Histamine as an endogenous antagonist of nicotinic ACh-receptor

Identification of histamine receptor subtypes in skeletal myogenesis

To date, conventional and/or novel histamine receptors (HRs) have not been investigated in mouse skeletal myogenesis. Therefore, the present study aimed to investigate the HR-subtypes in skeletal myogenesis. The myogenesis of C2C12 skeletal myoblasts was evaluated using desmin, myogenin and myosin heavy chain (Myh) as early, intermediate and late differentiation markers, respectively. Reverse transcription-quantitative polymerase chain reaction and immunostaining were performed and the messenger RNA (mRNA) expression levels of the HR-subtypes and markers were determined. H₁R mRNA was found to be highly expressed in myoblasts at day 0; however, the expression levels were reduced as differentiation progressed. By contrast, H₂R mRNA expression remained constant, while H₃R mRNA expression increased by 28-, 103- and 198-fold at days 2, 4 and 6 compared with the baseline level (day 0), respectively. In addition, Myh expression increased by 7,718-, 94,487- and 286,288-fold on days 2, 4 and 6 compared with the baseline expression level (day 0). Weak positive staining of the cells for H₃R protein was observed on day 2, whereas highly positive staining was observed on days 4 and 6. HR expression during myogenesis was, in part, regulated by the stage of differentiation. These results along with previous findings indicated possible involvement of H₁R in the regulation of progenitor cell mitogenesis and of H₂R in the relaxation of acetylcholine-stimulated contraction of muscle cells, following the activation of professional histamine-producing cells, including mast cells. By contrast, H₃R may participate in the regulation of specialized myocyte functions, potentially by maintaining the relaxed state under the influence of constitutive H₃R activity and low histamine concentrations, locally produced/released by non-professional histamine-producing cells.

Spontaneous smoking cessation before lung cancer diagnosis

INTRODUCTION

We have observed that many patients with lung cancer stop smoking before diagnosis, usually before clinical symptoms, and often without difficulty. This led us to speculate that spontaneous smoking cessation may be a presenting symptom of lung cancer.

METHODS

Patients from the Philadelphia Veterans Affairs Medical Center with lung cancer and for comparison, prostate cancer and myocardial infarction underwent a structured interview about their smoking habits preceding diagnosis. Severity of nicotine addiction was graded using the Fagerström Test for Nicotine Dependence. Among former smokers, dates of cessation, onset of symptoms, and diagnosis were recorded. Difficulty quitting was rated on a scale of 0 to 10. Distributions of intervals from cessation to diagnosis were compared between groups.

RESULTS

All 115 patients with lung cancer had been smokers. Fifty-five (48%) quit before diagnosis, and only six of these (11%) were symptomatic at quitting. Patients with lung cancer who quit were as dependent on nicotine, when smoking the most, as those who continued to smoke, unlike the other groups. Despite this, 31% quit with no difficulty. The median interval from cessation to diagnosis was 2.7 years for lung cancer, 24.3 years for prostate cancer, and 10.0 years for patients with myocardial infarction.

CONCLUSIONS

These results challenge the notion that patients with lung cancer usually quit smoking because of disease symptoms. The hypothesis that spontaneous smoking cessation may be a presenting symptom of lung cancer warrants further investigation.

Regulation of parasympathetic neurons by mast cells and histamine in the guinea pig heart

The potential interaction between the immune system and the autonomic nervous system was examined in the cardiac ganglia of guinea pigs. Intracellular voltage recordings were used to determine the effects of mast cell degranulation on the membrane properties of parasympathetic neurons in animals actively sensitized to ovalbumin. Stimulation of mast cell degranulation by perfusion with ovalbumin (10 micrograms/ml) produced a depolarization and increase in the excitability of intracardiac neurons. These effects could be mimicked by histamine application, either by perfusion (10 microM) or by local pressure application (100 microM, 1-2 s application). In either case, histamine application resulted in a similar membrane depolarization and increase in excitability. Immunohistochemical experiments demonstrated that histamine-immunoreactive mast cells are located in close proximity to parasympathetic postganglionic neurons. The histamine response was not due to release of other neurotransmitters from adjacent nerve terminals and both the depolarization and increase in excitability were inhibited by the H1 antagonist, pyrilamine (300 nM), and were unaffected by the H2 antagonist cimetidine (5 microM). Incubation of cardiac ganglion preparations from sensitized animals with pyrilamine prior to ovalbumin perfusion resulted in the inhibition of both the depolarization and increase in excitability. These results demonstrate that mast cell degranulation, and the subsequent release of histamine, results in the stimulation of intracardiac neurons via the activation of H1 receptors. Thus, local inflammatory reactions in the cardiac tissue can lead to the rapid activation of parasympathetic neurons, thereby altering cardiac function.

Histamine and related substances influence neurotransmission in the semicircular canal

Histamine and other imidazole-containing substances were found to increase ampullar nerve afferent firing rate while both H1 and H2 histamine antagonists effectively inhibited ampullar nerve activity. A specific inhibitor of histidine decarboxylase, the enzyme which catalyses the synthesis of histamine, reduced ampullar nerve firing in a dose-dependent manner. These observations suggest a physiological role for histamine in the inner ear. Maintenance of a response to histamine after de-efferentation of the crista ampullaris supports the hypothesis that the site of action is the hair cell; antagonism of the histamine response by a cholinergic antagonist, atropine, and antagonism of a cholinergically mediated facilitation by the histaminergic antagonist pyrilamine, indicate that the site of action may involve the acetylcholine receptor complex on the crista ampullaris hair cells. The observation that imidazole-containing compounds cause significant effects on semicircular canal neurotransmission provides an important finding with regard to the site of action of antihistamines used for the treatment of vertigo and motion sickness.

Endogenous anticonvulsant substances

Epileptic seizures will normally arrest abruptly and spontaneously, and the brain will remain refractory to further seizures for some time thereafter. This paper reviews the possible mechanisms underlying this seizure arrest and refractoriness. The data suggests that neuronal fatigue is not involved in either of these processes, whereas the role of ions and excitatory systems are unclear. Rather, seizure arrest and refractoriness may come about by the seizure-induced release and/or activity of multiple endogenous anticonvulsant substances. The spontaneous arrest of the seizure may involve the purine adenosine, in addition to other unknown mechanisms. Seizure refractoriness involves multiple systems, the most important of which, on the available evidence, are prostaglandins and opioid peptides and possibly benzodiazepine systems, although other neuropeptides and the purines may also be involved. The implications of these conclusions to anti-epileptic drug development and status epilepticus are discussed.

Histamine is an antagonist of the acetylcholine receptor at the frog endplate

The effects of histamine on the acetylcholine (ACh) receptor-channel complex were examined by means of voltage-clamp at the frog endplate. ACh was ionophoretically applied to the endplate. Histamine was added to the perfusate. Histamine (100 nM - 1 mM) reversibly depressed the peak amplitude of the ACh-induced inward current in a dose-dependent manner. The double reciprocal plot of the dose-response relationship between the peak ACh current and the amount of ACh applied suggested that histamine (100 microM) depressed the ACh-induced current in a competitive manner. Histamine prevented the specific ACh binding site within the receptor-channel complex from binding erabutoxin, a sea-snake venom, which binds irreversibly to the specific ACh binding site. Histamine had no detectable effects on the equilibrium potential of the endplate current but shortened the half-decay time of the endplate current in a voltage-dependent manner. It was therefore concluded that histamine blocks not only the specific ACh binding site but also interacts with the ACh-channel site. The present experiments strongly suggest that histamine can act as an antagonist to modulate nicotinic cholinergic transmission.