We have investigated the existence of neural connections between the duodenum 
and the sphincter of Oddi (SO). Stimulation of duodenal myenteric fiber bundles 
elicited synaptic responses in SO neurons, which included nicotinic fast 
excitatory postsynaptic potentials (EPSPs), slow EPSPs, and 
alpha(2)-adrenoreceptor-mediated inhibitory postsynaptic potentials. After 48 h 
in organ culture, when extrinsic fibers had diminished, only the fast EPSPs 
persisted. Duodenal mucosal stimulation also elicited nicotinic fast EPSPs in SO 
neurons. There was no association between the SO neurons that received duodenal 
input and their chemical coding. A reciprocal projection also exists from the SO 
to the duodenum. In acute and cultured preparations, duodenal myenteric 
stimulation caused antidromic responses in 20% of SO neurons. Furthermore, 45.6 
+/- 10.5 neurons in SO ganglia were retrogradely labeled from dye application 
sites in the duodenum. It is proposed that bidirectional neural communication 
occurs between the duodenum and the SO and that duodenal neurons provide 
excitatory fast synaptic input to SO neurons through a reflex that can be 
activated at the duodenal mucosa.