**4.1 Mechanism of action and toxic effects**

Opioids bind to specific endorphin system receptors located throughout the nervous system but not only. Opioid receptors are G-protein-coupled transmembrane receptors. These exist throughout the CNS, with particularly high concentration in thalamus and spinal cord. They are also present outside the CNS, and these are responsible for other opioids effects (gastrointestinal tract) and their postulated value in some peripheral anaesthetic techniques, such as intra-articular infiltrations [6]. The actions of various opioids are induced by the specific binding properties of the agent to the various receptors (**Table 5**).

