Acute pancreatitis decreases the sensitivity of pancreas-projecting dorsal motor nucleus of the vagus neurones to group II metabotropic glutamate receptor agonists in rats

Abstract

Recent studies have shown that pancreatic exocrine secretions (PES) are modulated by dorsal motor nucleus of the vagus (DMV) neurones, whose activity is finely tuned by GABAergic and glutamatergic synaptic inputs. Group II metabotropic glutamate receptors (mGluR) decrease synaptic transmission to pancreas-projecting DMV neurones and increase PES. In the present study, we used a combination of in vivo and in vitro approaches aimed at characterising the effects of caerulein-induced acute pancreatitis (AP) on the vagal neurocircuitry modulating pancreatic functions. In control rats, microinjection of bicuculline into the DMV increased PES, whereas microinjections of kynurenic acid had no effect. Conversely, in AP rats, microinjection of bicuculline had no effect, whereas kynurenic acid decreased PES. DMV microinjections of the group II mGluR agonist APDC and whole cell recordings of excitatory currents in identified pancreas-projecting DMV neurones showed a reduced functional response in AP rats compared to controls. Moreover, these changes persisted up to 3 weeks following the induction of AP. These data demonstrate that AP increases the excitatory input to pancreas-projecting DMV neurones by decreasing the response of excitatory synaptic terminals to group II mGluR agonist.

Figure 1

A, effect of intraduodenal Ensure infusions on PES in control (n = 5) and AP (n = 4) animals. Note that Ensure increased PES (expressed as normalized data) in control, but not in AP animals. B, time course of the effect of Ensure infusion into the duodenum on the total protein output, measured at 10 min intervals. Perfusion was started at time 0 and data are expressed as total protein output in 10 min. Note that Ensure increased PES in controls, but not AP animals. *P < 0.05 vs. baseline. AP, acute pancreatitis; PES, pancreatic exocrine secretions.

Figure 2

A, effect of BIC microinjections into the DVC on PES in controls (n = 4), AP (n = 4) and post-AP (n = 4) animals. B, effect of KYN microinjections into the DVC on PES in control (n = 5), AP (n = 5) and post-AP (n = 4) animals. C, location of a representative injection site into the DVC. Note that in control animals, microinjections of BIC elicit an increase in PES whereas KYN has no effect. In AP, BIC microinjections have no effect, whereas KYN elicits a decrease in PES. Two to 3 weeks post-AP, BIC microinjections elicited an increase in PES in only one of four animals tested, whereas microinjections of KYN elicited a decrease in PES in two of four animals tested. AP, acute pancreatitis; ap, area postrema; BIC, bicuculline; cc, central canal; DMV, dorsal motor nucleus of the vagus; DVC, dorsal vagal complex; KYN, kynurenic acid; NTS, nucleus tractus solitarius; PES, pancreatic exocrine secretions.

Figure 3

A, effect of APDC on PES in control, AP and post-AP animals (n = 3–6 for each data point). B, schematic drawing of the DVC showing the location of APDC injection sites in control, AP and post-AP rats. Note that all microinjections were done in the left DVC; however, for clarity, they are shown in both sides of the DVC. C, effect of group II metabotropic glutamate receptor antagonist EGLU on PES of control and AP animals (n = 5 for both groups). Note that in control animals, APDC induced a dose-dependent increase in PES. In AP, microinjections of low doses of APDC failed to induce an increase in PES, whereas 1036 pmol of APDC decreased PES. Two to 3 weeks post-AP, only the highest dose of APDC elicited a significant increase in PES. AP, acute pancreatitis; DMV, dorsal motor nucleus of the vagus; DVC, dorsal vagal complex; NTS, nucleus tractus solitarius; PES, pancreatic exocrine secretions; sol, solitary tract.

Figure 4

A, representative traces from pancreas-projecting DMV neurone showing the effect of APDC on mEPSCs in a control (left) and an AP animal (right). B, concentration–response curve of mEPSCs to APDC in control (n = 5–8 for each data point), AP (n = 4–7 for each data point) and post-AP (n = 3–7 for each data point) neurones. C, effect of 10 μm and 300 μm APDC on mEPSC frequency in DMV neurones from control (n = 7 and 5, respectively), AP (n = 4 for both concentrations) and post-AP (n = 6 and 4, respectively) rats. *P < 0.05 vs. control (P < 0.05 vs. control also at 3 and 100 μm, not shown). D, concentration–response of mIPSCs to APDC in neurones from control (n = 3–5 for each data point) and AP (n = 4–6 for each data point) animals. E, effect of 10 μm and 300 μm APDC on mIPSC frequency in neurones from control (n = 5 and 4, respectively) and AP (n = 5 and 4, respectively) animals. Note that AP shifts the concentration–response curve of mEPSCs to APDC to the right, whereas it has no effect on the response of mIPSCs to APDC. In addition, note that 2–3 weeks post-AP, the concentration–response curve to APDC remains shifted to the right compared to control animals. AP, acute pancreatitis; DMV, dorsal motor nucleus of the vagus; mEPSC miniature excitatory postsynaptic current; mIPSC, miniature inhibitory postsynaptic current.