PATCH-CLAMP RECORDINGS OF MEMBRANE CURRENTS EVOKED DURING NATURAL SYNAPTIC ACTIVITY IN SYMPATHETIC NEURONS

Membrane currents elicited by colonic distension and by electrical sti mulation of the intermesenteric nerve containing colonic afferent nerv e fibres were recorded from neurons of the mouse superior mesenteric g anglion at 20 degrees C with the whole-cell patch-clamp method. Electr ically-evoked excitatory postsynaptic currents reversed at - 3.5 mV. A t membrane holding voltages of - 70 mV and - 110 mV, the excitatory po stsynaptic currents were characterized by a single exponential decay w ith a mean (+/- S.E.M.) time-constant of 17.5 +/- 1.3 ms and 15.5 +/- 2.3 ms, respectively. Colonic distension evoked a series of the excita tory postsynaptic currents which ranged in amplitude from 10 to 700 pA (at a membrane holding voltage of - 70 mV). Hexamethonium (100 mu M) applied only to the ganglion abolished both electrically- and distensi on-evoked excitatory postsynaptic currents, suggesting activation of n icotinic acetylcholine receptors. The decay time-course of distension- evoked single excitatory postsynaptic currents was characterized by on e, or, less commonly, by two exponentials. The decay time-constant his tograms of distension-evoked single excitatory postsynaptic currents e xhibited main kinetic components of 8.1 +/- 2.3 ms and 8.2 +/- 2.5 ms (peak +/- S.D.) at - 70 and - 110 mV membrane holding voltages, respec tively. Longer time-constants ranging up to 51 ms were also observed. The number of the distension-evoked excitatory postsynaptic currents w ith a decay time-constant higher than 20 ms, as well as their mean amp litude, were significantly lower at - 110 mV than at - 70 mV membrane potential levels, in contrast to the currents with a decay time-consta nt lower than or equal to 20 ms. The results suggest that colonic affe rent nerve fibres activate in the mouse superior mesenteric ganglion n eurons a few populations of the postsynaptic nicotinic acetylcholine r eceptors with different channel kinetics, which are characterized by a lack of voltage sensitivity within - 70 to - 110 mV membrane potentia l range, except those with comparatively slow channel kinetics, which are possibly blocked by membrane hyperpolarization. (C) 1998 IBRO. Pub lished by Elsevier Science Ltd.