Electrical stimulation of nucleus paragigantocellularis induces opioid withdrawal-like behaviors in the rat

Goofballing of Opioid and Methamphetamine: The Science Behind the Deadly Cocktail

Globally, millions of people suffer from various substance use disorders (SUD), including mono-and polydrug use of opioids and methamphetamine. Brain regions such as the cingulate cortex, infralimbic cortex, dorsal striatum, nucleus accumbens, basolateral and central amygdala have been shown to play important roles in addiction-related behavioral changes. Clinical and pre-clinical studies have characterized these brain regions and their corresponding neurochemical changes in numerous phases of drug dependence such as acute drug use, intoxication, craving, withdrawal, and relapse. At present, many studies have reported the individual effects of opioids and methamphetamine. However, little is known about their combined effects. Co-use of these drugs produces effects greater than either drug alone, where one decreases the side effects of the other, and the combination produces a prolonged intoxication period or a more desirable intoxication effect. An increasing number of studies have associated polydrug abuse with poorer treatment outcomes, drug-related deaths, and more severe psychopathologies. To date, the pharmacological treatment efficacy for polydrug abuse is vague, and still at the experimental stage. This present review discusses the human and animal behavioral, neuroanatomical, and neurochemical changes underlying both morphine and methamphetamine dependence separately, as well as its combination. This narrative review also delineates the recent advances in the pharmacotherapy of mono- and poly drug-use of opioids and methamphetamine at clinical and preclinical stages.

Prolonged morphine exposure during adolescence alters the responses of lateral paragigantocellularis neurons to naloxone in adult morphine dependent rats

INTRODUCTION

Adolescence is a critical period in brain development, and it is characterized by persistent maturational alterations in the function of central nervous system. In this respect, many studies show the non-medical use of opioid drugs by adolescents. Although this issue has rather widely been addressed during the last decade, cellular mechanisms through which adolescent opioid exposure may induce long-lasting effects are not duly understood. The present study examined the effect of adolescent morphine exposure on neuronal responses of lateral paragigantocellularis nucleus to naloxone in adult morphine-dependent rats.

METHODS

Adolescent male Wistar rats (31 days old) received increasing doses of morphine (from 2.5 to 25 mg/kg, twice daily, s.c.) for 10 days. Control subjects were injected saline with the same protocol. After a drug-free interval (20 days), animals were rendered dependent on morphine during 10 days (10 mg/kg, s.c., twice daily). Then, extracellular single-unit recording was performed to investigate neural response of LPGi to naloxone in adult morphine-dependent rats.

RESULTS

Results indicated that adolescent morphine treatment increases the number of excitatory responses to naloxone, enhances the baseline activity and alters the pattern of firing in neurons with excitatory responses in adult morphine-dependent rats. Moreover, the intensity of excitatory responses is reduced following the early life drug intake.

CONCLUSION

It seems that prolonged opioid exposure during adolescence induces long-lasting neurobiological changes in LPGi responsiveness to future opioid withdrawal challenges.

Circadian rhythm influences naloxone induced morphine withdrawal and neuronal activity of lateral paragigantocellularis nucleus

Investigations have shown that the circadian rhythm can affect the mechanisms associated with drug dependence. In this regard, we sought to assess the negative consequence of morphine withdrawal syndrome on conditioned place aversion (CPA) and lateral paragigantocellularis (LPGi) neuronal activity in morphine-dependent rats during light (8:00-12:00) and dark (20:00-24:00) cycles. Male Wistar rats (250-300 g) were received 10 mg/kg morphine or its vehicle (Saline, 2 mL/kg/12 h, s.c.) in 13 consecutive days for behavioral assessment tests. Then, naloxone-induced conditioned place aversion and physical signs of withdrawal syndrome were evaluated during light and dark cycles. In contrast to the behavioral part, we performed in vivo extracellular single-unit recording for investigating the neural response of LPGi to naloxone in morphine-dependent rats on day 10 of morphine/saline exposure. Results showed that naloxone induced conditioned place aversion in both light and dark cycles, but the CPA score during the light cycle was larger. Moreover, the intensity of physical signs of morphine withdrawal syndrome was more severe during the light cycle (rest phase) compare to the dark one. In electrophysiological experiments, results indicated that naloxone evoked both excitatory and inhibitory responses in LPGi neurons and the incremental effect of naloxone on LPGi activity was stronger in the light cycle. Also, the neurons with the excitatory response exhibited higher baseline activity in the dark cycle, but the neurons with the inhibitory response showed higher baseline activity in the light cycle. Interestingly, the baseline firing rate of neurons recorded in the light cycle was significantly different in response (excitatory/inhibitory) -dependent manner. We concluded that naloxone-induced changes in LPGi cellular activity and behaviors of morphine-dependent rats can be affected by circadian rhythm and the internal clock.

Transcutaneous Auricular Neurostimulation (tAN): A Novel Adjuvant Treatment in Neonatal Opioid Withdrawal Syndrome

Maternal opioid use during pregnancy is a growing national problem and can lead to newborns developing neonatal opioid withdrawal syndrome (NOWS) soon after birth. Recent data demonstrates that nearly every 15 min a baby is born in the United States suffering from NOWS. The primary treatment for NOWS is opioid replacement therapy, commonly oral morphine, which has neurotoxic effects on the developing brain. There is an urgent need for non-opioid treatments for NOWS. Transcutaneous auricular neurostimulation (tAN), a novel and non-invasive form of electrostimulation, may serve as a promising alternative to morphine. tAN is delivered via a multichannel earpiece electrode worn on and around the left ear, targeting two cranial nerves—the vagus and trigeminal nerves. Prior research suggests that auricular neurostimulation exerts an anxiolytic effect on the body by releasing endogenous opioids and reduces withdrawal symptoms in adults actively withdrawing from opioids. In this first-in-human prospective, open-label trial, we investigated tAN as an adjuvant to morphine therapy in eight infants >33 weeks gestational age suffering from NOWS and receiving oral morphine treatment. Infants received tAN for 30 min 1 h before receiving a morphine dose. tAN was delivered at 0.1 mA below perception intensity at two different nerve targets on the ear: Region 1, the auricular branch of the vagus nerve; and Region 2, the auriculotemporal nerve. tAN was delivered up to four times daily for a maximum of 12 days. The primary outcome measures were safety [heart rate monitoring, Neonatal Infant Pain Scale (NIPS), and skin irritation] and morphine length of treatment (LOT). tAN was well-tolerated and resulted in no unanticipated adverse events. Comparing to the national average of 23 days, the average oral morphine LOT was 13.3 days (median 9 days) and the average LOT after tAN initiation was 7 days (median 6 days). These preliminary data suggest that tAN is safe and may serve as a promising alternative adjuvant for treating NOWS and reducing the amount of time an infant receives oral morphine.

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

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

Paternal exposure to morphine during adolescence potentiates morphine withdrawal in male offspring: Involvement of the lateral paragigantocellularis nucleus

BACKGROUND

Opiate exposure during adolescence perturbs the brain's maturation process and potentially confers long-term adverse consequences, not only in exposed individuals but also in their posterity. Here, we investigate the outcomes of adolescent paternal morphine exposure on morphine withdrawal profile in male offspring.

METHODS

Male Wistar rats were chronically subjected to 10 days of an escalating regimen of morphine during adolescence. After a 20-day washout period, adult males were allowed to copulate with naïve females. The adult male offspring were tested for somatic and affective components of naloxone-precipitated morphine withdrawal using conditioned place aversion. Moreover, electrical activity of the lateral paragigantocellularis (LPGi) nucleus, which is involved in development of opiate dependence, was recorded in response to a challenge dose of morphine via extracellular single-unit recordings.

RESULTS

Morphine-sired offspring exhibited augmented expression of naloxone-induced somatic and affective signs of opiate withdrawal compared to the control saline-sired counterparts. In vivo recording revealed that LPGi neurons displayed heterogeneous responses (inhibitory, excitatory, and no change) to acute morphine administration in both morphine- and saline-sired animals. The morphine-induced discharge inhibition was potentiated in morphine-sired offspring. However, the extent of discharge excitation in response to morphine did not reach significance in these subjects. Moreover, the lack of alteration in maternal behavior toward morphine-sired offspring indicates that this is due to germline-dependent transmission of epigenetic traits across generations.

CONCLUSIONS

Preconception paternal exposure to morphine during adolescence potentiates opiate withdrawal signs in male offspring which is mediated, at least in part, by epigenetic alteration of LPGi-related brain circuitry.

Possible involvement of the opioidergic system in the modulation of body temperature, jumping behavior and memory process in cholestatic and addicted mice

Cholestasis is related to an increased plasma level of endogenous opioid levels. Naloxone-induced withdrawal syndrome has been reported in a mouse model of cholestasis. Moreover, studies revealed that the memory process is affected by cholestasis. Thus, we aimed at determining whether pharmacological manipulation of the opioidergic system is involved in signs of cholestasis disease such as hypothermia and withdrawal behaviors such as jumping behavior as well as memory process in mice. Cholestasis was induced by bile duct resection in mice and physical dependence was induced by administration of morphine and/or tramadol three times daily (8, 12 and 16 h) at the doses of 25, 50 and 75 mg/kg during three consecutive days. The memory process was assessed by a step-down passive avoidance test. Our results indicated that cholestatic mice showed hypothermia whereas cholestatic- and drug dependent mice indicated hyperthermia. Moreover, administration of morphine (50 mg/kg) and/or tramadol (50 mg/kg) on the 4th day, 2 h before naloxone injection significantly decreased latency to first jumping but increased the number of jumping and rearing behavior as well as locomotor activity in BDL-vs. sham-operated mice. In addition, the latency time of the step-down test decreased in BDL-vs. sham-operated group, showing impairment of memory in BDL mice. The results of this study support the evidence that (1) the opioidergic system involved in thermoregulation of cholestasis mice, (2) μ-opioid receptors play an important role in withdrawal behaviors, and (3) memory process is affected by cholestasis and addiction in mice.

Discharge activities of neurons in the nucleus paragigantocellularis during the development of morphine tolerance and dependence: a single unit study in chronically implanted rats

The nucleus paragigantocellularis (PGi) has been proposed to play a role in opiate dependence/withdrawal. In the present study, we examined the discharge activity of PGi neurons before and after the development of morphine tolerance/dependence in rats. A multi-wire electrode was chronically implanted in the PGi, which allowed us to monitor the effects of both acute and chronic morphine treatments on the activity of PGi neurons recorded from the same site. We found that acute morphine excited, inhibited or had no effect on 36%, 35% or 29% of PGi neurons (N=556), respectively. After 3 days of continuous morphine infusion, which led to morphine tolerance/dependence, the firing rates of both excitatory and inhibitory PGi neurons returned to pre-morphine treatment levels, indicating that the PGi neurons developed tolerance to both excitatory and inhibitory effects of morphine. Naltrexone-precipitated withdrawal from chronic morphine treatment also induced heterogeneous responses in the PGi. On a site-by-site basis, most of the sites that showed excitatory response to acute morphine exhibited inhibitory response during withdrawal, and all the sites that showed inhibitory response to acute morphine exhibited excitatory response during withdrawal. Correlation analysis further quantitatively showed that PGi neurons' responses to acute morphine and that during withdrawal were inversely correlated with a correlation coefficient of 0.73, suggesting that adaptations in the PGi during the development of morphine dependence share common neural mechanisms with the acute effect of morphine. These results provide new insights into the role of the PGi in the development of morphine tolerance/dependence.

Alterations of BDNF and NT-3 genes expression in the nucleus paragigantocellularis during morphine dependency and withdrawal

Locus coeruleus (LC) plays a key role in opioid dependence and withdrawal. Chronic morphine administration induces neurochemical adaptations in the noradrenergic system. The nature of signal responsible for opiate-induced adaptations of noradrenergic neurons in LC is not well defined. Neurotrophins-signaling pathways such as brain derived neurotrophic factor (BDNF) and Neurotrophin-3 (NT-3) play a key role for regulating the noradrenergic response of LC neurons to opiates. The nucleus paragigantocellularis (PGi) is one of the two major afferents to LC. The present study was designed to evaluate the expression of BDNF and NT-3 in the context of opiate dependence and withdrawal in PGi. Such data are important because they could reveal the role of PGi as an additional source of BDNF and NT-3 in the neurochemical plasticity of LC neurons. Opiate dependence was induced by a progressive intraperitoneal treatment of morphine. In morphine dependent group PGi nucleus was extracted for gene expression assay 6h after the last injection of morphine. In spontaneous withdrawal, rats received the same chronic treatment as morphine group. PGi was extracted for gene expression assay 24, 48 and 72 h after the last injection of morphine. PGi nucleus was assayed for the expression of BDNF and NT-3 using semi-quantitative RT-PCR normalized to beta-actin gene expression. Results showed that chronic administration of morphine significantly increased BDNF and NT-3 gene expression in PGi. In spontaneous withdrawal, BDNF/NT-3 genes expression were high in comparison to control group. It seems that BDNF/NT-3 -signaling pathway originating from PGi is essential for opiate-induced adaptations of the LC neurons.

Changes in the brain kappa-opioid receptor levels of rats in withdrawal from physical dependence upon butorphanol

Changes in kappa-opioid receptor levels have been implicated in the development of physical dependence upon and withdrawal from the mixed agonist-antagonist opioid, butorphanol. Immunoblotting analysis was performed to determine the levels of kappa- and mu-opioid receptors in brain regions of rats in withdrawal from dependence upon butorphanol or morphine. Physical dependence was induced by a 72 h i.c.v. infusion with either butorphanol or morphine (26 nmol/microl/h). Withdrawal was subsequently precipitated by i.c.v. challenge with naloxone (48 nmol/5 microl/rat), administered 2 h following cessation of butorphanol or morphine infusion. Immunoblotting analysis of kappa-opioid receptors from butorphanol-withdrawal rats showed significant increases in 11 of 21 brain regions examined, including the nucleus accumbens, amygdala, dorsomedial hypothalamus, hypothalamus, paraventricular thalamus, thalamus, presubiculum, and locus coeruleus, when compared with saline treated, non-dependent controls. In addition, significant reductions were found in the hippocampus and in cortical brain regions, including the parietal cortex and temporal cortex from butorphanol-withdrawal rats. These findings contrasted with those from morphine-withdrawal rats, in which the only changes noted were increases in the thalamus and paraventricular thalamus. Changes in the levels of the mu-opioid receptor protein were observed in 11 of 21 brain regions examined in morphine-withdrawal rats, but only in three of 21 in butorphanol-withdrawal rats. These results implicate a substantive and largely unique role for kappa-opioid receptors in mediation of the development of physical dependence upon, and the expression of withdrawal from, butorphanol, as opposed to the prototypical opioid analgesic, morphine.

Butorphanol dependence and withdrawal decrease hippocampal kappa 2-opioid receptor binding

The present study examines the degree and distribution of alterations in the expression of kappa-opioid receptor subtypes using a model of chronic intracerebroventricular (i.c.v.) infusion of butorphanol. Autoradiographic characterization of binding for brain kappa(1) ([3H]CI-977)-, kappa(2) ([3H]bremazocine in the presence of DAMGO, DPDPE, and U-69,593)- and total kappa ([3H]bremazocine in the presence of only DAMGO and DPDPE)-opioid receptors was performed. Dependence was induced by a 72 h i.c.v. infusion with butorphanol (26 nmol/microl per hour) (butorphanol-dependent). Butorphanol withdrawal was produced by terminating the infusion of butorphanol in dependent animals. Responses were studied 7 h following termination (butorphanol-withdrawal). During both dependence and withdrawal phases, the binding signals for both kappa(1)- and kappa(2)-opioid receptors were significantly increased in certain regions, with especially marked increases in the frontal cortex, nucleus accumbens, parietal cortex, dorsomedial hypothalamus, ventral tegmental area and locus coeruleus. In contrast, a highly specific decrease in kappa(2)-, but increase in kappa(1)-, opioid receptor binding was noted in the hippocampus of rats in both butorphanol-dependent and-withdrawal groups. Therefore, alterations in kappa(1)- and kappa(2)-opioid receptors in the hippocampus may be differently involved in both adaptation to and recovery from chronic exposure to a mixed agonist/antagonist opioid analgesic. These results further illustrate the regional distribution of changes in binding characteristics of rat brain kappa(1)- and kappa(2)-opioid receptor subtypes in an established model of butorphanol dependence and withdrawal.

Focal kappa-opioid receptor-mediated dependence and withdrawal in the nucleus paragigantocellularis

The nucleus paragigantocellularis (PGi) has been hypothesized to play an important role in the development of physical dependence on opioids, including the prototype mu-opioid receptor agonist, morphine, and the mixed agonist/antagonist, butorphanol, which shows selective kappa-opioid receptor agonist activity, in rats. In confirmation of previous work, electrical stimulation of the PGi in opioid-nai;ve rats induced stimulus-intensity-related, withdrawal-like behaviors similar to those observed during naloxone-precipitated withdrawal from dependence upon butorphanol. Novel findings were made in rats surgically implanted with cannulae aimed at the lateral ventricle and the right PGi and made physically dependent by intracerebroventricular infusion of either morphine (26 nmol/microl/h) or butorphanol (26 nmol/microl/h) through an osmotic minipump for 3 days. Two hours following termination of the opioid infusion, microinjections of naloxone (11 nmol/400 nl), a nonselective opioid receptor antagonist, or nor-binaltorphimine (nor-BNI) (3.84 nmol/400 nl), a selective kappa-opioid receptor antagonist, were made into the PGi of morphine-dependent and butorphanol-dependent rats. Discrete PGi injections precipitated withdrawal behaviors, with significant (P<.05) increases noted in the incidence of teeth chattering, wet-dog shakes, and scratching. Composite scores for behavioral withdrawal were significantly higher in nor-BNI-precipitated, butorphanol-dependent rats (score=6.8+/-0.6), in naloxone-precipitated, butorphanol-dependent rats (8.9+/-0.8), and in naloxone-precipitated, morphine-dependent rats (11.5+/-0.9) than in all other groups. Both kappa- and mu-opioid receptor mediated dependence can be demonstrated at the level of a discrete medullary site, the PGi, which further supports a specific role for this nucleus in elicitation of behavioral responses during opioid withdrawal.

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.

Occurrence of morphine tolerance and dependence in the nucleus paragigantocellularis neurons

The occurrence of morphine tolerance and dependence in the nucleus paragigantocellularis neurons was investigated. The spontaneous activity was recorded from the nucleus paragigantocellularis neurons of urethane-anesthetized rats, using single unit recording. Morphine microinjected (20 mg/ml, 120-200 nl) into the nucleus paragigantocellularis of control rats had both excitatory and inhibitory effects. These effects were reversed by microinjection of naloxone, revealing the possible involvement of mu receptors. Morphine microinjected into morphine-dependent rats failed to change the spontaneous activity of the nucleus paragigantocellularis neurons that accounts for the occurrence of tolerance to morphine in these neurons. Microinjection of naloxone (25 mg/ml, 120-200 nl) in control rats had no effect on the spontaneous firing rate of the nucleus paragigantocellularis neurons but in morphine-dependent rats, either alone or after morphine microinjection, naloxone increased neuronal activity significantly, indicating the occurrence of dependence on morphine in the nucleus paragigantocellularis neurons. These data show that the nucleus paragigantocellularis neurons may play a role in physical dependence on morphine. This conclusion is consistent with the finding, that activation of the nucleus paragigantocellularis by electrical stimulation in morphine-naive rats can elicit behaviors similar to those observed during naloxone-precipitated morphine withdrawal.

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.

Endogenous opiates: 1999

This paper is the twenty-second installment of the annual review of research concerning the opiate system. It summarizes papers published during 1999 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 neurologic disorders; electrical-related activity; general activity and locomotion; gastrointestinal, renal, and hepatic function; cardiovascular responses; respiration and thermoregulation; and immunologic responses.

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.

Contribution of glutamatergic systems in locus coeruleus to nucleus paragigantocellularis stimulation-evoked behavior

The role of extracellular glutamate, within the locus coeruleus, in mediation of the behavioral signs elicited by electrical stimulation of the nucleus paragigantocellularis (PGi) was investigated in conscious, opioid-naive rats. Each rat was prepared with a chronically implanted unilateral electrode within the PGi and a microdialysis guide cannula directed at the ipsilateral locus coeruleus. Opioid withdrawal-like behaviors (rearing, teeth-chattering, wet-dog shakes, etc.) and increases in extracellular glutamate concentrations within the locus coeruleus were evoked, in a frequency-dependent (0.5-50 Hz) manner, during PGi stimulation. Reverse dialysis perfusion of the locus coeruleus with the nonspecific glutamate receptor antagonist, kynurenic acid (0.1, 1 mM), reduced the intensity of stimulation-induced behaviors by roughly 50%, but had no effect on the corresponding increases in glutamate concentrations. Perfusion of the locus coeruleus with the glutamate transporter inhibitor, L-trans-pyrrolidine dicarboxylic acid, at 1, but not at 0.1, mM significantly increased glutamate levels in dialysates. Neither concentration of the transporter inhibitor altered the behavioral score. The results indicate that the opioid withdrawal-like behaviors elicited by electrical stimulation of the brainstem at the site of the PGi are positively correlated with locus coeruleus levels of glutamate, and suggest further that the behaviors are partially mediated by release of glutamate within the locus coeruleus or its immediate vicinity.