P. Freeman

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].

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.

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).

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.

Analysis of the effects of histaminergic compounds on experimental nociception and body temperature in mice

Histaminergic neurones have been implicated in nociceptive processing in several species. Although the receptor subtypes involved have not been fully characterised, some histamine H1 antagonists have been used clinically as analgesics [1, 2]. Animal nociceptive tests often monitor responses to thermal stimuli. We have shown that mice swimming at 0–30°C have lower deep body temperatures which correlate with increased response latencies in the hotplate test [3]. This study investigated the effects of altering body temperature on other models of nociception in mice.

Factors affecting the extent of fetal fragility induced by histamine H1 antagonists.

Histamine has many roles in the body; one of which is associated with rapid mammalian cell growth, such as occurs during fetal development [1]. We have shown previously, using standard rat teratology tests that treatment of Lister Hooded rat dams with various histamine H 1 antagonists caused marked fragility of fetal joints [2, 3]. This effect was seen when the fetuses were stained for skeletal examination using a technique which clears soft tissues and stains ossified bone and cartilage with Alizarin red S and Alcian blue respectively [4]. This histaminergic effect has been found to be dose-dependent and time-related. To facilitate uptake of the stains, in particular Alcian blue, it is recommended that the specimens are skinned. As this process is time consuming, this double staining technique is not routinely used in standard teratology tests. Thus experiments were set up to see if the skinning process influenced the fetal fragility in any way. Other teratogens, notably thalidomide, have been shown to cause varying effects depending on the species and strain of animal used [5]. The experiments reported here were designed to investigate whether the mouse as well as different rat strains exhibited the fetal fragility effect seen in Lister Hooded rats when they were dosed in utero with histamine H1 antagonists.

Rat fetal and postnatal development after treatmentin utero with histamine H1 antagonists

Increase in tissue histamine is associated with rapid mammalian cell growth such as occurs during fetal development [1]. A wide range of antihistamines are available as OTC drugs and because of their antiemetic, antisecretory, and antiallergic properties, may be prescribed or used in pregnancy. Whilst setting up standard rat teratology tests, we observed that treatment of dams with histamine H1 antagonists caused marked fragility of fetal joints. Using three chemically different centrallypenetrating histamine H 1 antagonists, this effect was found to be dose-dependent and time-related [2]. Because these compounds have marked atropinic properties, we investigated the effects of another histamine H 1 antagonist with a different spectrum of pharmacological activity, notably lacking atropinic properties [3]. Recently, an intracellular histamine receptor associated with cell growth has been reported and an antagonist [DPPE, (N,N-diethyl-2-[4-(phenylmethyl) phenoxyl] ethanamine HC1)] selective for this receptor has become available [4]. The experiments reported here were designed to investigate receptor selectivity of this rat fetal skeletal fragility and to monitor the effects of the histamine H 1 antagonists on postnatal physical and behavioural development of pups dosed in utero.

Histaminergic (H1) modulation of antinociception in morphinetolerant mice

This study compares the actions of diphenhydramine (a centrally penetrating histamine H1-antagonist) and acrivastine (a peripherally acting H1-antagonist) in morphine-tolerant and morphine-naïve mice. Mice receivei morphine twice daily for 5–7 days to induce tolerance. Once tolerant, the mice again received morphine, this time after pre-treatment with diphenhydramine (30 mg/kg s.c.) or acrivastine (10 mg/kg s.c. or 5 μg intracerebroventricularly-i.c.v.). Diphenhydramine (s.c.) and acrivastine (i.c.v.) increased the action of morphine in the hot-plate test at 49±1°C. However, acrivastine (s.c.) had no effect. In morphine-naïve mice, diphenhydramine increased the antinociceptive action of morphine (5 mg/kg) as did acrivastine when given intracerebroventricularly but not subcutaneously. Therefore, H1-antagonists potentiate the antinociceptive action of morphine in both morphine-tolerant and morphine-naïve mice via a central, rather than peripheral, action.