Effects of reversible inactivation of the ventral tegmental area on the acquisition and expression of morphine-induced conditioned place preference in the rat

The reinstatement of the expression phase of morphine-induced conditioned place preference in male Wistar rats under ventral tegmental area stimulation and brief inactivation

Background and purpose

Previous research has found that the electrical stimulation of the ventral tegmental area (VTA) is involved in drug-dependent behaviors and plays a role in reward-seeking. However, the mechanisms remain unknown, especially the effect of electrical stimulation on this area. Therefore, this study aimed to investigate how the electrical stimulation and the temporary inactivation of VTA affect the morphine- dependent behavior in male rats.

Experimental approach

The adult Wistar male rats were anesthetized with ketamine and xylazine. The stimulation electrode (unilaterally) and the microinjection cannula (bilaterally) were implanted into the VTA, stereotaxically. Then, the rats underwent three-day of repeated conditioning with subcutaneous morphine (0.5 or 5 mg/kg) injections, in the conditioned place preference apparatus, followed by four-day forced abstinence, which altered their conditioning response to a morphine (0.5 mg/kg) priming dose on the ninth day. On that day, rats were given high- or low-intensity electrical stimulation or reversible inactivation with lidocaine (0.5 pL/site) in the VTA.

Findings/Results

Results showed that the electrical stimulation of the VTA with the high intensity (150 μA/rat), had a minimal effect on the expression of morphine-induced place conditioning in rats treated with a high dose (5 mg/kg) of morphine. However, the reversible inactivation of the VTA with lidocaine greatly increased place preference in rats treated with a low dose (0.5 mg/kg) of morphine. Additionally, the reinstatement of 0.5 mg/kg morphine-treated rats was observed after lidocaine infusion into the VTA.

Conclusion and implications

These results suggest that VTA electrical stimulation suppresses neuronal activation, but the priming dose causes reinstatement. The VTA may be a potential target for deep brain stimulation-based treatment of intractable disorders induced by substance abuse.

Effect of risperidone on morphine-induced conditioned place preference and dopamine receptor D2 gene expression in male rat hippocampus

BACKGROUND

Previous studies suggest the possible effect of risperidone on brain reward system and D1 and D2 dopamine receptors' involvement in morphine-induced conditioned place preference (CPP).

AIMS

The present study was designed to investigate the effect of risperidone as an atypical antipsychotic drug on morphine-induced CPP and D2-like dopamine receptor gene expression in rat.

MATERIALS AND METHODS

An unbiased CPP paradigm was used to study the effect of risperidone. Intraperitoneal (i.p.) injection of risperidone (1, 2, and 4 mg/kg) was performed 30 min before the morphine (10 mg/kg, i.p.) injection and just after the rat was placed in the CPP box. The open field test was used to assay the locomotor activity of animal. The gene expression of D2 dopamine receptor in hippocampus was measured by real-time PCR technique. The hippocampi of rats were also used for histology evaluation.

RESULTS

Morphine-produced (10 mg/kg) CPP and morphine-induced CPP were reversed only by the administration of a low dose of risperidone (1 mg/kg). Low dose of risperidone (1 mg/kg) showed no effect on locomotor activity but a higher dose of risperidone (2 and 4 mg/kg) decreased locomotor activity. Real-time PCR data analysis revealed that the gene expression of D2 dopamine receptor had significant difference between morphine and a 1 mg/kg dose of risperidone. Moreover, in histological evaluation, apoptosis was observed in the morphine group, whereas there was no evidence of apoptosis in the risperidone-treated groups.

CONCLUSION

Our results suggest that risperidone (1 mg/kg) reverses the morphine-induced CPP and may reduce the rewarding properties of morphine. It is also demonstrated that risperidone decreases the expression of D2 receptor in rat hippocampus. Therefore, risperidone can be considered potential adjunct therapy in morphine dependence.

Anterior cingulate cortex and its projections to the ventral tegmental area regulate opioid withdrawal, the formation of opioid context associations and context-induced drug seeking

Clinical evidence suggests that there are correlations between activity within the anterior cingulate cortex (ACC) following re-exposure to drug-associated contexts and drug craving. However, there are limited data contributing to our understanding of ACC function at the cellular level during re-exposure to drug-context associations as well as whether the ACC is directly related to context-induced drug seeking. Here, we addressed this issue by employing our novel behavioral procedure capable of measuring the formation of drug-context associations as well as context-induced drug-seeking behavior in male mice (8-12 weeks of age) that orally self-administered oxycodone. We found that mice escalated oxycodone intake during the long-access training sessions and that conditioning with oxycodone was sufficient to evoke conditioned place preference (CPP) and drug-seeking behaviors. Additionally, we found that thick-tufted, but not thin-tufted pyramidal neurons (PyNs) in the ACC as well as ventral tegmental area (VTA)-projecting ACC neurons had increased intrinsic membrane excitability in mice that self-administered oxycodone compared to controls. Moreover, we found that global inhibition of the ACC or inhibition of VTA-projecting ACC neurons was sufficient to significantly reduce oxycodone-induced CPP, drug seeking, and spontaneous opioid withdrawal. These results demonstrate a direct role of ACC activity in mediating context-induced opioid seeking among other behaviors, including withdrawal, that are associated with the DSM-V criteria of opioid use disorder.

Quetiapine attenuates the acquisition of morphine-induced conditioned place preference and reduces ERK phosphorylation in the hippocampus and cerebral cortex

Background: Quetiapine is an atypical antipsychotic that antagonizes dopamine and serotonin receptors. It has been suggested that quetiapine can be used to treat substance use disorders, including opioid use disorder. Opioids modulate dopaminergic functions associated with conditioned reinforcement and these effects can be measured via the conditioned place preference (CPP) paradigm. Opioids' unconditioned effects are regulated by several proteins, including extracellular signal-regulated kinase (ERK) and cAMP-responsive element-binding (CREB).Objective: To assess the effect of quetiapine on morphine-induced CPP and motor activity levels, and on the levels of ERK and CREB proteins in the hippocampus and cerebral cortex.Methods: 42 male rats were exposed to a CPP protocol, in which they underwent a conditioning paradigm with saline, quetiapine (40 mg/kg), morphine (10 mg/kg), morphine plus quetiapine (10, 20, or 40 mg/kg), or morphine plus memantine (7.5 mg/kg, a positive control drug) (n = 6 per group). The rats were tested for CPP and exploratory activity. Levels of ERK and CREB proteins in the hippocampus and cerebral cortex were also measured.Results: Quetiapine co-administered with morphine inhibited morphine-induced CPP [F (6, 70) = 11.67, p < .001] and morphine's effects on motor activity (p < .001). Morphine enhanced ERK phosphorylation in the hippocampus (p < .001) and cerebral cortex (p < .001), an effect inhibited by quetiapine.Conclusion: Quetiapine attenuates morphine-induced CPP and locomotion and these effects are associated with a reduction of ERK phosphorylation in the hippocampus and cerebral cortex. These results suggest that quetiapine should be further explored as a potential treatment for opioid use disorder.

Mu-opioid receptors in septum mediate the development of behavioural sensitization to a single morphine exposure in male rats

Behavioural sensitization (BS) is characterized by enhanced psychomotor responses to a dose of substance of abuse after prior repeated exposure. We previously reported that BS can be induced by a single injection of morphine in rats, whereas septal nuclei are specifically involved in the development phase of BS. Here, we demonstrated that intra-LS or intra-MS microinjections also incubated BS to a systemic morphine injection in a cross-sensitization fashion, whereas inactivation of either subdivision of septal nuclei (LS: lateral septum; MS: medial septum) can negate this ability of morphine. Then, non-selective (naloxone) and selective (μ-, δ- and κ-)opioid receptor antagonists were directly delivered into LS or MS, respectively, ahead of a morphine microinjection, whereas only μ-opioid receptors in both LS and MS play indispensable roles in mediating the BS development. Finally, there was a pronounced elevation in the levels of the monoamines (i.e. dopamine, homovanillic acid, 5-hydroxytryptamine and 5-hydroxyindoleacetic acid) in the septum, 8 h after a morphine injection detected with a HPLC-ECD method, suggesting that dopaminergi and serotoninergic systems are implicated in the BS formation. Our studies demonstrated that septal nuclei critically participate in the BS development. Essentially, μ- instead of δ- or κ-opioid receptors in LS and MS mediate sensitization to opiates.

Stimulation and transient inactivation of ventral tegmental area modify reinstatement of acquisition phase of morphine-induced conditioned place preference in male rats

For processing the development of psychological dependency, opioid reinforcement, and opiate-related associative reward, learning, and memory in the brain, the ventral tegmental area (VTA) is considered the key zone. As the responsible region for the morphine role in conditioned place preference (CPP), this area has an important role. So, the present research was conducted to investigate the effects of different intensities of electrical stimulation on VTA utilizing CPP, with two morphine doses. Reversible inactivation of VTA was performed via bilateral microinjection of Lidocaine into this area with two implanted separate cannulas. Our findings indicated that 5 mg/kg morphine-induced CPP was suppressed by 150 μA VTA electrical stimulation. The results also showed that bilateral Intra-VTA administration of Lidocaine significantly decreased the 5 mg/kg morphine-induced CPP acquisition phase in comparison with their respective sham group, which reversed in the reinstatement test. It should be concluded that these findings are important for the detection of mesolimbic nervous system ties and could help to find new ways to attenuate the rewarding action of morphine.

Microinjection of a Dopamine-D1 Receptor Agonist into the Ventral Tegmental Area Reverses the Blocked Expression of Morphine Conditioned Place Preference by N-Methyl-D-Aspartate Receptor Antagonist

BACKGROUND

The release of dopamine (DA) in the posterior ventral tegmental area (pVTA) plays an important role in cue-related learning, reward, and relapse. On the other hand, studies have shown that the use of N-methyl-D-aspartate receptor (NMDAR) antagonist (AP5) inhibits the expression of morphine (5 mg/kg, s. c) conditioned place preference (CPP). In this study, we have tried to show the interaction effect of the DA stimulatory agents through D1-like receptor (D1R) agonist (SKF38393) and D2-like receptor (D2R) antagonist (eticlopride; through disinhibition) with NMDAR antagonist into the pVTA on the expression of morphine CPP.

MATERIALS AND METHODS

The SKF38393 and eticlopride, individually and simultaneously (in ineffective doses), were injected into the pVTA with the AP5 in rats, and animals were then placed in a CPP apparatus.

RESULTS

Concomitant administration of D1R agonist (4 μg/rat) with NMDAR antagonist (1 μg/rat) induced the expression of morphine CPP, but the administration of D2R antagonist with NMDAR antagonist was unaffected on the expression of morphine CPP. Furthermore, concomitant administration of ineffective doses of D1R agonist and D2R antagonist with NMDAR antagonist had no effect on the expression of morphine CPP.

CONCLUSIONS

The results showed using higher doses of D1R agonist with NMDAR antagonist could reverse the blocked expression of morphine CPP by NMDAR antagonists, while, the use of D2R antagonist with NMDAR antagonist could not. Therefore, presynaptic receptors such as D1R probably through releasing other stimulatory neurotransmitters can play a vital role in the expression of morphine CPP and cue-related learning.

Drug-Induced Conditioned Place Preference and Its Practical Use in Substance Use Disorder Research

The conditioned place preference (CPP) paradigm is a well-established model utilized to study the role of context associations in reward-related behaviors, including both natural rewards and drugs of abuse. In this review article, we discuss the basic history, various uses, and considerations that are tied to this technique. There are many potential takeaway implications of this model, including negative affective states, conditioned drug effects, memory, and motivation, which are all considered here. We also discuss the neurobiology of CPP including relevant brain regions, molecular signaling cascades, and neuromodulatory systems. We further examine some of our prior findings and how they integrate CPP with self-administration paradigms. Overall, by describing the fundamentals of CPP, findings from the past few decades, and implications of using CPP as a research paradigm, we have endeavored to support the case that the CPP method is specifically advantageous for studying the role of a form of Pavlovian learning that associates drug use with the surrounding environment.

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 role of nucleus accumbens shell on acquisition and retrieval stages of morphine state dependent learning

AIM

In the present study, the effect of transient inactivation of the shell subregion of the nucleus accumbens (NAC shell) by lidocaine on the acquisition and retrieval stages of passive avoidance learning (PAL) and memory and morphine state-dependent learning (SDL) in male wistar rats was investigated.

METHODOLOGY

Adult male wistar rats weighing (220-250 g) were used. Lidocaine hydrochloride was bilaterally injected into the shell area of the nucleus accumbens 5 min before of subcutaneous morphine administration.

RESULTS

pre-training and pre-test infusion of lidocaine into the NAC shell significantly impaired PAL and memory. Furthermore, Pre-training administration of morphine (5 mg/kg, s.c.) in a step-through passive avoidance task induced state-dependent learning with impaired memory retrieval on the test day. The impairment of memory was restored after pre-test administration of the same dose of morphine. This phenomenon has been named as morphine state dependent learning (SDL). Moreover, Pre-training and pre-test inactivation of the NAC shell impaired morphine SDL.

CONCLUSIONS

The results suggest the role of NAC shell as a common structure in the PAL and morphine SDL. It is suggested that NAC shell as a common area plays a critical role in the acquisition and retrieval stages of PAL and also morphine SDL.

Opioid receptor signaling, analgesic and side effects induced by a computationally designed pH-dependent agonist

Novel pain killers without adverse effects are urgently needed. Opioids induce central and intestinal side effects such as respiratory depression, sedation, addiction, and constipation. We have recently shown that a newly designed agonist with a reduced acid dissociation constant (pK_a) abolished pain by selectively activating peripheral μ-opioid receptors (MOR) in inflamed (acidic) tissues without eliciting side effects. Here, we extended this concept in that pK_a reduction to 7.22 was achieved by placing a fluorine atom at the ethylidene bridge in the parental molecule fentanyl. The new compound (FF3) showed pH-sensitive MOR affinity, [³⁵S]-GTPγS binding, and G protein dissociation by fluorescence resonance energy transfer. It produced injury-restricted analgesia in rat models of inflammatory, postoperative, abdominal, and neuropathic pain. At high dosages, FF3 induced sedation, motor disturbance, reward, constipation, and respiratory depression. These results support our hypothesis that a ligand’s pK_a should be close to the pH of injured tissue to obtain analgesia without side effects.

The interactive effects of ketamine and ethanol on dopamine expression in the ventral tegmental area of rats

BACKGROUND

A number of studies have demonstrated the significant and rapid antidepressant effects of ketamine, which is also known as a neurotoxic and illicit drug. Ketamine and alcohol are increasingly used together in clubs by teenagers and young adults. Previous studies have proven that chronic ketamine consumption induces a delayed and persistent activation of the dopamine (DA) system. However, the rewarding properties of recreational ketamine abuse remain unclear, and the underlying mechanisms of the effects on the DA system after administration of ketamine with ethanol are yet to be explored.

METHODS

Here, we evaluated the effects of two different doses of ketamine (30 mg/kg and 60 mg/kg) with and without ethanol (0.3156 g/kg) on DA concentration in the rat's ventral tegmental area (VTA), a vital region in the reward and motivation system. We explored the effects of the combined drug treatment on the expression profiling of the DA metabolism genes, tyrosine hydroxylase, dopa decarboxylase, vesicular monoamine transporter 2, and synaptosomal-associated protein 25, as well as protein expression level of brain-derived neurotrophic factor in the rat's VTA.

RESULTS

We found that administration of ketamine with ethanol led to a significant increase of DA in the VTA associated with differential regulation of mRNA levels of the four DA metabolism genes and protein levels of brain-derived neurotrophic factor. Moreover, the rewarding properties of coadministration of ketamine and ethanol were related to dopaminergic neuron activation in the VTA.

CONCLUSION

These results indicated the possibility that combined drug treatment might positively affect the mesencephalic DA reward system.

A nontoxic pain killer designed by modeling of pathological receptor conformations

Indiscriminate activation of opioid receptors provides pain relief but also severe central and intestinal side effects. We hypothesized that exploiting pathological (rather than physiological) conformation dynamics of opioid receptor-ligand interactions might yield ligands without adverse actions. By computer simulations at low pH, a hallmark of injured tissue, we designed an agonist that, because of its low acid dissociation constant, selectively activates peripheral μ-opioid receptors at the source of pain generation. Unlike the conventional opioid fentanyl, this agonist showed pH-sensitive binding, heterotrimeric guanine nucleotide-binding protein (G protein) subunit dissociation by fluorescence resonance energy transfer, and adenosine 3',5'-monophosphate inhibition in vitro. It produced injury-restricted analgesia in rats with different types of inflammatory pain without exhibiting respiratory depression, sedation, constipation, or addiction potential.

Effects of Injection of Gamma-Aminobutyric Acid Agonists into the Nucleus Accumbens on Naloxone-Induced Morphine Withdrawal

Aims: This study was to investigate the effects of local administration of gamma-aminobutyric acid (GABA) agonists into the nucleus accumbens (NAc) on naloxone-induced morphine withdrawal symptoms. Methods: Bilateral guide cannulas were stereotaxically implanted in the shell or core regions of the NAc of Sprague-Dawley rats. After a recovery period, 3 morphine pellets, each consisting of 75 mg morphine base, were placed subcutaneously on the first and third days of the study with the rats under mild ether anaesthesia. The GABA agonists, baclofen hydrochloride or muscimol hydrobromide, were injected into the NAc, and morphine withdrawal was induced by naloxone on the fifth day. Results: Administration of baclofen to the shell or core regions of the NAc of Sprague-Dawley rats led to statistically significant decreases in both behavioural and locomotor activity parameters during the morphine withdrawal period, compared to the control group. However, there were no statistically significant changes in locomotor activity or withdrawal behavioural parameters, with the exception of wet dog shakes, between control and muscimol-treated groups. Conclusion: These findings show that GABAergic conduction in the NAc is effective on the morphine withdrawal symptoms, and that both the shell and core regions of the NAc are associated with this effect.

Transient inactivation of the nucleus accumbens (NAc) shell prominently ameliorates responses to acute stress in female rats

It is clear that the stress response differs between females and males. Moreover, the nucleus accumbens shell is considered a critical area in the brain's response to stress. In the present study we investigate the effect of the transient inactivation of the NAc shell on the metabolic and hormonal disturbance induced by acute stress in female rats. Female Wistar rats were canulated uni- or bilaterally in the NAc shell before stress induction. Electric foot shock was applied to the animals and lidocaine hydrochloride was administered intra-acumbally 5min before each stress session. Food and water intake, delay to eating time, plasma corticosterone, estradiol and progesterone were measured after stress administration. Our results indicate that food intake increased whereas water intake decreased in stress group, furthermore, delay in the onset of eating also was observed. In addition, plasma corticosterone and progesterone concentrations were increased whereas estradiol was decreased. Transient inactivation of right side of NAc shell inhibited the stress effect on food intake; however, the effect of stress was exacerbated when the left part of the NAc shell was inhibited. Transient inactivation of both NAc shell sides augmented the stress effect on water intake. The same results also obtained for the effect of stress on delay to eating time. Lidocaine administration inhibited the effects of stress on all of the hormones we tested. It can be concluded that the NAc shell plays an important role in the mediation of stress responses in female rats and this influence shows asymmetrical manner.

Involvement of ventral tegmental area ionotropic glutamate receptors in the expression of ethanol-induced conditioned place preference

The ventral tegmental area (VTA) is a well-established neural substrate of reward-related processes. Activity within this structure is increased by the primary and conditioned rewarding effects of abused drugs and its engagement is heavily reliant on excitatory input from structures upstream. In the case of drug seeking, it is thought that exposure to drug-associated cues engages glutamatergic VTA afferents that signal directly to dopamine cells, thereby triggering this behavior. It is unclear, however, whether glutamate input to VTA is directly involved in ethanol-associated cue seeking. Here, the role of intra-VTA ionotropic glutamate receptor (iGluR) signaling in ethanol-cue seeking was evaluated in DBA/2J mice using an ethanol conditioned place preference (CPP) procedure. Intra-VTA iGluRs α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPAR)/kainate and N-methyl-d-aspartate (NMDAR) were blocked during ethanol CPP expression by co-infusion of antagonist drugs 6,7-dinitroquinoxaline-2,3-dione (DNQX; AMPA/kainate) and d-(-)-2-Amino-5-phosphonopentanoic acid (AP5; NMDA). Compared to aCSF, bilateral infusion of low (1 DNQX+100 AP5ng/side) and high (5 DNQX+500 AP5ng/side) doses of the AMPAR and NMDAR antagonist cocktail into VTA blocked ethanol CPP expression. This effect was site specific, as DNQX/AP5 infusion proximal to VTA did not significantly impact CPP expression. An increase in activity was found at the high but not low dose of DNQX/AP5. These findings demonstrate that activation of iGluRs within the VTA is necessary for ethanol-associated cue seeking, as measured by CPP.

5-HT2A Serotonin Receptor Density in Adult Male Rats' Hippocampus after Morphine-based Conditioned Place Preference

INTRODUCTION

A close interaction exists between the brain opioid and serotonin (5-HT) neurotransmitter systems. Brain neurotransmitter 5-HT plays an important role in the regulation of reward-related processing. However, a few studies have investigated the potential role of 5-HT2A receptors in this behavior. Therefore, the aim of the present study was to assess the influence of morphine and Conditioned Place Preference (CPP) on the density of 5-HT2A receptor in neurons of rat hippocampal formation.

METHODS

Morphine (10 mg/kg, IP) was injected in male Wistar rats for 7 consecutive days (intervention group), but control rats received just normal saline (1 mL/kg, IP). We used a hotplate test of analgesia to assess induction of tolerance to analgesic effects of morphine on days 1 and 8 of injections. Later, two groups of rats were sacrificed one day after 7 days of injections, their whole brains removed, and the striatum and PFC immediately dissected. Then, the NR1 gene expression was examined with a semi-quantitative RT-PCR method.

RESULTS

Our data showed that the maximum response was obtained with 2.5 mg/kg of morphine. The density of 5-HT2A receptor in different areas of the hippocampus increased significantly at sham-morphine and CPP groups (P<0.05). On the other hand, the CPP groups had more 5-HT2A receptors than sham-morphine groups and also the sham-morphine groups had more 5-HT2A receptors than the control groups.

CONCLUSION

We concluded that the phenomenon of conditioned place preference induced by morphine can cause a significant increase in the number of serotonin 5-HT2A receptors in neurons of all areas of hippocampus.

Oxytocin enhances the expression of morphine-induced conditioned place preference in rats

Drug addiction is characterized by drug-seeking and drug-taking and has devastating consequences on addicts as well as on society. Environmental contexts previously associated with drug use can elicit continued drug use and facilitate relapse. Accumulating evidence suggests that the neuropeptide oxytocin might be a potential treatment for behavioral disorders, including drug addiction. Here, we investigated the effects of central oxytocin administration on the acquisition and expression of morphine-induced conditioned place preference (CPP), a model for measuring the rewarding effects of drugs of abuse, in male Wistar rats. Intracerebroventricular (ICV) administration of oxytocin (0.2μg) or the specific oxytocin receptor antagonist (OTA), desGly-NH2, d(CH2)5[Tyr(Me)(2), Thr(4)] OVT, (0.75μg), on the conditioning days did not affect the acquisition of morphine-induced CPP. By contrast, ICV oxytocin, but not OTA, administration immediately prior to the post-conditioning session enhanced the expression of morphine-induced CPP, possibly by activation of oxytocin receptors in the nucleus accumbens shell (NAcSh). The oxytocin enhancement of morphine-induced CPP was not associated with any changes in the locomotor activity of morphine-conditioned rats. Together, these data suggest that central administration of exogenous oxytocin enhances the expression of morphine-induced CPP, at least in part, via activation of oxytocin receptors within the NAcSh.

Direct bidirectional μ-opioid control of midbrain dopamine neurons

The ventral tegmental area (VTA) is required for the rewarding and motivational actions of opioids and activation of dopamine neurons has been implicated in these effects. The canonical model posits that opioid activation of VTA dopamine neurons is indirect, through inhibition of GABAergic inputs. However, VTA dopamine neurons also express postsynaptic μ-opioid peptide (MOP) receptors. We report here that in Sprague Dawley rat, the MOP receptor-selective agonist DAMGO (0.5-3 μM) depolarized or increased the firing rate of 87 of 451 VTA neurons (including 22 of 110 dopamine neurons). This DAMGO excitation occurs in the presence of GABAA receptor blockade and its EC50 value is two orders of magnitude lower than for presynaptic inhibition of GABA release on to VTA neurons. Consistent with a postsynaptic channel opening, excitations were accompanied by a decrease in input resistance. Excitations were blocked by CdCl2 (100 μM, n = 5) and ω-agatoxin-IVA (100 nM, n = 3), nonselective and Cav2.1 Ca(2+) channel blockers, respectively. DAMGO also produced a postsynaptic inhibition in 233 of 451 VTA neurons, including 45 of 110 dopamine neurons. The mean reversal potential of the inhibitory current was -78 ± 7 mV and inhibitions were blocked by the K(+) channel blocker BaCl2 (100 μM, n = 7). Blockade of either excitation or inhibition unmasked the opposite effect, suggesting that MOP receptors activate concurrent postsynaptic excitatory and inhibitory processes in most VTA neurons. These results provide a novel direct mechanism for MOP receptor control of VTA dopamine neurons.

Nucleus incertus inactivation impairs spatial learning and memory in rats

Nucleus incertus (NI) is a pontine nucleus which releases mainly GABA and relaxin-3 in rats. Its suggested functions include response to stress, arousal, and modulation of hippocampal theta rhythm. Since the role of NI in learning and memory has not been well characterized, therefore the involvement of this nucleus in spatial learning and memory and the aftermath hippocampal levels of c-fos and pCREB were evaluated. NI was targeted by implanting cannula in male rats. For reference memory, NI was inactivated by lidocaine (0.4 μl, 4%) at three stages of acquisition, consolidation and retrieval in Morris water maze paradigm. For working memory, NI was inactivated in acquisition and retrieval phases. Injection of lidocaine prior to the first training session of reference memory significantly increased the distance moved, suggesting that inactivation of NI delays acquisition in this spatial task. Inactivation also interfered with the retrieval phase of spatial reference memory, as the time in target quadrant for lidocaine group was less, and the escape latency was higher compared to the control group. However, no difference was observed in the consolidation phase. In the working memory task, with inter-trial intervals of 75 min, the escape latency was higher when NI was inactivated in the retrieval phase. In addition, c-fos and pCREB/CREB levels decreased in NI-inhibited rats. This study suggests that nucleus incertus might participate in acquisition of spatial reference, and retrieval of both spatial reference and working memory. Further studies should investigate possible roles of NI in the hippocampal plasticity.

The antero-posterior heterogeneity of the ventral tegmental area

The ventral tegmental area (VTA) is a brain region processing salient sensory and emotional information, controlling motivated behaviors, natural or drug-related reward, reward-related learning, mood, and participating in their associated psychopathologies. Mostly studied for its dopamine neurons, the VTA also includes functionally important GABA and glutamate cell populations. Behavioral evidence supports the presence of functional differences between the anterior VTA (aVTA) and the posterior VTA (pVTA), which is the topic of this review. This antero-posterior heterogeneity concerns locomotor activity, conditioned place preference and intracranial self-administration, and can be seen in response to ethanol, acetaldehyde, salsolinol, opioids including morphine, cholinergic agonists including nicotine, cocaine, cannabinoids and after local manipulation of GABA and serotonin receptors. It has also been observed after viral-mediated manipulation of GluR1, phospholipase Cγ (PLCγ) and cAMP response element binding protein (CREB) expression, with impact on reward and aversion-related responses, on anxiety and depression-related behaviors and on pain sensitivity. In this review, the substrates potentially underlying these aVTA/pVTA differences are discussed, including the VTA sub-nuclei and the heterogeneity in connectivity, cell types and molecular characteristics. We also review the role of the tail of the VTA (tVTA), or rostromedial tegmental nucleus (RMTg), which may also participate to the observed antero-posterior heterogeneity of the VTA. This region, partly located within the pVTA, is an inhibitory control center for dopamine activity. It controls VTA and substantia nigra dopamine cells, thus exerting a major influence on basal ganglia functions. This review highlights the need for a more comprehensive analysis of VTA heterogeneity.

NMDA receptors in the midbrain play a critical role in dopamine-mediated hippocampal synaptic potentiation caused by morphine

A single exposure to drugs of abuse produces an NMDAR (N-methyl-D-aspartate receptor)-dependent synaptic potentiation at excitatory synapses of dopamine (DA) neurons in the ventral tegmental area (VTA) of the midbrain. All addictive drugs can increase DA concentrations in projection areas of the midbrain, including the hippocampus. Hippocampal DA release subsequently modulates hippocampal plasticity and drug-associated memories. Using in vivo electrophysiological recording techniques in anesthetized rats, we show that systemic injection of morphine induced hippocampal synaptic potentiation in a dose-dependent manner. Intra-VTA but not intra-hippocampus injection of morphine evoked this potentiation. Local hippocampal dopamine D1 receptors (D1R) are required in the morphine-induced synaptic potentiation and conditioned place preference (CPP). Moreover, both NMDAR activation in the VTA and VTA/hippocampus dopaminergic connections are essential for the morphine-evoked potentiation and CPP. These findings suggest that NMDAR signalings in the midbrain play a key role in regulating dopamine-mediated hippocampal synaptic plasticity underlying drug-induced associative memory.

Role of dorsal hippocampal orexin-1 receptors in associating morphine reward with contextual stimuli

In this study, we evaluated the role of orexin receptors in the dorsal hippocampus (dHPC) in the development of morphine-induced conditioned place preference (CPP) and modification of hippocampal c-Fos and cyclic AMP response element-binding protein (CREB) levels. Orexin-A (0.5, 5, and 50 pmol) and the orexin-1 receptor antagonist, SB334867 (10, 20, and 40 nmol), were bilaterally infused into the dHPC immediately before conditioning with morphine (0.5 or 7.5 mg/kg) using the CPP task. Western blotting was then used to measure the protein levels of c-Fos, total CREB, and phosphorylated CREB (pCREB) in the hippocampus. Orexin did not enhance the rewarding efficacy of morphine (0.5 mg/kg), but caused a reduction in hippocampal c-Fos. Successful conditioning with morphine (7.5 mg/kg) was associated with increased levels of hippocampal c-Fos and CREB, but with decreased CREB phosphorylation. Intrahippocampal administration of SB334867 before conditioning sessions disrupted the rewarding effect of morphine (7.5 mg/kg) and blocked morphine-induced increases in hippocampal CREB protein levels. The results suggest that orexin signaling within the dHPC is necessary for the development of morphine CPP. Morphine reward is related to altered levels of hippocampal c-Fos and CREB. Inhibition of morphine-induced increases in CREB levels might be the underlying mechanism for the disruption of morphine CPP.

Brain stress system response after morphine-conditioned place preference

This study examined the involvement of the brain stress system in the reinforcing effects of morphine. One group of mice was conditioned to morphine using the conditioned place preference (CPP) paradigm and the other group received morphine in a home-cage (non-conditioned). Adrenocorticotropic hormone and corticosterone levels were measured by radioimmunoassay; phospho (p) CREB expression and the number of corticotropin-releasing factor (CRF) neurons and fibres were measured by immunohistochemistry in different brain areas. We observed that the number of CRF neurons in the paraventricular nucleus (PVN) was increased after morphine-induced CPP, which was paralleled with enhanced CRF-immunoreactivity fibres in the nucleus tractus solitarius (NTS) and ventral tegmental area (VTA) vs. home-cage group injected with morphine. Morphine exposure induced an increase in CREB phosphorylated at Ser133 in the PVN and central amygdale (CeA), whereas mice exhibiting morphine CPP had higher levels of pCREB in the PVN, CeA and bed nucleus of the stria terminalis (BNST). We also found that most of the CRF-positive neurons in the PVN, CeA and BNST co-express pCREB after morphine CPP expression, suggesting that the drug-associated environmental contexts can elicit neuronal activity in the brain stress system. From the present results it is clear that exposure to a drug-associated context remains a potent activator of signalling pathways leading to CRF activation in the brain stress system.

Withania somnifera prevents acquisition and expression of morphine-elicited conditioned place preference

Previous studies have reported that some of the central effects of morphine are counteracted by the administration of the methanolic extract of the root of Indian ginseng, Withania somnifera Dunal (WSE). The present study sought to determine whether WSE affects acquisition and expression of morphine-elicited conditioned place preference (CPP) in CD-1 mice. In CPP acquisition experiments, WSE (0, 25, 50, and 100 mg/kg) was administered, during conditioning, 30 min before morphine (10 mg/kg), whereas in expression experiments, WSE (0, 25, 50, and 100 mg/kg) was administered 30 min before the postconditioning test. The results demonstrate (i) that WSE was devoid of motivational properties; (ii) that WSE (100 mg/kg) was devoid of effects on spontaneous and morphine-stimulated motor activity and on spatial memory; and (iii) that WSE (50 and 100 mg/kg) significantly prevented the acquisition and expression of CPP. Further, to characterize the receptor(s) involved in these effects, we studied, by receptor-binding assay, the affinity of WSE for µ-opioid and γ-aminobutyric acid B receptors. These experiments revealed a higher affinity of WSE for γ-aminobutyric acid B than for µ-opioid receptors. Overall, these results point to WSE as an interesting alternative tool, worthy of further investigation, to study opiate addiction.

Evaluation of reward from pain relief

The human experience of pain is multidimensional and comprises sensory, affective, and cognitive dimensions. Preclinical assessment of pain has been largely focused on the sensory features that contribute to nociception. The affective (aversive) qualities of pain are clinically significant but have received relatively less mechanistic investigation in preclinical models. Recently, operant behaviors such as conditioned place preference, avoidance, escape from noxious stimulus, and analgesic drug self-administration have been used in rodents to evaluate affective aspects of pain. An important advance of such operant behaviors is that these approaches may allow the detection and mechanistic investigation of spontaneous neuropathic or ongoing inflammatory/nociceptive (i.e., nonevoked) pain that is otherwise difficult to assess in nonverbal animals. Operant measures may allow the identification of mechanisms that contribute differentially to reflexive hypersensitivity or to pain affect and may inform the decision to progress novel mechanisms to clinical trials for pain therapy. Additionally, operant behaviors may allow investigation of the poorly understood mechanisms and neural circuits underlying motivational aspects of pain and the reward of pain relief.

Involvement of the ventral tegmental area in a rodent model of post-traumatic stress disorder

Post-traumatic stress disorder (PTSD) is an anxiety disorder of considerable prevalence in individuals who have experienced a traumatic event. Studies of the neural substrate of this disorder have focused on the role of areas such as the hippocampus, the amygdala and the medial prefrontal cortex. We show that the ventral tegmental area (VTA), which directly modulates these areas, is part of this circuitry. Using a rat model of PTSD, we show that a brief but intense foot shock followed by three brief reminders can cause long-term behavioral changes as shown by anxiety-like, nociception, and touch-sensitivity tests. We show that an intraperitoneal injection of a dopamine (DA) antagonist or a bilateral inactivation of the VTA administered immediately before the traumatic event decrease the occurrence or intensity of these behavioral changes. Furthermore, we show that there is a significant decrease of baseline VTA dopaminergic but not GABAergic cell firing rates 2 weeks after trauma. Our data suggest that VTA DA neurons undergo long-term physiological changes after trauma and that this brain area is a crucial part of the circuits involved in PTSD symptomatology.

Pain relief produces negative reinforcement through activation of mesolimbic reward-valuation circuitry

Relief of pain is rewarding. Using a model of experimental postsurgical pain we show that blockade of afferent input from the injury with local anesthetic elicits conditioned place preference, activates ventral tegmental dopaminergic cells, and increases dopamine release in the nucleus accumbens. Importantly, place preference is associated with increased activity in midbrain dopaminergic neurons and blocked by dopamine antagonists injected into the nucleus accumbens. The data directly support the hypothesis that relief of pain produces negative reinforcement through activation of the mesolimbic reward-valuation circuitry.

Orexin A in the ventral tegmental area induces conditioned place preference in a dose-dependent manner: involvement of D1/D2 receptors in the nucleus accumbens

It has been shown that orexin A in the ventral tegmental area (VTA) is necessary for development of morphine place preference. Additionally, D1 and D2 dopamine receptors in the nucleus accumbens (NAc) have critical roles in motivation and reward. However, little is known about the function of orexin in conditioned place preference (CPP) in rats and involvement of D1/D2 receptors in the NAc. In the present study, we investigated the effect of direct administration of orexin A into the VTA, and examined the role of intra-accumbal dopamine receptors in development (acquisition) of reward-related behaviors in the rats. Adult male Wistar rats were unilaterally implanted by two separate cannulae into the VTA and NAc. The CPP paradigm was used, and, conditioning score and locomotor activity were recorded by Ethovision software. The results showed that unilateral intra-VTA administration of orexin A (27, 53 and 107 ng/0.3 μl saline) during conditioning phase induced CPP in a dose-dependent manner. The most effective dose of intra-VTA orexin-A in eliciting CPP was 107 ng. However, intra-NAc administration of SCH 23390 (0.25, 1 and 4 μg/0.5 μl saline), a D1 receptor antagonist, and sulpiride (0.25, 1 and 4 μg/0.5 μl DMSO), a D2 receptor antagonist, inhibited the development of orexin-induced CPP. The inhibitory effect of D2 but not D1 receptor antagonist was exerted in a dose-dependent manner. It is supposed that the activation of VTA dopaminergic neuron by orexin impresses the D2 receptors more than D1 receptors in the NAc.

Transient inactivation of the nucleus accumbens reduces both the expression and acquisition of morphine-induced conditioned place preference in rats

In the present study, the effects of transient inhibition of the shell and core parts of the nucleus accumbens by lidocaine on the expression and acquisition of morphine-induced conditioned place preference in male Wistar rats were investigated. In addition, the number of bouts of sniffing, rearing, and compartment crossing was scored. Lidocaine hydrochloride was injected into different parts of the nucleus accumbens 5 min before each morphine session for the transient inhibition of particular anatomical regions. Subcutaneous (s.c.) injection of morphine (2.5 and 5mg/kg) induced place preference. Transient inhibition of the left and/or right side of the shell part of nucleus accumbens reduced morphine place conditioning. However, when both sides of the nucleus were inhibited, inhibition was weaker when compared to the results when only one side was inhibited. Also, the number of compartment crossings in these animals reduced significantly. Nevertheless, the number of rearing occurrences was reduced only when both sides of the shell part of the nucleus accumbens were inhibited. In contrast, the number of sniffing bouts increased in all three groups. The results for the core part of the nucleus accumbens also indicated that place preference was inhibited after transient inhibition of the left, right, and both sides. However, although the number of total compartment crossings was reduced in all experimental groups, the reduction was not statistically significant. The data obtained was similar to the number of rearings, yet the number of sniffing bouts increased in the experimental groups compared to the control. In conclusion, these results confirmed the involvement of the left and right sides and core and shell parts of the nucleus accumbens in morphine place conditioning.

The role of nitric oxide in the effects of cumin (Cuminum Cyminum L.) fruit essential oil on the acquisition of morphine-induced conditioned place preference in adult male mice

OBJECTIVE: Nitric oxide is a neural messenger molecule in the central nervous system that is generated from L-arginine via the nitric oxide synthase (NOS) and is involved in many important oplold-induced effects. In Iranian ancient medicine, Cuminum cyminum L (green seed) has been used for the treatment of some diseases. In the present study, the effect of intraperitoneal (ip) administration of different doses of cumin fruit essential oil (FEO) on the acquisition of morphine-induced conditioned place preference (GPP) in L-arginine-treated mice was investigated. METHODS: A total of 213 adult male albino Wistar mice were used in these experiments. The CPP paradigm was carried out in 5 continuous days, pre-conditioning, conditioning and post-conditioning. Animals were randomly assigned to one of the two groups for place conditioning. CPP was induced by subcutaneous (sc) injection of morphine (5 mg/kg) in 3 days conditioning schedule. On the test day, conditioning scores and locomotor activity were recorded by Ethovision software. RESULTS: Sole administration of different doses of cumin FEO (0.01%, 0.1%, 0.5%, 1% and 2%; lp) or L-arginine (50, 100 and 200 mg/kg; lp) during the CPP protocol could not induce CPP. Nonetheless, morphine-induced CPP was decreased by different doses of cumin FEO (0.01%-2%), whereas it was increased by L-arginine (50-200 mg/kg) when they were injected before morphine (5 rug/kg) during a 3-day conditioning phase (acquisition period). Additionally, cumin FEO could interestingly attenuate the raising effect of L-arginine on morphine-induced CPP in a dose-dependent manner. CONCLUSIONS: It is suggested that some components of the Cuminum cyminum L. seed attenuate the excessive effect of L-arginine on morphine-induced CPP through the NOS inhibitory mechanism. It seems that cumin FEO possibly acts as a NOS inhibitor.

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.