Octopamine action, effect

Tyramine is produced by decarboxylation of tyrosine and is present in the CNS in higher (threefold) concentrations than m-tyramine, the hydroxylated derivative of phenylethylamine. In the periphery / -tyramine is easily hydroxylated to octopamine, which has some direct effects on ai adrenoceptors, unlike tyramine which functions by releasing NA. When tested on central neurons tyramine always produces the same effects as NA but they are slower and less marked, implying an indirect action. By contrast octopamine often produces the opposite effect to NA and it is probable that octopamine may have a functional role in the invertebrate CNS where it is found in higher concentrations (5pg/g) than in the mammalian brain (0.5ng/g). Neither tyramine nor octopamine have distinct behavioural effects, unlike phenylethylamine,... 

Table II. Potency of Octopamine and DCDM in the Crayfish Behavioral Assay and the Effect of Selected Octopaminergic Antagonists on their Action.

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... 

Central actions of octopamine. There is no doubt that, at the present time, more detail is known about the peripheral effects of octopamine than its central effects. This is almost certainly due to the amenability of peripheral preparations to experimentation. Table... 

A similar series of experiments has now been performed in insect visceral muscle (81). Octopamine elevates cyclic AMP content of the lateral oviduct of locusts. This effect is dose-dependent in the presence of IBMX, with a threshold dose at about 10"% and half-maximal stimulation at 7 x 10"%. Forskolin also elevates cyclic AMP content and IBMX potentiates the action of forskolin. 

An octopamine-sensitive adenylate cyclase is not the only means by which octopamine may exert its effects. Whilst DUMETi and octopamine modulate the amplitude and rate of relaxation of twitch tension in the extensor tibiae muscle of locusts via cyclic AMP, their effects upon the myogenic rhythm are mediated via a different route (86) Thus, elevated cyclic AMP does not induce a decrease in the frequency of myogenic rhythm as does octopamine, and the effects of octopamine are not potentiated by IBMX. In fact the mode of action of octopamine in reducing the frequency of myogenic contractions remains unknown. 

The studies discussed herein show that formamidines without the conventional lethal moiety can elicit dispersal behavior and can interfere with reproduction in twospotted spider mites. Walk-off dispersal elicited by formamidines was correlated with formamidine-induced mortality. Since these structure-activity relationships of formamidines are similar to those for pertubation of octopa-minergic transmission in insects, a similar mechanism could be involved in these actions in mites. Since structure-activity relationships for formamidine effects on spin-down and reproduction were not correlated with those for lethality, other mechanisms, which may or may not involve octopamine or other biogenic amines, are probably associated with these actions. 

Formamidines (such as chlordimeform (Figure 1) and amitraz) and imidazolines (such as naphazoline and XAMI Figure 1) cause a series of behavioral and lethal effects in insects and acarines that may be ascribed entirely or in part to the actions of these compounds and their common metabolites as powerful agonists at octopamine receptors... 

S0. This validates the stimulation of octopaminergic systems as an effective strategy for insecticidal/acaricidal action. However, there are several approaches to achieving this end other than by a direct stimulatory effect on the octopamine receptor. 

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