Naloxone-precipitated withdrawal enhances ERK phosphorylation in prefrontal association cortex and accumbens nucleus of morphine-dependent mice

The Inflammation/NF-κB and BDNF/TrkB/CREB Pathways in the Cerebellum Are Implicated in the Changes in Spatial Working Memory After Both Morphine Dependence and Withdrawal in Rat

We aimed to explore the impact of the cerebellum on the decline in spatial working memory following morphine dependence and withdrawal. Two groups of male Wistar rats received intraperitoneal injections of either saline (1 ml/kg) or morphine (10 mg/kg) twice daily for 10 days, serving as the control and dependent groups. Additionally, a withdrawal group underwent a 30-day withdrawal period after the dependence phase. Spatial working memory was assessed using a Y maze test. ELISA and western blot were used to assess protein levels in the cerebellum. On day 1, morphine impaired spatial working memory, deteriorated further after 10 days of morphine use, and nearly returned to its initial level following a 30-day withdrawal period. On day 10, significant increases in TNF-α, IL-1β, and CXCL12 and a notable decrease in IL-10 levels were detected in the morphine-dependent group, which did not completely restore in the withdrawal group. The protein levels of CXCR4, TLR4, P2X7R, and NF-κB sharply increased in the morphine-dependent group. However, these levels almost returned to normal after withdrawal. In the morphine-dependent group, BDNF decreased, while TrkB and CREB1 increased noticeably. Nevertheless, after withdrawal, TrkB and CREB1 but not BDNF levels returned to normal. In the morphine-dependent group, both CACNA1 and KCNMA1 decreased significantly and after withdrawal, only KCNMA1 showed partial restoration, while CACNA1 did not. It can be concluded that inflammation/NF-κB and BDNF/TrkB/CREB pathways play key roles in neural adaptation within the cerebellum, contributing to the decline in spatial working memory after both morphine dependence and withdrawal.

Nalbuphine Potentiates Reversal of Fentanyl Overdose by Naloxone

Developing an effective antidote for fentanyl-induced overdose to achieve timely reversal is an unmet public health need. Previously, we found that naloxone derivative NX90 with mild κ-opioid agonistic properties was three-fold more effective than the parent naloxone in reversing a fentanyl overdose in rats. To investigate whether κ-agonistic properties could indeed augment the robustness of overdose reversal, we evaluated a κ-agonist/µ-antagonist nalbuphine (NB) as well as its combinations with naloxone (NX) in a fentanyl overdose model in rodents. An administration of either NB or NX as single agents at 0.1 mg/kg doses produced a full recovery in 90 ± 9.9 min and 11.4 ± 2.7 min, respectively. A higher dose of NX at 0.2 mg/kg reversed an overdose within 4.8 ± 1.0 min. In contrast to that, the coadministration of NB and NX at 0.1 mg/kg each produced a synergistic effect, with overdose reversal in 3.4 ± 0.2 min. The coadministration of NX and NB at sub-therapeutic doses of 0.05 mg/kg each was also 1.2-fold more effective than NX at 0.2 mg/kg. We further found that co-administration of NB at different doses (0.025, 0.05, 0.1 mg/kg) and ratios (1:4 and 1:1) with NX had differential effects on overdose reversal, cardiorespiratory liabilities, and analgesia.

Minocycline alleviates the symptoms of morphine withdrawal via the CaMKII-Ras-ERK signaling pathway

OBJECTIVE

To investigate the effect of minocycline on morphine withdrawal symptoms.

METHODS

We established a rat model of morphine dependence, then injected the animals with naloxone to induce withdrawal symptoms. Minocycline was injected into the midbrain periaqueductal gray and its effect on withdrawal symptoms and Ca²⁺-dependent protein kinase (CaMKII), Ras, and phospho-extracellular signal-regulated kinase (p-ERK) expression was observed.

RESULTS

Minocycline inhibited withdrawal symptoms such as "wet dog" shakes, teeth chatter, and ptosis, perhaps by inhibiting the activation of microglia and the expression of CaMKII, Ras, and p-ERK. Minocycline had no effect on the behavior of control rats or on CaMKII, Ras, or p-ERK expression.

CONCLUSION

Minocycline alleviates morphine withdrawal symptoms by inhibiting the activation of microglia and downregulating the expression of CaMKII, Ras, and p-ERK.

Phosphorylation of GluN2B subunits of N-methyl-d-aspartate receptors in the frontal association cortex involved in morphine-induced conditioned place preference in mice

Morphine is one of the most abused drugs in the world, which has resulted in serious social problems. The frontal association cortex (FrA) has been shown to play a key role in memory formation and drug addiction. N-Methyl-d-aspartate receptors (NMDARs) are abundant in the prefrontal cortex (PFc) and much evidence indicates that GluN2B-containing NMDARs are involved in morphine-induced conditioned place preference (CPP). However, the function of GluN2B in the FrA during morphine-induced CPP has yet to be fully investigated. In the present work, a CPP animal model was employed to measure the expression of phosphorylated (p-) GluN2B (Serine; Ser 1303) in the FrA and NAc in different phases of morphine-induced CPP. We found that p-GluN2B (Ser 1303) was increased in the FrA during the development and reinstatement phases but unchanged in the extinction phase. The use of ifenprodil, a GluN2B-specific antagonist, to block the activity of GluN2B in the two phases attenuated morphine-induced CPP and reinstatement. Furthermore, ifenprodil also blocked morphine-induced upregulation of p-GluN2B (Ser 1303) in the FrA in both phases. These results indicate that GluN2B-containing NMDARs in the FrA may be involved in the regulation of morphine-induced CPP and reinstatement.

Very low dose naltrexone in opioid detoxification: a double-blind, randomized clinical trial of efficacy and safety

Withdrawal syndrome is one of the initial focuses of opioid detoxification. Very low dose naltrexone (VLNTX) has been found to reduce opioid tolerance and dependence in animal and human clinical studies. The aim of this study was to determine the safety and efficacy of VLNTX during early stages of detoxification. In a multi-arm parallel, double-blind, randomized controlled trial, 63 opioid-dependent male participants referring to Imam Reza Rehabilitation Center were allocated to three equal groups using block randomization method. They received 0.125 mg, 0.250 mg of VLNTX or placebo daily for 10 days, together with the routine clonidine-based protocol. Self-reported and observer ratings of withdrawal severity and adverse events were measured on the 1st, 4th and 10th day of treatment. Runny eyes (p = 0.006), anxiety (p = 0.031) and dehydration (p = 0.014) were reduced during the whole 10 days in the 0.125 mg VLNTX-treated group compared to placebo. Only drowsiness (p = 0.043) and dysphoric mood (p < 0.001) were reduced in the 0.250 mg VLNTX-treated group. Results of 1st, 4th, and 10th-day assessment showed that most symptoms reductions were for the 0.125 mg VLNTX and the placebo group in the 1st and 4th days, respectively. On the 10th day, there was not any significant difference between 0.250 mg VLNTX-treated group and placebo group. No adverse effect was observed. In the starting days of detoxification, VLNTX can reduce the withdrawal symptoms, but the efficacy declined by passing time. Further studies are needed to test the utility of this new therapeutic approach.

Prevention of morphine dependence and tolerance by Nepeta menthoides was accompanied by attenuation of Nitric oxide overproduction in male mice

ETHNOPHARMACOLOGICAL RELEVANCE

Repeated administration of morphine for chronic pain leads to dependence and tolerance that limits clinical usage. Nepeta menthoides is commonly known as Iranian Ustukhuddoos and are administered in traditional medicine for gastrodynia, bone pain, blood depurative and restlessness.

AIMS OF STUDY

To investigate the effects of Nepeta menthoides on expression and acquisition of morphine dependence and tolerance in mice with regard to oxidative stress.

MATERIALS AND METHODS

Morphine dependence in mice was developed by administration of gradually increasing doses of morphine twice daily for 7 consecutive days. In experimental groups, administration of Nepeta menthoides (200 and 400mg/kg), methadone and their combination were performed 60min prior to each morphine injection (for acquisition) or the last injection of morphine on test day (for expression). Morphine tolerance was measured by the tail-immersion test before and after the administration of a single dose of morphine (100mg/kg; i.p.) on the test day (8th day). Morphine dependence was also evaluated by counting the number of jumps after the injection of naloxone (5mg/kg; i.p.).

RESULTS

Nepeta menthoides, similar to methadone, significantly prevented the development (but not the expression) of morphine dependence, tolerance, and potentiated morphine antinociception and also reduced (23.23±1.15) Nitric oxide (NO) overproduction (35.23±3.36) (in compared with naloxone group (6.3±0.52)). However, single and repeated application of the extract could not change high single-dose morphine analgesia.

CONCLUSION

It appears that Nepeta menthoides and methadone prevented morphine dependence and tolerance, partly through inhibition of the NO overproduction.

Pleiotrophin modulates morphine withdrawal but has no effects on morphine-conditioned place preference

Pleiotrophin (PTN) is a neurotrophic factor with important functions in addiction and neurodegenerative disorders. Morphine administration induces an increase in the expression of PTN and Midkine (MK), the only other member of this family of cytokines, in brain areas related with the addictive effects of drug of abuse, like the Ventral Tegmental Area or the hippocampus. In spite of previous studies showing that PTN modulates amphetamine and ethanol rewarding effects, and that PTN is involved in morphine-induced analgesia, it was still unknown if the rewarding effects of morphine may be regulated by endogenous PTN. Thus, we aim to study the role of PTN in the reward and physical dependence induced by morphine. We used the Conditioned Place Preference (CPP) paradigm in PTN genetically deficient (PTN-/-) and wild type (WT) mice to assess the rewarding effects of morphine in absence of endogenous PTN. Second, to study if PTN may be involved in morphine physical dependence, naloxone-precipitated withdrawal syndrome was induced in PTN-/- and WT morphine dependent mice. Although the increase in the time spent in the morphine-paired compartment after conditioning tended to be more pronounced in PTN-/- mice, statistical significance was not achieved. The data suggest that PTN does not exert an important role in morphine reward. However, our results clearly indicate that PTN-/- mice develop a more severe withdrawal syndrome than WT mice, characterized as a significant increase in the time standing and in the total incidences of forepaw licking, forepaw tremors, wet dog shake and writhing. The data presented here suggest that PTN is a novel genetic factor that plays a role in morphine withdrawal syndrome.

Neuronal extracellular signal-regulated kinase (ERK) activity as marker and mediator of alcohol and opioid dependence

Early pioneering work in the field of biochemistry identified phosphorylation as a crucial post-translational modification of proteins with the ability to both indicate and arbitrate complex physiological processes. More recent investigations have functionally linked phosphorylation of extracellular signal-regulated kinase (ERK) to a variety of neurophysiological mechanisms ranging from acute neurotransmitter action to long-term gene expression. ERK phosphorylation serves as an intracellular bridging mechanism that facilitates neuronal communication and plasticity. Drugs of abuse, including alcohol and opioids, act as artificial yet powerful rewards that impinge upon natural reinforcement processes critical for survival. The graded progression from initial exposure to addiction (or substance dependence) is believed to result from drug- and drug context-induced adaptations in neuronal signaling processes across brain reward and stress circuits following excessive drug use. In this regard, commonly abused drugs as well as drug-associated experiences are capable of modifying the phosphorylation of ERK within central reinforcement systems. In addition, chronic drug and alcohol exposure may drive ERK-regulated epigenetic and structural alterations that underlie a long-term propensity for escalating drug use. Under the influence of such a neurobiological vulnerability, encountering drug-associated cues and contexts can produce subsequent alterations in ERK signaling that drive relapse to drug and alcohol seeking. Current studies are determining precisely which molecular and regional ERK phosphorylation-associated events contribute to the addiction process, as well as which neuroadaptations need to be targeted in order to return dependent individuals to a healthy state.

Hydrogen sulfide attenuates opioid dependence by suppression of adenylate cyclase/cAMP pathway

AIMS

The best-established mechanism of opioid dependence is the up-regulation of adenylate cyclase (AC)/cAMP pathway, which was reported to be negatively regulated by hydrogen sulfide (H2S), a novel endogenous neuromodulator. The present study was, therefore, designed to determine whether H2S is able to attenuate the development of opioid dependence via down-regulating AC/cAMP pathway.

RESULTS

We demonstrated that application of sodium hydrosulphide (NaHS) and GYY4137, two donors of H2S, significantly alleviated naloxone-induced robust withdrawal jumping (the most sensitive and reliable index of opioid physical dependence) in morphine-treated mice. Repeated treatment with NaHS inhibited the up-regulated protein expression of AC in the striatum of morphine-dependent mice. Furthermore, NaHS also attenuated morphine/naloxone-elevated mRNA levels of AC isoform 1 and 8, production of cAMP, and phosphorylation of cAMP response element-binding protein (CREB) in mice striatum. These effects were mimicked by the application of exogenous H2S or over-expression of cystathione-β-synthase, an H2S -producing enzyme, in SH-SY5Y neuronal cells on treatment with [D-Ala(2),N-Me-Phe(4),Gly(5)-ol]-Enkephalin, a selective μ-opioid receptor agonist. Blockade of extracellular-regulated protein kinase 1/2 (ERK1/2) with its specific inhibitor attenuated naloxone-induced CREB phosphorylation. Pretreatment with NaHS or stimulation of endogenous H2S production also significantly suppressed opioid withdrawal-induced ERK1/2 activation in mice striatum or SH-SY5Y cells.

INNOVATION

H2S treatment is important in prevention of the development of opioid dependence via suppression of cAMP pathway in both animal and cellular models.

CONCLUSION

Our data suggest a potential role of H2S in attenuating the development of opioid dependence, and the underlying mechanism is closely related to the inhibition of AC/cAMP pathway.

Opiate exposure and withdrawal induces a molecular memory switch in the basolateral amygdala between ERK1/2 and CaMKIIα-dependent signaling substrates

Opiate reward memories are powerful triggers for compulsive opiate-seeking behaviors. The basolateral amygdala (BLA) is an important structure for the processing of opiate-related associative memories and is functionally linked to the mesolimbic dopamine (DA) pathway. Transmission through intra-BLA DA D1-like and D2-like receptors independently modulates the formation of opiate reward memories as a function of opiate-exposure state. Thus, in the opiate-naive state, intra-BLA D1 transmission is required for opiate-related memory formation. Once opiate dependence and withdrawal has developed, a functional switch to a DA D2-mediated memory mechanism takes place. However, the downstream molecular signaling events that control this functional switch between intra-BLA DA D1 versus D2 receptor transmission are not currently understood. Using an unbiased place conditioning procedure in rats combined with molecular analyses, we report that opiate reward memory acquisition requires intra-BLA ERK1/2 signaling only in the previously opiate-naive state. However, following chronic opiate exposure and withdrawal, intra-BLA reward memory processing switches to a CaMKIIα-dependent memory substrate. Furthermore, the ability of intra-BLA DA D1 or D2 receptor transmission to modulate the motivational salience of opiates similarly operates through a D1-mediated ERK-dependent mechanism in the opiate-naive state, but switches to a D2-mediated CaMKIIα-dependent mechanism in the dependent/withdrawn state. Protein analysis of BLA tissue revealed a downregulation of ERK1/2 phosphorylation and a dramatic reduction in both total and phosphorylated CaMKIIα signaling, specifically in the opiate-dependent/withdrawn state, demonstrating functional control of ERK1/2-dependent versus CaMKIIα-dependent memory mechanisms within the BLA, controlled by opiate-exposure state.

Differential Regulation of MAPK Phosphorylation in the Dorsal Hippocampus in Response to Prolonged Morphine Withdrawal-Induced Depressive-Like Symptoms in Mice

Depression is one of the most frequent neuropsychiatric comorbidities associated with opiate addiction. Mitogen activated protein kinase (MAPK) and MAPK phosphatase (MKP) are involved in drug addiction and depression. However, the potential role of MAPK and MKP in depression caused by morphine withdrawal remains unclear. We utilized a mouse model of repeated morphine administration to examine the molecular mechanisms that contribute to prolonged withdrawal induced depressive-like behaviors. Depressive-like behaviors were significant at 1 week after withdrawal and worsened over time. Phospho-ERK (extracellular signal-regulated protein kinase) was decreased and MKP-1 was elevated in the hippocampus, and JNK (c-Jun N-terminal protein kinase), p38 (p38 protein kinase) and MKP-3 were unaffected. A pharmacological blockade of MKP-1 by intra-hippocampal sanguinarine (SA) infusion prevented the development of depressive-like behaviors and resulted in relatively normal levels of MKP-1 and phospho-ERK after withdrawal. Our findings support the association between hippocampal MAPK phosphorylation and prolonged morphine withdrawal-induced depression, and emphasize the MKP-1 as an negative regulator of the ERK phosphorylation that contributes to depression.

Phenylmethanesulfonyl fluoride, a serine protease inhibitor, suppresses naloxone-precipitated withdrawal jumping in morphine-dependent mice

We have previously shown that intracerebroventricular (i.c.v.) administration of cysteine protease inhibitors suppresses naloxone-precipitated withdrawal jumping in morphine-dependent mice, presumably through the inhibition of dynorphin degradation (see (Tan-No, K., Sato, T., Shimoda, M., Nakagawasai, O., Niijima, F., Kawamura, S., Furuta, S., Sato, T., Satoh, S., Silberring, J., Terenius, L., Tadano, T., 2010. Suppressive effects by cysteine protease inhibitors on naloxone-precipitated withdrawal jumping in morphine-dependent mice. Neuropeptides 44, 279-283)). In the present study, we examined the effect of phenylmethanesulfonyl fluoride (PMSF), a serine protease inhibitor, on naloxone-precipitated withdrawal jumping in morphine-dependent mice. The doses of morphine (mg/kg per injection) were subcutaneously given twice daily for 2 days [day 1 (30) and day 2 (60)]. On day 3, naloxone (8 mg/kg) was intraperitoneally administered 3h after the final injection of morphine (60 mg/kg), and the number of jumps was immediately recorded for 20 min. Naloxone-precipitated withdrawal jumping was significantly suppressed by i.c.v. administration of PMSF (4 nmol), given 5 min before each morphine treatment during the induction phase, with none given on the test day. The expression of tissue plasminogen activator (tPA), a serine protease that converts plasminogen to plasmin, in the prefrontal cortex was significantly increased in morphine-dependent and -withdrawal mice, as compared with saline-treated mice. Moreover, trans-4-(aminomethyl)-cyclohexanecarboxylic acid (300 pmol), an antiplasmin agent, and (Tyr(1))-thrombin receptor activating peptide 7 (0.45 and 2 nmol), an antagonist of protease activated receptor-1 (PAR-1), significantly suppressed naloxone-precipitated withdrawal jumping. The present results suggest that PMSF suppresses naloxone-precipitated withdrawal jumping in morphine-dependent mice, presumably through the inhibition of activities of tPA and plasmin belonging to the serine proteases family, which subsequently activates PAR-1.

Morphine withdrawal produces ERK-dependent and ERK-independent epigenetic marks in neurons of the nucleus accumbens and lateral septum

Epigenetic changes such as covalent modifications of histone proteins represent complex molecular signatures that provide a cellular memory of previously experienced stimuli without irreversible changes of the genetic code. In this study we show that new gene expression induced in vivo by morphine withdrawal occurs with concomitant epigenetic modifications in brain regions critically involved in drug-dependent behaviors. We found that naloxone-precipitated withdrawal, but not chronic morphine administration, caused a strong induction of phospho-histone H3 immunoreactivity in the nucleus accumbens (NAc) shell/core and in the lateral septum (LS), a change that was accompanied by augmented H3 acetylation (lys14) in neurons of the NAc shell. Morphine withdrawal induced the phosphorylation of the epigenetic factor methyl-CpG-binding protein 2 (MeCP2) in Ser421 both in the LS and the NAc shell. These epigenetic changes were accompanied by the activation of members of the ERK pathway as well as increased expression of the immediate early genes (IEG) c-fos and activity-regulated cytoskeleton-associated protein (Arc/Arg3.1). Using a pharmacological approach, we found that H3 phosphorylation and IEG expression were partially dependent on ERK activation, while MeCP2 phosphorylation was fully ERK-independent. These findings provide new important information on the role of the ERK pathway in the regulation of epigenetic marks and gene expression that may concur to regulate in vivo the cellular changes underlying the onset of the opioid withdrawal syndrome.

Non-Coding RNAs Regulating Morphine Function: With Emphasis on the In vivo and In vitro Functions of miR-190

Non-coding RNAs (ncRNAs), especially microRNAs, are reported to be involved in a variety of biological processes, including several processes related to drug addiction. It has been suggested that the biological functions of opioids, one typical type of addictive drugs, are regulated by ncRNAs. In the current review, we examine a variety of mechanisms through which ncRNAs could regulate μ-opioid receptor (OPRM1) activities and thereby contribute to the development of opioid addiction. Using miR-23b as an example, we present the possible ways in which ncRNA-mediated regulation of OPRM1 expression could impact opioid addiction. Using miR-190 as an example, we demonstrate the critical roles played by ncRNAs in the signal cascade from receptor to systemic responses, including the possible modulation of adult neurogenesis and in vivo contextual memory. After discussing the possible targets of ncRNAs involved in the development of opioid addiction, we summarize the mechanisms underlying the interaction between ncRNAs and opioid addiction and present suggestions for further study.

Endogenous opiates and behavior: 2010

This paper is the thirty-third consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2010 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration (Section 16); and immunological responses (Section 17).

Striatal signal transduction and drug addiction

Drug addiction is a severe neuropsychiatric disorder characterized by loss of control over motivated behavior. The need for effective treatments mandates a greater understanding of the causes and identification of new therapeutic targets for drug development. Drugs of abuse subjugate normal reward-related behavior to uncontrollable drug-seeking and -taking. Contributions of brain reward circuitry are being mapped with increasing precision. The role of synaptic plasticity in addiction and underlying molecular mechanisms contributing to the formation of the addicted state are being delineated. Thus we may now consider the role of striatal signal transduction in addiction from a more integrative neurobiological perspective. Drugs of abuse alter dopaminergic and glutamatergic neurotransmission in medium spiny neurons of the striatum. Dopamine receptors important for reward serve as principle targets of drugs abuse, which interact with glutamate receptor signaling critical for reward learning. Complex networks of intracellular signal transduction mechanisms underlying these receptors are strongly stimulated by addictive drugs. Through these mechanisms, repeated drug exposure alters functional and structural neuroplasticity, resulting in transition to the addicted biological state and behavioral outcomes that typify addiction. Ca²⁺ and cAMP represent key second messengers that initiate signaling cascades, which regulate synaptic strength and neuronal excitability. Protein phosphorylation and dephosphorylation are fundamental mechanisms underlying synaptic plasticity that are dysregulated by drugs of abuse. Increased understanding of the regulatory mechanisms by which protein kinases and phosphatases exert their effects during normal reward learning and the addiction process may lead to novel targets and pharmacotherapeutics with increased efficacy in promoting abstinence and decreased side effects, such as interference with natural reward, for drug addiction.

Changes in phosphorylation of CREB, ERK, and c-fos induction in rat ventral tegmental area, hippocampus and prefrontal cortex after conditioned place preference induced by chemical stimulation of lateral hypothalamus

Experimental evidence indicates that chemical stimulation of lateral hypothalamus (LH) by carbachol can produce conditioned place preference (CPP) in rats. Several lines of evidence have shown that cAMP-response element binding protein (CREB), extracellular signal-regulated kinase (ERK), and c-fos have pivotal role in CPP induced by drugs of abuse, such as morphine, cocaine, nicotine, and alcohol. Therefore, in the present study, we investigated the changes in phosphorylated-CREB (p-CREB) and -ERK (p-ERK), and c-fos induction within ventral tegmental area (VTA), hippocampus and prefrontal cortex (PFC) after the acquisition of CPP induced by intra-LH administration of carbachol. Animals were unilaterally implanted by cannula into LH. For chemical stimulation of LH, carbachol (250 nmol/0.5 μl saline) was microinjected once each day, during 3-day conditioning phase (acquisition period) of CPP paradigm. After the acquisition period, the brains were removed, and p-CREB and p-ERK, and c-fos induction in the ipsilateral VTA, hippocampus and PFC were measured by Western blot analysis. The results indicated a significant increase in level of phosphorylated CREB (P<0.01) in VTA, and PFC (P<0.05), during LH stimulation-induced CPP, while its level decreased in hippocampus (P<0.05). Also, in aforementioned regions, an increase in c-fos level was observed, but this enhancement in PFC was not significant. Moreover, p-ERK changed in these areas, but not significantly. Our findings suggest that studying the intracellular signals and their changes, such as phosphorylated-CREB, can elucidate a functional relationship between LH and other brain structures involved in reward processing in rats.