Increase in adenosine sensitivity in the nucleus accumbens following chronic morphine treatment

Cellular Tolerance Induced by Chronic Opioids in the Central Nervous System

Opioids are powerful analgesics that elicit acute antinociceptive effects through their action the mu opioid receptor (MOR). However opioids are ineffective for chronic pain management, in part because continuous activation of MORs induces adaptive changes at the receptor level and downstream signaling molecules. These adaptations include a decrease in receptor-effector coupling and changes to second messenger systems that can counteract the persistent activation of MORs by opioid agonists. Homeostatic regulation of MORs and downstream signaling cascades are viewed as precursors to developing tolerance. However, despite numerous studies identifying crucial mechanisms that contribute to opioid tolerance, no single regulatory mechanism that governs tolerance in at the cellular and systems level has been identified. Opioid tolerance is a multifaceted process that involves both individual neurons that contain MORs and neuronal circuits that undergo adaptations following continuous MOR activation. The most proximal event is the agonist/receptor interaction leading to acute cellular actions. This review discusses our understanding of mechanisms that mediate cellular tolerance after chronic opioid treatment that, in part, is mediated by agonist/receptor interaction acutely.

A Critical Role of Basolateral Amygdala-to-Nucleus Accumbens Projection in Sleep Regulation of Reward Seeking

BACKGROUND

Sleep impacts reward-motivated behaviors partly by retuning the brain reward circuits. The nucleus accumbens (NAc) is a reward processing hub sensitive to acute sleep deprivation. Glutamatergic transmission carrying reward-associated signals converges in the NAc and regulates various aspects of reward-motivated behaviors. The basolateral amygdala projection (BLAp) innervates broad regions of the NAc and critically regulates reward seeking.

METHODS

Using slice electrophysiology, we measured how acute sleep deprivation alters transmission at BLAp-NAc synapses in male C57BL/6 mice. Moreover, using SSFO (stabilized step function opsin) and DREADDs (designer receptors exclusively activated by designer drugs) (Gi) to amplify and reduce transmission, respectively, we tested behavioral consequences following bidirectional manipulations of BLAp-NAc transmission.

RESULTS

Acute sleep deprivation increased sucrose self-administration in mice and altered the BLAp-NAc transmission in a topographically specific manner. It selectively reduced glutamate release at the rostral BLAp (rBLAp) onto ventral and lateral NAc (vlNAc) synapses, but spared caudal BLAp onto medial NAc synapses. Furthermore, experimentally facilitating glutamate release at rBLAp-vlNAc synapses suppressed sucrose reward seeking. Conversely, mimicking sleep deprivation-induced reduction of rBLAp-vlNAc transmission increased sucrose reward seeking. Finally, facilitating rBLAp-vlNAc transmission per se did not promote either approach motivation or aversion.

CONCLUSIONS

Sleep acts on rBLAp-vINAc transmission gain control to regulate established reward seeking but does not convey approach motivation or aversion on its own.

Delta Opioid Receptors and Cardioprotection

The opioid receptor family, with associated endogenous ligands, has numerous roles throughout the body. Moreover, the delta opioid receptor (DORs) has various integrated roles within the physiological systems, including the cardiovascular system. While DORs are important modulators of cardiovascular autonomic balance, they are well-established contributors to cardioprotective mechanisms. Both endogenous and exogenous opioids acting upon DORs have roles in myocardial hibernation and protection against ischaemia-reperfusion (I-R) injury. Downstream signalling mechanisms governing protective responses alternate, depending on the timing and duration of DOR activation. The following review describes models and mechanisms of DOR-mediated cardioprotection, the impact of co-morbidities and challenges for clinical translation.

Prefrontal Cortex to Accumbens Projections in Sleep Regulation of Reward

Sleep profoundly affects the emotional and motivational state. In humans and animals, loss of sleep often results in enhanced motivation for reward, which has direct implications for health risks as well as potential benefits. Current study aims at understanding the mechanisms underlying sleep deprivation (SDe)-induced enhancement of reward seeking. We found that after acute SDe, mice had an increase in sucrose seeking and consumption but not food intake, suggesting a selective enhancement of motivation for reward. In the nucleus accumbens (NAc), a key brain region regulating emotional and motivational responses, we observed a decrease in the ratio of the overall excitatory over inhibitory synaptic inputs onto NAc principle neurons after SDe. The shift was partly mediated by reduced glutamatergic transmission of presynaptic origin. Further analysis revealed that there was selective reduction of the glutamate release probability at the medial prefrontal cortex (mPFC)-to-NAc synapses, but not those from the hippocampus, thalamus, or the basal lateral amygdala. To reverse this SDe-induced synaptic alteration, we expressed the stabilized step function opsin (SSFO) in the mPFC; optogenetic stimulation of SSFO at mPFC-to-NAc projection terminals persistently enhanced the action potential-dependent glutamate release. Intra-NAc optogenetic stimulation of SSFO selectively at mPFC-to-NAc terminals restored normal sucrose seeking in mice after SDe without affecting food intake. These results highlight the mPFC-to-NAc projection as a key circuit-based target for sleep to regulate reward-motivated behaviors.

SIGNIFICANCE STATEMENT

Sleep loss, a costly challenge of modern society, has profound physiological and psychological consequences, including altered reward processing of the brain. The current study aims at understanding the mechanisms underlying sleep deprivation-induced enhancement of reward seeking. We identify that the medial prefrontal cortex (mPFC)-to-nucleus accumbens (NAc) glutamatergic transmission is selectively weakened following acute sleep deprivation, whose restoration normalizes reward seeking in sleep-deprived mice. These results suggest a possibility of normalizing sleep deprivation-induced abnormal reward seeking by targeting specific neural projections, and they demonstrate the mPFC-to-NAc glutamatergic projection as a key circuit-based target for sleep to regulate reward-motivated behaviors.

Effects of the adenosinergic system on the expression and acquisition of sensitization to conditioned place preference in morphine-conditioned rats

In the presented study, we attempt to investigate if the sensitization to conditioned place preference (CPP) induced by low doses of morphine was developed in rats which have been previously conditioned with morphine. The experiments were performed in the CPP test. Firstly, it has been demonstrated that administration of ineffective dose of morphine on the 9th day induces the increase in time spent of rats at a morphine-paired compartment, confirming that sensitization to CPP has been developed in these animals. Secondly, it has been shown that stimulation of A1 receptor significantly inhibits the expression of morphine-induced of sensitization, and blockade of these receptors produces the opposite effect. Finally, it has been indicated that both stimulation and blockade of A1 and/or A2A receptors inhibit the acquisition of sensitization to CPP. The obtained results have strongly supported the significance of adenosinergic system in both expression and acquisition of studied sensitization. These results seem to be important for the identification of connections in the central nervous system which can help finding new strategies to attenuate rewarding action of morphine.

Principles of motivation revealed by the diverse functions of neuropharmacological and neuroanatomical substrates underlying feeding behavior

Circuits that participate in specific subcomponents of feeding (e.g., gustatory perception, peripheral feedback relevant to satiety and energy balance, reward coding, etc.) are found at all levels of the neural axis. Further complexity is conferred by the wide variety of feeding-modulatory neurotransmitters and neuropeptides that act within these circuits. An ongoing challenge has been to refine the understanding of the functional specificity of these neurotransmitters and circuits, and there have been exciting advances in recent years. We focus here on foundational work of Dr. Ann Kelley that identified distinguishable actions of striatal opioid peptide modulation and dopamine transmission in subcomponents of reward processing. We also discuss her work in overlaying these neuropharmacological effects upon anatomical pathways that link the telencephalon (cortex and basal ganglia) with feeding-control circuits in the hypothalamus. Using these seminal contributions as a starting point, we will discuss new findings that expand our understanding of (1) the specific, differentiable motivational processes that are governed by central dopamine and opioid transmission, (2) the manner in which other striatal neuromodulators, specifically acetylcholine, endocannabinoids and adenosine, modulate these motivational processes (including via interactions with opioid systems), and (3) the organization of the cortical-subcortical network that subserves opioid-driven feeding. The findings discussed here strengthen the view that incentive-motivational properties of food are coded by substrates and neural circuits that are distinguishable from those that mediate the acute hedonic experience of food reward. Striatal opioid transmission modulates reward processing by engaging frontotemporal circuits, possibly via a hypothalamic-thalamic axis, that ultimately impinges upon hypothalamic modules dedicated to autonomic function and motor pattern control. We will conclude by discussing implications for understanding disorders of "non-homeostatic" feeding.

Sustained ligand-activated preconditioning via δ-opioid receptors

We have previously described novel cardioprotection in response to sustained morphine exposure, efficacious in young to aged myocardium and mechanistically distinct from conventional opioid or preconditioning (PC) responses. We further investigate opioid-dependent sustained ligand-activated preconditioning (SLP), assessing duration of protection, opioid receptor involvement, additivity with conventional responses, and signaling underlying preischemic induction of the phenotype. Male C57BL/6 mice were treated with morphine (75-mg subcutaneous pellet) for 5 days followed by morphine-free periods (0, 3, 5, or 7 days) before ex vivo assessment of myocardial tolerance to 25-min ischemia/45-min reperfusion. SLP substantially reduced infarction (by ∼50%) and postischemic contractile dysfunction (eliminating contracture, doubling force development). Cardioprotection persisted for 5 to 7 days after treatment. SLP was induced specifically by δ-receptor and not κ- or μ-opioid receptor agonism, was eliminated by δ-receptor and nonselective antagonism, and was additive with adenosinergic but not acute morphine- or PC-triggered protection. Cotreatment during preischemic morphine exposure with the phosphoinositide-3 kinase (PI3K) inhibitor wortmannin, but not the protein kinase A (PKA) inhibitor myristoylated PKI-(14-22)-amide, prevented induction of SLP. This was consistent with shifts in total and phospho-Akt during the induction period. In summary, data reveal that SLP triggers sustained protection from ischemia for up to 7 days after stimulus, is δ-opioid receptor mediated, is induced in a PI3K-dependent/PKA-independent manner, and augments adenosinergic protection. Mechanisms underlying SLP may be useful targets for manipulation of ischemic tolerance in young or aged myocardium.

Adenosine A2A receptors and their role in drug addiction

The specific events between initial presumably manageable drug intake and the development of a drug- addicted state are not yet known. Drugs of abuse have varying mechanisms of action that create a complex pattern of behaviour related to drug consumption, drug-seeking, withdrawal and relapse. The neuromodulator adenosine has been shown to play a role in reward-related behaviour, both as an independent mediator and via interactions of adenosine receptors with other receptors. Adenosine levels are elevated upon exposure to drugs of abuse and adenosine A2A receptors are enriched in brain nuclei known for their involvement in the processing of drug-related reinforcement processing. A2A receptors are found in receptor clusters with dopamine and glutamate receptors. A2A receptors are thus ideally situated to influence the signalling of neurotransmitters relevant in the neuronal responses and plasticity that underlie the development of drug taking and drug-seeking behaviour. In this review, we present evidence for the role of adenosine and A2A receptors in drug addiction, thereby providing support for current efforts aimed at developing drug therapies to combat substance abuse that target adenosine signalling via A2A receptors.

The role of nucleus accumbens adenosine-opioid interaction in mediating palatable food intake

Nucleus accumbens micro-opioid stimulation leads to robust increases in the intake of highly palatable foods, such as a high-fat diet. While interactions between opioids and certain striatal neurotransmitters underlying this phenomenon have been explored, many potential interactions have not. Striatal adenosine has been shown to have a significant influence on striatal neurotransmission and locomotor activity behavior, however the interaction between opioids and adenosine on feeding behaviors has received less attention. The present study explored this interaction within the context of opioid-driven consumption of a high-fat diet. Specifically, intra-accumbens administration of selective A1 and A2(A) adenosine receptor ligands, with or without concurrent administration of the micro-opioid agonist (D)-Ala(2),N,Me-Phe(4),Gly-ol(5)-enkaphalin (DAMGO), on high-fat consumption and associated locomotor activity was examined. The A1 receptor agonist 2-Chloro-N6-cyclopentyladenosine (CCPA) had no effect on either baseline or DAMGO-induced locomotor or consumption behaviors associated with the high-fat diet. However, the A2(A) receptor agonist 2-p-(2 carboxyethyl)-phenethylamino-5'-N-ethylcarboxamido adenosine hydrochloride (CGS 21680) and the prodrug of the A2(A) receptor antagonist MSX-2, 3-(3-hydroxypropyl)-8-(m-methoxystyryl)-7-methyl-1-propargylxanthine phosphate disodium salt (MSX-3) produced the expected decrease and increase in locomotor activity, respectively. CGS 21680 had no effect on baseline or DAMGO-driven consumption of the high-fat diet. MSX-3 had no effect on DAMGO-induced locomotor activity but increased DAMGO-induced consumption. Lastly, the increased activity and consumption produced by MSX-3 alone was blocked by prior administration of the opioid antagonist naltrexone. In summary, these results suggest a potential role of striatal adenosine A2(A) receptors in mediating baseline and striatal opioid-mediated intake of a high-fat diet.

Up-regulation of AGS3 during morphine withdrawal promotes cAMP superactivation via adenylyl cyclase 5 and 7 in rat nucleus accumbens/striatal neurons

Effective medical treatment of opiate addiction is limited by a high relapse rate in abstinent addicts. Opiate withdrawal causes cAMP superactivation, but the underlying molecular mechanisms are not clear. Recent evidence implicates an activator of G-protein signaling 3 (AGS3) in opiate addiction. We found previously that during a 10-min activation of opioid receptors, AGS3 binds G alpha(i)-GDP to promote free G betagamma stimulation of adenylyl cyclase (AC) 2 and 4, and/or inactivate G alpha(i) inhibitory function, thereby transiently enhancing cAMP-dependent protein kinase A (PKA) activity. In contrast, we report here that in nucleus accumbens/striatal neurons, morphine withdrawal induces cAMP superactivation, which requires up-regulation of AGS3. cAMP increases as a function of withdrawal time, by approximately 20% at 10 min and 75% at 5 h. However, cAMP superactivation does not require G betagamma. Instead, adenosine A2A receptor activation of G alpha(s/olf) seems to initiate cAMP superactivation and promote AGS3 up-regulation. Elevated AGS3 binds to G alpha(i) to prevent its inhibition on AC activation. Moreover, withdrawal-induced increases in cAMP/PKA activate phospholipase C and epsilon protein kinase C to further stimulate AC5 and AC7, causing cAMP superactivation. Our findings identify a critical role for AC 5 and 7 and A2A receptors for up-regulation of AGS3 in morphine withdrawal-induced cAMP superactivation.

A differential role for the adenosine A2A receptor in opiate reinforcement vs opiate-seeking behavior

The adenosine A(2A) receptor is specifically enriched in the medium spiny neurons that make up the 'indirect' output pathway from the ventral striatum, a structure known to have a crucial, integrative role in processes such as reward, motivation, and drug-seeking behavior. In the present study we investigated the impact of adenosine A(2A) receptor deletion on behavioral responses to morphine in a number of reward-related paradigms. The acute, rewarding effects of morphine were evaluated using the conditioned place preference paradigm. Operant self-administration of morphine on both fixed and progressive ratio schedules as well as cue-induced drug-seeking was assessed. In addition, the acute locomotor response to morphine as well as sensitization to morphine was evaluated. Decreased morphine self-administration and breakpoint in A(2A) knockout mice was observed. These data support a decrease in motivation to consume the drug, perhaps reflecting diminished rewarding effects of morphine in A(2A) knockout mice. In support of this finding, a place preference to morphine was not observed in A(2A) knockout mice but was present in wild-type mice. In contrast, robust cue-induced morphine-seeking behavior was exhibited by both A(2A) knockout and wild-type mice after a period of withdrawal. The acute locomotor response to morphine in the A(2A) knockout was similar to wild-type mice, yet A(2A) knockout mice did not display tolerance to chronic morphine under the present paradigm. Both genotypes display locomotor sensitization to morphine, implying a lack of a role for the A(2A) receptor in the drug-induced plasticity necessary for the development or expression of sensitization. Collectively, these data suggest a differential role for adenosine A(2A) receptors in opiate reinforcement compared to opiate-seeking.

Adenosine receptors and the central nervous system

The adenosine receptors (ARs) in the nervous system act as a kind of "go-between" to regulate the release of neurotransmitters (this includes all known neurotransmitters) and the action of neuromodulators (e.g., neuropeptides, neurotrophic factors). Receptor-receptor interactions and AR-transporter interplay occur as part of the adenosine's attempt to control synaptic transmission. A(2A)ARs are more abundant in the striatum and A(1)ARs in the hippocampus, but both receptors interfere with the efficiency and plasticity-regulated synaptic transmission in most brain areas. The omnipresence of adenosine and A(2A) and A(1) ARs in all nervous system cells (neurons and glia), together with the intensive release of adenosine following insults, makes adenosine a kind of "maestro" of the tripartite synapse in the homeostatic coordination of the brain function. Under physiological conditions, both A(2A) and A(1) ARs play an important role in sleep and arousal, cognition, memory and learning, whereas under pathological conditions (e.g., Parkinson's disease, Alzheimer's disease, amyotrophic lateral sclerosis, stroke, epilepsy, drug addiction, pain, schizophrenia, depression), ARs operate a time/circumstance window where in some circumstances A(1)AR agonists may predominate as early neuroprotectors, and in other circumstances A(2A)AR antagonists may alter the outcomes of some of the pathological deficiencies. In some circumstances, and depending on the therapeutic window, the use of A(2A)AR agonists may be initially beneficial; however, at later time points, the use of A(2A)AR antagonists proved beneficial in several pathologies. Since selective ligands for A(1) and A(2A) ARs are now entering clinical trials, the time has come to determine the role of these receptors in neurological and psychiatric diseases and identify therapies that will alter the outcomes of these diseases, therefore providing a hopeful future for the patients who suffer from these diseases.

Morphine dependence protects rat kidney against ischaemia-reperfusion injury

Ischaemic preconditioning (IPC) protects the heart and kidneys against ischaemia-reperfusion (I/R) injury. It has been shown that opioid receptor activation can mimic cardiac IPC. In a kidney model of I/R, a single dose of morphine failed to mimic IPC. The aim of the present study was to determine the role of chronic morphine (dependence) in protection against renal I/R injury. Male Wistar rats were treated with increasing doses of morphine (20-30 mg/kg per day, s.c., for 5 days) to develop morphine dependence (MD). Three weeks before the I/R procedure, the right kidney was removed. Ischaemia-reperfusion injury was induced by clamping the left renal artery for 45 min, followed by 24 h reperfusion. Some MD rats were pretreated with naloxone (5 mg/kg, s.c.). Twenty-four hours later, creatinine and sodium concentrations were measured in serum and urine, then creatinine clearance (CCr) and the fractional excretion of sodium (FE(Na)) were calculated. Blood urea nitrogen (BUN) was measured only in serum samples. Kidneys were also assessed histologically for evidence of tissue injury. In the present study, MD decreased tissue injury (histological score), serum creatinine and BUN levels, increased CCr and decreased FE(Na) after I/R. Pretreatment with naloxone attenuated the protective effects of MD. Morphine dependence did not have any significant effect on urine volume. In conclusion, it seems that morphine dependence protects the kidney against I/R injury via opioid receptor-dependent pathways. Further studies are required to clearly determine the mechanisms involved.

Theophylline inhibits tolerance and sensitization induced by morphine: a conditioned place preference paradigm study in female mice

The effect of theophylline on reward properties of morphine was examined in the present study. A biased conditioned place preference paradigm was used to study the effects of theophylline on the development of conditioned place preference by morphine in sensitized and tolerant female mice. Subcutaneous injection of morphine (0.5-10 mg/kg) induced conditioned place preference in mice, while intraperitoneal administration of theophylline (2.5-100 mg/kg) did not induce conditioned place preference or conditioned place aversion. Theophylline (2.5-100 mg/kg) in combination with morphine (5 mg/kg), during conditioning sessions, decreased the acquisition of morphine conditioned place preference dose independently. Administration of theophylline (2.5-100 mg/kg) before testing also caused a significant reduction of the expression of morphine-induced conditioned place preference in a dose-independent manner. Administration of morphine (12.5, 25 or 50 mg/kg) daily, for 3 days, produced tolerance to conditioned place preference induced by the drug (5 mg/kg). Administration of theophylline (2.5 and 10 mg/kg) 1 h before morphine (12.5, 25 mg/kg), during development of tolerance, abolished morphine tolerance. A higher dose of theophylline (100 mg/kg), however, did not alter morphine tolerance. In addition, theophylline (2.5, 10 and 100 mg/kg) failed to reduce tolerance to a higher dose of morphine (50 mg/kg). Daily administration of morphine (5 mg/kg) for 3 days followed by a 5-day interval caused sensitization to morphine place conditioning. When theophylline was administered (2.5, 10 and 100 mg/kg) 1 h before morphine (5 mg/kg), during development of sensitization, inhibition of morphine-induced sensitization was demonstrated. The effect of theophylline was dose independent. It is concluded that while theophylline has no effect by itself, it reduced both the acquisition and expression of morphine conditioned place preference. In addition, theophylline reduced the acquisition of morphine conditioned place preference in morphine-sensitized and morphine-tolerant mice.

Interaction of adenosine and naloxone on regional cerebral blood flow in morphine-dependent rats

The present research aimed at investigating the opioid-adenosine interaction on regional cerebral blood flow (rCBF). Therefore rCBF in the sensory cortex of morphine-naive and -dependent rats was measured using the laser-Doppler flowmetry technique. The results showed that adenosine (10(-5), 10(-4), 10(-3) M) significantly increased rCBF in morphine-dependent rats (MDR) (P < 0.01). This effect was inhibited by theophylline (5 x 10(-5) M). Also systemic naloxone (0.5, 1.5 and 3 mg/kg, s.c.) significantly increased rCBF in MDR and it was accompanied by elevated blood pressure and heart rate. Local adenosine (10(-4) M) significantly augmented naloxone (0.5 mg/kg)-induced increase in rCBF of MDR but had no significant effect on naloxone's (1.5 and 3 mg/kg) increasing effect on rCBF. Theophylline also has no effect on naloxone increasing effect on rCBF. These data suggest that adenosine receptors responsiveness increase in sensory cortex of MDR. Naloxone also highly increased rCBF of MDR that probably not interfere with adenosine receptors. Also, it seems that adenosine acts as a modulator in rCBF regulation of morphine-dependent and morphine withdrawal rats.

Opioid-induced preconditioning: recent advances and future perspectives

Opioids, named by Acheson for compounds with morphine-like actions despite chemically distinct structures, have received much research interest, particularly for their central nervous system (CNS) actions involved in pain management, resulting in thousands of scientific papers focusing on their effects on the CNS and other organ systems. A more recent area which may have great clinical importance concerns the role of opioids, either endogenous or exogenous compounds, in limiting the pathogenesis of ischemia-reperfusion injury in heart and brain. The role of endogenous opioids in hibernation provides tantalizing evidence for the protective potential of opioids against ischemia or hypoxia. Mammalian hibernation, a distinct energy-conserving state, is associated with depletion of energy stores, intracellular acidosis and hypoxia, similar to those which occur during ischemia. However, despite the potentially detrimental cellular state induced with hibernation, the myocardium remains resilient for many months. What accounts for the hypoxia-tolerant state is of great interest. During hibernation, circulating levels of opioid peptides are increased dramatically, and indeed, are considered a "trigger" of hibernation. Furthermore, administration of opioid antagonists can effectively reverse hibernation in mammals. Therefore, it is not surprising that activation of opioid receptors has been demonstrated to preserve cellular status following a hypoxic insult, such as ischemia-reperfusion in many model systems including the intestine [Zhang, Y., Wu, Y.X., Hao, Y.B., Dun, Y. Yang, S.P., 2001. Role of endogenous opioid peptides in protection of ischemic preconditioning in rat small intestine. Life Sci. 68, 1013-1019], skeletal muscle [Addison, P.D., Neligan, P.C., Ashrafpour, H., Khan, A., Zhong, A., Moses, M., Forrest, C.R., Pang, C.Y., 2003. Noninvasive remote ischemic preconditioning for global protection of skeletal muscle against infarction. Am. J. Physiol. Heart Circ. Physiol. 285, H1435-H1443], the CNS [Borlongan, C.V., Wang, Y., Su, T.P., 2005. Delta opioid peptide (d-ala 2, d-leu 5) enkephalin: linking hiberation and neuroprotection. Front Biosci. 9, 3392-3398] and the myocardium [Romano, M.A., Seymour, E.M., Berry, J.A., McNish, R.A., Bolling, S.F., 2004. Relative contribution of endogenous opioids to myocardial ischemic tolerance. J Surg Res. 118, 32-37; Peart, J.N., Gross, G.J., 2004a. Exogenous activation of delta- and kappa-opioid receptors affords cardioprotection in isolated murine heart. Basic Res Cardiol. 99(1), 29-37]. For the purpose of this review, we will focus primarily on the protective effects of opioids against post-reperfusion myocardial stunning and infarction.

Chronic exposure to morphine produces a marked cardioprotective phenotype in aged mouse hearts

Aging is often associated with decreased myocardial ischemic tolerance. We recently reported that chronic preconditioning produced by continuous exposure to morphine affords a profound cardioprotective phenotype in young mice. In this study, we determined if chronic exposure to morphine retained its ability to precondition the myocardium in the young or aged heart. Young (10-14 weeks) or aged (24-26 months) C57/BL6 mice were untreated, administered morphine acutely (30 microM), or implanted with a morphine pellet (75 mg) for 5 days prior to heart isolation and perfusion. Following equilibration, perfused hearts were subjected to 25 min ischemia and 45 min reperfusion. Untreated hearts from both young and aged mice displayed marked contractile dysfunction and LDH release following reperfusion. Acute infusion of morphine improved recovery of end-diastolic pressure and developed pressure in young (P < 0.05 vs. untreated) but not senescent hearts. Hearts from mice exposed to morphine for 5 days displayed a further improvement in post-ischemic contractile function (P < 0.05 vs. acute treatment), and a marked reduction in post-ischemic LDH efflux (P < 0.05 vs. untreated) in both young and senescent hearts. These data demonstrate that aged hearts maintain the ability to be preconditioned by chronic exposure to morphine in the absence of acute protection.

Morphine-Tolerant Mice Exhibit a Profound and Persistent Cardioprotective Phenotype

Background— Morphine and other opioids continue to be used as the major treatment for acute pain both before and after surgery. In this regard, much research has focused on the mechanisms of morphine tolerance and dependence in the central nervous system; however, few studies have examined the effect of morphine on peripheral organs, such as the heart, in morphine-tolerant animals. Here, we examine the effect of tolerance to the analgesic effect of morphine on ischemic tolerance in mice after prolonged morphine exposure and withdrawal.

Methods and Results— Male C57/BL6 mice were implanted subcutaneously with either placebo or morphine pellets (25 or 75 mg). After prolonged exposure to and/or withdrawal from morphine or placebo, the hearts were excised and subjected to 25 minutes of ischemia and 45 minutes of reperfusion. Morphine-tolerant mice exhibited a markedly improved functional recovery compared with placebo and mice subjected to acute morphine. Lactate dehydrogenase release was also significantly reduced. The protection observed was equieffective 48 hours after withdrawal of pellet, whereas the onset of protection preceded analgesic tolerance.

Conclusions— These data demonstrate that chronic exposure to morphine unexpectedly results in a profound and persistent cardioprotective phenotype.

β-Phenylethylamines and the isoquinoline alkaloids

This review covers beta-phenylethylamines and isoquinoline alkaloids derived from them, including further products of oxidation. condensation with formaldehyde and rearrangement, some of which do not contain an isoquinoline system, together with naphthylisoquinoline alkaloids, which have a different biogenetic origin. The occurrence of the alkaloids, with the structures of new bases, together with their reactions, syntheses and biological activities are reported. The literature from July 2002 to June 2003 is reviewed, with 568 references cited.

Intermittent morphine treatment induces a long-lasting increase in cholinergic modulation of GABAergic synapses in nucleus accumbens of adult rats

Repeated exposure to drugs of abuse causes persistent behavioral sensitization and associated adaptations of striatal neurotransmission, which is thought to play an important role in certain aspects of drug addiction. Microdialysis and neurochemical studies suggest that intermittent morphine treatment may lead to a long-term increase in both ACh and dopaminergic neurotransmission in the nucleus accumbens (NAc). This implies that both cholinergic modulation of GABA synapses and their sensitivity to dopaminergic transmission might be changed, ultimately leading to a modified NAc output. Here we investigate to what extent cholinergic modulation and sensitivity to amphetamine, causing endogenous dopamine efflux, of GABAergic transmission in the nucleus accumbens are affected 3 weeks after a period of daily morphine injections in adult rats. To this end, we recorded medium spiny neurons using whole cell voltage clamp and monitored the frequency and amplitude of spontaneous GABAergic synaptic currents. We observed that the effect of nicotine on the frequency of spontaneous inhibitory postsynaptic currents (sIPSCs) was suppressed in rats pretreated with morphine, whereas the effects of mecamylamine and tetrodotoxin (TTX) were increased. These results indicate that the probability of GABA release was increased and that this effect resulted from an upregulation of the endogenous activation of presynaptic nicotinic receptors. In addition, we observed an increased sensitivity to in vitro application of amphetamine. This suggests that the long-term increase in dopaminergic transmission caused by the morphine treatment affects GABA synapses in the NAc. Hence, there may be two parallel synaptic mechanisms by which drugs of abuse may affect processing and integration of NAc inputs.

Addicting drugs utilize a synergistic molecular mechanism in common requiring adenosine and Gi-beta gamma dimers

The mesolimbic dopamine system and cAMP-dependent/protein kinase A (PKA) pathways are strongly implicated in addictive behaviors. Here we determine the role of dopamine D2 receptors (D2) in PKA signaling responses to delta-opioid (DOR) and cannabinoid (CB1) receptors. We find in NG108-15/D2 cells and in cultured primary neurons that a brief exposure to saturating concentrations of DOR and CB1 agonists increases cAMP, promotes PKA C alpha translocation and increases cAMP-dependent gene expression. Activation of PKA signaling is mediated by Gi-beta gamma dimers. Importantly, subthreshold concentrations of DOR or CB1 agonists with D2 agonists, which are without effect when added separately, together activate cAMP/PKA signaling synergistically. There is also synergy between DOR or CB1 with ethanol, another addicting agent. In all instances, synergy requires adenosine activation of adenosine A2 receptors and is mediated by beta gamma dimers. Synergy by this molecular mechanism appears to confer hypersensitivity to opioids and cannabinoids while simultaneously increasing the sensitivity of D2 signaling when receptors are expressed on the same cells. This mechanism may account, in part, for drug-induced activation of medium spiny neurons in the nucleus accumbens.

Antagonism of N-methyl-D-aspartate receptors reduces the vulnerability of the immune system to stress after chronic morphine

It has been shown that morphine-tolerant animals have an altered immunological sensitivity to stress. Although the glutamatergic system has been implicated in the neuroadaptive process underlying this tolerant state, its potential role in development of the altered immunological sensitivity consequent to chronic morphine treatment is not known. To determine this, a morphine-tolerant state was induced by 10-day administration of an escalating dose of morphine from 10 to 40 mg/kg (s.c., b.i.d.), and lymphocyte proliferative response to a T-cell mitogen was measured. Morphine challenge (10 mg/kg s.c.) after days of treatment was gradually less immunosuppressive, and this tolerance progression was delayed by concurrent administration of the N-methyl-D-aspartate (NMDA) receptor antagonist (-)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate (MK-801) (0.1 mg/kg s.c.) with chronic morphine. The effect was independent of glucocorticoid level changes and was not a result of an acute interaction of the drugs or the prolonged presence of the antagonist alone. Subsequent to chronic treatment, animals were subjected to opioid withdrawal and water stress. Both stressors induced 50% immunosuppression in morphine-tolerant animals compared with saline-treated controls. Increased immunological sensitivity to these stressors was attenuated when MK-801 was administered with chronic morphine as demonstrated by an accelerated recovery rate and lack of immunosuppression from opioid withdrawal and water stress, respectively. Together, these findings provide the first evidence that the neuroadapted state of the immune response after chronic morphine can be modified by NMDA receptor antagonism, as illustrated by a temporal deceleration of the development of immunological tolerance during chronic treatment that is associated with an attenuation of the immunological vulnerability of morphine-tolerant animals to stress.

Modulation of GABA release during morphine withdrawal in midbrain neurons in vitro

Chronic treatment with opioids induces adaptations in neurons leading to tolerance and dependence. Studies have implicated the midbrain periaqueductal gray (PAG) in the expression of many signs of withdrawal. Patch-clamp recording techniques were used to examine whether augmentation of adenylyl cyclase signalling produces hyperexcitation in GABAergic nerve terminals within the mouse PAG. Both the rate of mIPSCs and the amplitude of evoked IPSCs during naloxone-precipitated withdrawal was profoundly enhanced in chronically morphine treated mice, compared to vehicle treated controls, in the presence but not the absence an adenosine A(1) receptor antagonist DPCPX. Enhanced GABAergic transmission in the presence of DPCPX was abolished by blocking protein kinase A. Inhibitors of cAMP transport, phosphodiesterase and nucleotide transport mimicked the effect of DPCPX. Coupling efficacy of micro-receptors to presynaptic inhibition of GABA release was increased in dependent mice in the presence of DPCPX. The increased coupling efficacy was abolished by blocking protein kinase A, which unmasked an underlying micro-receptor tolerance. These findings indicate that enhanced adenylyl cyclase signalling following chronic morphine treatment produces (1) GABAergic terminal hyperexcitability during withdrawal that is retarded by a concomitant increase in endogenous adenosine, and (2) enhanced micro-receptor coupling to presynaptic inhibition that overcomes an underlying tolerance.

Functional tolerance and blockade of long-term depression at synapses in the nucleus accumbens after chronic cannabinoid exposure

The rewarding properties of the psychoactive constituents of marijuana, termed "cannabinoids," may reflect actions on synaptic transmission in the nucleus accumbens (NAc). Furthermore, long-term changes in these synapses may support the addictive process. Excitatory and inhibitory synapses are acutely inhibited by cannabinoids in the NAc, and endogenous cannabinoids (endocannabinoids) play a critical role in the expression of long-term depression (LTD) of excitatory cortical afferents in this structure. Because humans often use marijuana for prolonged periods, we examined the impact of long-term cannabinoid exposure on synaptic processes in an animal model. Electrophysiological recordings in rat brain slices containing the NAc were performed after chronic exposure to vehicle solution, Delta9-tetrahydrocannabinol (THC), or the cannabinoid agonist R(+)-[2,3-dihydro-5-methyl-3-[(morpholinyl)methyl]pyrrolo[1,2,3-de]-1,4-benzoxazin-6-yl]-(1-naphthalenyl)methanone mesylate (WIN55,212-2). Extracellular glutamatergic postsynaptic potentials and whole-cell GABAergic IPSCs were concentration-dependently inhibited by WIN55,212-2 in slices from naive or vehicle-treated animals. However, the sensitivity to WIN55,212-2 was diminished in chronic agonist-treated animals. In addition, cross-tolerance to the inhibitory effect of the mu-opioid agonist Tyr-D-Ala2, N-CH3-Phe4,Gly-ol-enkephalin was observed. Endocannabinoid-mediated LTD was initiated via electrical stimulation (5 min, 10 Hz) of glutamatergic afferents to the NAc and was completely blocked by the cannabinoid receptor antagonist SR141716A [N-piperidino-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methylpyrazole-3-carboxamide] in vehicle-treated animals. LTD was not observed in brain slices from rats chronically treated with Delta9-THC or WIN55,212-2. These data demonstrate that long-term exposure to the active ingredient of marijuana blocks synaptic plasticity in the NAc and reduces the sensitivity of GABAergic and glutamatergic synapses to both cannabinoids and opioids.

Endogenous opiates and behavior: 2002

This paper is the twenty-fifth consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over a quarter-century of research. It summarizes papers published during 2002 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

Dopamine and adenosine mediate substance P-induced depression of evoked IPSCs in the rat nucleus accumbens in vitro

The major projection cells of the nucleus accumbens (NAc) are under a strong inhibitory influence from GABAergic afferents and depend on afferent excitation to produce their output. We have earlier reported that substance P (SP), a peptide which is colocalized with GABA in these neurons, depresses excitatory synaptic transmission in this nucleus (Kombian, S.B., Ananthalakshmi, K.V.V., Parvathy, S.S. & Matowe, W.C. (2003) J. Neurophysiol., 89, 728-738). In order to better understand the role of this peptide in the synaptic physiology of the NAc, it is important to determine its effects on inhibitory synaptic responses. Using whole-cell recording in rat forebrain slices, we show here that SP also depresses evoked inhibitory postsynaptic currents (IPSCs) in the NAc via intermediate neuromodulators. SP caused a partially reversible, dose-dependent decrease in evoked IPSC amplitude. This effect was present without measurable changes in the holding current, input resistance of recorded cells or decay rate (tau) of IPSCs. It was mimicked by a neurokinin-1 (NK1) receptor-selective agonist, [Sar9, Met (O2)11]-SP, and blocked by an NK1 receptor-selective antagonist, L 732 138. The SP-induced IPSC depression was prevented by SCH23390, a dopamine D1-like receptor antagonist and by 8-cyclopentyltheophylline, an adenosine A1 receptor blocker. Furthermore, the SP effect was also markedly attenuated by exogenous adenosine, dipyridamole, rolipram and barium. These data show that SP, acting on NK1 receptors, depresses inhibitory synaptic transmission indirectly by enhancing extracellular dopamine and adenosine levels. SP therefore acts in the NAc to modulate both excitatory and inhibitory afferent inputs using the same mechanism(s).

Metabotropic glutamate receptor 2/3-dependent long-term depression in the nucleus accumbens is blocked in morphine withdrawn mice

The nucleus accumbens (NAc) plays a crucial role in addiction. We have recently shown that activation of presynaptic metabotropic glutamate 2/3 receptors (mGlu2/3) induces long-term depression (LTD) at glutamatergic synapses in the mouse nucleus accumbens (NAc) through the long lasting inhibition of P/Q-type Ca2+ channels and the cAMP/protein kinase A (PKA) pathway. Because presynaptic mGlu2/3 functions are augmented in the ventral tegmental area of morphine-withdrawn rats, we have evaluated the consequences of opiate treatment on mGlu2/3 LTD at prelimbic NAc glutamatergic synapses. Here we report that mGlu2/3 LTD is abolished after 1 week of withdrawal from chronic morphine treatment; in the morphine-withdrawn group LTD measured 5.99 +/- 4.84% (P < 0.05) compared with 21.13 +/- 5.42% in the sham group. In contrast, chronic morphine treatment did not alter the mechanisms normally underlying mGlu2/3 LTD, such as the cAMP/PKA pathway or P/Q-type Ca2+ channels. This study shows that one long-term consequence of morphine treatment is an alteration of synaptic plasticity at glutamatergic synapses in the NAc. Considering that mGlu2/3 agonists (e.g. LY-354740 used in the present study to induce LTD) reduce behavioural symptoms of morphine withdrawal, these findings could be important in the understanding of the cellular events underlying the dependence-inducing properties of opiates.

Adenosine receptors in the nervous system: pathophysiological implications

Adenosine is a ubiquitous homeostatic substance released from most cells, including neurones and glia. Once in the extracellular space, adenosine modifies cell functioning by operating G-protein-coupled receptors (GPCR; A(1), A(2A), A(2B), A(3)) that can inhibit (A(1)) or enhance (A(2)) neuronal communication. Interactions between adenosine receptors and other G-protein-coupled receptors, ionotropic receptors and receptors for neurotrophins also occur, and this might contribute to a fine-tuning of neuronal function. Manipulations of adenosine receptors influence sleep and arousal, cognition and memory, neuronal damage and degeneration, as well as neuronal maturation. These actions might have therapeutic implications for neurodegenerative diseases such as Parkinson's disease, Alzheimer's disease, as well as for other neurological situations such as epilepsy, idiopathic pain or even drug addition. Peripheral side effects associated with adenosine receptor agonists limit their usefulness in therapeutics; in contrast, adenosine receptor antagonists appear to have less side effects as it is the case of the well-known non-selective antagonists theophylline (present in tea) or caffeine (abundant in coffee and tea), and their emerging beneficial actions in Parkinson's disease and Alzheimer's disease are encouraging. A(1) receptor antagonism may also be useful to enhance cognition and facilitate arousal, as well as in the periphery when deficits of neurotransmitter release occur (e.g. myasthenic syndromes). Enhancement of extracellular adenosine levels through drugs that influence its metabolism might prove useful approaches in situations such as neuropathic pain, where enhanced activation of inhibitory adenosine A(1) receptors is beneficial. One might then consider adenosine as a fine-tuning modulator of neuronal activity, which via subtle effects causes harmonic actions on neuronal activity. Whenever this homeostasis is disrupted, pathology may be installed and selective receptor antagonism or agonism required.