Chemical signals specialization

I thank J. Heinze, M. Lindauer, C. Peeters, F. Roces, and J. Tautz for reading the manuscript. Special thanks are due to my former collaborator N. F. Carlin, with whom several of the concepts concerning anonymity and specificity of chemical signals have been developed in a joint paper (3). Some of the author s work presented in this essay has been made possible by grants from the National Science Foundation and the Leibniz-Prize from the Deutsche Forschungsgemein-schaft. 

Fig-1 Schematic view of the overall olfactory processing in insects. Pheromones and other semiochemicals are detected by specialized sensilla on the antennae, where the chemical signal is transduced into nervous activity. The olfactory receptor neurons in the semiochemi-cal-detecting sensilla are connected directly to the antennal lobe. Here the semiochemical-derived electrical signals are processed and sent out (through projection neurons) to the protocerebrum. Olfactory information is then integrated with other stimulus modalities, a decision is made, and the motor system is told what to do... 

Vertebrates, especially mammals, have evolved a bewildering variety of specialized glands that produce secretions which in turn carry chemical signals, independently of excretions. 

Signal specialization and evolution in mammals. In Chemical Signals in Vertebrates, vol. 8, ed. R. E. Johnston, D. Muller-Schwarze, and P. W. Sorensen, pp. 1-14. New York Plenum. 

Hormones are chemical signaling substances. They are synthesized in specialized cells that are often associated to form endocrine glands. Hormones are released into the blood and transported with the blood to their effector organs. In the organs, the hormones carry out physiological and biochemical regulatory functions. In contrast to endocrine hormones, tissue hormones are only active in the immediate vicinity of the cells that secrete them. 

The individual unit of the nervous system is the neuron, a specialized cell that both receives and transmits information. The nervous system contains more than 100 billion neurons and is a major user of metabolic energy in the human body. It is also a region particularly susceptible to injury from toxic chemicals, lack of oxygen, and other assaults. Depending on the nervous region in which they reside, neurons may have different anatomical features and may use different chemical transmitters. Neurons communicate with each other and with their end organs by these chemical signals, which are released from the nerve terminal and interact with specific receptors on adjacent neurons or cells. 

A further intercellular communication mechanism relies on electrical processes. The conduction of electrical impulses by nerve cells is based on changes in the membrane potential. The nerve cell uses these changes to communicate with other cells at specialized nerve endings, the synapses (see chapter 16). It is central to this type of intercellular commimication that electrical signals can be transformed into chemical signals (and vice versa, see chapter 16). 

Sorensen, P. W. and Scott, A. P., The evolution of hormonal sex pheromones in teleost fish. Poor correlation between the pattern of steroid release by goldfish and olfactory sensitivity suggests that these cues evolved as a result of chemical spying rather than signal specialization, ActaScand. Physiol., 152, 191, 1994. 

The axon is specialized to react to changes in membrane potential. When the cell s membrane potential reaches a certain threshold the axon responds with an action potential that rapidly transmits an electrical signal from the cell body to its terminals. Finally, the nerve terminal is specialized to convert the electrical signal of the action potential back into a chemical signal. It responds to depolarization by releasing a neurotransmitter that acts either upon the soma or dendritic membranes of the next neuron or, in the PNS, on an effector site (Figure 11.5). The specialized membrane is essential to the electrochemical properties of neurons. 

Electrochemical DNA biosensors are based on the use of nucleic acids or analogues as biorecognition element and electrochemical techniques for the transduction of the physical chemical signal. Two aspects are essential in the development of hybridization biosensors, sensitivity and selectivity. Traditional methods for detecting the hybridization event are too slow and require special preparation. Therefore, there is an enormous interest in developing new hybridization biosensors, and the electrochemical represent a very good alternative [108]. 

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