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Pheromone Advantage

Attract The Opposite Sex With Pheromones

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Tufted cell olfactory tract

FIGURE 2.2-2 (A) Anatomical schematic of the connections of various cells within the olfactory bulb. Anatomically, there are five layers in the olfactory bulb (OB). Shepherd (1970) observed that there is congruence between cells of the vertebrate retina and olfactory lobe cells. Retinal horizontal cells are analogous to OB periglomerular cells, and retinal amacrine cells are analogous to OB granule cells. There are no cells analogous to retinal bipolar cells in the OB, but the long primary dendrite of the mitral cell fills that position. Mitral cell axons also are functionally analogous to retinal ganglion cells. Granule cells and periglomerular cells are inhibitory interneurons. (B) Schematic interneuron connections in the OB. The olfactory receptor cell axons synapse with mitral cells, tufted cells, and periglomerular cells. The mitral cell dendrites also receive inhibitory inputs from the periglomerular cells. The granule cells make inhibitory synapses on the secondary dendrites of the mitral cells. Interesting, efferent control fibers from the CNS synapse on the granule and periglomerular cells. (From Kandel, E.R. et al., Principles of Neural Science, 3rd ed., Appleton & Lange, Norwalk, CT, 1991. With permission from the McGraw-Hill Companies.)

The periglomerular cells and granule cells in the olfactory lobe are known to be inhibitory on the mitral cells. Centrifugal (efferent) control fibers from the anterior olfactory nuclei stimulate these inhibitory cells. The receptor cell axons synapse excitatorily on the mitral, tufted, and periglomerular cells. The mitral and tufted interneurons send their (afferent) axons to the lateral olfactory tract. These axons synapse further in five different areas of the olfactory cortex.

It is well known that olfactory sensitivity can vary more than a factor of 103 between normal individuals. Some individuals are totally unable to smell certain odors, while others can. As persons age, there is a loss of threshold sensitivity for certain odors. Certain odors such as the "rotten egg" scent of H2S can be sensed in concentrations of several parts per trillion. The noses of trained dogs still remain the most reliable and most sensitive for detecting drugs and explosives. As noted, the feats of bloodhounds in tracking human trails is legendary. Certainly, there is a long way to go to invent an artificial nose with the sensitivity of a dog or a bear.

That olfaction in invertebrates is equally important in determining survival is discussed in the following section.

2.2.2 Olfaction in Arthropods

The first part of this section addresses olfaction in insects. Insects have evolved very efficient olfactory chemoreceptors with which they locate food, congenial substrate, and other insects of the opposite sex. Insect chemoreceptors are found all over their bodies; they are principally found on their antennae, but also on their heads, labial palps, legs, and feet. They take a variety of shapes, including the sensillum placo-deum (olfactory plate) found in the wasp Vespa and the honeybee; the sensillum basiconicum (olfactory cone) of Vespa, Locusta, and Necrophorus vespillio; the sensillum trichodeum (chemosensory hair) of Antheraea pernyi and Amorpha; the sensillum ampullaceum of the bee; the sensillum rhinarium from the antenna of Drepanosiphum (Homoptera); a chemosensory bristle from the wax moth, Galleria; a long sensillum basiconicum from a grasshopper. A collection of these insect chemosensors is illustrated in Figure 2.2-3. Note that some insect olfactory sensilla have only one sensory cell (e.g., sensillum ampullaceum in bees), others have two receptors (sensillum trichodeum in lepidoptera), while others have many receptors (sensillum placodeum of bees).

The first insect sex attractant (pheromone) was identified by Butenandt in 1959 for the silkworm moth, Bombyx mori L., and named bombykol. It has been discovered that Bombyx females actually emit three sex attractant pheromones: bombykol [(E,Z)-10,12-hexadecadien-1-ol], bombykal [(E,Z)-10,12-hexadecadienal], and (E,E)-10,12-hexadecadien-1-ol. At present, the sex pheromones of more than 1300 insect species have been identified. These pheromones, which number in the hundreds, have significant structural homologies (Pherolist, 1999).

The male Bombyx senses the pheromone with "tuned" receptors in hairs on its rather elaborate antennae. A male Bombyx may have as many as 2.5 x 104 hairs on each antenna. Each hair contains the dendrites of two bipolar chemoreceptor cells

Trichogen Cell

trichogen cell receptor cell basement membrane epidermal cell tormogen cell 'glia cell

FIGURE 2.2-3 Insect chemoreceptors: (a) sensillum trichodeum (chemosensory hair); (b) sensillum basiconicum; (c) sensillum coeliconicum of Locusta; (d) sensillim placodeum of Apis; (e) peg-shaped insect olfactory sensillum. (From Schneider, D. and Steinbrecht, R.A., in Invertebrate Receptors, McCarthy, J.D. and G.E. Newall, Eds., Academic Press, New York, 1968. © Academic Press. With permission from Academic Press.)

trichogen cell receptor cell basement membrane epidermal cell tormogen cell 'glia cell

FIGURE 2.2-3 Insect chemoreceptors: (a) sensillum trichodeum (chemosensory hair); (b) sensillum basiconicum; (c) sensillum coeliconicum of Locusta; (d) sensillim placodeum of Apis; (e) peg-shaped insect olfactory sensillum. (From Schneider, D. and Steinbrecht, R.A., in Invertebrate Receptors, McCarthy, J.D. and G.E. Newall, Eds., Academic Press, New York, 1968. © Academic Press. With permission from Academic Press.)

a at its base; one neuron has receptors for bombykol, the other for the related compound bombykal (Yoshimura, 1996), and probably some may respond to (E,E)-10,12-hexadecadien-1-ol, as well. When the female moth emits her sex pheromones, they travel downwind in an elongated plume. Airflow can be laminar or turbulent, depending on wind velocity; thus, the concentration of the pheromones as a function of distance from the female can decrease monotonically from diffusion effects (in slow, laminar airflow), or be "noisy" due to air turbulence. It appears that the male moth does not follow a simple scent gradient toward the female "emitter"; the pheromone concentration as a function of distance and time is too noisy. Instead, the male moth is stimulated by the reception of the pheromone to fly upwind (anemotaxis) in a sinusoidal, zigzag path to right and left of the wind vector. If the moth loses the odor plume, its flight behavior is modified to a search pattern of larger zigzags of up to 90° to the wind vector (the moth may even fly downwind) in an attempt to regain the scent. When it regains the plume, it again flies upwind in the smooth, zigzag pattern (Reike et al., 1997). The molecules of bombykol, bombykal, and (E,E)-10,12-hexadecadien-1-ol are shown in Figure 2.2-4. They are 16-carbon, long-chain, lipidlike molecules. It is estimated that the binding of only 20 bombykol molecules to their receptors within 100 m. will produce a behavioral response in a male B. mori.

There are many insects that are important because they present human health hazards (e.g., Anopheles mosquitos ^ malaria; tsetse flies ^ sleeping sickness; certain mosquitos ^ Eastern equine encephalitis; deer ticks ^ Lyme disease, etc.), or they damage crops or trees (many, many examples). The fact that phero-mones can be used to catch and kill harmful insects or to confuse their mating

Bombykol

Bombykal

(E,E)-10,12-Hcxadccadicn-1 -ol

FIGURE 2.2-4 Basic structure of the silkworm moth pheromones, bombykol, bombykal, and (E,E)-10,12-hexadecadien-1-ol. Each black ball is a carbon atom.

behavior has stimulated research in isolating these molecules from the insects in question, as well as leading to investigations on the molecular mechanisms of pheromone detection and olfactory cognition. Insects are also covered with other chemoreceptors (on legs, labial palps, etc.), which sense environmental molecules that enable them to select the correct food or to lay their eggs in the correct plant (or animal).

The tsetse fly (Glossina sp.) spreads sleeping sickness in central Africa (e.g., Zimbabwe). One means of poisoning these flies is to emit bait odors emulating cattle, which is their prey (other than humans). It has been discovered that tsetse flies locate their prey by optomotor-steered, upwind anemotaxis, following attractive kairomones. The main kairomones are 1-octen-3-ol, acetone, 3-methylphenol, 4-methylphenol, and CO2. The methylphenols were isolated from ox urine, and CO2, octenol, and acetone are found in cattle breath (Spath, 1995).

Fruitflies (Drosophila sp.) are perhaps an ideal model system in which to study olfaction. The maxillary palp of this insect contains only about 60 olfactory sensilla trichodeum, each with a pair of sensory neurons. The neurons fall into six functional classes, so, theoretically, there could be 21 different combinations of receptors in the hairs. The situation is made even more complex by the fact that a certain odorant can excite one class of receptor and inhibit another, and a particular receptor can be excited by one odor and inhibited by another. Thus, a complex odor composed of two or more odorants can produce a unique pattern or spatial distribution of receptor axon activities. The CNS of the insect must sort out this spatiotemporal pattern and generate an appropriate response (behavioral, biochemical, etc.) (de Bruyne et al., 1999). Some questions that should be asked: Do fruitflies carry a genetically determined neural template for the odors in their repertoire? Is there a multidimensional, AND operation between the odor-generated olfactory pattern and the stored template? Even olfaction in fruitflies appears very complex, in spite of their size. Perhaps researchers should try to find a simpler model system.

Threshold chemoreception of dissolved sugar by the fly, Calliphora sp., can be determined behaviorally by observing whether the fly extends its proboscis (as if to feed) when a drop of sugar solution is presented to the labellar taste hair. Such behavioral experiments have led to the determination of a threshold sensitivity of 2.5 mM for dissolved sucrose. (The human threshold is 20 mM, while the butterfly Dania can sense an amazing 8 |M.) The trichoid sensillae on the fly's labella also sense salt, water, and touch (Miller and Thompson, 1997).

Chemoreception (underwater olfaction) has also been extensively studied in lobsters and crabs. Lobsters are addressed here. Olfactory-related behavioral and nervous responses of both male and female lobsters of two species have been studied extensively (e.g., Homarus americanus and Panulirus argus). The olfactory chemoreceptors of these lobsters appear to be located principally on their antennules. They are called aesthetasc sensillae; each sensilla is innervated by an average of about 300 chemosensory neurons. A lobster has two main antennae (left and right), which project forward from universal pivots that allow them to be oriented forward, to the side, or to the rear over the animal's back. Medial to the main antennae, projecting forward from either side of the rostrum, are the shorter, paired antennules (two right, two left). Other parts of the lobster's head and mouth contain other types of chemosensors that are probably used to sense food.

By ablating the antennules on male and female Homarus lobsters and observing their mating behavior, Cowan (1999) was able to show that both male and female lobsters emit (perhaps in their urine) pheromones and that these chemical messengers affect behavior of the opposite sex. Male Homarus is normally agonistic toward members of its own species. That is, individuals compete for living space, food, and dominance. As with other animals of diverse species, a male Homarus strives for dominance in its territory, i.e., to become a dominant male. A dominant male evidently secretes a pheromone that advertises this fact. Female lobsters follow this pheromone trail upcurrent to the den of the dominant male. The female evidently secretes another pheromone that advertises her availability and inhibits the male's normally agonistic behavior toward her. She lives in his den with him (cohabits) for a few days until she molts her shell. At this point, she is food for any marine predator because of her soft body. The male shows admirable restraint, and guards her from predators during this vulnerable period; he does not touch her. Once her new carapace has partially hardened, she mates with the male; she remains in his company for a few days before going on her way. Cowan showed that removal of the antennules altered this normal courtship behavior, demonstrating that it was indeed regulated by the reception of chemical messengers from the opposite sex.

It is known that the California spiny lobster, P. interruptus, emits an aggregation pheromone that causes other solitary lobsters to gather in a group around it (ZimmerFaust et al., 1985). Such a pheremone, if isolated, could be used to increase the efficiency of baited traps.

The chemoreceptor (aesthetasc) sensillae on lobster antennules have been shown to respond to various chemicals. Cromarty and Derby (1997) showed that isolated, individual chemoreceptor cells detected (at least): taurine, p-alanine, hypotaurine, L-glutamate, glycine, proline, cysteine, NH4Cl, and adenosine-5'-monophosphate

(AMP). Some of the amino acids listed may be associated with damaged muscle, i.e., a food source. Cromarty and Derby concluded that individual chemoreceptor neurons from aethetasc sensillae express at least two types of receptors mediating excitation; one principal receptor class giving the strongest response and the greatest sensitivity to the principal input substance, and a second, minor class that also acts in an excitatory manner but which is present in a lower density on the receptor cell or which has a lower affinity to a second input substance.

Other invertebrates such as nematodes and even flagellated bacteria exhibit chemotaxis in response to a chemical "field" gradient. Space limitation prevents describing these interesting systems. The interested reader will be impressed by the plethora of information on these topics by doing World Wide Web searches.

2.2.3 Discussion

This section has discussed at the mammalian olfactory system and chemoreception in insects and crustaceans. The vertebrate olfactory system is most sensitive in certain species such as dogs (e.g., bloodhounds), bears, and predators such as wolves. Indeed, the nose of a trained dog can find hidden drugs or explosives better than any existing machine made by humans.

Arthropods were seen to make extensive use of surface, airborne, or waterborne chemical messengers to attract mates, mark territory, and establish paths to food. While mammalian chemoreception of external stimuli resides in the nose and tongue, arthropods have chemoreceptors all over their bodies (antennae, labial palps, legs, feet, etc.).

One of the problems in modeling chemoreception is that the exact chemical details of the processes, including by what reactions the odorant molecules are broken down is not known. Although it is known that mammalian olfactory neurons, unlike all other neurons, undergo programmed cell death (apoptosis) every month or so, we do not know whether the new receptors that grow to replace them have the same receptor specificity for odorants. If they do not, how is the neural "wiring" in the olfactory lobe reconfigured to accommodate the sensitivity of the new cell? Here is a ripe area for neural modeling to test hypotheses resulting from anatomical and neurophysiological (wet) work.

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