A high-fat diet combined with food deprivation increases food seeking and the expression of candidate biomarkers of addiction

Privileged proteins with a second residence: dual targeting and conditional re-routing of mitochondrial proteins

Almost all mitochondrial proteins are encoded by nuclear genes and synthesized in the cytosol as precursor proteins. Signals in the amino acid sequence of these precursors ensure their targeting and translocation into mitochondria. However, in many cases, only a certain fraction of a specific protein is transported into mitochondria, while the rest either remains in the cytosol or undergoes reverse translocation to the cytosol, and can populate other cellular compartments. This phenomenon is called dual localization which can be instigated by different mechanisms. These include alternative start or stop codons, differential transcripts, and ambiguous or competing targeting sequences. In many cases, dual localization might serve as an economic strategy to reduce the number of required genes; for example, when the same groups of enzymes are required both in mitochondria and chloroplasts or both in mitochondria and the nucleus/cytoplasm. Such cases frequently employ ambiguous targeting sequences to distribute proteins between both organelles. However, alternative localizations can also be used for signaling, for example when non-imported precursors serve as mitophagy signals or when they represent transcription factors in the nucleus to induce the mitochondrial unfolded stress response. This review provides an overview regarding the mechanisms and the physiological consequences of dual targeting.

Perinatal undernourishment provokes long-lasting alterations of clusterin and fumarate hydratase expression in the rat nucleus accumbens

BACKGROUND

Background: Perinatal malnutrition seems to provoke important neurochemical alterations in the brain that lead to higher vulnerability to develop neuropsychiatric disorders in the adulthood.

OBJECTIVES

We have examined the persistence and reversibility of the changes induced by perinatal undernourishment on the expression of fumarate hydratase in the rat nucleus accumbens, bearing in mind that this expression has been previously linked with addictive disorders. Clusterin, a multifunctional protein known to be neuroprotective and possibly related to addiction in humans, was studied in parallel.

METHODS

Female Wistar rats underwent a severe restriction of food during gestation and lactation. Upon weaning, a subgroup of undernourished animals was switched to normal chow and another one continued under food restriction. Control rats and their mothers were fed on chow along the experiment. Fumarate hydratase and clusterin were quantified by western blot after five months of postnatal life in the three experimental groups.

RESULTS

Food restriction along the whole experimental period provoked a marked upregulation of both clusterin and fumarate hydratase in the mitochondrial fraction of the nucleus accumbens. In the case of clusterin, this upregulation was also observed in the cytosolic fraction of the nucleus accumbens. When undernourishment was limited to gestation and lactation the two proteins appeared downregulated with respect to controls.

CONCLUSION

The results are consistent with the idea that perinatal malnutrition provokes marked changes in brain neurochemistry that are not fully corrected by the rehabilitation of normal feeding and could be linked to behavioural disturbances in the adulthood, that is, increased vulnerability to addiction.

Hippocampal-Dependent Inhibitory Learning and Memory Processes in the Control of Eating and Drug Taking

As manifestations of excessive and uncontrolled intake, obesity and drug addiction have generated much research aimed at identifying common neuroadaptations that could underlie both disorders. Much work has focused on changes in brain reward and motivational circuitry that can overexcite eating and drug-taking behaviors. We suggest that the regulation of both behaviors depends on balancing excitation produced by stimuli associated with food and drug rewards with the behavioral inhibition produced by physiological "satiety" and other stimuli that signal when those rewards are unavailable. Our main hypothesis is that dysregulated eating and drug use are consequences of diet- and drug-induced degradations in this inhibitory power. We first outline a learning and memory mechanism that could underlie the inhibition of both food and drug-intake, and we describe data that identifies the hippocampus as a brain substrate for this mechanism. We then present evidence that obesitypromoting western diets (WD) impair the operation of this process and generate pathophysiologies that disrupt hippocampal functioning. Next, we present parallel evidence that drugs of abuse also impair this same learning and memory process and generate similar hippocampal pathophysiologies. We also describe recent findings that prior WD intake elevates drug self-administration, and the implications of using drugs (i.e., glucagon-like peptide- 1 agonists) that enhance hippocampal functioning to treat both obesity and addiction are also considered. We conclude with a description of how both WD and drugs of abuse could initiate a "vicious-cycle" of hippocampal pathophysiology and impaired hippocampal-dependent behavioral inhibition.

Random access to palatable food stimulates similar addiction-like responses as a fixed schedule, but only a fixed schedule elicits anticipatory activation

Restricted intermittent food access to palatable food (PF) induces addiction-like behaviors and plastic changes in corticolimbic brain areas. Intermittent access protocols normally schedule PF to a fixed time, enabling animals to predict the arrival of PF. Because outside the laboratory the presence of PF may occur in a random unpredictable manner, the present study explored whether random access to PF would stimulate similar addiction-like responses as observed under a fixed scheduled. Rats were randomly assigned to a control group without chocolate access, to ad libitum access to chocolate, to fixed intermittent access (CH-F), or to random unpredictable access (CH-R) to chocolate. Only the CH-F group developed behavioral and core temperature anticipation to PF access. Both groups exposed to intermittent access to PF showed binge eating, increased effort behaviors to obtain chocolate, as well as high FosB/ΔFosB in corticolimbic areas. Moreover, FosB/ΔFosB in all areas correlated with the intensity of binge eating and effort behaviors. We conclude that both conditions of intermittent access to PF stimulate addiction-like behaviors and FosB/ΔFosB accumulation in brain reward areas; while only a fixed schedule, which provides a time clue, elicited anticipatory activation, which is strongly associated with craving behaviors and may favor relapse during withdrawal.

Comparison of the Time-Dependent Changes in Immediate Early Gene Labeling and Spine Density Following Abstinence From Contingent or Non-contingent Chocolate Pellet Delivery

Rationale: Incubation of craving is a phenomenon whereby responding for cues associated with a reward increases over extended periods of abstinence. Both contingent and non-contingent behavioral designs have been used to study the incubation of craving phenomenon with differing results. The present study directly compares behavioral and neural changes following contingent or non-contingent administration of chocolate flavored pellets.

Objective: The current study examined whether an incubation of craving response would be observed at the behavioral and neural levels following delays of abstinence from chocolate pellets in a contingent or non-contingent reinforcement design.

Methods: Rats were trained for 10 days to bar press for chocolate pellets (contingent) or received chocolate pellets in a non-contingent design (classical conditioning). Groups were then subjected to abstinence from the reward for 24 h, 7, 14 or 28 days at which point they were tested for responding for reward associated cues. Following the test, brains from all rats were processed and assessed for c-Fos and FosB labeling as well as dendritic spine density in the nucleus accumbens (NAc).

Results: Behavioral measures during the test (lever presses, food hopper entries and locomotor activity) revealed similar behavioral outcomes across all delays indicating the lack of an incubation of craving response on both the contingent and non-contingent designs. Overall, labeling of c-Fos in the NAc was lower for the non-contingent group compared to the operant-trained and food restricted control. Compared to the operant-trained and non-trained control groups, a significantly reduced FosB labeling was noted in the NAc of the classically conditioned groups across all abstinence periods. Spine density in the NAc was elevated in both the classically and operant conditioned compared to the food-restricted, non-trained controls.

Conclusions: Chocolate pellet reward did not result in incubation of craving but did produce behavioral learning that was associated with increased spine density. This suggests that chocolate pellet administration results in long-term structural and functional changes that are present for at least 28 days following abstinence. Contingent and non-contingent administration resulted in differential immediate early gene labeling in the NAc, but the functional significance of this has yet to be elucidated.

What Is the Evidence for "Food Addiction?" A Systematic Review

The diagnostic construct of “food addiction” is a highly controversial subject. The current systematic review is the first to evaluate empirical studies examining the construct of “food addiction” in humans and animals. Studies were included if they were quantitative, peer-reviewed, and in the English language. The 52 identified studies (35 articles) were qualitatively assessed to determine the extent to which their findings indicated the following addiction characteristics in relation to food: brain reward dysfunction, preoccupation, risky use, impaired control, tolerance/withdrawal, social impairment, chronicity, and relapse. Each pre-defined criterion was supported by at least one study. Brain reward dysfunction and impaired control were supported by the largest number of studies (n = 21 and n = 12, respectively); whereas risky use was supported by the fewest (n = 1). Overall, findings support food addiction as a unique construct consistent with criteria for other substance use disorder diagnoses. The evidence further suggests that certain foods, particularly processed foods with added sweeteners and fats, demonstrate the greatest addictive potential. Though both behavioral and substance-related factors are implicated in the addictive process, symptoms appear to better fit criteria for substance use disorder than behavioral addiction. Future research should explore social/role impairment, preoccupation, and risky use associated with food addiction and evaluate potential interventions for prevention and treatment.

Autism-Like Behaviours and Memory Deficits Result from a Western Diet in Mice

Nonalcoholic fatty liver disease, induced by a Western diet (WD), evokes central and peripheral inflammation that is accompanied by altered emotionality. These changes can be associated with abnormalities in social behaviour, hippocampus-dependent cognitive functions, and metabolism. Female C57BL/6J mice were fed with a regular chow or with a WD containing 0.2% of cholesterol and 21% of saturated fat for three weeks. WD-treated mice exhibited increased social avoidance, crawl-over and digging behaviours, decreased body-body contacts, and hyperlocomotion. The WD-fed group also displayed deficits in hippocampal-dependent performance such as contextual memory in a fear conditioning and pellet displacement paradigms. A reduction in glucose tolerance and elevated levels of serum cholesterol and leptin were also associated with the WD. The peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PPARGC1a) mRNA, a marker of mitochondrial activity, was decreased in the prefrontal cortex, hippocampus, hypothalamus, and dorsal raphe, suggesting suppressed brain mitochondrial functions, but not in the liver. This is the first report to show that a WD can profoundly suppress social interactions and induce dominant-like behaviours in naïve adult mice. The spectrum of behaviours that were found to be induced are reminiscent of symptoms associated with autism, and, if paralleled in humans, suggest that a WD might exacerbate autism spectrum disorder.