Gene knockout mice for opioid receptors and ligands
| Gene disrupted | Development | Phenotype (homozygous) | Reference |
| β-endorphin | No effect | No overt locomotor or behavioural changes; no analgesic response to swim stress | Rubinstein and colleagues2 |
| Pre-pro-enkephalin | No effect | Altered locomotor activity | Konig and colleagues3 |
| Increased pain perception to a variety of pain stimuli | |||
| Altered anxiety response | |||
| μ-opioid receptor | No effect | Altered locomotor activity | Mattes and colleagues4 |
| Analgesic effect to morphine abolished | |||
| Altered anxiety response | |||
| Response to thermal pain abolished | |||
| Normal response to visceral chemical pain | |||
| δ-opioid receptor | No effect | Awaiting full characterisation | Zhu and colleagues5 |
| κ-opioid receptor | No effect | No effect on locomotor activity | Simonin and colleagues |
| Anxiety response normal | |||
| Normal response to thermal, chemical and mechanical pain | |||
| Increased response to visceral chemical pain |