Introduction

“I should think we might fairly gauge the future of biological science, centuries ahead, by estimating the time it will take to reach a complete, comprehensive understanding of odor. It may not seem a profound enough problem to dominate all the life sciences, but it contains, piece by piece, all the mysteries.” — Lewis Thomas.

Smelling starts with a sniff. The process of breathing in air into the nose floods the nasal cavity with myriad odorous molecules, or simply put, odorants. These molecules may smell pleasant, repulsive, or act as carriers of critical biological or ecological messages.

Odorants communicating vital biological information typically elicit behavioral and physiological changes in animals, thus playing a pivotal role in the survival and the propagation of the species (Li and Liberles, 2015). In some cases, the same odorant delivers different biological messages to animals of different species. In others, the identity of these ecologically-relevant odorants may vary greatly among different species, ultimately driving evolutionary adaptations to distinct ecological niches (Bear, et al., 2016; Li, et al., 2013; Manoel, et al., 2019).

A major challenge in studying smell and odor-guided behaviors has been the understanding of the biological mechanisms that enable the discrimination of a large number of odor cues, which are typically presented to the animal’s nose in virtually infinite combinations of mixtures and concentrations.

This review presents a brief historical description of the key findings and early challenges surrounding odor coding in the mammalian nose. It discusses how recent advances in olfactory neurobiology fundamentally inform our understanding of the interactions between odorants and their receptors in the nose, and how this knowledge impacts theories of odor perception.

Organization of the mammalian olfactory system

The peripheral olfactory system of most mammalian species involves two major olfactory organs: the olfactory mucosa (OM) located at the top of the nasal cavity and the vomeronasal organ (VNO) sitting at its base (Buck, 2012). The anatomical structure of the olfactory system can vary significantly between species, with some mammalian lineages (e.g., catarrhine monkeys, apes, and humans) lacking a VNO (Keverne, 1999), and other species (e.g., rodents) displaying additional olfactory organs, such as the septal organ of Masera and the Grueneberg ganglion (Barrios, et al., 2014; Ma, 2010).

Our focus is on the OM of the nose, which is composed of the olfactory epithelium (OE) and a submucosa. The OE is mainly populated by sustentacular cells, horizontal and globose basal cells, immature and mature olfactory sensory neurons (Fig. 1). The submucosa sitting below contains olfactory ensheathing cells, glandular and cavernous tissues, blood, and lymph vessels (Cuschieri and Bannister, 1975; Huard, et al., 1998; Morrison and Costanzo, 1992; Sharma, et al., 2019). Odorant reception occurs primarily in the OE via the mature olfactory sensory neurons (hereafter referred to as OSNs). This cell type, with its molecular and physiological architectures, thus is at the center of this review.

Fig. 1
figure 1

The major cell types of the mammalian olfactory mucosa (OM). In mammals, the OM is composed by the olfactory epithelium (OE) and a submucosa. The OE is a pseudostratified epithelium composed mainly by sustentacular cells (SUCs), globose basal cells (GBCs), horizontal globose cells (HBCs), immature olfactory sensory neurons (iOSNs) and mature olfactory sensory neurons (mOSNs). The olfactory ensheathing cells (OECs) are an important cell type populating the submucosa

Early challenges in understanding odor coding and the discovery of the odorant receptors

“the olfactory imprint is collected in the mucosa by the peripheral expansion of the bipolar cells and is then transferred to the glomeruli where […] cells from the molecular layer collect said imprint to raise it to the brain.”

- Santiago Ramón y Cajal

The two pioneers of neuroscience (and eternal rivals), Camillo Golgi and Santiago Ramón y Cajal, had described the basic neuroanatomical structure of the olfactory system in the late nineteenth century (Golgi, 1875, Ramón y Cajal, 1892). However, the concept of odorant receptors was considered only mid-twentieth century (Jones and Jones, 1953; Ottoson, 1954; Pauling, 1946; Skouby and Zilstorff-Pedersen, 1954; Sviridenko, 1951), while the genes encoding odorant receptors remained incognito for almost the entire twentieth century.

The foundation for discovering the receptors genes was laid in the 1970s and ‘80 s, with an increase in molecular studies that suggested a second messenger mechanism in olfaction. First, high adenylate cyclase activity was found in olfactory ciliary preparations of dissociated frog OSNs (Kurihara and Koyama, 1972; Pace, et al., 1985), a biochemical finding later confirmed physiologically (Firestein, et al., 1991). This data was followed by the identification of cyclic AMP (cAMP) as the secondary messenger in olfactory reception (Gesteland, 1976; Minor and Sakina, 1973). Contemporary technological developments, such as electrophysiological recordings, revealed that distinct odorants evoke distinguishable activation patterns in the OE (Kauer and Moulton, 1974; Mackay-Sim, et al., 1982), and even suggested the existence of multiple OSNs subtypes (Gesteland, 1976; Holley and MacLeod, 1977; Lancet, 1986; Sicard, 1985; Sicard and Holley, 1984).

Towards the end of the 1980s, mounting evidence pointed to G-protein coupled receptors (GPCRs) as the strongest candidates for odorant receptors (Lancet, 1986). Especially the identification of an olfactory-specific gene coding for a Gα protein (Gαolf) and for a nucleotide-gated channel indicated that odorant activation involved G-protein mediated production of cAMP (Dhallan, et al., 1990; Jones and Reed, 1989). Around the same time, experiments using dissociated newt OSN provided evidence of intracellular calcium signaling during odorant binding. These experiments further implied a link to the mechanism of adenylate cyclase or gating of ion channels (Kurahashi and Shibuya, 1990).

The path looked paved for the discovery of the odorant receptors. Still, their identification as GPCRs would take several more years before transforming the field (Buck and Axel, 1991; Firestein, et al., 2014). Notable about this discovery was Buck's ingenious experimental design, which revealed a crucial feature of the OR family that would expand understanding of GPCRs: the mosaic character of the OR multigene family. ORs are highly conserved throughout evolution while also exhibiting striking structural diversity across their members. Instead of being defined by a specific set of shared amino acid sequences, the OR family relation is cross-cutting, meaning members share different sequences with various other members.

The mosaic character of OR genes had also made their discovery impracticable (Barwich, 2020; Buck, 2004). The standard discovery method of new gene families at the time was PCR. However, the amplification of genetic material with the known GPCR primer pair failed. Buck's use of RNA instead of DNA in combination in tandem with her design of 11 degenerate primers, amplifying related but not identical sequences (based on Buck's interest in genetic diversification), yielded the jackpot. Mammalian odorant receptors turned out to be the largest multigene family in the mammalian genome, containing ~ 400 intact genes in humans, ~ 1100 in mouse and ~ 2000 in elephants (Godfrey, et al., 2004; Malnic, et al., 2004; Niimura, et al., 2014; Zhang and Firestein, 2002).

Other chemosensory receptors in the OE

Since the discovery of the OR gene family in 1991, other evolutionary conserved families of chemosensory receptor genes were found to be expressed in the mammalian OE, including the trace-amine associated receptors (TAARs), two guanylyl cyclases (GUCY2D and GUCY1B2), and the membrane spanning 4-pass A (MS4A) receptors (Bear, et al., 2016; Fulle, et al., 1995; Greer, et al., 2016; Horowitz, et al., 2014; Leinders-Zufall, et al., 2007; Liberles and Buck, 2006; Omura and Mombaerts, 2015; Saraiva, et al., 2015b, 2019). These ‘atypical’ receptors feature in other reviews in this issue, or were recently covered in other review articles.

Odorant receptor expression in the OE

Following the discovery of the ORs, Buck and Axel’s laboratories deepened research into OR genetics and wiring. One topic of main interest were the expression patterns exhibited by this remarkable gene family in the OE. By performing RNA in situ hybridization experiments, they found that OSNs expressing the same OR gene are randomly distributed within spatially restricted zones in the OE (Fig. 2a) (Ressler, et al., 1993; Vassar, et al., 1993). Other studies confirmed the existence of different spatial patterns of expression, or zones, for mammalian ORs. However, the exact number of zones in the OE and their physiological function remains in debate (Bashkirova, et al., 2005; Tan and ** OR expression zones, but later studies identified as many as 9–12 partially overlap** zones. In the schematic, the 4 non-overlap** OR expression zones (blue, red, yellow, and green colors) are shown: left panel, lateral view of the olfactory mucosa (OM); right panel, a coronal section of the OM (including the respiratory epithelium, in grey). (b) In the nose, each mOSN expresses one allele of a single OR gene. This type of expression became known in the field as the ‘one neuron – one receptor rule’. (c) Recent RNA-seq experiments showed that most intact ORs are expressed in the OE across a large dynamic range, with only a minority being expressed at very high levels. As measured by RNA-seq, the abundance level of a given ORs in the OE correlates perfectly to the number of OSNs expressing it. The arrow depicts the position of the last OR plotted.