Acquisition Of Morphine-induced Place Preference Research Articles

Impact of P-glycoprotein at the blood-brain barrier on the uptake of heroin and its main metabolites: behavioral effects and consequences on the transcriptional responses and reinforcing properties

Transport across the BBB is a determinant of the rate and extent of drug distribution in the brain. Heroin exerts its effects through its principal metabolites 6-monoacetyl-morphine (6-MAM) and morphine. Morphine is a known substrate of P-glycoprotein (P-gp) at the blood-brain-barrier (BBB) however, little is known about the interaction of heroin and 6-MAM with P-gp. The objective of this paper is to study the role of the P-gp-mediated efflux at the BBB in the behavioral and molecular effects of heroin and morphine. The transport rates of heroin and its main metabolites, at the BBB, were measured in mice by in situ brain perfusion. We then examined the effect of inhibition of P-gp on the acute nociception, locomotor activity, and gene expression modulations induced by heroin and morphine. The effect of P-gp inhibition during the acquisition of morphine-induced place preference was also studied. Inhibition of P-gp significantly increased the uptake of morphine but not that of heroin nor 6-MAM. Inhibition of P-gp significantly increased morphine-induced acute analgesia and locomotor activity but did not affect the behavioral effects of heroin; in addition, acute transcriptional responses to morphine were selectively modulated in the nucleus accumbens. Increasing morphine uptake by the brain significantly increased its reinforcing properties in the place preference paradigm. The present study demonstrated that acute inhibition of P-gp not only modulates morphine-induced behavioral effects but also its transcriptional effects and reinforcing properties. This suggests that, in the case of morphine, transport across the BBB is critical for the development of dependence.

The NO/sGC/PKG signaling pathway in the NAc shell is necessary for the acquisition of morphine-induced place preference.

There is evidence that the nitric oxide (NO)/soluble guanylyl cyclase (sGC)/cGMP-dependent protein kinase (PKG) signaling pathway in the basal lateral amygdala and hippocampus plays a key role in memory processing, but it is not known if this NO signaling pathway in the nucleus accumbens (Gomes et al., 2006), a known pivotal region in reward memory, is essential for drug-associated reward memory. We therefore investigated the effect of the NO/sGC/PKG signaling pathway in the nucleus accumbens (NAc) on morphine-induced conditioned place preference (CPP). Results showed that a preconditioning microinjection of the NO synthase (NOS) inhibitor Nω-nitro-L-arginine methyl ester (L-NAME) into the NAc shell, but not into the core, significantly blocked the acquisition of morphine CPP. The blockage effect of L-NAME on the acquisition of CPP was imitated by the neuronal NOS inhibitor 7-nitroindazole, 3-bromo-, sodium salt (7-NI), the sGC inhibitor 1H-[1,2,4] oxadiazolo-[4,3-a]quinoxalin-1-one (ODQ), and the PKG inhibitor Rp-8Br-PET-cGMPS. The 7-NI- or ODQ-induced effect was reversed by premicroinjection of the sGC activator YC-1 or the PKG activator 8-Br-cGMP in the NAc shell. However, microinfusion of 7-NI, ODQ, or Rp-8Br-PET-cGMPS into the NAc shell or the core had no effect on the expression of morphine CPP. These findings indicate that the NO/sGC/PKG signaling pathway in the NAc shell is critical for the acquisition of morphine-induced place preference, whereas the same signaling pathway in the NAc shell or core is not involved in the retrieval of morphine-induced place preference.

Activation of phosphatidylinositol 3-kinase/Akt-mammalian target of Rapamycin signaling pathway in the hippocampus is essential for the acquisition of morphine-induced place preference in rats

Hippocampus is a critical structure for the acquisition of morphine-induced conditioned place preference (CPP), which is a usual learning paradigm for assessing drug reward. However, the precise mechanisms remain largely unknown. Phosphatidylinositol 3-kinase (PI3K) and its downstream targets, including Akt, mammalian target of Rapamycin (mTOR) and 70-kDa ribosomal S6 kinase (p70S6K), are critical molecules implicated in learning and memory. Here, we tested the role of PI3K/Akt-mTOR-p70S6K signaling pathway in morphine-induced CPP in the hippocampus. Our results showed that the acquisition of morphine CPP increased phosphorylation of Akt in the hippocampal CA3, but not in the nucleus accumbens (NAc), the ventral tegmental area (VTA) or the CA1. Moreover, the phosphorylated Akt exclusively expressed in the CA3 neurons. Likewise, levels of phosphorylated mTOR and p70S6K were significantly enhanced in the CA3 following morphine CPP. The alterations of these phosphorylated proteins are positively correlated with the acquisition of morphine CPP. More importantly, microinjection of PI3K inhibitor (LY294002) or mTOR inhibitor (Rapamycin) into the CA3 prevented the acquisition of CPP and inhibited the activation of PI3K-Akt signaling pathway. In addition, pre-infusion of β-FNA (β-funaltrexamine hydrochloride), a selective irreversible μ opioid receptor antagonist, into CA3 significantly prevented the acquisition of CPP and impaired Akt phosphorylation. All these results strongly implied that the PI3K-Akt signaling pathway activated by μ opioid receptor in hippocampal CA3 plays an important role in acquisition of morphine-induced CPP.

The influence of magnesium on morphine-induced stimulation of the reward system

The present study was designed to assess the influence of magnesium (Mg) as MgCl(2) (10 or 40 mg/kg b.wt/day i.p.) and of its interaction with morphine on the reward system (RS) in Wistar rats. For this purpose, we evaluated conditioning place preference on a 12 day experiment schedule. Our data show that MgCl(2) (10 mg/kg b.wt/day) has moderate but significant effects on stimulating RS (increasing the time spent in associated conditioned compartment) (327.75 +/- 11 s in the Mg (10 mg/kg b.wt) group vs 295.2 +/- 8 s in the control (saline) group, p < 0.05) but not at higher Mg doses (40 mg/kg b.wt/day). We tested the influence of MgCl(2) (10 mg/kg b.wt./day i.p.) upon naloxone (2 mg/kg b.wt/ i.p.)-induced place aversion. Administrated alone, naloxone has an aversive effect on place preference. MgCl(2) (10 mg/kg b.wt/day i.p.) has a significantly decreased aversive effect of naloxone (280.7 +/- 37 s in naloxone + MgCl(2) (10 mg/kg b.wt) group vs 189 +/- 21 s in naloxone group, p < 0.05). MgCl(2) at both tested doses, added to morphine (3 mg/kg b.wt/day i.p), decreased the acquisition of morphine-induced place preference (262.2 +/- 17 s) in morphine + MgCl(2) (40 mg/kg b.wt) group vs 462.15 +/- 28 s in morphine group, p < 0.05). MgCl(2), 10 mg/kg b.wt/day i.p. decreased both morphine-induced place preference and naloxone-induced place aversion.

GABA B receptors within the ventral tegmental area are involved in the expression and acquisition of morphine-induced place preference in morphine-sensitized rats

The influence of intra-ventral tegmental area administration of gamma-amino-butyric-acid-B (GABA B) receptor agonist and antagonist on the expression and acquisition of morphine-induced conditioned place preference (CPP) in morphine-sensitized female rats was examined. Our pilot experiment showed that subcutaneous administration of morphine-(2.5, 5 and 7.5 mg/kg) induced CPP. Administration of one dose daily morphine (5 mg/kg) for 3 days followed by 5 days rest, enhanced the conditioning induced by ineffective doses of morphine (0.25, 0.5 and 1 mg/kg). Injections of GABA B receptor agonist, baclofen, (1.5 and 12 µg/rat) reduced the expression of morphine CPP whereas the dose of 6 µg/rat of the drug increased it. Baclofen also significantly reduced the acquisition of morphine CPP in morphine-sensitized animals. Administration of GABA B receptor antagonist, CGP 35348, significantly reduced the expression (12 µg/rat) and acquisition (1.5, 6 and 12 µg/rat) of morphine CPP in morphine-sensitized animals. In conclusion, results confirmed the importance of GABA B receptors within the ventral tegmental area of morphine CPP in morphine-sensitized female rats.

Involvement of glucose and ATP-sensitive potassium (K+) channels on morphine-induced conditioned place preference

In the present study, the effects of glucose and ATP-sensitive K+ channel compounds on the acquisition of morphine-induced place preference in male mice were investigated. Subcutaneous administration of different doses of morphine (2.5-7.5 mg/kg) produced a dose-dependent conditioned place preference. With a 3-day conditioning schedule, it was found that glucose (100, 200, 500 and 1000 mg/kg), diazoxide (15, 30 and 60 mg/kg) or glibenclamide (3, 6 and 12 mg/kg) did not produce significant place preference or place aversion. Intraperitoneal administration of the glucose (1000 mg/kg) or glibenclamide (6 and 12 mg/kg) with a lower dose of morphine (0.5 mg/kg) elicited the significant conditioned place preference. The response of glibenclamide (6 mg/kg) was reversed by diazoxide (15, 30 and 60 mg/kg). Drug injections had no effects on locomotor activity during the test sessions. It is concluded that glucose and the ATP-sensitive K+ channel may play an active role in morphine reward.

Involvement Of μ‐Opioid Receptors Of Ventral pallidum In Morphine Conditioned Place Preference

The effects of repeated bilateral intra-vp injections of different doses of morphine on the acquisition of morphine-induced cpp in rats were investigated using an unbiased 3-days schedule of place conditioning. Animals receiving once-daily subcutaneous (s.c.) injections of morphine (0.5,2.5,5,or 7.5mg/kg) or saline (1.0ml/kg,s.c.) showed a significant place preference in a dose-dependent manner. The maximum response was observed with 5 mg/kg morphine. Three days intra-vp injections of morphine (1,3 or 5 microg/rat) followed by 5 days free of the drug, significantly elicited morphine-induced place preference in combination with an ineffective dose of morphine (0.5mg/kg,s.c.). Moreover, pre-treatment with the different doses of naloxone (0.5,1, or 2mg/kg,i.p.) reversed the morphine (5microg/rat) response on the acquisition of morphine (0.5mg/kg) cpp. On the other hand, our results showed that three days intraperitoneal injections of different doses of naloxone (0.5,1 or 2mg/kg) followed by 5 days free of the drug significantly decreased the acquisition of morphine (0.5mg/kg) cpp. It is concluded that repeated injections of morphine and/or naloxone in the ventral pallidum, followed by 5 days free of the drugs, can affect the acquisition of morphine-induced place preference.

Role of the cholinergic system in the rat basolateral amygdala on morphine-induced conditioned place preference

The effects of intra-basolateral amygdala (intra-BLA) injections of physostigmine, atropine, nicotine and/or mecamylamine on morphine-induced conditioned place preference (CPP) in rats was investigated by using an unbiased 3-day schedule of place conditioning design. Animals that received 3 daily injections of morphine (0.5–10 mg/kg) subcutaneously (s.c.) or saline (1.0 ml/kg, s.c.) showed a significant preference for compartment paired with morphine. The maximum response was observed with 7.5 mg/kg of the opioid. Administration of the anticholinesterase drug, physostigmine (1, 3 and 5 μg/rat) with an ineffective dose of morphine (0.5 mg/kg) elicited a significant CPP. Injections of antimuscarinic receptor agent, atropine (1, 4 and 7 μg/rat) dose-dependently inhibited the morphine (7.5 mg/kg)-induced place preference. The injections of nicotine (0.75, 1 and 2 μg/rat) potentiated the morphine (0.5 mg/kg)-induced place preference, while the nicotinic receptor antagonist, mecamylamine (1, 3 and 6 μg/rat) dose-dependently inhibited the morphine (7.5 mg/kg)-induced place preference. Furthermore, administration of atropine (7 μg/rat) but not mecamylamine (6 μg/rat) reduced the response induced by different doses of physostigmine plus morphine. Moreover, mecamylamine (6 μg/rat) but not atropine (7 μg/rat) reduced the response induced by different doses of nicotine plus morphine. It is concluded that the muscarinic and nicotinic receptor mechanisms in the BLA may be involved in the acquisition of morphine-induced place preference.

Effect of cyclosporin A on morphine-induced place conditioning in mice: involvement of nitric oxide

Cyclosporin A is shown to attenuate antinociceptive effects of morphine, development and expression of morphine-induced tolerance and dependency via nitric oxide (NO) pathway. In the present study, the effect of systemic cyclosporin A on morphine-induced conditioned place preference (CPP) and the probable involvement of nitric oxide were assessed in mice. Our data showed that administration of morphine (1, 2.5, 5, 7.5, 10 mg/kg) significantly increased the time spent in the drug-paired compartment in a dose-dependent manner. The maximum response was obtained with 5 mg/kg of morphine. Cyclosporin A (5, 10 mg/kg) and N G -nitro- l-arginine methyl ester ( l-NAME; 2.5, 5, 10 mg/kg), a nonselective nitric oxide synthase (NOS) inhibitor, did not induce either conditioned place preference or conditioned place aversion (CPA), while cyclosporin A (20 mg/kg) induced CPA. Both cyclosporin A (10, 20 mg/kg) and l-NAME (5, 10 mg/kg), in combination with morphine (5 mg/kg) during conditioning, significantly suppressed acquisition of morphine-induced place preference. Lower and per se noneffective doses of Cyclosporin A (1, 2.5, 5 mg/kg) and l-NAME (2.5 mg/kg), when coadministered, exerted a significant potentiating effect on the attenuation of morphine-induced place preference. Aminoguanidine (50, 100 mg/kg), the specific inducible nitric oxide synthase (iNOS) inhibitor, whether alone or in combination with cyclosporin A failed to show this inhibitory effect on morphine-induced place preference. In conclusion, decreasing nitric oxide production through inhibiting constitutive nitric oxide synthase may be a mechanism through which cyclosporin A attenuates morphine-induced place preference.

D1 receptor blockade stereospecifically impairs the acquisition of drug-conditioned place preference and place aversion.

The motivational effects of dopamine (DA) D1 receptor blockade and its influence on the motivational effects of amphetamine (1.0mg/kg s.c.), morphine (1.0mg/kg s.c.) and lithium (40mg/kg s.c.) were studied in a place-conditioning paradigm. Drugs tested were two potent D1 receptor antagonists, SCH 23390 and SCH 39166, that differ in the poor affinity of the latter for 5-HT(2) receptors, and SCH 23388, the inactive enantiomer of SCH 23390. SCH 23390 and SCH 39166, at low doses (12.5 and 25µg/kg s.c.), paired for 30min with one compartment, elicited place aversion. Higher doses of the D1 antagonists or pairing for 60min with one compartment failed to elicit place aversion. SCH 39166 (50µg/kg s.c.) paired with both compartments completely prevented the place-aversion elicited by SCH 23390 (12.5µg/kg s.c.). SCH 23390 and SCH 39166 at low doses (12.5 and 25µg/kg s.c. respectively), paired with both compartments, abolished amphetamine-induced place preference. The D1 antagonists also impaired the acquisition of morphine-induced place preference and lithium-induced place aversion but only at higher doses (50 and 100µg/kg s.c.). These effects were stereospecific as the inactive enantiomer SCH 23388, up to a dose of 500µg/kg s.c. failed to impair the acquisition of amphetamine and morphine-induced place preference. It is concluded that DA plays a dual role in motivation: one role is that of assigning motivational valence to stimuli in relation to changes in DA transmission; another role of DA relates to the learning process involved in the acquisition of positive as well as negative incentive properties by otherwise neutral stimuli (incentive learning).

Isradipine is able to separate morphine-induced analgesia and place conditioning

The effect of isradipine, a dihydropyridine calcium antagonist, on morphine-induced place preference and analgesia in rats and mice was studied. Isradipine (0.6–5.0 mg/kg s.c.) inhibited an acquisition of morphine-induced place preference in rats and mice in a dose-related manner. Isradipine did not affect or strengthen morphine-induced analgesia as measured by tail-clip and hot-plate tests in mice and tail-clip and tail-flick tests in rats. The results suggest that analgesic and reinforcing effects of morphine might be pharmacologically separated by isradipine.