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Exploring short term plasticity in guinea-pig myenteric neurons

R.L. Bertrand,1 K. Michel,2 M. Schemann2 and P.P. Bertrand,1 1Department of Physiology, School of Medical Sciences, University of New South Wales, NSW 2052, Australia and 2Department of Human Biology, Technische Universität München, Freising, Germany.

Enteric neurons undergo long-term increases in excitability during inflammation or infection, and can change the proportion of neurotransmitters responsible for fast synaptic transmission. We hypothesised that short-term, low frequency stimulation would be associated with an upregulation of non-nicotinic fast synaptic transmission (fast EPSPs). Two methods were used to monitor membrane potential changes in myenteric neurons from the guinea pig ileum: intracellular recordings, and fast CCD-based imaging with the potentiometric dye di-8-ANEPPS. Low frequency electrical stimulation of an interganglionic strand was used to stimulate activity in the myenteric network. Hexamethonium (200μM) was used to block nicotinic fast EPSPs. Imaging experiments revealed a control fast EPSP amplitude of 1.00±0.10ΔF/F (n=50 neurons). Following addition of hexamethonium for 10 minutes the fast EPSP was reduced to 0.55±0.07ΔF/F (55% of control). A train of electrical stimuli was then applied for 5 minutes (1Hz, 0.4ms) followed by a rest period of 2.5 minutes and fast EPSPs evoked again. After the first train fast EPSPs were still depressed (49% of control) and further trains of electrical stimulation did not improve this. Washout of hexamethonium resulted in fast EPSP amplitude returning to 67% of control (n=20). Electrophysiological experiments also showed that in the presence of hexamethonium fast EPSP amplitude remained depressed after stimulation (hexamethonium : 38% of control; after stimulation: 30% of control, n=5). In the presence of nicotinic blockade low frequency electrical stimulation of mixed excitatory/inhibitory fibres was not associated with an increase in fast EPSP amplitude. We predict that selective stimulation of excitatory fibres alone may be associated with an increase in non-nicotinic fast synaptic transmission.