Novel cues reinstate cocaine-seeking behavior and induce Fos protein expression as effectively as conditioned cues

Cue reinstatement of extinguished cocaine-seeking behavior is a widely used model of cue-elicited craving in abstinent human addicts. This study examined Fos protein expression in response to cocaine cues or to novel cues as a control for activation produced by test novelty. Rats were trained to self-administer cocaine paired with either a light or a tone cue, or received yoked saline and cue presentations, and then underwent daily extinction training. They were then tested for reinstatement of extinguished cocaine-seeking behavior elicited by response-contingent presentations of either the cocaine-paired cue or a novel cue (that is, tone for those trained with a light or vice versa). Surprisingly, conditioned and novel cues both reinstated responding and increased Fos similarly in most brain regions. Exceptions included the anterior cingulate, which was sensitive to test cue modality in saline controls and the dorsomedial caudate-putamen, where Fos was correlated with responding in the novel, but not conditioned, cue groups. In subsequent experiments, we observed a similar pattern of reinstatement in rats trained and tested for sucrose-seeking behavior, whereas rats trained and tested with the cues only reinstated to a novel, and not a familiar, light or tone. The results suggest that novel cues reinstate responding to a similar extent as conditioned cues regardless of whether animals have a reinforcement history with cocaine or sucrose, and that both types of cues activate similar brain circuits. Several explanations as to why converging processes may drive drug and novel cue reinforcement and seeking behavior are discussed.     

特麦角脲对线索诱发海洛因自身给药行为重建的影响

目的观察D2受体部分激动剂特麦角脲对线索诱发大鼠海洛因觅药行为复发的影响。方法建立大鼠海洛因固定比率自身给药行为模型,大鼠随机分为4组各8只:特麦角脲0.1mg/kg、0.2mg/kg、0.4mg/kg干预组(0.1mg/kg组、0.2mg/kg组、0.4mg/kg组)、生理盐水干预组(对照组)。戒断后第15d给予生理盐水或特麦角脲相应剂量腹腔注射,观察条件线索引燃海洛因觅药行为。结果①0.1mg/kg组各项行为指标与对照组相比,差异均无统计学意义(P>0.05);②0.2mg/kg组和0.4mg/kg组的总有效鼻触、第2h有效鼻触、获得条件刺激数均较对照组低(P<0.01);③大鼠活动度、无效鼻触各组之间差异均无统计学意义(P>0.05)。结论特麦角脲剂量依赖性地降低海洛因觅药行为重建,对运动功能无明显影响,有用于戒毒治疗的前景。     

c‐Fos expression associated with reinstatement of cocaine‐seeking behavior by response‐contingent conditioned cues

The capability of cocaine cues to generate craving in cocaine‐dependent humans, even after extended abstinence, is modeled in rats using cue reinstatement of extinguished cocaine‐seeking behavior. We investigated neural activity associated with incentive motivational effects of cocaine cues using c‐fos mRNA and Fos protein expression as markers. Unlike preceding studies, we used response‐contingent presentation of discrete cues to elicit cocaine seeking. Rats were first trained to press a lever, resulting in cocaine reinforcement and light and tone cues. Rats then underwent extinction training, during which lever presses decreased. On the test day, rats either received response‐contingent cocaine cues or received no cues. The cues reinstated extinguished cocaine‐seeking behavior on the test day. In general, cue‐elicited c‐fos mRNA and protein expression were similar and both were enhanced in the prefrontal cortex, ventral tegmental area (VTA), dorsal striatum, and nucleus accumbens. Cues elicited more widespread Fos protein expression relative to our previous research in which cues were presented noncontingently without prior extinction training, including increases in the VTA, substantia nigra, ventral subiculum, and lateral entorhinal cortex. We also observed a correlation between cocaine‐seeking behavior and Fos in the agranular insula (AgI) and basolateral amygdala (BLA). The findings suggest that connections between BLA and AgI play a role in cue‐elicited incentive motivation for cocaine and that reinstatement of cocaine seeking by response‐contingent cues activates a similar corticolimbic circuit as that observed with other modes of cue presentation; however, activation of midbrain and ventral hippocampal regions may be unique to reinstatement by response‐contingent cues. Synapse 63:823–835, 2009. © 2009 Wiley‐Liss, Inc.     

Homing with locale, taxon, and dead reckoning strategies by foraging rats: sensory hierarchy in spatial navigation.

Studies on foraging rats suggest that they can use visual, olfactory, and self-movement cues for spatial guidance, but their relative reliance on these different cues is not well understood. In the present study, rats left a hidden refuge to search for a large food pellet located somewhere on a circular table, and the accuracy with which they returned to the refuge with the food pellet was measured. Cue use was manipulated by administering probe trials from novel locations, blindfolding, moving the home cage relative to the table, rotating the table and using combinations of these manipulations. When visual cues were available and a consistent starting location used, a visual strategy dominated performance. When blindfolded, the rats used olfactory cues from the surface of the table and from the starting hole. When olfactory stimuli were made uninformative, by changing the starting hole and rotating the table, the rats still homed accurately, suggesting they used self-movement cues. In a number of cue combinations, in which cues gave conflicting information, performance degraded. The results suggest that rats display a hierarchical preference in using visual, olfactory and self-movement cues while at the same time being able to reaffirm or switch between various cue combinations. The results are discussed in relation to ideas concerning the neural basis of spatial navigation.     

Orexin‐1 receptor signaling increases motivation for cocaine‐associated cues

The orexin/hypocretin system is involved in multiple cocaine addiction processes that involve drug‐associated environmental cues, including cue‐induced reinstatement of extinguished cocaine seeking and expression of conditioned place preference. However, the orexin system does not play a role in several behaviors that are less cue‐dependent, such as cocaine‐primed reinstatement of extinguished cocaine seeking and low‐effort cocaine self‐administration. We hypothesized that cocaine‐associated cues, but not cocaine alone, engage signaling at orexin‐1 receptors (OX1Rs), and this cue‐engaged OX1R signaling increases motivation for cocaine. Motivation for cocaine was measured in Sprague–Dawley rats with behavioral‐economic demand curve analysis after pretreatment with the OX1R antagonist SB‐334867 (SB) or vehicle with and without light + tone cues. Demand for cocaine was higher when cocaine‐associated cues were present, and SB only reduced cocaine demand in the presence of these cues. We then investigated whether cocaine demand was linked to the cued reinstatement of cocaine seeking, as both procedures are partially driven by cocaine‐associated cues in an orexin‐dependent manner. SB blocked cue‐induced reinstatement behavior, and baseline demand predicted SB efficacy with the largest effect in high‐demand animals, i.e. animals with the greatest cue‐dependent behavior. We conclude that OX1R signaling increases the reinforcing efficacy of cocaine‐associated cues but not that of cocaine alone. This supports our view that orexin plays a prominent role in the ability of conditioned cues to activate motivational responses.     

Adaptive changes in a radial maze task: Efficient selection of baited arms with reduced foraging in senescent hooded rats

Qualitative differences in strategy selection during foraging in a partially baited maze were assessed in young and old rats. The baited and non-baited arms were at a fixed position in space and marked by a specific olfactory cue. The senescent rats did more re-entries during the first four-trial block but were more rapid than the young rats in selecting the reinforced arms during the first visits. Dissociation between the olfactory spatial cue reference by rotating the maze revealed that only few old subjects relied on olfactory cues to select the baited arms and the remainder relied mainly on the visuo-spatial cues.     

Food restriction and leptin impact brain reward circuitry in lean and obese Zucker rats

The rewarding effect produced by electrically stimulating certain sites in the lateral hypothalamus (LH) can be potentiated by food restriction and body weight loss in lean rats. Central leptin and insulin administration can suppress the rewarding impact of the stimulation. To determine whether there are additional peripheral signals that mediate the effect of weight loss on brain reward circuitry, we assessed changes in LH-self-stimulation following food restriction in the obese Zucker rat which develops resistance to circulating leptin and insulin. In addition, we examined the impact of acute food deprivation and leptin administration on LH self-stimulation in lean and obese Zucker rats. The number of brain stimulation rewards earned was measured over a range of LH stimulation frequencies that drove reward rates from zero to asymptotic levels. Restriction reduced frequency thresholds in a subset of lean and obese rats, whereas BSR was unaltered by acute food deprivation. Despite impairment in leptin signaling, intraventricular leptin (4 microg) increased thresholds in most lean and obese rats in which the rewarding effect was sensitive to restriction. These results show that brain reward circuitry in the obese Zucker rat is sensitive to weight loss and leptin.     

Dorsolateral neostriatum contribution to incentive salience: opioid or dopamine stimulation makes one reward cue more motivationally attractive than another

Pavlovian cues for rewards can become attractive incentives: approached and ‘wanted’ as the rewards themselves. The motivational attractiveness of a previously learned cue is not fixed, but can be dynamically amplified during re‐encounter by simultaneous activation of brain limbic circuitry. Here it was reported that opioid or dopamine microinjections in the dorsolateral quadrant of the neostriatum (DLS) of rats selectively amplify attraction toward a previously learned Pavlovian cue in an individualized fashion, at the expense of a competing cue. In an autoshaping (sign‐tracking vs. goal‐tracking) paradigm, microinjection of the mu opioid receptor agonist (DAMGO) or dopamine indirect agonist (amphetamine) in the DLS of sign‐tracker individuals selectively enhanced their sign‐tracking attraction toward the reward‐predictive lever cue. By contrast, DAMGO or amphetamine in the DLS of goal‐trackers selectively enhanced prepotent attraction toward the reward‐proximal cue of sucrose dish. Amphetamine also enhanced goal‐tracking in some sign‐tracker individuals (if they ever defected to the dish even once). That DLS enhancement of cue attraction was due to stronger motivation, not stronger habits, was suggested by: (i) sign‐trackers flexibly followed their cue to a new location when the lever was suddenly moved after DLS DAMGO microinjection; and (ii) DAMGO in the DLS also made sign‐trackers work harder on a new instrumental nose‐poke response required to earn presentations of their Pavlovian lever cue (instrumental conditioned reinforcement). Altogether, the current results suggest that DLS circuitry can enhance the incentive salience of a Pavlovian reward cue, selectively making that cue a stronger motivational magnet.     

Naloxone injections into CA3 disrupt pattern completion associated with relapse from cocaine seeking

The goal of the present research was to assess the degree to which a pattern completion process operates in cue‐induced relapse to cocaine‐seeking behavior. Using a novel cue‐preference version of the place preference task, rats were administered cocaine or saline, which resulted in a preference for the cocaine‐paired cues. After 21 days of abstinence and prior to the preference test, for one group, PBS or naloxone was injected into the CA3 subregion of the hippocampus and for a second group, saline or naloxone was injected systemically. The results indicated that infusions of naloxone into CA3 or systemic injections produced a marked disruption for one and two cues, but had minimal disruptive effect for three or four cues, suggesting that naloxone injections disrupt CA3 function and trigger a deficit in a pattern completion process. Thus, it appears that cue‐based activation of the dorsal CA3 might be a critical trigger via a pattern completion process. Based on additional analyses it appears that there is a disruption primarily for object touches for one cue naloxone injections into the CA3 or systemic injections, but no effect on time (spatial context). © 2016 Wiley Periodicals, Inc.     

Prelimbic cortex is a common brain area activated during cue‐induced reinstatement of cocaine and heroin seeking in a polydrug self‐administration rat model

Many preclinical studies examined cue‐induced relapse to heroin and cocaine seeking in animal models, but most of these studies examined only one drug at a time. In human addicts, however, polydrug use of cocaine and heroin is common. We used a polydrug self‐administration relapse model in rats to determine similarities and differences in brain areas activated during cue‐induced reinstatement of heroin and cocaine seeking. We trained rats to lever press for cocaine (1.0 mg/kg per infusion, 3‐hr/day, 18 day) or heroin (0.03 mg/kg per infusion) on alternating days (9 day for each drug); drug infusions were paired with either intermittent or continuous light cue. Next, the rats underwent extinction training followed by tests for cue‐induced reinstatement where they were exposed to either heroin‐ or cocaine‐associated cues. We observed cue‐selective reinstatement of drug seeking: the heroin cue selectively reinstated heroin seeking and the cocaine cue selectively reinstated cocaine seeking. We used Fos immunohistochemistry to assess cue‐induced neuronal activation in different subregions of the medial prefrontal cortex, dorsal striatum, nucleus accumbens, and amygdala. Fos expression results indicated that only the prelimbic cortex (PL) was activated by both heroin and cocaine cues; in contrast, no significant cue‐induced neuronal activation was observed in other brain areas. RNA in situ hybridization indicated that the proportion of glutamatergic and GABAergic markers in PL Fos‐expressing cells was similar for the heroin and cocaine cue‐activated neurons. Overall, the results indicate that PL may be a common brain area involved in both heroin and cocaine seeking during polydrug use.     

Development of a mouse test for repetitive, restricted behaviors: relevance to autism

Repetitive behavior, a core symptom of autism, encompasses stereotyped responses, restricted interests, and resistance to change. These studies investigated whether different components of the repetitive behavior domain could be modeled in the exploratory hole-board task in mice. Four inbred mouse strains, C57BL/6J, BALB/cByJ, BTBR T+tf/J, and FVB/NJ, and mice with reduced expression of Grin1, leading to NMDA receptor hypofunction (NR1neo/neo mice), were tested for exploration and preference for olfactory stimuli in an activity chamber with a 16-hole floor-board. Reduced exploration and high preference for holes located in the corners of the chamber were observed in BALB/cByJ and BTBR T+tf/J mice. All inbred strains had initial high preference for a familiar olfactory stimulus (clean cage bedding). BTBR T+tf/J was the only strain that did not demonstrate a shift in hole preference towards an appetitive olfactory stimulus (cereal or a chocolate chip), following home cage exposure to the food. The NR1neo/neo mice showed lower hole selectivity and aberrant olfactory stimulus preference, in comparison to wildtype controls. The results indicate that NR1neo/neo mice have repetitive nose poke responses that are less modified by environmental contingencies than responses in wildtype mice. 25-30% of NMDA receptor hypomorphic mice also show self-injurious responses. Findings from the olfactory studies suggest that resistance to change and restricted interests might be modeled in mice by a failure to alter patterns of hole preference following familiarization with an appetitive stimulus, and by high preference persistently demonstrated for one particular olfactory stimulus. Further work is required to determine the characteristics of optimal mouse social stimuli in the olfactory hole-board test.     

Reversible inactivation of the basolateral amygdala,but not the dorsolateral caudate putamen,attenuates consolidation of cocaine‐cue associative learning in a reinstatement model of drug‐seeking

Previous research has shown that the basolateral amygdala (BLA) mediates stimulus‐reward learning, including drug‐cue associations, whereas the dorsolateral caudate putamen (dlCPu) primarily mediates stimulus‐response (habit) learning. Recent evidence has indicated that the dlCPu may be critical in cocaine‐seeking following extended self‐administration, but it remains unknown whether the dlCPu plays a role in the early formation of drug‐cue associations. The current study used a model of Pavlovian learning to compare the roles of the BLA and dlCPu in the consolidation of cocaine‐cue associations that maintain cocaine‐seeking during cue‐induced reinstatement. Male Sprague‐Dawley rats self‐administered cocaine (0.2 mg/50 μL infusion, i.v.) in the absence of cues for 6 days (2 h/day). Immediately following a single 1‐h classical conditioning session in which passive cocaine infusions were paired with a light/tone cue, animals received bilateral infusions of the GABA receptor agonists, baclofen/muscimol (1.0/0.1 mm ), or vehicle into the BLA or dlCPu. Following additional cocaine self‐administration (5 days) and subsequent extinction (no cocaine or cues, 7 days), the ability of the previously cocaine‐paired cues to reinstate cocaine‐seeking was assessed. Inactivation of the BLA, but not the dlCPu, immediately following the classical conditioning session impaired the consolidation of cocaine‐cue associations as seen by decreased cue‐induced reinstatement. These results extend previous findings that the BLA mediates the consolidation of learned associations that drive cocaine‐seeking during subsequent reinstatement and indicate that the dlCPu does not play a role during initial stimulus‐drug associative learning.     

Hippocampal‐prefrontal theta phase synchrony in planning of multi‐step actions based on memory retrieval

Planning of multi‐step actions based on the retrieval of acquired information is essential for efficient foraging. The hippocampus (HPC) and prefrontal cortex (PFC) may play critical roles in this process. However, in rodents, many studies investigating such roles utilized T‐maze tasks that only require one‐step actions (i.e., selection of one of two alternatives), in which memory retrieval and selection of an action based on the retrieval cannot be clearly differentiated. In monkeys, PFC has been suggested to be involved in planning of multi‐step actions; however, the synchrony between HPC and PFC has not been evaluated. To address the combined role of the regions in planning of multi‐step actions, we introduced a task in rats that required three successive nose‐poke responses to three sequentially illuminated nose‐poke holes. During the task, local field potentials (LFP) and spikes from hippocampal CA1 and medial PFC (mPFC) were simultaneously recorded. The position of the first hole indicated whether the following two holes would be presented in a predictable sequence or not. During the first nose‐poke period, phase synchrony of LFPs in the theta range (4–10 Hz) between the regions was not different between predictable and unpredictable trials. However, only in trials of predictable sequences, the magnitude of theta phase synchrony during the first nose‐poke period was negatively correlated with latency of the two‐step ahead nose‐poke response. Our findings point to the HPC‐mPFC theta phase synchrony as a key mechanism underlying planning of multi‐step actions based on memory retrieval rather than the retrieval itself.     

Role of self‐generated odor cues in contextual representation

As first demonstrated in the patient H.M., the hippocampus is critically involved in forming episodic memories, the recall of “what” happened “where” and “when.” In rodents, the clearest functional correlate of hippocampal primary neurons is the place field: a cell fires predominantly when the animal is in a specific part of the environment, typically defined relative to the available visuospatial cues. However, rodents have relatively poor visual acuity. Furthermore, they are highly adept at navigating in total darkness. This raises the question of how other sensory modalities might contribute to a hippocampal representation of an environment. Rodents have a highly developed olfactory system, suggesting that cues such as odor trails may be important. To test this, we familiarized mice to a visually cued environment over a number of days while maintaining odor cues. During familiarization, self‐generated odor cues unique to each animal were collected by re‐using absorbent paperboard flooring from one session to the next. Visual and odor cues were then put in conflict by counter‐rotating the recording arena and the flooring. Perhaps surprisingly, place fields seemed to follow the visual cue rotation exclusively, raising the question of whether olfactory cues have any influence at all on a hippocampal spatial representation. However, subsequent removal of the familiar, self‐generated odor cues severely disrupted both long‐term stability and rotation to visual cues in a novel environment. Our data suggest that odor cues, in the absence of additional rule learning, do not provide a discriminative spatial signal that anchors place fields. Such cues do, however, become integral to the context over time and exert a powerful influence on the stability of its hippocampal representation. © 2014 The Authors. Hippocampus Published by Wiley Periodicals, Inc.     

Epigenetic and pharmacological regulation of 5HT3 receptors controls compulsive ethanol seeking in mice

Factors underlying individual vulnerability to develop alcoholism are largely unknown. In humans, the risk for alcoholism is associated with elevated cue reactivity. Recent evidence suggests that in animal models, reactivity to reward‐paired cues is predictive of addictive behaviors. To model cue reactivity in mice, we used a Pavlovian approach (PA) paradigm in which mice were trained to associate a cue with delivery of a food reinforcer. We then investigated the relationship between PA status with habitual and compulsive‐like ethanol seeking. After training mice to respond for 10% ethanol, habitual behavior was investigated using both an outcome devaluation paradigm, in which ethanol was devalued via association with lithium chloride‐induced malaise, and a contingency degradation paradigm in which the relationship between action and outcome was disrupted. Compulsive‐like behavior was investigated in a modified conditioned place preference paradigm in which footshock was paired with the reward‐paired chamber. PA was found to be predictive of habitual and compulsive‐like ethanol seeking. Additionally, innate risk status was related to epigenetic changes in the gene encoding the requisite subunit of the 5HT3 receptor, Htr3a, as well as 5HT3A protein expression in the amygdala. We then used pharmacological tools to demonstrate that risk status determines the ability of a 5HT3 antagonist to reduce compulsive ethanol seeking. These data indicate that risk status can be identified prior to any alcohol exposure by assessment of cue reactivity, and further that this endophenotype may be predictive of response to pharmacological treatment for components of alcoholism.     

Long‐lasting contribution of dopamine in the nucleus accumbens core,but not dorsal lateral striatum,to sign‐tracking

The attribution of incentive salience to reward‐paired cues is dependent on dopamine release in the nucleus accumbens core (NAcC). These dopamine signals conform to traditional reward‐prediction error signals and have been shown to diminish with time. Here we examined whether the diminishing dopamine signal in the NAcC has functional implications for the expression of sign‐tracking, a Pavlovian conditioned response indicative of the attribution of incentive salience to reward‐paired cues. Food‐restricted male Sprague Dawley rats were trained in a Pavlovian paradigm in which an insertable lever predicted delivery of food reward in a nearby food cup. After 7 or 14 training sessions, rats received infusions of saline, the dopamine antagonist flupenthixol, or the GABA agonists baclofen and muscimol into the NAcC or the dorsal lateral striatum (DLS). Dopamine antagonism within the NAcC attenuated sign‐tracking, whereas reversible inactivation did not affect sign‐tracking but increased non‐specific food cup checking behaviors. Neither drug in the DLS affected sign‐tracking behavior. Critically, extended training did not alter these effects. Although extended experience with an incentive stimulus may reduce cue‐evoked dopamine in the NAcC, this does not remove the dependence on dopamine in this region to promote Pavlovian cue approach nor result in the recruitment of dorsal lateral striatal systems for this behavior. These data support the notion that dopamine within the mesoaccumbal system, but not the nigrostriatal system, contributes critically to incentive motivational processes independent of the length of training.     

Orbitofrontal inactivation impairs reversal of Pavlovian learning by interfering with ‘disinhibition’ of responding for previously unrewarded cues

Orbitofrontal cortex (OFC) is critical for reversal learning. Reversal deficits are typically demonstrated in complex settings that combine Pavlovian and instrumental learning. Yet recent work has implicated the OFC specifically in behaviors guided by cues and the features of the specific outcomes they predict. To test whether the OFC is important for reversing such Pavlovian associations in the absence of confounding instrumental requirements, we trained rats on a simple Pavlovian task in which two auditory cues were presented, one paired with a food pellet reward and the other presented without reward. After learning, we reversed the cue–outcome associations. For half the rats, OFC was inactivated prior to each reversal session. Inactivation of OFC impaired the ability of the rats to reverse conditioned responding. This deficit reflected the inability of inactivated rats to develop normal responding for the previously unrewarded cue; inactivation of OFC had no impact on the ability of the rats to inhibit responding to the previously rewarded cue. These data show that OFC is critical to reversal of Pavlovian responding, and that the role of OFC in this behavior cannot be explained as a simple deficit in response inhibition. Furthermore, the contrast between the normal inhibition of responding, reported here, and impaired inhibition of responding during Pavlovian over‐expectation, reported previously, suggests the novel hypothesis that OFC may be particularly critical for learning (or behavior) when it requires the subject to generate predictions about outcomes by bringing together or integrating disparate pieces of associative information.     

Lesions of the paraventricular nucleus of the thalamus differentially affect sign‐ and goal‐tracking conditioned responses

Recently, evidence has emerged suggesting a role for the paraventricular nucleus of the thalamus (PVT) in the processing of reward‐associated cues. However, the specific role of the PVT in these processes has yet to be elucidated. Here we use an animal model that captures individual variation in response to discrete reward‐associated cues to further assess the role of the PVT in stimulus–reward learning. When rats are exposed to a Pavlovian conditioning paradigm, wherein a discrete cue predicts food reward, two distinct conditioned responses emerge. Some rats, termed sign‐trackers, approach and manipulate the cue, whereas others, termed goal‐trackers, approach the location of reward delivery upon cue presentation. For both sign‐ and goal‐trackers the cue is a predictor, but only for sign‐trackers is it also an incentive stimulus. We investigated the role of the PVT in the acquisition and expression of these conditioned responses using an excitotoxic lesion. Results indicate that PVT lesions prior to acquisition amplify the differences between phenotypes – increasing sign‐tracking and attenuating goal‐tracking behavior. Lesions of the PVT after rats had acquired their respective conditioned responses also attenuated the expression of the goal‐tracking response, and increased the sign‐tracking response, but did so selectively in goal‐trackers. These results suggest that the PVT acts to suppress the attribution of incentive salience to reward cues, as disruption of the functional activity within this structure enhances the tendency to sign‐track.     

Effects of intracerebroventricularly and intraperitoneally administered growth hormone on body weight and food intake in fa/fa Zucker rats.

Growth hormone (GH) possesses multiple metabolic effects, in particular with regard to glucose and lipid homeostasis. Studies on the effects of GH on body weight and food and water intake are scarce and have yielded controversial results. We investigated the effects of different modes of GH administration on the parameters of body weight and food intake as well as on insulin and leptin concentrations in fa/fa Zucker rats. In control experiments, aqua pro injection was given. GH was administered over a time period of 11 days at a daily dose of 250 microg intraperitoneally (i.p.) and 25 microg intracerebroventricularly (i.c.v.). While both food intake and body weight were found to be unaltered in the four groups after this observation period, there was an enhanced food intake and consecutively an increase in body weight over the day period when compared to the night period in the groups of rats that received GH i.c.v. or i.p. This tendency was also shown for water intake. Insulin and leptin concentrations were similar in all groups. Thus, injection of GH appears to modify food intake-related behavior, since the periods of enhanced food and water intake were shifted from night- to daytime. Thus, while in general the metabolic parameters remained unchanged, the activity pattern was clearly modified.     

Food restriction markedly increases dopamine D2 receptor (D2R) in a rat model of obesity as assessed with in-vivo muPET imaging ([11C] raclopride) and in-vitro ([3H] spiperone) autoradiography

Introduction: Dopamine (DA) regulates food intake by modulating food reward and motivation but its involvement in obesity is much less understood. Recent evidence points to the involvement of leptin in the DA‐related modulation of food intake. Here we assess DA D2 receptors (D2R) in a genetic rodent obesity model characterized by leptin‐receptor deficiency and assess the influence of food restriction on these receptors. Methods: We compared D2R levels between Zucker Obese (fa/fa) and Lean (Fa/Fa) rats at 1 and 4 months of age and in two different feeding conditions (restricted and unrestricted food access) using in‐vivo μPET imaging ([¹¹C] raclopride, which is a method sensitive to competition with endogenous DA) and in‐vitro ([³H] spiperone washed to ensure no competition with endogenous DA) autoradiography (ARG). Results: Both ARG and μPET showed that D2R were higher at 1 month than at 4 months of age and that food restricted animals had higher D2R than unrestricted animals. However there were significant differences in the results obtained at 4 months between ARG and μPET. ARG showed that at 1 month and at 4 months unrestricted lean rats (Le U) had significantly higher D2R binding than obese unrestricted rats (Ob U) but showed no differences between restricted obese (Ob R) and restricted lean rats (Le R). It also showed that D2R decline between 1 and 4 months of age was significantly attenuated in food restricted rats [both obese and lean]. In contrast, μPET showed that at 4 months of age, Ob U showed greater D2R availability than Le U rats but like ARG showed no differences between Ob R and Le R rats. Conclusion: The lower D2R binding in Ob U than Le U rats observed with ARG most likely reflects decreases in striatal D2 receptors levels whereas the increased availability observed with μPET is likely to reflect reduced DA release (resulting in decreased competition with endogenous DA). Lack of a significant difference between Ob R and Le R suggests that the differences in dopamine activity and D2R levels between Ob and Le Zucker rats are modulated by access to food. The ARG finding of an attenuation of the age‐related loss of D2R binding corroborates previous studies of the salutary effects of food restriction in the aging process. Because [¹¹C] raclopride is sensitive to competition with endogenous DA, the higher D2R binding in obese rats with raclopride despite the lower D2R levels shown with spiperone could reflect lower extracellular DA in the Ob rats and merits further investigation. Synapse 62:50–61, 2008. Published 2007 Wiley‐Liss, Inc.