Effect of Pargyline on Brain N-tele-Methylhistamine in Portocaval-Shunted Rats: Relation to Amine Neurotransmitters

Abstract: HPLC determination of histamine, serotonin, dopamine, and noradrenaline in the brain tissue of rats with portocaval anastomoses (PCA) has revealed a selective increase in histamine concentration. In the posterior hypothalamus, the steady-state level of the amine metabolites showed an inverse pattern; N-tele -methylhistamine(t-MeHA), as estimated by gas chromatography-mass spectrometry, was not changed significantly by portocaval shunting, whereas 5-hydroxyindoleacetic acid (5-HIAA) and homovanillic acid were more than doubled. Interestingly, the net increase in t-MeHA concentration in response to pargyline (80 mg/kg i.p.) was almost the same for PCA and sham-operated rats. This implies that the great enhancement of the histamine level in this area might be a consequence of the persistent stimulation of its synthesis and the unchanged activity of histaminergic neurons. In the rest of the brain, on the other hand, the steady-state level of t-MeHA was higher after PCA (3.8-fold), as were the levels of 5-HIAA and homovanillic acid. Surprisingly, t-MeHA remained unchanged after monoamine oxidase blockade. Of the pargyline-induced alterations in the concentrations of indoles and catechols, the most pronounced were those in the serotonin level; serotonin was elevated more than twofold in hypothalamus and more than 12-fold in the rest of the brain, with a concomitant 80% decrease in 5-HIAA. The dopamine and, to a much smaller extent, noradrenaline levels were also increased, and the levels of homovanillic acid and 3,4-dihydroxyphenylacetic acid fell below the detection limit. The study suggests that at least two different mechanisms operate in the brains of PCA rats to counteract the excessive synthesis of neuromediators, e.g., increased deposition and increased metabolism.     

Inhibition of Histamine Synthesis in Brain by α-Fluoromethylhistidine, a New Irreversible Inhibitor: In Vitro and In Vivo Studies

a-Fluoromethylhistidine (α-FMH), a new potent inhibitor of histidine decarboxylase (HD), has been used for in vitro and in vivo studies of brain HD. Following a preincubation with (+)-α-FMH, brain HD activity was inhibited in a time-dependent and concentration-dependent manner. The enzyme activity was not restored by overnight dialysis against standard buffer. The (–) antimer of α-FMH was ineffective. When injected intraperitoneally in a single dose of 20 mg/kg, (±)-α-FMH induced a complete loss in HD activity in cerebral cortex and hypothalamus as well as in peripheral tissues, such as stomach. At a dosage of 100 mg/kg (±)-α-FMH did not alter histamine-N-methyltransferase, DOPA decarboxylase, and glutamate decarboxylase activities. The maximal decrease of HD activity occurred after 2 h in both cerebral cortex and hypothalamus, but the time course of the recovery of enzyme activity was slower in the cerebral cortex. The enzyme activity reached control value within 3 days in hypothalamus and was not fully restored after 4 days in cerebral cortex. Contrasting with the diminished HD activity, a substantial concentration of histamine remained present in five regions of mouse brain. Thus, α-FMH is a highly specific irreversible inhibitor of brain HD activity and its efficacy makes it useful to study the physiological role of brain histamine.     

Lack of a Precursor-Product Relationship Between Histamine and Its Metabolites in Brain After Histidine Loading

Abstract: Levels of histamine and its major metabolites in brain, tele -methylhistamine (t-MH) and tele -methylimidazoleacetic acid (t-MIAA), were measured in rat brains up to 12 h after intraperitoneal administration of l -histidine (His), the precursor of histamine. Compared with saline-treated controls, mean levels of histamine were elevated at 1 h (+ 102%) after a 500 mg/kg dose; levels of t-MH did not increase. Following a 1,000 mg/kg dose; levels mean histamine levels were increased for up to 7 h, peaked at 3 h, and returned to control levels within 12 h. In contrast, levels of t-MH showed a small increase only after 3 h; levels of t-MIAA remained unchanged after either dose. Failure of most newly formed histamine to undergo methylation, its major route of metabolism in brain, suggested that histamine was metabolized by another mechanism possibly following nonspecific decarboxylation. To test this hypothesis, other rats were injected with α-fluoromethylhistidine (α-FMHis; 75 mg/kg, i.p.), an irreversible inhibitor of specific histidine decarboxylase. Six hours after rats received α-FMHis, the mean brain histamine level was reduced 30% compared with saline-treated controls. Rats given His (1,000 mg/kg) 3 h after α-FMHis (75 mg/kg) and examined 3 h later had a higher (+112%) mean level of histamine than rats given α-FMHis, followed by saline. Levels of t-MH and t-MIAA did not increase. These results imply that high doses of His distort the simple precursor-product relationship between histamine and its methylated metabolites in brain. The possibility that some His may undergo nonspecific decarboxylation in brain after His loading is discussed. These findings, and other actions of His independent of histamine, raise questions about the validity of using His loading as a specific probe of brain histaminergic function.     

Long-Term Effects of Portacaval Anastomosis on the 5-Hydroxytryptamine, Histamine, and Catecholamine Neurotransmitter Systems in Rat Brain

Abstract: Portacaval anastomosis (PCA) in the rat is used as a model for portal systemic encephalopathy. Changes in the serotonergic, histaminergic, and catecholaminergic neurotransmitter systems are often found shortly after PCA. We have examined the long-term effects of PCA on the aminergic systems in brains of male Wistar rats, which 8 months previously had been subjected to PCA. Precursors, amines, and metabolites were assayed by HPLC. Eight months after PCA, the catecholamine levels were unchanged in all brain regions. In contrast, tryptophan was evenly increased throughout the brain. The accumulation of 5-hydroxytryptophan after decarboxylase inhibition (NSD-1015; 100 mg/kg i.p.) and the endogenous levels of 5-hydroxyindoleacetic acid were significantly higher in PCA rats, particularly in the hypothalamus and midbrain, whereas 5-hydroxytryptamine concentrations were unchanged. Histamine levels were elevated throughout the brain with the greatest increase found in the hypothalamus and in the striatum. tele -Methylhistamine levels were significantly elevated in cortex and hypothalamus. We conclude that 8 months after PCA, catecholaminergic systems had reestablished their homeostasis, whereas serotonergic and histaminergic systems still show profound disturbances in their function. With histamine, this is reflected as an increase in the amounts of both transmitter and metabolite; serotonergic neurons respond by increasing only the level of the metabolite.     

Brain histamine and histamine H3 receptors following repeated L-histidine administration in rats

In order to assess the importance of the chronic increase in precursor availability on central histaminergic mechanisms in rats, nine male Wistar rats received L-histidine orally at a dose of 1000 mg/kg, twice daily (07.00 h and 19.00 h) for 1 week; 9 rats were used as controls. Brain tissue histamine and tele-methylhistamine levels, as well as plasma histamine concentration were assayed. Binding properties and regional distribution of the autoregulatory histamine H3 receptors in brain were studied with [3H]-R-alpha-methylhistamine receptor binding and autoradiography. In L-histidine loaded rats, tissue histamine levels in cortex, hypothalamus, and rest of the brain were significantly increased by 40%-70%. Histamine concentrations in cerebellum and plasma, and tele-methylhistamine concentrations in cortex and hypothalamus did not change. The binding properties of H3 receptors in cortex were not altered. However, there were changes in the regional distribution of [3H]-R-alpha-methylhistamine binding sites, suggestive of a region-selective up-/down-regulation of histamine H3 receptors or their receptor sub-types. These results imply that following repeated L-histidine administration in the rat (1) there is enhanced synthesis of brain histamine not reflected in its functional release; (2) the excess of histamine is sequestered and stored rather than being metabolized; (3) histamine H(3) receptor binding properties are not altered, whereas receptor density is changed in selected regions. In conclusion, these results demonstrate that the neuronal mechanisms controlling histamine synthesis, storage, and release are adaptable and allow the sequestration of the excess of histamine in order to prevent excessively high neuronal activity.     

Neuronal storage of histamine in the brain and tele-methylimidazoleacetic acid excretion in portocaval shunted rats

Rats with portocaval anastomosis (PCA), an animal model of hepatic encephalopathy (HE), have very high brain histamine concentrations. Our previous studies based on a biochemical approach indicated histamine accumulation in the neuronal compartment. In this study, immunohistochemical evidence is presented which further supports the amine localization in histaminergic neurons. These neurons become pathological in appearance with cisternae frequently seen along histaminergic fibres in many brain areas, including the hypothalamus, amygdala, substantia nigra and cerebral cortex. Such formations were not observed in sham-operated animals. The neuronal deposition is predominant, and unique for histamine. It serves as a mechanism to counterbalance excessive brain neurotransmitter formation evoked by PCA. However, there are other mechanisms. The data provided here show that there is also a significant increase in histamine catabolism in the shunted rats, as reflected by both the higher brain N-tele-methylhistamine (t-MeHA) concentration and urinary excretion of N-tele-methylimidazoleacetic acid (t-MelmAA), a major brain histamine end product. The stomach, in addition to the brain, is a site of enhanced histamine synthesis in portocavally shunted subjects. After gastrectomy or food deprivation to eliminate the contribution of the stomach, shunted rats excrete significantly more t-MelmAA, implying the role of the CNS. This last finding suggests that under strictly defined conditions, namely in parenterally fed HE patients with abnormal plasma L-histidine, the measurement of urinary t-MelmAA might provide valuable information concerning putative brain histaminergic activity.     

Sensitive Radioimmunoassays for Histamine and tele-Methylhistamine in the Brain

Serum albumin conjugates of histamine or tele-methylhistamine, a major catabolite, were prepared using 1,4-benzoquinone as the coupling agent and used to raise polyclonal antibodies in rabbits. The same reagent was used to prepare the [125I]iodinated tracer and treat tissue extracts submitted to the radioimmunoassays. The IC50 values of prederivatized histamine and tele-methylhistamine in the radioimmunoassays were 0.3 nM and 0.5 nM, respectively, whereas nonderivatized histamine or tele-methylhistamine, histidine, a variety of histamine derivatives, amines, etc., had at least 1,000-fold higher IC50 values. Application of the radioimmunoassays to nonpurified extracts of rat brain allowed the quantification of the two amine immunoreactivities in samples corresponding to less than 1 mg of hypothalamus. The tissue immunoreactivity corresponded to authentic histamine or tele-methylhistamine, as shown by (a) the parallel 125I-tracer displacement curves, (b) the similar elution patterns from HPLC columns, (c) the regional levels of histamine and tele-methylhistamine in brain, similar to those obtained with other methods, and (d) the clearcut effects of treatments with inhibitors of L-histidine decarboxylase or monoamine oxidase. The two radioimmunoassays appear as simple and sensitive tools to evaluate steady-state levels and turnover rates of histamine and tele-methylhistamine.     

pros-Methylimidazoleacetic Acid in Rat Brain: Its Regional Distribution and Relationship to Metabolic Pathways of Histamine

pros-Methylimidazoleacetic acid (p-MIAA; 1-methylimidazole-5-acetic acid), an isomer of the histamine metabolite, tele-methylimidazoleacetic acid (t-MIAA), is present in brain and CSF. Its relationship to histamine synthesis and catabolism was assessed in brains of rats. p-MIAA distribution in brain regions was heterogeneous although the concentrations in regions with the highest (hypothalamus) and the lowest (medulla-pons) levels differed less than four-fold. There was no significant correlation between the regional distributions of p-MIAA with those of histamine or its metabolites. pros-Methylhistidine (1 g/kg, i.p.) produced a 20-fold increase in mean levels of p-MIAA and up to a 50-fold increase in levels of pros-methylhistamine (p-MH), a putative intermediate; levels of histamine and its metabolites were unaltered. L-Histidine (1 g/kg, i.p.) or alpha-fluoromethylhistidine (100 mg/kg, i.p.), the irreversible inhibitor of histamine synthesis, did not alter the levels of p-MIAA in brain. Like the levels of t-MIAA, the levels of p-MIAA were unaltered after probenecid administration. Contrary to its effects in lowering t-MIAA levels, pargyline (75 mg/kg, i.p.) produced a slight rise in levels of p-MIAA in all regions. These findings suggest that, in brain, the metabolic pathways of histamine are independent of pathways that generate p-MIAA. Further, since brain is capable of p-MH formation, its use as an internal standard in analytical methods merits caution.     

L-histidine attenuates the effects of pentazocine on rewarding brain-stimulation

Previous research has indicated that the antihistamine tripelennamine potentiates the threshold lowering effects of pentazocine on brain stimulation reward, a model of drug-induced euphoria. To determine the importance of histamine in this interaction, we studied the effects of co-administration of L-histidine and pentazocine on the threshold for brain stimulation reward. Pentazocine (2.5-10.0 mg/kg) lowered the threshold for rewarding stimulation to the medial forebrain bundle-lateral hypothalamus in male F344 rats. L-histidine (500 and 750 mg/kg) by itself had no significant effects, yet antagonized the threshold lowering effects of pentazocine. These doses of L-histidine are known to significantly raise brain histamine concentrations. Our results suggest that histamine may play a tonic inhibitory role, at least in part, on the neural systems responsible for the reinforcing properties of pentazocine.     

Ontogenetic Development of Histamine H1- Receptor Binding in Rat Brain

Abstract: Histamine H_1- receptors labeled with [³H]mepyramine in rat brain show an age-dependent development. [³H]Mepyramine receptor density and histidine decarboxylase activity in whole rat brain reach adult levels at 25–30 days after birth and they attain 50% of adult level at day 10 and 17, respectively. The apparently later development of histidine decarboxylase activity in whole rat brain is partly accounted for by a biphasic developmental increase of this enzymatic activity in cerebral cortex. For all other brain regions examined, the development of histamine H_1- receptors parallels that of histidine decarboxylase. The increase in [³H]mepyramine binding can be accounted for by an absolute increase in the numbers of the receptor sites, with no change in affinity. Subcellular fractionation studies indicate that histamine H_1- receptors are predominantly associated with synaptosomal fractions derived from both newborn and adult rat.     

Brain Histaminergic System in Mast Cell-Deficient (Ws/Ws) Rats: Histamine Content, Histidine Decarboxylase Activity, and Effects of (S)α-Fluoromethylhistidine

Abstract: The mast cell-deficient [ Ws/Ws ( W hite spotting in the skin)] rat was investigated with regard to the origin of histamine in the brain. No mast cells were detected in the pia mater and the perivascular region of the thalamus of Ws/Ws rats by Alcian Blue staining. The histamine contents and histidine decarboxylase (HDC) activities of various brain regions of Ws/Ws rats were similar to those of +/+ rats except the histamine contents of the cerebral cortex and cerebellum. As the cerebral cortex and cerebellum have meninges that are difficult to remove completely, the histamine contents of these two regions may be different between Ws/Ws and +/+ rats. We assume that the histamine content of whole brain with meninges in Ws/Ws rats is <60% of that in +/+ rats. So we conclude that approximately half of the histamine content of rat brain is derived from mast cells. Next, the effects of ( S )α-fluoromethylhistidine (FMH), a specific inhibitor of HDC, on the histamine contents and HDC activities of various regions of the brain were examined in Ws/Ws rats. In the whole brain of Ws/Ws rats, 51 and 37% of the histamine content of the control group remained 2 and 6 h, respectively, after FMH administration (100 mg/kg of body weight). Therefore, we suggest that there might be other histamine pools including histaminergic neurons in rat brain.     

Disposition of Histamine, Its Metabolites, and pros-Methylimidazoleacetic Acid in Brain Regions of Rats Chronically Infused with α-Fluoromethylhistidine

Abstract: In mammalian brain, histamine is known to be metabolized solely by histamine methyltransferase (HMT), forming tele -methylhistamine (t-MH), then tele -methylimidazoleacetic acid (t-MIAA). We previously showed that imidazoleacetic acid (IAA), a GABA agonist, and histamine's metabolite in the periphery, is present in brain where its concentration increased after inhibition of HMT. Also, when [³H]histamine was given intracerebroventricularly to rats, a portion was converted to IAA, a process increased by inhibition of HMT. These results indicated that brain has the capacity to oxidize histamine but did not show whether this pathway is operative under physiological conditions. To address this question, rats were infused for >4 weeks with α-fluoromethylhistidine (α-FMHis), an irreversible inhibitor of histamine's synthetic enzyme, l -histidine decarboxylase. Compared with controls (untreated and saline-treated rats), brain levels of histamine, t-MH, and t-MIAA in all regions were markedly reduced in treated rats. As a percentage of controls, depletion of t-MIAA > t-MH > histamine in all regions, and regional depletions of histamine corresponded to its turnover rates in regions of rat brain. In contrast, levels of IAA were unchanged as were levels of pros -methylimidazoleacetic acid, an isomer of t-MIAA unrelated to histamine metabolism. Results suggest that in brains of rats, unlike in the periphery, most IAA may not normally derive from histamine. Because histamine in brain can be converted to IAA under certain conditions, direct oxidation of histamine may be a conditional phenomenon. Our results also support the existence of a very slow turnover pool of brain histamine and use of chronic α-FMHis infusion as a model to probe the histaminergic system in brain.     

Histamine: A Neurotransmitter Candidate for Drosophila Photoreceptors

Recent experimental evidence suggests that histamine might be the synaptic transmitter used by invertebrate photoreceptors. In the present study, we have examined whether histamine is a transmitter candidate for Drosophila photoreceptors. Our findings are as follows: (a) Large amounts of histamine are synthesized by wild-type heads, whereas heads from the eye-deficient mutants, eyes absent and sine oculis, show reduced histamine synthesis. (b) Histidine decarboxylase activity is approximately 10-fold higher in extracts of normal heads compared with that in the mutants. (c) Histamine taken up by fly heads is metabolized into N-acetylhistamine and imidazole-4-acetic acid. (d) Immunostaining of normal and sevenless heads with histamine-specific antisera demonstrates that histamine is present in photoreceptors R1-6 and R8. (e) Histamine synthesized from exogenously supplied [3H]histidine can be released by depolarization with 50 mM K+, and the release is Ca2+ dependent. These observations strongly suggest that histamine is a major neurotransmitter used by Drosophila photoreceptors.     

Effects of the NMDA antagonist MK-801 on radial maze performance in histidine-deficient rats

We examined the effects of a histidine-deficient diet on brain histamine contents as well as on learning and memory using the eight-arm radial maze in rats. A significant decrease in histamine content in the hippocampus was observed after long-term feeding of rats with a histidine-deficient diet. At the same time, significant enhancement of the acquisition process in radial maze performance was also observed. Pyrilamine did not show a significant effect on radial maze performance in histidine-deficient rats. On the other hand, pyrilamine caused a significant spatial memory deficit in control rats. Scopolamine was effective in inhibiting spatial memory in both histidine-deficient and control rats. MK-801 caused spatial memory deficits more potently in histidine-deficient rats than in controls. Brain glycine contents showed a significant increase in the hippocampus in histidine-deficient rats. These results indicated that the spatial memory deficits induced by MK-801 in histidine-deficient rats are closely related to increased glycine levels and activation of NMDA receptors.     

Phosphatidic Acid Accumulation and Catecholamine Release in Adrenal Chromaffin Cells: Stimulation by High Potassium and by Nicotine, and Effect of a Diacylglycerol Kinase Inhibitor R 59 022

The release of endogenous histamine (HA) from the hypothalamus of anesthetized rats was measured by in vivo microdialysis coupled with HPLC with fluorescence detection. Freshly prepared Ringer's solution was perfused at a rate of 1 microliter/min immediately after insertion of a dialysis probe into the medial hypothalamus, and brain perfusates were collected every 30 min into microtubes containing 0.2 M perchloric acid. The basal HA output was almost constant between 30 min and 7 h after the start of perfusion, with the mean value being 7.1 pg/30 min. Thus, the extracellular HA concentration was assumed to be 7.8 nM, by a calculation from in vitro recovery through the dialysis membrane. Perfusion with a high K+ (100 mM)-containing medium increased the HA output by 170% in the presence of Ca2+. Systemic administration of either thioperamide (5 mg/kg, i.p.), a selective H3 receptor antagonist, or metoprine (10 mg/kg, i.p.), an inhibitor of HA-N-methyltransferase, caused an approximately twofold increase in the HA output 30-60 min after treatment. The combined treatment with thioperamide and metoprine produced a marked increase (650%) in the HA output. The HA output decreased by approximately 70% 4-5 h after treatment with alpha-fluoromethylhistidine (alpha-FMH; 100 mg/kg, i.p.), an inhibitor of histidine decarboxylase. Furthermore, the effect of combined treatment with thioperamide and metoprine was no longer observed in alpha-FMH-treated rats. These results suggest that both HA-N-methyltransferase and H3 autoreceptors are involved in maintaining a constant level of extracellular HA and that their blockade effectively results in a higher activity level of the endogenous histaminergic system in the CNS.     

Effect of alpha-fluoromethylhistidine on the histamine content of the brain of W/Wv mice devoid of mast cells: turnover of brain histamine

In the brains of W/Wv mutant mice that have no mast cells, the histidine decarboxylase (HDC) level is as high as in the brain of congenic normal mice (+/+), but the histamine content is 53% of that of +/+ mice. The effects of alpha-fluoromethylhistidine (alpha-FMH) on the HDC activity and histamine content of the brain of W/Wv and +/+ mice were examined. In both strains, 30 min after i.p. injection of alpha-FMH the HDC activity of the brain had decreased to 10% of that in untreated mice. The histamine content decreased more gradually, and after 6 h about half of the control level remained in +/+ mice, whereas histamine had disappeared almost completely in W/Wv mice. It is concluded that the portion of the histamine content that was depleted by HDC inhibitor in a short time is derived from non-mast cells, probably neural cells. The half-life of histamine in the brain of W/Wv mice was estimated from the time-dependent decrease in the histamine content of the brain after administration of alpha-FMH: 48 min in the forebrain, 103 min in the midbrain, and 66 min in the hindbrain.     

New functions of histamine found in histidine decarboxylase gene knockout mice

Gene targeting techniques have revolutionized the investigation of the effects of bioactive substances in pathological and physiological conditions. Histamine synthesis is uniquely catalyzed by L-histidine decarboxylase. The knockout mice of this gene express no histamine-producing activity and lack histamine. These mice have been used to examine the mechanisms of histamine in several known phenotypes, e.g., gastric acid secretion, contraction of smooth muscles, vascular permeability, and awakening, and have also been used to explore unreported effects of histamine in the whole body. First, we will review the former mechanisms and then move to the latter, new effects. Especially, in the latter mechanisms, we focus on several important roles of histamine in angiogenesis, neutrophil and eosinophil recruitment, bacterial infection, and systemic anaphylaxis in this review. Moreover, to our surprise, the morphology of mast cells in the knockout mice was severely affected by the absence of histamine in terms of their granules.     

Regional distribution of the histamine metabolite, tele-methylimidazoleacetic acid, in rat brain: effects of pargyline and probenecid

Abstract: tele -Methylimidazoleacetic acid (t-MIAA), a major brain histamine metabolite, was measured in nine rat brain regions by a gas chromatography-mass spectrometric method that also measures the precursor amine, tele -methylhistamine (t-MH). The t-MIAA concentration of cerebellum, medulla-pons, midbrain, caudate nucleus, hypothalamus, frontal cortex, hippocampus, and thalamus varied 15-fold, hypothalamus showing the highest level (2.21 nmol/g) and cerebellum the lowest (0.15 nmol/ g). The concentrations of t-MIAA and t-MH were significantly correlated in all regions except midbrain, which had relatively more t-MIAA. Probenecid did not alter whole-brain t-MIAA levels. Treatment with pargyline, an inhibitor of monoamine oxidase, lowered the t-MIAA levels in all regions.     

Rat Brain Adenosine Deaminase After 2''-Deoxycoformycin Administration: Biochemical Properties and Evidence for Reduced Enzyme Levels Detected by 2''-[3H]Deoxycoformycin Ligand Binding

Near total inhibition of brain adenosine deaminase (ADA) activity in rats injected with the potent ADA inhibitor 2'-deoxycoformycin (DCF) was previously shown to reduce enzyme activity for up to 50 days during which time the enzyme exhibited reduced sensitivity to in vivo inhibition by DCF. Here, we investigated the biochemical properties of ADA and the basis for its reduced activity after DCF treatment. It was found that much higher doses of DCF were required to inhibit ADA in DCF-treated compared with drug-naive animals. Fourteen days after DCF administration, reduced ADA activity in brain homogenates was due to a decrease in Vmax, rather than to an altered Km of ADA for adenosine. DCF treatment had no effect on Ki values for erythro-9-(2-hydroxy-3-nonyl)adenine inhibition of ADA. The IC50 value for DCF inhibition of ADA in hypothalamus was unchanged. However, the Ki for DCF inhibition of ADA in whole brain increased by fivefold. Sucrose gradient analysis of brain ADA revealed only one corresponding peak of activity and [3H]DCF-labeled ADA in DCF-treated and control rats. A radioligand filtration assay with [3H]DCF was developed to assess the effects of DCF on ADA protein levels. Over a roughly 200-fold range of ADA activities the binding of [3H]DCF was highly correlated with deaminase activity (r = 0.99). In brain tissues taken 8 and 33 days after treatment of rats with DCF, [3H]DCF binding was reduced to 27% and 48% of control levels, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)     

Role of the Central Adrenergic System in the Regulation of Prostaglandin Biosynthesis in Rat Brain

The role of endogenous catecholamines in the regulation of brain prostaglandin (PG) synthesis was studied in the rat. Male rats were injected in the brain lateral ventricle or in the ventral noradrenergic bundle with either the catecholaminergic neurotoxin 6-hydroxydopamine or vehicle. Other groups of rats were injected intraperitoneally with the tyrosine hydroxylase inhibitor, alpha-methyl-p-tyrosine, or with the inhibitor of dopamine-beta-hydroxylase, FLA-63. All these drugs produced a significant depletion of norepinephrine (NE) content in the cortex and hypothalamus. The rats that had lower levels of NE exhibited reduced capacity to synthesize PGE2 but not thromboxane B2 and 6-keto-PGE1 alpha in the cortex and hypothalamus. However, induced production of PG, stimulated by the bacterial endotoxin lipopolysaccharide (LPS), remained unchanged, namely, a similar (2- to 2.5-fold) increase of PG synthesis was noted in control and in NE-depleted rats. We suggest that the regulation of PG synthesis under basal condition requires intact adrenergic input, whereas LPS-induced production of PG is independent of the adrenergic innervation.