Enigmatic receptors

The identification of new binding sites raises the problem of defining their role, if any. At times they are shown to be pharmacological receptors, in a strict sense, as they fulfill certain requirements, and a precise physiological role and function, and an endogenous ligand (neurotransmitter) are discovered. At other times, however, neither a clear physiological role nor an endogenous ligand are found, but the term “receptor” is still used, although it may not be a proper one in the conventional pharmacological sense. Furthermore, no clear intracellular signalling transduction pathway is defined and, as a consequence, it is not possible to determine whether drugs binding to these receptors act as agonists or antagonists. What their structure and biological function are and how they mediate the pharmacological effects of ligands may remain for a long time an enigma. The matter, in any case, is of great interest to researchers of different areas, especially to medicinal chemists who foresee novel potential targets for therapeutic interventions. In this meeting one section is dedicated to two examples of this kind of receptors: imidazoline (I) and sigma (σ) receptors.