Effect of 5-HT3 antagonists and a 5-HT(1A) agonist on fluoxetine-induced conditioned gaping reactions in rats

Infection, Learning, and Memory: Focus on Immune Activation and Aversive Conditioning

Here we review the effects of immune activation primarily via lipopolysaccharide (LPS), a cell wall component of Gram-negative bacteria, on hippocampal and non-hippocampal-dependent learning and memory. Rodent studies have found that LPS alters both the acquisition and consolidation of aversive learning and memory, such as those evoking evolutionarily adaptive responses like fear and disgust. The inhibitory effects of LPS on the acquisition and consolidation of contextual fear memory are discussed. LPS-induced alterations in the acquisition of taste and place-related conditioned disgust memory within bottle preference tasks and taste reactivity tests (taste-related), in addition to conditioned context avoidance tasks and the anticipatory nausea paradigm (place-related), are highlighted. Further, conditioned disgust memory consolidation may also be influenced by LPS-induced effects. Growing evidence suggests a central role of immune activation, especially pro-inflammatory cytokine activity, in eliciting the effects described here. Understanding how infection-induced immune activation alters learning and memory is increasingly important as bacterial and viral infections are found to present a risk of learning and memory impairment.

Toxin-induced aversive context conditioning: Assessing active aversive behaviors conditioned to the context of an automated activity monitor

Lithium chloride (LiCl) is an emetic drug that has been used to create animal models of anticipatory nausea and conditioned place aversion. In this study we examined escape behaviours from a context in which rats experienced the aversive effects of LiCl treatments. The experiment had two phases: acquisition of context conditioning, which consisted of pairing a distinct context with the pharmacological effects of a moderate dose of the toxin LiCl, and extinction of context conditioning, which consisted of placement in the distinct context in a drug free state. During context conditioning, 16 adult male Long-Evans rats were injected intraperitoneally with 96 mg/kg lithium chloride (LiCl; n = 8) or 0.9% saline (NaCl; n = 8) and placed individually in an automated locomotor activity apparatus for 30 min every other day for 4 days. During the extinction phase, rats were placed in the apparatus for 30 min every other day without injections during a 4 day extinction phase. A significant Drug x Trial interaction was found for the time spent in vertical position in the open field apparatus during trials 1-3 of the extinction phase. The LiCl treated rats exhibited significantly increased rearing behavior, relative to the control rats, indicative of conditioned aversion. The results of this study suggest that escape behavior (vertical activity) occurs in rats experiencing the aversive conditioned effects of LiCl in a distinct context. In the context of current theoretical accounts, the LiCl-conditioned increase in apparent escape behaviors can be considered a reflection of anticipatory nausea.

Fructose:glucose ratios--a study of sugar self-administration and associated neural and physiological responses in the rat

This study explored whether different ratios of fructose (F) and glucose (G) in sugar can engender significant differences in self-administration and associated neurobiological and physiological responses in male Sprague-Dawley rats. In Experiment 1, animals self-administered pellets containing 55% F + 45% G or 30% F + 70% G, and Fos immunoreactivity was assessed in hypothalamic regions regulating food intake and reward. In Experiment 2, rats self-administered solutions of 55% F + 42% G (high fructose corn syrup (HFCS)), 50% F + 50% G (sucrose) or saccharin, and mRNA of the dopamine 2 (D2R) and mu-opioid (MOR) receptor genes were assessed in striatal regions involved in addictive behaviors. Finally, in Experiment 3, rats self-administered HFCS and sucrose in their home cages, and hepatic fatty acids were quantified. It was found that higher fructose ratios engendered lower self-administration, lower Fos expression in the lateral hypothalamus/arcuate nucleus, reduced D2R and increased MOR mRNA in the dorsal striatum and nucleus accumbens core, respectively, as well as elevated omega-6 polyunsaturated fatty acids in the liver. These data indicate that a higher ratio of fructose may enhance the reinforcing effects of sugar and possibly lead to neurobiological and physiological alterations associated with addictive and metabolic disorders.

Effect of low doses of cannabidiolic acid and ondansetron on LiCl-induced conditioned gaping (a model of nausea-induced behaviour) in rats

BACKGROUND AND PURPOSE

To determine the minimally effective dose of cannabidiolic acid (CBDA) that effectively reduces lithium chloride (LiCl)-induced conditioned gaping reactions (nausea-induced behaviour) in rats and to determine if these low systemic doses of CBDA (5-0.1 μg·kg⁻¹) relative to those of CBD could potentiate the anti-nausea effects of the classic 5-hydroxytryptamine 3 (5-HT₃) receptor antagonist, ondansetron (OND).

EXPERIMENTAL APPROACH

We investigated the efficacy of low doses of CBDA to suppress acute nausea, assessed by the establishment of conditioned gaping to a LiCl-paired flavour in rats. The potential of threshold and subthreshold doses of CBDA to enhance the reduction of nausea-induced conditioned gaping by OND were then determined.

KEY RESULTS

CBDA (at doses as low as 0.5 μg·kg⁻¹) suppressed nausea-induced conditioned gaping to a flavour. A low dose of OND (1.0 μg·kg⁻¹) alone reduced nausea-induced conditioned gaping, but when it was combined with a subthreshold dose of CBDA (0.1 μg·kg⁻¹) there was an enhancement in the suppression of LiCl-induced conditioned gaping.

CONCLUSIONS AND IMPLICATIONS

CBDA potently reduced conditioned gaping in rats, even at low doses and enhanced the anti-nausea effect of a low dose of OND. These findings suggest that combining low doses of CBDA and OND will more effectively treat acute nausea in chemotherapy patients.

Evaluation of the potential of the phytocannabinoids, cannabidivarin (CBDV) and Δ(9) -tetrahydrocannabivarin (THCV), to produce CB1 receptor inverse agonism symptoms of nausea in rats

BACKGROUND AND PURPOSE

The cannabinoid 1 (CB1 ) receptor inverse agonists/antagonists, rimonabant (SR141716, SR) and AM251, produce nausea and potentiate toxin-induced nausea by inverse agonism (rather than antagonism) of the CB1 receptor. Here, we evaluated two phytocannabinoids, cannabidivarin (CBDV) and Δ(9) -tetrahydrocannabivarin (THCV), for their ability to produce these behavioural effect characteristics of CB1 receptor inverse agonism in rats.

EXPERIMENTAL APPROACH

In experiment 1, we investigated the potential of THCV and CBDV to produce conditioned gaping (measure of nausea-induced behaviour) in the same manner as SR and AM251. In experiment 2, we investigated the potential of THCV and CBDV to enhance conditioned gaping produced by a toxin in the same manner as CB1 receptor inverse agonists.

KEY RESULTS

SR (10 and 20 mg·kg(-1) ) and AM251 (10 mg·kg(-1) ) produced conditioned gaping; however, THCV (10 or 20 mg·kg(-1) ) and CBDV (10 or 200 mg·kg(-1) ) did not. At a subthreshold dose for producing nausea, SR (2.5 mg·kg(-1) ) enhanced lithium chloride (LiCl)-induced conditioned gaping, whereas Δ(9) -tetrahydrocannabinol (THC, 2.5 and 10 mg·kg(-1) ), THCV (2.5 or 10 mg·kg(-1) ) and CBDV (2.5 or 200 mg·kg(-1) ) did not; in fact, THC (2.5 and 10 mg·kg(-1) ), THCV (10 mg·kg(-1) ) and CBDV (200 mg·kg(-1) ) suppressed LiCl-induced conditioned gaping, suggesting anti-nausea potential.

CONCLUSIONS AND IMPLICATIONS

The pattern of findings indicates that neither THCV nor CBDV produced a behavioural profile characteristic of CB1 receptor inverse agonists. As well, these compounds may have therapeutic potential in reducing nausea.

Conditioned flavor avoidance and conditioned gaping: rat models of conditioned nausea

Although rats are incapable of vomiting, they demonstrate profound avoidance of a flavor previously paired with an emetic drug. They also display conditioned gaping reactions during re-exposure to the flavor. This robust learning occurs in a single trial and with long delays (hours) between consumption of a novel flavor and the emetic treatment. However, conditioned flavor avoidance learning is not a selective measure of the emetic properties of drugs, because non-emetic treatments (even highly rewarding treatments) produce conditioned avoidance, and anti-emetic treatments are generally ineffective in suppressing conditioned avoidance produced by an emetic drug. On the other hand, conditioned gaping reactions are consistently produced by emetic drugs and are prevented by anti-emetic drugs, indicating that they may be a more selective measure of conditioned malaise in rats. Here we review the literature on the use of conditioned flavor avoidance and conditioned gaping reactions as rat measures of conditioned nausea, as well as the neuropharmacology and neuroanatomy of conditioned gaping reactions in rats.

Suppression of lithium chloride-induced conditioned gaping (a model of nausea-induced behaviour) in rats (using the taste reactivity test) with metoclopramide is enhanced by cannabidiolic acid

We aimed to determine the potential of various doses of metoclopramide (MCP, a dopamine antagonist) to reduce lithium chloride (LiCl)-induced conditioned gaping (a nausea-induced behaviour) in rats, using the taste reactivity test. We then evaluated whether an ineffective low dose of cannabidiolic acid (CBDA, 0.1 μg/kg, Rock and Parker, 2013), the potent acidic precursor of cannabidiol (CBD, a non-psychoactive component of cannabis) could enhance the anti-nausea effects of an ineffective low dose of MCP. MCP (3.0 mg/kg) reduced conditioned gaping responses. Coadministration of ineffective doses of MCP (0.3 mg/kg) and CBDA (0.1 μg/kg) enhanced the suppression of conditioned gaping, over that of either drug alone, without interfering with conditioned taste avoidance. MCP dose-dependently reduced nausea-induced conditioned gaping in rats. As well, the suppression of conditioned gaping was enhanced when ineffective doses of MCP and CBDA were coadministered. These data suggest that CBDA could be a powerful adjunct treatment to anti-emetic regimens for chemotherapy-induced nausea.

Impairment of lithium chloride-induced conditioned gaping responses (anticipatory nausea) following immune system stimulation with lipopolysaccharide (LPS) occurs in both LPS tolerant and LPS non-tolerant rats

Anticipatory nausea is a classically conditioned response to a context that has been previously paired with toxin-induced nausea and/or vomiting. When injected with a nausea-inducing drug, such as lithium chloride (LiCl), rats will show a distinctive conditioned gaping response that has been suggested to be an index of nausea. Previous studies have found that immune system activation with an endotoxin, such as lipopolysaccharide (LPS), attenuates LiCl-induced conditioned gaping in rats. The present study examined the acquisition of LiCl-induced conditioned gaping in rats that were either LPS tolerant or LPS non-tolerant, as little is known about the effects of endotoxin tolerance on learning and memory. Male Long-Evan rats were given four systemic injections of LPS (200 μg/kg) or isotonic saline (NaCl) to induce LPS tolerance, indexed with 24 h changes in body weight following treatment. The animals were then given 4 acquisition trials in a LiCl-induced conditioned gaping paradigm. On conditioning days animals were treated with LPS (200 μg/kg) or saline followed 90 min later by injection of LiCl (127 mg/kg) or saline and then placed in a distinctive context for 30 min and their behavior video-recorded. On a drug free test day all animals were again placed in the distinctive context for 10 min and behavior was video-recorded. Gaping responses were scored for all acquisition days and the test day. Spleen and body weights were also obtained for all rats at the end of the experiment. Gaping responses were attenuated in rats treated with LPS in both the LPS tolerant and LPS non-tolerant groups. There were significant negative correlations between spleen weight as well as spleen/body weight ratios, and levels of conditioned gaping responses in LiCl treated rats, but not control rats. These results show that LPS interferes with learning/memory in the anticipatory nausea paradigm in rats that are both LPS tolerant and LPS non-tolerant.

5-HT(3) receptor mediates the dose-dependent effects of citalopram on pentylenetetrazole-induced clonic seizure in mice: involvement of nitric oxide

Citalopram is a selective serotonin reuptake inhibitor (SSRI), widely used in the treatment of depressive disorders. It has been shown that citalopram affects seizure susceptibility. Although the exact mechanism of these effects are not yet fully understood, recent data suggest that 5HT(3) receptors and nitric oxide (NO) might participate in the central effects of SSRIs. In this study in a mouse model of clonic seizure induced by pentylenetetrazole we investigated whether 5-HT(3) receptors are involved in the effects of citalopram on seizure threshold. In our experiments, citalopram at lower doses (0.5 and 1mg/kg, i.p) significantly increased the seizure threshold and at higher doses (≥25mg/kg) showed proconvulsive effects. Moreover, mCPBG (a 5-HT(3) receptor agonist) at low and non-effective doses augmented while non-effective doses of tropisetron prevented the anticonvulsive properties of citalopram. On the other hand, Low doses of nitric oxide synthase inhibitors l-NAME and 7-NI alone or in combination with lower doses of 5-HT(3) receptor agonist enhanced the anticonvulsive property of citalopram, while l-arginine (NO precursor) alone or in combination with tropisetron blocked the protective effect of citalopram. In summary, our findings demonstrate that 5-HT(3) receptor mediates the anticonvulsant properties of low doses of citalopram, whereas it seems that the proconvulsive effect is mostly mediated through the NO pathway and can be totally blocked by NOS inhibitors. This could propose a new approach toward finding the mechanism of citalopram activity, curtailing the adverse effects of citalopram and perhaps managing the convulsions as a vicious consequence of citalopram overdose.

Cannabidiol, a non-psychotropic component of cannabis, attenuates vomiting and nausea-like behaviour via indirect agonism of 5-HT(1A) somatodendritic autoreceptors in the dorsal raphe nucleus

BACKGROUND AND PURPOSE

To evaluate the hypothesis that activation of somatodendritic 5-HT(1A) autoreceptors in the dorsal raphe nucleus (DRN) produces the anti-emetic/anti-nausea effects of cannabidiol (CBD), a primary non-psychoactive cannabinoid found in cannabis.

EXPERIMENTAL APPROACH

The potential of systemic and intra-DRN administration of 5-HT(1A) receptor antagonists, WAY100135 or WAY100635, to prevent the anti-emetic effect of CBD in shrews (Suncus murinus) and the anti-nausea-like effects of CBD (conditioned gaping) in rats were evaluated. Also, the ability of intra-DRN administration of CBD to produce anti-nausea-like effects (and reversal by systemic WAY100635) was assessed. In vitro studies evaluated the potential of CBD to directly target 5-HT(1A) receptors and to modify the ability of the 5-HT(1A) agonist, 8-OH-DPAT, to stimulate [(35) S]GTPγS binding in rat brainstem membranes.

KEY RESULTS

CBD suppressed nicotine-, lithium chloride (LiCl)- and cisplatin (20 mg·kg(-1) , but not 40 mg·kg(-1) )-induced vomiting in the S. murinus and LiCl-induced conditioned gaping in rats. Anti-emetic and anti-nausea-like effects of CBD were suppressed by WAY100135 and the latter by WAY100635. When administered to the DRN: (i) WAY100635 reversed anti-nausea-like effects of systemic CBD, and (ii) CBD suppressed nausea-like effects, an effect that was reversed by systemic WAY100635. CBD also displayed significant potency (in a bell-shaped dose-response curve) at enhancing the ability of 8-OH-DPAT to stimulate [(35) S]GTPγS binding to rat brainstem membranes in vitro. Systemically administered CBD and 8-OH-DPAT synergistically suppressed LiCl-induced conditioned gaping.

CONCLUSIONS AND IMPLICATIONS

These results suggest that CBD produced its anti-emetic/anti-nausea effects by indirect activation of the somatodendritic 5-HT(1A) autoreceptors in the DRN.

LINKED ARTICLES

This article is part of a themed section on Cannabinoids in Biology and Medicine. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2012.165.issue-8. To view Part I of Cannabinoids in Biology and Medicine visit http://dx.doi.org/10.1111/bph.2011.163.issue-7.

Attenuation of cocaine-induced reinstatement of drug seeking in squirrel monkeys: kappa opioid and serotonergic mechanisms

RATIONALE

Kappa agonists can attenuate reinstatement of cocaine-seeking behavior induced by cocaine priming. The mechanisms underlying this effect have not been characterized fully but may have a serotonergic component as kappa agonists also increase the release of serotonin (5-hydroxytryptamine, 5-HT).

OBJECTIVES

This study investigated the role of kappa opioid receptor and 5-HT mechanisms in kappa agonist-induced attenuation of cocaine priming in monkeys.

METHODS

Squirrel monkeys were trained to self-administer cocaine (0.18-0.3 mg/kg/injection) under a second-order schedule in which drug seeking was maintained jointly by cocaine injections and a cocaine-paired visual stimulus. In extinction sessions, saline was substituted for cocaine, and the cocaine-paired stimulus was omitted. During test sessions, only saline was available for self-administration, and response-contingent presentations of the cocaine-paired stimulus were restored.

RESULTS

Priming injections of cocaine (0.1-1.0 mg/kg) induced reinstatement of drug seeking. Maximal levels of responding were similar to those maintained by active cocaine self-administration. Pretreatment with the kappa agonists enadoline (0.01 mg/kg) and spiradoline (0.3 mg/kg) or the 5-HT transport inhibitors fluoxetine (5.6 mg/kg) and citalopram (10.0 mg/kg) attenuated the priming effects of cocaine, shifting the cocaine dose-response function rightward and downward. Inhibition of cocaine-induced reinstatement of drug seeking by spiradoline and fluoxetine was reversed by R(+)8-hydroxy-2-(di-n-propylamino)tetralin (0.03 mg/kg), a 5HT(1A) agonist that inhibits 5-HT release. The effects of spiradoline also were reversed by the kappa antagonist nor-binaltorphimine (10.0 mg/kg).

CONCLUSIONS

Results suggest that the capacity of kappa opioid agonists to increase extracellular 5-HT levels may at least partially underlie kappa agonist-induced modulation of cocaine seeking.