Involvement of the serotonergic neuronal system in phencyclidine-induced place aversion in rats.

The possible involvement of the serotonergic neuronal system in aversive motivation produced by phencyclidine [1-(1-phenylcyclohexyl)piperidine; PCP] was investigated using a place-conditioning paradigm in rats. PCP (4 mg/kg, i.p.) produced place aversion in this task as reported previously (Kitaichi K, Noda Y, Hasegawa T, Furukawa H, Nabeshima T. Acute phencyclidine induces aversion, but repeated phencyclidine induces preference in the place conditioning test in rats. Eur J Pharmacol 1996;318:7-9). The blockade of serotonin2A (5-HT2A) receptors using the antagonist ritanserin (3 and 10 mg/kg, p.o.) significantly attenuated this aversive property of PCP whereas lesions of serotonergic neurons using 5,7-dihydroxytryptamine (5,7-DHT, 100 microg/animal, i.c.v.) failed to affect it. Repeated PCP treatment (10 mg/kg, i.p. for 14 days), which is enough to diminish the stereotyped 5-HT2A receptor-mediated head-twitch behavior, also decreased the place aversion. These results suggest that the serotonergic neuronal system, specifically the 5-HT2A receptor, may play a critical role in producing PCP-induced place aversion.     

Cocaine, but not morphine, induces conditioned place preference and sensitization to locomotor responses in CB1 knockout mice

The involvement of cannabinoid CB1 receptors in morphine and cocaine motivational effects was investigated using CB1 knockout mice. For this purpose, we evaluated the rewarding effects in the place conditioning paradigm and the sensitization to the locomotor responses induced by these drugs. The hyperlocomotion induced by acute morphine administration (15 mg/kg, s.c.) was preserved, but the sensitization to this locomotor response induced by chronic morphine treatment was abolished in CB1 mutant mice. Morphine (5 mg/kg, s.c.) induced conditioned place preference in wild-type mice but failed to produce any response in knockout mice, indicating the inability of morphine to induce rewarding effects in the absence of CB1 cannabinoid receptors. When the aversive effects of morphine withdrawal were investigated using the place aversion paradigm, no differences between genotypes were observed. Acute cocaine (10 mg/kg, i.p.) induced hyperlocomotor responses in wild-type and knockout mice and a chronic cocaine treatment produced a similar sensitization to this response in both genotypes. In the conditioning place preference paradigm, cocaine (20 mg/kg, i.p.) produced rewarding responses in both wild-type and knockout mice. These results demonstrate that CB1 receptors are essential for adaptive responses produced by chronic morphine but not by chronic cocaine treatment.     

Antagonism of the nitric oxide synthase inhibitor, L-NAME, of the effects of phencyclidine on latent inhibition in taste aversion conditioning

Latent inhibition (LI) is a behavioural procedure used to evaluate the potential propsychotic and antipsychotic properties of psychoactive drugs. In the present study, a conditioned taste aversion (CTA) procedure was used to investigate the effects of the nitric oxide (NO) synthase inhibitor, N(G)-nitro-L-arginine methyl ester (L-NAME), and the psychotomimetic drugs, phencyclidine (PCP) and d-amphetamine (d-AMP) on LI. PCP (2 mg/kg) and d-AMP (0.5 mg/kg) were both found to enhance LI in this procedure. The effect of d-AMP on LI was less pronounced and this drug also caused a weak disruption of taste aversion conditioning. Pretreatment with L-NAME (10 mg/kg) blocked the LI enhancing effect of PCP on LI but not that of d-AMP. L-NAME by itself caused an attenuation of LI. L-NAME has been shown to block also other behavioural and biochemical effects of PCP in previous studies and these results and the present findings suggest that at least some of the effects PCP are dependent on NO and possibly also that some NOS inhibitors may exert antipsychotic properties.     

Behavioural adaptations to addictive drugs in mice lacking the NMDA receptor epsilon1 subunit

N-methyl-D-aspartate (NMDA) receptors, a subtype of glutamate receptors (GluRs) formed by assembly of the GluRzeta subunit (called NR1 in rats) with any one of four GluRepsilon subunits (GluRepsilon1-4; NR2A-D), play an important role in excitatory neurotransmission, synaptic plasticity and brain development. Recent pharmacological studies have also indicated a role for NMDA receptors in drug addiction. In the present study, we investigated the behavioural adaptations to addictive drugs such as phencyclidine (PCP), methamphetamine (MAP) and morphine (MOR) in mice lacking the GluRepsilon1 subunit of the NMDA receptor. GluRepsilon1 mutant mice exhibited a malfunction of NMDA receptors, as evidenced by the reduction of [3H]MK-801 binding in an autoradiographic receptor binding assay. GluRepsilon1 mutant mice showed an attenuation of acute PCP- and MAP-induced hyperlocomotion. The development of sensitization by repeated treatment with PCP and MAP at a low, but not high, dose was also suppressed. The development of MOR-induced analgesic tolerance and naloxone-precipitated MOR withdrawal symptoms were attenuated in GluRepsilon1 mutant mice. In the place conditioning test, PCP-induced place aversion in naive mice and place preference in PCP-pretreated mice, as well as MOR-induced place preference, were diminished whereas MAP-induced place preference was not affected in GluRepsilon1 mutant mice. These findings provide genetic evidence that GluRepsilon1 subunit-containing NMDA receptors are involved in certain aspects of drug addiction.     

Antisense knockdown of neuronal nitric oxide synthase antagonizes nitrous oxide-induced behavior

The behavioral effects of nitrous oxide (N(2)O) were antagonized by non-specific inhibitors of nitric oxide synthase (NOS). To identify the isoform of NOS involved in this response, mice were pretreated with an antisense oligodeoxynucleotide (AS-ODN) against neuronal NOS, and then tested in a light/dark exploration paradigm. The AS-ODN but not the mismatch ODN significantly antagonized N(2)O induced behavior and also reduced NOS activity in the cerebellum and hippocampus. These results implicate neuronal NOS in the N(2)O response.     

The nitric oxide synthase inhibitor. L-NAME, blocks certain phencyclidine-induced but not amphetamine-induced effects on behaviour and brain biochemistry in the rat

1. 1. The present experiments examined the effects of the nitric oxide synthase (NOS) inhibitor, N^G-nitro-L-arginine methyl ester (L-NAME), on some behavioural and biochemical effects of phencyclidine (PCP) and d-amphetamine (AMPH) in rats. Observation of behaviour was performed using a subjective scoring system.

2. 2. PCP (4 mg/kg) increased locomotor activity, rearing, sniffing, grooming and stereotyped behaviour, and decreased stillness. PCP also increased forepaw myoclonus, forepaw treading, head weaving, licking and chewing behaviour. Most of these behaviours were significantly suppressed by pretreatment with L-NAME (10 mg/kg).

3. 3. AMPH (1 mg/kg) exerted different effects on behaviour. It increased locomotor activity, rearing, sniffing, and stereotyped behaviour, and decreased stillness and grooming, but failed to affect the other behavioural items observed. Pretreatment with L-NAME did not counteract these effects.

4. 4. Ex vivo biochemical analysis indicated that PCP increased the tissue concentration of the dopamine (DA) metabolites, dihydroxyphenyl acetic acid (DOPAC) and homovanillic acid (HVA), in the ventral striatum (i.e., the nucleus accumbens and olfactory tubercles) and frontal cortex but not in the dorsal striatum. DA was not significantly affected in any of these regions. Furthermore, PCP increased the tissue concentration of the serotonin (5-HT) metabolite, 5-hydroxyindole acetic acid (5-HIAA), in a similar manner, while 5-HT was not affected. These biochemical effects were significantly counteracted by pretreatment with L-NAME.

5. 5. AMPH decreased tissue DOPAC concentration in the dorsal striatum, an effect which was not sensitive to pretreatment with L-NAME. However, the combined treatment with L-NAME and AMPH increased tissue DA concentration in all three regions investigated.

6. 6. The neurochemical and behavioural effects of PCP and AMPH were further investigated in an experimental model which allowed measurement of prepulse inhibition (PPI) of acoustic startle in parallel with in vivo microdialysis sampling of extracellular DA concentration in the brain of awake, freely moving animals.

7. 7. Both PCP (2 mg/kg) and AMPH (2 mg/kg) caused a significant decrease in PPI of similar magnitude and duration. These behavioural effects were accompanied by a significant increase in extracellular DA concentration in the nucleus accumbens (PCP: 213 ± 21% of basal concentration; AMPH: 563±117%).

8. 8. The decrease in PPI caused by PCP was blocked by pretreatment with L-NAME, but the change in DA concentration was not. Neither of these measures were significantly affected by L-NAME in AMPH-treated rats.

9. 9. In conclusion, this study shows that several behavioural and biochemical effects of PCP are prevented by pretreatment with the NOS inhibitor, L-NAME, while the effects of AMPH are less sensitive to this pretreatment. These observations emphasise the involvement of nitric oxide in the pharmacological effects of PCP.

     

Treatment with the atypical antipsychotic,olanzapine, prevents the expression of amphetamine-induced place conditioning in the rat

Place conditioning (PC) experiments were conducted as a means to further elaborate the treatment potential of the atypical antipsychotic, olanzapine (OLZ), for stimulant abuse. The resulting preference/aversion provides an indirect measure of the incentive salience (i.e., euphoria/dysphoria) produced by a drug. Male Sprague-Dawley rats (n=48) were conditioned in two unique environments (i.e., vertical vs. horizontal stripped walls, large vs. small grid flooring) using injections (1.0 mg/kg ip) of either amphetamine (AMPH) or saline (SAL). On average, animals displayed a significant preference for the AMPH-paired location after 2.5 weeks of conditioning (five pairings each of AMPH and SAL). Once the preference was established, animals were pretreated (60 min) with a single dose of OLZ (0.0, 0.56, 1.0 or 1.5 mg/kg sc) given on the test (AMPH-free) day. For the following week's test, animals were injected with SAL (1.0 mg/kg ip) in an attempt to recapture the side preference exhibited before OLZ treatment. OLZ treatment prevented the expression of the AMPH-conditioned preference and reduced locomotor activity. Inhibition of preference resulted from the highest dose of OLZ (1.5 mg/kg), while the inhibition of locomotor activity occurred across all three doses. Additionally, while the effects on preference were no longer apparent by the SAL test the following week (reversible), the activity was still depressed during the SAL tests in animals that had experienced the highest dose of OLZ (1.5 mg/kg). Control experiments, in which OLZ was used as the conditioning drug, suggest that OLZ itself possesses no aversive effects in the PC paradigm, and may even produce a preference for the drug-paired chamber. Because the AMPH preference is dependent on dopamine (DA) release in the nucleus accumbens (NAcc), these experiments suggest that OLZ pretreatment interferes with the rewarding, as well as the subjective effects of AMPH.     

Drug reinforcement studied by the use of place conditioning in rat

Rats display a preference for an environment in which they previously received morphine. The present report provides behavioral and pharmacological data for this simple model of reinforcement produced by opiates and describes an aversion in rats for an environment in which they previously received naloxone. Preferences were produced with intravenous (i.v.) morphine sulfate at doses of 0.08-15 mg/kg and durations of the pairing between environment and morphine of 10 min to 1.5 h. Preferences were also seen with other opiate agonists (etorphine-HCl and levorphanol-tartrate), another route of drug administration (subcutaneous), and after 1-4 administrations of morphine. Cocaine-HCl (i.v.), a non-narcotic drug, known to be self-administered by humans, also produced a place preference. Lithium chloride (i.v.), an agent found to be a punishing stimulus in other situations, produced a place aversion. There was no appreciable preference for an environment paired with dextrorphan-tartrate and naloxone-HCl (2 mg/kg, i.p.) blocked the production of the preference produced by i.v. morphine. In contrast to the effect produced by morphine, aversions were produced with (-)-naloxone-HCl alone at doses of 0.1-45 mg/kg (i.v.). The aversion was not produced at (+)-naloxone. Implantation of rats with a 75 mg morphine pellet 3 days prior to place conditioning potentiated the aversive effect of naloxone. It was concluded that place conditioning produced by morphine and naloxone is mediated by specific opiate receptors and that stimulating and decreasing activity of the endogenous opioid peptide system with systemically administered drugs is positively reinforcing and aversive, respectively. The discussion emphasizes application of the simple and sensitive place conditioning model to drug reinforcement research, including analyses of reinforcement produced by microinjection of opiates into the brain.     

Effects of dopamine antagonists with different receptor blockade profiles on morphine-induced place preference in male mice

The effects of dopamine (DA) antagonists with different selectivity for the DA receptors (SCH 23390, 0.5, 0.25, 0.125 mg/kg; haloperidol, 0.2, 0.1 mg/kg; raclopride, 1.2, 0.6, 0.3 mg/kg; risperidone, 0.4, 0.2, 0.1 mg/kg; U-99194A maleate, 40, 20 mg/kg; clozapine, 2.5, 1.25, 0.625 mg/kg) on the acquisition of place conditioning and morphine-induced conditioned place preference (CPP) were explored in male mice. Morphine (40 mg/kg) produced CPP while SCH 23390, haloperidol and clozapine (highest dose) and risperidone (lowest dose) produced conditioned place aversion (CPA). Raclopride and U-99194A maleate did not produce CPP or CPA. Morphine-induced CPP was reversed by the administration of SCH 23390 and risperidone (all doses), haloperidol (highest dose) and raclopride and clozapine (intermediate and lowest doses). U-99194A maleate did not reverse morphine-induced CPP. These results suggest that the conditioned rewarding effects of morphine are mediated by the different subtypes of DA receptors.     

Effects of stress during reactivation on rewarding memory

When a stabilized memory is recalled or reactivated, it becomes labile and sensitive to disruptors such as protein-synthesis inhibitors. Previous evidence demonstrates that stress modulates different aspects of memory. The role of stress during reactivation on rewarding or aversive memory is not known, however. This study examines the effects of stress on rewarding or aversive memory using the conditioned place preference or conditioned place aversion paradigm. Rats were trained to acquire a sucrose and cocaine-conditioned place preference or naloxone-conditioned place aversion. Subsequently, rats were reexposed to the previous sucrose-paired, cocaine-paired and naloxone-paired chamber for 10 min before experiencing the stressful Morris water maze. All rats were tested for conditioned place preference or conditioned place aversion after the stressful water-maze task. After 5-day repeated exposure to the previously reward-paired chamber and experiencing stress, cocaine-conditioned place preference disappeared and sucrose-conditioned place preference was reversed; however, after 5-day repeated exposure to the previously naloxone-paired chamber and experiencing stress, naloxone-conditioned place aversion was not significantly changed. Our results provide the first evidence that the rewarding memory may have been reduced by exposing rats to stress during reexposure to the reward-paired context, which suggests that manipulations of drug memory during reactivation can provide a potential treatment for drug addiction.     

Effects of phencyclidine (PCP) on the visual evoked potentials in the rhesus monkey

Effects of acute and chronic administration of phencyclidine (PCP) on both neocortical and subcortical visual potentials (VEPs) and on spontaneous EEGs were studied in the rhesus monkeys with permanently implanted brain electrodes. VEPs were evoked by brief single photo-stimulator flashes (0.8 pps. 10 microsec duration). Injection of PCP (0.5 to 4.0 mg/kg doses, IV) in monkeys produced a significant inhibition on the peak-amplitude of major VEP components predominantly in the occipital lobe and hippocampus. The PCP-induced VEP inhibition persisted in the presence of occipital and hippocampal theta-activities. Nystagmus persisted throughout the 6 to 8 hours course of PCP-induced behavior. A biphasic pattern of inhibitory and excitatory effects on EEGs and behavior was also observed during the 6 to 8 hours observation period. Chronic administration of PCP (2.0 and 4.0 mg/kg dose IV daily) produced a significant decrease in its inhibitory effects on VEPs, suggesting tolerance development to the inhibitory effect on VEPs. The results suggest that the hippocampus has important implications in the modulation of PCP effects on CNS activities related to the visual function of the rhesus monkey.     

Maintenance of conditioned place preferences and aversion in C57BL6 mice: effects of repeated and drug state testing

In order to determine the factors affecting the expression of place conditioning (i.e. conditioned place preference or aversion), groups of C57BL6 mice were conditioned to morphine, cocaine or naloxone (all drugs 4 mg/kg s.c.) and tested intermittently (up to 4 weeks) or repeatedly (daily). When tested once only in a drug-free state, the expression of place conditioning to all drugs generally strengthened with the passing of time and was only marginally different when tested once more later. The expression of place conditioning was remarkably resistant to extinction during repeated daily testing, though decreases were observed over 2 weeks of testing. Subsequently, a small and non-significant degree of spontaneous recovery of the expression of place conditioning was observed when animals were undisturbed for an extended period of time. Administering animals the same drug to which they were conditioned 20 min prior to testing (i.e. drug state testing), either prior to or following repeated testing, enhanced expression of morphine-induced conditioned place preference, whereas expression of cocaine-induced conditioned place preference was unaffected. When vehicle was administered prior to testing, there was no evidence of naloxone-induced conditioned place aversion, whereas naloxone administration maintained conditioned place aversion. These results show that the expression of place conditioning in C57BL6 mice does not readily diminish over time, but on the contrary, tends to strengthen if tested only once or intermittently. Thus, the hedonic properties of drugs may be more clearly revealed during long-term, rather than short-tem testing, and in some instances, when tested in the drugged state.     

The failure of some rats to acquire intravenous cocaine self-administration is attributable to conditioned place aversion

Although cocaine administration in humans includes euphoric and anxiogenic effects, the latter are less well understood. Acute cocaine administration produces aversive effects including anxiogenic effects as well as appetitive effects in rats and mice. In the present study the self-administration and conditioned place preference paradigms were used to determine whether the failure of some rats to acquire intravenous cocaine self-administration is attributable to either an interference with learning or an aversion to cocaine. Rats were classified as self-administrators or non-self-administrators based on the mean number of cocaine self-infusions per session and whether or not rats exhibited either a stable high level of responding or a stable low level of responding. Intravenously administered cocaine produced place preference for the self-administrators, while intravenously administered cocaine produced place aversion for the non-self-administrators. The fact that the non-self-administrators showed place aversion is inconsistent with the interpretation that the failure of these rats to readily self-administer is attributable to cocaine-mediated interference of learning. This is the first study in which both the self-administration and the conditioned place preference paradigms have been used in the same animals to demonstrate that the effects of cocaine are appetitive for some rats and aversive for others, and are not an artifact of cocaine's interference with learning.     

Endogenous enkephalins, not endorphins, modulate basal hedonic state in mice

The aversive response to naloxone administration observed in human and animal studies suggests the presence of an endogenous opioid tone regulating hedonic state but the class(es) of opioid peptides mediating such opioid hedonic tone is uncertain. We sought to address this question using mice deficient in either beta-endorphin or pro-enkephalin in a naloxone-conditioned place aversion paradigm. Mice received saline in the morning in one chamber and either saline or naloxone (0.1, 1 or 10 mg/kg, s.c.) in the afternoon in another chamber, each day for 3 days. On the test day they were given free access to the testing chambers in the afternoon and the time spent in each chamber was recorded. Whereas wild-type and beta-endorphin-deficient mice exhibited a robust conditioned place aversion to naloxone, pro-enkephalin knockout mice failed to show aversion to naloxone at any dose tested. In contrast, these mice showed a normal conditioned aversion to the kappa opioid receptor agonist, U50,488 (5 mg/kg), and to LiCl (100 mg/kg) indicating that these mice are capable of associative learning. In a separate experiment, pro-enkephalin knockout mice, similar to wild-type and beta-endorphin-deficient mice, demonstrated a significant conditioned place preference to morphine (2.5, 5 and 10 mg/kg s.c.). These data suggest that enkephalins, but not endorphins, may mediate an endogenous opioid component of basal affective state and also indicate that release of neither endogenous enkephalins nor endorphins is critical for the acquisition or expression of the association between contextual cues and the rewarding effect of exogenously administered opiates.     

Effects of the serotonin2A/2C receptor agonist and antagonist on phencyclidine-induced dopamine release in rat medial prefrontal cortex

1. Systemic administration of PCP (7.5 mg/kg, i.p.) produced a greater increase in extracellular DA levels in the mPFC than in the STR and NAC, as determined by in vivo microdialysis of awake, freely moving rats. Preferential activation by PCP of prefrontal DA neurons may be, at least in part, the basis for the pathophysiology of PCP-induced psychosis as well as schizophrenia. 2. Recent studies suggest a possible involvement of 5-HT2A receptors in the pathophysiology and treatment of schizophrenia. This study was designed to examine whether and how 5-HT2A receptors modulate PCP-induced DA release in the mPFC. 3. The 5-HT2A/2C receptor agonist (+/-)-DOI (2.5 mg/kg, but not 0.75 mg/kg, i.p.), administered 60 min prior to PCP, significantly attenuated the PCP-induced increase in extracellular DA levels. Pretreatment of the 5-HT2A/2C receptor antagonist ritanserin (1.0 and 5.0 mg/kg, i.p.), administered 60 min prior to PCP, did not influence the PCP-induced increase. When administered alone, neither DOI (2.5 mg/kg) nor ritanserin (1.0 mg/kg) affected basal extracellular DA levels in the mPFC. 4. The NMDA receptor antagonist MK-801 (1.0 mg/kg, i.p.) also increased extracellular DA levels in the mPFC, but this effect was unaffected by pretreatment with DOI (2.5 mg/kg). 5. These results suggest that the stimulation of 5-HT2A/2C receptors may inhibit DA release in the mPFC when it is facilitated by PCP. Other than the NMDA receptor-mediated mechanism may also be involved in the neurochemical interaction between 5-HT2A receptors and PCP in the mPFC.     

Effects of oil intake in the conditioned place preference test in mice

We investigated the effects of corn oil intake in the conditioned place preference (CPP) test in mice. Voluntary intake of corn oil in the light box in the CPP test showed place preference, but its peroral administration 60 min before conditioning did not show either place preference or aversion. Acquisition of the place preference by corn oil intake was blocked by i.p. injection of SCH 23390 (0.03 mg/kg) and haloperidol (0.1 mg/kg), but not by that of (±)-sulpiride (100 mg/kg), (−)-sulpiride (100 mg/kg), and L-741,626 (1 mg/kg) 15 min before conditioning. These results suggest that stimulation of corn oil in the oral cavity, but not its postingestive effects, have positive reinforcing effects and the stimulation of corn oil is at least partly mediated via dopaminergic systems through the D₁ receptors. Moreover, the present results suggest that the CPP test is useful for the study of preferable stimulation and rewarding effects of food intake.     

Lithium chloride disrupts consolidation of morphine-induced conditioned place preference in male mice: the nitric oxide/cyclic GMP signaling pathway

Lithium effects on brain functions such as cognition, attention, learning and memory are well-established for ages; however, the way it affects these functions and its precise mechanism of action remains unknown. The purpose of this study was to determine the effects of lithium on the consolidation of morphine-associated conditioned place preference and the possible involvement of the NO/cGMP pathway. Using an unbiased conditioned place preference (CPP) model, the effects of lithium (1-100 mg/kg, i.p.), nitric oxide synthase inhibitor NG-nitro-L-arginine methyl ester (L-NAME) (5-100 mg/kg, i.p.), nitric oxide precursor L-arginine (50-150 mg/kg, i.p.) and phosphodiesterase inhibitor sildenafil (5-40 mg/kg, i.p.) on the consolidation of morphine-induced CPP were assessed. In addition, the possible interaction between lithium, L-arginine and sildenafil or subeffective doses of lithium and L-NAME on the consolidation of morphine-induced contextual memory was evaluated. NMRI mice were used in all studies. Lithium (5-30 mg/kg, i.p.), immediately after conditioning trials, significantly reduced the time spent by mice in the reward-paired compartment. Although post-training administration of L-arginine, sildenafil or L-NAME had no significant effect on the consolidation of CPP, concomitant administration of L-arginine (50-150 mg/kg) and sildenafil (5-10 mg/kg) with lithium (30 mg/kg) prevented the impairing effect of lithium. Also, co-administration of sub-effective doses of lithium (1 mg/kg) and L-NAME (5 mg/kg) disrupted consolidation of CPP. However, delayed administration of effective doses of lithium, which shows specific effect on memory consolidation, did not affect morphine-induced CPP. Lithium seems to inhibit consolidation of morphine-induced CPP and this impairing effect might be via nitric oxide/cyclic GMP pathway.     

Sulpiride injections into the medial septum reverse the influence of intra-medial septum injection of L-arginine on expression of place conditioning-induced by morphine in rats

Effects of intra-medial septum injections of L-arginine, a precursor of nitric oxide, N(G)-nitro-L-arginine methyl ester (L-NAME), an inhibitor of nitric oxide synthase, and sulpiride, a selective antagonist of dopamine D2 receptor on morphine-induced conditioned place preference (CPP) in male Wistar rats were examined. Using a 3-day schedule of conditioning, morphine (0.5-7.5 mg/kg, s.c.) produced a significant place preference in a dose-dependent manner. The maximum response was observed with 5.0 mg/kg of opioid. Sulpiride (0.3, 1.0 and 3.0 microg/rat), but not L-arginine (0.3, 1.0 and 3.0 microg/rat) or L-NAME (0.3, 1.0 and 3.0 microg/rat), in combination with morphine (5.0 mg/kg), during conditioning, significantly altered morphine-induced CPP. Single doses (0.3, 1.0 and 3.0 microg/rat) of either L-arginine or L-NAME, during conditioning, did not induce CPP. Sulpiride at 0.3-3.0 microg/rat, intra-medial septum, during conditioning, produced a significant conditioned place aversion. Intra-medial septum injections of L-arginine but not L-NAME or sulpiride, 1-2 min before testing, increased the expression of morphine-induced CPP. The administration of sulpiride (0.3, 1.0 and 3.0 microg/rat), but not L-NAME (0.3, 1.0 and 3.0 microg/rat), 1-2 min before the injection of L-arginine (0.3 microg/rat) on day of test, significantly attenuated the response to L-arginine. L-Arginine (0.3-3.0 microg/rat), during conditioning, showed a statistically significant increase in locomotor activity compared with that to control group. Moreover, sulpiride decreased locomotion by itself or in combination with morphine during conditioning and on the test day of morphine CPP. It can be concluded that L-arginine, a precursor of nitric oxide, in the rat median septum may play a role in expression of morphine conditioning due to dopamine release in this area.     

Effects of calcium channel antagonists on the motivational effects of nicotine and morphine in conditioned place aversion paradigm

The motivational component of drug withdrawal may contribute to drug seeking and relapse through the negative reinforcement-related process; thus, it is important to understand the mechanisms that mediate affective withdrawal behaviors. The present study was undertaken to examine the calcium-dependent mechanism of negative motivational symptoms of nicotine and morphine withdrawal using the conditioned place aversion (CPA) paradigm. Rats were chronically treated with nicotine (1.168 mg/kg, free base, s.c., 11 days, three times daily) or morphine (10 mg/kg, s.c., 11 days, twice daily). Then, during conditioning, rats pre-treated with nicotine or morphine received a nicotinic receptor antagonist mecamylamine (3.5 mg/kg) or an opioid receptor antagonist naloxone (1 mg/kg) to precipitate withdrawal in their initially preferred compartment, or saline in their non-preferred compartment. Our results demonstrated that after three conditioning sessions, mecamylamine induced a clear place aversion in rats that had previously received nicotine injections, and naloxone induced a significant place aversion in rats that had previously received morphine injections. Further, the major findings showed that calcium channel antagonists, i.e., nimodipine, verapamil and flunarizine (5 and 10 mg/kg, i.p.), injected before the administration of mecamylamine or naloxone, attenuated nicotine or morphine place aversion.As an outcome, these findings support the hypothesis that similar calcium-dependent mechanisms are involved in aversive motivational component associated with nicotine a morphine withdrawal. We can suggest that calcium channel blockers have potential for alleviating nicotine and morphine addiction by selectively decreasing the incentive motivational properties of both drugs, and may be beneficial as smoking cessation or opioid dependence pharmacotherapies.     

Involvement of nitric oxide pathway in the acute anticonvulsant effect of melatonin in mice

Melatonin, the major hormone produced by the pineal gland, is shown to have anticonvulsant effects. Nitric oxide (NO) is a known mediator in seizure susceptibility modulation. In the present study, the involvement of NO pathway in the anticonvulsant effect of melatonin in pentylenetetrazole (PTZ)-induced clonic seizures was investigated in mice. Acute intraperitoneal administration of melatonin (40 and 80 mg/kg) significantly increased the clonic seizure threshold induced by intravenous administration of PTZ. This effect was observed as soon as 1 min after injection and lasted for 30 min with a peak effect at 3 min after melatonin administration. Combination of per se non-effective doses of melatonin (10 and 20 mg/kg) and nitric oxide synthase (NOS) substrate L-arginine (30, 60 mg/kg) showed a significant anticonvulsant activity. This effect was reversed by NOS inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME, 30 mg/kg), implying an NO-dependent mechanism for melatonin effect. Pretreatment with L-NAME (30 mg/kg) and N(G)-nitro-L-arginine (L-NNA, 10 mg/kg) inhibited the anticonvulsant property of melatonin (40 and 80 mg/kg) and melatonin 40 mg/kg, respectively. Specific inducible NOS (iNOS) inhibitor aminoguanidine (100 and 300 mg/kg) did not affect the anticonvulsant effect of melatonin, excluding the role of iNOS in this phenomenon, while pretreatment of with 7-NI (50 mg/kg), a preferential neuronal NOS inhibitor, reversed this effect. The present data show an anticonvulsant effect for melatonin in i.v. PTZ seizure paradigm, which may be mediated via NO/L-arginine pathway by constitutively expressed NOS.