 5) or dexmedetomidine (100 micrograms/mL; total dose 300 micrograms; DEX group, n = 6). Cerebral perfusion pressure, Paco2 and arterial oxygen content were controlled and regional cerebral blood flow (CBF; microspheres) and global cerebral metabolic rate for oxygen consumption (CMRO2) were measured. In another group (n = 5), drug distribution under the experimental conditions was assessed by 3H-clonidine administered by ventricular-cisternal perfusion. In the mock CSF group, flow to the cerebral hemispheres increased during hypoxia under baseline conditions and after CSF infusion: 66 +/- 8 to 170 +/- 15 mL.min-1.100 g-1 (265% +/- 24% of baseline value), 83 +/- 9 to 154 +/- 14 mL.min-1.100 g-1 (201% +/- 54% of post-CSF infusion value). DEX decreased normoxic flow in the cerebral hemispheres from 76 +/- 6 to 44 +/- 4 ml.min-1.100 g-1 with decreases in other regions of similar magnitude. After DEX, the absolute flow in all regions during hypoxia was 52%-55% of that prior to DEX (P < 0.05). However, because DEX also decreased normoxic CBF, the percent increase in flow during hypoxia was similar before and after DEX. CMRO2 was not affected by hypoxia prior to DEX. However, after DEX, hypoxia caused a marked reduction in cerebral oxygen delivery (5.2 +/- 1.0 vs 13.7 +/- 2.3 ml.min-1.100 g-1 for the CSF group) and CMRO2 (2.5 +/- 0.6 vs 3.9 +/- 0.6 ml.min-1.100 g-1). Regional accumulation of intraventricularly administered 3H-clonidine was greatest in periventricular brain structures (e.g., caudate nucleus, dorsal brainstem), and the concentration in the cerebral cortex was approximately 1% of the concentration in the ipsilateral caudate nucleus. We conclude that centrally administered DEX reduces CBF during normoxia and prevents adequate oxygen delivery during hypoxia. The mechanism of DEX-induced CBF reduction is not metabolically mediated, since CMRO2 is maintained at control values during normoxia despite the significant blood flow reduction. We believe that the reduction in CMRO2 during hypoxia in DEX-treated dogs is the result of a reduction of oxygen delivery rather than the underlying mechanism for the observed reduction in CBF during hypoxia.

Influence of cAMP on cerebrospinal fluid opioid concentration: role in cAMP-induced pial artery dilation

Previously, it has been observed that cGMP analogs and agents that elevate cGMP levels markedly increase the concentration of the opioids [Met5]enkephalin and [Leu5]enkephalin in cortical periarachnoid cerebrospinal fluid (CSF) of the newborn pig. However, such agents had no effect on CSF dynorphin-(1-13) concentration. The present study was designed to: (1) investigate the influence of cAMP on the CSF concentration of the opioids [Met5]enkephalin, [Leu5]enkephalin and dynorphin-(1-13); and (2) determine the role of these opioids in cAMP-induced pial artery vasodilation. Piglets equipped with closed cranial windows were used to measure pial artery diameter and collect cortical periarachnoid CSF for assay of opioids. The cAMP analog, 8-Bromoadenosine-3',5'-cyclic monophosphate (8-Bromo cAMP) elicited pial dilation that was blunted by a cAMP antagonist, Rp 8-Bromoadenosine-3',5'-cyclic monosphorothioate (10(-5) M) (11 +/- 1 and 19 +/- 1 vs. 1 +/- 1 and 1 +/- 1 for 10(-8) M, 10(-6) M 8-Bromo cAMP before and after Rp 8-Bromoadenosine-3',5'-cyclic monosphorothioate, respectively). The dilation produced by 8-Bromo cAMP was accompanied by modest increases in CSF [Met5]enkephalin and co-administration of Rp 8-Bromoadenosine-3',5'-cyclic monosphorothioate with 8-Bromo cAMP blocked these increases in CSF opioid concentration (1179 +/- 48, 1593 +/- 92 and 2079 +/- 88 vs. 1054 +/- 32, 1038 +/- 15 and 1071 +/- 17 pg/ml for control, 10(-8) M and 10(-6) M 8-Bromo cAMP before and after Rp 8-Bromoadenosine-3',5'-cyclic monosphorothioate, respectively). The release of CSF [Leu5]enkephalin by 8-Bromo cAMP was also blocked by Rp 8-Bromoadenosine-3',5'-cyclic monosphorothioate. In contrast 8-Bromo cAMP produced marked increases in CSF dynorphin-(1-13) (38 +/- 3, 61 +/- 3 and 88 +/- 6 vs. 27 +/- 3, 28 +/- 3 and 30 +/- 4 pg/ml for control, 10(-8) M and 10(-6) M 8-Bromo cAMP before and after Rp 8-Bromoadenosine-3',5'-cyclic monosphorothioate, respectively). Similar blunted vascular and biochemical responses were observed with the co-administration of Sp 8-Bromoadenosine-3',5'-cyclic monophosphorothioate, another analog of cAMP, with Rp 8-Bromoadenosine-3',5'-cyclic monosphorothioate. The opioid receptor antagonist naloxone (1 mg/kg i.v.) attenuated 8-Bromo cAMP-induced dilation (9 +/- 1 and 17 +/- 1 vs. 5 +/- 1 and 8 +/- 1 for 10(-8) M, 10(-6) M 8-Bromo cAMP before and after naloxone). These data show that cAMP contributes to the release of the CSF opioids [Met5]enkephalin, [Leu5]enkephalin and dynorphin-(1-13), and suggest that, while cGMP is more important relative to cAMP in elevating CSF [Met5]enkephalin and [Leu5]enkephalin concentration, the converse is true for dynorphin-(1-13). Further, these data indicate that opioids contribute to cAMP-induced pial artery vasodilation.

Relationship between opioids and activation of phospholipase C and protein kinase C in brain injury induced pial artery vasoconstriction

Previously, it has been observed that newborn pig pial artery constriction after fluid percussion brain injury was associated with elevated CSF dynorphin and beta endorphin concentration. Additionally, brain injury reversed dynorphin-induced pial artery vasodilation to vasoconstriction. The present study was designed to characterize the relationship between opioids and activation of phospholipase C (PLC) and protein kinase C (PKC) in brain injury-induced pial vasoconstriction. Anesthetized newborn pigs equipped with a closed cranial window were connected to a percussion device consisting of a saline-filled cylindrical reservoir with a metal pendulum. Brain injury of moderate severity (1.9-2.3 atm) was produced by allowing the pendulum to strike a piston on the cylinder. Brain injury decreased pial arteriolar diameter within 10 min of injury and continued to fall progressively for 3 h (130 +/- 5, 108 +/- 4 and 102 +/- 5 microns for 0, 10 and 180 min postinjury). In contrast, the PLC inhibitor, neomycin (10(-4) M), blunted brain injury-induced pial vasoconstriction (133 +/- 4, 129 +/- 4 and 135 +/- 5 microns for 0, 10 and 180 min postinjury, respectively). Similarly, staurosporine (10(-7) M), a PKC inhibitor, also blunted brain injury-induced vasoconstriction. beta endorphin (10(-8), 10(-6) M)-induced pial artery vasoconstriction was blunted by neomycin (12 +/- 1, 19 +/- 1 vs. 2 +/- 1, 4 +/- 2% constriction before and after neomycin, respectively). Staurosporine similarly blunted beta endorphin pial constriction (10 +/- 1, 15 +/- 1 vs. 1 +/- 1, 1 +/- 1% constriction before and after staurosporine, respectively). The constrictor potential for dynorphin was also inhibited by neomycin and staurosporine.(ABSTRACT TRUNCATED AT 250 WORDS)

Regional cerebral blood flow responses to neurochemical stimulation of the substantia innominata in the anaesthetized rat

Since electrical stimulation of neurones may activate not only cell bodies but also neuronal fibres, this study aimed to test a selectively cholinergic neurochemical stimulation of the rat substantia innominata (SI) by the local microinjection of carbachol; the effects of this acetylcholine agonist were compared with glutamate. Cortical and subcortical cerebral blood flow (CBF) were measured in anaesthetized rats with the [14C]iodoantipyrine method by the tissue sampling technique immediately following the intracerebral (SI) microinjection of saline, 50 nmol of carbachol or glutamate. Carbachol microinjection into the SI induced a transient but significant vasodilation in frontoparietal motor (+28%) and temporal (+41%) cortices, that lasted for less than 10 min. Glutamate did not elicit any significant CBF modifications when compared to control rats although a significant interhemispheric asymmetry after microinjection was observed in the frontoparietal motor cortex. This latter observation would suggest that the glutamate-induced cortical response is less pronounced than that elicited by carbachol. Overall, these results demonstrate that a selective cholinergic stimulation of the SI can induce a transient cortical vasodilation and further confirms the hypothesis of a muscarinic modulation of CBF via this basal structure.

The contribution of delta 1- and delta 2-opioid receptors to hypoxia-induced pial artery dilation in the newborn pig

Previously, it has been observed that mu-opioid receptors contribute to while kappa-opioid receptors oppose pial artery dilation in response to hypoxia. The present study was designed to investigate the contribution of delta 1- and delta 2-opioid receptor activation to hypoxia-induced pial vasodilation. Newborn pigs equipped with a closed cranial window were used to measure pial artery diameter and collect cortical periarachnoid CSF for assay of opioids. Hypoxia increased CSF leucine enkephalin (a delta -agonist) from 36 +/- 6 to 113 +/- 17 pg/ml (n = 5). Hypoxia-induced pial artery vasodilation was attenuated during moderate hypoxia (PaO2 approximately 35 mm Hg), while this response was blunted during severe hypoxia (PaO2 approximately 25 mm Hg), by the delta 1-opioid receptor antagonist 7-benzylidenenaltrexone (BNTX; 10(-8) M) (23 +/- 2 vs. 13 +/- 2 and 34 +/- 6 vs. 10 +/- 3% for moderate and severe hypoxia in the absence and presence of BNTX, respectively; n = 5). In contrast, the delta 2-opioid receptor antagonist naltrindole (10(-9) M) blunted pial vasodilation during moderate hypoxia, but only attenuated the vasodilator response during severe hypoxia (22 +/- 2 vs. 8 +/- 2 and 33 +/- 4 vs. 23 +/- 4% for moderate and severe hypoxia in the absence and presence of naltrindole, respectively; n = 5).(ABSTRACT TRUNCATED AT 250 WORDS)

The role of nitric oxide in opioid-induced pial artery vasodilation

The present study was designed to investigate the role of nitric oxide (NO) and the production of cGMP in the vasodilator response to opioid agonists in newborn pigs equipped with a closed cranial window. Methionine-enkephalin (10(-8), 10(-6) M), an endogenous mu opioid agonist, produced pial artery dilation that was attenuated by L-nitroarginine (LNA, 10(-6) M), an NO synthase inhibitor (10 +/- 1 vs. 4 +/- 1 and 16 +/- 1 vs. 7 +/- 1% for 10(-8), 10(-6) M methionine-enkephalin, respectively). Methionine-enkephalin-induced vasodilation was associated with increased cortical periarachnoid CSF cGMP and these changes in CSF cGMP were attenuated by LNA (354 +/- 11 and 596 +/- 32 vs. 278 +/- 13 and 266 +/- 19 fmol/ml for control and methionine-enkephalin 10(-6) M before and after LNA, respectively). Leucine enkephalin, an endogenous delta agonist, elicited similar changes in pial diameter and CSF cGMP while dynorphin, an endogenous k agonist, produced dilation associated with large increases in CSF cGMP (374 +/- 18 vs. 1054 +/- 45 fmol/ml for control and dynorphin 10(-6) M, respectively). Vascular and biochemical changes for these two opioids were similarly attenuated by LNA. The synthetic selective opioid receptor agonists, DAMGO, DPDPE, deltorphin, and U50,488H (10(-8), 10(-6) M) mu, delta 1, delta 2, and kappa agonists, respectively, also elicited increases in pial artery diameter and CSF cGMP that were similarly attenuated by LNA.(ABSTRACT TRUNCATED AT 250 WORDS)

Cholinergic and vasoactive intestinal polypeptidergic innervation of the cerebral arteries

Acetylcholine and vasoactive intestinal polypeptide are not only two vasoactive agonists that predominantly induce a vasodilatation of the cerebral arteries, but also correspond to neurotransmitters that innervate the various anatomical segments of the cerebral vasculature. The distinct patterns of the cerebrovascular cholinergic and vasoactive intestinal polypeptidergic innervation, their neurochemistry, in vitro and in vivo pharmacology, as well as the putative pathophysiological implications of these neurotransmission systems are critically summarized on the basis of the most recently published literature.

Potential mechanisms of the age-related changes in the blood-brain barrier

A variety of age-related changes in the blood-brain barrier transport processes have been identified. These include reduced hexose and butyrate transport, reduced choline transport, reduced triiodothyronine transport without a change in the transport of neutral and basic amino acids. The potential mechanisms underlying these age-related changes include hemodynamic alterations in the cerebral circulation of aged rats, notably increased occurrence of arteriovenous shunting. Additional age-related changes in cerebral microvessels include alterations in protein composition, and increased accumulation of lipid peroxidation byproducts, along with changes in membrane fluidity of isolated cerebral microvessels. In addition, neurotransmitter activity notably beta adrenergic neurotransmission, is significantly reduced in cerebral microvessels of aged rats. These alterations taken together may account for some of the age-related changes in the blood-brain barrier.

The role of opioids in newborn pig fluid percussion brain injury

The present study was designed to characterize the relationship between cerebral opioid concentration, cerebral hemodynamics, and cerebral oxygenation following percussion brain injury in neonatal pigs. Previous research found that opioids represent a significant vasoactive component in the regulation of the neonatal piglet cerebral circulation. Anesthetized newborn (1-5 days old) pigs equipped with a closed cranial window were connected to a percussion device consisting of a saline-filled cylindrical reservoir with a metal pendulum. Brain injury of moderate severity (1.9-2.3 atm.) was produced by allowing the pendulum to strike a piston on the cylinder. Fluid percussion brain injury decreased pial arteriolar diameter (132 +/- 5 to 110 +/- 5 microns within 10 min). Cerebral blood flow also fell within 10 min of injury and continued to fall progressively for 3 h, resulting in a 46 +/- 4% decrease. Within 30 s of brain injury, there was a transient increase in cerebral hemoglobin-O2 saturation that was reversed to a progressive profound decrease in cerebral hemoglobin-O2 saturation for the next 3 h, as measured by near infrared spectroscopy. CSF opioid concentrations were increased 10 min after brain injury; dynorphin showed the largest proportional increase (5.8 +/- 0.9 fold). The CSF concentration for other opioids continued to increase over 180 min while the dynorphin concentration progressively decreased with time. In naloxone (1 mg/kg i.v.) pretreated piglets, the brain injury induced decrease in arteriolar diameter was attenuated (129 +/- 5 to 121 +/- 5 microns within 10 min). Similarly, the decrease in regional cerebral blood flow and cerebral hemoglobin-O2 saturation observed following brain injury were also blunted by naloxone.(ABSTRACT TRUNCATED AT 250 WORDS)

Stimulation of phospholipase C in cultured microvascular endothelial cells from human frontal lobe by histamine, endothelin and purinoceptor agonists

1. Cultures of endothelial cells derived from the microvasculature of human frontal lobe have been investigated for phospholipase C (PLC) responses to histamine, endothelins and purinoceptor agonists. 2. Using cells prelabelled with [3H]-inositol and measuring total [3H]-inositol (poly)phosphates, histamine acting at H1 receptors stimulated a substantial response with an EC50 of about 10 microM. 3. Endothelin-1 also gave a clear stimulation of phosphoinositide-specific phospholipase C. Both concentration-response curves and binding curves showed effective responses and binding in the rank order of endothelin-1 > sarafotoxin S6b > endothelin-3, suggesting an ETA receptor. 4. Assay of total [3H]-inositol (poly)phosphates showed no response to the purinoceptor agonists, 2-methylthioadenosine 5'-trisphosphate (2MeSATP), adenosine 5'-O-(3-thiotrisphosphate) (ATP gamma S) or beta,gamma-methylene ATP. Both ATP and UTP gave a small PLC response. 5. Similarly, when formation of [32P]-phosphatidic acid from cells prelabelled with 32Pi was used as an index of both PLC and phospholipase D, a small response to ATP and UTP was seen but there was no response to the other purinoceptor agonists tested. 6. Study by mass assay of stimulation by ATP of inositol (1,4,5) trisphosphate accumulation revealed a transient response in the first few seconds, a decline to basal, followed by a small sustained response. 7. These results show that human brain endothelial cells in culture are responsive to histamine and endothelins in a manner which may regulate brain capillary permeability. Purines exert a lesser influence.

Calcium-dependence of histamine- and carbachol-induced inositol phosphate formation in human U373 MG astrocytoma cells: comparison with HeLa cells and brain slices

1. Histamine (1 mM) induced an accumulation of inositol monophosphate ([3H]-IP1) in the U373 MG human astrocytoma cell line which increased with time in the presence of 30 mM Li+. After a 30 min incubation period with 1 mM histamine [3H]-IP1 was the major product detected (84 +/- 1% of total [3H]-IPx) and was present at a level 11 (+/- 1) fold of basal accumulation. 2. Concentration-response curves for histamine-induced [3H]-IP1 accumulation in U373 MG cells (EC50 5.4 +/- 0.5 microM) were shifted to the right in a parallel fashion by mepyramine (slope of a Schild plot 0.99 +/- 0.08), yielding a Kd for mepyramine of 3.5 +/- 0.3 nM, consistent with the involvement of histamine H1-receptors. 3. The temelastine-sensitive binding of [3H]-mepyramine to a membrane fraction from U373 MG cells was hyperbolic and had a mean Kd of 2.5 +/- 1.0 nM. The maximum amount of temelastine-sensitive binding was 86 +/- 19 pmol g-1 membrane protein. 4. Carbachol also induced [3H]-IP1 accumulation in U373 MG cells, 2.8 (+/- 0.1) fold of basal with 1 mM carbachol, with an EC50 of 48 +/- 8 microM. Pirenzepine shifted carbachol concentration-response curves to the right (slope of Schild plot 0.89 +/- 0.07) giving a Kd for pirenzepine of 0.10 +/- 0.01 microM, suggesting that phosphoinositide hydrolysis in U373 MG cells is mediated by the M3-, rather than the M1-, muscarinic receptor subtype. 5. [3H]-IP1 accumulation induced by both 1 mM histamine and by 1 mM carbachol increased when the Ca2+ concentration of the medium was increased from 'zero' (no added Ca2+) to 0.3 mM. Histamine-stimulated [3H]-IP1 accumulation was further increased, although not so markedly, as the Ca2+ was raised to 4 mM. The same pattern was apparent with histamine-induced accumulations of [3H]-IP2 and [3H]-IP3. In contrast, [3H]-IPx accumulation in response to carbachol increased between 0.3 and 1.3 mM, but thereafter remained unchanged ([3H]-IP1) or declined ([3H]-IP2 and [3H]-IP3). 6. In HeLa cells, [3H]-IP1 accumulations induced by 1 mM histamine and 1 mM carbachol showed the same pattern of Ca2+ dependence and were independent of extracellular Ca2+ above 0.3 mM (histamine) or 1.3 mM (carbachol). The response to carbachol appeared to be mediated by an M3-muscarinic receptor (apparent Kd for pirenzepine 0.09 microM). 7. In cross-chopped slices of guinea-pig cerebral cortex and guinea-pig cerebellum, [3H]-IPI accumulation induced by 1 mM histamine in the presence of 10 mM Li+ increased as the extracellular Ca2+ was increased from 0.3 to 2.5 mM, but a further increase to 4 mM had no further effect. In contrast the response to histamine in rat cerebral cortex increased markedly between 1.3 and 4 mM Ca2+. Accumulations of [3H]-IP1 induced by carbachol in guinea-pig or rat cerebral cortical slices were not increased as extracellular Ca2+ was raised from 0.3 to 4 mM.8. Nimodipine (100 nM) and w-conotoxin (3 microM) had no significant effect on histamine-induced [3H]-IP1accumulation in rat cerebral cortical slices or in U373 MG cells. 9. We conclude that histamine-induced [3H]-IP1 accumulation in U373 MG cells does appear to have a component dependent on the extracellular Ca2+ concentration. The degree of Ca2+-dependence approaches that observed in guinea-pig cerebral cortex but is much less than in rat cerebral cortex.Whether U373 MG cells will be of use as a model system for the apparent Ca2+-entry component observed in guinea-pig or rat brain slices remains to be established.

Effect of histamine on the blood-tumor barrier in transplanted rat brain tumors

We studied the effect of intracarotid administration of histamine on the blood-tumor barrier permeability and also on the blood-brain barrier permeability in transplanted rat C6 glioma. There was no definite Evans blue (EB) extravasation either in normal or tumor tissue after intracarotid saline infusion. In contrast, histamine at doses of 1 and 10 micrograms/kg/min produced slight to moderate EB extravasation in the tumor without any significant extravasation in the normal brain tissue. Intravenously administered H1 and H2 receptor antagonists (5 mg/kg each) reduced the histamine (10 micrograms/kg/min) induced extravasation of EB in the tumor tissue. These results indicated that brain tumor vessels responded to histamine in a different fashion from normal brain capillaries. Histamine could thus be utilized for selective drug delivery to brain tumors without affecting normal brain tissue.

Nitric oxide and endothelin secretion by brain microvessel endothelial cells: regulation by cyclic nucleotides

Endothelin (ET)-1 was originally characterized as a potent vasoconstrictor peptide secreted by vascular endothelial cells. It possesses a wide range of biological activities within the cardiovascular system and in other organs, including the brain. Also secreted by endothelial cells, nitric oxide (NO), has recently been identified as a relaxing factor, as well as a pleiotropic mediator, second messenger, immune defence molecule, and neurotransmitter. Most of the data concerning the secretion of these two agents in vitro has been collected from studies on macrovascular endothelial cells. Given the remarkable heterogeneity of endothelia in terms of morphology and function, we have analyzed the ability of brain microvessel endothelial cells in vitro to release ET-1 and NO, which, at the level of the blood-brain barrier, have perivascular astrocytes as potential targets. The present study was performed with immortalized rat brain microvessel endothelial cells, which display in culture a non transformed phenotype. Our data demonstrate that: (1) these cells release NO when induced by IFN gamma and TNF alpha, (2) they constitutively secrete ET-1, and (3) cAMP potentiates the cytokine-induced NO release and exerts a biphasic regulation on ET-1 secretion: micromolar concentrations of 8-Br-cAMP inhibit and higher doses stimulate ET-1 secretion. This stimulation is blocked by EGTA and the calmodulin antagonist W7, but not by protein kinase C inhibitors, suggesting the involvement of the calmodulin branch of the calcium messenger system. These results suggest that cerebral microvessel endothelial cells may participate in vivo to the regulation of glial activity in the brain through the release of NO and ET-1.

Potassium transport at the blood-brain and blood-CSF barriers

Figure 5 gives a summary of K transporters at the BBB based on the available evidence. It appears that the cerebral endothelial cells have an array of potassium channels, although the degree to which each is open under physiological conditions is uncertain. Different channels are present on the luminal and abluminal membranes, and the opening and closing of these channels may allow modulation of the brain K influx and efflux rates and play a role in brain K homeostasis. These channels may also play a role in hyperosmotic brain volume regulation by increasing the entry rate of potassium into brain and may be involved in volume regulation of the endothelial cell itself. The nature of fluid transport at the BBB remains to be fully elucidated, with the presence of a Na/K/2Cl co-transporter being uncertain. The abluminal inwardly-rectifying channel may act as a leak pathway to allow modulation of fluid secretion by the Na/K ATPase without altering the K concentration of that fluid. Finally, there is some evidence that K transport at the BBB is under hormonal and neuronal control. The cerebral capillaries possess receptors for many of the hormones present in blood and brain.