Ryanodine-Sensitive Stores Regulate the Excitability of AH Neurons in the Myenteric Plexus of Guinea-Pig Ileum

Abstract

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.

Characterization of Myenteric Sensory Neurons in the Mouse Small Intestine

“…stanford.edu/ϳcpatton/maxc.html) free [Ca 2ϩ ] of 0.09 M at 37°C (Bers et al 1994). This value is close to the resting free intracellular [Ca 2ϩ ] as estimated using Ca 2ϩ -sensitive dyes in guinea pig Dogiel type II neurons (Hillsley et al 2000;Tatsumi et al 1988).…”

“…Alternatively, dialysis with patch pipette solutions, including Ca 2ϩ chelators such as EGTA might reduce Ca 2ϩ -dependent K ϩ currents (Staley et al 1992;Velumian and Carlen 1999). Therefore we expected that sharp electrodes might have artifactually facilitated the recording of single-spike AHPs because the slow AHP appears to depend on the priming of intracellular Ca 2ϩ stores (Hillsley et al 2000) and because impalement produces shunt currents that can load the cell with calcium (Clements and Redman 1989; Spruston and Johnston 1992;Staley et al 1992;Thurbon et al 1998). Furthermore, measurements made using the Ca 2ϩ indicator fura-2 in hippocampal neurons have demonstrated an increase of Յ1 M in intracellular [Ca 2ϩ ] that was caused by impalement with a fine microelectrode (Kudo and Ogura 1986).…”

Histamine promotes excitability in bovine adrenal chromaffin cells by inhibiting an M-current

“…The afterhyperpolarization was not affected by thapsigargin pretreatment indicating that CICR is not important for activation of Ca 2+ -activated K + channels during the afterhyperpolarization, in contrast to several central and peripheral neurons (Sah & McLachlan, 1991;Jobling et al 1993;Hillsley et al 2000;Shah & Haylett, 2000;Vogalis et al 2001). Further, as the afterhyperpolarization was unaffected by apamin, it is unlikely that SK channels play a significant role in the afterhyperpolarization.…”

Control of IsAHP in mouse hippocampus CA1 pyramidal neurons by RyR3-mediated calcium-induced calcium release

“…In some peripheral neurons [20,29,40,54,60] and also in some central neurons [6,44,48], the ryanodine-sensitive intracellular Ca 2+ store is an important source of Ca 2+ for the activation of IsAHP.…”