Septal nuclei critically mediate the development of behavioral sensitization to a single morphine injection in rats

Behavioral sensitization to psychostimulants and opioids: What is known in rodents and what still needs to be explored in humans?

Repeated exposure to substances of abuse results in an increase in some behavioral responses. This phenomenon, called behavioral sensitization (BS), is well described in preclinical models. However, its existence in humans is still a matter of debate. After a review of preclinical evidence of BS and its mechanisms in animal models, we reviewed the evidence supporting the existence of BS in humans, despite the limited research available in this regard. We focused our review on opioids and psychostimulants, since they share the ability to promote addictive behaviors. Further, they induce BS despite their distinct sedative and stimulant properties. Moreover, we proposed future research perspectives in this review to address the remaining unsolved questions, especially regarding BS in humans using a harm reduction approach.

The 5-HT7 receptors in the VLO contribute to the development of morphine-induced behavioral sensitization in rats

The 5-hydroxytryptamine 7 receptor (5-HT₇R) is one of the most recently cloned serotonin receptors which have been implicated in many physiological and pathological processes including drug addiction. Behavioral sensitization is the progressive process during which re-exposure to drugs intensified the behavioral and neurochemical responses to drugs. Our previous study has demonstrated that the ventrolateral orbital cortex (VLO) is critical for morphine-induced reinforcing effect. The aim of the present study was to investigate the effect of 5-HT₇Rs in the VLO on morphine-induced behavioral sensitization and their underlying molecular mechanisms. Our results showed that a single injection of morphine, followed by a low challenge dose could induce behavioral sensitization. Microinjection of the selective 5-HT₇R agonist AS-19 into the VLO during the development phase significantly increased morphine-induced hyperactivity. Microinjection of the 5-HT₇R antagonist SB-269970 suppressed acute morphine-induced hyperactivity and the induction of behavioral sensitization, but had no effect on the expression of behavioral sensitization. In addition, the phosphorylation of AKT (Ser 473) was increased during the expression phase of morphine-induced behavioral sensitization. Suppression of the induction phase could also block the increase of p-AKT (Ser 473). In conclusion, we demonstrated that 5-HT₇Rs and p-AKT in the VLO at least partially contribute to morphine-induced behavioral sensitization.

A Prolonged Treatment Associated With Absent Septum Pellucidum in Neonatal Abstinence Syndrome: A Case Report

Neonatal abstinence syndrome (NAS) involves a widely variable treatment course among affected individuals. Prognostic indicators that would help predict length of hospital stay and individualize treatment would be valuable to newborns, parents, and hospital staff, including advanced practice registered nurses. We describe a newborn with a prolonged NAS treatment course necessitating high doses of opioids and phenobarbital, found to have an isolated absent septum pellucidum (ASP). We hypothesize a mechanism for an association between an ASP and a difficult NAS treatment course. Should this be substantiated by other cases, it could provide a valuable prognosticator and indicate alternate treatment pathways.

Molecular chaperone heat shock protein 70 inhibitors suppress conditioned place preference induced by morphine exposure in male rats

Previous studies have indicated a role for molecular chaperone heat shock protein 70 (Hsp70) in the development of behavioural sensitization to morphine in rodents, suggesting that Hsp70 expression following morphine exposure is involved in molecular changes that may underlie addiction vulnerability. The current study was carried out to investigate the role of Hsp70 in the positive reinforcing properties of morphine using conditioned place preference (CPP) in male rats. An unbiased CPP procedure of three phases (pre-conditioning: d1-d3; conditioning: d4-d6; and testing: d7) was used. During the conditioning phase, morphine injections (5 mg/kg, subcutaneously) were administered to induce significant place preference. To explore the effect of Hsp70 on the development and expression of morphine CPP, Hsp70 inhibitors (PES, KNK437 and methylene blue) were administered into the lateral ventricle prior to either morphine conditioning sessions or a morphine challenge on the test day. Furthermore, Hsp70 expression within the mesocorticolimbic system was measured after the treatment with KNK437, a transcriptional inhibitor. We found that PES and KNK437, respectively, injected intracerebroventricularly dose-dependently attenuated both the development and expression of morphine CPP. Methylene blue treatment demonstrated an attenuation of the development, but had no effect on the expression of morphine CPP. Following KNK437 treatment, Hsp70 expression was significantly inhibited in the shell of nucleus accumbens (NAc) during both the development and expression of morphine CPP. The findings suggest that Hsp70 in the NAc shell plays an important role in the reinforcing effects of morphine and may be involved in the development of morphine dependence.

Mu-opioid receptors in septum mediate the development of behavioural sensitization to a single morphine exposure in male rats

Behavioural sensitization (BS) is characterized by enhanced psychomotor responses to a dose of substance of abuse after prior repeated exposure. We previously reported that BS can be induced by a single injection of morphine in rats, whereas septal nuclei are specifically involved in the development phase of BS. Here, we demonstrated that intra-LS or intra-MS microinjections also incubated BS to a systemic morphine injection in a cross-sensitization fashion, whereas inactivation of either subdivision of septal nuclei (LS: lateral septum; MS: medial septum) can negate this ability of morphine. Then, non-selective (naloxone) and selective (μ-, δ- and κ-)opioid receptor antagonists were directly delivered into LS or MS, respectively, ahead of a morphine microinjection, whereas only μ-opioid receptors in both LS and MS play indispensable roles in mediating the BS development. Finally, there was a pronounced elevation in the levels of the monoamines (i.e. dopamine, homovanillic acid, 5-hydroxytryptamine and 5-hydroxyindoleacetic acid) in the septum, 8 h after a morphine injection detected with a HPLC-ECD method, suggesting that dopaminergi and serotoninergic systems are implicated in the BS formation. Our studies demonstrated that septal nuclei critically participate in the BS development. Essentially, μ- instead of δ- or κ-opioid receptors in LS and MS mediate sensitization to opiates.

Sirtuin 1 inhibitor EX527 suppresses morphine-induced behavioral sensitization

Morphine addiction is categorized as a chronic recurrent brain disease which always results in mental disturbance, concomitant diseases and early death. Recent evidence suggested that Sirtuin 1 (SIRT1) played a crucial role in learning, memory and reward, nevertheless, its role in morphine addiction is still unclear. We explored whether SIRT1 in the ventrolateral orbital cortex (VLO) is associated with morphine addiction and its potential mechanism. We applied the morphine-induced behavioral sensitization paradigm to investigate whether microinjection of EX527, a SIRT1 inhibitor, into the VLO could affect the rat behaviors. Furthermore, we focused on the expression of extracellular signal-regulated protein kinases (ERK) and brain-derived neurotrophic factor (BDNF), potential downstream targets of SIRT1. Microinjecting EX527 into the VLO significantly suppressed morphine-induced behavioral sensitization. We found that the expression of SIRT1, phosphorylated ERK (p-ERK) and BDNF in the VLO were markedly up-regulated by morphine administrations in expression phase. These positive changes were significantly inhibited by microinjecting EX527 into the VLO. These results suggest that SIRT1 in the VLO may mediate morphine-induced behavioral sensitization and the overexpression of SIRT1, p-ERK and BDNF could be the potential mechanism. Taken together, the results of our research provide evidence to support that SIRT1 play an important role in morphine vulnerability and microinjecting EX527 into the VLO could significantly suppress morphine addiction in rats.

The day-night differences in ERK1/2, GSK3β activity and c-Fos levels in the brain, and the responsiveness of various brain structures to morphine

As with other drugs or pharmaceuticals, opioids differ in their rewarding or analgesic effects depending on when they are applied. In the previous study, we have demonstrated the day/night difference in the sensitivity of the major circadian clock in the suprachiasmatic nucleus to a low dose of morphine, and showed the bidirectional effect of morphine on pERK1/2 and pGSK3β levels in the suprachiasmatic nucleus depending on the time of administration. The main aim of this study was to identify other brain structures that respond differently to morphine depending on the time of its administration. Using immunohistochemistry, we identified 44 structures that show time-of-day specific changes in c-Fos level and activity of ERK1/2 and GSK3β kinases in response to a single dose of 1 mg/kg morphine. Furthermore, comparison among control groups revealed the differences in the spontaneous levels of all markers with a generally higher level during the night, that is, in the active phase of the day. We thus provide further evidence for diurnal variations in the activity of brain regions outside the suprachiasmatic nucleus indicated by the temporal changes in the molecular substrate. We suggest that these changes are responsible for generating diurnal variation in the reward behavior or analgesic effect of opioid administration.

Targeting Phosphodiesterases in Pharmacotherapy for Substance Dependence

Substance dependence is a chronic relapsing brain disorder associated with adaptational changes in synaptic plasticity and neuronal functions. The high levels of substance consumption and relapse rate suggest more reliable medications are in need to better address the underlying causes of this disease. It has been well established that the intracellular second messengers cyclic AMP (cAMP) and cyclic GMP (cGMP) and their signaling systems play an important role in the molecular mechanisms of substance taking behaviors. On this basis, the phosphodiesterase (PDE) superfamily, which crucially controls cyclic nucleotide levels by catalyzing their hydrolysis, has been proposed as a novel class of therapeutic targets for substance use disorders. This chapter reviews the expression patterns of PDEs in the brain with regard to neural structures underlying the dependent process and highlights available evidence for a modulatory role of PDEs in substance dependence.

Altered Acoustic Startle Reflex, Prepulse Inhibition, and Peripheral Brain-Derived Neurotrophic Factor in Morphine Self-Administered Rats

BACKGROUND

Previous studies suggested that opiate withdrawal may increase anxiety and disrupt brain-derived neurotrophic factor function, but the effects of i.v. morphine self-administration on these measures remain unclear.

METHODS

Adult male Sprague-Dawley rats were implanted with a catheter in the jugular vein. After 1 week of recovery, the animals were allowed to self-administer either i.v. morphine (0.5 mg/kg per infusion, 4 h/d) or saline in the operant conditioning chambers. The acoustic startle reflex and prepulse inhibition were measured at a baseline and on self-administration days 1, 3, 5, and 7 (1- and 3-hour withdrawal). Blood samples were collected on self-administration days 3, 5, and 7 from separate cohorts of animals, and the levels of brain-derived neurotrophic factor and corticosterone were assayed using the enzyme-linked immunosorbent assay method.

RESULTS

Compared with the saline group, the morphine self-administration group showed hyper-locomotor activity and reduced defecation during the self-administration. The morphine self-administration increased acoustic startle reflex at 1-hour but not 3-hour withdrawal from morphine and disrupted prepulse inhibition at 3-hour but not 1-hour withdrawal. The blood brain-derived neurotrophic factor levels were decreased in the morphine self-administration group at self-administration days 3 and 5, while the corticosterone levels remained unchanged throughout the study.

CONCLUSIONS

The current findings suggest that spontaneous withdrawal from i.v. morphine self-administration may have transient effects on acoustic startle, sensorimotor gating, and peripheral brain-derived neurotrophic factor levels, and these changes may contribute to the adverse effects of opiate withdrawal.

Inflammatory pain memory facilitates occlusal interference-induced masticatory muscle hyperalgesia in rats

BACKGROUND

Patients with an orofacial pain history appear to be more susceptible to occlusal interference pain in dental practice for unknown reasons. Pain memory has a critical function in subsequent pain perception. This study aims to explore whether orofacial pain memory could affect the masticatory muscle pain perception for occlusal interference.

METHODS

Cross-injection of 2% carrageenan into bilateral masseters in male rats was carried out to establish the inflammatory pain memory model. The effects of pain memory on masseter muscle nociception were tested by applying crowns with heights beyond the occlusal plane by 0.2 or 0.4 mm onto a maxillary molar 2 weeks after inflammation in the right masseter. The 0.4-mm crowns were removed on day 2 or day 4 after application to further confirm the effects of pain memory. Moreover, memory impairment was established using ibotenic acid (IBO) infusion into the bilateral hippocampus, followed by behaviour tests, including the Morris water maze test and the locomotor activity test. The relationship between pain memory and occlusal interference-induced masseter muscle pain perception was subsequently re-examined. The head withdrawal thresholds of masseters on both sides were measured to reflect the perception.

RESULTS

Inflammatory pain memory aggravated the 0.2-mm crown-induced mechanical hyperalgesia of the masseters, but not in the 0.4-mm crown group. However, the recovery of the 0.4-mm crown-induced mechanical hyperalgesia was postponed. The effects of pain memory were reversed in rats with impaired mnemonic function of the hippocampus.

CONCLUSIONS

Inflammatory pain memory facilitated occlusal interference-induced masseter muscle pain.

Endogenous opiates and behavior: 2013

This paper is the thirty-sixth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2013 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior, and the roles of these opioid peptides and receptors in pain and analgesia; stress and social status; tolerance and dependence; learning and memory; eating and drinking; alcohol and drugs of abuse; sexual activity and hormones, pregnancy, development and endocrinology; mental illness and mood; seizures and neurologic disorders; electrical-related activity and neurophysiology; general activity and locomotion; gastrointestinal, renal and hepatic functions; cardiovascular responses; respiration and thermoregulation; and immunological responses.

Brucine suppresses ethanol intake and preference in alcohol-preferring Fawn-Hooded rats

AIM

Brucine (BRU) extracted from the seeds of Strychnos nux-vomica L is glycine receptor antagonist. We hypothesize that BRU may modify alcohol consumption by acting at glycine receptors, and evaluated the pharmacodynamic profiles and adverse effects of BRU in rat models of alcohol abuse.

METHODS

Alcohol-preferring Fawn-Hooded (FH/Wjd) rats were administered BRU (10, 20 or 30 mg/kg, sc). The effects of BRU on alcohol consumption were examined in ethanol 2-bottle-choice drinking paradigm, ethanol/sucrose operant self-administration paradigm and 5-d ethanol deprivation test. In addition, open field test was used to assess the general locomotor activity of FH/Wjd rats, and conditioned place preference (CPP) was conducted to assess conditioned reinforcing effect.

RESULTS

In ethanol 2-bottle-choice drinking paradigm, treatment with BRU for 10 consecutive days dose-dependently decreased the ethanol intake associated with a compensatory increase of water intake, but unchanged the daily total fluid intake and body weight. In ethanol/sucrose operant self-administration paradigms, BRU (30 mg/kg) administered before each testing session significantly decreased the number of lever presses for ethanol and the ethanol intake, without affecting the number of sucrose (10%) responses, total sucrose intake, and the number of lever presses for water. Acute treatment with BRU (30 mg/kg) completely suppressed the deprivation-induced elevation of ethanol consumption. Treatment with BRU (10, 20, and 30 mg/kg) did not alter locomotion of FH/Wjd rats, nor did it produce place preference or aversion.

CONCLUSION

BRU selectively decreases ethanol consumption with minimal adverse effects. Therefore, BRU may represent a new pharmacotherapy for alcoholism.