The role of alpha-adrenoceptor mechanism(s) in morphine-induced conditioned place preference in female mice

The α1 adrenoceptor antagonist prazosin potentiates morphine induced conditioned place preference in rats

The norepinephrine (NE) system is involved in pathways that regulate morphine addiction. Here, we investigated the role of α1 adrenoceptor in the ventrolateral orbital cortex (VLO) of rats with repeated morphine treatment and underlying molecular mechanisms. The rewarding properties of morphine were assessed by the conditioned place preference (CPP) paradigm. Prazosin, an α1 adrenoceptor antagonist, was microinjected into the VLO. The expression of α1 adrenoceptor, p-CaMKII/CaMKII, CRTC1, BDNF and PSD95 in the VLO were determined by immunohistochemistry or western blotting. Neurotransmitter NE in the VLO and inflammatory factors in serum were detected separately through high-performance liquid chromatography and enzyme-linked immunosorbent assay. Our experimental results showed that repeated morphine administration induced stable CPP and prazosin promoted the morphine-induced CPP. Microinjection of prazosin in the VLO not only blocked the activity of α1 adrenoceptor, decreased CaMKII phosphorylation and CRTC1, which eventually resulted in a regression of synaptic plasticity-related proteins, but also was accompanied by significantly decreasing of NE in the VLO and increasing of inflammatory cytokines in peripheral blood. These findings suggested that prazosin potentiates the addictive effects of morphine. The effect of increased CPP through reducing α1 adrenoceptor and NE was associated with the CaMKII-CRTC1 pathway and synaptic plasticity-related proteins in the VLO and inflammatory cytokines in the peripheral blood. The NE system may therefore be an underlying therapeutic target in morphine addiction. Additionally, we believe that the clinical use of prazosin in hypertensive patients with morphine abuse may be a potential risk because of its reinforcing effect on addiction.

Alpha1-adrenergic receptor blockade in the ventral tegmental area attenuates acquisition of cocaine-induced pavlovian associative learning

Activity of the alpha₁-adrenergic receptor (α₁-AR) in the ventral tegmental area (VTA) modulates dopaminergic activity, implying its modulatory role in the behavioral functions of the dopamine (DA) system. Indeed, intra-VTA α₁-AR blockade attenuates conditioned stimulus dependent behaviors such as drug seeking responses signifying a role of the noradrenergic signaling in the VTA in conditioned behaviors. Importantly, the role of the VTA α₁-AR activity in Pavlovian associative learning with positive outcomes remains unknown. Here, we aimed to examine how intra-VTA α₁-AR blockade affects acquisition of cocaine-induced Pavlovian associative learning in the conditioned place preference (CPP) paradigm. The impact of α₁-AR blockade on cocaine-reinforced operant responding and cocaine-evoked ultrasonic vocalizations (USVs) was also studied. In addition, both α₁-AR immunoreactivity in the VTA and its role in phasic DA release in the nucleus accumbens (NAc) were assessed. We demonstrated cellular localization of α₁-AR expression in the VTA, providing a neuroanatomical substrate for the α₁-AR mechanism. We showed that prazosin (α₁-AR selective antagonist; 1 μg/0.5 μl) microinfusion attenuated electrically evoked DA transients in the NAc and dose-dependently (0.1-1 μg/0.5 μl) prevented the acquisition of cocaine CPP but did not affect cocaine-reinforced operant responding nor cocaine-induced positive affective state (measured as USVs). We propose that the VTA α₁-AR signaling is necessary for the acquisition of Pavlovian associative learning but does not encode hedonic value. Thus, α₁-AR signaling in the VTA might underlie salience encoding of environmental stimuli and reflect an ability of alerting/orienting functions, originating from bottom-up information processing to guide behaviors.

Sex Differences in Animal Models of Opioid Reward

PURPOSE OF REVIEW

This review aims to discuss sex differences observed in preclinical rodent models of opioid reward.

RECENT FINDINGS

Utilizing a variety of methodological approaches and drug regimens, no clear consensus has emerged regarding the effects of opiates between males and females. This is quite dissimilar to work examining psychostimulants, where female animals reliably exhibit stronger behavioral responses.

SUMMARY

With opioid research quickly expanding to determine the neural underpinnings of opioid addiction, further research is essential to determine the conditions wherein sex differences may occur and how they may relate to the human condition.

GABAB receptors within the central nucleus of amygdala may involve in the morphine-induced incentive tolerance in female rats

Objective(s):

Central nucleus of amygdala (CeA) is the most important region for morphine-induced reward, and GABAergic system plays an important role on morphine reinforcement. The influence of CeA administration of GABA_B receptor agonist and antagonist on the expression and acquisition of morphine-induced incentive tolerance using conditioned place preference (CPP) paradigm was investigated in the present study. Our purpose was to evaluate the role of CeA GABA_B receptors in morphine tolerance.

Materials and Methods:

Seven days after surgery and cannulation, the experiments were begun. Subcutaneous (SC) injections of morphine induced CPP. Administration of one daily dose of morphine (12.5 mg/kg) for 3 days in order to develop tolerance to the drug reduced the conditioning induced by morphine (7.5 mg/kg, SC). GABA_B receptor agonist, baclofen (1.5, 6 and 12 µg/rat) or GABA_B receptor antagonist, CGP35348 (1.5, 6 and 12 µg/rat) were injected into the CeA 5 min before the experiments in the test day (expression of tolerance) or 5 min before each injection of morphine (12.5 mg/kg) (acquisition of tolerance).

Results:

It was shown that injections of baclofen (1.5 and 12 µg/rat) reduced acquisition, whereas the dose of 6 µg/rat of the drug exacerbated the acquisition of morphine tolerance. Baclofen at all doses significantly increased the expression of tolerance to morphine. Administration of CGP35348 (1.5, 6 and 12 µg/rat) reduced the acquisition and expression of morphine tolerance.

Conclusion:

These results confirmed the importance of GABA_B receptors with in the CeA in morphine tolerance in female rats.

Effects of paclitaxel on mechanical sensitivity and morphine reward in male and female C57Bl6 mice

This study evaluated the hypothesis that a paclitaxel treatment regimen sufficient to produce mechanical allodynia would alter sensitivities of male and female mice to the conditioned rewarding and reinforcing effects of morphine. Saline or paclitaxel were administered on Days 1, 3, 5, and 7 in male and female C57Bl/6 mice to induce morphine-reversible mechanical allodynia as measured by the Von Frey filament test. Paclitaxel treatment did not change sensitivity to morphine conditioned place preference (CPP) relative to saline treatment in either male or female mice. Morphine produced peak self-administration under a fixed ratio-1 (FR1) schedule of reinforcement for 0.03 mg/kg morphine per infusion in female mice and 0.1 mg/kg morphine per infusion in male mice. During the progressive ratio experiments, saline treatment in male mice decreased the number of morphine infusions for 12 days whereas the paclitaxel-treated male mice maintained responding for morphine similar to baseline levels during the same time period. However, paclitaxel did not have an overall effect on the reinforcing efficacy of morphine assessed over a limited dose range during the course of the repeated self-administration. These results suggest that the reward-related behavioral effects of morphine are overall not robustly altered by the presence of paclitaxel treatment under the current dosing regimen, with the exception of maintaining a small yet significant higher baseline than saline treatment during the development of allodynia in male mice. (PsycINFO Database Record

Norepinephrine at the nexus of arousal, motivation and relapse

Arousal plays a critical role in cognitive, affective and motivational processes. Consistent with this, the dysregulation of arousal-related neural systems is implicated in a variety of psychiatric disorders, including addiction. Noradrenergic systems exert potent arousal-enhancing actions that involve signaling at α1- and β-noradrenergic receptors within a distributed network of subcortical regions. The majority of research into noradrenergic modulation of arousal has focused on the nucleus locus coeruleus. Nevertheless, anatomical studies demonstrate that multiple noradrenergic nuclei innervate subcortical arousal-related regions, providing a substrate for differential regulation of arousal across these distinct noradrenergic nuclei. The arousal-promoting actions of psychostimulants and other drugs of abuse contribute to their widespread abuse. Moreover, relapse can be triggered by a variety of arousal-promoting events, including stress and re-exposure to drugs of abuse. Evidence has long-indicated that norepinephrine plays an important role in relapse. Recent observations suggest that noradrenergic signaling elicits affectively-neutral arousal that is sufficient to reinstate drug seeking. Collectively, these observations indicate that norepinephrine plays a key role in the interaction between arousal, motivation, and relapse. This article is part of a Special Issue entitled SI: Noradrenergic System.

Regulation of neurological and neuropsychiatric phenotypes by locus coeruleus-derived galanin

Decades of research confirm that noradrenergic locus coeruleus (LC) neurons are essential for arousal, attention, motivation, and stress responses. While most studies on LC transmission focused unsurprisingly on norepinephrine (NE), adrenergic signaling cannot account for all the consequences of LC activation. Galanin coexists with NE in the vast majority of LC neurons, yet the precise function of this neuropeptide has proved to be surprisingly elusive given our solid understanding of the LC system. To elucidate the contribution of galanin to LC physiology, here we briefly summarize the nature of stimuli that drive LC activity from a neuroanatomical perspective. We go on to describe the LC pathways in which galanin most likely exerts its effects on behavior, with a focus on addiction, depression, epilepsy, stress, and Alzheimer׳s disease. We propose a model in which LC-derived galanin has two distinct functions: as a neuromodulator, primarily acting via the galanin 1 receptor (GAL1), and as a trophic factor, primarily acting via galanin receptor 2 (GAL2). Finally, we discuss how the recent advances in neuropeptide detection, optogenetics and chemical genetics, and galanin receptor pharmacology can be harnessed to identify the roles of LC-derived galanin definitively. This article is part of a Special Issue entitled SI: Noradrenergic System.

Adrenaline rush: the role of adrenergic receptors in stimulant-induced behaviors

Psychostimulants, such as cocaine and amphetamines, act primarily through the monoamine neurotransmitters dopamine (DA), norepinephrine, and serotonin. Although stimulant addiction research has largely focused on DA, medication development efforts targeting the dopaminergic system have thus far been unsuccessful, leading to alternative strategies aimed at abating stimulant abuse. Noradrenergic compounds have shown promise in altering the behavioral effects of stimulants in rodents, nonhuman primates, and humans. In this review, we discuss the contribution of each adrenergic receptor (AR) subtype (α1, α2, and β) to five stimulant-induced behaviors relevant to addiction: locomotor activity, conditioned place preference, anxiety, discrimination, and self-administration. AR manipulation has diverse effects on these behaviors; each subtype profoundly influences outcomes in some paradigms but is inconsequential in others. The functional neuroanatomy and intracellular signaling mechanisms underlying the impact of AR activation/blockade on these behaviors remain largely unknown, presenting a new frontier for research on psychostimulant-AR interactions.

Effect of yohimbine stress on reacquisition of oxycodone seeking in rats

RATIONALE

Stress, a powerful precipitant of drug seeking during abstinence, may also accelerate the return to pathological patterns of intake after initial instances of drug reuse.

OBJECTIVE

To explore the effect of stress on a learning process underlying relapse, this study assessed the effect of yohimbine on reacquisition of oxycodone seeking.

METHODS

One hundred thirty-two male Sprague-Dawley rats underwent place conditioning with oxycodone (2 mg/kg, SC; ×6 days), extinction (vehicle × 6 days), and reconditioning with 0, 0.25, 2, or 5 mg/kg oxycodone (2 days). Yohimbine (0, 2.5, or 5 mg/kg, IP) was administered 30 min prior to reconditioning.

RESULTS

Pretreatment with 2.5 mg/kg yohimbine increased, while 5 mg/kg yohimbine decreased, reacquisition of oxycodone-induced place preference. A follow-up study (n = 30) further indicated that the effect of yohimbine was specific to reacquisition.

CONCLUSION

The observation that yohimbine can enhance reacquisition of oxycodone seeking supports the hypothesis that stress can facilitate learning processes involved in the unfolding of relapse.

Study of adrenergic, imidazoline, and endothelin receptors in clonidine-, morphine-, and oxycodone-induced changes in rat body temperature

OBJECTIVES

The potentiation of morphine or oxycodone analgesia by endothelin-A (ET(A)) receptor antagonists and imidazoline/α(2)-adrenergic agonists is well documented. However, the effect of morphine or oxycodone in combination with an ET(A) receptor antagonist or an imidazoline/α(2) adrenergic agonist on body temperature is not known. The present study was carried out to study the role of ET(A) and imidazoline/α(2) adrenergic receptors in body temperature effects of morphine, oxycodone, and clonidine in rats.

METHODS

Body temperature was determined in male Sprague-Dawley rats treated with morphine, oxycodone, or clonidine. Yohimbine, idazoxan, and BMS182874 were used to determine the involvement of α(2)-adrenergic, imidazoline, and ET(A) receptors, respectively.

KEY FINDINGS

Morphine and oxycodone produced hyperthermia which was not affected by α(2)-adrenergic antagonist yohimbine, imidazoline/α(2)-adrenergic antagonist idazoxan, or ET(A) receptor antagonist BMS182874. Clonidine alone produced hypothermia that was comparable to the hypothermia observed with clonidine plus morphine or oxycodone. The hypothermic effect of clonidine was blocked by idazoxan and yohimbine. The blockade by idazoxan was more pronounced compared to yohimbine. Clonidine hypothermia was not affected by BMS182874.

CONCLUSIONS

This is the first report demonstrating that ET(A) receptors do not influence morphine- and oxycodone- induced hyperthermia or clonidine-induced hypothermia. Imidazoline receptors and α(2)-adrenergic receptors are involved in clonidine-induced hypothermia, but not in morphine- and oxycodone-induced hyperthermia.

Determination of α(2)-adrenoceptor and imidazoline receptor involvement in augmentation of morphine and oxycodone analgesia by agmatine and BMS182874

Studies have demonstrated that clonidine (α(2)-adrenoceptor and imidazoline receptor agonist) and BMS182874 (endothelin ET(A) receptor antagonist) potentiate morphine and oxycodone analgesia. Agmatine, an endogenous clonidine-like substance, enhances morphine analgesia. However, its effect on oxycodone analgesia and its interaction with endothelin ET(A) receptor antagonists are not known. The present study was performed to determine the effect of agmatine on morphine and oxycodone analgesia and the involvement of α(2)-adrenoceptors, imidazoline receptors, opioid receptors, and endothelin receptors. Antinociception at various time intervals was determined by the tail-flick latency method in mice. Agmatine produced dose-dependent increase in tail-flick latency, while BMS182874 did not produce any change over the 360-min observation period. Agmatine significantly potentiated morphine as well as oxycodone analgesia which was not altered by BMS182874. BMS182874 pretreatment did not increase the analgesic effect produced by agmatine alone. Agmatine-induced potentiation of morphine and oxycodone analgesia was blocked by idazoxan (imidazoline receptor/α(2)-adrenoceptor antagonist) and yohimbine (α(2)-adrenoceptor antagonist). BMS182874-induced potentiation of morphine or oxycodone analgesia was not affected by yohimbine. However, idazoxan blocked BMS182874-induced potentiation of oxycodone but not morphine analgesia. This is the first report demonstrating that agmatine potentiates not only morphine but also oxycodone analgesia in mice. Potentiation of morphine and oxycodone analgesia by agmatine appears to involve α(2)-adrenoceptors, imidazoline receptors, and opioid receptors. In addition, imidazoline receptors may be involved in BMS182874-induced potentiation of oxycodone but not morphine analgesia. It is concluded that agmatine may be used as an adjuvant in opiate analgesia.

Enhanced tyrosine hydroxylase phosphorylation in the nucleus accumbens and nucleus tractus solitarius-A2 cell group after morphine-conditioned place preference

Although dopamine (DA) has been extensively implicated in the morphine-induced conditioned place preference (CPP; a measure of reward), noradrenaline (NA) and other systems may play a larger role than previously suspected. The mesolimbic DA system, comprised of projections from the ventral tegmental area (VTA) to the nucleus accumbens (NAc), receives noradrenergic innervations from the nucleus tractus solitaries (NTS)-A2 cell group and is modulated by NA. The purpose of the present study was to evaluate the turnover of DA and NA in the NAc and the site-specific phosphorylation of TH in the NAc, VTA, and NTS on the CPP mice conditioned by morphine. A dose-effect curve for morphine-induced CPP (0.5-8 mg/kg, s.c.) was obtained using 6-day conditioning sessions followed by a CPP test. TH phosphorylation was determined by quantitative blot immunolabeling and immunohistochemistry using phosphorylation state-specific antibodies; NA and DA turnover was evaluated by high-performance liquid chromatography. Morphine-induced CPP phosphorylates TH at serine (Ser)40 but not Ser31 in NAc, which is associated with an enhanced of DA and NA turnover. We also found that morphine-induced CPP increased levels of TH phosphorylated at Ser31 and Ser40 in the NTS. The present study demonstrates that morphine-induced CPP might stimulate TH activity and accelerate DA and NA turnover in the NAc via a mechanism involving phosphorylation of TH.

Involvement of alpha1-adrenoceptors in conditioned place preference supported by nicotine in rats

RATIONALE

The noradrenergic system might be a critical mediator of psychostimulants and opiates hedonic value.

OBJECTIVES

The objective of this study is to evaluate the involvement of alpha1-adrenoceptors (alpha1-ARs) in nicotine incentive learning.

MATERIALS AND METHODS

Rats, subjected to an unbiased conditioned place preference (CPP) procedure, received eight 30-min alternating nicotine (0.06 mg/kg) and saline pairings with distinct floor textures. The alpha1-AR antagonist, prazosin (0.125, 0.25, 0.5, or 1 mg/kg), was administered 30 min before nicotine pairings (acquisition) or one of the two 20-min test sessions conducted 24 h and 3 weeks after conditioning (expression).

RESULTS

Pre-pairing injections of prazosin (0.5-1 mg/kg) prevented the acquisition of nicotine-CPP. On pre-test administration, prazosin (0.5 mg/kg) abolished the short-term expression of nicotine-CPP; whereas, none of the tested doses impaired its long-term expression. During a drug-free 3-week test session, nicotine-CPP was also weakened in rats given prazosin (0.5 mg/kg) before the 24-h test, while nicotine-CPP was reduced neither in animals given prazosin immediately after the first test session nor in those not subjected to the 24-h test.

CONCLUSIONS

The activation of alpha1-ARs is one of the mechanisms that code for the incentive motivational value of nicotine. It participates also in the short-term, but not the long-term, control of behavior by nicotine-paired stimuli. The latter effect does not result from disruption by prazosin of either memory for the nicotine-cue association or reconsolidation processes at recall. Thus, differences exist in the neurobiological mechanisms that contribute to the incentive motivational value of nicotine and the short- and long-term "memory" of the incentive salience acquired by nicotine-paired cues.

Estrogen pretreatment modulates morphine-induced conditioned place preference in ovariectomized mice

Estrogen is known to modulate the neurotransmission in the brain. The main aim of this study was to investigate the effects of estrogen on the rewarding properties of morphine using conditioned place preference (CPP) paradigm in adult female mice. The possible rewarding effect of estrogen was also examined in ovariectomized mice. Following a 6-day conditioning procedure, sham operated animals showed a significant preference towards the side previously paired with a range of morphine doses (2, 5 and 10--but not 20--mg/kg, SC). However, ovariectomized mice showed decreased CPP compared to gonadally intact mice with a right shift in their morphine dose-response curve. These effects were reversed by chronic daily administration of estradiol benzoate (EB; 20 microg/kg, SC). Furthermore, in ovariectomized mice, EB per se was able to induce CPP. In conclusion, our findings indicate that estradiol has a facilitating effect on morphine reward while its deficiency increases the threshold dose of morphine to induce CPP.

Comparative study of alpha2-adrenoceptors in Fischer 344 and Lewis rats. Evidence for clonidine-induced place aversion

Fischer 344 (F344) and Lewis rat strains have been shown to exhibit different vulnerability to development or maintenance of opioid seeking behaviours probably due to differences in the endogenous opioid system. Since opioid and alpha(2)-adrenergic mechanisms closely interact in nociception and substance abuse, strain differences may be expected to affect alpha(2)-adrenoceptor-mediated events. The sensitivity of these two strains to alpha(2)-adrenoceptor-mediated antinociception has been reported to be markedly different. In this work we have further studied the function of alpha(2)-adrenoceptors in F344 and Lewis rats by means of several in vivo and in vitro procedures. Comparative studies of [(3)H]RX821002 and [(35)S]GTPgammaS binding revealed that alpha(2)-adrenoceptors could be slightly more responsive to agonist stimulation in the brain cortex of F344 rats, which is in agreement with previous antinociception studies. However, these differences were modest, not observed in the spinal cord and did not translate into functional differences concerning the effects of clonidine on vas deferens contractility and body temperature. Conditioning experiments showed that a moderate dose of clonidine, which is relevant in antinociceptive and opioid antiwithdrawal studies, induces a robust place aversion which is also equivalent in F344 and Lewis rats. This finding underlies the consistency of the effect and its independency of genetic differences between both rat strains. It seems therefore that the pharmacological properties of alpha(2)-adrenoceptors are similar in F344 and Lewis rats, and thus the previously reported differences in clonidine-induced antinociception could be attributed to other factors such as dissimilar endogenous function of specific noradrenergic pathways.

Lobeline attenuates progressive ratio breakpoint scores for intracranial self-stimulation in rats

The alkaloid lobeline inhibits the function of vesicular monoamine and dopamine transporters and diminishes the behavioral and neurochemical effects of nicotine and amphetamines. In the present study, we examined the interaction of systemic administration of lobeline on breakpoint scores on a progressive ratio (PR) schedule of intracranial self-stimulation (ICSS) of the medial forebrain bundle (MFB). Rats were run in two 30 min sessions, separated by a 10 min timeout period. At the end of the first session, each rat was injected with either 0, 0.5, 1.0 or 2.0 mg/kg (i.p.) lobeline. Positive controls known to suppress and to augment ICSS responding included the adrenergic antagonist prazosin (0, 0.5 and 2.0 mg/kg, i.p.) and the psychostimulant cocaine (0, 1.25, and 5.0 mg/kg, i.p.). Analyses of changes in average PR breakpoint scores between the 2 sessions revealed that lobeline significantly suppressed PR scores at doses of 0.5, 1.0 and 2.0 mg/kg, as did 0.5 mg/kg and 2.0 mg/kg prazosin. These changes are unlikely to reflect motoric effects of these drugs inasmuch as neither lobeline nor prazosin alter locomotion at these doses. In contrast, PR breakpoint scores were significantly increased at 5.0 mg/kg cocaine, a dose that is sufficient to elevate locomotion in the rat. These results are consistent with the view that lobeline modulates brain reinforcement processes.

Measuring reward with the conditioned place preference (CPP) paradigm: update of the last decade

Conditioned place preference (CPP) continues to be one of the most popular models to study the motivational effects of drugs and non-drug treatments in experimental animals. This is obvious from a steady year-to-year increase in the number of publications reporting the use this model. Since the compilation of the preceding review in 1998, more than 1000 new studies using place conditioning have been published, and the aim of the present review is to provide an overview of these recent publications. There are a number of trends and developments that are obvious in the literature of the last decade. First, as more and more knockout and transgenic animals become available, place conditioning is increasingly used to assess the motivational effects of drugs or non-drug rewards in genetically modified animals. Second, there is a still small but growing literature on the use of place conditioning to study the motivational aspects of pain, a field of pre-clinical research that has so far received little attention, because of the lack of appropriate animal models. Third, place conditioning continues to be widely used to study tolerance and sensitization to the rewarding effects of drugs induced by pre-treatment regimens. Fourth, extinction/reinstatement procedures in place conditioning are becoming increasingly popular. This interesting approach is thought to model certain aspects of relapse to addictive behavior and has previously almost exclusively been studied in drug self-administration paradigms. It has now also become established in the place conditioning literature and provides an additional and technically easy approach to this important phenomenon. The enormous number of studies to be covered in this review prevented in-depth discussion of many methodological, pharmacological or neurobiological aspects; to a large extent, the presentation of data had to be limited to a short and condensed summary of the most relevant findings.

Neurobiology of addiction. An integrative review

Evidence that psychoactive substance use disorders, bulimia nervosa, pathological gambling, and sexual addiction share an underlying biopsychological process is summarized. Definitions are offered for addiction and addictive process, the latter being the proposed designation for the underlying biopsychological process that addictive disorders are hypothesized to share. The addictive process is introduced as an interaction of impairments in three functional systems: motivation-reward, affect regulation, and behavioral inhibition. An integrative review of the literature that addresses the neurobiology of addiction is then presented, organized according to the three functional systems that constitute the addictive process. The review is directed toward identifying candidate neurochemical substrates for the impairments in motivation-reward, affect regulation, and behavioral inhibition that could contribute to an addictive process.

There and back again: a tale of norepinephrine and drug addiction

Fueled by anatomical, electrophysiological, and pharmacological analyses of endogenous brain reward systems, norepinephrine (NE) was identified as a key mediator of both natural and drug-induced reward in the late 1960s and early 1970s. However, reward experiments from the mid-1970s that could distinguish between the noradrenergic and dopaminergic systems resulted in the prevailing view that dopamine (DA) was the primary 'reward transmitter' (a belief holding some sway still today), thereby pushing NE into the background. Most damaging to the NE hypothesis of reward were studies demonstrating that NE receptor antagonists and NE reuptake inhibitors failed to impact drug self-administration. In recent years new tools, such as genetically engineered mice, and new experimental paradigms, such as reinstatement of drug seeking following withdrawal, have propelled NE back into the awareness of addiction researchers. Of particular interest is disulfiram, an inhibitor of the NE biosynthetic enzyme dopamine beta-hydroxylase, which has demonstrated promising efficacy in the treatment of cocaine dependence in preliminary clinical trials. The purpose of this review is to synthesize the new data linking NE to critical aspects of DA signaling and drug addiction, with a focus on psychostimulants (eg, cocaine), opiates (eg, morphine), and alcohol.

Role of alpha(1)-adrenoceptors of the locus coeruleus in self-stimulation of the medial forebrain bundle

The present experiments were undertaken to clarify the role of central alpha(1)-adrenoceptors in reward processes. Rats, trained to self-stimulate via electrodes in the medial forebrain bundle of the lateral hypothalamus, were administered alpha(1)-selective drugs near the locus coeruleus (LC), a site of a dense concentration of alpha(1)-receptors. Effects on reward potency were assessed from shifts in rate-frequency curves while effects on motor response capacity were judged from changes in the maximal rates of responding. It was found that local blockade of LC alpha(1)-receptors with terazosin produced a significant dose-dependent and site-dependent rightward shift of 0.08 log units and a significant decrease of 16.3% in the maximum response rate. Both effects were completely reversed by coadministration of the alpha(1)-agonist, phenylephrine and were not attributable to terazosin's weak action at alpha(2)-adrenoceptors. It is concluded that LC alpha(1)-adrenoceptors are involved both in reward/motivational processes and operant response elaboration which are postulated to work together to facilitate goal attainment.

Effects of RGH-237 [N-{4-[4-(3-Aminocarbonyl-phenyl)-piperazin-1-yl]-butyl}-4-bromo-benzamide], an Orally Active, Selective Dopamine D3 Receptor Partial Agonist in Animal Models of Cocaine Abuse

Dopamine D(3) receptor partial agonism has been suggested as a potential therapeutic intervention in cocaine addiction. RGH-237 [N-{4-[4-(3-aminocarbonyl-phenyl)-piperazin-1-yl]-butyl}-4-bromo-benzamide] was identified as a novel selective dopamine D(3) receptor partial agonist and used for testing this hypothesis in animal models. The compound showed nanomolar affinity to human (K(i) = 6.7 nM) and rat (K(i) = 1.6 nM) D(3) receptors with an intrinsic activity of approximately 50%. It possessed several hundredfold selectivity over the D(2) receptor. The molecule bound with moderate (100-250 nM) affinity to 5-hydroxytryptamine 1A (5-HT(1A)) and nonselectively labeled opiate receptors. RGH-237 proved to be practically inactive on more than 40 other targets, including monoaminergic, cholinergic, GABAergic, and glutamatergic receptors. In rats orally administered RGH-237 was well and rapidly absorbed yielding 41% oral bioavailability. At its pharmacologically active dose (10 mg/kg p.o.), the brain concentration of RGH-237 reached 110 ng/g. Its blood and brain levels were sustained for 3 h. RGH-237 at the oral dose of 10 mg/kg moderately but significantly inhibited the acquisition of cocaine-induced place preference, although by itself, it had no place-conditioning effect. The compound did not affect fixed ratio 1 cocaine self-administration. In a reinstatement paradigm of cocaine self-administration, the compound potently and dose-dependently blocked the cue-induced cocaine-seeking behavior of rats at 10 and 30 mg/kg oral doses. RGH-237 did not affect seeking activity for natural rewards, such as sucrose and water. It did not exert notable effect on spontaneous motor activity of rats. Our results demonstrate that selective D(3) partial agonists may be an effective therapeutic means in the treatment of cocaine abuse.

Effects of ultra-low doses of nicotine on the expression of morphine-induced conditioned place preference in mice

In the present study, the effects of acute administration of nicotine, as well as nicotinic and muscarinic acetylcholine receptor antagonists, on the expression of morphine-induced conditioned place preference, have been investigated in male Swiss-Webster mice. Animals received different doses of morphine 5 days after surgical cannulation in the lateral ventricle. Subcutaneous injections of morphine (2-5 mg/kg) in mouse produced place preference in a dose-dependent manner. Furthermore, both intraperitoneal (0.0006-0.1 mg/kg) and intracerebroventricular (0.007-25 ng) nicotine administration significantly reduced the expression of morphine-induced place preference, in a dose-dependent manner. Nicotine, however, was effective over narrow ultra-low dose ranges (0.0012, 0.0025, 0.005 and 0.01 mg/kg; intraperitoneal) and (0.03, 0.1, 0.3 and 0.6 ng/mouse; intracerebroventricular). In addition, locomotor activity was reduced when higher doses of nicotine [both intraperitoneal (0.02, 0.03 and 0.1 mg/kg) and intracerebroventricular (10 and 24 ng/mouse)] were used. Nicotine alone, however, did not cause motivational effects. Intracerebroventricular injection of hexamethonium (0.03, 0.1 and 0.3 mug/mouse; 10 min before nicotine) diminished the effects of nicotine on morphine-induced conditioned place preference. This effect could neither be obtained by intraperitoneal administration of hexamethonium (1, 5 and 10 mg/kg; 30 min before nicotine), nor be reproduced after either intracerebroventricular or intraperitoneal injection of atropine (a muscarinic receptor antagonist). The antagonists, themselves, did not show any motivational effects when used alone and were unable to affect the expression of morphine-induced conditioned place preference. It appears that ultra-low doses of nicotine can reduce the expression of morphine-induced place preference, and that central nicotinic acetylcholine receptors play a role in this regard.

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.

Central alpha1-adrenergic system in behavioral activity and depression

Central alpha(1)-adrenoceptors are activated by norepinephrine (NE), epinephrine (EPI) and possibly dopamine (DA), and function in two fundamental and opposed types of behavior: (1) positively motivated exploratory and approach activities, and (2) stress reactions and behavioral inhibition. Brain microinjection studies have revealed that the positive-linked receptors are located in eight to nine brain regions spanning the neuraxis including the secondary motor cortex, piriform cortex, nucleus accumbens, preoptic area, lateral hypothalamic area, vermis cerebellum, locus coeruleus, dorsal raphe and possibly the C1 nucleus of the ventrolateral medulla, whereas the stress-linked receptors are present in at least three areas including the paraventricular nucleus of the hypothalamus, central nucleus of the amygdala and bed nucleus of the stria terminalis. Recent studies utilizing c-fos expression and mitogen-activated protein kinase activation have shown that various diverse models of depression in mice produce decreases in positive region-neural activity elicited by motivating stimuli along with increases in neural activity of stress areas. Both types of change are attenuated by various antidepressant agents. This has suggested that the balance of the two networks determines whether an animal displays depressive behavior. A central unresolved question concerns how the alpha(1)-receptors in the positive-activity and stress systems are differentially activated during the appropriate behavioral conditions and to what extent this is related to differences in endogenous ligands or receptor subtype distributions.

Role of noradrenergic signaling by the nucleus tractus solitarius in mediating opiate reward

Norepinephrine (NE) is widely implicated in opiate withdrawal, but much less is known about its role in opiate-induced locomotion and reward. In mice lacking dopamine beta-hydroxylase (DBH), an enzyme critical for NE synthesis, we found that NE was necessary for morphine-induced conditioned place preference (CPP; a measure of reward) and locomotion. These deficits were rescued by systemic NE restoration. Viral restoration of DBH expression in the nucleus tractus solitarius, but not in the locus coeruleus, restored CPP for morphine. Morphine-induced locomotion was partially restored by DBH expression in either brain region. These data suggest that NE signaling by the nucleus tractus solitarius is necessary for morphine reward.

Effects of Papaver rhoeas extract on the acquisition and expression of morphine-induced conditioned place preference in mice

In the present study, the effects of water-alcohol extract of Papaver rhoeas on the acquisition and expression of morphine-induced conditioned place preference (CPP) in mice were investigated. Subcutaneous (s.c.) administration of morphine (1, 10 and 20 mg/kg) produced place preference. On the other hand, intraperitoneal (i.p.) administration of the plant extract (25, 50 and 100 mg/kg) did not show any effect. Injection of extract (25, 50 and 100 mg/kg, i.p.) 30 min before the morphine administration decreased the acquisition of morphine CPP. Administration of the plant extract (25, 50 and 100 mg/kg, i.p.) 30 min before the test did not change the expression of morphine-induced CPP. It could be concluded that Papaver rhoeas reduced the acquisition but not the expression of morphine-induced conditioned place preference.

Endogenous opiates and behavior: 2004

This paper is the 27th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over 30 years of research. It summarizes papers published during 2004 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, and the roles of these opioid peptides and receptors in pain and analgesia; stress and social status; tolerance and dependence; learning and memory; eating and drinking; alcohol and drugs of abuse; sexual activity and hormones, pregnancy, development and endocrinology; mental illness and mood; seizures and neurologic disorders; electrical-related activity and neurophysiology; general activity and locomotion; gastrointestinal, renal and hepatic functions; cardiovascular responses; respiration and thermoregulation; and immunological responses.