Chronic water restriction reduces sensitivity to brain stimulation reward in male and female rats

States of physiological need motivate individuals to seek and consume stimuli that restore homeostatic balance. This goal-directed behavior is driven, in part, by pathways that process reward and are sensitive to changes in physiological state, including the mesolimbic dopamine system. The effects of hunger and its physiological markers have been more widely studied for their role in modulating reward signaling pathways. However, fluid need produces robust goal-directed behavior and has also been shown to affect neural substrates of reward processing. To test how acute and chronic states of thirst might alter reward sensitivity, we used the intracranial self-stimulation (ICSS) rate-frequency paradigm (Carlezon & Chartoff, 2007) with male and female Long Evans rats. We hypothesized that sensitivity to ICSS would increase under an acute need state for water and would decrease under chronic deprivation. We found that acute water deprivation for 22-hours prior to the ICSS session did not alter any parameters of reward sensitivity. To elicit motivated behavior toward water in the absence of physiological need, we chemogenetically activated glutamatergic neurons of the subfornical organ (SFO). Despite eliciting more water consumption than acute deprivation, acute chemogenetic activation of SFO neurons also did not alter reward sensitivity. Finally, subjects underwent a five-day chronic water restriction protocol with daily ICSS sessions to determine the effects of sustained physiological need. Chronic water restriction resulted in reduced sensitivity to ICSS. Together, these results indicate that persistent changes in physiological state alter the responsiveness of reward circuitry that could potentially exacerbate maladaptive reward-seeking behaviors.

Mood, Food, and Fertility: Adaptations of the Maternal Brain

Successfully rearing young places multiple demands on the mammalian female. These are met by a wide array of alterations in maternal physiology and behavior that are coordinated with the needs of the developing young, and include adaptations in neuroendocrine systems not directly involved in maternal behavior or lactation. In this article, attenuations in the behavioral and neuroendocrine responses to stressors, the alterations in metabolic pathways facilitating both increased food intake and conservation of energy, and the changes in fertility that occur postpartum are described. The mechanisms underlying these processes as well as the factors that contribute to them and the relative contributions of these stimuli at different times postpartum are also reviewed. The induction and maintenance of the adaptations observed in the postpartum maternal brain are dependent on mother-young interaction and, in most cases, on suckling stimulation and its consequences for the hormonal profile of the mother. The peptide hormone prolactin acting on receptors within the brain makes a major contribution to changes in metabolic pathways, suppression of fertility and the attenuation of the neuroendocrine response to stress during lactation. Oxytocin is also released, both into the circulation and in some hypothalamic nuclei, in response to suckling stimulation and this hormone has been implicated in the decrease in anxiety behavior seen in the early postpartum period. The relative importance of these hormones changes across lactation and it is becoming increasingly clear that many of the adaptations to motherhood reviewed here reflect the outcome of multiple influences. © 2016 American Physiological Society. Compr Physiol 6:1493-1518, 2016.

Nutritional controls of food reward

The propensity to select and consume palatable nutrients is strongly influenced by the rewarding effects of food. Neural processes integrating reward, emotional states and decision-making can supersede satiety signals to promote excessive caloric intake and weight gain. While nutritional habits are influenced by reward-based neural mechanisms, nutrition and its impact on energy metabolism, in turn, plays an important role in the control of food reward. Feeding modulates the release of metabolic hormones that have an important influence on central controls of appetite. Nutrients themselves are also an essential source of energy fuel, while serving as key metabolites and acting as signalling molecules in the neural pathways that control feeding and food reward. Along these lines, this review discusses the impact of nutritionally regulated hormones and select macronutrients on the behavioural and neural processes underlying the rewarding effects of food.

Many mouths to feed: the control of food intake during lactation

Providing nutrients to their developing young is perhaps the most energetically demanding task facing female mammals. In this paper we focus primarily on studies carried out in rats to describe the changes in the maternal brain that enable the dam to meet the energetic demands of her offspring. In rats, providing milk for their litter is associated with a dramatic increase in caloric intake, a reduction in energy expenditure and changes in the pattern of energy utilization as well as storage. These behavioral and physiological adaptations result, in part, from alterations in the central pathways controlling energy balance. Differences in circulating levels of metabolic hormones such as leptin, ghrelin and insulin as well as in responsiveness to these signals between lactating and nonlactating animals, contribute to the modifications in energy balance pathways seen postpartum. Suckling stimulation from the pups both directly, and through the hormonal state that it induces in the mother, plays a key role in facilitating these adaptations.

Appetite and reward

The tendency to engage in or maintain feeding behaviour is potently influenced by the rewarding properties of food. Affective and goal-directed behavioural responses for food have been assessed in response to various physiological, pharmacological and genetic manipulations to provide much insight into the neural mechanisms regulating motivation for food. In addition, several lines of evidence tie the actions of metabolic signals, neuropeptides and neurotransmitters to the modulation of the reward-relevant circuitry including midbrain dopamine neurons and corticolimbic nuclei that encode emotional and cognitive aspects of feeding. Along these lines, this review pulls together research describing the peripheral and central signalling molecules that modulate the rewarding effects of food and the underlying neural pathways.

Time course effects of adrenalectomy and food intake on cocaine- and amphetamine-regulated transcript expression in the hypothalamus

Cocaine- and amphetamine-regulated transcript (CART) has been implicated in the feeding behavior and the regulation of hypothalamic-pituitary-adrenal axis activity. In this study we investigated the expression of CART mRNA in the hypothalamus at several intervals after adrenalectomy or sham surgery in basal conditions or after a fasting-refeeding regimen. Male Wistar rats, with free access to food and drinking, were subjected to bilateral adrenalectomy (ADX) or sham surgery. Plasma corticosterone, ACTH, and leptin levels, epididymal and perirenal fat content, and CART expression were determined 1, 3, 7 and 14 days after surgery. Another set of rats was subjected to a 48-h fasting period followed by refeeding during 4 h on the 7th day after ADX or sham surgery. On the day of the experiment, rats were anesthetized and perfused and the brain was processed for CART mRNA in situ hybridization. We observed that long-term but not short-term adrenalectomy decreased leptin plasma levels and CART expression in the arcuate and paraventricular nuclei. Furthermore, we showed that CART expression was reduced by fasting and it was increased after refeeding in the sham group, however, CART expression was not changed by fasting or refeeding after ADX. In conclusion, the present data indicate that following long-term ADX, under freely feeding conditions, there is a decrease of CART expression in the hypothalamus that is associated with a decrease of leptin secretion. CART expression induced by feeding seems to be modulated by glucocorticoid.

Calculating utility: preclinical evidence for cost-benefit analysis by mesolimbic dopamine

RATIONALE

Throughout our lives we constantly assess the costs and benefits of the possible future outcomes of our actions and use this information to guide behavior. There is accumulating evidence that dopamine contributes to a fundamental component of this computation-how rewards are compared with the costs incurred when obtaining them.

OBJECTIVE

We review the evidence for dopamine's role in cost-benefit decision making and outline a simple mathematical framework in which to represent the interactions between rewards, costs, behavioral state and dopamine.

CONCLUSIONS

Dopamine's effects on cost-benefit decision making can be modeled using simple utility-function curves. This approach provides a useful framework for modeling existing data and generating experimental hypotheses that can be objectively and quantitatively tested by observing choice behavior without the necessity to account for subjective psychological states such as pleasure or desire. We suggest that dopamine plays a key role in overcoming response costs and enabling high-effort behaviors. A particularly important anatomical site of this action is the core of the nucleus accumbens. Here, dopamine is able to modulate activity originating from the frontal cortical systems that also assess costs and rewards. Internal deprivation states (e.g., hunger and thirst) also help to energize goal-seeking behaviors, probably in part by their rich influence on dopamine, which can in turn modify decision making policies.

Increased mRNA levels of tyrosine hydroxylase and dopamine transporter in the VTA of male rats after chronic food restriction

Dieting as a strategy to reduce body weight often fails as it causes food cravings leading to bingeing and weight regain. Evidence from several lines of research suggests the presence of shared elements for neural regulation of food and drug craving. We quantified the expression of eight genes involved in dopamine signalling in brain regions related to the mesolimbic and nigrostriatal dopamine system in male rats subjected to chronic food restriction using quantitative real-time polymerase chain reaction. Food restriction strongly increased mRNA levels of tyrosine hydroxylase and the dopamine transporter in the ventral tegmental area. Quantitative autoradiography indicated that the dopamine transporter was also upregulated at the protein level in the shell of the nucleus accumbens. However, these effects were not observed after acute food deprivation. We suggest that the results reflect a sensitization of the mesolimbic dopamine pathway characterized by increased clearance of extracellular dopamine in the nucleus accumbens shell. Such sensitization of the mesolimbic dopamine system may be one of the underlying causes for the food cravings that interfere with dietary compliance.

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.

Does neuropeptide Y contribute to the modulation of brain stimulation reward by chronic food restriction?

The rewarding effect produced by electrically stimulating particular sites in the lateral hypothalamus (LH) can be enhanced by chronic food restriction and body weight loss. The impact on brain stimulation reward (BSR) of certain hormones involved in the regulation of energy balance, such as leptin and corticotropin-releasing hormone, depends upon the sensitivity of BSR to food restriction. The present investigation assessed the influence of neuropeptide Y (NPY), a potent orexigenic peptide, on BSR generated by stimulating restriction-sensitive and -insensitive sites in the LH. Twelve male Long Evans rats were trained to press a lever for a rewarding train of stimulation. Rate-frequency curves, reflecting the number of rewards earned as a function of the stimulation frequency, were collected during free-feeding and then again following a period of food restriction and 20-25% body weight loss. NPY (4 microg) was administered intraventriculary during the food restriction condition. Alterations in the rewarding effect of the stimulation were assessed by measuring changes in the frequency required to maintain half-maximal rewards earned (M-50). In half of the subjects, food restriction produced significant decreases in M-50 values, indicating that the reward effectiveness of the stimulation was potentiated. In contrast, M-50 values were unaltered by food restriction in the remaining six animals. In most of the subjects in which M-50 values decreased following chronic food restriction, NPY failed to alter BSR. Similarly, BSR was unchanged by NPY administration in most of the rats with restriction-insensitive stimulation sites. These findings suggest that NPY does not take part in the process whereby food restriction and leptin modulate reward circuitry activated by stimulating restriction-sensitive sites.