Electrophysiological and muscarinic properties of celiac ganglion neurons in primary culture

Abstract

This dissertation comprises studies of the responsiveness to cholinergic agonists and the electrical membrane properties of neurons of the prevertebral, sympathetic, celiac ganglion of the guinea pig in primary culture. More specifically, the major content of this work is the investigation of the mechanisms of slow cholinergic or muscarinic excitation in these cells. The primary mechanisms of muscarinic responses in these cells resulted from the inhibition of two potassium currents: the M-current and a muscarine-sensitive leak-current. During intracellular current-clamp recording, cells were assessed for their passive and active membrane properties including: resting potential, time constant, input resistance, rheobase, spike amplitude, afterhyperpolarization amplitude and duration, and degree of accommodation. These cells responded to acetylcholine with both fast nicotinic and slow muscarinic depolarizations. The results indicated that celiac neurons in primary culture retain certain electrophysiological properties similar to those in intact ganglia, and produce varied responses to cholinergic stimulation. Responses to muscarine were studied in the dissociated neurons using the whole-cell voltage-clamp technique. Muscarine, administered to cells for 1.5 seconds, evoked inward shifts in holding current which were concentration-dependent. The amplitude of the inward current transients decreased with hyperpolarization toward the equilibrium potential for potassium, where the null or reversal potential was reached. Evidence for the presence of the M-potassium current was obtained in eighty percent of the neurons. Further investigation, however, revealed that these cells exhibit two ubiquitous and independent mechanisms of muscarinic excitation. These two mechanisms were the inhibition of two potassium currents: the M-current and a muscarine-sensitive leakage current. These currents could be expressed together in the same neuron or separately. The cation potassium channel blockers, cesium and barium, had dissimilar effects on these currents. While barium effectively blocked both types of potassium channels equally, cesium preferentially blocked M-current. Thus, cesium was able to differentiate between the two currents.