Potassium-induced release of tritiated histamine from rat brain tissue slices

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.

Metabolism of the dorsal cochlear nucleus in rat brain slices

In vitro brain slices of the cochlear nucleus have been used for electrophysiological and pharmacological studies. More information is needed about the extent to which the slice resembles in vivo tissue, since this affects the interpretation of results obtained from slices. In this study, some chemical parameters of the dorsal cochlear nucleus (DCN) in rat brain slices were measured and compared to the in vivo state. The activities of malate dehydrogenase and lactate dehydrogenase were reduced in some DCN layers of incubated slices compared to in vivo brain tissue. The activities of choline acetyltransferase and acetylcholinesterase were increased or unchanged in DCN layers of slices. Adenosine triphosphate (ATP) concentrations for in vivo rat DCN were similar to those of cerebellar cortex. Compared with in vivo values, ATP concentrations were decreased in the DCN of brain slices, especially in the deep layer. Vibratome-cut slices had lower ATP levels than chopper-cut slices. Compared with the in vivo data, there were large losses of aspartate, glutamate, glutamine, gamma-aminobutyrate and taurine from incubated slices. These amino acid changes within the slices correlated with the patterns of release from the slices.

[3H]histamine uptake and release by astrocytes from rat brain: effects of sodium deprivation, high potassium, and potassium channel blockers

Histamine transport has been characterized in cultured astroglial cells of rat brain. The kinetics of [3H]-histamine uptake yielded a Km of 0.19 +/- 0.03 microM and a Vmax of 3.12 +/- 0.75 pmol X mg protein-1 X min-1. Transport system revealed high affinity for histamine and an approximately ten times higher capacity than that shown in cultured glial cells of chick embryonic brain. Ouabain which interferes with utilization of ATP to generate ion gradients, and the replacement of Na+ with choline inhibited the initial rate of uptake showing a strong Na(+)-dependency and suggesting the presence of a tightly coupled sodium/histamine symporter. Dissipation of K(+)-gradient (in > out) by high K+ or by K(+)-channel blockers, BaCl2, (100 microM), quinine (100 microM) or Sparteine (20 microM) produced also remarkable inhibitions in the uptake of [3H]-histamine. Impromidine, a structural histamine-analogue could inhibit the uptake non-competitively in a range of concentrations of 1 to 10 microM with a Ki value of 2.8 microM, indicating the specificity of the uptake. [3H]histamine uptake measurements carried out by using a suspension of dissociated hypothalamic cells, of rat brain showed a strong gliotoxin-sensitivity and yielded a Km of 0.33 +/- 0.08 microM; and a Vmax of 2.65 +/- 0.35 pmoles x mg protein-1 x min-1. The uptake could be reversed by incubating the cells in histamine-free Krebs medium. The [3H]histamine efflux was sensitive to Na+ omission, ouabain treatment and high K+ or K+ channel blockers, resulting in marked elevations in the efflux.(ABSTRACT TRUNCATED AT 250 WORDS)

Serotonin release from rat brain mast cells in vitro

Mast cells are primarily localized in connective tissues, where they secrete numerous mediators. They have also been identified in the mammalian central nervous system on the basis of their histochemical and morphological properties, but their role there remains unknown. A perfusion system was used to investigate in vitro mediator release from rat brain mast cells. Compound 48/80, the classic mast cell secretagogue of connective tissue mast cells, induced dose-dependent and non-cytotoxic release of serotonin, histamine and beta-hexosaminidase from mast cells in the rat thalamus and hypothalamus, but not in the cerebellum which was used as a negative control. Detailed studies were performed on thalamic mast cells, which were identified on the basis of metachromasia with Toluidine Blue and Safranin-positive staining with the Alcian Blue/Safranin technique. Their secretion was characterized by: (a) parallel release of serotonin, histamine and beta-hexosaminidase; (b) lack of dependence on extracellular calcium; (c) susceptibility to inhibition by disodium cromoglycate; and (d) lack of lactate dehydrogenase release. These results indicate that the morphology and secretory characteristics of thalamic mast cells resemble those of connective tissue mast cells. The ability of brain mast cells to secrete their mediators is discussed in the context of their possible involvement in brain pathophysiology.

Autoinhibition of histamine synthesis mediated by presynaptic H3-receptors

The regulation of histamine synthesis was studied on rat brain slices or synaptosomes labeled with L-[3H]histidine. Depolarization by increased extracellular K+ concentration enhanced by about twofold the [3H]histamine formation in slices of cerebral cortex. This stimulation was also observed, although to a lesser extent, in synaptosomes from cerebral cortex and slices from the posterior hypothalamus where most histaminergic cell-bodies are located, suggesting that it may occur in nerve endings as well as in perikarya. In the presence of exogenous histamine in increasing concentrations the K+-induced stimulation was progressively reduced by up to 60-70%. The effect of exogenous histamine appears to be receptor-mediated as shown by its saturable character, high pharmacological specificity and competitive reversal by histamine antagonists. The EC50 value of histamine for synthesis reduction (0.34 +/- 0.03 microM) was similar to its EC50 value for release inhibition known to be mediated by H3-receptors. In addition, whereas mepyramine and tiotidine, two potent antagonists at H1- and H2-receptors, respectively, were poorly effective, the H3-receptor antagonists burimamide and impromidine reversed the histamine effect in an apparently competitive manner. These effects were observed in slices of cerebral cortex or posterior hypothalamus as well as in cortical synaptosomes. Furthermore, even in the absence of added histamine, H3-receptor antagonists enhanced the depolarization-induced stimulation of [3H]histamine synthesis, indicating a participation of released endogenous histamine in the synthesis control process. The potencies of H3-receptor antagonists were similar to those of these agents at presynaptic autoreceptors controlling [3H]histamine release. It is concluded that H3-receptors control not only release but also synthesis of histamine at the level of nerve endings and also, presumably, of perikarya. A relationship between the two regulatory processes, possibly via intracellular calcium, seems likely but remains to be investigated at the molecular level.

Growth hormone neurosecretory dysfunction

The basis for understanding clinical disorders in the neuroregulation of GH secretion is derived from the complexity of the CNS-hypothalamic-pituitary axis. Studies in animals and humans demonstrate an anatomic, physiological and pharmacological evidence for neurosecretory control over GH secretion including neurohormones (GRH, somatostatin), neurotransmitters (dopaminergic, adrenergic, cholinergic, serotonergic, histaminergic, GABAergic), and neuropeptides (gut hormones, opioids, CRH, TRH, etc). The observation of a defect in the neuroregulatory control of GH secretion in CNS-irradiated humans and animals led to the hypothesis of a disorder in neurosecretion, GHND, as a cause for short stature. We speculate that in this heterogeneous group of children a disruption in the neurotransmitter-neurohormonal functional pathway could modify secretion ultimately expressed as poor growth velocity and short stature.

Histaminergic axons in the neostriatum and cerebral cortex of the rat: a correlated light and electron microscopic immunocytochemical study using histidine decarboxylase as a marker

Histaminergic nerve fibers and their axonal varicosities in the neostriatum and cerebral cortex were light and electronmicroscopically examined by means of peroxidase-antiperoxidase immunocytochemistry with histidine decarboxylase (HDC) as a marker. A majority of HDC-like immunoreactive axonal varicosities observed in serial thin sections for electron microscopy exhibited no synaptic contacts in either the neostriatum or cerebral cortex. The remaining small proportion of immunoreactive axonal varicosities formed synaptic contacts with non-immunoreactive dendritic shafts and spines.

Autoregulation of histamine release in brain by presynaptic H3-receptors

Regulation of histamine release was studied mainly on brain slices prelabeled with L-[3H]-histidine and depolarized by increased extracellular K+ concentration or veratridine in a non-superfused system. The released 3H-labeled amines, isolated by ion-exchange chromatography from a large excess of 3H-labeled precursor consisted by more than 95% of unchanged [3H]histamine. Exogenous histamine reduced the release of neosynthesized [3H]histamine via stimulation of previously characterized H3-receptors whereas it did not modify the 3H-labeled amine release from slices prelabeled with preformed [3H]histamine. The maximal inhibitory effect of exogenous histamine progressively diminished as the strength of the depolarizing stimulus or the external Ca2+ concentration were elevated. On the contrary H3-receptor antagonists like impromidine or burimamide enhanced the depolarization-induced release of [3H]histamine, an effect which was particularly marked when slices were loaded with histamine by preincubation with [3H]histidine in high concentration. These results suggest that the inhibition of [3H]histamine release by exogenous histamine acting via H3-receptor stimulation is mediated by a restricted access of Ca2+ and that its extent is influenced by the degree of autostimulation by endogenous histamine as well as, possibly, by actual internal Ca2+ concentration. In addition the decrease in external Ca2+ concentration shifted rightwards the concentration-response curve to histamine. The autoinhibitory effect of exogenous histamine was found on slices from various regions, known from lesion studies to contain terminals of extrinsic histaminergic neurons. It did not apparently involve interneurones, not being prevented in slices in which the traffic of action potentials was blocked by tetrodotoxin. It also remained unaffected in striatal slices in which the neuronal cell-bodies were selectively destroyed by prior local infusion of kainic acid. Finally exogenous histamine inhibited [3H]histamine release from depolarized synaptosomes of rat cerebral cortex, with an EC50 value similar to that found with slices and was antagonised by impromidine with an apparent Ki value similar to that displayed at H3-receptors. It is concluded that histamine modulates its own release from cerebral neurones by interacting with H3-presynaptic autoreceptors and via mechanisms similar to those previously evidenced on other aminergic systems.

Rat brain mast cells: contribution to brain histamine levels

Recent studies have shown that mast cells (MCs) are present in rat brain, that they have a predominantly thalamic localization, and that they contain histamine (HA). However, the degree to which these cells contribute to brain HA levels has remained unclear. Our recent studies of the precise distribution of rat brain MCs permitted us to develop a method to determine both the MC numbers and HA content from the same brain. Thalamic MC numbers were highly correlated with both the amount (ng) and the concentration (ng/g) of thalamic HA in both sexes (p less than 0.005). Slopes of these regression lines, suggestive of the HA content of thalamic MCs, were 2.5 and 1.3 pg/cell in males and females, respectively, substantially less than the HA levels in peritoneal MCs. Thalamic MC numbers were not correlated with HA (ng) outside of thalamus, but were significantly (p less than 0.005) correlated with whole brain HA amounts (ng) and levels (ng/g). These results are direct biochemical evidence for a contribution by MCs to brain HA levels, and indicate that thalamic MCs contribute up to 90% of the HA in thalamus, and up to 50% of whole brain HA levels.

Fine structure of histaminergic neurons in the caudal magnocellular nucleus of the rat as demonstrated by immunocytochemistry using histidine decarboxylase as a marker

The morphology of histamine-containing neurons in the caudal magnocellular nucleus was light and electron microscopically examined by means of peroxidase-antiperoxidase (PAP) immunocytochemistry with histidine decarboxylase (HDC) as a marker. HDC-like immunoreactive (HDCI) neurons had large (25-30 microns in diameter) perikarya from which two to four primary dendrites arose. The perikarya had a nearly round nucleus and well-developed Golgi apparatus in addition to a large number of mitochondria and rough endoplasmic reticulum. Immunoreactive endproducts were found diffusely throughout the perikarya, dendrites, and axons. HDCI neurons made synaptic contact with nonreactive axon terminals on the perikarya and dendrites. In addition, the HDCI neurons very frequently formed puncta adherentia with neuronal elements, either HDCI or nonreactive, or glial cells. Most of the HDCI axon terminals serially observed under electron microscopy did not exhibit typical synaptic contact in the caudal magnocellular nucleus. These findings suggest the nonsynaptic release of histamine in the caudal magnocellular nucleus.

Histamine and the hypothalamus

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

Auto-inhibition of brain histamine release mediated by a novel class (H3) of histamine receptor

Although histaminergic neurones have not yet been histochemically visualized, there is little doubt that histamine (HA) has a neurotransmitter role in the invertebrate and mammalian central nervous system. For example, a combination of biochemical, electrophysiological and lesion studies in rats have shown that histamine is synthesized in and released from a discrete set of neurones ascending through the lateral hypothalamic area and widely projecting in the telencephalon. Histamine acts on target cells in mammalian brain via stimulation of two classes of receptor (H1 and H2) previously characterized in peripheral organs and probably uses Ca2+ and cyclic AMP, respectively, as second messengers. It is well established that several neurotransmitters affect neuronal activity in the central nervous system through stimulation not only of postsynaptic receptors, but also of receptors located presynaptically which often display distinct pharmacological specificity and by which they may control their own release. Such 'autoreceptors' have been demonstrated (or postulated) in the case of noradrenaline, dopamine, serotonin, acetylcholine and gamma-aminobutyric acid (GABA) neurones but have never been demonstrated for histamine. We show here that histamine inhibits its own release from depolarized slices of rat cerebral cortex, an action apparently mediated by a class of receptor (H3) pharmacologically distinct from those previously characterized, that is, the H1 and H2 receptors.

Effects of pargyline on tele-methylhistamine and histamine in rat brain

Rapid and complete inhibition of brain MAO produced linear increases in brain t-MH levels from 30 min to 4 hr after drug treatment at a rate of 0.26 nmole/g X hr, resulting in a 3-fold increase which persisted for at least 12 hr. HA levels were slightly elevated 1 and 2 hr after drug administration but quickly returned to control levels, suggestive of sensitive regulatory mechanisms in brain. Although the slight change in HA levels precludes steady-state assumptions, the rate of increase in brain t-MH levels after MAO inhibition provides a novel estimate of the half-life of endogenous brain HA (50 min). Despite the transient effect of pargyline on brain HA content, the effect of pargyline on brain t-MH levels suggests that MAO inhibitors may produce long-term alterations in brain histaminergic dynamics.

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

The hypothalamus of anaesthetized cats and conscious, freely moving rabbits was superfused with CSF through double-walled, push-pull cannulae and the release of endogenous histamine was determined in the superfusates by a radioenzymatic assay. In the posterior hypothalamic area of the anaesthetized cat, the rate of release of endogenous histamine varied rhythmically; phases of high rate of release appeared at 60 min cycles. The release of histamine was increased by electrical stimulation of the superfused area, as well as by hypothalamic superfusion with potassium-rich CSF. In the conscious rabbit, the anterior hypothalamic area and the posterior hypothalamic nucleus were superfused simultaneously. In both regions, the resting release of histamine varied rhythmically at approximately 70 min cycles. Phases of high or low-rate of release in the anterior hypothalamic area coincided with the corresponding phases in the posterior hypothalamic nucleus. The rhythmic release of endogenous histamine in the hypothalamus, as well as the ability of depolarizing stimuli to enhance the release of the amine support the idea that histamine acts as a neurotransmitter in the central nervous system.

Measurement of tele-methylhistamine and histamine in human cerebrospinal fluid, urine, and plasma

A gas chromatographic-mass spectrometric method described by us to measure tele-methylhistamine (t-MH) in brain was used to measure t-MH in human cerebrospinal fluid (CSF), urine and plasma. The presence of t-MH in these body fluids was rigorously established. No pros-methyl-histamine could be detected, and it was used as internal standard to quantify t-MH in the fluids. The mean levels of t-MH were: urine, 943 pmol/mg creatinine; plasma, 12.3 pmol/ml; and CSF, 2.2 pmol/ml. Parallel measurements of histamine by a radioenzymatic method showed, respectively, 182 pmol/mg creatinine; 19.5 pmol/ml; and 388 pmol/ml. The levels of HA in CSF, much higher than those of its metabolite, t-MH, are high enough to stimulate HA receptors in the central nervous system.

Histamine and some of its metabolites in human body fluids

The concentrations of histamine, t-methylhistamine and t-methylimidazoleacetic acid were measured in human cerebrospinal fluid, plasma and urine, Especially noteworthy are the levels of histamine in cerebrospinal fluid which are far higher than those of t-methylhistamine and of t-methylimidazoleacetic acid, and high enough to stimulate histamine receptors in the central nervous system. It is suggested that mast cells, which surround the subarachnoid space, may contribute histamine to the cerebrospinal fluid and may offer a target for drugs and for immunologic actions. The t-methylhistamine and t-methylimidazoleacetic acid levels in cerebrospinal fluid may reflect central histaminergic activity, although a source of these metabolites in addition to histamine needs to be considered.

Electrically induced release of radiolabeled histamine from rat hippocampal slices: opposing roles for H1- and H2-receptors

Rat hippocampal slices were preloaded with 3H-histamine and superfused with physiological medium and electrically stimulated in the absence (S1) and in the presence (S2) of drugs. The electrically evoked 3H-overflow consisted mainly of histamine, was Ca++ dependent and completely blocked by tetrodotoxin, all pointing towards an impulse triggered neuronal release. Mepyramine, promethazine and diphenhydramine the H1-antagonists, inhibited the stimulation evoked histamine release in a dose dependent manner. Burimamide and cimetidine, the H2-antagonists, enhanced the stimulation induced release of histamine whereas dimaprit, the H2-antagonist, had the opposite effect. Histamine by itself did not influence its own release. The observations indicate an opposing role for H1- and H2-receptors in modulating spike induced histamine release and represents a functional consequence of the stimulation of the receptors.

Release of vasoactive intestinal peptide from rat brain slices by various depolarizing agents

The release of vasoactive intestinal peptide (VIP) from rat brain cortical and amygdala slices was studied by using various depolarizing agents such as potassium (K+), veratridine (VER) and batrachotoxin (BTX). The basal release of VIP observed is of the same order of magnitude for both structures and represents less than 0.1% of the tissue content per minute measured by a specific radioimmunoassay. Maximal stimulation obtained with 56 mMK+, 50 microM VER and 1 microM BTX corresponds to a mean 3-fold increase above the basal release of VIP in both cortex and amygdala. When the incubation medium did not contain any calcium, the action of potassium on the release of VIP was suppressed. When tetrodotoxin (1 microM) was added to the incubation medium, the veratridine- and batrachotoxin-induced release of VIP was inhibited whereas K+-induced release was unaffected. These results support the hypothesis that VIP can be a neurotransmitter in the central nervous system.

Evidence for neuronal localization of histamine-N-methyltransferase in rat brain

There is evidence that histamine may be a neurotransmitter in mammalian brain. Histamine in neurons of the central nervous system is easily released and rapidly turned over. The cellular localization of histamine-N-methyltransferase, the proposed histamine-inactivating enzyme, was investigated by measuring its activity in rat striatum after applying neurochemical or electrolytic lesions. The results indicate a major neuronal localization of the enzyme in this area.

Tetrahydronorharmane modulates the depolarisation-induced efflux of 5-hydroxytryptamine and dopamine and is released by high potassium concentration from rat brain slices

Tetrahydronorharmane (THN), a compound which occurs in man and in rats is supposed to act as a modulator of 5-hydroxytryptaminergic neurones. It is reported here that THN stimulated the potassium-induced release of the neurotransmitter from brain slices. This suggests enhancement of the efflux as a mechanism for the 5-hydroxytryptamine-like action of THN in pharmacological experiments. Furthermore, THN decreased the potassium-evoked efflux of (3H)-dopamine. This is consistent with the inhibition of dopaminergic neurones by THN as seen previously in various in vivo experiments. Finally it is demonstrated that a high concentration of potassium evokes the efflux of (14C)-THN from brain slices. This finding suggests that THN is taken up into neurones and exerts its modulating action on serotonergic neurones either inside or outside the cell.

Ethanol-induced alterations in histamine content and release in the rat hypothalamus

In the rat hypothalamus, histamine content and histidine decarboxylas: activity are enhanced significantly after acute administration (80--160 mg/100g body weight) of ethanol. The effects are less pronounced after chronic treatment (15% v/v in drinking water for 4 weeks). Histamine methyltransferase is unaffected in either case. In hypothalamic slices preloaded with 3H-histamine and superfused with amine free solution the basal and K+-induced efflux of 3H-histamine are inhibited by alcohol. The inhibition of histamine release along with the increased levels of histamine may play an important role in the central effects of alcohol.

Influence of enkephalin on K+-evoked efflux of putative neurotransmitters in rat brain. Selective inhibition of acetylcholine and dopamine release

In rat brain slices preincubated with various radiolabelled putative neurotransmitters, methionine-enkephalin diminished the potassium-evoked release of dopamine and acetylcholine. The effect was antagonised by naloxone. The potassium-induced effux of three other neurotransmitters, histamine, 5-hydroxy-tryptamine and gamma-aminobutyric acid, were unaffected by methionine-enkephalin. A probable physiological function for the endogenous ligands in specifically affecting the catecholaminergic and cholinergic transmission is suggested.

Modulation by histamine of the efflux of radiolabeled catecholamines from rat brain slices

Histamine induced a dose-dependent stimulation of 3H-catecholamine(CA) efflux (superfusion procedure) from hypothalamic, striatal, hippocampal and cortical slices. The extra-hypothalamic regions were the most sensitive to histamine. Efflux of 14C-GABA and 14C-(acetyl)choline was not affected. The effect of histamine on 3H-CA efflux developed slowly, reaching its maximum after 15-20 min. Histamine was inefffective with tissue from reserpinized animals. The major part of the radioactivity released by histamine consisted of CA metabolites. Histamine apparently does not enter catecholaminergic neurons, since tyramine and the CA had no effect on the efflux of 3H-histamine previously taken up by brain slices. After incubation of slices with 3H-CA in the presence of histamine and subsequent superfusion, tyramine or K+ -depolarization induced much less 3H-CA release than from control slices not incubated with histamine. It is suggested that histamine may act as a modulator of presynaptic catecholaminergic processes in the central nervous system by causing a depletion of the transmitter stores in the nerve terminals.