Alpha 2-adrenoceptor-mediated inhibition of histamine release from rat cerebral cortical slices

Are the pharmacology and physiology of α₂ adrenoceptors determined by α₂-heteroreceptors and autoreceptors respectively?

α(2)-Adrenoceptors are important mediators of physiological responses to the endogenous catecholamines noradrenaline and adrenaline. In addition, α(2)-adrenoceptors are pharmacological targets for the treatment of hypertension, sympathetic overactivity and glaucoma. α(2)-Adrenoceptors are also targeted to induce sedation and analgesia in anaesthesia and intensive care. α(2)-Adrenoceptors were first described as presynaptic receptors inhibiting the release of various transmitters from neurons in the central and peripheral nervous systems. In addition to these presynaptic neuronal receptors, α(2)-adrenoceptors were also identified in many non-neuronal cell types of the body. Gene-targeting in mice provided a comprehensive assignment of the physiological and pharmacological functions of these receptors to specific α(2A)-, α(2B) - and α(2C)-adrenoceptor subtypes. However, the specific cell types and signalling pathways involved in these subtype-specific α(2)-adrenoceptor functions were largely unexplored until recently. This review summarizes recent findings from transgenic mouse models, which were generated to define the role of α(2)-adrenoceptors in adrenergic neurons, that is, α(2)-autoreceptors, versus α(2)-adrenoceptors in non-adrenergic neurons, termed α(2)-heteroreceptors. α(2)-Autoreceptors are primarily required to limit release of noradrenaline from sympathetic nerves and adrenaline from adrenal chromaffin cells at rest. These receptors are desensitized upon chronic activation as it may for instance occur due to enhanced sympathetic activity during chronic heart failure. In contrast, pharmacological effects of acutely administered α(2)-adrenoceptor agonist drugs essentially require α(2)-heteroreceptors in non-adrenergic neurons, including analgesia, sedation, hypothermia and anaesthetic-sparing as well as bradycardia and hypotension. Thus a clear picture has emerged of the significance of auto- versus heteroreceptors in mediating the physiological functions of α(2)-adrenoceptors and the pharmacological functions of α(2)-adrenoceptor agonist drugs respectively.

Histamine neuronal system as a therapeutic target for the treatment of cognitive disorders

Much has been learned over the past 20 years about the role of histamine as a neurotransmitter. This brief article attempts to evaluate the progress accomplished in this field, and discusses the therapeutic potential of the H3 receptor (H3R). All histaminergic neurons are localized in the tuberomammillary nucleus of the posterior hypothalamus and project to almost all regions of the CNS. Histamine exerts its effect via interaction with specific receptors (H1R, H2R, H3R and H4R). Antagonists of both H1R and H2R have been successful as blockbuster drugs for treating allergic conditions and gastric ulcers. H4R is still awaiting better functional characterization, but the H3R is an attractive target for potential therapies of CNS disorders. Indeed, considerable interest was raised by reports that pharmacological blockade of H3Rs exerted procognitive effects in a variety of animal tasks analyzing different types of memory. In addition, blockade of H3Rs increased wakefulness and reduced bodyweight in animal models. Such findings hint at the potential use of H3R antagonists/inverse agonists for the treatment of Alzheimer’s disease and other dementias, attention-deficit hyperactivity disorder, obesity and sleep disorders. As a result, an increasing number of H3R antagonists/inverse agonists progress through the clinic for the treatment of a variety of conditions, including attention-deficit hyperactivity disorder, cognitive disorders, narcolepsy and schizophrenia. Moreover, the use of H3R antagonists/inverse agonists that weaken traumatic memories may alleviate disorders such as post-traumatic stress syndrome, panic attacks, specific phobias and generalized anxiety. The use of H3R ligands for the treatment of neurodegenerative disorders is demonstrated in several studies, indicating a role of the histamine neurons and H3Rs in neuroprotection. Recently, direct evidence demonstrated that histaminergic neurons are organized into functionally distinct circuits, impinging on different brain regions, and displaying selective control mechanisms. This could imply independent functions of subsets of histaminergic neurons according to their respective origin and terminal projections. The possibility that H3Rs control only some of those functions implies that H3R-directed therapies may achieve selective effects, with minimal side effects, and this may increase the interest regarding this class of drugs.

Effects of histamine H1 receptor antagonists on depressive-like behavior in diabetic mice

We previously reported that streptozotocin-induced diabetic mice showed depressive-like behavior in the tail suspension test. It is well known that the central histaminergic system regulates many physiological functions including emotional behaviors. In this study, we examined the role of the central histaminergic system in the diabetes-induced depressive-like behavior in the mouse tail suspension test. The histamine contents in the hypothalamus were significantly higher in diabetic mice than in non-diabetic mice. The histamine H(1) receptor antagonist chlorpheniramine (1-10 mg/kg, s.c.) dose-dependently and significantly reduced the duration of immobility in both non-diabetic and diabetic mice. In contrast, the selective histamine H(1) receptor antagonists epinastine (0.03-0.3 microg/mouse, i.c.v.) and cetirizine (0.01-0.1 microg/mouse, i.c.v.) dose-dependently and significantly suppressed the duration of immobility in diabetic mice, but not in non-diabetic mice. Spontaneous locomotor activity was not affected by histamine H(1) receptor antagonists in either non-diabetic or diabetic mice. In addition, the number and affinity of histamine H(1) receptors in the frontal cortex were not affected by diabetes. In conclusion, we suggest that the altered neuronal system mediated by the activation of histamine H(1) receptors is involved, at least in part, in the depressive-like behavior seen in diabetic mice.

Selective histamine H3 receptor antagonists for treatment of cognitive deficiencies and other disorders of the central nervous system

Evidence exists to implicate the monoamine histamine in the control of arousal and cognitive functions. Antagonists of H(3) receptors are postsynaptic and presynaptic modulators of neural transmission in a variety of neuronal circuits relevant to cognition. Accumulating neuroanatomical, neurochemical, pharmacological, and behavioral data support the idea that H(3) receptor antagonists may function to improve cognitive performances in disease states (e.g., Alzheimer's disease and mild cognitive impairment states). Thus, H(3) receptor antagonists have been shown to increase performance in attention and memory tests in nonhuman experiments and prevent the degradation in performances produced by scopolamine, MK-801, or age. In contrast, agonists of the H(3) receptor generally produce cognitive impairing effects in animal models. The role of H(3) receptors in these behavioral effects is substantiated by data indicating a central origin for their effects, the selectivity of some of the H(3) receptor antagonists studied, and the pharmacological modification of effects of H(3) receptor antagonists by selective H(3) receptor agonists. Data and issues that challenge the potential role for H(3) receptor antagonists in cognitive processes are also critically reviewed. H(3) receptor antagonists may also have therapeutic value in the management of obesity, pain, sleep disorders, schizophrenia, and attention deficit hyperactivity disorder.

α2-Adrenergic receptor-mediated presynaptic inhibition of GABAergic IPSPs in rat histaminergic neurons

Nuclei of the brainstem involved in behavioral state control are mutually interconnected. Histaminergic neurons of the posterior hypothalamus receive inputs from brainstem noradrenergic cell groups as well as from the locus coeruleus. The role of adrenergic inputs in histaminergic function is unclear. We examined the actions of adrenergic agonists on histaminergic neurons of the tuberomamillary nucleus (TM) using electrophysiological methods in a brain slice preparation. Evoked GABAergic inhibitory postsynaptic potentials (IPSPs) in histaminergic neurons were reduced in amplitude following the application of norepinephrine (NE) (2-20 microM) or clonidine (10 microM) but were not affected by isoproterenol (10 microM). Norepinephrine application caused no changes in membrane properties of TM neurons. Responses to exogenously applied GABA were unaffected by adrenergic agonists. Clonidine reduced the frequency of spontaneous IPSPs, an action that was blocked by yohimbine. Norepinephrine did not alter the amplitude distribution of bicuculline-sensitive miniature inhibitory postsynaptic currents (mIPSCs). Thus, GABA release onto TM neurons is modulated presynaptically by adrenergic alpha(2)-receptors. Inputs from noradrenergic neurons of the brainstem will reduce the inhibitory actions of GABAergic inputs resulting in disinhibition of histaminergic neurons.

Role of H3-receptor-mediated signaling in anxiety and cognition in wild-type and Apoe-/- mice

Increasing evidence supports a role for histamine as a neurotransmitter and neuromodulator in emotion and cognition. The H(3) receptor was first characterized as an autoreceptor that modulates histamine release and synthesis via negative feedback. Mice deficient in apoE (Apoe(-/-)) have been used to define the role of apoE in brain function. In the present study, we investigated the possible role of histamine H(3)-receptor-mediated signaling in anxiety and cognition in mice Apoe(-/-) and wild-type mice. H(3) antagonists increased measures of anxiety in wild-type, but not Apoe(-/-), mice. In contrast, H(3) antagonists similarly impaired object recognition in wild-type and Apoe(-/-) mice. In Apoe(-/-) mice, reduced negative feedback via H(3) receptors could contribute to increased signaling of H(1) receptors. Apoe(-/-) mice showed higher sensitivity to the anxiety-reducing effects of the H(1) receptor antagonist mepyramine than wild-type mice. These effects were dissociated from effects of mepyramine on the HPA axis. Compared to saline controls, mepyramine reduced plasma ACTH and corticosterone levels in wild-type, but not Apoe(-/-), mice. These data support a role for apoE in H(3) receptor signaling. H(3) antagonists were proposed as a treatment for cognitive disorders such as Alzheimer's disease, which is associated with increased anxiety and cognitive impairments. As H(3) antagonists increase measures of anxiety and impair object recognition in wild-type mice, the use of H(3) antagonists in cognitive disorders may be counterproductive and should be carefully evaluated.

Effects of alpha-adrenoceptor agonists and antagonists on histamine-induced impairment of memory retention of passive avoidance learning in rats

The effect of alpha-adrenoceptor agents on the impairment induced by histamine was measured for memory retention of passive avoidance learning in rats. Post-training intracerebroventricular (i.c.v.) injection was carried out in all the experiments. Histamine (5, 10 and 20 microg/rat) reduced, while a histamine H(1) receptor antagonist, chlorpheniramine (0.1, 1 and 10 microg/rat), increased memory retention. The histamine H(2) receptor antagonist, ranitidine (0.1, 1, 10 and 20 microg/rat), did not elicit any response in this respect. Different doses of chlorpheniramine but not ranitidine reversed the histamine-induced impairment of memory. Clonidine and prazosin decreased, but yohimbine and phenylephrine increased, memory retention. Yohimbine decreased the inhibitory response to histamine. Phenylephrine, clonidine and prazosin did not alter the histamine effect. It is concluded that a histamine-induced impairment of memory retention through histamine H(1) receptors and an alpha(2)-adrenoceptor mechanism may be involved in the histamine response.

High concentrations of adrenergic antagonists prolong sciatic nerve blockade by tetrodotoxin

BACKGROUND

Millimolar-range concentrations of some adrenergic antagonists have been shown to have local anesthetic-like properties, and to stimulate GTPase activity in vitro. In this report, we investigate whether these agents can potentiate the effect of tetrodotoxin (TTX) and bupivacaine, a conventional local anesthetic, and whether GTPase activation plays a role.

METHODS

Rats received sciatic nerve blockade with tetrodotoxin or bupivacaine co-injected with adrenergic antagonists and/or agonists, or pertussis toxin. Thermal nociceptive blockade was quantified with modified hotplate testing.

RESULTS

Nerve block from TTX alone lasted 153 (99-223) min (median and 25th and 75th percentiles). Co-injection with 20 mM phentolamine, propranolol, and yohimbine prolonged TTX block to 856 (765-862), 486 (444-510), and 465 (413-495) min respectively (P<0.005 in all cases, compared to TTX alone). Micromolar concentrations of adrenergic antagonists (which inhibited the prolongation of TTX block by epinephrine) did not prolong TTX block. Injection of adrenergic antagonists alone did not produce specific nerve block. They did not prolong TTX block when injected at a remote subcutaneous site. Prolongation of TTX block by phentolamine was not inhibited by co-injection with pertussis toxin. Adrenergic antagonists did not prolong bupivacaine block.

CONCLUSIONS

High concentrations of adrenergic antagonists markedly prolonged TTX block, but not bupivacaine block. This locally mediated action does not appear to be adrenergic-receptor-specific, or mediated by GTPase activation.

In vivo modulation of rat hypothalamic histamine release by the histamine H3 receptor ligands, immepip and clobenpropit. Effects of intrahypothalamic and peripheral application

We investigated the effect of the new potent and selective histamine H3 receptor agonist, immepip, and the histamine H3 receptor antagonist, clobenpropit, on in vivo neuronal histamine release from the anterior hypothalamic area of urethane-anesthetized rats, using microdialysis. Intrahypothalamic perfusion with immepip at concentrations of 1 and 10 nM reduced histamine release to 75% and 35% of its basal level, respectively. Peripheral injection of immepip (5 mg/kg) caused a sustained decrease in histamine release of 50%. Clobenpropit potently increased histamine release after intrahypothalamic perfusion. The maximal increase in histamine release was 2-fold, observed at a concentration of 10 nM clobenpropit. Peripheral injection of clobenpropit (5-15 mg/kg) increased histamine release to about 150% of the basal value. A more marked increase in histamine release was found after injection of the histamine H3 receptor antagonist, thioperamide (5 mg/kg). In conclusion, intrahypothalamic perfusion of the histamine H3 receptor agonist, immepip and the histamine H3 receptor antagonist, clobenpropit, potently and oppositely modulated in vivo histamine release from the anterior hypothalamic area. The decreased histamine release after peripheral injection of immepip indicates that this novel agonist readily crosses the blood-brain barrier, making it a potential candidate for in vivo histamine H3 receptor studies. The differential increase in histamine release after peripheral injection of clobenpropit and thioperamide is discussed.

Effects of a selective alpha 2-adrenoceptor antagonist, atipamezole, on hypothalamic histamine and noradrenaline release in vivo

In vivo microdialysis was used to study the effects of a potent and selective alpha 2-adrenoceptor antagonist, atipamezole, on histamine and noradrenaline release from the medial hypothalamus in anesthetized rats. Local perfusion with atipamezole via the microdialysis probe increased histamine release significantly and dose-dependently. However, the effect of systemic administration of atipamezole (1 mg/kg) was opposite: it significantly decreased histamine release. Local and systemic administration of atipamezole produced an approx. 2-fold increase in noradrenaline release. To study the modulatory effect of noradrenergic neurons on histamine release, noradrenaline synthesis was inhibited with alpha-methyl-p-tyrosine. In the microdialysis experiment, rats that received alpha-methyl-p-tyrosine exhibited no decrease, but rather a slight increase in histamine release in response to systemic atipamezole administration. These results show clearly that atipamezole enhances noradrenaline release in vivo from rat hypothalamus and its effects on histamine release are dependent on the route of drug administration.

Histamine metabolites in cerebrospinal fluid of patients with chronic schizophrenia: their relationships to levels of other aminergic transmitters and ratings of symptoms

Levels of the histamine metabolites, tele-methylhistamine (t-MH) and tele-methylimidazoleacetic acid (t-MIAA), and metabolites of other aminergic transmitters and of norepinephrine were measured in cerebrospinal fluid of 36 inpatients with chronic schizophrenia and eight controls. The mean t-MH level from controls was nearly identical to the levels seen previously in healthy volunteers. Compared with controls, the mean level of t-MH in the schizophrenic patients was 2.6-fold higher (p = 0.006); 21 of the patients had levels exceeding the range of controls. There was no significant difference (p > 0.05) in levels of other analytes, although the levels of t-MH correlated significantly with those of t-MIAA, homovanillic acid, 3,4-dihydroxyphenylacetic acid, norepinephrine, 3-methoxy-4-hydroxyphenylglycol and 5-hydroxyindoleacetic acid. The difference in levels of t-MH were not attributable to medication, since those taking (n = 10) or withdrawn from (n = 26) neuroleptic drugs had nearly the same mean levels of t-MH; each group had higher levels than controls (ANOVA: p < 0.05). Patients with or without tardive dyskinesia showed no significant differences in means of any analyte. Only levels of t-MH among those with schizophrenia correlated with positive symptom scores on the Psychiatric Symptom Assessment Scale (rs = 0.45, p < 0.02). The elevated levels of t-MH in cerebrospinal fluid, which represent histamine that was released and metabolized, suggest increased central histaminergic activity in patients with chronic schizophrenia.

Effects of systemic clonidine on auditory event-related potentials in squirrel monkeys

Event-related potential (ERP), electroencephalographic (EEG), and behavioral data were collected from squirrel monkeys (Saimiri sciureus) in a 90-10 auditory oddball paradigm. Background or target tones were presented once every 2 s, and responses to the targets were rewarded. ERPs were recorded from epidural electrodes following systemic administration of clonidine (0.1 mg/kg) or a saline placebo. EEG power spectral and behavioral performance were assessed simultaneously as indices of behavioral state. Clonidine significantly decreased the area and increased the latency of a P300-like potential. The amplitude and areas of the earlier P1, N1, and P2 components and a later slow wave-like potential were not reduced, nor were ther latencies altered. Clonidine produced increased EEG power in the alpha range (7.5-12 Hz) and decreased power in the upper beta range (20-40 Hz) but did not affect performance in the oddball task. Because two major effects of clonidine are to substantially reduce activity in the noradrenergic nucleus locus coeruleus (LC) and to reduce norepinephrine (NE) release from axons, the present results support the hypothesis that the LC and its efferent projection system are important in modulating the activity of P300-like potentials.

Modulation by dopamine receptors of the histamine release in the rat hypothalamus

The involvement of dopaminergic neurons of the hypothalamus in the modulation of histamine release was studied by the push-pull technique. The posterior hypothalamus of the conscious, freely moving rat was superfused with artificial cerebrospinal fluid (CSF) and the release of histamine was determined radioenzymatically in the superfusate. Agonists and antagonists of dopamine D1-, D2- and D3-receptors were dissolved in CSF and applied to the hypothalamus through the push-pull cannula. Hypothalamic superfusion with the D1-, D2- and D3-receptor agonists dopamine or R(-)-apomorphine enhanced the release rate of histamine. (+/-)-Apomorphine also enhanced the release of histamine, but to a lesser extent than did equimolar concentration of R(-)-apomorphine. The D3-agonist quinpirole inhibited the release of histamine, while the D1-receptor agonist SKF 82958 [(+-)-6-chloro-7,8-dihydroxy-3-allyl-1-phenyl-2,3,4,5-tetrahydro-1H-3- benzazepine] did not virtually influence the release of the neurotransmitter. On the other hand, [-]-sulpiride which predominantly blocks D2-receptors, decreased histamine release. Hypothalamic superfusion with SKF 83566 [(+-)-7-bromo-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3- benzazepine], which seems to be a selective antagonist of D1-receptors, enhanced the release rate of histamine. These findings suggest that dopaminergic neurons of the hypothalamus influence the release of histamine from its neurons in a dual way. D2-heteroreceptors stimulate the release of histamine, while D3-heteroreceptors seem to inhibit the release of this neurotransmitter. Both types of dopamine receptors might be located presynaptically on histaminergic neurons.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of histamine turnover by 8-OH-DPAT, buspirone and 5-hydroxytryptophan in the mouse and rat brain

The effects of 5-hydroxytryptamine (5-HT) receptor agonists on histamine turnover in mouse and rat brains were examined. The histamine turnover rate was estimated from the accumulation of tele-methylhistamine 90 min after i.p. injection of pargyline (65 mg/kg). In whole mouse brains, the histamine turnover was significantly inhibited by the 5-HT1A agonists, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) (greater than 0.5 mg/kg) and buspirone (greater than 2 mg/kg) injected s.c. 10 min before pargyline treatment. 5-hydroxytryptophan (20 mg/kg) also significantly inhibited histamine turnover. Injections of the 5-HT1B agonist m-trifluoromethylphenylpiperazine (10 and 20 mg/kg) or the 5-HT2 agonist (1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (1, 2 and 5 mg/kg), however, did not affect histamine turnover. The inhibitory effect of 8-OH-DPAT (1 mg/kg) on histamine turnover was significantly antagonized (by 40%) by pindolol (20 mg/kg) and slightly antagonized (by 29%) by spiperone (10 mg/kg), while methysergide (20 mg/kg) and ketanserin (10 mg/kg) demonstrated no antagonistic effects. 8-OH-DPAT (0.3 and 1 mg/kg) also showed an inhibiting effect on histamine turnover in various regions of rat brains. Although the extent of inhibition was slightly larger in the striatum and cerebral cortex, there was no marked regional difference. These results suggest that histaminergic activity in the brain is regulated by 5-HT1A receptors.

Glutamatergic regulation of histamine release from rat hypothalamus

A microdialysis method was used to study the effects of glutamate on the in vivo release of histamine from the anterior hypothalamic area of rats anesthetized with urethane. Infusion of 1 mM glutamate through a microdialysis probe increased histamine release to about 150% of the basal release. Infusion of N-methyl-D-aspartate (NMDA, 0.1 mM) caused a similar increase. Glutamate-evoked histamine release was completely blocked by D-(-)-2-amino-5-phosphonopentanoic acid (AP5, 0.1 mM), a specific antagonist of NMDA receptors. AP5 alone also reduced histamine release to about 60% of the basal level. Infusion of tetrodotoxin (100 nM) reduced histamine release to about 30% of the basal release, but had no effect on glutamate-evoked release. These results clearly indicate that glutamate enhances histamine release through NMDA receptors located on histaminergic nerve terminals, and suggest that there is a tonic glutamatergic regulation of this release.

Regulation of histamine release in rat hypothalamus and hippocampus by presynaptic galanin receptors

The effect of galanin, a peptide present in a subpopulation of histaminergic neurons emanating from the rat posterior hypothalamus, was investigated on K(+)-evoked [3H]histamine release in slices and synaptosomes from rat cerebral cortex, striatum, hippocampus and hypothalamus. Porcine galanin (0.3 microM) significantly inhibited histamine release induced by 25 mM K+ in slices from hypothalamus and hippocampus, but not from cerebral cortex and striatum, i.e., only in regions in which a colocalization of histamine and galanin has been described. The inhibitory effect of galanin was concentration dependent, with an EC50 value of 5.8 +/- 1.9 nM. The maximal inhibition was of 30-40% in hypothalamic and hippocampal slices depolarized with 25 mM K+. The galanin-induced inhibition observed in hypothalamic slices was not prevented in the presence of 0.6 microM tetrodotoxin and also occurred in hippocampal and hypothalamic synaptosomes, strongly suggesting the activation by galanin of presynaptic receptors located upon histaminergic nerve endings. The maximal inhibitory effect of galanin in slices or synaptosomes was lower than that previously reported for histamine acting at H3-autoreceptors, possibly suggesting that not all histaminergic axon terminals, even in the hypothalamus and hippocampus, are endowed with galanin receptors. It increased progressively in hypothalamic and hippocampal synaptosomes as the strength of the depolarizing stimulus was reduced. It is concluded that galanin modulates histamine release via presynaptic receptors, presumably autoreceptors located upon nerve terminals of a subpopulation of cerebral histaminergic neurons.

In vivo modulation of the histamine release in the hypothalamus by adrenoreceptor agonists and antagonists

The modulation of the histamine release from histaminergic neurons by noradrenergic neurons was investigated by the push-pull technique. The posterior hypothalamus of the conscious, freely moving rat was superfused with artificial CSF through a push-pull cannula and the release of endogenous histamine was determined in the superfusate. Hypothalamic superfusion with a potassium-rich CSF enhanced the release rate of histamine. Superfusion with the alpha 2-agonists noradrenaline or clonidine diminished the release rate of histamine. Moreover, clonidine abolished the potassium-induced increase in the histamine release. Superfusion with the alpha 2-antagonists yohimbine or idazoxan enhanced the release rate of histamine. It is concluded that noradrenaline released from noradrenergic neurons of the hypothalamus modulates the release of histamine from histaminergic neurons by stimulating alpha 2-adrenoreceptors located on histaminergic nerve terminals.

Characterization of extracellular histamine in the striatum and bed nucleus of the stria terminalis of the rat: an in vivo microdialysis study

The intracerebral microdialysis technique, coupled with a sensitive radioenzymatic assay, was employed to study histamine release in the striatum and in the bed nucleus of the stria terminalis (BNST) in conscious, freely moving rats. In these brain regions, extracellular histamine concentrations decreased by 20% when calcium was omitted from the perfusion solution. Extracellular histamine was insensitive to the addition of tetrodotoxin to the perfusion medium. In striatum, extracellular histamine concentrations declined in an apparent biexponential manner after the administration of alpha-fluoromethylhistidine, an inhibitor of histamine synthesis. The half-lives for the disappearance of histamine were 32 min and 7.7 h, indicating the presence of at least two histamine pools. Histidine loading resulted in a nearly twofold increase in histamine outflow in striatum. In the BNST, yohimbine increased the extracellular histamine content by 50%, suggesting that histamine release is subject to alpha 2-adrenergic regulation in vivo. The extent to which histamine detected in cerebral microdialysis samples is of neurogenic origin remains to be established.

Influence of catecholamines on the in vivo release of histamine in the hypothalamus

The hypothalamus of conscious, freely moving rats was superfused with artificial CSF through push-pull cannulae and the release of endogenous histamine was determined radioenzymatically in the superfusate. Superfusion with potassium chloride enhanced the release rate of histamine. The effect of potassium chloride was abolished by alpha-fluoromethylhistidine. Noradrenaline (alpha 1- and alpha 2-agonist) and clonidine (alpha 2-agonist) decreased the release rate of histamine and inhibited the potassium-induced histamine release. In preliminary experiments, yohimbine (alpha 2-antagonist) seemed to increase the release rate of histamine in the superfusate. beta-Agonists and antagonists (isoprenaline, salbutamol, propranolol) did not influence the release of histamine.

Noradrenergic potentiation of excitatory transmitter action in cerebrocortical slices: evidence for mediation by an alpha 1 receptor-linked second messenger pathway

Considerable evidence from intact, anesthetized preparations suggests that norepinephrine (NE) can modulate the efficacy of synaptic transmission within local circuits of the mammalian neocortex; i.e. both iontophoretic application of NE and activation of the coeruleocortical pathway are capable of facilitating cortical neuronal responses to non-noradrenergic synaptic inputs and putative transmitter agents. In the present study, the effects of NE on somatosensory cortical neuronal responses to putative excitatory transmitters were characterized using in vitro tissue slice preparations. Somatosensory unit responses to iontophoretic pulses of acetylcholine (ACh) or glutamate (Glu) (10-60 nA; 5-25 s duration) were examined before, during and after a period of continuous NE (1-35 nA; 4-25 min duration) microiontophoresis. Quantitative analysis of per-event histograms indicated that both Glu- and ACh-evoked excitatory discharges were routinely (Glu 94%, n = 54; ACh 67%, n = 9) potentiated above control levels during NE administration. In 8 cells, NE revealed robust excitatory discharges to otherwise subthreshold iontophoretic doses of Glu. The alpha-specific agonist, phenylephrine, mimicked (n = 3), NE-induced potentiation of Glu-evoked discharges whereas the alpha antagonist phentolamine blocked (n = 5) enhancement of these responses. Moreover, activation of protein kinase C by iontophoretic application of phorbol 12,13-diacetate (5-15 nA, n = 4) mimicked the potentiating actions of NE on Glu-evoked excitatory responses. Results from other experiments further indicated that these facilitating actions of NE on Glu-evoked responses do not involve beta receptor activation or intracellular increases in cyclic AMP. In summary, these results demonstrate that NE can facilitate cortical neuronal responses to threshold and subthreshold level applications of putative excitatory transmitter agents. Moreover, it appears that, unlike noradrenergic facilitating influences on GABA-induced inhibition, these actions are mediated by an alpha adrenoceptor mechanism which may be linked to intracellular activation of protein kinase C. Overall, these findings reinforce the idea that noradrenergic modulatory actions on excitatory and inhibitory neuronal responses may involve the activation of separate receptor-linked second messenger systems.

In vivo release of neuronal histamine in the hypothalamus of rats measured by microdialysis

Using an in vivo intracerebral microdialysis method coupled with an HPLC-fluorometric method, we investigated the extracellular level of endogenous histamine in the anterior hypothalamic area of urethane-anaesthetized rats. The basal rate of release of endogenous histamine in the anterior hypothalamic area measured by this method was 0.09 +/- 0.01 pmol/20 min. When the anterior hypothalamic area was depolarized by infusion of 100 mM K+ through the dialysis membrane or electrical stimulation at 200 mu A was applied through an electrode implanted into the ipsilateral tuberomammillary nucleus, histamine release increased to 175% and 188%, respectively, of the basal level. These increases were completely suppressed by removal of extracellular Ca2+. The basal release of histamine was also suppressed after infusion of 10(-6) M tetrodotoxin or i.p. administration of 100 mg/kg of alpha-fluoromethylhistidine. On the other hand, 3-fold increase in the basal release was observed after i.p. administration of 5 mg/kg thioperamide. These results clearly indicate that both the basal and evoked release of histamine measured by our method are of neuronal origin.

Involvement of histaminergic neurons in arousal mechanisms demonstrated with H3-receptor ligands in the cat

The effects of histamine H3-receptor ligands on sleep-waking parameters were studied in freely moving cats. Oral administration of (R)alpha-methylhistamine (alpha MHA), a H3-agonist, caused a significant increase in deep slow wave sleep while that of thioperamide, a H3-antagonist, enhanced wakefulness in a marked and dose-dependent manner. The arousal effects of thioperamide were prevented by pretreatment with alpha MHA or mepyramine, a H1-receptor antagonist. The findings support the hypothesis that the histaminergic neurons are critically involved in arousal mechanisms and suggest that H3-receptors play an active part in these mechanisms by regulating histamine transmission.

Modulation of histamine release in the rat brain by kappa-opioid receptors

The opioid modulation of histamine release was studied in rat brain slices labeled with L-[3H]histidine. The K(+)-induced [3H]histamine release from cortical slices was progressively inhibited by the preferential kappa-agonists ketocyclazocine, dynorphin A (1-13), Cambridge 20, spiradoline, U50,488H, and U69,593 in increasing concentrations. In contrast, the mu-agonists morphine, morphiceptin, and Tyr-D-Ala-Gly-(NMe)Phe-Gly-ol (DAGO) were ineffective as were the preferential delta-agonists [D-Ala2,D-Leu5]enkephalin (DA-DLE) and [D-Pen2,D-Pen5]enkephalin (DPDPE). Nor-binaltorphimine (nor-BNI) and MR 2266, two preferential kappa-antagonists, reversed the inhibitory effect of the various kappa-agonists more potently than did naloxone, with mean Ki values of 4 nM and 25 nM, respectively. The effects of ketocyclazocine and naloxone also were seen in slices of rat striatum, another brain region known to contain histaminergic nerve endings. We conclude that kappa-opioid receptors, presumably located on histaminergic axons, control histamine release in the brain. However, nor-BNI and naloxone failed, when added alone, to enhance significantly [3H]histamine release from cerebral cortex or striatum, and bestatin, an aminopeptidase inhibitor, failed to decrease K(+)-evoked [3H]histamine release. These two findings suggest that under basal conditions these kappa-opioid receptors are not tonically activated by endogenous dynorphin peptides. The inhibition of cerebral histamine release by kappa-agonists may mediate the sedative actions of these agents in vivo.