Action potential-dependent calcium transients in myenteric S neurons of the guinea-pig ileum

Simultaneous intracellular microelectrode recording and Fura-2 imaging was used to investigate the relationship between intracellular calcium ion concentration ([Ca2+](i)) and excitability of tonic S neurons in intact myenteric plexus of the guinea-pig ileum, S neurons were impaled in myenteric ganglia, at locations near connections with internodal strands. The calcium indicator Fura-2 was loaded via the recording microelectrode. The estimated [Ca2+](i) of these neurons was approximately 95 nM (n = 25). Intracellular current injection (200 ms pulses, 0.2 nA, delivered at 0.05 Hz) resulted in action potential firing throughout the stimulus pulse, accompanied by transient increases in [Ca2+](i) (to approximately 240 nM, n = 12), Increasing the number of evoked action potentials by increasing stimulus duration (100-500 ms) or intensity (0.05-0.3 nA) produced correspondingly larger [Ca2+](i) transients. Single action potentials rarely produced resolvable [Ca2+](i) events, while short bursts of action potentials (three to five events) invariably produced resolvable [Ca2+](i) increases. Some neurons demonstrated spontaneous action potential firing, which was accompanied by sustained [Ca2+](i) increases. Action potential firing and [Ca2+](i) increases were also observed by activation of slow synaptic input to these neurons, in cases where the slow depolarization initiated action potential firing. Action potentials (evoked or spontaneous) and associated [Ca2+](i) transients were abolished by tetrodotoxin (1 mu M). omega-conotoxin GVIA (100 nM) reduced [Ca2+](i) transients by approximately 67%, suggesting that calcium influx through N-type calcium channels contributes to evoked [Ca2+](i) increases. The S neurons in this study showed prominent afterhyperpolarizations following bursts of action potential firing. The time-course of afterhyperpolarizations was correlated with the time-course of evoked [Ca2+](i) transients. Afterhyperpolarizations were blocked by tetrodotoxin and reduced by omega-conotoxin GVIA, suggesting that calcium influx through N-type channels contributes to these events. The electrical properties of Fura-2-loaded neurons were not significantly different from properties of neurons recorded without Fura-2 injection, suggesting that Fura-2 injection alone does not significantly influence the electrical properties of these cells. These data indicate that myenteric S neurons in situ show prominent, activity-dependent increases in [Ca2+](i). These events can be generated spontaneously, or be evoked by intracellular current injection or synaptic activation. [Ca2+](i) transients in these neurons appear to involve action potential-dependent opening of N-type calcium channels, and the elevation in [Ca2+](i) increase may underlie afterhyperpolarizations and regulate excitability of these enteric neurons. (C) 1999 IBRO. Published by Elsevier Science Ltd.