Noradrenergic and dopaminergic activity in the hypothalamic paraventricular nucleus after naloxone-induced morphine withdrawal

Opioid withdrawal: role in addiction and neural mechanisms

Withdrawal from opioids involves a negative affective state that promotes maintenance of drug-seeking behavior and relapse. As such, understanding the neurobiological mechanisms underlying withdrawal from opioid drugs is critical as scientists and clinicians seek to develop new treatments and therapies. In this review, we focus on the neural systems known to mediate the affective and somatic signs and symptoms of opioid withdrawal, including the mesolimbic dopaminergic system, basolateral amygdala, extended amygdala, and brain and hormonal stress systems. Evidence from preclinical studies suggests that these systems are altered following opioid exposure and that these changes mediate behavioral signs of negative affect such as aversion and anxiety during withdrawal. Adaptations in these systems also parallel the behavioral and psychological features of opioid use disorder (OUD), highlighting the important role of withdrawal in the development of addictive behavior. Implications for relapse and treatment are discussed as well as promising avenues for future research, with the hope of promoting continued progress toward characterizing neural contributors to opioid withdrawal and compulsive opioid use.

Dopamine Supersensitivity: A Novel Hypothesis of Opioid-Induced Neurobiological Mechanisms Underlying Opioid-Stimulant Co-use and Opioid Relapse

Emergent harms presented by the co-use of opioids and methamphetamine highlight the broader public health challenge of preventing and treating opioid and stimulant co-use. Development of effective therapeutics requires an understanding of the physiological mechanisms that may be driving co-use patterns, specifically the underlying neurobiology of co-use and how they may facilitate (or be leveraged to prevent) continued use patterns. This narrative review summarizes largely preclinical data that demonstrate clinically-meaningful relationships between the dopamine and opioid systems with direct implications for opioid and stimulant co-use. Synthesized conclusions of this body of research include evidence that changes in the dopamine system occur only once physical dependence to opioids develops, that the chronicity of opioid exposure is associated with the severity of changes, and that withdrawal leaves the organism in a state of substantive dopamine deficit that persists long after the somatic or observed signs of opioid withdrawal appear to have resolved. Evidence also suggests that dopamine supersensitivity develops soon after opioid abstinence and results in increased response to dopamine agonists that increases in magnitude as the abstinence period continues and is evident several weeks into protracted withdrawal. Mechanistically, this supersensitivity appears to be mediated by changes in the sensitivity, not quantity, of dopamine D2 receptors. Here we propose a neural circuit mechanism unique to withdrawal from opioid use with implications for increased stimulant sensitivity in previously stimulant-naïve or inexperienced populations. These hypothesized effects collectively delineate a mechanism by which stimulants would be uniquely reinforcing to persons with opioid physical dependence, would contribute to the acute opioid withdrawal syndrome, and could manifest subjectively as craving and/or motivation to use that could prompt opioid relapse during acute and protracted withdrawal. Preclinical research is needed to directly test these hypothesized mechanisms. Human laboratory and clinical trial research is needed to explore these clinical predictions and to advance the goal of developing treatments for opioid-stimulant co-use and/or opioid relapse prevention and withdrawal remediation.

Hormonal and neural responses to restraint stress in an animal model of perimenopause in female rats

The present study investigated the hormonal and neural responses to stress in a perimenopause animal model induced by 4-vinylcyclohexene diepoxide (VCD), which induces progressive follicular depletion in rodents, allowing studies on the transition to ovarian failure. Female rats, aged 28 days old, were s.c. injected for 15 consecutive days with corn oil or VCD. At 85 ± 5 days after the onset of treatment, the jugular vein was cannulated in the afternoon of metoestrus and in next morning (dioestrus) at 10.00 am, rats were subjected to 30 minutes of restraint stress. Blood samples were withdrawn before (-5 minutes), during (2, 5, 15 and 30 minutes) and after (45, 60 and 90 minutes) stress and plasma prolactin, progesterone and corticosterone levels were measured. Animals were perfused, brains processed for c-Fos/tyrosine hydroxylase (TH) in the locus coeruleus (LC) and c-Fos/corticotrophin-releasing factor (CRF) in the paraventricular nucleus (PVN). In unstressed rats the density of β-endorphin fibres was assessed in LC and PVN. In VCD-treated rats, stress-induced prolactin peak was higher, basal and peak progesterone levels were lower, and both levels of corticosterone were similar to controls. However, the recovery period was longer for both adrenal hormones. In VCD-treated rats the number of c-Fos/TH and c-Fos/CRF-immunoreactive neurones was higher whereas the density of β-endorphin fibres was lower in LC and PVN. We surmise that the hyperactivity of the LC and PVN neurones in VCD-treated rats may be a result of the lower progesterone levels that resulted in the decrease of β-endorphin content in both nuclei, thus impairing the negative-feedback mechanism in the recovery period.

Bee venom stimulation of a lung meridian acupoint reduces inflammation in carrageenan-induced pleurisy: an alternative therapeutic approach for respiratory inflammation

Respiratory inflammation is a frequent and fatal pathologic state encountered in veterinary medicine. Although diluted bee venom (dBV) has potent anti-inflammatory effects, the clinical use of dBV is limited to several chronic inflammatory diseases. The present study was designed to propose an acupoint dBV treatment as a novel therapeutic strategy for respiratory inflammatory disease. Experimental pleurisy was induced by injection of carrageenan into the left pleural space in mouse. The dBV was injected into a specific lung meridian acupoint (LU-5) or into an arbitrary non-acupoint located near the midline of the back in mouse. The inflammatory responses were evaluated by analyzing inflammatory indicators in pleural exudate. The dBV injection into the LU-5 acupoint significantly suppressed the carrageenan-induced increase of pleural exudate volume, leukocyte accumulation, and myeloperoxidase activity. Moreover, dBV acupoint treatment effectively inhibited the production of interleukin 1 beta, but not tumor necrosis factor alpha in the pleural exudate. On the other hand, dBV treatment at non-acupoint did not inhibit the inflammatory responses in carrageenan-induced pleurisy. The present results demonstrate that dBV stimulation in the LU-5 lung meridian acupoint can produce significant anti-inflammatory effects on carrageenan-induced pleurisy suggesting that dBV acupuncture may be a promising alternative medicine therapy for respiratory inflammatory diseases.

Changes in Expression of Dopamine, Its Receptor, and Transporter in Nucleus Accumbens of Heroin-Addicted Rats with Brain-Derived Neurotrophic Factor (BDNF) Overexpression

BACKGROUND The aim of this study was to explore how changes in the expression of BDNF in MLDS change the effect of BDNF on dopamine (DA) neurons, which may have therapeutic implications for heroin addiction. MATERIAL AND METHODS We established a rat model of heroin addiction and observed changes in the expression of BDNF, DA, dopamine receptor (DRD), dopamine transporter (DAT), and other relevant pathways in NAc. We also assessed the effect of BDNF overexpression in the NAc, behavioral changes of heroin-conditioned place preference (CPP), and naloxone withdrawal in rats with high levels of BDNF. We established 5 adult male rat groups: heroin addiction, lentivirus transfection, blank virus, sham operation, and control. The PCR gene chip was used to study gene expression changes. BDNF lentivirus transfection was used for BDNF overexpression. A heroin CPP model and a naloxone withdrawal model of rats were established. RESULTS Expression changes were found in 20 of the 84 DA-associated genes in the NAc of heroin-addicted rats. Weight loss and withdrawal symptoms in the lentivirus group for naloxone withdrawal was less than in the blank virus and the sham operation group. These 2 latter groups also showed significant behavioral changes, but such changes were not observed in the BDNF lentivirus group before or after training. DRD3 and DAT increased in the NAc of the lentivirus group. CONCLUSIONS BDNF and DA in the NAc are involved in heroin addiction. BDNF overexpression in NAc reduces withdrawal symptoms and craving behavior for medicine induced by environmental cues for heroin-addicted rats. BDNF participates in the regulation of the dopamine system by acting on DRD3 and DAT.

Hypothalamic Norepinephrine Mediates Acupunctural Effects on Hypothalamic-Pituitary-Adrenal Axis During Ethanol Withdrawal

A previous study demonstrated that acupuncture at ST36 (Zu-San-Li) attenuates ethanol withdrawal (EW)-induced hyperactivation of the hypothalamic-pituitary-adrenal axis in rats. The current study investigated the involvement of hypothalamic norepinephrine (NE) in that process. Rats were intraperitoneally treated with 3 g/kg/d of ethanol or saline for 28 days. After 24 hours of EW, acupuncture was applied to rats at bilateral ST36 points or at nonacupoints (tail) for 1 minute. A high-performance liquid chromatography analysis showed that EW significantly increased both the NE and the 3-methoxy-4-hydroxy-phenylglycol (MHPG) levels in the hypothalamic paraventricular nucleus (PVN). Western blot analysis also revealed that EW markedly elevated the phosphorylation rates of tyrosine hydroxylase (TH), but spared TH protein expression in the PVN. However, acupuncture at ST36, but not at nonacupoints, greatly inhibited the increase in the hypothalamic NE, MHPG, and phosphorylation rates of TH. Additionally, postacupuncture infusion of NE into the PVN significantly attenuated the inhibitory effects of acupuncture at ST36 on the oversecretion of plasma corticosterone during EW. These results suggest that acupuncture at ST36 inhibits EW-induced hyperactivation of the hypothalamic NEergic system to produce therapeutic effects on the hypothalamic-pituitary-adrenal axis.

Role of corticotropin-releasing factor (CRF) receptor-1 on the catecholaminergic response to morphine withdrawal in the nucleus accumbens (NAc)

Stress induces the release of the peptide corticotropin-releasing factor (CRF) into the ventral tegmental area (VTA), and also increases dopamine (DA) levels in brain regions receiving dense VTA input. Since the role of stress in drug addiction is well established, the present study examined the possible involvement of CRF1 receptor in the interaction between morphine withdrawal and catecholaminergic pathways in the reward system. The effects of naloxone-precipitated morphine withdrawal on signs of withdrawal, hypothalamo-pituitary-adrenocortical (HPA) axis activity, dopamine (DA) and noradrenaline (NA) turnover in the nucleus accumbens (NAc) and activation of VTA dopaminergic neurons, were investigated in rats pretreated with vehicle or CP-154,526 (selective CRF1R antagonist). CP-154,526 attenuated the increases in body weight loss and suppressed some of withdrawal signs. Pretreatment with CRF1 receptor antagonist resulted in no significant modification of the increased NA turnover at NAc or plasma corticosterone levels that were seen during morphine withdrawal. However, blockade of CRF1 receptor significantly reduced morphine withdrawal-induced increases in plasma adrenocorticotropin (ACTH) levels, DA turnover and TH phosphorylation at Ser40 in the NAc. In addition, CP-154,526 reduced the number of TH containing neurons expressing c-Fos in the VTA after naloxone-precipitated morphine withdrawal. Altogether, these results support the idea that VTA dopaminergic neurons are activated in response to naloxone-precipitated morphine withdrawal and suggest that CRF1 receptors are involved in the activation of dopaminergic pathways which project to NAc.

Opioids inhibit visceral afferent activation of catecholamine neurons in the solitary tract nucleus

Brainstem A2/C2 catecholamine (CA) neurons within the solitary tract nucleus (NTS) influence many homeostatic functions, including food intake, stress, respiratory and cardiovascular reflexes. They also play a role in both opioid reward and withdrawal. Injections of opioids into the NTS modulate many autonomic functions influenced by catecholamine neurons including food intake and cardiac function. We recently showed that NTS-CA neurons are directly activated by incoming visceral afferent inputs. Here we determined whether opioid agonists modulate afferent activation of NTS-CA neurons using transgenic mice with EGFP expressed under the control of the tyrosine hydroxylase promoter (TH-EGFP) to identify catecholamine neurons. The opioid agonist Met-enkephalin (Met-Enk) significantly attenuated solitary tract-evoked excitatory postsynaptic currents (ST-EPSCs) in NTS TH-EGFP neurons by 80%, an effect reversed by wash or the mu opioid receptor-specific antagonist D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH(2) (CTOP). Met-Enk had a significantly greater effect to inhibit afferent inputs onto TH-EGFP-positive neurons than EGFP-negative neurons, which were only inhibited by 50%. The mu agonist, DAMGO, also inhibited the ST-EPSC in TH-EGFP neurons in a dose-dependent manner. In contrast, neither the delta agonist DPDPE, nor the kappa agonist, U69,593, consistently inhibited the ST-EPSC amplitude. Met-Enk and DAMGO increased the paired pulse ratio, decreased the frequency, but not amplitude, of mini-EPSCs and had no effect on holding current, input resistance or current-voltage relationships in TH-EGFP neurons, suggesting a presynaptic mechanism of action on afferent terminals. Met-Enk significantly reduced both the basal firing rate of NTS TH-EGFP neurons and the ability of afferent stimulation to evoke an action potential. These results suggest that opioids inhibit NTS-CA neurons by reducing an excitatory afferent drive onto these neurons through presynaptic inhibition of glutamate release and elucidate one potential mechanism by which opioids could control autonomic functions and modulate reward and opioid withdrawal symptoms at the level of the NTS.

Morphine withdrawal activates hypothalamic-pituitary-adrenal axis and heat shock protein 27 in the left ventricle: the role of extracellular signal-regulated kinase

The negative affective states of withdrawal involve the recruitment of brain and peripheral stress circuitry [e.g., noradrenergic activity, induction of the hypothalamo-pituitary-adrenocortical (HPA) axis, and the expression and activation of heat shock proteins (Hsps)]. The present study investigated the role of extracellular signal-regulated protein kinase (ERK) and β-adrenoceptor on the response of stress systems to morphine withdrawal by the administration of [amino[(4-aminophenyl)thio]methylene]-2-(trifluoromethyl)benzeneacetonitrile (SL327), a selective inhibitor of ERK activation, or propranolol (a β-adrenoceptor antagonist). Dependence on morphine was induced by a 7-day subcutaneous implantation of morphine pellets. Morphine withdrawal was precipitated on day 8 by the injection of naloxone (2 mg/kg s.c.). Plasma concentrations of adrenocorticotropin and corticosterone were determined by radioimmunoassay; noradrenaline (NA) turnover in left ventricle was determined by high-performance liquid chromatography; and catechol-O-methyl transferase (COMT) and Hsp27 expression and phosphorylation at Ser82 were determined by quantitative blot immunolabeling. Morphine-withdrawn rats showed an increase of NA turnover and COMT expression in parallel with an enhancement of adrenocorticotropin and plasma corticosterone concentrations. In addition, we observed an enhancement of Hsp27 expression and phosphorylation. Pretreatment with SL327 or propranolol significantly reduced morphine withdrawal-induced increases of plasma adrenocorticotropin and Hsp27 phosphorylation at Ser82 without any changes in plasma corticosterone levels. The present findings demonstrate that morphine withdrawal is capable of inducing the activation of HPA axis in parallel with an enhancement of Hsp27 expression and Hsp27 phosphorylation at Ser82 and suggest a role for β-adrenoceptors and ERK pathways in mediating morphine-withdrawal activation of the HPA axis and cellular stress response.

Behavioral effects of fatty acid amide hydrolase inhibition on morphine withdrawal symptoms

Chronic morphine exposure causes tolerance and dependence. The cessation of morphine consumption induces a withdrawal syndrome that may involve cannabinoids and is characterized by undesirable psychological and physical signs. The present study examined whether augmentation of the endocannabinoid system by inhibition of fatty acid amide hydrolase could suppress the morphine withdrawal syndrome in morphine-addicted rats. Morphine dependency was induced by 7 consecutive days of morphine injection. The morphine-addicted rats received URB597 (1, 0.5, 0.3, 0.1, 0.03 mg/kg), a fatty acid amide hydrolase inhibitor, before the precipitation of morphine withdrawal syndromes by naloxone. Withdrawal symptoms including jumping, teeth chattering, paw tremor, wet dog shakes, face grooming, penis licking, standing, rearing, sniffing and percent of weight loss were recorded during 30 min after naloxone injection. The results showed that the morphine withdrawal precipitated rats had significantly more withdrawal symptoms than naive control rats and the administration of URB597 (all doses except 0.03 mg/kg) reduced most of the morphine withdrawal symptoms. We conclude that the administration of URB597 modulated morphine withdrawal symptoms. This finding shows that endocannabinoids interact with the opioid system during the morphine withdrawal period and that potentiation of the endogenous cannabinoid system by URB597 may be a new target strategy for the management of morphine addiction.

Social influences on plasma testosterone levels in morphine withdrawn adolescent mice and their drug-naïve cage-mates

Opioid administration in males results in opioid-induced androgen deficiency which persists throughout the treatment. In adults, this quickly reverses once opioid administration is suspended. However, less is known about the duration of the effect following drug discontinuation in adolescents. Given the significant implications to sexual maturation in adolescent males, this study examined plasma testosterone levels in both morphine withdrawn mice and their drug-naïve (saline-injected) cage-mates as compared to drug-naïve mice housed physically and visually separate from the morphine-treated mice ('saline only'). Consistent with the literature, plasma testosterone levels in morphine withdrawn adults were reduced on withdrawal day 1 (WD1) and returned to baseline levels by WD9. No significant effects were observed in their saline cage-mates. In the adolescents, no significant differences were observed on WD1 between the morphine withdrawn mice, their saline cage-mates, and the saline only mice - all of which had significantly lower plasma testosterone levels than adults. By WD9, testosterone levels in the saline only adolescent mice had reached adult levels. Notably, plasma testosterone levels were reduced in both the morphine withdrawn adolescent mice and their saline cage-mates, as compared to saline only mice. The effect was not a drug effect per se, given that reduced plasma testosterone levels were not observed in individually housed morphine withdrawn mice. Moreover, our results also suggest that these social effects are not solely explained by stress. These results have numerous implications to the short term and long term health of both adolescents requiring pain management and of adolescent drug addicts.

CRF₂ mediates the increased noradrenergic activity in the hypothalamic paraventricular nucleus and the negative state of morphine withdrawal in rats

BACKGROUND AND PURPOSE

Recent evidence suggests that corticotropin-releasing factor (CRF) receptor signalling is involved in modulating the negative symptoms of opiate withdrawal. In this study, a series of experiments were performed to further characterize the role of CRF-type 2 receptor (CRF₂) signalling in opiate withdrawal-induced physical signs of dependence, hypothalamus-pituitary-adrenal (HPA) axis activation, enhanced noradrenaline (NA) turnover in the hypothalamic paraventricular nucleus (PVN) and tyrosine hydroxylase (TH) phosphorylation (activation), as well as CRF₂ expression in the nucleus of the solitary tract-A₂ noradrenergic cell group (NTS-A₂).

EXPERIMENTAL APPROACH

The contribution of CRF₂ signalling in opiate withdrawal was assessed by i.c.v. infusion of the selective CRF₂ antagonist, antisauvagine-30 (AS-30). Rats were implanted with two morphine (or placebo) pellets. Six days later, rats were pretreated with AS-30 or saline 10 min before naloxone and the physical signs of abstinence, the HPA axis activity, NA turnover, TH activation and CRF₂ expression were measured using immunoblotting, RIA, HPLC and immunohistochemistry.

KEY RESULTS

Rats pretreated with AS-30 showed decreased levels of somatic signs of naloxone-induced opiate withdrawal, but the corticosterone response was not modified. AS-30 attenuated the increased production of the NA metabolite, 3-methoxy-4-hydroxyphenylglycol, as well as the enhanced NA turnover observed in morphine-withdrawn rats. Finally, AS-30 antagonized the TH phosphorylation at Serine40 induced by morphine withdrawal.

CONCLUSIONS AND IMPLICATIONS

These results suggest that physical signs of opiate withdrawal, TH activation and stimulation of noradrenergic pathways innervating the PVN are modulated by CRF₂ signalling. Furthermore, they indicate a marginal role for the HPA axis in CRF₂-mediation of opiate withdrawal.

Hindbrain noradrenergic A2 neurons: diverse roles in autonomic, endocrine, cognitive, and behavioral functions

Central noradrenergic (NA) signaling is broadly implicated in behavioral and physiological processes related to attention, arousal, motivation, learning and memory, and homeostasis. This review focuses on the A2 cell group of NA neurons, located within the hindbrain dorsal vagal complex (DVC). The intra-DVC location of A2 neurons supports their role in vagal sensory-motor reflex arcs and visceral motor outflow. A2 neurons also are reciprocally connected with multiple brain stem, hypothalamic, and limbic forebrain regions. The extra-DVC connections of A2 neurons provide a route through which emotional and cognitive events can modulate visceral motor outflow and also a route through which interoceptive feedback from the body can impact hypothalamic functions as well as emotional and cognitive processing. This review considers some of the hallmark anatomical and chemical features of A2 neurons, followed by presentation of evidence supporting a role for A2 neurons in modulating food intake, affective behavior, behavioral and physiological stress responses, emotional learning, and drug dependence. Increased knowledge about the organization and function of the A2 cell group and the neural circuits in which A2 neurons participate should contribute to a better understanding of how the brain orchestrates adaptive responses to the various threats and opportunities of life and should further reveal the central underpinnings of stress-related physiological and emotional dysregulation.

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.

Induction of FosB/DeltaFosB in the brain stress system-related structures during morphine dependence and withdrawal

The transcription factor DeltaFosB is induced in the nucleus accumbens (NAc) by drugs of abuse. This study was designed to evaluate the possible modifications in FosB/DeltaFosB expression in both hypothalamic and extrahypothalamic brain stress system during morphine dependence and withdrawal. Rats were made dependent on morphine and, on day 8, were injected with saline or naloxone. Using immunohistochemistry and western blot, the expression of FosB/DeltaFosB, tyrosine hydroxylase (TH), corticotropin-releasing factor (CRF) and pro-dynorphin (DYN) was measured in different nuclei from the brain stress system in morphine-dependent rats and after morphine withdrawal. Additionally, we studied the expression of FosB/DeltaFosB in CRF-, TH- and DYN-positive neurons. FosB/DeltaFosB was induced after chronic morphine administration in the parvocellular part of the hypothalamic paraventricular nucleus (PVN), NAc-shell, bed nucleus of the stria terminalis, central amygdala and A(2) noradrenergic part of the nucleus tractus solitarius (NTS-A(2)). Morphine dependence and withdrawal evoked an increase in FosB/DeltaFosB-TH and FosB/DeltaFosB-CRF double labelling in NTS-A(2) and PVN, respectively, besides an increase in TH levels in NTS-A(2) and CRF expression in PVN. These data indicate that neuroadaptation to addictive substances, observed as accumulation of FosB/DeltaFosB, is not limited to the reward circuits but may also manifest in other brain regions, such as the brain stress system, which have been proposed to be directly related to addiction.

Effects of galanin on monoaminergic systems and HPA axis: Potential mechanisms underlying the effects of galanin on addiction- and stress-related behaviors

Like a number of neuropeptides, galanin can alter neural activity in brain areas that are important for both stress-related behaviors and responses to drugs of abuse. Accordingly, drugs that target galanin receptors can alter behavioral responses to drugs of abuse and can modulate stress-related behaviors. Stress and drug-related behaviors are interrelated: stress can promote drug-seeking, and drug exposure and withdrawal can increase activity in brain circuits involved in the stress response. We review here what is known about the ability of galanin and galanin receptors to alter neuronal activity, and we discuss potential mechanisms that may underlie the effects of galanin on behaviors involved in responses to stress and addictive drugs. Understanding the mechanisms underlying galanin's effects on neuronal function in brain regions related to stress and addiction may be useful in developing novel therapeutics for the treatment of stress- and addiction-related disorders.

Effects of rolipram and diazepam on the adaptive changes induced by morphine withdrawal in the hypothalamic paraventricular nucleus

A role for the cyclic AMP systems in the development of morphine dependence has been previously reported. In this study we investigated whether morphine dependence was inhibited by phosphodiesterase (PDE) 4 inhibitors rolipram and diazepam. Dependence on morphine was induced by a 7-day s.c. implantation of morphine pellets. On day 8, morphine withdrawal was precipitated by an injection of naloxone. In order to determine the effect of rolipram and diazepam rats were injected with these drugs once daily for seven days as well as 30 min before of naloxone injection. When opioid withdrawal was precipitated, an enhanced noradrenaline turnover and increased level of cyclic AMP and cyclic GMP in the hypothalamic paraventricular nucleus (PVN) were observed 30 min after naloxone administration. Moreover, c-Fos expression was induced in the PVN after naloxone-precipitated morphine withdrawal. Co-administration of rolipram or diazepam with morphine during the pre-treatment period, significantly reduced the signs of withdrawal, the enhancement of noradrenaline turnover and the increase in cyclic AMP. However, these inhibitors did not modify either levels of cyclic GMP or c-Fos expression in the PVN. These findings demonstrate that co-administration of rolipram or diazepam with morphine attenuate the withdrawal syndrome and suggest that these compounds may prevent the up-regulation of the cyclic AMP pathway and the associated increase in cyclic AMP level in morphine-withdrawn rats.

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

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

Regulation of extracellular signal-regulated kinases (ERKs) by naloxone-induced morphine withdrawal in the brain stress system

Our previous studies have shown that morphine withdrawal increases the hypothalamic-pituitary-adrenocortical axis activity, which is dependent on a hyperactivity of noradrenergic pathways (nucleus tractus solitarius-A(2)) innervating the hypothalamic paraventricular nucleus. The extracellular signal-regulated kinase has been implicated in drug addiction, but its role in activation of paraventricular nucleus and nucleus tractus solitarius during morphine dependence remain poorly understood. We have determined the activation of extracellular signal-regulated kinase during morphine dependence and withdrawal as well as its involvement in morphine withdrawal-induced gene expression. We show that naloxone-induced morphine withdrawal activates extracellular signal-regulated kinases(1/2) and increases c-Fos expression in rat paraventricular nucleus and nucleus tractus solitarius-A(2) neurons. Activated extracellular signal-regulated kinases(1/2) was colocalized with c-Fos in both nuclei, and this response was blocked by SL327, a drug that prevents extracellular signal-regulated kinase activation. In the paraventricular nucleus from morphine-withdrawn rats, the number of neurons expressing CRF was increased. Immunohistochemical study showed a dramatic increase in c-Fos immunoreactivity within CRF-positive cells. These results suggest that extracellular signal-regulated kinases1/2 signaling pathway is necessary for morphine withdrawal-induced activation of brain areas associated with the stress system.

L-type calcium channel blockade attenuates morphine withdrawal: in vivo interaction between L-type calcium channels and corticosterone

Both opioids and calcium channel blockers could affect hypothalamic-pituitary-adrenal (HPA) axis function. Nifedipine, as a calcium channel blocker, can attenuate the development of morphine dependence; however, the role of the HPA axis in this effect has not been elucidated. We examined the effect of nifedipine on the induction of morphine dependency in intact and adrenalectomized (ADX) male rats, as assessed by the naloxone precipitation test. We also evaluated the effect of this drug on HPA activity induced by naloxone. Our results showed that despite the demonstration of dependence in both groups of rats, nifedipine is more effective in preventing of withdrawal signs in ADX rats than in sham-operated rats. In groups that received morphine and nifedipine concomitantly, naloxone-induced corticosterone secretion was attenuated. Thus, we have shown the involvement of the HPA axis in the effect of nifedipine on the development of morphine dependency and additionally demonstrated an in vivo interaction between the L-type Ca2+ channels and corticosterone.

Enhanced immune sensitivity to stress following chronic morphine exposure

Chronic administration of escalating doses ofmorphine leads to neuroadaptive changes precipitating development of tolerance to many of the acute effects of morphine, such as analgesia, activation of the hypothalamic-pituitary-adrenal (HPA) axis and suppression of immune cell activities. Interestingly, morphine tolerance has also been shown to be accompanied by heightened immunosuppressive effects of restraint stress using a rodent model. These observations have led to the hypothesis that the altered neuronal state accompanying opioid tolerance may contribute to this enhanced immune sensitivity to stress. To further test this hypothesis using different stressors, Sprague-Dawley rats were treated chronically with morphine for at least 8 days and then challenged with either psychological (water stress) or systemic stressors [morphine withdrawal, lipopolysaccharide (10 mug/kg i.p. challenge)]. It was found that, independent of the type of stress employed, morphine-tolerant animals displayed significantly lower mitogen-stimulated blood lymphocyte responses when compared to the responses of similarly treated saline controls. To determine whether direct activation of central stress pathways may also lead to enhanced immune sensitivity, morphine-tolerant animals were centrally injected with IL-1beta (1 ng/mul i.c.v.), a cytokine that activates the HPA axis by central mechanisms. Similar to the other types of stress, this direct central challenge was also found to be more immunosuppressive in morphine-tolerant animals compared to controls. Collectively, these studies demonstrate that morphine-tolerant animals have an enhanced susceptibility to the debilitating effects of a variety of stressors on immune cell function, an effect that is likely due to the neuroadaptive changes that develop during chronic morphine exposure.

Regulation of serine (Ser)-31 and Ser40 tyrosine hydroxylase phosphorylation during morphine withdrawal in the hypothalamic paraventricular nucleus and nucleus tractus solitarius-A2 cell group: role of ERK1/2

Our previous studies have shown that naloxone-induced morphine withdrawal increases the hypothalamic-pituitary-adrenocortical (HPA) axis activity, which is dependent on a hyperactivity of noradrenergic pathways [nucleus tractus solitarius (NTS) A(2)] innervating the hypothalamic paraventricular nucleus (PVN). Short-term regulation of catecholamine biosynthesis occurs through phosphorylation of tyrosine hydroxylase (TH), which enhances enzymatic activity. In the present study, the effect of morphine withdrawal on site-specific TH phosphorylation in the PVN and NTS-A(2) was determined by quantitative blot immunolabeling and immunohistochemistry using phosphorylation state-specific antibodies. We show that naloxone-induced morphine withdrawal phosphorylates TH at Serine (Ser)-31 but not Ser40 in PVN and NTS-A(2), which is associated with both an increase in total TH immunoreactivity in NTS-A(2) and an enhanced TH activity in the PVN. In addition, we demonstrated that TH neurons phosphorylated at Ser31 coexpress c-Fos in NTS-A(2). We then tested whether pharmacological inhibition of ERK activation by ERK kinase contributes to morphine withdrawal-induced phosphorylation of TH at Ser31. We show that the ability of morphine withdrawal to stimulate phosphorylation at this seryl residue is reduced by SL327, an inhibitor of ERK(1/2) activation. These results suggest that morphine withdrawal increases noradrenaline turnover in the PVN, at least in part, via ERK(1/2)-dependent phosphorylation of TH at Ser31.

Activation of the ERK signalling pathway contributes to the adaptive changes in rat hearts during naloxone-induced morphine withdrawal

BACKGROUND AND PURPOSE

We have previously demonstrated that morphine withdrawal induced hyperactivity of the heart by activation of noradrenergic pathways innervating the left and right ventricle, as evaluated by noradrenaline turnover and c-Fos expression. The extracellular signal-regulated kinase (ERK) has been implicated in drug addiction, but its role in activation of the heart during morphine dependence remains poorly understood. Here, we have looked for activation of ERK during morphine withdrawal and if this activation induced gene expression.

EXPERIMENTAL APPROACH

Dependence on morphine was induced by s.c. implantation of morphine pellets for 7 days. Morphine withdrawal was precipitated on day 8 by injection of naloxone (2 mg kg(-1), s.c.). ERK1/2, their phosphorylated forms and c-Fos were measured by western blotting and immunohistochemistry of cardiac tissue.

KEY RESULTS

Naloxone-induced morphine withdrawal activated ERK1/2 and increased c-Fos expression in cardiac tissues. c-Fos expression was blocked by SL327, a drug that prevents ERK activation.

CONCLUSIONS AND IMPLICATIONS

These results indicate that signalling through the ERKs is necessary for morphine withdrawal-induced hyperactivity of the heart and suggest that this pathway may also be involved in activation of immediate-early genes in both cytosolic and nuclear effector mechanisms that have the potential to bring about long-term changes in the heart.

Activation of stress‐related hypothalamic neuropeptide gene expression during morphine withdrawal

Morphine withdrawal results in serious affective and somatic symptoms including activation of the hypothalamo-pituitary-adrenocortical (HPA) axis. To reveal secretory, activational and transcriptional changes in the hypothalamus of morphine-dependent rats during naloxone precipitated opioid withdrawal, we measured corticosterone secretion, c-Fos induction and heteronuclear (hn)RNA levels of corticotropin-releasing hormone (CRH) and arginine vasopressin (AVP) in naïve and morphine dependent animals injected with saline or 5 mg/kg naloxone. Naloxone precipitated morphine withdrawal resulted in a significant increase in corticosterone secretion and induction of neuronal activation in the hypothalamic paraventricular nucleus (PVH) 2 h after challenge. Using probes complementary to intronic sequences of genes encoding neuropeptides in parvocellular neurosecretory neurons of the PVH, we found robust increases in CRH and AVP hnRNAs in morphine dependent rats during naloxone precipitated withdrawal. Naïve rats and animals that were implanted with morphine pellets for 8 days did not display significant up-regulation of ongoing neuropeptide expression in the parvocellular compartment of the PVH. In addition to hypophyseotropic neurons, naloxone precipitated withdrawal resulted in a marked activation in autonomic-related projection neurons in PVH and in the magnocellular neurons in the PVH and supraoptic nuclei. These activations however were not associated with induction of CRH or AVP hnRNAs.

Locus ceruleus control of slow-wave homeostasis

Sleep intensity is regulated by the duration of previous wakefulness, suggesting that waking results in the progressive accumulation of sleep need (Borbely and Achermann, 2000). In mammals, sleep intensity is reflected by slow-wave activity (SWA) in the nonrapid eye movement (NREM) sleep electroencephalogram, which increases in proportion to the time spent awake. However, the mechanisms responsible for the increase of NREM SWA after wakefulness remain unclear. According to a recent hypothesis (Tononi and Cirelli, 2003), the increase in SWA occurs because during wakefulness, many cortical circuits undergo synaptic potentiation, as evidenced by the widespread induction of long-term potentiation (LTP)-related genes in the brain of awake animals. A direct prediction of this hypothesis is that manipulations interfering with the induction of LTP-related genes should result in a blunted SWA response. Here, we examined SWA response in rats in which cortical norepinephrine (NA) was depleted, a manipulation that greatly reduces the induction of LTP-related genes during wakefulness (Cirelli and Tononi, 2004). We found that the homeostatic response of the lower-range SWA was markedly and specifically reduced after NA depletion. These data suggest that the wake-dependent accumulation of sleep need is causally related to cellular changes dependent on NA release, such as the induction of LTP-related genes, and support the hypothesis that sleep SWA homeostasis may be related to synaptic potentiation during wakefulness.

Neonatal animal models of opiate withdrawal

The symptoms of opiate withdrawal in infants are defined as neonatal abstinence syndrome (NAS). NAS is a significant cause of morbidity in term and preterm infants. Factors, such as polysubstance abuse, inadequate prenatal care, nutritional deprivation, and the biology of the developing central nervous system contribute to the challenge of evaluating and treating opiate-induced alterations in the newborn. Although research on the effects of opiates in neonatal animal models is limited, the data from adult animal models have greatly contributed to understanding and treating opiate tolerance, addiction, and withdrawal in adult humans. Yet the limited neonatal data that are available indicate that the mechanisms involved in these processes in the newborn differ from those in adult animals, and that neonatal models of opiate withdrawal are needed to understand and develop effective treatment regimens for NAS. In this review, the behavioral and neurochemical evidence from the literature is presented and suggests that mechanisms responsible for opiate tolerance, dependence, and withdrawal differ between adult and neonatal models. Also reviewed are studies that have used neonatal rodent models, the authors' preliminary data based on the use of neonatal rat and mouse models of opiate withdrawal, and other neonatal models that have been proposed for the study of neonatal opiate withdrawal.

Role of PKC-α,γ isoforms in regulation of c-Fos and TH expression after naloxone-induced morphine withdrawal in the hypothalamic PVN and medulla oblongata catecholaminergic cell groups

We previously demonstrated that morphine withdrawal induced hyperactivity of the hypothalamus-pituitary-adrenocortical axis by activation of noradrenergic pathways innervating the hypothalamic paraventricular nucleus (PVN), as evaluated by Fos expression and corticosterone release. The present study was designed to investigate the role of protein kinase C (PKC) in this process by estimating changes in PKCalpha and PKCgamma immunoreactivity, and whether pharmacological inhibition of PKC would attenuate morphine withdrawal-induced c-Fos expression and changes in tyrosine hydroxylase (TH) immunoreactivity levels in the PVN and nucleus tractus solitarius/ ventrolateral medulla (NTS/VLM). Dependence on morphine was induced in rats by 7 day s.c. implantation of morphine pellets. Morphine withdrawal was induced on day 8 by an injection of naloxone. The protein levels of PKCalpha and gamma were significantly down-regulated in the PVN and NTS/VLM from the morphine-withdrawn rats. Morphine withdrawal induced c-Fos expression in the PVN and NTS/VLM, indicating an activation of neurons in those nuclei. TH immunoreactivity was increased in the NTS/VLM after induction of morphine withdrawal, whereas there was a decrease in TH levels in the PVN. Infusion of calphostin C, a selective protein kinase C inhibitor, produced a reduction in the morphine withdrawal-induced c-Fos expression. Additionally, the changes in TH levels in the PVN and NTS/VLM were significantly modified by calphostin C. The present results suggest that activated PKC in the PVN and catecholaminergic brainstem cell groups may be critical for the activation of the hypothalamic-pituitary adrenocortical axis in response to morphine withdrawal.

Involvement of 3',5'-cyclic adenosine monophosphate-dependent protein kinase in regulation of Fos expression and tyrosine hydroxylase levels during morphine withdrawal in the hypothalamic paraventricular nucleus and medulla oblongata catecholaminergic cell groups

Morphine withdrawal stimulates the hypothalamic-pituitary-adrenocortical axis activity by activation of nucleus tractus solitarius (NTS)/ventrolateral medulla (VLM) noradrenergic pathways innervating the hypothalamic paraventricular nucleus (PVN). We investigated whether cAMP-dependent protein kinase (PKA) plays a role in this process by estimating changes in PKA immunoreactivity and the influence of inhibition of PKA on Fos protein expression and tyrosine hydroxylase (TH) immunoreactivity levels in the PVN and NTS/VLM 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.). When opioid withdrawal was precipitated, an increase in PKA immunoreactivity levels was observed 90 min after naloxone administration in the PVN and NTS/VLM areas. Morphine withdrawal induced expression of Fos in the PVN and NTS/VLM, indicating an activation of neurones in those nuclei. TH immunoreactivity in NTS/VLM was increased 90 min after induction of morphine withdrawal, whereas there was a decrease in TH levels in the PVN at the same time point. When the selective PKA inhibitor HA-1004 was infused it greatly diminished the Fos expression observed in morphine-withdrawn rats. Furthermore, the changes in TH immunoreactivity were significantly modified by infusion of HA-1004. The present findings suggest that an up-regulated PKA-dependent transduction pathway might contribute to the activation of the hypothalamic-pituitary-adrenocortical axis in response to morphine withdrawal.

Increase of tyrosine hydroxylase levels and activity during morphine withdrawal in the heart

Our previous studies have shown an enhanced activity of the noradrenergic pathways innervating the heart in rats withdrawn from morphine. However, the possible adaptive changes that can occur in these pathways during morphine dependence are not known. We studied the alterations in tyrosine hydroxylase (the rate-limiting enzyme in catecholamines biosynthesis) and tyrosine hydroxylase activity in the heart (right and left ventricle) during morphine withdrawal. In the same paradigm, we measured Fos expression as a marker of neuronal activation and the normetanephrine/noradrenaline ratio (an index of noradrenaline turnover). We evaluated the levels of tyrosine hydroxylase and Fos by quantitative Western blot analysis, and noradrenaline turnover using high-performance liquid chromatography (HPLC). 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.). The results show a significant increase in tyrosine hydroxylase levels and activity in the right and left ventricle 30 or 90 min after naloxone precipitated withdrawal in parallel with an increase in noradrenaline turnover. Morphine withdrawal also induced an increase in the Fos expression, which indicates an activation of cardiac cellular activity. Our results suggest that an increase in tyrosine hydroxylase protein levels and tyrosine hydroxylase enzyme activity might contribute to the enhanced noradrenergic activity in the heart in response to morphine withdrawal.

Locus ceruleus control of state-dependent gene expression

Wakefulness and sleep are accompanied by changes in behavior and neural activity, as well as by the upregulation of different functional categories of genes. However, the mechanisms responsible for such state-dependent changes in gene expression are unknown. Here we investigate to what extent state-dependent changes in gene expression depend on the central noradrenergic (NA) system, which is active in wakefulness and reduces its firing during sleep. We measured the levels of approximately 5000 transcripts expressed in the cerebral cortex of control rats and in rats pretreated with DSP-4 [N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine], a neurotoxin that removes the noradrenergic innervation of the cortex. We found that NA depletion reduces the expression of approximately 20% of known wakefulness-related transcripts. Most of these transcripts are involved in synaptic plasticity and in the cellular response to stress. In contrast, NA depletion increased the expression of the sleep-related gene encoding the translation elongation factor 2. These results indicate that the activity of the central NA system during wakefulness modulates neuronal transcription to favor synaptic potentiation and counteract cellular stress, whereas its inactivity during sleep may play a permissive role to enhance brain protein synthesis.

Cellular mechanisms underlying neuronal excitability during morphine withdrawal in physical dependence: lessons from the magnocellular oxytocin system

Opiates are used clinically as analgesics, but their euphoric actions can lead to continued use and to dependence and addiction. While there are many factors involved in drug abuse, avoidance of stressful withdrawal symptoms is a key feature of addiction and its treatment. Fundamental to this is the need to understand the cellular processes that induce dependence and lead to the withdrawal syndrome. Many neurones in the brain express opioid receptors but only a few types of neurone develop dependence during chronic morphine exposure. The physiology of opiate-dependent cells is altered such that they require the continued presence of the drug to function normally and this is revealed, in cells that are inhibited by initial acute exposure to opiate, by a rebound hyperexcitation upon opiate withdrawal. Hypothalamic oxytocin neurones robustly develop morphine dependence and provide an exceptional opportunity to probe the cellular mechanisms underlying morphine dependence and withdrawal excitation. Although expression of morphine withdrawal excitation by oxytocin cells requires afferent inputs, the underlying mechanisms appear to reside within the oxytocin neurones themselves and probably involve changes in the intrinsic membrane properties of these neurones.

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).

Effects of U-50488H and U-50488H withdrawal on c-fos expression in the rat paraventricular nucleus. Correlation with c-fos in brainstem catecholaminergic neurons

1. In the present work, we have studied the expression of Fos during acute and chronic administration of the kappa-opioid receptor agonist U-50488H and after U-5088H withdrawal in the rat hypothalamic paraventricular nucleus (PVN). Fos production was also studied in brainstem regions that innervate the PVN: the A(2) cell group of the nucleus of solitary tract (NTS-A(2)) and the A(1) cell group of the ventrolateral medulla (VLM-A(1)), combined with immunostaining for tyrosine hydroxylase (TH) for immunohistochemical identification of active neurons after acute U-50488H administration. 2. For acute experiments, male rats were treated with saline i.p. for 4 days. On day 5, rats were given saline or U-50488H (15 mg x kg(-1), i.p.). Other groups of rats were rendered tolerant/dependent on U-50488H by injecting the drug twice daily (15 mg x kg(-1), i.p.) for 4 days. Control animals received saline i.p. on the same time schedule. On day 5, rats were treated with vehicle i.p., with U-50488H (15 mg x kg(-1)) or with the selective kappa opioid-receptor antagonist nor-binaltorphimine (Nor-BNI, 5 mg x kg(-1), i.p.). 3. Using immunohistochemical staining of Fos, present results indicate that acute administration of U-50488H produced an increase in Fos expression in the PVN and in the noradrenergic A(1) and A(2) cell groups. Moreover, when double-label immunohistochemistry was used to identify Fos and catecholaminergic-positive neurons in the brainstem, it was found that catecholaminergic-positive neurons in the NTS and VLM showed a significant increase in Fos expression in response to acute U-50488H injection. Chronic application of U-50488H leads to the development of tolerance towards their effects on Fos expression in the PVN as well as in the NTS and VLM. However, administration of Nor-BNI to U-50488H-dependent rats did not induce any changes in Fos immunoreactivity in the PVN or in the brainstem. 4. These findings demonstrate that acute activation of kappa-opioid receptors results in different altered patterns of immediate-early gene expression in the PVN, which occurs concurrently with an increased activity of their inputs from the brainstem. Interestingly in contrast to morphine withdrawal, present results demonstrate that rats withdrawn from U-50488H did show no changes in Fos-immunoreactivity in the PVN, NTS or VLM, indicating the absence of dependence on the kappa-agonist under the present experimental conditions.

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.

Effect of naloxone-precipitated morphine withdrawal on CRH and vasopressin mRNA expression in the rat hypothalamic paraventricular nucleus

Morphine withdrawal is characterized by an increase in the hypothalamus-pituitary-adrenocortical (HPA) axis activity. Here, by means of in situ hybridization, the changes in CRH and vasopressin (AVP) mRNAs have been analysed within the rat hypothalamic paraventricular nucleus (PVN) during morphine dependence and after naloxone-precipitated morphine withdrawal. CRH and AVP mRNA expression were analysed 30 min following administration of saline or naloxone to control groups and to morphine dependent rats. The data for in situ hybridization analysis of PVN neurons show that there were no changes in the total size of labelled area for CRH or AVP mRNA during morphine withdrawal, indicating that dependence on morphine does not involve alterations in the number of neurons expressing CRH or AVP mRNA. However, levels of mRNA encoding for CRH were decreased in the PVN during morphine dependence and withdrawal. By contrast, injection of saline or naloxone to morphine dependent rats did not affect the intensity of AVM mRNA expression. All these findings are discussed in term of cellular events that couple morphine dependence-increased HPA axis activity with changes in gene expression in selective neurons of the PVN.

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.

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.

Effects of morphine and morphine withdrawal on brainstem neurons innervating hypothalamic nuclei that control the pituitary-adrenocortical axis in rats

Different data support a role for brainstem noradrenergic inputs to the hypothalamic paraventricular nucleus (PVN) in the control of hypothalamus - pituitary - adrenocortical (HPA) axis. However, little is known regarding the functional adaptive changes of noradrenergic afferent innervating the PVN and supraoptic nucleus (SON) during chronic opioid exposure and upon morphine withdrawal. Here we have studied the expression of Fos after administration of morphine and during morphine withdrawal in the rat hypothalamic PVN and SON. Fos production was also studied in brainstem regions that innervate hypothalamic nuclei: the nucleus of solitary tract (NTS - A2) and the ventrolateral medulla (VLM - A1) and combined with immunostaining for tyrosine hydroxylase (TH) for immunohistochemical identification of active neurons during morphine withdrawal. Male rats were implanted with s.c. placebo or morphine (tolerant/dependent) pellets for 7 days. On day 8 rats received an injection of saline i.p., morphine i.p., saline s.c. or naloxone s.c. Acute morphine administration produced an increase in Fos expression at hypothalamic nuclei and in the brainstem regions, and tolerance developed towards this effect. Precipitated morphine withdrawal induced marked Fos immunoreactivity within the PVN and SON. Concomitantly, numerous neurons in the brainstem were stimulated by morphine withdrawal. Moreover, catecholaminergic-positive neurons in the brainstem showed a significant increase in Fos expression in response to morphine withdrawal. These findings demonstrate that chronic activation of opioid receptors results in altered patterns of immediate-early genes (IEG) expression in the PVN and SON, which occurs concurrently with an increased activity of their inputs from the brainstem.

Effects of prenatal protein malnutrition and neonatal stress on CNS responsiveness

Maturation of the nervous system and consequent behavior depends in part on prenatal nutritional factors and postnatal environmental stimulation. In particular, the hypothalamus and the hippocampus are two important CNS areas that are vulnerable to such pre- and postnatal manipulations. Therefore, the present study was undertaken to explore the effects of both prenatal protein malnutrition and neonatal isolation stress on hypothalamic and hippocampal functioning in infant rats. Specifically, we assessed the levels of plasma corticosterone, as well as dopamine, serotonin and their metabolites in both the hypothalamus and hippocampus in rat pups that had been prenatally malnourished (6% casein diet) and isolated from nest, dam, and siblings for 1 h daily during postnatal days (PND) 2 through 8. We found that on PND 9 malnourished pups weighed less, had smaller hypothalami and a suppressed corticosterone response to acute and chronic isolation stress. However, their dopamine metabolism in the hypothalamus was increased following acute isolation on PND 9 as seen in isolated controls. Prenatal protein malnutrition also resulted in a significant elevation in serotonin in both brain areas, increased 5HIAA in the hypothalamus, and decreased dopamine in the hippocampus. Repeated isolation caused a reduction in 5HIAA in both brain parts, but only in control pups. These pre- and postnatal challenges may each cause a specific pattern of modifications in the CNS and, in combination, may be additive, particularly in the hypothalamic-pituitary-adrenal (HPA) stress response and the serotonergic functioning in both the hypothalamus and hippocampus, a finding with important clinical implications.

Endogenous opiates: 2000

This paper is the twenty-third installment of the annual review of research concerning the opiate system. It summarizes papers published during 2000 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; learning, memory, and reward; eating and drinking; alcohol and other drugs of abuse; sexual activity, pregnancy, and development; mental illness and mood; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; gastrointestinal, renal, and hepatic function; cardiovascular responses; respiration and thermoregulation; and immunological responses.

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.

Differential responsivity of the hypothalamic-pituitary-adrenal axis to glucocorticoid negative-feedback and corticotropin releasing hormone in rats undergoing morphine withdrawal: possible mechanisms involved in facilitated and attenuated stress responses

Chronic morphine treatment produces profound and long-lasting changes in the pituitary-adrenal responses to stressful stimuli. The purpose of the present study was to explore the mechanisms involved in these altered stress responses. Chronic morphine administration increased basal plasma concentrations of corticosterone and adrenocorticotropic hormone (ACTH), which peaked at 36 h after the final morphine injection and returned to normal levels within 84-h. Whole brain glucocorticoid receptor protein expression was reduced (approximately 70%) in morphine-treated rats 4-h after the final morphine injection and these levels recovered within 16-h. Twelve hours following morphine withdrawal, rats displayed normal ACTH, but potentiated and prolonged corticosterone responses to restraint stress. Both the ACTH and corticosterone responses to restraint in acutely withdrawn rats were insensitive to dexamethasone. Furthermore, acutely withdrawn rats displayed reduced ACTH but prolonged corticosterone responses to peripheral corticotropin releasing hormone (CRH) administration. These findings suggest that the normal ACTH and enhanced corticosterone responses to stress in acutely withdrawn rats involved decreased sensitivity of negative-feedback systems to glucocorticoids, reduced pituitary responsivity to CRH, and enhanced sensitivity of the adrenals to ACTH. Eight days following morphine withdrawal, rats displayed dramatically reduced ACTH, but normal corticosterone responses to restraint stress. These rats displayed enhanced sensitivity to dexamethasone and normal pituitary-adrenal responses to CRH. These data suggest that the reduced ACTH responses to stress in 8-day withdrawal rats involved increased sensitivity of negative-feedback systems to glucocorticoids as well as reduced CRH and/or AVP function in response to stress. Taken together, the results of this study illustrate some of the mechanisms mediating altered stress responsivity in rats that have received chronic morphine treatment.

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.