5,7-dihydroxytryptamine lesions of the dorsal and median raphe nuclei interfere with lithium-induced conditioned gaping, but not conditioned taste avoidance, in rats

Activation of multiple neuromodulatory systems in alert rats acquiring conditioned taste aversion revealed by positron emission tomography

Conditioned taste aversion (CTA) is an essential ability for animals to consume food safely and is regulated by neuromodulatory systems including the dopamine, noradrenaline, serotonin, and acetylcholine systems. However, because few studies focused on a comprehensive understanding of whole-brain activities, how these neuromodulators contribute to the process of CTA remains an open issue. 18F-fluorodeoxyglucose (FDG)-positron emission tomography (PET) can visualize activated regions within the whole brain simultaneously and noninvasively. This study aimed to understand the mechanisms of CTA, especially focusing on the retrieval process after CTA acquisition by FDG-PET imaging. CTA was established in rats who received an intraoral application of saccharin solution (IOAS) on the first day (Day 1), a LiCl i.p. injection after an IOAS on Day 2, and an IOAS on Day 3 (CTA group). The subtraction images of Day 3 of the SHAM group, which received a 0.9 % NaCl (saline) injection instead of a LiCl on Day 2, from those of Day 3 of the CTA group revealed increases in FDG signals in multiple brain regions including the substantia nigra, ventral tegmental area, locus coeruleus, dorsal raphe, and nucleus basalis magnocellularis, in addition to the hippocampus and nociception-related regions, including the parabrachial nucleus and solitary nucleus. On the other hand, the visceral pain induced by the LiCl injection increased FDG signals in the primary and secondary somatosensory and insular cortices in addition to the parabrachial nucleus and solitary nucleus. These results suggest that the retrieval process of CTA induces brain regions producing neuromodulators and pain-related brainstem.

The Role of Serotonin in Fear Learning and Memory: A Systematic Review of Human Studies

Fear is characterized by distinct behavioral and physiological responses that are essential for the survival of the human species. Fear conditioning (FC) serves as a valuable model for studying the acquisition, extinction, and expression of fear. The serotonin (5-hydroxytryptamine, 5-HT) system is known to play a significant role in emotional and motivational aspects of human behavior, including fear learning and expression. Accumulating evidence from both animal and human studies suggests that brain regions involved in FC, such as the amygdala, hippocampus, and prefrontal cortex, possess a high density of 5-HT receptors, implicating the crucial involvement of serotonin in aversive learning. Additionally, studies exploring serotonin gene polymorphisms have indicated their potential influence on FC. Therefore, the objective of this work was to review the existing evidence linking 5-HT with fear learning and memory in humans. Through a comprehensive screening of the PubMed and Web of Science databases, 29 relevant studies were included in the final review. These studies investigated the relationship between serotonin and fear learning using drug manipulations or by studying 5-HT-related gene polymorphisms. The results suggest that elevated levels of 5-HT enhance aversive learning, indicating that the modulation of serotonin 5-HT2A receptors regulates the expression of fear responses in humans. Understanding the role of this neurochemical messenger in associative aversive learning can provide insights into psychiatric disorders such as anxiety and post-traumatic stress disorder (PTSD), among others.

Descending projections from the insular cortex to the trigeminal spinal subnucleus caudalis facilitate excitatory outputs to the parabrachial nucleus in rats

ABSTRACT

Nociceptive information from the orofacial area projects to the trigeminal spinal subnucleus caudalis (Sp5C) and is then conveyed to several nuclei, including the parabrachial nucleus (PBN). The insular cortex (IC) receives orofacial nociceptive information and sends corticofugal projections to the Sp5C. The Sp5C consists of glutamatergic and GABAergic/glycinergic interneurons that induce excitatory postsynaptic currents and inhibitory postsynaptic currents, respectively, in projection neurons. Therefore, quantification of glutamatergic IC inputs in combination with identifying postsynaptic neuronal subtypes is critical to elucidate IC roles in the regulation of Sp5C activities. We investigated features of synaptic transmission from the IC to glutamatergic and GABAergic/glycinergic Sp5C neurons of laminae I/II using vesicular GABA transporter-Venus transgenic rats that received an injection of adeno-associated virus-channelrhodopsin-2-mCherry into the IC. Selective stimulation of IC axon terminals in Sp5C slice preparations induced monosynaptic excitatory postsynaptic currents in both excitatory glutamatergic and inhibitory GABAergic/glycinergic Sp5C neurons with a comparable amplitude. Paired whole-cell patch-clamp recordings showed that unitary inhibitory postsynaptic currents from inhibitory neurons influencing excitatory neurons, including neurons projecting to the PBN, exhibited a high failure rate and were suppressed by both bicuculline and strychnine, suggesting that excitatory neurons in the Sp5C receive both GABAergic and glycinergic inhibition with low impact. Moreover, selective stimulation of IC axons increased the firing rate at the threshold responses. Finally, we demonstrated that selective stimulation of IC axons in the Sp5C by a chemogenetic approach decreased the thresholds of both mechanical and thermal nociception. Thus, IC projection to the Sp5C is likely to facilitate rather than suppress excitatory outputs from the Sp5C.

Conditioned taste aversion versus avoidance: A re-examination of the separate processes hypothesis

Rats not only avoid ingesting a substance associated with LiCl toxicosis, but they display rejection reflexes (e.g., gapes) to its taste; this latter response is thought to reflect disgust or taste aversion. Prior work has shown that rats also avoid consuming foods/fluids associated with other adverse gastrointestinal (GI) effects like lactose indigestion but without the concomitant change in oromotor responses (taste reactivity; TR) indicative of aversion. Because of interpretive limitations of the methods used in those studies, we revisited the taste aversion-avoidance distinction with a design that minimized non-treatment differences among groups. Effects on intake and preference (Experiments 1a, 1b, and 2), as well as consummatory (TR, Experiment 1a and 1b) and appetitive (Progressive Ratio, Experiment 2) behaviors to the taste stimulus were assessed after training. In both experiments, rats were trained to associate 0.2% saccharin (CS) with intraduodenal infusions of LiCl, Lactose, or NaCl control. Rats trained with 18% lactose, 0.3 and 1.5 mEq/kg dose of LiCl subsequently avoided the taste CS in post-training single-bottle intake tests and two-bottle choice tests. However, only those trained with 1.5 mEq/kg LiCl displayed post-conditioning increases in taste CS-elicited aversive TR (Experiment 1a and 1b). This dose of LiCl also led to reductions in breakpoint for saccharin. The fact that conditioned avoidance is not always accompanied by changes in other common appetitive and/or consummatory indices of ingestive motivation further supports a functional dissociation between these processes, and highlights the intricacies of visceral influences on taste-guided ingestive motivation.

The Functional and Neurobiological Properties of Bad Taste

The gustatory system serves as a critical line of defense against ingesting harmful substances. Technological advances have fostered the characterization of peripheral receptors and have created opportunities for more selective manipulations of the nervous system, yet the neurobiological mechanisms underlying taste-based avoidance and aversion remain poorly understood. One conceptual obstacle stems from a lack of recognition that taste signals subserve several behavioral and physiological functions which likely engage partially segregated neural circuits. Moreover, although the gustatory system evolved to respond expediently to broad classes of biologically relevant chemicals, innate repertoires are often not in register with the actual consequences of a food. The mammalian brain exhibits tremendous flexibility; responses to taste can be modified in a specific manner according to bodily needs and the learned consequences of ingestion. Therefore, experimental strategies that distinguish between the functional properties of various taste-guided behaviors and link them to specific neural circuits need to be applied. Given the close relationship between the gustatory and visceroceptive systems, a full reckoning of the neural architecture of bad taste requires an understanding of how these respective sensory signals are integrated in the brain.

Sensory Cortical Activity Is Related to the Selection of a Rhythmic Motor Action Pattern

Rats produce robust, highly distinctive orofacial rhythms in response to taste stimuli-responses that aid in the consumption of palatable tastes and the ejection of aversive tastes, and that are sourced in a multifunctional brainstem central pattern generator. Several pieces of indirect evidence suggest that primary gustatory cortex (GC) may be a part of a distributed forebrain circuit involved in the selection of particular consumption-related rhythms, although not in the production of individual mouth movements per se. Here, we performed a series of tests of this hypothesis. We first examined the temporal relationship between GC activity and orofacial behaviors by performing paired single-neuron and electromyographic recordings in awake rats. Using a trial-by-trial analysis, we found that a subset of GC neurons shows a burst of activity beginning before the transition between nondistinct and taste-specific (i.e., consumption-related) orofacial rhythms. We further showed that shifting the latency of consumption-related behavior by selective cueing has an analogous impact on the timing of GC activity. Finally, we showed the complementary result, demonstrating that optogenetic perturbation of GC activity has a modest but significant impact on the probability that a specific rhythm will be produced in response to a strongly aversive taste. GC appears to be a part of a distributed circuit that governs the selection of taste-induced orofacial rhythms.

SIGNIFICANCE STATEMENT

In many well studied (typically invertebrate) sensorimotor systems, top-down modulation helps motor-control regions "select" movement patterns. Here, we provide evidence that gustatory cortex (GC) may be part of the forebrain circuit that performs this function in relation to oral behaviors ("gapes") whereby a substance in the mouth is rejected as unpalatable. We show that GC palatability coding is well timed to play this role, and that the latency of these codes changes as the latency of gaping shifts with learning. We go on to show that by silencing these neurons, we can change the likelihood of gaping. These data help to break down the sensory/motor divide by showing a role for sensory cortex in the selection of motor behavior.

Endocannabinoid regulation of nausea is mediated by 2-arachidonoylglycerol (2-AG) in the rat visceral insular cortex

Cannabinoid (CB) agonists suppress nausea in humans and animal models; yet, their underlying neural substrates remain largely unknown. Evidence suggests that the visceral insular cortex (VIC) plays a critical role in nausea. Given the expression of CB1 receptors and the presence of endocannabinoids in this brain region, we hypothesized that the VIC endocannabinoid system regulates nausea. In the present study, we assessed whether inhibiting the primary endocannabinoid hydrolytic enzymes in the VIC reduces acute lithium chloride (LiCl)-induced conditioned gaping, a rat model of nausea. We also quantified endocannabinoid levels during an episode of nausea, and assessed VIC neuronal activation using the marker, c-Fos. Local inhibition of monoacylglycerol lipase (MAGL), the main hydrolytic enzyme of 2-arachidonylglycerol (2-AG), reduced acute nausea through a CB1 receptor mechanism, whereas inhibition of fatty acid amide hydrolase (FAAH), the primary catabolic enzyme of anandamide (AEA), was without effect. Levels of 2-AG were also selectively elevated in the VIC during an episode of nausea. Inhibition of MAGL robustly increased 2-AG in the VIC, while FAAH inhibition had no effect on AEA. Finally, we demonstrated that inhibition of MAGL reduced VIC Fos immunoreactivity in response to LiCl treatment. Taken together, these findings provide compelling evidence that acute nausea selectively increases 2-AG in the VIC, and suggests that 2-AG signaling within the VIC regulates nausea by reducing neuronal activity in this forebrain region.

Nausea: current knowledge of mechanisms, measurement and clinical impact

Nausea is a subjective sensation, which often acts as a signal that emesis is imminent. It is a widespread problem that occurs as a clinical sign of disease or as an adverse effect of a drug therapy or surgical procedure. The mechanisms of nausea are complex and the neural pathways are currently poorly understood. This review summarises the current knowledge of nausea mechanisms, the available animal models for nausea research and the anti-nausea properties of commercially available anti-emetic drugs. The review also presents subjective assessment and scoring of nausea. A better understanding of the underlying mechanisms of nausea might reveal potential clinically useful biomarkers for objective measurement of nausea in species of veterinary interest.

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.

The synchronous activity of lateral habenular neurons is essential for regulating hippocampal theta oscillation

Lateral habenula (LHb) has attracted growing interest as a regulator of serotonergic and dopaminergic neurons in the CNS. However, it remains unclear how the LHb modulates brain states in animals. To identify the neural substrates that are under the influence of LHb regulation, we examined the effects of rat LHb lesions on the hippocampal oscillatory activity associated with the transition of brain states. Our results showed that the LHb lesion shortened the theta activity duration both in anesthetized and sleeping rats. Furthermore, this inhibitory effect of LHb lesion on theta maintenance depended upon an intact serotonergic median raphe, suggesting that LHb activity plays an essential role in maintaining hippocampal theta oscillation via the serotonergic raphe. Multiunit recording of sleeping rats further revealed that firing of LHb neurons showed significant phase-locking activity at each theta oscillation cycle in the hippocampus. LHb neurons showing activity that was coordinated with that of the hippocampal theta were localized in the medial LHb division, which receives afferents from the diagonal band of Broca (DBB), a pacemaker region for the hippocampal theta oscillation. Thus, our findings indicate that the DBB may pace not only the hippocampus, but also the LHb, during rapid eye movement sleep. Since serotonin is known to negatively regulate theta oscillation in the hippocampus, phase-locking activity of the LHb neurons may act, under the influence of the DBB, to maintain the hippocampal theta oscillation by modulating the activity of serotonergic neurons.

The anti-nausea effects of CB1 agonists are mediated by an action at the visceral insular cortex

BACKGROUND AND PURPOSE

Conditioned gaping reactions reflect nausea-induced behaviour in rats. Cannabinoid 1 receptor (CB(1) ) agonists interfere with the establishment of nausea-induced conditioned gaping; however, it is not known if their effects are mediated by an action at peripheral or central CB(1) receptors.

EXPERIMENTAL APPROACH

We utilized the conditioned gaping model of nausea to evaluate the effect of peripheral and central administration of the peripherally restricted CB(1) agonist, CB13, on the establishment of LiCl-induced gaping in rats. We further evaluated the ability of HU-210 administered to the gustatory insular cortex (GIC) or visceral insular cortex (VIC) to interfere with LiCl-induced conditioned gaping and determined if this effect was mediated by CB(1) receptors.

KEY RESULTS

Central, but not peripheral, CB13 suppressed LiCl-induced conditioned gaping. Central administration of the potent CB(1) agonist, HU-210, delivered to the VIC, but not the GIC, suppressed the establishment of LiCl-induced gaping reactions, but not LiCl-induced suppression of hedonic reactions or conditioned taste avoidance. This pattern of results suggests that HU-210 delivered to the VIC prevented LiCl-induced nausea, but not learning per se. The suppression of LiCl-induced conditioned gaping by HU-210 was mediated by CB(1) receptors because it was prevented by co-administration of CB(1) antagonist/inverse agonist, AM-251, into the VIC. A high dose of AM-251 (20 µg) administered alone into the VIC did not produce conditioned gaping reactions.

CONCLUSIONS AND IMPLICATIONS

The nausea-relieving effects of CB(1) agonists, but not the nausea-inducing effects of CB(1) inverse agonists, are mediated, at least in part, by their action at the VIC in rats.

Double dissociation between regulation of conditioned disgust and taste avoidance by serotonin availability at the 5-HT(3) receptor in the posterior and anterior insular cortex

A taste associated with emetic drugs produces conditioned disgust reactions in rats (predominantly gaping), unlike nonemetic drugs that can still produce conditioned taste avoidance but not conditioned disgust. That difference suggests nausea is a prerequisite for learning disgust reactions to tastes. Depletion of forebrain serotonin (5-HT) by 5,7-dihydroxytryptamine (5,7-DHT) lesions of the dorsal raphe nucleus and median raphe nucleus prevents LiCl-induced conditioned disgust reactions (Limebeer et al., 2004). Here we demonstrate that partial depletion of 5-HT in the insular cortex (IC) prevents LiCl-induced conditioned disgust reactions. Furthermore, a double dissociation occurred in the partial regulation of disgust and taste avoidance by selective 5-HT(3) receptor antagonism/agonism in the posterior (granular) region of the IC and the anterior (dorsal agranular) region of the IC, respectively. Intracranial administration of the 5-HT(3) receptor antagonist, ondansetron (OND), to the posterior IC impaired the establishment of LiCl-induced conditioned gaping reactions, but not LiCl-induced conditioned taste avoidance (CTA). Likewise, posterior IC administration of the 5-HT(3) receptor agonist m-chlorophenylbiguanide (mCPBG) enhanced the establishment of LiCl-induced conditioned gaping and produced conditioned gaping on its own (which was prevented by intracranially administered OND), with no effect on CTA. On the other hand, anterior IC administration of OND partially reduced the establishment of LiCl-induced CTA, and mCPBG produced a weak CTA, both without effect on gaping. These results suggest that activation of 5-HT(3) receptors in the posterior IC is important for the production of nausea-induced conditioned disgust reactions, while activation of 5-HT(3) receptors in the anterior IC are involved in the production of CTA.

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.

Ondansetron interferes with unconditioned lying-on belly and acquisition of conditioned gaping induced by LiCl as models of nausea-induced behaviors in rats

Rats selectively display conditioned gaping reactions when re-exposed to flavours previously paired with nausea-inducing treatments and drugs that reduce nausea also reduce these reactions, suggesting that they represent a model of nausea-induced behavior in rats. However, these reactions rely upon learning, they are not unconditional malaise-induced reactions. Here we compared the effectiveness of the anti-nausea drug, ondansetron (OND) to interfere with the establishment of conditioned gaping reactions and the unconditional malaise-induced reaction of lying on belly (LOB). Pretreatment with OND significantly reduced both LiCl-induced LOB and conditioned gaping reactions, without modifying conditioned taste avoidance. The frequency of gaping and duration of LOB were highly correlated. These results provide additional support for the validity of the conditioned gaping model as a rodent model of nausea-induced behavior.

Regulation of nausea and vomiting by cannabinoids

Considerable evidence demonstrates that manipulation of the endocannabinoid system regulates nausea and vomiting in humans and other animals. The anti-emetic effect of cannabinoids has been shown across a wide variety of animals that are capable of vomiting in response to a toxic challenge. CB(1) agonism suppresses vomiting, which is reversed by CB(1) antagonism, and CB(1) inverse agonism promotes vomiting. Recently, evidence from animal experiments suggests that cannabinoids may be especially useful in treating the more difficult to control symptoms of nausea and anticipatory nausea in chemotherapy patients, which are less well controlled by the currently available conventional pharmaceutical agents. Although rats and mice are incapable of vomiting, they display a distinctive conditioned gaping response when re-exposed to cues (flavours or contexts) paired with a nauseating treatment. Cannabinoid agonists (Δ(9) -THC, HU-210) and the fatty acid amide hydrolase (FAAH) inhibitor, URB-597, suppress conditioned gaping reactions (nausea) in rats as they suppress vomiting in emetic species. Inverse agonists, but not neutral antagonists, of the CB(1) receptor promote nausea, and at subthreshold doses potentiate nausea produced by other toxins (LiCl). The primary non-psychoactive compound in cannabis, cannabidiol (CBD), also suppresses nausea and vomiting within a limited dose range. The anti-nausea/anti-emetic effects of CBD may be mediated by indirect activation of somatodendritic 5-HT(1A) receptors in the dorsal raphe nucleus; activation of these autoreceptors reduces the release of 5-HT in terminal forebrain regions. Preclinical research indicates that cannabinioids, including CBD, may be effective clinically for treating both nausea and vomiting produced by chemotherapy or other therapeutic treatments.

Inverse agonism of cannabinoid CB1 receptors potentiates LiCl-induced nausea in the conditioned gaping model in rats

BACKGROUND AND PURPOSE

Cannabinoid CB(1) receptor antagonists/inverse agonists, potentiate toxin-induced nausea and vomiting in animal models. Here, we sought to determine if this potentiated nausea was mediated by inverse agonism or neutral antagonism of the CB(1) receptor, and if the potentiated nausea would be produced by intracerebroventricular (icv) administration of an inverse agonist.

EXPERIMENTAL APPROACH

The conditioned gaping model of nausea in rats was used to compare the CB(1) receptor antagonist/inverse agonist, AM251, and the CB(1) receptor neutral antagonists, AM6527 (centrally and peripherally active) and AM6545 (peripherally active), in potentiating conditioned gaping produced by lithium chloride (LiCl) solution. The effect of icv (lateral ventricle and 4th ventricle) administration of AM251 on LiCl-induced gaping in this model was also evaluated.

KEY RESULTS

At a dose that did not produce conditioned gaping on its own, systemically administered AM251 (1.25 mg.kg(-1)) potentiated LiCl-induced conditioned gaping and reduced sucrose palatability; however, even doses as high as 8 mg.kg(-1) of AM6545 and AM6527 neither potentiated LiCl-induced conditioned gaping nor reduced sucrose palatability. Infusions of AM251 into the lateral ventricles (1.25, 12.5 and 125 microg) or the 4th ventricle (2.5, 12.5 and 125 microg) did not potentiate LiCl-induced conditioned gaping reactions, but all doses attenuated saccharin palatability during the subsequent test.

CONCLUSIONS AND IMPLICATIONS

Inverse agonism, but not neutral antagonism, of CB(1) receptors potentiated toxin-induced nausea. This effect may be peripherally mediated or may be mediated centrally by action on CB(1) receptors, located distal to the cerebral ventricles.

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

RATIONALE

The effect of manipulation of the serotonin (5-HT) system on conditioned gaping (presumably reflective of nausea in rats) was evaluated.

OBJECTIVE

The potential of the selective serotonin reuptake inhibitor (SSRI), fluoxetine (which produces nausea in the clinic), to produce conditioned gaping in rats and of the 5-HT(3) antagonists (ondansetron and palonosetron) and the 5-HT(1A) autoreceptor agonist (8-OH-DPAT) to reverse this effect were evaluated.

MATERIALS AND METHODS

In each of four experiments, rats received three pairings of intraorally delivered 17% sucrose solution and fluoxetine (0, 2, 10 or 20 mg/kg) and 72 h later were given a drug-free test trial. In experiment 2, rats were pretreated with the 5-HT(3) antagonists, ondansetron (0, 0.1 or 1.0 mg/kg) or the longer acting palonosetron (0.1 mg/kg), 30 min before each of three sucrose-fluoxetine (20 mg/kg) pairings. In experiment 3, rats were injected with palonosetron (0.1 mg/kg) 2 h before each of three sucrose-fluoxetine (20 mg/kg) or sucrose-lithium chloride (LiCl, 25 mg/kg) pairings. In experiment 4, rats were pretreated with the 5-HT(1A) autoreceptor agonist, 8-OH-DPAT (DPAT, 0.1 mg/kg) 30 min before each of three sucrose-fluoxetine (20 mg/kg) pairings.

RESULTS

After two sucrose-fluoxetine pairings, the highest dose of fluoxetine tested (20 mg/kg) produced conditioned gaping reactions. These conditioned gaping reactions were prevented by pretreatment with DPAT, but not with the 5-HT(3) antagonists. On the other hand, palonosetron administered 2 h prior to sucrose-LiCl pairings attenuated conditioned gaping reactions.

CONCLUSIONS

These results suggest that the conditioned nausea produced by SSRIs, but not LiCl, may be resistant to treatment with 5-HT(3) antagonists, but not 5-HT(1A) autoreceptor agonists.

The novel cannabinoid CB1 receptor neutral antagonist AM4113 suppresses food intake and food-reinforced behavior but does not induce signs of nausea in rats

Drugs that interfere with cannabinoid CB1 transmission suppress various food-motivated behaviors, and it has been suggested that such drugs could be useful as appetite suppressants. Biochemical studies indicate that most of these drugs assessed thus far have been CB1 inverse agonists, and although they have been shown to suppress food intake, they also appear to induce nausea and malaise. The present studies were undertaken to characterize the behavioral effects of AM4113, which is a CB1 neutral antagonist, and to examine whether this drug can reduce food-reinforced behaviors and feeding on diets with varying macronutrient compositions. Biochemical data demonstrated that AM4113 binds to CB1 receptors, but does not show inverse agonist properties (ie no effects on cyclic-AMP production). In tests of spontaneous locomotion and analgesia, AM4113 reversed the effects of the CB1 agonist AM411. AM4113 suppressed food-reinforced operant responding with rats responding on fixed ratio (FR) 1 and 5 schedules of reinforcement in a dose-dependent manner, and also suppressed feeding on high-fat, high-carbohydrate, and lab chow diets. However, in the same dose range that suppressed feeding, AM4113 did not induce conditioned gaping, which is a sign of nausea and food-related malaise in rats. These results suggest that AM4113 may decrease appetite by blocking endogenous cannabinoid tone, and that this drug may be less associated with nausea than CB1 inverse agonists.

Differential effects of neurotoxin-induced lesions of the basolateral amygdala and central nucleus of the amygdala on lithium-induced conditioned disgust reactions and conditioned taste avoidance

When rats are intraorally exposed to saccharin solution that has previously been paired with lithium chloride (LiCl), they display Pavlovian conditioned disgust reactions. When exposed to LiCl-paired saccharin solution by bottle, they display suppressed instrumental approach to the bottle resulting in suppressed consumption. The present experiments demonstrated that while neither neurotoxin-induced lesions of the basolateral amygdala (BLA) nor the central nucleus of the amygdala (CeA) attenuated the display of Pavlovian conditioned disgust reactions, lesions of the BLA (but not the CeA) attenuated instrumental conditioned avoidance of the taste. The results are discussed in light of current models of the role of the amygdala in aversive learning.

Cannabinoid CB1 receptor inverse agonists and neutral antagonists: effects on food intake, food-reinforced behavior and food aversions

Drugs that interfere with cannabinoid CB1 receptor transmission suppress a number of food-related behaviors, and these compounds are currently being assessed for their potential utility as appetite suppressants. In addition to rimonabant (SR141716A), several other compounds have been evaluated, including AM251 and AM1387. Biochemical studies indicate that most of the drugs assessed thus far have been CB1 inverse agonists, and these drugs all act to suppress food intake and disrupt food-reinforced behavior. Behavioral tests involving intake of different diets (i.e., high fat, high carbohydrate, laboratory chow) indicate that consumption of all three food types is disrupted by CB1 inverse agonists, and that, expressed as a percent of baseline intake, the effect is roughly comparable across different diets. Although CB1 inverse agonists do not appear to produce severe motor impairments that disrupt feeding behavior, there is evidence that they can induce nausea and malaise. Recent studies have been undertaken to characterize the behavioral effects of CB1 receptor neutral antagonists such as AM4113 to determine if these drugs can reduce feeding and food-reinforced behaviors. Across a variety of different tests, AM4113 produces effects on food-motivated behavior that are very similar to those produced by CB1 inverse agonists. Moreover, this drug did not induce conditioned gaping in rats or vomiting in ferrets. These results suggest that CB1 receptor neutral antagonists may decrease appetite by blocking endogenous cannabinoid tone, and that these drugs may be less associated with nausea than is the case for CB1 inverse agonists.

Effects of the FAAH inhibitor, URB597, and anandamide on lithium-induced taste reactivity responses: a measure of nausea in the rat

RATIONALE

The endogenous cannabinoid system plays a vital role in the control of nausea and emesis. Because of the rapid breakdown and hydrolysis of endocannabinoids, such as anandamide, the therapeutic effects may be enhanced by prolonging their duration of action.

OBJECTIVE

The present experiment evaluated the potential of various doses of URB597, a fatty acid amide hydrolase (FAAH) inhibitor, alone and in combination with systemic administration of anandamide to modulate the establishment of lithium-induced conditioned taste reactivity responses in rats.

MATERIALS AND METHODS

In experiment 1, on the conditioning day, rats first received an injection of 0.3 mg/kg URB597, 0.15 mg/kg URB597, or vehicle and then received a second injection of anandamide (5 mg/kg) or vehicle, before a 3-min exposure of 0.1% saccharin by intraoral infusion. Immediately after the saccharin exposure, the rats were injected with lithium chloride. On each of three test days, rats received a 3-min intraoral infusion of saccharin solution, and the taste reactivity responses were videotaped and monitored. In experiment 2, the effects of pretreatment with the CB(1) antagonist, AM-251, on URB597 and anandamide-induced suppressed aversion was evaluated.

RESULTS

Administration of URB597 alone and in combination with anandamide reduced active rejection reactions elicited by a LiCl-paired saccharin solution; both effects were reversed by pretreatment with AM-251, suggesting that they were CB(1) receptor mediated.

CONCLUSIONS

The results suggest that prolonging the action of anandamide by pretreatment with the FAAH inhibitor, URB597, suppresses lithium-induced nausea in the rat.

Conditioned gaping in rats: a selective measure of nausea

When intraorally infused with a flavored solution previously paired with an emetic drug, rats display a characteristic gaping reaction that reflects conditioned nausea in this species that is unable to vomit. The commonly used conditioned taste avoidance measure, is not a selective measure of nausea because nearly every drug tested (even rewarding drugs) is capable of producing a conditioned taste avoidance. In contrast, only emetic drugs produce conditioned gaping reactions in rats, and anti-emetic drugs interfere with the establishment and the expression of conditioned gaping reactions but do not interfere with conditioned taste avoidance. The conditioned gaping reaction can be used as a pre-clinical tool to evaluate the side effects of nausea that might result from newly developed pharmaceutical agents.

The role of nausea in taste avoidance learning in rats and shrews

When paired with a novel flavoured solution, the injection of an emetic drug, such as lithium chloride, produces avoidance of that solution in both non-emetic rats and in emetic shrews. On the other hand, the pairing of a novel flavour with a drug with rewarding properties results in conditioned taste avoidance in rats, but in conditioned taste preference in shrews. It, therefore, appears that nausea may be necessary for the establishment of conditioned taste avoidance in the emetic shrew, but not in the non-emetic rat. Indeed, pre-treatment with the anti-emetic agents, ondansetron or Delta9-tetrahydrocannabinol, interferes with the establishment of lithium-induced conditioned taste avoidance in the shrew, but does not even attenuate the establishment of lithium-induced conditioned taste avoidance in the rat. The results of a number of studies suggest that the nature of flavour-drug associations varies on the basis of the emetic capacity of the species.

The cannabinoid CB1 antagonist AM 251 produces food avoidance and behaviors associated with nausea but does not impair feeding efficiency in rats

RATIONALE

A growing body of evidence suggests that cannabinoid CB1 receptor antagonists have potential therapeutic utility as appetite suppressants. However, the specific mechanisms underlying the reduction in food intake produced by these drugs are not well understood.

OBJECTIVE

Considering the known antiemetic and motor-suppressive effects of CB1 agonists, the present studies were conducted to determine if the reductions in food intake induced by the CB1 antagonist AM 251 could result from nausea or impairments in intake-related motor control, rather than solely from appetite suppression.

METHODS

Three experiments were conducted to examine the effects of AM 251 (2.0, 4.0, or 8.0 mg/kg or vehicle) on detailed parameters of food intake, on the development of conditioned taste avoidance, and on taste reactivity.

RESULTS

In the first experiment, acute administration of AM 251 dose-dependently decreased food intake; nevertheless, feeding rate (grams consumed per time spent eating) and food handling were unaffected, which suggests that food intake was not reduced because of severe motor impairments. In the second experiment, AM 251 dose-dependently reduced intake of a flavor with which it had previously been associated, indicating that conditioned taste avoidance had developed. Lastly, AM 251 was found to induce conditioned rejection reactions in a dose-dependent manner.

CONCLUSIONS

The CB1 antagonist AM 251 may reduce food intake in part by inducing nausea or malaise, but not because of incoordination or motor slowing related to feeding.

Ondansetron and Delta-9-Tetrahydrocannabinol Interfere With the Establishment of Lithium-Induced Conditioned Taste Avoidance in the House Musk Shrew (Suncus murinus)

Considerable evidence suggests that rats can learn to avoid a taste in the absence of nausea. The current experiments evaluated the potential of the antiemetic agents, ondansetron (OND) and delta-9-tetrahydrocannabinol (THC), to interfere with lithium chloride (LiCl)-induced taste avoidance in the house musk shrew, Suncus murinus, an insectivore that, unlike rats, is capable of vomiting. At a dose that did not modify saccharin (Experiment 1) or sucrose (Experiment 2) intake, OND prevented the establishment of LiCl-induced taste avoidance in the shrew. A low dose of THC (1 mg/kg), which did not modify sucrose intake during conditioning, also prevented the establishment of LiCl-induced taste avoidance in the shrew. Higher doses of THC were also effective, but they also suppressed sucrose consumption during conditioning. These results suggest that nausea is a necessary component of the unconditioned stimulus for the establishment of conditioned taste avoidance in the shrew, unlike the rat, which does not vomit when injected with a toxin.