Even if we have lost faith in our noses, we are still strongly influenced by smells - even if only subconsciously. Smells can evoke memories and emotions, influence our mood and are important to our enjoyment when eating. We often use the word "taste" erroneously, when we actually mean smell. For we can only taste something if it is salty, sour, sweet or bitter. All the delicate nuances of an epicurean cuisine or of a noble glass of wine are, in the final analysis, savoured through our sense of smell.

How do we register a smell? The fact that our knowledge of the molecular backgound is still so scant is due to the complexity of this world of smell. More than 10.000 odours can be distinguished - even in extremely low concentrations. The sense of smell is thus exceptionally specific and sensitive, and might best be compared in its complexity with the immune system.

What happens when a molecule of some odour is absorbed by the olfactory mucus membrane and transformed into an electric impulse of the nerve fibre? New electrophysiological and molecular-biological methods are now enabling us to follow the individual steps of signal transduction at the molecular level. So what are the processes which trigger the olfactory stimuli in a single cell? Odorous substances are constantly emitting low quantities of specific molecules into the ambient air. During inspiration, these penetrate into the nose as far as the olfactory epithelium, where they interact with the receptor protein molecules in the membranes of the olfactory cells via interposed proteins, the so-called "G-proteins". The receptors activated in this way produce a large number of messenger substances known as "second messenger" molecules. These substances - some 2.000 per receptor - then open ion channels in the cell membrane, through which positively charged molecules, cations, stream in and depolarize the cell. This creates a bioelectric voltage gradient between stimulated areas in the nerve cell - the receptor potential. With this cascade-like amplification mechanism even a very small number of odor molecules will be recognized.

Transduction Cascade elucidated in Detail

All the molecular components of the transduction cascade have now been isolated and the DNA of the genes involved, has been analyzed. More than 30 types of olfactory receptors have been described, sequenced and analyzed. These receptors are members of a large gene family comprising several hundred members, which makes it presumably one of the largest in the entire vertebrate genome. The receptor proteins belong to the "G-protein-coupled receptors" and consist of some 300 amino acids. the receptors span the cell membrane in seven hydrophobic, transmembrane domains which constitute the region of maximal similarity to other members of this superfamily. The variability in the external domains (loops) between the third, fourth and fifth regions spanned by the receptors is consistent with the idea that there the odorant molecules can bind. This binding sets the following intracellular reaction cascade in motion: the receptor activates a specific G-protein, which in its turn acts on a ATP-splitting enzyme (adenylate cyclase), which enhances the concentration of the cyclic adenosine monophosphate (cAMP) in the cell. cAMP is a chemical compound which is involved in the body's energy metabolism. The cAMP, "second messenger", concentration increases very rapidly (in just a few milliseconds). cAMP molecules are then able to open ion channels in the membrane. The genetic structure of this ion channel has now also been elucidated.

During the last three years, the biophysical and pharmacological properties of this channel have been intensively studied. We succeeded in isolating individual olfactory cells of salamanders, rabbits and human beings simply by mechanical dissection, altthough greter success was achieved after weak enzymatic pretreatment. Measurements with odorant application showed that ion channels opened after a latency of 200 to 300 milliseconds. Experiments in witch the cAMP concentration in the cell was kept artificially high through pharnacological influences produced a markedly stronger and longer-lasting activation of the ion channels. These discoveries were the first indication that cAMP-controled ion channels are involved in the formation of the odour-induced generator current.

A detailed study of this channel was conducted on so-called "inside-out patches" of isolated olfactory cells, with which the piece of membrane stamped out of the cell is so formed that the former inside of the cell is turned outside. This exposed the binding side for cAMP in the membrane, enabling different second messenger substances to be applied in the course of the experiments. The very first experiments revealed that this odour-activated ion channel is a non-specific cation channel, in other words, a channel which permits the passage of positively charged ions such as those from sodium., potassium, or calcium. The channel can be directly opened by binding of cAMP. The specificity of the channel protein to cAMP was high but, interestingly, cyclic guanosine monophosphate (cGMP) was also able to open the channel. This explains why a channel of similar structure is involved in the transduction process of vision, where cGMP is the second messenger stimulated by light-induced rhodopsin activation. We were also able to identify a similar type of channel on human sperm, where its function is presumably to find the egg cell.

One important discovery regarding the functioning of the system concerned the differentiated effects of the calcium concentration on the channel both inside and outside the cell. If the calcium inside the cell is increased, the probability that the channel will open is dramatically reduced by lowering the affinity to cAMP. Since calcium ions flow through the channel, this calcium block might be employed as a possible means of extinguishing the odorant response of the cell. Increasing the calcium concentration outside the cell, on the other hand, reduces the conductivity of the channel. All the data so far collected support the idea of a high extra-cellular calcium concentration, which would be logical because al large number of small channel openings reduce the background noise of the odour signal, thus enhancing the sensitivity of the cell. This is a desirable property as animals have to be able to react to even al low number of odour molecules.

Human Kidney Tumor Cells acted as "olfactory Cells"

Under normal physiological conditions, olfactory receptors are activated by rapid fluctuations in stimulant concentrations. In order to reproduce these functionally important conditions, stimulants were applied with a specifically developed ultra-rapid drug application system which produces a rise in the stimuli concentration on the receptor within less than one millisecond. The first measurements on isolated olfactory cells, once again on "inside-out patches", produced a surprise. The rise time of the channel response occupied, as expected, just a few milliseconds but, even after removal of the stimulant , the channel still remained open for approximately half a second until it closed with a time-constant of around 200 milliseconds. Unfortunately, the stamped out membranes mostly contained only one channel or just a few. In order to facilitate more detailed examination of the unique reaction mechanism a preparation was employed which had a very high channel density, so that even minor effect would also be directly visible. For this purpose we used human embryonic kidney tumor cells, changing their function to that of "olfactory cells". This was achieved by introducing c-DNA from the olfactory channel of a rat with the aid of bacterial plasmid DNA. So-called "stop genes" suppressed the normal protein synthesis of the cell, so that the tumor cell produced just the olfactory channel protein in very high densities. On stamped-out membrane patches from such cells the typical cAMP/cGMP-activated olfactory channels, in densities often exceeding 1,000/µm, could now really be seen. Brief stimulatory pulses of only 30 milliseconds duration in a saturating concentration of cAMP or cGMP initiated the almost simultaneous opening of all these channels. In the case of these channels, too, the current thus generated persisted, as had already been observed on natural olfactory cells, for nearly half a second after the cAMP had been removed, until it slowly returned to its rest-state level.

This unique kinetic property just identified in ion channels activated by "second messengers" could be of great functional significance. As biochemical studies have shown, the rise in cAMP in the cell triggered by the binding of an odour molecule to a receptor reaches its maximum very rapidly and then drops off within 20 to 30 milliseconds, irrespective of whether any odour remains on the receptor or not. But the electrical cell response lasts for a half to one second. Our data have now, for the first time, furnished an explanation for this process. Independently of all this, the biological system embracing receptor molecules, G-protein, adenylate cyclase and cAMP can in the meanwhile be regenerated for the next stimulus.

From this a model of olfactory transduction can be reproduced which has a surprising similarity to the synaptic mechanisms, in other words, to the transduction of transmitter substances between two cells. Evidently, certain chemical odour classes (flowery, fruity) use the cAMP path described above and others (putrefying, reeking, but also aetherial) take a second path which has only recently been identified in vertebrates, the so-called IP₃ transduction cascade.

Personal Scent based on "Main Histo-compatibility Complex"

Odorants intervene at many levels in our life. Thus recent exciting discoveries have revealed that there is a personal scent, which is based on genetically anchored principles of immunological self-nonself distinction, or "main histo-compatibility complex" (MHC). The closer the relationship, the closer the similarity of the personal scents. Odours can also influence our mood, can equally well provoke passion or aversion, sympathy or antipathy and extend right into the sexual field. "I think it stinks" or "he's a proper stinker" are traditional turns of phrase which have now received scientific validation.

Personal judgment, in other words, wheter one likes an odour or not, so-called "hedonics", is to a certain extent inherited; it is positive for natural/floral odours, negative for the smell of decomposition. This is true of all population groups all over the world. But there is an additional, powerful, acquired component which begins even in the womb, as animal experiments have shown. A foetus will react to olfactory stimuli from the 28th week on. Thus young rabbits prefer the vegetable matter ingested by their mother during her pregnancy. There are also many examples of culturally very disparate preferences for odours: Japanese much prefer the smell of soya sauce to that of a pizza, and the opposite is true of Europeans. Reactions to specific odours are frequently overlayed and concealed, for example, by visual, acoustic and cognitive processes.

This complicates identification of the effects of odours. hence we studied the influence of odours on sleeping human beings. For the first time scientists succeeded, through continual, simultaneous monitoring of a range of physiological parameters (heart rate, respiration rate, EEG, electro-oculograms, skin resistance), in demonstrating that odours are indeed registered in sleep and, moreover, that physiological parameters will change in response to specific odours. The smell of an orange, for example, will raise the heart and respiration frequency rate, whereas  the faeces-like smell of skatole lowers the heart frequency rate without producing any measurable effect on respiration.

In order to approach a step nearer to an answer to the old question of whether human beings also have so-called pheromones - odours which are generated by ourselves for use as a chemical "language" of communication with our fellow human beings - we also examined the effect of human body odours (female underarm perspiration and vaginal secretions) on male test subjects. It was discovered that whereas some individuals reacted with a significant rise in heart rate, in others no changes were recorded. In addition to this, analysis of the contents of dreams (the odours were always applied for two minutes in threshold concentration at the beginning of a REM phase) revealed that orange an human body odours produced a highly significant correlation with pleasant dreams, while skatole, in contrast, produced correlations with unpleasant dreams.

The sense of smell, which for a long time was held to be the "lost sense" amongst us human beings, is acquiring increasing importance, especially today when we are oversaturated with visual and acoustic stimuli, devoting increasing attention to our bodies and are living more consciously in harmony with nature.

A dog (red line) tracks a pheasant (yellow line). As the dog keeps leaving the odour to prevent receptor adaptation, it zigzags.

The vital links for odorant recognition between the nose and the cerebral processing centres.

Longitudinal section through a human olfactory epithelium. The olfactory sense cells are labeled in yellow by a specific antibody. Between these lie the unlabelled supporting cells