Alarm pheromones as an exponent of emotional state shortly before death—Science fiction or a new challenge?

Attractiveness of illness-associated odorant cues in female rats is modulated by ovarian hormones, but not associated with pro-inflammatory cytokine levels

Odorant cues released by rodents play a key role in mate preference/selection. The goal of the following series of studies was to determine the impact of acute illness, and the potential role of the inflammatory response, on the release of illness-associated odor cues from female rats. Adult female Sprague-Dawley rats were injected with lipopolysaccharide (LPS, 100 μg/kg) and their soiled bedding was used as a stimulus to naïve male odor recipients. While odored bedding from sick males elicited a robust avoidance response evidenced by decreased sniffing, avoidance and burying behavior, odored bedding from sick females elicited only a reduction in sniffing, indicating a reduction in odor attractiveness. Odor cues from ovariectomized, but not sham-operated females decreased sniffing behavior and increased avoidance in male odor recipients. Acute estradiol benzoate (EB, 20 μg/kg) replacement into ovariectomized females restored the investigatory response of male recipients toward odor cues, while LPS administration into ovariectomized oil or EB treated females had little impact on odor attractiveness. Measurement of cytokines in both brain (the paraventricular nucleus of the hypothalamus) and blood from female odor donors indicated increased expression of TNF-α, IL-1β, and IL-6 following LPS, which was not affected by EB treatment. These findings illustrate a critical sexual dimorphism by demonstrating that acute illness reduces the attractiveness of female odor, whereas odor cues from sick males are highly aversive. Moreover, the attractiveness of female odor appears to be associated with circulating ovarian hormone levels, but not central or peripheral inflammatory cytokines.

Olfactory and tissue markers of fear in mammals including humans

Pheromones are a mysterious world of chemical signals involved in conspecific communication. They play a number of key functions important for preservation of life of individual organisms, for their defence, survival of offspring and preservation of species. The best-known groups of pheromones include: trail pheromones, territorial pheromones, sex pheromones, aggregation pheromones, dispersion pheromones, repellent pheromones, social pheromones and alarm pheromones. Alarm pheromones are pheromones that are emitted by animals in threatening situations and inform members of the same species of danger. The identified alarm pheromones are synthesised by insects and aquatic organisms. Also humans are able to emit and perceive pheromones. Although alarm pheromones have not been isolated and identified in man so far, there is presumably evidence for their presence in humans. Pinpointing human alarm pheromones, determinants of experienced stress and inductors of provoked fear could have widespread consequences. Their identification could also be of significant importance for the practical utilisation of results by institutions responsible for safety and defence as well as law enforcement/crime detection and antiterrorist activities.

Chemical communication and mother-infant recognition

Fifty years after the term "pheromone" was coined by Peter Karlson and Martin Lüsher the search for these semiochemicals is still an elusive goal of chemical ecology and communication studies. Contrary to what appears in the popular press, the race is still on to capture and define human scents. Over the last several years, it became increasingly clear that pheromone-like chemical signals probably play a role in offspring identification and mother recognition. Recently, we analyzed the volatile compounds in sweat patch samples collected from the para-axillary and nipple-areola regions of women during pregnancy and after childbirth. We hypothesized that, at the time of birth and during the first weeks of life, the distinctive olfactory pattern of the para-axillary area is probably useful to newborns for recognizing and distinguishing their own mother, whereas the characteristic pattern of the nipple-areola region is probably useful as a guide to nourishment.

The role of neuroinflammation in the release of aversive odor cues from footshock-stressed rats: Implications for the neural mechanism of alarm pheromone

Stressed animals have been known to release aversive chemosignals toward which conspecifics show avoidance-like responses. The present studies assessed whether inflammatory cytokine responses provoked by footshock stress modulate odor signals released from male rats. Male rats were exposed to 30min of intermittent footshock (60 shocks, 1.0mA, 100ms each, variable ITI of 30s) or remained in their home cages as non-stressed controls. Real time RT-PCR analysis of brain tissues indicated that footshock increased the pro-inflammatory cytokine, IL-1β and hnCRH as well as c-fos mRNA expressions in the paraventricular nucleus, and the bed nucleus of the stria terminalis, and increased plasma corticosterone levels. Soiled bedding collected from rats exposed to 30-min, but not 5-min, of footshock elicited a differential response, as expressed by decreased sniffing and increased avoidance in male test subjects. Soiled bedding from rats given corticosterone injection (s.c. 1.25 or 3.75mg/ml) 3h before bedding collection evoked no avoidance response in odor-recipients. Furthermore, ICV infusion of the anti-inflammatory cytokine IL-10 (20 or 200ng) into the stimulus animals 30-min before a 30-min footshock session, had no effect on plasma corticosterone levels in the stimulus animals, but attenuated the release of aversive odor as indicated by dose-dependently diminished avoidance in odor-recipient rats. These results demonstrated that stressed rats release odorant cues that cause other rats to move away from the source of the signal. Such stress-induced chemosignals may be mediated by inflammatory cytokine responses in the brain.

Alarm pheromones-chemical signaling in response to danger

Many animals respond to the threat of predation by producing alarm signals that warn other individuals of the presence of danger or otherwise reduce the success of predators. While alarm signals may be visual or auditory as well as chemical, alarm pheromones are common, especially among insects and aquatic organisms. Plants too emit chemical signals in response to attack by insect herbivores that recruit the herbivores' natural enemies and can induce preparations for defense in neighboring plants (or other parts of the same plant). In this chapter, we discuss our current understanding of chemical alarm signaling in a variety of animal groups (including social and presocial insects, marine invertebrates, fish, and mammals) and in plants. We also briefly discuss the exploitation of alarm pheromones as foraging cues for natural enemies. We conclude with a brief discussion of the potential exploitation of alarm signaling to achieve the applied goal of managing pest species.

Grueneberg ganglion cells mediate alarm pheromone detection in mice

Alarm pheromones (APs) are widely used throughout the plant and animal kingdoms. Species such as fish, insects, and mammals signal danger to conspecifics by releasing volatile alarm molecules. Thus far, neither the chemicals, their bodily source, nor the sensory system involved in their detection have been isolated or identified in mammals. We found that APs are recognized by the Grueneberg ganglion (GG), a recently discovered olfactory subsystem. We showed with electron microscopy that GG neurons bear primary cilia, with cell bodies ensheathed by glial cells. APs evoked calcium responses in GG neurons in vitro and induced freezing behavior in vivo, which completely disappeared when the GG degenerated after axotomy. We conclude that mice detect APs through the activation of olfactory GG neurons.