Histamine: action on single hypothalamic neurones

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.

The physiology of brain histamine

Histamine-releasing neurons are located exclusively in the TM of the hypothalamus, from where they project to practically all brain regions, with ventral areas (hypothalamus, basal forebrain, amygdala) receiving a particularly strong innervation. The intrinsic electrophysiological properties of TM neurons (slow spontaneous firing, broad action potentials, deep after hyperpolarisations, etc.) are extremely similar to other aminergic neurons. Their firing rate varies across the sleep-wake cycle, being highest during waking and lowest during rapid-eye movement sleep. In contrast to other aminergic neurons somatodendritic autoreceptors (H3) do not activate an inwardly rectifying potassium channel but instead control firing by inhibiting voltage-dependent calcium channels. Histamine release is enhanced under extreme conditions such as dehydration or hypoglycemia or by a variety of stressors. Histamine activates four types of receptors. H1 receptors are mainly postsynaptically located and are coupled positively to phospholipase C. High densities are found especially in the hypothalamus and other limbic regions. Activation of these receptors causes large depolarisations via blockade of a leak potassium conductance, activation of a non-specific cation channel or activation of a sodium-calcium exchanger. H2 receptors are also mainly postsynaptically located and are coupled positively to adenylyl cyclase. High densities are found in hippocampus, amygdala and basal ganglia. Activation of these receptors also leads to mainly excitatory effects through blockade of calcium-dependent potassium channels and modulation of the hyperpolarisation-activated cation channel. H3 receptors are exclusively presynaptically located and are negatively coupled to adenylyl cyclase. High densities are found in the basal ganglia. These receptors mediated presynaptic inhibition of histamine release and the release of other neurotransmitters, most likely via inhibition of presynaptic calcium channels. Finally, histamine modulates the glutamate NMDA receptor via an action at the polyamine binding site. The central histamine system is involved in many central nervous system functions: arousal; anxiety; activation of the sympathetic nervous system; the stress-related release of hormones from the pituitary and of central aminergic neurotransmitters; antinociception; water retention and suppression of eating. A role for the neuronal histamine system as a danger response system is proposed.

Histamine mediates fast synaptic inhibition of rat supraoptic oxytocin neurons via chloride conductance activation

Axons from the histaminergic neurons of the tuberomammillary nucleus project to both the anterior and tuberal portions of the supraoptic nucleus. Histamine is known to activate vasopressin neurons via a histamine receptor subtype 1 and to increase release of vasopressin, but effects on oxytocin neurons have been previously unexplored. Here we investigated the effects of tuberomammillary nucleus electrical stimulation as well as of histamine antagonists on supraoptic nucleus oxytocin and vasopressin neurons in slices of rat hypothalamus. Electrical stimulation evoked short constant latency (approximately 5 ms), fast (4-6 ms onset to peak) inhibitory postsynaptic potentials in oxytocin neurons and, as shown previously, fast excitatory postsynaptic potentials in vasopressin neurons. These synaptic responses followed paired-pulse stimulus frequencies up to 100 Hz and were, thus, probably reflecting monosynaptic connections. Inhibitory postsynaptic potentials were selectively blocked by histamine receptor subtype 2 antagonists (either cimetidine or famotidine) and by picrotoxin but not by histamine receptor subtype 1 antagonists or bicuculline. Similar synaptic responses to tuberomammillary nucleus stimulation were found in 16 of 16 neurons immunocytochemically identified as oxytocinergic and in seven putative oxytocin neurons. Perifusion of the slice with low chloride medium (4.8 mM) reversed stimulus-evoked inhibitory postsynaptic potentials. We conclude that histaminergic neurons monosynaptically contact both oxytocin and vasopressin cells of the supraoptic nucleus and inhibit the former via activation of chloride channels which can be blocked by the histamine receptor subtype 2 antagonists, famotidine and cimetidine.

Central cardiovascular responses of histamine and homodimaprit in normal and hypophysectomized rats

Central administration of histamine in conscious rats produced half the pressor response in hypophysectomized rats compared to normal animals. The pressor response was blocked completely by hexamethonium in hypophysectomized rats but not at all in normal rats. Homodimaprit given centrally, gave pressor effects of equal magnitude in both groups. In normal rats, it caused significantly higher blood blood pressure following ganglionic blockade, whereas in hypophysectomized rats hexamethonium did not block the response to homodimaprit. These results, plus previous findings, show that histamine, given centrally, raises blood pressure in normal conscious animals by releasing vasoactive substances from the brain. In hypophysectomized rats it stimulates sympathetic output. Homodimaprit, on the other hand, produces its pressor action by the release of vasoactive substances from the brain both in normal and hypophysectomized animals. A hypothesis is proposed to suggest that parallel neuronal processing occurs in the brain when histamine is given.

Emerging concepts of structure-function dynamics in adult brain: the hypothalamo-neurohypophysial system

As the first known of the mammalian brain's neuropeptide systems, the magnocellular hypothalamo-neurohypophysial system has become a model. A great deal is known about the stimulus conditions that activate or inactivate the elements of this system, as well as about many of the actions of its peptidergic outputs upon peripheral tissues. The well-characterized actions of two of its products, oxytocin and vasopressin, on mammary, uterine, kidney and vascular tissues have facilitated the integration of newly discovered, often initially puzzling, information into the existing body of knowledge of this important regulatory system. At the same time, new conceptions of the ways in which neuropeptidergic neurons, or groups of neurons, participate in information flow have emerged from studies of the hypothalamo-neurohypophysial system. Early views of the SON and PVN nuclei, the neurons of which make up approximately one-half of this system, did not even associate these interesting, darkly staining anterior hypothalamic cells with hormone secretion from the posterior pituitary. Secretion from this part of the pituitary, it was thought, was neurally evoked from the pituicytes that made the oxytocic and antidiuretic "principles" and then released them upon command. When these views were dispelled by the demonstration that the hormones released from the posterior pituitary were synthesized in the interesting cells of the hypothalamus, the era of mammalian central neural peptidergic systems was born. Progress in developing an ever more complete structural and functional picture of this system has been closely tied to advancements in technology, specifically in the areas of radioimmunoassay, immunocytochemistry, anatomical tracing methods at the light and electron microscopic levels, and sophisticated preparations for electrophysiological investigation. Through the judicious use of these techniques, much has been learned that has led to revision of the earlier held views of this system. In a larger context, much has been learned that is likely to be of general application in understanding the fundamental processes and principles by which the mammalian nervous system works.(ABSTRACT TRUNCATED AT 400 WORDS)

Histamine excites arcuate neurons in vitro through H1 receptors

The electrophysiological effects of histamine on neurons of the hypothalamic arcuate nucleus of the female rat were tested with extracellular single unit recordings in an in vitro slice preparation. Histamine increased the spontaneous neuronal firing rate in 63% of the arcuate cells tested. An inhibitory response to histamine was seen in only one of the 117 neurons tested. The excitatory response to histamine showed dose dependency and was stable during synaptic blockade (by high magnesium and low calcium concentrations) and across a temperature range of 29-37 degrees C. Administration of histaminergic type 1 (pyrilamine and chlorpheniramine) and type 2 (cimetidine) receptor blockers revealed that the excitatory responses to histamine were mediated by type 1 receptors. The same neurons were also tested for responses to norepinephrine, serotonin, acetylcholine and substance P. A significant correlation was found between responses to histamine and substance P: all units excited by substance P were also excited by histamine. This subclass of histamine-responsive arcuate neurons may play a role in the regulation of the anterior pituitary, since histamine and substance P have similar effects on LH and prolactin secretion.

Effects of histamine on thermosensitive neurons in rat preoptic slice preparations

Single neuronal activities were recorded extracellularly from slice preparations of the rat preoptic area and effects of histamine (0.01 10 microM) on the activities were examined with regard to thermosensitivies of the neurons. Superfusion of histamine increased the firing rate in 52 of 75 warm-sensitive neurons and in 22 of 41 thermally insensitive neurons in a dose-dependent manner. Ten (3%) warm-sensitive neurons and 6 (15%) thermally insensitive neurons were inhibited by histamine. Mepyramine (10 microM) (H1-antagonist), but not famotidine (H2-antagonist), blocked the histamine (10 microM) induced excitation in 19 (76%) of 25 warm-sensitive neurons and in 6 (75%) of 8 thermally insensitive neurons. These results suggest that histamine excites both warm-sensitive and thermally insensitive neurons in the preoptic area mainly via the H1-receptor.

Analysis of histamine as a hair-cell transmitter in the lateral line of Xenopus laevis

The actions of histamine and histamine antagonists on afferent nerve activity were investigated in the lateral line of Xenopus laevis. Histamine (0.002-2.0 mM) had no effect on spontaneous activity or excitatory responses to water motion. In contrast, pyrilamine, an H1 receptor antagonist, suppressed spontaneous activity beginning at 0.01-0.05 mM. Below 0.3 mM the suppression was often preceded by a small excitatory response and responses to high (24-30 dB re threshold), but not low (0-18 dB) levels of water motion were selectively suppressed. Higher concentrations (0.3-2.0 mM) abolished spontaneous activity and suppressed responses at all levels of water motion. Cimetidine, an H2 receptor antagonist, had similar actions but was one-tenth as potent as pyrilamine. Tetrodotoxin (0.001-0.1 microM), which blocks voltage-sensitive Na+ channels, mimicked the suppressive effects of the histamine antagonists. Histamine (2.0 mM) failed to block the actions of pyrilamine (0.1 mM) indicating its effects are mediated through a mechanism other than histamine receptors. In addition, pyrilamine (0.05-0.1 mM) non-selectively suppressed excitation to exogenously applied L-glutamate (1.0-2.0 mM), L-aspartate (1.0-2.0 mM), kainate (0.005-0.01 mM), and quisqualate (0.002-0.005 mM) and altered responses to N-methyl-D-aspartate (0.5-1.0 mM). The results are inconsistent with histamine being a transmitter in the Xenopus lateral line and reveal that the actions of histamine antagonists are nonspecific, possibly due, in part, to blockade of voltage-sensitive Na+ channels.

Evidence for histaminergic arousal mechanisms in the hypothalamus of cat

Polygraphic 23-hr recordings were carried out in 25 adult cats in order to examine the effects of both systemic and local injections of various histaminergic and antihistaminergic drugs on sleep-waking cycles. alpha-Fluoromethylhistidine (alpha-FMH), a specific inhibitor of histidine decarboxylase, when injected intraperitoneally at a dose of 20 mg/kg, induced a significant increase in deep slow wave sleep (S2) and a decrease in wakefulness (W), without modifying light slow wave sleep (S1) and paradoxical sleep (PS). Intraperitoneal injections of mepyramine (1 mg and 5 mg/kg), a well-known histamine H1-receptor antagonist, increased deep slow wave sleep and decreased wakefulness, as well as paradoxical sleep. Bilateral injections of alpha-FMH (50 micrograms/1 microliter) into the ventrolateral posterior hypothalamus, where histamine immunoreactive neurones have been recently identified, resulted in a significant decrease in wakefulness and increase in deep slow wave sleep. Similarly, injections of mepyramine (120 micrograms/1 microliter) in the same structures caused a significant decrease in wakefulness and an increase in deep slow wave and paradoxical sleep as well. In contrast, local injections of SKF-91488 (50 micrograms/1 microliter), a specific inhibitor of histamine-N-methyltransferase, led to a significant increase in wakefulness and decrease in both slow wave sleep (SWS) and paradoxical sleep. Injections of histamine, at doses of 5, 30 and 60 micrograms/1 microliter, also increased wakefulness and decreased slow wave sleep dose dependently, while these effects were completely blocked by pretreatment with mepyramine. The results suggest that histaminergic systems in the hypothalamus play an important role in arousal mechanisms and their actions are mediated through H1-receptors.

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.

Induction of prolactin release by H1- or H2-histamine agonists in maturing male and female rats

Previous evidence has shown that histamine (HA) increases prolactin (PRL) release in adult male and female rats. This response is depending on the age of the animals and some differences were appreciated between male and female rats. In addition, at 20 days of age the PRL response was greater than earlier ages. The purpose of the present communication was to study further this response and to define the type of HA receptor that may be involved in the mediation of this PRL response. Animals of both sexes, at 5 or 20 days of age, were injected into the lateral brain ventricles with three log-doses of histamine (HA), 2,2, pyridilethylamine (2 PEA, and H1-histamine agonist), 4 methyl-histamine (4 MHA, an H2-histamine agonist) or saline as control. Fifteen minutes later a sample of blood was taken from the jugular vein and plasma PRL levels were determined by RIA. In complete agreement with previous work, results obtained indicate that HA is able to induce an increased PRL release in 20-day-old animals. This response was not identical in rats of both sexes. Female rats appeared to be more sensitive to HA than male rats. 2 PEA, was not able to reproduce the response in male rats but it was effective in female animals. 4 MHA induced the response in both sexes in a similar way than HA. At 5 days of age in male rats HA had no effect on PRL release. In female rats, instead, only the greater dose was able to induce a small but statistically significant response. In general, nor 2 PEA or 4 MHA were able to stimulate the PRL release at this age. The present results suggest that H2-histamine receptors are present in male rats and in female animals both types of receptors may be involved in the control of PRL release.

Effect of histamine on the hippocampal neurons in guinea-pigs

Histamine produced an excitatory effect on CA3 pyramidal cells in hippocampal slices of guinea-pigs. The amplitude of population spikes and EPSPs was augmented by histamine at concentrations of 10(-7) to 10(-6) M. Cimetidine but not pyrilamine prevented the stimulatory effect. The histamine effect was mimicked by dimaprit but not by 2-pyridylethylamine. It is concluded that histamine has a facilitatory influence on the hippocampal excitatory system via H2 receptor.

The effects of iontophoretic application of putative neurotransmitters on the electrical activity of rat mediobasal hypothalamic neurons in relation to their steroid sensitivity

In order to characterize the neurons responsible for ACTH release and the suppression produced by adrenocortical steroids, we have studied the pharmacological sensitivity of neurons in the region containing the highest concentration of corticotropin releasing factor (CRF) in the brain and probably responsible for most of these activities. The effects of 8 putative neurotransmitters applied iontophoretically to more than 400 mediobasal hypothalamic (MBH) neurons in the rat were examined, and compared with cortisol sensitivity of the same neurons. Glutamate was the only agent that produced excitation exclusively, while GABA, serotonin, glycine and dopamine were inhibitory in action. Mixed excitation and inhibition were produced by histamine, acetylcholine (ACh) and norepinephrine (NE). All 8 cells excited by ACh were inhibited by cortisol and constitute only about 10% of neurons sampled; these neurons are considered to be strong candidates for CRF-releasing cells. On 7 of these neurons histamine was strongly excitatory and its action greatly outlasted the iontophoretic application. Histamine, however, excited many other neurons, including those neither excited by ACh nor steroids.

Histamine and the hypothalamus

The chemical tools that could be used to examine the function of histamine in the brain are considered together with the evidence linking histamine specifically with the hypothalamus. The distribution of histamine and the enzymes responsible for its synthesis and metabolism is consistent with there being both mast cells and histaminergic nerve terminals within the hypothalamus. Iontophoresis, mepyramine binding and histamine-stimulated adenylate cyclase studies suggest that both histamine H1- and H2- receptors are present in the hypothalamus. In addition, intracerebroventricularly injected histamine receptor agonists and antagonists affect many functions associated with the hypothalamus such as cardiovascular control, food intake, body temperature control, and pituitary hormones whose release is mediated via the hypothalamus, such as corticotropin, growth hormone, thyroid stimulating hormone, prolactin, gonadotropins and vasopressin. However, only in the case of thyroliberin release, prolactin release, body fluid control and blood pressure control is there evidence yet that such effects are mediated via histamine receptors actually in the hypothalamus. The effects of enzyme inhibitors suggest endogenous histamine may be involved in the physiological control of thyroid stimulating hormone, growth hormone and blood pressure, and the effects of receptor antagonists support a role for endogenous histamine in prolactin control. Otherwise, there is little evidence for a physiological role for endogenous, as against exogenous, histamine whether it be from histaminergic terminals or mast cells. In addition, few studies have tried to distinguish possible effects on presynaptic receptors, postsynaptic receptors, hypothalamic blood vessels or the hypophyseal portal blood vessels. It is concluded that although there is good evidence now linking histamine and the hypothalamus more specific studies are required, for instance using microinjection or in vitro techniques and the more specific chemical tools now available, to enable a clearer understanding of the physiological role of histamine in the hypothalamus.

Effects of histamine microinjections into the hippocampus on open-field behavior in rats

Increasing evidence supports the concept that histamine would subserve neurotransmitter functions in the brain. In hippocampus, some cases of histamine receptors coupled to adenylate cyclase has been described. In our laboratory we were interested in studying the possible physiological role of these histamine receptors and behavior. Unilateral cannulae were stereotaxically implanted in the caudal hippocampus in adult male rats. Forty-eight hours later, after an acute microinjection of histamine (1 microgram), 3-methyl-histamine (1 microgram) or saline (1 microliter), the following behavioral parameters were measured by an observer in an open-field: locomotor, rearing, sniffing, grooming, social sniffing and holding activities. It was observed that histamine inhibited locomotor and rearing activities; whereas, 3-methyl-histamine induced an increase in almost all behaviors. Results support the concept of a probable function of histamine in exploratory behavior in the rat.

Effect of diphenhydramine on stress-induced changes in brain histidine decarboxylase activity, histamine and plasma corticosterone levels

Exposure of rats to platform stress induced a significant elevation in hypothalamic histamine levels. Air blast-stress resulted in a significant increase in hypothalamic histamine concentration and in histidine decarboxylase activity. No significant changes were noted either in the enzyme activity or in histamine levels in the midbrain or cortex of stressed rats. In the nonstressed rats, diphenhydramine (7.5 mg/kg intragastrically), a H1-receptor antagonist, did not influence histidine decarboxylase activity or histamine concentration in any of the three brain regions investigated. However, diphenhydramine pretreatment prevented the increase in histidine decarboxylase activity induced by air blasts. In untreated rats, plasma corticosterone levels were significantly elevated following either platform stress (4.5-fold) or air blasts (7.8-fold). A significant increase was also noted in saline and diphenhydramine-treated animals following these stressors, however, the increase in saline or diphenhydramine treated rats following air blasts was significantly less than that seen in untreated stressed controls.

Histamine: correlative studies in nucleus accumbens

The role of histamine as a neurotransmitter has been the subject of considerable controversy. Recent evidence suggests it to be involved in such complex activities as arousal and affect. The purpose of the present study is to examine the possible source, function, and pharmacology of histamine in the nucleus accumbens, an area of the brain also implicated in complex activities such as affect. The anatomical studies suggest that the most probable source of the histamine in nucleus accumbens is the complex region lateral to the mammillary nuclei. These areas are the intercalated nucleus and the tuberomammillary nucleus (nuclei gemini hypothalami). To a lesser degree, the supramammillary complex may also contribute histamine-containing axons to the accumbens area. Adenylate cyclase in the rabbit nucleus accumbens displayed activation in response to histamine agonists (histamine, 2-Me-histamine, and 4-Me-histamine). The action of the H1 antagonist promethazine was greater than the H2 antagonist metiamide in reducing enzyme activation by histamine and 2-Me-histamine. In contrast, metiamide was more potent than promethazine toward antagonism of the action of 4-Me-histamine. However, no additive effects were noted when agonists were added in combination. Based upon these data, it is suggested that activation of adenylate cyclase in the rabbit nucleus accumbens is mediated in part by mixed H1 and H2 receptors or cellular disruption reflects the loss of receptor specificity. Physiological studies demonstrated that the H2 agonist 4-Me-histamine had an inhibitory effect on the activity of neurons driven by stimulation of the fimbria. The magnitude of the effect was frequency dependent. The H1 agonist 2-Me-histamine had no significant effect. Iontophoretic application of 4-Me-histamine had minimal effect upon low frequency volleys (0.5 Hz) but had a pronounced effect upon higher frequency volleys (6.0 Hz). These effects were antagonized by metiamide. Iontophoretic application of metiamide alone produced an effect only upon the P component of the field response, which is also bicuculline sensitive. Bicuculline coadministration was also effective in antagonizing the 4-Me-histamine effect. The physiological data suggest that histamine works through H2 receptors in nucleus accumbens, perhaps by potentiating the effects of gamma-aminobutyric acid (GABA). Thus, histamine in nucleus accumbens appears to function as a modulatory substance whose effect is dependent upon the activity of other transmitter and afferent systems.

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.

Electrically-evoked release of [3H]-histamine from the guinea-pig hypothalamus

1 [3H]-histamine was taken up by slices of guinea-pig hypothalamus against a concentration gradient. 2 Electrical field stimulation of the superfused slices resulted in an increased efflux of radioactivity, the major part of which was shown to be associated with histamine by paper chromatography. 3 The evoked release of histamine was dependent on calcium ions in the superfusate and was increased by 56% when the frequency of stimulation was doubled from 5 to 10 Hz.

Prolactin release induced by histamine in maturing male and female rats

It has been previously shown that histamine (HA) increases PRL release in adult rats. The purpose of the present work was to determine the ontogeny of the HA-induced PRL response in male and female rats. Animals from day 1 to day 40 after birth were injected, under ether anesthesia, into the lateral brain ventricles with 9.0 micrograms HA (free base) per rat. Fifteen minutes later a sample of blood was taken from the jugular vein and plasma PRL levels were determined by RIA. Results obtained indicate that HA is able to induce increased PRL release in developing rats. Sex differences were observed. In male rats, the earliest PRL response was observed at day 10 whereas female rats showed clearcut responses at day 1. During development, males showed a quite regular pattern of responses but in female rats there were highly irregular; positive responses did present a 5-day periodicity. In general terms, the magnitude of the PRL responses in developing rats was lower than observed in adult rats. However, in 20-day old male and female rats and 40-day old female rats, a marked increase of PRL took place after the HA injection.

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.

Effects of a histamine synthesis inhibitor and antihistamines on the sexual behavior of female rats

Intraventricular administration of alpha-hydrazinohistidine, a histamine synthesis inhibitor, at different doses and times before testing produced a significant decrease of lordotic responses and sexual receptivity in ovariectomized estrogen plus progesterone-primed female rats. The H1-antihistamines pyrilamine and chlorfeniramine and the H2-antihistamine metiamide, injected in the lateral ventricle, significantly decreased the lordosis quotient but did not modify receptivity; antihistamine-injected rats showed no soliciting behavior. Exploratory activity was decreased by both alpha-hydrazinohistidine and metiamide but not by the H1-antihistamines. It is concluded that treatments which either deplete histamine or block their receptors can alter female copulatory responsiveness. The mechanism of this antihistamine effect appears to be unrelated to that of other side effects, such as motor impairment, sedation, or local anesthesia.

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.

Central actions of histamine: microelectrophoretic studies

Responses of neurones in different regions of the rat and cat brain to micro-electrophoretically applied histamine and some related substances including antagonists are reported. Histamine excites most neurones in the hypothalamus but depresses the vast majority of neurones in other structures. Depressant actions of histamine and some inhibitions in the cortex and hippocampus of the rat are antagonized by metiamide, an H2-receptor antagonist. The results lend strong support to the idea of histaminergic transmission in the mammalian brain.

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

The activity of single neurons in the vestibular neuronal complex of midcollicular decerebrate, decerebellectomized cats were recorded and their responsiveness to iontophoretically applied histamine and other agents determined. The majority of the cells tested were inhibited by iontophoresis of histamine while 24% were excited by this agent. Neurons exhibiting inhibitory responses were widely distributed throughout the four vestibular nuclei and adjacent reticular formation whereas excitatory responses to histamine were obtained mainly in the region of the lateral vestibular nucleus. The H2-receptor blocking agents metiamide and cimetidine were examined as to their actions on spontaneously firing cells and cells affected by histamine. Metiamide was selective in blocking histamine-induced inhibition but not excitation while cimetidine was ineffective in blocking either response. These results suggest that histamine has both inhibitory and excitatory actions on brain stem neurons and metiamide is an effective antagonist of histamine-induced inhibition.

Antagonism by some antihistamines of the amino acid-evoked responses recorded from the lobster muscle fibre and the frog spinal cord

1 The effects of some antihistamines on the lobster muscle fibre and the frog spinal cord were investigated using intracellular and extracellular recordings, respectively. 2. On lobster muscle, histamine H1-blockers reversibly antagonized responses to bath-applied glutamate, aspartate and quisqualate but not responses to gamma-aminobutyric acid (GABA). Iontophoretic glutamate potentials were also reduced. Histamine (up to 1 mM) had no effect on this preparation. 3 The H1-antagonists produced a small increase in muscle membrane conductance and a slight hyperpolarization. These effects were largely unchanged in a low C1- bathing solution. Procaine (1 mM) decreased membrane conductance and did not affect responses to GABA or glutamate. 4 The H2-antagonist burimamide blocked both glutamate and GABA-evoked responses on the lobster muscle without affecting resting potential or conductance. 5 In the frog cord, bath-applied histamine produced ventral root depolarizations and dorsal root hyperpolarizations (sometimes biphasic responses). These effects were reduced by tetrodotoxin (TTX) but not by antazoline (H1-blocker) or burimamide; the latter reversibly antagonized responses to both glutamate and GABA on TTX-treated cords while antazoline was ineffective. 6 It is suggested that antihistamines can act as non-specific amino acid antagonists by interacting at the level of the receptor-coupled ionophores.

Hypothalamic releasing factors: physiological evidence for a regulatory action on central neurons and pathways for their distribution in brain

Recent observations based on single cell recordings obtained in various areas of the brain indicate that TRH, LH-RH and somatostatin have potent effects on the activity of central neurons. There is also electrophysiological evidence for the existence of a system of hypothalamic tuberoinfundibular neurons with widespread extrahypothalamic connections. These connections may indicate possible pathways for the observed widespread distribution of these peptides in the brain. These findings, coupled with behavioral studies and subcellular localization data support the postulate that hypothalamic peptides may have an important role inthe modulation of central neuronal activity.