 Miosis and reduced bowel motility tend to persist; little tolerance develops to these effects.The canonical MOR1 isoform is responsible for morphine-induced analgesia, whereas the alternatively spliced MOR1D isoform (through heterodimerization with the gastrin-releasing peptide receptor) is required for morphine-induced itching.As with other G protein-coupled receptors, signalling by the μ-opioid receptor is terminated through several different mechanisms, which are upregulated with chronic use, leading to rapid tachyphylaxis.