Opiate-induced Changes Research Articles

Opiate-induced changes in brain adenosine levels and narcotic drug responses

We have very little information about the metabolomic changes that mediate neurobehavioral responses, including addiction. It was possible that opioid-induced metabolomic changes in brain could mediate some of the pharmacodynamic effects of opioids. To investigate this, opiate-induced brain metabolomic responses were profiled using a semi-targeted method in C57BL/6 and 129Sv1 mice, which exhibit extreme differences in their tendency to become opiate dependent. Escalating morphine doses (10–40mg/kg) administered over a 4-day period selectively induced a twofold decrease (p<0.00005) in adenosine abundance in the brainstem of C57BL/6 mice, which exhibited symptoms of narcotic drug dependence; but did not decrease adenosine abundance in 129Sv1 mice, which do not exhibit symptoms of dependence. Based on this finding, the effect of adenosine on dependence was investigated in genetically engineered mice with alterations in adenosine tone in the brain and in pharmacologic experiments. Morphine withdrawal behaviors were significantly diminished (p<0.0004) in genetically engineered mice with reduced adenosine tone in the brainstem, and by treatment with an adenosine receptor1 (A1) agonist (2-chloro-N6-cyclopentyladenosine, 0.5mg/kg) or an A2a receptor (A2a) antagonist (SCH 58261, 1mg/kg). These results indicate that adenosine homeostasis plays a crucial role in narcotic drug responses. Opiate-induced changes in brain adenosine levels may explain many important neurobehavioral features associated with opiate addiction and withdrawal.

Effects of Opiates and HIV Proteins on Neurons: The Role of Ferritin Heavy Chain and a Potential for Synergism

Human immunodeficiency virus 1 (HIV-1) and its associated proteins can have a profound impact on the central nervous system. Co-morbid abuse of opiates, such as morphine and heroin, is often associated with rapid disease progression and greater neurological dysfunction. The mechanisms by which HIV proteins and opiates cause neuronal damage on their own and together are unclear. The emergence of ferritin heavy chain (FHC) as a negative regulator of the chemokine receptor CXCR4, a co-receptor for HIV, may prove to be important in elucidating the interaction between HIV proteins and opiates. This review summarizes our current knowledge of central nervous system damage inflicted by HIV and opiates, as well as the regulation of CXCR4 by opiate-induced changes in FHC protein levels. We propose that HIV proteins and opiates exhibit an additive or synergistic effect on FHC/CXCR4, thereby decreasing neuronal signaling and function.

Role of NK-1 neurotransmission in opioid-induced hyperalgesia

Opiates are among the most important drugs for treatment of moderate to severe pain and prolonged opiate administration is often required to treat chronic pain states. We investigated the neurobiological actions of sustained opiate administration revealing paradoxical pronociceptive adaptations associated with NK-1 receptor function. Sustained morphine delivered over 6 days elicited hyperalgesia in rats and mice during the period of opiate delivery. Sustained morphine administration increased substance P (SP) and NK-1 receptor expression in the spinal dorsal horn. Sustained morphine treatment also enhanced capsaicin-evoked SP release in vitro, and increased internalization of NK-1 receptors in response to noxious stimulation. While NK-1 receptor internalization was observed primarily in the superficial laminae of placebo-treated rats, NK-1 receptor internalization was seen in both superficial and deep lamina of the dorsal horn in morphine-treated animals. Morphine-induced hyperalgesia was reversed by spinal administration of an NK-1 receptor antagonist in rats and mice, and was observed in wildtype (NK-1 +/+), but not NK-1 receptor knockout (NK-1 −/−), mice. These data support a critical role for the NK-1 receptor in the expression of sustained morphine-induced hyperalgesia. Additionally, these data indicate that sustained opiate administration induces changes reminiscent of those associated with inflammatory pain. These opiate-induced changes might produce unintended deleterious actions in the course of pain treatment in patients. Understanding of sustained morphine-induced neurochemical changes will help identify approaches that limit the deleterious actions of opiates.

Genetic variation in heroin‐induced changes in behaviour: effects of B6 strain dose on conditioned reward and locomotor sensitization in 129‐B6 hybrid mice

Substantial interindividual variability exists in the propensity to develop opiate addiction. Genetic variation in opiate reward may contribute to this variability. A large body of evidence indicates genetic variation in mice for several effects of opiate drugs. The present study examined heroin-induced place conditioning and locomotor sensitization in the two strains of mice employed most frequently in the generation of transgenic animals, C57BL/6J (B6) and 129X1/sVJ (129), as well as in groups of B6-129 hybrid mice, differing in their amount of B6 genetic background. Four pairings of 100 microg/kg of heroin elicited robust place conditioning and locomotor sensitization in B6 controls and in N(10) congenic B6-129 hybrid mice. In comparison, the identical treatment produced no locomotor sensitization and induced place aversion in 129 controls. No heroin-induced changes in the behaviour of N(3) congenic B6-129 hybrid mice or F5-8 non-congenic B6-129 hybrid mice were observed. The expression of place conditioning was not facilitated in any group by the administration of a heroin-priming injection prior to testing. These data indicate that genetic variation exists in mice for the rewarding and locomotor-sensitizing effects of heroin and that the capacity of heroin to induce conditioned reward and locomotor sensitization can be modulated in a B6 strain dose-dependent manner in B6-129 hybrid mice. Thus, strain differences in heroin responsiveness should be considered when examining transgenic lines on B6-129 backgrounds for opiate-induced changes in behaviour that may be relevant for addiction.

A cell biologist’s perspective on physiological adaptation to opiate drugs

Opiate drugs such as morphine and heroin are among the most effective analgesics known but are also highly addictive. The clinical utility of opiates is limited by adaptive changes in the nervous system occurring after prolonged or repeated drug administration. These adaptations are believed to play an important role in the development of physiological tolerance and dependence to opiates, and to contribute to additional changes underlying the complex neurobehavioral syndrome of drug addiction. All of these adaptive changes are initiated by the binding of opiate drugs to a subfamily of G protein-coupled receptors that are also activated by endogenously produced opioid neuropeptides. It is increasingly evident that opiate-induced adaptations occur at multiple levels in the nervous system, beginning with regulation of opioid receptors themselves and extending to a complex network of direct and indirect modifications of “downstream” signaling machinery. Efforts in my laboratory are directed at understanding the biochemical and cell biological basis of opiate adaptations. So far, we have focused primarily on adaptations occurring at the level of opioid receptors themselves. These studies have contributed to defining a set of membrane trafficking mechanisms by which the number and functional activity of opioid receptors are controlled. The role of these mechanisms in affecting adaptation of “downstream” neurobiological substrates, and in mediating opiate-induced changes in whole-animal physiology and behavior, are exciting questions that are only beginning to be explored.

Essential role for RGS9 in opiate action.

Regulators of G protein signaling (RGS) are a family of proteins known to accelerate termination of effector stimulation after G protein receptor activation. RGS9-2, a brain-specific splice variant of the RGS9 gene, is highly enriched in striatum and also expressed at much lower levels in periaqueductal gray and spinal cord, structures known to mediate various actions of morphine and other opiates. Morphine exerts its acute rewarding and analgesic effects by activation of inhibitory guanine nucleotide-binding regulatory protein-coupled opioid receptors, whereas chronic morphine causes addiction, tolerance to its acute analgesic effects, and profound physical dependence by sustained activation of these receptors. We show here that acute morphine administration increases expression of RGS9-2 in NAc and the other CNS regions, whereas chronic exposure decreases RGS9-2 levels. Mice lacking RGS9 show enhanced behavioral responses to acute and chronic morphine, including a dramatic increase in morphine reward, increased morphine analgesia with delayed tolerance, and exacerbated morphine physical dependence and withdrawal. These findings establish RGS9 as a potent negative modulator of opiate action in vivo, and suggest that opiate-induced changes in RGS9 levels contribute to the behavioral and neural plasticity associated with chronic opiate administration.

Are NMDA receptors involved in opiate-induced neural and behavioral plasticity? A review of preclinical studies.

Research over the past decade demonstrating that NMDA receptor antagonists have the ability to inhibit opiate tolerance, sensitization and physical dependence has led to the suggestion that NMDA receptors may have a critical role in opiate-induced neural and behavioral plasticity. However, there have been suggestions that the effects of NMDA receptor antagonists on these phenomena result from non-specific behavioral or pharmacological effects, rather than from a specific inhibition of plasticity. To review the literature in order to explore whether the effects of NMDA receptor antagonists on opiate-induced changes in behavior are best accounted for by an inhibition of neural and behavioral plasticity, or if alternative explanations might better account for the results. The effects of NMDA receptor antagonists on the development of tolerance to opiate analgesia and the development of opiate physical dependence do not appear to be due to confounding behavioral effects produced by high doses of NMDA receptor antagonists, "side-effects" of a particular drug or drug class, blockade of associative learning processes, or state-dependency. Results on tolerance and sensitization to the locomotor effects of morphine are more mixed and controversial; however, there is evidence suggesting that NMDA receptor antagonists may inhibit these phenomena in a similar manner. NMDA receptor antagonists appear to inhibit the neural plasticity underlying some forms of opiate tolerance, sensitization and physical dependence, suggesting that NMDA receptors are involved in the development of these drug-induced changes in behavior. Further research will help to determine the neural mechanisms responsible for these phenomena, and the therapeutic potential for drugs acting on the NMDA receptor complex in the treatment of pain and addiction.

Opiates transdeactivate chemokine receptors: delta and mu opiate receptor-mediated heterologous desensitization.

An intact chemotactic response is vital for leukocyte trafficking and host defense. Opiates are known to exert a number of immunomodulating effects in vitro and in vivo, and we sought to determine whether they were capable of inhibiting chemokine-induced directional migration of human leukocytes, and if so, to ascertain the mechanism involved. The endogenous opioid met-enkephalin induced monocyte chemotaxis in a pertussis toxin-sensitive manner. Met-enkephalin, as well as morphine, inhibited IL-8-induced chemotaxis of human neutrophils and macrophage inflammatory protein (MIP)-1alpha, regulated upon activation, normal T expressed and secreted (RANTES), and monocyte chemoattractant protein 1, but not MIP-1beta-induced chemotaxis of human monocytes. This inhibition of chemotaxis was mediated by delta and micro but not kappa G protein-coupled opiate receptors. Calcium flux induced by chemokines was unaffected by met-enkephalin pretreatment. Unlike other opiate-induced changes in leukocyte function, the inhibition of chemotaxis was not mediated by nitric oxide. Opiates induced phosphorylation of the chemokine receptors CXCR1 and CXCR2, but neither induced internalization of chemokine receptors nor perturbed chemokine binding. Thus, inhibition of chemokine-induced chemotaxis by opiates is due to heterologous desensitization through phosphorylation of chemokine receptors. This may contribute to the defects in host defense seen with opiate abuse and has important implications for immunomodulation induced by several endogenous neuropeptides which act through G protein-coupled receptors.

Alterations in diaphragm EMG activity during opiate-induced respiratory depression

While opiate-induced increases in thoracic muscle tone may contribute to impaired ventilation during opiate anesthesia, the effects of high-dose opiates on diaphragm muscle activity have not been elucidated. The effects of the opiate agonist alfentanil (ALF, 500 μg/kg subcutaneously) on diaphragm (DIA) and intercostal (IC) electromyographic (EMG) activity in spontaneously ventilating adult rats were studied. EMG segments corresponding to inspiration and expiration were selected using an impedance plethysmographic respiratory waveform. Total EMG activity over a respiratory cycle was significantly greater in the DIA than in the IC. ALF produced a decrease in inspiratory and an increase in expiratory DIA EMG activity. These changes in diaphragm function following ALF were accompanied by significant respiratory depression. The effects of the alpha-2 agonist dexmedetomidine on ALF-induced changes in diaphragm and intercostal EMG activity were also examined. While dexmedetomidine alone had minimal effects on DIA activity, it significantly attenuated the ALF-induced increase in expiratory DIA EMG. The potential etiology and implications of these opiate-induced changes in diaphragm muscle function are discussed.

μ- and κ-opioid receptor agonists produce peripheral inhibition of neurogenic plasma extravasation in rat skin

Extravasation elicited in rat skin by antidromic electrical stimulation of the saphenous nerve was dose dependently inhibited by the intravenous (i.v.) application of the μ-opioid receptor agonists, morphine and [D-Ala 2,Me-Phe 4,Gly-ol 5]enkephalin (DAGO), and the κ-opioid receptor agonists (−)-U 50488H, (−)-ICI 197067 and ICI 204448. This inhibition was antagonised by naloxone, and the lack of action of (+)-U 50488H (5 mg/kg) indicated that the κ effect was stereoselective. The δ-opioid receptor agonist, [D-Pen 2, D-Pen 5]enkephalin (DPDPE), and the σ-receptor agonist, (+)-SKF 10047, had no effect on neurogenic extravasation at a dose of 5 mg/kg. Opiate-induced changes in cutaneous blood flow were not important for opiate inhibition since extravasation produced by exogenous substance P (the putative neurotransmitter) was not influenced by the opiates. An indirect opiate action via the adrenal glands was excluded since the effectiveness of DAGO, (−) - ICI 197067 and ICI 204448 was unchanged after adrenalectomy. Finally, the cutaneous locus of the opiate effect was shown by blocking the activity of systematically applied DAGO and ICI 204448 with naloxone injected into paw. These results indicate that specific μ-opioid and κ-opioid, but not δ-opioid and σ receptor agonists, inhibit neurogenic plasma extravasation in rat skin, probably via opioid receptors on peripheral terminals of sensory C-fibres.

Mu- and kappa-opiate receptor control of prolactin secretion in rats: ontogeny and interaction with serotonin.

The present study explores developmental changes in mu- and kappa-opiate receptor control of PRL secretion. The ontogeny of mu- and kappa-receptor function was determined by assessing the PRL response to the mu-agonist sufentanil (SUF) and the kappa-agonist U50488 in 5-, 10-, 15-, 20-, and 60-day-old rats. Both mu- and kappa-agonists stimulated PRL secretion at all ages. The selectivity of the mu- and kappa-agonists was confirmed by selective blockade with their respective antagonists (beta-funaltrexamine and nor-binaltorphimine). Serotonin mediation of opiate-induced changes in PRL secretion was explored across ontogeny by testing cyproheptadine (CYPRO) blockade of agonist responses in 5-, 10-, 20-, and 60-day-old rats. CYPRO attenuated the PRL response to the mu-agonist SUF in 20- and 60-day-old rats, but not in the 5- or 10-day-old pups. CYPRO did not block the kappa-agonist U50488 at any age. Similarly, pretreatment with parachlorophenylalaine lowered the PRL response to SUF in 60-day-old rats, but not in 10-day-old rats. These results support previous reports of a serotonin-mediated mu control of PRL secretion that develops by day 20 and a kappa control of PRL secretion that is independent of serotonin at all ages. These findings also suggest that a previously reported serotonin-independent mu-receptor-mediated control of PRL secretion can be stimulated early in ontogeny.

Chronic treatment with naltrexone enhances morphine-stimulated dopamine neurotransmission: Neurochemical and behavioral evidence

Rats were implanted for 10 days with a slow-release pellet of naltrexone or were given sham surgery. At one of various different intervals during or after implantation of the pellet, the synthesis of dopamine (DA) was assessed in the nigrostriatal and mesolimbic systems. The results indicated that naltrexone alone was without effect on the synthesis of DA. However, one day after removal of the pellet, naltrexone-treated animals displayed an enhanced response to the DA-stimulatory action of morphine (15 mg/kg) in both the nigrostriatal and mesolimbic systems. This change was accompanied by an increase in specific binding of the μ-specific radioligand [ 3H]DAGO in whole brain and by an increase in the depressant action of morphine on locomotor activity. In contrast, at 10 days after removal of the pellet, naltrexone was without effect on morphine-induced changes in the synthesis of DA and locomotor activity, thus indicating that the supersensitivity to morphine was transient. These results support the idea that opioids modulate DAergic neurotransmission in the nigrostriatal and mesolimbic pathways and that this modulatory role may underlie opiate-induced changes in locomotor behavior.

The effects of the acute administration of buprenorphine hydrochloride on the release of anterior pituitary hormones in the rat: Evidence for the involvement of multiple opiate receptors

Multiple opiate receptor agonists and antagonists have been found to produce different patterns of anterior pituitary hormone release. The present studies examined the pattern of anterior pituitary hormone release produced by buprenorphine. The effects of the kappa agonist ethylketocyclazocine on thyroid stimulating hormone release were also examined. Following buprenorphine, serum levels of corticosterone and luteinizing hormone were not changed while growth hormone release was stimulated in a dose-dependent manner. Prolactin release was stimulated after the lowest dose of buprenorphine while the highest dose induced a fall in serum prolactin. Similar biphasic effects on hydroid stimulating hormone were seen after either buprenorphine or ethylketocyclazocine. The results provide support for the role of multiple opiate receptors in opiate-induced changes in anterior pituitary hormone release.

Stereoselectivity of opiate antagonists in rat hippocampus and neocortex: Responses to (+) and (−) isomers of naloxone

The relative potencies of the (+) and (−) isomers of naloxone in antagonizing electrophysiological responses to d-alanine 2-methionine enkephalinamide were compared in rat frontal cortex and hippocampus. In the in vitro hippocampus, the (−) isomer was found to be at least a 100 times more potent than the (+) isomer in antagonizing opiate-induced changes in field potentials. Similar stereoselectivity was observed in vivo in both frontal cortex and hippocampus in terms of the antagonism of enkephalin-induced changes in spontaneous cell firing. The direct effects of (+) and (−)-naloxone were examined as well. In hippocampus both in vivo and in vitro, no differential effect was observed, whereas in the neocortex (−)-naloxone was considerably more potent than the (+) isomer in eliciting depressions of spontaneous activity. These direct effects of naloxone in the cortex do not appear to be due to an antagonism of the effects of endogenously released opioids. These results demonstrate that the stereoselectivity of naloxone isomers in antagonizing electrophysio logical responses to opiates in the cortex and hippocampus parallels that previously observed in other brain regions and in other tissues. In addition, they suggest that naloxone may have interactions with other unknown opiate (or possibly non-opiate) receptors which are of physiological significance.

Facilitation of self-stimulation in rats by methadone.

The effects of morphine and its derivatives on self-stimulation behavior have been widely studied. In those experiments which have used multiple injections (over days) and multiple post-injection tests (within days), the typical findings includes a depression of responding after the initial injections followed by a facilitation of responding on subsequent days. There have been only a few reports which have tested the effects of methadone in this paradigm. Some investigators have observed only depression of self-stimulation while others have reported both the transient depression and the subsequent facilitation generally obtained with morphine. In the present experiment we administered either 5 mg/kg or 10 mg/kg methadone IP over a five day period and tested MFB-LH self-stimulation at 2, 4, 6, 8, 10 and 23 hours post-injection. Compared to saline controls, the 10 mg/kg dose produced the typical opiate-induced changes in self-stimulation, i.e., an initial depression which lasted for two hours on the first two days but was replaced by significant facilitation by hour 4 of day 3. This facilitation persisted for at least 10 hours on all 5 days of the experiment. Except for a transient (days 2-3) depression of self-stimulation, 5 mg/kg was without effects. The present experiment demonstrates that methadone does facilitate self-stimulation but that its ability to do so is highly dose-dependent.

A study of the quantal (all-or-none) change in reflex latency produced by opiate analgesics

The properties of opiate-induced changes of tail-flick latency were studied in the rat. (1) Morphine and pentazocine produced a stepwise increase in latency which rose from near baseline to cut-off (usually greater than 20 sec) in less than 30 sec. Abrupt return to pre-treatment latencies was observed either spontaneously or when the rat was back-titrated with the narcotic antagonist naloxone. (2) The proportion of rats showing this stepwise change increase with increasing dose; however, the step itself was independent of dose. The same step was produced by a slow, constant infusion of morphine but was not produced by ice-water stress or barbiturate administration. (3) Increasing heat intensity to the tail shortened the baseline latency and raised the mean dose of morphine required to produce a step latency increase. (4) A step increase in latency was also observed when paw withdrawal instead of tail-flick was measured. We hypothesize that the analgesic behavior described partly defines the operating characteristics of an intrinsic endorphin-mediated analgesia system which mediates narcotic suppression of withdrawal reflexes.