Effects of environmental novelty on fear-related behavior and stress responses of rats to emotionally relevant odors

Sucrose preference test: A systematic review of protocols for the assessment of anhedonia in rodents

Anhedonia is described as a decreased ability to experience rewarding and enjoyable activities, a core symptom of major depressive disorder. The sucrose preference test (SPT) is a widely used and reliable behavioural test to assess anhedonia in rodents, based on a two-bottle choice paradigm. To date, different protocols are in use, inducing variability between researchers and hampering comparisons between studies. We performed a systematic review of the SPT protocols used in 2021 to identify the parameters in which they differ and their potential impact. We searched a total of four databases (PubMed, Scopus, Web of Science and Science Direct), from 1st January 2021 to 31st December 2021, and screened a total of 1066 articles. After screening by title and abstract, a total of 415 articles were included in this review. We extracted and analysed the different procedures used, the type of sweet solution and the habituation, deprivation, and testing protocols. The overall quality of the studies was considered very good, however, SPT protocols were extremely variable between studies with a total of 65 different habituation protocols and 104 combinations of food/water deprivation and preference testing duration. As the SPT is one of the most used tests to assess anhedonia in rodents, this work raises awareness of the great variability in SPT protocols being currently used. Furthermore, we call for standardization in the protocol used, and overall improvement of data reporting of methodologies and results, to increase the consistency between studies and allow a better comparison of results between different labs.

Non-invasive assessment of positive affective state using infra-red thermography in rats

With recent increased focus on positive welfare in animal welfare science, there is demand for objective positive welfare indicators. It is unclear whether changes in body surface temperature can be used to non-invasively identify and quantify positive states in mammals. We recorded continuous measurements of tail surface temperature using infra-red thermography (IRT) and concurrent behavioural observations in male and female Wistar rats (Rattus norvegicus). If tail surface temperature can differentiate between positive and negative experiences, we expect a qualitatively different response compared to negative experiences. Three groups of rats were presented with increasing magnitudes of food rewards (neutral/none, one and three rewards). The rats were placed in an arena to which they were habituated and filmed for 30 s before and 30 min after exposure to different rewards. Tail temperature initially decreased from the pre-reward baseline and subsequently returned towards baseline temperature. The overall pattern of the change was the same as for rats subjected to negative stimuli in previous studies. Nevertheless, dynamic changes in tail temperature, specifically the rate of recovery and the behavioural response (exploration), differed between neutral and rewarded rats but failed to distinguish reward magnitude. Sex differences were found in both thermal and behavioural responses, unrelated to reward magnitudes. Female rats exhibited a greater initial response with a slower recovery than male rats, emphasising the value of using of both sexes in animal welfare research. This study improves our understanding of the effects of positive emotions induced by food reward on peripheral body temperature and behaviour.

Are Predator Smell (TMT)-Induced Behavioral Alterations in Rats Able to Inhibit Seizures?

We aimed to evaluate the chemical and behavioral effects of 2,5-dihydro-2,4,5-trimethylthiazoline (TMT) after olfactory exposure and to verify their influence in the expression of acute audiogenic seizures in the Wistar Audiogenic Rat (WAR) strain. PROTOCOL 1: TMT gas chromatography was applied to define odor saturation in a chamber to different concentrations, time required for saturation and desaturation, and if saturation was homogeneous. Also, male Adult Wistar rats were exposed to saline (SAL) or to different TMT concentrations and their behaviors were evaluated (neuroethology). PROTOCOL 2: Male adult WARs were exposed for 15 s to SAL or TMT, followed by sound stimulation for 1 min or until tonic–clonic convulsion. Behavioral analysis included latencies (wild running and tonic–clonic convulsion), seizure severity indexes, and neuroethology. Gas chromatography established a saturation homogeneous to different concentrations of TMT, indicating that saturation and desaturation occurred in 30 min. TMT triggered fear-like or aversion-like reactions associated with reduction in motor activity and in grooming behavior, in the 2 highest concentrations. Pure TMT presented anticonvulsant properties, such as less-severe seizure phenotype, as well as a decrease in tonic–clonic convulsion expression. TMT elicited fear-like or aversion-like behaviors in Wistar and WAR and can be utilized in a quantifiable and controllable way. Our results suggested possible antagonism between “fear-related” or “aversion-related” and “seizure-related” networks.

Markov Networks of Collateral Resistance: National Antimicrobial Resistance Monitoring System Surveillance Results from Escherichia coli Isolates, 2004-2012

Surveillance of antimicrobial resistance (AMR) is an important component of public health. Antimicrobial drug use generates selective pressure that may lead to resistance against to the administered drug, and may also select for collateral resistances to other drugs. Analysis of AMR surveillance data has focused on resistance to individual drugs but joint distributions of resistance in bacterial populations are infrequently analyzed and reported. New methods are needed to characterize and communicate joint resistance distributions. Markov networks are a class of graphical models that define connections, or edges, between pairs of variables with non-zero partial correlations and are used here to describe AMR resistance relationships. The graphical least absolute shrinkage and selection operator is used to estimate sparse Markov networks from AMR surveillance data. The method is demonstrated using a subset of Escherichia coli isolates collected by the National Antimicrobial Resistance Monitoring System between 2004 and 2012 which included AMR results for 16 drugs from 14418 isolates. Of the 119 possible unique edges, 33 unique edges were identified at least once during the study period and graphical density ranged from 16.2% to 24.8%. Two frequent dense subgraphs were noted, one containing the five β-lactam drugs and the other containing both sulfonamides, three aminoglycosides, and tetracycline. Density did not appear to change over time (p = 0.71). Unweighted modularity did not appear to change over time (p = 0.18), but a significant decreasing trend was noted in the modularity of the weighted networks (p < 0.005) indicating relationships between drugs of different classes tended to increase in strength and frequency over time compared to relationships between drugs of the same class. The current method provides a novel method to study the joint resistance distribution, but additional work is required to unite the underlying biological and genetic characteristics of the isolates with the current results derived from phenotypic data.

Oral administration of green plant-derived chemicals and antioxidants alleviates stress-induced cellular oxidative challenge

BACKGROUND

This study examined the efficacy of the combination antioxidant, Formula 42 (F42), on cellular stress indicators in animal and human models of stress-induced oxidative stress.

METHODS

A sub-chronic psychological stress model in rodents was used to induce stress and oxidative stress indicators over a 10-day period during which animals received oral doses of F42 or water. Following treatment, body weight, plasma stress hormone corticosterone, and oxidative capacity were evaluated. In healthy human subjects, a randomized double-blind crossover study was used to examine the antioxidant effect of F42 or placebo in an exercise-induced oxidative stress model. Erythrocyte and plasma oxidative status was evaluated using the fluorescent activation of 2',7'-dichlorofluorescin (DCF) as an indicator.

RESULTS

Oral administration of F42 reduced the corticosterone response to acute stress compared to vehicle but did not differ at the conclusion of the 10-day study. However, F42 administration did reduce stress-induced growth restriction and alleviate DCF activation in circulating erythrocytes by approximately 10% following 10 days of stress exposure. Oral administration of F42 also significantly reduced DCF activation by approximately 10% in healthy human subjects undergoing exercise-induced oxidative stress.

CONCLUSIONS

Oral administration of F42 in rodents produces transient reductions in stress hormones and reduces stress indicators following sub-chronic psychological stress exposure. In humans, F42 acts as an early and potent antioxidant capable of scavenging free radicals within 30 min of ingestion.

The smell of fear: innate threat of 2,5-dihydro-2,4,5-trimethylthiazoline, a single molecule component of a predator odor

In the last several years, the importance of understanding what innate threat and fear is, in addition to learning of threat and fear, has become evident. Odors from predators are ecologically relevant stimuli used by prey animals as warnings for the presence of danger. Of importance, these odors are not necessarily noxious or painful, but they have innate threat-like properties. This review summarizes the progress made on the behavioral and neuroanatomical fundamentals of innate fear of the predator odor, 2,5-dihydro-2,4,5-trimethylthiazoline (TMT), a component of fox feces. TMT is one of several single molecule components of predator odors that have been isolated in the last several years. Isolation of these single molecules has allowed for rapid advances in delineating the behavioral constraints and selective neuroanatomical pathways of predator odor induced fear. In naïve mice and rats, TMT induces a number of fear and defensive behaviors, including robust freezing, indicating it is an innate threat stimulus. However, there are a number of behavioral constraints that we do not yet understand. Similarly, while some of the early olfactory sensory pathways for TMT-induced fear are being delineated, the pathways from olfactory systems to emotional and motor output regions are less well understood. This review will focus on what we know and what we still need to learn about the behavior and neuroanatomy of TMT-induced fear.

A combination of plant-derived odors reduces corticosterone and oxidative indicators of stress

In this study, we measured typical stress markers in addition to oxidative status and reduced glutathione in erythrocytes, and plasma lipid peroxidation of restraint-stressed animals exposed to a combination of plant-derived odors (0.03% Z-3-hexen-1-ol, 0.03% E-2-hexenal, and 0.015% α-pinene in triethyl citrate). Male Wistar rats aged 6-7 weeks postnatal were exposed to vehicle (triethyl citrate, n = 12), plant-derived odors (n = 12), or 1% propionic acid odor (n = 12) under control or stress conditions, and blood samples were collected. Restraint stress increased plasma glucose and plasma corticosterone concentrations by approximately 10% (P < 0.01) and 125% (P < 0.001), respectively, in vehicle-exposed animals. Similar increases were observed in animals exposed to a 1% propionic acid odor, indicating the novelty of odor exposure does not alter stress responsiveness. There was also an increase of approximately 15% in both erythrocytic oxidative status (P < 0.001) and plasma lipid peroxidation (P < 0.05), and a decrease of approximately the same magnitude in reduced glutathione (P < 0.05) in restrained animals with vehicle exposure. There were no differences observed between control and stress treatment with plant-derived odor exposure in any of the measured parameters. It was concluded that exposure to plant-derived odors reduce corticosterone, glucose, and redox responses elicited by psychological stress.

A pleasant familiar odor influences perceived stress and peripheral nervous system activity during normal aging

Effects of smells on stress have been demonstrated in animals and humans, suggesting that inhaling certain odorants may counteract the negative effects of stress. Because stress plays a key role in cerebral aging, the present study set out to examine whether positive odor effects on perceived stress can be achieved in elderly individuals. To this end, two groups of aged individuals (n = 36 women, aged from 55 to 65 years), were tested. The first group was exposed for 5 days to a pleasant and, by end of exposure, familiar odor (“exposure odor”), whereas the other was exposed to a non-scented control stimulus. Stress and mood states were assessed before and after the 5-day odor exposure period. Psychophysiological markers were also assessed at the end of exposure, in response to the “exposure odor” and to a “new odor.” Results revealed that stress on this second exposure was decreased and zygomatic electromyogram activity was increased specifically in the group previously exposed to the odor (p < 0.05). Taken as a whole, these findings offer a new look at the relationship between perceived stress, olfaction and normal aging, opening up new research perspectives on the effect of olfaction on quality of life and well-being in aged individuals.

Order of exposure to pleasant and unpleasant odors affects autonomic nervous system response

When mammals are exposed to an odor, that odor is expected to elicit a physiological response in the autonomic nervous system. An unpleasant aversive odor causes non-invasive stress, while a pleasant odor promotes healing and relaxation in mammals. We hypothesized that pleasant odors might reduce a stress response previously induced by an aversive predator odor. Rats were thus exposed to pleasant and unpleasant odors in different orders to determine whether the order of odor exposure had an effect on the physiological response in the autonomic nervous system. The first trial examined autonomic nerve activity via sympathetic and parasympathetic nerve response while the second trial examined body temperature response. Initial exposure to a pleasant odor elicited a positive response and secondary exposure to an unpleasant odor elicited a negative response, as expected. However, we found that while initial exposure to an unpleasant odor elicited a negative stress response, subsequent secondary exposure to a pleasant odor not only did not alleviate that negative response, but actually amplified it. These findings were consistent for both the autonomic nerve activity response trial and the body temperature response trial. The trial results suggest that exposure to specific odors does not necessarily result in the expected physiological response and that the specific order of exposure plays an important role. Our study should provide new insights into our understanding of the physiological response in the autonomic nervous system related to odor memory and discrimination and point to areas that require further research.

Comparison between low doses of TMT and cat odor exposure in anxiety- and fear-related behaviors in mice

Few comparisons were made between cat odor and synthetic fox odor (TMT) to study fear and anxiety in rodents. TMT is frequently used are at high concentration while the stimulus should be as possible closer to natural conditions. The aim of this work was to compare behavioral responses of mice exposed to cat odor and low doses of TMT (i.e. 10 μl of a solution containing 1%, 0.1% or 0.01% TMT). Behavioral parameters were recorded in elevated plus-maze and in open field. Results showed that 1% TMT and 0.1% TMT induced similar responses to cat odor, contrary to water and 0.01% TMT which failed to elicit fear or anxiety-related behaviors. Additionally, behavioral changes were more marked in EPM - e.g. time spent in open arms - than in open field - e.g. freezing. These findings are discussed in terms of a possible continuum of mild anxiety-like behaviors to strong fear-like behaviors linked to predator odor intensity.

Different patterns of neuronal activities in the infralimbic and prelimbic cortices and behavioral expression in response to two affective odors, 2,5-dihydro-2,4,5-trimethylthiazoline and a mixture of cis-3-hexenol and trans-2-hexenal, in the freely moving rat

The medial prefrontal cortex (mPFC) is involved in stimulus perception, attentional control, emotional behavior, and the stress response. These functions are thought to be mediated by the infralimbic (IL) and prelimbic (PL) subregions of mPFC; however, few studies have examined the roles of IL and PL cortices in olfactory cognition. In the present study, we investigated the acute effects of two odors, 2,5-dihydro-2,4,5-trimethylthiazoline (TMT) and a mixture of cis-3-hexenol and trans-2-hexenal (green odor: GO), on behavioral responses and IL and PL neuronal activities using extracellular single-unit recordings in a freely moving rat. We found that the total number of spike firings in IL and PL neurons did not change with 10s presentation of odors. TMT presentation induced significant changes in burst firing activity in IL and PL neurons, while GO presentation induced changes in burst firing only in IL neurons. In the temporal profile of the firing activity of IL neurons, TMT exposure induced transient activation and GO exposure induced sustained activation. Those of PL neurons showed sustained activation during TMT exposure and transient activations during GO exposure. GO exposure induced a stretch-attend posture, whereas TMT exposure induced immobility. Furthermore, multiple regression analysis indicated that the property of the odor and neuronal activities of IL and PL regions were correlated with behavioral responses. These findings reveal that olfaction-related neurons exist in IL and PL regions, and that the neurons in these regions might temporarily encode odor information in order to modulate motor outputs by tuning firing properties in the early stage of cognition according to the odor property.