dCA1-NAc shell glutamatergic projection mediates context-induced memory recall of morphine

Opioid relapse is generally caused by the recurrence of context-induced memory reinstatement of reward. However, the internal mechanisms that facilitate and modify these processes remain unknown. One of the key regions of the reward is the nucleus accumbens (NAc) which receives glutamatergic projections from the dorsal hippocampus CA1 (dCA1). It is not yet known whether the dCA1 projection to the NAc shell regulates the context-induced memory recall of morphine. Here, we used a common model of addiction-related behavior conditioned place preference paradigm, combined with immunofluorescence, chemogenetics, optogenetics, and electrophysiology techniques to characterize the projection of the dCA1 to the NAc shell, in context-induced relapse memory to morphine. We found that glutamatergic neurons of the dCA1 and gamma aminobutyric acidergic (GABA) neurons of the NAc shell are the key brain areas and neurons involved in the context-induced reinstatement of morphine memory. The dCA1-NAc shell glutamatergic input pathway and the excitatory synaptic transmission of the dCA1-NAc shell were enhanced via the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) when mice were re-exposed to environmental cues previously associated with drug intake. Furthermore, chemogenetic and optogenetic inactivation of the dCA1-NAc shell pathway decreased the recurrence of long- and short-term morphine-paired context memory in mice. These results provided evidence that the dCA1-NAc shell glutamatergic projections mediated the context-induced memory recall of morphine.

Lateral habenula electrical stimulation with different intensities in combination with GABAB receptor antagonist reduces acquisition and expression phases of morphine-induced CPP

The lateral habenula (LHb) plays a principal role in response to aversive stimuli and negative emotional states. In this study, we have evaluated the effects of unilateral electrical stimulation (e-stim) of the LHb on morphine-conditioned place preference (CPP), before or after bilateral injections of Gamma-aminobutyric acid-B receptor (GABA_BR) antagonist, phaclofen, in male rats. Morphine (5 mg/kg; s.c.) induced a significant CPP, using a 5-day CPP paradigm. Intra-LHb microinjection of phaclofen or the LHb e-stim decreased only the acquisition of CPP. The 150 μA stimulation plus phaclofen significantly suppressed the expression phase but induced aversion in the acquisition of CPP, and an e-stim of 25 μA in combination with the antagonist, significantly prevented only the acquisition phase. The findings of this study confirm the possible role of GABA_BRs in the LHb on the acquisition and the expression of CPP. These results show that e-stim of LHb alone or plus phaclofen may change the GABA transmission, involving into CPP. Therefore, the GABAergic system, especially through GABA_BRs, may play a prominent role in the behavioral responses to morphine-induced CPP by LHb stimulation.

Early life maternal deprivation attenuates morphine induced inhibition in lateral paragigantocellularis neurons in adult rats

Early life stress can serve as one of the principle sources leading to individual differences in confronting challenges throughout the lifetime. Maternal deprivation (MD), a model of early life stress, can cause persistent alterations in brain function, and it may constitute a risk factor for later incidence of drug addiction. It is becoming more apparent that early life MD predisposes opiate abuse in adulthood. Although several behavioral and molecular studies have addressed this issue, changes in electrophysiological features of the neurons are yet to be understood. The lateral paragigantocellularis (LPGi) nucleus, which participates in the mediation of opiate dependence and withdrawal, may be susceptible to modifications following MD. This study sought to find whether early life MD can alter the discharge activity of LPGi neurons and their response to acute morphine administration in adult rats. Male Wistar rats experienced MD on postnatal days (PNDs) 1-14 for three h per day. Afterward, they were left undisturbed until PND 70, during which the extracellular activities of LPGi neurons were recorded in anesthetized animals at baseline and in response to acute morphine. In both MD and control groups, acute morphine administration induced heterogeneous (excitatory, inhibitory, and no effect) responses in LPGi neurons. At baseline recording, the interspike interval variability of the LPGi neurons was attenuated in both inhibitory and excitatory responses in animals with the history of MD. The extent of morphine-induced discharge inhibition was also lower in deprived animals compared to the control group. These findings suggest that early life MD induces long-term alterations in LPGi neuronal activity in response to acute administration of morphine. Therefore, the MD may alter the vulnerability to develop opiate abuse in adulthood.

Sex differences in the expression of morphine withdrawal symptoms and associated activity in the tail of the ventral tegmental area

Recent studies, in male rodents, have begun to elucidate a role for the GABAergic neurons in the tail of the ventral tegmental area (tVTA) in morphine withdrawal. To date, the mechanisms underlying morphine withdrawal have been studied almost exclusively in male animals. As a result, there is a considerable gap in our current understanding of the processes underlying sex differences in morphine withdrawal behaviors and its effects on cellular activity in the tVTA in females. The purpose of the present study was to investigate the influence of sex on the expression and duration of spontaneous somatic morphine withdrawal syndrome, and to characterize the relationship between spontaneous somatic withdrawal symptoms and cellular activation (measured as phosphorylated CREB; pCREB), in the GABAergic tVTA in male and female rats. Morphine-dependent adult male and female Long Evans rats underwent 72 h of spontaneous withdrawal, and somatic withdrawal symptoms were assessed every 12 h. Male morphine-dependent rats expressed more severe symptoms during the early phases of withdrawal compared to females. Although, females demonstrated lower overall symptom severity, their symptoms persisted for a longer period of time, thus demonstrating higher withdrawal-symptom severity than males during late withdrawal. pCREB activity in the tVTA was elevated in morphine-withdrawn rats and was positively correlated with the severity of withdrawal symptoms. These results demonstrate sex differences in the timing of the expression of somatic withdrawal. Our data add to the growing body of evidence demonstrating a role for the tVTA in morphine withdrawal and begin to establish a sex-dependent behavioral and molecular profile within this brain region.

Brain Reward Circuits in Morphine Addiction

Morphine is the most potent analgesic for chronic pain, but its clinical use has been limited by the opiate's innate tendency to produce tolerance, severe withdrawal symptoms and rewarding properties with a high risk of relapse. To understand the addictive properties of morphine, past studies have focused on relevant molecular and cellular changes in the brain, highlighting the functional roles of reward-related brain regions. Given the accumulated findings, a recent, emerging trend in morphine research is that of examining the dynamics of neuronal interactions in brain reward circuits under the influence of morphine action. In this review, we highlight recent findings on the roles of several reward circuits involved in morphine addiction based on pharmacological, molecular and physiological evidences.

STUDYING THE PHYSICAL DEPENDENCE ON AND TOLERANCE TO THE ANTINOCICEPTIVE EFFECT OF RU-1205 SUBSTANCE

We have studied the physical dependence on and tolerance to the analgesic activity of compound RU-1205. It is established that this compound does not cause side effects typical of morphine and butorphanol including the development of withdrawal syndrome upon naloxone provocation and tolerance to analgesic activity upon 14-day administration.

[EEG characteristics of medial prefrontal cortex in rats with morphine dependent place preference under shuttling condition]

OBJECTIVE

To study the correlation between EEG characteristics of medial prefrontal cortex (mPFC) and drug-seeking behavior of rats with morphine dependent place preference under shuttling condition.

METHODS

Forty rats were randomly divided into four groups (n = 10): morphine PL group, NS PL group, morphine IL group and NS IL group. After embeding the electrode in prelimbic (PL) or infralimbic (IL) cortex of each group by brain stereotaxic operation, the model of morphine dependent conditioned place preference (CPP) in rats was established. The differences of EEG wave percentage in mPFC were telemetered and analyzed when rats shuttled before and after the model.

RESULTS

After the model, the withdrawal symptoms were evident in morphine PL and IL group, and the activity time and distance in white box were increased obviously. Compared with control group, after the model, the EEG in morphine PL group showed that: when the rats shuttled to white box, 8 wave decreased obviously, P wave increased obviously. When the rats shuttled to black box, brain waves showed opposite changes. The EEG in morphine IL group showed that: when the rats shuttled to white box, a wave increased obviously, P and a wave decreased obviously. When the rats shuttled to black box, the brain wave had no significant differences compared with control group.

CONCLUSION

The EEG changes are different in PL and IL cortex of morphine CPP rats under shuttling condition, and the EEG changes are also different when rats shuttling to white or black box. There is possibly different mechanism, when different drug-seeking environmental cues caused EEG changes in different regions of mPFC.

[Changes of telemetry electrical activity in the infralimbic cortex of morphine-dependent rats with extinguished drug-seeking behavior]

OBJECTIVE

To investigate the changes of telemetry electrical activity in the infralimbic cortex (IL) of morphine-dependent rats with extinguished drug-seeking behavior.

METHODS

SD rats were randomly divided into model group and control group and received operations of brain stereotaxic electrode embedding in the IL. The rats in the model group were induced to acquire morphine dependence and then received subsequent extinction training, and the changes of electrical activity in the IL were recorded with a physical wireless telemetry system.

RESULTS

In rats with morphine dependence, the time staying in the white box was significantly longer on days 1 and 2 after withdrawal than that before morphine injection and that of the control rats, but was obviously reduced on days 1 and 2 after extinction training to the control level. Compared with the control group, the morphine-dependent rats on day 2 following withdrawal showed significantly increased β wave and decreased δ wave when they stayed in the white box but significantly increased δ wave and decreased α wave and β wave when they shuttled from the black to the white box. On day 2 of extinction, the model rats, when staying in the white box, showed significantly decreased θ wave compared with that of the control rats group but decreased β wave and θ wave and increased δ wave compared with those in the withdrawal period. When they shuttled from black to white box, the model rats showed decreased δ wave and increased α wave and β wave compared with those in the withdrawal period.

CONCLUSION

Morphine-dependent rats have abnormal changes of electrical activity in the IL in drug-seeking extinction to affect their drug-seeking motive and inhibit the expression and maintenance of drug-seeking behaviors.

Morphine and nicotine addiction and withdrawal influence baroreflex sensitivity and blood pressure in two-kidney one clip hypertensive (2K1C) rats

OBJECTIVE

This study aimed to investigate the effects of addiction to morphine and nicotine as well as their withdrawal on both baroreflex sensitivity and blood pressure in hypertensive rats.

METHODS

In this experimental study 40 male rats were divided into two main groups as follows: in group I, hypertensive rats received saline for 8 weeks; in group II, hypertensive rats were treated with morphine and nicotine for 8 weeks. At the end of 8 weeks group II rats were divided into four sub-groups including, 3 sub-groups of those were put on drug withdrawal protocol. At the end of experiment, blood pressure, heart rate, plasma renin activity (PRA), serum NO concentration and baroreflex sensitivity (BRS) were measured.

RESULTS

RESULTS demonstrated that BP and BRS were significantly lower in addicted to morphine and nicotine hypertensive rats compared to control (p < 0.05). Addiction withdrawal (in morphine and nicotine withdrawal rats) completely reversed BP and BRS to the pre-addiction levels (p < 0.05). Withdrawal in the only nicotine treated group lowered BP and BRS compared to group that had received morphine and nicotine together (p < 0.05).

CONCLUSION

RESULTS of current study may propose simultaneous morphine and nicotine withdrawal can prevent cardiovascular complications raised due to withdrawal (Fig. 5, Ref. 58).

Toll-like receptor 4 mutant and null mice retain morphine-induced tolerance, hyperalgesia, and physical dependence

The innate immune system modulates opioid-induced effects within the central nervous system and one target that has received considerable attention is the toll-like receptor 4 (TLR4). Here, we examined the contribution of TLR4 in the development of morphine tolerance, hyperalgesia, and physical dependence in two inbred mouse strains: C3H/HeJ mice which have a dominant negative point mutation in the Tlr4 gene rendering the receptor non-functional, and B10ScNJ mice which are TLR4 null mutants. We found that neither acute antinociceptive response to a single dose of morphine, nor the development of analgesic tolerance to repeated morphine treatment, was affected by TLR4 genotype. Likewise, opioid induced hyperalgesia and opioid physical dependence (assessed by naloxone precipitated withdrawal) were not altered in TLR4 mutant or null mice. We also examined the behavioural consequence of two stereoisomers of naloxone: (-) naloxone, an opioid receptor antagonist, and (+) naloxone, a purported antagonist of TLR4. Both stereoisomers of naloxone suppressed opioid induced hyperalgesia in wild-type control, TLR4 mutant, and TLR4 null mice. Collectively, our data suggest that TLR4 is not required for opioid-induced analgesic tolerance, hyperalgesia, or physical dependence.

[Inhibition of morphine intake by antibodies to serotonin-modulating anticonsolidation protein in model of self-administration in rats]

The article concerns study of effects of polyclonal antibodies to serotonin-modulating anticonsolidation protein (SMAP) being in direct dependence on serotonin level and providing intracellular transduction of serotonergic signal, on positive reinforcement effect of morphine in rats. The task was formed in Wistar male rats in the model of morphine self-administration as a result of pressing of one of two levers attached to the wall, joined to the pump delivering each time 100 μg of morphine directly into the vena jugularis. In the 1st series of studies brain cingulate cortex and hypothalamus were taken from the rats achieved stable level of morphine intake and SMAP level was measured with indirect immune-enzyme assay. It was shown that in the morphine-self-injected rats SMAP level in the cingulate cortex is significantly upregulated (p = 0.01), while in the hypothalamus it was left unchanged. In the 2nd series of studies the rats with stable level of morphine intake were administered intraperitoneally with anti-SMAP rabbit polyclonal antibodies (experimental group) or non-immune γ-globulins (control group). Soon after antibodies administration the animals of the experimental group demonstrated manifold decrease of morphine intake lasted for 8 days (p < 0.008), whereas it did not change in the controls. SMAP upregulation in the brain cingulate cortex in the rats with stable morphine intake, obviously, indicates to its engagement in positive reinforcement effect of morphine. Blockade of SMAP activity with anti-SMAP antibodies in the nerve cells induced sharp decrease of morphine intake due to disturbances of transduction through intracellular serotonin's signal channels.

Splicing factor transformer-2β (Tra2β) regulates the expression of regulator of G protein signaling 4 (RGS4) gene and is induced by morphine

Regulator of G protein signaling 4 (RGS4) is a critical modulator of G protein-coupled receptor (GPCR)-mediated signaling and plays important roles in many neural process and diseases. Particularly, drug-induced alteration in RGS4 protein levels is associated with acute and chronic effects of drugs of abuse. However, the precise mechanism underlying the regulation of RGS4 expression is largely unknown. Here, we demonstrated that the expression of RGS4 gene was subject to regulation by alternative splicing of the exon 6. Transformer-2β (Tra2β), an important splicing factor, bound to RGS4 mRNA and increased the relative level of RGS4-1 mRNA isoform by enhancing the inclusion of exon 6. Meanwhile, Tra2β increased the expression of full-length RGS4 protein. In rat brain, Tra2β was co-localized with RGS4 in multiple opioid action-related brain regions. In addition, the acute and chronic morphine treatment induced alteration in the expression level of Tra2β in rat locus coerulus (LC) in parallel to that of RGS4 proteins. It suggests that induction of this splicing factor may contribute to the change of RGS4 level elicited by morphine. Taken together, the results provide the evidence demonstrating the function of Tra2β as a new mediator in opioid-induced signaling pathway via regulating RGS4 expression.

Extinction of aversive memories associated with morphine withdrawal requires ERK-mediated epigenetic regulation of brain-derived neurotrophic factor transcription in the rat ventromedial prefrontal cortex

Recent evidence suggests that histone deacetylase (HDAC) inhibitors facilitate extinction of rewarding memory of drug taking. However, little is known about the role of chromatin modification in the extinction of aversive memory of drug withdrawal. In this study, we used conditioned place aversion (CPA), a highly sensitive model for measuring aversive memory of drug withdrawal, to investigate the role of epigenetic regulation of brain-derived neurotrophic factor (BDNF) gene expression in extinction of aversive memory. We found that CPA extinction training induced an increase in recruiting cAMP response element-binding protein (CREB) to and acetylation of histone H3 at the promoters of BDNF exon I transcript and increased BDNF mRNA and protein expression in the ventromedial prefrontal cortex (vmPFC) of acute morphine-dependent rats and that such epigenetic regulation of BDNF gene transcription could be facilitated or diminished by intra-vmPFC infusion of HDAC inhibitor trichostatin A or extracellular signal-regulated kinase (ERK) inhibitor U0126 (1,4-diamino-2,3-dicyano-1,4-bis(methylthio)butadiene) before extinction training. Correspondingly, disruption of the epigenetic regulation of BDNF gene transcription with U0126 or suppression of BDNF signaling with Trk receptor antagonist K252a or BDNF scavenger tyrosine kinase receptor B (TrkB)-Fc blocked extinction of CPA behavior. We also found that extinction training-induced activation of ERK and CREB and extinction of CPA behavior could be potentiated or suppressed by intra-vmPFC infusion of d-cycloserine, a NMDA receptor partial agonist or aminophosphonopentanoic acid, a NMDA receptor antagonist. We conclude that extinction of aversive memory of morphine withdrawal requires epigenetic regulation of BDNF gene transcription in the vmPFC through activation of the ERK-CREB signaling pathway perhaps in a NMDA receptor-dependent manner.

[Repeated morphine pretreatment reduces glutamatergic synaptic potentiation in the nucleus accumbens induced by acute morphine exposure]

Repeated exposure to morphine leads to the addiction, which influences its clinical application seriously. The glutamatergic projection from prefrontal cortex (PFC) to the nucleus accumbens (NAc) plays an important role in rewarding effects. It is still unknown whether morphine exposure changes PFC-NAc synaptic transmission. To address this question, in vivo field excitatory postsynaptic potentials (fEPSPs) induced by electric stimulating PFC-NAc projection fibers were recorded to evaluate the effect of acute morphine exposure (10 mg/kg, s.c.) on glutamatergic synaptic transmission in NAc shell of repeated saline/morphine pretreated rats. It was showed that acute morphine exposure enhanced fEPSP amplitude and reduced paired-pulse ratio (PPR) in saline pretreated rats, which could be reversed by following naloxone injection (1 mg/kg, i.p.), an opiate receptor antagonist. However, repeated morphine pretreatment significantly inhibited both the enhancement of fEPSP amplitude and reduction of PPR induced by acute morphine exposure. Those results indicate that the initial morphine exposure enhances PFC-NAc synaptic transmission by pre-synaptic mechanisms, whereas morphine pretreatment occludes this effect.

Ketorolac prevents recurrent withdrawal induced hyperalgesia but does not inhibit tolerance to spinal morphine in the rat

Chronic use of opioid is associated with pro-nociceptive phenomena such as hyperalgesia or tolerance. The interaction between opioid and non-steroidal anti-inflammatory drugs (NSAIDs) with respect to opioid-associated hyperalgesia and tolerance remains largely unknown. This study examines the effect of subcutaneous or intrathecal administration of ketorolac, an NSAID, on recurrent withdrawal induced hyperalgesia and tolerance to spinal morphine in rats. Animals were infused with morphine intrathecally, and daily subcutaneous naloxone was used for recurrent withdrawal purpose. We observed that escape latencies on hot box were decreased in animals subjected to withdrawal, and this decrease was reversed by subcutaneous ketorolac pretreatment. In addition, we observed that recurrent withdrawal did not significantly affect the magnitude of spinal morphine tolerance. Compared to controls, all morphine infused animals showed similar changes in their dose responses to spinal morphine, effective dose 50 values and tolerance ratios; and these changes were not affected by the ketorolac given subcutaneously. The effect of ketorolac on tolerance was further examined by directly delivering ketorolac to the spinal cord, and again we observed similar changes in the daily latency, percentage of area under the curve and percentage of maximal possible effects among groups infused with morphine, regardless of intrathecal ketorolac treatment. Together, our results demonstrate that recurrent withdrawal is associated with hyperalgesia but this has no effect on the tolerance development; ketorolac protects against recurrent withdrawal induced hyperalgesia without significantly altering spinal morphine tolerance.

Role of music in morphine rewarding effects in mice using conditioned place preference method

OBJECTIVES

This research aims at studying the neuroendocrine effects of music on creating morphine dependence in mice using conditioned place preference (CPP).

METHODS

The mice treated with 10 mg/kg morphine subcutaneously, fast music and slow music. Morphine was used to create dependence. In order to recognize the morphine rewarding effects, CPP technique was used. In the conditioning stage that lasted for 8 days, different groups of mice, after receiving the treatment were randomly placed in compartment for 30 minutes. The post-conditioning stage included the fourth day, the ninth day, the 12th day and the 16th day.

RESULTS

Comparing place preference between morphine group and the control group, a significant increase (p<0.05) was observed in the place preference of morphine group, while a significant decrease (p<0.05) was demonstrated in the place preferences of morphine + taxi girl music group compared with morphine group alone. In addition morphine + alone in the rain music group demonstrated a significantly increased conditioned place preference (p<0.05) compared with the morphine group.

CONCLUSIONS

Alone in the rain music acts as a positive pleasant emotion increasing the dopaminergic activity in the Nucleus Accumbens (NAc) and Ventral Tegmental Area (VTA) and through associated learning mechanisms of reward-related behavior increases morphine addiction. However, taxi girl music may act as unpleasant experiences producing negative emotions and reducing morphine addiction.

Influence of morphine sensitization on the responsiveness of mesolimbic and mesocortical dopamine transmission to appetitive and aversive gustatory stimuli

RATIONALE

Repeated treatment with morphine has been shown to sensitize rats to its stimulant effects on motor activity and mesolimbic dopamine (DA) transmission.

OBJECTIVES

The aim of this study is to investigate if morphine sensitization is associated to changes in the behavioral reactions to appetitive and aversive taste stimuli and in the response of in vivo DA transmission in the nucleus accumbens (NAc) shell and core and medial prefrontal cortex (PFCX) to the same stimuli.

METHODS

Rats were administered twice a day for three consecutive days with increasing doses of morphine [10, 20, and 40 mg/kg, subcutaneously (sc)] or with saline. After 15 days of withdrawal, rats were infused intraorally with either an appetitive (sweet chocolate, 1 ml) or an aversive solution (quinine HCl 5 × 10(-4) M, 1 ml). The behavioral taste reactions were recorded during microdialysis of DA in the NAc shell and core and PFCX.

RESULTS

Opiate sensitization did not affect behavioral reactions to intraoral chocolate or quinine. In rats naive to the taste stimuli, morphine sensitization was associated to potentiation of stimulatory DA response to appetitive and aversive taste stimuli in the NAc core. Morphine sensitization reciprocally affected habituation of DA responsiveness after one trial exposure to appetitive and aversive taste stimuli (abolition it in the shell, induction in the PFCX). No habituation of DA responsiveness to taste was observed in the NAc core in controls as well as in morphine-sensitized rats.

CONCLUSIONS

These results suggest that opiate sensitization is associated to differential adaptive changes of the responsiveness of DA transmission to taste stimuli in DA terminal areas.

Acute morphine administration and withdrawal from chronic morphine increase afterdepolarization amplitude in rat supraoptic nucleus neurons in hypothalamic explants

Supraoptic nucleus (SON) neurons secrete either oxytocin or vasopressin into the bloodstream from their axon terminals in the posterior pituitary gland. SON neurons are powerfully inhibited by the classical μ-opioid receptor agonist, morphine. Oxytocin neurons develop morphine dependence when chronically exposed to this opiate, and undergo robust withdrawal excitation when morphine is subsequently acutely antagonized by naloxone. Morphine withdrawal excitation is evident as an increased firing rate and is associated with an increased post-spike excitability that is consistent with the expression of an enhanced post-spike afterdepolarization (ADP) during withdrawal. Here, we used sharp electrode recording from SON neurons in hypothalamic explants from morphine naïve and morphine treated rats to determine the effects of morphine on the ADP, and to test the hypothesis that morphine withdrawal increases ADP amplitude in SON neurons. Acute morphine administration (0.05-5.0 μM) caused a dose-dependent hyperpolarization of SON neurons that was reversed by concomitant administration of 10 μM naloxone, or by washout of morphine; counter-intuitively, acute exposure to 5 μM morphine increased ADP amplitude by 78 ± 11% (mean ± SEM). Naloxone-precipitated morphine withdrawal did not alter baseline membrane potential in SON neurons from morphine treated rats, but increased ADP amplitude by 48 ± 11%; this represents a hyper-activation of ADPs because the basal amplitude of the ADP was similar in SON neurons recorded from explants prepared from morphine naïve and morphine treated rats. Hence, an enhanced ADP might contribute to morphine withdrawal excitation of oxytocin neurons.

Effects of Opioid and α2-Adrenoceptor Agonists on the Isolated Ileum of Morphine-dependent Guinea-pigs During Withdrawal and After Clonidine Treatment

The present study was undertaken to investigate the effect of clonidine administration to opiate-dependent guinea-pigs after morphine withdrawal on subsequent twitch responses of the longitudinal muscle-myenteric plexus preparations to electrical field stimulation. The results indicate that clonidine, administered immediately after morphine removal, causes tolerance to the inhibition exerted by opioid and α2-adrenoceptor agonists on the electrically-evoked twitches. Such a finding suggests that the mechanism of action of clonidine involves not only its well-known effects on locus coeruleus neurons but also that it has specific actions on the myenteric plexus. This work shows the existence of interactions between opioid and α-adrenoceptor on the cholinergic neurons present in the isolated ileum.

Drug-sensitive reward in crayfish: an invertebrate model system for the study of SEEKING, reward, addiction, and withdrawal

In mammals, rewarding properties of drugs depend on their capacity to activate appetitive motivational states. With the underlying mechanisms strongly conserved in evolution, invertebrates have recently emerged as a powerful new model in addiction research. In crayfish natural reward has proven surprisingly sensitive to human drugs of abuse, opening an unlikely avenue of research into the basic biological mechanisms of drug addiction. In a series of studies we first examined the presence of natural reward systems in crayfish, then characterized its sensitivity to a wide range of human drugs of abuse. A conditioned place preference (CPP) paradigm was used to demonstrate that crayfish seek out those environments that had previously been paired with the psychostimulants cocaine and amphetamine, and the opioid morphine. The administration of amphetamine exerted its effects at a number of sites, including the stimulation of circuits for active exploratory behaviors (i.e., SEEKING). A further study examined morphine-induced reward, extinction and reinstatement in crayfish. Repeated intra-circulatory infusions of morphine served as a reward when paired with distinct visual or tactile cues. Morphine-induced CPP was extinguished after repeated saline injections. Following this extinction phase, morphine-experienced crayfish were once again challenged with the drug. The priming injections of morphine reinstated CPP at all tested doses, suggesting that morphine-induced CPP is unrelenting. In an exploration of drug-associated behavioral sensitization in crayfish we concurrently mapped measures of locomotion and rewarding properties of morphine. Single and repeated intra-circulatory infusions of morphine resulted in persistent locomotory sensitization, even 5 days following the infusion. Moreover, a single dose of morphine was sufficient to induce long-term behavioral sensitization. CPP for morphine and context-dependent cues could not be disrupted over a drug free period of 5 days. This work demonstrates that crayfish offer a comparative and complementary approach in addiction research. Serving as an invertebrate animal model for the exposure to mammalian drugs of abuse, modularly organized and experimentally accessible nervous systems render crayfish uniquely suited for studying (1) the basic biological mechanisms of drug effects, (2) to explore how the appetitive/seeking disposition is implemented in a simple neural system, and (3) how such a disposition is related to the rewarding action of drugs of abuse. This work aimed to contribute an evolutionary, comparative context to our understanding of a key component in learning, and of natural reward as an important life-sustaining process.

Nitric oxide and renal protection in morphine-dependent rats

Morphine treatment for 5 days protects heart against ischemia-reperfusion (IR) injury. This study evaluated the involvement of nitric oxide (NO) in morphine-induced renal protection. Three weeks after right nephrectomy, increasing doses of morphine were administered (20-30 mg kg(-1)day(-1), 5 days) to develop dependence in rats. The left kidney underwent 45-min ischemia and 24-h reperfusion. Some rats were pretreated with naloxone (5 mg kg(-1)) or L-NAME (20 mg kg(-1)). In one group, IR was induced 24h after the last dose of morphine during the withdrawal period. Plasma nitrite/nitrate levels and serum creatinine and BUN were measured. Creatinine clearance and fractional excretion of sodium (FE(Na)) were calculated. Myeloperoxidase (MPO) activity, malondialdehyde (MDA) level, and inducible NO synthase (iNOS) expression were determined and histopathology was studied in the left kidney. IR increased serum creatinine and BUN, plasma NO (p<0.01), FE(Na), iNOS expression (p<0.001), MPO activity, MDA level, and tissue damage and decreased creatinine clearance. Morphine decreased plasma NO (p<0.05 vs IR), serum creatinine and BUN (p<0.01), FE(Na), MPO activity, MDA level, iNOS expression, and tissue damage (p<0.05), but increased creatinine clearance (p<0.05). Pretreatment with naloxone significantly increased NO production and iNOS expression in morphine-treated rats after IR (p<0.01 vs morphine dependence+IR). Pretreatment with L-NAME in morphine-treated rats decreased NO production (10.7+/-1.9, p<0.01 vs morphine dependence+IR) but could not change iNOS expression after IR. Both naloxone and L-NAME significantly abolished the protective effects of morphine dependence on functional and histological factors. The protective effect of morphine dependence on serum creatinine, BUN, FE(Na), and creatinine clearance persisted during the withdrawal period, whereas iNOS expression decreased. NO production was not decreased during the withdrawal period (p>0.1 vs morphine dependence+IR group). Morphine dependence provided renal protection in the acute phase and during withdrawal. Excessive increase or decrease in NO production abolished the effects of morphine, which suggested a role for balanced NO production and iNOS expression.

Suppressive effects by cysteine protease inhibitors on naloxone-precipitated withdrawal jumping in morphine-dependent mice

The effects of various protease inhibitors on naloxone-precipitated withdrawal jumping were examined in morphine-dependent mice. The doses of morphine were subcutaneously given twice daily for 2 days (day 1, 30 mg/kg; day 2, 60 mg/kg). On day 3, naloxone (8 mg/kg) was intraperitoneally administered 3h after final injection of morphine (60 mg/kg), and the number of jumping was immediately recorded for 20 min. Naloxone-precipitated withdrawal jumping was significantly suppressed by the intracerebroventricular administration of N-ethylmaleimide (0.5 nmol) and Boc-Tyr-Gly-NHO-Bz (0.4 nmol), inhibitors of cysteine proteases involved in dynorphin degradation, 5 min before each morphine treatment during the induction phase, with none given on the test day, as well as by dynorphin A (62.5 pmol) and dynorphin B (250 pmol). However, amastatin, an aminopeptidase inhibitor, phosphoramidon, an endopeptidase 24.11 inhibitor, and captopril, an angiotensin-converting enzyme inhibitor, caused no changes. The present results suggest that cysteine protease inhibitors suppress naloxone-precipitated withdrawal jumping in morphine-dependent mice, presumably through the inhibition of dynorphin degradation.

The C-terminal amidated analogue of the substance P (SP) fragment SP(1-7) attenuates the expression of naloxone-precipitated withdrawal in morphine dependent rats

We previously demonstrated that intracerebroventricular (i.c.v.) administration of the substance P (SP) aminoterminal fragment SP(1-7) attenuates the expression of morphine withdrawal in the male rat. In this study we have used a synthetic analogue of this peptide, i.e. the SP(1-7) amide showing higher binding potency than the native heptapeptide, in a similar experimental set-up. Thus, Wistar male rats were made tolerant to morphine by daily injections of the opiate during 8 days. Following peptide administration (i.c.v.) and a subsequent naloxone challenge a variety of physical syndromes of withdrawal were recorded. We observed that the SP(1-7) amide potently and dose-dependently reduced several signs of reaction to morphine withdrawal. Interestingly, the effect of the peptide amide was significantly attenuated by the addition of the sigma agonist (+)-SKF-10047. We conclude that the SP(1-7) amide mimics the effect of the native SP fragment and that the mechanisms for its action involve a sigma receptor site.

[Effects of clobenpropit and histidine on reinstatement of morphine-induced conditioned place preference]

OBJECTIVE

To investigate the effects of clobenpropit and histidine on reinstatement of morphine-induced conditioned place preference (CPP) in rats.

METHODS

The persistence, extinction and reinstatement of morphine-induced CPP were established.In clobenpropit group three different doses of clobenpropit (2, 5 and 10 microg/rat, i.c.v.) were administered 15 min after morphine (1 mg/kg, i.p.) was injected. In histidine group histidine (100, 200, 500 mg/kg) was given 1 h prior to morphine treatment (1 mg/kg i.p).

RESULT

The CPP was reinstated by priming injection of 1 mg/kg morphine. Clobenpropit (5, 10 microg/rat) significantly inhabited the reinstatement by a priming dose of morphine-induced CPP compared with the morphine control group; histidine (100, 200, 500 mg/kg) significantly inhibited the reinstatement in a dose-dependent manner.

CONCLUSION

Clobenpropit and histidine inhibit the revival of morphine-induced CPP in a dose dependent manner, indicating that endogenous histamine may inhibit relapse of morphine to some extent.

Effects of Morphine on: Spontaneous, Dorsal Raphe, Spinal Tract of Trigeminal Nucleus, Medial Lemniscus and Reticular Lateral Magnocellular Evoked Responses of Hypothalamic Units, in Naive and Morphine Physically Dependent Rats

The spontaneous activity and the inputs to the medial basal hypothalamus (MBH) following dorsal raphe (DR), spinal tract of the trigeminal nerve (SpV), medial lemniscus (ML), reticular lateral magnocellular nucleus ( RLM ) and acoustic (Ac) stimulation and the effects of morphine and the opioid antagonist, naloxone, on these inputs, were investigated in morphine-naive and morphine-dependent animals. The observations were obtained in freely behaving animals previously implanted with permanent electrodes. The spontaneous activity of MBH neurons exhibits heterogenic spontaneous firing rates. This spontaneous activity is affected by acute and chronic morphine treatment. The MBH neuronal population exhibits neurophysiological patterns of tolerance of morphine dependence and withdrawal. The central input exerts a marked influence on MBH neurons in both naive and morphine-dependent animals. These inputs are modified by morphine challenge dose in both preparations, i.e., in morphine-naive and morphine-dependent animals, and are reversed by naloxone. The DR and Ac inputs affect the MBH neuronal activity differently from that observed following SpV, ML and RLM stimuli. The effects of morphine and naloxone on the DR and Ac input in morphine-naive and morphine-dependent animals differ from those observed following SpV, ML and RLM inputs. The MBH neurons exhibited a high percentage of convergence to Ac, DR, SpV, ML and RLM stimulation.

Elevated glucocorticoid levels are responsible for induction of tyrosine hydroxylase mRNA expression, phosphorylation, and enzyme activity in the nucleus of the solitary tract during morphine withdrawal

Chronic opiate exposure induces neurochemical adaptations in the noradrenergic system. Enhanced responsiveness of the hypothalamo-pituitary-adrenal axis after morphine withdrawal has been associated with hyperactivity of ascending noradrenergic input from the nucleus of the solitary tract (NTS-A(2)) cell group to the hypothalamic paraventricular nucleus (PVN). This study addressed the role of morphine withdrawal-induced corticosterone (CORT) release in regulation of tyrosine hydroxylase (TH), the rate-limiting enzyme of catecholamine biosynthesis in adrenalectomized (ADX) rats supplemented with low CORT pellet (ADX plus CORT). Present results show that in sham-ADX rats, noradrenergic neurons in the NTS-A(2) became activated during morphine withdrawal, as indicated by increased TH mRNA expression. However, this induction of TH expression is not detected in ADX plus CORT rats that are unable to mount CORT secretory response to morphine withdrawal. Total TH protein levels were elevated in the NTS-A(2) from sham-operated rats during morphine dependence and withdrawal, whereas we did not find any alteration in ADX plus CORT animals. Furthermore, high levels of TH phosphorylated (activated) at Ser31 (but not at Ser40) were found in the A(2) area from sham-morphine withdrawn rats. Consistent with these effects, we observed an increase in the enzyme activity of TH in the PVN. However, induction of morphine withdrawal to ADX plus CORT animals did not alter the phosphorylation (activation) of TH in NTS-A(2) and decreased TH activity in the PVN. These results suggest the existence of a positive reverberating circle in which elevated glucocorticoids during morphine abstinence play a permissive role in morphine withdrawal-induced activation of noradrenergic pathway innervating the PVN.

Disposition of Ethanol and its Effect on Rectal Temperature in Morphine-dependent, Morphine-abstinent and Morphine-naive Rats

Rats were treated chronically with twice daily injections of morphine hydrochloride in gradually increasing doses from 20 to 200 mg/kg for 22 consecutive days. Rats in a control group were injected with 0.9% NaCl. At 1 hour after the last injection of morphine (dependent state) and at 3 and 16 days after abrupt withdrawal (abstinent state) the animals were injected intraperitoneally with 2.0 g/kg ethanol. Blood ethanol concentrations (tail blood) and rectal temperatures were determined at 30-60 min. intervals for up to 7 hours. The absorption of ethanol was slower in rats treated with morphine and the time taken to reach the end of the blood concentration curve was increased. This implies a slower turnover of ethanol in morphine-dependent and abstinent rats. At 16 days after withdrawal, the blood ethanol profiles were the same as in control rats not exposed to morphine. Injection of morphine (200 mg/kg) intolerant animals caused a pronounced hyperthermia which lasted for about 4 hours. Ethanol treatment rapidly counteracted the rise in body temperature. Morphine abstinent rats showed a hypothermic response to ethanol. The altered disposition of ethanol in acute withdrawal may result from physiological disturbances such as impaired fluid balance, dehydration, altered peripheral blood flow and poor nutritional status. There was no evidence for a faster rate of ethanol metabolism in the hypermetabolic state associated with morphine tolerance and dependence.

Chronic deep brain stimulation in the rat nucleus accumbens and its effect on morphine reinforcement

In order to explore a novel method for the treatment of drug abuse, we evaluated the effect of chronic deep brain stimulation (DBS) of the rat nucleus accumbens (NAc) on morphine reinforcement, using a DBS apparatus and an implant method we developed. Thirty-two adult rats weighing 240-260 g were divided into three groups, which included a DBS group (n = 10, administration of surgery, morphine and DBS), a sham DBS group (n = 12, administration of surgery and morphine) and a control group (n = 10, administration of physiological saline). The DBS electrode was stereotaxically implanted into the core of unilateral NAc and connected to an implantable pulse generator. Then, they were fixed to the rat skull. One week later, the rats in each group were intraperitoneally injected with morphine at an increasing dose (10-60 mg/kg) once daily. The rats in the DBS group were administered a 130-Hz high-frequency stimulation (HFS) once daily. A 900-second conditioned place preference (CPP) paradigm was used for determining the effect of electrical stimulation on morphine reinforcement in rats. The data showed that 7-10 days later, the preference score of the DBS group was significantly lower than that of the sham DBS group. The results suggest that chronic HFS of the rat NAc can block CPP induced by morphine and attenuate morphine reinforcement.

Effect of chronic morphine exposure on response properties of rat barrel cortex neurons

Chronic exposure to morphine can impair performance in tasks which need sensory processing. Using single unit recordings we investigate the effect of chronic morphine exposure on the firing properties of neurons in layers IV and V of the whisker-related area of rat primary somatosensory cortex. In urethane-anesthetized animals, neuronal activity was recorded in response to principal and adjacent whisker deflections either stimulated independently or in a conditioning test paradigm. A condition test ratio (CTR) was calculated for assessing the inhibitory receptive field. In layer IV, chronic morphine treatment did not change the spontaneous discharge activity. On responses to principal and adjacent whisker deflections did not show any significant changes following chronic morphine exposure. The magnitude Off responses to adjacent whisker deflection decreased while its response latency increased. In addition, there was a significant increase in the latency of Off responses to principal whisker deflection. CTR did not change significantly following morphine exposure. Layer V neurons, on the other hand, did not show any significant changes in their spontaneous activity or their evoked responses following morphine exposure. Our results suggest that chronic morphine exposure has a subtle modulatory effect on response properties of neurons in barrel cortex.

[The expression of BDNF and PSD-95 in hippocampal CA1 region of morphine-withdrawn rat with different dependent times]

OBJECTIVE

To observe the expression of brain-derived neurotrophic factor (BDNF) and postsynaptic density-95 (PSD-95) in hippocampal CA1 region of rat with morphine dependence for different times and withdrawn for 1 week, and investigate the influence of that morphine dependence is withdrawn on rat hippocampal CA1 area.

METHODS

Animal models of rats with morphine withdrawal for 1 week and different morphine dependent times(1 week, 2 weeks, 4 weeks) were established. The expression of BDNF and PSD-95 in hippocampal CA1 were identified with RT-PCR.

RESULTS

The expression of BDNF and PSD-95 in hippocampal CA1 decreased in the withdrawn group with morphine dependence for 1 week as compared with that in normal saline (NS) group (P < 0.01), and it increased in the withdrawn group with morphine dependence for 2 weeks as compared with that in morphine-dependent group for 1 week (P < 0.05) but still decreased as compared with that in NS group (P < 0.01), and it decreased in the withdrawn group with morphine dependence for 4 weeks as compared with the other three groups (P < 0.01).

CONCLUSION

The expression of BDNF and PSD-95 in hippocampal CA1 decreases in morphine-depended rats withdrawn for 1 week. Morphine withdrawal has a manifest harm in hippocampal CA1 region of rat.

Valproic acid sodium inhibits the morphine-induced conditioned place preference in the central nervous system of rats

The present study was performed to investigate the effects of valproic acid sodium (VPA), a widely utilized antiepileptic drug, on the establishment of chronic morphine-induced conditioned place preference (CPP). The rat model of morphine-induced CPP was conditioned with alternating intracerebroventricular (i.c.v.) injections of morphine (60 microg/6 microl) and saline for 5 days. To investigate the influence of VPA on morphine-induced CPP, rats received chronic pretreatment of i.c.v. VPA (500 microg/rat) 10 min previous to the daily morphine injection. The results demonstrated that the morphine-induced CPP was significantly attenuated by VPA pretreatment, while the VPA itself could not induce any CPP or conditioned place aversion (CPA) effects. The results of the present study not only confirmed the reliability of establishing morphine-induced CPP model by i.c.v. injection, but also suggest that the antiepileptic drug VPA may be utilized as potential therapeutic medications for drug abuse in the future.

Differential effects of dopamine on pain-related electric activities in normal rats and morphinistic rats

OBJECTIVE

To investigate the influence of dopamine (DA) and DA receptor's antagonist on the transmission of noxious information in the central nervous system of normal rats or morphinistic rats.

METHODS

The influence of DA on the electric activity of the pain-excited neuron (PEN) in the caudate nucleus (Cd) of normal rats or morphinistic rats was recorded after the sciatic nerve was noxiously stimulated.

RESULTS

DA shortened the average latency of the evoked discharge of PEN in the Cd of normal rats, indicating that DA could increase the activity of PEN and pain sensitivity in normal rats. This effect could be inhibited by Droperidol. DA increased the average latency of the evoked discharge of PEN in the Cd of morphinistic rats, indicating that DA could inhibit the activity of PEN and pain sensitivity in morphinistic rats.

CONCLUSION

The responses to painful stimulation were completely opposite between normal rats and morphinistic rats after the intracerebroventricular injection of DA.

Naloxone rapidly evokes endogenous kappa opioid receptor-mediated hyperalgesia in naïve mice pretreated briefly with GM1 ganglioside or in chronic morphine-dependent mice

Low-dose naloxone-precipitated withdrawal hyperalgesia is a reliable indicator of physical dependence after chronic morphine treatment. A remarkably similar long-lasting (>3-4 h) hyperalgesia is evoked by injection of a low dose of naloxone (10 microg/kg, s.c.) in naïve mice after acute pretreatment with the glycolipid, GM1 ganglioside (1 mg/kg) (measured by warm-water-immersion tail-flick assays). GM1 treatment markedly increases the efficacy of excitatory Gs-coupled opioid receptor signaling in nociceptive neurons. Co-treatment with an ultra-low-dose (0.1 ng/kg, s.c.) of the broad-spectrum opioid receptor antagonist, naltrexone or the selective kappa opioid receptor antagonist, nor-binaltorphimine, blocks naloxone-evoked hyperalgesia in GM1-pretreated naïve mice and unmasks prominent, long-lasting (>4 h) inhibitory opioid receptor-mediated analgesia. This unmasked analgesia can be rapidly blocked by injection after 1-2 h of a high dose of naltrexone (10 mg/kg) or nor-binaltorphimine (0.1 mg/kg). Because no exogenous opioid is administered to GM1-treated mice, we suggest that naloxone may evoke hyperalgesia by inducing release of endogenous bimodally acting opioid agonists from neurons in nociceptive networks by antagonizing putative presynaptic inhibitory opioid autoreceptors that "gate" the release of endogenous opioids. In the absence of exogenous opioids, the specific pharmacological manipulations utilized in our tail-flick assays on GM1-treated mice provide a novel bioassay to detect the release of endogenous bimodally acting (excitatory/inhibitory) opioid agonists. Because mu excitatory opioid receptor signaling is blocked by ultra-low doses of naloxone, the higher doses of naloxone that evoke hyperalgesia in GM1-treated mice cannot be mediated by activation of mu opioid receptors. Co-treatment with ultra-low-dose naltrexone or nor-binaltorphimine may selectively block signaling by endogenous GM1-sensitized excitatory kappa opioid receptors, unmasking inhibitory kappa opioid receptor signaling, and converting endogenous opioid receptor-mediated hyperalgesia to analgesia. Co-treatment with kelatorphan stabilizes putative endogenous opioid peptide agonists released by naloxone in GM1-treated mice, so that analgesia is evoked rather than hyperalgesia. Acute treatment of chronic morphine-dependent mice with ultra-low-dose naltrexone (0.1 ng/kg) results in remarkably similar rapid blocking of naloxone (10 microg/kg)-precipitated withdrawal hyperalgesia and unmasking of prominent opioid analgesia. These studies may clarify complex mechanisms underlying opioid physical dependence and opioid addiction.

Effect of parental morphine addiction on hippocampal long-term potentiation in rats offspring

Attention to addiction of women alone for fetus and infant's health has caused the possible role of father's status was less considered, while some developmental impairments including decrease of liter size, weight loss, congenital deficiencies, behavioral disorders, and learning and memory impairments in offspring with addicted father have been reported. In this study the effects of addiction of one or both parents to morphine on male and female offspring hippocampal long-term potentiation (LTP), were assessed. One hundred twenty female and 48 male rats (4-5 months, 250-270 g) were used. Forty females and 16 males were addicted by oral administration of morphine (32 mg/kg twice daily) for 5 days before mating. Then each two males with five females were housed (coupled) per cage as five groups for coupling: (A) addicted females+5% dextrose males (add.F); (B) addicted males+5% dextrose females (add.M); (C) addicted females+addicted males (add.MF); (D) 5% dextrose females+intact males (dex.F); (E) 5% dextrose males+intact females (dex.M). In puberty offspring LTP was induced in hippocampal dentate gyrus by stimulation of perforant path (pp). Changes of population spikes (PS) amplitude and LTP slope at 0, 5, 30, 60 and 120 min were evaluated. Slope of LTP at 30, 60 and 120 min, and amplitude of PS at 60 and 120 min in add.F and add.M offspring were significantly lower than dextrose groups (P<0.01). LTP slope and PS amplitude of male and female offspring did not different between add.F and add.M groups. Our results suggest that both parental and paternal addiction to morphine may cause memory deficiency through reduction of LTP in hippocampus.

Effects of morphine and its withdrawal on Y-maze spatial recognition memory in mice

Effects of morphine on acquisition and retrieval of memory have been proven in the avoidance paradigms. In present study, we used a two-trial recognition Y-maze to test the effects of acute morphine and morphine withdrawal on spatial recognition memory. The Y-maze is based on the innate tendency of rodents to explore novel environments and therefore avoid punishment and reward.

RESULTS

1) Pre-training morphine 10 mg/kg impaired the recognition spatial memory of acquisition after a 1 h inter-trial interval (ITI), whereas morphine 2.5, 5 and 10 mg/kg showed impairment after 2 h ITI. 2) Pre-retention morphine 5, 10 mg/kg disrupted the retrieval of memory after 1 h ITI. 3) Morphine 5 and 10 mg/kg caused hyper-locomotor activity depending on the state. 4) Mice withdrawn from morphine 40 mg/kg but not 10 mg/kg for 3 days showed amnesia in Y-maze. Our data suggested that acute morphine impaired the acquisition and retrieval of spatial recognition memory and increased the locomotor activity in the Y-maze depending on the dose and state. Moreover, withdrawal from chronic morphine also impaired acquisition in the Y-maze depending on the dose and state.

[Dynamics of hippocampal sensory gating during the chronic morphine administration, withdrawal and re-exposure to morphine in rats]

Drug addiction is considered as a chronic, recurrent brain disease characterized by relapse. Repeated exposure to certain drugs, such as morphine, can produce deleterious sequelae, such as drug dependence, tolerance and compulsive drug seeking. In the present study, we investigated the dependence and psychological craving for morphine in rats using the conditioned place preference (CPP) paradigm. On the other hand, to study the effect of morphine on hippocampal sensory gating (N40), double click auditory-evoked potential was recorded during the chronic morphine administration, withdrawal and re-exposure to morphine in rats. The rats in morphine group received a course of morphine (10 mg/kg, i.p.) injection for 12 d, followed by 12 d of withdrawal, 1 d of re-exposure to morphine (2.5 mg/kg, i.p.) and 2 d of the second withdrawal. The rats in the control group were treated in the same way except that saline was applied instead of morphine. CPP test demonstrated that the method of drug administration in the present study induced dependence and psychological craving for morphine in rats. The results in the double click auditory-evoked potential experiment showed that during the chronic morphine administration, hippocampal N40 gating was damaged. In the initial 2 d of the first withdrawal hippocampal N40 gating in morphine group was reduced compared with that in the control group and it was significantly greater on the 3rd day, and then recovered gradually to the normal level from day 4 to day 12. After re-exposure to morphine, hippocampal N40 gating in morphine group was significantly reduced compared with that in the control group, and it remained at a lower level during the following 2 d, suggesting that hippocampal N40 gating in rats was more sensitive to morphine during re-exposure. Our results suggest that long-term repeated morphine administration and re-exposure to morphine disrupt hippocampal N40 gating, and that the effect of morphine addiction on the brain is possibly long-term.

Drugs of Abuse-Induced Hyperthermia, Blood-Brain Barrier Dysfunction and Neurotoxicity: Neuroprotective Effects of a New Antioxidant Compound H-290/51

The psychostimulants, morphine and methamphetamine are well known drugs of abuse that induce brain pathology and/or neurodegeneration resulting in a huge burden on our society. The possible mechanisms of psychostimulants induced neuropathology and neurodegeneration are still not well known. The drugs of abuse results in profound hyperthermia and widespread alterations in neurochemical metabolism in the central nervous system (CNS). It appears that psychostimulants induced hyperthermia and/or release of neurochemicals influence the blood-brain barrier (BBB) dysfunction leading to brain pathology. The drugs of abuse also induce oxidative stress resulting in generation of free radicals and lipid peroxidation. Thus, further research is needed to understand the basic function of BBB disruption and temperature regulation by psychostimulants and to modify them pharmacologically to attenuate brain dysfunction and neuropathology. This review is focused on the problems of morphine and methamphetamine induced hyperthermia and their effects on breakdown of the BBB function leading to brain damage. Work done in our laboratory suggest that hyperthermia caused by these drugs is responsible for BBB disruption and neurodegeneration. This hypothesis is further supported by our observation that pretreatment with a potent antioxidant compound H-290/51 attenuates the BBB disruption and induces marked neuroprotection following morphine induced withdrawal and methamphetamine induced neurotoxicity. The possible mechanisms and functional significance of these findings are discussed.

Nucleus accumbens NMDA receptor subunit expression and function is enhanced in morphine-dependent rats

We have previously shown, using radioligand binding studies, that N-methyl-d-aspartate (NMDA) NR1 and NR2A receptor subunits density was decreased in the forebrain of morphine-dependent rats. We have now determined if morphine-dependent rats display regional differences in NMDA receptor expression and whether such changes are functionally relevant. In morphine-dependent rats, the expression of NR1 and NR2A subunits protein, as determined by Western blotting with NMDA receptor subunit antibodies, were decreased in frontal cortex and hippocampus but significantly increased in the nucleus accumbens. The expression of the NR2B subunit was unchanged in all regions examined. In separate groups of morphine-dependent rats, MK-801-induced hyperactivity (thought to be mediated via modulation of nucleus accumbens dopamine release) was significantly enhanced in morphine-dependent animals. Similarly, the MK-801-induced increase of dopamine metabolism was significantly increased in the nucleus accumbens of morphine-dependent animals as compared to sham controls. Results provide both biochemical and behavioural evidence to suggest that NMDA receptor function in the nucleus accumbens, at least with respect to an interaction with the limbic dopamine system, is markedly enhanced in morphine-dependent rats. This increase in function may be associated with an enhanced expression of NMDA receptors, particularly those in the nucleus accumbens containing the NR2A subunit. Taken together, these data support several studies in the literature indicating that NMDA receptors in the nucleus accumbens are involved in the process of opiate dependence.

Lesions of the medial prefrontal cortex prevent the induction but not expression of morphine-induced behavioral sensitization in mice

Repetitive exposure to morphine induces behavioral sensitization, which is supposed to involve in the process of addiction to drugs. As the underlying neuropharmacological mechanisms and anatomical substrates are considerably different between different drugs and different phases of behavioral sensitization, this study was designed to investigate the roles of the medial prefrontal cortex in the induction and expression of morphine-induced behavioral sensitization in mice. In experiments 1 and 2, mice were dosed with morphine (10 mg/kg, i.p.) once daily for 7 continuous days after induction of sham or kainic acid lesions of the medial prefrontal cortex. Locomotor activity was measured on days 1 and 7 to test acute morphine-induced hyperactivity and the induction of behavioral sensitization. In experiment 3, mice were subjected to surgery after behavioral sensitization was achieved. The mice were challenged with morphine 7 days later to evaluate the expression of behavioral sensitization. The results showed that lesions of the medial prefrontal cortex (mPFC) blocked acute morphine-induced hyperactivity and the induction of behavioral sensitization, but the lesions had no effect on the expression of behavioral sensitization. These results provide the first direct evidence that the mPFC may be involved in the induction, but not the expression, of morphine-induced behavioral sensitization.

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

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

Effect of agmatine on brain l-citrulline production during morphine withdrawal in rats: A microdialysis study in nucleus accumbens

Agmatine, an endogenous nitric oxide (NO) synthase inhibitor and ligand for imidazoline receptors, has been previously shown to prevent morphine dependence in rats. The present study was designed to investigate NO formation in nucleus accumbens core region (NAcc) during naloxone (NL)-precipitated morphine withdrawal in rats treated with agmatine or l-NAME by using intracerebral microdialysis in freely moving rats, through measuring extracellular l-citrulline concentrations, an indirect sign of NO production since equal amounts of l-citrulline and NO are produced from l-arginine. l-Citrulline levels in the NAcc core did not change following administration of agmatine (40 mg/kg i.p.) or l-NAME (100 mg/kg i.p.) in control rats. Both agmatine and l-NAME attenuated withdrawal symptoms of morphine in NL (2 mg/kg i.p.)-precipitated withdrawal. l-Citrulline levels showing the release of NO increased in morphine-dependent rats during NL-precipitated withdrawal. Agmatine and l-NAME treatments significantly suppressed the increase in l-citrulline levels compared to physiological saline-treated rats in this setting. The results suggest that the release of l-citrulline in NAcc may be involved in the processes of morphine withdrawal and agmatine as an endogenous inhibitor of NO synthase may be one of the factors involved in the changes in the physiology and behavioral state during opioid withdrawal and may have pharmacological importance.

Does the presence of morphine counteract adaptive changes in expression of G-protein alpha subunits mRNA induced by chronic morphine treatment?

Opiate dependence develops due to changes in intracellular signaling caused by long-term exposure to morphine. Here we investigated changes in the mRNA expression of the main classes of G-protein alpha (Galpha) subunits in various brain regions in morphine-dependent rats. Rats received increasing doses of morphine, 10-50 mg/kg, b.i.d., for 14 days. G-protein alpha-subunit mRNA expression was determined shortly following the conclusion of chronic morphine administration (2 h after the final dose) and during withdrawal (48 h after the final dose). Significant changes in mRNA expression for Galpha subunits were observed in several brain areas during withdrawal, while the changes were much less evident or absent 2 h after the final drug injection. Changes in mRNA expression were particularly evident in the nucleus accumbens (increases in Galpha(12), Galpha(q), Galpha(11), and Galpha(o) during withdrawal, increase in Galpha(i) and decrease in Galpha(s) just following treatment). The direction of the changes, which were not all significant, for Galpha(12), Galpha(q), and Galpha(11) was generally consistent in the amygdala and prefrontal cortex; changes in G proteins coupled to the adenylyl cyclase cascade were less consistent. These results suggest that morphine dependence leads to alterations in intracellular signaling, which are reflected in changes in the expression of genes encoding various G proteins. The results may explain why signs of opiate dependence are not expressed during chronic administration of morphine, but only after cessation of the treatment.

Post-treatment of dextromethorphan reverses morphine effect on conditioned place preference in rats

Previously we showed that coadministration of dextromethorphan (DM) with morphine attenuates morphine-rewarding effect. Here we further investigated if DM is effective in reversing or treating drug-seeking effect when given after subchronic morphine treatment. The conditioned place preference (CPP) test was used to investigate the rewarding and drug-seeking effects of morphine. Rats were injected and conditioned with morphine for 6 days and then withdrawn from morphine for 4 days and treated with saline or DM during this period. Subchronic morphine induced a significant place preference for the drug-paired compartment. DM treatment during the morphine withdrawal period significantly reversed the preference from the drug-paired compartment to saline-paired compartment. Both nucleus accumbens (NAc) and VTA were proven to be the sites involved in the action of DM. Behavioral sensitization occurred in both morphine group and DM treatment group determined by the locomotor activity before and after subchronic morphine treatment. The dopamine (DA) turnover rate in the NAc, dorsal striatum (DS) and medial prefrontal cortex (mPFC) was increased after subchronic morphine treatment. DM treatment reversed the increase of DA turnover rate in the NAc but not in the DS or mPFC. These data suggest that DM might have potential in the treatment of morphine addiction.

Cholecystokinin is necessary for the expression of morphine conditioned place preference

There is evidence that the neuropeptide cholecystokinin (CCK) is important for the rewarding effects of drugs of abuse. However, less is known regarding the role of CCK in drug seeking and craving. The present study investigated whether the CCK(B) antagonist L-365, 260 could block morphine-induced drug seeking using the conditioned place preference paradigm and whether the dopaminergic reward pathway contributes to the effect of L-365, 260 on expression of morphine place preference. We found that systemic administration of the CCK(B) antagonist L-365, 260 attenuates the expression of morphine-induced drug seeking as assessed using conditioned place preference (CPP) and shows that this effect is mediated by CCK(B) receptors in the anterior nucleus accumbens (NAcc). Additionally, we demonstrate that this effect is dependent on D(2) receptor activation in the anterior nucleus accumbens (NAcc). These results indicate that endogenous CCK modulates the incentive-salience of morphine-associated cues and suggest that CCK antagonists may be useful in the treatment of drug craving.

The molecular mechanisms of morphine addiction

Addiction to opiates such as morphine is a major public health concern. A more thorough understanding of the molecular mechanisms of opiate addiction can lead to better treatment options in the future. Many of the changes in neuronal activity that occur upon morphine exposure have been known for some time, but until recently, little was known about the changes in gene expression that underlie these effects. Recent advances in molecular biology such as microarray analysis and quantitative (real time) PCR have allowed us to examine the gene expression changes that occur in response to morphine treatments and during morphine withdrawal. This review summarizes many of the known molecular and cellular actions of morphine, and some of the important gene expression changes that occur in response to morphine treatment. Many of these gene expression changes underlie the alterations in neuronal excitability, cell morphology and cell birth or death responsible for producing morphine's rewarding effects, the development of dependence, and withdrawal symptoms after treatment ends.

Morphine conditioned place preference depends on glucocorticoid receptors in both hippocampus and nucleus accumbens

Learned association between drugs of abuse and context is essential for the formation of drug conditioned place preference (CPP), which is believed to engage many brain regions including hippocampus and nucleus accumbens (NAc). The underlying mechanisms are not fully understood. Here, we examined whether glucocorticoid receptors (GRs) of hippocampus and NAc influenced the formation of morphine CPP in Sprague Dawley rats. We found that systemic or intrahippocampal infused DMSO vehicle (DMSO 20% in saline) 30 min before daily morphine (10 mg/kg, s.c.) conditioning did not affect the formation of morphine CPP. In contrast, systemic administration (5 mg/kg, s.c.) or intrahippocampal infusion (0, 0.1, 1.0, 10, 20 microg per side) of the GR antagonist RU38486 blocked or impaired the formation of CPP in a dose-dependent manner, respectively. Furthermore, intra-NAc infused RU38486 (10 microg per side) but not DMSO vehicle also prevented the formation of CPP. These results demonstrate that both the GRs of hippocampus and NAc are necessary for the formation of morphine CPP, suggesting a neural network function of the GRs in forming the opiate-associated memory.

[Effects of Caulis Sinomenii and sinomenine on morphine-induced place preference and brain histamine level in mice]

OBJECTIVE

To evaluate the effects of Caulis Sinomenii and sinomenine on conditioned place preference (CPP) induced by morphine and brain histamine level in mice.

METHODS

Sixty mice were randomized into 6 equal groups and morphine (Mor) was injected subcutaneously (9 mg/kg) for 6 consecutive days to induce CPP using a shuttle box. Since the 4th day of training, the mice in 5 of the groups were treated for 3 consecutive days with Caulis Sinomenii (10 g/kg), sinomenine (60 mg/kg), diphenhydramine (30 mg/kg), CP48/80 (5 mg/kg) and L-histidine (750 mg/kg) in addition to morphine (9 mg/kg) treatment, respectively, leaving the other group with exclusive morphine treatment. Another 10 mice received saline injection to serve as saline control group. The content of histamine (HA) in the mouse brain was measured by fluorospectrophotometry.

RESULTS

In morphine group, the mice showed significantly extended stay in morphine-paired compartment whose HA content in the brain was markedly increased (P<0.01). Treatment with Caulis Sinomenii and sinomenine resulted in significantly reduced time of stay in morphine-paired compartment and brain HA level (P<0.01).

CONCLUSION

CPP induced by morphine in mice is associated with increased HA level in the brain. Caulis Sinomenii and sinomenine can suppress the acquisition of place preference induced by morphine and modulate HA level in the central nervous system in morphine-dependent mice.

Activation in extended amygdala corresponds to altered hedonic processing during protracted morphine withdrawal

Previously we reported that during protracted morphine abstinence rats show reduced conditioned place preferences (CPP) for food-associated environments, compared to non-dependent subjects. To determine the brain regions involved in this altered reward behavior, we examined neural activation (as indexed by Fos-like proteins) induced by a preference test for a food-associated environment in 5-week morphine-abstinent versus non-dependent animals. The results indicate that elevated Fos expression in the anterior cingulate cortex (Cg) and basolateral amygdala (BLA) correlated positively with preference behavior in all groups. In contrast, Fos expression in stress-associated brain areas, including the ventral lateral bed nucleus of the stria terminalis (VL-BNST), central nucleus of the amygdala (CE), and noradrenergic (A2) neurons in the nucleus tractus solitarius (NTS) was significantly elevated only in morphine-abstinent animals. Furthermore, the number of Fos positive neurons in these areas was found to correlate negatively with food preference in abstinent animals. These results indicate that the altered hedonic processing during protracted morphine withdrawal leading to decreased preference for cues associated with natural rewards may involve heightened activity in stress-related brain areas of the extended amygdala and their medullary noradrenergic inputs.