Supersensitivity of sigma receptors after repeated administration of cocaine.

We investigated the role of sigma receptors in the expression of behavioral sensitization induced by cocaine. Rats received intraperitoneal injections of either 20 mg/kg cocaine or saline once daily for 14 consecutive days. Cocaine-treated rats became sensitized. After a 5-day abstinence period, a challenge dose of (+)-3-[3-hydroxyphenyl]-N-(1-propyl)piperidine ((+)-3-PPP), a sigma receptor agonist, was administered. (+)-3-PPP at doses of 12 and 24 mg/kg induced significantly more frequent rearing and more potent stereotypy consisting of repetitive head movement and sniffing in cocaine-sensitized rats than in saline-pretreated rats. These enhanced responses to (+)-3-PPP lasted for at least a month. The enhanced responses to (+)-3-PPP were attenuated by 30 mg/kg BMY 14802, a putative sigma antagonist, and also attenuated by 100 mg/kg (+/-)-sulpiride, a D2 dopamine antagonist. These findings show that repeated administration of cocaine produces lasting supersensitivity of simga receptors, which may induce subsequent activation of dopaminergic transmission.     

Sensitization of anorexia and locomotion induced by chronic administration of ephedrine in rats.

Repeated daily administration of the sympathomimetic agent ephedrine (EPH) leads to an augmentation (sensitization) of locomotor activity in rats. The present experiments examined the impact of repeated administration of the (-)- and (+)-EPH enantiomers on feeding in rats to assess whether the anorexic activity of EPH exhibits tolerance or sensitization during chronic exposure and whether the time course of these effects follows that observed in studies of locomotion. Adult male rats were injected once daily for 12 days with either vehicle or 5, 10 or 20 mg/kg (-)-EPH or with 10 or 20 mg/kg (+)-EPH. Horizontal locomotion and diet consumption were assessed for 60 min in an activity chamber. Suppression of feeding and the induction of locomotion were augmented over the first four days of administration of either 10 mg/kg or 20 mg/kg of the (-)-EPH enantiomer. In contrast, repeated administration of 20 mg/kg (+)-EPH resulted in augmentation of appetite suppression but not locomotion. These results confirm and extend the phenomenon of locomotor and feeding sensitization for ephedrine, but suggest that these effects may differ for the two enantiomers of ephedrine.     

Differentiation of phencyclidine and sigma receptor types affecting the central inspiratory termination mechanism in cat

The effects of 1) the phencyclidine receptor ligand TCP, 2) sigma receptor ligands (+)3-PPP and DTG, and 3) N-methyl-D-aspartate receptor blockers MK-801 and dextrorphan were determined on a brainstem mechanism which controls the termination of the inspiratory phase of the breathing cycle. Inspiratory bursts were recorded from the phrenic nerve in decerebrate paralyzed cats ventilated by means of a phrenic driven servoventilator. The central mechanism which terminates inspiration was tested by withholding lung inflation, thus suppressing the contribution of the vagal feedback from the lungs to inspiratory termination. TCP increased the duration of test inspiration (tTi) by 17% at 0.03 mg/kg and by 14-fold (from 1.6 to 23 s) at 1 mg/kg. With dextrorphan, tTi was significantly increased at 3 mg/kg. In contrast, (+)3-PPP and DTG did not increase tTi at doses up to 10 mg/kg, although MK-801 (0.03 mg/kg), given after the sigma ligands, increased tTi by 59-90%. It is concluded that phencyclidine but not sigma receptor ligands block the central mechanism which terminates inspiration and that the likely site of action is the NMDA receptor complex.     

CGS 15943, a nonxanthine adenosine receptor antagonist: effects on locomotor activity of nontolerant and caffeine-tolerant rats

CGS 15943 (0.1-10 mg/kg, IP) dose-dependently increased the locomotor activity of rats to the same extent as caffeine (1.0-100 mg/kg, IP) did and was approximately 26 times more potent than caffeine. N-Ethylcarboxamidoadenosine (0.001-0.01 mg/kg, SC), an analog of adenosine, dose-dependently decreased locomotor activity; this effect was antagonized surmountably by concurrent administration of CGS 15943. The apparent pA2 value for this interaction, 6.57, was approximately 1.5 log-units (28-fold) higher than the pA2 for caffeine-NECA reported previously. Rats consuming 70 mg/kg/day of caffeine via their drinking water were tolerant to the stimulation of locomotor activity induced by both caffeine and CGS 15943. These results suggest that caffeine and CGS 15943 increase locomotor activity by a common mechanism of action possibly involving adenosine receptors or a cellular element conformationally similar to adenosine receptors.     

Evidence that the potential antipsychotic agent rimcazole (BW 234U) is a specific, competitive antagonist of sigma sites in brain

Rimcazole (BW 234U) is a potential antipsychotic agent which in open-clinical trials appears to be effective in acute schizophrenic patients. In the present study, rimcazole was found to block the specific binding of [3H]-(+)-SKF 10,047 to sigma sites in rat and guinea pig brain (IC50 = 5.0 X 10(-7) M). The compound was 100 times weaker as a blocker of phencyclidine sites (IC50 = 4.3 X 10(-5) M). At 1 X 10(-5) M, rimcazole had only weak effects on mu, delta, kappa and epsilon opioid receptors. Scatchard analysis of the binding data from guinea pig brain revealed an apparent KD for [3H]-(+)-SKF 10,047 of 85 +/- 5 nM and a Bmax of 824 +/- 27 fmole/mg protein. In the presence of 5 X 10(-7) M BW 234U, the apparent KD was 165 +/- 35 nM, but the Bmax (892 +/- 146 fmoles/mg protein) was not affected. This suggests that rimcazole is a competitive inhibitor of sigma sites. The agent was also capable of blocking sigma sites in vivo (ID50 = 6 mg/kg i.p., mice) as judged by an in vivo sigma receptor binding assay. Thus, if the antipsychotic activity of rimcazole is confirmed in double-blind, placebo-controlled trials, it would be the first compound whose mechanism of antipsychotic activity may best be explained by a direct blockade of sigma sites and not by a direct blockade of dopamine (D2) receptors in brain.     

Involvement of central serotonergic systems in dextromethorphan-induced behavioural syndrome in rats

Dextromethorphan, a noncompetitive blocker of the N-methyl-D-aspartate (NMDA) type of glutamate receptor, at 45, 60 and 75 mg/kg, ip doses induced a behavioural syndrome characterised by reciprocal forepaw treading, lateral head-weaving, hind-limb abduction and flat body posture. Such type of behavioural syndrome is induced by 8-hydroxy-2- (di-n-propylamino) tetralin (8-OH-DPAT) by directly stimulating the central postsynaptic 5-hydroxytryptamine (5-HT, serotonin) receptors of the 5-HT1A type. Pretreatment with buspirone (5, 10 mg/kg, ip) and l-propranolol (10, 20 mg/kg, ip) antagonised the behavioural syndrome induced by 8-OH-DPAT and dextromethorphan. Pretreatment with p-chlorophenylalanine (100 mg/kg/day x 4 days) antagonised the behavioural syndrome induced by dextromethorphan and dexfenfluramine but had no significant effect on 8-OH-DPAT induced behavioural syndrome. This indicates that dextromethorphan induces the behavioural syndrome by releasing 5-HT from serotonergic neurons with resultant activation of the postsynaptic 5-HT1A receptors by the released 5-HT. Pretreatment with fluoxetine (10 mg/kg, ip) significantly potentiated the behavioural syndrome induced by dextromethorphan and 5-hydroxytryptophan but significantly antagonised dexfenfluramine induced behavioural syndrome. This indicates that dextromethorphan releases 5-HT by a mechanism which differs from that of dexfenfluramine. Dextromethorphan may be releasing 5-HT by blocking the NMDA receptors and thereby counteracting the inhibitory influence of l-glutamate on 5-HT release.     

Endogenous opioid peptides in the control of food intake in cats

In this report, we investigated the role of exogenous and endogenous enkephalins on food intake in the cat, using, respectively, exogenous [D-Ala2-Met5]-enkephalin (DAME) and acetorphan (Ac) in order to inhibit the degradation of endogenous enkephalins. In addition, the selective peripheral antagonist naltrexone methylbromide (NTxMB) and the nonselective antagonist naloxone (Nx) were used in an attempt to discriminate central and peripheral opioid receptors. In 18-hours food-deprived animals, Ac (5 mg/kg IV) increased milk intake during sham feeding (+18%, p less than 0.05), but did not modify it in feeding conditions. Nx (1 mg/kg SC) reduced milk intake in sham-feeding experiments (-67%, p less than 0.01) more than in milk-feeding conditions (-30%, p less than 0.01). NTxMB (1 mg/kg SC) did not modify milk intake in sham-feeding but decreased it in feeding experiments. In nonfasted animals, Ac did not modify food intake. IV infusion of DAME (50 micrograms/kg) resulted in a reduction of daily food intake (-32%, p less than 0.01). Nx (1 mg/kg SC) decreased the earlier 30 min intake followed by reduction of daily intake (-30%, p less than 0.01). NTxMB (1 and 4 mg/kg SC) increased the 30-min intake dose dependently, without significant change in daily intake. In conclusion, Ac increases food intake in sham-feeding conditions, suggesting that endogenous enkephalins are likely to be involved in the stimulation of food intake. The effects of Nx and NTxMB furthermore suggest both a central activation, and a peripheral inhibition of food intake by opiates when food is allowed to proceed normally through the digestive tract.     

The effect of thyrotropin releasing hormone and morphine on gastrointestinal transit

The effect of thyrotropin releasing hormone (TRH) alone and in combination with morphine on the gastrointestinal transit was investigated by using the charcoal meal test in mice. The intraperitoneal (IP) administration of TRH decreased the transit when given in a dose of 1.0 mg/kg 10 min prior to the meal. The intracerebroventricular (ICV) administration of TRH (10 μg/mouse) also inhibited the transit when given just prior to the charcoal meal. Subcutaneous (SC) administration of morphine (5, 10 and 20 mg/kg) inhibited gastrointestinal transit in a dose dependent manner. When TRH (1, 3 and 10 mg/kg, IP as well as 0.3 μg, ICV) which had no effect on the transit by itself was combined with morphine (10 mg/kg, SC), an enhancement in the inhibition of the transit was observed. TRH-induced inhibition of the transit was antagonized by naloxone (0.1 mg/kg, SC). It is concluded that TRH inhibits gastrointestinal transit in the mouse possibly via the opiate receptor system.     

The anthelmintic activity of a novel organic arsenical, R7/45, upon Brugia pahangi in Meriones unguiculatus

The new organic arsenical R7/45 is a rapidly acting and very potent anthelmintic against adult Brugia pahangi in jirds. Against adult worms implanted into the peritoneal cavity 5 subcutaneous (SC) injections at 2.5 mg/kg of R7/45 killed 100% of adult worms. A single dose SC of 20 mg/kg was 100% effective and 10 mg/kg 76% effective against adult worms. When jirds were autopsied at different times after treatment at 20 mg/kg SC 89% of worms were dead within three days. R7/45 was not active when given by stomach intubation. Pretreatment of jirds with R7/45 had no effect on adult worms subsequently implanted into jirds. R7/45 was highly active against third and fourth stage larvae of B. pahangi in jirds.     

Amphetamine-induced inhibition of central noradrenergic neurons: a pharmacological analysis

The amphetamine-induced inhibition of brain noradrenaline (NA) containing neurons in the rat locus coeruleus (LC) was pharmacologically analyzed utilizing single unit recording techniques. The presynaptic α-receptor blocking agent yohimbine (10 mg/kg i.p., 30 min before) largely prevented the amphetamine-induced depression of LC units in contrast to prazosin (0.6 mg/kg i.p., 30 min) or phenoxybenzamine (20 mg/kg, 30 min) which both slow preference for postsynaptic α-receptors. The β-receptor blocking agent, propranolol (10 mg/kg, 30 min), as well as the peripherally but not centrally active α-receptor blocking drug phentolamine (10 mg/kg, i.p., 30 min), also did not block the amphetamine effect. The LC inhibition by amphetamine was blocked by pretreatment with reserpine (10 mg/kg, i.p., 5 h), which caused almost total depletion of brain catecholamines. However, unlike the amphetamine-induced inhibition of central dopamine (DA) neurons the NA cell inhibition was not blocked by pretreatment with a tyrosine hydroxylase inhibitor (α-MT, 50 or 250 mg/kg i.p., 30 min). These results suggest that the amphetamine-induced inhibition of NA neurons in the LC is an indirect effect, mediated via activation of central α-receptors of presynaptic character. The lack of antagonism by α-MT indicate that the NA release by amphetamine, unlike its effect on brain DA, is not critically dependent on the rate of tyrosine hydroxylation. Thus the euphoriant action of amphetamine, which is blocked by α-MT, may be associated with release of DA rather than NA in brain.     

In vivo evaluation of (+)-MR200 as a new selective sigma ligand modulating MOP, DOP and KOP supraspinal analgesia

The compound (+)-MR200 [(+)-methyl (1R,2S)-2-{[4-(4-chlorophenyl)-4-hydroxypiperidin-1-yl]methyl}-1-phenylcyclopropanecarboxylate] is a sigma ligand with increased affinity and selectivity compared to the structurally related ligand haloperidol. From the results of a previous study on the modulation of a systemically injected KOP opioid agonist analgesia by (+)-MR200, we analysed the influence of this sigma ligand on the antinociceptive effect of centrally injected MOP, DOP, and KOP selective agonists using the tail-flick test in rats. The results obtained confirmed that systemic administration of (+)-MR200 (1mg/Kg s.c.) did not modify basal tail-flick latency. Pre-treatment with 1mg/Kg s.c. of (+)-MR200 provided a significant increase in the antinociceptive effect of DAMGO (100ng/rat i.c.v.) and DPDPE (20 microg/rat i.c.v.). Conversely to previous reports, pre-treatment with (+)-MR200 reversed, in these experimental conditions, U-50488H (100 microg/rat i.c.v.) analgesia. The mechanism involved in these effects was not clear, but provided additional data on a diverging modulator role of selective sigma-1 antagonists on KOP analgesia.     

Dexamethasone induces biphasic effect on morphine hypermotility in mice: a dose-related phenomenon

The present study examined interaction between dexamethasone (DEX) and morphine on the locomotor activity in groups of mice by using the activity cage test. Morphine administration (30-75-150 mg/kg, ip) induced a dose-related increase of the locomotor activity of mice, whereas DEX per se (0.1-1.0-10 mg/kg, ip) did not modify the activity of control mice. Pretreatment of mice with DEX 0.1 mg did not alter the hyperactivity produced by the three doses of morphine. In contrast, DEX administered at 1.0 mg reduced the morphine effects on locomotor activity, whereas DEX at 10 mg potentiated the morphine hypermotility. Our results suggest that DEX may play an important regulatory role on the central effects of morphine through a differential modulation of brain excitability systems.     

Pupillary effects of leucine and methionine enkephalin in rats after intraperitoneal administration

Changes in pupil size after peripheral administration of met-enkephalin, leu-enkephalin, or morphine were studied in the rat. With a simple pupillographic technique, the pupil diameter of male, S.D. rats (250–300 g) was measured by a series of photographs taken every 60 sec for at least 45 min after the last drug injection. Morphine (8 mg/kg, SC) caused mydriasis characterized by rapid and marked fluctuations of pupil size. Mydriasis also occurred after leu-enkephalin (5 and 10 mg/kg, IP) and met-enkephalin (20 mg/kg, IP). Both peptides induced morphine-like fluctuations. When given 15 min after morphine, leu-enkephalin (5 and 10 mg/kg) increased the mydriatic effect of morphine from 172 percent of control to 224 and 272 percent, respectively. Met-enkephalin (20 mg/kg, but not 10 mg/kg) also enhanced the mydriatic response of morphine, to 244 percent of control. These interactions appear to represent simple addition rather than potentiation. The effects of both peptides were reversed by naloxone (1 mg/kg, SC), suggesting an opiate receptor interaction for the pupillary effects of the enkephalins. The rat pupil thus provides one of the few in vivo models permitting quantification of enkephalin action after parenteral administration.     

Chiral liquid chromatography resolution and stereoselective pharmacokinetic study of the enantiomers of a novel anticonvulsant, N-(4-chlorophenyl)-1-(4-pyridyl)ethylamine, in rats

A selective chiral high performance liquid chromatographic (HPLC) method was developed and validated to separate and quantify the enantiomers of a novel anticonvulsant agent, N-(4-chlorophenyl)-1-(4-pyridyl)ethylamine (AAP-Cl), in rat plasma. After extraction of the plasma samples with ethyl acetate, the separation was accomplished by an HPLC system consisting of a Chirex chiral column (250 mm x 4.6 mm i.d.) and a mobile phase of hexane:ethanol:tetrahydrofuran (280:20:40 (v/v)) containing trifluroacetic acid (0.3% (v/v)) and triethylamine (0.018% (v/v)) at a flow rate of 0.8 ml/min with UV detection. Male Sprague-Dawley rats were given (+)-AAP-Cl (10 and 20 mg/kg), (-)-AAP-Cl (10 mg/kg) or the racemic mixture (20 mg/kg) by i.v. bolus injection and serial blood samples were collected at different times after drug administration. (+)-AAP-Cl and (-)-AAP-Cl were separated with a resolution factor, Rs, of at least 1.4, and a separation factor, alpha, greater than 1.09. Linear calibration curves were obtained over the concentration range of 0.5-30 microg/ml in plasma for both (+)-AAP-Cl and (-)-AAP-Cl (R2 > or = 0.996) with a limit of quantitation of 100 ng/ml and the recovery was greater than 80% for both enantiomers. The accuracy and precision for both enantiomers ranged from 96 to 102% (+/-0.2-7%) at upper and lower concentrations. The plasma concentration-time profiles of the enantiomers of AAP-Cl were best described by a two-compartment open model with a mean terminal half-life of about 5h, volume of distribution at steady state of 3 l/kg and clearance of about 0.6l/(hkg) in rats. There was no significant difference between the pharmacokinetic parameters of (+)-AAP-Cl and (-)-AAP-Cl, suggesting that the disposition of AAP-Cl in rats is not enantioselective. In addition, no chiral inversion of (+)-AAP-Cl to (-)-AAP-Cl or vice versa was observed. The results of this investigation have shed some light on the mechanism of action and disposition of AAP-Cl in rats.     

Inhibition of apomorphine-induced yawning and penile erection by neurotensin.

The yawns and penile erection elicited in rats by apomorphine (100 micrograms/kg SC) are dose-dependently suppressed by the enkephalinase-resistant analog of NT, [D-Trp11]NT, intracerebroventricularly (ICV) injected (10-120 ng per rat). This antagonistic effect was shared by NT (0.75-3 micrograms per rat) administered ICV. The yawns induced by pilocarpine (2 mg/kg IP) were similarly antagonized by [D-Trp11]NT (30-120 ng per rat). The enkephalinase inhibitor acetorphan (5 mg/kg IV) reduced in a naloxone (2 mg/kg, SC)-resistant manner the apomorphine-induced penile erection or yawning.     

NE-100, a novel sigma receptor ligand: In vivo tests

It has been suggested that sigma receptor antagonists may be useful as antipsychotic drugs. N, N-dipropyl-2-[4-methoxy-3-(2-phenylethoxy)phenyl]-ethylamine monohydrochloride (NE-100) is a novel compound with high affinity for the sigma receptor (IC50 = 4.16 nM), but low affinity (IC50 > 1000 nM) for D1, D2, 5-HT1A, 5-HT2 and phencyclidine (PCP) receptors. The head-weaving behavior induced by either (+)SKF10047 or PCP was dose-dependently antagonized by NE-100 with oral ED50 at 0.27 and 0.12 mg/kg, respectively. NE-100 did not affect dopamine agonists-induced stereotyped behavior and/or hyperactivity. NE-100 failed to induce catalepsy in rats. These findings indicate that NE-100 may have antipsychotic activity without the liability of motor side effects typical of neuroleptics.     

Piracetam modifies morphine and related drug-induced elevation of serum corticosterone concentration in rats

Piracetam (2-oxo-l-pyrrolidine acetamide, UCB 6215) or physiological saline solution was injected intravenously to female rats; after 60 min the animals were decapitated and blood was collected. Piracetam in doses of 100, 300 and 600 mg/kg resulted in a progressive suppression of serum corticosterone concentration (Cpd B) as compared to the controls. Morphine (5 and 10 mg/kg), nalorphine (5 and 10 mg/kg) and naloxone (0.5 mg/kg) induced a significant rise of Cpd B 30 min after subcutaneous injection, however, this could be prevented by 300 mg/kg piracetam given intraperitoneally 60 min prior to decapitation. Piracetam was ineffective in reducing the effects of high doses of morphine (20 mg/kg) and nalorphine (20 mg/kg). The drug had no effect on either ether stress or electric footshocks induced activation of the pituitary-adrenocortical system. In vitro the drug had no effect on pituitary ACTH release following exposure to crude hypothalamic extract. It is concluded that the effect of piracetam on the pituitary-adrenocortical axis is mediated through hypothalamic or extrahypothalamic brain structures and influences one of the effects of morphine and related drugs.     

Effect of flavone in a canine model of myocardial stunning

Putative cardioprotective action of flavone (10, 20 and 30 mg/kg) was investigated in a canine model of regional ischemia (20 min) followed by 60 min of reperfusion. In animals pretreated with vehicle, myocardial stunning was evidenced by significant changes in hemodynamic parameters (depressed mean arterial pressure, LV peak (+) dP/dt, LV peak (-) dP/dt and elevated LV end-diastolic pressure) and biochemical parameters (decreased myocardial ATP and rise in plasma malondialdehyde or MDA; a marker of free radical-induced injury). A reduction in plasma MDA was noted with 20 and 30 mg/kg flavone, although attenuation of myocardial dysfunction was evident with all the three doses. The results suggest that besides a significant dose-dependent antioxidant effect, flavone may also have some cardioprotective actions per se, which needs to be further investigated.     

Role of Brain Regional GABA: Aldrin-Induced Stimulation of Locomotor Activity in Rat

Aldrin, a chlorinated hydrocarbon group of pesticide, is a well known central nervous system (CNS) stimulant. The CNS stimulating effect of aldrin is manifested in the form of an increase in locomotor activity (LA) of animals. Maximum increase in LA was observed at 2 h following aldrin (2-10 mg/kg, p.o.) treatment and this aldrin-induced increase in LA attained a peak at a dose of 10 mg/kg, p.o. Administration of aldrin (2 or 5 mg/kg/day, p.o.) enhanced LA of rats and reached a maxima after 12 consecutive days of treatment following which aldrin-induced LA was gradually reduced and restored to control value after 20 consecutive days of aldrin treatment. A single administration of aldrin (2-10 mg/kg, p.o.) reduced the GABA system in cerebellum, hypothalamus and pons-medulla. The treatment with aldrin (2 mg/kg/day, p.o.) for 12 consecutive days produced more inhibition in those brain regional GABA system than that observed with a single dose of aldrin. These results, thus, suggest that aldrin-induced inhibition of central GABA may be a cause of stimulation of LA with aldrin either at a single dose or for 12 consecutive days.     

Effects of AH-9700, (+)-pentazocine, DTG and oxybutynin on micturition in anesthetized rats with acetone-induced cystitis

We investigated the effects of AH-9700 (1-[2-(3,4-dihydro-6,7-dimethyl-2-naphthalenyl)ethyl] pyrrolidine fumarate; a novel sigma receptor ligand), (+)-pentazocine and 1,3-di-o-tolylguanidine (DTG) (two typical sigma receptor ligands), and oxybutynin (a currently used anti-pollakiuria drug) on cystometrograms in anesthetized rats with 30% acetone-induced cystitis. Compared to sham-treated rats, acetone-treated cystitis models exhibited an increase in urinary frequency during continuous filling cystometry. Intravenous administration of AH-9700 (1-5 mg/kg), (+)-pentazocine or DTG to the rats with cystitis dose-dependently prolonged micturition intervals and increased the micturition threshold pressure. Oxybutynin (1 mg/kg. i.v.) also extended micturition intervals, but decreased the micturition pressure. These results indicate that AH-9700, (+)-pentazocine and DTG improve abnormal frequent urination caused by acetone-induced cystitis in a manner different from that of oxybutynin.