Ryanodine-sensitive stores regulate the excitability of AH neurons in the myenteric plexus of guinea-pig ileum.

Myenteric afterhyperpolarizing (AH) neurons are primary afferent neurons within the gastrointestinal tract. Stimulation of the intestinal mucosa evokes action potentials (AP) that are followed by a slow afterhyperpolarization (AHP(slow)) in the soma. The role of intracellular Ca(2+) ([Ca(2+)](i)) and ryanodine-sensitive Ca(2+) stores in modulating the electrical activity of myenteric AH neurons was investigated by recording membrane potential and bis-fura-2 fluorescence from 34 AH neurons. Mean resting [Ca(2+)](i) was approximately 200 nM. Depolarizing current pulses that elicited APs evoked AHP(slow) and an increase in [Ca(2+)](i), with similar time courses. The amplitudes and durations of AHP(slow) and the Ca(2+) transient were proportional to the number of evoked APs, with each AP increasing [Ca(2+)](i) by approximately 50 nM. Ryanodine (10 microM) significantly reduced both the amplitude and duration (by 60%) of the evoked Ca(2+) transient and AHP(slow) over the range of APs tested (1-15). Calcium-induced calcium release (CICR) was graded and proportional to the number of APs, with each AP triggering a rise in [Ca(2+)](i) of approximately 30 nM Ca(2+) via CICR. This indicates that CICR amplifies Ca(2+) influx. Similar changes in [Ca(2+)](i) and AHP(slow) were evoked by two APs in control and six APs in ryanodine. Thus, the magnitude of the change in bulk [Ca(2+)](i) and not the source of the Ca(2+) is the determinant of the magnitude of AHP(slow). Furthermore, lowering of free [Ca(2+)](i), either by reducing extracellular Ca(2+) or injecting high concentrations of Ca(2+) buffer, induced depolarization, increased excitability, and abolition of AHP(slow). In addition, activation of synaptic input to AH neurons elicited a slow excitatory postsynaptic potential (sEPSP) that was completely blocked in ryanodine. These results demonstrate the importance of [Ca(2+)](i) and CICR in sensory processing in AH neurons. Activity-dependent CICR may be a mechanism to grade the output of AH neurons according to the intensity of sensory input.