Barbiturate-induced inhibition of rat brain histamine turnover

Morphine-induced increases of extracellular histamine levels in the periaqueductal grey in vivo: a microdialysis study

The effect of morphine on extracellular histamine levels in two regions of the rat midbrain was studied in vivo by microdialysis. Morphine (5.6 and 12.8 mg/kg, s.c.) significantly and dose-dependently increased extracellular histamine levels in the periaqueductal grey, while no significant effect was observed in the reticular formation. In addition, no significant effect of sequential saline injections was observed on extracellular histamine levels in the periaqueductal grey. Since morphine has no effect on histamine catabolism, these results suggest that morphine increases histamine release in the rat PAG, a site where morphine and histamine are known to have analgesic action. Taken with earlier studies showing the ability of H2 antagonists to block morphine analgesia, these results support the hypothesis that histamine and H2 receptors are important in mediating morphine analgesia in the rat periaqueductal grey. The cellular origin of the extracellular histamine, and the mechanism of this morphine effect remain to be determined.

Regulation of histamine turnover via muscarinic and nicotinic receptors in the brain

To clarify the regulation of central histaminergic (HAergic) activity by cholinergic receptors, the effects of drugs that stimulate the cholinergic system on brain histamine (HA) turnover were examined, in vivo, in mice and rats. The HA turnover was estimated from the accumulation of tele-methylhistamine (t-MH) during the 90-min period after administration of pargyline (65 mg/kg, i.p.). In the whole brain of mice, oxotremorine, at doses higher than 0.05 mg/kg, s.c., significantly inhibited the HA turnover, this effect being completely antagonized by atropine but not by methylatropine. A large dose of nicotine (10 mg/kg, s.c.) also significantly inhibited the HA turnover. This inhibitory effect was antagonized by mecamylamine but not by atropine or hexamethonium. A cholinesterase inhibitor, physostigmine, at doses higher than 0.1 mg/kg, s.c., significantly inhibited the HA turnover. This effect was antagonized by atropine but not at all by mecamylamine. None of these cholinergic antagonists used affected the steady-state t-MH level or HA turnover by themselves. In the rat brain, physostigmine (0.1 and 0.3 mg/kg, s.c.) also decreased the HA turnover. This inhibitory effect of physostigmine was especially marked in the striatum and cerebral cortex where muscarinic receptors are present in high density. Oxotremorine (0.2 mg/kg, s.c.) and nicotine (1 mg/kg, s.c.) also decreased the HA turnover in the rat brain. However, these effects showed no marked regional differences. These results suggest that the stimulation of central muscarinic receptors potently inhibits the HAergic activity in the brain and that strong stimulation of central nicotinic receptors can also induce a similar effect.

Effects of the histamine H3-agonist (R)-alpha-methylhistamine and the antagonist thioperamide on histamine metabolism in the mouse and rat brain

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)-alpha-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)-alpha-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)-alpha-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 approximately 70% 1 and 2 h after treatment. Lower doses of (R)-alpha-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)-alpha-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.

Actions of the brain-penetrating H2-antagonist zolantidine on histamine dynamics and metabolism in rat brain

The effects of zolantidine, the first brain-penetrating H2-receptor antagonist, on the brain levels of histamine (HA) and the HA metabolite tele-methylhistamine (t-MH), the activity of histamine methyltransferase (HMT) and the brain HA turnover rates were investigated in rats. Zolantidine dimaleate (0.1 to 100 mg/kg, s.c.) had no effect on whole brain levels of HA or t-MH and no effect on brain HMT activity, when measured 30 min after administration. Furthermore, brain t-MH levels in pargyline-treated animals were unaffected by zolantidine (0.1 to 25 mg/kg), indicating the absence of an effect on brain HA turnover. In vitro, zolantidine was a potent competitive inhibitor of both brain and kidney HMT, with Ki values of 2.3 and 2.7 microM respectively. These results show that, despite the ability of zolantidine to inhibit HMT in vitro, large doses of this drug did not alter brain HA methylation or turnover in vivo, and they imply that blockade of post-synaptic H2-receptors does not change brain HA dynamics.

Histamine metabolites in cerebrospinal fluid of the rhesus monkey (Macaca mulatta): cisternal-lumbar concentration gradients

Similar to metabolites of other aminergic transmitters, histamine metabolites of brain, tele-methylhistamine (t-MH) and tele-methylimidazoleacetic acid (t-MIAA), could have a concentration gradient between rostral and caudal sites of CSF. To test this hypothesis, cisternal and lumbar CSF samples were collected in pairs from eight monkeys (Macaca mulatta), and levels of t-MH and t-MIAA were measured by gas chromatography-mass spectrometry. pros-Methylimidazoleacetic acid (p-MIAA), an endogenous isomer of t-MIAA that is not a histamine metabolite, was also measured. Cisternal levels (in picomoles per milliliter, mean +/- SEM) of t-MH (9.9 +/- 1.4) and t-MIAA (40.8 +/- 7.6), but not of p-MIAA (9.7 +/- 1.2), exceeded those in lumbar CSF (t-MH, 1.8 +/- 0.3; t-MIAA, 6.8 +/- 0.9; p-MIAA, 8.6 +/- 0.6) in every monkey. The magnitudes of the mean cisternal-lumbar concentration gradients for t-MH (6.6 +/- 1.1) and t-MIAA (6.5 +/- 1.3) were indistinguishable. These gradients exceed those of metabolites of most other transmitters. There was no gradient for the levels of p-MIAA. The cisternal, but not lumbar, levels of t-MH and t-MIAA were correlated. There was no significant difference between the means of the metabolite concentration ratios (t-MIAA/t-MH) in cisternal (4.0 +/- 0.4) and lumbar (4.4 +/- 0.9) CSF. The steepness of these gradients suggests that levels of t-MH and t-MIAA in lumbar CSF might be useful probes of histaminergic metabolism in brain.