G. Sturman

Modulation of morphine-induced antinociception in mice by histamine H3-receptor ligands

The effects of compounds active at histamine H3-receptors on morphine-induced antinociception have been investigated in thermal and chemical tests in mice; tail-immersion (50°C) and hot-plate (49° and 55°C) tests and acetic acid-induced writhing. Neither (R)α-methylhistamine, a specific agonist, (S)α-methylhistamine, a chemical control, nor thioperamide, an antagonist, had any antinociceptive action alone but thioperamide (3 mg kg−1) attenuated the effects of morphine in the tailimmersion test wile (R)α-methylhistamine (1 mg kg−1), but not the (S) isomer, potentiated its effects in the hot-plate test at 55°C. These results are consistent with the morphine potentiation seen with H1-antagonists and suggest that central histaminergic mechanisms can modulate opioid actions.

Modulation of the intracellular and H3-histamine receptors and chemically-induced seizures in mice

Central histaminergic modulation of H1 rather than H2-receptors has been shown to modify epileptic activity. Compounds acting on the HIC- and H3-receptors were tested against chemically-induced seizures in mice. Compounds antagonising the microsomal and nuclear intracellular receptors (HIC) only modified seizures at doses where toxicity was observed. Antagonists of the histamine H3-receptor (thioperamide and burimamide) only potentiated the severity of clonic convulsions induced by picrotoxin, while impromidine (i.c.v.), an antagonist with H2-agonist activity, inhibited leptazol-induced seizures. The H3-agonist, (R)α-methylhistamine, potentiated chemically-induced seizures, but at lower doses there was slight inhibition.

Histamine H1-antagonists potentiate seizures in the EL (epilepsy-like) mouse model of temporal lobe epilepsy

The EL (epilepsy-like) mouse (an inbred mutant strain derived from DDY mice) has been accepted internationally as a murine model of human hereditary, sensory precipitated, temporal lobe epilepsy [1]. When these mice are subjected to twice weekly rhythmic vestibular stimulation between 5–8 weeks of age, they develop full tonic-clonic seizures after several weeks. Initially the seizures are not fatal but susceptibility increases with age [2]. Increases in brain histamine concentrations in rodents have been shown to increase seizure threshold and protect against convulsive activity [3] while centrally(such as diphenhydramine and mepyramine) but not peripherallyacting (such as acrivastine) histamine H1 antagonists potentiate chemically-induced and electroshock-induced seizures in mice [4–6]. Similarly histamine H1 antagonists have been shown to precipitate seizure attacks in epileptic patients [7].

Histaminergic drugs as modulators of CNS function

The first indication that histamine might be important in the functioning of the brain was the finding that the centrally penetrating histamine H1 antagonists had marked sedative properties. Subsequently with the development of more specific compounds and drugs for the H1, H2 and H3 receptors a greater understanding of the neurotransmitter/modulator role of histamine in the CNS has been possible. Histamine is now associated with wakefulness, suppression of seizures, hypothermia and emesis. The histamine H1 antagonists have been shown to potentiate opioid-induced analgesia, and modify eating and drinking patterns as well as endocrine secretions from the pituitary gland. Additionally, clinically useful antidepressants have been shown to inhibit histamine-sensitive adenylate cyclase from the mammalian brain. Recently, a possible role for both histamine H1 and H2 receptors in schizophrenia has been reported. As more specific and centrally-penetrating histaminergic compounds are developed, so the roles of histamine as a neurotransmitter/modulator in the brain will be better understood.

Determination of histamine in tissues using reversed phase-HPLC with post column fluorimetric detection.

A fluorimetric method for the measurement of histamine using ortho-pthaldialdehyde (OPA) in biological tissues was described in 1959 [1]. This method is still widely used but a number of drugs [2] as well as naturally occurring substances such as histidine, glutathione and spermidine interfere with this assay. Thus it is preferable that a separation method such as HPLC together with a suitable detection system is used for measuring histamine in biological tissues. However, there are many variations in the chromatographic HPLC conditions used; both in terms of the method of detection and the type of columns. Fluorimetric detection methods for histamine use either pre[3] or post-column [4, 5] derivatisation procedures. Due to the apparent instability of the histamine/OPA fluorescence, it was decided to develop a post-column HPLC method. Whilst post-column derivatisation methods [4, 5] have been successful in analysing histamine, both used nonstandard column packing materials, namely cation exchange. Thus a combination of ion-paired RP-HPLC with fluorimetric detection using post-column derivatisation has been developed.

Loss of skeletal integrity in rat fetuses from dams treated with histamine H1 antagonists

Pregnant Lister Hooded rats were dosed orally with chlorocyclizine (60 mg/kg), promethazine (5 and 10 mg/kg) or doxylamine (500 and 750 mg/kg) (histamine H1 antagonists prescribed for nausea of pregnancy) over the period of organogenesis (days 7–13). Fetuses were removed by caesarian section on day 21 and stained with Alizarin Res S and Alcian Blue for calcified bone and cartilage respectively. There were no significant differences in fetal size with any of the treatments; however, all 3 antagonists caused dose-dependent loss of skeletal integrity and marked fragility compared to the controls. By dosing dams on selected days, skeletal fragility was seen in all fetuses; the most marked effects occurring after dosing on days 10–12. Prolonging the gestation period with progesterone (10 mg/dam orally) resulted in normal sekeletal calcification but fetuses were still very fragile. Therefore, histamine H1 antagonists may interfere with the development of rat fetal joints.

Histamine H2-receptor modulation in two mouse models of seizure susceptibility.

Results and discussion Dimaprit (0.3‐3 mg/kg) produced a bell shaped dose-related decrease in the s.c. leptazol-seizure model in male BK/TO mice (n = 30‐60) (Fig. 1), with significant anticonvulsant effects at 0.3 and 3.0 mg/kg (p < 0.01) and at 1 mg/kg (p < 0.001) in the occurrence of seizure incidence. The severity (total of seizures observed within each of the groups) of the seizures was also reduced in a dose-related manner with both 0.3 and 3 mg/kg showing significant differences (p < 0.05) and 1 mg/kg (p < 0.01). However, zolantidine (3 and 10 mg/kg) was ineffective in this model in male BK/TO mice (n = 20‐50). In the leptazol infusion model in female BK/TO mice, dimaprit (1 mg/kg) increased the leptazol dose needed to evoke tonic seizures by nearly 50% (65.73 ± 4.66 mg/kg in the controls compared to 92.46 ± 9.86 mg/kg (p < 0.05)).

3. Histaminergic mechanisms in the CNSBrain histamine turnover and seizure behaviour in two mouse strains: Preliminary findings

Epilepsy-prone strains of animal are often used as models for epilepsy. Susceptibility of the DBA/2 mouse to audiogenic seizures is age-related, beginning as soon as they can hear (~day 14) and is virtually lost by day 40 [1]. Controversy exists over the relationship between whole brain levels of 5-HT and noradrenaline and the audiogenic seizure susceptibility in the DBA/2 mouse [2]. Alterations in CNS histamine levels and function have been associated with epileptic seizures in rodents [3] and to date investigations into CNS histamine levels in this mouse strain have not been published. Thus we have measured levels of histamine and its major metabolite in 4 distinct brain regions in male DBA/2 mice compared with a nonaudiogenic-sensitive mouse strain (BKTO).

Effect of amthamine, a histamine H2-agonist, in two mouse chemical-induced seizure models

A decrease in CNS histamine levels and function has been associated with epileptic seizure [1]. The involvement of the histamine H1-receptor in modulating epileptic seizures is well documented and histamine H1-antagonists such as diphenhydramine are known to precipitate seizures, both clinically and in animal models [2]. However, to date the role and understanding of the histamine H2-receptor in epileptic seizures is still uncertain [2, 3]. Histamine H2-antagonists, such as cimetidine and ranitidine, are widely used in the treatment of gastrointestinal disorders, but they penetrate the brain poorly. However, convulsions in critically ill and multiple-drug-treated patients, such as the elderly and patients with chronic hepatic and renal failure, have been reported [4]. Also from experimental work, it has been suggested that histamine H2-receptors may modulate effects in epilepsy [5]. Previously we have reported that in both leptazol (pentylenetetrazol) induced seizures in ‘normal’ BK/TO mice and audiogenic seizures in DBA/2 mice, the centrally-penetrating histamine H2-antagonist zolantidine potentiated the incidence of seizures, whilst the histamine H2-receptor agonist dimaprit reduced them. Dimaprit also has some activity on other histamine receptors. However, amthamine has a potency slightly greater than that of histamine and significantly higher than that of dimaprit (approximately ten fold) on the histamine H2-receptor and is without action on the histamine H3-receptor [6]. Thus the aim of this study was to examine the effects of the centrally-penetrating novel histamine H2receptor agonist, amthamine in two chemical-induced seizure models in mice.

Effects of histamine H2-antagonists on pre- and post-natal development in the rat.

Maternal and fetal histamine-forming capacities are increased many-fold during pregnancy [1]. Our studies of the effects on fetal rats of dosing the dams with histamine H1 antagonists during pregnancy, particularly during the period of organogenesis, showed an increase in fetal fragility and delays in the development of some reflexes in the offspring of dams allowed to litter down normally [2, 3]. It seemed logical to extend this work to investigate effects of histamine H2 antagonists since these compounds are increasingly being licensed for self-medication of gastric conditions, especially gastritis associated with non-steroidal anti-inflammatory therapy. Having been developed relatively recently, these compounds have been tested according to standard toxicology protocols (including reproductive toxicology and teratology tests) without demonstrating notable effects, although cimetidine is reported to have some anti-androgenic activity [4]. Clinical reports suggest that there is little evidence of adverse effects when these drugs are taken during pregnancy [5]. We have now studied the effects of two histamine H2 antagonists, cimetidine and ranitidine, on the development of rats dosed in utero.