Cholecystokinin depolarizes guinea pig sphincter of Oddi neurons by activating CCK-A receptors

The mucosal folds at the pancreaticobiliary junction

PURPOSE

The structure and function of the mucosal folds in the terminal bile and pancreatic ducts and hepatopancreatic ampulla are poorly characterised. The distribution, muscularity, and innervation of these folds were investigated.

METHODS

The pancreaticobiliary junction was excised from ten cadavers (five male, 66-90 years) and examined histologically by serially sectioning (4-μm thickness) along the length of the terminal bile and pancreatic ducts from the tip of the major duodenal papilla. Three surgical specimens (two male, 63-72 years) were also evaluated. Sections were stained with haematoxylin and eosin, anti-actin (smooth muscle), anti-S100 (innervation), and anti-cholecystokinin (CCK)-A receptor antibodies. ImageJ software was used to compare relative radial fold projection and semi-quantitatively assess the smooth muscle and nerve content. In one additional cadaver specimen, folds were examined by scanning electron microscopy.

RESULTS

Mucosal folds in the terminal bile duct were arranged circumferentially in a lattice-like arrangement and were distributed over an average distance of 7.3 mm along the terminal bile duct compared to 4.2 mm along the pancreatic duct (P = 0.001), projected further into the lumen, and were more densely innervated than those in the terminal pancreatic duct. Folds in both ducts contained smooth muscle which was more prominent in folds nearest to the major duodenal papilla. Mucosal folds in cadaver and surgical specimens showed no evidence of CCK-A receptor immunoreactivity.

CONCLUSIONS

This study demonstrates that the mucosal folds of the terminal bile and pancreatic ducts contain muscle and nerve fibres, suggesting an active rather than purely passive function.

Tachykinins mediate slow excitatory postsynaptic transmission in guinea pig sphincter of Oddi ganglia

Intracellular recording techniques were used to test whether tachykinins could be mediators of slow excitatory postsynaptic potentials (EPSPs) in guinea pig sphincter of Oddi (SO) ganglia. Application of the tachykinin substance P (SP) onto SO neurons caused a prolonged membrane depolarization that was reminiscent of the slow EPSP in these cells. Pressure ejection of the neurokinin 3 (NK3) receptor-specific agonist senktide caused a similar depolarization; however, no responses were detected on application of NK1 or NK2 receptor agonists. The NK3 receptor antagonist SR-142801 (100 nM) significantly inhibited both SP-induced depolarization and the stimulation-evoked slow EPSP, as did NK3 receptor desensitization with senktide. Capsaicin, which causes the release of SP from small-diameter afferent fibers, induced a depolarization that was similar to the evoked slow EPSP in both amplitude and duration. The capsaicin-induced depolarization was significantly attenuated in the presence of SR-142801. These data indicate that tachykinins, released from extrinsic afferent fibers, act via NK3 receptors to provide slow excitatory synaptic input to SO neurons.

Duodenal sensory neurons project to sphincter of Oddi ganglia in guinea pig

Retrograde labeling of duodenum-sphincter of Oddi (SO) preparations in vitro with the carbocyanine dye DiI revealed that duodenal neurons project to the SO. The duodenum-SO-projecting neurons were immunoreactive (IR) for choline acetyltransferase but not nitric oxide synthase or calretinin, indicating that this is a cholinergic projection and that this pathway is distinct from the circuitry involved in the ascending limb of the peristaltic reflex. Approximately 20% of the duodenum-SO projection neurons were IR for calbindin. Calbindin-IR nerves within SO ganglia degenerated when the SO was maintained in organ culture alone, but persisted when the SO was cultured with the duodenum intact. Therefore, SO ganglia are a target of the calbindin-positive duodenum-SO projection. Because calbindin is a marker of intrinsic sensory neurons that have processes that pass to the mucosa, these neurons are in position to detect the release of a compound from the mucosa and signal its release to SO ganglia. When applied to retrogradely labeled neurons, cholecystokinin (CCK) elicited a prolonged depolarization, indicating that duodenum-SO-projecting neurons could be capable of detecting CCK released from the mucosa. It is proposed that the role of the intrinsic sensory neurons that project to the SO may be to signal the postprandial release of CCK, thus providing an instruction to decrease SO resistance and facilitate the flow of bile into the duodenum.