Illustration showing a delta opioid membrane receptor (coloured ribbons) with naltrindole (red, blue and white spheres) bound to it. The delta opioid receptor is expressed in the basal ganglia and neocortical regions of the brain. It usually binds to enkephalins, opioid peptide molecules that regulate nociception (detection of harmful stimuli). Naltrindole is a highly potent delta opioid receptor antagonist that suppresses enkephalin binding. Scientists use it as a probe for delta opioid receptors.