Octopamine arthropods

It is the purpose of this chapter to discuss the neuroeffector, octopamine, and its related biochemistry in the arthropod nervous system as a target area for the rational discovery of control agents in the light of the criteria listed in Table I. 

Octopamine in the CNS. It is reasonable to suppose that this handful of varied systems where OA has effector functions is only the tip of the iceberg and that more will be discovered. All of these known systems are located peripherally since the demonstration of a specific transmitter role for any compound within the CNS is very challenging. However, there is every reason to believe that OA has important transmitter or modulator functions in the CNS of arthropods. It is synthesized and stored there in plausible amounts (8,9f13) and OA-sensitive adenylate cyclase activity has been found within the CNS of several arthropods. In addition to the examples cited in the recent review by Bodnaryk (26), this activity has been demonstrated in the tobacco hornworm (27f28), Drosophila melanogaster (29)t the tick, Amblyomma hebraeum (27) and the crayfish (30,31) Neurones specifically sensitive to OA have been... 

General Functions of Octopamine in Arthropods. The actions of OA in invertebrates therefore are multiple and probably involve both central and peripheral sites. Moreover, as pointed out previously, many of the known actions of OA are comparable to those of NE and E in the vertebrate central and sympathetic nervous systems. These multiple effects are concerned with arousal and stress responses, increasing the responsiveness to outside stimuli, and alerting the resting animal and priming it for action and movement. It is interesting, though inconclusive, in this context to note that ants... 

Table III. Comparative Activity of Substituted Phenylethylamines in Three Octopamine-sensitive Systems from Arthropods.

The action of tyramine on nerve receptors is mainly indirect by release of norepinephrine and dopamine from neuronal storage sites (363, 384). Tyramine and its /3-oxidized counterpart octopamine have been referred to as false neurotransmitters because these compounds can be taken up, stored, and released from nerve endings in a way similar to those of the principal neurotransmitters norepinephrine and dopamine (385). Octopamine was first discovered in salivary glands of octopods (386). The compound is widely distributed in the animal kingdom and is present in high amounts in the nervous system of several species of invertebrates such as molluscs and arthropods, where it acts as a specific transmitter substance (387). Octopamine may also play a role in the regulation of adrenergic neurotransmission in mammals (387). Administration of octopamine to intact animals produces a transient rise in blood pressure (388). 

Chlordimeform (2,36) was introduced in the early 1970s to control mites, ticks, and moths in the field. Chemically it is iV-(2-methyl-4-chlorophenyl)A iV -dimethylformamidine, and it has shown no cross-resistance with other insecticides. Its selectivity is attributed to its mimicry of octopamine 2.35), which is an important neurotransmitter in arthropods but not in vertebrates (Evans and Gee, 1980). Death occurs partly by exhaustion after overstimulation, partly by starvation. The only effect on mammals appears to be an inhibition of monoamine oxidase, 300 times weaker than that evoked by the much-used anti-depressant drug, tranylcypromine (9.47) (Aziz and Knowles, 1973). The suggestion that inhibition of this enzyme also contributes to the effect on insects and mites is not tenable (Neumann and Voss, 1977). 

Chlordimeform (2.3 ), a much used insecticide which gently inhibits mammalian MAO, seems to owe its usefulness in the field to mimicry of the arthropod neurotransmitter, octopamine 2.35) (see Sections 2.6.1 and 6.4.1). 

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