Postischemic regulation of central histamine receptors

This study characterizes changes occurring in the central histaminergic system associated with ischemia-reperfusion pathology in the rat. Specifically, after a postocclusion time period of 48 h, we have analyzed histamine H(1) receptor mRNA expression, histamine H(2) receptor protein amount and binding densities, and histamine H(3) receptor mRNA expression and binding densities in brain regions that have been suggested to be selectively vulnerable to transient global ischemia, i.e. hippocampus, thalamus, caudate-putamen, and cerebral cortex. We found an increase in H(1) receptor mRNA expression in the caudate-putamen: given that ischemia reduces glucose uptake and H(1) receptor activation has been shown to decrease this effect, an increase of expression levels may result in mitigating tissue damage due to energy failure observed in ischemia. A decrease in H(2) receptor binding densities in the caudate-putamen was also observed; the ischemia-induced decrease in H(2) receptor protein was also detectable by Western blot analysis. This phenomenon may underlie the previously reported ischemia induced striatal dopamine release. H(3) receptor mRNA expression was increased in the caudate putamen of the postischemic brain but was decreased in the globus pallidus and the thalamus; in association with this, H(3) receptor binding densities were increased in the cortex, caudate-putamen, globus pallidus, and hippocampus. The upregulation of H(3) receptor ligand binding may be involved in the previously reported continuous neuronal histamine release. Our data suggest that central histamine receptor expression and ligand binding are altered in brain ischemia in distinct areas, and may participate in neuroprotection and/or ischemia-associated neuronal damage.     

Measurements of cerebral blood flow and metabolism in patients with Alzheimer's disease

Positron emission tomography (PET) was used to measure cerebral blood flow (CBF), cerebral metabolic rate of oxygen (CMRO2), and cerebral metabolic rate of glucose (CMRglc) in patients with Alzheimer's disease. In the patients, values for CBF, CMRO2, and CMRglc have been shown to drop by 30-50% in comparison to age-matched normal controls. In the early stage (stage I), reductions in CBF and CMRO2 are prominent in the temporal and the temporoparietal cortices. In stage II, reduction in the parietal cortex also become quite prominent, and in the late stage (stage III) reduction begins prominently in the frontal cortex as well. These PET findings in Alzheimer's disease differ from those in vascular dementia, Pick's disease, and Huntington's disease. In the interrelationship among CBF, CMRO2 and higher brain function, CBF and CMRO2 decrease especially in the left frontal, the left temporal and the left parietal cortices in patients with marked language disability. On the contrary, CBF and CMRO2 decrease in the right temporal and the right parietal cortices in patients with marked apraxia and visuospatial deficits. Cerebral blood flow and metabolism are closely related to the functioning of nerve cells. Therefore we can isolate the region responsible for higher brain dysfunction and similarly evaluate the effects of treatment using cerebral blood flow and metabolism measurements.     

Histamine modulates heat stress-induced changes in blood-brain barrier permeability, cerebral blood flow, brain oedema and serotonin levels: an experimental study in conscious young rats.

The possibility that endogenous histamine plays an important role in modulating the pathophysiology of heat stress was examined in young rats using a pharmacological approach. Subjection of young animals (six to seven weeks old) to heat stress at 38 degrees C for 4 h in a biological oxygen demand incubator (relative humidity 47-50%, wind velocity 20-25 cm/s) resulted in a profound increase in blood-brain barrier permeability to Evans Blue albumin (whole brain 375%) and [131I]sodium (whole brain 478%) along with a significant reduction in the cerebral blood flow (mean 34%). The water content of the whole brain was elevated by 4.5% (about 19% volume swelling) from the control. At this time-period, the plasma and whole brain 5-hydroxytryptamine levels were elevated by 656% and 328%, respectively, from the control group. Pretreatment with cimetidine (a histamine H2 receptor antagonist) significantly thwarted the increases in the brain water content and the blood-brain barrier permeability. In cimetidine-pretreated animals, the cerebral blood flow was significantly elevated and the plasma and brain 5-hydroxytryptamine (serotonin) levels were slightly but significantly reduced as compared with the untreated stressed group. However, prior treatment with mepyramine (a histamine H1 receptor antagonist) neither attenuated the changes in water content and the blood-brain barrier permeability nor altered the cerebral blood flow and 5-hydroxytryptamine levels. In fact, there was a significantly higher permeation of the tracers across the cerebral vessels in these drug-treated animals along with a greater accumulation of the brain water content as compared with the untreated stressed group. The cerebral blood flow and 5-hydroxytryptamine levels showed only minor changes from the untreated stressed group. These results show, probably for the first time, that (i) the endogenous histamine plays an important role in the pathophysiology of heat stress, and (ii) this effect appears to be mediated via specific histamine H2 receptors.     

The pattern of function-related regional cerebral blood flow investigated by single photon emission tomography with 99mTc-HMPAO in patients with presenile Alzheimer's disease and Korsakoff's psychosis

Single photon emission tomography (SPET) with the lipophilic blood flow marker 99mTc-hexamethyl propyleneamine oxime (99mTc-HMPAO) has been used to determine regional uptake of radiolabel into brain regions of patients with presenile Alzheimer's disease and Korsakoff's psychosis, and age-matched controls. Using occipital cortical uptake as reference area, the pattern of relative regional cerebral blood flow (rCBF) was determined in other cortical areas and basal ganglia. In Alzheimer's disease, reduction in rCBF occurred most strikingly in posterior temporal and parietal areas. By contrast, in Korsakoff's psychosis, posterior temporal rCBF was maintained, although there was a trend to reduced tracer uptake in other cortical areas. These impairments of flow were correlated with impairments of neuropsychological function. In Alzheimer's disease, left posterior temporal and left parietal regions in particular showed rCBF to be strongly correlated with most aspects of cognitive function. In Korsakoff's psychosis, however, impaired flow in frontal regions was correlated with impaired performance on tests of memory and orientation. The findings in Alzheimer's disease show quantitative parallels with those from studies using Positron Emission Tomography (PET), and extend our understanding of the relationship between cognition and regional brain function in dementia. The findings in Korsakoff's psychosis offer the first direct evidence linking frontal lobe dysfunction with the cognitive impairment seen in the disorder.     

Specific immunotherapy stimulates the binding of histamine H2 receptor antagonist by lymphocytes.

The binding of antagonists of histamine receptors H1 (promethazine) and H2 (ranitidine) by peripheral blood lymphocytes from pollinotics was determined before and after the course of immunotherapy. We found that lymphocytes from atopic subjects showed significant decrease in the binding of H2 receptor antagonist as compared to control subjects. Specific immunotherapy induced statistically significant increase in H2 receptor antagonist binding, which correlated with the improvement of clinical symptoms.     

Binding of antagonists of H1 and H2 histamine receptors to peripheral blood lymphocytes of atopic and healthy subjects.

The binding of the antagonists of histamine H1 and H2 receptors by peripheral blood lymphocytes from atopic and healthy subjects was investigated. We found that lymphocytes from atopic subjects showed statistically significant decrease in the binding of H2 receptor antagonist - ranitidine. In addition, lymphocytes from atopic and control subjects had similar capacity of binding of H1 receptor antagonist - promethazine. The ratio of the amount of H1 and H2 antagonists, bound to lymphocytes from atopic and healthy subjects, was calculated. The difference between the values in the group of atopic (2.55) and control subjects (1.55) was statistically significant.     

Muscarinic acetylcholine receptor subtype mRNA expression and ligand binding in the aged rat forebrain

Previous studies indicate that a 20-30% decline in muscarinic acetylcholine receptor binding occurs in localized areas of rat brain during aging. In this study, reduced [3H]-quinuclidinyl benzilate binding was observed in striata from 24-25-month-old rats relative to 5-6-month-old animals using homogenate binding assays. To determine if the decline in receptor concentration occurs as a result of decreased receptor synthesis, the expression of the m1, m3, and m4 muscarinic receptor mRNAs as well as [3H]-QNB binding were determined in adjacent sections of young and old male rats using in situ hybridization and in vitro receptor autoradiography respectively. A significant decline in collective muscarinic receptor binding as assessed by [3H]-QNB was observed in the caudate putamen, olfactory tubercle, nucleus accumbens, and several frontal and parietal cortical areas. The only difference observed in muscarinic mRNA expression for any of the three subtypes examined was a decline in m1 hybridization in the olfactory tubercle. The results of this study demonstrate that the regional brain areas displaying age-related decreases in receptor binding do not correlate with those areas showing a decrease in muscarinic receptor expression. Apparently, the decline in muscarinic acetylcholine receptor density with age does not result from a decline in receptor gene expression.     

Regional expression of the histamine H(2) receptor in adult and developing rat brain

Histamine H(2) receptor expression was studied in adult and developing rat brain. Northern blot and in situ hybridizations indicated that histamine H(2) receptor messenger RNA expression is widespread and not limited to neurons in the adult rat brain. Prominent H(2) receptor expression in the adult brain was seen in the dentate gyrus, hippocampal subfields CA1-CA3, piriform cortex and in some diencephalic nuclei, e.g. in the suprachiasmatic nucleus and the red nucleus. Most of the adult brain nuclei displayed a very low H(2) receptor expression. Histamine H(2) receptor was also expressed during development in widespread areas of the central nervous system, coinciding with the transient production of histamine in the raphe neurons at embryonic day 15. From embryonic days 16 and 17 until birth, histamine H(2) receptor expression in the cortical plate coincided with the development and sprouting of histaminergic fibers into the cerebral cortex. The widespread and diffuse expression of histamine H(2) receptors in the adult rat brain suggests that the H(2) receptor modulates the excitability of neuron and astrocyte functions in many brain areas rather than mediating targeted cell-to-cell signals. During development, histamine H(2) receptor expression is seen in several target areas for the histaminergic fibers. This could indicate that histamine, through the H(2) receptor, regulates fetal development of the brain.     

Brain metabolic alterations in Cushing's syndrome as monitored by proton magnetic resonance spectroscopy.

Proton magnetic resonance spectroscopy ((1)H MRS) was used to evaluate changes in cerebral metabolites in 13 patients with Cushing's syndrome (including seven with pituitary corticotroph adenomas and six with primary adrenal disease) as compared to 40 normal subjects. Data were recorded in the frontal, thalamic and temporal areas; quantification of the MRS signals demonstrated a statistically significant decrease of the Cho/Cr ratio in the frontal and thalamic areas but not in the temporal area for patients with Cushing's syndrome. The largest decrease in Cho/Cr was measured in the thalamic area of patients with a Cushing's syndrome secondary to an adrenal disease. No statistically significant changes in the NAA/Cr ratio were measured in any of the areas studied. These results suggest that the quantification of choline levels could be helpful for monitoring the cerebral metabolite alterations in patients with hypercortisolism.     

3H]vesamicol binding in human brain cholinergic deficiency disorders

We measured the binding of the vesicular acetylcholine transport blocker [3H]vesamicol (2-[4-phenylpiperidino] cyclohexanol; AH-5183) to autopsied frontal cortex and amygdala of patients from 4 disorders having a marked brain cholinergic reduction, namely Alzheimer's disease, Parkinson's disease with dementia, dominantly inherited olivopontocerebellar atrophy and Down's syndrome. Although mean activity of the specific cholinergic marker enzyme choline acetyltransferase (ChAT) was markedly reduced by about 60% in frontal cortex in the 4 patient groups and by 80% or greater in amygdala of the Alzheimer's and Down's syndrome patients, [3H]vesamicol binding density was, on average, either normal or only slightly reduced as compared with the controls. This discrepancy suggests that in human brain [3H]vesamicol binding is either not preferentially localized to cholinergic nerve endings or, in these cholinergic deficiency syndromes, a substantial proportion of the vesamicol binding sites persist on cholinergic nerve terminals despite loss of ChAT activity.     

125I]-omega conotoxin binding to human frontal cortex from normal, Alzheimer's and non-Alzheimer's dementia patients

We studied the binding of the calcium antagonist neurotoxin [125I]-omega conotoxin (GVIA) in age-matched human brains from normal, Alzheimer's disease and non-Alzheimer's dementia patients. Crude preparations of plasmalemmal membranes from frontal cortex were utilized. Saturation isotherms were subjected to Scatchard analysis to determine maximal binding capacity (Bmax) and binding affinity (Kd). In all brain samples tested, [125I]-GVIA binding was homogenous to a single class of high affinity binding sites. Scatchard analysis of saturation isotherms gave the following estimates for normal brains (mean +/- S.D., n = 7): Bmax = .630 +/- .200 pmol/mg and Kd = .177 +/- .054 nM. No significant change was observed in the Kd or Bmax estimates for [125I]-omega conotoxin binding in Alzheimer's disease or non-Alzheimer's dementia brains when compared to normal brains. Although these findings do not rule out the existence of localized changes in calcium channel receptor binding in the frontal cortex of Alzheimer's disease patients, the results do suggest that the neuronal voltage sensitive calcium channel may be unaltered in Alzheimer's disease.     

Preservation of 5-hydroxytryptamine1A receptor-G protein interactions in the cerebral cortex of patients with Alzheimer's disease.

The coupling of 5-hydroxytryptamine1A (5-HT1A) receptors to guanine nucleotide binding (G) proteins was investigated in membranes prepared from frontal and parietal cortices of control and Alzheimer's disease brains by characterising the effect of guanosine 5'-[beta gamma-imido]diphosphate (Gpp[NH]p) on [3H]8-hydroxy-2-(di-n-propylamino)-tetralin ([3H]8-OH-DPAT) binding parameters. In the absence of guanine nucleotides, [3H]8-OH-DPAT bound to a single high affinity binding site in all membrane types. The number of [3H]8-OH-DPAT binding sites was significantly decreased in the parietal cortex of Alzheimer's disease samples compared with controls, whereas in the frontal cortex the number of binding sites remained unchanged. Gpp[NH]p reduced the [3H]8-OH-DPAT binding affinity and the number of binding sites to the same degree in both regions in control and Alzheimer's disease cases. [3H]8-OH-DPAT binding was inhibited in a concentration dependent manner with an IC50 value of approximately 1 microM in all cases. These results suggest that the 5-HT1A receptor-G protein complex is functionally intact in these regions in Alzheimer's disease brain.     

Alterations in the ryanodine receptor calcium release channel correlate with Alzheimer's disease neurofibrillary and beta-amyloid pathologies.

Investigation of the integrity of the ryanodine receptor in Alzheimer's disease is important because it plays a critical role in the regulation of calcium release from the endoplasmic reticulum in brain, impairment of which is believed to contribute to the pathogenesis of Alzheimer's disease. The present study compared ryanodine receptor levels and their functional modulation in particulate fractions from control and Alzheimer's disease temporal cortex, occipital cortex and putamen. Relationships between ryanodine receptor changes and the progression of Alzheimer's disease pathology were determined by examining autoradiographic [3H]ryanodine binding in entorhinal cortex/anterior hippocampus sections from 22 cases that had been staged for neurofibrillary changes and beta-amyloid deposition. A significant (P < 0.02) 40% decrease in the Bmax for [3H]ryanodine binding and significantly higher IC50 values for both magnesium and Ruthenium Red inhibition of [3H]ryanodine binding were detected in Alzheimer's disease temporal cortex particulate fractions compared to controls. Immunoblot analyses showed Type 2 ryanodine receptor holoprotein levels to be decreased by 20% (P < 0.05) in these Alzheimer's disease cases compared to controls. No significant differences were detected in [3H]ryanodine binding comparing control and Alzheimer's disease occipital cortex or putamen samples. The autoradiography study detected increased [3H]ryanodine binding in the subiculum, CA2 and CA1 regions in cases with early (stage I-II) neurofibrillary pathology when compared to Stage 0 cases. Analysis of variance of data with respect to the different stages of neurofibrillary pathology revealed significant stage-related declines of [3H]ryanodine binding in the subiculum (P < 0.02) with trends towards significant decreases in CA1, CA2 and CA4. Post-hoc testing with Fisher's PLSD showed significant reductions (74-94%) of [3H]ryanodine binding in the subiculum, and CA1-CA4 regions of the late isocortical stage (V-VI) cases compared to the early entorhinal stage I-II cases. [3H]Ryanodine binding also showed significant declines with staging for beta-amyloid deposition in the entorhinal cortex (P < 0.01) and CA4 (P < 0.05) with trends towards a significant decrease in the dentate gyrus. We conclude that alterations in ryanodine receptor binding and function are very early events in the pathogenesis of Alzheimer's disease, and may be fundamental to the progression of both neurofibrillary and beta-amyloid pathologies.     

Regulation of antibody-dependent cellular cytotoxicity in multiple sclerosis by central nervous system hormones

Using oligodendrocytes from primary brain cultures as targets in an antibody-dependent cellular cytotoxicity (ADCC) assay, we have examined the effects of insulin and histamine on killer cells in multiple sclerosis (MS) and other neurological disease (OND) controls compared to normal healthy controls. The effects were shown to be specific for effectors by preincubation experiments. MS patients' ADCC to primary oligodendrocytes was depressed, but could be boosted to normal control levels by histamine binding to the H1 receptor. Significant elevation of MS ADCC by cimetidine alone suggested that endogenous histamine production and H2 receptor binding could be mediating a suppressive effect on MS ADCC to oligodendrocytes. In addition, MS ADCC could be boosted significantly by insulin. MS killer cells were more sensitive in vitro to the boosting effects of both histamine and insulin than either OND or normal controls, both in dose response and magnitude of the increased ADCC.     

Density and pharmacology of alpha5 subunit-containing GABA(A) receptors are preserved in hippocampus of Alzheimer's disease patients

The anatomical localization and pharmacology of alpha5 subunit-containing GABA type-A receptors in the human hippocampal formation of Alzheimer's disease patients were studied with an alpha5 receptor selective ligand, [3H]L-655,708 and compared to age-matched human controls. Autoradiographic analyses revealed a heterogeneous distribution of [3H]L-655,708 binding sites in CA1-CA3 areas with high levels in stratum oriens, stratum pyramidale and stratum radiatum contrasting with low levels in stratum lacunosum. The highest quantity of alpha5 receptors was found in the molecular layer of the dentate gyrus. This pattern of expression was identical in both hippocampus from control and Alzheimer's disease subjects. Quantitative studies demonstrated that the number of [3H]L-655,708 binding sites is well preserved in Alzheimer's disease with only a moderate reduction (25-30%) in the CA1 subfield and entorhinal cortex. Furthermore, saturation and competition experiments with [3H]L-655,708 and representative benzodiazepine site ligands revealed that alpha5 receptors in Alzheimer's hippocampus have an alpha5beta2/3gamma2 pharmacology and structure as in control human brain.Overall, the data reported here provide evidence for a specific expression and relative sparing of alpha5 subunit-containing gamma-aminobutyric acid type-A receptors in the hippocampus of Alzheimer's patients.     

Alzheimer's disease: a review concerning immune response and microischemia

Alzheimer's disease (AD), as we think of it today, is the idiopathic progressive loss of cognitive function over a period of several years. The risk of late onset dementia increases significantly with each decade of life such that half of the population over the age of 80 is vulnerable to this disease (1). We know that proper functioning of the central nervous system is dependent on adequate blood flow to remove harmful metabolic products and supply nutrients such as glucose and oxygen to the brain. It has been suggested that cerebral hypoperfusion causes AD (2). Mean cerebral blood flow decreases with age and with sclerosis of cerebral blood vessels. Blood flow appears to increase in stimulated areas of the brain during different activities. However, there is a derangement of blood flow in disease states; this has been documented in the temporal lobes of AD patients, (3,4). English language journal articles located by a MEDLINE search (1960-1999) were reviewed with consideration to the hypothesis that Alzheimer's disease is an autoimmune disease initiated by low oxygen tension and microischemia. Inflammation is thought to be a known contributor to the pathology of AD (5,6). Recent reports support the concept of autoimmunity as a final common pathway of neuron death, particularly for cholinergic in Alzheimer's disease (6). A model of Alzheimer's disease is proposed and related research and treatment modalities are discussed.     

Peripheral benzodiazepine binding sites in Alzheimer's disease frontal and temporal cortex.

Much evidence has accumulated to suggest that the peripheral type benzodiazepine (PBZ) binding site has a predominantly glial localization. Elevated PBZ binding densities have been reported in various models of brain damage, apparently reflecting glial proliferation in response to neurodegeneration. In the present study, PBZ receptor densities were examined in frontal and temporal cortex of Alzheimer's disease (AD) patients using the ligand [3H]PK 11195. There was a highly significant (p less than 0.01) increase in PBZ binding sites in the temporal cortex from AD patients. In the frontal cortex, a moderate increase was observed that approached statistical significance (p = 0.07). Decreased choline acetyltransferase activity was observed in both regions. These findings offer support for the potential use of the PBZ binding site as a marker for gliosis associated with neuronal cell death.     

Muscarinic and nicotinic contributions to cognitive function and cortical blood flow.

Muscarinic receptor blockade in humans induces a transient memory deficit claimed to mimic aspects of Alzheimer's disease (AD). AD is also strongly associated with a specific blood flow abnormality in parietotemporal cortex; we previously showed that, despite induction of a dementia-like state, scopolamine does not produce these blood flow changes. In the present study, we administered both muscarinic and nicotinic receptor blockade (using scopolamine and mecamylamine) to seven elderly healthy subjects and measured subsequent changes in cognition and cortical perfusion, using the 133Xe inhalation method to quantify regional cerebral blood flow (rCBF). Results confirmed earlier findings of scopolamine-induced memory deficit and frontal cortex flow reduction. Only mecamylamine, however, produced a perfusion deficit in parietotemporal cortex. All effects were transient and dose-dependent. These findings demonstrate the safety and feasibility of differential and combined blockade of nicotinic and muscarinic cholinergic blockade in human subjects. Furthermore, the nicotinic antagonist mecamylamine yields rCBF changes similar to those seen in AD, despite producing only minimal cognitive effects on its own. The rCBF and behavioral manifestations in AD may therefore reflect the functional loss of nicotinic receptors in addition to alterations in other receptor systems.     

Increased alpha 2-adrenergic receptor binding in locus coeruleus projection areas in dementia with Lewy bodies.

Clinical studies suggest involvement of brain noradrenergic systems in the pathophysiology of disruptive agitation in Alzheimer's disease (AD). This behavioral problem is even more prevalent in dementia with Lewy bodies (DLB). Here we used receptor autoradiography with [(125)I]para-iodoclonidine to estimate alpha-2 adrenergic receptor (A2R) density in locus coeruleus (LC) projection areas in postmortem brain tissue from age and gender comparable groups of DLB (n = 6), AD (n = 5) and normal (n = 7) subjects. LC neuronal loss was substantial and equivalent in DLB and AD. A2R density was greater in DLB than in normals in the deep layers of the frontal cortex. A2R density was greater in DLB than in AD in hippocampus (CA-1, CA-3 and dentate hilus) and in the granule layer of the cerebellum. Increased A2R binding in DLB is consistent with expression of presynaptic A2R on fibers from surviving LC neurons involved in reinnervation of LC projection areas. These areas develop compensatory noradrenergic hyperinnervation in a rat model of partial LC ablation. It is also consistent with upregulation of post-synaptic A2R in response to loss of LC noradrenergic innervation. Either mechanism could lower the threshold for increased agitation in response to noradrenergic outflow in these dementing disorders.     

Effects of histamine on alpha adrenergic receptor expression on the lymphocytes of normal and asthmatic subjects

Low levels of [3H]-dihydroergocryptine (DHE) (alpha adrenergic receptor) binding to peripheral blood mononuclear leukocytes from asthmatic subjects were significantly increased following incubation with histamine. In contrast, [3H]-DHE binding was undetectable both before and after histamine incubation using cells from nonasthmatic subjects. The change in binding showed a biphasic dose dependence on histamine and was not associated with changes in DHA (beta-2 receptor) or yohimbine (alpha-2 receptor) binding. The induction of alpha adrenergic receptors by the ubiquitous inflammatory mediator histamine may help explain the alpha adrenergic hyperresponsiveness seen in asthmatic patients.