Effects of CB1 receptor antagonist within the nucleus accumbens on the acquisition and expression of morphine-induced conditioned place preference in morphine-sensitized rats

It has been shown that cannabinoids interact with the opiate system in reward-related behaviors and in animal models of addiction. In the present study, the effects of bilateral intra-accumbal administration of AM251, a CB1 receptor antagonist, on the acquisition and expression of ineffective dose of morphine-induced conditioned place preference (CPP) in morphine-sensitized rats were investigated. 158 adult male albino Wistar rats were used in these experiments. Subcutaneous (s.c.) administration of morphine (0.25, 0.5, 0.75, 1, 2.5 and 5mg/kg) induced CPP only at the dose of 5mg/kg. In addition, repeated administration of morphine (5mg/kg; s.c.), once daily for 3 days followed by 5 days free of the opioid (sensitization period), increased conditioning response induced by ineffective doses of morphine (0.25, 0.5 and 0.75 mg/kg). Bilateral intra-accumbal administration of AM251 (5, 25 and 125 ng/0.5 microl per side) dose-dependently reduced the acquisition and expression of morphine-induced CPP in morphine-sensitized rats, while bilateral intra-accumbal administration of neither saline nor DMSO (0.5 microl/side) had effects on the acquisition and expression of morphine-induced CPP in sensitized rats. The results indicated that CB1 receptors within the nucleus accumbens are involved in the acquisition and expression of morphine-induced CPP in sensitized rats. Our findings also suggest the existence of cross-talk between cannabinoids and opiates on the sensitization to morphine and the implication of endocannabinoid system in the process of sensitization to opiates.     

Influence of morphine- or apomorphine-induced sensitization on histamine state-dependent learning in the step-down passive avoidance test

Effects of morphine- or apomorphine-induced sensitization on histamine state-dependent memory of passive avoidance task were examined in mice. Pre-training intracerebroventricular (i.c.v.) administration of histamine (20 microg/mouse) decreased the learning of a one-trial passive avoidance task. Pre-test administration of histamine (10 and 20 microg/mouse) reversed amnesia induced by pre-training of histamine, with maximum response at 20 microg/mouse. Pre-training histamine-induced amnesia was also reversed in morphine- or apomorphine-sensitized mice that had previously received once daily injections of morphine (20 and 30 mg/kg) or apomorphine (0.5 and 1 mg/kg) for 3 days. The reversion of histamine-induced amnesia in morphine-sensitized mice was decreased by once daily administration of naloxone (0.5 and 1 mg/kg), SCH 23390 (0.05 and 0.1 mg/kg) or sulpiride (25, 50 and 100 mg/kg) prior to injection of morphine (30 mg/kg/day, 3 days). Furthermore, once daily administration of sulpiride (50 and 100 mg/kg) but not SCH 23390 (0.01, 0.05 and 0.1 mg/kg) prior to apomorphine (1 mg/kg, for 3 days) decreased the reversion of pre-training histamine-induced amnesia by apomorphine. The results suggest that apomorphine or morphine sensitization affects the impairment of memory induced by histamine and thus it is postulated that opioid and dopamine receptors may play an important role in this effect.     

Synergistic improvement effect of nicotine-ghrelin co-injection into the anterior ventral tegmental area on morphine-induced amnesia

In the present study the effect of ghrelin or ghrelin/nicotine injection into the anterior ventral tegmental area (aVTA) on morphine-induced amnesia in passive avoidance learning have been evaluated. Also, the role of the aVTA nicotinic receptors in possible ghrelin-induced effects has been investigated. All animals were bilaterally implanted with chronic cannulas in the aVTA. A step-through type passive avoidance task was used for measurement of memory. We found that post-training subcutaneous (s.c.) injection of morphine (0.5–7.5 mg/kg) dose-dependently reduced the step-through latency, indicating morphine-induced amnesia. Post-training bilateral infusion of ghrelin (0.3, 1.5 and 3 nmol/μl) in a dose-dependent manner reversed amnesia induced by morphine (7.5 mg/kg, s.c.). Furthermore, reversal effect of ghrelin (3 nmol/μl) was blocked by pre-treatment of intra-aVTA administration of mecamylamine (1–3 μg/rat), a nicotinic acetylcholine receptor antagonist. Intra-aVTA administration of the higher dose of mecamylamine (3 μg/rat) into the aVTA by itself decreased the step-through latency and induced amnesia. In addition, post-training intra-aVTA administration of nicotine (0.25, 0.5, 1 μg/rat) which alone cannot affect memory consolidation, decreased significantly the amnesia induced by morphine (7.5 mg/kg, s.c.). Co-treatment of an ineffective dose of ghrelin (0.3 nmol/μl) with an ineffective dose of nicotine (0.25 μg/rat) significantly increased step-through latency of morphine (7.5 mg/kg, s.c.) treated animals, indicating the synergistic effect of the drugs. Taken together, our results suggest that intra-aVTA administration of ghrelin reversed morphine-induced amnesia and that ghrelin interacts synergistically with nicotine to mitigate morphine-induced amnesia.     

The effect of dopamine receptor blockade on the development of sensitization to the locomotor activating effects of amphetamine and morphine

The effect of dopamine (DA) receptor blockade on the development of sensitization to the locomotor activating effects of systemic amphetamine and intra-ventral tegmental area (intra-VTA) morphine was investigated. Rats were pretreated with the D-1 DA receptor antagonist, SCH-23390 (0.04 or 0.2 mg/kg, i.p.) or one of two D-2 DA receptor antagonists, pimozide (0.5 mg/kg, i.p.) and Ro 22-2586 (0.2 mg/kg, i.p.), prior to each of 5 exposures to the sensitizing drug. SCH-23390 blocked the development of sensitization to amphetamine but not to intra-VTA morphine. Pimozide had the opposite effect and Ro 22-2586 had no effect on the development of sensitization to either amphetamine or intra-VTA morphine. All 3 antagonists, at the doses tested, completely blocked the acute locomotor activating effects of these two drugs. Pretreatment in separate animals with low autoreceptor doses of sulpiride (25 mg/kg, i.p. with amphetamine and 10 mg/kg, i.p. with intra-VTA morphine) slightly potentiated the acute locomotor effect and produced a slight enhancement of the sensitized response to amphetamine and intra-VTA morphine. Pretreatment with a higher dose of sulpiride (50 mg/kg, i.p.) blocked the acute locomotor effect of intra-VTA morphine but had no effect on the development of sensitization to this drug. These results suggest that the mechanisms underlying the development of sensitization to the locomotor activating effects of amphetamine and intra-VTA morphine are different even though these may ultimately result in similar changes in the activity of mesencephalic DA neurons. Implications of these findings for the differential involvement of D-1 and D-2 DA receptors and for neural hypotheses of behavioral sensitization are discussed.     

Repeated pre-exposure to morphine into the ventral pallidum enhances morphine-induced place preference: involvement of dopaminergic and opioidergic mechanisms

In the present study, the effect of repeated administration of morphine into the ventral pallidum (intra-VP) on the conditioned place preference (CPP) induced by systemic morphine injection was investigated in male Wistar rats. Subcutaneous (s.c.) administration of morphine (2.5, 5 and 7.5mg/kg), during conditioning, induced conditioned place preference (CPP). The maximum response was obtained with 5mg/kg of morphine. Lower dose of morphine (0.5mg/kg) did not induce CPP, but in the animals which had previously, received 3 days intra-VP repeated injections of morphine (3 or 5microg/rat) followed by 5 days free of the drug, elicited a significant CPP. Moreover, 3 days intraperitoneal (i.p.) pretreatment with different doses of naloxone (0.5, 1 and 2mg/kg), SCH 23390 (0.012, 0.025 and 0.05mg/kg) or sulpiride (6.2, 12.5 and 25mg/kg) in combination with repeated injections of morphine (5microg/rat), blocked the opioid response on the acquisition of morphine (0.5mg/kg) CPP. On the other hand, our results showed that 3 days single repeated administration of different doses of naloxone (0.5, 1 or 2mg/kg, i.p.), SCH 23390 but not sulpiride followed by 5 days free of the drug, significantly decreased the acquisition of morphine (0.5mg/kg) CPP and also induced place aversion. Furthermore, the drugs' injections had no effect on locomotor activity on the testing phase of CPP. It is concluded that repeated intra-VP injections of morphine induces behavioral sensitization, which may be due to the opioidrgic and/or dopaminergic mechanism(s).     

Differential behavioural sensitization to intermittent morphine treatment in alcohol-preferring AA and alcohol-avoiding ANA rats: role of mesolimbic dopamine

Alcohol-preferring AA (Alko Alcohol) and alcohol-avoiding ANA (Alko Non-Alcohol) rats have well-documented differences in their voluntary ethanol consumption and brain opioidergic systems. The aim of the present study was to investigate whether these rat lines differ in their susceptibility to morphine-induced behavioural and neurochemical sensitization. The rats were given 15 injections of morphine (10 mg/kg, s.c.) or saline every other day. Locomotor activity and release of dopamine in the nucleus accumbens were monitored after a challenge with additional morphine injections (10 mg/kg) 1 and 5 weeks after withdrawal from the repeated treatment. Morphine increased locomotion more in the previously morphine-treated rats than in the saline-treated controls. Furthermore, AA rats were more sensitive to this effect of morphine than ANA rats. Accumbal morphine-induced dopamine release was significantly higher in the morphine-treated AA than ANA rats after the first challenge injection 1 week from withdrawal, but no differences were observed after the second challenge. The brain and plasma concentrations of morphine were similar among the lines suggesting that the differences in the effects of morphine cannot be explained in terms of differential pharmacokinetics of morphine in these lines. These data show that AA rats are more susceptible to morphine-induced behavioural sensitization than ANA rats. Furthermore, it suggests that mesolimbic dopamine has at best only a transient role in the expression of opioid-induced behavioural sensitization. The relationship between the mechanisms underlying the differential sensitivity of these rat lines to the effects of repeated morphine and voluntary ethanol drinking remains to be determined.     

AMPA antagonist LY293558 blocks the development, without blocking the expression, of behavioral sensitization to morphine

Morphine (3.0 mg/kg, s.c.) stimulates locomotor activity in rats, and this effect sensitizes with repeated intermittent treatment. We examined the ability of the AMPA antagonist LY293558, administered systemically over a range of doses (0.1-3.0 mg/kg), to alter morphine sensitization. Pretreatment with 3.0 mg/kg LY293558 attenuated the acute (session 1) locomotor-stimulating actions of morphine, whereas 1.0, 0.3, and 0.1 mg/kg were without effect. No sensitization was observed after repeated morphine treatment (3.0 mg/kg, s.c., every other day for 9 days) when morphine injections were preceded by 0.3, 1.0, or 3.0 mg/kg LY293558, whereas significant sensitization was observed when morphine injections were preceded by vehicle or 0.1 mg/kg of the antagonist. When all rats were challenged with morphine (3.0 mg/kg, s.c.) alone on day 11, the locomotor activity of rats previously exposed to LY293558 at 3.0, 1.0, or 0.3 mg/kg--but not at 0.1 mg/kg--was significantly lower than that of rats previously given morphine preceded by vehicle. On day 13, pretreatment with 1.0 mg/kg LY293558 failed to alter preestablished morphine sensitization in rats previously pretreated with vehicle. These data indicate that LY293558 blocks the development but not the expression of morphine sensitization, confirming a role for AMPA receptors in the initiation of neurobiological adaptations that occur with chronic morphine treatment.     

Repeated administration of dopaminergic agents in the nucleus accumbens and morphine-induced place preference

In the present study, the effects of repeated intra nucleus accumbens (intra-NAc) injections of dopamine receptor agents on morphine-induced conditioned place preference (CPP) in rats were investigated by using an unbiased 3-days schedule of place conditioning design. The animals receiving once daily subcutaneous (s.c.) injections of morphine (0.5-7.5mg/kg) or saline (1.0 ml/kg, s.c.) showed a significant place preference in a dose-dependent manner. The maximum response was observed with 5mg/kg of the opioid. Three days intra-NAc injections of apomorphine (0.5 and 1 microg/rat) followed by 5 days free of the drug, increased or decreased, respectively CPP induced by the lower dose of morphine (0.5mg/kg, s.c.). Morphine-induced CPP was also significantly increased in the animals that had previously received the 3-days intra-NAc injections of SKF 38393 (4 and 8 microg/rat) or quinpirole (2 and 4 microg/rat, intra-NAc). The CPP induced by a higher dose of morphine (5mg/kg, s.c.) was significantly decreased in the animals that had previously received the 3-days SCH 23390 (0.005 and 0.01 microg/rat; intra-NAc). On the other hand, the CPP induced by morphine (5mg/kg, s.c.) was significantly increased in the animals that had previously received the 3-days sulpiride administration (5 microg/rat, intra-NAc). The 3-days administration of apomorphine, SKF 38393 or quinpirole, but not SCH 23390 and sulpiride reduced the locomotor activity in the test session. It is concluded that repeated injections of dopamine receptors agents followed by 5 days free of the drugs in the NAc can affect morphine reward.     

Morphine-induced place preference: involvement of the central amygdala NMDA receptors

In the present study, the effects of bilateral injections of N-methyl-d-aspartate (NMDA) receptor agonist and/or antagonist into the central amygdala (CeA) on the acquisition and expression of morphine-induced conditioned place preference (CPP) were investigated in male Wistar rats. Animals that received 3 daily subcutaneous (s.c.) injections of morphine (1-9 mg/kg) or saline (1.0 ml/kg) indicated a significant preference for compartment paired with morphine in a dose dependent manner. Intra-CeA administration of the NMDA (0.01, 0.1 or 1 microg/rat) with an ineffective dose of morphine (1 mg/kg, s.c.) elicited a significant CPP. Administration of the non-competitive NMDA receptor antagonist, MK-801 (0.1, 0.3 or 0.5 microg/rat), into the central amygdala dose-dependently inhibited the morphine (6 mg/kg, s.c.)-induced place preference. Furthermore, intra-CeA administration of MK-801 (0.25, 0.5 or 1 microg/rat) reduced the response induced by NMDA (1 microg/rat, intra-CeA) plus morphine (1 mg/kg, s.c.). Neither NMDA nor MK-801 alone produce a significant place preference or place aversion. Moreover, intra-CeA injection of NMDA but not MK-801 before testing significantly increased the expression of morphine (6 mg/kg, s.c.)-induced place preference. NMDA or MK-801 injections into the CeA had no effects on locomotor activity on the testing sessions. These results suggest that the NMDA receptor mechanisms in the central amygdala may be involved in the acquisition and expression of morphine-induced place preference.     

Evidence for opiate-activated NMDA processes masking opiate analgesia in rats

The acute interaction between opioid receptors and N-methyl-D-aspartate (NMDA) receptors on nociception was examined in rats using tail-flick and paw-pressure vocalisation tests. When injected at various times (1 to 6 h) after morphine (5 to 20 mg/kg, i.v.) or fentanyl (4x40 microgram/kg, i.v.), the opioid receptor antagonist naloxone (1 mg/kg, s.c.) not only abolished the opiate-induced increase in nociceptive threshold, but also reduced it below the basal value (hyperalgesia). The noncompetitive NMDA receptor antagonist MK-801 (0.15 or 0.30 mg/kg, s.c.) prevented the naloxone-precipitated hyperalgesia and enhanced the antinociceptive effects of morphine (7.5 mg/kg, i.v.) and fentanyl (4x40 microgram/kg, i.v.). These results indicate that the antinociceptive effects of morphine and fentanyl, two opiate analgesics widely used in humans in the management of pain, are blunted by concomitant NMDA-dependent opposing effects which are only revealed when the predominant antinociceptive effect is sharply blocked by naloxone. This study provides new rationale for beneficial adjunction of NMDA receptor antagonists with opiates for relieving pain by preventing pain facilitatory processes triggered by opiate treatment per se.     

Opioid receptor modulation of feeding-evoked dopamine release in the rat nucleus accumbens

Feeding is associated with increases in the activity of the mesolimbic dopamine (DA) system which originates in the ventral tegmental area (VTA) and projects heavily to the nucleus accumbens. The present study used in vivo brain microdialysis to assess the contribution of opioid receptors in feeding-evoked DA release in the nucleus accumbens. Feeding in 18 h food-deprived rats increased DA release by about 50% above baseline. Systemic injection of the opioid receptor antagonist naltrexone (1 mg/kg, s.c.) blocked the effect of feeding on DA release and reduced the amount of food consumed. Unilateral application of naltrexone (100 μM) in the VTA via a microdialysis probe failed to affect the DA response to feeding, the amount of food consumed, or the latency to eat. In contrast, intra-VTA naltrexone significantly reduced the effect of systemic heroin (0.5 mg/kg, s.c.) on accumbal DA release. These results indicate that: (1) opioid receptor activation is a component of the neural substrates of deprivation-induced feeding: (2) opioid receptors in the VTA do not contribute significantly to feeding-associated increases in DA release in the nucleus accumbens; and (3) heroin-induced increases in accumbal DA release are mediated, at least in part, by opioid receptors in the VTA.     

Ventral pallidal injections of a mu antagonist block the development of behavioral sensitization to systemic morphine

Acute activation of opioid receptors in the ventral pallidum increases motor behaviors in rats. The present study was designed to investigate the possibility that the ventral pallidum influences motor responses induced by chronic opiate treatments and to examine the receptors that may be involved in such an effect. For five consecutive days, ambulations were quantified after rats received once-daily intraperitoneal (i.p.) injections of morphine (10 mg/kg) or saline following bilateral intra-ventral pallidal injections of either saline (0.5 microl/hemisphere), the mu antagonist CTOP (2. 1 microg/0.5microl/hemisphere), or the D1 antagonist SCH23390 (0.25 microg/0.5microl/hemisphere). Behavioral sensitization to an acute morphine challenge (10 mg/kg i.p.) was assessed 72 h after terminating the repeated treatment regimen. Rats who repeatedly received the intra-ventral pallidal saline + i.p. morphine exhibited increases in ambulations during the chronic treatment protocol and this effect was greatly enhanced (i.e., sensitized) following the post withdrawal acute morphine challenge. Rats repeatedly treated with intra-ventral pallidal CTOP + i.p. morphine did not display a motor response either during the chronic treatment regime or to the acute morphine challenge; an effect not seen when CTOP was injected into brain structures located dorsal to the ventral pallidum. The rats repeatedly treated with intra-ventral pallidal injections of SCH23390 + i.p. morphine demonstrated a motor response during the chronic protocol but the magnitude of this response was not significantly enhanced by the acute morphine challenge. These results demonstrate that: 1) mu opioid and D1-like dopamine receptors in the ventral pallidum influence the increase in locomotion that occurs during repeated morphine treatments; and 2) mu opioid (but not D1) receptors in the ventral pallidum are important in the postwithdrawal sensitized response to morphine. Such observations indicate that the ventral pallidum plays a critical role in morphine-induced behavioral sensitization.     

Inhibition of nitric oxide synthase in the ventral tegmental area attenuates cocaine sensitization in rats

1. Male Sprague-Dawley rats were pretreated via bilateral infusion of the VTA with the selective neuronal nitric oxide synthase inhibitor, 7-nitroindazole (0, 8, or 40 ng/hemisphere), prior to each of 7 daily systemic cocaine (30 mg/kg, i.p.) or saline (1 ml/kg) treatments. 2. After a 7-day treatment withdrawal period, rats received a final systemic challenge with either cocaine (30 mg/kg, i.p.) or saline (1 ml/kg). 3. Locomotor and stereotypic activity were measured following the first and last treatments. 4. Daily cocaine treatment led to the development of sensitization to its stereotypic effects as revealed upon drug challenge. 5. The development of sensitization of cocaine-induced stereotypy was completely blocked by daily intra-VTA pretreatment with 7-nitroindazole. 6. In addition, attenuation of the locomotor effects of cocaine challenge was also observed in animals that received daily intra-VTA 7-nitroindazole (40 ng/hemisphere) infusions. 7. The results indicate that VTA nitric oxide is necessary for the development of sensitization of cocaine-induced stereotypic behavior, and that its repeated inhibition may produce lasting effects on the locomotor response to the drug.     

Involvement of dopamine D1 receptors of the central amygdala on the acquisition and expression of morphine-induced place preference in rat

In the present study, the effects of intra-central amygdala (CeA) injection of dopamine D1 receptor agonist and antagonist on morphine-induced conditioned place preference (CPP) were investigated in male Wistar rats. Our data showed that subcutaneous (s.c.) injection of morphine sulphate (0.5-10 mg/kg) significantly increased the time spent in the drug-paired compartment in a dose-dependent manner. Intra-CeA administration of the dopamine D1 receptor agonist, SKF 38393 (2 and 4 micro g/rat) with an ineffective dose of morphine (0.5 mg/kg), elicited a significant conditioned place preference. On the other hand, a single dose of SKF 38393 (2 micro g/rat, intra-CeA) in combination with the lower doses (0.5 and 2.5 mg/kg), but not with the higher doses of morphine potentiated morphine-induced CPP. Furthermore, intra-CeA administration of the dopamine D1 receptor antagonist, SCH 23390 (0.5-1 micro g/rat) decreased the acquisition of conditioned place preference induced by morphine (7.5 mg/kg). The response of SKF 38393 was decreased by SCH 23390 (0.75 micro g/rat). SKF 38393 or SCH 23390 by themselves did not elicit any effect on place conditioning. On the other hand, intra-CeA administration of SKF 38393 or SCH 23390 significantly decreased the expression of morphine (7.5 mg/kg)-induced place preference. SKF 38393 or SCH 23390 injections into the CeA had no effects on the locomotor activity on the test sessions. The results indicate that the dopamine D1 receptors in the CeA may be involved in the acquisition and expression of morphine-induced place preference.     

Sensitization does not develop to cocaine-induced potentiation of the antinociceptive effect of morphine

Repeated intermittent cocaine administration produces a progressive increase (sensitization) in the motor stimulatory action of cocaine. Previous studies have shown that cocaine produces antinociception and also enhances the antinociceptive effect of opioid analgesics. The present study was designed to investigate if sensitization to these effects of cocaine develops. In the first part of the study, we determined if acute cocaine administration (3, 10, 30 mg/kg, intraperitoneal [i.p.]) increases the antinociceptive effect of morphine (5 mg/kg, subcutaneous [s.c.]) in rats using the hot plate test. Cocaine (30 mg/kg, i.p.), alone, produced a small but significant antinociceptive effect at 15 min after drug administration. When administered 15 min prior to morphine, cocaine dose-dependently enhanced the effect of morphine (5 mg/kg, s.c.) at the time (45 min post-cocaine) when cocaine by itself did not significantly change the hot plate latency. In the second part of the study, we examined if sensitization develops to cocaine-induced antinociception and its ability to increase the antinociceptive effect of morphine. Na?ve rats were injected with either saline or cocaine (30 mg/kg) once daily for 3 days and tested on the hot plate apparatus either 24 h or 1 wk after the last cocaine injection. Some of the rats from each group were also tested for motor stimulation induced by cocaine (5 mg/kg, i.p.) 24 h after the hot plate test to confirm that sensitization had occurred to the motor stimulatory action of the drug. Additional rats were treated with saline or cocaine for 3 days, but neither treated with morphine nor tested on the hot plate apparatus, and tested for behavioral sensitization to the motor stimulatory action of cocaine (5 mg/kg, i.p.) 24 h or 1 wk later. Sensitization developed to the motor stimulatory effect of cocaine in both groups, regardless of morphine treatment on the prior day. Sensitization also developed to the antinociceptive effect of cocaine 24 h but not 1 wk after the last cocaine injection. No sensitization was observed in the ability of cocaine to enhance the antinociceptive effect of morphine. Overall, our data suggest that while cocaine enhanced the antinociceptive effect of morphine, sensitization did not develop to this action of cocaine.     

Selective psychostimulant sensitization by food restriction: differential changes in accumbens shell and core dopamine

We have recently reported that behavioural sensitization to morphine, amphetamine, cocaine and nicotine is associated with an increased response of dialysate dopamine to the same drugs in the nucleus accumbens core and/or a reduced response in the shell. Prolonged exposure to stressful stimuli also induces behavioural sensitization to drugs of abuse. We therefore investigated the effect of different drugs of abuse on behaviour and on dopamine transmission in the nucleus accumbens shell and core of rats stressed by 1 week schedule of food restriction. Food-restricted rats (80% of their initial body weight) were implanted with microdialysis probes in the nucleus accumbens shell and core and challenged with cocaine (5 and 10 mg/kg i.p.), amphetamine (0.25 and 0.5 mg/kg s.c.), morphine (1 and 2 mg/kg s.c.), nicotine (0.2 and 0.4 mg/kg s.c.) and the changes in dialysate dopamine transmission were monitored together with the behaviour. Food restricted rats showed strong behavioural sensitization to cocaine and amphetamine but not to morphine or nicotine as compared to ad libitum fed controls. Behavioural sensitization to psychostimulants was associated with an increased response of dialysate dopamine in the core and with an unchanged or even reduced response in the shell. No significant differences were observed between controls and food-restricted animals in the ability of morphine and nicotine to stimulate dopamine transmission in the shell and core. The present results indicate that a sensitized dopamine response in the nucleus accumbens core is a general feature of the expression of behavioural sensitization.     

Simultaneous activation of spinal antiopioid system (neuropeptide FF) and pain facilitatory circuitry by stimulation of opioid receptors in rats

Neuropeptide FF (NPFF) is a mammalian FMRFamide-like octapeptide with antiopioid properties that inhibits morphine-induced analgesia but also produces hyperalgesia. In the present study, a series of three experiments was carried out to investigate the interactions between opioid receptor stimulation and antiopioid systems. First, by using an in vitro superfusion system with rat spinal cord slices, we showed that morphine stimulated NPFF release in a dose-dependent manner. The stimulating effect which was observed with morphine concentrations as low as 100 fM reached a maximum at 0.1 nM, then decreased and was ineffective at 10 μM. The morphine-induced release of NPFF was abolished by naloxone (1 μM) but unaltered by tetrodotoxin. Second, by an in vivo approach, we showed that a single heroin administration (2.5 mg/kg, s.c.) elicited in 30 min a drastic drop (38%) in spinal NPFF content. In a third experiment, we evaluated the capacity of naloxone in revealing an antiopioid component associated with opioid receptor stimulation. The administration of naloxone (1 mg/kg, s.c.) 25 min following that of heroin (2.5 mg/kg, s.c.), not only abolished the heroin-induced increase of tail-flick latency, but also lowered it under the basal value by 30%. These results indicate that opioid receptor stimulation activates both pain inhibitory and pain facilitatory systems in which NPFF may play a significant role and that opiate-induced analgesia is always partly masked.     

The potential anti-addictive agent, 18-methoxycoronaridine, blocks the sensitized locomotor and dopamine responses produced by repeated morphine treatment

18-Methoxycoronaridine (18-MC), a novel synthetic iboga congener, attenuates the reinforcing efficacy of morphine, disrupts some signs of morphine withdrawal in physically dependent rats and attenuates the dopamine response in the nucleus accumbens to acute morphine. The present study further investigated the interactions between 18-MC and morphine by examining the effects of 18-MC (40 mg/kg, i.p., 19 h earlier) on the expression of dopamine sensitization in the nucleus accumbens in response to morphine (20 mg/kg, i.p.) and on the dose-effect curves for morphine-induced locomotion (0-30 mg/kg, i.p.) in rats treated either acutely or repeatedly (five, once daily, injections of 20 mg/kg, i.p.) with morphine. Compared to vehicle pretreated controls, 18-MC increased the potency of morphine, shifting the dose-response curve to the left, in acute morphine treated rats; however, 18-MC did not alter the potency of morphine in rats treated repeatedly with morphine. Repeated morphine administration induced locomotor sensitization in approximately 50% of the rats tested; in vehicle pretreated rats, the morphine dose-response curve was shifted to the left in sensitized as compared to non-sensitized rats. In 18-MC pretreated rats, sensitized and non-sensitized rats responded similarly to morphine, revealing a blockade of sensitization by 18-MC. Consistent with this behavioural finding, 18-MC pretreatment completely abolished the sensitized dopamine response in the nucleus accumbens expressed by rats repeatedly treated with morphine. It is suggested that the potential anti-addictive efficacy of 18-MC might be related to an ability to restore normal functioning to a hypersensitive mesolimbic dopamine system produced by previous repeated morphine administration.     

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.     

Ethanol enhances naloxone sensitization and disrupts morphine discrimination--comparison to dizocilpine and pentobarbital: explanation of enhancing acute and attenuating chronic effects

1. Ethanol affects ligand-gated ion channels as a positive modulator of gamma-aminobutyric acid (GABA(A)) receptor function and an N-methyl-D-aspartate (NMDA) antagonist. NMDA antagonists attenuate chronic drug effects. Accordingly, we found that ethanol decreased morphine dependence and locomotor sensitization. We now test whether ethanol alters sensitization to the disrupting effects of naloxone on schedule-controlled responding after morphine administration or affects the acute stimulus effects of morphine. 2. Groups of rats, trained to lever-press for food, were co-administered ethanol (1 g/kg; i.p.), the NMDA antagonist dizocilpine (DZ; 0.05 mg/kg; i.p.), the GABA(A) agonist pentobarbital (PB; 3 mg/kg i.p.), or vehicle with morphine (5 mg/kg s.c.). Separate groups received naloxone (0.1-1 mg/kg s.c.) 4-hrs later, prior to food sessions (FR15; 30 min) on three consecutive days. Ethanol enhanced the suppressive effects of higher naloxone doses on all three days. DZ and PB altered this behavior differentially by day and naloxone dose. 3. Next, we examined the effects of ethanol, DZ, PB, and naloxone (0.3 mg/kg; s.c.) on morphine discrimination. Rats, trained to discriminate morphine (3.2 mg/kg s.c.) from saline in a two-lever, food-reinforced procedure, were tested with morphine (0, 1-5.6 mg/kg) after vehicle and drug administrations. Naloxone blocked dose-related responding to morphine, demonstrating pharmacological specificity, and altered response rates. Both ethanol and DZ, but not PB, disrupted morphine-appropriate responding. 4. The paradox that ethanol and DZ attenuate chronic morphine effects while enhancing acute effects may reflect a temporal pattern of primary mu opiate receptor function followed by secondary NMDA-mediated processes induced by morphine administration.