P2Y1 and Cysteinyl Leukotriene Receptors Mediate Purine and Cysteinyl Leukotriene Co-Release in Primary Cultures of Rat Microglia

Inflammation is widely recognized as contributing to the pathology of acute and chronic neurodegenerative conditions. Microglial cells are pathologic sensors in the brain and activated microglia have been viewed as detrimental. Leukotriene, including cysteinyl leukotrienes (CysLTs) are suggested to be involved in brain inflammation and neurological diseases and ATP, by its receptors is a candidate for microglia activation. A23187 (10μM) stimulated microglia to co-release CysLTs and [3H]adenine based purines ([3H]ABPs), mainly ATP. The biosynthetic production of CysLTs was abolished by 10μM MK-886, an inhibitor of 5-lipoxygenase-activating protein activity. RT-PCR analysis showed that microglia expressed both CysLT1 / CysLT2 receptors, P2Y1 ATP-receptors and several members of the ATP binding cassette (ABC) transporters including MRP1, MRP4 and Pgp. The increase in [Ca2+]i elicited by LTD4 (0.1 μM) and 2MeSATP (100μM), agonists for CysLT- and P2Y1-receptors, was abolished by the respective antagonists, BAYu9773 (0.5 μM) and suramin (50 μM). The stimulation of both receptor subtypes, induced a concomitant increase in the release of both [3H]ABPs and CysLTs that was blocked by the antagonists and significantly reduced by a cocktail of ABC transporter inhibitors, BAPTA/AM (intracellular Ca2+ chelator) and staurosporine (0.1 μM, PKC blocker). P2Y antagonist was unable to antagonise the effects of LTD4 and BAYu9773 did not reduce the effects of 2MeSATP. These data suggest that: i) the efflux of purines and cysteinyl-leukotrienes is specifically and independently controlled by the two receptor types, ii) calcium, PKC and the ABC transporter system can reasonably be considered common mechanisms underlying the release of ABPs and CysLTs from microglia. The blockade of P2Y1 or CysLT1/CysLT2 receptors by specific antagonists that abolished the raise in [Ca2+]i and drastically reduced the concomitant efflux of both compounds, as well as the effects of BAPTA and staurosporine support this hypothesis. In conclusion, the data of the present study suggest a cross talk between the purine and leukotriene systems in a possible autocrine/paracrine control of the microglia-mediated initiation and progression of an inflammatory response.

P2X7 receptor activation in rat brain cultured astrocytes increases the biosynthetic release of cysteinyl leukotrienes

Astrocytes have been recognized as important elements in controlling inflammatory as well as immune processes in the central nervous system (CNS). Recently, glial cells have been shown to produce cysteinyl leukotrienes (CysLTs) which are known lipid mediators of inflammation and whose extracellular concentrations rise under different pathological conditions in the brain. In the same conditions also extracellular concentrations of ATP dramatically increase reaching levels able to activate P2X7 ionotropic receptors for which an emerging role in neuroinflammation and neurodegeneration has been claimed. RTPCR analysis showed that primary cultures of rat brain astrocytes express P2X7 receptors. Application of the selective P2X7 agonist benzoyl benzoly ATP (BzATP) markedly increased [Ca2+]i which was mediated by a calcium influx from the extracellular milieu. The P2X7 antagonist, oATP, suppressed the BzATP-induced calcium increase. Consistent with the evidence that increased calcium levels activate the leukotriene biosynthetic pathway, challenge of astrocytes with either the calcium ionophore A23187 or BzATP significantly increased CysLT production and the cell pre-treatment with EGTA abolished these effects. Again the P2X7 antagonist prevented the BzATP-mediated CysLT efflux, whereas the astrocyte pretreatment with MK-571, a CysLT1 receptor antagonist, was ineffective. The astrocyte pre-treatment with a cocktail of inhibitors of ATP binding cassette (ABC) proteins reduced the BzATP-mediated CysLT production confirming that ABC transporters are involved in the release of CysLTs. The astrocyte P2X7- evoked rise of CysLT efflux was abolished in the presence of MK-886, an inhibitor of 5-lipoxygenase activating protein (FLAP) whose expression, along with that of 5-lipoxygenase (5-LO) was reported by Northern Blot analysis. The stimulation of P2X7 induced an up-regulation of FLAPmRNA that was reduced by the antagonist oATP. These data suggest that in rat brain cultured astrocytes P2X7ATP receptors may participate in the control of CysLT release thus further supporting a role for extracellular ATP as an integral component of the inflammatory brain response.

Reversible changes in goldfish brain polyamine concentrations and synthetic enzymes after cold exposure

Exposure of goldfish to the cold (5 degrees C) caused a sharp increase in brain putrescine level during the first week. Such increase continued at a minor rate for the whole period of exposure (2 months). In contrast, the content of spermidine and spermine remained unchanged. Putrescine increase was concomitant with a remarkable rise in ornithine decarboxylase activity (ODC), which reached a maximum stimulation after 1 week of cold exposure, and declined thereafter, remaining significantly higher than the control for the entire period of study. Cold exposure caused also a reduction of S-adenosylmethionine decarboxylase (AdoMetDC) activity and an increase of ornithine level, whereas methionine content was unchanged. When fish exposed to cold temperature were returned to 20 degrees C, the modifications observed on brain polyamine metabolism were completely reversed. Supported by previous observations, our results suggest that the changes in the polyamine metabolism induced in goldfish brain by cold exposure could represent an homeostatic mechanism carried out by the goldfish to minimize the possible effects of thermal changes.

Distribution and expression of A1 adenosine receptors, adenosine deaminase and adenosine deaminase-binding protein (CD26) in goldfish brain

The expression patterns of adenosine A(1) receptors (A(1)Rs), adenosine deaminase (ADA) and ADA binding protein (CD26) were studied in goldfish brain using mammalian monoclonal antibody against A(1)R and polyclonal antibodies against ADA and CD26. Western blot analysis revealed the presence of a band of 35 kDa for A(1)R in membrane preparations and a band of 43 kDa for ADA in both cytosol and membranes. Immunohistochemistry on goldfish brain slices showed that A(1) receptors were present in several neuronal cell bodies diffused in the telencephalon, cerebellum, optic tectum. In the rhombencephalon, large and medium sized neurons of the raphe nucleus showed a strong immunopositivity. A(1)R immunoreactivity was also present in the glial cells of the rhombencephalon and optic tectum. An analogous distribution was observed for ADA immunoreactivity. Tests for the presence of CD26 gave positive labelling in several populations of neurons in the rhombencephalon as well as in the radial glia of optic tectum, where immunostaining for ADA and A(1)R was observed. In goldfish astrocyte cultures the immunohistochemical staining of A(1)R, ADA and CD26, performed on the same cell population, displayed a complete overlapping distribution of the three antibodies. The parallel immunopositivity, at least in some discrete neuronal areas, for A(1)Rs, ADA and CD26 led us to hypothesize that a co-localization among A(1)R, ecto-ADA and CD26 also exists in the neurons of goldfish since it has been established to exist in the neurons of mammals. Moreover, we have demonstrated for the first time, that A(1)R, ecto-ADA and CD26 co-localization is present on the astroglial component of the goldfish brain. This raises the possibility that a similar situation is also shown in the glia of the mammalian brain.

The calcium-dependent [3H]acetylcholine release from synaptosomes of brown trout (Salmo trutta) optic tectum is inhibited by adenosine A1 receptors: effects of enucleation on A1 receptor density and cholinergic markers

Presynaptic inhibition is one of the major control mechanisms in the CNS. Previously we reported that A1 adenosine receptors are highly concentrated in the brain, including optic tectum, of trout and that they inhibited the release of glutamate. The optic tectum is heavily innervated by cholinergic nerve terminals. We have investigated whether A1 receptors inhibit the presynaptic release of acetylcholine and whether the inhibition is triggered by calcium. The release of [3H]ACh evoked by 30 mM KCl was Ca2+ dependent and it was dose-dependently inhibited by the A1 adenosine receptor agonist 2-chloro-N(6)-cyclopentyladenosine (CCPA) ranging between 10 nM to 100 microM. The maximum of inhibition was reached at 10 microM. The A1 receptor antagonist 8-cyclopentyltheopylline (CPT, 10 microM), reversed almost completely the inhibition induced by CCPA 10 microM. In Fura-2/AM loaded synaptosomes, K(+) depolarization raised [Ca2+](i) by about 64%. CCPA (10 microM) reduced the K(+)-evoked Ca2+ influx increase by about 48% and this effect was completely antagonised by CPT 10 microM. Synaptosome pretreatment with different Ca2+ channel blockers differently affected K(+)-evoked Ca2+ influx. This was not significantly modified by nifedipine (1 microM, L-type blocker) nor by omega-agatoxin IVA (0.3 microM, P/Q-type blocker), whereas about 50% reduction was shown by 0.5 microMomega-conotoxin GVIA (N-type blocker). Neurochemical parameters associated with cholinergic transmission and the density of A(1) adenosine receptors were measured in the trout optic tectum 12 days after unilateral eye ablation. A significant drop of both acetylcholinesterase (AChE) activity (24%) and choline acetyltransferase (CAT) activity (32%) was observed in deafferentated optic tectum, whereas the high affinity choline uptake did not parallel the decrease in enzyme activity. Eye ablation caused a marked decrease (43%) of A1 receptor density without changing the affinity. The K(+)-evoked release of [3H]ACh from synaptosomes of deafferentated was not modify as well as the efficacy of 10 microMCCPA in decreasing [3H]ACh release was not apparently modified.

Mechanisms of inhibitory effects of zinc and cadmium ions on agonist binding to adenosine A1 receptors in rat brain

The dose-dependent inhibition of zinc and cadmium ions of agonist binding to A1 adenosine receptors in rat brain is prevented by histidine and cysteine, respectively. In the present study, the possible different mechanisms of Zn2+ and Cd2+ inhibitions were examined. The effects of Zn2+ and Cd2+ on equilibrium binding parameters of the agonists N6-cyclohexyl-[2,8-3H]-adenosine ([3H]CHA) or chloro-N6-cyclopentyl-adenosine ([3H]CCPA) and the antagonist cyclopentyl-1,3-dipropylxanthine ([3H]DPCPX) were compared with those effects of reagents or binding conditions which altered histidyl or cysteinyl residues of the A1 receptor. Zn2+ pretreatment did not change A1 agonist or antagonist affinity, but did reduce the Bmax. The inhibitory effects of Zn2+ pretreatments were also maintained after several membrane washings. Diethylpyrocarbonate, a histidine-specific alkylating reagent, behaved like zinc ions: pretreatment with A1 agonist protected the histidyl residues of the [3H]CHA binding site against modification by Zn2+, while the modification of the protonation state of the nitrogen of the imidazole group of histidines by changing pH indicated that the interactions of Zn2+ with the histidyl residues were feasible with their unprotonated form. These findings suggest the formation of coordination bonds between Zn2+ and histidines critical for [3H]CHA or [3H]DPCPX binding, which may prevent the ligand interaction with the specific sites without modifying the binding kinetics of radioligand to the non-chelated recognition sites. Cd2+ pretreatment reduced the [3H]CCPA affinity, but did not modify the affinity of the antagonist [3H]DPCPX, the Bmax remaining unaffected. As with cadmium effects, the oxidation of the thiol group of cysteine by dithionitrobenzoic acid (DTNB) reduced [3H]CCPA affinity without changing the number of binding sites. The reducing reagent dithiothreitol, which alone was unable to modify [3H]CCPA binding, overcame the inhibiting effects of both Cd2+ and DTNB. These findings suggest that cadmium ions may oxidize SH groups of cysteines localized on the A1 receptor molecule or a cysteine localized in the region of G(i)alpha subunit involved in the coupling with receptors. This mechanism can justify potential conformational modifications of the receptor molecule producing the decrease in affinity.

Pharmacological characterization of adenosine A1 receptors and its functional role in brown trout (Salmo trutta) brain

The adenosine receptor agonist N(6)-cyclohexyl[(3)H]adenosine ([(3)H]CHA) was used to identify and pharmacologically characterize adenosine A1 receptors in brown trout (Salmo trutta) brain. In membranes prepared from trout whole brain, the A1 receptor agonist [(3)H]CHA bound saturably, reversibly and with high affinity (K(d)=0. 69+/-0.04 nM; B(max)=0.624+/-0.012 pmol/mg protein) to a single class of binding sites. In equilibrium competition experiments, the adenosine agonists and antagonists all displaced [(3)H]CHA from high-affinity binding sites with the rank order of potency characteristic for an adenosine A1 receptors. A1 receptor density appeared not age-related (from 3 months until 4 years), and was similar in different brain areas. The specific binding was inhibited by guanosine 5'-triphosphate (IC(50)=0.778+/-0.067 microM). GTP (5 microM) induced a low affinity state of A1 receptors. In superfused trout cerebral synaptosomes, 30 mM K(+) stimulated the release of glutamate in a calcium dependent manner. Glutamate-evoked release was dose-dependently reduced by CHA, and the inhibition was reversed by the A1 antagonist 8-cyclopentyltheophylline (CPT). In the same synaptosomal preparation, 30 mM K(+) as well as 1 mM glutamate stimulated the release of adenosine in a Ca(2+)-independent manner and tetrodotoxin insensitive. These findings show that in trout brain adenosine A1 receptors are present which are involved in the modulation of glutamate transmitter release. Moreover, the stimulation of adenosine release by K(+) depolarisation or glutamate support the hypothesis that, as in mammalian brain, a cross-talk between adenosine and glutamate systems exists also in trout brain.

Transport of doxorubicin in mast cell granules and the effect of the calcium antagonist verapamil

P-glycoprotein has been identified in mast cells stabilized in culture as well as in rat peritoneal mast cells, and is primarily concentrated on the granular membrane. This study aimed to define the role of this protein in the transport and accumulation of doxorubicin in mast cell granules and in its histamine releasing effect. The reverting agent verapamil, that is a substrate for P-glycoprotein, inhibited doxorubicin uptake in intact mast cells in a dose and time dependent manner, but had no effect on the exocytotic action of the antineoplastic drug. Doxorubicin was also concentrated in granules with intact membranes and the uptake was dependent on temperature and showed a trend for saturation. Verapamil and vinblastine, another substrate for P-glycoprotein, significantly reduced doxorubicin concentrations in intact granules. Similar results were obtained with the metabolic inhibitors sodium metavanadate, N-ethylmaleimide, and sodium azide, whereas ouabain, an inhibitor of sodium-potassium ATPase, was without effect. Doxorubicin was taken also up in granule remnants, consisting of a proteoglycan matrix without membrane, that are extruded from mast cells upon stimulation. However, the uptake was not dependent on temperature and was not modified by P-glycoprotein substrates or metabolic inhibitors. Rat peritoneal mast cells were examined for the expression of P-glycoprotein at the protein level with C219 monoclonal antibody, using Western blot, confirming that P-glycoprotein was expressed in mast cells. These data suggest the presence of a P-glycoprotein active in the transport of doxorubicin, in mast cell granules.

Increased density of M1 receptors in the hippocampus of juvenile rats chronically deprived of NGF

Binding studies were used to assess the changes in affinity and/or number of M1 muscarinic receptors in hippocampi from juvenile rats chronically deprived of NGF. NGF deprivation was obtained by implanting into right ventricle at postnatal day 2 (P2) hybrydoma cells secreting high levels of monoclonal antibodies against NGF (alphaD11). Parenteral myeloma cells (P3U) were used as controls. Competition experiments were used to characterise the [3H]-PNZ binding sites in membrane preparations of hippocampi from rats sacrificed at P15. [3H]-PNZ bound M1 receptors both in P3U and alphaD11 group as shown by displacing potency order of antagonists: TLZ=4-DAMP>PNZ>p-F-HHSiD>MTC. The deprivation of NGF for two weeks significantly increased the number of M1 receptors without changing the Ki values of antagonists with exception of methoctramine which showed an increase in affinity in alphaD11 group. Similar changes in binding parameters were already observed after the first week of anti-NGF treatment. In contrast, a treatment for a week with implant at postnatal day 15 failed to produce any changes in M1 binding parameters. These results provide further physiological evidence for developmentally regulated modulatory role of NGF in the cholinergic function in the hippocampus.

Zinc and cadmium ions differently modulates A1 adenosine receptors

The results suggest that: 1) Zinc ions may chelate the histidines critical for the agonist binding preventing hydrogen bonds between nonprotonated nitrogen atom of His-251 and the exocyclic N6-H in CHA or CCPA molecule and between His-278 and -OH of the ribose ring. This mechanism can explain the reduction in the number of binding sites without changing the affinity. 2) Cadmium ions may oxidize cysteine SH-groups. The redox reaction between Cd2+ and receptor thiols may result in binding of the metal into stable (di)thiol-cadmium complexes rather than in the formation of disulfide and liberation of the reduced metal. This mechanism can justify the conformational modifications of the receptor molecule producing the decrease in affinity.

Effects of nicergoline on rabbit electroretinogram during recovery after ischaemia in light and dark

Nicergoline is an ergot alkaloid derivative acting as a neuroprotective agent. In the present investigation, b-wave time-course recovery profiles under both light- and dark-adapted conditions, were studied in order to evaluate the possible effectiveness of nicergoline in the protection of the rabbit retina. Retinal ischaemia was induced by bilateral occlusion of common carotid artery in male rabbit of the Dutch strain. Groups of animals were subjected to 15-, 30- and 60-min periods of ischaemia under pentobarbital anaesthesia. Electroretinogram recordings were simultaneously obtained from both eyes, using, as the stimulus, the brightest flash from a stimulator positioned 15 cm in front of each eye. The treatment with nicergoline, administered immediately before the carotid occlusion, induced a significant protection only when the ischaemia seemed to cause retinal damage that the reperfusion alone was not able to recover completely. Nicergoline did not modify the recovery rate after 15-min or 30-min light-adapted and 15-min dark-adapted ischaemia; in these conditions the controls showed a full recovery. After 30-min dark-adapted ischaemia, the maximum recovery of the controls was 82%, and nicergoline significantly improved b-wave amplitude at all time points of reperfusion up to the complete recovery. Rabbit retina was irreversibly damaged by a 60-min ischaemia. In these conditions nicergoline significantly increased the percentage of b-wave recovery both in light- and dark-adapted ERG. Nicergoline, probably on the basis of its metabolic actions, seems to be effective in severe conditions of hypoxia and is more potent in dark than in light-adapted conditions. Its effectiveness in these experimental conditions could be justified by the different oxygen consumption of the photoreceptors in light and dark and the different sensitivity of cones and rods to the ischaemia.

Inhibitory and excitatory effects of adenosine antagonists on spontaneous locomotor activity in mice

The behavioral effect of the adenosine antagonists CPT, PACPX, DPCPX and PD 115,199 on spontaneous locomotor activity was investigated in mice after parenteral administration. CPT, PACPX and PD 115,199 affected locomotor activity in a biphasic way. Doses in the nanomolar/kg range significantly reduced locomotion (PACPX> or =PD 115,199>>CPT). Higher doses were progressively less active until they became ineffective or slightly stimulated locomotion. NECA, a mixed A1/A2 agonist, and CCPA, a highly selective A1 agonist, also induced a biphasic behavior, with low doses stimulating and high doses inhibiting locomotion. The stimulant effect of 1 nmol/kg NECA was antagonized by depressant doses of antagonists, whereas antagonists-induced hypomotility was potentiated by a depressant dose of NECA (20 nmol/kg). It is suggested that the blockade of A1 receptors by antagonists is probably responsible for reducing locomotor activity, whereas the activation of A2 receptors by agonists is likely responsible for reducing locomotion in mice.

An NK1 receptor-dependent component of the slow excitation recorded intracellularly from rat motoneurons following dorsal root stimulation

Intracellular recording from lumbar motoneurons of the neonatal rat spinal cord in vitro was used to study how recently developed non-peptide antagonists such as SR-140333 and SR-48698, known to block distinct subtypes of tachykinin receptors peripherally, might affect synaptic transmission elicited by electrical stimulation of dorsal root fibres. SR-140333 (1 microM) preferentially antagonized responses mediated by an exogenously applied agonist acting on the NK1 receptor subclass, while SR-48968 (0.5 microM) preferentially reduced responses mediated by an exogenously applied agonist acting on the NK2 receptor subclass. SR-48968 did not affect fast or slow excitatory postsynaptic potentials (EPSPs) or 'wind-up' responses induced by repetitive, low-frequency stimulation (mimicking certain types of nociceptive input); binding studies using this radiolabelled ligand disclosed specific binding activity (21 fmol/mg protein) selectively displaced by an NK2 receptor agonist. SR-140333 reduced the late component of fast and slow EPSPs, and of wind-up. Pharmacological block of ionotropic glutamate receptors abolished all dorsal root-evoked EPSPs. In comparison to glutamate receptor blockers, SR-140333 was a weaker antagonist of slow synaptic responses, though it displayed preferential antagonism towards some components of the wind-up phenomenon. The present results provide evidence obtained with a novel NK1 antagonist that a neuropeptide (presumably substance P), although not directly released by primary afferents onto motoneurons, is a neurotransmitter (acting via NK1 receptors) in the pathway mediating slow synaptic responses of motoneurons, and is presumably involved in signalling nociceptive inputs from the periphery.

Biochemical and pharmacological evidence for the presence of A1 but not A2a adenosine receptors in the brain of the low vertebrate teleost Carassius auratus (goldfish)

In whole brain membranes of goldfish, 3H-chlorocyclopentyladenosine bound to adenosine A1 receptors. The A1 receptors were ubiquitously distributed in the brain with a maximum in the hypothalamus and a minimum in the spinal cord. In superfused goldfish cerebellar slices, cyclohexyladenosine inhibited the cyclic AMP accumulation stimulated by forskolin and the selective adenosine A1 receptor antagonist, 8-cyclopentyltheophylline, reversed this effect. In the same brain preparation, 30 mM K+ stimulated the release of glutamate, glutamine, glycine and GABA in a Ca(2+)-dependent manner, whereas the aspartate and taurine release was Ca(2+)-independent. Cyclohexyladenosine, in a dose-dependent manner, inhibited the 30 mM K(+)-evoked release of glutamate whereas that of aspartate was unaffected. The CHA inhibition of glutamate-evoked release was reversed by 8-cyclopentyltheophylline. The adenosine A2a receptors were not detectable in whole brain membranes of goldfish either using the specific agonist 3H-CGS 21680 or 3H-5'-N-ethylcarboxamidoadenosine. The presence of A2b seems to be suggested by the NECA stimulation of cyclic AMP accumulation, which was reversed by 8-cyclopentyltheophylline. The results, taken together, indicate that adenosine has a neuromodulatory function in the nervous system of lower vertebrates which is comparable to that described in mammalian brain.

Effects of chronic administration of adenosine antagonists on adenosine A1 and A2a receptors in mouse brain

In the present study the effects of chronic in vivo treatments with caffeine and more selective antagonists (PACPX and PD115,199) on the binding parameters of 3H-CHA and 3H-NECA to cortical A1 and striatal A2a adenosine receptors of mouse brain are shown. The drugs were injected intraperitoneally once a day for 6 or 20 days. Treatment for 20 days with caffeine (250 mumol/kg), PD115,199 (50-250 nmol/kg) and PACPX (250 nmol/kg) shifted the A1 low affinity receptors into an agonist-specific high affinity state. Moreover, after 20 days of treatment, the antagonists decreased the affinity of 3H-CHA to A1 receptors in the high affinity state. Antagonist treatments for 6 days did not modify the 3H-CHA binding parameters. The A2a striatal receptors were dose- and time-dependently up-regulated by caffeine and PD115,199, whereas PACPX displayed an up-regulation independent of dose or length of treatment. Moreover, PD115,199 decreased the affinity of 3H-NECA to A2a striatal receptors. This effect on affinity was visible after 20 days of treatment with 50 and 250 nmol/kg. This study provides evidence for a sensitivity of A2a receptors greater than that of A1 receptors and for a different regulation of cortical A1 and striatal A2a adenosine receptors of mouse brain after chronic treatment with antagonists.

Strain-related differences in adenosine receptor density and in behavioral sensitivity to adenosine analogs in mice

The behavioral effects of the adenosine agonists 5'-N-ethylcarboxamidoadenosine (NECA) was investigated in two strains of inbred mice, CD1 and CBA. NECA dose dependently reduced spontaneous locomotor activity with similar potency (ED50 = 36 +/- 1.5 and 36 +/- 1.1 nmol/kg IP for CBA and CD1 mice, respectively) and efficacy (> 90% at 100 nmol/kg) in the two strains. One nmol/kg NECA, an ineffective dose in CBA mice, exerted a significant stimulant action in CD1 mice. In saturation experiments, no differences were found in the density or in the affinity of striatal A2a receptors labeled with [3H]NECA. A strain-related difference was found in the density of striatal A1 receptors labeled with [3H]CCPA. In CBA mice, the Bmax value was 32% less than in CD1 mice (0.646 +/- 0.037 and 0.951 +/- 0.073 pmol bound/mg protein, respectively, p < 0.05). No differences in [3H]CCPA binding parameters were found in cortical and hippocampal membranes obtained from the two strains, whereas a higher density of A1 binding sites was found in the cerebellum of CBA mice. The present results show a close correlation between binding studies and the depressant action of NECA and present evidence for strain-related differences in regional distribution of central adenosine receptors and in behavioral response to purinergic drugs.

Effects of divalent cations on adenosine agonist binding to A1 receptors and non-A1/non-A2 sites in rat cerebral cortex

In the present study results are reported concerning the effects of several divalent cations on the binding characteristics of [3H]-cyclohexyladenosine on A1 adenosine receptors and of [3H]-N-ethylcarboxamidoadenosine on non-A1/non-A2 sites in membranes from cerebral cortex of the rat. The [3H]-cyclohexyladenosine binding to A1 receptors was dose-dependently increased by Mn2+, Co2+, Ca2+. The binding characteristics of the agonist were differently affected by Ca2+/Mn2+ and Mg2+. Ca2+ and Mn2+ increased the Bmax value without any change in Kd, whereas Mg2+ decreased the Kd value without changing the Bmax. In the presence of Ca2+ and Mg2+ the Kd value was similar to that obtained in the presence of Mg2+, whereas the Bmax value was similar to the apparent number of binding sites calculated in the presence of Ca2+. The cations, Cu2+, Cd2+, Zn2+, decreased the A1 binding with IC50 values of 19.6 microM, 39.2 microM and 103.9 microM, respectively. The binding characteristics of [3H]-N-ethylcarboxamidoadenosine to non-A1/non-A2 sites were affected by Ca2+, Mn2+, Co2+ and Mg2+ in the opposite manner to A1 receptors. They decreased the binding with IC50 values of 20.1 mM, 22.8 mM, 93.0 mM and 18.1 mM, respectively. This occurs through an enhancement in Kd values without changes in the number of binding sites. The findings on A1 receptor and non-A1/non-A2 binding site, taken together, suggest that cations could also exert a modulatory action via specific interactions with divalent cation binding sites on the receptor molecule.

Functional adenosine A1 receptors in goldfish brain: regional distribution and inhibition of K(+)-evoked glutamate release from cerebellar slices

In goldfish brain, [3H]cyclohexyladenosine binding sites are ubiquitously distributed with a maximum in the hypothalamus and a minimum in the spinal cord. The binding parameters measured in cerebellar membranes (Kd = 0.88 +/- 0.08 nM; Bmax = 59.65 +/- 2.62 fmol/mg protein) are not significantly different from those of the whole brain. In perfused goldfish cerebellar slices, stimulation of cyclic AMP accumulation by 10(-5) M forskolin was markedly reduced (58.7%) by treatment with 10(-4) M cyclohexyladenosine, an adenosine A1 receptor agonist, and the reduction was reversed in the presence of 10(-4) M 8-cyclopentyltheophylline, a selective A1 receptor antagonist. In the same brain preparation, 30 mM K+ stimulated the release of glutamate, glutamine, glycine and GABA in a Ca(2+)-dependent manner, whereas the aspartate and taurine release was Ca(2+)-independent. Cyclohexyladenosine inhibited the 30 mM K(+)-evoked release of glutamate in a dose-related manner. This effect was reversed by 8-cyclopentyltheophylline. These results support the hypothesis that adenosine A1 receptors present in goldfish cerebellum are involved in the modulation of glutamate transmitter release.

Adenosine A1 receptor-mediated inhibition of evoked glutamate release is coupled to calcium influx decrease in goldfish brain synaptosomes

Binding of [3H]cyclohexyladenosine (CHA) to the cellular fractions and P2 subfractions of the goldfish brain was studied. The A1 receptor density was predominantly in synaptosomal membranes. In goldfish brain synaptosomes (P2), 30 mM K+ stimulated glutamate, taurine and GABA release in a Ca(2+)-dependent fashion, whereas the aspartate release was Ca(2+)-independent. Adenosine, R-phenylisopropyladenosine (R-PIA) and CHA (100 microM) inhibited K(+)-stimulated glutamate release (31%, 34% and 45%, respectively). All of these effects were reversed by the selective adenosine A1 receptor antagonist, 8-cyclopentyltheophylline (CPT). In the same synaptosomal preparation, K+ (30 mM) stimulated Ca2+ influx (46.8 +/- 6.8%) and this increase was completely abolished by pretreatment with 100 nM omega-conotoxin. Pretreatment with 100 microM R-PIA or 100 microM CHA, reduced the evoked increase of intra-synaptosomal Ca2+ concentration, respectively by 37.7 +/- 4.3% and 39.7 +/- 9.0%. A possible correlation between presynaptic A1 receptor inhibition of glutamate release and inhibition of calcium influx is discussed.

Circadian rhythm of cortical and striatal adenosine receptors

In order to investigate diurnal variations in binding parameters of A1 and A2 adenosine receptors, Kd and Bmax were calculated in mice that had been housed under controlled light-dark cycles for 4 weeks (light on from 7.00 to 19.00 h). A1 cortical receptors were labelled by N6-cyclohexyl-[3H]adenosine, and A1 and A2 striatal receptors by 5'-N-ethylcarboxamido[8-3H]adenosine. Significant differences were found for Bmax values measured at 3-h intervals across a 24-h period. Cyclic variations of the number of binding sites were shown, with a minimum number of A1 and A2 receptors during the light period and a maximum during the dark period. The amplitude for cortical A1 receptors, between 03.00 and 18.00 h, was 39%, while for striatal A1 receptors, between 03.00 and 15.00 h, was 92%. The amplitude for the A2 striatal receptors, between 03.00 and 15.00 h, was 147%. No substantial rhythm was found in the Kd values. These differences in the amplitude could suggest a different physiological modulation of the two adenosine receptor subtype moiety which could reflect a physiologically-relevant mechanism by which adenosine exerts its modulatory role in the central nervous system.

Circadian rhythm in adenosine A1 receptor of mouse cerebral cortex

In order to investigate diurnal variation in adenosine A1 receptors binding parameters, Bmax and Kd values of specifically bound N6 - cyclohexyl-[3H]adenosine were determined in the cerebral cortex of mice that had been housed under controlled light-dark cycles for 4 weeks (light on from 7.00 to 19.00 h). Significant differences were found for Bmax values measured at 3-hr intervals across a 24-h period, with low Bmax values during the light period and high Bmax values during the dark period. The amplitude between 03.00 and 18.00 hr was 33%. No substantial rhythm was found in the Kd values. It is suggested that the changes in the density of A1 receptors could reflect a physiologically-relevant mechanism by which adenosine exerts its modulatory role in the central nervous system.

Adenosine receptors in cortical-derived vesicles of the rat: studies on binding sites and accumulation of cyclic AMP

A vesicular preparation derived from the cerebral cortex of the rat was used to obtain, under the same experimental condition, binding parameters and stimulation data for cyclic AMP. Two analogues of adenosine were employed in the binding studies: [3H]NECA, a mixed A1/A2 agonist and [3H]CHA, a more selective A1 agonist. The [3H]CHA seemed to bind to a single high affinity site (Kd = 1.31 nM, Bmax = 0.327 pmol bound); saturation data for [3H]NECA were resolved for the presence of a high and a low affinity binding site (Kd1 = 3.08 nM, Bmax1 0.115 pmol bound; Kd2 = 204 nM, Bmax2 1.59 pmol bound), but only when calcium ions were omitted from the incubation medium. At 0 degree C, [3H]NECA bound to a single, low affinity site; the presence of calcium ions (1 mM) significantly reduced the affinity of [3H]NECA (Kd 419 nM), with respect to the absence of calcium (Kd 208 nM), without affecting the Bmax value. The influence of calcium ions was also investigated on the binding of [3H]CHA and a reduction of the Bmax value (36%) was found. Regardless of the presence or the absence of calcium ions, NECA stimulated accumulation of cyclic AMP in a dose-dependent way with an EC50 of 2.79 microM; this value did not correlate with the Kd of the low affinity binding site for [3H]NECA. Thus, the purpose of establishing a correlation between binding sites for analogues of adenosine and the site in the cerebral cortex through which the accumulation of cyclic AMP is induced, was not achieved. It is concluded that the stimulatory effect of analogues of adenosine on adenylate cyclase might not be a receptor-mediated effect. The complex influence of calcium ions on affinity and binding capacity of analogues of adenosine is discussed.

5'-N-ethylcarboxamido[8-3H]adenosine binds to two different adenosine receptors in membranes from the cerebral cortex of the rat

In the present study it is reported that [3H]NECA binds in a specific and saturable manner to membranes from the cerebral cortex of the rat. Scatchard analysis revealed two binding sites. The high affinity binding site (Kd 10.66 +/- 5 nM, Bmax 0.305 +/- 0.05 pmol/mg prot) was characterized by the following features: maximum binding at 25 degrees C, sensitivity to pretreatment with NEM and regulation by Gpp[NH]p, enhancing of binding in the presence of 1.0 mM MgCl2 and 1.5 mM CaCl2; the rank order of potency of several analogues of adenosine in competing with [3H]NECA for binding, was CHA greater than L-PIA greater than NECA greater than CADO. The low affinity binding site (Kd261.8 +/- 50 nM, Bmax 4.19 +/- 0.33 pmol/mg prot) showed maximum binding at 0 degrees C, insensitivity to pretreatment with NEM up to 1 mM and to regulation by Gpp[NH]p, and inhibition of binding in the presence of MgCl2 and CaCl2. The low affinity site was also present in membranes not pretreated with adenosine deaminase and, even in this condition, an IC50 of 188.5 +/- 36 nM for NECA and an IC50 of 4.35 +/- 0.20 microM for adenosine were found. It is concluded that the high affinity binding site is similar to the A1 adenosine receptors. The low affinity binding site is not classifiable among the A-type adenosine receptors, although it shows peculiar features shared both with the human platelet A2 receptor and the adenosine receptor formerly studied with [3H]adenosine in membranes from the brain of the rat; these results could reflect heterogeneity of adenosine receptors in central nervous system.

Effects of an atypical barbiturate (valofan) on spontaneous and stimulated locomotor activity and on brain serotonin metabolism in mice

The effects of an atypical barbiturate, 2-allophanyl-2-allyl-4-valerolactone (valofan), have been studied on spontaneous locomotor activity and amphetamine-induced hypermotility in mice. Some serotonergic agents were employed in order to evaluate the possible involvement of serotonin on its mechanism of action. Valofan (250 and 500 mg/kg) always reduced spontaneous locomotion and amphetamine-induced hypermotility; after the administration of L-tryptophan (10 mg/kg) and tranylcypromine (1 mg/kg) both locomotions decreased; when administered in combination with valofan they resulted in an additive effect. Similar results have been obtained by administration of 5-methoxy-N,N-dimethyltryptamine (1 mg/kg) and pindolol (5 mg/kg) alone or combined with valofan. Valofan affected the serotonin metabolism increasing 5-HT and 5-HIAA concentrations in a dose dependent way in the cortex, in the brainstem or in total brain. No correlation seems to exist between the effects of valofan, on its own or combined with serotonergic agents, on motor activity and its effects on the accumulation of 5-HT and 5-HIAA. We can therefore suggest that valofan manifests its central effects by an unclear mechanism which cannot completely be accounted for its metabolic effects on cerebral serotonin and dopamine.

Effects of 2-allophanyl-2-allyl-4-valerolactone (valofan) on cerebral energy metabolism of rat in normoxia and hypobaric hypoxia

The effects of an atypical barbiturate, valofan, and of a classical barbiturate, exobarbital, on the cerebral energy metabolism of rat have been evaluated under normoxic and hypobaric hypoxia conditions. In hypobaric hypoxia, the survival time was significantly increased by valofan 1.5 g/kg and exobarbital 0.05 g/kg in respect to the controls. In normoxia, valofan 1.5 g/kg (dose weakly sedative) significantly increased creatin-P level and reduced pyruvate and lactate concentrations; exobarbital 0.05 g/kg (anesthetic dose) showed a similar behavior except for glucose which was increased. In hypobaric hypoxia, valofan and exobarbital did not cause any change in the pool of labile phosphates, nor in the levels of pyruvate and lactate in respect to the controls. Repeated treatment (20 days) with the two drugs significantly altered the cerebral energy metabolism during normoxia conditions. The concentrations of ATP, creatin-P, glucose, pyruvate and lactate were increased, while L/P ratio was unaffected.

Effects of caffeine and chlor-desmethyldiazepam on fighting behavior of mice with different reactivity baselines

The effects of various doses of caffeine and of chlor-desmethyldiazepam on footshock-induced aggressive behavior were examined in mice with different baselines of aggressiveness. Caffeine significantly increased the number of fighting episodes with all the doses tested. This was more evident in mice with low rather than in those with high basal rates of agonistic response. Caffeine caused the appearance of minimal convulsive signs in mice subjected to a threshold electroshock which did not produce any seizure in the controls; it also increased metrazol toxicity. Chlor-desmethyldiazepam enhanced fighting behavior at doses of 0.04 and 0.08 mg/kg, but decreased it at 1.25 mg/kg. The first two doses produced the same effects as caffeine on electroshock test, but did not influence metrazol toxicity.

Effect of an atypical barbiturate, the 2-allophanyl-2-allyl-4-valerolactone (valofan), on exploratory behaviour and brain serotonin concentrations in mice

This study investigates, both behaviourally and biochemically, the action of 2-allophanyl-2-allyl-4-valerolactone (Valofan) in the mouse, after acute or repeated administration. The exploratory behaviour was measured in different experimental conditions by a hole-board test modified in observation length: 10 min instead of original 5. The variations in 5-HT and 5-HIAA levels were measured in cortex and brainstem. Acute administration of Valofan (50 to 500 mg/kg os) did not change exploration during the first 5 min period, while at higher doses (200-500 mg/kg) it did produce a significant increase in basal exploratory behaviour, measured by prolonging hole-board test to 10 min. These data were confirmed by the habituation test to he thole-board whereby, after three days of exposure, low exploratory baseline of mice was constant. The elevation of the exploration was consistant with a significant net increase in 5-HT levels (greater than 5-HT; less than 5-HIAA) in brainstem, and with a relative enhance in amine levels (= 5-HT; less than 5-HIAA) in cortex for higher doses of the drug. Modifications of serotonin mechanisms positively affect behaviour in an unfamiliar environment. Surprisingly repeated treatment (for 8 days) with Valofan did not change exploration in respect of controls. The biochemical pattern of repeated treatments showed that higher doses of drug increased 5-HT without affecting 5-HIAA levels in brainstem and cortex. Repeatedly handheld mice showed control values significantly higher than those of acute treatment. This increase in activity corresponded to a biochemical pattern similar to that obtained after acute administration of 500 mg/kg Valofan, indicating that handling affected the basal 5-HT content. Thus Valofan could stimulate exploration in mice with a low baseline, while it did not change the activity of animals with a higher baseline. Further evidence for a possible involvement of 5-HT mechanisms in the action of Valofan was given by the fatigue test. Mice, submitted to hole-board after 24 h of forced walking, showed, with 500 mg/kg, a significant decrease in exploration: action of Valofan seemed to potentiate the effects exerted by stress on serotonin turnover.

Regional distribution of high affinity binding of 3H-adenosine in rat brain

The high and low affinity adenosine binding sites with Kd values ranging respectively from 0.8 to 1.65 microM and from 3.1 to 13.86 microM were demonstrated in the following rat brain areas: cortex, hippocampus, striatum, cerebellum, diencephalon, and pons-medulla. Adenosine receptors involved in the high affinity binding seem to be mainly Ra-type. The analysis of the regional distribution of 3H-Adenosine showed the highest levels of specific binding in striatum and hippocampus; somewhat smaller values in cortex, cerebellum, and diencephalon, and even lower in pons-medulla.

Interaction of caffeine with benzodiazepines: behavioral effects in mice

Three series of behavioral experiments, i.e. hole-board, open-field and spontaneous motor activity test, were carried out on mice with the goal of determining the effects of the interaction between caffeine and two peculiar benzodiazepines (BDZ). The drugs, desmethyldiazepam (DMDZ) and chlor-desmethyldiazepam (Cl-DMDZ), were administered acutely by the intraperitoneal route. In preliminary experiments, the effects of several doses of caffeine were examined in the three in order to obtain some indication of the doses to use for interaction experiments. Therefore, stimulant doses of caffeine (5, 10 and 20 mg/kg) were associated with the two benzodiazepines. It was found that BDZ counteracted the effect of caffeine in the hole-board and in the spontaneous motor activity tests. By contrast, in the open-field situation, the association always gave rise to an increase in exploration activity. Furthermore, the present investigations showed, once again, that chlor-desmethyldiazepam is more active than the other benzodiazepine.

Tissue distribution of 14C-heteronium bromide: radioactivity levels at different time intervals after oral administration in the rat

The tissue distribution of radioactivity after oral administration to rats of 14C-heteronium bromide is measured by liquid scintillation counting and the results expressed as specific activity and percentage of administered radioactivity. From the data obtained in blood, liver, kidney. stomach, duodenum, cecum, large intestine and stool some conclusions can be drawn. Heteronium bromide undergoes a rapid systemic absorption, the radioactivity being present as early as 15 min from the administration, in all the tested organs. The blood levels show two peaks: one at 120 min and a second at 360 min. This diphasic behaviour can be explained either by the presence of an active enterohepatic circulation, as indirectly indicated by the data from liver and duodenum, or by a transient shift of the molecule from blood to other tissues, rich in polysulfuronic acids. The principal route of excretion is represented by the kidney, where consistent levels are reached at 120 min, while the intestinal route becomes evident at 240 min and reaches its maximum at 720 min. The complete metabolic cycle of the compound is long lasting, since in all the tested tissues, marked radioactivity levels are still present after 720 min. The pharmacokinetic profile obtained, suggesting a long persistence of the drug and/or of its metabolites in the organism, is in agreement with previous pharmacodynamic data showing a long lasting action for heteronium bromide.