Opioid agonists inhibit excitatory neurotransmission in ganglia and at the neuromuscular junction in Guinea pig gallbladder

BACKGROUND & AIMS: Opiates administered therapeutically could have an inhibitory effect on the neuromuscular axis of the gallbladder, and thus contribute to biliary stasis and acalculous cholecystitis. METHODS: Intracellular recordings were made from gallbladder neurons and smooth muscle, and tension measurements were made from muscle strips. Opioid receptor-specific agonists tested: delta, DPDPE; kappa, U-50488H; and mu, DAMGO. RESULTS: Opioid agonists had no effect on gallbladder neurons or smooth muscle. Each of the opioid agonists potently suppressed the fast excitatory synaptic input to gallbladder neurons, in a concentration-dependent manner with half-maximal effective concentration values of about 1 pmol/L. Also, each agonist caused a concentration-dependent reduction in the amplitude of the neurogenic contractile response (half-maximal effective concentration values: DPDPE, 189 pmol/L; U-50488H, 472 pmol/L; and DAMGO, 112 pmol/L). These ganglionic and neuromuscular effects were attenuated by the highly selective opioid-receptor antagonist, naloxone. Opioid-receptor activation also inhibited the presynaptic facilitory effect of cholecystokinin in gallbladder ganglia. Immunohistochemistry with opioid receptor-specific antisera revealed immunostaining for all 3 receptor subtypes in nerve bundles and neuronal cell bodies within the gallbladder, whereas opiate-immunoreactive nerve fibers are sparse in the gallbladder. CONCLUSIONS: These results show that opiates can cause presynaptic inhibition of excitatory neurotransmission at 2 sites within the wall of the gallbladder: vagal preganglionic terminals in ganglia and neuromuscular nerve terminals. These findings support the concept that opiates can contribute to gallbladder stasis by inhibiting ganglionic activity and neurogenic contractions.