Impaired synthesis or cellular storage of norepinephrine, dopamine, and 5-hydroxytryptamine in human inflammatory bowel disease

The present study was aimed at evaluating the extent of dysfunction of the enteroendocrine and enteric nervous system, as indicated by changes in tissue levels of monoamines (dopamine, DA; norepinephrine, NE; 5-hydroxytryptamine, 5-HT) and their precursors and metabolites in the colonic mucosa of patients afflicted with ulcerative colitis (UC, N = 21) and Crohn's disease (CD, N = 22). In CD, but not in UC, NE tissue levels in both the noninflamed and inflamed colonic mucosa were markedly lower than in control subjects (N = 16). In the inflamed mucosa of CD and in UC patients levels of L-DOPA were twice those in controls. DA levels in the inflamed mucosa of CD and UC patients were markedly lower than in controls. This resulted in significant reductions in DA/L-DOPA tissue ratios, a rough measure of L-amino acid decarboxylase activity. 5-HT levels in the inflamed mucosa of CD and UC patients were markedly lower than in controls. In conclusion, intestinal cellular structures responsible for the synthesis and storage of DA, NE, and 5-HT may have been affected by the associated inflammatory process in both CD and UC.

Anticonvulsant and sodium channel-blocking properties of novel 10,11-dihydro-5H-dibenz[b,f]azepine-5-carboxamide derivatives

A series of esters of the major metabolite of oxcarbazepine (2), 10, 11-dihydro-10-hydroxy-5H-dibenz[b,f]azepine-5-carboxamide, were synthesized and evaluated for their anticonvulsant and brain sodium channel-blocking properties. The compounds were assayed intraperitoneally and per os in rats against seizures induced by maximal electroshock (MES). Neurologic deficit was evaluated by the rotarod test. The enantiomeric acetates (R)-11 and (S)-12 were the most active of the series against MES-induced seizures with oral ED(50) values at t(max) of 10.9 +/- 2.3 and 4.7 +/- 0.9 mg/kg, respectively. After intraperitoneal administration, carbamazepine (1) behaved more potently than 2 and all other new dibenz[b, f]azepine-5-carboxamide derivatives in the MES test; compounds 2 and 12 were equally potent. In the rotarod test, low doses of 1 produced considerable motor impairment, which did not occur with 2, enantiomeric alcohols (S)-6, (R)-7, and racemic alcohol 8, or racemic acetate 10 or (R)-11. The potencies of the racemic and enantiomerically pure alcohols 8, (S)-6, and (R)-7 derived from 2 in the MES and rotarod test were found to be similar between them, and consequently they exhibit similar protective index values. All three forms of the alcohol and their corresponding acetates (pairs 8 & 10, 6 & 12, and 7 & 11) were found to differ in the MES or rotarod tests; the ED(50) value for (S)-6 against MES-induced seizures was nearly 3-fold that for (S)-12. The protective index also differed markedly between all stereoisomers of the alcohol and their corresponding acetates, most pronouncedly for compound (S)-12 which attained the highest value (12.5) among all compounds tested. Blockade of voltage-sensitive sodium channels was studied by investigating [(3)H]batrachotoxinin A 20-alpha-benzoate ([(3)H]BTX) binding. Acetates (R)-11 and (S)-12 were more potent than the standards 1 and 2 at inhibiting the binding of [(3)H]BTX to sodium channels and the influx of (22)Na(+) into rat brain synaptosomes. It is concluded that acetates (R)-11 and (S)-12 are not simple metabolic precursors of alcohols (R)-7 and (S)-6 in rodents but that they possess anticonvulsant and sodium channel-blocking properties in their own right.

Efficacy and safety of 800 and 1200 mg eslicarbazepine acetate as adjunctive treatment in adults with refractory partial-onset seizures

OBJECTIVES

To evaluate the efficacy and safety of eslicarbazepine acetate (ESL) as adjunctive therapy in adults with partial-onset seizures.

MATERIAL AND METHODS

Double-blind, placebo-controlled, parallel-group, multicenter study consisting of an 8-week baseline period, after which patients were randomized to placebo (n = 87) or once-daily ESL 800 mg (n = 85) or 1200 mg (n = 80). Patients received half dose during 2 weeks preceding a 12-week maintenance period.

RESULTS

Seizure frequency over the maintenance period was significantly (P < 0.05) lower than placebo in both ESL groups. Responder rate was 23% (placebo), 35% (800 mg), and 38% (1200 mg). Median relative reduction in seizure frequency was 17% (placebo), 38% (800 mg), and 42% (1200 mg). The most common adverse events (AEs) (>10%) were dizziness, somnolence, headache, and nausea. The majority of AEs were of mild or moderate severity.

CONCLUSIONS

Once-daily treatment with ESL 800 and 1200 mg was effective and generally well tolerated.

Oxidative and non-oxidative mechanisms of neuronal cell death and apoptosis by L-3,4-dihydroxyphenylalanine (L-DOPA) and dopamine

1. The present study was designed to evaluate the nature of intervening agents in L-DOPA- and dopamine-induced neurotoxicity in Neuro-2A cells. 2. In the absence of cells and in conditions of light protection, at 37 degrees C, L-DOPA or dopamine (1 mM) in culture medium degraded spontaneously in a time-dependent manner, this being prevented by ascorbic acid (200 microM) and other antioxidants, namely glutathione (1 mM), N-acetyl-L-cysteine (1 mM), sodium metabisulphite (200 microM), but not N-ter-butyl-alpha-phenylnitrone (1 mM) and deferoxamine (100 microM). 3. The viability of Neuro-2A cells declined following treatment with L-DOPA or dopamine in a concentration- and time-dependent manner. The decrease in cell viability by L-DOPA (10+/-4% of control) or dopamine (15+/-4% of control) was markedly attenuated by antioxidants (ascorbic acid, glutathione, N-acetyl-L-cysteine and sodium metabisulphite). Autoxidation of L-DOPA or dopamine was accompanied by the formation of H(2)O(2) in a time-dependent manner, this being completely prevented by ascorbic acid at 24 h or markedly reduced at 48 h. 4. Protective effects of 100 U ml(-1) catalase (40+/-1% of control) against L-DOPA-induced cell death were lower than those conferred by 200 microM ascorbic acid (70+/-3% of control). Catalase-induced protection (59+/-5% of control) against dopamine-induced cell death was similar to that conferred by 200 microM ascorbic acid (57+/-4% of control). L-DOPA-induced neuronal cell death was also accompanied by increases in caspase-3 activity, this being insensitive to ascorbic acid. Dopamine-induced increase in caspase-3 activity occurred only when autoxidation of the amine was prevented by ascorbic acid. 5. It is suggested that in addition to generation of H(2)O(2) and quinone formation, L-DOPA- and dopamine-induced cell death may result from induction of apoptosis, as evidenced by increases in caspase-3 activity. Dopamine per se induces apoptosis by a mechanism independent of oxidative stress, as evidenced by the fact that increases in caspase-3 activity occurred only when autoxidation of the amine was prevented.

Metabolism of two new antiepileptic drugs and their principal metabolites S(+)- and R(-)-10,11-dihydro-10-hydroxy carbamazepine

BIA 2-093 and BIA 2-059 are two stereoisomers under development as new antiepileptic drugs. They act as prodrugs for the corresponding hydroxy derivatives (S(+)- or R(-)-10,11-dihydro-10-hydroxy carbamazepine, respectively) which are known to be the active metabolites of the antiepileptic drug oxcarbazepine (OXC). The purpose of this study was to define the metabolic pathway especially in terms of stereoselectivity, and to estimate the possibility of racemization in humans. For in vivo studies, the rat, mouse and rabbit were chosen as models in order to cover a broad spectrum of metabolic activity. In addition, incubations with liver microsomes from these three species plus dog and monkey were compared to results obtained with human liver microsomes. It was found that both drugs were almost instantly hydrolysed to the corresponding 10-hydroxy compounds in mice, rats and rabbits. Mice and rabbits were not able to oxidize the 10-hydroxy compounds to OXC in significant amounts. In the rat, BIA 2-093 also gave origin to OXC, whereas BIA 2-059 resulted in the formation of OXC and the trans-diol metabolite in equal amounts. It could be shown that the rat is able to reduce the formed OXC in liver to S(+)-10-hydroxy metabolite, resulting in a loss of enantiomeric purity after treatment with BIA 2-059 rather than in the case of BIA 2-093. Human liver microsomes hydrolysed BIA 2-093 and BIA 2-059 to their corresponding 10-hydroxy compounds and to OXC in a very small extent with BIA 2-093 only. Therefore, BIA 2-093 and BIA 2-059 seem to be preferable drugs over OXC since they most likely exhibit a 'cleaner' metabolism. From a therapeutic point of view BIA 2-059 would be less appropriate than BIA 2-093 for the purpose of treating epileptic patients due to its propensity to undergo inactivation to the trans-diol.

Interaction of the novel anticonvulsant, BIA 2-093, with voltage-gated sodium channels: comparison with carbamazepine

PURPOSE

BIA 2-093 [(S)-(-)-10-acetoxy-10,11-dihydro-5H-dibenz/b,f/azepine-5-carboxamide] is endowed with an anticonvulsant potency similar to that of carbamazepine (CBZ), but produces less cognitive and motor impairment. This study evaluated whether voltage-gated sodium channels (VGSCs) are a primary locus for the action of BIA 2-093.

METHODS

We used the whole-cell voltage-clamp technique in the mouse neuroblastoma cell line N1E-115 to investigate the effects of BIA 2-093 and CBZ on VGSCs, displacement of [3H]-batrachotoxinin A 20-alpha-benzoate ([3H]-BTX), and [3H]-saxitoxin to define their relative potency to bind to rat brain sodium channels, and inhibition of uptake of 22Na by rat brain cortical synaptosomes stimulated by veratridine as a measure of sodium entry.

RESULTS

The inhibitory potencies of BIA 2-093 and CBZ increased as the holding potential was made less negative (-100, -90, -80, and -70 mV) with median inhibitory concentration (IC50) values (in microM) of, respectively, 4,337, 618, 238, and 139 for BIA 2-093, and 1,506, 594, 194, and 101 for CBZ. BIA 2-093 displayed a similar potency in displacing [3H]-BTX (IC50 values, 222 vs. 361 microM; p > 0.05) and inhibiting the uptake of 22Na (IC50 values, 36 vs. 138 microM; p > 0.05). Both drugs failed to displace [3H]-saxitoxin in concentrations up to 300 microM.

CONCLUSIONS

BIA 2-093, like CBZ, inhibits sodium currents in a voltage-dependent way by an interaction predominantly with the inactivated state of the channel and interacts with neurotoxin receptor site 2, but not with receptor site 1. BIA 2-093 displayed a potency blocking VGSCs similar to that of CBZ.

Carbamazepine inhibits L-type Ca2+ channels in cultured rat hippocampal neurons stimulated with glutamate receptor agonists

In order to better understand the mechanism(s) of action of carbamazepine (CBZ), we studied its effects on the increase in [Ca2+]i and [Na+]i stimulated by glutamate ionotropic receptor agonists, in cultured rat hippocampal neurons, as followed by indo- or SBFI fluorescence, respectively. CBZ inhibited the increase in [Ca2+]i stimulated either by glutamate, kainate, alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionate (AMPA), or N-methyl-D-aspartate (NMDA), in a concentration-dependent manner. In order to discriminate the effects of CBZ on the activation of glutamate receptors from possible effects on Ca2+ channels, we determined the inhibitory effects of Ca2+ channel blockers on [Ca2+]i changes in the absence or in the presence of CBZ. The presence of 1 microM nitrendipine, 0.5 microM omega-conotoxin GVIA (omega-CgTx GVIA), or of both blockers, inhibited the kainate-stimulated increase in [Ca2+]i by 51.6, 32.9 or 68.7%, respectively. In the presence of both 100 microM CBZ and nitrendipine, the inhibition was similar (54.1%) to that obtained with nitrendipine alone, but in the presence of both CBZ and omega-CgTx GVIA, the inhibition was greater (54%) than that caused by omega-CgTx GVIA alone. However, CBZ did not inhibit the increase in [Na+]i stimulated by the glutamate receptor agonists, but inhibited the increase in [Na+]i due to veratridine. Tetrodotoxin, or MK-801, did not inhibit the influx of Na+ stimulated by kainate, indicating that Na+ influx occurs mainly through the glutamate ionotropic non-NMDA receptors. Moreover, LY 303070, a specific AMPA receptor antagonist, inhibited the [Na+]i response to kainate or AMPA by about 70 or 80%, respectively, suggesting that AMPA receptors are mainly involved. Taken together, the results suggest that CBZ inhibits L-type Ca2+ channels and Na+ channels, but does not inhibit activation of glutamate ionotropic receptors.

L-DOPA transport properties in an immortalised cell line of rat capillary cerebral endothelial cells, RBE 4

The present study aimed to determine the kinetics of L-3, 4-dihydroxyphenylalanine (L-DOPA) uptake in an immortalised cell line of rat capillary cerebral endothelial cells (clones RBE 4 and RBE 4B), to define the type of inhibition produced by L-5-hydroxytryptophan (L-5-HTP), 2-aminobicyclo(2,2,1)-heptane-2-carboxylic acid (BHC) and N-(methylamino)-isobutyric acid (MeAlB) and its sodium dependence. Non-linear analysis of the saturation curves for L-DOPA and L-5-HTP revealed in RBE 4 cells Km values (in microM) of 72 and 102 and in RBE 4B cells Km values (in microM) of 60 and 118, respectively. IC50 values for L-5-HTP (RBE 4, 1026 microM; RBE 4B, 831 microM) obtained in the presence of a nearly saturating (250 microM) concentration of L-DOPA were almost 5-fold those obtained when non-saturating (25 microM) concentrations of L-DOPA were used. IC50 values for BHC obtained in the presence of a nearly saturating (250 microM) concentration of L-DOPA were also 6- to 5-fold those obtained when non-saturating (25 microM) concentrations of L-DOPA were used. MeAlB (up to 2.5 mM) was found not to interfere with the uptake of L-DOPA. In RBE 4 cells, Vmax values for L-DOPA uptake were identical in the absence and the presence of 150 microM L-5-HTP or 150 microM BHC, but Km values (microM) were significantly greater (P<0.05) when L-DOPA uptake was studied in the presence of L-5-HTP or BHC. Similar findings were observed when RBE 4B cells were used. Uptake of (250 microM) L-DOPA in the absence of sodium in the incubation medium was similar to that observed in the presence of increasing concentrations of sodium (20 to 140 mM). It is concluded that RBE 4 and RBE 4B cells are endowed with the L-type amino acid transporter through which L-DOPA and L-5-HTP can be taken up, and suggested that this immortalised cell line of rat capillary cerebral endothelium might constitute an interesting in vitro model for the study of BBB mechanisms, namely those concerning solute and nutrient transfer across the brain capillary endothelium.

Regulation of dopamine synthesis in the rat kidney

1. Both animal and clinical studies clearly suggest a close link between dopamine production and renal delivery of sodium. 2. It appears that tubular transport of sodium and DOPA could represent the rate limiting step for the renal synthesis of dopamine; the membrane transport mechanism is sensitive to the nucleotide cyclic GMP. 3. In sodium overloaded animals AAAD activity appears to be greater than that in controls, but the tubular transport of L-DOPA might be decreased. 4. Since the final product of an increased sodium delivery to the kidney is an increased urinary excretion of dopamine, it might be hypothesized that the outward transport of newly formed dopamine in tubular epithelial cells is also affected by high tubular sodium.

Cell inward transport of L-DOPA and 3-O-methyl-L-DOPA in rat renal tubules

1. The present study has determined the kinetics of the uptake of L-3,4-dihydroxyphenylalanine (L-DOPA) and 3-O-methyl-L-DOPA (3-OMDOPA) in rat renal tubules and examined the effect of 3-OMDOPA on the inward transport of L-DOPA and on its conversion into dopamine in kidney homogenates. 2. The accumulation of both L-DOPA and 3-OMDOPA in renal tubules was found to occur through non-saturable and saturable mechanisms. The kinetics of the saturable component of L-DOPA and 3-OMDOPA uptake in renal tubules were as follows: L-DOPA, Vmax = 11.1 nmol mg-1 protein h-1 and Km = 216 microM (n = 6); 3-OMDOPA, Vmax = 8.1 nmol mg-1 protein h-1 and Km = 231 microM (n = 5). The diffusion constant of the non-saturable component for the accumulation of L-DOPA and 3-OMDOPA was 0.0010 and 0.0014 mumol-1, respectively. 3. 3-OMDOPA (100 to 2000 microM) was found to produce a concentration-dependent decrease (29% to 81% reduction) of the saturable component of the tubular uptake of L-DOPA; the Ki value of 3-OMDOPA for inhibition of L-DOPA uptake was found to be 181 microM (n = 5). The accumulation of L-DOPA obtained in experiments conducted at 4 degrees C was not affected by 3-OMDOPA. 4. In experiments conducted in kidney homogenates only L-DOPA (10 to 5000 microM) was found to be decarboxylated. The Vmax and Km values for aromatic L-amino acid decarboxylase determined in the absence of 3-OMDOPA (Vmax = 14.1 nmol mg-1 protein h-1; Km =62 MicroM) were not significantly different from those observed when the decarboxylation of L-DOPA was carried out in the presence of 1000 MicroM 3-OMDOPA (Vmax = 15.7 nmol mg-1 protein h-1; Km = 68 MicroM).5. It is concluded that the tubular uptake of both L-DOPA and 3-OMDOPA occur through nonsaturable and saturable mechanisms; only the saturable tubular uptake of L-DOPA was found to be inhibited by 3-OMDOPA. It is further shown that 3-OMDOPA neither undergoes decarboxylation into 3-MT nor affects the decarboxylation of L-DOPA.

Effects of tolcapone upon soluble and membrane-bound brain and liver catechol-O-methyltransferase

The present study was aimed to evaluate the sensitivity of soluble (S) and membrane bound (MB) catechol-O-methyltransferase (COMT) from rat brain and liver to inhibitors which interact with the enzyme as competitive (tropolone), non-competitive (S-adenosyl-l-homocysteine; SAHC) and tight-binding (tolcapone and 3,5-dinitrocatechol) inhibitors. COMT activity was evaluated by the ability to methylate adrenaline (0.1 to 2000 microM) to metanephrine in the presence of a saturating concentration of the methyl donor (S-adenosyl-l-methionine). When using a fixed amount of total protein (2 micrograms/ml), but variable concentrations of COMT, the inhibitory potency of tolcapone upon S- and MB-COMT activity in the brain was in the low nM range (IC50's of 2 and 3 nM, respectively), whereas in liver the IC50 values for tolcapone against liver MB- and S-COMT (IC50's of 123 and 795 nM, respectively) were markedly higher than those observed in the brain. By contrast, when inhibition studies were performed with a fixed concentration of COMT (15 nM), as determined by the Ackermann-Potter equation, tolcapone was found to be endowed with the same potency (in the low nM range) in inhibiting S- and MB-COMT from both brain and liver. As for tolcapone, 3,5-dinitrocatechol was more potent against MB- than against S-COMT when a fixed amount of total protein was used, but showed the same potency when a fixed concentration of COMT was used. Tropolone, a competitive inhibitor, was much less potent than tolcapone and 3,5-dinitrocatechol in inhibiting S- and MB-COMT from both brain and liver and its potency was found not to depend on enzyme concentration. SAHC, a non-competitive inhibitor, behaved similarly to tight-binding inhibitors when a fixed amount of total protein was used. By contrast, when a fixed amount of enzyme was used, SAHC was found to be endowed with the same potency against S- and MB-COMT from brain and liver. In the final series of experiments the inhibitory effect of tolcapone was examined under in vitro ex vivo conditions, using the same concentration of COMT (15 nM). One hour after its oral administration, tolcapone (0.3 to 30 mg/kg) was found to be much more potent against MB-COMT than against S-COMT. In the liver, 0.3 mg/kg tolcapone resulted in 82% inhibition of MB-COMT and 31% inhibition of S-COMT. In the brain, 3.0 mg/kg tolcapone inhibited 78% MB-COMT, whereas S-COMT activity was reduced by 38% only. In conclusion, the results reported here show that tolcapone is particularly potent in inhibiting MB-COMT from liver and brain under in vivo experimental conditions, though it does not discriminate between MB- and S-COMT under in vitro experimental conditions when using the same amount of enzyme in the assay.

Assessment of renal dopaminergic system activity in the nitric oxide-deprived hypertensive rat model

1. The present paper reports changes in the urinary excretion of dopamine, 5-hydroxytryptamine and amine metabolites in nitric oxide deprived hypertensive rats during long-term administration of NG-nitro-L-arginine methyl ester (L-NAME). Aromatic L-amino acid decarboxylase (AAAD) activity in renal tissues and the ability of newly-formed dopamine to leave the cellular compartment where the synthesis of the amine has occurred were also determined. 2. Twenty four hours after exposure to L-NAME, both systolic (SBP) and diastolic (DBP) blood pressure were increased by 20 mmHg; heart rate was slightly decreased. During the next 13 days both SBP and DBP increased progressively reaching 170 +/- 3 and 116 +/- 3 mmHg, respectively. 3. Baseline urinary excretion of L-DOPA, dopamine, 3,4-dihydroxyphenylacetic acid (DOPAC), 3-methoxytyramine (3-MT) and homovanillic acid (HVA) during the 4 day period of stabilization averaged 4.4 +/- 0.5, 13.8 +/- 0.3, 37.4 +/- 0.8, 180.0 +/- 2.7 and 206.1 +/- 6.7 nmol day-1, respectively. The urinary excretion of L-DOPA, dopamine and DOPAC, but not that of 3-MT and HVA, were increased from day 6-8 of L-NAME administration onwards (L-DOPA, up to 13.4 +/- 2.1; dopamine, up to 23.0 +/- 1.6; DOPAC, up to 62.8 +/- 3.7 nmol day-1). Baseline daily urinary excretion of 5-hydroxytryptamine and 5-hydroxyindolacetic acid (5-HIAA) averaged 73.5 +/- 1.1 and 241.7 +/- 5.4 nmol day-1, respectively. During the first week of L-NAME administration, the urinary excretion of both 5-hydroxytryptamine and 5-HIAA did not change significantly; however, as was found with dopamine and DOPAC, changes in the urinary excretion of 5-hydroxytryptamine were evident during the second week of L-NAME administration. 4. In experiments performed on homogenates of isolated renal tubules, the decarboxylation of L-DOPA to dopamine was dependent on the concentration of L-DOPA used (10 to 5000 microM) and saturable at 1000 microM. AAAD activity as determined in homogenates (Vmax, in nmol mg-1 protein h-1; Km in microM) was significantly (P < 0.01) higher in rats given L-NAME for 14 days (Vmax = 25 +/- 2; Km = 72 +/- 10) than in control rats (Vmax = 14 +/- 1; Km = 63 +/- 7), rats given L-NAME for 7 days (Vmax = 15 +/- 1; Km = 69 +/- 5) and rats given L-NAME plus L-arginine (Vmax = 13 +/- 1; Km = 60 +/- 3) for 14 days. 5. A considerable amount of the total dopamine formed from added L-DOPA in kidney slices escaped into the incubation medium. The application of the Michaelis-Menten equation to the net transport of newly-formed dopamine allowed the identification of a saturable (carrier-mediated transfer) and a non-saturable component (diffusion). No significant differences in the diffusional rate of transfer(0.14 +/- 0.02 micro mol-1) were observed between the four experimental groups. However, the saturable outward transfer of dopamine (Vmax, in micromol mg-1 protein h-1; Km in microM) was higher in control animals(Vmax= 2.3 +/- 0.2; Km = 568 +/- 67) than that in rats treated with L-NAME for 14 days (Vmax = 0.8 +/- 0.02;Km = 241 +/- 21), but similar to that observed in rats receiving L-NAME plus L-arginine (Vmax= 2.4+/- 0.2; Km= 618 +/- 61); the saturable dopamine outward rate of transfer in rats given L-NAME for 7days (Vmax = 3.9 +/- 0.2; Km = 1006 +/- 32) was higher than in controls.6. In conclusion, the present studies show that the hypertensive response resulting from the long-term administration of L-NAME is accompanied by an increased urinary excretion of dopamine and 5-hydroxytryptamine, which appears to follow an enhanced activity of renal AAAD. The observation that the increased AAAD activity can be reversed by the administration of L-arginine to L-NAME treated rats favours the view that the adaptational response which results in an enhanced AAAD activity probably involves a decrease in the generation of nitric oxide.

Defective D1-like receptor-mediated inhibition of the Cl-/HCO3- exchanger in immortalized SHR proximal tubular epithelial cells

The sensitivity of the Cl(-)/HCO(3)(-) exchanger to dopamine D(1)- and D(2)-like receptor stimulation in immortalized renal proximal tubular epithelial cells from the spontaneous hypertensive rat (SHR) and Wistar-Kyoto rat (WKY) was examined. The activity of the Cl(-)/HCO(3)(-) exchanger (in pH U/s) in SHR cells (0.00191) was greater than in WKY cells (0.00126). The activity of Cl(-)/HCO(3)(-) exchanger was exclusively observed at the apical cell side and probably occurs through the SLC26A6 anion transporter that is expressed in both WKY and SHR cells. Stimulation of D(1)-like receptors with SKF-38393 markedly attenuated the HCO(3)(-)-dependent intracellular pH recovery in WKY cells but not in SHR cells. Stimulation of D(2)-like receptors with quinerolane did not alter Cl(-)/HCO(3)(-) exchanger activity in both WKY and SHR cells. The selective D(1)-like receptor antagonist SKF-83566 prevented the effect of SKF-38393. Both WKY and SHR cells responded to dibutyryl-cAMP (DBcAMP) with inhibition of the Cl(-)/HCO(3)(-) exchanger, and downregulation of PKA (overnight exposure to DBcAMP) abolished the inhibitory effect of both DBcAMP and SKF-38393 in WKY cells. Both SHR and WKY cells responded to forskolin with increases in the formation of cAMP. However, only WKY responded to SKF-38393 with increases in the formation of cAMP that was prevented by SKF-83566. It is concluded that WKY cells respond to D(1)-like dopamine receptor stimulation with inhibition of the apical Cl(-)/HCO(3)(-) (SLC26A6) exchanger and SHR cells have a defective D(1)-like dopamine response.

Increased cerebrospinal fluid dopamine and 3,4-dihydroxyphenylacetic acid levels in Huntington's disease: evidence for an overactive dopaminergic brain transmission

Levels of dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), noradrenaline (NA), 3-methoxy-4-hydroxyphenylglycol (MHPG), and 5-hydroxyindoleacetic acid (5-HIAA) in the CSF of patients with Huntington's disease (HD) were measured by HPLC. CSF DA, DOPAC, and MHPG levels were found to be increased in HD patients. Levels of HVA, 5-HIAA, and NA in the CSF of HD patients did not differ from those of controls. Changes in CSF DA and DOPAC levels were consistent with previous findings of increased DA tissue content in some brain areas of patients with HD. These results suggest that CSF DOPAC levels could be a more reliable index of overactive dopaminergic brain systems in HD than CSF HVA levels.

Dopamine formation, from its immediate precursor 3,4-dihydroxyphenylalanine, along the rat digestive tract

The formation of dopamine, from L-3,4-dihydroxyphenylalanine (L-dopa), in fragments of non-glandular and glandular stomach, duodenum, jejunum, ileum and proximal and distal colon of the rat was examined. The deamination of newly-formed dopamine into 3,4-dihydroxyphenylacetic acid (dopac) was also studied. The synthesis of dopamine in tissues incubated with 500 microM L-dopa for 20 min in conditions of catechol-O-methyltransferase (Comt) inhibition was found to be in the duodenum, jejunum and ileum 2-fold that in the proximal colon, 6-fold that in the glandular stomach and 120-fold that in the non-glandular stomach and distal colon. The formation of dopac in these tissues followed the pattern of amine formation. In the jejunum, the formation of dopamine and dopac was found to be dependent on the concentration of L-dopa (50 to 5000 microM) used. In another set of experiments, it was found that the formation of dopamine in jejunal segments loaded with increasing concentrations of L-dopa (50, 100 and 500 microM) was a time-dependent process. The rate constant (k) of formation of dopamine as a function of time was found to be similar (0.050 +/- 0.005) with either concentration of L-dopa; the rate constant of dopac formation in these experiments was, in contrast, found to be greater at the highest concentrations of L-dopa (100 and 500 microM). Aromatic L-amino acid decarboxylase (Aaad) activity determined in homogenates of the jejunal mucosa was found to be twice that observed in homogenates of the remaining jejunal wall (muscular).(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacological profile of opicapone, a third-generation nitrocatechol catechol-O-methyl transferase inhibitor, in the rat

BACKGROUND AND PURPOSE

Catechol-O-methyltransferase (COMT) is an important target in the levodopa treatment of Parkinson's disease; however, the inhibitors available have problems, and not all patients benefit from their efficacy. Opicapone was developed to overcome those limitations. In this study, opicapone's pharmacological properties were evaluated as well as its potential cytotoxic effects.

EXPERIMENTAL APPROACH

The pharmacodynamic effects of opicapone were explored by evaluating rat COMT activity and levodopa pharmacokinetics, in the periphery through microdialysis and in whole brain. The potential cytotoxicity risk of opicapone was explored in human hepatocytes by assessing cellular ATP content and mitochondrial membrane potential.

KEY RESULTS

Opicapone inhibited rat peripheral COMT with ED50 values below 1.4 mg⋅kg(-1) up to 6 h post-administration. The effect was sustained over the first 8 h and by 24 h COMT had not returned to control values. A single administration of opicapone resulted in increased and sustained plasma levodopa levels with a concomitant reduction in 3-O-methyldopa from 2 h up to 24 h post-administration, while tolcapone produced significant effects only at 2 h post-administration. The effects of opicapone on brain catecholamines after levodopa administration were sustained up to 24 h post-administration. Opicapone was also the least potent compound in decreasing both the mitochondrial membrane potential and the ATP content in human primary hepatocytes after a 24 h incubation period.

CONCLUSIONS AND IMPLICATIONS

Opicapone has a prolonged inhibitory effect on peripheral COMT, which extends the bioavailability of levodopa, without inducing toxicity. Thus, it exhibits some improved properties compared to the currently available COMT inhibitors.

A kinetic study of the rate of formation of dopamine, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in the brain of the rat: implications for the origin of DOPAC

The rates of disappearance of dopamine, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in the brain of rats treated with the tyrosine hydroxylase inhibitor alpha-methyl-p-tyrosine were measured. Levels of dopamine. DOPAC and HVA in tissue were quantified in the striatum, nucleus accumbens and frontal cortex of the rat by means of high pressure liquid chromatography with electrochemical detection. Rats were sacrificed 1, 2 or 3 hr after the administration of alpha-methyl-p-tyrosine (200 mg/kg, i.p.). Levels of dopamine, DOPAC and HVA in tissue were logarithmically transformed, plotted against the duration of inhibition of tyrosine hydroxylase and the rate constant of the decline of dopamine and of its metabolites calculated. The rate constant of decline of DOPAC in each of brain studied was found to be greater than that for dopamine. The rate constant for the decline of dopamine was found to be greater in areas of the brain presenting the largest DOPAC/dopamine tissue ratios (frontal cortex greater than nucleus accumbens greater than striatum). The present results suggest that a substantial amount of DOPAC in brain derives from a newly formed pool of dopamine.

Ontogenic aspects of liver and kidney catechol-O-methyltransferase sensitivity to tolcapone

1. The present work describes the catechol-O-methyltransferase (COMT) activities in the liver and kidney of developing and adult rats (aged 3, 6, 9, 18, 30 and 60 days; n = 5 per group) and evaluates the enzyme sensitivity to inhibition by tolcapone, a reversible COMT inhibitor. 2. COMT activity, evaluated by the ability to methylate adrenaline to metanephrine, was determined in liver and kidney homogenates prepared in 0.5 mM phosphate buffer (pH = 7.8) containing pargyline (0.1 mM), MgCl2 (0.1 mM), EGTA (1 mM) and S-adenosyl-L-methionine (0.1 mM). Vmax (in nmol mg-1 protein h-1) of liver COMT was found to decrease gradually with age, from 5.3 +/- 0.5 at the age of 3 days up to 2.9 +/- 0.2 at the age of 60 days; for the same age range, Km values (in microM; geometric means with 95% confidence limits) increased from 3.3 (1.0, 7.5) up to 13.1 (2.1, 24.1). At the age of 3 days, Vmax values for kidney COMT (2.6 +/- 0.1) were lower than those for the liver COMT. However, Vmax values for kidney COMT were found to increase up to 6.2 +/- 0.6 at the age of 18 days and then declined by 44% at the age of 30 and 60 days. In kidney, aging was also accompanied by an increase in Km values for COMT (from 2.7 [1.1, 4.3] up to 24.0 [11.7, 36.3]). 3. The sensitivity of liver and renal COMT activity to tolcapone was markedly dependent on the age, 3-days old rats being more sensitive to tolcapone than older animals. The IC50 values (in nM) for inhibition of liver COMT by tolcapone increased gradually with age, from 41 (26, 65) at the age of 3 days up to 720 (640, 800) at the age of 60 days. As was found in the liver, IC50 values (in nM) for inhibition of kidney COMT by tolcapone also increased with age, from 8 (6, 10) at the age of 3 days up to 177 (131, 240) at the age of 60 days. In all experimental groups, the IC50 values for inhibition of liver COMT by tolcapone was higher than those for kidney COMT. 4. In conclusion, these results suggest that aging is accompanied by a decrease in liver and kidney COMT affinity for the substrate (evidenced by the increase in Km values) and a decrease in sensitivity towards inhibition by tolcapone (evidenced by the increase in IC50 values). Furthermore, kidney COMT is shown to be more sensitive to inhibition by tolcapone than liver COMT, irrespective of the age of the animal.

Endogenous dopamine modulates jejunal sodium absorption during high-salt diet in young but not in adult rats

BACKGROUND/AIMS

This study was designed to investigate the contribution of endogenous catecholamines to the regulation of small intestinal sodium transport during postnatal development.

METHODS

Jejunal permeability was determined by a constant perfusion, nonabsorbable marker technique in weanling, adolescent, and adult rats fed either a high-salt diet or normal-salt diet. Tissue catecholamine levels were determined by high-performance liquid chromatography with electrochemical detection.

RESULTS

In 20-day-old but not in 40-day-old rats, a significantly lower net sodium absorption was observed during high-salt diet compared with age-matched controls on normal-salt diet. Inhibition of dopamine synthesis significantly increased the net sodium absorption in 20-day-old rats on high-salt diet compared with untreated 20-day-old rats on high-salt diet. The basal levels of dopamine in 20-day-old rats were twofold higher than in 40-day-old rats. During high-salt diet, both age groups responded with an increase in dopamine production. Norepinephrine levels were significantly higher (30-fold) in 20-day-old rats than in 40-day-old rats, but norepinephrine content was not significantly changed during high-salt diet in either groups.

CONCLUSIONS

The results indicate that weanling animals have a greater jejunal sodium absorption than older animals, probably because of higher noradrenergic tonus. A challenge with a high-salt diet results in a decrease of the intestinal sodium absorption in weaning rats but not in adult rats; endogenous dopamine appears to play an important role in this regulation.

Expression of LAT1 and LAT2 amino acid transporters in human and rat intestinal epithelial cells

The present study evaluated the presence of LAT1 and LAT2 amino acid transporters in human Caco-2 cells and rat IEC-6 cells along the mucosa of the rat digestive tract. The LAT1 cDNA was amplified by PCR using two sets of primers (one specific for rat LAT1 and another simultaneously specific for human, rat and mice). The LAT2 cDNA was amplified by PCR using one set of primers simultaneously specific for human, rat and mice LAT2. The presence of LAT1 and LAT2 protein was examined by means of immunoblotting using an antibody raised against the rat LAT1 and mouse LAT2. Caco-2 and IEC-6 cells, as well as the rat intestinal mucosa, are endowed with both LAT1 and LAT2 transporter transcripts and protein. LAT1 protein is most abundant in IEC-6 cells, which is in agreement with functional data previously reported. The findings in the rat intestinal mucosa indicate that LAT1 protein is most abundant in the colon and its abundance markedly decreases at the level of jejunum and ileum, which contrast with relative homogeneous presence of LAT2 across the digestive tract. In conclusion, Caco-2 and IEC-6 cells, as well as the rat intestinal mucosa, are endowed with both LAT1 and LAT2 amino acid transporter transcripts and protein.

Comparative study of ortho- and meta-nitrated inhibitors of catechol-O-methyltransferase: interactions with the active site and regioselectivity of O-methylation

In this work, we present a comparative case study of "ortho-" and "meta-nitrated" catecholic inhibitors of catechol-O-methyltransferase (COMT), with regard to their interaction with the catalytic site of the enzyme and the in vitro regioselective formation of their mono-O-methyl ether metabolites. In particular, the effects of altering the attachment position of the inhibitors' side-chain substituent, within the classic nitrocatechol pharmacophore, were investigated. For this purpose, we compared two simple regioisomeric nitrocatechol-type inhibitors of COMT, BIA 3-228 and BIA 8-176, which contain the benzoyl substituent attached at the meta and ortho positions, respectively, relative to the nitro group. The two compounds were slowly O-methylated by COMT in vitro, but the particular substitution pattern of each compound was shown to have a profound impact on the regioselectivity of their O-methylation. To provide a plausible interpretation of these results, a comprehensive analysis of the protein-inhibitor interactions and of the relative chemical susceptibility to O-methylation of the catechol hydroxyl groups was performed by means of docking simulations and ab initio molecular orbital calculations. The major structural and chemical factors that determine the enzyme regioselectivity of O-methylation were identified, and the X-ray structure of the complex of COMT with S-adenosyl-l-methionine and BIA 8-176 is herein disclosed. This is the first reported structure of the soluble form of COMT complexed with a nitrocatecholic inhibitor having a bulky substituent group in adjacent position (ortho) to the nitro group. Structural and dynamic aspects of this complex are analyzed and discussed, in the context of the present study.

A comparison between the pattern of dopamine and noradrenaline release from sympathetic neurones of the dog mesenteric artery

The release of dopamine and noradrenaline (NA), from the main trunk of the mesenteric artery and its proximal branches elicited by electrical nerve stimulation and K+, has been measured by using high pressure liquid chromatography with electrochemical detection. Both stimuli released dopamine and NA. With the main trunk of the mesenteric artery, dopamine represented 8% of the NA tissue content; the dopamine/NA ratio in the catecholamine overflow caused by nerve stimulation or K+-induced depolarization also averaged 8%. For the proximal branches the tissue dopamine/NA ratio was significantly greater than that observed to occur in the overflow caused by nerve stimulation and K+. When the perifusion with a K+-enriched medium was extended to 120 min the amount of NA released from both the main trunk and the proximal branches progressively declined. The same pattern of release was observed for dopamine in the main trunk, whereas for the proximal branches dopamine overflow did not decline throughout the perifusion period. The addition of alpha-methyl-p-tyrosine did not change the pattern of amine overflow. Our interpretation of these results is that both dopamine and NA are derived from the same sympathetic neurone. In the proximal branches of the mesenteric artery dopamine and NA appear to be in two different storage structures, whereas in the main trunk both dopamine and NA are located in only one storage structure.