Inhibitory effect of fruit extracts on the formation of heterocyclic amines

Natural extracts have attracted considerable attention for development into effective inhibitors against the formation of genotoxic heterocyclic amines (HAs) in processed foods. In this study, four fruit extracts (apple, elderberry, grape seed, and pineapple) were evaluated for their effects on HA formation in fried beef patties. Apple and grape seed extracts were found to be the most effective in both the degree of inhibition in the formation of individual HAs (2-amino-3,8-dimethylimidazo [4,5-f]quinoxaline (MeIQx), 2-amino-3,4,8-trimethylimidazo[4,5-f]quinoxaline (4,8-DiMeIQx), and 2-amino-1-methyl-6-henylimidazo [4,5-b]pyridine (PhIP)) and in the reduction of total HA content (approximately 70% relative to the control). Activity-guided analysis of apple extract using model systems (PhIP- and MeIQx-producing models) showed that the proanthocyanidins, phloridzin and chlorogenic acid were responsible for reducing the amount of HAs formed. Proanthocyanidins were identified as the dominant inhibitors because they were strongly active against HA formation in both the PhIP and MeIQx model systems. For phloridzin, the inhibitory effect was observed only on the formation of PhIP. In contrast, chlorogenic acid, although effective against the formation of MeIQx, significantly enhanced the formation of PhIP. This is the first report showing the inhibitory activities of apple phenolics on the formation of heterocyclic amines. The findings provide valuable information for the development of effective strategies to minimize HA content of cooked meats and to identify several new natural products that may have new applications in the food industry.

Effects of brimonidine ingestion on cardiovascular responses and renal function in conscious dogs

The effects of brimonidine, an alpha(2)-adrenoceptor agonist, on blood pressure, heart rate, respiratory rate, renal function and some blood parameters were investigated in 10 dogs. Dogs were divided into two groups, low dose (LD; 0.2 mg/kg) and high dose (HD; 0.5 mg/kg) of brimonidine given orally. The alpha(2)-adrenergic antagonist yohimbine hydrochloride was injected to dogs at a dose of 0.1 mg/kg in both groups at the fifth hour after brimonidine administration. The results demonstrated that after administration of brimonidine, mean arterial blood pressure decreased dramatically at 2 h by 23% and 20% in LD and HD groups, respectively. Heart rate was decreased in a similar manner and both remained low at 5 h after brimonidine administration. Respiratory rate was decreased by 50%, while the electrocardiogram showed prolongation of the PR interval. Glomerular filtration rate (GFR) and effective renal blood flow were reduced when measured at 4 h after brimonidine ingestion in both groups, but the effect was more pronounced in the LD group. Brimonidine caused natriuresis and kaliuresis in both LD and HD groups. The packed cell volume was decreased and hyperglycaemia was detected. Most of the effects can be reversed completely after administration of yohimbine. However, yohimbine can restore GFR only partially. These data suggest that brimonidine caused cardiovascular and respiratory depression. The adverse effects of this drug can be antagonized by yohimbine, suggesting that these effects were mediated via the alpha(2)-adrenoceptor.

Loss of endothelial KATP channel-dependent, NO-mediated dilation of endocardial resistance coronary arteries in pigs with left ventricular hypertrophy

The influence of left ventricular hypertrophy (LVH) on the endothelial function of resistance endocardial arteries is not well established. The aim of this study was to characterise the mechanisms responsible for UK-14,304 (alpha(2)-adrenoreceptor agonist)-induced endothelium-dependent dilation in pig endocardial arteries isolated from hearts with or without LVH. LVH was induced by aortic banding 2 months before determining endothelial function. Following euthanasia, hearts were harvested and endocardial resistance arteries were isolated and pressurised to 100 mmHg in no-flow conditions. Vessels were preconstricted with acetylcholine (ACh) or high external K(+) (40 mmol l(-1) KCl). Results are expressed as mean+/-s.e.m. UK-14,304 induced a maximal dilation representing 79+/-6% (n=8) of the maximal diameter. NO synthase (l-NNA, 10 micromol l(-1), n=7) or guanylate cyclase (ODQ, 10 micromol l(-1), n=4) inhibition reduced (P<0.05) UK-14,304-dependent dilation to 35+/-6 and 18+/-7%, respectively. Apamin and charybdotoxin reduced (P<0.05) to 39+/-8% (n=4) the dilation induced by UK-14,304. In depolarised conditions, however, this dilation was prevented (P<0.05). UK-14,304-induced dilation was reduced (P<0.05) by glibenclamide (Glib, 1 micromol l(-1)), a K(ATP) channel blocker, either alone (35+/-10%, n=5) or in combination with l-NNA (34+/-9%, n=4). In LVH, UK-14,304-induced maximal dilation was markedly reduced (25+/-4%, P<0.05) compared to control; it was insensitive to l-NNA (21+/-5%) but prevented either by the combination of l-NNA, apamin and charybdotoxin, or by 40 mmol l(-1) KCl. Activation of endothelial alpha(2)-adrenoreceptor induces an endothelium-dependent dilation of pig endocardial resistance arteries. This dilation is in part dependent on NO, the release of which appears to be dependent on the activation of endothelial K(ATP) channels. This mechanism is blunted in LVH, leading to a profound reduction in UK-14,304-dependent dilation.

Role of alpha2-adrenergic receptors and cyclic adenosine monophosphate-dependent protein kinase in the regulation of norepinephrine release in the central nervous system of spontaneously hypertensive rats

There has been much evidence showing that the central sympathetic nervous system may be involved in the control of blood pressure. In the present study, we investigated the role of the presynaptic alpha2-adrenergic receptors and the cyclic adenosine monophosphate-dependent protein kinase (protein kinase A) in the regulation of norepinephrine release in the central nervous system in hypertension. The alpha2-adrenergic receptor agonists UK 14, 304 and clonidine inhibited the stimulation-evoked [3H]norepinephrine release in a dose-dependent manner in the medulla oblongata of Sprague-Dawley rats. Pretreatment of pertussis toxin (a potent inhibitor of the Gi-protein) attenuated the suppression of NE release by UK 14, 304. The protein kinase A inhibitor H-8 also reduced the stimulation-evoked [3H]norepinephrine release in rat medulla oblongata. In spontaneously hypertensive rats, the inhibitory effect of UK 14, 304 on the stimulation-evoked norepinephrine release was significantly less than in age-matched normotensive Wistar-Kyoto rats. By contrast, the protein kinase A inhibitor H-8 reduced the stimulation-evoked norepinephrine release to a greater extent in hypertension than in normotensive controls. The results of the present study showed that the alteration in the presynaptic alpha2-receptor-protein kinase A system might actively participate in the regulation of norepinephrine release in the central nervous system in hypertension.

Vascular natriuretic peptide receptor-linked particulate guanylate cyclases are modulated by nitric oxide-cyclic GMP signalling

(1) The sensitivity of the particulate guanylate cyclase-cyclic guanosine-3',5'-monophosphate (cGMP) system to atrial (ANP) and C-type (CNP) natriuretic peptides was investigated in aortae and mesenteric small arteries from wild-type (WT) and endothelial nitric oxide synthase (eNOS) knockout (KO) mice. (2) ANP and CNP produced concentration-dependent relaxations of mouse aorta that were significantly attenuated by the natriuretic peptide receptor (NPR)-A/B antagonist HS-142-1 (10(-5) M). Both ANP and CNP were more potent in aortae from eNOS KO mice compared to WT. (3) The potency of ANP and CNP in aortae from WT animals was increased in the presence of the NOS inhibitor, N(G)-nitro-L-arginine (3 x 10(-4) M) and the soluble guanylate cyclase inhibitor 1H-[1,2,4]oxadiazolol[4,3,a]quinoxalin-1-one (5 x 10(-6) M). (4) In contrast, the potency of ANP and CNP in aortae from eNOS KO animals was reduced following pretreatment of tissues with supramaximal concentrations of the NO-donor, glyceryl trinitrate (3 x 10(-5) M, 30 min) or ANP (10(-7) M, 30 min). (5) Responses to acetylcholine in aortae from WT mice (dependent on the release of endothelium-derived NO) were significantly reduced following pretreatment of tissues with GTN (3 x 10(-5) M, 30 min) and ANP (10(-7) M, 30 min). (6) CNP and the NO-donor, spermine-NONOate caused concentration-dependent relaxations of mesenteric small arteries from WT animals that were significantly increased in eNOS KO mice compared to WT. ANP was unable to significantly relax mesenteric arteries from WT or eNOS KO animals. (7) In conclusion, both NPR-A- and NPR-B-linked pGC pathways are modulated by NO-cGMP in murine aorta and mesenteric small arteries and crossdesensitisation occurs between NPR subtypes. The biological activity of endothelium-derived NO is also influenced by the ambient concentration of NO and natriuretic peptides. Such an autoregulatory pathway may represent an important physiological homeostatic mechanism and link the paracrine activity of NO and CNP with the endocrine functions of ANP and BNP in the regulation of vascular tone and blood pressure.

Effect of high sucrose diet on cytochrome P450 1A and heterocyclic amine mutagenesis

To assess the effect of dietary sucrose on liver cytochrome P450 1A content and activity, male F344 rat weanlings were randomized into two diet groups for a period of 90 days. One group was fed a diet containing 65% of total calories from sucrose (HSD) while the other was fed standard lab chow (0% sucrose). Microsomal fractions from each of 10 animals in each group were used in Western immunoblot, mutagenesis and 7-alkoxyresorufin-0-deethylase (AROD) assays. No statistically significant difference in the mean quantity of liver CYP 1A2 was detected by Western blot analysis, while a significant decrease in mean liver CYP 1A1 was observed in the rats fed the HSD. Liver microsomal-dependent mutagenesis of two heterocyclic amines (HCAs), 2-amino-3,8-dimethyl-imadazo[4,5-f]quinoxaline (MeIQx) and 2-amino-3-methyl-imidazo[4,5-f]quinoline (IQ), in Salminella typhimurium TA98 was decreased in animals on the HSD compared to those on the control diet by 33% (p < 0.001) and 25% (p < 0.001), respectively. In addition, rats on the HSD had significantly decreased ethoxyresorufin-0-deethylase (EROD) and methoxyresorufin-0-deethylase (MROD) activity over a range of substrate concentrations. These results show that a HSD alters hepatic CYP 1A content and activity and suggests that the metabolism of substrates for this P450 subfamily may be significantly altered.

Effects of acute and subchronic administration of dexefaroxan, an alpha(2)-adrenoceptor antagonist, on memory performance in young adult and aged rodents

The present study examined the influence of dexefaroxan, a potent and selective alpha(2)-adrenoceptor antagonist, on cognitive performance in rodents. In young adult rats, dexefaroxan reversed the deficits induced by UK 14304 [5-bromo-N-(4,5-dihydro-1-H-imidazol-2-yl)-6-quinoxalinamine], scopolamine, and diazepam in a passive avoidance task. In this test, dexefaroxan also attenuated the spontaneous forgetting induced by a 15-week training-testing interval. Moreover, dexefaroxan, given immediately after training, increased the memory performance of rats trained with a weak electric footshock in the passive avoidance test, facilitated spatial memory processes in the Morris water maze task in rats, and increased the performance of mice in an object recognition test. Thus, dexefaroxan appears to have a promnesic effect in these tests by facilitating the processes of memory retention, rather than acquisition or other noncognitive influences. The facilitatory effects of dexefaroxan in young adult rats persisted even after a 21- to 25-day constant subcutaneous infusion by using osmotic minipumps, indicating that tolerance to the promnesic effect of the drug did not occur during this prolonged treatment interval. Furthermore, in the passive avoidance and Morris water maze tests, dexefaroxan ameliorated the age-related memory deficits of 24-month-old rats to a level that was comparable to that of young adult animals, and reversed the memory deficits induced by excitotoxin lesions of the nucleus basalis magnocellularis region. Together, these findings support a potential utility of dexefaroxan in the treatment of cognitive deficits occurring in Alzheimer's disease.

Blockade of AMPA/kainate receptors can either decrease or increase the survival of cultured neocortical cells depending on the stage of maturation

Neurotoxicity has often been associated with glutamate receptor stimulation and neuroprotection with glutamate receptor blockade. However, the relationship may be much more complex. We dissociated cells from the rat neocortical anlage at an early stage of prenatal development (embryonic day 14). The cells were exposed in vitro to agonists and antagonists of alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate (AMPA)/kainate and N-methyl-D-aspartate (NMDA) receptors and the effects on differentiation and survival have been quantitatively and qualitatively evaluated. NMDA and the non-competitive antagonist (5R,10S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]-cyclohepten-5,10-imine hydrogen maleate (MK-801) had the expected effects (the agonist decreasing and the antagonist increasing neuronal survival) when applied at a relatively advanced stage of in vitro maturation, but no significant effect in either direction at earlier stages. Kainate also had an effect on cell survival only at an advanced stage (where it decreased the number of cells). However, this cannot be attributed to the absence of functional AMPA/kainate receptors at earlier stages, since: (1) cells could be loaded with cobalt; and (2) early application of kainate dramatically reduced the number of cobalt-positive cells. Furthermore, exposure at early stages to 6,7-dinitroquinoxaline-2,3-dione (DNQX), or GYKI 53655, (competitive and non-competitive AMPA receptor antagonists, respectively) strongly reduced cell survival. The effects were concentration- and time-dependent with a complex time--curve. The decrease in cell number was maximal after antagonist application from 2 to 5 days in vitro. The effects of DNQX could be cancelled by co-application of kainate. When exposed to an antagonist at later stages of development, the number of surviving cells gradually approached control values and finally became significantly higher. Our results suggest that cells of the developing neocortex (and perhaps newly generated cells in the adult brain) require at different stages of their development, an appropriate level of AMPA/kainate receptor activation.

Effects of citrus flavonoids on the mutagenicity of heterocyclic amines and on cytochrome P450 1A2 activity

Heterocyclic amines (HCA's) are promutagens produced by high temperature cooking of meat products and are activated by cytochrome P450 (CYP) lA2. Using Aroclor 1254 induced rat liver S9 we tested four citrus flavonoids diosmin, naringenin, naringin and rutin for their effects on the mutagenicity of HCA's MeIQx, Glu-P-1*, IQ and PhIP in Salmonella typhimurium TA98. The effects of the citrus flavonoids on CYPlA2 activity was determined by measuring demethylation of methoxyresorufin (MROD). MeIQx induced mutagenesis in S. typhimurium was significantly inhibited by all four flavonoids in a concentration dependent manner at 0.25, 0.5 and 1.0 mumole. Glu-P-1 induced mutagenesis was inhibited by rutin and naringenin. IQ induced mutagenesis was significantly inhibited by each flavonoid except diosmin at all three doses. With the exception of diosmin and naringin at 0.25 mumole all four flavonoids at all three doses significantly inhibited PhlP induced mutagenesis. The inhibition of MROD activity by the citrus flavonoids correlated best with the inhibition of MeIQx induced mutagenesis but also correlated with the inhibition of IQ induced mutagenesis except for diosmin and with the inhibition of PhlP induced mutagenesis except for the 0.25 mumole dose of diosmin and naringin. Our data suggest a chemopreventive potential for diosmin, naringin, naringenin and rutin towards CYPlA2 mediated mutagenesis of HCA's.

Effect of brimonidine on rabbit trabecular meshwork hyaluronidase activity

PURPOSE

To study the presence of hyaluronidase activity in the rabbit trabecular meshwork and its regulation by brimonidine.

METHODS

A spectrophotometric assay that consists of the assessment of N-acetylhexosamine groups released from hyaluronic acid was used to examine hyaluronidase activity. Cyclic adenosine monophosphate (cAMP) levels were assessed by radioimmunoassay.

RESULTS

Hyaluronidase activity was detected in the rabbit trabecular meshwork. Its optimal activity was in the acid range of pH 3.8. Brimonidine significantly increased trabecular hyaluronidase-specific activity and decreased cAMP accumulation. Yohimbine significantly inhibited the effect of brimonidine on both hyaluronidase activity and cAMP accumulation.

CONCLUSIONS

The finding of endogenous hyaluronidase activity in rabbit trabecular meshwork supports the hypothesis that this tissue can metabolize its own glycosaminoglycan (GAG) products. The present results suggest, however, that the hypotensive effect of brimonidine could be mediated, at least in part, by its ability to increase GAG catabolism, probably through a cAMP-independent mechanism.

Application of a single-cell gel electrophoresis (comet) assay to screen the antimutagenic activity in foods

Three cell lines (HL60, U937 and RAW264.7) were studied for their sensitivity against mutagens by using a single-cell gel electrophoresis (comet) assay. RAW264.7, the most sensitive one, was chosen to screen the antimutagenic activity in swine and bovine offal. Aqueous extracts of the swine stomach (0.2 mg/ml) and heart (10 mg/ml) were found to have antimutagenic activity against MeIQx (+ S9mix)-treated cells.

Phenolics: blocking agents for heterocyclic amine-induced carcinogenesis

Chemopreventive effects of synthetic and naturally occurring antioxidants on heterocyclic amine (HCA)-induced rat carcinogenesis and mechanisms of inhibition were assessed. In a medium-term liver bioassay, combined treatment with 0.03% 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx) and synthetic antioxidants such as 1-O-hexyl-2,3,5-trimethylhydroquinone (HTHQ), butylated hydroxyanisole (BHA), butylated hydroxutoluene (BHT), tert-butylhydroquinone (TBHQ) or propyl gallate, each at a dose of 0.25%, inhibited development of preneoplastic glutathione S-transferase placental form (GST-P) positive foci as compared with MeIQx alone, after initiation with diethylnitrosamine (DEN). Of these antioxidants, HTHQ showed the greatest activity. 8-Hydroxydeoxyguanosine (8-OHdG), a marker for DNA damage induced by active oxygen species, and malonedialdehyde and 4-hydroxynonenal levels were not largely influenced by the treatment with MeIQx or antioxidants, either alone or in combination. In the same medium-term liver bioassay, effects of some naturally occurring antioxidants, such as green tea catechins (GTC), hesperidin, chlorogenic acid, quercetin, rutin, curcumin, daidzin, ferulic acid and genistein were also examined. Of these antioxidants, only GTC tended to inhibit GST-P positive foci development, while quercetin, rutin, curcumin, daidzin, ferulic acid and genistein all exerted significant enhancing effects. Examination of HTHQ influence in a medium term liver bioassay with HCA Glu-P-1, in which the experimental period was extended for up to 26 weeks, also demonstrated a significant decrease in the incidence of liver tumours to 40% in the group treated with 0.5% HTHQ and 0.03% 2-amino-6-methyldipyrido[1,2-a:3',2'-d]imidazole (Glu-P-1) as compared with the Glu-P-1 alone value of 89%. Effects of HTHQ on colon carcinogenesis induced by 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) were evaluated in a two-stage colon carcinogenesis model using 1,2-dimethylhydrazine (DMH) as an initiator. At week 36, the multiplicity of colon tumours induced by 0.02% PhIP after DMH initiation (9.1+/-6.2/rat) was dose-dependently decreased by the combined treatment with 0.5% HTHQ (3.6+/-1.8, P < 0.001) and 0.125% HTHQ (6.2+/-3.2, not significant). Similarly, the incidence of mammary carcinomas in female F344 rats induced by oral administration of 0.02% PhIP (40%) for 52 weeks was significantly decreased by simultaneous treatment with 0.5% HTHQ (5%). Alpha-tocopherol and chlorophyllin only reduced the multiplicity of carcinomas. Analysis of the influence of HTHQ on metabolic activation of Glu-P-1 or PhIP after incubation with rat S9 mixture and NADPH by HPLC, revealed that each major metabolite was strongly reduced by the addition of HTHQ. Immunohistochemically detected PhIP-DNA adduct positive nuclei in the colon induced by continuous oral treatment with 0.02% PhIP for 2 weeks decreased by the combined treatment with 0.5 or 0.125% HTHQ. These results indicate that synthetic antioxidant HTHQ is a very strong chemopreventor of heterocyclic amine (HCA)-induced carcinogenesis and that depressed metabolic activation rather than antioxidant activity is responsible for the observed effect.

Inhibitory effects of antioxidants on formation of heterocyclic amines

It is important to search for effective antioxidants to suppress formation of mutagenic and carcinogenic heterocyclic amines (HCAs), like 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx) and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), because these HCAs are considered to be probable human carcinogens. The effects of various food-derived antioxidants on MeIQx formation were examined by their addition (0.2 mmol each) to mixtures of creatine (0.4 mmol), glycine (0.4 mmol) and glucose (0.2 mmol), and heating at 128 degreesC for 2 h. Glycine was replaced by l-phenylalanine in the case of PhIP formation. Among the 14 kinds of antioxidants tested, green tea catechins and the major component [(-)-epigallocatechin gallate], two flavonoids (luteolin and quercetin) and caffeic acid were found to clearly suppress the formation of both MeIQx and PhIP, being 3.2-75% of the level of the controls. These phenolic antioxidants also reduced the total mutagenicity of the heated mixtures. The results suggest that foodstuffs containing catechins, flavonoids and caffeic acid may suppress the formation of HCAs in cooked foods.

The influence of two carotenoid food dyes on clastogenic activities of cyclophosphamide and dioxidine in mice

The influence of the food dyes E160e (beta-apo-8'-carotenal in an oil suspension) and E160a (beta-carotene in an oil suspension) on clastogenic effects of cyclophosphamide (CP) and dioxidine (DN) was investigated. Chromosome damage in the bone marrow of C57BL/6 mice was reported. The following protocols were used: (1) simultaneous single administration of the dye and the mutagen and the subsequent animal sacrifice within 24 hr; (2) a 4-day pretreatment with the dye (daily administrations) followed with simultaneous injection of the dye and the mutagen on the 5th day 24 hr before sacrifice; (3) daily co-administration of the dye and the mutagen for 5 days with sacrifice 6 hr after the last administration. CP at a dose of 30 mg/kg and DN at 300 mg/kg were injected intraperitoneally; the dyes at doses of 0.5, 5 and 50 mg/kg were given orally. Under all the protocols applied, E160e at a dose of 50 mg/kg caused a significant reduction of both DN and CP effects. At 5 mg/kg this dye reduced the effects of the mutagens only under the pretreatment regimen. Pretreatment with E160a at doses of 5 and 50 mg/kg resulted in a meaningful reduction of the DN effect. Under the combined treatment with mutagens this dye reduced both CP and DN effects.

ACEA 1021, a glycine site antagonist with minor psychotomimetic and amnestic effects in rats

Antagonists of the allosteric glycine site of the NMDA receptor complex have been suggested to be beneficial in the treatment of neurodegenerative disorders. However, unwanted side effects like psychomotor stimulation and amnesia must be expected. ACEA 1021 (5-nitro-6,7-dichloro-1,4-dihydroquioxaline-2,3dione) is one of the first high-selective glycine site antagonists which passes the blood-brain barrier and which has promising anticonvulsive and neuroprotective properties. In the present study the effects of ACEA 1021 (5, 7.5, 8, 10, 15 and 20 mg/kg i.p.) on sniffing stereotypy, locomotor activity, prepulse inhibition of the acoustic startle response, the anti-cataleptic properties and spatial learning were tested. Only 7.5 mg/kg ACEA 1021 induced a sniffing stereotypy which was antagonized by the partial glycine site agonist D-cycloserine (D-4-amino-3-isoxazolidinone). ACEA 1021 had neither an effect on motor behavior measured in the open field nor on the acoustic startle response in the prepulse inhibition paradigm nor on the acquisition of spatial learning in the 8-arm-radial maze. Anti-cataleptic properties of ACEA 1021 in dopamine D2 (haloperidol (4'fluoro-4-(1-(4-hydroxy-4-p-chlorophenyl-piperidino)-butyrophe non)) or D1 (SCH 23390 (7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzaze pin e hydrochloride)) receptor antagonist-pretreated rats were only minor. Thus, ACEA 1021 is a glycine site antagonist with minimal psychotomimetic side effects and with no amnesia properties. However, it has only minor anti-parkinsonian effects.

Formation of the mutagenic/carcinogenic imidazoquinoxaline-type heterocyclic amines through the unstable free radical Maillard intermediates and its inhibition by phenolic antioxidants

Generation of the imidazoquinoxaline-type heterocyclic amines in the heated model system composed of glucose/glycine/creatinine in aqueous diethylene glycol was effectively prevented by phenolic antioxidants, butylated hydroxyanisole (BHA), propyl gallate (PG), sesamol, esculetin and epigallocatechin gallate (EGCG) in a dose-dependent manner. Generation of the mutagens in heated-and-dried bonito meat was effectively prevented on pretreatment with EGCG or green tea extract. Electron spin resonance (ESR) studies showed that the heated model mixture of glucose/glycine generated the unstable pyrazine cation radical, and its formation was inhibited by BHA, sesamol and EGCG. ESR-spin trapping studies using 5,5-dimethyl-1-pyrroline N-oxide (DMPO) and N-tert-butyl-alpha-phenylnitrone (PBN) showed that the heated model mixture of glucose/glycine or glucose/glycine/creatinine generated unstable carbon-centred radical(s), and their formation was effectively inhibited by BHA, sesamol and EGCG. It is likely that the unstable free radical Maillard intermediates played an important role in the formation of the imidazoquinoxaline-type heterocyclic amines, and the phenolic antioxidants effectively scavenged the radical species to prevent the mutagen formation.

A tyrosine kinase regulates alpha-adrenoceptor-stimulated contraction and phospholipase D activation in the rat aorta

Since previous studies had indicated a role for tyrosine kinases in alpha 2-adrenoceptor-induced contractile responses in other blood vessels, as well as in the activation of phospholipase D, we examined the sensitivity of these responses in rat aorta to the tyrosine kinase inhibitor genistein. Contractions induced by both noradrenaline and the alpha 2-adrenoceptor-selective agonist UK14304 (5-bromo-6-[2-imidazolin-2-yl-amino]-quinoxaline) were fully inhibited by genistein, with the latter responses being more sensitive. Contractions induced by high K+ buffer were also inhibited, but to a lesser extent. Both agonists caused a stimulation of phospholipase D activity, which could be blocked by pretreatment with pertussis toxin, indicating involvement of either Gi or Go. Genistein completely inhibited the agonist-induced phospholipase D activity and also substantially reduced the basal level of phospholipase D activity. Pretreatment with either the alpha 1-adrenoceptor antagonist prazosin or the alpha 2-adrenoceptor antagonist rauwolscine was also effective in eliminating the agonist-induced increase of phospholipase D. These results indicate that a tyrosine kinase-regulated phospholipase D plays a critical role in alpha-adrenoceptor-induced contractions of the rat aorta and that stimulation of both alpha 1- and alpha 2-adrenoceptors is essential to allow phospholipase activation.

Inhibition of 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline-mediated DNA-adduct formation by chlorophyllin in Drosophila

The effect of chlorophyllin on 2-amino-3,8-dimethyimidazo[4,5-f]quinoxaline (MeIQx)-mediated DNA-adduct formation in Drosophila was studied. Third-instar larvae of Drosophila were fed MeIQx at 1 mg/6.5 g-feed/bottle, with or without chlorophyllin (100-300 mg). After a 6 h feeding exposure to MeIQx, the larvae were divided into 2 groups. The first group was examined for covalent DNA adducts by 32P-postlabeling assay. The second group was assayed for DNA damage by allowing the larvae to develop to adults and measuring the male/female ratio (males, DNA repair-deficient; females, DNA repair-proficient). The 32 P-postlabeling results indicated a significant decrease in DNA adduct levels in larvae treated with MeIQx and 300 mg chlorophyllin (1.7 +/- 0.7 adducts/10(7) nucleotides) as compared with MeIQx-treated larvae 6.5 +/- 2.1 adducts/10(7) nucleotides). The results on male/female sex ratios also indicated a chlorophyllin-induced decrease in DNA damage by exposure to MeIQx. The suppressive effect of chlorophyllin on the genotoxic actions of a polycyclic mutagen, MeIQx, may be a result of complex formation between chlorophyllin and the mutagen.

The mechanism by which NBQX enhances NMDA currents in retinal ganglion cells

When the quinoxaline NBQX (2,3-dihydroxy-6-nitro-7-sulfamoylbenzo (F) quinoxaline), a KA/AMPA antagonist, is bath applied to the tiger salamander retina, a paradoxical action is evident in the light-evoked synaptic responses of ganglion cells: NBQX enhances excitatory synaptic currents at light onset observed under whole-cell voltage-clamp conditions in a perfused retinal slice preparation. This observation was surprising because synaptic inputs into ganglion cells that are mediated by KA/AMPA receptors are entirely blocked by NBQX. Thus, the NBQX-enhanced current is entirely mediated by NMDA receptors. The purpose of this study was to determine the mechanism(s) by which blocking KA/AMPA receptors appears to enhance NMDA currents. Using hyperosmotic sucrose stimulation to activate neurotransmitter release from the inner retina, we observed that NBQX augmented the sucrose-evoked response, suggesting that at least a component of this enhancement may reside in the inner retina. NBQX does not enhance NMDA currents activated by bath applied NMDA, demonstrating that the NBQX-induced enhancement does not result from modulation of NMDA receptors. Voltage-clamp studies, carried out at the appropriate holding potential, indicate that NBQX enhances glutamatergic transmission and reduces inhibitory inputs onto ganglion cells. In the presence of strychnine and picrotoxin, the NBQX-induced enhancement of NMDA currents is eliminated, suggesting that NBQX facilitates the expression of NMDA currents by a selective and partial reduction of inhibitory mechanisms. Additional studies suggest that part of the NMDA enhancement by NBQX is evident at the postsynaptic level, but a presynaptic component probably also participates, perhaps at the level of bipolar cell terminals. One way to account for this observation is to assume that a subpopulation of inhibitory amacrine cells requires KA/AMPA receptors exclusively for their synaptic activation: previous studies of sustained amacrine cells support this interpretation. Thus the NBQX-induced enhancement phenomenon may reflect a network-selective distribution of NMDA and KA/AMPA receptors among third-order neurons.

Involvement of alpha 2-adrenoceptors in metabolic and hormonal responses to a mixed meal in beagle dogs

The effects of alpha 2-adrenoceptor blockade or activation on glucose, insulin, free fatty acids (FFA), and glycerol responses to a mixed meal were studied in the beagle dog. The alpha 2-adrenoceptor antagonist deriglidole (1 mg/kg po), administered 45 min before feeding, significantly reduced glycemia and increased insulin, FFA, and glycerol levels. Although the alpha 2-adrenoceptor agonist UK-14.304 (3 micrograms/kg sc), administered 15 min before feeding, had no effect per se, it completely blocked meal-induced insulin release, thus promoting a mild increase in glycemia, and prolonged the meal-induced FFA decrease. Deriglidole antagonized the reduction of insulin secretion and the hyperglycemia induced by UK-14.304. The meal-induced fall in FFA levels was still observed after deriglidole treatment and was markedly amplified when UK-14.304 was administered with deriglidole. These results suggest that, in the dog, insulin release and lipolysis are very sensitive to alpha 2-adrenoceptor stimulation. It is also suggested that the meal-evoked decrease in lipid mobilization results from an increase in alpha 2-adrenoceptor stimulation rather than from an increase in insulin secretion.

Sodium ions attenuate the inhibitory effects of neuropeptide Y on norepinephrine release in rat hypothalamus

Neuropeptide Y (NPY) has a wide and specific distribution both in the central and peripheral nervous systems. In the present study, we have investigated the effects of NPY on norepinephrine release in rat hypothalamus, and further examined the interaction of NPY with alpha 2-adrenergic receptors, as well as the influence of sodium ions on the modulation of norepinephrine release. In an in vitro study, NPY significantly inhibited the stimulation-evoked norepinephrine release from hypothalamic slices in a dose-dependent manner. The alpha 2-adrenergic receptor agonist, UK 14,304, also reduced the stimulation-evoked norepinephrine release. A low concentration of NPY, which had no effects on its own, significantly potentiated the inhibitory effect of UK 14,304 on the stimulation-evoked [3H]norepinephrine release. The blockade of alpha 2-adrenergic receptors by RX 781094 diminished the inhibitory effects of NPY on norepinephrine release. Pretreatment of slices with pertussis toxin (a potent inhibitor of the Gi-proteins) significantly attenuated the suppressive effects of NPY and UK 14,304 on norepinephrine release. When the sodium concentration of the perfusion medium was increased, the inhibitory effects of NPY and UK 14,304 on norepinephrine release were significantly reduced. These results show that NPY might inhibit norepinephrine release that is partially mediated by alpha 2-adrenergic receptors and the pertussis toxin-sensitive Gi-proteins in rat hypothalamus. Moreover, less suppressive effects of NPY and UK 14,304 on norepinephrine release in the presence of excess sodium ions suggest that sodium ions might actively participate in regulating the NPY and alpha 2-adrenergic receptor mediated functions in the central nervous system.

Dietary caffeine reduces the genotoxicity of MeIQx in the host-mediated assay in mice

The influence of dietary caffeine on the genotoxicity of the cooked food mutagen 2-amino-3,8-dimethylimidazo[4,5-f]-quinoxaline (MeIQx) was evaluated using the host-mediated assay in mice. For four weeks, BALB/c mice were fed a purified diet with or without caffeine (0.01% wt/wt in the diet). In the host-mediated assay, Salmonella typhimurium TA98 was given intravenously immediately before an oral dose of MeIQx (1.5 mg/kg body wt). After one hour, the mice were killed, the Salmonellae were recovered from the liver, and the number of mutants (his+ revertants) were determined. Consumption of caffeine led to a 47% reduction in the number of mutants induced by MeIQx (p < 0.001). Subsequent in vitro experiments using S. typhimurium TA98 revealed that the capacity of hepatic S-9 fractions from the caffeine-fed mice to covert MeIQx to an active mutagen was reduced by approximately 35%. This effect was not attributable to caffeine in the S-9 preparation. These data suggest that consumption of caffeine modifies MeIQx mutagenicity by altering the spectrum of enzymes involved in its activation.

Presynaptic alpha 2A-adrenoceptors inhibit the release of endogenous dopamine in rabbit caudate nucleus slices

alpha 2-Adrenoceptors modulating the release of dopamine were identified and characterized in slices of the head of the rabbit caudate nucleus. Release of endogenous dopamine was measured by fast cyclic voltammetry as the increase in the extracellular concentration of dopamine elicited by electrical stimulation. The electrochemical signal was identified as dopamine by means of the oxidation potential, the voltammogram and the fact that the signal was not changed by desipramine, which inhibits the high affinity uptake of noradrenaline, but was greatly increased by nomifensine, which in addition inhibits the high affinity uptake of dopamine. Stimulation by 6 pulses/100 Hz increased the extracellular concentration of dopamine by about 85 nM. The selective alpha 2-adrenoceptor agonist 5-bromo-6-(2-imidazolin-2-ylamino)-quinoxaline (UK 14,304) reduced this release with an EC50 of 173 nM and by maximally 75%. The alpha 2-adrenoceptor agonists clonidine and oxymetazoline only tended to cause a decrease. Six drugs, including oxymetazoline, were tested as antagonists against UK 14,304. Their order of antagonist potency (pKD values in brackets) was rauwolscine (8.0) > oxymetazoline (7.5) > 2-(2,6-dimethoxyphenoxyethyl)aminomethyl-1,4-benzodioxane (WB 4101; 7.3) > phentolamine (7.1) > corynanthine (5.1) approximately prazosin (<6). Given alone, the antagonists did not change the release of dopamine elicited by 6 pulses/100 Hz, and the same was true for the dopamine receptor antagonist sulpiride. When caudate slices were stimulated by 10 pulses/1 Hz, sulpiride increased the release of dopamine. Desipramine and rauwolscine, in contrast, again caused no change. It is concluded that dopaminergic axons in the rabbit caudate nucleus possess release-inhibiting alpha 2-adrenoceptors.2+ off

[Pharmacological characterization of alpha 2-adrenoceptor regulated 5-HT release in the rat hippocampus]

The purpose of the present study is to confirm the functional regulation of alpha 2-adrenoceptor on the release of serotonin (5-HT) from the rat hippocampus in vivo. Under several pharmacological conditions, extracellular levels of 5-HT were estimated by assaying its concentrations in the perfusion fluid through the use of high-performance liquid chromatography with electrochemical detection. Extracellular 5-HT in the hippocampus was reduced by tetrodotoxin, 10 microM co-perfusion and was increased by perfusion with a selective 5-HT reuptake inhibitor, fluoxetine, 10 microM. Addition of potassium (K+; 120 mM) to the perfusion fluid evoked an approximately 3-fold increase in 5-HT release, and a calcium free medium completely prevented this K(+)-evoked 5-HT release. Potassium-evoked 5-HT release from the hippocampus of freely moving rats was significantly and concentration-dependently inhibited when alpha 2-adrenoceptor agonist, UK14,304, 0.1 microM to 10 microM was added to the perfusion solution, while the output of a 5-HT major metabolite, 5-hydroxyindoleacetic acid (5-HIAA), remained unchanged. This action of UK14,304 was prevented by pretreatment with idazoxan, 5 mg/kg, i. p., an alpha 2-adrenoceptor antagonist. In rats that were catecholaminergically denervated with 6-hydroxydopamine, UK14,304, 10 microM also inhibited the potassium-evoked 5-HT release, but had no effect on the 5-HIAA output. The UK14,304-induced inhibition of 5-HT release was prevented by pretreatment with pertussis toxin (PTX). These findings suggest that 5-HT release is functionally modulated via alpha 2-adrenoceptors located on the serotonergic nerve terminals in the rat hippocampus. They also indicate the possibility that the inhibition of 5-HT release via alpha 2-adrenoceptors is linked to G-proteins which are substrates of PTX.

CNQX infused into entorhinal cortex blocks memory expression, and AMPA reverses the effect

Rats were trained in a step-down inhibitory avoidance task using a 0.8-mA foot shock and tested for retention 26 days later. Three to five days prior to the retention test they were bilaterally implanted with cannulae aimed at the entorhinal cortex. Ten minutes before testing they received an infusion, into the entorhinal cortex, of vehicle, ciano-nitro-quinoxaline-dione (CNQX; 0.5 micrograms), amino-hydroxy-methyl-isoxalone-propionate (AMPA; 1.0 or 2.5 micrograms), or AMPA (1.0 micrograms) plus CNQX (0.5 micrograms). CNQX blocked memory expression; the effect lasted less than 90 min. AMPA had no effect of its own, but at the lower dose level it counteracted the depressant influence of CNQX. It is not likely that the effect of CNQX could have been due to an influence on performance: In separate sets of experiments the bilateral intraentorhinal infusion of CNQX (0.5 micrograms) 10 min before training did not affect either acquisition or retention of the avoidance task or general activity during 3 min of free exploration in the training box. The results indicate that the integrity of AMPA receptors in the entorhinal cortex is necessary for memory expression.

Alpha-2 adrenoceptor mediated changes in aqueous dynamics: effect of pertussis toxin

Because alpha-2 (alpha 2) adrenoceptor-mediated inhibition of signal transduction mechanisms is regulated, in part, by inhibitory G (Gi) protein, we studied the effects of pertussis toxin (PTX) pretreatment on alpha 2 adrenergic agonist, UK 14,304-18 (UK; brimonidine)-induced: (1) changes in intraocular pressure (IOP) and aqueous flow rate of rabbits; (2) modulation of 3H-norepinephrine (NE) overflow of rabbit iris-ciliary bodies (ICBs) and (3) accumulation of cyclic AMP in rabbit ICBs and cultured non-pigmented ciliary epithelial (NPE) cells. Results showed that UK (50 micrograms) lowered the IOP of normal rabbits by 8 +/- 1 mmHg (n = 8) at 3 hr when treated topically and that the reduction of IOP was accompanied by a decrease in aqueous humor inflow (35%, n = 5). Therefore, it was postulated that these UK-induced effects involve activation of a Gi protein linked to alpha 2 adrenoceptors. In PTX-pretreated (2.5 micrograms kg-1, i.a.) rabbits, hypotensive responses to UK were reduced by 50% and 70% (n = 8) at days 4 and 7 post PTX treatment, respectively. The suppression of aqueous humor inflow induced by UK was also prevented by the PTX treatment. In isolated, perfused rabbit ICBs, UK (1 microM) caused 50% inhibition of 3H-NE overflow from electrical field stimulation. Pretreatment with PTX (150 ng ml-1, 4 hr) partially prevented UK-induced inhibition of NE overflow. In in vitro assays of postjunctional alpha 2 adrenoceptor activity, UK (1 microM) inhibited isoproterenol (ISO, 1 microM)-stimulated cAMP accumulation by 45% and 48% in ICBs and NPE cells, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

In vitro effect of vegetable and fruit juices on the mutagenicity of 2-amino-3-methylimidazo[4,5-f]quinoline, 2-amino-3,4-dimethylimidazo[4,5-f]quinoline and 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline

The antimutagenic potencies of the juices of 28 fruits and 34 vegetables commonly consumed in Germany were investigated with respect to the mutagenic activities induced by 2-amino-3-methyl[4,5-f]-quinoline (IQ), and in part by 2-amino-3,4-dimethylimidazo[4,5-f]quinoline (MeIQ) or 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx) in Salmonella typhimurium TA98 and TA100. With IQ, weak to strong antimutagenic activities were found in 68% of the fruits and 73% of the vegetables that were tested. In fruits, strong antimutagenic activities were detected in bananas, blackberries, blueberries, sweet and sour cherries, blackcurrants and redcurrants, pineapple and watermelon. Moderate antimutagenic activities were detected in greengage, kiwi, mangos, honeydew melons and plums. Weak antimutagenic activities were detected in apple, apricot, mirabelle, pears, peaches and strawberries, whereas white and red grapes and raspberries were inactive, and gooseberries and citrus fruits in general possessed marginal or no antimutagenic activities. In vegetables, strong to moderate antimutagenic activities were found for all cruciferous vegetables, except Chinese cabbage, which had only weak antimutagenic activity. Other vegetables with strong antimutagenic activities were beets, chives, horseradish, onions, rhubarb and spinach. Moderate antimutagenic activities were found with green beans and tomatoes, weak activities in eggplant, garden cress, many lettuces, leeks, mangold, cucumber, pumpkin, radish and summer squash. Asparagus, carrots, fennel leaves, parsley, green peppers and radishes were inactive. When fruit and vegetable juices were heated, a considerable reduction of antimutagenic potencies was seen with apple, apricot, kiwi, pineapple, beets, cabbage (Chinese, Savoy, red and white), cauliflower, leafy lettuce, cucumber, onions, radish and rhubarb. Antimutagenic factors in blackberries, blueberries, sweet and sour cherries, honeydew melons, mirabelle, plums, strawberries, Brussels sprouts, chicory greens, eggplant, garden cress, mangold, pumpkin, lamb's lettuce and spinach were, however, remarkably heat stable. Antimutagenic potencies in bananas, blackcurrants and redcurrants, greengages, gooseberries, mangos, watermelon, green beans, kohlrabi, horseradish, tomatoes and chives were partially reduced. Antimutagenic activities in the juices of eight apple cultivars were moderate in two, weak in four, and marginal or absent in two. No major differences, however, were detected in five batches of oranges and three batches each of grapefruits, asparagus, green beans, broccoli, cauliflower, spinach and tomatoes. No (or only minor) differences were seen between IQ, MeIQ and MeIQx and tester strains TA98 and TA100. Pineapple and celeriac juices inhibited the enzymatic system responsible for the activation of IQ, but had no desmutagenic activity. Peroxidase activity found to be present in broccoli, cauliflower, green beans and tomatoes may contribute to antimutagenic activities in these vegetables.

Effect of chronic antidepressant treatment on 5-HT1B presynaptic heteroreceptors inhibiting acetylcholine release

The effect of long term treatment with two tricyclic antidepressants on the sensitivity of 5-HT1B presynaptic heteroreceptors inhibiting acetylcholine (ACh) release was investigated. Groups of male rats received during 14 days either saline, citalopram (20 mg/kg), a serotonin (5-HT) uptake blocker, or tianeptine (2 x 10 mg/kg), an antidepressant that enhances 5-HT uptake. The efficacy of the 5-HT1B selective agonist 7-trifluoromethyl-4-(4-1-piperazinyl)-pyrrolo[1,2-a]quinoxaline (CGS 12066B) in reducing K(+)-evoked [3H]acetylcholine release from hippocampal synaptosomes was determined 24 hr after the last administration. The chronic treatment with citalopram or tianeptine modified neither the basal nor the K(+)-evoked release of [3H]acetylcholine. In contrast, these treatments significantly reduced the efficacy of CGS 12066B to inhibit the release of [3H]acetylcholine induced by K+ depolarization. These data suggest that chronic antidepressant treatment desensitizes 5-HT1B presynaptic heteroreceptors through a mechanism which seems to be independent of the synaptic availability of 5-HT.

Antagonism by citalopram and tianeptine of presynaptic 5-HT1B heteroreceptors inhibiting acetylcholine release

The interactions of citalopram and tianeptine, two antidepressants having opposite effects on serotonin (5-HT) uptake, with 5-HT1B presynaptic heteroreceptors located on cholinergic terminals were investigated. In rat hippocampal synaptosomes, citalopram (0.01 or 0.1 microM) or tianeptine (0.01-10 microM) did not modify the basal or the K(+)-evoked release of [3H]acetylcholine. Only at the concentration of 100 microM did tianeptine significantly decrease (-18%) the K(+)-evoked release of [3H]acetylcholine without affecting the spontaneous outflow of radioactivity. The inhibitory effect of 7-trifluoromethyl-4-(4-1-piperazinyl)-pyrrolo[1,2-a]quinoxaline (CGS 12066B), a 5-HT1B receptor agonist, on the stimulation-induced release of [3H]acetylcholine was reduced in a concentration-dependent manner by citalopram and tianeptine. Both drugs completely reversed the inhibitory effects of CGS 12066B at concentrations that did not modify by themselves the release of [3H]acetylcholine. In contrast, tianeptine, up to a concentration of 1 microM, failed to antagonise the inhibitory effect of the muscarinic receptor agonist carbachol on K(+)-evoked [3H]acetylcholine release. Finally, the administration of tianeptine ex vivo (10 or 20 mg/kg) modified neither the depolarisation-induced release of [3H]acetylcholine nor the inhibitory effect of CGS 12066B on this presynaptic process. These findings further confirm that antidepressants interact in vitro with presynaptic 5-HT1B heteroreceptors.

2,3-dihydroxy-6-nitro-7-sulfamoylbenzo(F)quinoxaline enhances the protective activity of common antiepileptic drugs against maximal electroshock-induced seizures in mice

NBQX (2,3-dihydroxy-6-nitro-7-sulfamoylbenzo(F)quinoxaline), a novel and selective AMPA antagonist, was tested to evaluate its influence upon anticonvulsant activity of common antiepileptic drugs in mice. NBQX (10, 20, 40 mg/kg, i.p.) had no influence upon the threshold for electroconvulsions. NBQX (10 mg/kg) enhanced the activity of anticonvulsant drugs decreasing their ED50S against maximal electroshock from 321 to 190 mg/kg for valproate, from 19.5 to 14.5 mg/kg for carbamazepine, from 31.0 to 21.4 mg/kg for phenobarbital, from 17.8 to 9.5 mg/kg for diphenylhydantoin and from 19.5 to 10.5 mg/kg for diazepam. In addition, NBQX (10 mg/kg) failed to impair motor performance and long-term memory determined in the chimney test and passive avoidance task. The combinations of NBQX (10 mg/kg) and carbamazepine, diphenylhydantoin or phenobarbital resulted in no adverse effects. Diazepam (10.5 mg/kg) alone impaired the motor performance and long-term memory and so it did when combined with NBQX. Also retention of the passive avoidance task and motor performance were impaired by valproate alone or given together with NBQX. Finally, NBQX (10 mg/kg) did not affect the plasma level of any antiepileptic drug. It is concluded that non-NMDA glutamate receptor blockade results in the considerable enhancement of the efficacy of common antiepileptic drugs.

Anisodamine at higher concentrations in inhibiting alpha-adrenergic responses in isolated canine blood vessels

The present study was designed to examine the effect of higher concentration of anisodamine on alpha-adrenergic responses in isolated canine blood vessels. Up to 10(-3) mol/L, anisodamine did not significantly affect the responses of saphenous vein to alpha 2-adrenergic agonist UK-14, 304. In contrast, anisodamine (10(-5), 10(-4), 10(-3) mol/L) caused the concentration-response curves of femoral artery to norepinephrine (pA2 = 4.81 +/- 0.11) to phenylephrine (pA2 = 4.86 +/- 0.20) shift markedly. However, the antagonism on the alpha 1-adrenergic responses of canine femoral artery to norepinephrine and phenylephrine by higher concentrations of anisodamine produces dose ratios which yield a linear Schild regression with a slope less than unity, indicating an inequilibrium between agonist, antagonist, and receptors. The probable mechanisms involved are discussed.

GYKI 52466 protects against non-NMDA receptor-mediated excitotoxicity in primary rat hippocampal cultures

Glutamate excitotoxicity is mediated by both N-methyl-D-aspartate (NMDA)-receptor and non-NMDA receptor (alpha-amino-3-hydroxy-5-methyl-isoxazolepropionate (AMPA)/kainate (KA)) mechanisms but the lack of specific antagonists has limited the characterization of AMPA/KA receptor-mediated excitotoxicity. The 2,3-benzodiazepine GYKI 52466 is a newly described non-competitive AMPA/KA receptor antagonist. We have investigated the neuroprotective efficacy of GYKI 52466 in an embryonic rat hippocampal culture model of non-NMDA receptor-mediated excitotoxicity using KA as an agonist at the AMPA/KA receptor. Overnight treatment with 500 microM KA resulted in prominent neuronal excitotoxicity as assessed by lactate dehydrogenase efflux. GYKI 52466 attenuated KA excitotoxicity in a dose-dependent manner with an IC50 of 9 microM. Together with competitive antagonists (e.g., various quinoxalinediones), non-competitive antagonists like GYKI 52466 can now be used to dissect mechanisms of non-NMDA receptor mediated excitotoxicity.

Aniracetam reverses the anticonvulsant action of NBQX and GYKI 52466 in DBA/2 mice

Aniracetam (1-p-anisoyl-2-pyrrolidinone) selectively reverses the anticonvulsant activities of the non-NMDA receptor antagonists, GYKI 52466 (1-(4-aminophenyl)-4-methyl-7,8-methylenedioxy-5H-2,3- benzodiazepine.HCl) and, to a lesser extent, NBQX (2,3-dihydroxy-6-nitro-7-sulfamoylbenzo(F)quinoxaline), without affecting the anticonvulsant activity of the competitive NMDA receptor antagonist, D(-)-CPPene, in DBA/2 mice. Pretreatment with aniracetam (50 nmol i.c.v., 15 min before drugs) increases the ED50 values (mumol/kg i.p., 15 min) for GYKI 52466-induced protection against sound-induced clonic seizures in DBA/2 mice 7 fold, from 20.1 (11.9-33.9) to 142 (91.7-219), and for NBQX-induced protection 2 fold, from 39.7 (33.8-46.7) to 85.6 (63.9-115), respectively. Aniracetam on its own (12.5-100 nmol i.c.v.) has no convulsant activity, but reverses the anticonvulsant effect of GYKI 52466 (60 mumol/kg i.p., 15 min) in a dose-dependent manner.

Cerebroprotective effect of a non-N-methyl-D-aspartate antagonist, NBQX, after focal ischaemia in the rat

The cerebroprotective effect of the non-N-methyl-D-aspartate antagonist, NBQX (2,3-dihydroxy-6-nitro-7-sulfamoylbenzo(f)-quinoxaline.NA) on neurological deficit and infarct volume (visualised with 2,3,5-triphenyltetrazolium) 24 h after permanent middle cerebral artery (MCA) occlusion in Fischer rats (n = 6 per dose) was studied. NBQX (10, 20 or 30 mg Kg-1, i.p., 0,30,60 min) immediately after MCA occlusion reduced cortical infarct volume by 45% (not significant), 70% (p < 0.05) or 75% (p < 0.05) respectively. NBQX (30 mg Kg-1, i.p., 60, 90, 120 min) reduced cortical infarct volume by 58% (p < 0.05). With a 2 h delay NBQX was ineffective. Neurological deficits (with blinded assessment) were improved with immediate or delayed NBQX (3 x 30 mg Kg-1, i.p.). The main adverse behavioral effect of NBQX (3 x 20 or 3 x 30 mg Kg-1, p.i.) was ataxia. The cerebroprotective effect of NBQX in rats suggests a possible therapeutic role for non-N-methyl-D-aspartate antagonists given shortly after stroke onset.

The insulin-secreting cell line, RINm5F, expresses an alpha-2D adrenoceptor and nonadrenergic idazoxan-binding sites

The pharmacological properties of alpha-2 adrenoceptors and the existence of nonadrenergic idazoxan-binding sites (NAIBS) were investigated in the insulin-secreting cell-line, RINm5F, using [3H]RX821002 and [3H]idazoxan. Analysis of [3H]RX821002 saturation isotherms revealed the presence of a single class of binding sites (Bmax = 47.5 +/- 3.5 fmol/mg protein) having high affinity (Kd = 1.26 +/- 0.18 nM). Inhibition of [3H]RX821002 binding by adrenergic compounds showed that the labeled sites displayed the properties expected for an alpha-2 adrenoceptor. Based on competition data with drugs having alpha-2 adrenoceptor subtype selectivity, the receptor from RINm5F is neither an alpha-2B nor an alpha-2C. It resembles the alpha-2A, but deviates from this subtype because of a weak affinity for yohimbine and rauwolscine. In this respect, RINm5F alpha-2 adrenoceptor is identical to the receptor previously described in rat intestinal mucosa and corresponds to a fourth subtype: alpha-2D. Agonist inhibition curves were better fitted by a two-site model and indicated that about half of the receptor population was under a high-affinity state corresponding to G protein-coupled receptors. [32P]ADP-ribosylation with pertussis toxin and immunodetection with specific antibodies permitted the identification of three distinct G proteins: Gi2, Gi3 and G0. Binding experiments with [3H]idazoxan showed that this imidazoline labeled two types of sites corresponding to alpha-2 adrenoceptors and NAIBS. Analysis of saturation isotherms under binding conditions allowing to discriminate between the two site populations indicated that the density of NAIBS (44 +/- 2 fmol/mg protein) was fairly identical to that of alpha-2 adrenoceptors. The pharmacological properties of NAIBS, as assessed by determining the relative affinity of imidazolinic and nonimidazolinic compounds, reasonably matched that reported in other tissues. Taken together, these data make the RINm5F cell-line 1) the first model in permanent culture known as expressing an alpha-2 adrenoceptor of the alpha-2D subtype; 2) a good system for studying in vitro the respective role of alpha-2 adrenoceptors and NAIBS in the regulation of insulin secretion by beta cells.

Contraction-mediating alpha 2-adrenoceptors in the mouse vas deferens

The question of the existence of postjunctional, contraction-mediating alpha 2-adrenoceptors, in addition to the known alpha 1-adrenoceptors, was studied in the mouse isolated vas deferens. Both the alpha 1-selective agonist phenylephrine and the alpha 2-selective agonist 5-bromo-6-(2-imidazolin-2-ylamino)-quinoxaline (UK 14,304) caused contraction of the vas deferens. In the presence of the alpha 1-selective antagonist prazosin (added in order to prevent an alpha 1 component in the effect of high concentrations of UK 14,304), the alpha 2-selective antagonist yohimbine and idazoxan shifted the concentration-response curve of UK 14,304 to the right in a manner compatible with competitive antagonism and with dissociation constants KB indicating the involvement of alpha 2-adrenoceptors. The maximal contraction elicited by UK 14,304 (in the presence of prazosin) was much lower than the maximal contraction elicited by phenylephrine. The effect of UK 14,304 was not changed by the P2-purinoceptor agonist alpha,beta-methylene-ATP and was reduced by neuropeptide Y, but was markedly enhanced by relatively low concentrations of phenylephrine. When the sympathetic fibres of the vas deferens were stimulated by trains of ten widely spaced (0.5 Hz) electric pulses, the tissue responded with ten separate twitches in which purinergic and adrenergic components were isolated by prazosin and suramin, respectively. Prazosin reduced the first adrenergic twitch in these trains at concentrations close to its KB value at alpha 1-adrenoceptors, whereas yohimbine and idazoxan reduced the first adrenergic twitch at concentrations far lower than their KB values at alpha 1-adrenoceptors.(ABSTRACT TRUNCATED AT 250 WORDS)

The pharmacology of fluparoxan: a selective alpha 2-adrenoceptor antagonist

1. This paper describes the pharmacology of the novel alpha 2-adrenoceptor antagonist fluparoxan (GR 50360) which is currently being studied clinically as a potential anti-depressant. Idazoxan and yohimbine were included in many studies for comparison. 2. In the rat isolated, field-stimulated vas deferens and the guinea-pig isolated, field-stimulated ileum preparations, fluparoxan was a reversible competitive antagonist of the inhibitory responses to the alpha 2-adrenoceptor agonist UK-14304 with pKB values of 7.87 and 7.89 respectively. In the rat isolated anococcygeus muscle, fluparoxan was a much weaker competitive antagonist of the contractile response to the alpha 1-adrenoceptor agonist phenylephrine with a pKB of 4.45 giving an alpha 2: alpha 1-adrenoceptor selectivity ratio of greater than 2500. 3. In the conscious mouse, fluparoxan (0.2-3.0 mg kg-1) was effective by the oral route and of similar potency to idazoxan in preventing clonidine-induced hypothermia and antinociception. In the rat, UK-14304-induced hypothermia (ED50 = 1.4 mg kg-1, p.o. or 0.5 mg kg-1, i.v.) and rotarod impairment (ED50 = 1.1 mg kg-1 p.o. or 1.3 mg kg-1, i.v.) were antagonized by fluparoxan. Fluparoxan, 0.67-6 mg kg-1, p.o., also prevented UK-14304-induced sedation and bradycardia in the dog. 4. In specificity studies fluparoxan had low or no affinity for a wide range of neurotransmitter receptor sites at concentrations up to at least 1 x 10(-5) M. It displayed weak affinity for 5-HT1A (pIC50 = 5.9) and 5-HT1B (pKi = 5.5) binding sites in rat brain. 5. We conclude that fluparoxan is a highly selective and potent alpha 2-adrenoceptor antagonist. The density of rat brain [3H]-dihydroalprenolol binding sites was reduced by 26% when fluparoxan was administered chronically for 6 days at a dose of 12 mg kg- 1 orally twice daily. The down-regulation of beta-adrenoceptors by fluparoxan is consistent with its antidepressant potential.

Potentiation of synaptic reflexes by D-serine in the rat spinal cord in vitro

6-Cyano-7-nitroquinoxaline-2,3-dione (CNQX) (10 microM) depressed dorsal root-evoked ventral and dorsal root potentials of the in vitro immature rat spinal cord to 26.3 +/- 5.2 S.E.M. and 40.8 +/- 2.7% of control values respectively. These depressant effects of CNQX were partially reversed by D-serine (EC50 values 39.7 microM +/- 8.7 S.E.M. N = 6 and 34.9 +/- 12.5 microM, N = 5 for ventral root potential and dorsal root potential respectively). Under our experimental conditions, which included the presence of Mg2+ (0.75 mM) in the bathing medium, no measurable potentiation of these synaptic reflexes by D-serine was recorded in the absence of CNQX. These data indicate that CNQX, in addition to its depressant effect at non-NMDA receptors, depresses an NMDA receptor-mediated component of segmental transmission through its action at the glycine site of the NMDA receptor complex.

The effectiveness of alpha 2-adrenoceptor activation increases from the distal to the proximal part of the veins of canine limbs

1. The effectiveness of alpha 1- and alpha 2-adrenoceptor activation was compared at different levels of the saphenous and cephalic vein of the dog in vitro. 2. Helically cut strips were used to determine concentration-response curves to phenylephrine, noradrenaline, UK-14,304 (5-bromo-6-(imidazoline-2-ylamino)-quinoxaline) and B-HT 920 (2-amino-6-allyl-5,6,7,8-tetra-hydro-4H-(thiazo)-4,5-d-azepine). The effect of prazosin and yohimbine on these curves was also studied. 3. At the distal level, the maximum response to UK-14,304 amounted to 33 and 50% of those to noradrenaline in the saphenous and cephalic vein, respectively, while at the proximal level the maximum response to UK-14,304 amounted to 72 and 78% of those to noradrenaline, in the saphenous and cephalic vein, respectively. 4. In both vessels, the results obtained with B-HT 920 were very similar to those for UK-14,304. 5. The pD2 values for UK-14,304 - which were identical at the three levels of both vessels - and the pA2 values for the antagonism exerted by either prazosin or yohimbine against the responses to UK-14,304 indicate that the alpha 2-adrenoceptors are identical at the different levels of both vessels. 6. These results show that the effectiveness of alpha 2-adrenoceptor stimulation increases from the distal to the proximal regions of canine limb veins. Apparently, this is due to a greater density of alpha 2-adrenoceptors in the proximal regions. 7. Yohimbine is much more potent against phenylephrine distally than proximally in both vessels. However, after 30 nm phenoxybenzamine - a concentration which eliminates the vast majority of alpha,-adrenoceptors without affecting alpha 2-adrenoceptors - yohimbine became equally potent at both levels, suggesting that the difference existing before phenoxybenzamine depended on alpha,-adrenoceptors. Hence it is concluded that alpha,-adrenoceptors in distal and proximal portions may differ.

Alpha-adrenoceptor reserve in the canine cephalic vein

In the canine cephalic vein, the pD2 for the selective alpha 1-agonist, phenylephrine, was 6.08 +/- 0.08 (n = 14) and that for the selective alpha 2-agonist, UK-14,304, was 8.32 +/- 0.06 (n = 10). The pA2 values for the antagonism exerted by the selective alpha 1-antagonist, prazosin, against phenylephrine and UK-14,304 were 7.74 +/- 0.05 (n = 14) and 6.28 +/- 0.03 (n = 8), respectively, while those for the antagonism exerted by the selective alpha 2-antagonist, yohimbine, against phenylephrine and UK-14,304 were 7.40 +/- 0.02 (n = 14) and 8.93 +/- 0.05 (n = 14), respectively. Furthermore, the concentration-response curve for UK-14,304 was typically biphasic, the first phase being antagonized by yohimbine and the second phase by prazosin and phenoxybenzamine. These results show that there are postsynaptic alpha 1- and alpha 2-adrenoceptors in the canine cephalic vein. In such a preparation, only one concentration of phenoxybenzamine (1 nM) shifted the concentration-response curves for noradrenaline, adrenaline and isoprenaline to the right without reducing the maximum. However, at the concentrations tested, phenoxybenzamine did not shift the concentration-response curve for phenylephrine to the right without depressing its maximum. It is concluded that: (1) the canine cephalic vein is a suitable preparation to study postsynaptic alpha 1- and alpha 2-adrenoceptors; (2) according to the original definition of 'spare receptors', there is no alpha 1-adrenoceptor reserve in canine cephalic vein; (3) UK-14,304 is a partial agonist at alpha 1-adrenoceptors.

[Pharmacological studies of celiprolol: II. Alpha 2-Adrenoceptor blocking effects of a cardioselective beta-blocker, celiprolol]

alpha-Adrenoceptor blocking effects of the cardioselective beta-blocker celiprolol were tested. 1. Celiprolol antagonized the contractions induced by UK-14304, but not those by phenylephrine, in isolated rat tail arteries with a pA2 value of 4.95. 2. In the isolated rat vas deferens, twitch contractions elicited by the transmural electrical stimulation (TS) were almost blocked by TTX (10(-7) M). Clonidine inhibited the TTX-sensitive contraction in a concentration-dependent manner. Celiprolol produced little effect on the inhibitory effects of clonidine. The concentration-inhibition curve of clonidine was shifted to the right by yohimbine (10(-8) M), but not by prazosin (10(-8) M). 3. Although celiprolol slightly increased the 3H-efflux to TS from the [3H]-norepinephrine-loaded rat vas deferens, the increment was not significant. Yohimbine (10(-7) M) significantly increased the 3H-efflux. TTX (10(-7) M) and bretylium (3 x 10(-5) M) blocked the 3H-efflux. 4. In the spinal rat treated with propranolol (3 mg/kg, i.v.) and prazosin (0.1 mg/kg, i.v.), celiprolol (30 and 100 mg/kg, i.v.) and yohimbine (0.1-1 mg/kg, i.v.) inhibited the pressor response to clonidine. 5. These results indicate that celiprolol may have a weak alpha 2-blocking effect which was more effective on postsynaptic alpha 2-receptors than presynaptic ones.

Autoradiographic characterization and localization of quisqualate binding sites in rat brain using the antagonist [3H]6-cyano-7-nitroquinoxaline-2,3-dione: comparison with (R,S)-[3H]alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid binding sites

Using quantitative autoradiography, we have investigated the binding sites for the potent competitive non-N-methyl-D-aspartate (non-NMDA) glutamate receptor antagonist [3H]6-cyano-7-nitro-quinoxaline-2,3-dione ([3H]-CNQX) in rat brain sections. [3H]CNQX binding was regionally distributed, with the highest levels of binding present in hippocampus in the stratum radiatum of CA1, stratum lucidum of CA3, and molecular layer of dentate gyrus. Scatchard analysis of [3H]CNQX binding in the cerebellar molecular layer revealed an apparent single binding site with a KD = 67 +/- 9.0 nM and Bmax = 3.56 +/- 0.34 pmol/mg protein. In displacement studies, quisqualate, L-glutamate, and kainate also appeared to bind to a single class of sites. However, (R,S)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) displacement of [3H]CNQX binding revealed two binding sites in the cerebellar molecular layer. Binding of [3H]AMPA to quisqualate receptors in the presence of potassium thiocyanate produced curvilinear Scatchard plots. The curves could be resolved into two binding sites with KD1 = 9.0 +/- 3.5 nM, Bmax = 0.15 +/- 0.05 pmol/mg protein, KD2 = 278 +/- 50 nM, and Bmax = 1.54 +/- 0.20 pmol/mg protein. The heterogeneous anatomical distribution of [3H]CNQX binding sites correlated to the binding of L-[3H]glutamate to quisqualate receptors and to sites labeled with [3H]AMPA. These results suggest that the non-NMDA glutamate receptor antagonist [3H]CNQX binds with equal affinity to two states of quisqualate receptors which have different affinities for the agonist [3H]AMPA.

Effect of tryptamine on the mutagenic activity of 2-amino-3-methylimidazo(4,5-f) quinoline (IQ) and related azaarenes in the Ames test

The bioactivation of the azaarenes 2-amino-3-methylimidazo(4,5-f) quinoline (IQ), 2-amino-3,4-dimethylimidazo(4,5-f) quinoline (MeIQ) and 2-amino-3,8-dimethylimidazo(4,5-f) quinoxaline (MeIQx) to mutagens by hepatic S9 preparations derived from Aroclor-pretreated Wistar rats was inhibited by tryptamine (2-50 microM). However, with similar preparations derived from Sprague-Dawley rats, bioactivation of IQ and MeIQx was less markedly inhibited by tryptamine while metabolic activation of MeIQ was enhanced. In the absence of cytosol, activation of IQ by microsomal preparations of both rat strains was inhibited by tryptamine. Cytosolic fractions from both rat strains were incapable of activation of IQ per se but increased the mutagenicity of the microsomal metabolite(s). This potentiation of the mutagenic activity by cytosol derived from Wistar rats was also inhibited by tryptamine whereas no significant inhibition was observed with cytosolic preparations from Sprague-Dawley rats. There appear to be two alternative pathways of microsomal metabolism of IQ: a tryptamine-sensitive pathway, probably involving the formation of the N-hydroxymetabolite; and a tryptamine-insensitive pathway producing weakly mutagenic or non-mutagenic metabolites which are activated to a potent mutagen by the cytosol. The tryptamine-insensitive pathway appears to be the major route of activation of the azaarenes in microsomal preparations from Sprague-Dawley rats and the principal activation route for MeIQ in both rat strains.

A difference in the affinity of some selective alpha 2-adrenoceptor antagonists when compared on isolated vasa deferentia of rat and rabbit

The alpha 2-adrenoceptor antagonist potencies of the benzoquinolizines (Wy 26 703, Wy 25 309, Wy 26 392), the benzodioxans (RX 781 094, RS 21 361), yohimbine and rauwolscine have been compared at presynaptic alpha 2-adrenoceptors in the isolated vasa deferentia of the rat and rabbit. Yohimbine and rauwolscine are of equal potency as antagonists in both the rat and rabbit against the agonists clonidine or UK 14304. The benzoquinolizines and benzodioxans are very weak antagonists of clonidine or UK 14304 at the presynaptic alpha 2-adrenoceptors of the rabbit vas deferens when compared to their potency at the presynaptic alpha 2-adrenoceptors of the rat vas deferens. This suggests that the presynaptic alpha 2-adrenoceptors present in the rat vasa deferentia may be different from those present in the rabbit vasa deferentia.

Blockade and postjunctional vascular alpha 1- and alpha 2-adrenoceptors in pithed rat by the enantiomers of WB-4101

The enantiomers of WB-4101 were evaluated for their ability to antagonize the alpha 1-adrenoceptor mediated pressor effect of cirazoline and the alpha 2-adrenoceptor mediated pressor effect of UK-14,304 in pithed rats. The (S)-stereoisomer of WB-4101 was more potent than the enantiomeric (R)-isomer in antagonizing both cirazoline and UK-14,304. The difference in potency between the enantiomers in blocking cirazoline was 37-fold in contrast to an enantiomeric difference of less than 3-fold for antagonizing UK-14,304. Based on DR2 values (i.e., dose of WB-4101 isomer in mg/kg required to produce a 2-fold rightward shift in the dose-response curves of cirazoline and UK-14,304) obtained in vivo from Schild regressions, alpha 1/alpha 2 selectivity ratios were calculated. While the (S)-enantiomer displays a 187-fold selectivity for alpha 1-adrenoceptors, the (R)-enantiomer is only approximately 13-fold selective for alpha 1-adrenoceptors. These results indicate that the alpha 1- and alpha 2-adrenoceptor blocking activity of WB-4101 resides predominantly in the (S)-enantiomer and that both enantiomers of WB-4101 are selective alpha 1-adrenoceptor antagonists in vivo. However the degree of alpha 1-adrenoceptor selectivity differs from each enantiomer, and the enantiomeric activity ratios differ for each alpha-adrenoceptor subtype.