Vagal parasympathetic preganglionic efferents and their effects on gastric motility in the rat

Tayo, Emmanuel Korede (1989) Vagal parasympathetic preganglionic efferents and their effects on gastric motility in the rat. University of Southampton, Doctoral Thesis.

Abstract

The vagus nerve plays an important role in the physiological regulation of upper gastrointestinal function. The complexity of the role of the vagus is reflected in the heterogeneity of cell types at all levels of the vagal axis and in particular in the range of neurotransmitters they employ. This study examines the neurochemical heterogeneity of vagal preganglionic efferents projecting to the stomach and their role in the control of gastric motility in the rat. Vagal efferent cell bodies in the brain stem, projecting to the abdomen were identified by retrograde tracing of the fluorescent dye, True Blue, following its injection into the stomach wall. Using a combination of acetylcholinesterase histochemistry and tyrosine hydroxylase immunofluorescence, a neurochemical classification of these neurones was established. Cells which expressed acetylcholinesterase, constituted less than 40% of identified vagal efferents. Cells which expressed tyrosine hydroxylase-like immunoreactivity, constituted less than 4% of identified vagal efferents and the majority of (60%), of identified vagal efferents could not be identified on the basis of either acetylcholinesterase histochemistry or tyrosine hydroxylase immunocytochemistry, thus inferring that they might be noncholinergic and nonadrenergic. These morphological observations were used as the basis for the design of physiological experiments to re-examine the role of the vagus in the control of intragastric pressure. In the urethane anesthetised rate, the effect of stimulation of the peripheral end of the cut vagus nerve on intragastric pressure, was assessed in the presence of systemically administered hexamethomium. Hexamethonium alone, blocked all responses to vagal stimulation. However in the presence of an increased gastric tone by carbachol or bethanechol, vagal stimulation produced a hexamethonium resistant inhibition. This effect was partially but not completely blocked by propranolol and domperidone. It was not mimicked by serotin in which in this preparation produced excitation. However, brain natriuretic peptide was shown to produce an inhibition similar in magnitude to that elicited by vagal stimulation. The morphological and physiological observations are consistent and indicate the existence of a vagal catecholaminergic inhibitory effect on intragastric pressure and the presence of a significant vagal nonadrenergic noncholinergic preganglionic efferent input to the stomach.

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