Loperamide

Enteric neurons synthesize and release endogenous opioid peptides and other neurotransmitters, such as acetylcholine and substance P. Endogenous opioids bind to opioid receptors expressed on these neurons to regulate gastrointestinal signalling, motility, and balance of fluids and electrolytes.

Loperamide acts on the mu (μ)-opioid receptor expressed on the circular and longitudinal intestinal muscle. Receptor binding leads to the recruitment of G-protein receptor kinases and the activation of downstream molecular cascades that inhibit enteric nerve activity. By inhibiting the excitability of enteric neurons, loperamide suppresses neurotransmitter release, pre-synaptic and post-synaptic inhibition of transmission of excitatory and inhibitory motor pathways, and secretomotor pathways. Loperamide inhibits the release of acetylcholine and prostaglandins, thereby reducing propulsive peristalsis and increasing intestinal transit time. Loperamide stimulates the intestinal absorption of water and electrolytes by inhibiting calmodulin. Loperamide can bind to and hyperpolarize submucosal secretomotor neurons, promoting dry, hard stools.