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  1. Introduction
  2. Serotonin receptors
    1. The 5-HT1 receptors
    2. The 5-HT3 receptor
    3. Investigations into the functions of recently identified 5-HT4
    4. The D1 receptor
    5. The dopamine D2 receptor
    6. The D3 and D4 receptors
    7. The a2 receptors
  3. Histamine receptors
    1. H2 receptors
  4. Cholinergic receptors
  5. Conclusion
  6. References

Ultimately the effects of monoamines on CNS function and behavior depend upon their interactions with receptor molecules. The binding of monoamines to these plasma membrane proteins initiates a series of intracellular events that modulate neuronal excitability. Unlike the transporters, multiple receptor subtypes exist for each monoamine transmitter. The initial classification of many receptor subtypes was based on radioligand binding studies. Receptor binding sites were identified on the basis of the rank order of binding affinities for a number of agonist and antagonist compounds. More recently, the molecular cloning of monoamine receptors has confirmed that many of the sites initially defined by these binding studies did indeed correspond to distinct receptor proteins encoded by unique genes. Molecular cloning has also led to the identification of previously unknown receptors, and to the introduction of powerful tools to characterize receptor structure and function.

[...] The functional roles of the peripheral adrenergic receptors are better understood than are its central functions. Cardiac b1 receptors play a major role in the regulation of heart function, and b2 receptors regulate bronchial muscle contraction. b3 receptors are found in adipose tissue, where they stimulate fat catabolism. Although b1 and b2 receptors are widely distributed in the CNS, their contributions to catecholamine function are not well understood. Propranolol (Inderal) is a widely used nonspecific antagonist of both b1 and b2 receptors. [...]

[...] The D3 and D4 receptors are considered to be D2-like on the basis of similarities in their gene structures, sequence homologies, and pharmacology. These receptors are expressed in lower abundance than the D2 receptor and their regional distributions are distinct. Whereas D3 receptor expression is high in the nucleus accumbens, highest levels of D4 receptors are expressed in the frontal cortex, midbrain, amygdala, and medulla. Whereas little D3 receptor expression has been detected outside the nervous system, D4 receptors are more abundant in the heart than in the brain. [...]

[...] In the wake of the recent proliferation of known receptors subtypes, much work needs to be done to determine the functional roles of individual receptors. In many instances this effort is hampered by the paucity of selective agonist and antagonist drugs. Recently, a molecular genetic approach to examining receptor function has been applied to complement pharmacological studies. Gene targeting procedures have enabled the generation of mouse strains with disruptions in genes that encode individual receptor subtypes; the resulting mutant mice have a complete and specific absence of the targeted receptor. [...]

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