Participation of noradrenergic pathways in the expression of opiate withdrawal: biochemical and pharmacological evidence

Potentiated startle and hyperalgesia during withdrawal from acute morphine: effects of multiple opiate exposures

RATIONALE

Administration of an opiate antagonist following acute morphine exposure elevates the startle response in rodents, a phenomenon that may reflect the anxiogenic effects of withdrawal. Previous acute dependence studies have demonstrated escalated withdrawal severity following multiple withdrawal episodes.

OBJECTIVES

To examine the effects of prior opiate exposure on the magnitude of withdrawal-potentiated startle and an additional measure of acute dependence, withdrawal-induced hyperalgesia.

METHODS

The effects of repeated naloxone-precipitated morphine withdrawals on acoustic startle responding were evaluated in experiments that varied either the dose of the opiate antagonist (8-day, repeated measures procedure) or agonist (3-day procedure). Additional experiments examined withdrawal-induced hyperalgesia utilizing either a single-day dependence paradigm or the same 3-day procedure as in the startle experiment.

RESULTS

Repeated naloxone-precipitated withdrawals from acute morphine exacerbated withdrawal severity in both startle procedures, although this effect varied biphasically (inverted-U function) with morphine dose in the 3-day dependence paradigm. Withdrawal from a single morphine exposure also induced hyperalgesia, and this effect was intensified by prior withdrawal episodes.

CONCLUSIONS

These data demonstrate that repeated withdrawals from acute morphine exacerbate the severity of potentiated startle and hyperalgesia. These paradigms may be useful in examining the neural plasticity underlying the development of opiate dependence.

Chronic very low dose naltrexone administration attenuates opioid withdrawal expression

BACKGROUND

Different regimens of agonist and antagonist drugs have been used in opioid withdrawal management, with variable results. We examined whether administering extremely small quantities of opiate antagonists in the presence of opiate agonist drugs reduces withdrawal expression.

METHODS

Forty-one male Sprague-Dawley rats were implanted with morphine or placebo pellets for eight days. Starting on day 3, some rats received naltrexone in their drinking water (5 mg/L), or unadulterated water. On day 8, rats were injected with saline or naltrexone (100 mg/kg) and evaluated for behavioral signs of withdrawal. Next, sections through the locus coeruleus (LC) and nucleus of the solitary tract (NTS), brainstem areas exhibiting cellular activation following opiate withdrawal, were processed for c-Fos to detect early gene expression. Finally, the same nuclei were examined for protein kinase A regulatory subunit II (PKA) and phosphorylated cyclic adenosine monophosphate response element binding protein (pCREB), using Western blot analysis.

RESULTS

Withdrawal was attenuated and c-Fos, PKA, and pCREB expression was decreased in the NTS and LC of rats receiving chronic very low doses of naltrexone.

CONCLUSIONS

Reduction of withdrawal upon chronic very low naltrexone administration may be due in part to decreased activation of brainstem noradrenergic neurons in morphine dependent rats.

Effects of the selective norepinephrine uptake inhibitor nisoxetine on prodynorphin gene expression in rat CNS

Cocaine binds to dopamine (DA), serotonin (5-HT) and norepinephrine (NE) transporters blocking the reuptake of these monoamines into presynaptic terminals. As previously reported, continuous infusion of cocaine for seven days or GBR 12909, a selective dopamine uptake inhibitor, produced significant decreases in prodynorphin (PDYN) gene expression in the hypothalamus. Cocaine also produced a significant increase in PDYN mRNA in the caudate putamen, whereas GBR12909 has no effect and the selective serotonin uptake inhibitor fluoxetine decreases PDYN mRNA in the same brain region. The effect of the selective norepinephrine uptake inhibitor nisoxetine was examined on PDYN gene expression. Nisoxetine or vehicle was infused continuously for 7 days via osmotic minipump into male rats. This treatment produced significant increases in PDYN gene expression in the hypothalamus (183% of control), nucleus accumbens (142% of control) and hippocampus (124% of control) and a significant decrease in the caudate putamen (69% of control). These data suggest that nisoxetine affects PDYN gene expression and support a role for NE in the mechanisms underlying the effects of chronic exposure to psychoactive drugs. Moreover, nisoxetine, as well as fluoxetine, decreases PDYN mRNA in the caudate putamen, in contrast to the up-regulation produced by cocaine. Thus, the inhibition of NE uptake alone cannot account for the cocaine-induced increase of PDYN gene expression. These findings suggest that PDYN gene expression regulation by cocaine in the caudate putamen might be due to a combination of effects on two or three monoamine transporters, or to a mechanism unrelated to transporters inhibition.

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

It has been shown that the alpha-adrenergic system is involved in some effects of opioids, including analgesia and reward. Gender differences also exist between males and females in response to alpha-adrenergic agents. This study was designed to determine the effects of alpha-adrenoceptor agonists and antagonists on the acquisition or expression of morphine-induced conditioned place preference (CPP) in female mice. The experiments showed that subcutaneous injections of morphine (0.5-8 mg/kg) induced CPP in a dose-dependent manner in mice. Intrapritoneal administration of the alpha-1-adrenoceptor agonist, phenylephrine (0.03, 0.1 and 0.3 mg/kg), and alpha-2 adrenoceptor agonist, clonidine (0.0001, 0.0005 and 0.001 mg/kg), as well as alpha-1-adrenoceptor antagonist, prazosin (0.01, 0.05 and 0.1 mg/kg) or alpha-2 adrenoceptor antagonist, yohimbine (0.005, 0.01 and 0.05 mg/kg) did not induce motivational effects and also did not alter locomotor activity in the animals. In the second set of experiments, the drugs were used before testing on Day 5, to test their effects on the expression of morphine-induced CPP. Intrapritoneal administration of phenylephrine and clonidine decreased the expression of morphine-induced CPP. In contrast, after application of prazosin or yohimbine, the expression of morphine-induced CPP was increased. Administration of lower (0.03 mg/kg) and higher doses of phenylephrine (0.1 and 0.3 mg/kg) during acquisition of morphine CPP decreased and increased the morphine CPP, respectively. Similarly, the administration of prazosin and clonidine decreased while yohimbine increased the morphine CPP. It may be concluded that alpha-adrenoceptor mechanism(s) influence morphine-induced CPP in female mice.

Morphine withdrawal-induced c-fos expression in the heart: a peripheral mechanism

We previously demonstrated that hyperactivity of cardiac noradrenergic pathways observed during morphine withdrawal is mediated by peripheral mechanisms. In the present study, naloxone methiodide (quaternary derivative of naloxone that does not cross the blood-brain barrier) and naloxone were administered to morphine-dependent rats and Fos immunostaining was used as a reflection of neuronal activity. Dependence on morphine was induced by 7-day chronic subcutaneous (s.c.) implantation of six morphine pellets (75 mg). Morphine withdrawal was precipitated by administration of naloxone methiodide (5 mg/kg, s.c.) or naloxone (5 mg/kg, s.c.) on day 8. Using immunohistochemical staining of Fos, present results indicate that the administration of naloxone methiodide or naloxone to morphine-dependent rats induced marked Fos immunoreactivity within the cardiomyocyte nuclei. Moreover, Western blot analysis revealed a peak expression of c-fos in the right and left ventricles after naloxone methiodide- or naloxone-precipitated withdrawal. In addition, in the hypothalamic paraventricular nucleus (PVN), Fos expression was increased after naloxone-but not after naloxone methiodide-administration to morphine-dependent rats. These results suggest that the activation of c-fos expression observed during morphine withdrawal in the heart is due to intrinsic mechanisms outside the central nervous system (CNS).

Elevated startle during withdrawal from acute morphine: a model of opiate withdrawal and anxiety

RATIONALE

An elevated startle response has been observed in humans and animals during withdrawal from multiple substances of abuse, a phenomenon thought to reflect the anxiogenic effects of withdrawal. Although anxiety is a common symptom of opiate withdrawal, few studies have examined the effects of morphine withdrawal on acoustic startle.

OBJECTIVE

To develop a procedure for assessing opiate dependence through measurement of the startle reflex in rats.

METHODS

The effects of opiate withdrawal on startle were evaluated using both spontaneous and naloxone-precipitated withdrawal from an acute dose of morphine. The ability of the treatment drugs clonidine and chlordiazepoxide to block withdrawal-induced increases in startle was also tested.

RESULTS

Spontaneous withdrawal from an injection of morphine sulfate produced a significant increase in acoustic startle 2 h (3.2 mg/kg) or 4 h (10 mg/kg) after drug administration. Morphine withdrawal (10 mg/kg morphine sulfate) precipitated by the opiate antagonist naloxone (2.5 mg/kg) also produced a significant increase in startle magnitude. This elevation of startle was blocked by both clonidine (35 microg/kg) and chlordiazepoxide (10 mg/kg).

CONCLUSIONS

These data demonstrate that both spontaneous and precipitated withdrawal from an acutely administered opiate produce anxiety-like effects on acoustic startle. This paradigm may be useful in the study of anxiety and the early mechanisms of drug dependence.

Cocaine and other indirect-acting monoamine agonists differentially attenuate a naltrexone discriminative stimulus in morphine-treated rhesus monkeys

Monoaminergic drugs can modify opioid withdrawal in nonhumans, and cocaine is reported to attenuate opioid withdrawal in humans. Drug discrimination was used to examine whether s.c. cocaine or other indirect-acting monoamine agonists attenuate morphine (3.2 mg/kg/day) withdrawal induced by naltrexone and by 27 h of morphine deprivation. Naltrexone-precipitated withdrawal was attenuated not only by morphine but also by cocaine, amphetamine, and imipramine. However, reversal of naltrexone-precipitated withdrawal was greater for morphine than for any of the indirect-acting monoamine agonists. Attenuation of the naltrexone discriminative stimulus by indirect-acting monoamine agonists was pharmacologically selective insofar as drugs lacking affinity for monoamine transporters (ketamine and triazolam) were without effect. Twenty-seven hours of morphine deprivation occasioned naltrexone-lever responding and decreased response rate, and both effects were reversed by morphine, cocaine, and amphetamine and not by imipramine, desipramine, ketamine, and triazolam. Thus, indirect-acting monoamine agonists attenuate some (e.g., discriminative) aspects of naltrexone-precipitated withdrawal, whereas only indirect-acting agonists with high affinity for dopamine transporters attenuate deprivation-induced withdrawal. These results suggest that dopamine is differentially involved in naltrexone- and deprivation-induced withdrawal and support the notion that opioid-dependent individuals use stimulants, in part, to attenuate withdrawal.

Agmatine reduces only peripheral-related behavioral signs, not the central signs, of morphine withdrawal in nNOS deficient transgenic mice

Agmatine inhibits morphine tolerance/dependence and potentiates morphine analgesia. This study was designed to investigate whether neuronal nitric oxide mediates the actions of agmatine in morphine dependence by using mice lacking a functional form of this enzyme. Mice received agmatine just after the morphine pellet implantation for 3 days twice daily or single injection 30 min before naloxone. In both genotypes treated for 3 days with morphine pellets, naloxone administration precipitated clear signs of withdrawal. Both acute and chronic administration of agmatine reduced withdrawal signs in wild type mice and reduced only peripheral signs of morphine dependence in neuronal nitric oxide synthase knockout mice. Withdrawal signs, that are related to central nervous system activity were not affected. These findings indicate that neuronal nitric oxide synthase partly mediates the effects of agmatine in morphine physical dependence.

EEG spectral power and mean frequencies in early heroin abstinence

The purpose of the present study was to investigate cumulative heroin effects on brain functioning by studying relationships among electroencephalography (EEG) spectral power and mean frequencies and heroin abuse history. Eyes-closed resting EEG data were collected from the 19 monopolar electrode sites in 33 heroin abusers and 13 age-matched healthy volunteers. The mean age of the patients was 23.1+/-4.5 years, the duration of daily heroin abuse (DDHA) ranged from 4 to 44 months, the intravenous doses of heroin ranged from 0.04 to 1.00 g/day, the abstinence length ranged from 6 days to 4.5 months. General linear model (GLM) repeated measures procedure revealed a significant group effect on the distribution of the mean power spectrum between bands and mean frequencies in almost all analyzed derivations. Further analysis demonstrated that these intergroup differences were diversely related to at least three aspects of heroin-taking history. Frequency shifts in alpha2 range, most prominent in frontal and central derivations, were related to duration of daily heroin consumption. Slowing of alpha1 mean frequency, most prominent in central, temporal, and occipital derivations, was registered mainly in heroin addicts who abused high doses of the drug. Spectral power characteristics of brain electrical activities in our patient population were strongly predicted by abstinence length. The present results give grounds to suppose that chronic heroin-taking induces neuronal oscillation frequency changes, which may contribute to the development of antisocial trends and some semantic processes disturbances in these patients. Supplementary neurophysiological deficit is characteristic for heroin addicts who takes high doses of the drug, however, its relation to heroin abuse remains unclear. Pronounced desynchronization is observed in acute heroin withdrawal, and spectral power characteristics tend to normalize almost completely during several weeks of abstinence.

Roles of alpha1- and alpha2-adrenoceptors in the nucleus raphe magnus in opioid analgesia and opioid abstinence-induced hyperalgesia

Noradrenaline and alpha-adrenoceptors have been implicated in the modulation of pain in various behavioral conditions. Noradrenergic neurons and synaptic inputs are present in neuronal circuits critical for pain modulation, but their actions on neurons in those circuits and consequently the mechanisms underlying noradrenergic modulation of pain remain unclear. In this study, both recordings in vitro and behavioral analyses in vivo were used to examine cellular and behavioral actions mediated by alpha1- and alpha2-adrenoceptors on neurons in the nucleus raphe magnus. We found that alpha1- and alpha2-receptors were colocalized in the majority of a class of neurons (primary cells) that inhibit spinal pain transmission and are excited during opioid analgesia. Activation of the alpha1-receptor depolarized whereas alpha2-receptor activation hyperpolarized these neurons through a decrease and an increase, respectively, in potassium conductance. Blockade of the excitatory alpha1-receptor or activation of the inhibitory alpha2-receptor significantly attenuated the analgesia induced by local opioid application, suggesting that alpha1-receptor-mediated synaptic inputs in these primary cells contribute to their excitation during opioid analgesia. In the other cell class (secondary cells) that is thought to facilitate spinal nociception and is inhibited by analgesic opioids, only alpha1-receptors were present. Blocking the alpha1-receptor in these cells significantly reduced the hyperalgesia (increased pain) induced by opioid abstinence. Thus, state-dependent activation of alpha1-mediated synaptic inputs onto functionally distinct populations of medullary pain-modulating neurons contributes to opioid-induced analgesia and opioid withdrawal-induced hyperalgesia.

Endocrine withdrawal syndromes

Hypersecretion of endogenous hormones or chronic administration of high doses of the same hormones induces varying degrees of tolerance and dependence. Elimination of hormone hypersecretion or discontinuation of hormone therapy may result in a mixed picture of two syndromes: a typical hormone deficiency syndrome and a generic withdrawal syndrome. Thus, hormones with completely different physiological effects may produce similar withdrawal syndromes, with symptoms and signs reminiscent of those observed with drugs of abuse, suggesting shared mechanisms. This review postulates a unified endocrine withdrawal syndrome, with changes in the hypothalamic-pituitary-adrenal axis and the central opioid peptide, in which noradrenergic and dopaminergic systems of the brain act as common links in its pathogenesis. Long-term adaptations to hormones may involve relatively persistent changes in molecular switches, including common intracellular signaling systems, from membrane receptors to transcription factors. The goals of therapy are to ease withdrawal symptoms and to expedite weaning of the patient from the hormonal excess state. Clinicians should resort to the fundamentals of tapering hormones down over time, even in the case of abrupt removal of a hormone-producing tumor. In addition, the prevention of stress and concurrent administration of antidepressants may ameliorate symptoms and signs of an endocrine withdrawal syndrome.

Effects of imipramine on the expression and development of morphine dependence in mice

In the present study, the effects of imipramine and/or alpha-adrenoceptor agents on naloxone-induced jumping in morphine-dependent mice were examined. In the first set of experiments, the drugs were used before naloxone injection, to test their effects on the expression of jumping. Administration of imipramine (10-60 mg/kg) 15 min before naloxone increased the number of jumping in mice. Injection of the alpha2-adrenoceptor agonist, clonidine (0.1 mg/kg), or alpha1-adrenoceptor agonist, phenylephrine (4 mg/kg), themselves neither altered naloxone-induced jumping nor influenced the imipramine response. The alpha2-adrenoceptor antagonist, yohimbine (4 mg/kg), itself but not the alpha1-adrenoceptor antagonist, prazosin (1 mg/kg), increased jumping and decreased the imipramine effect. In the second set of experiments, imipramine and/or the alpha-adrenoceptor drugs were injected during the development of morphine dependence. Imipramine (10-40 mg/kg) increased the development of dependence and increased jumping was seen. Clonidine did not influence the imipramine effect. Phenylephrine was lethal in combination with imipramine. Both yohimbine and prazosin decreased the effect of imipramine. Imipramine and phenylephrine but not clonidine, yohimbine or prazosin decreased locomotion. It is concluded an alpha2-adrenoceptor mechanism may be involved in the influence of imipramine on the expression and development of naloxone-induced withdrawal signs in mice.

Prolonged activation of mesolimbic dopaminergic neurons by morphine withdrawal following clonidine: participation of imidazoline and norepinephrine receptors

The alpha2 adrenoceptor (alpha2R) agonist clonidine is used as a treatment for heroin addiction. Substantial evidence indicates that dopaminergic and noradrenergic systems have key roles in opiate dependence and withdrawal but the possible interactions between these two pathways remain unclear. The objective of this study was to establish the effects of clonidine pretreatment on ventral tegmental area dopaminergic (VTA DA) neuronal activity during morphine withdrawal. Responses of VTA DA neurons to withdrawal precipitated by naltrexone were characterized in anesthetized rats using extracellular recordings. As expected, withdrawal produced a marked inhibition of VTA DA neuronal activity. However, pretreatment with clonidine prevented this inhibition induced by withdrawal, and instead produced a long-lasting activation of firing rate (+50%) and burst firing (+19%). In contrast, pretreatment with a more selective alpha2R agonist, UK14304, did not prevent the inhibition of VTA DA neuron activity during withdrawal. We tested whether the high affinity of clonidine for imidazoline-1 receptors (I1Rs) was responsible for the difference between these two alpha2R agonists. In morphine-dependent rats pretreated with rilmenidine (mixed alpha2R/I1R agonist), precipitation of withdrawal elicited a 22% increase of VTA DA impulse activity. The action of clonidine on I1Rs was studied by coadministering clonidine with RX821002, a specific alpha2R antagonist. Pretreatment with RX821002 plus clonidine prevented the inhibition of VTA DA activity during withdrawal but failed to produce excitation. These results indicate that the pharmacological effects of clonidine on VTA DA neurons during morphine withdrawal is related to actions on I1Rs as well as alpha2Rs.

Autoradiographic evidence that prenatal morphine exposure sex-dependently alters mu-opioid receptor densities in brain regions that are involved in the control of drug abuse and other motivated behaviors

The present study examined the effects of prenatal morphine exposure on mu-opioid receptor density in young adult male and female rats to assess the long-term alterations in several brain areas including the nucleus accumbens (NAc), bed nucleus of stria terminalis (BNST), and the basolateral (BLA), lateral (LA), central (CeA), and posteromedial cortical (PMCoA) amygdaloid nuclei. These brain areas are involved in motivating and rewarding behaviors of opiates and other drugs of abuse. The reinforcing actions of opiates appear to be mu-opioid receptor dependent. The results demonstrate that in male rats, prenatal morphine exposure significantly increases the density of mu-opioid receptors in the NAc and PMCoA. In contrast, the same prenatal morphine exposure reduces the density of mu-opioid receptors in the BLA, while increasing it in the CeA and without effects in the LA or BNST. In female rats, prenatal morphine exposure has no effects on the density of mu-opioid receptors in the above six brain areas, but the density of these receptors is dependent on the presence or absence of ovarian hormones. Thus, the present study demonstrates that mid- to late gestational morphine exposure induces long-term, sex-specific alterations in the density of mu-opioid receptors in the NAc and amygdala. Moreover, this prenatal morphine exposure also eliminates sex differences in the density of mu-opioid receptors in the NAc, CeA, and PMCoA but not in the BLA, LA, and BNST.

The locus coeruleus-noradrenergic system: modulation of behavioral state and state-dependent cognitive processes

Through a widespread efferent projection system, the locus coeruleus-noradrenergic system supplies norepinephrine throughout the central nervous system. Initial studies provided critical insight into the basic organization and properties of this system. More recent work identifies a complicated array of behavioral and electrophysiological actions that have in common the facilitation of processing of relevant, or salient, information. This involves two basic levels of action. First, the system contributes to the initiation and maintenance of behavioral and forebrain neuronal activity states appropriate for the collection of sensory information (e.g. waking). Second, within the waking state, this system modulates the collection and processing of salient sensory information through a diversity of concentration-dependent actions within cortical and subcortical sensory, attention, and memory circuits. Norepinephrine-dependent modulation of long-term alterations in synaptic strength, gene transcription and other processes suggest a potentially critical role of this neurotransmitter system in experience-dependent alterations in neural function and behavior. The ability of a given stimulus to increase locus coeruleus discharge activity appears independent of affective valence (appetitive vs. aversive). Combined, these observations suggest that the locus coeruleus-noradrenergic system is a critical component of the neural architecture supporting interaction with, and navigation through, a complex world. These observations further suggest that dysregulation of locus coeruleus-noradrenergic neurotransmission may contribute to cognitive and/or arousal dysfunction associated with a variety of psychiatric disorders, including attention-deficit hyperactivity disorder, sleep and arousal disorders, as well as certain affective disorders, including post-traumatic stress disorder. Independent of an etiological role in these disorders, the locus coeruleus-noradrenergic system represents an appropriate target for pharmacological treatment of specific attention, memory and/or arousal dysfunction associated with a variety of behavioral/cognitive disorders.

Prostaglandin E2 attenuates SR141716A-precipitated withdrawal in tetrahydrocannabinol-dependent mice

The present study aimed to clarify the role of the arachidonic acid cascade in mediating the expression of withdrawal signs in cannabinoid-dependent mice. Mice were injected with Delta(8)-tetrahydrocannabinol (THC) at 20 mg/kg (i.p.) every 12 h, 11 times. When SR141716A, a specific cannabinoid CB1 receptor antagonist, at 10 mg/kg (i.p.) was given 4 h after the last THC injection, withdrawal signs such as forepaw licking, facial preening, grooming, forepaw tremor, head shakes and weight loss were clearly observed. PGE(2) at 0.1, 1.0 and 3.2 microg (per animal; i.c.v.) given prior to SR141716A (10 mg/kg, i.p.) dose-dependently decreased the number of forepaw licking, facial preening, grooming and forepaw tremor episodes. Instead of SR141716A, a cyclooxygenase inhibitor diclofenac at 10 mg/kg (i.p.) also precipitated these withdrawal signs. The results suggest that the expression of THC withdrawal is due to a decrease in prostaglandin levels through inactivation of the arachidonic acid cascade in the brain.

Role of the bed nucleus of the stria terminalis versus the amygdala in fear, stress, and anxiety

The bed nucleus of the stria terminalis is a limbic forebrain structure that receives heavy projections from, among other areas, the basolateral amygdala, and projects in turn to hypothalamic and brainstem target areas that mediate many of the autonomic and behavioral responses to aversive or threatening stimuli. Despite its strategic anatomical position, initial attempts to implicate the bed nucleus of the stria terminalis in conditioned fear were largely unsuccessful. Recent studies have shown, however, that the bed nucleus of the stria terminalis does participate in certain types of anxiety and stress responses. In this work, we review these findings and suggest from the emerging pattern of evidence that, although the bed nucleus of the stria terminalis may not be necessary for rapid-onset, short-duration behaviors which occur in response to specific threats, the bed nucleus of the stria terminalis may mediate slower-onset, longer-lasting responses that frequently accompany sustained threats, and that may persist even after threat termination.

Morphine state-dependent learning: interactions with alpha2-adrenoceptors and acute stress

The interactions of -adrenoceptors and acute restraint stress with morphine state-dependent memory of passive avoidance were examined in mice. Memory acquired following pre-training morphine administration (5 mg/kg, i.p.) was dose- and time-dependently retrieved by pre-test morphine; this effect was reversible by yohimbine (1 mg/kg). Pre-test clonidine (0.005-0.1 mg/kg) was also effective in restoring morphine-induced memory. Pre-training clonidine (2 mg/kg) induced an amnestic effect that was restorable by pre-test clonidine or morphine; this effect was also blocked by yohimbine. Acute pre-training stress for 2 h induced an amnestic effect that was reversible by pre-test morphine (1 and 5 mg/kg) or clonidine (0.01 and 0.1 mg/kg). Finally, acute pre-test stress could restore the impairment of memory induced by pre-training morphine. The data are suggestive of a functional interaction between -opioid, -adrenergic receptors and stress in modulating state-dependent learning and memory.

Morphine withdrawal is modified in pituitary adenylate cyclase-activating polypeptide type I-receptor-deficient mice

The pituitary adenylate cyclase-activating polypeptide type I-receptor (PAC1) is a G-protein-coupled receptor that is widely expressed in neurons of the central and peripheral nervous system. The strong expression of PAC1 in the second sensory neuron as well as in brainstem regions such as the locus coeruleus prompted us to elucidate the potential in vivo role of PAC1-mediated signalling in pain perception and opioid addiction using a PAC1-deficient mouse line. We observed a selective involvement of PAC1 in the mediation of visceral pain. While there was no impairment in acute somatic pain perception, PAC1-mutants exhibited a dramatically decreased response in the abdominal writhing test. These data in concert with data from the literature implicate PAC1 in the mediation of visceral and chronic pain. In addition, we observed that PAC1 did not influence the motivational aspects of opioid addictive properties, since morphine-induced rewarding effects and sensitization to locomotor responses were completely maintained in PAC1-deficient mice. However, there was a dramatic increase in physical withdrawal signs after naloxone-precipitated morphine withdrawal in PAC1 mutants. At the cellular level, electrophysiological examinations in locus coeruleus neurons from morphine-dependent wild-type and PAC1-deficient mice did not reveal any differences in firing rates. These data therefore suggested that most likely disruption of PAC1-mediated signalling in afferents towards the locus coeruleus but not within the intrinsic locus coeruleus system led to the enhancement of somatic withdrawal signs.

Influence of prazosin and clonidine on morphine analgesia, tolerance and withdrawal in mice

Rapid development of tolerance and dependence limits the usefulness of morphine in long-term treatment. We examined the effects of clonidine (alpha(2)-adrenoceptor agonist) and prazosin (alpha(1)-adrenoceptor antagonist) on morphine analgesia, tolerance and withdrawal. Morphine tolerance was induced using a 3-day cumulative twice-daily dosing regimen with s.c. doses up to 120 mg/kg. Tolerance was assessed on day 4, as loss of the antinociceptive effect of a test dose of morphine (5 mg/kg). After 10 h, morphine withdrawal was precipitated with naloxone (1 mg/kg). Prazosin had no analgesic effect alone but dose-dependently potentiated morphine analgesia in morphine-naive mice. Another alpha(1)-adrenoceptor antagonist, corynanthine, had similar effects. Prazosin also increased the analgesic potency of the morphine test dose in morphine-tolerant mice. Naloxone-precipitated vertical jumping was not affected, but weight loss was reduced by prazosin. Acutely administered clonidine potentiated morphine analgesia and alleviated opioid withdrawal signs, as expected. We conclude that in addition to the already established involvement of alpha(2)-adrenoceptors in opioid actions, also alpha(1)-adrenoceptors have significant modulatory role in opioid analgesia and withdrawal.

Understanding polydrug use: review of heroin and cocaine co-use

The use of cocaine by heroin-dependent individuals, or by patients in methadone or buprenorphine maintenance treatment, is substantial and has negative consequences on health, social adjustment and outcome of opioid-addiction treatment. The pharmacological reasons for cocaine use in opioid-dependent individuals, however, are poorly understood and little is known about the patterns of heroin and cocaine co-use. We reviewed anecdotal evidence suggesting that cocaine is co-used with opioid drugs in a variety of different patterns, to achieve different goals. Clinical and preclinical experimental evidence indicates that the simultaneous administration of cocaine and heroin (i.e. 'speedball') does not induce a novel set of subjective effects, nor is it more reinforcing than either drug alone, especially when the doses of heroin and cocaine are high. There is mixed evidence that the subjective effects of cocaine are enhanced in individuals dependent on opioids, although it is clear that cocaine can alleviate the severity of symptoms of withdrawal from opioids. We also reviewed preclinical studies investigating possible neurobiological interactions between opioids and cocaine, but the results of these studies have been difficult to interpret mainly because the neurochemical mechanisms mediating the motivational effects of cocaine are modified by dependence on, and withdrawal from, opioid drugs. Our analysis encourages further systematic investigation of cocaine use patterns among opioid-dependent individuals and in laboratory animals. Once clearly identified, pharmacological and neuroanatomical methods can be employed in self-administering laboratory animals to uncover the neurobiological correlates of specific patterns of co-use.

Regulation of tyrosine hydroxylase levels and activity and Fos expression during opioid withdrawal in the hypothalamic PVN and medulla oblongata catecholaminergic cell groups innervating the PVN

Morphine withdrawal increases the hypothalamic-pituitary-adrenocortical (HPA) axis activity, which is dependent on an hyperactivity of noradrenergic pathways innervating the hypothalamic paraventricular nucleus (PVN). However, the possible adaptive changes that can occur in these pathways during morphine dependence are not known. We studied the alterations in tyrosine hydroxylase (TH; the rate-limiting enzyme in catecholamines biosynthesis) immunoreactivity levels and TH enzyme activity in the rat NTS-A2/VLM-A1 noradrenergic cell groups and in the PVN during morphine withdrawal. In the same paradigm, we measured Fos expression as a marker of neuronal activation. TH and Fos immunoreactivity was determined by quantitative Western blot analysis, combined with immunostaining for TH and Fos for immunohistochemical identification of active neurons during morphine withdrawal. Dependence on morphine was induced by a 7-day s.c. implantation of morphine pellets. Morphine withdrawal was precipitated on day 8 by an injection of naloxone (5 mg/kg s.c.). Morphine withdrawal induced the expression of Fos in the PVN and NTS/VLM, which indicates an activation of neurons in these nuclei. TH immunoreactivity in the NTS/VLM was increased 90 min after morphine withdrawal, whereas there was a decrease in TH levels in the PVN at the same time point. Following withdrawal, Fos immunoreactivity was present in most of the TH-positive neurons of the A2 and A1 neurons. TH activity was measured in the PVN, a projection area of noradrenergic neurons arising from NTS-A2/VLM-A1. Morphine withdrawal was associated with an increase in the enzyme activity at different time points after naloxone-precipitated morphine withdrawal. The present results suggest that an increase in TH protein levels and TH enzyme activity might contribute to the enhanced noradrenergic activity in the PVN in response to morphine withdrawal.

Extremely long recovery time for the sedative effect of clonidine in male type 1 alcohol-dependent subjects in full sustained remission

The possible relation between alpha-2-adrenoceptor function-as assessed by changes in systolic and diastolic blood pressure and heart rate, as well as level of sedation, after administration of clonidine (2.0 microg/kg, i.v.)-and length of time of alcohol dependence or duration of remission was investigated in 17 male subjects with alcohol dependence in full sustained remission. Six healthy males were used as control subjects. The clonidine-induced scores for level of sedation were found to correlate with duration of time in remission (r = 0.60; P <.02). Median split of duration of remission revealed that subjects with short-term (2 +/- 1 years) duration of remission had significantly lower scores for clonidine-induced level of sedation than the scores for both subjects with long-term (12 +/- 5 years) duration of remission (P <.004) and control subjects (P <.02). There was also a significant correlation between duration of remission and values for clonidine-induced reduction of systolic blood pressure (r = 0.51; P <.05). Results indicate an extremely long recovery period in some aspects of alpha-2-adrenoceptor function, especially for clonidine-induced increase in level of sedation, with a normalization time of 4 to 5 years.

Morphine withdrawal-induced c-fos expression in the hypothalamic paraventricular nucleus is dependent on the activation of catecholaminergic neurones

We previously demonstrated that morphine withdrawal induced hyperactivity of noradrenergic pathways innervating the hypothalamic paraventricular nucleus (PVN) in rats, in parallel with an increase in the neurosecretory activity of the hypothalamus-pituitary-adrenocortical (HPA) axis, as evaluated by corticosterone release. These neuroendocrine effects were dependent on stimulation of alpha-adrenoceptors. In the present study, Fos immunostaining was used as a reflection of neuronal activity and combined with immunostaining for tyrosine hydroxylase (TH) for immunohistochemical identification of active neurones during morphine withdrawal. Dependence on morphine was induced by 7-day chronic subcutaneous implantation of six morphine pellets (75 mg). Morphine withdrawal was precipitated by administration of naloxone (5 mg/kg subcutaneously) on day 8. Fos immunoreactivity in the PVN and also in the nucleus tractus solitarius (NTS)-A2 and ventrolateral medulla (VLM)-A1 cell groups, which project to the PVN, increased during morphine withdrawal. Following withdrawal, Fos immunoreactivity was present in most of the TH-positive neurones of the A2 and A1 neurones. In a second study, the effects of administration of adrenoceptor antagonists on withdrawal-induced Fos expression in the PVN were studied. Pre-treatment with alpha1- or alpha2-adrenoceptor antagonists, prazosin (1 mg/kg intraperitoneally) and yohimbine (1 mg/kg intraperitoneally), respectively, 20 min before naloxone administration to morphine-dependent rats markedly reduced Fos expression in the PVN. Similarly, pre-treatment with the beta antagonist, propranolol (3 mg/kg intraperitoneally), significantly prevented withdrawal-induced Fos expression. Collectively, these results suggest the hypothesis that noradrenergic neurones in the brainstem innervating the PVN are active during morphine withdrawal, and that activation of transcriptional responses mediated by Fos in the HPA axis following withdrawal are dependent upon hypothalamic alpha- and beta-adrenoceptors.

Neural correlates of the motivational and somatic components of naloxone-precipitated morphine withdrawal

In morphine-dependent rats, low naloxone doses have been shown to induce conditioned place aversion, which reflects the negative motivational component of opiate withdrawal. In contrast, higher naloxone doses are able to induce a 'full' withdrawal syndrome, including overt somatic signs. The c-fos gene is commonly used as a marker of neuronal reactivity to map the neural substrates that are recruited by various stimuli. Using in situ hybridization, we have analysed in the brain of morphine-dependent rats the effects of acute withdrawal syndrome precipitated by increasing naloxone doses on c-fos mRNA expression. Morphine dependence was induced by subcutaneous implantation of slow-release morphine pellets for 6 days and withdrawal was precipitated by increasing naloxone doses inducing the motivational (7.5 and 15 micro g/kg) and somatic (30 and 120 micro g/kg) components of withdrawal. Our mapping study revealed a dissociation between a set of brain structures (extended amygdala, lateral septal nucleus, basolateral amygdala and field CA1 of the hippocampus) which exhibited c-fos mRNA dose-dependent variations from the lowest naloxone doses, and many other structures (dopaminergic and noradrenergic nuclei, motor striatal areas, hypothalamic nuclei and periaqueductal grey) which were less sensitive and recruited only by the higher doses. In addition, we found opposite dose-dependent variations of c-fos gene expression within the central (increase) and the basolateral (decrease) amygdala after acute morphine withdrawal. Altogether, these results emphasize that limbic structures of the extended amygdala along with the lateral septal nucleus, the basolateral amygdala and CA1 could specifically mediate the negative motivational component of opiate withdrawal.

Inhibition of protein kinase C but not protein kinase A attenuates morphine withdrawal excitation of rat hypothalamus-pituitary-adrenal axis

Our previous studies have shown an enhanced activity of the hypothalamus-pituitary-adrenocortical axis response in rats withdrawn from morphine, which results from an increase in the hypothalamic paraventricular nucleus noradrenergic activity that is dependent on alpha-adrenoceptor activation. The first objective of this work was to examine the effect of protein kinase A (PKA) and protein kinase C (PKC) inhibitors on morphine withdrawal-induced changes in corticosterone release (an index of the hypothalamus-pituitary-adrenocortical axis activity) and in catecholaminergic turnover in the paraventricular nucleus. Plasma corticosterone levels as well as the concentration of noradrenaline, 3-methoxy-4-hydroxyphenylethyleneglycol (MHPG), dopamine and 3,4-dihydroxyphenylacetic acid (DOPAC) in the paraventricular nucleus were determined. The second purpose of the study was to assess whether kinase inhibitors, administered continuously through s.c. osmotic minipumps, get into the brain. Chronic pretreatment for 7 days with the selective PKA inhibitor N-(2'guanidinoethyl)-5-isoquinolinesulfonamide (HA-1004) concomitantly with morphine did not affect the increase in corticosterone release observed after naloxone-precipitated morphine withdrawal. However, pretreatment with the selective PKC inhibitor, calphostin-C significantly antagonized the corticosterone hypersecretion in morphine-withdrawn rats. Neither HA-1004 nor calphostin-C co-administered with morphine for 7 days did modify the morphine withdrawal-induced increase in noradrenaline turnover. Pretreatment with HA-1004 inhibits the increase in dopamine turnover during morphine withdrawal, whereas calphostin-C did not affect the DOPAC/dopamine ratio. Our results might indicate that expression of morphine dependence for hypothalamus-pituitary-adrenocortical axis hyperactivity involves PKC but not PKA signaling mechanisms. It is suggested that in rats PKC may be up-regulated during morphine dependence. High-performance liquid chromatography (HPLC) analysis of hypothalamic tissue from rats perfused with kinase inhibitors demonstrates that both calphostin-C and HA-1004 can cross the blood-brain barrier when administered peripherally.

The effects of dopamine receptor agents on naloxone-induced jumping behaviour in morphine-dependent mice

In the present study, the effects of dopamine receptor agonists and antagonists on naloxone-induced jumping in morphine-dependent mice were examined. Mice were rendered dependent as described in the methods section. Naloxone was injected to elicit jumping (as withdrawal sign). The first group received dopamine receptor drugs before naloxone injection to test the effects of the drugs on the expression of jumping. Administration of the dopamine D1/D2 receptor agonist, apomorphine (0.25, 0.5 and 1 mg/kg), decreased jumping, but not diarrhoea, induced by naloxone. The effect of apomorphine on jumping was reduced by the dopamine D2 receptor antagonist, sulpiride. The dopamine D2 receptor agonist, quinpirole (0.1, 0.3 and 0.5 mg/kg), increased jumping, while it decreased diarrhoea in mice. Different doses of sulpiride did not alter jumping, but one dose of the drug (12.5 mg/kg) decreased jumping. Neither the dopamine D1 receptor agonist, SKF38393 (1-phenyl-7,8-dihydroxy-2,3,4,5-tetrahydro-1H-3-benzazepine hydrochloride; 8 and 16 mg/kg), nor the dopamine D1 receptor antagonist, SCH23390 (R-(+)-8-chloro-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-benzazepine-7-ol maleate; 5, 10 and 25 mg/kg), altered jumping, but they decreased diarrhoea. The second group of animals received the drugs during the development of dependence. Administration of quinpirole (0.1, 0.3 and 0.5 mg/kg), but not bromocriptine (4, 8 and 16 mg/kg), apomorphine (0.25, 0.5, 1 and 2 mg/kg) or sulpiride (12.5, 25 and 50 mg/kg) decreased naloxone-induced jumping and diarrhoea. A dose of SKF38393 (8 mg/kg) decreased jumping, while both SKF38393 (4 and 16 mg/kg) and SCH23390 (5 and 10 microg/kg) increased diarrhoea. It is concluded that activation of both dopamine D1 and D2 receptors may suppress naloxone-induced jumping in morphine-dependent mice, and that stimulation of dopamine D1 receptors during development of morphine dependence may increase diarrhoea through peripheral mechanism.

The role of alpha2-adrenoceptors in the modulatory effects of morphine on seizure susceptibility in mice

PURPOSE

To evaluate the effect of the alpha2-adrenoceptor agonist clonidine and the antagonist yohimbine on the dual modulation of seizure susceptibility induced by morphine and the anticonvulsant effect of acute stress in mice.

METHODS

The thresholds for the clonic seizures induced after intravenous administration of pentylenetetrazole (PTZ) or bicuculline were assessed in mice weighing 23-30 g. Acute stress was induced by restraining mice for 2 h in a restrainer.

RESULTS

Morphine at lower doses (0.5, 1, and 3 mg/kg) increased and, at higher doses (15, 30, and 75 mg/kg), decreased the seizure threshold. Pretreatment with clonidine (0.001-0.1 mg/kg) inhibited the anticonvulsant effect of morphine, while potentiating its proconvulsant effect. Conversely, yohimbine (0.5-2 mg/kg) potentiated the anticonvulsant effect of morphine but inhibited its proconvulsant effects. Acute stress induced an anticonvulsant effect that was reversible by naloxone (1 mg/kg) or clonidine (0.05-0.1 mg/kg) or a combination of their lower doses (0.3 and 0.01 mg/kg, respectively), while being potentiated by yohimbine (1 mg/kg).

CONCLUSIONS

alpha2-Adrenoceptors play a dual role in the anticonvulsant effects of morphine. The activation of these receptors also can decrease the anticonvulsant effect of acute restraint stress in mice.

Cocaine and amphetamine attenuate the discriminative stimulus effects of naltrexone in opioid-dependent rhesus monkeys

This study tested the hypothesis that stimulants (indirect dopamine agonists) attenuate the discriminative stimulus of naltrexone in monkeys chronically treated with L-alpha-acetylmethadol (LAAM). Four rhesus monkeys (Macaca mulatta) received LAAM (1.0 mg/kg s.c.) twice daily and discriminated a withdrawal-precipitating dose of naltrexone (0.0178 mg/kg s.c.) from saline. Cocaine (0.1-1.78 mg/kg), amphetamine (0.32-1.78 mg/kg), haloperidol (0.01-0.1 mg/kg), sulpiride (1.0-10.0 mg/kg), propranolol (0.32-3.2 mg/kg), clonidine (0.001-0.1 mg/kg), desipramine (0.32-3.2 mg/kg), and imipramine (1.0-10.0 mg/kg) were given s.c. before cumulative doses of naltrexone. Cocaine and amphetamine antagonized the discriminative stimulus effects of naltrexone, each shifting the naltrexone dose-effect curve significantly (e.g., 100-fold) rightward or downward. In contrast, the dopamine antagonist haloperidol shifted the naltrexone dose-effect curve 5-fold leftward. Sulpiride, desipramine, clonidine, and propranolol had comparatively less effect on the naltrexone discriminative stimulus, whereas some doses of imipramine attenuated the naltrexone stimulus in a manner similar to that of cocaine and amphetamine. These results support the notion that multiple neurotransmitter systems are involved in the discriminative stimulus effects of opioid withdrawal. Furthermore, these data are consistent with reports that dopamine levels decrease during opioid withdrawal and provide evidence that enhancing dopamine or other monoamine levels may attenuate subjective effects of opioid withdrawal.

Brain-derived neurotrophic factor is essential for opiate-induced plasticity of noradrenergic neurons

Chronic opiate exposure induces numerous neurochemical adaptations in the noradrenergic system, including upregulation of the cAMP-signaling pathway and increased expression of tyrosine hydroxylase (TH), the rate-limiting enzyme in catecholamine biosynthesis. These adaptations are thought to compensate for opiate-mediated neuronal inhibition but also contribute to physical dependence, including withdrawal after abrupt cessation of drug exposure. Little is known about molecules that regulate the noradrenergic response to opiates. Here we report that noradrenergic locus ceruleus (LC) neurons of mice with a conditional deletion of BDNF in postnatal brain respond to chronic morphine treatment with a paradoxical downregulation of cAMP-mediated excitation and lack of dynamic regulation of TH expression. This was accompanied by a threefold reduction in opiate withdrawal symptoms despite normal antinociceptive tolerance in the BDNF-deficient mice. Although expression of TrkB, the receptor for BDNF, was high in the LC, endogenous BDNF expression was absent there and in the large majority of other noradrenergic neurons. Therefore, a BDNF-signaling pathway originating from non-noradrenergic sources is essential for opiate-induced molecular adaptations of the noradrenergic system.

The effects of simultaneous administration of alpha(2) -adrenergic agents with L-NAME or L-arginine on the development and expression of morphine dependence in mice

Both alpha(2)-adrenoceptors and the L-arginine/nitric oxide (NO) pathway have been implicated in the modulation of morphine dependence. This study examined the effects of simultaneous administration of the alpha(2)-adrenoceptor agonist clonidine or the antagonist yohimbine together with the NO precursor L-arginine or the NO synthase (NOS) inhibitor NG-nitro-L-arginine methyl ester (L-NAME) on the induction and expression of morphine dependence as assessed by naloxone-precipitated withdrawal jumping and diarrhoea. Male NMRI mice weighing 20-30 g were used. In the induction phase, clonidine (0.01-0.1 mg/kg) intensified and yohimbine (0.5-2 mg/kg) attenuated the degree of morphine dependence. Yohimbine reversed the effect of clonidine. L-NAME (5 and 10 mg/kg) did not affect the development of morphine dependence, but significantly potentiated the effects of both subeffective (0.01 mg/kg) and effective (0.03 mg/kg) doses of clonidine. L-Arginine did not alter morphine dependence but inhibited the effect of clonidine. The effects of yohimbine in the induction phase were attenuated by L-NAME, but were not significantly affected by L-arginine. In the expression phase, clonidine attenuated and yohimbine intensified the signs of dependence. The effect of clonidine was inhibited by yohimbine. In the expression phase, L-NAME attenuated the withdrawal syndrome at 10 mg/kg and showed potentiation with clonidine in suppressing withdrawal signs. L-Arginine did not alter morphine dependence, but at 20 mg/kg inhibited and at 100 mg/kg potentiated the attenuating effect of clonidine on the expression of withdrawal syndrome. The effect of yohimbine on the expression phase was also attenuated by L-NAME, but was not significantly affected by L-arginine. In conclusion, alpha(2)-adrenergic and NO pathways seem to be functionally linked in the modulation of opioid dependence.

Noradrenaline in the bed nucleus of the stria terminalis is critical for stress-induced reactivation of morphine-conditioned place preference in rats

The effect of noradrenaline in the bed nucleus of the stria terminalis and locus coeruleus on maintenance and reactivation of morphine-conditioned place preference induced by footshock stress was investigated in rats. After receiving alternate injection of morphine (10 mg/kg) and saline for 6 consecutive days, the rats spent more time in the drug-paired compartment (morphine-conditioned place preference) on day 7. These animals did not show morphine-conditioned place preference on day 37 following sham-footshock once every 3 days from days 8 to 36 (28 days drug-free). However, 15 min of intermittent footshock once every 3 days could induce the maintenance of morphine-conditioned place preference on day 37 with significantly more time spent in the drug-paired compartment than on day 0. Microinjection of the alpha(2)-adrenoceptor agonist, clonidine (0.1 or 1 microg), into the locus coeruleus 30 min before footshock did not affect stress-induced maintenance of conditioned place preference. However, infusions of clonidine (1 microg) into the bed nucleus of the stria terminalis significantly attenuated the maintenance of conditioned place preference induced by footshock stress. In another experiment, after a 21-day extinction of morphine-conditioned place preference, a single footshock could reactivate the morphine place preference that was significantly blocked by pretreatment with infusion of clonidine (0.1 or 1 microg) into the bed nucleus of the stria terminalis but not the locus coeruleus. Reactivation of morphine-conditioned place preference elicited by footshock stress was significantly inhibited by 6-hydroxydopamine-induced lesions in the ventral noradrenergic bundle, most of the norepinephrine input to the bed nucleus of the stria terminalis arising from caudal brain stem noradrenergic cell groups. In contrast, chemical lesions of the dorsal noradrenergic bundle that arises from the locus coeruleus had no such effects. These findings suggest that noradrenergic neurons in locus coeruleus are not involved in stress-induced reinstatement of drug-seeking and further clearly demonstrate that noradrenaline in the bed nucleus of the stria terminalis plays a critical role in mediating this effect. Comprehension of the neurochemical events underlying the stress-induced and the bed nucleus of the stria terminalis-mediated reinstatement of drug-seeking may, therefore, throw more light on the biological bases of drug dependence and addictive behavior

Neurobiology of the nicotine withdrawal syndrome

The aversive aspects of withdrawal from chronic nicotine exposure are thought to be an important motivational factor contributing to the maintenance of the tobacco habit in human smokers. Much emphasis has been placed on delineating the underlying neurobiological mechanisms mediating different components of the nicotine withdrawal syndrome. Recent studies have shown that both central and peripheral populations of nicotinic acetylcholine receptors (nAChRs) are involved in mediating somatic signs of nicotine withdrawal as measured by the rodent nicotine abstinence scale. However, only central populations of nAChRs are involved in mediating affective aspects of nicotine withdrawal, as measured by elevations in brain-stimulation reward thresholds and conditioned place aversion. Nicotine interacts with several neurotransmitter systems, including acetylcholine, dopamine, opioid peptides, serotonin, and glutamate systems. Evidence so far suggests that these neurotransmitters play a role in nicotine dependence and withdrawal processes. The available evidence also suggests that different underlying neurochemical deficits mediate somatic and affective components of nicotine withdrawal. The aim of the present review is to discuss preclinical findings concerning the neuroanatomical and neurochemical substrates involved in these different aspects of nicotine withdrawal.

Characterization of the signal transduction pathways mediating morphine withdrawal-stimulated c-fos expression in hypothalamic nuclei

The transcription factor, Fos, is considered as a functional marker of activated neurons. We have shown previously that acute administration of morphine induces the expression of Fos in hypothalamic nuclei associated with control of the hypothalamus-pituitary-adrenocortex axis, such as the paraventricular nucleus and the supraoptic nucleus. In the current study, we examined the role of protein kinase A, protein kinase C and Ca2+ entry through L-type Ca2+ channels in naloxone-precipitated Fos expression in the paraventricular and supraoptic nuclei. After 7 days of morphine treatment, we did not observe any modification in Fos production. However, when opioid withdrawal was precipitated with naloxone a dramatic increase in Fos immunoreactivity was observed in the parvocellular division of the paraventricular nucleus and in the supraoptic nucleus. Chronic co-administration of chelerythrine (a selective protein kinase C inhibitor acting at its catalytic domain) with morphine did not affect the increase in Fos expression observed in nuclei from morphine withdrawn rats. In addition, infusion of calphostin C (another protein kinase C inhibitor, which interacts with its regulatory domain) did not modify the morphine withdrawal-induced expression of Fos. In contrast, when the selective protein kinase A inhibitor, N-(2'guanidinoethyl)-5-isoquinolinesulfonamide (HA-1004), was infused it greatly diminished the increased Fos production observed in morphine-withdrawn rats. Furthermore, chronic infusion of the selective L-type Ca2+ channel antagonist, nimodipine, significantly inhibited the enhancement of Fos induction in the paraventricular and supraoptic nuclei from morphine-withdrawn animals. Taken together, these data might indicate that protein kinase A activity is necessary for the expression of Fos during morphine withdrawal and that an up-regulated Ca2+ system might contribute to the activation of Fos. The present findings suggest that protein kinase A and Ca2+ influx through L-type Ca2+ channels might contribute to the activation of neuroendocrine cells in the paraventricular and supraoptic nuclei.

Neurotrophin-3 modulates noradrenergic neuron function and opiate withdrawal

Somatic symptoms and aversion of opiate withdrawal, regulated by noradrenergic signaling, were attenuated in mice with a CNS-wide conditional ablation of neurotrophin-3. This occurred in conjunction with altered cAMP-mediated excitation and reduced upregulation of tyrosine hydroxylase in A6 (locus coeruleus) without loss of neurons. Transgene-derived NT-3 expressed by noradrenergic neurons of conditional mutants restored opiate withdrawal symptoms. Endogenous NT-3 expression, strikingly absent in noradrenergic neurons of postnatal and adult brain, is present in afferent sources of the dorsal medulla and is upregulated after chronic morphine exposure in noradrenergic projection areas of the ventral forebrain. NT-3 expressed by non-catecholaminergic neurons may modulate opiate withdrawal and noradrenergic signalling.

Subcellular localization of alpha-2A-adrenergic receptors in the rat medial nucleus tractus solitarius: regional targeting and relationship with catecholamine neurons

alpha-2A-adrenergic receptor (alpha2A-AR) agonists modulate diverse autonomic functions. These actions are believed to involve functionally specialized, second-order neurons in catecholamine-containing portions of the medial nucleus tractus solitarius (mNTS) at both intermediate (NTSi) and caudal (NTSc) levels. However, the cellular mechanisms subserving alpha2A-AR-mediated actions within the mNTS have yet to be established. Immunocytochemistry was employed to examine the subcellular distribution of alpha2A-AR in both the intermediate and caudal mNTS and its association with cells containing the catecholamine-synthesizing enzyme tyrosine hydroxylase (TH). Quantitative regional comparison using immunogold showed that this receptor was distributed differentially to dendrites (NTSi, 46%; NTSc, 31%) and glia (NTSi, 29%; NTSc, 48%) at different levels of the NTS. Somata, axons, and terminals less frequently contained alpha2A-AR. The subcellular distribution of alpha2A-AR relative to catecholaminergic neurons also was similar within both subregions. Approximately 50% of alpha2A-AR-labeled somata also contained TH. In somatic profiles, alpha2A-AR labeling was often found in the cytosol and in association with endoplasmic reticulum and Golgi complexes, sites of receptor synthesis and trafficking. Approximately 20% of alpha2A-AR-immunoreactive dendrites also contained TH, where the receptor was often found on extrasynaptic portions of the plasma membrane near unlabeled terminals, some of which made symmetric contacts. However, TH-labeled terminals and dendrites usually were detected in the neuropil at a short distance (<10 microm) from alpha2A-AR-labeled neurons. alpha2A-AR-labeled glia frequently apposed unlabeled dendrites and terminals and were often located near TH-immunoreactive dendrites. These results indicate that, within the mNTS, alpha2A-AR is involved in a variety of autonomic processes, including postsynaptic modulation of mostly noncatecholaminergic dendrites, as well as influencing glia functions.

Sensitivity to naloxone of the behavioral signs of morphine withdrawal and c-Fos expression in the rat CNS: a quantitative dose-response analysis

Several studies have used c-Fos expression to delineate the neural substrate underlying naloxone-precipitated morphine withdrawal (MW). However, because behavioral manifestations of MW depend on both the degree of dependence and the doses of naloxone (NAL), a comprehensive study would require examining c-Fos expression in relation with the degree of MW. Here, changes in behavior and in c-Fos-like immunoreactivity (FLI) were studied in the same rats after injection of three doses of NAL to precipitate various degrees of MW. Fifteen established signs of MW were examined for 1 hour after NAL injection, and FLI was quantified in 52 regions of the brain and in the lumbosacral spinal cord. Linear regression analyses were used to examine changes in numbers of signs and FLI neurons with the doses of NAL, and data were considered dose-related for a statistical level of significance of P < 0.05. In summary, autonomic signs of MW increased in a dose-related manner, whereas somatomotor signs did not. After MW, 33 central nervous system regions exhibited significant increases in FLI and were, thus, considered as important neural correlates of MW. Twenty of them displayed dose-related increases in c-Fos expression and correspond to regions related to autonomic functions. Low c-Fos expression was detected in some regions involved in motor control or in reward, suggesting either their minor role in MW or a limitation of the technique. This dose-response analysis suggests that the increase in the severity of autonomic manifestations of MW is associated with a gradual activation of major structures of the autonomic nervous system.

Injections of an opioid antagonist into the locus coeruleus and periaqueductal gray but not the amygdala precipitates morphine withdrawal in the 7-day-old rat

Opiate withdrawal behaviors in the infant differ from those of the adult. The neural circuitry underlying opioid withdrawal in the adult rat is well defined and includes the locus coeruleus (LC) and periaqueductal gray (PAG), with a minor role of the amygdala. Because the different behaviors that constitute the infant syndrome may be mediated by different neural circuits, we tested the hypothesis that these three sites are involved in opiate withdrawal. Pups were injected with morphine from day 1-6 after birth (b.i.d.) and on the morning of the seventh day. Withdrawal was then elicited by local injection of the opioid antagonist methylnaloxonium into the LC, PAG, or amygdala. Withdrawal signs were precipitated in a dose-dependent manner following injection into the LC or PAG, but not the amygdala. The withdrawal behaviors elicited from the LC and PAG included both the same and different behaviors. The results support the hypothesis that the neural circuitry mediating opiate withdrawal behaviors is similar in infant and adult animals, but the behaviors expressed are age-specific.

Cellular and synaptic adaptations mediating opioid dependence

Although opioids are highly effective for the treatment of pain, they are also known to be intensely addictive. There has been a massive research investment in the development of opioid analgesics, resulting in a plethora of compounds with varying affinity and efficacy at all the known opioid receptor subtypes. Although compounds of extremely high potency have been produced, the problem of tolerance to and dependence on these agonists persists. This review centers on the adaptive changes in cellular and synaptic function induced by chronic morphine treatment. The initial steps of opioid action are mediated through the activation of G protein-linked receptors. As is true for all G protein-linked receptors, opioid receptors activate and regulate multiple second messenger pathways associated with effector coupling, receptor trafficking, and nuclear signaling. These events are critical for understanding the early events leading to nonassociative tolerance and dependence. Equally important are associative and network changes that affect neurons that do not have opioid receptors but that are indirectly altered by opioid-sensitive cells. Finally, opioids and other drugs of abuse have some common cellular and anatomical pathways. The characterization of common pathways affected by different drugs, particularly after repeated treatment, is important in the understanding of drug abuse.

Chronic morphine treatment and withdrawal increase extracellular levels of norepinephrine in the rat bed nucleus of the stria terminalis

Extracellular levels of norepinephrine (NE) and glutamate (Glu) in the ventral bed nucleus of the stria terminalis (vBNST) of saline- and chronic morphine-treated rats, with or without withdrawal, were studied by means of the in vivo microdialysis technique in anesthetized rats. In addition, the tissue concentration of NE was studied at different rostrocaudal levels of the vBNST. Chronic morphine treatment significantly increased extracellular levels of NE, but not Glu, in vBNST. At 48 h after naloxone-induced morphine withdrawal there was a further significant increase in the extracellular levels of NE, but not Glu, in vBNST. The presence of UK 14304, an alpha(2)-adrenergic agonist, induced a significant decrease in NE extracellular levels in all experimental groups. In contrast, UK 14304 induced a significant decrease in Glu extracellular levels only in saline-treated rats. The results also show that the vBNST presents a rostrocaudal gradient of NE and contains 9.4% of total brain NE. The increase in NE extracellular levels in vBNST induced by chronic morphine treatment and the further increase in NE levels 48 h after naloxone-induced morphine withdrawal suggest that NE in vBNST may be involved in the pharmacological effects of chronic morphine and withdrawal.

Effects of chronic oral nicotine treatment and its withdrawal on locomotor activity and brain monoamines in mice

The effects of chronic nicotine and its withdrawal on locomotor activity and brain monoamines were studied using a new animal model of administering nicotine in the drinking water to male NMRI mice as the sole source of fluid. Locomotor activity as well as cerebral concentrations of dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), 5-hydroxytryptamine (5-HT), 5-hydroxyindoleacetic acid (5-HIAA), noradrenaline (NA) and 3-methoxy-4-hydroxyphenylethyleneglycol (MOPEG) were measured post mortem on the 50th day of nicotine administration or at 12-14 or 23-25 h after withdrawal. On the 50th day of drug administration the chronically nicotine-treated mice were more active than the control mice drinking tap water and after withdrawal from nicotine the locomotor activity dropped to the level of the controls. In chronically nicotine-treated mice the striatal concentrations of DOPAC, HVA and 5-HIAA, hypothalamic 5-HIAA and NA as well as cortical NA were elevated. The concentrations of DOPAC, HVA and 5-HIAA reversed to control levels within 23-25 h after withdrawal from nicotine. The nicotine-induced elevation of the hypothalamic NA concentration was still significant at 23-25 h after withdrawal. At 12-14 h after withdrawal the hypothalamic concentration of MOPEG was increased. In conclusion, our findings on locomotor activity suggest that administration of nicotine in the drinking water to mice for several weeks seems to be a relevant method to study nicotine dependence. Furthermore, the alterations found in cerebral DA, NA and 5-HT metabolism during chronic nicotine administration indicate that all three cerebral transmitter monoamines might be involved in nicotine dependence and withdrawal.

Changes in catecholaminergic pathways innervating the rat heart ventricle during morphine dependence. Involvement Of alpha(1)- and alpha(2)-adrenoceptors

In the present study, we examined the effects of alpha(1)- and the alpha(2)-adrenoceptors blockade on the changes in the ventricular content of catecholamines in rats withdrawn from morphine. Rats were given morphine by s.c. implantation of morphine pellets for 5 days. On the seventh day, morphine withdrawal was induced by s.c. administration of naloxone (1 mg/kg), and rats were killed 30 min later. Pretreatment with yohimbine (alpha(2)-adrenoceptor) or prazosin (alpha(1)-adrenoceptor) 15 min prior to naloxone administration attenuated some of the behavioural signs of morphine withdrawal. In addition, biochemical analysis indicated that yohimbine completely abolished the withdrawal-induced increase in noradrenaline and dopamine turnover in the right ventricle. By contrast, prazosin did not block the hyperactivity of catecholaminergic neurons in the heart during withdrawal. These data suggest that the hyperactivity of catecholaminergic neurons in the heart during morphine withdrawal is dependent upon alpha(2)-adrenoceptor activation. In addition, the present results rule out the involvement of alpha(1)-adrenoceptors.

The nucleus paragigantocellularis and opioid withdrawal-like behavior

Participation of the nucleus paragigantocellularis (PGi) in mediation of opioid withdrawal was examined in conscious, unrestrained, non-opioid-dependent rats, using electrical stimulation of the PGi. A characteristic series of behaviors, which resembled those seen during naloxone-precipitated withdrawal from dependence on the opioid agonist, butorphanol, was elicited during 30 min of PGi stimulation. Thus, the behavioral syndrome has been termed opioid withdrawal-like. Simultaneous microdialysis measurement of glutamate within the locus ceruleus indicated a positive correlation between extracellular glutamate concentrations and behavioral responses. Behavioral responses were inhibited by 50% during reverse dialysis perfusion of the locus ceruleus with the glutamate receptor antagonist, kynurenic acid, without any effect on glutamate concentrations. Thus, increases in locus ceruleus glutamate partially mediate opioid withdrawal-like behavior. Intracerebroventricular (i.c.v.) injections of the opioid antagonist, naloxone, or of the mu-selective (beta-funaltrexamine) or the delta-selective (naltrindole) opioid antagonists decreased, but did not abolish, stimulation-induced behavioral responses. Similar i.c.v. injections of the kappa-selective antagonist, nor-binaltorphimine, had no effect on behavioral responses to PGi stimulation. Activation of the PGi by electrical stimulation can elicit behaviors similar to those observed during opioid withdrawal. Moreover, additional levels of complexity are evident in the neuropharmacology of PGi stimulation-induced opioid withdrawal-like behavior.

Possible involvement of the locus coeruleus in inhibition by prostanoid EP(3) receptor-selective agonists of morphine withdrawal syndrome in rats

We examined the mechanism of the inhibitory effect of prostanoid EP(3) receptor agonists on naloxone-precipitated withdrawal syndrome in morphine-dependent rats. Rats were rendered morphine dependent by subcutaneous (s.c.) implantation of two pellets containing 75 mg morphine for 5 days. Morphine withdrawal syndrome was precipitated by i.p. injection of naloxone (3 mg/kg). Intracerebroventricular (i.c.v.) administration of (+/-)-15alpha-hydroxy-9-oxo-16-phenoxy-17,18, 19,20-tetranorprost-13-trans-enoic acid (M&B28,767: prostanoid EP(3) receptor agonist) or sulprostone (prostanoid EP(1)/EP(3) receptor agonist) significantly suppressed many withdrawal signs. Northern blotting and in situ hybridization studies revealed that i.c.v. administration of M&B28,767 (1 pg/rat) attenuated the elevation of c-fos mRNA during naloxone-precipitated withdrawal in many brain regions, including the cerebral cortex, thalamus, hypothalamus and locus coeruleus. Double in situ hybridization analysis revealed that in the locus coeruleus most of the tyrosine hydroxylase mRNA-positive neurons expressed mu-opioid receptor mRNA and more than half of these neurons were positive for prostanoid EP(3) receptor mRNA. These results indicate that the suppression by prostanoid EP(3) receptor agonists of naloxone-precipitated morphine withdrawal syndrome can be attributed to the inhibition of neuronal activity in several brain regions, including the locus coeruleus, the largest source of central noradrenergic neurons.

Noradrenaline in the ventral forebrain is critical for opiate withdrawal-induced aversion

Cessation of drug use in chronic opiate abusers produces a severe withdrawal syndrome that is highly aversive, and avoidance of withdrawal or associated stimuli is a major factor contributing to opiate abuse. Increased noradrenaline in the brain has long been implicated in opiate withdrawal, but it has not been clear which noradrenergic systems are involved. Here we show that microinjection of beta-noradrenergic-receptor antagonists, or of an alpha2-receptor agonist, into the bed nucleus of the stria terminalis (BNST) in rats markedly attenuates opiate-withdrawal-induced conditioned place aversion. Immunohistochemical studies revealed that numerous BNST-projecting cells in the A1 and A2 noradrenergic cell groups of the caudal medulla were activated during withdrawal. Lesion of these ascending medullary projections also greatly reduced opiate-withdrawal-induced place aversion, whereas lesion of locus coeruleus noradrenergic projections had no effect on opiate-withdrawal behaviour. We conclude that noradrenergic inputs to the BNST from the caudal medulla are critically involved in the aversiveness of opiate withdrawal.

The bed nucleus of the stria terminalis. A target site for noradrenergic actions in opiate withdrawal

Hyperactivity of brain norepinephrine (NE) systems has long been implicated in mechanisms of opiate withdrawal (OW). However, little is known about where elevated NE may act to promote OW. Here we report that the bed nucleus of the stria terminalis (BNST), the densest NE target in the brain, is critical for NE actions in OW. (1) Many BNST neurons become Fos+ after OW. Pretreatment with the beta antagonist, propranolol, markedly reduces OW symptoms and the number of Fos+ cells in the BNST. (2) Numerous neurons in the nucleus tractus solitarius (A2 neurons) and the A1 cell group are triple labeled for tyrosine hydroxylase, a retrograde tracer from the BNST, and Fos after OW, revealing numerous NE neurons that project to the BNST from the medulla that are stimulated by OW. Fewer such triple-labeled neurons were found in the locus caeruleus. (3) Behavioral studies reveal that local microinjections of selective beta-adrenergic antagonists into the BNST attenuate OW symptoms. In particular, withdrawal-induced place aversion is abolished by bilateral microinjection of a cocktail of selective beta 1 (betaxolol) plus the beta 2 (ICI 181,555) antagonists (1.0 nmol each/0.5 microL per side) into the BNST. Similar results were obtained with neurochemically selective lesions of the ventral ascending NE bundle, the pathway for A1 and A2 projections to the BNST. Similar lesions of the dorsal NE bundle of projections from the locus caeruleus had no effect on either aversive or somatic withdrawal symptoms. Together, these results indicate that beta-receptor activation in the BNST is critical for aversive withdrawal symptoms, and that A1 and A2 neurons in the medulla are the source of this critical NE.

Origin of noradrenergic afferents to the shell subregion of the nucleus accumbens: anterograde and retrograde tract-tracing studies in the rat

The nucleus accumbens (NAcc) can be subdivided into 'core' and 'shell' based on anatomical connections and histochemical markers. Previous studies have demonstrated dopamine-beta-hydroxylase immunoreactive (DBH-ir) fibers in the NAcc shell, but the source of these noradrenergic (NE) afferents has not been determined. Therefore, we have investigated in detail the anatomy of NE afferents to this subregion. Dual immunohistochemistry for DBH and substance P demonstrated numerous DBH-ir fibers in the caudal NAcc shell. Neurons projecting to the NAcc were identified with Fluoro-Gold (FG) or cholera toxin B (CTb) retrograde tracing and tyrosine hydroxylase (TH) immunohistochemistry. Single- and double-labeled neurons were observed in the A2 and A1 NE cell groups following FG injections into the caudal NAcc shell. Numerous FG and CTb single-labeled neurons were found in the rostral locus coeruleus (LC), subcoeruleus and pericoerulear dendritic region, with an occasional double-labeled neuron in the LC. Few labeled neurons were seen in the brainstem after FG injections into the NAcc core, consistent with the lack of DBH-ir in this subterritory. To confirm these results, injections of Phaseolus vulgaris leucoagglutinin or biotinylated dextran amine were made into the LC or nucleus tractus solitarius (NTS). Virtually no labeled fibers were observed in the NAcc following injections into central LC. However, fibers were observed in the NAcc shell after injections in the NTS. These results indicate that the primary source(s) of NE afferents to the NAcc shell is the A2 region of the NTS, with lesser contributions from A1 and LC.

Prevention of morphine-induced muscarinic (M2) receptor adaptation suppresses the expression of withdrawal symptoms

Treatment of opiate addiction is generally directed at the suppression of withdrawal symptoms through maintenance of the 'addicted' state with methadone. Yet relatively little is known regarding the neural substrates that contribute to, and maintain the prolonged state of withdrawal experienced by addicts. Opiates can profoundly alter the dynamics of brain and peripheral cholinergic systems, and central administration of anticholinergic drugs in dependent rats has been shown to decrease the expression of precipitated withdrawal symptoms. The purpose of this study was to determine whether the adaptive changes to M2 muscarinic receptors in autonomic centers are linked to the expression of withdrawal phenomena. During the peak period of withdrawal, there was a significant increase in both the expression of M2 muscarinic receptors and its corresponding mRNA within the rostral ventrolateral medulla, a primary vasomotor region. That most of these changes in receptor expression were adaptive in nature was suggested by the fact that when the acetylcholinesterase inhibitor DFP was co-administered with morphine, both the increased mRNA expression and the appearance of withdrawal symptoms were inhibited. Thus, interference with morphine-induced M2 muscarinic receptor adaptation in critical brain regions was correlated with a reduction in the development of physical dependence.

Endogenous opiates: 1997

This paper is the twentieth installment of our annual review of research concerning the opiate system. It summarizes papers published during 1997 that studied the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects, although stress-induced analgesia is included. The specific topics covered this year include stress; tolerance and dependence; eating and drinking; alcohol; gastrointestinal, renal, and hepatic function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurologic disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunologic responses; and other behaviors.