Relative contribution of SK_Ca and TREK1 channels in purinergic and nitrergic neuromuscular transmission in the rat colon

Abstract

Purinergic and nitrergic neurotransmission predominantly mediate inhibitory neuromuscular transmission in the rat colon. We studied the sensitivity of both purinergic and nitrergic pathways to spadin, a TWIK-related potassium channel 1 (TREK1) inhibitor, apamin, a small-conductance calcium-activated potassium channel blocker and 1H-[1,2,4]oxadiazolo[4,3-α]quinoxalin-1-one (ODQ), a specific inhibitor of soluble guanylate cyclase. TREK1 expression was detected by RT-PCR in the rat colon. Patch-clamp experiments were performed on cells expressing hTREK1 channels. Spadin (1 μM) reduced currents 1) in basal conditions 2) activated by stretch, and 3) with arachidonic acid (AA; 10 μM). l-Methionine (1 mM) or l-cysteine (1 mM) did not modify currents activated by AA. Microelectrode and muscle bath studies were performed on rat colon samples. l-Methionine (2 mM), apamin (1 μM), ODQ (10 μM), and N(ω)-nitro-l-arginine (l-NNA; 1 mM) depolarized smooth muscle cells and increased motility. These effects were not observed with spadin (1 μM). Purinergic and nitrergic inhibitory junction potentials (IJP) were studied by incubating the tissue with l-NNA (1 mM) or MRS2500 (1 μM). Both purinergic and nitrergic IJP were unaffected by spadin. Apamin reduced both IJP with a different potency and maximal effect for each. ODQ concentration dependently abolished nitrergic IJP without affecting purinergic IJP. Similar effects were observed in hyperpolarizations induced by sodium nitroprusside (1 μM) and nitrergic relaxations induced by electrical stimulation. We propose a pharmacological approach to characterize the pathways and function of purinergic and nitrergic neurotransmission. Nitrergic neurotransmission, which is mediated by cyclic guanosine monophosphate, is insensitive to spadin, an effective TREK1 channel inhibitor. Both purinergic and nitrergic neurotransmission are inhibited by apamin but with different relative sensitivity.

Activation of TREK-1 by morphine results in analgesia without adverse side effects

“…Interestingly, contrary to its antinociceptive effect, morphineinduced reduction in gastroinstestinal tract function was unchanged in TREK-1 KO mice; all parameters tested were identical in both genotypes. This result is consistent with the recent observations that the nitrergic-induced inhibition of neuromuscular transmission in the gastrointestinal tract, an important factor in morphine-induced constipation 47 , does not involve TREK-1 channels 48 . It demonstrates that analgesia and constipation, two usual effects of opioids, do not involve the same mechanisms downstream from mOR.…”

Interstitial cells of Cajal mediate nitrergic inhibitory neurotransmission in the murine gastrointestinal tract

“…, and ICC-GCKO (13 cells from 6 animals). In the global GCKO, a nitrergic IJP was not measurable; on the contrary, the purinergic IJP was followed by a slight depolarization (10 cells from 5 animals Studies investigating the downstream targets of cGMP revealed the involvement of cGMP-dependent protein kinase I (PKG), but the identity of further downstream mediators is uncertain (14,26,32). In a recent publication, Klein et al (24) proposed PKG to be the exclusive mediator of the nitrergic IJP in colonic ICC, based on an apparently abolished nitrergic hyperpolarization in ICC-specific PKG KO mice.…”

Estrogen Regulates the Expression of Small-Conductance Ca²⁺-Activated K⁺ Channels in Colonic Smooth Muscle Cells

“…On the other hand, the activation of apamin-sensitive SK channels by low-frequency field stimulation has been associated with the relaxation of isolated mouse stomach [23], andpurinergic stimulation of mouse ileum [24] and colon [8] triggers the opening of apamin-sensitive SK channels, which also participate in the relaxation of mouse distal colon induced by nitric oxide treatment [25]. In our study, after the CCh-induced maximal contraction of colonic circular muscle, administration of apamin led to the contraction of the muscle again quickly, which confirmed that the activation of apamin-sensitive SK channels might inhibit colon contraction.…”

“…The SK family in mammals includes SK1, SK2, and SK3. In addition, SK2 and SK3 are expressed in the colon, and apamin-sensitive SK channels underlie the relaxing effect of purinergic stimulation on the colon and might also play a role on the nitrergic relaxation pathway [8]. …”