Electrical properties and synaptic potentials of rabbit pancreatic neurons

Pancreatic ganglia receive innervation from a wide variety of extrinsic nerves and supply the predominant innervation to pancreatic acini, islets, and ducts. This study used intracellular recordings to investigate the electrical properties and synaptic potentials of rabbit pancreatic neurons. Neurons had a mean resting membrane potential of −54±0.4 mV and generated action potentials with a mean overshoot of 10±0.4 mV and a mean after-spike hyperpolarization (ASH) of 11±0.5 mV with duration of 210±19 ms. Action potentials exhibited a high threshold (−15±1 mV) for intracellular stimulation and a phasic firing pattern was observed in response to prolonged depolarizing currents. Stimulation of attached nerve bundles evoked multiple fast excitatory postsynaptic potentials (fEPSPs) which were abolished by hexamethonium in 75% of neurons, while a non-cholinergic fEPSP was observed in 25% of the neurons. Repetitive stimulation (3–30 Hz) evoked muscarinic slow EPSPs with a mean amplitude of 8±2 mV and duration of 5±1 s in a small subset (21%) of neurons. Exogenous muscarine evoked a mean slow depolarization of 10±1 mV amplitude in 22% of neurons tested. Following repetitive nerve stimulation non-cholinergic late, slow EPSPs with a mean amplitude of 4.3±0.4 mV were recorded in 32% of neurons. Nicotinic transmission was subject to inhibition mediated by presynaptic muscarinic receptors at low (0.5 Hz) stimulus frequencies in 80% of neurons. At higher frequencies (≥1 Hz), either facilitation or depression of nicotinic transmission was observed depending on the ganglion studied. A population (9%) of neurons exhibited spontaneous, low-amplitude pacemaker-like potentials. Spontaneous fEPSPs and action potentials were also observed and these occasionally occurred in rhythmically timed bursts. Thus, distinct subpopulations of pancreatic neurons could be identified on the basis of both their intrinsic electrical properties and the receptors mediating and/or modulating synaptic transmission. These neurons function as critical sites of integration for synaptic input from extrinsic pancreatic nerves and thereby determine the postganglionic firing patterns presented to the pancreatic exocrine and endocrine secretory cells.