Chronic psychosocial stress causes delayed extinction and exacerbates reinstatement of ethanol-induced conditioned place preference in mice

Rationale

We have shown previously, using an animal model of voluntary ethanol intake and ethanol-conditioned place preference (EtOH-CPP), that exposure to chronic psychosocial stress induces increased ethanol intake and EtOH-CPP acquisition in mice.

Objective

Here, we examined the impact of chronic subordinate colony (CSC) exposure on EtOH-CPP extinction, as well as ethanol-induced reinstatement of CPP.

Methods

Mice were conditioned with saline or 1.5 g/kg ethanol and were tested in the EtOH-CPP model. In the first experiment, the mice were subjected to 19 days of chronic stress, and EtOH-CPP extinction was assessed during seven daily trials without ethanol injection. In the second experiment and after the EtOH-CPP test, the mice were subjected to 7 days of extinction trials before the 19 days of chronic stress. Drug-induced EtOH-CPP reinstatement was induced by a priming injection of 0.5 g/kg ethanol.

Results

Compared to the single-housed colony mice, CSC mice exhibited increased anxiety-like behavior in the elevated plus maze (EPM) and the open field tests. Interestingly, the CSC mice showed delayed EtOH-CPP extinction. More importantly, CSC mice showed increased alcohol-induced reinstatement of the EtOH-CPP behavior.

Conclusion

Taken together, this study indicates that chronic psychosocial stress can have long-term effects on EtOH-CPP extinction as well as drug-induced reinstatement behavior and may provide a suitable model to study the latent effects of chronic psychosocial stress on extinction and relapse to drug abuse.     

Interactive effects of methylphenidate and alcohol on discrimination, conditioned place preference and motor coordination in C57BL/6J mice

Introduction

Prior research indicates methylphenidate (MPH) and alcohol (ethanol, EtOH) interact to significantly affect responses humans and mice. The present studies tested the hypothesis that MPH and EtOH interact to potentiate ethanol-related behaviors in mice.

Methods

We used several behavioral tasks including: drug discrimination in MPH-trained and EtOH-trained mice, conditioned place preference (CPP), rota-rod and the parallel rod apparatus. We also used gas chromatographic methods to measure brain tissue levels of EtOH and the d- and l-isomers of MPH and the metabolite, ethylphenidate (EPH).

Results

In discrimination, EtOH (1 g/kg) produced a significant leftward shift in the MPH generalization curve (1–2 mg/kg) for MPH-trained mice, but no effects of MPH (0.625–1.25 mg/kg) on EtOH discrimination in EtOH-trained mice (0–2.5 g/kg) were observed. In CPP, the MPH (1.25 mg/kg) and EtOH (1.75 g/kg) combination significantly increased time on the drug paired side compared to vehicle (30.7 %), but this was similar to MPH (28.8 %) and EtOH (33.6 %). Footslip errors measured in a parallel rod apparatus indicated that the drug combination was very ataxic, with footslips increasing 29.5 % compared to EtOH. Finally, brain EtOH concentrations were not altered by 1.75 g/kg EtOH combined with 1.25 mg/kg MPH. However, EtOH significantly increased d-MPH and l-EPH without changing l-MPH brain concentrations.

Conclusions

The enhanced behavioral effects when EtOH is combined with MPH are likely due to the selective increase in brain d-MPH concentrations. These studies are consistent with observations in humans of increased interoceptive awareness of the drug combination and provide new clinical perspectives regarding enhanced ataxic effects of this drug combination.     

Involvement of 2-arachidonoyl glycerol in the increased consumption of and preference for ethanol of mice treated with neurotoxic doses of methamphetamine

Background and purpose:

Methamphetamine (METH) is a psychostimulant amphetamine that causes long-term dopaminergic neurotoxicity in mice. Hypodopaminergic states have been demonstrated to increase voluntary ethanol (EtOH) consumption and preference. In addition, the endocannabinoid system has been demonstrated to modulate EtOH drinking behaviour. Thus, we investigated EtOH consumption in METH-lesioned animals and the role of cannabinoid (CB) signalling in this EtOH drinking.

Experimental approach:

Mice were treated with a neurotoxic regimen of METH, and 7 days later exposed to increasing concentrations of drinking solutions of EtOH (3, 6, 10 and 20%). Seven days after neurotoxic METH, the following biochemical determinations were carried out in limbic forebrain: CB₁ receptor density and stimulated activity, 2-arachidonoyl glycerol (2-AG) and monoacylglycerol lipase (MAGL) activity, dopamine levels and dopamine transporter density.

Key results:

EtOH consumption and preference were increased in METH-treated mice. Seven days after METH, a time at which both dopamine levels and density of dopamine transporters in limbic forebrain were decreased, CB₁ receptor density and activity were unaltered, but 2-AG levels were increased. At this same time-point, MAGL activity was reduced. The CB₁ receptor antagonist AM251 prevented the METH-induced increase in EtOH consumption and preference, while N-arachidonoyl maleimide, an inhibitor of MAGL, increased EtOH consumption and preference in both saline- and METH-treated mice.

Conclusions and implications:

An increase in endocannabinoid tone may be involved in the increased consumption of and preference for EtOH displayed by METH-lesioned mice as blockade of the CB₁ receptor decreased EtOH-seeking behaviours, whereas the MAGL inhibitor increased EtOH consumption.This article is part of a themed issue on Cannabinoids. To view the editorial for this themed issue visit http://dx.doi.org/10.1111/j.1476-5381.2010.00831.x     

Differential susceptibility to ethanol and amphetamine sensitization in dopamine D3 receptor-deficient mice

Rationale

Dopamine D3 receptors (D3Rs) have been implicated in behavioral sensitization to various drugs of abuse, but their role in ethanol (EtOH) sensitization has not been directly examined. We used D3R knockout (D3 KO) mice to examine whether the D3R plays a permissive role in EtOH and amphetamine (AMPH) sensitization. We also investigated whether EtOH sensitization is accompanied by alterations in D3R mRNA expression or binding.

Materials and methods

After comparing EtOH sensitization in C57Bl/6 mice and DBA/2 mice, D3 KO, wild type (WT), and for comparison, D1 and D2 KOs received five biweekly injections of EtOH (2.2 g/kg, i.p.) or saline. Another group of D3 KOs and WT controls received six times AMPH (1.5 mg/kg, i.p.). D3R mRNA and binding were measured in EtOH-sensitized DBA/2 mice with in situ hybridization and [¹²⁵I]-7-OH-PIPAT autoradiography, respectively.

Results

C57Bl/6 mice expressed EtOH sensitization albeit to a lesser extent than DBA/2 mice. Compared to WT mice, D3 KOs were resistant to EtOH sensitization. The behavioral profile of D3 KOs was more similar to D1 KOs than D2 KOs, which also failed to develop EtOH sensitization. However, D3 KOs developed AMPH sensitization normally. EtOH sensitization was not accompanied by changes in either D3R mRNA or D3R binding in the islands of Calleja, nucleus accumbens, dorsal striatum, or cerebellum.

Conclusions

These results suggest a necessary role for the D3R in EtOH but not AMPH sensitization, possibly through postreceptor intracellular mechanisms. Results also suggest that different neurochemical mechanisms underlie sensitization to different drugs of abuse.     

Ethanol drinking-in-the-dark facilitates behavioral sensitization to ethanol in C57BL/6J, BALB/cByJ, but not in mu-opioid receptor deficient CXBK mice

Background

Neuroplasticity associated with drug-induced behavioral sensitization has been associated with excessive drug pursuit and consumption characteristic of addiction. Repeated intraperitoneal (ip) injections of ethanol (EtOH) can induce psychomotor sensitization in mice. In terms of its clinical relevance, however, it is important to determine whether this phenomenon can also be produced by voluntary EtOH consumption.

Methods

The present investigation used a drinking-in-the-dark (DID) methodology to induce high levels of EtOH drinking in mice; EtOH replaces water for 2 or 4 h, starting 3 h after the beginning of the dark cycle. Animals followed a 3-week DID protocol prior to an evaluation of EtOH-induced locomotor activity (acute and repeated EtOH). For the first week, animals had access to 20% EtOH. On weeks 2 and 3, different concentrations of EtOH (10, 20 or 30%) were used. Three different inbred strains of mice were used: C57BL/6J (B6), BALB/cByJ (BALB), and CXBK. The CXBK mouse line was used because of its reduced expression and functioning of brain mu-opioid receptors, which have been suggested to participate in the development of EtOH-induced sensitization. B6 and BALB mice were used as controls.

Results

B6 and CXBK mice presented comparable levels of EtOH drinking (approx. 3 g/kg in 2 h), that were higher than those showed by BALB. All animals, regardless of genotype, adjusted volume of EtOH intake to obtain stable g/kg of EtOH across concentrations. Previous EtOH DID produced (B6) or potentiated (BALB) sensitization to EtOH; this effect was not seen in CXBK. Western blot analysis showed a reduced number of mu-opioid receptors in several brain regions of CXBK as compared to that of B6 and BALB mice.

Conclusions

In summary, here we show that the DID methodology can be used to trigger EtOH-induced neuroplasticity supporting psychomotor sensitization, a process that might require participation of mu-opioid receptors.     

JNJ-39220675, a novel selective histamine H3 receptor antagonist, reduces the abuse-related effects of alcohol in rats

Rationale

A few recent studies suggest that brain histamine levels and signaling via H₃ receptors play an important role in modulation of alcohol stimulation and reward in rodents.

Objective

The present study characterized the effects of a novel, selective, and brain penetrant H₃ receptor antagonist (JNJ-39220675) on the reinforcing effects of alcohol in rats.

Methods

The effect of JNJ-39220675 on alcohol intake and alcohol relapse-like behavior was evaluated in selectively bred alcohol-preferring (P) rats using the standard two-bottle choice method. The compound was also tested on operant alcohol self administration in non-dependent rats and on alcohol-induced ataxia using the rotarod apparatus. In addition, alcohol-induced dopamine release in the nucleus accumbens was tested in freely moving rats.

Results

Subcutaneous administration of the selective H₃ receptor antagonist dose-dependently reduced both alcohol intake and preference in alcohol-preferring rats. JNJ-39220675 also reduced alcohol preference in the same strain of rats following a 3-day alcohol deprivation. The compound significantly and dose-dependently reduced alcohol self-administration without changing saccharin self-administration in alcohol non-dependent rats. Furthermore, the compound did not change the ataxic effects of alcohol, alcohol elimination rate, nor alcohol-induced dopamine release in nucleus accumbens.

Conclusions

These results indicate that blockade of H₃ receptor should be considered as a new attractive mechanism for the treatment of alcoholism.     

Assessment of the impact of pattern of cocaine dosing schedule during conditioning and reconditioning on magnitude of cocaine CPP, extinction, and reinstatement

Rationale and objective

We sought to examine the impact of differing cocaine administration schedules and dosing on the magnitude of cocaine conditioned place preference (CPP), extinction, and stress- and cocaine-induced reinstatement of CPP.

Methods

First, in C57Bl/6J mice, we investigated whether total cocaine administration or pattern of drug exposure could influence the magnitude of cocaine CPP by conditioning mice with a fixed-low dose (F_L; 7.5 mg/kg; total of 30 mg/kg), a fixed-high dose (F_H; 16 mg/kg; total of 64 mg/kg), or an ascending dosing schedule (Asc; 2, 4, 8, and 16 mg/kg; total of 30 mg/kg). Next, we investigated if cocaine or saline is more effective at extinguishing preference by reconditioning mice with either a descending dosing schedule (Desc; 8, 4, 2, and 1 mg/kg) or saline. Finally, we examined if prior conditioning and reconditioning history alters stress (~2–3-min forced swim test) or cocaine-induced (3.5 mg/kg) reinstatement.

Results

We replicated and extended findings by Itzhak and Anderson (Addict. Biol. 17(4): 706–16, 2011) demonstrating that Asc conditioning produces a greater CPP than either the F_L or F_H conditioning schedules. The magnitude of extinction expressed was similar in the Desc reconditioned and saline groups. Moreover, only the saline, and not the Desc reconditioned mice, showed stress and cocaine-induced reinstatement of CPP.

Conclusions

Our results suggest that the schedule of cocaine administration during conditioning and reconditioning can have a significant influence on the magnitude of CPP and extinction of preference and the ability of cocaine or a stressor to reinstate CPP.     

The role of acetaldehyde in ethanol reinforcement assessed by Pavlovian conditioning in newborn rats

Rationale

Animal studies indicate that central acetaldehyde, dependent on catalase metabolism of ethanol (EtOH), modulates ethanol reinforcement. Brain catalase activity and acetaldehyde (ACD) production are significantly higher in rat pups compare d with adults. Interestingly, infant rats show high EtOH affinity for alcohol consumption and are particularly sensitive to the drug’s reinforcing effects.

Objectives

We tested whether central ACD is necessary and sufficient to induce appetitive conditioning in newborn rats through the artificial nipple technique.

Methods

Vehicle, EtOH (100 mg%), and acetaldehyde (0.35 μmol) were administered into the cisterna magna (1 μl). Half of the animals also received a central administration of 75 μg (experiment 1) or 40 μg of d-penicillamine (experiment 2). Afterwards, pups were exposed to an olfactory cue (conditioned stimulus). One hour later, neonates were tested with an artificial nipple in the presence of the conditioned cue. Nipple attachment duration, mean grasp duration, and number of nipple disengagements served as dependent variables.

Results

Positive responses to the scented nipple occurred in neonates conditioned with EtOH or ACD (experiments 1 and 2). In experiment 1, there were indications that d-penicillamine weakened the reinforcing effects of EtOH and ACD. In experiment 2, d-penicillamine (40 μg) significantly inhibited appetitive conditioned responses dependent upon EtOH or ACD.

Conclusions

Appetitive conditioning was observed when employing either central EtOH or ACD as unconditioned stimuli. Central abduction of ACD inhibited conditioned appetitive responsiveness to the surrogate nipple. Central ACD is involved in the determination or modulation of EtOH’s motivational properties during early stages in development.     

Gastric mucosal preventive effects of prostacyclin and β-carotene,and their biochemical effects in rats treated with ethanol and HCl at different doses and time intervals after administration of necrotizing agents

Aims

The aims of these studies were to evaluate the biochemical mechanisms involving energy metabolism of:

1. Gastric mucosal damage affected by oral administration of 0.6 mol/L HCl (representing an acid-dependent model) and 96% ethanol (EtOH) (as a non-acid-dependent model);
2. PGI₂-induced (ED₅₀=5 μg/kg po) and β-carotene-induced (ED₅₀=1 mg/kg po) gastroprotection on the gastric mucosal damage produced by HCl and EtOH at different times and doses.

Methods

Sprague-Dawley rats were used. After 24 h starvation (with tap water ad libitum), gastric mucosal damage was induced by oral administration of 1 ml 0.6 mol/L HCl or 96% EtOH. Rats were pretreated with oral saline, PGI₂ (5 and 50 μg/kg) and β-carotene (1 and 10 mg/kg) and killed at 0, 1, 5, 15, 30 and 60 min after administration of the necrotizing agents. The number and severity of gastric mucosal lesions, measurement of adenosine triphosphate (ATP), adenosine diphosphate (ADP), adenosine monophosphate (AMP), lactate (enzymatically), and cyclic adenosine monophosphate (cAMP) by RIA were carried out at different time intervals, after the necrotizing agents were administered. The ratio of ATP/ADP, adenylate pool (ATP+ADP+AMP) and ‘energy charge’ [(ATP+0.5 ADP)/(ATP+ADP+AMP)] were calculated.

Results

The results showed that:

1. The mucosal damage (number and severity) reached about 50% of that obtained 60 min after administration of the necrotizing agents in both models;
2. PGI₂ prevented in the early period (0–15 min), while β-carotene inhibited in the later period (15–60 min) the gastric mucosal damage produced by EtOH and HCl;
3. The ATP-ADP transformation was decreased in the first (early; 0–15 min) by PGI₂ and in the late period (15–60 min) by β-carotene;
4. ATP-cAMP transformation was increased in the early period by PGI₂ and in the late phase by β-carotene;
5. No significant change was obtained in the ‘energy charge’ and lactate by PGI₂ or β-carotene administration;
6. The changes in adenine nucleotides were the same in the EtOH or HCl models with and without treatment with PGI₂ and β-carotene; however, the mucosal protective action of PGI₂ and β-carotene, and the energy metabolism, differed significantly dependent on dose and time after

administration of EtOH and HCl.

Conclusions

1. The development of gastric mucosal damage and its prevention can be discriminated into early and late phases;
2. The early phase of gastric mucosal damage can be prevented by PGI₂, and the late phase by β-carotene;
3. The β-carotene effect only partly depends on its presumptive scavenging properties; and
4. PGI₂ prevents the development of gastric mucosal damage, while β-carotene stimulates the repair mechanisms.

     

Effects of the beta-lactam antibiotic ceftriaxone on nicotine withdrawal and nicotine-induced reinstatement of preference in mice

Rationale

Several studies suggest that repeated nicotine administration causes alterations in glutaminergic transmission that may play an important role in developing and maintaining nicotine addiction. Chronic nicotine administration in rats decreases the expression of the glutamate transporter-1 (GLT-1) and cysteine–glutamate exchanger (system x_C?) in the nucleus accumbens. We hypothesized that ceftriaxone, a GLT-1 and system x_C? activator, would decrease murine behavioral aspects of nicotine dependence.

Objective

This study aimed to investigate the effect of repeated ceftriaxone administration on the behavioral effects of nicotine using mouse models of conditioned reward and withdrawal.

Method

Using male ICR mice, the ability of repeated ceftriaxone injections to modulate the development and reinstatement of a nicotine-conditioned place preference (CPP) was evaluated. Additionally, nicotine withdrawal-associated signs were assessed. These included both physical (somatic signs and hyperalgesia) and affective (anxiety-related behaviors) withdrawal signs in mice. Finally, the effects of ceftriaxone on nicotine-induced antinociception and hypothermia after acute nicotine injection were measured.

Result

Ceftriaxone had no effect on the development of nicotine preference but significantly attenuated nicotine-induced reinstatement of CPP. Furthermore, ceftriaxone reversed all nicotine withdrawal signs measured in mice.

Conclusion

Altogether, these findings show that a β-lactam antibiotic reduces nicotine withdrawal and nicotine-seeking behavior. Our results suggest that the documented efficacy of ceftriaxone against cocaine and morphine dependence-related behaviors effects extends to nicotine.     

Involvement of nucleus accumbens dopamine D1 receptors in ethanol drinking, ethanol-induced conditioned place preference, and ethanol-induced psychomotor sensitization in mice

Rationale

Dopamine D1 receptor (D1R) signaling has been associated to ethanol consumption and reward in laboratory animals.

Objectives

Here, we hypothesize that this receptor, which is located within the nucleus accumbens (NAc) neurons, modulates alcohol reward mechanisms.

Methods

To test this hypothesis, we measured alcohol consumption and ethanol-induced psychomotor sensitization and conditioned place preference (CPP) in mice that received bilateral microinjections of small interference RNA (siRNA)-expressing lentiviral vectors (LV-siD1R) producing D1R knock-down. The other group received control (LV-Mock) viral vectors into the NAc.

Results

There were no differences in the total fluid consumed and also no differences in the amount of ethanol consumed between groups prior to surgery. However, after surgery, the LV-siD1R group consumed less ethanol than the control group. This difference was not associated to taste neophobia. In addition, results have shown that down-regulation of endogenous D1R using viral-mediated siRNA in the NAc significantly decreased ethanol-induced behavioral sensitization as well as acquisition, but not expression, of ethanol-induced place preference.

Conclusions

We conclude that decreased D1R expression into the NAc led to reduced ethanol rewarding properties, thereby leading to lower voluntary ethanol consumption. Together, these findings demonstrate that the D1 receptor pathway within the NAc controls ethanol reward and intake.     

Assessment of the abuse liability of ABT-288, a novel histamine H3 receptor antagonist

Rationale

Histamine H₃ receptor antagonists, such as ABT-288, have been shown to possess cognitive-enhancing and wakefulness-promoting effects. On the surface, this might suggest that H₃ antagonists possess psychomotor stimulant-like effects and, as such, may have the potential for abuse.

Objectives

The aim of the present study was to further characterize whether ABT-288 possesses stimulant-like properties and whether its pharmacology gives rise to abuse liability.

Methods

The locomotor-stimulant effects of ABT-288 were measured in mice and rats, and potential development of sensitization was addressed. Drug discrimination was used to assess amphetamine-like stimulus properties, and drug self-administration was used to evaluate reinforcing effects of ABT-288. The potential development of physical dependence was also studied.

Results

ABT-288 lacked locomotor-stimulant effects in both rats and mice. Repeated administration of ABT-288 did not result in cross-sensitization to the stimulant effects of d-amphetamine in mice, suggesting that there is little overlap in circuitries upon which the two drugs interact for motor activity. ABT-288 did not produce amphetamine-like discriminative stimulus effects in drug discrimination studies nor was it self-administered by rats trained to self-administer cocaine. There were no signs of physical dependence upon termination of repeated administration of ABT-288 for 30 days.

Conclusions

The sum of these preclinical data, the first of their kind applied to H₃ antagonists, indicates that ABT-288 is unlikely to possess a high potential for abuse in the human population and suggests that H₃ antagonists, as a class, are similar in this regard.     

The cannabinoid CB2 receptor is necessary for nicotine-conditioned place preference, but not other behavioral effects of nicotine in mice

Rationale

Whereas cannabinoid CB₁ receptors have long been known to contribute to the rewarding effects and dependence liability of many drugs of abuse, recent studies have implicated the involvement of cannabinoid CB₂ receptors.

Objective

Here, we evaluated the role of CB₂ receptors in the rewarding properties of nicotine, as assessed in the conditioned place preference (CPP) paradigm and mecamylamine-precipitated withdrawal in nicotine dependent mice.

Methods

Using complementary pharmacological and genetic approaches, we investigated the involvement of CB₂ receptors in nicotine- and cocaine-induced CPP in mice and mecamylamine-precipitated withdrawal in nicotine-dependent mice. We also determined whether deletion of CB₂ receptors affects nicotine-induced hypothermia and hypoalgesia.

Results

Nicotine-induced (0.5 mg/kg) CPP was completely blocked by selective CB₂ antagonist, SR144528 (3 mg/kg) in wild-type mice, and was absent in CB₂ (?/?) mice. Conversely, the CB₂ receptor agonist, O-1966 (1, 3, 5, 10, 20 mg/kg) given in combination with a subthreshold dose of nicotine (0.1 mg/kg) elicited a place preference. In contrast, O-1966 (20 mg/kg) blocked cocaine (10 mg/kg)-induced CPP in wild type mice, while CB₂ (?/?) mice showed unaltered cocaine CPP. CB₂ (+/+) and (?/?) nicotine-dependent mice showed almost identical precipitated withdrawal responses and deletion of CB₂ receptor did not alter acute somatic effects of nicotine.

Conclusions

Collectively, these results indicate that CB₂ receptors are required for nicotine-induced CPP in the mouse, while it is not involved in nicotine withdrawal or acute effects of nicotine. Moreover, these results suggest that CB₂ receptors play opposing roles in nicotine- and cocaine-induced CPP.     

Allopregnanolone does not influence ethanol-induced conditioned place preference in DBA/2J mice

Rationale The neurosteroid allopregnanolone (ALLOP; 3-hydroxy-5-pregnan-20-one) produces behavioral and discriminative characteristics similar to that of ethanol (EtOH) and can modulate some of the behavioral and electrophysiological effects of EtOH.Objective The present experiments investigated ALLOP modulation of the effects of EtOH in a place conditioning procedure in male DBA/2J mice.Methods In a series of experiments examining different EtOH doses (1, 2 g/kg) and ALLOP administration times, ALLOP (0, 3.2, 10, 17 mg/kg, IP) was administered four times with EtOH prior to placement on a distinctive floor (CS+). On alternate days, vehicle was administered prior to a saline injection paired with the other floor stimulus (CS–). In a separate experiment, finasteride (0, 50, 100 mg/kg, IP), a 5-reductase inhibitor that blocks ALLOP synthesis, was administered prior to both CS+ and CS– trials. In a final experiment, animals were place conditioned to EtOH alone, and ALLOP (0, 3.2, 10, 17 mg/kg, IP) was administered prior to the preference test only.Results During conditioning, ALLOP increased and finasteride decreased EtOH-stimulated activity compared with vehicle pretreatment. Acquisition of 2 g/kg EtOH-induced conditioned place preference was observed in all mice, regardless of treatment with either ALLOP or finasteride. Similarly, ALLOP did not modulate the expression of EtOH-induced place preference. EtOH increased brain ALLOP levels compared with saline; however, ALLOP administration produced dose-dependent elevations in brain ALLOP levels that were not further augmented by EtOH (2 g/kg) administration.Conclusions These findings indicate that ALLOP does not modulate EtOH-induced place conditioning in male DBA/2J mice.     

Activation of postsynaptic 5-HT1A receptors improve stress adaptation

Rationale

Serotonin-1A (5-HT_1A) receptors modulate the stress response and have been implicated in the etiology and treatment of depression and anxiety disorders. A reduction in postsynaptic 5-HT_1A receptor function in limbic areas has consistently been observed following exposure to chronic stress.

Objectives

To investigate the hypothesis that increased activation of 5-HT_1A receptors in rats having reduced 5-HT function may improve stress adaptation and the behavioral sequelae commonly associated with chronic stress.

Methods

One hundred forty-four Sprague–Dawley rats received injections of para-chlorophenylalanine to partially deplete 5-HT then were given daily systemic pretreatment with the 5-HT_1A receptor agonist, 8-hydroxy-2- (di-n-propylamino) tetralin (8-OH-DPAT), the antagonist, WAY 100635, or vehicle prior to either restraint stress (6 h/day for 10 daily sessions) or control conditions. Anxiety- and depressive-like behaviors were then assessed using the open field and sucrose preference tests. Protein level of hippocampal glucocorticoid receptors (GR) and mineralocorticoid receptors was detected by immunohistochemistry and brain-derived neurotrophic factor (BDNF) was determined by in situ hybridization.

Results

8-OH-DPAT pretreatment prior to stress exposure attenuated later stress-induced anxiety- and depression-like behaviors and increased GR and BDNF mRNA expression in the hippocampus relative to vehicle- and WAY 100635-pretreated, stressed animals.

Conclusion

The stress-related impairments associated with 5-HT deficiency can be improved by 8-OH-DPAT pretreatment prior to stress exposure and are associated with an augmentation of GR-like immunoreactivity and BDNF mRNA expression in the hippocampus. It suggested that selective activation of 5-HT_1A receptors may be a potential treatment strategy for stress-related disorders such as anxiety and depression.     

Acute 5-HT1A autoreceptor knockdown increases antidepressant responses and serotonin release in stressful conditions

Rationale

Identifying the etiological factors in anxiety and depression is critical to develop more efficacious therapies. The inhibitory serotonin_1A receptors (5-HT_1AR) located on 5-HT neurons (autoreceptors) limit antidepressant responses and their expression may be increased in treatment-resistant depressed patients.

Objectives

Recently, we reported that intranasal administration of modified small interference RNA (siRNA) molecules targeting 5-HT_1AR in serotonergic neurons evoked antidepressant-like effects. Here we extended this finding using marketed siRNAs against 5-HT_1AR (1A-siRNA) to reduce directly the 5-HT_1A autoreceptor expression and evaluate its biological consequences under basal conditions and in response to stressful situations.

Methods

Adult mice were locally infused with vehicle, nonsense siRNA, and 1A-siRNA into dorsal raphe nucleus (DR). 5-HT_1AR knockout mice (1A-KO) were also used. Histological approaches, in vivo microdialysis, and stress-related behaviors were performed to assess the effects of 5-HT_1A autoreceptor knockdown.

Results

Intra-DR 1A-siRNA infusion selectively reduced 5-HT_1AR mRNA and binding levels and canceled 8-OH-DPAT-induced hypothermia. Basal extracellular 5-HT in medial prefrontal cortex (mPFC) did not differ among treatments. However, 1A-siRNA-treated mice displayed less immobility in the tail suspension and forced swim tests, as did 1A-KO mice. This was accompanied by a greater increase in prefrontal 5-HT release during tail suspension test. Moreover, intra-DR 1A-siRNA infusion augmented the increase of extracellular 5-HT in mPFC evoked by fluoxetine, up to the level in 1A-KO mice.

Conclusion

Together with our previous report, the present results indicate that acute suppression of 5-HT_1A autoreceptor expression evokes robust antidepressant-like effects, likely mediated by an increased capacity of serotonergic neurons to release 5-HT in stressful conditions.     

Enhanced alcohol-seeking behavior by nicotine in the posterior ventral tegmental area of female alcohol-preferring (P) rats: modulation by serotonin-3 and nicotinic cholinergic receptors

Rationale

Alcohol and nicotine co-use can reciprocally promote self-administration and drug-craving/drug-seeking behaviors. To date, the neurocircuitry in which nicotine influences ethanol (EtOH) seeking has not been elucidated. Clinical and preclinical research has suggested that the activation of the mesolimbic dopamine system is involved in the promotion of drug seeking. Alcohol, nicotine, and serotonin-3 (5-HT₃) receptors interact within the posterior ventral tegmental area (pVTA) to regulate drug reward. Recently, our laboratory has reported that systemic administration of nicotine can promote context-induced EtOH seeking.

Objectives

The goals of the current study were to (1) determine if microinjections of pharmacologically relevant levels of nicotine into the pVTA would enhance EtOH seeking, (2) determine if coadministration of nicotinic cholinergic receptor antagonist (nACh) or 5-HT₃ receptor antagonists would block the ability of nicotine microinjected into the pVTA to promote EtOH seeking, and (3) determine if 5-HT₃ receptors in the pVTA can modulate EtOH seeking.

Results

Nicotine (100 and 200 μM) microinjected into the pVTA enhanced EtOH seeking. Coinfusion with 200 μM mecamylamine (nACh antagonist) or 100 and 200 μM zacopride (5-HT₃ receptor antagonist) blocked the observed nicotine enhancement of EtOH seeking. The data also indicated that microinjection of 1 μM CPBG (5-HT₃ receptor agonist) promotes context-induced EtOH seeking; conversely, microinjection of 100 and 200 μM zacopride alone reduced context-induced EtOH seeking.

Conclusions

Overall, the results show that nicotine-enhanced EtOH-seeking behavior is modulated by 5-HT₃ and nACh receptors within the pVTA and that the 5-HT₃ receptor system within pVTA may be a potential pharmacological target to inhibit EtOH-seeking behaviors.     

Modulation of 5-HT7 receptor: effect on object recognition performances in mice

Objective

Recent data suggest that 5-HT₇ receptors (5-HT₇R) are involved in memory processes and, particularly, those related to novelty-induced arousal, even though this remains so far speculative and controversial. In order to assess the role of 5-HT₇R in episodic-like memory, mice were administered 5-carboxamidotryptamine (5-CT, a 5-HT_1A/1B/1D/7R agonist) and/or SB-269970 (a selective 5-HT₇R antagonist) immediately after the acquisition session of the novel object recognition test.

Materials and methods

The object recognition test was performed in order to assess the effects of modulation of 5-HT₇R during consolidation phase on episodic-like memory performances in mice. A protocol including 3 days of familiarisation to the apparatus has been realised in order to decrease the effect of novelty-induced arousal.

Results

With a 2-h delay, SB-269970 (3 and 10 mg/kg, administered subcutaneously) impaired the discrimination of the novel object. With a 4-h delay, while control mice were not able to discriminate the novel object, mice treated with 5-CT (1 mg/kg) showed a significant discrimination. This promnesic effect with a long delay is effectively mediated by 5-HT₇R activation since it was blocked by SB-269970 (10 mg/kg), but not by WAY-100135 (10 mg/kg) or by GR-127935 (10 mg/kg).

Conclusion

These data suggest that 5-HT₇R tonically modulates cognitive processes involved in consolidation performances in object recognition. Therefore, 5-HT₇R could be a promising target to treat memory dysfunctions (especially episodically related deficits) related to normal or pathological ageing.     

Choice for response alternatives differing in reinforcement frequency in dopamine D2 receptor mutant and Swiss-Webster mice

Rationale

A previous study showed that dopamine (DA) D₂ receptors (D₂Rs) are involved in the reinforcing effectiveness of food, but the specific involvement of DA D₂Rs in choice among food reinforcers remains unclear.

Objectives

The current study used genetic and pharmacological approaches to assess the role of D₂Rs in choice among food-reinforcement frequencies using the generalized matching law (GML), which specifies that logged response and time allocation ratios vary linearly with logged reinforcer ratios.

Methods

Congenic D₂R knockout (KO) and wild-type (WT) mice were exposed to concurrent variable-interval schedules of reinforcement with scheduled relative-reinforcement rates from 4:1 to 1:4. Effects of the D₂R antagonist (?)-eticlopride (0.1–1.0 mg/kg) were assessed in Swiss-Webster mice.

Results

Response and time allocation ratios were related to obtained reinforcement ratios as predicted by the GML. GML fits accounted for ≥92 % of the variance in allocation ratios and did not differ in D₂R KO and WT mice. Similarly, there were no significant effects of (?)-eticlopride dose on GML fits, despite effects on overall response rates.

Conclusions

The current results demonstrate that neither deletion nor acute blockade of D₂Rs affects choice among response alternatives varying in food-reinforcement frequencies. Because previously published results suggest a role of D₂Rs in choice between response alternatives differing in reinforcer magnitude and delay or magnitude and probability, the current findings suggest that D₂Rs play a role in choice only among certain parameters of reinforcement. Furthermore, these findings suggest parameters of reinforcement may only be fungible in a complex manner.