Effects of histamine on dentate granule cells in vitro

Analyses of rapid estrogen actions on rat ventromedial hypothalamic neurons

Rapid estrogen actions are widely diverse across many cell types. We conducted a series of electrophysiological studies on single rat hypothalamic neurons and found that estradiol (E2) could rapidly and independently potentiate neuronal excitation/depolarizations induced by histamine (HA) and N-Methyl-d-Aspartate (NMDA). Now, the present whole-cell patch study was designed to determine whether E2 potentiates HA and NMDA depolarizations - mediated by distinctly different types of receptors - by the same or by different mechanisms. For this, the actions of HA, NMDA, as well as E2, were investigated first using various ion channel blockers and then by analyzing and comparing their channel activating characteristics. Results indicate that: first, both HA and NMDA depolarize neurons by inhibiting K(+) currents. Second, E2 potentiates both HA and NMDA depolarizations by enhancing the inhibition of K(+) currents, an inhibition caused by the two transmitters. Third, E2 employs the very same mechanism, the enhancement of K(+) current inhibition, thus to rapidly potentiate HA and NMDA depolarizations. These data are of behavioral importance, since the rapid E2 potentiation of depolarization synergizes with nuclear genomic actions of E2 to facilitate lordosis behavior, the primary female-typical reproductive behavior.

International Union of Basic and Clinical Pharmacology. XCVIII. Histamine Receptors

Histamine is a developmentally highly conserved autacoid found in most vertebrate tissues. Its physiological functions are mediated by four 7-transmembrane G protein-coupled receptors (H1R, H2R, H3R, H4R) that are all targets of pharmacological intervention. The receptors display molecular heterogeneity and constitutive activity. H1R antagonists are long known antiallergic and sedating drugs, whereas the H2R was identified in the 1970s and led to the development of H2R-antagonists that revolutionized stomach ulcer treatment. The crystal structure of ligand-bound H1R has rendered it possible to design new ligands with novel properties. The H3R is an autoreceptor and heteroreceptor providing negative feedback on histaminergic and inhibition on other neurons. A block of these actions promotes waking. The H4R occurs on immuncompetent cells and the development of anti-inflammatory drugs is anticipated.

Differential effects of the histamine H(3) receptor agonist methimepip on dentate granule cell excitability, paired-pulse plasticity and long-term potentiation in prenatal alcohol-exposed rats

BACKGROUND

We previously reported that prenatal alcohol-induced deficits in dentate gyrus (DG) long-term potentiation (LTP) are ameliorated by the histamine H3 receptor inverse agonist ABT-239. ABT-239 did not enhance LTP in control rats, suggesting that the possibility of a heightened H3 receptor-mediated inhibition of LTP in prenatal alcohol-exposed (PAE) offspring.

METHODS

To further investigate this mechanism, we examined the effect of methimepip, a selective histamine H3 receptor agonist, on DG granule cell responses and LTP in saccharin control and PAE rats. Long-Evans rat dams voluntarily consumed either a 0 or 5% ethanol solution 4 hours each day throughout gestation. Adult male offspring from these dams were anesthetized with urethane and electrodes implanted into the entorhinal cortical perforant path and the DG.

RESULTS

In control offspring, methimepip reduced the coupling of fast excitatory postsynaptic field potentials to population spikes (E-S coupling), the probability of glutamate release, as measured by paired-pulse ratio (PPR) and diminished DG LTP. Similar reductions in E-S coupling and LTP were observed in saline-treated PAE offspring. In contrast to the control group, methimepip did not exacerbate PAE-induced reductions in E-S coupling or LTP.

CONCLUSIONS

While the effects of methimepip in control offspring were consistent with speculation of a PAE-induced enhancement of H3 receptor-mediated inhibition of E-S coupling and LTP, the absence of an added effect of methimepip in PAE offspring could indicate either an inability to further inhibit these responses with methimepip in PAE rats or the presence of more complex regulatory neural interactions with in vivo recordings in PAE rats. Follow-up studies of H3 receptor-mediated responses in DG slice preparations are under way to provide clearer insights into the role of the H3 receptor regulation of excitatory transmission in PAE rats.

Mechanism of H₂ histamine receptor dependent modulation of body temperature and neuronal activity in the medial preoptic nucleus

Histamine is involved in the central control of arousal, circadian rhythms and metabolism. The preoptic area, a region that contains thermoregulatory neurons is the main locus of histamine modulation of body temperature. Here we report that in mice, histamine activates H(2) subtype receptors in the medial preoptic nucleus (MPON) and induces hyperthermia. We also found that a population of glutamatergic MPON neurons express H(2) receptors and are excited by histamine or H(2) specific agonists. The agonists decreased the input resistance of the neuron and increased the depolarizing "sag" observed during hyperpolarizing current injections. Furthermore, at -60 mV holding potential, activation of H(2) receptors induced an inward current that was blocked by ZD7288, a specific blocker of the hyperpolarization activated cationic current (I(h)). Indeed, activation of H(2) receptors resulted in increased I(h) amplitude in response to hyperpolarizing voltage steps and a depolarizing shift in its voltage-dependent activation. The neurons excited by H(2) specific agonism expressed the HCN1 and HCN2 channel subunits. Our data indicate that at the level of the MPON histamine influences thermoregulation by increasing the firing rate of glutamatergic neurons that express H(2) receptors.

Histamine influences body temperature by acting at H1 and H3 receptors on distinct populations of preoptic neurons

The preoptic area/anterior hypothalamus, a region that contains neurons that control thermoregulation, is the main locus at which histamine affects body temperature. Here we report that histamine reduced the spontaneous firing rate of GABAergic preoptic neurons by activating H3 subtype histamine receptors. This effect involved a decrease in the level of phosphorylation of the extracellular signal-regulated kinase and was not dependent on synaptic activity. Furthermore, a population of non-GABAergic neurons was depolarized, and their firing rate was enhanced by histamine acting at H1 subtype receptors. In our experiments, activation of the H1R receptors was linked to the PLC pathway and Ca(2+) release from intracellular stores. This depolarization persisted in TTX or when fast synaptic potentials were blocked, indicating that it represents a postsynaptic effect. Single-cell reverse transcription-PCR analysis revealed expression of H3 receptors in a population of GABAergic neurons, while H1 receptors were expressed in non-GABAergic cells. Histamine applied in the median preoptic nucleus induced a robust, long-lasting hyperthermia effect that was mimicked by either H1 or H3 histamine receptor subtype-specific agonists. Our data indicate that histamine modulates the core body temperature by acting at two distinct populations of preoptic neurons that express H1 and H3 receptor subtypes, respectively.

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.

Functional genomics of sex hormone-dependent neuroendocrine systems: specific and generalized actions in the CNS

Sex hormone effects on hypothalamic neurons have been worked out to a point where receptor mechanisms are relatively well understood, a neural circuit for a sex steroid-dependent behavior has been determined, and several functional genomic regulations have been discovered and conceptualized. With that knowledge in hand, we approach deeper problems of explaining sexual arousal and generalized CNS arousal. After a brief summary of arousal mechanisms, we focus on three chemical systems which signal generalized arousal and impact hormone-dependent hypothalamic neurons of behavioral importance: histamine, norepinephrine and enkephalin.

Changes in hippocampal histamine receptors across the hibernation cycle in ground squirrels

Hibernation is a physiological state characterized by a dramatic reduction in various functions, such as body temperature, heart rate, and metabolism. The hippocampus is thought to be important for regulation of the hibernation bout because it remains electrophysiologically active throughout this extremely depressed state. The question arises as to what neuronal influences act within the hippocampus during hibernation to sustain its activity. We hypothesized that histaminergic input might be an important contributor. Brain histamine is involved in functions relevant to hibernation, such as the regulation of diurnal rhythms, body temperature, and energy metabolism. Furthermore, we have previously shown that the histaminergic system appears to be activated during the hibernating state. In this study, we used receptor binding autoradiography, in situ hybridization, and GTP-gamma-S binding autoradiography to study changes in histamine receptors across the hibernation bout. We were able to demonstrate an increase in histamine H1 and H2 receptors in the hippocampus during hibernation, whereas the mRNA expression and receptor density of the inhibitory H3 receptor decreased. Histamine H3 receptors were shown to exhibit both histamine-activated and constitutive GTP-gamma-S-binding activity in the ground squirrel hippocampus, both of which decreased during hibernation, indicating a decrease in H3 receptor G-protein activation. Taken together, our results indicate that histamine may be involved in maintaining hibernation by sustaining hippocampal activity, possibly through H1 and H2 receptor activity and decreased inhibition by H3 receptors. The involvement of brain histamine, which is generally thought of as an arousal molecule, in maintaining a depressed state of the brain suggests a more general role for the amine in controlling arousal state.

Excitatory effects of histamine on cerebellar interpositus nuclear cells of rats through H(2) receptors in vitro

Neuroanatomical studies have revealed a direct hypothalamocerebellar histaminergic pathway, and our previous studies have demonstrated an excitatory effect of histamine on granule and Purkinje cells of the cerebellar cortex. In this study, we further investigated the effect of histamine on the neuronal firing of cerebellar interpositus nucleus (IN) by using cerebellar slice preparations. Eighty-seven IN cells were recorded from 38 slices. The vast majority of the cells responded to histamine stimulation with an excitatory response (79/87, 90.8%), and the rest of them showed no reaction (8/87, 9.2%). The histamine-induced excitation was not blocked by application of low-Ca(2+)/high-Mg(2+) medium (n=8), supporting a direct postsynaptic action of histamine. The histamine H(2) receptor antagonist ranitidine effectively blocked the excitatory response of IN cells to histamine (n=23), but the histamine H(1) receptor antagonist triprolidine could not significantly block the histamine-induced excitation, or only very slightly decreased the excitatory effect of histamine on the cells (n=21). On the other hand, the highly selective histamine H(2) receptor agonist dimaprit mimicked the excitatory effect of histamine on IN cells and the dimaprit-induced excitation was also blocked by ranitidine (n=14). Successively perfusing slices with the medium containing ranitidine and triprolidine, respectively, we found that ranitidine exhibited the same blocking effect on the dimaprit-induced excitation, but triprolidine had no such effect (n=8). Moreover, the histamine H(1) receptor agonist 2-pyridylethylamine did not show any effect on the IN cells (n=9). These results demonstrate that histamine excites cerebellar IN cells via the histamine H(2) receptor mechanism. Together with our previous results, we suggest that the hypothalamocerebellar histaminergic fibers may modulate neuronal activities of the cerebellar cortex and deep nuclei in parallel. The significance of the excitatory effect of histamine on the cerebellar nuclear cells is discussed.

Regional expression of the histamine H(2) receptor in adult and developing rat brain

Histamine H(2) receptor expression was studied in adult and developing rat brain. Northern blot and in situ hybridizations indicated that histamine H(2) receptor messenger RNA expression is widespread and not limited to neurons in the adult rat brain. Prominent H(2) receptor expression in the adult brain was seen in the dentate gyrus, hippocampal subfields CA1-CA3, piriform cortex and in some diencephalic nuclei, e.g. in the suprachiasmatic nucleus and the red nucleus. Most of the adult brain nuclei displayed a very low H(2) receptor expression. Histamine H(2) receptor was also expressed during development in widespread areas of the central nervous system, coinciding with the transient production of histamine in the raphe neurons at embryonic day 15. From embryonic days 16 and 17 until birth, histamine H(2) receptor expression in the cortical plate coincided with the development and sprouting of histaminergic fibers into the cerebral cortex. The widespread and diffuse expression of histamine H(2) receptors in the adult rat brain suggests that the H(2) receptor modulates the excitability of neuron and astrocyte functions in many brain areas rather than mediating targeted cell-to-cell signals. During development, histamine H(2) receptor expression is seen in several target areas for the histaminergic fibers. This could indicate that histamine, through the H(2) receptor, regulates fetal development of the brain.

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.

H2 histamine receptor-phosphorylation of Kv3.2 modulates interneuron fast spiking

Histamine-containing neurons of the tuberomammilary nucleus project to the hippocampal formation to innervate H1 and H2 receptors on both principal and inhibitory interneurons. Here we show that H2 receptor activation negatively modulates outward currents through Kv3.2-containing potassium channels by a mechanism involving PKA phosphorylation in inhibitory interneurons. PKA phosphorylation of Kv3.2 lowered the maximum firing frequency of inhibitory neurons, which in turn negatively modulated high-frequency population oscillations recorded in principal cell layers. All these effects were absent in a Kv3.2 knockout mouse. These data reveal a novel pathway for histamine-dependent regulation of high-frequency oscillations within the hippocampal formation.

In vivo electrophysiological investigations into the role of histamine in the dentate gyrus of the rat

Drugs acting at the three known classes of histamine receptors were injected intracerebroventricularly into the rat. The effects of these drugs upon synaptic potentials recorded from the dentate gyrus of the freely-moving rat were determined. Population spikes and field excitatory postsynaptic potentials were recorded from the granule cell layer of the dentate gyrus following stimulation of the perforant path. Drugs, dissolved in 0.9% NaCl were applied into the lateral cerebral ventricle in a volume of 5 microl over a period of 6 min. The histamine H1 receptor antagonist mepyramine (0.4 or 0.8 microg) had no significant effect on population spikes or field excitatory postsynaptic potentials. In contrast the H2 receptor antagonist cimetidine (3.25, 6.5 or 13 microg) showed a biphasic effect. At the lower doses (3.25 or 6.5 microg) a small (15%) depression of the field excitatory postsynaptic potentials and population spikes was observed beginning about 1 h following the infusion. At the highest dose tested (13 microg) a marked increase of the population spike was observed beginning immediately following the infusion and lasting for 90 min. Application of the H3 receptor agonist R-alpha-methylhistamine (0.2 microg) depressed the field excitatory postsynaptic potentials (15% at 4 h post-injection) and even more strongly the population spike (50%). Surprisingly, at higher doses (0.4 and 0.8 microg) no effect was seen. The H3 receptor antagonist thioperamide (0.41 and 0.82 microg) did not cause an increase in synaptic potentials but rather at the highest dose a small depression occurred at later time points (2-4 h following the infusion). At the lower dose (0.41 microg) thioperamide blocked the effect of R-alpha-methylhistamine (0.2 microg). These results show that the histaminergic system modulates information flow through the dentate gyrus in a complex manner involving both histamine H2 and H1 receptors.

Differential modulation of hippocampal signal transfer by tuberomammillary nucleus stimulation in freely moving rats dependent on behavioral state

Tuberomammillary histamine neurons (TM) in the posterior hypothalamus project to extensive parts of the brain, including the hippocampal formation. The purpose of the present experiments was to investigate whether activation of the TM modulates signal transfer from the perforant pathway (PP) or ventral hippocampal commissure (VHC) to the dentate gyrus (DG) in freely moving rats. Paired pulses of electrical stimulation were delivered to PP or VHC, and evoked field potentials (fEPSPs and pop spikes) were recorded in the DG. Before activating PP or VHC, the TM was triggered by electrical stimulation. Experimentation was performed during four behavioral conditions: exploration, grooming, awake immobility, and slow-wave sleep. Electrical activation of the TM was found to modify dentate fEPSPs evoked by PP or VHC stimulation without generating a field potential by itself. Train stimulation of the TM (100 Hz, 500 ms) preceding paired pulses on the hippocampus by 50 ms decreased dentate fEPSPs in dependence of the ongoing behavior and the pathway stimulated. During exploration but not consummatory behavior, the PP signal was reduced when preceded by TM stimulation; during consummatory behavior but not exploration, the VHC signal was reduced. In contrast to other hippocampal afferents which increase pop spikes but leave fEPSPs unchanged, TM stimulation decreased dentate fEPSPs without affecting pop-spike activity. Thus, the TM-histaminergic system seems to modulate signal processing in the dentate gyrus in a specific way, exerting an inhibitory action on the entorhinal input only during learning-related exploratory behavior.

Histaminergic system in co-cultures of hippocampus and posterior hypothalamus: a morphological and electrophysiological study in the rat

Neurons of the tuberomammillary nucleus in the posterior hypothalamus diffusely project to most parts of the central nervous system, where their main transmitter, histamine, modulates the excitability of the target neurons. The development of a histaminergic hypothalamo-hippocampal pathway and its function were studied in organotypic co-cultures. Immunocytochemistry for histidine decarboxylase, the specific synthesizing enzyme, stained clusters of neurons in the hypothalamic tuberomammillary area. Immunolabelled varicose processes innervated the co-cultured hippocampus and established a few synaptic contacts on dendrites. Cultured tuberomammillary neurons displayed their typical membrane properties and were spontaneously active. In hippocampal pyramidal cells of the CA3 region the long-lasting afterhyperpolarization was reduced by histamine or impromidine and increased by the H2 antagonist cimetidine, but not by the H1 antagonist mepyramine. The membrane potential was depolarized in presence of an H2 agonist and hyperpolarized by an H2 antagonist. In single hippocampal cultures histamine antagonists did not affect afterhyperpolarization and membrane potential. Histaminergic neurons retain their main morphological and physiological characteristics in slice cultures and establish a functional connection with co-cultured target cells.

Histamine H3 receptor-mediated depression of synaptic transmission in the dentate gyrus of the rat in vitro

1. The effects of histamine on excitatory synaptic transmission in the dentate gyrus region of rat hippocampal slices were examined using extracellular and whole-cell patch-clamp recording techniques. The GABAA receptor antagonist picrotoxin (50 microM) was present in the bath in all experiments. 2. Histamine (0.7-70 microM) reversibly depressed field excitatory postsynaptic potentials (fEPSPs) or excitatory postsynaptic currents (EPSCs) recorded intracellularly by up to 30%. The presynaptic fibre volley and EPSC reversal potential were unaffected by histamine, as were responses following pressure ejection of the glutamate receptor agonist S-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (S-AMPA) into the slice. 3. Histamine (7 microM) depressed equally the AMPA and N-methyl-D-aspartate (NMDA) components of the dual-component EPSC, recorded at -40 mV. 4. In addition to depressing synaptic transmission, histamine also reduced the magnitude of paired-pulse depression (PPD; 40 ms interpulse interval) of the medial perforant path EPSC. 5. Histamine depressed medial perforant path EPSCs more strongly than lateral perforant path EPSCs. Paired-pulse facilitation (PPF; 40 ms interpulse interval) in the lateral perforant path was enhanced by histamine. 6. The effects of histamine on synaptic transmission and PPD were mimicked by the selective H3 receptor agonist R-alpha-methylhistamine (0.1-10 microM) but not by the selective H2 receptor agonist dimaprit (10 microM). Similarly, the H3 receptor antagonist thioperamide (10 microM) blocked the effect of histamine whereas the H1 antagonist mepyramine (1 microM) and the H2 receptor antagonist cimetidine (50 microM) were ineffective. 7. Histamine actions on synaptic transmission and PPD were not occluded by application of the metabotropic glutamate agonist L-2-amino-4-phosphonobutyrate (AP4). 8. The results indicate that histamine depresses synaptic transmission in the dentate gyrus by binding to histamine H3 receptors located on perforant path terminals. The mechanism by which histamine depresses transmission is independent of that used by class III metabotropic glutamate receptors.

pH-dependent facilitation of synaptic transmission by histamine in the CA1 region of mouse hippocampus

The action of histamine on excitatory synaptic transmission in the CA1 region of the mouse hippocampus was investigated. In a medium at pH 7.4 population spikes were increased for approximately 60 min after a brief (5 min) perfusion with histamine (5 microM) but excitatory postsynaptic potentials (EPSPs) were unaffected, as previously reported in the rat. At pH 7.0, however, a late component of extra- and intracellularly registered EPSPs was enhanced in two phases: a shorter (by 30%) and a longer lasting one (> 1 h by 10%). The NMDA antagonist amino-phosphonovalerate (60 microM) blocked this late component and prevented the EPSP broadening by histamine. In some cells histamine induced burst-firing and prolonged EPSPs. The actions on EPSPs were not mediated by any of the known histamine receptors as they were not mimicked by histamine H1, H2 and H3 agonists or blocked by H1, H2 or H3 antagonists. We conclude that histamine enhances a late (NMDA) component of hippocampal EPSPs when protonization is increased by a slight shift of the pH in the acidic direction. Such shifts occur during intense nervous discharges, e.g. in epileptic tissue or following tetanic stimulation, and in hypoxic conditions.

Subcortical modulation of synaptic plasticity in the hippocampus

A two-stage model of memory trace formation in the hippocampus considers exploratory behavior associated with theta activity and consumatory behaviors or immobility associated with the occurrence of synchronous pyramidal cell discharges. Threshold, frequency and extent of CA3 bursts differ between differently performing strains of mice and are subject to subcortical modulation. Mechanisms for such actions are described: interference with potassium and other cation channels through second messengers and a direct action of histamine on the NMDA receptor channel. Furthermore long-lasting potentiations of synaptic transmission and excitability following brief exposures of hippocampal slices to histamine, noradrenaline, and serotonin are mediated by cyclic AMP and presumably protein kinase A.

Histamine potentiates N-methyl-D-aspartate responses in acutely isolated hippocampal neurons

N-methyl-D-aspartate (NMDA)-evoked currents were recorded from acutely isolated rat hippocampal neurons, using the whole-cell patch-clamp technique and a rapid perfusion system. Histamine, at concentrations from 0.5 to 100 microM, reversibly enhanced NMDA currents by up to 50%. The effect cannot be ascribed to activation of the known histamine receptors (H1, H2, H3) but is occluded by spermine. These results suggest an interaction of histamine with the polyamine-binding site on the NMDA receptor complex. This modulatory action could allow the histaminergic system to determine time and loci of NMDA receptor-mediated events, such as memory formation according to behavioral state.

Histamine-containing nerve fibers innervate human cerebellum

Histamine is found in nerve cell bodies of the tuberomammillary nucleus in mammalian brain. This nucleus is prominent in human brain. Samples of human cerebelli obtained from neurosurgical operations were examined for the presence of histamine-containing nerve fibers. In all samples, a moderately dense network of histamine-immunoreactive fibers was seen in the molecular layer. These fibers ran parallel to the Purkinje cell layer after traversing it perpendicularly. Numerous fibers were also seen in the granular cell layer. The results suggest that the human cerebellar cortex receives a direct input from histamine-synthesizing hypothalamic neurons, as no other histamine-containing neurons have been found in human brain.

Comparison between the membrane and synaptic properties of human and rodent dentate granule cells

We have compared the cellular and synaptic properties of rodent dentate granule cells with those of humans. The human tissue was obtained from neurosurgical procedures which necessitated removal of the hippocampus for treatment of extra-hippocampal tumors which presented clinically with seizures. The hippocampi studied here were neuroanatomically similar to autopsy controls. The present studies have demonstrated that there are few differences between rodent and human granule cells as regards either their membrane properties or their synaptic physiology and pharmacology. The differences we noted were (1) less spike frequency adaptation in the human relative to rodent cells; and (2) perforant path stimulation reliably elicited both feedforward and feedback inhibition in the rodent cells, while in the human tissue feedback inhibition appeared to predominate. It is unclear if these changes are due to the seizure experience or if they represent true species differences.

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).