Opioids induce constipation by prompting alpha-Synuclein hyperphosphorylation in the colonic myenteric plexus

Opioid-induced constipation (OIC) is the most common gastrointestinal disorder associated with opioid use. It is linked to impaired neurotransmitter release. Alpha-Synuclein (α-Syn) plays a crucial role in maintaining neurotransmitter homeostasis and regulating synaptic plasticity in the nervous system. However, its role in the disease progression remains unclear. In the present study, we investigated the impact of α-Syn hyperphosphorylation on colonic dysmotility and constipation symptoms using a Sprague-Dawley rat model of OIC. Our results suggest that α-Syn expression at the Ser129 phosphorylation site (pS129-α-Syn) is significantly increased in the colonic myenteric layer of OIC rats. Conversely, inhibiting pS129-α-Syn reversed the colonic dysmotility and increased the expression of synaptic functional proteins, such as Synapsin-1, Synaptotagmin-1, vesicle-associated membrane protein 2 (VAMP-2), and Syntaxin-1, as well as enteric neurotransmitter synthases, including neuronal nitric oxide synthase (nNOS) and adenosine triphosphate synthase (ATPB). Additionally, we found that opioids downregulate GSK3β protein expression at the Ser9 site by activating the μ-opioid receptors (MOR). This increases GSK3β kinase activity, ultimately inducing pS129-α-Syn overexpression. In summary, the development of OIC correlates with α-Syn hyperphosphorylation in myenteric plexus neurons in the colon. Opioids can inhibit synaptic vesicle trafficking and enteric neurotransmitter release via the GSK3β/α-Syn hyperphosphorylation signaling axis, ultimately leading to colonic dysmotility and constipation.