Effect of formulation parameters on the release characteristics of propranolol from asymmetric membrane coated tablets.

Controlled delivery of drugs has been achieved successfully by use of asymmetric membranes. In our study, we have evaluated the influence of various preparation parameters such as concentration of the polymer, concentration of the pore former and temperature of the precipitation bath on the permeability and the release characteristics of propranolol. Propranolol tablets were prepared by direct compression and were coated with varying concentrations of cellulose acetate and glycerin. The coat was precipitated in water, maintained at various temperatures, followed by air drying of the coat. Scanning Electron Microscopy (SEM) was used to characterize the asymmetric structure of the membrane. The influence of various preparation parameters on the release of propranolol from asymmetric coated tablets was evaluated. SEM confirmed the asymmetric nature of the membrane. A zero order release of propranolol was obtained from the coated tablets of propranolol. Various preparation parameters studied significantly affected (p<0.05) the release of propranolol hydrochloride from the asymmetric membrane coated tablets and the release was independent of the pH and the rate of agitation of the dissolution medium (p>0.05). Asymmetric membranes can be successfully utilized in the controlled delivery of highly water soluble drugs like propranolol and by modifying preparation parameters like polymer concentration, pore former concentration and temperature of the precipitation bath, desired release rates can be obtained.

Development and validation of a high-performance liquid chromatographic method for the analysis of propylthiouracil in pharmaceuticals.

A simple, rapid, and stability-indicating high-performance liquid chromatographic (HPLC) method was developed and validated for the assay of propylthiouracil (PTU). The method was used to quantify PTU in topical formulations and in tablets. Excellent linearity was observed between PTU concentration and the peak area (R2= 0.999). The limit of detection was 1 ng, and the limit of quantitation was 1.2 ng. The method proved to be selective. Selectivity was validated by subjecting a stock solution of PTU to acidic, basic, and oxidative degradations. The peaks of the degradation products did not interfere with the peak of PTU. Excipients present in the dosage forms did not interfere with the analysis, and the recovery of PTU from each dosage form was quantitative.

Synthesis and opiate receptor binding properties of 17-methyl-6,7-dehydro-3,14-dihydroxy-4,5alpha-epoxy-6,7:4',5'-pyrimidin omorphinans.

A class of opioid receptor active derivatives of oxymorphone has been synthesized using a common enaminone intermediate. The derivatives have heterocyclic groups fused to the 6,7-positions of the morphinan system and all were synthesized in high yield. A pyrazolo derivative is an agonist for the mu and delta receptors and an antagonist for the kappa receptor.

Comparison of in vitro dissolution and permeation of fluconazole from different suppository bases.

Fluconazole suppositories were prepared in hydrophilic, lipophilic, and amphiphilic bases. In vitro evaluation was conducted to compare the effect of different bases on the release and permeation of fluconazole. Four types of suppository bases were evaluated; hydrophilic (polyethylene glycol, PEG), lipophilic (cocoa butter, CB; Witepsol W45 WW45), and amphiphilic (Suppocire AP SAP, a polyglycolized glyceride). The uniformity of dosage units prepared with each base was determined by ultraviolet (UV) spectroscopy. The influence of suppository base on the release of fluconazole was studied using USP dissolution apparatus I. Rate constants for each release pattern were determined and compared using a one-way analysis of variance (ANOVA) on ranks. The order of in vitro dissolution of fluconazole from the bases was as follows: PEG > (SAP = WW45) > CB. Results suggest that in vitro release of fluconazole is greater from a hydrophilic base (PEG). Preliminary permeation studies were conducted on each type of base using Franz diffusion cells. Permeation was studied through the rat rectal membrane, and normal saline was used as the receptor medium. A modified reverse-phase high-performance liquid chromatography (HPLC) method was used and validated for analyzing fluconazole. Flux values (microgram/cm2/hr) were calculated and compared using a one-way ANOVA (p < .001). The order of permeation was as follows: SAP > (PEG = WW45) > CB. The increased permeation characteristics seen with the SAP base are probably due to an alteration of the membrane characteristics due to the surface active properties of the base.

Sensitization to the locomotor stimulant effect of cocaine modifies the binding of [3H]MK-801 to brain regions and spinal cord of the mouse.

The effects of chronic administration of cocaine and its subsequent withdrawal on the binding of [3H]MK-801, an antagonist of the N-methyl-D-aspartate (NMDA) receptor, to brain regions and spinal cord of the mouse was determined. Chronic administration of cocaine (10 mg/kg, SC) twice a day for 7 days sensitized male Swiss-Weber mice to its locomotor activity, as evidenced by a greater locomotor activity in comparison with vehicle-injected controls. In nonwithdrawing mice, the binding of [3H]MK-801 was increased in the cerebellum and spinal cord but was decreased in the cortex and hypothalamus. On the other hand, during withdrawal from cocaine, a significant decrease in the binding of [3H]MK-801 was observed only in the cortex, but the changes in the cerebellum, spinal cord and hypothalamus were normalized. It is concluded that, in the mouse, sensitization to the locomotor activating action of cocaine is associated with differential changes in the NMDA receptors of the central nervous system.

Enhancement of morphine antinociception by ibogaine and noribogaine in morphine-tolerant mice.

The effects of ibogaine, an alkaloid isolated form the bark of the African shrub, Tabernathe iboga, and noribogaine, a metabolite of ibogaine, on morphine antinociception were determined in male Swiss-Webster mice. Mice were rendered tolerant to morphine by implanting them with a pellet containing 25 mg of morphine base for 3 days. Placebo pellet-implanted mice served as controls. The antinociception of morphine (10 mg/kg, s.c.) was determined alone or in combination with an appropriate dose of ibogaine or noribogaine. Tolerance to morphine developed as a result of morphine pellet implantation as evidenced by decreased antinociceptive response to morphine. Both ibogaine and noribogaine dose-dependently enhanced morphine antinociception in morphine-tolerant but not in morphine-naive mice. It is concluded that ibogaine and noribogaine enhance morphine antinociception in morphine-tolerant mice.

Effects of acute administration of L-arginine on morphine antinociception and morphine distribution in central and peripheral tissues of mice.

The effect of acute treatment with L-arginine, a substrate for nitric oxide synthase (NOS) that forms NO, an important second messenger, on morphine antinociception and distribution of morphine in central and peripheral tissues of male Swiss-Webster mice was determined. The antinociception activity of morphine (10 mg/kg, s.c.) was attenuated by 400 and 800 mg/kg doses of L-arginine, but a lower dose (200 mg/kg) had no effect. D-Arginine (200-800 mg/kg) did not modify morphine antinociception. The dose of L-arginine (200 mg/kg) that did not modify morphine antinociception also did not alter the distribution of morphine in brain regions and spinal cord. A dose of 800 mg/kg of L-arginine produced a significant decrease in the concentration of morphine in midbrain and spinal cord. The highest dose of L-arginine (800 mg/kg) also increased the concentration of morphine in spleen. None of the doses of L-arginine modified the concentration of morphine in serum or urine. The results suggests that acute activation of the NO system attenuates morphine antinociception possibly by inhibiting its uptake in central sites (midbrain and spinal cord) involved in antinociceptive actions.

Kinetic properties of nitric oxide synthase in cerebral cortex and cerebellum of morphine tolerant mice.

The effects of chronic morphine administration on the kinetics of nitric oxide synthase (NOS) activity were determined in the mouse cerebral cortex and cerebellum. Male Swiss-Webster mice were implanted subcutaneously with morphine pellets each containing 25 mg of morphine base for 3 days. Mice implanted with placebo pellets served as controls. NOS activity was determined by measuring the rate of conversion of [3H]arginine to [3H]citrulline. Morphine pellet implantation increased NOS activity in the cerebral cortex and cerebellum. Analysis of the Eadie-Hofstee plot indicated that the Vmax of NOS in the cortex and cerebellum was 140 and 228 pmol [3H]citrulline formed/min/mg protein and the Km values were 9.3 and 10.1 mumol/l, respectively. Mice implanted with morphine pellets had higher Vmax values in both the cortex and cerebellum, but the Km values did not differ from those of control mice implanted with placebo pellets. It is concluded that chronic treatment with morphine increases NOS activity in the brain without modifying its substrate affinity.

Analgesic and thermic effects, and cerebrospinal fluid and plasma pharmacokinetics, of intracerebroventricularly administered morphine in normal and sensitized rats.

The relationship between asthma and opioids has barely been investigated. This study examines whether active sensitization of rats changes the analgesic and thermic effects of intracerebroventricular morphine or the pharmacokinetics of the drug. Morphine (5, 10 and 20 microg) was given intracerebroventricularly to sensitized (active immunization to ovalbumin and Al(OH)3 then airway challenge with ovalbumin after 12 days) and normal (i.e. non-sensitized) male Sprague-Dawley rats. The tail-flick latencies and changes in colon temperature were determined before morphine injection and at 30 min intervals for a period of 300 min afterwards. Results were expressed as the area under the time-response curve. The analgesic and hyperthermic response to morphine for sensitized rats was less than that obtained for normal rats. Cerebrospinal fluid and blood samples were collected periodically for a period of 240 min and morphine levels were determined by a highly sensitive radioimmunoassay. The pharmacokinetic parameters half-life, terminal elimination rate constant and the mean residence time were determined in both cerebrospinal fluid and plasma by non-compartmental analysis. The area under the cerebrospinal fluid concentration-time curve from time zero to infinity was higher for sensitized rats than for normal rats for all three doses of morphine but these differences did not correspond with similar changes in pharmacological responses. In conclusion, the attenuated analgesic and thermic responses to intracerebroventricular morphine in the sensitized rats might be a result of pharmacodynamic alterations rather than to pharmacokinetic changes.

Effect of chronic administration of L-arginine, NG-nitro-L-arginine or their combination on morphine concentration in peripheral tissues and urine of the mouse.

1. Chronic administration of L-arginine (200 mg/kg, i.p) twice a day for 4 days decreased the antinociceptive response to subcutaneously, but not to intracerebroventricularly, administered morphine in male Swiss-Webster mice, as measured by the tail-flick test. 2. The decreased antinociceptive response to morphine was reversed by concurrent administration of NG-nitro-L-arginine (L-NNA) (5 mg/kg, IP), an inhibitor of nitric oxide synthase. 3. The concentrations of morphine in mice treated chronically with L-arginine and then given morphine (10 mg/kg, SC) were determined in the peripheral tissues. L-Arginine treatment significantly increased the concentration of morphine in spleen and lungs, did not modify it in liver, kidneys and urine. L-NNA by itself had no effect on the distribution of morphine in peripheral tissues but reversed the changes induced by chronic treatment with L-arginine. 4. Acute administration of L-arginine (200 mg/kg, IP) did not modify either the morphine antinociception or the morphine distribution in peripheral tissues. 5. Previous studies from this laboratory indicated that chronic treatment with L-arginine decreases the concentration of morphine in several brain regions and spinal cord of mice. 6. The facts that chronic treatment with L-arginine does not alter antinociception induced by ICV administered morphine and it increases the concentration of morphine in peripheral tissues while decreasing it in brain regions after peripheral administration of morphine suggest that the decreased antinociception induced by subcutaneously administered morphine may be related to its decreased entry into the brain.

Interactions of cocaine with morphine, U-50,488H and [D-Pen2, D-Pen5]enkaphalin.

The effects of acute and chronic administration of cocaine on the antinociception and tolerance to the antinociceptive actions of mu-(morphine), kappa-(U-50,488H), and delta-([D-Pen2,D-Pen5]enkephalin; DPDPE), opioid receptor agonists were determined in male Swiss-Webster mice. Intraperitoneal injection of 40 mg/kg of cocaine by itself produced weak antinociceptive response as measured by the tail-fick test but the lower doses were ineffective. Administration of morphine (10 mg/kg, SC), U-50,488H (25 mg/kg, IP) or DPDPE (10 microg/mouse, ICV) produced antinociception in mice. Cocaine (20 mg/kg) potentiated the antinociceptive action of morphine and DPDPE but had no effect on U-50,488H-induced antinociception. Administration of morphine (20 mg/kg, SC), U-50,488H (25 mg/kg, IP) or DPDPE (20 microg/mouse, ICV) twice a day for 4 days resulted in the development of tolerance to their antinociceptive actions. Tolerance to the antinociceptive actions of morphine and U-50,488H was inhibited by concurrent treatment with 20 or 40 mg/kg doses of cocaine; however, tolerance to the antinociceptive action of DPDPE was not modified by cocaine. It is concluded that cocaine selectively potentiates the antinociceptive action of mu- and delta- but not of the kappa-opioid receptor agonist. On the other hand, cocaine inhibits the development of tolerance to the antinociceptive actions of mu- and kappa- but not of delta-opioid receptor agonists in mice.

Evidence for a role of N-methyl-D-aspartate receptors in L-arginine-induced attenuation of morphine antinociception.

Twice daily injections of L-arginine (50, 100 or 200 mg/kg, i.p.) for 4 days dose-dependently, decreased morphine antinociception in male Swiss-Webster mice as measured by the tail-flick test. To determine the possible role of N-methyl-D-aspartate (NMDA) receptor in the action of L-arginine, the effects of MK-801, a noncompetitive antagonist of the NMDA receptor and of LY 235959, a competitive antagonist of the NMDA receptor on L-arginine-induced attenuation of morphine antinociception were determined. MK-801 (0.01-0.10 mg/kg, i.p.) or LY 235959 (1.0-4.0 mg/kg, i.p.) given 10 min before each injection of L-arginine (200 mg/kg, i.p.) reversed the action of the letter in a dose-dependent manner on morphine antinociception. It is concluded that NMDA receptors are involved in the action of L-arginine in attenuating morphine antinociception.

Effect of chronic administration of [D-Pen2, D-Pen5] enkephalin on the activity of nitric oxide synthase in brain regions and spinal cord of mice.

The effect of multiple intracerebroventricular (i.c.v.) injections of [D-Pen2, D-Pen5]enkephalin (DPDPE), a delta-opioid receptor agonist, on the activity of nitric oxide synthase (NOS) was determined in the brain regions and spinal cord of the mouse. Male Swiss Webster mice were injected twice daily with DPDPE (20 micrograms/mouse, i.c.v.) or its vehicle for 4 days. This procedure has previously been shown to induce tolerance to the antinociceptive actions of DPDPE in mice. On day 5, the animals treated with DPDPE were either sacrificed 20 min after an i.c.v. injection of DPDPE (tolerant) or without any injection (abstinent i.e., 16 h after the last injection of DPDPE). NOS activity in brain regions (cortex, striatum, hippocampus, midbrain, pons/medulla, hypothalamus and cerebellum) and spinal cord was determined by the rate of conversion of arginine into citrulline. Tolerance to DPDPE was associated increases in NOS activity in midbrain (49%) and pons/medulla (32%) and decreases in cerebellum (28%) and spinal cord (44%). However, NOS activity was unchanged in the cortex, corpus striatum, hippocampus and hypothalamus. On the other hand, during abstinence from DPDPE, NOS activity increased in midbrain (84%) and hypothalamus (35%) but decreased in cerebral cortex (27%) cerebellum (27%) and spinal cord (20%). NOS activity was unchanged in the corpus striatum, hippocampus and pons/medulla. Previous studies from this laboratory had demonstrated that chronic administration of mu- and kappa-opioid receptor agonists results in increases NOS activity in certain brain regions and that tolerance to mu- and kappa-, but not to delta-opioid receptor agonists, is attenuated by NOS inhibitors. The present studies, for the first time, demonstrate decreases in NOS activity in certain brain regions and spinal cord of mice treated chronically with delta-opioid receptor agonist. Furthermore, these findings may explain the inability of NOS inhibitors to attenuate tolerance to DPDPE in mice.

Effects of noribogaine on the development of tolerance to antinociceptive action of morphine in mice.

The effects of noribogaine, a metabolite of ibogaine, on the development of tolerance to the antinociception action of morphine was determined in male Swiss-Webster mice. Ibogaine is an alkaloid isolated from the bark of the African shrub, Tabernanthe iboga. Morphine tolerance in mice was developed by two different methods. Mice were rendered tolerant to morphine either by subcutaneous implantation of a pellet containing 25 mg morphine free base for 4 days or by injecting morphine (20 mg/kg, s.c.) twice a day for 4 days. Placebo pellet implanted mice or vehicle injected mice served as controls. To determine the effect of intraperitoneally administered noribogaine on tolerance development, the drug was injected in the appropriate dose twice a day. In pellet implanted mice, a dose of 20 mg/kg of noribogaine attenuated the tolerance to morphine whereas lower doses had no effect. Similarly, in mice given multiple injections of morphine, noribogaine attenuated tolerance development at 20 and 40 mg/kg doses. Previous studies from this laboratory had shown that ibogaine at 40 and 80 mg/kg doses inhibited tolerance to morphine. Because noribogaine could attenuate morphine tolerance at lower doses than ibogaine, it is concluded that the attenuating effect of ibogaine on morphine tolerance may be mediated by its conversion to noribogaine, a more active metabolite.

Effects of acute and chronic administration of L-arginine on the antinociceptive action of morphine-6-beta-D-glucuronide.

Chronic administration of L-arginine (200 mg/kg i.p.) but not of D-arginine (200 mg/kg i.p.) twice a day for 4 days decreased the antinociceptive response to subcutaneously administered morphine-6-beta-D-glucuronide (M6G), a potent metabolite of morphine, in male Swiss-Webster mice as measured by the tail-flick test. However, the antinociceptive response to intracerebroventricularly administered M6G was unaffected by chronic treatment with L-arginine. The decreased antinociceptive response to M6G (s.c.) was reversed by concurrent administration of NG-nitro-L-arginine (5 mg/kg i.p.), an inhibitor of nitric oxide synthase. Acute administration of L-arginine (200 mg/kg i.p.) had no effect on M6G-induced antinociception, but higher doses (400 and 800 mg/kg i.p.) decreased it. Since the antinociceptive response to centrally administered M6G was unaffected by chronic L-arginine treatment, the decreased antinociceptive response of peripherally administered M6G may be related to a decrease of M6G entry into brain structures responsible for antinociceptive action.

Sensitization to the locomotor stimulant activity of cocaine is associated with increases in nitric oxide synthase activity in brain regions and spinal cord of mice.

Effects of multiple administrations of cocaine and subsequent cocaine withdrawal on the activity of nitric oxide synthase (NOS) in brain regions and spinal cord of male Swiss-Webster mice were determined. Chronic administration of cocaine resulted in the development of sensitization to its locomotor activity in the mouse. Chronic administration of cocaine was associated with increases in NOS activity in cerebral cortex, cerebellum, midbrain, hypothalamus, hippocampus, amygdala and spinal cord, but NOS activity was unaffected in pons/medulla and corpus striatum. Forty-eight hours after withdrawal from cocaine, NOS activity was increased only in the cerebral cortex. Recent studies have shown that cocaine-induced sensitization to behavioral effects can be inhibited by NOS inhibitors. The present studies provide the first evidence that chronic treatment with cocaine alters NOS activity in brain regions and spinal cord, and are consistent with behavioral studies with cocaine and NOS inhibitors.

Effects of NMDA receptor antagonists on delta1- and delta2-opioid receptor agonists-induced changes in the mouse brain [3H]DPDPE binding.

Male Swiss-Webster mice were injected intracerebroventricularly (i.c.v.) with [D-Pen2,D-Pen5]enkephalin (20 microg/mouse) twice a day for 2 days. This procedure resulted in down-regulation of binding sites for [3H][D-Pen2,D-Pen5]enkephalin as evidenced by a 52% decrease in the Bmax value. Twice daily injections of (+)-5-methyl-10,11-dihydro-5H-dibenzo-[a,d]-cyclohepten-5,10-imine (MK-801) (0.1 mg/kg, i.p.) or [(-)3-SR,4a-RS,8a-SR-6-(phosphonomethyl)-1,2,3,4,4a,5,6,7,8,8a-decahy droisoquinoline-3-carboxylic acid] (LY 235959) (2 mg/kg, i.p.), the noncompetitive and competitive antagonist of the N-methyl-D-aspartate (NMDA) receptor, respectively, for 2 days did not alter the Bmax or Kd value of [3H][D-Pen2,D-Pen5]enkephalin binding to the mouse brain. Concurrent treatment of MK-801, but not of LY 235959 with [D-Pen2,D-Pen5]enkephalin, reversed the decreases in Bmax value of [3H][D-Pen2,D-Pen5]enkephalin. Twice daily injections of [D-Ala2,Glu4] deltorphin II (20 microg/mouse) for 2 days caused an increase in the Kd value, but not the Bmax value of [3H][D-Pen2,D-Pen5]enkephalin to bind to brain membranes. Concurrent treatment of [D-Ala2,Glu4]deltorphin II with LY 235959 reversed the increase in Kd value of [3H][D-Pen2,D-Pen5]enkephalin binding induced by multiple injections of [D-Ala2,Glu4]deltorphin II, but MK-801 had no effect. The results suggest that multiple injections of delta1- and delta2-opioid receptor agonists down-regulate delta1-opioid receptors of the brain by modifying Bmax and Kd values of [3H][D-Pen2,D-Pen5]enkephalin binding, respectively. MK-801 and LY 235959 reverse delta1- and delta2-opioid receptor agonists-induced down-regulation of brain delta1-opioid receptor, respectively, apparently by different mechanisms. It is concluded that short term treatment of mice with delta1-opioid receptor agonist down-regulates brain delta1-opioid receptors by decreasing Bmax of the ligand which is partially reversed by concurrent treatment with MK-801 but not by LY 235959. On the other hand, short term treatment of mice with delta2-opioid receptor agonist down-regulates brain delta1-opioid receptors by increasing Kd of the ligand which is partially reversed by concurrent treatment with LY 235959 but not by MK-801.

Effect of multiple injections of U-50, 488H, a kappa-opioid receptor agonist, on the activity of nitric oxide synthase in brain regions and spinal cord of mice.

1. The time course of the effects of multiple injections of U-50,488H, a kappa-opioid receptor agonist, and its subsequent termination on its analgesic action and nitric oxide synthase (NOS) activity was determined in the brain regions and spinal cord of the mouse. 2. Male Swiss-Webster mice were rendered tolerant to U-50,488H by twice-daily injections of the drug (25 mg/kg, IP) for 4 days. Vehicle-injected mice served as controls. 3. In tolerant mice, NOS activity was unchanged in brain regions and the spinal cord after treatment with U-50,488H. During abstinence from U-50,488H, NOS activity was found to be increased in the cortex and remainder of the brain, but no change was noted in the cerebellum, midbrain and spinal cord. 4. These studies demonstrate that withdrawal from the short-term treatment with U-50,488H in mice causes induction of NOS in certain brain regions. However, long-term treatment and withdrawal from U-50,488H are not associated with changes in the central NOS activity and indicate a possible adaptation in the NOS activity.

Time course of the changes in central nitric oxide synthase activity following chronic treatment with morphine in the mouse: reversal by naltrexone.

1. The time course of the effect of chronic administration of morphine on the activity of nitric oxide synthase (NOS) in the brain regions and spinal cord of the mouse was determined. The effect of naltrexone by itself on the NOS activity and that induced by morphine also were determined. 2. Male Swiss Webster mice were implanted subcutaneously with a pellet containing 25 mg of morphine free base for 4 days. Placebo pellet implanted mice served as controls. 3. Twenty-four hours after treatment with morphine, NOS activity decreased in the cerebellum, midbrain, cortex and remainder of the brain as well as in the spinal cord. Forty-eight and 72 hr after the treatment with morphine, NOS activity increased in the cerebellum and cortex, but no change was observed in the other brain regions and spinal cord. Twenty-four hours after morphine pellet removal (withdrawal), NOS activity in all brain regions and the spinal cord has returned to normal. 4. Implantation of a pellet containing 10 mg of naltrexone did not alter NOS activity in any brain region or spinal cord for 24, 48 and 72 hr or 24 hr after removal of the pellet. 5. Implantation of a naltrexone pellet in conjunction with a morphine pellet blocked the changes in NOS activity in the brain region and spinal cord induced by morphine. 6. It is concluded that the initial decrease in NOS activity by morphine may be related to enhanced motor activity, whereas the increase in NOS activity in certain brain regions may be associated with tolerance-physical dependence development. Additionally, the changes in central NOS activity by morphine appear to be mediated by opioid receptors because they were blocked by concurrent treatment with naltrexone.

Mechanism of attenuation of morphine antinociception by chronic treatment with L-arginine.

The effects of twice-daily injections of L-arginine or D-arginine (200 mg/kg i.p.) for 4 days on morphine-induced antinociception, brain nitric oxide synthase activity and brain and serum distribution of morphine and brain mu-opioid receptors labeled with [3H][D-Ala2,MePhe4,Gly5-ol]enkephalin were determined in male Swiss-Webster mice. Chronic treatment with L-arginine, but not D-arginine, decreased the antinociceptive response to morphine in mice, increased the activity of nitric oxide synthase in the midbrain and decreased brain levels of morphine, compared with vehicle-injected controls. Significant decreases in morphine levels were observed in midbrain, pons and medulla, hippocampus, striatum and spinal cord of L-arginine-treated mice, in comparison with vehicle-injected mice. However, the levels of morphine in cortex, amygdala and hypothalamus of L-arginine- or D-arginine-treated mice did not differ from those of vehicle-injected controls. Acute treatment with L-arginine (200 mg/kg i.p.) or D-arginine (200 mg/kg i.p.) did not modify either morphine antinociception or morphine distribution in brain regions or the spinal cord. Chronic administration of L-arginine or D-arginine did not alter the Bmax or Kd values of [3H][D-Ala2,MePhe4,Gly5-ol]enkephalin binding to the mouse brain membranes. These results suggest that chronic treatment with L-arginine reduces the antinociceptive effect of morphine by increasing brain nitric oxide synthase activity and by decreasing the concentration of morphine in certain brain regions and spinal cord.