Circadian rhythms of hedonic drinking behavior in mice

Circadian variations influence anxiety-related behaviour, olfaction, and hedonic response in male Sprague-Dawley rats

INTRODUCTION

Despite the acknowledged impact of circadian rhythms on various aspects of life, behavioural tests with laboratory animals often overlook alignment with their natural activity patterns. This study aims to evaluate the influence of circadian variations on the results, validity, and reliability of different behavioural tests in rats.

METHODS

Three behavioural tests, the Light-Dark Box Test (LDB), assessing anxiety-related behaviour and locomotor activity; the Buried Pellet Test (BPT), revealing olfactory abilities and motivation issues; and the Sucrose Preference Test (SPT), studying the anhedonic response, were employed to encompass multiple daytime-dependent behavioural aspects in male Sprague-Dawley rats.

RESULTS

Our findings underscore distinct circadian effects on locomotor activity, exploratory behaviour, olfactory acuity, motivation, and hedonic response. Notably, anxious behaviour remained unaffected by daytime conditions. Furthermore, decreased data variance was found to be correlated with conducting behavioural tests during the subjects' active phase.

DISCUSSION

This study demonstrates extensive circadian influences on nearly all parameters investigated, coupled with a significant reduction in data variability during the active phase. Emphasising the importance of aligning experimental timing with rats' natural activity patterns, our results suggest that conducting tests during the active phase of the animals not only refines test sensitivity , reduces stress, and provides more representative data, but also contributes to ethical animal research (3 R) and improves test relevance. This, in turn, enhances the reliability and validity of experimental outcomes in behavioural research and promotes animal welfare.

An open-source platform for head-fixed operant and consummatory behavior

Head-fixed behavioral experiments in rodents permit unparalleled experimental control, precise measurement of behavior, and concurrent modulation and measurement of neural activity. Here, we present OHRBETS (Open-Source Head-fixed Rodent Behavioral Experimental Training System; pronounced 'Orbitz'), a low-cost, open-source platform of hardware and software to flexibly pursue the neural basis of a variety of motivated behaviors. Head-fixed mice tested with OHRBETS displayed operant conditioning for caloric reward that replicates core behavioral phenotypes observed during freely moving conditions. OHRBETS also permits optogenetic intracranial self-stimulation under positive or negative operant conditioning procedures and real-time place preference behavior, like that observed in freely moving assays. In a multi-spout brief-access consumption task, mice displayed licking as a function of concentration of sucrose, quinine, and sodium chloride, with licking modulated by homeostatic or circadian influences. Finally, to highlight the functionality of OHRBETS, we measured mesolimbic dopamine signals during the multi-spout brief-access task that display strong correlations with relative solution value and magnitude of consumption. All designs, programs, and instructions are provided freely online. This customizable platform enables replicable operant and consummatory behaviors and can be incorporated with methods to perturb and record neural dynamics in vivo.

A computational analysis of mouse behavior in the sucrose preference test

The sucrose preference test (SPT) measures the relative preference of sucrose over water to assess hedonic behaviors in rodents. Yet, it remains uncertain to what extent the SPT reflects other behavioral components, such as learning, memory, motivation, and choice. Here, we conducted an experimental and computational decomposition of mouse behavior in the SPT and discovered previously unrecognized behavioral subcomponents associated with changes in sucrose preference. We show that acute and chronic stress have sex-dependent effects on sucrose preference, but anhedonia was observed only in response to chronic stress in male mice. Additionally, reduced sucrose preference induced by optogenetics is not always indicative of anhedonia but can also reflect learning deficits. Even small variations in experimental conditions influence behavior, task outcome and interpretation. Thus, an ostensibly simple behavioral task can entail high levels of complexity, demonstrating the need for careful dissection of behavior into its subcomponents when studying the underlying neurobiology.

An Open-Source Platform for Head-Fixed Operant and Consummatory Behavior

Head-fixed behavioral experiments in rodents permit unparalleled experimental control, precise measurement of behavior, and concurrent modulation and measurement of neural activity. Here we present OHRBETS (Open-Source Head-fixed Rodent Behavioral Experimental Training System; pronounced 'Orbitz'), a low-cost, open-source ecosystem of hardware and software to flexibly pursue the neural basis of a variety of motivated behaviors. Head-fixed mice tested with OHRBETS displayed operant conditioning for caloric reward that replicates core behavioral phenotypes observed during freely moving conditions. OHRBETS also permits for optogenetic intracranial self-stimulation under positive or negative operant conditioning procedures and real-time place preference behavior, like that observed in freely moving assays. In a multi-spout brief-access consumption task, mice displayed licking as a function of concentration of sucrose, quinine, and sodium chloride, with licking modulated by homeostatic or circadian influences. Finally, to highlight the functionality of OHRBETS, we measured mesolimbic dopamine signals during the multi-spout brief-access task that display strong correlations with relative solution value and magnitude of consumption. All designs, programs, and instructions are provided freely online. This customizable ecosystem enables replicable operant and consummatory behaviors and can be incorporated with methods to perturb and record neural dynamics in vivo .

Impact Statement

A customizable open-source hardware and software ecosystem for conducting diverse head-fixed behavioral experiments in mice.

Oral oxycodone self-administration leads to features of opioid misuse in male and female mice

Use of prescription opioids, particularly oxycodone, is an initiating factor driving the current opioid epidemic. There are several challenges with modelling oxycodone abuse. First, prescription opioids including oxycodone are orally self-administered and have different pharmacokinetics and dynamics than morphine or fentanyl, which have been more commonly used in rodent research. This oral route of administration determines the pharmacokinetic profile, which then influences the establishment of drug-reinforcement associations in animals. Moreover, the pattern of intake and the environment in which addictive drugs are self-administered are critical determinants of the levels of drug intake, of behavioural sensitization and of propensity to relapse behaviour. These are all important considerations when modelling prescription opioid use, which is characterized by continuous drug access in familiar environments. Thus, to model features of prescription opioid use and the transition to abuse, we designed an oral, homecage-based oxycodone self-administration paradigm. Mice voluntarily self-administer oxycodone in this paradigm without any taste modification such as sweeteners, and the majority exhibit preference for oxycodone, escalation of intake, physical signs of dependence and reinstatement of seeking after withdrawal. In addition, a subset of animals demonstrate drug taking that is resistant to aversive consequences. This model is therefore translationally relevant and useful for studying the neurobiological substrates of prescription opioid abuse.

Variability in Sleep Timing and Dietary Intake: A Scoping Review of the Literature

The objective of this scoping review was to summarize previous studies which examined the effect of day-to-day variability in sleep timing and social jetlag (SJL) on dietary intake. A systematic literature search was conducted in PubMed, Embase, and Clarivate Analytics Web of Science and we identified 22 records. No difference in caloric and macronutrient intake between SJL groups was observed in studies that enrolled healthy young adults. However, studies that enrolled participants with obesity and obesity-related chronic conditions reported a higher caloric intake and a higher intake of carbohydrates, total fat, saturated fats, and cholesterol in participants with SJL than in those without. Most studies reported a lower quality of diet, a delayed mealtime, and eating jetlag in participants with SJL vs. those without SJL. No correlation of day-to-day variability in sleep timing with average caloric intake was observed, but bed-time variability was negatively associated with diet quality. Methodological issues have been identified in sources assessed including study design, power calculation, population enrolled, and tools/metrics used for sleep timing variability assessment. Future well powered longitudinal studies, with clear protocols, standardized metrics, including all age groups from general population are needed to clarify the dietary intake consequences of variability in sleep timing.

Chronic Unpredictable Mild Stress Model of Depression: Possible Sources of Poor Reproducibility and Latent Variables

Major depressive disorder (MDD) is one of the most common mood disorders worldwide. A lack of understanding of the exact neurobiological mechanisms of depression complicates the search for new effective drugs. Animal models are an important tool in the search for new approaches to the treatment of this disorder. All animal models of depression have certain advantages and disadvantages. We often hear that the main drawback of the chronic unpredictable mild stress (CUMS) model of depression is its poor reproducibility, but rarely does anyone try to find the real causes and sources of such poor reproducibility. Analyzing the articles available in the PubMed database, we tried to identify the factors that may be the sources of the poor reproducibility of CUMS. Among such factors, there may be chronic sleep deprivation, painful stressors, social stress, the difference in sex and age of animals, different stress susceptibility of different animal strains, handling quality, habituation to stressful factors, various combinations of physical and psychological stressors in the CUMS protocol, the influence of olfactory and auditory stimuli on animals, as well as the possible influence of various other factors that are rarely taken into account by researchers. We assume that careful inspection of these factors will increase the reproducibility of the CUMS model between laboratories and allow to make the interpretation of the obtained results and their comparison between laboratories to be more adequate.

Afferent nerve activity in a mouse model increases with faster bladder filling rates in vitro, but voiding behavior remains unaltered in vivo

Storage and voiding functions in urinary bladder are well-known, yet fundamental physiological events coordinating these behaviors remain elusive. We sought to understand how voiding function is influenced by the rate at which the bladder fills. We hypothesized that faster filling rates would increase afferent sensory activity and increase micturition rate. In vivo, this would mean animals experiencing faster bladder filling would void more frequently with smaller void volumes. To test this hypothesis, we measured afferent nerve activity during different filling rates using an ex vivo mouse bladder preparation and assessed voiding frequency in normally behaving mice noninvasively (UroVoid). Bladder afferent nerve activity depended on the filling rate, with faster filling increasing afferent nerve activity at a given volume. Voiding behavior in vivo was measured in UroVoid cages. Male and female mice were given access to tap water or, to induce faster bladder filling rates, water containing 5% sucrose. Fluid intake increased dramatically in mice consuming 5% sucrose. As expected, micturition frequency was elevated in the sucrose group. However, even with the greatly increased rate of urine production, void volumes were unchanged in both genders. Although faster filling rates generated higher afferent nerve rates ex vivo, this did not translate into more frequent, smaller-volume voids in vivo. This suggests afferent nerve activity is only one factor contributing to the switch from bladder filling to micturition. Together with afferent nerve activity, higher centers in the central nervous system and the state of arousal are likely critical to coordinating the micturition reflex.

Development of eight wireless automated cages system with two lick-o-meters each for rodents

Drinking behavior has been used in basic research to study metabolism, motivation, decision-making and different aspects of health problems, such as anhedonia and alcohol use disorders. In the majority of studies, liquid intake is measured by weighing the bottles before and after the experiment. This method does not tell much about the drinking microstructure, e.g., licking bouts and periods of preference for each liquid, which could be valuable to understand drinking behavior. To improve the data acquisition of drinking microstructure, companies have developed lick-o-meters devices that acquire timestamps when animals approach or drink from a specific sipper. Nevertheless, commercially available devices have elevated costs. Here, we present a low-cost alternative for a lick-o-meter system that allows wireless data acquisition of licking from eight cages with two sippers each. We run a three-phase validation protocol to ensure 1) proper choice of the sensor to detect licks; 2) adaptation of the device to a wireless transmission and realistic in silico tests; and 3) in vivo tests to correlate the amount of licks measured by the prototype and the bottle weight. The capacitive sensor presented appropriate recall and precision for our device. After adaptation to wireless transmission, the in silico validation demonstrated low reading and transmission errors for the device even when tested in extreme simultaneous licking conditions. Finally, a positive correlation between volume consumption and lick's count in the in vivo test was observed, showing that the prototype can be used for in vivo studies interested in rodent drinking microstructure.Significant StatementThis study presents an innovative and low-cost solution for drinking behavioral studies: a lick-o-meter system based on an open-source hardware platform with a user-friendly interface software, capable of simultaneously receiving data from eight automated cages with two drinking bottles each. The lick-o-meter brings an accessible device to acquire high-quality and detailed data. This device also has the possibility to be adaptable to new types of sensors or other neuroscience tools capable of measuring brain activity simultaneously to the behavior.

Daily changes in light influence mood via inhibitory networks within the thalamic perihabenular nucleus

Exposure to irregular lighting schedules leads to deficits in affective behaviors. The retino-recipient perihabenular nucleus (PHb) of the dorsal thalamus has been shown to mediate these effects in mice. However, the mechanisms of how light information is processed within the PHb remains unknown. Here, we show that the PHb contains a distinct cluster of GABAergic neurons that receive direct retinal input. These neurons are part of a larger inhibitory network composed of the thalamic reticular nucleus and zona incerta, known to modulate thalamocortical communication. In addition, PHbGABA neurons locally modulate excitatory-relay neurons, which project to limbic centers. Chronic exposure to irregular light-dark cycles alters photo-responsiveness and synaptic output of PHbGABA neurons, disrupting daily oscillations of genes associated with inhibitory and excitatory PHb signaling. Consequently, selective and chronic PHbGABA manipulation results in mood alterations that mimic those caused by irregular light exposure. Together, light-mediated disruption of PHb inhibitory networks underlies mood deficits.

Adolescent THC exposure: effects on pain-related, exploratory, and consummatory behaviors in adult male vs. female rats

RATIONALE

Adolescent cannabinoid exposure has been shown to alter cognitive, reward-related, and motor behaviors as well as mesocorticolimbic dopamine (DA) function in adult animals. Pain is also influenced by mesocorticolimbic DA function, but it is not known whether pain or cannabinoid analgesia in adults is altered by early exposure to cannabinoids.

OBJECTIVE

To determine whether adolescent Δ⁹-tetrahydrocannabinol (THC) exposure alters pain-related behaviors before and after induction of persistent inflammatory pain, and whether it influences antinociceptive of THC, in adult rats, and to compare the impact of adolescent THC exposure on pain to its effects on known DA-dependent behaviors such as exploration and consumption of a sweet solution.

METHODS

Vehicle or THC (2.5 to 10 mg/kg s.c.) was administered daily to male and female rats on post-natal day (PND) 30-43. In adulthood (PND 80-88), sensitivity to mechanical and thermal stimuli before and after intraplantar injection of complete Freund's adjuvant (CFA) was determined. Antinociceptive, exploratory, and consummatory effects of 2.0 mg/kg THC were then examined.

RESULTS

Adolescent THC exposure did not significantly alter adult sensitivity to non-noxious or noxious stimuli either before or after CFA injection, nor did it alter the antinociceptive effect of THC. In contrast, adolescent THC exposure altered adult exploratory and consummatory behaviors in a sex-dependent manner: when tested as adults, adolescent THC-treated males showed less hedonic drinking than adolescent vehicle-treated males, and females but not males that had been THC-exposed as adolescents showed reduced sensitivity to THC-induced suppression of activity and THC-induced hedonic drinking as adults.

CONCLUSIONS

Adolescent THC exposure that altered both exploratory and consummatory behaviors in adults did not alter pain-related behaviors either before or after induction of inflammatory pain, suggesting that cannabinoid exposure during adolescence is not likely to substantially alter pain or cannabinoid analgesia in adulthood.

The sleep-wake distribution contributes to the peripheral rhythms in PERIOD-2

In the mouse, Period-2 (Per2) expression in tissues peripheral to the suprachiasmatic nuclei (SCN) increases during sleep deprivation and at times of the day when animals are predominantly awake spontaneously, suggesting that the circadian sleep-wake distribution directly contributes to the daily rhythms in Per2. We found support for this hypothesis by recording sleep-wake state alongside PER2 bioluminescence in freely behaving mice, demonstrating that PER2 bioluminescence increases during spontaneous waking and decreases during sleep. The temporary reinstatement of PER2-bioluminescence rhythmicity in behaviorally arrhythmic SCN-lesioned mice submitted to daily recurring sleep deprivations substantiates our hypothesis. Mathematical modeling revealed that PER2 dynamics can be described by a damped harmonic oscillator driven by two forces: a sleep-wake-dependent force and an SCN-independent circadian force. Our work underscores the notion that in peripheral tissues the clock gene circuitry integrates sleep-wake information and could thereby contribute to behavioral adaptability to respond to homeostatic requirements.

Circadian rhythms are daily cycles in behavior and physiology which repeat approximately every 24 hours. The master regulator of these rhythms is located in a small part of the brain called the supra-chiasmatic nucleus. This brain structure regulates the timing of sleep and wakefulness and is also thought to control the daily rhythms of cells throughout the body on a molecular level. It does this by synchronizing the activity of a set of genes called clock genes.

Under normal conditions, the levels of proteins coded for by clock genes change throughout the day following a rhythm that matches sleep-wake patterns. However, keeping animals and humans awake at their preferred sleeping times affects the protein levels of clock genes in many tissues of the body. This suggests that, in addition to the supra-chiasmatic nucleus, sleep-wake cycles may also influence clock-gene rhythms throughout the body.

To test this theory, Hoekstra, Jan et al. measured the levels of PERIOD-2, a protein coded for by the clock gene Period-2, while tracking sleep-wake states in mice. They did this by imaging a bioluminescent version of the PERIOD-2 protein in the brain and the kidneys, at the same time as they recorded the brain activity, movement and muscle response of animals. Results showed that PERIOD-2 increased on waking and decreased when mice fell asleep. Additionally, in mice lacking a circadian rhythm in sleep-wake behavior – whose changes in PERIOD-2 levels with respect to time were greatly reduced – imposing a regular sleep-wake cycle restored normal PERIOD-2 rhythmicity.

Next, Hoekstra, Jan et al. developed a mathematical model to understand how sleep-wake cycles together with circadian rhythms affect clock-gene activity in the brain and kidneys. Computer simulations suggested that sleep-wake cycles and circadian factors act as forces of comparable strength driving clock-gene dynamics. Both need to act in concert to keep clock-genes rhythmic. The model also predicted the large and immediate effects of sleep deprivation on PERIOD-2 levels, giving further credence to the idea that waking accelerated clock-gene rhythms while sleeping slowed them down. Modelling also suggested that having regular clock-gene rhythms protects against sleep disturbances.

In summary, this work shows how sleep patterns contribute to the daily rhythms in clock genes in the brain and body. The findings support the idea that well-timed sleep-wake schedules could help people to adjust to new time zones. It might also be useful to inform other strategies to reduce the health impacts of shift work.

The circadian clock in the mouse habenula is set by catecholamines

Circadian rhythms are those variations in behavioral and molecular processes of organisms that follow roughly 24 h cycles in the absence of any external cue. The hypothalamic suprachiasmatic nucleus (SCN) harbors the principal brain pacemaker driving circadian rhythms. The epithalamic habenula (Hb) contains a self-sustained circadian clock functionally coupled to the SCN. Anatomically, the Hb projects to the midbrain dopamine (DA) and serotonin (5-HT) systems, and it receives inputs from the forebrain, midbrain, and brainstem. The SCN is set by internal signals such as 5-HT or melatonin from the raphe nuclei and pineal gland, respectively. However, how the Hb clock is set by internal cues is not well characterized. Hence, in the present study, we determined whether DA, noradrenaline (NA), 5-HT, and the neuropeptides orexin (ORX) and vasopressin influence the Hb circadian clock. Using PER2::Luciferase transgenic mice, we found that the amplitude of the PER2 protein circadian oscillations from Hb explants was strongly affected by DA and NA. Importantly, these effects were dose-and region (rostral vs. caudal) dependent for NA, with a main effect in the caudal part of the Hb. Furthermore, ORX also induced a significant change in the amplitude of PER2 protein oscillations in the caudal Hb. In conclusion, catecholaminergic (DA, NA) and ORXergic transmission impacts the clock properties of the Hb clock likely contributing to the circadian regulation of motivated behaviors. Accordingly, pathological conditions that lead in alterations of catecholamine or ORX activity (drug intake, compulsive feeding) might affect the Hb clock and conduct to circadian disturbances.

Reduction in vasopressin cells in the suprachiasmatic nucleus in mice increases anxiety and alters fluid intake

Central vasopressin (AVP) has been implicated in the control of multiple behaviors, including social behavior, anxiety-like behavior, and sickness behavior. The extent to which the different AVP-producing cell groups contribute to regulating these behaviors has not been extensively investigated. Here we test the role of AVP cells in the suprachiasmatic nucleus (SCN) in these behaviors by ablating these cells using viral-mediated, Cre-dependent caspase in male and female AVP-Cre + mice and Cre-controls. We compared anxiety and social behaviors, as well as sickness behaviors (lethargy, anhedonia (indexed by sucrose consumption), and changes in anxiety-like- and social behavior) induced via injection of bacterial lipopolysaccharide (LPS). We found that SCN AVP cell ablation increased anxiety-like behavior and sucrose consumption in both sexes, as well as increased urine marking by males in a non-social context, but did not alter behavioral responses to sickness. Our data suggest that SCN AVP does not strongly affect LPS-induced behavioral changes, but may contribute to anxiety-like behavior, and may play a role in ingestive reward/motivation and fluid intake.

Brain stimulation rewarding experience attenuates neonatal clomipramine-induced adulthood anxiety by reversal of pathological changes in the amygdala

Major depressive disorder (MDD) is associated with enhanced anxiety and reduced reward processing leading to impaired cognitive flexibility. These pathological changes during depression are accompanied by dysfunctional hypothalamic-pituitary-adrenal (HPA) axis and its impaired regulation by the amygdala. Notably, the electrical stimulation of brain reward areas produces an antidepressant effect in both MDD patients and animal models of depression. However, the effects of chronic electrical self-stimulation of lateral hypothalamus - medial forebrain bundle (LH-MFB) on depression-associated anxiety and accompanying changes in plasma corticosterone levels, structural, and neurochemical alterations in the amygdala are unknown. Here, we used the neonatal clomipramine (CLI) model of depression. During adulthood, neonatal CLI and vehicle administered rats were subjected to bilateral electrode implantation at LH-MFB and trained to receive intracranial self-stimulation (ICSS) for 14 days. Rats were then tested for anhedonic and anxiety-like behaviors, followed by estimation of plasma corticosterone levels, assessment of amygdalar volumes and neuronal/glial numbers, levels of monoamines and their metabolites in the amygdala. We found that chronic ICSS of LH-MFB reverses CLI-induced anhedonia and anxiety. Interestingly, amelioration of CLI-induced enhanced anhedonia and anxiety in ICSS rats was associated with partial reversal of enhanced plasma corticosterone levels, hypertrophy of basolateral amygdala (BLA), and altered noradrenaline (NA) metabolism in the amygdalar complex. We suggest that beneficial effects of ICSS on CLI-induced anxiety at least in part mediated by the restoration of amygdalar and HPA axis functioning. Our results support the hypothesis that brain stimulation rewarding experience might be evolved as a therapeutic strategy for reversal of amygdalar dysfunction in depression.

Circadian modulation of motivation in mice

Most living organisms have a circadian timing system adapted to optimize the daily rhythm of exposure to the environment. This circadian system modulates several behavioral and physiological processes, including the response to natural and drug rewards. Food is the most potent natural reward across species. Food-seeking is known to be mediated by dopaminergic and serotonergic transmission in cortico-limbic pathways. In the present work, we show evidence of a circadian modulation of motivation for food reward in young (4-months old) and aged (over 1.5 years old) C57BL/6 mice. Motivation was assayed through the progressive ratio (PR) schedule. Mice under a 12:12 light/dark (LD) cycle exhibited a diurnal rhythm in motivation, becoming more motivated during the night, coincident with their active phase. This rhythm was also evident under constant dark conditions, indicating the endogenous nature of this modulation. However, circadian arrhythmicity induced by chronic exposure to constant light conditions impaired the performance in the task causing low motivation levels. Furthermore, the day/night difference in motivation was also evident even without caloric restriction when using a palatable reward. All these results were found to be unaffected by aging. Taken together, our results indicate that motivation for food reward is regulated in a circadian manner, independent of the nutritional status and the nature of the reward, and that this rhythmic modulation is not affected by aging. These results may contribute to improve treatment related to psychiatric disorders or drugs of abuse, taking into account potential mechanisms of circadian modulation of motivational states.

Adverse Effects of Circadian Disorganization on Mood and Molecular Rhythms in the Prefrontal Cortex of Mice

Disturbance of the daily cycles in sleep and wakefulness induced by conditions such as shift work and jet lag can increase the risk of affective disorders including anxiety and depression. The way such circadian disorganization disrupts the regulation of mood, however, is not well understood. More specifically, the impact of circadian disorganization on the daily rhythms of the neuronal function that controls mood remains unclear. We therefore investigated the effects of circadian disorganization on expression rhythms of clock genes as well as immediate early genes (IEGs) in several mood-controlling regions of the brain. To introduce circadian disorganization of behaviors, we exposed male C57BL/6J mice to chronic reversal of the light-dark cycle and we found a marked negative mood phenotype in these mice. Importantly, the most adverse effect of circadian disorganization on expression rhythms of clock and IEGs was observed in the prefrontal cortex (PFC) when compared to that in other mood-related areas of the brain. Dysregulation of molecular rhythms in the PFC is therefore suggested to be associated with the development of mood disorders in conditions including shift work and jet lag.

An Open-Source, Automated Home-Cage Sipper Device for Monitoring Liquid Ingestive Behavior in Rodents

Measuring ingestive behavior of liquids in rodents is commonly used in studies of reward, metabolism, and circadian biology. Common approaches for measuring liquid intake in real time include computer-tethered lickometers or video-based systems. Additionally, liquids can be measured or weighed to determine the amount consumed without real-time sensing. Here, we built a photobeam-based sipper device that has the following advantages over traditional methods: (1) it is battery powered and fits in vivarium caging to allow home-cage measurements; (2) it quantifies the intake of two different liquids simultaneously for preference studies; (3) it is low cost and easily constructed, enabling high-throughput experiments; and (4) it is open source so that others can modify it to fit their experimental needs. We validated the performance of this device in three experiments. First, we calibrated our device using time-lapse video-based measurements of liquid intake and correlated sipper interactions with liquid intake. Second, we used the sipper device to measure preference for water versus chocolate milk, demonstrating its utility for two-bottle choice tasks. Third, we integrated the device with fiber photometry, establishing its utility for measuring neural activity in studies of ingestive behavior. This device requires no special equipment or caging, and is small, battery powered, and wireless, allowing it to be placed directly in rodent home cages. The total cost of fabrication is less than $100, and all design files and code are open source. Together, these factors greatly increase scalability and utility for a variety of behavioral neuroscience applications.

Non-invasive Strategies for Chronic Manipulation of DREADD-controlled Neuronal Activity

Chemogenetic strategies have emerged as reliable tools for remote control of neuronal activity. Among these, designer receptors exclusively activated by designer drugs (DREADDs) have become the most popular chemogenetic approach used in modern neuroscience. Most studies deliver the ligand clozapine-N-oxide (CNO) using a single intraperitoneal injection, which is suitable for the acute activation/inhibition of the targeted neuronal population. There are, however, only a few examples of strategies for chronic modulation of DREADD-controlled neurons, the majority of which rely on the use of delivery systems that require surgical intervention. Here, we expand on two non-invasive strategies for delivering the ligand CNO to chronically manipulate neural population in mice. CNO was administered either by using repetitive (daily) eye-drops, or chronically through the animal's drinking water. These non-invasive paradigms result in robust activation of the designer receptors that persisted throughout the CNO treatments. The methods described here offer alternatives for the chronic DREADD-mediated control of neuronal activity and may be useful for experiments designed to evaluate behavior in freely moving animals, focusing on less-invasive CNO delivery methods.

Eating Rewards the Gears of the Clock

Eating behavior is regulated by metabolic and hedonic brain networks, which interact with each other to balance the physiological regulation of hunger and satiety. The daily balance of this regulation is controlled by the central circadian clock. Importantly, metabolic and reward properties of food impact the functioning of circadian clocks, altering the oscillatory activity of the molecular clockwork and circadian rhythms. However, when feeding (metabolic or reward) is timed, the whole circadian system is entrained. Furthermore, besides synchronizing the clock, the timing of both metabolic and reward eating might be crucial for health, to improve circadian physiology, as well as to treat metabolic (e.g., diabetes, obesity) and neurological diseases (e.g., mental, neurodegenerative).

A Free-Choice High-Fat High-Sugar Diet Alters Day-Night Per2 Gene Expression in Reward-Related Brain Areas in Rats

Under normal light-dark conditions, nocturnal rodents consume most of their food during the dark period. Diets high in fat and sugar, however, may affect the day-night feeding rhythm resulting in a higher light phase intake. In vitro and in vivo studies showed that nutrients affect clock-gene expression. We therefore hypothesized that overconsuming fat and sugar alters clock-gene expression in brain structures important for feeding behavior. We determined the effects of a free-choice high-fat high-sugar (fcHFHS) diet on clock-gene expression in rat brain areas related to feeding and reward and compared them with chow-fed rats. Consuming a fcHFHS diet for 6 weeks disrupted day-night differences in Per2 mRNA expression in the nucleus accumbens (NAc) and lateral hypothalamus but not in the suprachiasmatic nucleus, habenula, and ventral tegmental area. Furthermore, short-term sugar drinking, but not fat feeding, upregulates Per2 mRNA expression in the NAc. The disruptions in day-night differences in NAc Per2 gene expression were not accompanied by altered day-night differences in the mRNA expression of peptides related to food intake. We conclude that the fcHFHS diet and acute sugar drinking affect Per2 gene expression in areas involved in food reward; however, this is not sufficient to alter the day-night pattern of food intake.

Neural Mechanisms of Circadian Regulation of Natural and Drug Reward

Circadian rhythms are endogenously generated near 24-hour variations of physiological and behavioral functions. In humans, disruptions to the circadian system are associated with negative health outcomes, including metabolic, immune, and psychiatric diseases, such as addiction. Animal models suggest bidirectional relationships between the circadian system and drugs of abuse, whereby desynchrony, misalignment, or disruption may promote vulnerability to drug use and the transition to addiction, while exposure to drugs of abuse may entrain, disrupt, or perturb the circadian timing system. Recent evidence suggests natural (i.e., food) and drug rewards may influence overlapping neural circuitry, and the circadian system may modulate the physiological and behavioral responses to these stimuli. Environmental disruptions, such as shifting schedules or shorter/longer days, influence food and drug intake, and certain mutations of circadian genes that control cellular rhythms are associated with altered behavioral reward. We highlight the more recent findings associating circadian rhythms to reward function, linking environmental and genetic evidence to natural and drug reward and related neural circuitry.

Effects of a free-choice high-fat high-sugar diet on brain PER2 and BMAL1 protein expression in mice

The suprachiasmatic nucleus (SCN) times the daily rhythms of behavioral processes including feeding. Beyond the SCN, the hypothalamic arcuate nucleus (ARC), involved in feeding regulation and metabolism, and the epithalamic lateral habenula (LHb), implicated in reward processing, show circadian rhythmic activity. These brain oscillators are functionally coupled to coordinate the daily rhythm of food intake. In rats, a free choice high-fat high-sugar (fcHFHS) diet leads to a rapid increase of calorie intake and body weight gain. Interestingly, under a fcHFHS condition, rats ingest a similar amount of sugar during day time (rest phase) as during night time (active phase), but keep the rhythmic intake of regular chow-food. The out of phase between feeding patterns of regular (chow) and highly rewarding food (sugar) may involve alterations of brain circadian oscillators regulating feeding. Here, we report that the fcHFHS diet is a successful model to induce calorie intake, body weight gain and fat tissue accumulation in mice, extending its effectiveness as previously reported in rats. Moreover, we observed that whereas in the SCN the day-night difference in the PER2 clock protein expression was similar between chow-fed and fcHFHS-fed animals, in the LHb, this day-night difference was altered in fcHFHS-exposed animals compared to control chow mice. These findings confirm previous observations in rats showing disrupted daily patterns of feeding behavior under a fcHFHS diet exposure, and extend our insights on the effects of the diet on circadian gene expression in brain clocks.

Circadian neurons in the lateral habenula: Clocking motivated behaviors

The main circadian clock in mammals is located in the hypothalamic suprachiasmatic nucleus (SCN), however, central timing mechanisms are also present in other brain structures beyond the SCN. The lateral habenula (LHb), known for its important role in the regulation of the monoaminergic system, contains such a circadian clock whose molecular and cellular mechanisms as well as functional role are not well known. However, since monoaminergic systems show circadian activity, it is possible that the LHb-clock's role is to modulate the rhythmic activity of the dopamine, serotonin and norephinephrine systems, and associated behaviors. Moreover, the LHb is involved in different pathological states such as depression, addiction and schizophrenia, states in which sleep and circadian alterations have been reported. Thus, perturbations of circadian activity in the LHb might, in part, be a cause of these rhythmic alterations in psychiatric ailments. In this review the current state of the LHb clock and its possible implications in the control of monoaminergic systems rhythms, motivated behaviors (e.g., feeding, drug intake) and depression (with circadian disruptions and altered motivation) will be discussed.