Ryozo Oishi

CHARACTERIZATION OF HISTAMINE RELEASE FROM THE RAT HYPOTHALAMUS AS MEASURED BY IN VIVO MICRODIALYSIS

Abstract: 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 Ringers solution was perfused at a rate of 1 μl/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 ∼70% 4–5 h after treatment with α‐fluoromethylhistidine (α‐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 α‐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.

Regional differences in the turnover of neuronal histamine in the rat brain

The turnover rate of histamine (HA) and the half-life of neuronal HA were estimated in 9 regions of the rat brain following pargyline-induced accumulation of tele-methylhistamine (t-MH). The turnover rate was the highest in the hypothalamus (108.7 ng/g/hr). The striatum also showed a high turnover rate (80.2 ng/g/hr) despite much lower levels of HA and t-MH, as compared with the levels in the hypothalamus. The turnover rate was relatively high in the thalamus, cerebral cortex, amygdala and midbrain, but it was very low in the cerebellum. t-MH accumulation in the spinal cord was nil. The HA levels were reduced to various degrees (from nil to less than 40% of the control) by (S)-alpha-fluoromethylhistidine, depending on the regions studied. The neuronal HA content of each brain region was subsequently estimated, and the half-life of neuronal HA in each region was calculated. The half-life of neuronal HA was the shortest (7.7 min) in the striatum, while it was long (about 50 min) in the hypothalamus and thalamus. Half-life values of about 20 min were obtained in other regions. These results show the high levels of histaminergic activity in some parts of the telencephalon, thalamus and midbrain as well as the hypothalamus.

A highly sensitive assay for histamine using ion-pair HPLC coupled with postcolumn fluorescent derivatization : its application to biological specimens

Abstract: A simple and highly sensitive method for the determination of histamine (HA) was developed using ion‐pair, reversed‐phase HPLC coupled with postcolumn o‐phthalal‐dehyde derivatization fluorometry, and it was applied to the unpurified extracts of human and rat plasma, and brains of rats and mice. The HA concentrations both in the plasma and brains determined by the present method were well consistent with the values obtained by cation‐exchange HPLC with postcolumn fluorescent derivatization currently in use. The present method was more advantageous than the assay using cation‐exchange HPLC: (1) it was three to four times more sensitive (the detection limit was 0.5 pg of HA), and (2) it enabled the measurement of HA in samples containing C/?)a‐methylhistamine, a potent and specific H3‐receptor agonist, which could not be separated from HA by cation‐exchange chromatography. Using the present method coupled with intracerebral microdialysis, we found in the rat hypo‐thalamus that (R)α‐methylhistamine (5 mg/kg i.p.) markedly decreased the extracellular concentration of HA with a maximal effect (83% reduction) during 30‐60 min after injection, suggesting that most of HA in the microdialysate fraction is neuronal in origin.

Galanin Inhibits Noradrenaline‐Induced Accumulation of Cyclic AMP in the Rat Cerebral Cortex

Abstract: The effect of galanin on poradrenaline (NA)‐induced accumulation of cyclic AMP was investigated in slices of rat cerebral cortex. NA (10−4M) increased cyclic AMP levels during a 20‐min observation period. Galanin (3 × 10−7M) significantly inhibited this response at all time points examined, although it did not change the basal levels of cyclic AMP. Galanin (10−3× 10−6M) inhibited the cyclic AMP response to NA (10−4M) in a dose‐dependent manner, with an IC50 of ∼5.6 × 10−5M and a maximum inhibition of 59%. These results suggest that galanin, devoid of any detectable effects by itself, modulates the cyclic AMP response to NA in the rat cerebral cortex.

EFFECTS OF THE HISTAMINE H3 AGONIST R- ALPHA-METHYLHISTAMINE AND THE ANTAGONIST THIOPERAMIDE ON HISTAMINE METABOLISM IN THE MOUSE AND RAT BRAIN

Abstract: To study the feedback control by histamine (HA) H3‐receptors on the synthesis and release of HA at nerve endings in the brain, the effects of a potent and selective H3‐agonist, (R)‐α‐methylhistamine, and an H3‐antagonist, thioperamide, on the pargyline‐induced accumulation of tele‐methylhistamine (t‐MH) in the brain of mice and rats were examined in vivo. (R)‐α‐Methylhistamine dihydrochloride (6.3 mg free base/kg, i.p.) and thioperamide (2 mg/kg, i.p.), respectively, significantly decreased and increased the steady‐state t‐MH level in the mouse brain, whereas these compounds produced no significant changes in the HA level. When administered to mice immediately after pargyline (65 mg/kg, i.p.), (R)‐α‐methylhistamine (3.2 mg/kg, i.p.) inhibited the pargyline‐induced increase in the t‐MH level almost completely during the first 2 h after treatment. Thioperamide (2 mg/kg, i.p.) enhanced the pargyline‐induced t‐MH accumulation by ∼70% 1 and 2 h after treatment. Lower doses of (R)‐α‐methylhistamine (1.3 mg/kg) and thioperamide (1 mg/kg) induced significant changes in the pargyline‐induced t‐MH accumulation in the mouse brain. In the rat, (R)‐α‐methylhistamine (3.2 mg/kg, i.p.) and thioperamide (2 mg/kg, i.p.) also affected the pargyline‐induced t‐MH accumulation in eight brain regions and the effects were especially marked in the cerebral cortex and amygdala. These results indicate that these compounds have potent effects on HA turnover in vivo in the brain.

Morphine-induced changes in histamine dynamics in mouse brain

Abstract: The effect of the acute morphine treatment on histamine (HA) pools in the brain and the spinal cord was examined in mice. Morphine (1–50 mg/kg, s.c.) administered alone caused no significant change in the steadystate levels of HA and its major metabolite, tele‐methylhistamine (t‐MH), in the brain. However, depending on the doses tested, morphine significantly enhanced the pargyline (65 mg/kg, i.p.)‐induced accumulation of t‐MH and this effect was antagonized by naloxone. A specific inhibitor of histidine decarboxylase, α‐fluoromethylhistidine (α‐FMH) (50 mg/kg, i.p.), decreased the brain HA level in consequence of the almost complete depletion of the HA pool with a rapid turnover. Morphine further decreased the brain HA level in α‐FMH‐pretreated mice. Morphine administered alone significantly reduced the HA level in the spinal cord, an area where the turnover of HA is very slow. These results suggest that the acute morphine treatment increases the turnover of neuronal HA via opioid receptors, and this opiate also releases HA from a slowly turning over pool(s).

Neuronal histamine inhibits methamphetamine-induced locomotor hyperactivity in mice

Whether central histaminergic (HAergic) neurons mediate the regulation of methamphetamine (MAMP)-induced hyperactivity was clarified. L-histidine (HIS; 500 and 1000 mg/kg i.p.) reduced the locomotor hyperactivity induced by MAMP (1 mg/kg i.p.) in mice, and the effect was significant only at 1000 mg/kg. HIS significantly elevated brain histamine (HA) levels, in both doses, whereas telemethylhistamine (t-MH) levels were elevated only at 1000 mg/kg. Pretreatment with alpha-fluoromethylhistidine, a histidine decarboxylase inhibitor, suppressed both behavioral and biochemical effects of HIS. Metoprine, a HA-N-methyltransferase inhibitor, increased brain HA levels, decreased t-MH levels and suppressed the MAMP-induced locomotor hyperactivity. It is concluded that central HAergic systems may play an inhibitory role on the MAMP-induced locomotor hyperactivity.

Direct Evidence for Increased Continuous Histamine Release in the Striatum of Conscious Freely Moving Rats Produced by Middle Cerebral Artery Occlusion

Extracellular histamine in the striatum of conscious freely moving rats collected by intracerebral microdialysis 1 day after implantation of a U-shaped dialysis probe was measured by HPLC coupled with postcolumn o-phthalaldehyde derivatization fluorometry. The basal fractional histamine outputs were almost constant from 1 to 7 h after the start of perfusion (5.9–8.4 pg/30 min). Depolarization by perfusion with a high K+ (100 mM)-containing medium produced a significant (124%) increase and neuronal blockade by perfusion with a tetrodotoxin (1 μM)-containing medium resulted in a 68% reduction in the histamine output. The histamine output was markedly reduced by intraperitoneal injection of α-fluoromethylhistidine (100 mg/kg), an irreversible inhibitor of histidine decarboxylase, or (R)-α-methylhistamine (5 mg/kg), a potent and specific H3-receptor agonist. After middle cerebral artery (MCA) occlusion, the histamine output gradually increased, and reached four times the control value 8 h later. When rats were pretreated with metoprine (10 mg/kg), a histamine N-methyltransferase inhibitor, there was no significant difference in the histamine output between the MCA-occluded and the sham-operated groups during the first 3.5 h after the operation, but the histamine output gradually increased thereafter in the MCA-occluded group. In rats treated with α-fluoromethylhistidine, MCA occlusion failed to cause an increase in the histamine output. These results demonstrate that MCA occlusion induces a long-lasting increase in neuronal histamine release in the rat striatum.

Feeding-Related Circadian Variation in tele-Methylhistamine Levels of Mouse and Rat Brains

Circadian changes in the brain histamine (HA) and tele‐methylhistamine (t‐MH) levels were studied in mice and rats after adaptation to an alternating 12‐h light/dark cycle (lights on at 0600). Although there was no significant circadian fluctuation of the brain HA levels, the levels of t‐MH, a major metabolite of brain HA, showed a marked circadian variation. In mice, the t‐MH levels were about 80 ng/g from 1200 to 1800 but about two times higher values were obtained from 2400 to 0600 of the next morning. In rats, the t‐MH levels ranged from 24 to 28 ng/g at 0600 and 1200, slightly increased at 1800, and reached at 2400 a peak twice as high as the levels seen during the light period. The t‐MH levels again rapidly decreased during the subsequent 3 h. In mice fasted from 1200, the t‐MH levels did not increase during the period of darkness. When mice were fed at 1200 after a 24‐h fast, a significant increase in the t‐MH levels was observed at 1800. There was no significant circadian variation of the HA and t‐MH levels in the plasma of mice and rats. These results suggest that circadian variation in brain t‐MH levels is related to feeding and possible subsequent changes in elimination of t‐MH from the brain and/or turnover of HA in the brain. This phenomenon should be given due attention when HA dynamics in the brain are being assessed.

Histamine turnover in the brain of different mammalian species: implications for neuronal histamine half-life.

The turnover of neuronal histamine (HA) in nine brain regions and the spinal cord of the guinea pig and the mouse was estimated and the values obtained were compared with data previously obtained in rats. The size of the neuronal HA pool was determined from the decrease in HA content, as induced by (S)‐α‐fluoromethylhistidine (α‐FMH), a suicide inhibitor of histidine decarboxylase. The ratios of neuronal HA to the total differed with the brain region. Pargyline hydrochloride increased the tele‐methylhistamine (t‐MH) levels linearly up to 2 h after administration in both the guinea pig and the mouse whole brain. Regional differences in the turnover rate of neuronal HA, calculated from the pargylineinduced accumulation of t‐MH, as well as in the size of the neuronal HA pool, were more marked in the mouse than in the guinea pig brain. The hypothalamus showed the highest rate in both species. There was a good correlation between the steady‐state t‐MH levels and the turnover rate in different brain regions. Neither the elevation of the t‐MH levels by pargyline nor the reduction of HA by α‐FMH was observed in the spinal cord, thereby suggesting that the HA present in this region is of mast cell origin. The half‐life of neuronal HA in different brain regions was in the range of 13‐38 min for the mouse and 24‐37 min for the guinea pig, except for HA from the guinea pig hypothalamus, which had an extraordinarily long value of 87 min. These results suggest that there are species differences in the function of the brain histaminergic system.