Signal transduction by histamine in the cerebellum and its modulation by <Emphasis Type="Italic">N</Emphasis>-methyltransferase

Histamine has been suggested to have roles as a neurotransmitter or a neuromodulator. Direct fiber connections between the hypothalamus and the cerebellum have recently been demonstrated and it is suggested that the cerebellum is involved in the control of autonomic and emotional functions. These fibers include histaminergic fibers. The components of histaminergic signal transmission are demonstrated in the cerebellum as follows: (1) the histaminergic fibers are visualized immunohistochemically in the cerebellar cortex of rat, guinea pig and human; (2) histamine H1 receptors are visualized by autoradiographic studies in the molecular layer of mouse and guinea pig. In situ hybridization study also detects the expression of H1 receptors in the Purkinje cells. H2 receptors are expressed in the Purkinje cells and granule cells of guinea pig; and (3) the application of histamine to the slices of guinea pig or rat cerebellar cortex elicits an increase in the turnover of phosphoinositides, so H1 receptors in the cerebellum are functional. Additionally, we have recently shown in the guinea pig that Purkinje cells express one of the histamine inactivating enzymes, and that inhibition of this enzyme enhances phosphoinositide turnover by histamine. Therefore, all the components of histaminergic neurotransmission are demonstrated in the cerebellum. These data suggest that histamine is involved in the signal transmission from the hypothalamus to the cerebellum. Here we review each component of histaminergic neurotransmission in the cerebellum.     

The histaminergic system in the guinea pig central nervous system: an immunocytochemical mapping study using an antiserum against histamine

Using an antiserum against conjugated histamine we mapped the histaminergic somata and their fiber projection areas in carbodiimide-fixed guinea pig central nervous system. The neurons were large and they were found exclusively in the posterior hypothalamus, as in the rat, but in the guinea pig they were more numerous and distributed more widely in thin layer around the posterior mammillary nucleus, scattered between and within the medial mammillary nuclei, and in a dense cell cluster emerging from the caudal magnocellular nucleus and extending to the medial preoptic area. The density of histamine-immunopositive fibers was very high in the olfactory tubercle, diagonal band of Broca, nucleus accumbens, medial and cortical amygdaloid nuclei, periventricular and lateral basal hypothalamus, paraventricular thalamus, and in a region from the medial central gray to the locus coeruleus and the parabrachial nucleus. Dense fiber networks were found in the piriform and entorhinal cortex, septum, dentate gyrus, and subiculum, in most parts of amygdala, and in many areas of the hypothalamus, thalamus, substantia nigra, raphe nuclei, inferior olivary, solitary tract and medial vestibular nuclei, and neurohypophysis. Medium fiber density was observed in the internal layers of the olfactory bulb, anterior olfactory nuclei, neocortex, zone CA1 of hippocampus, and many midbrain and hindbrain regions. Low density was present in the outer layers of the olfactory bulb, other parts of hippocampus, the globus pallidus, most of the caudatus-putamen, the cerebellar cortex, and the dorsal horn of the spinal cord. The retina and most of the myelinated white matter had single or no histaminergic fibers. It may be concluded from the results that most fibers seem to follow a ventromedial route to the forebrain, reaching the amygdala ventral to the medial forebrain bundle, the hippocampus via subiculum, and the hindbrain structures via the medial central gray. As compared to the rat, the fiber projections in the guinea pig brain were denser, particularly in the hippocampus, thalamus, pons-medulla, and neurohypophysis. The fiber densities in various regions of the guinea pig brain are compared to histamine receptor densities and the possible functions of histamine are discussed.     

Cyclic AMP-generating systems: regional differences in activation by adrenergic receptors in rat brain

Catecholamine, histamine, and adenosine-mediated accumulations of radioactive cyclic AMP were assessed in adenine-labeled slices from eight rat brain regions. 2-Fluoronorepinephrine, a selective beta-adrenergic agonist, elicited an an accumulation of cyclic AMP in cerebral cortex, cerebellum, hippocampus, striatum, superior colliculi, thalamus, hypothalamus, and medulla-pons. In cerebral cortex and most other brain regions, the beta-adrenergic-mediated response appeared to involve primarily beta 1-adrenergic receptors, while in cerebellum, there was a significant involvement of beta 2-adrenergic receptors. 6-Fluoronorepinephrine, a selective alpha-adrenergic agonist, elicited accumulations of cyclic AMP in all regions except cerebellum. Combinations of the two fluoro derivatives afforded in all brain regions an accumulation of cyclic AMP identical with that elicited by norepinephrine. In hypothalamus, the alpha- and beta-adrenergic responses were significantly greater than additive. In cerebral cortex, the alpha-adrenergic receptor-mediated response appeared to involve alpha 1-adrenergic receptors and to be nearly completely dependent on adenosine, while in other brain regions, the dependence of the alpha-adrenergic response on adenosine was less or absent. Combinations of 6-fluoronorepinephrine and histamine had greater than additive effects in cortex and hippocampus. The results indicate that the interactive control of cyclic AMP-generating systems by alpha-adrenergic, beta-adrenergic, adenosine, and histamine receptors differs significantly among rat brain regions.     

Pharmacological analysis of immepip and imetit homologues. Further evidence for histamine H(3) receptor heterogeneity?

Following a previous report by our research group on discriminative properties of a series of aliphatic histamine homologues, we now studied immepip, imetit and its lower and higher sidechain homologues as ligands for the histamine H(3) receptor in a [(125)I]-iodophenpropit binding assay using rat cerebral cortex membranes, and two functional H(3) receptor models (inhibition of the neurogenic contraction of the guinea pig jejunum and inhibition of [(3)H]-noradrenaline release in rat cerebral cortex slices). The immepip homologues behaved as competitive H(3)-receptor antagonists in both functional systems. The potencies (pA(2) values) observed at the guinea pig jejunum were 8.4 and 6.2 for the immepip homologues VUF 4929 and VUF 4735, respectively, whereas on the electrically evoked release of [(3)H]-noradrenaline from cortical slices the pA(2) values were 7.1 and 5.5 for VUF 4929 and VUF 4735, respectively. Moreover, immepip, but not the (R)-alpha-methylhistamine, showed almost a tenfold higher agonistic potency in the rat cerebral cortex than in the guinea pig jejunum. For imetit and its homologues important discrepancies in the potencies in the two functional assays were noticed as well. VUF 8328 acts as a potent (pD(2)=8.0) partial agonist in the brain, but as a very active (pA(2)=9.4) competitive antagonist in the guinea pig jejunum. The partial agonistic activity of VUF 8328 in the brain was confirmed by GTP gamma S-sensitive, biphasic displacement of [(125)I]-iodophenpropit binding to rat cerebral cortex membranes. The differences in potencies shown by the various ligands are discussed in relation to H(3) receptor heterogeneity.     

Localization and characterization of binding sites for vasopressin and oxytocin in the brain of the guinea pig.

Using autoradiography on film, specific binding sites for arginine-vasopressin (AVP) and for oxytocin (OT) were localized in various areas of the brain of adult male guinea pigs. Vasopressin binding sites were detected with [3H]AVP or with [125I]VPA, a recently synthetized linear vasopressin antagonist radiolabeled with 125I. [125I]VPA and [3H]AVP yielded similar results, thus suggesting that AVP binding sites present in the guinea pig brain are V1 type receptors. Supporting evidence on this was obtained in competing studies using structural analogues allowing to discriminate V1 receptors from V2 and from OT receptors. Oxytocin binding sites were labeled with [3H]OT or with the iodinated OT antagonist [125I]OTA; both ligands yielded similar results. The localization in the guinea pig brain of AVP binding sites differed from that of OT binding sites. AVP binding sites were mainly detected in the olfactory bulb and throughout the cerebral cortex. Oxytocin binding sites were most noticeable in the hypothalamic ventromedial nucleus, in the amygdaloid complex and in restricted areas of the cerebral cortex. A comparison of the present data with those previously described in the rat, the mouse, the human and the hamster brain suggests that similar binding sites are present in these species, but that their anatomical distribution differs markedly. These data are discussed in relation to immunocytochemical and electrophysiological data which suggest that binding sites detected by autoradiography may represent, at least in part, functional neuronal receptors.     

Modulation of neuronal firing mode in cat and guinea pig LGNd by histamine: possible cellular mechanisms of histaminergic control of arousal.

The thalamus is innervated by histaminergic fibers presumably arising from neurons in the tuberomammillary nucleus of the hypothalamus. The possible function of this histaminergic projection was addressed through investigation of the cellular actions of histamine on guinea pig and cat dorsal lateral geniculate (LGNd) relay neurons maintained as a slice in vitro. Local application of histamine to LGNd relay neurons resulted in a slow depolarization that was associated with a decrease in membrane conductance and was blocked by the H1-antagonists pyrilamine, triprolidine, or diphenhydramine. Current versus voltage relationships revealed that the slow depolarization was associated with an inward current that reversed near EK, indicating that it was due to a decrease in a potassium current. The slow depolarizing response to histamine was occluded by maximal activation of the slow depolarizing responses resulting from stimulation of alpha 1-adrenergic or muscarinic receptors, suggesting that they are all mediated by reduction in the same potassium current and/or alteration of a common second messenger. In the presence of H1-receptor antagonists, application of histamine resulted in a small depolarization that was associated with a marked increase in apparent membrane conductance. Voltage-clamp recordings revealed that this response was associated with enhancement of the hyperpolarization-activated cation current Ih. This response to histamine was blocked by local or bath application of the H2-antagonists cimetidine or tiotidine. The functional consequences of these actions of histamine were addressed with extracellular and intracellular recordings in guinea pig and cat LGNd relay neurons. Extracellular recordings in cat LGNd revealed the occurrence of highly regular 1-4 Hz rhythmic burst discharges. Application of histamine halted rhythmic bursting and replaced it with a prolonged period of single-spike activity. Intracellular recordings indicate that the histamine-induced switch in firing mode is due largely to the slow depolarizing response mediated by H1-receptors, but is also facilitated by the enhancement of Ih mediated by H2-receptors. These postsynaptic actions indicate that increased activity in the tuberomammillary histaminergic system may result in a switch of thalamic neuronal activity from rhythmic burst firing to single-spike activity and thereby promote the accurate transmission and processing of sensory information and cognition.     

Distribution of the histamine H(2) receptor in monkey brain and its mRNA localization in monkey and human brain

The distribution of histamine H(2) receptor mRNA was determined by in situ hybridization histochemistry in human and monkey brain. In the case of monkey brain, we combined this technique with receptor ligand autoradiography to compare the distribution of mRNA and receptor binding sites. [(125)I]Iodoaminopotentidine ([(125)I]-APT), a reversible, high specific activity antagonist with high affinity and selectivity for the H(2) receptor, was used for receptor autoradiography. Radiolabeled oligonucleotides derived from the human mRNA sequence encoding this receptor were used as hybridization probes. The highest density of the H(2) receptor mRNA in human and monkey brain was found in caudate and putamen nuclei and external layers of cerebral cortex. Moderate levels were seen in the hippocampal formation and lower densities in the dentate nucleus of cerebellum. Areas such as globus pallidus, amygdaloid complex, cerebellar cortex, and substantia nigra were devoid of hybridization signal. The distribution of H(2) receptor mRNA in monkey brain is generally in good agreement with that of the corresponding binding sites: prominent in caudate, putamen, accumbens nuclei, and cortical areas. The hippocampus showed lower densities of receptors and low levels were detected in the globus pallidus pars lateralis. No binding sites were seen in amygdaloid complex and substantia nigra. The distribution of histaminergic innervation is in good correlation with the areas of high density for H(2) receptors: caudate, putamen, and external layers of cerebral cortex in monkey and human brain. The presence of mRNA in caudate and putamen nuclei, together with its absence from substantia nigra, suggests that the H(2) receptors found in the striatum are synthesized by intrinsic cells and not by nigral dopaminergic cells. These striatal H(2) receptors may be located on short circuit striatal interneurons or somatodendritically on striatal projection neurons which project to the globus pallidus pars lateralis. In conclusion, the present results, which constitute, to our knowledge, the first report of the regional distribution of mRNA encoding H(2) receptors detected by in situ hybridization, define the sites of synthesis of H(2) receptors and are the basis for future, more detailed studies that should result in a better understanding of H(2) receptor function.     

Receptors for VIP and PACAP in guinea pig cerebral cortex

Receptors for vasoactive intestinal peptide (VIP) and pituitary adenylate cyclase-activating polypeptide (PACAP) in guinea pig cerebral cortex were characterized by (1) radioreceptor binding of ¹²⁵I-labeled VIP (human/rat/porcine), and (2) cyclic AMP (cAMP) formation. Saturation analysis of ¹²⁵I-VIP binding to membranes of guinea pig cerebral cortex resulted in a linear Scatchard plot, suggesting the presence of a single class of high-affinity receptor-binding sites, with a Kd of 0.63 nM and a Bmax of 77 fmol/mg protein. Various peptides from the PACAP/VIP/secretin family displaced the specific binding of ¹²⁵I-VIP to guinea pig cerebrum with the relative rank order of potency: chicken VIP (cVIP) ≥ PACP38 ∼ PACAP27 ∼ guinea pig VIP (gpVIP) ≥ mammalian (human/rat/porcine) VIP (mVIP) > peptide histidine-methionine (PHM) > peptide histidine-isoleucine (PHI) > secretin. Analysis of the competition curves revealed displacement of ¹²⁵I-VIP from high- and lower-affinity binding sites, with IC50 values in the picomolar and the nanomolar range, respectively. About 70% of the specific ¹²⁵I-VIP-binding sites in guinea pig cerebral cortex were sensitive to Gpp(NH)p, a nonhydrolyzable analog of GTP. Pituitary adenylate cyclase-activating polypeptide 38 (PACAP38), PACAP27, cVIP, gpVIP, mVIP, PHM, and PHI stimulated cAMP production in [³H]adenine-prelabeled slices of guinea pig cerebral cortex in a concentration-dependent manner. Of the tested peptides, the most effective were PACAP38 and PACAP27, which at a 1 μM concentration produced a 17- to 19-fold rise in cAMP synthesis, increasing the nucleotide production to approx 11% conversion above the control value. The three forms of VIP (cVIP, mVIP, and gpVIP) at the highest concentration used, i.e., 3 μM, produced net increases in cAMP production in the range of 8–9% conversion, whereas 5 μM PHM and PHI, by, respectively, 6.7% and 4.9% conversion. It is concluded that cerebral cortex of guinea pig contains VPAC- type receptors positively linked to cAMP formation. In addition, the observed stronger action of PACAP (both PACAP38 and PACAP27), when compared to any form of VIP, on cAMP production in this tissue, suggests its interaction with both PAC1 and VPAC receptors.     

Species differences in cerebral taurine concentrations correlate with brain water content

The notion that taurine (Tau) has an osmoregulatory function in the mammalian brain has not been established, although it has been reported that the severity of hyponatremic edema is proportional to cerebral [Tau]. Tau pools are not easily altered in vivo, but the fact that there are large differences in cerebral taurine levels between mice, rats and guinea pigs offers an opportunity to determine whether endogenous Tau affects volume regulation of the brain in hyposmolal conditions. This issue was investigated by injecting saline or distilled water intraperitoneally at 150 ml/kg in anesthetized mice, rats and guinea pigs. The animals were decapitated 4 h later, and blood osmolality, cortical specific gravity, Na+, K+ and amino acid concentrations were determined. In controls, blood osmolality and specific gravity of the cortex were highest in the mouse (304 +/- 3 mmol/kg; 1.0488 +/- 0.0003 kg/l), followed by the rat (294 +/- 1; 1.0462 +/- 0.0002) and the guinea pig (285 +/- 2; 1.0445 +/- 0.0002). There was a correlation between these measures and cortical Tau levels which were 10.31 +/- 0.36 mmol/kg in mouse cortex, 6.31 +/- 0.18 in rat cortex and 1.37 +/- 0.06 in guinea pig cortex. Despite these differences, water-induced cerebrocortical swelling did not differ between the species studied. Interspecies variation in cortical osmolality did not relate to [Na+] and [K+], since the levels of these electrolytes were higher in the guinea pig cortex than in the rat and mouse cortex. After administration of water, the levels of Na+ and K+ were reduced in rat and guinea pig cortex, while only [Na+] was significantly decreased in mouse cortex.(ABSTRACT TRUNCATED AT 250 WORDS)     

Heterogeneity of histaminergic neurons in the tuberomammillary nucleus of the rat

Histaminergic neurons of the hypothalamic tuberomammillary nuclei (TMN) send projections to the whole brain. Early anatomical studies described histaminergic neurons as a homogeneous cell group, but recent evidence indicates that histaminergic neurons are heterogeneous and organized into distinct circuits. We addressed this issue using the double‐probe microdialysis in freely moving rats to investigate if two compounds acting directly onto histaminergic neurons to augment cell firing [thioperamide and bicuculline, histamine H₃‐ and γ‐aminobutyric acid (GABA)_A‐receptor (R) antagonists, respectively] may discriminate groups of histaminergic neurons impinging on different brain regions. Intra‐hypothalamic perfusion of either drug increased histamine release from the TMN and cortex, but not from the striatum. Thioperamide, but not bicuculline, increased histamine release from the nucleus basalis magnocellularis (NBM), bicuculline but not thioperamide increased histamine release from the nucleus accumbens (NAcc). Intra‐hypothalamic perfusion with thioperamide increased the time spent in wakefulness. To explore the local effects of H₃‐R blockade in the histaminergic projection areas, each rat was implanted with a single probe to simultaneously administer thioperamide and monitor local changes in histamine release. Thioperamide increased histamine release from the NBM and cortex significantly, but not from the NAcc or striatum. The presence of H₃‐Rs on histaminergic neurons was assessed using double‐immunofluorescence with anti‐histidine decarboxylase antibodies to identify histaminergic cells and anti‐H₃‐R antibodies. Confocal analysis revealed that all histaminergic somata were immunopositive for the H₃‐R. This is the first evidence that histaminergic neurons are organized into functionally distinct circuits that influence different brain regions, and display selective control mechanisms.     

Effects of simvastatin and 6-hydroxydopamine on histaminergic H1 receptor binding density in rat brains

Statins have been widely used for the treatment of a variety of medical conditions including psychoneurological disorders beyond their original use in lowering cholesterol. Histamine receptors play an important role in the regulation of neural activity, however, it is unknown whether statins act on histamine receptors, particularly for their neural regulatory effects. This study examined the effects of simvastatin and 6-hydroxydopamine (6-OHDA) lesions on histamine H1 receptors using [³H] pyrilamine binding autoradiography. Compared to the saline group, simvastatin (1 mg/kg/day) significantly decreased H1 receptor bindings in the primary motor cortex (M1), ventromedial hypothalamic nucleus (VMH), caudate putamen (CPu), accumbens core (AcbC) and prefrontal cortex (PfC) (all p < 0.05); however 10 mg/kg/day simvastatin increased H1 receptor density only in the medial amygdaloid nucleus (Mep) (p < 0.05), but had no significant effect in other regions examined. The 6-OHDA lesion did not alter H1 receptor binding density in most brain areas, except a trend decrease in the hippocampus (p = 0.07) and a trend increase in the cingulate cortex (p = 0.06). These results suggested that simvastatin has different effects on the H1 receptors in different rat brain regions depending on the doses. Therefore, simvastatin can modulate histaminergic neurotransmission in the brain, and support the role of H1 receptors in psychoneurological disorders.     

Differential localization ofH-[Pro]SP binding sites in the guinea pig and rat brain

Due to the existence of differences in the pharmacological properties of tachykinin NK-1 receptors in the rat and the guinea pig, the autoradiographic distribution of NK-1 binding sites was compared in the brain of the two species using the selective NK-1 ligand³H-[Pro⁹]SP. If a good similarity in the distribution of NK-1 binding sites could be seen in basal ganglia, a relative absence of correlation was observed between the estimated optical densities in other brain structures of the two species. For instance, the interpeduncular nucleus, the lateral habenular nucleus and the deep layers of the cerebral cortex were labeled in the guinea pig but not in the rat while the reverse was observed for the columns of the vermis lobules 9–10, the dorsal raphe nucleus, the medial habenular nucleus, the superficial cortical layers and the dorsal hippocampus. Furthermore, the high similarity found in the localization of¹²⁵I-BHSP (a non selective ligand) and³H-[Pro⁹]SP binding sites, does not suggest the existence of NK-1 binding site subtypes in the guinea pig brain.     

Chronic prenatal ethanol exposure alters hippocampal GABA(A) receptors and impairs spatial learning in the guinea pig

Chronic prenatal ethanol exposure (CPEE) can injure the developing brain, and may lead to the fetal alcohol syndrome (FAS). Previous studies have demonstrated that CPEE upregulates γ-aminobutyric acid type A (GABA_A) receptor expression in the cerebral cortex, and decreases functional synaptic plasticity in the hippocampus, in the adult guinea pig. This study tested the hypothesis that CPEE increases GABA_A receptor expression in the hippocampus of guinea pig offspring that exhibit cognitive deficits in a hippocampal-dependent spatial learning task. Timed, pregnant guinea pigs were treated with ethanol (4 g/kg maternal body weight per day), isocaloric-sucrose/pair-feeding, or water throughout gestation. GABA_A receptor subunit protein expression in the hippocampus was measured at two development ages: near-term fetus and young adult. In young adult guinea pig offspring, CPEE increased spontaneous locomotor activity in the open-field and impaired task acquisition in the Morris water maze. CPEE did not change GABA_A receptor subunit protein expression in the near-term fetal hippocampus, but increased expression of the β2/3-subunit of the GABA_A receptor in the hippocampus of young adult offspring. CPEE did not change either [³H]flunitrazepam binding or GABA potentiation of [³H]flunitrazepam binding, but decreased the efficacy of allopregnanolone potentiation of [³H]flunitrazepam binding, to hippocampal GABA_A receptors in adult offspring. Correlational analysis revealed a relationship between increased spontaneous locomotor activity and growth restriction in the hippocampus induced by CPEE. Similarly, an inverse relationship was found between performance in the water maze and the efficacy of allopregnanolone potentiation of [³H]flunitrazepam binding in the hippocampus. These data suggest that alterations in hippocampal GABA_A receptor expression and pharmacological properties contribute to hippocampal-related behavioral and cognitive deficits associated with CPEE.     

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.     

Neurotransmitter receptor localizations: Brain lesion induced alterations in benzodiazepine, GABA, β-adrenergic and histamine H1-receptor binding

Selective neuronal lesions have been utilized in efforts to localize binding sites in rat brain for β-adrenergic, γ-aminobutyric acid (GABA), histamine H₁ and benzodiazepine receptors. The various receptors respond differentially to lesions both in extent of change and in time course. After kainate lesions in the corpus striatum, benzodiazepine receptors are depleted up to 45% at 45–78 days but are unaffected after 7 days. By contrast striatal GABA receptors are increased at 7 days but depleted at later times. Thus both striatal benzodiazepine and GABA receptors appear to be associated at least in part with intrinsic neurons.In the cerebellum both benzodiazepine and GABA receptors are reduced in kainate treated rats and in Nervous mice, mutants which lack Purkinje cells. The most pronounced dissimilarity between benzodiazepine and GABA receptors occurs in Weaver mice, which selectively lack granule cells and display a 60% reduction in GABA receptors but a 40% augmentation in benzodiazepine receptors. A major portion of cerebellar GABA receptors, therefore, appear to be localized to granule cells.Striatal β-adrenergic receptors are reduced following intrastriatal kainate injections but are unaffected by cerebral cortex ablation, suggesting an association with intrinsic neurons but not with axon terminals of the corticostriate pathway. While intraventricular injections of 6-hydroxydopamine enhance [³H]dihydroalprenolol binding to β-adrenergic receptors in the cerebral cortex and hippocampus, such binding is not augmented in the corpus striatum, brain stem, midbrain or thalamus-hypothalamus by this treatment. Moreover, medial forebrain bundle lesions, which destroy ascending adrenergic neurons, fail to alter cerebral cortical or striatal β-adrenergic receptors. Thus denervation-elicited increase in β-adrenergic receptors vary with brain region and the type of denervating lesion.Histamine H₁-receptors are the most resistant of all to neuronal lesions. In the corpus striatum [³H]mepyramine binding is unaffected by cerebral cortex ablation, nigral injections of 6-hydroxydopamine or brain stem hemisection. In the hippocampus, medial forebrain bundle lesions, intrahippocampal kainate injection, and fimbria and fornix transection largely fail to alter [³H]mepyramine binding. Accordingly, a major portion of these receptors may be associated with nonneuronal elements such as glia or blood vessels.     

Structure of the human histamine H3 receptor gene (HRH3) and identification of naturally occurring variations

Summary. Neurotransmitter release is modulated by presynaptic histamine H₃ receptors located on histaminergic, noradrenergic and other nonhistaminergic neurons of the central and peripheral nervous system. Here, we report the determination of the structure of the human histamine H₃ receptor gene (HRH3) and the identification of a missense mutation (Ala280Val) in a patient with Shy-Drager syndrome. The coding region of the gene consists of three exons interrupted by two introns of approximately 1 kb in size. Exon boundaries only partly correspond to transmembrane domain organization. The homozygous Ala280Val variation in the third intracellular loop of the histamine H₃ receptor of a patient with Shy-Drager syndrome may be related to the etiology of the illness due to altered norepinephrine release. Furthermore, knowledge of the gene structure allows the verification of alternative splicing of the receptor. The corresponding histamine H₃ receptor isoforms as reported for the guinea pig and rat histamine H₃ receptor in different brain regions are not found in the human brain. Received July 11, 2001; accepted November 30, 2001     

Benzodiazepine receptor in brain.

The evidence that the brain possesses specific receptors for benzodiazepines is summarized. Further we present a series of brain lesion experiments in rats showing that specific neuronal destructions by 6-hydroxydopamine, kainic acid in the striatum, X-ray irradiation of the hippocampus, intraperitoneal 3-acetyl-pyridine or hemisection at the thalamic level do not reduce the level of benzodiazepine receptors in striatum, hippocampus, cortex or cerebellum. These results show that the benzodiazepines are not positioned on dopamine or noradrenaline terminals, cholergic or GABA-ergic neurons in the striatum, granular cells in the hippocampus or climbing fibers in the cerebellum.     

Histaminergic system in the tree shrew brain

This study mapped the histamine-immunoreactive neuronal system in the brain of the tree shrew (Tupaia belangeri) and compared its structure with that of the rat and guinea pig. The histamine-containing cell bodies lay in the posterior ventral hypothalamus in the tuberomammillary complex, as in the rodents. The morphology of this complex resembled that of the rat. The histaminergic axons projected to nearly all parts of the brain. The main ascending bundle ran ventromedially: the densest innervation was found in the ventral hypothalamus, preoptic area, septum, medial part of nucleus accumbens, and bed nucleus of the stria terminalis. High fiber densities were present in the amygdaloid nuclei and claustrum. Another pathway ran dorsomedially along the periventricular hypothalamus and sent fibers to all parts of the diencephalon. Part of these fibers followed the central gray to the midbrain and spread laterally below the inferior colliculus. Another descending pathway ran through the interfascicular and medial raphe nuclei to meet the pontine central gray. The densest fiber networks were seen in the dorsal tegmental and parabrachial nuclei, and around the locus coeruleus. Also the substantia nigra, interpeduncular and mesencephalic reticular nuclei, colliculi, and vestibular and raphe nuclei received a dense histaminergic innervation. The organization of the fibers in the tree shrew brain resembled more that in the guinea pig than that in the rat. As compared with the guinea pig, more fibers were present, particularly in the globus pallidus, central thalamus, and deep cerebellar nuclei. No fibers were seen in the outer layer of the piriform cortex. In Tupaia, a laminar organization of the fibers was evident in the hippocampus, in contrast to the rodents. Also, a dense periventricular fiber plexus was prominent.     

Expression of the three opioid receptor subtypes mu, delta and kappa in guinea pig and rat brain cell cultures and in vivo

Expression of the three opioid receptor subtypes mu, delta and kappa in aggregating cell cultures prepared from embryonic guinea pig or rat brains was compared with the in vivo expression of the receptors in the brain of developing and adult animals of the same species. At the day of culturing, one third of the receptors in the brain of guinea pig embryos were of the kappa type. In culture, however, the aggregating brain cells acquired within 14 days a high percentage (75%) of kappa receptors. As only 28% of the receptors in the adult guinea pig brain are of this subtype, an attempt was made to further analyse the specificity of this developmental process. In guinea pig, the 2-fold increase in kappa receptors in culture was accompanied with a decline in both the percentage and the density (per protein) of mu and delta subtypes. In contrast, a marked increase in delta receptors was observed in rat whole brain, forebrain or hindbrain cultures. Thus, the developmental pattern of the three receptor subtypes in rat brain cultures, but not in guinea pig, was similar to that in vivo. These and additional experiments suggest that at the developmental stage taken to prepare the cultures, neurons expressing opioid receptors were already programmed in the rat but not in guinea pig brain.     

Transient changes in the limbic histaminergic system after systemic kainic acid-induced seizures

Increased brain histamine is reported to protect against convulsions. We used systemic kainic acid (KA) administration to study possible changes of the histaminergic system in rat brain in status epilepticus (SE). Robust increases in brain histamine concentrations and numbers of histamine-immunoreactive nerve fibers were detected in the piriform cortex (Pir) and amygdala after KA injection, suggesting a reactive increase, which is opposite to other published aminergic transmitter responses. These changes, lasting several weeks, might be coupled to a mechanism unrelated to the anticonvulsive function of histamine. Transient increases in mRNA expression of H(3) receptor isoforms with a full-length third intracellular loop, coupled to mitogen-activated protein kinase pathway, were detected first in the hippocampal CA3c area, followed by the Pir and amygdala and then the hippocampal CA1 area. These results suggest that histamine and H3 receptors, which also control the release of GABA and glutamate, might be involved in convulsive SE.