Modulatory effects of the basolateral amygdala α2-adrenoceptors on nicotine-induced anxiogenic-like behaviours of rats in the elevated plus maze

The effect of low- and moderate-intensity interval training on cognitive behaviors of male and female rats with VPA-induced autism

Introduction

This study was performed to evaluate the effects of low and moderate treadmill exercise for one month on social interaction, anxiety-like behaviors, and spatial learning and memory in male and female autistic rats.

Methods

Pregnant rats received valproic acid (VPA) (600 mg/kg/i.p) once on gestational day 12.5 to induce autism-like symptoms in the offspring. After delivery, the offspring were divided into six main groups, each with male and female subgroups: Control (CTL, prenatal normal saline), autism (prenatal VPA), low-intensity training (LIT,normal saline + low treadmill exercise), moderate -intensity training (MIT, normal saline + moderate treadmill exercise), VPA + LIT, and VPA + MIT. On the 60th day, the offspring were tested by the elevated plus maze (EPM), open field test (OFT), social interaction test (SIT), and Morris water maze (MWM).

Results

The results showed that both LIT and MIT could partly alleviate anxiety-like behaviors induced by prenatal VPA exposure in two sexes. Social impairment was observed in the autistic rats and was improved by LIT in both sexes and MIT in females. No significant change was seen in the spatial learning and memory of autistic rats by exercise.

Conclusion

The findings suggest that treadmill exercise can be helpful for improving some autism-like behaviors. Further studies are needed to investigate the involved mechanisms.

Activation of mediodorsal thalamic dopamine receptors inhibited nicotine-induced anxiety in rats: A possible role of corticolimbic NMDA neurotransmission and BDNF expression

The present study aimed to evaluate the functional interaction between the dopaminergic and glutamatergic systems of the mediodorsal thalamus (MD), the ventral hippocampus (VH), and the prefrontal cortex (PFC) in nicotine-induced anxiogenic-like behaviors. Brain-derived neurotrophic factor (BDNF) level changes were measured in the targeted brain areas following the drug treatments. The percentage of time spent in the open arm (% OAT) and open arm entry (% OAE) were calculated in the elevated plus maze (EPM) to measure anxiety-related behaviors in adult male Wistar rats. Systemic administration of nicotine at a dose of 0.5 mg/kg induced an anxiogenic-like response associated with decreased BDNF levels in the hippocampus and the PFC. Intra-MD microinjection of apomorphine (0.1-0.3 μg/rat) induced an anxiogenic-like response, while apomorphine inhibited nicotine-induced anxiogenic-like behaviors associated with increased hippocampal and PFC BDNF expression levels. Interestingly, the blockade of the VH or the PFC NMDA receptors via the microinjection of D-AP5 (0.3-0.5 μg/rat) into the targeted sites reversed the inhibitory effect of apomorphine (0.5 μg/rat, intra-MD) on the nicotine response and led to the decrease of BDNF levels in the hippocampus and the PFC. Also, the microinjection of a higher dose of D-AP5 (0.5 μg/rat, intra-PFC) alone produced an anxiogenic effect. These findings suggest that the functional interaction between the MD dopaminergic D1/D2-like and the VH/PFC glutamatergic NMDA receptors may be partially involved in the anxiogenic-like effects of nicotine, likely via the alteration of BDNF levels in the hippocampus and the PFC.

Electronic cigarettes can damage lingual papillae and taste buds. Can vitamins C and E supplementation reverse this damage?

AIMS

This histological study focuses on the impact of electronic cigarette liquid (EC) on lingual papillae, especially taste buds, compare it to nicotine, and investigates the potential of vitamins in reversing these unwanted changes.

MAIN METHODS

40 adult male rats were allocated into 5 groups. Control injected saline intraperitoneally, electronic cigarettes group injected EC-liquid containing nicotine of dose (0.75 mg/kg), electronic cigarette group injected EC-liquid then supplemented orally with vitamins C and E, nicotine group injected pure nicotine of dose (0.75 mg/kg) and lastly nicotine group injected with pure nicotine of dose (0.75 mg/kg) then supplemented orally with vitamins C and E. Keratin surface area and the ratio between taste buds and its epithelial covering surface areas in fungiform papillae were measured.

KEY FINDINGS

Histological examination of EC group revealed abnormal epithelial stratification and mitotic figs. EC plus V group showed intact basal cell layer. N group showed better histological stratification than EC group. Fungiform and circumvallate papillae in EC and N groups showed distorted appearance of taste buds. Histomorphometry analysis showed a significant decrease in taste buds to epithelium surface areas in EC, nicotine, and EC plus V groups, p-value (<0.05). There was no significant difference between control and N plus V groups.

SIGNIFICANCE

Administration of vitamins C and E showed preservation of normal histological features of the lingual mucous membrane. EC caused striking damage to taste buds even after the administration of vitamins. The negative effects of electronic cigarettes are not confined only to the presence of nicotine.

Reduction of nicotine and ethanol intake in alcohol-preferring (UChB) female rats by the α4β2 nicotinic acetylcholine receptor partial agonists 5-bromocytisine and cytisine

RATIONALE

Neuronal nicotinic acetylcholine receptors (nAChRs) are implicated in the reinforcing effects of nicotine and ethanol. Previous studies have shown that cytisine and its 5-bromo derivative are partial agonists at the α4β2 nAChRs and that the parent molecule cytisine is effective in reducing both nicotine- and ethanol-self-administration in rats. However, whether 5-bromocytisine affects nicotine or ethanol self-administration was unknown.

OBJECTIVES

The present study compared the effects of 5-bromocytisine and cytisine on nicotine self-administration and further assessed the effect of daily drug injection on voluntary ethanol consumption in alcohol-preferring female rats. Animals were administered a 1.5mg/kg i.p. dose of 5-bromocytisine or cytisine every day for 15-16 days.

RESULTS

The initial efficacy of 5-bromocytisine and cytisine in reducing nicotine intake was similar (-80%) while for voluntary ethanol intake 5-bromocytisine was a superior inhibitor over cytisine (-78% and -40% respectively). The efficacy of cytisine began to diminish after 10 days of daily administration, which was attributed to tolerance development to its inhibitory effects both on nicotine and ethanol self-administration. Tolerance did not develop for 5-bromocytisine.

CONCLUSION

5-Bromocytisine, a weaker α4β2 nAChR partial agonist than cytisine, also produces a sustained inhibition of both nicotine and ethanol self-administration, and unlike cytisine, it does not develop tolerance.

Ameliorating age-dependent effects of resveratrol on VPA-induced social impairments and anxiety-like behaviors in a rat model of Neurodevelopmental Disorder

Although anxiety disorders, as well as difficulties in social interaction, are documented in children with autism spectrum disorder (ASD) as a neurodevelopmental disorder, the effectiveness of potential therapeutic procedures considering age and sex differences is under serious discussion. The present study aimed to investigate the effect of resveratrol (RSV) on anxiety-like behaviors and social interaction in juvenile and adult rats of both sex in a valproic acid (VPA)-induced autistic-like model. Prenatal exposure to VPA was associated with increased anxiety, also causing a significant reduction in social interaction in juvenile male subjects. Further administration of RSV attenuated VPA-induced anxiety symptoms in both sexes of adult animals and significantly increased the sociability index in male and female juvenile rats. Taken together, it can be concluded that treatment with RSV can attenuate some of the harsh effects of VPA. This treatment was especially effective on anxiety-like traits in adult subjects of both sexes regarding their performance in open field and EPM. We encourage future research to consider the sex and age-specific mechanisms behind the RSV treatment in the prenatal VPA model of autism.

High-Intensity Interval Training-Induced Hippocampal Molecular Changes Associated with Improvement in Anxiety-like Behavior but Not Cognitive Function in Rats with Type 2 Diabetes

(1) Background: Exercise exerts many neuroprotective effects in diabetes-induced brain disorders. In this study, we investigated the effect of high-intensity interval training (HIIT) on brain molecular changes and cognitive and anxiety-like behaviors in rats with type 2 diabetes. (2) Methods: Twenty-eight adult male rats were divided into four groups (n = 7): control (C), exercise + control (C+EX), diabetes (DM), and diabetes + exercise (DM+EX). Diabetes was induced using a two-month high-fat diet and a single dose of streptozotocin (35 mg/kg) in the DM and DM+EX groups. After, the C+EX and DM+EX groups performed HIIT for eight weeks (five sessions per week, running at 80-100% of VMax, 4-10 intervals) on a motorized treadmill. Then, the elevated plus maze (EPM) and open field test (OFT) were performed to evaluate anxiety-like behaviors. The Morris water maze (MWM) and shuttle box were used to assess cognitive function. The hippocampal levels of beta-amyloid and tau protein were also assessed using Western blot. (3) Results: The hippocampal levels of beta-amyloid and tau protein were increased in the DM group, but HIIT restored these changes. While diabetes led to a significant decrease in open arm time percentage (%OAT) and open arm enters percentage (%OAE) in the EPM, indicating anxiety-like behavior, HIIT restored them. In the OFT, grooming was decreased in diabetic rats, which was restored by HIIT. No significant difference between groups was seen in the latency time in the shuttle box or for learning and memory in the MWM. (4) Conclusions: HIIT-induced hippocampal molecular changes were associated with anxiety-like behavior improvement but not cognitive function in rats with type 2 diabetes.

Under or Absent Reporting of Light Stimuli in Testing of Anxiety-Like Behaviors in Rodents: The Need for Standardization

Behavioral neuroscience tests such as the Light/Dark Test, the Open Field Test, the Elevated Plus Maze Test, and the Three Chamber Social Interaction Test have become both essential and widely used behavioral tests for transgenic and pre-clinical models for drug screening and testing. However, as fast as the field has evolved and the contemporaneous involvement of technology, little assessment of the literature has been done to ensure that these behavioral neuroscience tests that are crucial to pre-clinical testing have well-controlled ethological motivation by the use of lighting (i.e., Lux). In the present review paper, N = 420 manuscripts were examined from 2015 to 2019 as a sample set (i.e., n = ~20–22 publications per year) and it was found that only a meager n = 50 publications (i.e., 11.9% of the publications sampled) met the criteria for proper anxiogenic and anxiolytic Lux reported. These findings illustrate a serious concern that behavioral neuroscience papers are not being vetted properly at the journal review level and are being released into the literature and public domain making it difficult to assess the quality of the science being reported. This creates a real need for standardizing the use of Lux in all publications on behavioral neuroscience techniques within the field to ensure that contributions are meaningful, avoid unnecessary duplication, and ultimately would serve to create a more efficient process within the pre-clinical screening/testing for drugs that serve as anxiolytic compounds that would prove more useful than what prior decades of work have produced. It is suggested that improving the standardization of the use and reporting of Lux in behavioral neuroscience tests and the standardization of peer-review processes overseeing the proper documentation of these methodological approaches in manuscripts could serve to advance pre-clinical testing for effective anxiolytic drugs. This report serves to highlight this concern and proposes strategies to proactively remedy them as the field moves forward for decades to come.

The Effect of Oral Mucosal Mesenchymal Stem Cells on Pathological and Long-Term Outcomes in Experimental Traumatic Brain Injury

Background

Neuroprotective effects of stem cells have been shown in some neurologic diseases. In this study, the effect of oral mucosal mesenchymal stem cells (OMSCs) on traumatic brain injury (TBI) was evaluated in long term.

Materials and Methods

TBI was induced by Marmarou's method. The number of 2 × 10⁶ OMSCs was intravenously injected 1 and 24 h after the injury. Brain edema and pathological outcome were assessed at 24 h and 21 days after the injury. Besides, long-term neurological, motor, and cognitive outcomes were evaluated at days 3, 7, 14, and 21 after the injury.

Results

OMSCs administration could significantly inhibit microglia proliferation, and reduce brain edema and neuronal damage, at 24 h and 21 days after the injury. Neurological function improvement was observed in the times evaluated in OMSCs group. Cognitive and motor function dysfunction and anxiety-like behavior were prevented especially at 14 and 21 days after the injury in the treatment group.

Conclusion

According to the results of this study, OMSCs administration after TBI reduced brain edema and neuronal damage, improved neurologic outcome, and prevented memory and motor impairments and anxiety-like behavior in long term.

The effect of early handling on anxiety-like behaviors of rats exposed to valproic acid pre-and post-natally

INTRODUCTION

Autism spectrum disorder (ASD) is a complex, behaviorally defined disorder of the immature brain as a result of genetic and environmental risk factors, such as prenatal exposure to valproic acid (VPA). This syndrome is known for its high prevalence. On the other hand, postnatal manipulations have been shown to affect brain development, cortical neuroscience, and pituitary-adrenal activity. In early handling (EH) procedure, pups are removed from their mother on a daily basis from birth to lactation, are physically touched, and exposed to the (a) new environment. In the present study, the effect of EH on anxiety-like behaviors in rats exposed pre- and post-natally to valproic acid was investigated.

METHODS

Pregnant Wistar rats were randomly separated into six groups which are prenatal saline, Prenatal VPA, Prenatal VPA + EH and postnatal saline, Postnatal VPA, Postnatal VPA + EH. VPA administration was performed either on ED12.5 (600 mg/kg, i.p.) or PD 2-4 (400 mg/kg, s.c.). In the groups receiving EH, pups underwent physical handling from PD 1 to 21. On postnatal day 21 all offspring were weaned and the behavioral tests were performed on 30 and 31 days of age. Elevated plus maze and open field tests were used to investigate anxiety-like behaviors.

RESULTS

The results revealed that intraperitoneal injection of valrpoic acid (600 mg.kg) during pregnancy significantly reduced OAT% in males (p < 0.01) and females in a non-significant manner (p > 0.05). In comparison, rearing counts of prenatal VPA groups significantly increased in female sex (p < 0.05) in the EPM test. Following postnatal VPA administration (400 mg/kg), decrease in the time spent in central zone occurred in female rats in the open filed (p < 0.05), as well as a significant increase in the number of grooming of the male sex (p < 0.05). Applying Early Handling to male and female Wistar rats receiving prenatal VPA significantly reversed the OAT% fall (p < 0.05). EH in postnatally VPA exposed animals significantly decreased the OAT% and OAE% criteria, while increasing the locomotor activity of the female sex (p < 0.05). Compared with the postnatal VPA group, no significant change was reported in the EPM performance of postnatal VPA + EH group in neither of sexes (p > 0.05).

CONCLUSION

The findings of this study suggest that injections of valproic acid during pregnancy lead to anxiety-like behaviors in male offspring, which EH can improve (attenuate) to some extent. VPA injections on the second to the fourth day of infancy did not have a profound effect on anxiety level. Further behavioral studies need to be performed using other devices to investigate anxiety-like behaviors and to determine the mechanisms involved in these behaviors.

Improved spatial memory, neurobehavioral outcomes, and neuroprotective effect after progesterone administration in ovariectomized rats with traumatic brain injury: Role of RU486 progesterone receptor antagonist

OBJECTIVES

The contribution of classic progesterone receptors (PR) in interceding the neuroprotective efficacy of progesterone (P4) on the prevention of brain edema and long-time behavioral disturbances was assessed in traumatic brain injury (TBI).

MATERIALS AND METHODS

Female Wistar rats were ovariectomized and apportioned into 6 groups: sham, TBI, oil, P4, vehicle, and RU486. P4 or oil was injected following TBI. The antagonist of PR (RU486) or DMSO was administered before TBI. The brain edema and destruction of the blood-brain barrier (BBB) were determined. Intracranial pressure (ICP), cerebral perfusion pressure (CPP), and beam walk (BW) task were evaluated previously and at various times post-trauma. Long-time locomotor and cognitive consequences were measured one day before and on days 3, 7, 14, and 21 after the trauma.

RESULTS

RU486 eliminated the inhibitory effects of P4 on brain edema and BBB leakage (P<0.05, P<0.001, respectively). RU486 inhibited the decremental effect of P4 on ICP as well as the increasing effect of P4 on CPP (P<0.001) after TBI. Also, RU486 inhibited the effect of P4 on the increase in traversal time and reduction in vestibulomotor score in the BW task (P<0.001). TBI induced motor, cognitive, and anxiety-like disorders, which lasted for 3 weeks after TBI; but, P4 prevented these cognitive and behavioral abnormalities (P<0.05), and RU486 opposed this P4 effect (P<0.001).

CONCLUSION

The classic progesterone receptors have neuroprotective effects and prevent long-time behavioral and memory deficiency after brain trauma.

Administration of N-acetylcysteine Plus Acetylsalicylic Acid Markedly Inhibits Nicotine Reinstatement Following Chronic Oral Nicotine Intake in Female Rats

Background

Nicotine is the major addictive component of cigarette smoke and the prime culprit of the failure to quit smoking. Common elements perpetuating the use of addictive drugs are (i) cues associated with the setting in which drug was used and (ii) relapse/reinstatement mediated by an increased glutamatergic tone (iii) associated with drug-induced neuroinflammation and oxidative stress.

Aims

The present study assessed the effect of the coadministration of the antioxidant N-acetylcysteine (NAC) plus the anti-inflammatory acetylsalicylic acid (ASA) on oral nicotine reinstatement intake following a post-deprivation re-access in female rats that had chronically and voluntarily consumed a nicotine solution orally. The nicotine-induced oxidative stress and neuroinflammation in the hippocampus and its effects on the glutamate transporters GLT-1 and XCT mRNA levels in prefrontal cortex were also analyzed.

Results

The oral coadministration of NAC (40 mg/kg/day) and ASA (15 mg/kg/day) inhibited by 85% of the oral nicotine reinstatement intake compared to control (vehicle), showing an additive effect of both drugs. Acetylsalicylic acid and N-acetylcysteine normalized hippocampal oxidative stress and blunted the hippocampal neuroinflammation observed upon oral nicotine reinstatement. Nicotine downregulated GLT-1 and xCT gene expression in the prefrontal cortex, an effect reversed by N-acetylcysteine, while acetylsalicylic acid reversed the nicotine-induced downregulation of GLT-1 gene expression. The inhibitory effect of N-acetylcysteine on chronic nicotine intake was blocked by the administration of sulfasalazine, an inhibitor of the xCT transporter.

Conclusion

Nicotine reinstatement, following post-deprivation of chronic oral nicotine intake, downregulates the mRNA levels of GLT-1 and xCT transporters, an effect reversed by the coadministration of N-acetylcysteine and acetylsalicylic acid, leading to a marked inhibition of nicotine intake. The combination of these drugs may constitute a valuable adjunct in the treatment of nicotine-dependent behaviors.

E2-BSA and G1 exert neuroprotective effects and improve behavioral abnormalities following traumatic brain injury: The role of classic and non-classic estrogen receptors

The role of classical and non-classical estrogen receptors (ERs) in mediating the neuroprotective effects of this hormone on brain edema and long-term behavioral disorders was evaluated after traumatic brain injury (TBI). Ovariectomized rats were divided as follows: E2 (17 β-estradiol), E2-BSA (E2 conjugated to bovine serum albumin), G1 [G-protein-coupled estrogen receptor agonist (GPER)] or their vehicle was injected following TBI, whereas ICI (classical estrogen receptor antagonist), G15 (GPER antagonist), ICI + G15, and their vehicle were injected before the induction of TBI and the injection of E2 and E2-BSA. Brain water (BWC) and Evans blue (EB) contents were measured 24 h and 5 h after TBI, respectively. Intracranial pressure (ICP) and cerebral perfusion pressure (CPP) were measured before and at different times after TBI. Locomotor activity, anxiety-like behavior, and spatial memory were assessed on days 3, 7, 14, and 21 after injury. E2, E2-BSA, and G1 prevented the increase of BWC and EB content after TBI, and these effects were inhibited by ICI and G15. ICI and G15 also inhibited the beneficial effects of E2, E2-BSA on ICP, as well as CPP, after trauma. E2, E2-BSA, and G1 prevented the cognitive deficiency and behavioral abnormalities induced by TBI. Similar to the above parameters, ICI and G15 also reversed this E2 and E2-BSA effects on days 3, 7, 14, and 21. Our findings indicated that the beneficial effects of E2-BSA and E2 were inhibited by both ICI and G15, suggesting that GPER and classic ERs were involved in mediating the long-term effects of E2.

Synergistic effect between citalopram and citicoline on anxiolytic effect in non-sensitized and morphine-sensitized mice: An isobologram analysis

In the present study, the effects of intraperitoneal (i.p.) injections of citalopram and citicoline on morphine-induced anxiolytic effects were investigated in non-sensitized and morphine-sensitized mice using elevated plus-maze (EPM). Subcutaneous (s.c.) administration morphine (5 mg/kg) increased the percentage of open arm time (%OAT, in morphine-sensitized mice), and open arm entries (%OAE, in non-sensitized mice), but not a locomotor activity, indicating an anxiolytic response to morphine. On the other hand, i.p. administration of naloxone decreased %OAT (morphine-sensitized mice), and %OAE (non-sensitized and morphine-sensitized mice), but not a locomotor activity, showing an anxiogenic effect to naloxone. Moreover, i.p.co-administration of citalopram (5 and 10 mg/kg) and citicoline (75 mg/kg) induced the anxiolytic effect. Interestingly, i.p. co-administration of low doses of citalopram (0.5, 1 and 2.5 mg/kg) and citicoline (25 mg/kg) significantly increased %OAT and %OAE in non-sensitized as well as %OAT in morphine-sensitized mice, indicating an anxiolytic effect. An isobolographic analysis of data was performed, presenting a synergistic interaction between citalopram and citicoline upon the production of anxiolytic effect in non-sensitized and morphine-sensitized mice. In conclusion, it seems that (1) morphine sensitization affects the anxiety behavior in the EPM, (2) μ-opioid receptors play an important role in morphine anxiolytic effect, (3) citalopram and citicoline induced anti-anxiety effect, (4) a synergistic effect of citalopram and citicoline upon induction of anti-anxiety behavior in non-sensitized and morphine-sensitized mice.

Effects of precondition α2-adrenoceptor agents on memory- and anxiety-related processes in the transient cerebral ischemic rats

Neurological evidence for the neuroprotective function of α₂-adrenoceptors in the cerebral ischemia is inconsistent. It is not examined how pretreatment with a single dose of α₂-adrenoceptor agents can affect motor function and anxiety- and memory-related responses in the cerebral ischemic animals. The transient forebrain ischemia model was provided, using a bilateral common carotid arterial occlusion (two-vessel occlusion, 2VO) in male Wistar rats. The 2VO rats impaired motor functions in the Rota-rod and wire grip tests and also decreased the step-through latency and the percentage of time spent on the open arms (%OAT), the percentage of entries into the open arms (%OAE) as well as locomotion in the elevated plus maze (EPM), indicating a memory deficit and anxiety-like behavior. Intraperitoneal single administration of yohimbine (0, 0.001, 0.01, and 0.1 mg/kg) before the 2VO did not alter these parameters while the higher and middle doses of clonidine (0.01 and 0.1 mg/kg) prevented the memory deficit and hypo-locomotion and its middle dose abrogated Rota-rod dysfunction and anxiety-like response. Meanwhile, both drugs did not influence on the measured behaviors in the sham groups by themselves. Moreover, yohimbine (0.001 mg/kg) abolished the beneficial effects of clonidine (0.01 and 0.1 mg/kg) on motor function in the Rota-rod and memory retention and also at its middle dose on the %OAT and locomotion in the 2VO rats. Our findings show a neuroprotective role for clonidine in motor function and memory- and anxiety-related behaviors of 2VO rats and the importance of α₂-adrenoceptors in these processes.

Chemogenetic Silencing of the Locus Coeruleus-Basolateral Amygdala Pathway Abolishes Pain-Induced Anxiety and Enhanced Aversive Learning in Rats

BACKGROUND

Pain affects both sensory and emotional aversive responses, often provoking anxiety-related diseases when chronic. However, the neural mechanisms underlying the interactions between anxiety and chronic pain remain unclear.

METHODS

We characterized the sensory, emotional, and cognitive consequences of neuropathic pain (chronic constriction injury) in a rat model. Moreover, we determined the role of the locus coeruleus (LC) neurons that project to the basolateral amygdala (BLA) using a DREADD (designer receptor exclusively activated by designer drugs).

RESULTS

Chronic constriction injury led to sensorial hypersensitivity in both the short term and long term. Otherwise, long-term pain led to an anxiety-like profile (in the elevated zero maze and open field tests), as well as increased responses to learn aversive situations (in the passive avoidance and fear conditioning tests) and an impairment of nonemotional cognitive tasks (in the novel object recognition and object pattern of separation tests). Chemogenetic blockade of the LC-BLA pathway and intra-BLA or systemic antagonism of beta-adrenergic receptors abolished both long-term pain-induced anxiety and enhanced fear learning. By contrast, chemogenetic activation of this pathway induced anxiety-like behaviors and enhanced the aversive learning and memory index in sham animals, although it had little effect on short- and long-term chronic constriction injury animals. Interestingly, modulation of LC-BLA activity did not modify sensorial perception or episodic memory.

CONCLUSIONS

Our results indicate that dimensions associated with pain are processed by independent pathways and that there is an overactivation of the LC-BLA pathway when anxiety and chronic pain are comorbid, which involves the activity of beta-adrenergic receptors.

Stress-Induced Reinstatement of Nicotine Preference Requires Dynorphin/Kappa Opioid Activity in the Basolateral Amygdala

The dynorphin (DYN)/kappa-opioid receptor (KOR) system plays a conserved role in stress-induced reinstatement of drug seeking for prototypical substances of abuse. Due to nicotine's high propensity for stress-induced relapse, we hypothesized that stress would induce reinstatement of nicotine seeking-like behavior in a KOR-dependent manner. Using a conditioned place preference (CPP) reinstatement procedure in mice, we show that both foot-shock stress and the pharmacological stressor yohimbine (2 mg/kg, i.p.) induce reinstatement of nicotine CPP in a norbinaltorphimine (norBNI, a KOR antagonist)-sensitive manner, indicating that KOR activity is necessary for stress-induced nicotine CPP reinstatement. After reinstatement testing, we visualized robust c-fos expression in the basolateral amygdala (BLA), which was reduced in mice pretreated with norBNI. We then used several distinct but complementary approaches of locally disrupting BLA KOR activity to assess the role of KORs and KOR-coupled intracellular signaling cascades on reinstatement of nicotine CPP. norBNI injected locally into the BLA prevented yohimbine-induced nicotine CPP reinstatement without affecting CPP acquisition. Similarly, selective deletion of BLA KORs in KOR conditional knock-out mice prevented foot-shock-induced CPP reinstatement. Together, these findings strongly implicate BLA KORs in stress-induced nicotine seeking-like behavior. In addition, we found that chemogenetic activation of Gαi signaling within CaMKIIα BLA neurons was sufficient to induce nicotine CPP reinstatement, identifying an anatomically specific intracellular mechanism by which stress leads to reinstatement. Considered together, our findings suggest that activation of the DYN/KOR system and Gαi signaling within the BLA is both necessary and sufficient to produce reinstatement of nicotine preference.

SIGNIFICANCE STATEMENT

Considering the major impact of nicotine use on human health, understanding the mechanisms by which stress triggers reinstatement of drug-seeking behaviors is particularly pertinent to nicotine. The dynorphin (DYN)/kappa-opioid receptor (KOR) system has been implicated in stress-induced reinstatement of drug seeking for other commonly abused drugs. However, the specific role, brain region, and mechanisms that this system plays in reinstatement of nicotine seeking has not been characterized. Here, we report region-specific engagement of the DYN/KOR system and subsequent activation of inhibitory (Gi-linked) intracellular signaling pathways within the basolateral amygdala during stress-induced reinstatement of nicotine preference. We show that the DYN/KOR system is necessary to produce this behavioral state. This work may provide novel insight for the development of therapeutic approaches to prevent stress-related nicotine relapse.