Agonist indirect

All three receptors are of the seven-transmembrane superfamily and are positively coupled to adenylyl cyclase. In addition to 3-adrenoceptor agonists, indirect SYMPATHOMIMETICS may cause the eventual activation of 3-adrenoceptors (or a-adrenoceptors). depending on tissue factors, by causing release of noradrenaline (e.g. cphedrine, pseudoephedrine) or preventing noradrenaline reuptake (e.g. cocaine). 

Beta-agonists, indirectly via c-AMP, act on mast cell p2-receptors inhibiting the release of bronchoconstrictor mediators such as histamine. Slow-reacting substance of anaphylaxis (SRS-A), now known to be leukotriene D3 and D4 (Fig. 5-5), and eosinophil chemo-tactic factor of anaphylaxis (ECF-A) may also have their release inhibited by c-AMP. Leukotriene D, which is a mediator of hypersensitivity reactions, may still have an important role in causing asthmatic symptoms. In fact, it is 100 times more effective than histamine in producing vascular permeability. 

Direct agonist, indirect agonist A direct agonist binds and activates the receptor an indirect one brings about receptor activation by binding to some other molecule, eg, a reuptake carrier, and causing an increase in the synaptic concentration of the normal transmitter... 

Antagonist An antagonist is a receptor ligand preventing the action of an agonist, in a direct (competitive) or indirect (allosteric) manner. 

The term opioid refers to any exogenous substance that acts as an agonist at any of several receptors. Opioid antagonists are drugs that bind to a receptor but produce no actions. The poppy plant, Papaver somniferum, from which opium is obtained, is grown in many areas of the world. Morphine constitutes 10% of opium, and codeine can be obtained direcdy from opium. Semisynthetic opioids such as heroin and oxycodone are obtained directly or indirectly from morphine. There are other distinct chemical classes of drugs with opioid actions, including the methadones. 

Figure 7.7 Dopamine-induced rotation in the rat in which one (left) nigrostriatal dopamine pathway from the substantia nigra (SN) to the caudate putamen (CP) has been lesioned by a prior injection (14 days) of 6-hydroxydopamine. Amphetamine, an indirectly acting amine, releases DA and so can only act on the right side. Since the animal moves away from the dominating active side it induces ipsilateral rotation (i.e. towards the lesioned side). By contrast, the development of postS5maptic supersensitivity to DA on the lesioned side ensures that apomorphine, a directly acting agonist, is actually more active on that side and so the animal turns away from it (contralateral rotation)...

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