Receptors, which locate on both the cell surface and within the cell, are drug targets where medicine produce their beneficial effects in various disease states. Receptors are typically envisaged as cell surface recognition sites for endogenous hormones, neurotransmitters, and neuromodulators. They are coupled to various signal transduction
systems located both within the membrane and intracellular, and can therefore regulate responses to the cellular/tissue microenvironment. Receptors can be defined in terms of their selectivity, the saturability and reversibility of ligand binding, and functionality. The definition of a receptor in both pharmacological and physiological terms requires that it has specific interactions with ligands that belong to a given pharmacological class. Receptors are complex proteins with multiple potential ligand recognition sites, including sites that may be distinct from the endogenous agonist recognition site and may actually reside on distinct proteins that are part of the receptor complex. Such receptor modulatory sites may represent novel drug targets, e.g., allosteric or modulatory sites. The effect of benzodiazepines (BZs) on GABAA receptor function illustrates the conceptualization of ancillary drug targets and the elusive nature of the proposed endogenous modulator, presumed to be a "BZ-like" substance. In septic shock, the induction of a toxic cytokine receptor-mediated cascade has significantly complicated the search for new drugs
to treat this condition. This emphasizes the need to define key targets in critical pathways rather than attempt to treat their sequelae.
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