Chemosensation

Chemosensation - taste and smell

Our chemical senses, taste and smell, are our oldest senses. Long before organisms could hear or see, they needed to find nutrients and had to avoid toxic substances, and they were attracted by signaling molecules that were released by their mating partners. Right from the beginning of evolution, the task of our chemical senses was not to merely identify the chemical compounds encountered by the organism, but to directly initiate behavioural responses. The smell of fire or of a predator must rapidly initiate a flight reaction or aggressive behaviour, the smell of a mating partner initiates mating behaviour. In vertebrates the chemical senses were tightly connected to the limbic system, a brain region that controls behaviour, emotions, and feelings. We are unaware of how strongly the chemical senses control our feelings, yet this tight connection still exists in humans and is even reflected in our daily language: we think that small children are "sweet" and call them "honey pie". Occasionally, we have to swallow a "bitter" pill and some things leave a "sour taste in our mouth", and finally, we all pursue the "sweet smell of success".

Our gustatory system - the sense of taste

Although served by anatomically and morphologically distinct systems, the sensations of smell and taste often function in concert, especially when we eat and drink. In fact, when we talk about the "taste" of food, in the strict scientific meaning we often mean the smell of odorants that are set free in our mouth and reach the nose via a direct connection, the choanae. The sensation "hot" (in the sense of chili or pepper) is the stimulation of pain receptors in our mouth. Taste in the strict scientific meaning covers only five different sensations: salty, sour, bitter, sweet, and "umami". These taste qualities are detected by specialized sensory cells, the taste cells, and are conveyed to the brain by three different nerves (the cranial nerves VII (N. facialis), IX (N. glossopharyngeus), and X (N. vagus)).

The most important function of taste is to control the quality of our food. The sensation umami is elicited by glutamate (an amino acid needed for the synthesis of protein) and by ribonucleotides (needed for the synthesis of nucleic acids), both of which are found in high concentration in muscle tissue and meat products. If you chew a "medium" steak, you experience umami taste. The sensation umami indicates protein-rich food. The sensation sweet guides us to food that is rich in carbohydrates. Both protein and carbohydrates are essential nutrients, hence, evolution took care that sweet and umami evoke pleasant sensations and stimulate food uptake. This behavioural pattern is especially obvious already in new born children: if you put some sugar on their tongue, they smile and immediately start to suck. If uncontrolled, unfortunately, in our civilized world this behaviour leads to increased body weight and obesity. Many toxic plants contain bitter tasting substances, thus bitter is a warning signal. Strong bitter taste initiates vomiting, saving us from intoxication. The threshold for the detection of certain dangerous bitter substances is a thousand fold lower than that for sugar or salt. Sour indicates unripe fruits or spoiled food, salty is most important for our mineral and water balance. Our taste system also controls reflexes in the digestive tract, such as secretion of saliva or enzymes.

The taste cells are clustered in structures called taste buds, which are located primarily in the papillae of the tongue (but some are also found in the cheeks, the palate or other places in the mouth). In humans, there are three types of sensory papillae. Up to 400 fungiform papillae that look like small mushrooms are located in the frontal two-thirds of the tongue. Foliate papillae (15-30) form leaf-like folds on the back sides of the tongue. Circumvallate papillae (7-15) appear as large round structures surrounded by a groove and are located in the back third of the tongue. In total, an adult human may have several thousand taste buds. Their number drops with age. Being highly exposed to toxic and other environmental influence, taste cells are in great danger of being damaged. Hence, taste cells are routinely replaced by newly born cells after about ten days. In mammals such as human, the continuous replacement of sensory cells throughout life is most unusual; it is only found in the chemical senses.

	Figure 1: Different kinds of sensory papillae are found on our tongue. Taste buds are drawn in white.

Taste cells are elongated cells with two poles. One pole reaches the surface of the tongue by an opening in the taste bud called the taste pore. Small processes, the microvilli, extend from the surface of the taste cell and are exposed to tastants in the saliva on the tongue. It is here, that sensory transduction takes place. At the opponent pole, taste cells transmit their information through synaptic contacts to innervating nerves that convey the information to the brain (Central representation of taste sensation).

	Figure 2: Scheme of a taste bud.

How do taste cells detect and respond to tastants?

Two criteria need to be fulfilled to stimulate a taste cell with a tastant, e.g. sugar. First, the cell needs a specific receptor molecule that binds the tastant. Second, the cell`s interior, which at rest is negatively charged, must become more positive (physiologists call this "depolarization"). This is usually achieved by the opening of ion channels that allow the influx of positively charged ions, such as Na<sup>+</sup> or Ca<sup>2+</sup>. We know a great deal about the receptors and ion channels involved in taste transduction. Roughly 30 genes in our genome code for taste receptor proteins. In the case of salt and sour taste, the cellular response is quite simple. The cells harbour proteins that serve as receptor and ion channel at the same time. Salt (NaCl) dissociates into Na<sup>+</sup> and Cl<sup>-</sup> in water. Some taste cells express an ion channel that conducts Na<sup>+</sup> into the cell. If the Na<sup>+</sup> concentration in the saliva is high, many Na<sup>+</sup> ions flow into the cell and the cell becomes strongly excited. Sour taste is elicited by protons (H<sup>+</sup>). Two mechanisms were suggested for sour taste. In one model, taste cells express an ion channel that is always open and is permeable to K<sup>+</sup> ions. Through this channel, K<sup>+</sup> ions leave the cell, making the cell`s interior negative. Protons were suggested to bind to this channel and to block the K⁺ outflow, which would make the cell more positive. Recently, we suggested an alternative model: we found a special kind of ion channel, a so-called pacemaker channel, in a population of taste cells of the tongue. We could show that protons bind to and activate this channel, leading to the influx of Na⁺ ions and to excitation of the cell. We could show that protons bind to and activate this channel, leading to the influx of Na⁺ ions and to excitation of the cell.

The cellular responses to sugars (sweet), amino acids (umami), and bitter substances are more complicated. For each of these tastants, specific receptor molecules were identified, that belong to the large family of G-protein coupled receptors (the receptors for odorants, for many neurotransmitters and hormones, and rhodopsin also fall into this family, probably the largest family of proteins known). Upon binding of the tastant to the receptor, a biochemical cascade is initiated that involves the activation of a G-protein and of an enzyme called phospholipase β2. The cascade finally results in the opening of ion channels called TRPM5, that allow the influx of positively charged ions. Animals, in which either the phospholipase β2 or the TRPM5 are not functional, do neither taste sugar, umami, nor bitter substances. On the other hand, salty and sour taste sensations can still be elicited, because they are mediated by other mechanisms that do not depend on these two proteins.

Coding of taste

An engineer would design our gustatory system such, that a given taste cell (and therefore its postsynaptic nerve fibre that transmits the information to the brain) responds to one taste quality only (say sweet). Indeed, most recent evidence suggests that this may be true for the majority of taste cells. However, many gustatory nerve fibres can be stimulated by several taste qualities (say sweet, bitter and salty). Different fibres show different combinations of taste qualities with different sensitivity profiles. Our brain identifies the taste by comparing the pattern of activity over many such fibres. Fibres that are "broadly tuned" are not unique to taste, but are also found in other sensory systems. Compare to our colour vision: three broadly tuned colour receptors are sufficient to convey the myriad hues that we can discriminate.

Our olfactory system - the sense of smell

We are continuously bombarded by airborne molecules released by a variety of sources in our environment. It is the extraordinary discriminative capability of our olfactory system that allows to extract important information from these molecules. With our olfactory system, we can detect signals from distant sources. You smell a fire much earlier than you see it! Smell controls our emotions and feelings even stronger than taste does. You only need to smell the flavour of a certain biscuit and within one instant you recall last Christmas evening in much detail! Trained humans (such as perfume designers) can distinguish thousands of odorants. The sense of smell is carried by receptor cells that lie deep within the nasal cavity. In humans, roughly 10 million receptor cells are confined to a patch of specialized tissue, the olfactory epithelium, covering roughly 5 cm² of the upper nasal cavity. Our olfactory capabilities are impressive, but poor compared to those of many animals, e.g. dogs. For certain substances, dogs are a thousand fold more sensitive than humans. In line with this, the olfactory epithelium of a dachshund covers an area of 75 cm² and contains roughly 120 million receptor cells.

	Figure 3: Section through the nasal cavity of a rat. Folded structures, the turbinates, serve to increase the surface and carry the olfactory epithelium.

Olfactory receptor cells are bipolar neurons. They send a single dendrite to the surface of the epithelium, where it ends in a knob. The knob gives rise to several fine processes, the olfactory cilia. The cilia are embedded in the mucus and cover the epithelium with a dense meshwork. The cilia are the site of sensory transduction. At their basal pole, olfactory neurons form an axon. Up to 1000 axons are clustered in bundles that leave the nasal cavity through a thin bone to enter the adjacent olfactory bulb, a phylogenetically old region of the brain. Like taste cells, olfactory neurons are being generated and replaced throughout life; they have a lifespan of roughly 60 days. That means that every day some 100,000 axons have to find their way from the nasal cavity to the bulb and have to establish new synaptic contacts with their target neurons.

	Figure 4: Olfactory epithelium of a rat with a schematic representation of an olfactory sensory neuron.

	Figure 5: Olfactory cilia form a dense meshwork on the surface of the olfactory epithelium.

Sensory transduction - how do our olfactory neurons respond to odorants?

The application of odorants to the cilia of olfactory neurons generates a depolarization. At the soma, the depolarization initiates so-called action potentials, which are transmitted via the axon to the brain. As in many other sensory systems, olfactory signal transduction employs specific receptors, a G-protein and the activation of ion channels. Our genome harbours approximately 1000 (!) genes that code for olfactory receptor proteins, all of which are G-protein-coupled receptors. Yet, most of the genes are so-called pseudogenes and do not yield functional proteins. Probably, the high number of pseudogenes indicates that the sense of smell, once necessary for survival in the early evolution of mankind has become less important in more recent time. We are left with about 350 different olfactory receptors. The receptor proteins differ in the molecular properties of their odorant binding pocket. It is thought that they are only activated by a family of odorants that fit into this pocket. Each olfactory neuron expresses only one of these genes and is thus specific for that particular family of odorants. Upon binding of the odorant, the receptor activates a G-protein (G_olf), which in turn activates an enzyme called adenylate cyclase III. This enzyme synthesizes the internal messenger cAMP. The signal is amplified, as one receptor activates many G_olf molecules, and the cAMP concentration rapidly increases. Olfactory neurons express a certain class of ion channels that are activated by the binding of cAMP, the CNG channels (closely related channels are found in photoreceptors of the retina). Upon opening of the channels, Na⁺ and Ca²⁺ ions enter the cell and the cell gets excited. It is currently believed that most olfactory neurons show the same signal transduction cascade, irrespective of the odorant they respond to. If one of the proteins involved is not functional, e.g. the G_olf or the CNG channel, the animal looses its olfactory capabilities; it becomes completely "anosmic". Nevertheless, we have good evidence that one class of olfactory neurons utilizes an alternative signaling pathway.

Getting used to smell: how olfactory neurons adapt

Everybody knows from his own experience that our sensory systems rapidly and effectively adapt to persistent stimuli. This is also true for smell: if you enter a bar, you smell the mixture of smoke, alcohol, perfume etc. Later, however, the sensation is virtually lost and it takes a short break outside in fresh air before you regain your sensitivity. This adaptation is in part achieved by a smart feedback mechanism within the olfactory neuron. When the cell is stimulated and the CNG channels open, they allow the influx of Ca²⁺ ions into the cilia. Ca²⁺ binds to a small calcium-binding protein called calmodulin (CaM). The CNG channel protein harbours a binding site for the Ca²⁺/CaM complex. As soon as Ca²⁺/CaM binds to the CNG channels, the channels reduce their sensitivity to cAMP, and close again. Moreover, Ca²⁺/CaM activates an enzyme called phosphodiesterase (PDE) that destroys cAMP. Thus, although the odorant is still present, the excitation of the cell is strongly reduced. Further adaptational mechanisms exist at different stages of olfactory information processing in the brain.

	Figure 6: Signal transduction and adaptation in olfactory sensory neurons.

Coding in the olfactory system

How can we discriminate thousands of odorants, if we have only 350 different olfactory receptors? Clearly, each receptor must be more or less broadly tuned and must bind more than one type of odorant. As in other sensory systems, our brain compares the pattern of activity in many fibres to identify the odorant. The first stage of this analysis is the olfactory bulb. The axons of the 10 million olfactory neurons end in the olfactory bulb in structures called "glomeruli". About 1000 olfactory neurons converge onto one glomerulum. All of these 1000 olfactory neurons express the same olfactory receptor, and thus, show the same specificity. A mixture of odorants, e.g. the odour of a banana, will activate several different receptor cells and hence, a characteristic ensemble of glomeruli. This ensemble differs from the ensemble activated by the odour of a strawberry.

Smell, sex, and social life

Olfactory neurons are not the only sensory neurons in the nasal cavity of vertebrates. First, there are free nerve endings of the trigeminal nerve that serve as pain receptors and also detect harmful substances. They respond to acid, chlor gas and other unpleasant compounds. Secondly, vertebrates have a specialized region in the nasal cavity that differs in shape and size between the species. Often it is tube-shaped. It is called the "vomeronasal organ" (VNO), or the "organ of Jacobson". In animals, the sensory cells in this organ do not respond to conventional odorants. Instead, they detect "pheromones". Pheromones are substances secreted by animals to chemically communicate with other individuals of their own species. Pheromones can induce and control certain patterns of behaviour. Often this behaviour is related to reproduction. Pheromones serve to attract the mating partner and control mating behaviour or milk-feeding behaviour. The VNO is present in humans, but it is still a matter of debate, whether it is functional and whether pheromones exist in humans, and if so, whether they control human behaviour. It was reported that women who slept for a long time in the same room, synchronized their menstrual cycle, probably due to the action of pheromone-like substances in their sweat. On the other hand, in humans all but one of the genes that are essential for signal transduction in the VNO, i.e. the genes coding for VNO receptors and for the TRP2 ion channel, are pseudogenes that yield no functional proteins. The human VNO remains an enigma.

Unexpected functions: the olfactory system seems to utilize molecules of our immune system

Many animals, such as dogs and mice, can identify individuals solely based on their individual smell. Two classes of proteins seem to be important for this function: the "major urinary proteins" (MUPs) and, most interestingly, proteins of the "major histocompatibility complex" (MHC). MHC molecules are important proteins in the immune system. They are involved in cell-cell interactions, in the generation of immune responses and in the rejection of transplanted organs (that is why they are called MHC). A mouse of an inbred strain can smell, whether another mouse belongs to the same strain, or to a strain that is identical in all genes, except one single gene in the MHC locus! MUP and MHC molecules are much too large to serve as odorants. MUPs together with short versions of MHC molecules, which are found in the sweat and urin might serve as carriers for small, volatile molecules that originate from our cell metabolism. Thus, individuals that differ in their MHC inventory could have different cocktails of endogenous odorous substances. Mice try to avoid mating with individuals that have an MHC composition similar to their own, and thus, are closely related. This may serve as a kind of natural behavioural means to avoid incest. Some scientists think that similar mechanisms may still work in humans. During ovulation, women were reported to prefer the smell of men that differ strongly from their own MHC, but such reports have to be interpreted with great caution. It is well established, however, that mothers and their babies can identify each other solely on the basis of their smell.

Publications on chemosensation from our institute: