The development of sniffing

Abstract

Sniffing is a commonly displayed behavior in rodents, yet how this important behavior adjusts throughout development to meet the sensory demands of the animals has remained largely unexplored. In this issue of Chemical Senses, Boulanger-Bertolus et al. investigates the ontogeny of odor-evoked sniffing through a longitudinal study of rats engaged in several olfactory paradigms from infancy to adulthood. The results of this study yield a cohesive picture of sniffing behavior across three developmental stages, while also providing direct comparisons within subjects between these timepoints. As we discuss herein, these results advance the field in relation to existing literature on the development of odor-evoked sniffing behavior in several important ways.

Sniffing, or the rapid inspiration and expiration of air through the nose, which often coincides with the rhythmic coordination of head, nose, and whisker movements, is a ubiquitously displayed behavior among terrestrial vertebrates, including rodents. This behavior is considered important for exploring olfactory scenes and olfactory perception (Mainland and Sobel 2006; Wachowiak 2011). Sniffing can also be used as a behavioral readout of olfactory performance as well as affective and behavioral states (Clarke and Trowill 1971; Hegoburu et al. 2011; Wesson et al. 2008). While the above understandings stem largely from studies in adult rodents, we know very little regarding how sniffing behavior differs across the lifespan.

In this issue of Chemical Senses, Boulanger-Bertolus et al. investigate the ontogeny of sniffing through the first longitudinal study of rats examined from infancy through adulthood (Boulanger-Bertolus et al. 2023). To accomplish this, the authors employed a powerful combination of several well-established olfactory paradigms to assess sniffing in the context of olfactory novelty, habituation, and conditioned affective responses at three different age points. These included infants (postnatal day [P] 12–15), juveniles (P22–24), and adults (P > 75). The authors first confirmed that odors were perceived, since rats across all age groups increased their respiratory frequency or sniffing in response to odor presentation, with peak frequencies reaching 10 Hz, 8 Hz, and 7 Hz in adults, juveniles, and infants, respectively. Interestingly, the latencies to reach peak frequency differed between age groups, as adults and juveniles reached peak frequency in 9–10 s while infants took 17 s. Upon analyzing the time period following odor offset, the authors found that while adults and juveniles slowed their respiratory response to pre-odor rates relatively quickly, infants continued to engage in high frequency sniffing for a prolonged period of time. These results track with previous work exemplifying that infants display heighted arousal to novel stimuli and environments (Alberts and May 1980a; Bolles and Woods 1964; Campbell and Spear 1972). Therefore, the results of this study provide a potential measure whereby sniffing can be used as a behavioral readout of exploration, novelty, and arousal even at an age where fine motor control is limited.

Upon examination of sniffing responses to repeated presentations of a neutral odor, the authors found that the ­odor-induced sniffing response in older rats takes longer to habituate. Interestingly, while infants show sustained investigatory sniffing to the first presentation of an odor, this response is absent upon subsequent presentations, suggesting that infants identify an odor as familiar more rapidly than juveniles and adults.

When odors are conditioned to predict an aversive stimulus such as a shock, infant, and adult rats display a modest but consistent increase in respiratory frequency upon repeated odor presentations. This result is to be expected as the animals learn, and the odor acquires salience as an alarm signal. Interestingly, however, while juvenile rats display a similar increase in respiratory frequency upon initial odor presentation, this response quickly subsides. Why then is this increase not seen in juveniles? Importantly, previous work from the Mouly lab has confirmed that juvenile rats have no deficits in learning the odor-shock pairing (Boulanger-Bertolus et al. 2014). Instead, as the authors detail, the ontogeny of contextual fear learning may be at play. Since contextual fear learning emerges around P17–23 (Brasser and Spear 1998; Raineki et al. 2009; Sullivan et al. 2000), during odor-shock conditioning juvenile rats learn that not only the odor cues, but also their environment is predictive of shock. This may ultimately lead to a conflicting response in their respiratory response and a loss of sniffing to the odor cue. Regardless of age, however, all rats show an increase in respiratory frequency upon being shocked, indicating that it is perceived similarly.

While few studies have investigated with such technical precision and longitude the ontogeny of sniffing as Boulanger-Bertolus et al., other reports have highlighted the developmental onset of high frequency sniffing behavior. Work by Alberts and May (1980) investigated sniffing in rats from P1 to P20, and reported that high frequency sniffing (>4 Hz) is rare and unsustainable within the first week of life (Alberts and May 1980a). Between P11 and P20, however, quintessential “sniffing” begins to emerge, and it is during this time period where rats begin to showcase the coordinated control of head, nose, and whisker movements during periods of rhythmic polypnea as eloquently detailed by Welker in his seminal 1964 study (Alberts and May 1980a; Welker 1964). More recent work by Zhang et al. confirmed the early findings of Alberts and May, reporting that high frequency sniffing is largely absent until P15 (Zhang et al. 2021). Importantly, Zhang et al. also examined olfactory cortex network activity across this same age range by monitoring local field potentials. Zhang et al. found that whereas 10–15 Hz oscillations are predominantly present from birth through P15 in the ­piriform cortex, adult-like high frequency beta and gamma oscillations quickly appear by P15–P16, aligning with the emergence of adult-like sniffing patterns (Zhang et al. 2021).

While the work of Boulanger-Bertolus et al. covered the entire developmental span of the rat, an understudied developmental stage hidden within this span is that of adolescence. During this pivotal developmental period, the brain undergoes a series of rather drastic changes with subsequent alterations in social and cognitive behaviors (Spear 2000). The olfactory system similarly experiences dynamic changes during this time period, and olfactory preferences and sensitivity differ between children, adolescents, and adults (Alberts and May, 1980b; Apfelbach et al. 1991; Hummel et al. 2011; Kraemer and Apfelbach 2004). Furthermore, variations in odor sensitivity exist even between early and late stage adolescence (Herz et al. 2020). It is interesting to consider whether odor-induced sniffing behavior during adolescence differs from that seen in infants, juveniles, and adults.

Undoubtedly, the results of this study by Boulanger-Bertolus et al. lay a solid foundation in the ontogeny of sniffing. It is important to note that alterations in sniffing behavior also occur in olfactory-independent contexts, such as social interactions (Wesson 2013), during anticipation of a reward (Clarke and Trowill 1971; Ikemoto and Panksepp 1994), and even in response to auditory or visual stimuli (Wesson et al. 2008). Based on this, sniffing can serve as a valuable behavioral readout of not only olfactory performance, but also arousal and motivational state. Therefore, having an in-depth understanding of the ontogeny of this behavior can better inform work utilizing sniffing in both olfactory and non-olfactory based studies at any age point.