An open-source lickometer and microstructure analysis program

High-throughput low-cost digital lickometer system for the assessment of licking behaviours in mice

BACKGROUND

Proper hydration is essential for maintaining health and supports various biological processes, including temperature regulation, immune function, nutrient delivery, and organ function. Visual assessment has traditionally been used to quantify liquid intake, although technological advances in optical and electrical sensors now offer higher accuracy and larger potential for automatic operation with millisecond precision and individual lick resolution.

NEW METHOD

We describe an inexpensive electronic sensor board to monitor mouse licking behavior. The system is equipped with integrated filtering and data preprocessing steps. It measures lick count, frequency, width and interlick intervals with high resolution, allowing the real-time monitoring of complex licking patterns in several mice in their respective home cages over prolonged periods.

RESULTS

Our lickometer provides two-millisecond resolution, efficiently detecting variations in licking behaviors in mice. The system is adapted to monitor licking behaviors in up to 12 mice simultaneously. Lick count, duration and interlick intervals, along with preference for sweet water were monitored over two days, revealing variations in licking patterns across light and dark phases extended over prolonged periods.

COMPARISON WITH EXISTING METHODS

Our lickometer allows for monitoring licking behaviors and dynamics. It can be adapted to conventional mouse cages using electrical circuits. It is open-source, cost-effective, efficient, and can be utilized in real-time for large cohorts, representing an ideal tool for studying ingestive dynamics in different environmental and pathological contexts.

CONCLUSION

We have developed a novel, cost-effective, and efficient device to monitor ingestive behaviors in mice. The throughput of our device allows for monitoring several mice simultaneously while it can be applied directly to a conventional mouse cage, simplifying its implementation into pre-existing experimental setups.

High-Precision Optical Fiber-Based Lickometer

Quantifying and analyzing licking behavior can offer valuable insights into fundamental neurobiological mechanisms controlling animal consummatory behaviors. Lickometers are typically based on electrical properties, a strategy that comes with limitations, including susceptibility to electrical interference and generation of electrical disturbances in electrophysiological measurements. While optical lickometers offer an alternative method to measure licks and quantify fluid intake in animals, they are prone to false readings and susceptibility to outside light sources. To overcome this problem, we propose a low-cost open-source lickometer that combines a restricted infrared beam defined by optical fibers, with a poke design that allows easy access to the tongue while limiting access of other body parts and external light sources. This device also includes features for detecting nose pokes and presenting visual cues during behavioral tasks. We provide validation experiments that demonstrate the optical lickometer's reliability, high-sensitivity and precision, and its application in a behavioral task, showcasing the potential of this tool to study lick microstructure in combination with other techniques, such as imaging of neural activity, in freely moving mice.

AgRP neuron activity promotes associations between sensory and nutritive signals to guide flavor preference

OBJECTIVE

The learned associations between sensory cues (e.g., taste, smell) and nutritive value (e.g., calories, post-ingestive signaling) of foods powerfully influences our eating behavior [1], but the neural circuits that mediate these associations are not well understood. Here, we examined the role of agouti-related protein (AgRP)-expressing neurons - neurons which are critical drivers of feeding behavior [2; 3] - in mediating flavor-nutrient learning (FNL).

METHODS

Because mice prefer flavors associated with AgRP neuron activity suppression [4], we examined how optogenetic stimulation of AgRP neurons during intake influences FNL, and used fiber photometry to determine how endogenous AgRP neuron activity tracks associations between flavors and nutrients.

RESULTS

We unexpectedly found that tonic activity in AgRP neurons during FNL potentiated, rather than prevented, the development of flavor preferences. There were notable sex differences in the mechanisms for this potentiation. Specifically, in male mice, AgRP neuron activity increased flavor consumption during FNL training, thereby strengthening the association between flavors and nutrients. In female mice, AgRP neuron activity enhanced flavor-nutrient preferences independently of consumption during training, suggesting that AgRP neuron activity enhances the reward value of the nutrient-paired flavor. Finally, in vivo neural activity analyses demonstrated that acute AgRP neuron dynamics track the association between flavors and nutrients in both sexes.

CONCLUSIONS

Overall, these data (1) demonstrate that AgRP neuron activity enhances associations between flavors and nutrients in a sex-dependent manner and (2) reveal that AgRP neurons track and rapidly update these associations. Taken together, our findings provide new insight into the role of AgRP neurons in assimilating sensory and nutritive signals for food reinforcement.

AgRP neuron activity promotes associations between sensory and nutritive signals to guide flavor preference

Objective

The learned associations between sensory cues (e.g., taste, smell) and nutritive value (e.g., calories, post-ingestive signaling) of foods powerfully influences our eating behavior [1], but the neural circuits that mediate these associations are not well understood. Here, we examined the role of agouti-related protein (AgRP)-expressing neurons - neurons which are critical drivers of feeding behavior [2; 3] - in mediating flavor-nutrient learning (FNL).

Methods

Because mice prefer flavors associated with AgRP neuron activity suppression [4], we examined how optogenetic stimulation of AgRP neurons during intake influences FNL, and used fiber photometry to determine how endogenous AgRP neuron activity tracks associations between flavors and nutrients.

Results

We unexpectedly found that tonic activity in AgRP neurons during FNL potentiated, rather than prevented, the development of flavor preferences. There were notable sex differences in the mechanisms for this potentiation. Specifically, in male mice, AgRP neuron activity increased flavor consumption during FNL training, thereby strengthening the association between flavors and nutrients. In female mice, AgRP neuron activity enhanced flavor-nutrient preferences independently of consumption during training, suggesting that AgRP neuron activity enhances the reward value of the nutrient-paired flavor. Finally, in vivo neural activity analyses demonstrated that acute AgRP neuron dynamics track the association between flavors and nutrients in both sexes.

Conclusions

Overall, these data (1) demonstrate that AgRP neuron activity enhances associations between flavors and nutrients in a sex-dependent manner and (2) reveal that AgRP neurons track and update these associations on fast timescales. Taken together, our findings provide new insight into the role of AgRP neurons in assimilating sensory and nutritive signals for food reinforcement.

LIQ HD (Lick Instance Quantifier Home Cage Device): An Open-Source Tool for Recording Undisturbed Two-Bottle Drinking Behavior in a Home Cage Environment

Investigation of rodent drinking behavior has provided insight into drivers of thirst, circadian rhythms, anhedonia, and drug and ethanol consumption. Traditional methods of recording fluid intake involve weighing bottles, which is cumbersome and lacks temporal resolution. Several open-source devices have been designed to improve drink monitoring, particularly for two-bottle choice tasks. However, beam-break sensors lack the ability to detect individual licks for bout microstructure analysis. Thus, we designed LIQ HD (Lick Instance Quantifier Home cage Device) with the goal of using capacitive sensors to increase accuracy and analyze lick microstructure, building a device compatible with ventilated home cages, increasing scale with prolonged undisturbed recordings, and creating a design that is easy to build and use with an intuitive touchscreen graphical user interface. The system tracks two-bottle choice licking behavior in up to 18 rodent cages, or 36 single bottles, on a minute-to-minute timescale controlled by a single Arduino microcontroller. The data are logged to a single SD card, allowing for efficient downstream analysis. LIQ HD accuracy was validated with sucrose, quinine, and ethanol two-bottle choice tasks. The system measures preference over time and changes in bout microstructure, with undisturbed recordings tested up to 7 d. All designs and software are open-source to allow other researchers to build on the system and adapt LIQ HD to their animal home cages.

Balancing the scale

Traditional discussions involving 'basic' and 'applied' behavioral research often focus on the differences, or gaps, between these areas. They take place in different environments, use different methods, ask different questions, and have different objectives. Applied animal behavior is no exception. Focusing on the differences in these areas is to the detriment of a cohesive and complete understanding of animal behavior. This paper instead focuses on the similarities between these two sides, and presents them as a matter of scale. A series of real-life examples experienced by the authors is used to highlight how the skills and knowledge of both the applied and the basic sides are valuable and necessary to not only further both fields independently, but to develop a comprehensive understanding of animal behavior.

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.

Optogenetic Stimulation of Type I GAD65+ Cells in Taste Buds Activates Gustatory Neurons and Drives Appetitive Licking Behavior in Sodium-Depleted Mice

Mammalian taste buds are comprised of specialized neuroepithelial cells that act as sensors for molecules that provide nutrition (e.g., carbohydrates, amino acids, and salts) and those that are potentially harmful (e.g., certain plant compounds and strong acids). Type II and III taste bud cells (TBCs) detect molecules described by humans as "sweet," "bitter," "umami," and "sour." TBCs that detect metallic ions, described by humans as "salty," are undefined. Historically, type I glial-like TBCs have been thought to play a supportive role in the taste bud, but little research has been done to explore their role in taste transduction. Some evidence implies that type I cells may detect sodium (Na⁺) via an amiloride-sensitive mechanism, suggesting they play a role in Na⁺ taste transduction. We used an optogenetic approach to study type I TBCs by driving the expression of the light-sensitive channelrhodopsin-2 (ChR2) in type I GAD65⁺ TBCs of male and female mice. Optogenetic stimulation of GAD65⁺ TBCs increased chorda tympani nerve activity and activated gustatory neurons in the rostral nucleus tractus solitarius. "N neurons," whose NaCl responses were blocked by the amiloride analog benzamil, responded robustly to light stimulation of GAD65⁺ TBCs on the anterior tongue. Two-bottle preference tests were conducted under Na⁺-replete and Na⁺-deplete conditions to assess the behavioral impact of optogenetic stimulation of GAD65⁺ TBCs. Under Na⁺-deplete conditions GAD65-ChR2-EYFP mice displayed a robust preference for H₂O illuminated with 470 nm light versus nonilluminated H₂O, suggesting that type I glial-like TBCs are sufficient for driving a behavior that resembles Na⁺ appetite.SIGNIFICANCE STATEMENT This is the first investigation on the role of type I GAD65⁺ taste bud cells (TBCs) in taste-mediated physiology and behavior via optogenetics. It details the first definitive evidence that selective optogenetic stimulation of glial-like GAD65⁺ TBCs evokes neural activity and modulates behavior. Optogenetic stimulation of GAD65⁺ TBCs on the anterior tongue had the strongest effect on gustatory neurons that responded best to NaCl stimulation through a benzamil-sensitive mechanism. Na⁺-depleted mice showed robust preferences to "light taste" (H₂O illuminated with 470 nm light vs nonilluminated H₂O), suggesting that the activation of GAD65⁺ cells may generate a salt-taste sensation in the brain. Together, our results shed new light on the role of GAD65⁺ TBCs in gustatory transduction and taste-mediated behavior.

Optogenetic Stimulation of Type I GAD65+ Cells in Taste Buds Activates Gustatory Neurons and Drives Appetitive Licking Behavior in Sodium-Depleted Mice

Mammalian taste buds are comprised of specialized neuroepithelial cells that act as sensors for molecules that provide nutrition (e.g., carbohydrates, amino acids, and salts) and those that are potentially harmful (e.g., certain plant compounds and strong acids). Type II and III taste bud cells (TBCs) detect molecules described by humans as "sweet," "bitter," "umami," and "sour." TBCs that detect metallic ions, described by humans as "salty," are undefined. Historically, type I glial-like TBCs have been thought to play a supportive role in the taste bud, but little research has been done to explore their role in taste transduction. Some evidence implies that type I cells may detect sodium (Na⁺) via an amiloride-sensitive mechanism, suggesting they play a role in Na⁺ taste transduction. We used an optogenetic approach to study type I TBCs by driving the expression of the light-sensitive channelrhodopsin-2 (ChR2) in type I GAD65⁺ TBCs of male and female mice. Optogenetic stimulation of GAD65⁺ TBCs increased chorda tympani nerve activity and activated gustatory neurons in the rostral nucleus tractus solitarius. "N neurons," whose NaCl responses were blocked by the amiloride analog benzamil, responded robustly to light stimulation of GAD65⁺ TBCs on the anterior tongue. Two-bottle preference tests were conducted under Na⁺-replete and Na⁺-deplete conditions to assess the behavioral impact of optogenetic stimulation of GAD65⁺ TBCs. Under Na⁺-deplete conditions GAD65-ChR2-EYFP mice displayed a robust preference for H₂O illuminated with 470 nm light versus nonilluminated H₂O, suggesting that type I glial-like TBCs are sufficient for driving a behavior that resembles Na⁺ appetite.SIGNIFICANCE STATEMENT This is the first investigation on the role of type I GAD65⁺ taste bud cells (TBCs) in taste-mediated physiology and behavior via optogenetics. It details the first definitive evidence that selective optogenetic stimulation of glial-like GAD65⁺ TBCs evokes neural activity and modulates behavior. Optogenetic stimulation of GAD65⁺ TBCs on the anterior tongue had the strongest effect on gustatory neurons that responded best to NaCl stimulation through a benzamil-sensitive mechanism. Na⁺-depleted mice showed robust preferences to "light taste" (H₂O illuminated with 470 nm light vs nonilluminated H₂O), suggesting that the activation of GAD65⁺ cells may generate a salt-taste sensation in the brain. Together, our results shed new light on the role of GAD65⁺ TBCs in gustatory transduction and taste-mediated behavior.

Movement assay for the undergraduate neuroscience laboratory

Described is a design for easy-to-construct apparatus that measures movement of flying insects suitable for the undergraduate teaching laboratory. The system does not require the purchase of specialized scientific equipment or software. The apparatus can be constructed and operated without advanced knowledge in electronics or programming. The goal of this apparatus was to expand upon previous research detecting insect flight in response to radiation. We improved upon the quantification and resolution of flight across differing intensities of white light. All of this was achieved using low-cost and commonly available materials and open-source software to collect and analyze data. The only substantial prerequisites for this design are a PC with a 3.5 mm microphone input and an understanding of basic electrical connections. The apparatus was validated with comparative physiological data from two different species of butterfly.