Histamine H1 antagonists block M-currents in dissociated rat cortical neurons

Monoaminergic control of brain states and sensory processing: Existing knowledge and recent insights obtained with optogenetics

Monoamines are key neuromodulators involved in a variety of physiological and pathological brain functions. Classical studies using physiological and pharmacological tools have revealed several essential aspects of monoaminergic involvement in regulating the sleep-wake cycle and influencing sensory responses but many features have remained elusive due to technical limitations. The application of optogenetic tools led to the ability of monitoring and controlling neuronal populations with unprecedented temporal precision and neurochemical specificity. Here, we focus on recent advances in revealing the roles of some monoamines in brain state control and sensory information processing. We summarize the central position of monoamines in integrating sensory processing across sleep-wake states with an emphasis on research conducted using optogenetic techniques. Finally, we discuss the limitations and perspectives of new integrated experimental approaches in understanding the modulatory mechanisms of monoaminergic systems in the mammalian brain.

Effects of antihistamines on the function of human α7-nicotinic acetylcholine receptors

Effects of the histamine H₁ receptor (H1R) antagonists (antihistamines), promethazine (PMZ), orphenadrine (ORP), chlorpheniramine (CLP), pyrilamine (PYR), diphenhydramine (DPH), citerizine (CTZ), and triprolidine (TRP) on the functional properties of the cloned α7 subunit of the human nicotinic acetylcholine receptor expressed in Xenopus oocytes were investigated. Antihistamines inhibited the α7-nicotinic acetylcholine receptor in the order PYR>CLP>TRP>PMZ>ORP≥DPH≥CTZ. Among the antihistamines, PYR showed the highest reversible inhibition of acetylcholine (100 µM)-induced responses with IC₅₀ of 6.2 µM. PYR-induced inhibition was independent of the membrane potential and could not be reversed by increasing the concentration of acetylcholine. Specific binding of [¹²⁵I] α-bungarotoxin, a selective antagonist for α7-nicotinic acetylcholine receptor, was not changed in the presence of PYR suggesting a non-competitive inhibition of nicotinic receptors. In line with functional experiments, docking studies indicated that PYR can potentially bind allosterically with the α7 transmembrane domain. Our results indicate that the H₂-H₄ receptor antagonists tested in this study (10 µM) showed negligible inhibition of α7-nicotinic acetylcholine receptors. On the other hand, H₁ receptor antagonists inhibited the function of human α7-nicotinic acetylcholine receptor, with varying potencies. These results emphasize the importance of α7-nicotinic acetylcholine receptor for future pharmacological/toxicological profiling.

Effect of hyperthermia on histamine blood level and convulsive behavior in infant rats

Febrile seizures (FS), which have been extensively studied using animal models, are the most common type of convulsive events in children, but the cellular mechanisms causing FS are still unclear. Histamine has been suggested to participate in seizure control. This study investigated the effect of hyperthermia (HT) on histamine blood level (HBL) and convulsive behavior in prepubertal rats. Forty Wistar rat pups were assigned to 5 groups (n=8), namely, control, HT, cromolyn, chlorpheniramine, and ranitidine. Two groups of adult rats were also used as control and HT adults. The control rats were placed in a hyperthermic chamber, and a room temperature current of air was blown on them. In all other groups, the rats were placed in the chamber for 30 min, and a current of warm air was applied to them. In the pretreatment groups, the rats received an injection of 68-mg/kg cromolyn sodium, 4-mg/kg chlorpheniramine, or 80-mg/kg ranitidine intraperitoneally 30 min prior to HT. Body temperature and convulsive behaviors were recorded. Then, the rats were anesthetized with ether, and their blood sample was obtained through direct heart puncture. Hyperthermia initiated convulsive behaviors in infant rats but not in the adult ones. Pretreatment with chlorpheniramine significantly potentiated convulsive behaviors (p=0.017). Hyperthermia led to a significant decrease in the HBL of both infant (p<0.001) and adult (p=0.003) rats. Pretreatments led to more decrease in the HBL of infant rats (p<0.001). It was concluded that HT could lead to a decrease in HBL, which in turn increases the seizure susceptibility of animals. Histamine may have a pivotal role in hyperthermia-induced seizures.

Induction of prolonged, continuous slow-wave sleep by blocking cerebral H₁ histamine receptors in rats

BACKGROUND AND PURPOSE

Classic H(1) histamine receptor (H(1)R) antagonists are non-selective for H(1)R and known to produce drowsiness. Modern antihistamines are more selective for H(1)R, and are 'non-drowsy' presumably due to reduced permeability through the blood-brain barrier. To characterize both histaminergic sleep regulation and the central actions of antihistamines, in the present study we analysed the effect of classic and modern antihistamines on rats' sleep using continuous i.c.v. infusions.

EXPERIMENTAL APPROACH

Effects of classic (d-chlorpheniramine; d-CPA) and second-generation (cetirizine) antihistamines on sleep were compared after i.p. injections or continuous i.c.v. infusions into rats. Fluorescent cetirizine/DBD-pz was synthesized to trace the approximate distribution of cerebral cetirizine. Furthermore, the effects of H(1) R antagonists on cultured preoptic neurons were examined using calcium imaging.

KEY RESULTS

d-CPA 4 mg·kg(-1) i.p. increased non-rapid eye movement (REM) sleep whereas 10-40 mg·kg(-1) d-CPA decreased non-REM sleep at dark onset time. Nocturnal i.c.v. infusions of d-CPA (10 µmol·100 µL(-1)·10 h(-1)) increased drowsiness but not non-REM sleep, whereas the same i.c.v. infusions of cetirizine significantly increased non-REM sleep, abolished REM sleep, and decreased wakefulness for more than 10 h. The medial preoptic area contained the greatest fluorescent labelling after i.c.v. cetirizine/DBD-pz infusions. Histamine-induced Ca(2+) increases in medial preoptic neurons were blocked by d-CPA or cetirizine, whereas d-CPA, but not cetirizine, increased Ca(2+) irrespective of antihistaminergic activity at ≥ 100 µM.

CONCLUSION AND IMPLICATIONS

The excitatory action of d-CPA may explain the seemingly inconsistent actions of d-CPA on sleep. Cerebral H(1)R inhibition by cetirizine induces synchronization of cerebral activity and prolonged, continuous slow-wave sleep.

First and second generation H₁ histamine receptor antagonists produce different sleep-inducing profiles in rats

First generation H₁ histamine receptor antagonists, such as d-chlorpheniramine (d-CPA) and diphenhydramine, produce drowsiness in humans. They are currently used as over-the-counter sleep aids. However, the mechanisms underlying drowsiness induced by these H₁ histamine receptor antagonists remain obscure because they produce heterogeneous receptor-independent actions. Ketotifen is a second generation H₁ histamine receptor antagonist which is more permeable to the brain than newer H₁ histamine receptor antagonists. Therefore, to access sleep-inducing profiles by H₁ histamine receptor blocking actions, the present study compared the dose-dependent effects of diphenhydramine and ketotifen (1-40 mg/kg, intraperitoneal injection at dark onset time) on daily sleep-wake patterns in rats. Ketotifen dose-dependently decreased rapid-eye-movement (REM) sleep and increased non-REM sleep by amplifying slow-wave electroencephalogram powers. Diphenhydramine at 4 mg/kg transiently increased non-REM sleep and reduced REM sleep similar to the effects of ketotifen. The larger injections of diphenhydramine (10-40 mg/kg), however, reduced non-REM sleep, abolished slow-wave enhancements and facilitated wakefulness. The bi-directional action of diphenhydramine on sleep is similar to our former results using d-CPA. Taken together, the arousal effects caused by over-dose administrations of the first generation H₁ histamine receptor antagonists may be mediated by H₁ histamine receptor-independent actions. To further examine the tolerance of ketotifen-induced sleep, 3 mg/kg ketotifen was injected daily for 5 days 3 h before light onset time. These experiments consistently enhanced non-REM-sleep at the end of the active phase of rats, suggesting that ketotifen may function as a desirable sleep aid although the coincidental REM sleep reduction requires attention.

Beta-phenylethylamine inhibits K+ currents in neocortical neurons of the rat: a possible mechanism of beta-phenylethylamine-induced seizures

beta-Phenylethylamine (beta-PEA), an endogenous amine synthesized in the brain, serves as a neuromodulator and is involved in the pathophysiology of various neurological disorders such as depression, schizophrenia, and attention-deficit hyperactivity disorder. beta-PEA fully exerts the physiological effects within the nanomolar concentration range via the trace amine receptors, but beta-PEA also causes convulsions at much higher concentrations via an as yet unknown mechanism. To investigate the electrophysiological mechanism by which beta-PEA induces convulsions, we examined the effect of beta-PEA on ionic currents passing through the cell membrane of dissociated rat cerebral cortical neurons, using a patch-clamp technique. The external application of beta-PEA suppressed ionic currents which continuously flowed when the membrane potential was held at -25 mV. The suppression was in a concentration-dependent manner and a half-maximal effective concentration was 540 muM. These currents suppressed by beta-PEA consisted of two K(+) currents: a time- and voltage-dependent K(+) current (M-current) and a leakage K(+) current. The suppression of the M-current reduces the efficacy of the current in limiting excessive neuronal firing, and the suppression of the leakage K(+) current can cause membrane depolarization and thus promote neuronal excitation. Reducing both of these currents in concert may produce neuronal seizing activity, which could conceivably underlie the convulsions induced by high-dose beta-PEA.

Antihistamine mepyramine directly inhibits KCNQ/M channel and depolarizes rat superior cervical ganglion neurons

The first-generation antihistamines are widely prescribed medications that relieve allergic reactions and urticaria by blocking the peripheral histamine H(1) receptor. Overdose of these drugs often results in serious neuronal toxic effects, including seizures, convulsions and worsening of epileptic symptoms. The KCNQ/M K(+) channel plays a crucial role in controlling neuron excitability. Here, we demonstrate that mepyramine and diphenhydramine, two structurally related first-generation antihistamines, can act as potent KCNQ/M channel blockers. Extracellular application of these drugs quickly and reversibly reduced KCNQ2/Q3 currents heterologously expressed in HEK293 cells. The current inhibition was concentration and voltage dependent. The estimated IC(50) (12.5 and 48.1 microM, respectively) is within the range of drug concentrations detected in poisoned patients (30-300 microM). Both drugs shifted the I-V curve of KCNQ2/Q3 channel to more depolarized potentials and altered channel gating properties by prolonging activation and shortening deactivation kinetics. Mepyramine also inhibited the individual homomeric KCNQ1-4 and heteromeric KCNQ3/Q5 currents. Moreover, mepyramine inhibited KCNQ2/Q3 current in an outside-out patch excised from HEK293 cells and the inhibitory effect was neither observed when it was applied intracellularly nor affected by blocking phospholipase C (PLC) activity, indicating an extracellular and direct channel blocking mechanism. Finally, in cultured rat superior cervical ganglion (SCG) neurons, mepyramine reduced the M type K(+) current in a concentration-dependent manner and led to marked membrane potential depolarization. It is likely that these effects may be involved in the adverse neuroexcitatory effects observed in patients experiencing an overdose of antihistamines.

Selective inhibition of Kir currents by antihistamines

In the present study, the effects of antihistamines on inwardly rectifying potassium (Kir) channels expressed in Xenopus oocyte were investigated using two-electrode voltage clamp technique. Firstly, effects of antihistamines on two members of Kir2.0 sub-family, Kir2.1 and Kir2.3 were compared. For antihistamines that selectively block histamine H(1) receptor, the first-generation antihistamines mepyramine and diphenhydramine inhibited Kir2.3 current by 25.0+/-2.9% and 17.3+/-0.7% at concentrations of 100 microM, respectively. In contrast, the second- and third-generation antihistamines astemizole and desloratadine were completely devoid of any inhibitory effect on Kir2.3 current. Histamine H(2) receptor antagonist cimetidine, at 100 microM, failed to inhibit Kir2.3 current. On the other hand, Kir2.1 current was not sensitive to any of these drugs. The mepyramine-induced inhibition of Kir2.3 current was significantly reduced by a single point mutation in Kir2.3 (Kir2.3(I213L)), which enhances Kir2.3-PIP(2) interaction. Secondly, the effect of mepyramine was also tested on Kir3.4*, another member of Kir family. 100 microM mepyramine produced a 30.3+/-4.6% inhibition on Kir3.4* current. These results suggest that the first-generation histamine H(1) receptor antagonists selectively inhibit Kir currents. The inhibitory effect of antihistamines on Kir currents may be involved in their neuronal and cardiac toxic effects caused by drug overdosing.

Mast cell stabilizer ketotifen [4-(1-methyl-4-piperidylidene)-4h-benzo[4,5]cyclohepta[1,2-b]thiophen-10(9H)-one fumarate] prevents mucosal mast cell hyperplasia and intestinal dysmotility in experimental Trichinella spiralis inflammation in the rat

Trichinella spiralis infection in rats induces hypermotility and an abnormal response to cholecystokinin (CCK) similar to motor disturbances observed in irritable bowel syndrome. Mast cell hyperplasia is also characteristic of this experimental model. The aim of our study was to correlate mast cell activity with the development of dysmotility and to demonstrate whether the mast cell stabilizer ketotifen [4-(1-methyl-4-piperidylidene)-4H-benzo[4,5]cyclohepta[1,2-b]thiophen-10(9H)-one fumarate] could prevent the development of intestine hypermotility. Sprague-Dawley rats were infected with T. spiralis and, 5 days after infection, treated with the mast-cell stabilizer ketotifen (10 mg/kg/day). Twelve days after infection, intestinal spontaneous motor activity and response to CCK were evaluated by means of strain-gauge transducers. Immunohistochemistry for rat mast cell protease II (RMCPII), cyclooxygenase (COX)-2, and inducible nitric-oxide synthase (iNOS) was performed in intestinal specimens. In addition, RMCPII and myeloperoxidase were determined in serum. Infected control rats showed hypermotility, mast cell hyperplasia, increased RMCPII levels, increased myeloperoxidase, and overexpression of COX-2 and iNOS. In contrast, ketotifen-treated rats showed spontaneous intestinal motility and CCK response similar to the noninfected control rats. Mast cell hyperplasia and RMCPII were reduced in ketotifen-treated rats. Inflammatory parameters were less modified by ketotifen, but those animals that received the longest ketotifen treatment showed a slight amelioration in these parameters. These results indicate that mast cells are implicated in the development of hypermotility. The treatment with ketotifen prevented hypermotility and mast cell hyperplasia and diminished mucosal mast cell activity.