Iontophoretic studies of histamine and histamine antagonists in the feline vestibular nuclei

Histamine excites rat lateral vestibular nuclear neurons through activation of post-synaptic H2 receptors

Through whole-cell patch recordings in brainstem slices, the effects of histamine on neuronal activity of the lateral vestibular nucleus (LVN) were investigated. Bath application of histamine elicited a concentration-dependent excitation of both spontaneous firing (n=19) and silent (n=7) LVN neurons. Moreover, histamine induced a stable inward current in the LVN neurons (n=5) and the histamine-induced depolarization of membrane potential persisted in the presence of tetrodotoxin (n=4), indicating a direct post-synaptic effect of the histamine on the LVN neurons. Selective histamine H2 receptor antagonist ranitidine effectively blocked the histamine-evoked excitatory responses on the LVN neurons (n=4), but selective histamine H1 receptor antagonist triprolidine did not (n=4). In addition, selective histamine H2 receptor agonist dimaprit (n=3) rather than 2-pyridylethylamine (n=4), a selective histamine H1 receptor agonist, mimicked the excitatory action of histamine on LVN neurons. The results demonstrate that histamine excites the LVN neurons via post-synaptic histamine H2 receptors and suggest that the central histaminergic projection arising from the hypothalamus may modulate LVN neurons activity and actively influence the vestibular reflexes and functions.

Histamine in the nervous system

Histamine is a transmitter in the nervous system and a signaling molecule in the gut, the skin, and the immune system. Histaminergic neurons in mammalian brain are located exclusively in the tuberomamillary nucleus of the posterior hypothalamus and send their axons all over the central nervous system. Active solely during waking, they maintain wakefulness and attention. Three of the four known histamine receptors and binding to glutamate NMDA receptors serve multiple functions in the brain, particularly control of excitability and plasticity. H1 and H2 receptor-mediated actions are mostly excitatory; H3 receptors act as inhibitory auto- and heteroreceptors. Mutual interactions with other transmitter systems form a network that links basic homeostatic and higher brain functions, including sleep-wake regulation, circadian and feeding rhythms, immunity, learning, and memory in health and disease.

Changes in the histaminergic system during vestibular compensation in the cat

To determine how the histaminergic system is implicated in vestibular compensation, we studied the changes in histidine decarboxylase (HDC; the enzyme synthesizing histamine) mRNA regulation in the tuberomammillary (TM) nuclei of cats killed 1 week, 3 weeks and 3 months after unilateral vestibular neurectomy (UVN). We also used one- and two-step bilateral vestibular neurectomized (BVN) cats to determine whether HDC mRNA regulation depended on the asymmetrical vestibular input received by the TM nuclei neurons. In addition, we analysed the HDC mRNA changes in the TM nuclei and the recovery of behavioural functions in UVN cats treated with thioperamide, a pure histaminergic drug. Finally, we quantified binding to histamine H3 receptors (H3Rs) in the medial vestibular nucleus (VN) by means of a histamine H3R agonist ([3H]N-alpha-methylhistamine) in order to further investigate the sites and mechanisms of action of histamine in this structure. This study shows that UVN increases HDC mRNA expression in the ipsilateral TM nucleus at 1 week. This increased expression persisted 3 weeks after UVN, and regained control values at 3 months. HDC mRNA expression was unchanged in the one-step BVN cats but showed mirror asymmetrical increases in the two-step BVN compared to the 1 week UVN cats. Three weeks' thioperamide treatment induced a bilateral HDC mRNA up-regulation in the UVN cats, which was higher than in the untreated UVN group. Binding to histamine H3Rs in the MVN showed a strong bilateral decrease after thioperamide treatment, while it was reduced ipsilaterally in the UVN cats. That such changes of the histaminergic system induced by vestibular lesion and treatment may play a functional role in vestibular compensation is strongly supported by the behavioural data. Indeed, spontaneous nystagmus, posture and locomotor balance were rapidly recovered in the UVN cats treated with thioperamide. These results demonstrate that changes in histamine levels are related to vestibular compensation.

Spontaneous activity in rat vestibular nuclei in brain slices and effects of acetylcholine agonists and antagonists

Extracellular recording was used to investigate spontaneously active neurons in all four major nuclei of the rat vestibular nuclear complex (VNC) in brainstem slices. The density of spontaneously active neurons was highest in the medial vestibular nucleus (MVN), slightly lower in the superior (SuVN) and spinal (SpVN) nuclei, and lowest in the lateral vestibular nucleus (LVN). We compared the effects of acetylcholine agonists and antagonists on spontaneously discharging neurons in MVN, SuVN, and SpVN with those in the nearby dorsal cochlear nucleus (DCN). The proportion of neurons responding to carbachol was greatest in DCN and smallest in SpVN. Unlike in DCN, some neurons in MVN, SuVN, and SpVN showed decreased firing during carbachol or muscarine. Magnitudes of responses to carbachol and muscarine were closely correlated (P<0.01). MVN neurons possessed nicotinic as well as muscarinic receptors. Activation of either type was unaffected by blocking synaptic transmission. The IC(50) values for the muscarinic subtype-preferential antagonists were compared, and tropicamide, preferential for M(4), was the most potent. Our results suggest that: (1) the relative numbers of spontaneously active neurons in rat VNC differ among nuclei; (2) acetylcholine agonists elicit changes in mean firing rates of neurons in MVN, SuVN and SpVN, but fewer neurons respond, and responses are smaller than in DCN; (3) both muscarinic and nicotinic acetylcholine receptors are present on MVN neurons, but muscarinic receptors may be more prominent.

Histamine immunoreactivity changes in vestibular-lesioned and histaminergic-treated cats

Histamine is likely involved in vestibular function recovery since histaminergic medications are effective in vestibular-related syndromes. We investigated the histamine immunoreactivity changes after unilateral vestibular neurectomy and the effects of betahistine (a partial histamine H1 receptor agonist and an histamine H3 receptor antagonist) and thioperamide (a pure histamine H3 receptor antagonist) treatment in cats. Histamine staining was analyzed in the tuberomammillary and vestibular nuclei through immunohistochemical methods and quantification techniques in light microscopy. Unilateral vestibular neurectomy induced a strong bilateral decrease in histamine immunoreactivity in the vestibular nuclei and a smaller reduction in the tuberomammillary nuclei in both acute (1 week) and compensated (3 weeks, 1 year) cats. One-week thioperamide or betahistine treatment led to a near-total lack of staining in these structures in both lesioned and control cats. One-month betahistine treatment had weaker effects in the compensated cats. We conclude that vestibular lesions reduce histamine staining because of an increase in histamine release in the vestibular and tuberomammillary nuclei, promoting vestibular functions recovery, and betahistine could contribute to this process by acting on both the presynaptic histamine H3 and postsynaptic histamine H1 receptors.

Medial vestibular nucleus in the guinea-pig: histaminergic receptors. I. An in vitro study

Antihistaminergic drugs are currently used for the symptomatic treatment of vestibular-related syndromes such as vertigo and motion sickness. We therefore investigated whether histamine could modulate the firing of medial vestibular nuclei neurons (MVNn). Recently, we have demonstrated that different cell types are present among MVNn in guinea-pig brainstem slices. Bath-application of histamine at 10(-4) or 10(-5) M induced a small membrane depolarization accompanied by a slight decrease in membrane resistance and a reversible increase in spontaneous firing in all MVN cell types. These effects were presumably postsynaptic as they persisted in a low-calcium/high-magnesium solution. Using a variety of agonists and antagonists of histamine receptors (H1, H2 and H3), we conclude that these effects are mediated by H2 receptors. The companion paper is concerned with an in vivo study of the histaminergic modulation of the vestibular function (Yabe et al., in press).

Medial vestibular nucleus in the guinea-pig: histaminergic receptors. II. An in vivo study

In a companion paper (Serafin et al. 1992) we have demonstrated in vitro that histamine depolarizes three previously described medial vestibular nucleus neuron (MVNn) types (Serafin et al. 1991a, b). It has also been shown that this effect was exclusively mediated through postsynaptic H2 receptors. All the same, the eventual contribution of presynaptic H3 receptors to the physiological response of the MVNn to histamine remained an open question since, during the slicing procedure, any histaminergic axons projecting to the vestibular nuclei would have been interrupted. This rendered our study of H3-mediated effects of histamine difficult. Hence, in the present in vivo study our aim was three-fold: (1) to investigate the presence of H3 receptors at the vestibular nuclei level; (2) to evaluate the functional importance of MVNn H2 receptors; and (3) to explore whether H3 ligands, when injected intraperitoneally (i.p.), could modulate dynamic vestibular functions. In order to address the first two questions, we investigated postural changes induced by perfusion of the guinea-pig's vestibular nuclear complex with specific ligands of the H2 and H3 receptors. Our data extend the conclusions of our in vitro study and suggest that lateral vestibular nuclei neurons and the MVNn are endowed with both H2 and H3 receptors. Our results indicate furthermore that histamine can modulate, quite effectively, static vestibular reflexes. Finally, the present study demonstrates that i.p. injection of thioperamide, an H3 antagonist, induces a significant decrease in the horizontal vestibular-ocular reflex gain and, by contrast to most of the clinically used antihistaminics, has no detrimental effect on the alertness level. Our results may thus lead to clinical testing and use of H3 antagonists as antivertigo or anti motion-sickness drugs.

Neurochemical mechanisms of recovery from peripheral vestibular lesions (vestibular compensation)

This paper reviews the literature relating to the neurochemical basis of vestibular compensation, a process of behavioral recovery which occurs following the removal of afferent input from one labyrinth (unilateral labyrinthectomy, UL). Although vestibular compensation is known to be correlated with a return of resting activity to the vestibular nucleus (VN) ipsilateral to the UL (the deafferented VN), the neurochemical mechanisms by which this neuronal recovery occurs, are unknown. At present, there is little evidence to support the hypothesis that denervation supersensitivity of excitatory amino acid, dopamine, norepinephrine or acetylcholine receptors in the deafferented VN, is responsible for vestibular compensation: binding studies for glutamate or acetylcholine do not support an upregulation of these receptor types. However, changes in the affinity or efficacy of these receptor complexes cannot be ruled out. There are still many neurotransmitter systems, such as serotonergic and histaminergic systems, which have not been investigated in relation to vestibular compensation. In several species it has been shown that treatment with adrenocorticotropic hormone, fragment 4-10 (ACTH-(4-10], can accelerate vestibular compensation. It is not clear how these drugs exert their effects. In vitro electrophysiological studies have shown that VN neurons are capable of generating resting activity in the absence of their normal afferent inputs and it is possible that these neurons have pacemaker-like membrane characteristics which contribute to the regeneration of activity following UL. Recent biochemical studies have revealed changes in the phosphorylation patterns of a number of proteins during compensation. The possible relationship between these phosphorproteins and the synaptic or membrane changes which are responsible for vestibular compensation remains to be determined.

Histamine depolarizes rat medial vestibular nucleus neurons recorded intracellularly in vitro

The effects of histamine (HA) on the resting membrane potential and input resistance of rat medial vestibular nucleus (MVN) neurons were investigated using intracellular recording techniques from a submerged brain slice preparation. The exogenous application of HA predominantly produced a concentration-dependent membrane depolarization and induction of action potential firing. The depolarization exhibited a rapid onset, a slow recovery, and usually occurred in the absence of any apparent change in conductance. These effects of HA could be mimicked by the H2-agonist impromidine and were reversibly blocked by the H2-antagonist cimetidine. Tetrodotoxin (TTX) or low calcium/high magnesium-containing media failed to block completely the HA-induced depolarization supporting a direct postsynaptic receptor mediated action of HA. The diminished HA-induced depolarization observed following pretreatment with TTX cannot exclude an additional presynaptic action by HA. The present findings reveal that HA exerts a novel direct excitation of rat MVN neurons through an H2-receptor.

Adverse reactions and interactions with H2-receptor antagonists

Histamine H2-receptor antagonists have been used in the treatment of gastrointestinal diseases for more than a decade and during this period have become one of the most commonly prescribed groups of drugs in the world. The deserved popularity of the H2-receptor antagonists reflects, in part, their therapeutic efficacy, which has revolutionised the treatment of peptic ulcer disease. An equally, or more, important reason for the widespread use of H2-receptor antagonists is their remarkably low toxicity. We have attempted, in this review, to present a detailed account of the minor and more serious adverse reactions, while emphasising the low incidence of the former and the rarity of the latter. The toxicology of the H2-receptor antagonists is discussed under two main headings: adverse effects; and drug interactions. The latter category is potentially the more significant, since the frequent use of therapy with multiple drugs may give rise to drug interactions, some of which are serious and may even be lethal. These drug interactions occur especially in the gastrointestinal tract, the liver and the kidneys. Thus, the absorption of other drugs may be altered because the H2-receptor antagonists inhibit gastric secretion--an effect illustrated by ketoconazole, the absorption of which is reduced when given in combination with cimetidine. Very important drug interactions are caused by inhibition of the hepatic microsomal enzyme cytochrome P450 by some of the H2-receptor antagonists. This effect appears to be related to the chemical structure of the individual H2-receptor antagonists and is not attributable to histamine H2-receptor blockade. For example, cimetidine is a powerful inhibitor of cytochrome P450, while the interaction of ranitidine with this system is weaker. Consequently, cimetidine reduces the metabolism of many drugs which are normally degraded by phase I reactions, leading to potentially toxic plasma concentrations of therapeutic agents such as some oral anticoagulants, beta-blockers, anticonvulsants, benzodiazepines and xanthines. Some of the H2-receptor antagonists are actively secreted by the renal tubules and may thus compete with other drugs for cationic tubular transport mechanisms, resulting in reduced urinary excretion and hence potentially toxic plasma concentrations. This type of drug interaction has been reported after administration of both cimetidine and ranitidine with procainamide or quinidine.(ABSTRACT TRUNCATED AT 400 WORDS)

Histamine-induced excitation of spontaneously active medullary neurones in the rat brain is mediated by H2-receptors. A microiontophoretic study using H1- and H2-agonists and antagonists

The effects of histamine, applied by microiontophoresis onto spontaneously-active medullary neurones were investigated in the rat. Histamine caused current-dependent excitation of these neurones, an action that is at variance with previous studies in the cat. The nature of the receptor mediating these effects was examined using a number of agonists with differing potencies at peripheral H1- and H2-receptors. The precursor of histamine, L-histidine and the metabolite, N-telemethylhistamine did not mimic the effects of histamine while the H2-agonist, 4-methylhistamine caused similar but weaker excitation. The extent of excitations produced by the H1-agonists, 2-pyridylethylamine, 2-methylhistamine and 2-thiazolylethylamine could be related to their activity at H2-receptors. Metiamide was ineffective in antagonising responses to histamine and related agonists as was mepyramine. The H2-antagonist ranitidine, however, proved a good antagonist of responses to histamine and the H1- and H2-agonists, despite an unrelated excitatory action which may be linked to inhibition of cholinesterase. It is concluded that the excitatory effects of microiontophoretically-applied histamine and the agonists on medullary neurones in the rat is probably a result of activation of H2-receptors.

Mental status, the intensive care unit, and cimetidine

A blinded, prospective study of the association between cimetidine treatment and altered mental status (MS) was evaluated in 217 surgical intensive care patients stratified by the degree of renal or hepatic dysfunction or both. The cimetidine and noncimetidine groups did not differ in blood chemistries, physiologic-metabolic data, or concurrent medications. There was a correlation between treatment with cimetidine and changes in MS (p less than 0.0001). The MS changes with cimetidine ranged from 33% for neither hepatic nor renal dysfunction to 80% for both renal and liver impairment. The magnitude of MS change was influenced by disease and serum concentration, with patients with both renal and liver disease having the greatest MS change at the highest serum concentrations. The type of MS change was also disease specific. All MS change cleared within 24 to 36 hours of stopping the drug. It appears that both dosage and underlying disease state are important factors determining the nature and severity of cimetidine-associated mental status changes.

Histamine actions on activity of cultured hypothalamic neurons: evidence for mediation by H1- and H2-histamine receptors

Tuberal hypothalamic tissue cultures were used to investigate the actions of histaminergic agents on neuronal activity using extracellular glass micropipettes. Histamine and H1- and H2-agonists were applied locally onto single active neurons by iontophoresis, while histaminergic antagonists were perfused through the bathing medium. Peri-event histogram and ratemeter analysis showed histamine to both excite and depress unit activity. Excitations were only antagonized by putative H1- and not H2-histamine antagonists, whereas inhibitions were antagonized by H2- and H2-antagonists. Dimaprit, a specific H2-agonist, elicited inhibitions of activity, while 2-(2-pyridyl)ethylamine, a putative H1-agonist, elicited both excitations and inhibitions. Two pharmacologically distinct populations of histamine receptors may exit in the hypothalamus: excitatory H2-receptors and inhibitory H2-receptors.

Are histamine H2-receptor depressions of neuronal activity in tissue cultures from rat hypothalamus mediated through cyclic adenosine monophosphate?

Iontophoretic application of histamine H2-agonists depressed spontaneous firing of tuberal hypothalamic neurons in tissue culture. Perfusion with inhibitors of cyclic nucleotide phosphodiesterase increased the strength of these depressions, which persisted during perfusion with Ca2+-free medium. These results support a role of cyclic AMP as a mediator of histamine H2-elicited depressions of neuronal activity.

Pharmacokinetic and clinical studies in patients with cimetidine-associated mental confusion

15 cases of cimetidine-associated mental confusion have been reported. In order that this syndrome might be investigated changes in mental status (M.S.) were correlated with serum concentrations and renal and hepatic function in 36 patients, 30 patients had no M.S. change on cimetidine and 6 had moderate to severe changes. These 6 patients had both renal and liver dysfunction (P less than 0.05), as well as cimetidine trough-concentrations of more than 1.25 microgram/ml (P less than 0.05). The severity of M.S. changes increased as trough-concentrations rose, 5 patients had lumbar puncture. The cerebrospinal fluid: serum ratio of cimetidine concentrations was 0.24:1 and indicates that cimetidine passes the blood-brain barrier; it also raises the possibility that M.S. changes are due to blockade of histamine H2-receptors in the central nervous system. Patients likely to have both raised trough-concentrations and mental confusion are those with both severe renal and hepatic dysfunction. They should be closely observed and should be given reduced doses of cimetidine.

Hypersensitivity to histamine in the guinea-pig brain: microiontophoretic and biochemical studies

Electrolytic lesions of the medial forebrain bundle induce a fall in histidine decarboxylase activity (the specific synthetic enzyme of brain histamine) in the ipsilateral cerebral cortex and hippocampus of the guinea pig brain; these results suggest the presence of an ascending histaminergic pathway in the guinea pig brain similar to that described in the rat. Possible alterations in the sensitivity of histaminergic receptors present in the target areas were studied following this type of lesion by combining electrophysiological and biochemical approaches. Microiontophoretic applications of histamine or noradrenaline reveal a hypersensitivity (lower ejecting currents for threshold and maximal responses) in cortical neurons ipsilateral but not contralateral to the lesion, whereas responses to iontophoretically applied GABA are not modified. In contrast the responsiveness of histamine-sensitive cyclic AMP generating systems is not modified, neither in the cerebral cortex nor in the hippocampus after this type of lesion. Similar conclusions are reached from the data obtained with specific agonists of the two classes of histaminergic receptors and measurements in the presence of a phosphodiesterase inhibitor. Several hypotheses are discussed in order to reconcile the finding of a denervation hypersensitivity revealed by iontophoresis contrasting with an unaltered responsiveness of the histaminergic receptors linked to the adenylate cyclase.