Social interactions and olfactory cues are required for contagious itch in mice

Abstract

The phenomenon of contagious itch, observed in both humans and rodents, remains a topic of ongoing debate concerning its modulators and underlying pathways. This study delves into the relationship between contagious itch and familiar olfactory cues, a non-visual factor contributing to this intriguing behavior. Our findings showed that contagious itch in observer mice occurs during physical interaction with the cagemate itch-demonstrator but not with a stranger demonstrator or in a non-physical encounter condition. Notably, itch-experienced observer mice displayed an increased contagious itch behavior, highlighting the relevance of itch-associated memory in this phenomenon. Furthermore, anosmic observer mice, whether itch-naïve or itch-experienced, displayed no contagious itch behavior. These results demonstrate that the familiar olfactory cues, specifically cagemate body odors, are required for contagious itch behaviors in mice. In line with these behavioral findings, our study reveals increased activity in brain regions associated with olfaction, emotion, and memory during contagious itch, including the olfactory bulb, the amygdala, the hypothalamus, and the hippocampus, with this activity diminished in anosmic mice. In conclusion, our study unveils the critical role of familiar olfactory cues in driving contagious itch in mice, shedding light on the interplay between social factors, sensory perception, and memory in this phenomenon.

Keywords: Contagious itch; Emotional contagion; Olfaction; Olfactory system.

Figure 1

Contagious itch behavior in mice: the role of social modulation and physical encounter with a cagemate. (A) Cagemate mice were habituated for three days before the experiment. Physical Encounter (PE) involved direct contact, Non-Physical Encounter (NPE) utilized a transparent barrier, and the third condition exposed mice to a video of the scratching demonstrator. Bar graphs represent the number of (B) look-scratch and (C) looks exhibited by observer mice observing a scratching cagemate demonstrator that has been injected with the effective dose of C48/80 (100 µg/site) in the physical encounter (PE) and non-physical encounter (NPE) groups relative to their corresponding control (Ctrl) groups injected with normal saline. Significant differences are indicated by asterisks (^**** P < 0.0001, ^xxxx P < 0.0001, and ^* P < 0.05 for comparisons with C48/80 (PE). (D) The familiar group of mice shared a cage for two weeks. In the unfamiliar group, mice from different family cages were habituated separately before the experiment. The mean number of (E) look-scratch and (F) looks in observer mice when placed next to a cagemate or non-cagemate (Correction in all similar positions: non-cagemate) scratching demonstrator. Values are presented as mean ± SEM.

Figure 2

The importance of olfactory cues in itch communication between mice. (A) Anosmia in observer mice was induced by intranasal zinc sulfate post-lidocaine anesthesia for four days. Auditory impairment was achieved with kanamycin (1000 mg/kg) and furosemide. Anosmic (PE) animals exhibited significantly fewer contagious scratching behaviors (^*** P < 0.001 compared to PE controls without anosmia) (B) but a similar number of looking behaviors (C) as the physical encounter (PE) group. The mean number of (D) look-scratch and (E) looks of deaf observer mice relative to the physical encounter (PE) group. Values are presented as mean ± SEM. NS: not significant.

Figure 3

Prior itch experience modulates look-scratch behavior. (A) Primed observer mice, pre-exposed to itch stimuli, with 100 µg/site of C48/80 one day before the experiment. Representative bar graphs show the mean number of (B) look-scratch, (C) looks, and (D) total scratching bouts of primed observer mice in comparison to mice without prior itch experience in the physical encounter (PE) and non-physical encounter (NPE) conditions (^**** P < 0.0001 shows the comparison between primed and non-primed mice in the PE condition). E) Group 1: Observers got no pruritogen, paired with demonstrators receiving an effective C48/80 dose (100 µg/site). Group 2: Both observers and itch demonstrators got a sub-effective C48/80 dose (30 µg/site). Group 3: Observers got the sub-effective dose, while adjacent itch demonstrators received the effective dose (100 µg/site) of C48/80. Representative bar graphs of the mean number of (F) look-scratch, (G) looks, and (H) total scratching bouts of observer mice in different groups. The observer mice injected with the pruritogen exhibited a significant increase in both look-scratch responses compared to the observer that received no pruritogen (^**** P < 0.0001 and ^#### P < 0.0001 pairwise comparisons). The total scratching bouts of pruritogen-injected mice were higher when paired with mice exhibiting scratching behavior (^$$$$ P < 0.0001 compared C48/80 (30 µg/site) injected mice placed next to the demonstrator received 30 or 100 µg/site of C48/80, ^**** P < 0.0001 and ^#### P < 0.0001 compared C48/80-injected mice placed alone or next to a demonstrator with scratching activity. Values are presented as mean ± SEM. NS: not significant.

Figure 4

Olfactory inputs regulate enhanced contagious itch by past and current experiences of itch. (A) The primed anosmic mice exhibited a significant reduction in look-scratch behavior, an effect potentiated in non-anosmic primed mice (^**** P < 0.0001 pairwise comparisons). The number of looks (B) and total scratching bouts (C) are comparable between the groups. Itch priming was induced by the injection of 100 µg/site of C48/80 one day before the designated experimental day. (D) Anosmic mice with concurrent experience of itch induced by the sub-effective dose of C48/80 exhibited significantly lower counts of look-scratch responses but a comparable number of (E) looks and (F) total scratching bouts compared to their non-anosmic counterparts. Values are presented as mean ± SEM.

Figure 5

Activated brain regions in contagious scratching behavior. (A–F) Western blot depicting c-Fos protein expression in various brain regions of observer mice after physical (PE) and non-physical encounters (NPE) with scratching demonstrators, as well as anosmic observers exposed to adjacent itch demonstrators (n = 4). (G) Illustration representing the findings of our experiment. The contagious itch can be seen in mice models of the same sex, age, strain, and home cage while the olfactory system is intact, and the physical encounter is not restrained. This behavior is strengthened in mice with previous itch exposure and originates from empathetic behaviors. The olfactory bulb (containing the main olfactory bulb (MOB) and the accessory olfactory bulb (AOB)), hippocampus (HPC), thalamus (TH), amygdala (AG), and hypothalamus (HTH) are involved in the signal transmission of contagious itch behavior. Created with BioRender.com.