MicroSweat: A Wearable Microfluidic Patch for Noninvasive and Reliable Sweat Collection Enables Human Stress Monitoring

Stress affects cognition, behavior, and physiology, leading to lasting physical and mental illness. The ability to detect and measure stress, however, is poor. Increased circulating cortisol during stress is mirrored by cortisol release from sweat glands, providing an opportunity to use it as an external biomarker for monitoring internal emotional state. Despite the attempts at using wearable sensors for monitoring sweat cortisol, there is a lack of reliable wearable sweat collection devices that preserve the concentration and integrity of sweat biomolecules corresponding to stress levels. Here, a flexible, self-powered, evaporation-free, bubble-free, surfactant-free, and scalable capillary microfluidic device, MicroSweat, is fabricated to reliably collect human sweat from different body locations. Cortisol levels are detected corresponding to severe stress ranging from 25 to 125 ng mL^-1 averaged across multiple body regions and 100-1000 ng mL^-1 from the axilla. A positive nonlinear correlation exists between cortisol concentration and stress levels quantified using the perceived stress scale (PSS). Moreover, owing to the sweat variation in response to environmental effects and physiological differences, the longitudinal and personalized profile of sweat cortisol is acquired, for the first time, for various body locations. The obtained sweat cortisol data is crucial for analyzing human stress in personalized and clinical healthcare sectors.

Open-source, cost-effective system for low-light in vivo fiber photometry

Fiber photometry uses genetically encoded optical reporters to link specific cellular activity in stereotaxically targeted brain structures to specific behaviors. There are still a number of barriers that have hindered the widespread adoption of this approach. This includes cost, but also the high-levels of light required to excite the fluorophore, limiting commercial systems to the investigation of short-term transients in neuronal activity to avoid damage of tissue by light. Here, we present a cost-effective optoelectronic system for in vivo fiber photometry that achieves high-sensitivity to changes in fluorescence intensity, enabling detection of optical transients of a popular calcium reporter with excitation powers as low as 100 nW. By realizing a coherent detection scheme and by using a photomultiplier tube as a detector, the system demonstrates reliable study of in vivo neuronal activity, positioning it for future use in the experiments inquiring into learning and memory processes. The system was applied to study stress-evoked calcium transients in corticotropin-releasing hormone neurons in the mouse hypothalamus.

Canadian Association of Neurosciences Review: Learning at a Snail's Pace

While learning and memory are related, they are distinct processes each with different forms of expression and underlying molecular mechanisms. An invertebrate model system, Lymnaea stagnalis, is used to study memory formation of a non-declarative memory. We have done so because: 1) We have discovered the neural circuit that mediates an interesting and tractable behaviour; 2) This behaviour can be operantly conditioned and intermediate-term and long-term memory can be demonstrated; and 3) It is possible to demonstrate that a single neuron in the model system is a necessary site of memory formation. This article reviews how Lymnaea has been used in the study of behavioural and molecular mechanisms underlying consolidation, reconsolidation, extinction and forgetting.

The participation of NMDA receptors, PKC, and MAPK in the formation of memory following operant conditioning in Lymnaea

Background

Memory is the ability to store, retain, and later retrieve information that has been learned. Intermediate term memory (ITM) that persists for up to 3 h requires new protein synthesis. Long term memory (LTM) that persists for at least 24 h requires: DNA transcription, RNA translation, and the trafficking of newly synthesized proteins. It has been shown in a number of different model systems that NMDA receptors, protein kinase C (PKC) and mitogen activated protein kinase (MAPK) are all involved in the memory formation process.

Results

Here we show that snails trained in control conditions are capable of forming, depending on the training procedure used, either ITM or LTM. However, blockage of NMDA receptors (MK 801), inhibition of PKC (GF109203X hydrochloride) and MAPK activity (UO126) prevent the formation of both ITM and LTM.

Conclusions

The injection of either U0126 or GF109203X, which inhibit MAPK and PKC activity respectively, 1 hour prior to training results in the inhibition of both ITM and LTM formation. We further found that NMDA receptor activity was necessary in order for both ITM and LTM formation.

Ecologically relevant stressors modify long-term memory formation in a model system

Stress can alter adaptive behaviours, and as well either enhance or diminish learning, memory formation and/or memory recall. We focus attention on how environmentally relevant stressors (e.g. predator detection, crowding, and low concentrations of environmental Ca(++)) alter memory formation in the pond snail, Lymnaea stagnalis. We specifically look at operant conditioning of aerial respiration and whether or not long-term memory forms following the acquisition of the learned event, not performing aerial respiration. We will also examine the strain differences in Lymnaea which allow or cause isolated populations to possess different heritable cognitive capabilities, as manifested by differing abilities to form long-term memory.

A quantitative proteomic analysis of long-term memory

BACKGROUND

Memory is the ability to store, retain, and later retrieve learned information. Long-term memory (LTM) formation requires: DNA transcription, RNA translation, and the trafficking of newly synthesized proteins. Several components of these processes have already been identified. However, due to the complexity of the memory formation process, there likely remain many yet to be identified proteins involved in memory formation and persistence.

RESULTS

Here we use a quantitative proteomic method to identify novel memory-associated proteins in neural tissue taken from animals that were trained in vivo to form a long-term memory. We identified 8 proteins that were significantly up-regulated, and 13 that were significantly down-regulated in the LTM trained animals as compared to two different control groups. In addition we found 19 proteins unique to the trained animals, and 12 unique proteins found only in the control animals.

CONCLUSIONS

These results both confirm the involvement of previously identified memory proteins such as: protein kinase C (PKC), adenylate cyclase (AC), and proteins in the mitogen-activated protein kinase (MAPK) pathway. In addition these results provide novel protein candidates (e.g. UHRF1 binding protein) on which to base future studies.

The perception of stress alters adaptive behaviours in Lymnaea stagnalis

Stress can alter adaptive behaviours, and as well either enhance or diminish learning, memory formation and/or memory recall. We show here that two different stressors have the ability to alter such behaviours in our model system, Lymnaea stagnalis. One, a naturally occurring stressor - the scent of a predator (crayfish) - and the other an artificially controlled one - 25 mmol l(-1) KCl - significantly alter adaptive behaviours. Both the KCl stressor and predator detection enhance long-term memory (LTM) formation; additionally predator detection alters vigilance behaviours. The predator-induced changes in behaviour are also accompanied by specific and significant alterations in the electrophysiological properties of RPeD1 - a key neuron in mediating both vigilance behaviours and memory formation. Naive lab-bred snails exposed to crayfish effluent (CE; i.e. the scent of the predator) prior to recording from RPeD1 demonstrated both a significantly reduced spontaneous firing rate and fewer bouts of bursting activity compared with non-exposed snails. Importantly, in the CE experiments we used laboratory-reared snails that have not been exposed to a naturally occurring predator for over 250 generations. These data open a new avenue of research, which may allow a direct investigation from the behavioral to the neuronal level as to how relevant stressful stimuli alter adaptive behaviours, including memory formation and recall.

Modulation of aerial respiratory behaviour in a pond snail

Aerial respiratory in Lymnaea is driven by a three-neuron CPG whose sufficiency and necessity has been directly demonstrated. While this CPG is 'hard-wired' it displays a tremendous amount of plasticity. That is, it is possible by employing specific training procedures to alter how it functions in a specific hypoxic environment. Thus, it is possible to study directly the causal mechanisms of long-term memory formation, forgetting, and modulation of the memory at a single cell level. Thus, it is possible to use a relatively simple three-neuron CPG to study not only important questions concerning regulation of important homeostatic mechanisms but to also use it to study how learning and non-declarative memory are mediated at a cellular level.

Operant conditioning of an in vitro CNS-pneumostome preparation of Lymnaea

Operant conditioning of aerial respiratory behaviour and its consolidation into long-term memory in Lymnaea has been previously studied in both intact, freely moving snails and in in vitro preparations made from previously trained snails. Here, we show in previously untrained semi-intact in vitro Lymnaea preparations that aerial respiratory behaviour can also be operantly conditioned. Neither yoked control nor 'run-down' control procedures in these in vitro preparations result in an alteration of aerial respiratory behaviour. Memory in the operantly trained semi-intact preparations persists for at least 1h after training. Intracellular recordings made from RPeD1, one of the 3-CPG neurons and the neuron that initiates CPG activity; show that there are specific changes in central excitatory input to this neuron concurrent with learning and its consolidation into memory. In addition following the acquisition of learning and its consolidation into memory the ability of RPeD1 and VI/J neurons when depolarized to cause a pneumostome opening is significantly decreased. Thus, previously untrained in vitro semi-intact preparations can be used to study changes in neuronal activity in a neuron known to be both necessary for the behaviour and for memory formation.

Learning and memory inLymnaeaare negatively altered by acute low-level concentrations of hydrogen sulphide

Hydrogen sulphide (H2S) is a common industrial pollutant as well as an endogenous neural transmitter/neural modulator. Experiments were performed on the pond snail Lymnaea stagnalis to determine the acute effects of low-level exposure to H2S (50–100 μmol l–1) on aerial respiratory behaviour, associative learning,and its subsequent consolidation into long-term memory (LTM). A 3-neuron network whose sufficiency and necessity have been demonstrated drives aerial respiratory behaviour in Lymnaea. In the presence of 100 μmol l–1 H2S the number of bouts of aerial respiration and the total breathing time were significantly increased compared to the control hypoxic situation, but were equivalent to those observed in snails that had been subjected to a `more intense hypoxic challenge'. In addition, at a concentration of 100 μmol l–1 H2S neither associative learning nor long-term memory (LTM) were observed. However, snails subjected to a `more intense hypoxic challenge' still had the capacity to learn and form LTM. These snails, in fact, showed statistically the best learning and memory performance of any group. While learning and memory were observed at 50 and 75 μmol l–1 H2S,respectively, they were statistically poorer than the learning and memory exhibited by snails in the standard hypoxia condition. Hence the ability to learn and form memory was compromised by H2S. Thus an invertebrate model system with a well-defined neural network can be used to study of the effects of H2S on the processes of learning and memory.

A molluscan model system in the search for the engram

A 3-neuron central pattern generator, whose sufficiency and necessity has been directly demonstrated, mediates aerial respiratory behaviour in the pond snail, Lymnaea stagnalis. This behaviour can be operantly conditioned, and this associative learning is consolidated into long-lasting memory. Depending on the operant conditioning training procedure used the learning can be consolidated into intermediate term (ITM) or long-term memory (LTM). ITM persists for only 2-3 h, whilst LTM persists for days to weeks. LTM is dependent on both altered gene activity and new protein synthesis while ITM is only dependent on new protein synthesis. We have now directly established that one of the 3-CPG neurons, RPeD1, is a site of LTM formation and storage. We did this by ablating the soma of RPeD1 and leaving behind a functional primary neurite capable of mediating the necessary synaptic interactions to drive aerial respiratory behaviour by the 3-neuron CPG. However, following soma ablation the neuronal circuit is only capable of mediating learning and ITM. LTM can no longer be demonstrated. However, if RPeD1's soma is ablated after LTM consolidation memory is still present. Thus the soma is not needed for the retention of LTM. Using a similar strategy it may be possible to block forgetting.

Memory, Reconsolidation and Extinction in Lymnaea Require the Soma of RPeD1

The central pattern generator (CPG) that drives aerial respiratory behaviour in Lymnaea consists of 3 neurons. One of these, RPeD1--the cell that initiates activity in the circuit, plays an absolutely necessary role as a site for memory formation, memory reconsolidation, and extinction. Using an operant conditioning training procedure that results in a long-term non-declarative memory (LTM), we decrease the occurrence of aerial respiratory behaviour. Since snails can still breathe cutaneously learning this procedure is not harmful. Concomitant with behavioural memory are changes in the spiking activity of RPeD1. Going beyond neural correlates of memory we directly show that RPeD1 is a necessary site for LTM formation. Expanding on this finding we show that this neuron is also a necessary site for memory reconsolidation and 'Pavlovian' extinction. As far as we can determine, this is the first time a single neuron has been shown to be a necessary site for these different aspects memory. RPeD1 is thus a key neuron mediating different hierarchical aspects of memory. We are now in a position to determine the necessary neuronal, molecular and proteomic events in this neuron that are causal to memory formation, reconsolidation and extinction.

Associative learning and memory in Lymnaea stagnalis: how well do they remember?

The search for 'the how and the where' of memory formation in the brain, the engram, is still one of the unattained 'Holy Grails' of neuroscience. Over the years, various paths have been trodden in attempts to attain this goal, and while tantalizing glimpses appear now and then on the scientific horizon, the Grail still has not been grasped. One of the paths that investigators have walked is the invertebrate 'model system' approach. Some invertebrates possess relatively simple nervous systems that mediate relatively simple behaviours that are both interesting and trainable. In this commentary, we would like to shed light on a relatively new player, the pond snail Lymnaea stagnalis L., that is being used in the quest to illuminate 'the how and the where' the nervous systems encode and store memory. We will show that it is possible to demonstrate that a single neuron is a site of memory formation and storage for a form of associative learning in this lowly snail. It may be that the Grail is a little closer to being grasped.