The perception of smell begins with the activation of odorant receptors (OR) in the olfactory sensory neurons. The OR family has one of the largest gene pool with around 900 genes in humans and 1,500 in mouse. ORs are thus one of the largest known gene families in the human and mouse genomes, representing 3-5% of all genes. These receptors belong to the superfamily of G protein-coupled receptors and consist of seven hydrophobic membrane-spanning regions. However, they differ in their amino acid sequence in the transmembrane domain III, IV, and V, which suggest that these parts are responsible for the discrimination of odorant species [1]. In the mouse olfactory epithelium, there are about 1,000 genes encoding different types of odorant receptors. Each OSN expresses only one type of odorant receptor gene in its ciliary membrane. A single OR responds to many types of odorants and aditionally, a single type of odorant can stimulate several types of ORs [2]. The binding of the odorant to the receptor changes the conformation of its plasma membrane receptors. The activated receptors catalyzes the exchange of guanosine 5-diphosphate (GDP) for guanosine 5-triphosphate (GTP) on multiple trimeric G proteins causing the dissociation of Gαolf from Gβ and γ [3]. Each Gαolf in turn activates adenyl cyclase type III that converts adenosine triphosphate (ATP) into Cyclic Adenosine Monophosphate (cAMP). cAMP plays an important role in olfaction as a second messenger and me-diates signal transduction for a wide variety of odorants [4,5] (Fig. 1).

Olfactory cilia contains the major components of olfactory signal transduction including high levels of Cyclic Nucleotide-Gated Channels (CNG).These channels play a significant role in mediating chemoelectrical energy conversion in olfactory cilia. The information about the odorant molecules is first transmitted as chemical information and then the CNG channel convert it into an electrical signal by the activation of ion channels across the plasma membrane. These channels consists of four subunits containing two CNGA2, one CNGA4, and one CNGB1b [6]. Each subunit has six transmembrane domains (S1-S6) and a pore region between S5 and S6 domains. A cyclic nucleotide binding site is present near the C-terminal at the cytoplasmic side in each subunit [7]. CNG channels are permeable to monovalent ions, such as Na⁺ and K⁺, and also to Ca²⁺ (Fig. 1).

Historically, the olfactory transduction was regarded as a system that worked similarly to the vertebrate phototransduction in a sense that it involved the activation of G protein-coupled receptors that triggered an electrical response by generating a current through CNG channels. This view was altered when Kleene and Gesteland reported the presence of Ca²⁺-activated Cl^- conductance in olfactory cilia. This Ca²⁺-activated Cl^- current was then found to be part of the odorant-induced current in amphibian [8,9] and mammalian [10] OSNs. This finding provided an explanation for the observation that the olfactory response persisted even in the absence of external Na⁺ [11,12]. Apparently, olfactory cilia did not contain Ca²⁺ stores because application of pharmacological agents that caused Ca²⁺ release from intracellular stores failed to elicit a ciliary Ca²⁺ transient current [13]. Additionally, Ca²⁺ signaling in olfactory receptor cells is regulated separately in the cilia and in the cell body, which allows the generation of compartmentalized Ca²⁺ transient current [14]. This eliminates the cell body as a source of Ca²⁺ in the cilia and shows that ciliary Ca²⁺ is regulated exclusively by the CNG channel. Odor stimulation opens the CNG channels and increases the ciliary Ca²⁺ concentration [15] and this primary Ca²⁺ signal generates an additional secondary Cl^- current that is excitatory under physiological conditions [9,16]. Thus, generation of the receptor response of OSNs is thought to involve the sequential activation of two currents: a primary cationic inward current through CNG channels, which is activated by cAMP and is mainly carried by Na⁺ and Ca²⁺, and a secondary Cl^- inward current through Ca²⁺-activated Cl channels (CACC) that is triggered by the influx of Ca²⁺ (Fig. 1).

Recently, the molecular identity of the Ca²⁺-activated Cl^- channels was discovered independently by three research teams [17,18,19]. Studies on gene expression in the olfactory system support the concept that TMEM16B (ANO2) represents the only Anoctamin at the plasma membrane of OSNs [17]. Ano2 tops a list of genes highly enriched in OSNs over other cells of the Main Olfactory Epithelium [20], and it is the only Anoctamin consistently identified in high amounts in mass spectrometric screens in olfactory membranes [21,22]. No such enrichment was shown for the other Anoc-tamins while Ano1, Ano6, Ano8, and Ano10 are known to be expressed in the MOE [21,23].

A number of studies have shown that calmodulin (CaM) drives regulatory feedback mechanisms that control several key steps of the transduction pathway. During odor detection, CaM is trigger-ed by Ca²⁺ influx and it reduces ciliary cAMP levels through activation of phosphodiesterase [24] and by inhibition of adenylyl cyclase (AC III), which is a substrate for the CaM-dependent protein kinase CaMK II [25]. Moreover, CaM deactivates the cAMP-gated channel to its ligand and in this way promotes channel closure and rapid adaptation [26]. CaM serves an important link between the two transduction channels by coupling a depolarizing Cl^- current to the odor-induced Ca²⁺ signal. The transduction model illustrates the dual effect of Ca-CaM. The inhibitory effects of Ca-CaM restrict the activity of the cAMP-gated channels to a short time period. The transient Ca²⁺ influx causes a prolonged elevation of the ciliary Ca²⁺ concentration and opens multiple Cl^- channels, as there are eight times more Cl^- channels in the ciliary membrane than cation channels. As a result, the initial small Ca²⁺ influx triggers a much larger Cl^- efflux and generates an excitatory receptor current with Cl^- as the main charge carrier. Thus, the presence of this anion-based signal amplification in OSNs [27] points to the evolutionary adaptation of the cells that have to convert small Ca²⁺ signals into sizeable depolarizations. In this case, Cl^- accumulation provides the driving force for a depolarizing Cl^- current that is triggered by a Ca²⁺ signal. CaM plays an important role in this amplification mechanism, as it coordinates both its excitatory and inhi-bitory processes (Fig. 1).

The performance of the olfactory system is highly remarkable because of its odorant detection efficiency and its ability to discriminate between chemically similar odors. However, the olfactory system has to cope with low sensitivity of the receptors that puts serious pressure on the ciliary transduction machinery. Since the production of the second messenger cAMP is limited due to the low efficiency of the initial transduction step, a different process is required in downstream of the transduction cascade. To solve this dilemma sensory cilia of OSN have designed a unique alternative mechanism, that is the accumulation of chloride ions at rest and liberation of a chloride current upon odor detection. This chloride current amplifies the initial neuronal activation (Fig. 1).

Cl^- accumulation in ORNs are mediated by the Na⁺/K⁺/2Cl^- cotransporter, NKCC1 [28,29]. Accordingly, research has shown that genetic knockout of NKCC1 following suppression of Cl^- accumulation or Cl^- efflux, significantly inhibited the sensory response of ORNs [28,30,31]. Expression of NKCC1 can be found in the ciliary layer of the olfactory epithelium. It seems that other kinases support the normal function of NKCC1 such as SPAK ("STE20/SPS1-related proline/alanine-rich kinase") and OSR1 ("oxidative stress-responsive kinase-1") [32], as well as WNK1 and WNK4 [WNK = "with no lysine (K) kinase"] [33]. NKCC1 serves as a substrate for the two protein kinases SPAK and OSR1 [32]. These kinases are in turn activated by the protein kinases WNK1 and WNK4 [33]. Thus, WNK1 and WNK4 participate indirectly in the control of NKCC1. Recent studies have showed that WNK1 and WNK4 phosphorylate OSR1 and SPAK [34,35,36] promote phosphorylation and activation of NKCC1. All four kinases, SPAK, OSR1, WNK1, and WNK4 are present in ORN cilia and are required for maximal activation of the transporter. The five proteins form a supramolecular complex: SPAK and OSR1 bind toNKCC1 [37], whereasWNK1 andWNK4 bind to SPAK/OSR1 [35,36]. It is interesting to note that the kinase WNK3 is not expressed in ORNs. WNK3 shows the highest expression in the brain, and it is thought to act as a Cl^- sensor involved in protecting cells from excessive Cl^- accumulation [37,38,39]. The absence of WNK3 from ORNs all points to the fact that these neurons are at dispense with means for intracellular Cl^- reduction and thus instead optimize Cl^- accumulation (Fig. 1).