Pheromone

Bombicol (silkworm hormone, Bombyx mori).

The pheromones are chemical substances secreted by living beings in order to provoke specific behaviors in other individuals of the same species. They are a means of transmitting volatile signals produced in liquid form, which are then dispersed in the environment. In the case of molecules for interspecific communication, the term allelomones is used.

Many species of plants and animals use different scents or chemical messages as a means of communication and almost all send one or more codes by this means, both to attract or reject each other sexually and for other purposes. Some butterflies, such as the male Saturnia pyri, are capable of detecting the scent of the female 20 km away.

The term pheromone was coined in the late 1950s, from the Greek roots φέρω, 'to carry', and ὁρμόνη, 'stimulus, hormone'.

In 2006, a subclass of receptors found in the olfactory epithelium called Trace Amine-Associated Receptor 1 was shown to be activated by volatile amines found in mouse urine, including a pheromone.

Evolution

Olfactory chemical signal processes have evolved in all taxonomic groups of organisms, including bacteria, and are thus the oldest phylogenetically receptor system. They are considered to serve survival functions by generating appropriate responses to danger signals, sex, and dominance status among members of the same species.

In addition, it has also been suggested that in the evolution from unicellular prokaryotes to multicellular eukaryotes, paracrine and endocrine primordial signals evolved within individual organisms.

Pheromone receptors

In the olfactory epithelium

In humans there is a group of six G protein-coupled receptors (i.e., TAAR1, TAAR2ls, TAARs) in the olfactory epithelium that function to identify volatile amine odorants, including certain pheromones; these TAARs are thought to function as receptors for pheromones that identify social messages.

In the vomeronasal organ

In reptiles, amphibians and mammals (except primates) pheromones are detected by the vomeronasal organ or Jacobson's organ in addition to ordinary olfactory membranes. This organ is located at the base of the nasal septum between the nose and the mouth. This organ is present in most amphibians, reptiles, and non-primate mammals, but is absent in birds, adult catarrhine monkeys, and apes. The existence of the Jacobson's organ in humans is under discussion. It is present in the fetus, but is reduced afterwards. Three types of G protein-coupled receptors have been identified, although they differ from typical receptors, they must nonetheless have some function.

In bees

Pheromones in domestic bees are produced in special glands and act through smell as a rule. Worker bees also have them. Nasonov's glands, on the back of the abdomen, which emit Nasonov's pheromone, are especially well known. They fan them by raising the abdomen and flapping the wings.

Some pheromones can work by oral exchange. The queen uses them to control the workers and, in the nuptial flight, to attract the males. It stimulates the aggregation in the swarms, prevents the construction of royal cells, transmits its presence, which maintains the tranquility of the hive, promotes the collection of nectar.

There are pheromones produced by mandibular glands that permeate the body and are picked up by the workers with their tongues, and thus they are transmitted to generalize the knowledge that the queen is present.

The pheromones that make the construction of new queen cells (those used to raise new queens) are produced in tarsal glands (in the legs).

In ants

Ants are arthropods of the Formicidae family that use pheromones as a sign of recruitment, recognition, territoriality, and alarm.

Anatomy

Fig. 1. Location of some of the exocrine glands in the ants.

These glands are modifications of epidermal cells of the integument and can be unicellular or an aggregate of several cells. There are approximately 50 total glands in an individual and they can be of one or two types. The first group are epithelial glands they secrete directly into the cuticle. The second group is a group of glands composed of different secretory units, each with a secretory cell and a duct cell. The secretions of these glands can go directly to the outside or have internal reservoirs. Regarding the hormone release sites, in the case of sex hormones, these are released from the gaster. Some ants like Rhytidoponera metallica use the pygidial gland and others like Monomorium pharaonis use the Dufour's gland while others produce sex hormones in their venom glands. The alarm pheromones used in case of danger can be produced in the mandibular, pygidial or Dufour glands. Dufour's gland is the source of chemicals that can cause attraction, orientation, colony migration, alarm recruitment, or establishment. Insects have two main chemosensory systems, taste and smell, and pheromone signals could be detected by either of the two systems, however, findings in this regard have been made by olfactory detection. In insects, odors are detected by sensilla. olfactory cells that can have different sizes and shapes, with a single or double cuticular wall and always with pores that penetrate the cuticle so that odor molecules can reach the chemoreceptor neurons. Olfactory neurons of adults project to the olfactory lobes of the cerebrum while in larvae these neurons project to a location in the brain homologous to the adult olfactory lobe. The dendrites of olfactory neurons are lined with cuticular hairs which comprise the olfactory sensilla found on the antennae or some insects also on maxillary palps

Electrophysiology

Pheromones are perceived by olfactory sensilla located on the antennae. Sensillae have one or more sensory dendrites found within the cuticular layer that contains numerous pores. Pheromone reception is divided into the following steps (according to Jackson and Morgan, 1993):

1. The pheromone molecules are absorbed through the surface of the sensilia and are diffused by the tubular pore system of the surface.
2. The molecules reach the dendrite membrane where they join the protein receptors causing an electrical impulse.
3. After pheromones react with acceptors are quickly converted into inactive compounds by enzymes in the antennas.
4. Potential graduates accumulate on the surface of the dendrite until added to form an action potential that spreads along the neural axon to the brain.
5. Different types of receptors respond to different volatile compounds which carries different types of signals to the brain and generates different physiological and/or behavioral responses.

Vogt, 2005 gives a more detailed description of the molecular basis of insect chemoreception which can be applied to the processing of many hormones. For many years pheromone detection was seen as a distinct system from odor detection but pheromone detection at the sensilla level seems to be an adaptive version of odor detection in general. Once inside, odors bind to proteins known as soluble odorant binging proteins (OBPs) and are transported by these proteins to transmembrane olfactory receptor proteins, ORs. Odor molecules are broken down by a variety of enzymes that are in the lumen of the sensilla. The activation of the ORs that are receptors associated with G proteins generates an increase in a compound known as inositol triphosphate (IP3) that directly activates the ion channels of neuronal membranes. Pheromones can also occur in structurally different ways. If one isomer of the molecule is produced, then the other isomer of the molecule will be less active or totally inactive. In ants of the Atta texana species, the alarm pheromone in the form of one of the isomers ((S)-(+)-4-methyl-3-heptanone) produces a response threshold 100 times lower than its R isomer.

Electrophysiological Experiments

There are different techniques to measure the electrical activity of olfactory receptors in insects such as electroanthenography and single cell recording techniques. However, these techniques have hardly been applied in studies in ants due to their very hard cuticle and their low potential responses to them. However, some studies have been carried out where the electrophysiological responses of Camponotus atriceps ants were measured in the recognition of conspecifics and non-conspecifics. Simultaneous antenna-brain activity was recorded by immobilizing the head and antennae of the ants inserted into pipette tips, the head and antennae were immobilized with beeswax. A window was opened in the capsule and cephalic glands. The trachea and muscles were removed to gain access to the antennal lobes and cetiform bodies. The antennal sensitivity of ants C. atriceps was determined by means of an electroanthenogram, inserting the distal terminal portion of the intact antenna into the saline-filled glass capillary electrode. Brain activity was recorded using tungsten microelectrodes on the ipsilateral antennal lobe and on the cetiform bodies. The reference glass capillary electrode was placed on the head behind the brain. The signals generated by the antennae and the brain were transferred to an amplifier and an audio monitor and later digitized to be viewed on a monitor (Fig.2).

Fig.2. Amount of an electroantenogram to measure the anteno-cerebral electrical activity in the ants.

As a result of this electrophysiological experiment, it was found that in 90% of the tests carried out, the ants responded aggressively to the filter paper with non-conspecific extract (Fig. 3).

Fig. 3. Electrical activity recorded in the antenal lobes (ALs) and cetiform bodies (MBs), using an electroantenogram (EAG). A: Conspective response. B: Response to non-conspectives.

There are also other electrophysiological experiments that have been carried out to evaluate the response of ants to stimuli such as seed odor or carbon dioxide detection, where it was discovered that ampullary sensilla are responsible for CO2 perception. It was confirmed that the neurons of this organ are continuously active during CO2 stimulation, which allows the ants to detect the concentration of carbon dioxide inside their nests.

Behavior

Different types of pheromones produced by ants have been described (1):

• Sexuals: They're attraction hormones. Female ants are located outside the nest and release sexual pheromones to attract males. Another type of approach is given when the male releases pheromones from the jaw glands to attract the female, the aggregation pheromones play an important role in mating as they group members of a colony in the same area for that purpose.
• Dispersion or spacing: They are pheromones that increase spacing among individuals thus diminishing intra-specific competition.
• Alarm: Used to alert conscientists in case of danger, the most common are alphatic ketones. Although alarm signals often involve an increase in the locomotion of individuals there is a special case in the species Zacryptocerus varians that lives in mangroves where the alarm signal makes the ants stay still and close to the substratum not to accidentally fall into the water.
• Trace: They are used by ants to be followed by conscientists either to food sources or to new places to build nests. When an ant finds a new source of food marks the path with pheromones, the path will later be reinforced with the accumulation of more pheromones. The pheromones are volatile therefore after a time of exhausting the food are scattered and the ants stop following that path.
• Surface: They are secretions that stimulate the exchange of food. They are produced on the surface of the body of the ants and are perceived at short distances or by direct contact. They serve for the recognition of conspecies.

It has been shown that thanks to the use of pheromones, ants have the ability to choose the shortest path from their nest to a food source and back from the food source to the nest thanks to a positive feedback system in that the probability of an ant choosing any of the possible paths is modified by the previous ant that has discharged the pheromone in the shortest path, so that after a while all the ants will follow the same path. It has also been found that ants can leave pheromone trails to indicate where the best quality food is, and that when they find better quality sources than others they deposit pheromones at a higher rate, making a U-shaped path with pheromone establishment from the food source to the nest (21). Likewise, it was believed that once a path has been established thanks to a reinforcement by deposition of pheromones, it could not be changed for another, however it has been suggested that there may be some flexibility on the part of the ants to choose new paths if, for example, the original path they follow is blocked. It has also been shown that ants that make a U path (that is, on the way from food to the nest, return to the food before reaching the nest) are the ones that have the most pheromones. deposited during a path towards a better quality food. Likewise, within social insects such as ants, the queen ant has the ability to produce pheromones responsible for regulating the development of the ovaries of the females in the colony, the greatest effect on the ovaries of the other females being the inhibition of the oogenesis. Pheromones as a signal of aggression and defense have also been used as ant pest control methods, more specifically formic acid that serves as a toxic repellent and alarm pheromone has also been used as a fumigant for other arthropods that affect crops in the world..

In silkworm

Its name is bombicol, one of the most studied pheromones, produced by the female silkworm moth, with very small amounts of it it is possible to attract males over 1 km. Scientists are interested in synthesizing it as an alternative to pesticides, since using it as a trap could isolate the males or releasing it into the field to prevent the encounter of males and females, thus interrupting the reproductive cycle.

In mice

It has been shown in laboratories that, in some female rodents, the mere fact of smelling the pheromones of a male other than the one that fertilized them, even indirectly through their urine or the smell of their bedding, is capable of inducing them abortions. This is known as the Bruce effect, after H. M. Bruce, who discovered it in 1959.

In the urine of male Mus musculus mice there are non-volatile substances, presumably sex pheromones, which are innately attractive to females of their species. On the other hand, the volatile odors that this urine gives off are not innately attractive, but rather acquire this property when they are associated with pheromones in what constitutes an appetitive emotional learning model. Non-volatile pheromones stimulate the vomeronasal system, while the volatiles associated with them are detected by the olfactory system. In the reinforcement system, nonvolatile pheromones first perceived activate the basolateral nucleus of the amygdala and the nucleus accumbens, but not the ventral tegmental area or the prefrontal cortex. The reinforcement system is activated in a different way when animals perceive volatile odors that have acquired the attractive property by association with pheromones. The detection of these stimuli with acquired attractiveness stimulates the activity of the basolateral amygdala, the ventral tegmental area, and the prefrontal cortex, but not the nucleus accumbens. The association between non-volatile pheromones and their associated odors therefore occurs in the basolateral nucleus of the amygdala.

In humans

There are scientific studies that point to the possible existence of pheromones in humans. Even so, these studies continue to be subject to debate due to their methodology and their inconclusive conclusions. There is currently no definitive consensus within the scientific community about the existence of human pheromones. In humans, TAAR5 receptors are presumably averse to trimethylamine, which is known to be an hTAAR5 antagonist.