Morphine- and cocaine-induced c-Fos levels in Lewis and Fischer rat strains

A novel cholinergic projection from the lateral parabrachial nucleus and its role in methamphetamine-primed conditioned place preference

Drug relapse is a big clinical challenge in the treatment of addiction, but its neural circuit mechanism is far from being fully understood. Here, we identified a novel cholinergic pathway from choline acetyltransferase-positive neurons in the external lateral parabrachial nucleus (eLPB^ChAT) to the GABAergic neurons in the central nucleus of the amygdala (CeA^GABA) and explored its role in methamphetamine priming-induced reinstatement of conditioned place preference. The anatomical structure and functional innervation of the eLPB^ChAT–CeA^GABA pathway were investigated by various methods such as fluorescent micro-optical sectioning tomography, virus-based neural tracing, fibre photometry, patch-clamp and designer receptor exclusively activated by a designer drug. The role of the eLPB^ChAT–CeA^GABA pathway in methamphetamine relapse was assessed using methamphetamine priming-induced reinstatement of conditioned place preference behaviours in male mice. We found that the eLPB^ChAT neurons mainly projected to the central nucleus of the amygdala. A chemogenetic activation of the eLPB^ChAT neurons in vitro or in vivo triggered the excitabilities of the CeA^GABA neurons, which is at least in part mediated via the cholinergic receptor system. Most importantly, the chemogenetic activation of either the eLPB^ChAT neurons or the eLPB^ChAT neurons that project onto the central nucleus of the amygdala decreased the methamphetamine priming-induced reinstatement of conditioned place preference in mice. Our findings revealed a previously undiscovered cholinergic pathway of the eLPB^ChAT–CeA^GABA and showed that the activation of this pathway decreased the methamphetamine priming-induced reinstatement of conditioned place preference.

Nucleus Tractus Solitarius Neurons Activated by Hypercapnia and Hypoxia Lack Mu Opioid Receptor Expression

Impaired chemoreflex responses are a central feature of opioid-induced respiratory depression, however, the mechanism through which mu opioid receptor agonists lead to diminished chemoreflexes is not fully understood. One brainstem structure involved in opioid-induced impairment of chemoreflexes is the nucleus of the solitary tract (NTS), which contains a population of neurons that express mu opioid receptors. Here, we tested whether caudal NTS neurons activated during the chemoreflex challenge express mu opioid receptors and overlap with neurons activated by opioids. Using genetic labeling of mu opioid receptor-expressing neurons and cFos immunohistochemistry as a proxy for neuronal activation, we examined the distribution of activated NTS neurons following hypercapnia, hypoxia, and morphine administration. The main finding was that hypoxia and hypercapnia primarily activated NTS neurons that did not express mu opioid receptors. Furthermore, concurrent administration of morphine with hypercapnia induced cFos expression in non-overlapping populations of neurons. Together these results suggest an indirect effect of opioids within the NTS, which could be mediated through mu opioid receptors on afferents and/or inhibitory interneurons.

Non-genetic factors that influence methamphetamine intake in a genetic model of differential methamphetamine consumption

RATIONALE

Genetic and non-genetic factors influence substance use disorders. Our previous work in genetic mouse models focused on genetic factors that influence methamphetamine (MA) intake. The current research examined several non-genetic factors for their potential influence on this trait.

OBJECTIVES

We examined the impact on MA intake of several non-genetic factors, including MA access schedule, prior forced MA exposure, concomitant ethanol (EtOH) access, and gamma-aminobutyric acid type B (GABAB) receptor activation. Selectively bred MA high drinking (MAHDR) and low drinking (MALDR) mice participated in this research.

RESULTS

MAHDR, but not MALDR, mice increased MA intake when given intermittent access, compared with continuous access, with a water choice under both schedules. MA intake was not altered by previous exposure to forced MA consumption. Male MAHDR mice given simultaneous access to MA, EtOH, and an EtOH+MA mixture exhibited a strong preference for MA over EtOH and EtOH+MA; MA intake was not affected by EtOH in female MAHDR mice. When independent MAHDR groups were given access to MA, EtOH, or EtOH+MA vs. water in each case, MA intake was reduced in the water vs. EtOH+MA group, compared with the water vs. MA group. The GABAB receptor agonist R(+)-baclofen (BAC) not only reduced MA intake but also reduced water intake and locomotor activity in MAHDR mice. There was a residual effect of BAC, such that MA intake was increased after termination of BAC treatment.

CONCLUSIONS

These findings demonstrate that voluntary MA intake in MAHDR mice is influenced by non-genetic factors related to MA access schedule and co-morbid EtOH exposure.

Central GLP-1 receptors: Novel molecular targets for cocaine use disorder

Given that the search for effective pharmacotherapies for cocaine use disorder has, thus far, been fruitless, there remains a critical need for conceptually innovative approaches toward identifying new medications to treat this disease. A better understanding of the neurocircuits and neurobiological mechanisms underlying cocaine taking and seeking may identify molecular substrates that could serve as targets for novel pharmacotherapies to treat cocaine use disorder. Recent preclinical evidence suggests that glucagon-like peptide-1 (GLP-1) receptor agonists could be re-purposed to treat cocaine craving-induced relapse. This review endeavors to comprehensively summarize the current literature investigating the efficacy of GLP-1 receptor agonists in reducing the rewarding and reinforcing effects of cocaine in animal models of cocaine use disorder. The role of central endogenous GLP-1 circuits in voluntary cocaine taking and seeking is also discussed. Behavioral, neurochemical, electrophysiological and molecular biology studies indicate that central GLP-1 receptor activation functionally modulates the mesolimbic reward system and decreases addiction-like phenotypes in rodents. Overall, an emerging preclinical literature provides compelling evidence to advance GLP-1 receptor agonists into clinical trials testing the efficacy of these medications in preventing cocaine craving-induced relapse.

Amphetamine-induced activation of neurons within the rat nucleus of the solitary tract

Despite generally being a reinforcing drug of abuse, amphetamine (amph) also produces effects such as hypophagia and conditioned taste avoidance (CTA), which may indicate that amph acts as an aversive homeostatic stressor. Stress-responsive prolactin-releasing peptide (PrRP)-positive noradrenergic and glucagon-like peptide-1 (GLP-1)-positive neurons in the caudal nucleus of the solitary tract (cNTS) are modulated by metabolic state, and are prime candidates for mediating amph-induced hypophagia and CTA. The present study used dual immunolabeling and fluorescent in situ hybridization (RNAscope) to examine acute amph-induced activation of cFos expression in phenotypically-identified cNTS neurons in ad lib-fed vs. overnight-fasted male Sprague Dawley rats. We also examined the impact of food deprivation on amph-induced CTA. Compared to control saline treatment, amph activated significantly more cNTS neurons, including PrRP-negative noradrenergic (NA) neurons, GABAergic neurons, and glutamatergic neurons, but not PrRP or GLP-1 neurons. Amph also increased neural activation within a subset of central cNTS projection targets, including the lateral parabrachial nucleus and central amygdala, but not the paraventricular hypothalamus. Food deprivation did not alter amph-induced neural activation or impact the ability of amph to support CTA. These findings indicate that PrRP-negative NA and other cNTS neurons are recruited by acute amph treatment regardless of metabolic state, and may participate in amph-induced hypophagia and CTA.

Regional influence of cocaine on evoked dopamine release in the nucleus accumbens core: A role for the caudal brainstem

Cocaine increases dopamine concentration in the nucleus accumbens through competitive binding to the dopamine transporter (DAT). However, it also increases the frequency of dopamine release events, a finding that cannot be explained by action at the DAT alone. Rather, this effect may be mediated by cocaine-induced modulation of brain regions that project to dopamine neurons. To explore regional contributions of cocaine to dopamine signaling, we administered cocaine to the lateral or fourth ventricles and compared the effects on dopamine release in the nucleus accumbens evoked by electrical stimulation of the ventral tegmental area to that of systemically-delivered cocaine. Stimulation trains caused a sharp rise in dopamine followed by a slower return to baseline. The magnitude of dopamine release ([DA]max) as well as the latency to decay to fifty percent of the maximum (t(1/2); index of DAT activity) by each stimulation train were recorded. All routes of cocaine delivery caused an increase in [DA]max; only systemic cocaine caused an increase in t(1/2). Importantly, these data are the first to show that hindbrain (fourth ventricle)-delivered cocaine modulates phasic dopamine signaling. Fourth ventricular cocaine robustly increased cFos immunoreactivity in the nucleus of the solitary tract (NTS), suggesting a neural substrate for hindbrain cocaine-mediated effects on [DA]max. Together, the data demonstrate that cocaine-induced effects on phasic dopamine signaling are mediated via actions throughout the brain including the hindbrain.

Glucagon-Like Peptide-1 Receptor Activation in the Ventral Tegmental Area Decreases the Reinforcing Efficacy of Cocaine

Cocaine addiction continues to be a significant public health problem for which there are currently no effective FDA-approved treatments. Thus, there is a clear need to identify and develop novel pharmacotherapies for cocaine addiction. Recent evidence indicates that activation of glucagon-like peptide-1 (GLP-1) receptors in the ventral tegmental area (VTA) reduces intake of highly palatable food. As the neural circuits and neurobiological mechanisms underlying drug taking overlap to some degree with those regulating food intake, these findings suggest that activation of central GLP-1 receptors may also attenuate cocaine taking. Here, we show that intra-VTA administration of the GLP-1 receptor agonist exendin-4 (0.05 μg) significantly reduced cocaine, but not sucrose, self-administration in rats. We also demonstrate that cocaine taking is associated with elevated plasma corticosterone levels and that systemic infusion of cocaine activates GLP-1-expressing neurons in the nucleus tractus solitarius (NTS), a hindbrain nucleus that projects monosynaptically to the VTA. To determine the potential mechanisms by which cocaine activates NTS GLP-1-expressing neurons, we microinjected corticosterone (0.5 μg) directly into the hindbrain fourth ventricle. Intraventricular corticosterone attenuated cocaine self-administration and this effect was blocked in animals pretreated with the GLP-1 receptor antagonist exendin-(9-39) (10 μg) in the VTA. Finally, AAV-shRNA-mediated knockdown of VTA GLP-1 receptors was sufficient to augment cocaine self-administration. Taken together, these findings indicate that increased activation of NTS GLP-1-expressing neurons by corticosterone may represent a homeostatic response to cocaine taking, thereby reducing the reinforcing efficacy of cocaine. Therefore, central GLP-1 receptors may represent a novel target for cocaine addiction pharmacotherapies.

Sex differences in morphine-induced behavioral sensitization and social behaviors in ICR mice

Gender and genetic strain are two prominent variants that influence drug abuse. Although certain sex-related behavioral responses have been previously characterized in ICR mice, little is known about the effects of sex on morphine-induced behavioral responses in this outbred strain. Therefore, in this study, we investigated the sex differences of morphine-induced locomotion, anxiety-like and social behaviors in ICR mice. After morphine or saline exposure for four consecutive days (twice daily), increased locomotion, more time spent in the central area, as well as attenuated rearing and self-grooming behaviors were found in morphine-treated females in an open field; no differences were found in locomotion and the time spent in the central area between male and female controls. When interacting with the same-sex individuals, female controls were engaged in more social investigation, following, body contacting and self-grooming behaviors than controls; morphine exposure reduced contacting and self-grooming behaviors in females; in contrast, these effects were not found in males. These results indicate that female ICR mice are more prosocial and are more susceptible to morphine exposure than males.

Cocaine self-administration and extinction alter medullary noradrenergic and limbic forebrain cFos responses to acute, noncontingent cocaine injections in adult rats

Central noradrenergic (NA) signaling contributes critically to multiple behavioral effects of cocaine administration, particularly stress- and anxiety-related effects. The present study examined the ability of acute cocaine to induce the immediate early gene product, cFos, in NA neurons and stress-related neural circuits in rats that were cocaine-naïve, or had a history of cocaine self-administration with or without extinction. Rats implanted with jugular catheters were trained to self-administer cocaine (0.5-mg/kg/infusion), with a subset subsequently trained on extinction. Cocaine-naïve controls were handled daily. After a final day of self-administration, extinction, or handling, rats received an i.p. injection of either cocaine (20-mg/kg) or saline, and 90min later were anesthetized and perfused. Tissue sections were processed for immunoperoxidase labeling of nuclear cFos with either immunoperoxidase or immunofluorescent cytoplasmic labeling of dopamine beta hydroxylase or tyrosine hydroxylase. Acute cocaine increased the number of activated NA neurons within the caudal nucleus of the solitary tract (NTS; A2 cell group) in cocaine-naïve and extinguished rats, but not in rats that only self-administered. Extinction attenuated cocaine-induced cFos activation in NA neurons of the caudal ventrolateral medulla (A1/C1 cell groups), and attenuated cFos within the paraventricular nucleus of the hypothalamus, the apex of the central neuroendocrine stress axis. Cocaine consistently increased cFos in the bed nucleus of the stria terminalis, regardless of history. NA neurons of the locus coeruleus (A6 cell group) were not activated after cocaine administration in any experimental group. Thus, the ability of cocaine to activate central stress circuitry is altered after cocaine self-administration. Our results suggest a unique role for the NTS in cocaine-induced reinstatement, as extinction training enhanced the ability of cocaine to activate NA neurons within this region. These findings suggest central NA systems originating in the caudal brainstem as potential targets for the treatment of cocaine addiction.

Integration of vestibular and emetic gastrointestinal signals that produce nausea and vomiting: potential contributions to motion sickness

Vomiting and nausea can be elicited by a variety of stimuli, although there is considerable evidence that the same brainstem areas mediate these responses despite the triggering mechanism. A variety of experimental approaches showed that nucleus tractus solitarius, the dorsolateral reticular formation of the caudal medulla (lateral tegmental field), and the parabrachial nucleus play key roles in integrating signals that trigger nausea and vomiting. These brainstem areas presumably coordinate the contractions of the diaphragm and abdominal muscles that result in vomiting. However, it is unclear whether these regions also mediate the autonomic responses that precede and accompany vomiting, including alterations in gastrointestinal activity, sweating, and changes in blood flow to the skin. Recent studies showed that delivery of an emetic compound to the gastrointestinal system affects the processing of vestibular inputs in the lateral tegmental field and parabrachial nucleus, potentially altering susceptibility for vestibular-elicited vomiting. Findings from these studies suggested that multiple emetic inputs converge on the same brainstem neurons, such that delivery of one emetic stimulus affects the processing of another emetic signal. Despite the advances in understanding the neurobiology of nausea and vomiting, much is left to be learned. Additional neurophysiologic studies, particularly those conducted in conscious animals, will be crucial to discern the integrative processes in the brain stem that result in emesis.

Rewarding effects of the electrical stimulation of the parabrachial complex: taste or place preference?

The lateral parabrachial complex has been related to various emotional-affective processes. It has been shown that electrical stimulation of the external Lateral Parabrachial (LPBe) nucleus can induce reinforcing effects in place preference and taste discrimination tasks but does not appear to support self-stimulation. This study examined the relative relevance of place and taste stimuli after electrical stimulation of the LPBe nucleus. A learning discrimination task was conducted that simultaneously included both sensory indexes (taste and place) in order to determine the preference of animals for one or the other. After a taste stimulus reversal task, the rewarding effect of stimulation was found to be preferentially associated with place. These results are discussed in the context of the rewarding action and biological constraints induced by different natural and artificial reinforcing agents.

Conditioned taste avoidance induced by Δ(9)-tetrahydrocannabinol in the Fischer (F344) and Lewis (LEW) rat strains

Although Fischer (F344) and Lewis (LEW) rats differ in their sensitivity to the rewarding effects of ∆(9)-tetrahydrocannabinol (THC), no data have been reported on differences in their sensitivity to the drug's aversive effects, a limiting factor in drug use and abuse. Examining the degree of differences (if any) in such effects in these strains may help further characterize possible genetic factors important to abuse vulnerability. Accordingly, the aversive effects of THC (1-5.6 mg/kg; intraperitoneal) were examined in 32F344 and 32 LEW subjects using the conditioned taste avoidance (CTA) procedure. Thermoregulation was assessed following an acute injection of THC (same as CTA groups) after a week washout period following the last trial. Subjects in both strains displayed dose-dependent THC-induced taste avoidance, with no significant strain difference. THC induced dose-dependent decreases in core body temperature in both strains. LEW subjects displayed lower core body temperatures than F344 rats, although this effect was independent of THC and was likely stress related. These results were discussed in terms of the nature of THC-induced taste avoidance and the basis of strain differences in the aversive effects of drugs of abuse.

Prepubertal Fischer 344 rats display stronger morphine-induced taste avoidance than prepubertal Lewis rats

The present report asked if the previously reported differences in morphine-induced conditioned taste avoidance between adult F344 and LEW rats (F344 > LEW) are also evident in prepubescence (early adolescence). To assess this possibility, adult (Experiment 1) and prepubertal (Experiment 2) F344 and LEW rats were assessed for their ability to acquire morphine-induced taste avoidance (0, 3.2, 10, or 18 mg/kg) in a modified taste avoidance procedure. In each experiment, rats of both strains were given repeated pairings of saccharin and morphine followed by a final two-bottle avoidance test. Adult and prepubertal F344 subjects displayed a more rapid acquisition of the avoidance response as well as stronger suppression of consumption than their LEW counterparts. These data suggest the strains differ in their sensitivity to the aversive effects of morphine and that this differential sensitivity is evident early in development and is developmentally stable. The basis for these strain differences in morphine-induced avoidance was discussed.

Fischer rats are more sensitive than Lewis rats to the suppressive effects of morphine and the aversive kappa-opioid agonist spiradoline

Data have suggested that rats avoid intake of an otherwise palatable saccharin cue when paired with a drug of abuse, at least, in part, because the value of the taste cue pales in anticipation of the availability of the highly rewarding drug. Earlier support for this hypothesis was provided by the finding that, relative to the less sensitive Fischer rats, Lewis rats exhibit greater avoidance of a saccharin cue when paired with a rewarding sucrose or cocaine unconditioned stimulus (US), but not when paired with the aversive agent, lithium chloride. More recent data, however, have shown that Fischer rats actually exhibit greater, not less, avoidance of the same saccharin cue when morphine serves as the US. Therefore, Experiment 1 evaluated morphine-induced suppression of intake of the taste cue in Lewis and Fischer rats when the morphine US was administered subcutaneously, rather than intraperitoneally. Experiment 2 examined the effect of strain on the suppression of intake of the saccharin cue when paired with spiradoline, a selective kappa-opioid receptor agonist. The results confirmed that Fischer rats are more responsive to the suppressive effects of morphine than Lewis rats, and that Fischer rats also exhibit greater avoidance of the saccharin cue when paired with spiradoline, despite the fact that spiradoline is devoid of reinforcing properties. Taken together, the data suggest that the facilitated morphine-induced suppression observed in Fischer rats, compared with Lewis rats, may reflect an increased sensitivity to the aversive, kappa-mediated properties of opiates.

Cocaine self-administration differentially modulates the expression of endogenous cannabinoid system-related proteins in the hippocampus of Lewis vs. Fischer 344 rats

The endocannabinoids anandamide and 2-arachidonyl glycerol (2-AG) are modulators of glutamate and γ-aminobutyric acid (GABA), two transmitters involved in cocaine addiction. However, little is known on the effects of cocaine on the enzymes that produce and degrade endocannabinoids. The present work addresses the effects of cocaine self-administration on the immunohistochemical expression of endocannabinoid signalling (ECS)-related proteins in the hippocampus. The study has been performed on two different strains of rats, Lewis (Lew) and Fischer 344 (F344), which are characterized for displaying a differential sensitivity to cocaine, thus making them suitable in the study of vulnerability to drug addiction. Both strains showed differences in the expression of ECS-related proteins in the hippocampus, i.e. Lew rats exhibited lower CB1 expression but higher CB2 expression than F344 rats. After setting similar cocaine self-administration, both strains showed clear differences in the expression of ECS-related proteins, which were differentially restricted to either the 2-AG or anandamide signalling pathways in a self-administration training/drug-dependent manner. The decreases observed in CB1 expression and N-acyl phosphatidylethanolamine phospholipase D:fatty acid amino hydrolase ratio after saline self-administration were enhanced only in cocaine self-administered Lew rats. CB2 expression increase and diacylglycerol lipase α:monoacylglycerol lipase ratio decrease detected after saline self-administration were blocked only in cocaine self-administered F344 rats. These findings indicate that cocaine may regulate hippocampal GABA/glutamate synapses by directly modulating endocannabinoid production/degradation enzymes and that these actions are strain-dependent. This differential response suggests that the endogenous cannabinoid system might contribute to genotype/strain differences on the sensitivity to self-administration training and cocaine addiction.

Cocaine-induced c-Fos expression in rats selectively bred for high or low saccharin intake and in rats selected for high or low impulsivity

Sweet preference and impulsivity are predictors of cocaine self-administration; however, no research has been conducted to investigate neuronal activation in key brain reward areas after first time exposure to cocaine in rats that differ in their propensity for cocaine-seeking and -taking behavior. In this study we used rats that had been selectively bred for high vs. low saccharin intake and rats selected for high vs. low impulsivity for food. The goal of this study was to investigate whether there are differences of c-Fos reactivity between high and low phenotypes and determine whether these differences are similar between the two animal models. A group of rats was bred for high or low saccharin intake. Another group of rats was selected as high or low impulsive based on performance in a delay-discounting task. Subsequently, rats were given an acute injection of cocaine or saline and then c-Fos expression was observed and analyzed in several brain regions. The low reward-seeking phenotypes showed higher cocaine-induced c-Fos expression in several of these regions. Low saccharin preferring rats showed higher cocaine-induced c-Fos expression in the nucleus accumbens shell, and low impulsive rats showed higher cocaine-induced c-Fos expression in the orbitofrontal cortex and cingulate gyrus 1 area. In addition, both low impulsive and low saccharin rats had higher cocaine-induced c-Fos in the dorsal medial and dorsal lateral caudate putamen. The results indicate that individual differences in neuronal reactivity exist prior to chronic exposure to drugs of abuse. Furthermore, similar differences between the two animal models may be indicative of a common mechanism underlying vulnerability to drugs of abuse.

Exposure to nicotine during periadolescence or early adulthood alters aversive and physiological effects induced by ethanol

The majority of smokers begin their habit during adolescence, which often precedes experimentation with alcohol. Interestingly, very little preclinical work has been done examining how exposure to nicotine during periadolescence impacts the affective properties of alcohol in adulthood. Understanding how periadolescent nicotine exposure influences the aversive effects of alcohol might help to explain why it becomes more acceptable to this preexposed population. Thus, Experiment 1 exposed male Sprague Dawley rats to either saline or nicotine (0.4mg/kg, IP) from postnatal days 34 to 43 (periadolescence) and then examined changes in the aversive effects of alcohol (0, 0.56, 1.0 and 1.8g/kg, IP) in adulthood using the conditioned taste aversion (CTA) design. Changes in blood alcohol concentration (BAC) as well as alcohol-induced hypothermia and locomotor suppression were also assessed. To determine if changes seen were specific to nicotine exposure during periadolescence, the procedures were replicated in adults (Experiment 2). Preexposure to nicotine during periadolescence attenuated the acquisition of the alcohol-induced CTAs (at 1.0g/kg) and the hypothermic effects of alcohol (1.0g/kg). Adult nicotine preexposure produced similar attenuation in alcohol's aversive (at 1.8g/kg) and hypothermic (1.8g/kg) effects. Neither adolescent nor adult nicotine preexposure altered BACs or alcohol-induced locomotor suppression. These results suggest that nicotine may alter the aversive and physiological effects of alcohol, regardless of the age at which exposure occurs, possibly increasing its overall reinforcing value and making it more likely to be consumed.

The paradox of drug taking: the role of the aversive effects of drugs

In 1991, Woods described the paradoxical nature of eating, specifically, that it produced aversive and negative effects. He noted in this analysis the multiple physiological and behavior adaptations, both learned and unlearned, that were aimed at regulating food intake and reducing its aversive, disruptive effects. From this position, he argued that consumption reflected a balance of the positive and aversive effects of eating. The present review extends this analysis to drug use and abuse, i.e., that drug taking itself also is a balance of reward and aversion. Although traditionally the analysis of drug use and abuse has focused on a drug's positive and negative rewarding effects, the present review highlights the aversive effects of these same drugs, e.g., cocaine, morphine, alcohol, and describes such effects as protective in nature. This balance and the manner by which it can be impacted by subject and experiential factors are described with a focus on genetic models of drug abuse using the Lewis and Fischer inbred rat strains.

Conditioned taste aversion learning: implications for animal models of drug abuse

Drugs of abuse are typically discussed in terms of their rewarding effects and how these effects mediate drug taking. However, these drugs produce aversive effects that could have an important role in the overall acceptability of a drug and its likelihood of being self-administered. Rewarding and aversive effects, then, could be interpreted as separate behavioral effects, with the balance of the two determining overall drug acceptability. Interestingly, the role of aversive effects on drug acceptability in the self-administration preparation has received limited attention in this context. This chapter examines the aversive effects of drugs and discusses their role in drug taking. If these aversive effects serve a protective function, manipulations that alter or decrease these effects could have implications for drug taking. Several factors have been reported to alter conditioned taste aversion (CTA) learning, a preparation used in the assessment of the aversive effects of drugs in general. Two of these factors, drug history and strain, are reviewed here. By reviewing these, we intend to demonstrate the protective nature of aversive effects in the initiation and escalation of drug taking and to provide evidence that reductions in aversive effects could produce changes in patterns of drug self-administration that could lead to an increased vulnerability to abuse drugs by altering the reward-aversion balance. The aim of this chapter is not to question the importance of rewarding effects in self-administration but rather to provide evidence that aversive effects are an important factor that needs to be considered in discussions of drug-taking behavior.

Naloxone-precipitated conditioned taste aversions in morphine-dependent Fischer (F344) and Lewis rat strains

The Fischer 344 (F344) and Lewis (LEW) rat strains are genetically divergent populations that are used to study the effects of and responses to drugs of abuse. In this context, LEW rats display faster acquisition of drug self-administration than F344 rats. Interestingly, these strains have also been reported to differ in their somatic responses to morphine withdrawal. To address possible strain differences in the affective response to withdrawal, the present study assessed the ability of naloxone-precipitated withdrawal from morphine to induce conditioned taste aversions in male F344 and LEW rats. Specifically, subjects from each of these strains were given chronic morphine to induce dependence and then given access to a novel saccharin solution followed by naloxone. These pairings were given every fourth day for a total of two conditioning trials after which subjects were given access to saccharin but without naloxone administration to assess extinction of the naloxone-induced aversion. Behavioral assays of withdrawal were also performed after each naloxone administration. Both F344 and LEW subjects acquired aversions to the naloxone-associated taste with no significant differences in the rate of acquisition of the aversions. Differences did appear during extinction with LEW animals extinguishing the taste aversion significantly faster than F344 animals. The data were discussed in terms of the relative strength of the affective responses during withdrawal and the role of such responses to drug use and abuse.

Early maternal separation and sex differences in the aversive effects of amphetamine in adult rats

Early life stress increases vulnerability to psychostimulant abuse, sometimes in a sex-dependent manner. These effects are presumably produced by modulation of neurobiological reward mechanisms; however, drug intake is also influenced by sensitivity to the drug's aversive properties, and little is known about the effects of early life stress on stimulant aversion. To assess this possibility, Sprague-Dawley rat litters experienced daily separation from the dam for 3 h (MS) or 15 min (H) on post-natal days 2-14; control litters (AFR) experienced twice-weekly routine animal facility care only. As adults, the animals were tested for conditioned taste aversions (CTA) induced by 1.5, 2.0 or 3.0 mg/kg d-amphetamine administered intraperitoneally over three acquisition trials followed by six drug-free extinction trials (n=7-8/sex/dose/rearing group). All groups acquired significant aversions compared to their vehicle-treated controls, but differential rearing had no effect on CTA acquisition or extinction. Collapsed across rearing groups, females exhibited significantly stronger aversions and slower extinction than their male counterparts at the low 1.5 mg/kg dose, and unlike the males, failed to fully extinguish relative to their vehicle controls at 1.5 and 2.0 mg/kg. These data underscore the importance of sex differences in assays of stimulant abuse liability, and further support the hypothesis that the effects of post-natal stress on the reinforcing efficacy of psychostimulants may be predominantly due to modulation of reward mechanisms.

Dissociation between the aversive and pharmacokinetic effects of ethanol in female Fischer and Lewis rats

In humans and laboratory animal models, vulnerability to alcohol abuse is influenced by endogenous factors such as genotype. Using the inbred Fischer and Lewis rat strains, we previously reported stronger conditioned taste aversions (CTA) in male Fischer rats that could not be predicted by genotypic differences in alcohol absorption [Roma PG, Flint WW, Higley JD, Riley AL. Assessment of the aversive and rewarding effects of alcohol in Fischer and Lewis rats. Psychopharmacology (Berl) 2006;189:187-99]. The present study made similar assessments in Fischer and Lewis females via four-trial CTA induced by 1 or 1.5 g/kg intraperitoneal (IP) ethanol (n=10-12/strain/dose) as well as measures of blood alcohol concentrations (BAC) at 15, 60 and 180 min post-injection with 1.5 g/kg IP ethanol or saline (n=7-8/strain/dose). Dose-dependent CTAs were produced, but the strains did not differ from each other in these measures; however, BACs in the Lewis females were significantly higher than Fischer at all three time points. As with males of the Fischer and Lewis genotypes, a dissociation between BACs and the aversive effects of alcohol was observed. These data are the first assessments of these particular phenotypes in Fischer and Lewis females, and when considered with the historical data, suggest a Genotype x Sex interaction in the centrally mediated sensitivity to alcohol's aversive effects.

Morphine differentially regulates hsp90beta expression in the nucleus accumbens of Lewis and Fischer 344 rats

We have comparatively studied hsp90beta gene and protein expression in the nucleus accumbens of Lewis and Fischer 344 (F344) rats, two inbred strains that exhibit prominent behavioural differences in drug-seeking behaviours. Phenotypical studies confirmed that Lewis rats developed a higher preference for morphine-paired environments after conditioning. RT-PCR assays did not reveal strain-related differences in hsp90beta gene expression in basal conditions; however, acute morphine treatment provoked an increase of hsp90beta mRNA 2h after injection only in the case of Lewis rats. We also found a significant upregulation of the Hsp90beta protein in both strains 8h after morphine injection, this increase being significantly higher in Lewis rats. Taking into account the suggested roles for Hsp90 in the brain, the data suggest that Lewis and F344 strain differences concerning opioid-seeking behaviours could be related to differential sensitivity to opioid-induced neuronal plasticity within the brain reward system, an effect that could be mediated (at least partially) by stress proteins.

Effects of cross-fostering on cocaine-induced conditioned taste aversions in Fischer and Lewis rats

The systematic comparison between Fischer and Lewis rats is a popular animal model of genetic factors in drug abuse. Although genetic and environmental factors interact to affect drug abuse in humans, analogous effects have not yet been reported within the Fischer-Lewis model. In order to assess the contributions and interaction of genotype and early maternal environment on responses to a drug of abuse, the present study employed a cross-fostering design, where male and female Fischer and Lewis pups were reared by unrelated dams of their own strain (in-fostered) or of the other strain (cross-fostered). As adults, rats from both strains were tested for their ability to acquire a conditioned taste aversion to a novel saccharin solution that had been repeatedly paired with an injection of cocaine (32 mg/kg, subcutaneous). In-fostered Fischer females acquired significantly weaker aversions than in-fostered Lewis females across the multiple saccharin-cocaine pairings. However, cross-fostered Fischer females exhibited aversions that were not only significantly stronger than their in-fostered Fischer counterparts, but identical to all groups of the Lewis genotype. No strain differences or cross-fostering effects were observed in the males. The data with the female subjects cannot be accounted for simply by the genetic strain of the subjects and demonstrate a clear gene-environment interaction effect on responses to the aversive effects of cocaine in Fischer and Lewis rats. Implications for studying maternal behavior as a source of epigenetic modulation of drug abuse vulnerability were discussed.

Learned preferences induced by electrical stimulation of a food-related area of the parabrachial complex: Effects of naloxone

Electrical stimulation of the External Lateral Parabrachial Subnucleus (LPBe), a food-related area, induced behavioral preferences for associated stimuli in a taste discrimination learning task. Although this stimulation appeared to be ineffective to elicit standard lever press self-stimulation, it induced place preference for one of two training compartments of a rectangular maze in which animals (adult male Wistar rats) received concurrent electrical brain stimulation. In subjects that consistently showed a preference behavior in different trials, administration of the opioid antagonist naloxone (4 mg/ml/kg) blocked concurrent learning when the test was made in a new maze but not in the same maze in which animals had learned the task. These results are discussed in terms of the possible participation of the LPBe subnucleus in different natural and artificial brain reward systems.

Morphine-induced place conditioning in Fischer and Lewis rats: acquisition and dose-response in a fully biased procedure

The Fischer (F344) and Lewis (LEW) rat strains differ on a variety of behavioral assays examining the effects of morphine, with many of the differences observed during acquisition of behavioral responses. The results of these studies and others examining endogenous physiology and the biochemical effects of morphine suggest that F344 rats are more sensitive to morphine than LEW rats. However, LEW animals have shown greater conditioned place preferences (CPP) to 4 mg/kg than F344 rats. CPP is a popular assay of drug reward in which acquisition of the preference can be measured across multiple conditioning cycles, yet this aspect of CPP has not been assessed in F344 and LEW rats. As part of an ongoing effort to fully characterize the conditioned rewarding effects of abused drugs in these strains, the present study assessed the effects of 0, 1, 4 and 10 mg/kg subcutaneous (SC) morphine in adult male F344 and LEW rats (n=12/strain/dose). A fully biased place conditioning procedure was employed where morphine's effects were paired with the initially non-preferred chamber on Day 1, saline was paired with the preferred chamber on Day 2 and drug-free access to the entire apparatus was allowed on Day 3. This conditioning and testing regimen was repeated for four consecutive cycles. The F344 animals acquired CPP at 1 mg/kg only; this effect emerged after only two conditioning cycles. LEW rats never acquired a CPP at any dose tested. Peak blood morphine levels following SC injections of 1, 4 or 10 mg/kg revealed no significant strain or dose effects. These behavioral data are consistent with the hypothesis that F344 rats are more sensitive to the rewarding effects of morphine than LEW rats. Additional implications for the Fischer-Lewis model of drug abuse and the utility of CPP acquisition procedures are discussed.

Region-specific mechanisms for testosterone-induced Fos in hamster brain

Hamsters self-administer androgens. Previously, we determined that testosterone (T) activates select steroid- and opiate-sensitive brain regions. Is T-stimulated neuronal activation androgenic? Thirty-five castrated males with physiologic T replacement (n=7/group) were pre-treated with the androgen antagonist flutamide (15 mg/kg sc) or ethanol (0.25 ml) and infused into the lateral ventricle (ICV) for 4 h with 40 microg T (TF and TE, respectively) or 40 microl vehicle (VF and VE). To determine if androgens and opiates activate overlapping brain areas, 7 additional males received 20 mug morphine sulfate ICV following ethanol injection (ME). Immediately after ICV infusion, animals were perfused. Sixty-micrometer coronal brain slices were stained for Fos. Fos-positive neurons were counted in a 0.3-mm(2) area from 5 regions previously shown to express T-induced Fos: the posteromedial bed nucleus of the stria terminalis (BSTPM), posteromedial amygdala (MeP), lateral habenula (LHb), ventral tegmental area, and lateral pontine nucleus. T induced Fos in all areas reported previously (TE vs. VE, p<0.05), except LHb (p>0.05). Morphine induced Fos in all 5 brain regions (ME vs. VE, p<0.05), indicating that androgens and opiates activate overlapping brain regions. Flutamide alone did not induce Fos (VF vs. VE, p>0.05). Moreover, flutamide treatment blocked T-induced Fos expression only in the steroid-sensitive BSTPM, suggesting that androgens mediate neuronal activation in this area (mean+/-SEM: TF: 68.4+/-13.2 vs. TE: 137.9+/-17.6, p<0.05). The absence of flutamide effects on T-induced Fos in the steroid-sensitive MeP (TE: 210.6+/-50.0 vs. TF: 215.3+/-28.2, p>0.05) suggests that distinct mechanisms activate Fos in individual androgen-responsive nuclei.

Assessment of the aversive and rewarding effects of alcohol in Fischer and Lewis rats

RATIONALE

Application of the Fischer-Lewis genetic model of drug abuse to the study of alcohol's motivational properties has been limited.

OBJECTIVES

To assess the aversive and rewarding effects of ethanol in Fischer and Lewis rats.

MATERIALS AND METHODS

Fischer and Lewis rats underwent a four-trial combined conditioned taste aversion/conditioned place preference procedure (CTA/CPP; 0, 1, 1.25, or 1.5 g/kg IP ethanol). Others received 0, 1, or 1.5 g/kg followed by tail blood sampling at 15-, 60- and 180-min post-injection. In additional groups, hypothermia to 0, 1.5, and 3 g/kg was assessed before and 30- and 60-min post-injection.

RESULTS

All alcohol-treated groups except low-dose Lewis acquired CTA after one trial. Fischer rats developed stronger CTAs than Lewis at 1.25 and 1.5 g/kg. Ethanol-induced reward in taste or place conditioning was not evident in either strain. Lewis animals showed overall higher peak blood alcohol concentrations, but hypothermia did not vary by strain.

CONCLUSION

Compared to Fischer, Lewis rats are less sensitive to alcohol's aversive effects as assessed in the CTA paradigm. The behavioral differences observed are not due to hypothermia, but pharmacokinetic differences may contribute. These data underscore the importance of genetic factors and the aversive effects of initial drug exposures in modeling vulnerability to abuse. In addition to its application with other drugs, the Fischer-Lewis model may be useful for investigating the biobehavioral bases of alcohol abuse.

The effects of apomorphine and D-amphetamine on striatal c-Fos expression in Sprague-Dawley and Long Evans rats and their F1 progeny

We previously reported that Sprague-Dawley (SD) rats are significantly more sensitive than Long Evans (LE) rats to disruption of prepulse inhibition (PPI) of the startle reflex by the dopamine agonists, apomorphine (APO) and D-amphetamine (AMPH). This susceptibility is inherited through F1 (SD x LE) and N2 backcross (F1 x SD) generations via an orderly pattern (SD>N2>F1>LE). Here we examined systemic APO (0.5 mg/kg) and AMPH (4.5 mg/kg) modulation of neural activity in four regions of the striatum suspected to be involved in the dopaminergic regulation of PPI - dorsolateral (dlCPu) and medial (mCPu) caudate/putamen and core (NACc) and medial shell (NACms) regions of nucleus accumbens - under conditions that mimicked those used to assess PPI. Immunohistochemical quantification of c-Fos protein expression was used as the surrogate measure of neural activity in SD and LE rats and their F1 crosses. Vehicle-treatment showed significant regional differences in Fos expression, particularly between the dlCPu and the other three areas, but no strain-related differences were observed. Three of four brain areas examined (dlCPu, mCPu and NACc) exhibited drug-induced changes in Fos expression--APO decreased and AMPH increased Fos expression in each region. The aggregate effect across these three regions revealed Fos expression to be significantly greater in LE compared to SD rats for both drugs, with F1 rats intermediate. This pattern of inheritance (LE>F1>SD) reveals an inverse relationship between striatal Fos expression and PPI sensitivity for these drugs; and a positive relationship with reported heritable differences in D2-linked G-protein binding in the CPu and NACc, and with locomotor activation/suppression by AMPH and APO.

Differences in the brain expression of c-fos mRNA after restraint stress in Lewis compared to Sprague–Dawley rats

In order to study the contribution of genetic factors to the pattern of stress-induced brain activation, we studied the expression of c-fos mRNA, a marker of neuronal activity, in male Sprague-Dawley and Lewis strains, the latter being known to have a deficient responsiveness of the hypothalamic-pituitary-adrenal (HPA) axis. Immobilization (IMO) alone or combined with the immersion into water at 21 degrees C was applied for 15 or 60 min. The expression of c-fos mRNA was quantified by in situ hybridization in those brain areas that represent important parts of neuronal circuits activated by stress: medial prefrontal cortex, medial amygdala, lateral septum ventral part, paraventricular nucleus of the hypothalamus and locus coeruleus. While in controls, c-fos mRNA was not detectable in tested brain areas, both types of stressors induced a strong expression of this immediate early gene. There were only small differences in c-fos mRNA expression related to the type of stressor or the length of exposure to them. However, there were remarkable differences in the expression between the two rat strains. When compared to Sprague-Dawley rats, Lewis rats showed a reduced c-fos mRNA expression after both stressors in most brain areas, which may be related to the reduced responsiveness of HPA axis and also with other abnormal responses in this strain. However, this hyporesponsiveness was not observed in all brain areas studied, suggesting that there is not a generalized defective c-fos response to stress in Lewis rats and that some responses to stress may be normal in this strain.

The effects of cocaine, alcohol and cocaine/alcohol combinations in conditioned taste aversion learning

We have recently reported that alcohol attenuates cocaine place preferences. Although the basis for this effect is unknown, alcohol may attenuate cocaine reward by potentiating its aversive effects. To examine this possibility, these experiments assessed the effects of alcohol on cocaine-induced taste aversions under conditions similar to those that resulted in attenuated place preferences. Specifically, Experiments 1 and 2 assessed the effects of alcohol (0.5 g/kg) on taste aversions induced by 20, 30 and 40 mg/kg cocaine. Experiment 3 examined the role of intertrial interval in the effects of alcohol (0.5 g/kg) on cocaine (30 mg/kg) taste aversions. In Experiments 1 and 2, cocaine was effective at conditioning aversions. Alcohol produced no measurable effect. Combining cocaine and alcohol produced no greater aversion than cocaine alone (and, in fact, weakened aversions at the lowest dose of cocaine). In Experiment 3, varying the intertrial interval from 3 days (as in the case of Experiments 1 and 2) to 1 day (a procedure identical to that in which alcohol attenuated cocaine place preferences) resulted in significant alcohol- and cocaine-induced taste aversions. Nonetheless, alcohol remained ineffective in potentiating cocaine aversions. Thus, under these conditions alcohol does not potentiate cocaine's aversiveness. These results were discussed in terms of their implication for the effects of alcohol on cocaine-induced place preferences. Further, the effects of alcohol on place preferences conditioned by cocaine were discussed in relation to other assessments of the effects of alcohol on the affective properties of cocaine and the implications of these interactions for alcohol and cocaine co-use.

Strain differences in the acquisition of nicotine-induced conditioned taste aversion

Lewis (LEW) and Fischer (F344) rat strains differ on a variety of physiological and behavioral endpoints, including reactivity to drugs of abuse. Although they differ in drug reactivity, such assessments are generally limited to morphine and cocaine. To determine if these differences generalize to other drugs, the present study examined these strains for their reactivity to the affective properties of nicotine, specifically their sensitivity to nicotine in the conditioned taste aversion preparation. For four or five conditioning cycles given every other day, rats from both strains were allowed access to saccharin and injected with nicotine (0.1, 0.4, 0.8 mg/kg) or vehicle. On intervening days, all rats were given access to water and injected with vehicle. Under this one-bottle training and testing procedure, neither strain displayed aversions at the lowest dose of nicotine (0.1 mg/kg). Aversions were evident for both strains at 0.4 and 0.8 mg/kg, although the F344 rats acquired the aversions at 0.4 mg/kg faster and displayed a significantly greater aversion at 0.8 mg/kg than subjects from the LEW strain. For both strains, aversions were evident at all doses (and in a dose-dependent manner) when subjects were given access to saccharin and water in a two-bottle test. There were, however, no strain differences on this test. Differences between the two strains in their acquisition of nicotine-induced taste aversions were discussed in the context of aversion assessments with other compounds as well as in relation to differences in the self-administration of nicotine in the two strains.

Perinatal exposure to delta(9)-tetrahydrocannabinol alters heroin-induced place conditioning and fos-immunoreactivity

In the present study, the effects of perinatal exposure to Delta(9)-tetrahydrocannabinol (THC) on heroin-induced place conditioning and Fos-immunoreactivity (Fos-IR) were examined. Male albino Wistar rats (N=104) were pretreated with vehicle (n=52) or 5 mg/kg THC (n=52) from postnatal days 4 through 14. At approximately 8 weeks of age, 72 rats were divided into six equal groups (n=12 per group) and injected subcutaneously (s.c.) with vehicle, 0.5, or 2.0 mg/kg heroin and tested in an unbiased two-compartment place conditioning task. In vehicle-pretreated rats, 2.0 mg/kg but not 0.5 mg/kg heroin produced a significant place preference. Perinatal THC exposure significantly enhanced the rewarding properties of both doses of heroin. In the second experiment, 32 rats were divided into four equal groups (n=8 per group) and injected with vehicle or 0.5 mg/kg heroin s.c. and perfused 2-h later. Fos-IR was examined in several brain regions directly or indirectly involved in reward. Acute administration of heroin in vehicle pretreated rats increased Fos-IR in the central, medial, and dorsomedial caudate putamen (CPu), nucleus accumbens (NAC, core and shell regions), lateral septum, islands of Calleja-major (ICjM), bed nucleus of the stria terminalis (BNST), central nucleus of the amygdala (CEA), dorsolateral and dorsomedial periaqueductal gray (PAG), ventral tegmental area (VTA), Edinger-Westphal nucleus (EW). Perinatal THC exposure significantly increased heroin-induced Fos-IR in the dorsomedial CPu. Conversely, perinatal THC exposure reduced heroin-induced Fos-IR in the NAC (shell), BNST, CEA, dorsolateral and lateral PAG, VTA, and EW. The present study demonstrates an increase in the rewarding properties of heroin following exposure to THC at an early age and provides new evidence regarding possible neural correlates underlying this behavioral alteration. Neuropsychopharmacology (2006) 31, 58-69. doi:10.1038/sj.npp.1300770; published online 25 May 2005.

Suppression of natural killer cell activity by morphine is mediated by the nucleus accumbens shell

Despite a wealth of data indicating that morphine modulates immune status by acting at mu-opioid receptors in the brain, there is little known about how the opioid system interacts with other neurotransmitter systems to modulate specific immune parameters. The aim of the present study was to investigate whether dopaminergic projections to the nucleus accumbens are involved in morphine-induced suppression of splenic natural killer (NK) cell activity. The results indicate that administration of the dopamine D1 antagonist SCH-23390 into the nucleus accumbens shell, but not core, blocked morphine's suppressive effect on NK activity in male Lewis rats. In support of these findings, the effect of morphine was also prevented by intra-accumbens microinfusions of the dopaminergic immunotoxin anti-DAT-saporin. Additionally, administration of the D1 agonist SKF-38393 into the nucleus accumbens shell produced reductions in splenic NK activity comparable to morphine, suggesting a critical role for D1 receptors in the modulation of NK activity. Collectively, these findings demonstrate that dopaminergic inputs to the nucleus accumbens are critically involved in opioid-induced immunosuppression and suggest that opioid-induced increases in D1 receptor activation may have adverse consequences on immune status.

Endogenous opiates and behavior: 2004

This paper is the 27th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over 30 years of research. It summarizes papers published during 2004 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior, and the roles of these opioid peptides and receptors in pain and analgesia; stress and social status; tolerance and dependence; learning and memory; eating and drinking; alcohol and drugs of abuse; sexual activity and hormones, pregnancy, development and endocrinology; mental illness and mood; seizures and neurologic disorders; electrical-related activity and neurophysiology; general activity and locomotion; gastrointestinal, renal and hepatic functions; cardiovascular responses; respiration and thermoregulation; and immunological responses.

Repeated exposure to Δ9-tetrahydrocannabinol alters heroin-induced locomotor sensitisation and Fos-immunoreactivity

The present study examined the effect of chronic exposure to Delta(9)-tetrahydrocannabinol (THC) on heroin-induced locomotor sensitisation and Fos-immunoreactivity (Fos-IR). Adult male albino Wistar rats (n=60) were injected intraperitoneally (i.p.) 21 times with vehicle, 0.05, 0.5, or 5.0mg/kg THC (once every 48 h for 41 days). Locomotor activity was assessed for 180 min on pre-exposure days 1, 21, and 41. Following a 2-week washout period, rats were divided into five equal groups (n=12) and injected subcutaneously (s.c.) with vehicle or heroin (0.5mg/kg). Locomotor activity was recorded for 240 min. In drug-naïve rats, heroin significantly increased locomotor activity. THC pre-exposure further increased heroin-induced locomotion. After an interval of 2 weeks, rats pre-exposed to vehicle and 5.0mg/kg THC in the first part of the experiment were randomly assigned to one of four treatment groups (n=6) and injected s.c. with vehicle or 0.5mg/kg heroin and perfused 2h later. Fos-IR was examined in several brain regions. Acute heroin increased Fos-IR in drug-naïve rats in the caudate-putamen (CPu; central, medial and dorsomedial regions), nucleus accumbens (NAC; core and shell regions), bed nucleus of the stria terminalis (BNST), lateral septum, central nucleus of the amygdala (CEA), periaqueductal grey (PAG; dorsolateral, dorsomedial, and lateral), and the Edinger-Westphal nucleus. Pre-exposure to THC significantly increased heroin-induced Fos-IR in the dorsomedial CPu and the NAC (core). Conversely, THC pre-exposure reduced heroin-induced Fos-IR in the BNST, CEA, and the PAG (dorsolateral and lateral). The present study demonstrates that THC pre-exposure increases the locomotor stimulating effects of heroin and provides new evidence for the neural correlates that may underlie cannabinoid and opioid cross-sensitisation.

Lithium-chloride-induced conditioned taste aversions in the Lewis and Fischer 344 rat strains

Conditioned taste aversions (CTAs) are differentially induced by cocaine and morphine in the Lewis and Fisher 344 (LEW and F344, respectively) rat strains. Although the acquisition of LiCl-induced aversions has recently been reported to be comparable between the two strains, these aversions were induced by the oral consumption of LiCl, and the possibility exists that, given their different weights, that differential doses were functionally administered. To address the issue of LiCl-induced aversions in LEW and F344 rats (and to control for this possible confound), the present study assessed the ability of intraperitoneally (ip) administered LiCl to induce aversions in the two strains. Specifically, rats from both strains were given 20-min access to saccharin and injected immediately, thereafter, with 0.3, 0.6, 0.9 or 1.2 mEq/kg, 0.15 M LiCl (or its distilled water vehicle). Under these conditions, both strains acquired dose-dependent aversions that increased over repeated trials. Although there was no overall strain difference in LiCl-induced aversions, LEW rats displayed a stronger aversion at the 0.3 mEq/kg dose (on Trial 3) and acquired the aversion at this dose more rapidly than the F344 rats did (by Trial 2 vs. Trial 3). Although evident, this strain difference with LiCl does not parallel that reported with morphine (in which F344 rats are more sensitive than LEW rats) or with cocaine (in which the differences between LEW and F344 rats are larger and occur at more doses and on more trials). These cross-drug comparisons suggest that strain differences in aversion learning are drug dependent. Because drug acceptability has been reported to be a function of the balance between the reinforcing and aversive effects of various compounds, the examination of possible strain differences in aversion learning with a range of such compounds may provide insight into drug acceptability (and use) in these strains.

Specific features of learning with nociceptive electrical reinforcement in rats of different genetic strains: Role of brain neurotransmitter systems

The interaction between neurotransmitter systems and opioid system in rats of different strains was studied during learning and emotional stress. The interaction between noradrenergic, serotoninergic, and opioid systems in the brain is the main neurochemical mechanism underlying learning reinforced by nociceptive electrical stimulation, which determine individual differences in the rate and type of learning.