The experiments described here show that radiolabclIed formaldemethonc is formed from the radiolabelled methyl group of S-adenosyl-L-methionine in the course of the enzymic conversion of histamine to W-methylhistamine in the presence of dimedone suggesting that the formation of HCHO is probably linked to the enzymic trdnsmethylation of histamine. Histamine S-~denosyi-L-methioni~ Formaide~yde Enzymie transmethyiation Hotline-N-methy~transferase TLC

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)

Labelled histamine was taken up into cultured glial cells of chick embryonic brain by a system with high affinity for histamine and diffusion. The active uptake, occurring at low concentrations of the amine, was Na+ dependent and gave an apparent Km of 0.24 microM and a Vmax of 0.31 pmol x mg protein-1 x min-1. The uptake was completely blocked by desmethylimipramine (Ki = 2.5 microM) and partially by the histamine agonists and histamine-N-methyltransferase blockers 4-methylhistamine and 2-methylhistamine (I30 values obtained were 2 microM and 5 microM). Other psychoactive drugs were either ineffective (imipramine) or they showed moderate inhibitory effects (amitriptyline and cocaine). Ouabain (100 microM) inhibited uptake by approximately 50%. Diffusion occurred at high concentrations of the amine, was insensitive to extracellular Na+, and was proportional to histamine concentration up to 1 mM. [3H]-Histamine, taken up into the cells, was metabolized and/or released. The spontaneous efflux of the radioactivity measured after 10 min of exposure to [3H]-histamine (when most of it was still unmetabolized), was moderately Ca++ dependent, accelerated by both reduced concentrations of extracellular Na+ and enhanced concentrations of K+ and inhibited by desmethylimipramine. After prolonged (60 min) incubation, histamine metabolites detected in the cells presented 78% of the chromatogram radioactivity and consisted of N tau-methylhistamine and N tau-methylimidazole acetic acid. These results indicate that at low nM concentrations, histamine is taken up and metabolized by (and released from) glial cells by an Na(+)-dependent system, and the intracellular metabolism seems to serve an increased uptake of the amine.

The ability of glial cells to take up histamine in vitro suggests that these cells may be involved in histamine inactivation. This prompted us to study the possible interactions between neuronal and glial processes which determine the histamine concentration in the synaptic cleft. In vitro experiments showed that the glial metabolic toxin, fluoroacetate (20 and 40 mmol/l) depressed histamine uptake into cultured astroglial cells and dissociated hypothalamic cells of rats. For in vivo experiments, the push-pull superfusion technique was used. In anaesthetized rat, the anterior hypothalamic area was superfused through the push-pull cannula with artificial cerebrospinal fluid (aCSF) or with aCSF which contained fluoroacetate and the release of endogenous histamine was determined in the superfusate. Hypothalamic superfusion with fluoroacetate (20 mmol/l) led to a pronounced increase in extracellular histamine. The effect of fluoroacetate was inhibited by 5 micromol/l tetrodotoxin. Superfusion with Ca++-free, Mg++-rich (12 mmol/l) aCSF inhibited the basal release rate of histamine. Under these conditions, 20 mmol/l fluoroacetate did not modify the level of the amine in the superfusate. These data demonstrate that depression of glial function enhances the concentration of histamine in the extracellular space by slowing down the uptake of the amine into the glial cells. Thus, under in vivo conditions, glial cells are directly involved in the continuous removal of neuronal histamine from the synaptic cleft.

Potassium chloride induced a dose-dependent release of histamine from rat peritoneal mast cells at concentrations from 5 to 150 mM in the absence of extracellular Ca2+. Potassium concentrations greater than 150 mM produced less histamine release. The release was energy-dependent and was complete within one minute. The histamine liberating effect of KCl could be inhibited by NaCl and by preincubation with ketotifen. The monovalent cations, Rb+ and Cs+ also evoked histamine release, whereas Na+ and Li+ were ineffective.

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IntroductionThe main function of various transport systems is to maintain the cellular concentrations of the physiological substances. However, transport also contributes to various mechanisms in clearing and scavenging substances from the interstitial spaces. In the CNS, the termination of many neurotransmitters involves rapid removal from the synapse; mainly by uptake either into presynaptic terminal or into the surrounding glia through specialised transport systems.Histamine (HA) is a regulator of the vascular permeability through cerebrovascular endothelium as well as being a central neurotransmitter. However, no neuronal uptake of HA has been detected in vertebrates [1]. In invertebrates, HA is taken up into the photoreceptor terminals and surrounding glia and replenishes HA stores by an activitydependent fashion [2]. In mammalian neurones, a polyspecific cation transporter (hOCT 2 ) can transport HA along with monoamines [3] but as it shows a low affinity for HA (K M in mM range), it is not assumed to play a considerable role in HA inactivation.Besides inactivation, the possibility of a clearance mechanism in which glial and cerebral endothelial HA uptake are co-operatively involved, has also been considered. We have focused our research on HA uptake by CNS-resident nonneuronal cells (including mast cells) bearing physiological significance. Histamine uptake by astroglial cellsAstrocytes perform a variety of CNS functions, including uptake of a number of neurotransmitters. In 1985, we showed that HA is taken up and slowly metabolised by chick astroglial cells [4]. In chick astroglial cells (grown as monolayer culture), uptake was found to be saturable, with high affinity for HA and relatively low capacity (K M = 0.24 mM; V max = 0.31 pmol/mg protein/min), showing energy-requirements and strong sensitivity to extracellular Na + concentration. The uptake vs [Na + ] curve was saturable, fitted the Michaelis-Menten equation and resulted in a K M of 172 mM for Na + . HA taken up into these cells was slowly metabolised and/or released ('reversed uptake'), dependent on the shift of the Na + /K + gradient [5].Bidirectonal uptake could also be detected in astroglial cultures from postnatal (P 0 -P 6 ) rat brains with similar affinity for HA (K M = 0.19 mM) but with 10 times higher capacity (V max = 3.12 pmol/mg protein/min) [6]. It was noteworthy that partial dissipations of the inward oriented Na + (in < out) and the outward oriented K + (in > out) gradient could reverse the inward-directed HA uptake resulting in a carrier-mediated release of HA. The release of HA was increased by high [K + ] and K + channel blockers which might depolarise the cells, suggesting an electrogenic transporter for the uptake. The HA analogues, 2-meHA and 4-me-HA showed significant inhibition while well-known uptake inhibitors (imipramine and cocaine) did not show much effect on the uptake [6]. Impromidine and the impromidine derivative, VUF 8407 [9] showed potent inhibitory effects on HA uptake and antagonistic actions on the H 3 auto-r...

Repeated administration of histamine H1 and H2 receptor antagonists resulted in significant alterations in the hypothalamic histamine level of rats and similar changes in the HD activity of the hypothalami were observed after these treatment. The changes in the level run parallel with the modifications in the HD activity. The effects of classical antihistaminergic agents were however opposite to that of the specific H2 antagonists, namely chloropyramine and tripelennamine markedly enhanced while metiamide and cimetidine diminished the hypothalamic level of histamine.