Octopamine antagonists

False neurotransmitters are amines which are similar enough in structure to normal amine neurotransmitters that they bind to receptors but are much less active or totally inactive (i.e. they are antagonists). One such false neurotransmitter is octopamine, which is formed from tyrosine by decarboxylation followed by side-chain hydroxylation. 

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

Hirashima, A., Kuwano, E. and Eto, M. (2003) Comparative receptor surface analysis of octopaminergic antagonists for the locust neuronal octopamine receptor. Comp. Biol. Chem., 27, 531-540. 

One of the general features of octopamine receptors appears to be their susceptibility to vertebrate -adrenergic receptor antagonists, and their insensitivity to -adrenergic antagonists. 

The agonists and antagonists displayed a wide range of potencies, with the rank order of both similar to that found for the octopamine-2 receptors in the locust extensor tibiae muscle. The results support the proposed role of octopamine as an endogenous neuromodulator of moth olfactory perception. 

Octopamine is present in C. elegans (181), but its cellular distribution is not yet known. It acts antagonistically to 5-HT, to produce suppression of egg-laying and stimulation of pharyngeal pumping. 

The chemical mediators in the central nervous system of insects are far from completely explored, but acetylcholine (ACh) and octopamine (2.35) play important roles. The neurotransmitter at insect ganglia is acetylcholine, but that at the neuromuscular junction is not acetylcholine but L-glutamic acid (Usherwood and Machili, 1968), and for this neurotransmitter, no selective antagonist has yet been found. The receptors for the neurotransmitters are well protected by selectively permeable membranes. 

Figure 3. Dose-response curves of octopamine and tyramine in attenuating forskolin-stimulated cAMP production in B96Bom/HEK-293 cells, Forskolin (10 pMJ-stimulated cAMP levels are shown as 100%. Yohimbine (10 pM) was tested as an antagonist. Data represent means SE of at least four independent experiments. p < 0.01 vs control p < 0.05 vs control (paired t test) p < 0.01 vs tyramine-attenuated levels (unpaired t test). (Reproduced with permission from reference 9. Copyright 2003 Blackwell.)...

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