Reversal of acute theophylline toxicity by calcium channel blockers in dogs and rats

Theophylline, widely used in the treatment of pulmonary diseases, has a narrow therapeutic index; the recommended plasma levels being 10–20 μg/ml in humans. The misuse or abuse of theophylline can cause life-threatening central nervous system and cardiovascular effects. Increased intracellular Ca²⁺ levels are thought to play an important role in theophylline toxicity and death. The objective of this study was to determine whether Ca²⁺ channel blockers, e.g. verapamil, nifedipine, or diltiazem, prevent sudden death caused by theophylline treatment in rats and dogs. Groups of Sprague-Dawley rats were treated with theophylline alone (150 mg/kg i.p.) or with theophylline pretreatment followed by administration of verapamil (0.25 to 0.5 mg/kg i.p.), nifedipine (0.25 to 1.0 mg/kg i.p.), or diltiazem (0.5 to 1.0 mg/kg i.p.), 2.5 to 15 min later. The rats were observed for toxic signs and survival over a period of 15 days. All three calcium channel blockers significantly reduced the theophylline-induced sudden death in rats. In a separate study, neither verapamil (0.5 mg/kg i.p.) nor nifedipine (1.0 mg/kg i.p.) prevented the theophylline-induced myocardial necrosis in the rat. In beagle dogs, verapamil (0.5 mg/kg i.v.) prevented theophylline (15 mg/kg/min i.v. for 10 min)-induced hypotension, arrhythmias, and sudden death. Our results support previously reported findings that calcium plays a major role in theophylline-induced toxicity and death.     

Seizures induced by theophylline and isoniazid in mice.

Isoniazid-induced seizures respond poorly to anticonvulsants but well to pyridoxine (Vitamin B6); theophylline produces difficult-to-treat seizures with substantial morbidity and mortality. Theophylline therapy depresses plasma pyridoxal-5'-phosphate (PLP), the active metabolite of pyridoxine, suggesting that theophylline-induced seizures might be amenable to treatment with pyridoxine. Our study established the dose-response relationship for convulsions due to isoniazid and theophylline in mice and determined if pyridoxine antagonized such seizures. Female CD-1 outbred mice weighing 25 to 30 g were used. Clonic seizures had clonic activity lasting 5 sec; tonic seizures had loss of the righting reflex with tonic hindlimb extension. Groups of 10 mice received single doses of 50, 100, 150, 200, 250 or 300 mg aminophylline/kg i.p. or 100, 150, 200, 250, 300 or 350 mg isoniazid/kg i.p. and were observed for seizures or death. Pyridoxine or saline with aminophylline or isoniazid were administered simultaneously. The LD50 for aminophylline was 266 mg/kg; for isoniazid it was 160 mg/kg. Doses of 150 mg aminophylline/kg or 100 mg isoniazid/kg did not induce seizures. Pyridoxine with aminophylline or isoniazid did not alter the frequency or time of onset of seizures or death. This was unexpected because pyridoxine antagonizes theophylline-induced seizures in mice and reverses isoniazid-induced seizures in humans. We found no evidence that PLP depletion in mice is a mechanism for seizures induced by isoniazid or aminophylline in a fashion similar to isoniazid in humans.     

Antagonism of caffeine-induced convulsions by ethanol and dizocilpine (MK-801) in mice

Ethanol (1 and 2 g/kg, i.p.) and MK-801 (1 mg/kg, i.p.), N-methyl-D-aspartate (NMDA) receptor antagonist, offered protection against caffeine-induced convulsions in mice. Subeffective doses of ethanol (0.5-g/kg, i.p.) and MK-801 (0.5 mg/kg, i.p.) when administered concurrently, did not provide a facilitatory anticonvulsant action against caffeine-induced convulsions. Ethanol (0.5 g/kg, i.p.) when administered concurrently with adenosine (100 mg/kg, i.p.) or dipyridamole (5 mg/kg, i.p.) elicited a facilitatory anticonvulsant action. However, concurrent administration of subeffective doses of MK-801 (0.5 mg/kg i.p.) with adenosine (100 mg/kg, i.p.) did not offer a facilitatory anticonvulsant action against caffeine-induced convulsions. The protective effect of ethanol (1 g/kg, i.p.) against caffeine-induced convulsions was reversed by an imidazobenzodiazepine, Ro 15-4513 (4 mg/kg, i.p.). Ro 15-4513 did not produce any proconvulsant effect with caffeine. It is suggested that ethanol and MK-801 elicit their anticonvulsant actions against caffeine-induced convulsions through different receptor mechanisms and that the anticonvulsant action of ethanol may be partly attributed to its ability to act via central adenosinergic mechanisms.     

Influence of adenosine agonists and antiepileptic drugs on theophylline-induced seizures in rats

Seizures is a major toxicity of theophylline. The mechanism of theophylline-induced seizures is not known, but antagonism at adenosine receptors may be a possibility. The effect of pretreatment with different doses of adenosine (100, 500 and 1000 mg/kg, i.p.), and the adenosine A1 receptor agonist N6-cyclopentyladenosine (CPA), 1, 5 and 10 mg/kg, i.p., was studied against seizures induced by theophylline in rats. Both these drugs, at all dose levels tested, failed to protect theophylline seizures. Thus adenosinergic system is unlikely to be involved in mediating the convulsant action of theophylline. On the other hand, the conventional antiepileptic drugs, i.e. diazepam (4 mg/kg), sodium valproate (300 mg/kg) and phenobarbitone (50 mg/kg), but not carbamazepine, afforded some protection. The modification of course of seizures, by the antiepileptic drugs suggests the involvement of some other alternate mechanism in theophylline-induced seizures.     

Tolerance to the convulsions induced by daily nicotine treatment in rats.

Development of tolerance to the nicotine-induced convulsions in rats was examined. Acute intraperitoneal (i.p.) administration of nicotine (2.5, 3.75 and 5 mg/kg) produced convulsions in a dose-dependent manner. Mecamylamine (1 mg/kg, i.p.) antagonized the convulsions, but hexamethonium (5 mg/kg, i.p.) did not modify them. Daily nicotine administration (2.5, 3.75 and 5 mg/kg, i.p.) once a day for 6 days developed tolerance to the convulsions induced by nicotine. After the daily administrations of nicotine for 6 days, the effects of a challenge administration of nicotine (2 mg/kg) on the nicotine-induced convulsions were tested on the 7th-day. Further tolerances were also developed by the 7th-day challenge administration. After the 7th-day test, nicotine levels of the brain and blood 15 min after the challenge injection were measured. With nicotine (5 mg/kg once a day)-treatment, nicotine levels of all the brain regions were increased. In contrast, a similar challenge injection had no effect on blood nicotine level. These results indicate that the development of tolerance to the nicotine-induced convulsions is produced relatively earlier and day by day by daily administrations to rats, which is closely related with the increase in brain nicotine level.     

Effects of single and long-term theophylline treatment on the threshold of mechanical nociception: contribution of adenosine A1 and alpha2-adrenoceptors

The effects of single and long-term treatment with theophylline as well as the influence of adenosine A1 receptor agonist cyclopentyladenosine (CPA) and a-adrenergic receptor antagonists prazosin and yohimbine were assessed in the paw pressure test in rats. Both single (37.5 and 75 mg/kg) and long-term (75 mg/kg/day, 14 days i.p.) theophylline treatments exerted antinociceptive effect by increasing the mechanical pain threshold. Single treatment of theophylline (75 mg/kg) antagonized the antinociceptive effect of CPA (0.1 mg/kg); CPA (0.1 mg/kg) abolished the theophylline-induced antinociception. Chronic treatment with theophylline did not change the antinociceptive effect of CPA, while CPA decreased the theophylline antinociception. Yohimbine (0.5 mg/kg), an a 2-adrenoceptor antagonist, diminished the antinociception of a single dose (75 mg/kg) of theophylline, whereas prazocin, an a 1-adrenoceptor antagonist, did not affect it. These results suggest that adenosine A1 and a 2-adrenoceptors take part in the antinociception induced by a single dose of theophylline. The antinociception induced by chronic theophylline treatment probably has a more complex mechanism in which the involvement of adenosine A1.     

The adenosine receptor antagonist theophylline induces a monoamine-dependent increase of the anticataleptic effects of NMDA receptor antagonists

Previous work revealed that adenosine antagonists as theophylline reversed neuroleptic-induced catalepsy and potentiated anticataleptic effects of dopamine agonists reflecting specific adenosine-dopamine receptor interactions in the central nervous system. We tested whether similar functional interactions exist between adenosine receptors and glutamate receptors of the N-methyl-D-asparte (NMDA) subtype. The present study demonstrates that the anticataleptic effects of the competitive NMDA receptor antagonist CGP37849 and the non-competitive NMDA receptor antagonist dizocilpine can be potentiated by coadministration of a threshold dose of the adenosine receptor antagonist theophylline (2.5 mg/kg, i.p.) in haloperidol (0.5 mg/kg, i.p.)-pretreated rats. This potentiation was elicited only with higher doses of CGP37849 (4 and 8 mg/kg, i.p.) or dizocilpine (0.16 mg/kg, i.p.) in haloperidol (0.5 mg/kg, i.p.), but not in reserpine (5 mg/kg, i.p.) plus -methyl-ptyrosine (100 mg/kg, i.p.)-pretreated animals. Therefore, these synergistic interactions seem to be brought about by indirect monoamine-dependent mechanisms rather than direct functional interrelationships between NMDA and adenosine A_2a receptors.     

Protective effect of resveratrol against pentylenetetrazole-induced seizures and its modulation by an adenosinergic system

The effect of trans-resveratrol (resveratrol), a polyphenolic compound with potent antioxidant activity, was investigated against pentylenetetrazole (PTZ) induced seizures in rats. Resveratrol (20, 40, and 80 mg/kg i.p.) administered 20 min prior to convulsive challenge with PTZ (60 mg/kg i.p.) dose dependently reduced the percent incidence of generalized tonic-clonic convulsions. Resveratrol (40 mg/kg) also potentiated the effect of sodium valproate (150 mg/kg) and diazepam (2 mg/kg) against PTZ-induced seizures. Since adenosine, an endogenous anticonvulsant, has been demonstrated to modulate the action of various antiepileptics, experiments were also carried out to determine whether an adenosinergic mechanism is involved in the anticonvulsant action of resveratrol. When a subanticonvulsant dose of adenosine (500 mg/kg) was administered together with resveratrol, a significant reduction in the percent incidence of generalized tonic-clonic convulsions was observed. Moreover, the nonspecific adenosine receptor antagonist theophylline (50 mg/kg i.p.) significantly reversed the resveratrol-induced protection, whereas the specific adenosine A2 receptor antagonist 3,7-dimethyl-1-propargylxanthine (1 mg/kg i.p.) could not reverse the resveratrol-induced protection. The findings of the present study suggest an antiepileptic potential of resveratrol and that an adenosinergic mechanism may play a role in its anticonvulsant activity.     

Role of nitric oxide in systemic effect of theophylline on mouse body temperature

In the present study, the interaction of nitric oxide synthase (NOS) inhibitors, L-NAME (N(G)-nitro-L-arginine methyl ester HCl) and L-NA (N(omega)-nitro-L-arginine), and its precursor, L-arginine (2-(S)-2-amino-5-[(aminoiminomethyl)amino] pentatonic acid), with theophylline on mouse body temperature was studied. Intraperitoneal (i.p.) injection of different doses of theophylline altered body temperature. Lower doses of theophylline (12.5 and 25 mg/kg) increased, but a higher dose (100 mg/kg) reduced, the animals' body temperature. The combination of L-arginine (20 and 40 mg/kg) with the highest dose of theophylline potentiated the hypothermic effect induced by the latter drug, while L-arginine by itself did not alter body temperature. L-NAME (10-80 mg/kg) or L-NA (10 mg/kg) plus a lower dose of theophylline (12.5 mg/kg) reduced the theophylline-induced hyperthermic response. L-NA (1, 5, and 10 mg/kg) in combination with the high dose of theophylline (100 mg/kg) also induced greater hypothermia. Both L-NAME and L-NA by themselves reduced body temperature. It is concluded that nitric oxide (NO) may be involved in the effects of theophylline on body temperature in mice.     

The anti-immobility effect of hyperoside on the forced swimming test in rats is mediated by the D2-like receptors activation

The crude extracts of HYPERICUM species native to South Brazil showed analgesic and antidepressant-like effects in rodents. The chemical characterization of these species revealed that they are rich in flavonoids and phloroglucinol derivatives. In the present study a detailed investigation was performed on the activities of hyperoside (HYP), a common flavonoid in the genus HYPERICUM. Hyperoside was obtained from the aerial parts of H. CAPRIFOLIATUM by chromatographic procedures. Mice treated with single doses (10, 20 and 40 mg/kg i.p.) did not present signs of toxicity or weight loss. At 20 and 40 mg/kg i.p. the mice exploratory behavior in the open field test was reduced. At 20 mg/kg i. p. the pentobarbital sleeping time increased, but not the sleeping latency. No activity was found on the hot-plate (10 and 20 mg/kg i.p.) or in the acetic acid-induced writhing test (20 and 40 mg/kg p.o.). Nevertheless, an antidepressant-like effect in the forced swimming test in mice and rats was observed (HYP 10 and 20 mg/kg i.p. in mice; HYP 1.8 mg/kg/day p.o. in rats). The antidepressant-like effect in rats was prevented by the administration of sulpiride (50 mg/kg i.p.) a D2 antagonist. In conclusion, hyperoside was found to present a depressor effect on the central nervous system as well as an antidepressant-like effect in rodents which is, at least in part, mediated by the dopaminergic system.     

The effects of rubidium, caesium and quinine on 5-HT-mediated behaviour in rat and mouse--2. Caesium.

Rats and mice were given either CsCl (3 mmol/kg, s.c.) or saline (as control), twice daily for 3 days. The administration of tranylcypromine (TCP) (15 mg/kg, i.p.) to rats pretreated with CsCl produced the 5-HT behavioural syndrome. Pretreatment with CsCl also enhanced the syndrome induced by p-chloroamphetamine (3 mg/kg, i.p.) or by TCP (15 mg/kg, i.p.) plus L-tryptophan (50 mg/kg, i.p.). p-Chlorophenylalanine (300 mg/kg, i.p., daily on 2 consecutive days) or (-)-propranolol (20 mg/kg, i.p.), pindolol (4 mg/kg, i.p.) and ritanserin (0.4 mg/kg, s.c.), all prevented the behavioural syndrome induced by CsCl and TCP in rats. Pretreatment of rats with CsCl potentiated the 5-HT syndrome, elicited by the 5-HT agonists, 8-OH-DPAT (0.5 mg/kg, s.c.), 5-MeODMT (2 mg/kg, s.c.) and quipazine (25 mg/kg, i.p.). Pretreatment with CsCl potentiated the 5-HT2-mediated head-twitches in the mouse but had no effects on hypothermia in the mouse induced by 8-OH-DPAT (0.5 mg/kg, s.c.). The rate of synthesis of 5-HT in the whole brain (excluding cerebellum) was enhanced by pretreatment with CsCl. The enhancement of 5-HT neuronal function by caesium may be related to its ability to block K(+)-channels in neuronal membranes.     

Involvement of NMDA receptors in thiopental-induced loss of righting reflex, antinociception and anticonvulsion effects in mice

Potentiation of GABA(A) receptor-mediated inhibitory neurotransmission contributes to the anesthetic action of thiopental. However, the inhibiting action of general anesthetic on excitatory neurotransmission also purportedly underlies its effects. The aim of the study was to elucidate the role of glutamate receptors (NMDA and AMPA receptors) in thiopental-induced anesthesia. Intracerebroventricular (i.c.v.) NMDA (50 ng) significantly increased the induction time of loss of righting reflex and decreased sleep time induced by intraperitoneal injection (i.p.) of thiopental (50 mg/kg). Furthermore, NMDA at 50 ng i.c.v. increased the 50% effective dose values for thiopental to produce loss of righting reflex and immobility in response to noxious tail clamp by 25% and 21% (p < 0.05), respectively. However, intrathecal (IT) administration of NMDA or both of i.c.v. or IT administration of AMPA did not show such antagonizing effects on thiopental action at subconvulsive dose. Finally, thiopental (25 mg/kg i.p.) inhibited convulsions induced by NMDA (0.4 microg i.c.v.) or bicuculline (0.6 microg i.c.v.). However, i.p. muscimol (1 mg/kg) blocked the convulsions induced by bicuculline, but not those induced by NMDA at 3 mg/kg. Similarly, i.p. MK-801 (0.1 mg/kg) antagonized NMDA-induced convulsions, but not bicuculline-induced convulsions at 0.3 mg/kg. Therefore, we suggest that the effects of the selective GABA(A) and NMDA receptors on convulsive behavior are special to their sites of action, and that the inhibitory action of thiopental on NMDA receptors is possibly not mediated by secondary effects of its GABA(A) receptors agonism. These results above indicate the involvement of NMDA receptors in thiopental-induced anesthesia in mice.     

The role of nitrergic system in lidocaine-induced convulsion in the mouse

The effects of N-nitro-L-arginine-methyl ester (L-NAME) a nitric oxide (NO) synthase inhibitor and L-arginine, a NO precursor, were investigated on lidocaine-induced convulsions. In the first experiment, four groups of mice received physiological saline (0.9%), L-arginine (300 mg/kg, i.p.), L-NAME (100 mg/kg, i.p.) and diazepam (2 mg/kg), respectively. Thirty minutes after these injections, all mice received lidocaine (50 mg/kg, i.p.). In the second experiment, four groups of mice received similar treatment in the first experiment, and 30 min after these injections, all mice received a higher dose of lidocaine (80 mg/kg). L-NAME (100 mg/kg, i.p.) and diazepam (2 mg/kg) significantly decreased the incidence of lidocaine (50 mg/kg)-induced convulsions. In contrast, the L-arginine treatment increased the incidence of lidocaine (80 mg/kg, i.p.)-induced convulsions significantly. These results may suggest that NO is a proconvulsant mediator in lidocaine-induced convulsions.     

The protective effect of adenosinergic agents, Ro 5-4864 and carbamazepine against hypoxic stress-induced neurotoxicity in mice

Mice subjected to hypoxic stress resulted in increased respiratory rate, tremor and convulsions followed by death. The latencies for convulsion and death following hypoxic stress were 33.29 +/- 1.20 and 34.36 +/- 1.16 min, respectively. In the present study effects of adenosinergic agents Ro 5-4864, a "peripheral-type" benzodiazepine receptor agonist, and carbamazepine were studied on hypoxic stress-induced neurotoxicity. Adenosinergic agents such as adenosine, 2-chloroadenosine, N6-cyclohexyladenosine and dipyridamole increased the latencies for convulsions and death due to hypoxia. Theophylline (50 mg/kg i.p.), an adenosine receptor antagonist, reversed this protective effect of adenosinergic agents. Pretreatment with Ro 5-4864 (10, 20 mg/kg i.p.) also offered theophylline (50 mg/kg, i.p.)-sensitive protection against hypoxic stress. Similarly, carbamazepine treatment (10-30 mg/kg, i.p.) significantly prolonged the latencies for convulsion and death following hypoxic stress. Prior treatment with theophylline (50 mg/kg, i.p.) reversed this protective effect of carbamazepine, indicating the possible involvement of adenosinergic mechanism in the observed protective effect of carbamazepine. These results indicate that the adenosinergic mechanism may be responsible for the observed neuroprotective effect of these agents against hypoxia.     

Acute toxicity of nimbolide and nimbic acid in mice, rats and hamsters

Nimbolide and nimbic acid are toxic to mice only when given i.p. and i.v. but they are less toxic to rats and hamsters. The LD50 values of a single i.p. administration of nimbolide to adult male, female and weanling mice were 225, 280 and 240 mg/kg body wt, respectively, and its i.v. LD50 value was decreased to 24 mg/kg body wt in adult male mice. No fatality was observed when nimbolide was given i.g., i.m. and s.c. to adult male mice. Estimated LD50 values of nimbolide in rats and hamsters were somewhat higher than 600 and 500 mg/kg body wt. After 12-23 h i.p. administration of a lethal dose, most animals died of possible dysfunctions in kidney (tubular necrosis), small intestine (hemorrhagic necrosis), pancreas (acinar cell necrosis) and liver (mild fatty infiltration and focal necrosis). In contrast, mice and rats given a lethal dose of nimbolide (i.v.) died of a marked and sudden drop in arterial blood pressure and respiratory paralysis within about 1-18 min. Nimbic acid was less toxic to mice with an i.v. LD50 value of 265 mg/kg body wt and i.p. and i.g. LD50 values of higher than 600 mg/kg body wt. The possible cause of death induced by nimbic acid may be similar to that of nimbolide given i.v. and this is a sudden hypotensive shock.     

Anticonvulsant and sodium channel blocking activity of higher doses of clenbuterol

Clenbuterol, a lipophilic beta2-adrenoceptor agonist, was investigated in various seizure models of experimental epilepsy. In the maximal electroshock seizure threshold test, clenbuterol (> or =4 mg/kg i.p.) increased the electroconvulsive threshold for tonic seizures in mice. In the traditional maximal electroshock seizure (MES) test in mice, ED50 values of 11 mg/kg i.p. or s.c. were determined. In both models, the beta2-receptor antagonist ICI 118.551 did not antagonize the anticonvulsant activity of clenbuterol. Combinations of clenbuterol with standard antiepileptics revealed additive anticonvulsant effects. Repeated administration of clenbuterol (5 mg/kg s.c., twice daily for 14 days) to mice did not significantly influence its anticonvulsant potency or the effectiveness of phenobarbital in the MES test. In various chemically-induced seizure tests with tonic convulsions, clenbuterol inhibited or tended to suppress the tonic phase. However, this drug was not effective in preventing clonic seizures in the pentylenetetrazol (85 mg/kg s.c.) seizure threshold test. In the rotarod ataxia test (mice), a minimal "neurotoxic" dose (TD50) of 41 mg/kg i.p. was determined. In unrestrained rats with chronically implanted electrodes in the dorsal hippocampus, clenbuterol (2 mg/kg and 4 mg/kg i.p.) significantly reduced the duration of electrically evoked hippocampal afterdischarges. In amygdala-kindled rats, clenbuterol (5 mg/kg and 10 mg/kg i.p.) reduced the seizure severity to stage 3. Additional studies indicated that clenbuterol (6 mg/kg i.p.) increased the heart rate and decreased the blood pressure, but this drug did not alter the plasma level of the two tested antiepileptics phenobarbital and carbamazepine. Furthermore, in whole-cell voltage-clamp experiments on cultured neonatal rat cardiomyocytes, clenbuterol (1-100 microM) depressed the fast sodium current in a concentration- and frequency-dependent manner. In conclusion, the anticonvulsant effects of higher doses of clenbuterol against generalized tonic-clonic and complex partial seizures seem to be related to the inhibition of voltage-dependent sodium channels and not to the modulation of beta-adrenoceptors.     

The central action of pizotifen

The central action of the potential antidepressant drug pizotifen (Sandomigran) was studied in mice, rats and rabbits. Pizotifen in doses up to 10 mg/kg i.p. was ineffective in classic tests for antidepressant activity. It neither antagonized the effects of reserpine in rats (hypothermia, ptosis) nor potentiated the effects of amphetamine (in mice and rats), nialamide or L-dopa (in mice) on locomotor activity. However, its antidepressant activity was found in the despair test in rats.On the other hand, pizotifen inhibited the head twitch reaction induced by L-5-hydroxytryptophan in mice (ED₅₀=0.009 mg/kg, i.p.) and by 5-methoxytryptamine (+tranylcypromine) in rats (ED₅₀=0.45 mg/kg, i.p.). It also antagonized tryptamine-induced clonic convulsions of fore-paws in rats (ED₅₀=0.35 mg/kg, i.p.), and in doses of 5–10 mg/kg s.c. inhibited hyperthermia produced by LSD in rabbits. Finally, pizotifen (0.1–0.3 mg/kg, i.v.) inhibited or abolished LSD- or quipazine-induced stimulation of the hind limb flexor reflex of spinal rats; the above effect was not due to noradrenolytic action of the drug. These results suggest that pizotifen strongly blocks the central postsynaptic serotonin receptors.     

Nitric oxide signaling pathway in the anti-convulsant effect of adenosine against pentylenetetrazol-induced seizure threshold in mice

The present study was performed to examine the involvement of nitric oxide (NO) signaling pathway in the anti-convulsant effect of adenosine against pentylenetetrazol seizure threshold in mice. Minimal dose of pentylenetetrazol (i.v., mg/kg) needed to induce different phases (myoclonic jerks, generalized clonus and tonic extension) of convulsions was recorded as an index of seizure threshold. Adenosine (100 or 200 mg/kg i.p.) produced a significant increase in the seizure threshold for convulsions induced by pentylenetetrazol i.v. infusion. The anti-convulsant effect of adenosine (100 mg/kg i.p.) was prevented by either L-arginine (50 mg/kg i.p.) [substrate for nitric oxide synthase (NOS)] or sodium nitroprusside (3 mg/kg i.p.) [a NO donor]. On the other hand, N(G)-nitro-L-arginine methyl ester (L-NAME, 2.5 mg/kg i.p.) [a non-selective NOS inhibitor] or 7-nitroindazole (7-NI) (25 mg/kg i.p.) [a specific neuronal nitric oxide synthase (nNOS) inhibitor] potentiated the anti-convulsant action of sub-effective dose of adenosine (50 mg/kg i.p.). Aminoguanidine (100 mg/kg i.p.) [a specific inducible NOS (iNOS) inhibitor] pre-treatment was not effective in inducing anti-convulsant effect with sub-effective dose of adenosine (50 mg/kg i.p.). Furthermore, the increase in seizure threshold elicited by adenosine (100 mg/kg i.p.) was also inhibited by concomitant administration with sildenafil (5 mg/kg i.p.) [phosphodiesterase 5 inhibitor]. In contrast, treatment of mice with methylene blue (1 mg/kg i.p.) [a direct inhibitor of both nitric oxide synthase (NOS) and soluble guanylate cyclase (sGC)] failed to induce anti-convulsant action with adenosine (50 mg/kg i.p.) against pentylenetetrazol i.v. infusion. The results demonstrated that the anti-convulsant action of adenosine in the pentylenetetrazol i.v. seizure threshold paradigm may possibly involve an interaction with the L-arginine-NO-cGMP pathway which may be secondary to the activation of adenosine receptors.     

Endotoxin stimulates fecal pellet output in rats through a neural mechanism

The effects of endotoxin on fecal pellet output and the neural mechanisms involved in this response were investigated in conscious rats. E. coli endotoxin (40 micro g/kg i.p.) significantly increased fecal excretion for 3 h after the injection. Water content in feces was not modified by endotoxin. Ablation of primary afferent neurons by systemic administration of high doses of capsaicin (20+30+50 mg/kg s.c.) to adult rats prevented the stimulatory effect of endotoxin and so did abdominal vagotomy. Adrenoceptor blockade with phentolamine (5 mg/kg i.p.) + propranolol (3 mg/kg i.p.) did not modify pellet output in endotoxin-treated rats while muscarinic receptor blockade with atropine (1 mg/kg i.p.) abolished the stimulatory effect of endotoxin. Finally, the increase in pellet output induced by endotoxin was prevented in animals receiving the substance P receptor antagonist D-Pro2, D-Trp7,9-substance P (2 mg/kg i.p.) or the NO-synthase inhibitor L-NAME (10 mg/kg i.p.). None of the above treatments modified pellet output in saline-treated rats.These observations indicate that endotoxin increases fecal pellet output through a nervous reflex in which capsaicin-sensitive afferent neurons and the release of excitatory (acetylcholine and substance P) and inhibitory (NO) neurotransmitters in the colonic wall are involved.     

Some correlations between procaine-induced convulsions and monoamines in the spinal cord of rats

The relationships between the convulsions induced by the local anesthetic procaine and monoamines in the spinal cord were investigated in rats. The levels of dopamine (DA) and serotonin (5-HT) in the spinal cord were time-dependently increased after procaine (170 mg/kg, i.p.), which induced clonic convulsions, but the level of norepinephrine (NE) was unchanged. The rats that died during convulsions had a marked increase in DA. Phenobarbital (25 mg/kg, s.c.) produced both depletion of DA and elevation of 5-HT in the spinal cord and completely protected rats against convulsions. A 5-HT precursor, 5-hydroxytryptophan (5-HTP; 20-80 mg/kg, i.p.), suppressed the convulsions in a dose-dependent manner as shown by a decrease in the incidence and a prolongation of the latency, but a DA precursor, 3,4-dihydroxyphenylalanine (L-DOPA; 20-80 mg/kg, i.p.), markedly shortened the latency. Furthermore, the effect of L-DOPA on the convulsions was inhibited by the combination of 5-HTP. alpha-Methyl-p-tyrosine (20-80 mg/kg, i.p.) or DL-p-chlorophenylalanine (20-80 mg/kg, i.p.), an inhibitor of NE and DA or 5-HT biosynthesis, had a slight effect on the convulsions. These results suggest that procaine causes significant elevations of rat spinal DA and 5-HT in the convulsive process and suggest that dopaminergic and serotonergic neurons may be associated with procaine-induced convulsions.