Release of endogenous histamine in the hypothalamus of anaesthetized cats and conscious, freely moving rabbits

Neurotransmitters are released in brain areas according to ultradian rhythms: Coincidence with ultradian oscillations of EEG waves

Use of the push-pull superfusing technique has shown that in the brain the release rates of endogenous catecholamines, GABA, glutamate and histamine are not constant but fluctuate temporally according to ultradian rhythms. Rhythmic fluctuations have been found in the posterior and anterior hypothalamus, the locus coeruleus, the nucleus of the solitary tract, the mammillary body and the medial amygdaloid nucleus of cats and rats. Similar fluctuations appear in the nitric oxide signal registered in the nucleus accumbens, as well as in the power of delta and theta waves of the EEG in the posterior hypothalamus. The EEG rhythmic fluctuations are generated in the arcuate nucleus because they disappear after its electrocoagulation. The frequency of the EEG fluctuations is increased, decreased or even abolished when catecholamine or histamine receptor agonists and antagonists are centrally applied showing that the EEG ultradian rhythm is controlled by catecholaminergic and histaminergic neurons. Moreover, the rhythmic fluctuations of delta and theta waves corelate negatively with those of histamine in the rat posterior hypothalamus. The possible role of these rhythmic fluctuations is discussed. Their potential importance for pharmacotherapy is still unknown.

Use of Push-Pull Superfusion Technique for Identifying Neurotransmitters Involved in Brain Functions: Achievements and Perspectives

The push-pull superfusion technique (PPST) is a procedure for in vivo examination of transmitter release in distinct brain areas. This technique allows to investigate dynamics of transmitter release both under normal and experimentally evoked conditions. The PPST can be modified so that it is possible to determine release of endogenous transmitters simultaneously with electroencephalogram (EEG) recordings, recordings of evoked potentials or the on-line determination of endogenous nitric oxide (NO) released into the synaptic cleft. Because of the good time resolution, the method provides further the possibility to modify the collection periods of superfusates depending on the neuronal function that is analyzed. For instance, investigation of central cardiovascular control, behavioral tasks or mnemonic processes requires very short collection periods, because changes in transmitter release occur within seconds. Even more important is the time resolution when rates of transmitter release are correlated with evoked extracellular potentials or EEG recordings. This review provides an overview of the different devices which might be combined with the PPST and perspectives for future work.

Origin of endogenous nitric oxide released in the nucleus accumbens under real-time in vivo conditions

AIMS

Nitric oxide (NO), is a simple but multifarious molecule. It is implicated in physiological and pathological processes within the striatum, mainly in the nucleus accumbens (NAc). The aim of the present study was to determine the origin of NO in the NAc of anaesthetized rats by applying various compounds known to modulate the release of NO when applied either systemically or locally.

MAIN METHODS

Real-time monitoring of NO was carried out by introducing an amperometric NO sensor into the outer tubing of a push-pull cannula. For local application of substances, the push-pull superfusion technique was used.

KEY FINDINGS

An overdose of urethane (i.p.) or superfusion of the NAc with tetrodotoxin (TTX) led to a fall of NO release in the NAc. The NO synthase (NOS) inhibitors 7-nitroindazolmonosodiumsalt (7-NINA, neuronal NOS selective) and N-nitro-L-arginine (L-NNA, NOS selective) decreased release of NO when applied i.p. or locally. Superfusion of the NAc with N-methyl-D-aspartate (NMDA) elicited a dose dependent increase of NO release.

SIGNIFICANCE

Combination of an amperometric NO sensor for real-time monitoring of NO release with the push-pull superfusion technique showed that NO released in the NAc is, at least to a great extent, of neuronal origin. The enhanced release of NO elicited by locally applied NMDA demonstrates that activation of NMDA receptors facilitates NO synthesis, thus underlining the functionality of NO targets within the NAc.

Evidence for the presence of histamine uptake into the synaptosomes of rat brain

Histamine has many physiological roles in the brain and periphery. Neuronal histamine is metabolized almost exclusively by histamine N-methyltransferase. Although several neurotransmitter systems such as dopamine and 5-hydroxytryptamine have their specific reuptake system in their neurons and glial cells, a specific histamine reuptake system into the corresponding nerve terminals or glial cells has not yet been clearly elucidated. We characterized the uptake of histamine into the P2 fractions of rat brain homogenized in 0.32 mol/l sucrose using in vitro uptake techniques. [3H]histamine uptake increased with the increment of added protein amount and elapsed time. [3H]histamine uptake was also temperature-dependent. The uptake of [3H]histamine into the P2 fractions occurs by two saturable processes, a high-affinity and a low-affinity, characterized by K(m) values of 0.16 and 1.2 micromol/l, respectively. Na(+), Cl(-) and HCO(3)(-) ions were essential for the uptake of histamine in P2 fractions. [3H]histamine uptake was inhibited in the presence of several tricyclic antidepressants. In accordance with this, the endogenous release of histamine from brain slices evoked by 100 mmol/l K(+) was augmented in the presence of 20 micromol/l imipramine. These results further support the existence of a specific histamine uptake system in the brain, although the precise molecular entities have not been identified until now.

Enhancing effect of zinc on astroglial and cerebral endothelial histamine uptake

We have studied the effect of zinc ion on the uptake of histamine (HA) into cultured astroglial and cerebral endothelial cells and established that Zn(2+) enhances the uptake of the amine dose-dependently and in remarkable extents by increasing the V(max) to about 3-fold (from 3.25 +/- 0.42 to 8.50 +/- 0.97 pmol/mg protein/min in astroglial cells) without altering the K(M) (0.20 +/- 0.03 microM) significantly. The stimulatory effect of zinc ion showed strong sensitivity for VUF 8407, an inhibitory compound of astroglial and cerebral endothelial uptake of HA. In the presence of 20 microM VUF 8407 the zinc-enhanced uptake was reduced by about 50% in both cell types. Binding measurements revealed increased capacities of the zinc-exposed HA binding (B(max)= 0.41 +/- 0.05 increased to 1.21 +/- 0.16 pmol/mg protein in astroglial membranes and B(max) = 0.25 +/- 0.03 enhanced to 1.05 +/- 0.12 pmol/mg protein in cerebral endothelial membranes) but statistically unchanged affinity of the ligand for HA carrier (K(D) values calculated as 35.2 +/- 3.4 nM and 45.1 +/- 3.8 nM for astroglial bindings; whereas 25 +/- 2.1 nM and 30 +/- 2.6 nM for cerebral endothelial bindings of the amine). The compound VUF 8407 reduced the B(max) of zinc-exposed HA binding of astroglial membranes but did not modify the K(D) of the zinc-exposed membrane significantly. The ex vivo experiments confirmed our in vitro findings; an i.c.v. dose of 0.4 micromol/kg ZnSO(4,) 24 hr after the injection, enhanced the uptake of [(3)H]HA into dissociated hypothalamic and cerebellar cells to about 2- and 3-fold, respectively. Present data clearly showed that zinc exposures enhance the astroglial and the cerebral endothelial uptake of HA in vitro and it might be considered that zinc produces similar effects in vivo. Free zinc may participate in the regulation of the extraneuronal HA concentration and this metal ion (endogenous or exogenous) might be favored in the removal of the amine from the interstitial space especially in conditions with relatively high HA.

Influence of mediobasal hypothalamic lesion and catecholamine receptor antagonists on ultradian rhythm of EEG in the posterior hypothalamus of the rat

The delta and theta frequency bands of the EEG in the posterior hypothalamic area (PH) of the urethane-anaesthetized rat vary according to an ultradian rhythm with a frequency of approximately one cycle per 100 min. Injected into the lateral ventricle, prazosin (150 nmol) abolished the rhythmic changes, propranolol (150 nmol) increased, while yohimbine, SKF-83566 and sulpiride (150 nmol each) decreased the cycle duration. Electrocoagulation of the rostral arcuate nucleus and median eminence (Arc-ME) of medial basal hypothalamus abolished the rhythmic EEG changes in the PH. Our results indicate that the ultradian EEG rhythm in the PH is susceptible to regulatory influences mediated by noradrenergic and dopaminergic neurons. For the generation of the ultradian rhythm, the functional integrity of the Arc-ME is required.

Release of endogenous GABA in the posterior hypothalamus of the conscious rat; effects of drugs and experimentally induced blood pressure changes

Push-pull superfusion was used to investigate the release of endogenous GABA in the posterior hypothalamus of the conscious, freely moving rat at basal conditions and in response to centrally applied drugs or to peripherally induced blood pressure changes. After an initial, exponential decline, the release rate of GABA remained fairly constant for many hours. Fluctuations in the release rate of GABA point to the existence of an ultradian rhythm with an approximate frequency of 1 cycle/65 min. Hypothalamic superfusion with a potassium-rich (50 or 90 mmol/l) artificial cerebrospinal fluid led to a concentration-dependent increase in the GABA release. The release of GABA was also enhanced by veratridine (1 or 10 mumol/l) in a concentration-dependent way. Hypothalamic superfusion with the neutrotoxin tetrodotoxin (1 mumol/l) led to a long-lasting decrease in the GABA release. The rise in blood pressure (45 mmHg) elicited by an intravenous infusion of noradrenaline was associated with an increased release rate of GABA in the hypothalamus. Hypotension produced by nitroprusside (25 mmHg) led to a counteracting decrease in hypothalamic GABA outflow. The findings suggest that approximately 45% of the basal outflow of GABA found in the superfusate are released from GABA-ergic neurons of the posterior hypothalamus. The release rate of GABA fluctuates according to an ultradian rhythm. The modified release of GABA in response to experimentally induced blood pressure changes suggests that, in the posterior hypothalamus of the conscious rat, GABAergic neurons are involved in cardiovascular control and possess a hypotensive function.

Pulsatile release of histamine in the hypothalamus of conscious rats

The pattern of histamine release was investigated in the hypothalamus of the conscious, freely moving rat over 20 h. Under anaesthesia, a guide cannula was stereotaxically inserted into the posterior hypothalamus. In the conscious animal, the stylet of the guide cannula was replaced by a push-pull cannula, and the hypothalamus was superfused with artificial cerebrospinal fluid. Histamine was determined radioenzymatically in the superfusate which was continuously collected in time periods of 20 min. The release rate of histamine fluctuated according to an ultradian rhythm (frequency: 1 cycle per 83 min) and a circadian rhythm with the highest release rate of histamine between 11:00 p.m. and 1:00 a.m. The release rate of histamine during darkness was higher than that during the light period. The results demonstrate that, in the brain, neuronal histamine is released according to rhythms with various frequencies.

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.

Patterns of histamine release in the brain

The pattern of histamine release has been investigated in various brain areas of anaesthetized cats and conscious, freely moving rats by the push-pull technique. In the hypothalamus, medial amygdaloid nucleus and mamillary body of the anaesthetized cat, histamine was found to be released according to an ultradian rhythm with a frequency of 1 cycle per 1-2 h. Additionally, oscillations have been observed in the medial amygdaloid nucleus and mamillary body with a frequency of 1 oscillation per 10 min. In the posterior hypothalamus of the conscious rat, histamine is also released rhythmically with a frequency of 1 cycle per 1.5 h. Moreover, the release rate of histamine is increased in the night.

Characterization of histamine release from the rat hypothalamus as measured by in vivo microdialysis

The release of endogenous histamine (HA) from the hypothalamus of anesthetized rats was measured by in vivo microdialysis coupled with HPLC with fluorescence detection. Freshly prepared Ringer's solution was perfused at a rate of 1 microliter/min immediately after insertion of a dialysis probe into the medial hypothalamus, and brain perfusates were collected every 30 min into microtubes containing 0.2 M perchloric acid. The basal HA output was almost constant between 30 min and 7 h after the start of perfusion, with the mean value being 7.1 pg/30 min. Thus, the extracellular HA concentration was assumed to be 7.8 nM, by a calculation from in vitro recovery through the dialysis membrane. Perfusion with a high K+ (100 mM)-containing medium increased the HA output by 170% in the presence of Ca2+. Systemic administration of either thioperamide (5 mg/kg, i.p.), a selective H3 receptor antagonist, or metoprine (10 mg/kg, i.p.), an inhibitor of HA-N-methyltransferase, caused an approximately twofold increase in the HA output 30-60 min after treatment. The combined treatment with thioperamide and metoprine produced a marked increase (650%) in the HA output. The HA output decreased by approximately 70% 4-5 h after treatment with alpha-fluoromethylhistidine (alpha-FMH; 100 mg/kg, i.p.), an inhibitor of histidine decarboxylase. Furthermore, the effect of combined treatment with thioperamide and metoprine was no longer observed in alpha-FMH-treated rats. These results suggest that both HA-N-methyltransferase and H3 autoreceptors are involved in maintaining a constant level of extracellular HA and that their blockade effectively results in a higher activity level of the endogenous histaminergic system in the CNS.

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.

Release of histamine in rat hypothalamus and corpus striatum in vivo

Histamine has remained a putative neurotransmitter for many years, partially because some of the criteria necessary to define it as a central nervous system neurotransmitter have not been established. The demonstration of in vitro release and the quantification of turnover as an indirect measure of release have been complicated by the histological evidence for multiple histamine pools in the central nervous system. In brain, there are multiple cell types which probably contain histamine. These cells include mast cells, neurolipomastocytoid cells, microvascular endothelial cells, and a histaminergic neuronal system which has been visualized using immunocytochemical methods. Using in situ brain microdialysis and a sensitive and specific radioenzymatic assay for histamine, we have identified histamine in the extracellular space of the rat hypothalamus and corpus striatum in vivo. Following neuronal selective stimuli, significant increases in extracellular histamine levels only were observed in the posterior hypothalamus, where dense histaminergic neuronal terminals have been described. However, after manipulations targeted towards histamine-containing mast cells, such increases were seen in both the posterior hypothalamus and corpus striatum. In summary, this study demonstrates that endogenous histamine can be released from the posterior hypothalamus by stimuli targeted towards histamine neurons and that histamine may also be released by non-neuronal mast cell elements.

The release of endogenous histamine in distinct brain areas is modified by electrical stimulation

In anaesthetized cats, mamillary bodies, hypothalamic areas and medial amygdaloid nuclei were bilaterally superfused through push-pull cannulae and the effects of the electrical stimulation on the release of endogenous histamine were investigated. Electrical stimulation of the mamillary body increased the release of histamine in the stimulated area, as well as in the contralateral mamillary body. Electrical stimulation of the lateral hypothalamic area enhanced the histamine release in the contralateral hypothalamic area. Stimulation of the posterior hypothalamic area led to a delayed increase in the histamine release in the stimulated area. Stimulation of the medial amygdaloid nucleus reduced the release if histamine in the ipsilateral posterior hypothalamic area, while the histamine release in the contralateral lateral hypothalamic area was enhanced. The results demonstrate that electrical stimulation of distinct brain areas rich in histaminergic neurons may either increase, or decrease the release rate of histamine in the stimulated area and/or in remote brain areas.

Diurnal fluctuation in levels of histamine metabolites in cerebrospinal fluid of rhesus monkey

In samples of ventricular cerebrospinal fluid (CSF) that were collected from a conscious, restrained rhesus monkey at intervals of 30 90 min, levels of the histamine metabolites, tele-methylhistamine (t-MH) and tele-methylimidazoleacetic acid (t-MIAA), were determined by gas chromatography-mass spectrometry. Levels of t-MH and t-MIAA each showed time-related fluctuations. Peak and trough concentrations of t-MIAA, the product of t-MH, paralleled, but lagged about 2 h behind, the levels of t-MH. Within the first 3 h of illumination, metabolite levels increased more than 3-fold; they fell sharply within the first 3 h of darkness. Mean levels of t-MH and t-MIAA were significantly higher during periods of illumination than of darkness. Fluctuations in the levels of pros-methylimidazoleacetic acid (p-MIAA), an endogenous isomer of t-MIAA that is not a histamine metabolite, were markedly different from those of t-MH or t-MIAA; p-MIAA levels peaked only at the middle of the dark period. The time-related fluctuations in levels of t-MH and t-MIAA, but not p-MIAA, are similar to the daily rhythmic changes observed in monkey CSF for the levels of other central neurotransmitters and peptide neurohormones.

Determination of the endogenous and evoked release of catecholamines from the hypothalamus and caudate nucleus of the conscious and unrestrained rat

The action of catecholamines within the CNS is important for the expression of numerous vegetative and behavioral functions. To understand the role these amines play, it is necessary to measure changes in the levels of these transmitter substances by utilizing new developments and methodology in the behaving animal. Utilizing new developments in methodology, it is possible to measure the release of amines into perfusates obtained from specific sites in the brain of the rat under basal and evoked conditions without prior purification or concentration. Using the push-pull perfusion technique, perfusates were obtained from the hypothalamus and caudate nucleus and analyzed by liquid chromatography with electrochemical detection. It is possible to readily determine basal release of dopamine from the caudate nucleus. Detection of both dopamine and noradrenaline is possible under ephedrine stimulated conditions from both the caudate nucleus and the hypothalamus. Although levels of serotonin (5-HT) were detected in brain perfusates, it may not be of neuronal origin. It may be possible to use these techniques to delineate the roles these amines play in various physiological functions.

Effects of short-lasting and long-lasting blood pressure changes on the release of endogenous catecholamines in the hypothalamus of the conscious, freely moving rabbit

In anaesthetized rabbits guide cannulae were stereotaxically inserted into the anterior hypothalamic area and into the posterior hypothalamic nucleus. Additionally, catheters were inserted into the carotid artery and the jugular vein. Some days after the operation push-pull cannulae were inserted through the guide cannulae into the hypothalamic regions of the conscious, freely moving animal. The areas were superfused with artificial CSF and the release of the endogenous catecholamines dopamine, noradrenaline and adrenaline was determined in the superfusates. In the two hypothalamic regions the resting release of the catecholamines rhythmically varied with time at approximately 70 min cycles. A rise in blood pressure induced by the intravenous injection of noradrenaline or tramazoline enhanced the rates of release of the catecholamines in the anterior hypothalamic area. A fall of blood pressure elicited by the intravenous injection of nitroprusside or chlorisondamine decreased the release of the catecholamines in this hypothalamic area. The changes in blood pressure coincided with the changes in the rates of release of the catecholamines in the anterior hypothalamic area. These and previous results concerning changes in the release of endogenous catecholamines in the posterior hypothalamic nucleus also indicate that in the conscious, freely moving rabbit changes in blood pressure lead to counteracting alterations in the release of catecholamines in the two hypothalamic regions.