Purinergic neuromuscular transmission in the gastrointestinal tract; functional basis for future clinical and pharmacological studies

Abstract

Nerve-mediated relaxation is necessary for the correct accomplishment of gastrointestinal (GI) motility. In the GI tract, NO and a purine are probably released by the same inhibitory motor neuron as inhibitory co-transmitters. The P2Y1 receptor has been recently identified as the receptor responsible for purinergic smooth muscle hyperpolarization and relaxation in the human gut. This finding has been confirmed in P2Y1 -deficient mice where purinergic neurotransmission is absent and transit time impaired. However, the mechanisms responsible for nerve-mediated relaxation, including the identification of the purinergic neurotransmitter(s) itself, are still debatable. Possibly different mechanisms of nerve-mediated relaxation are present in the GI tract. Functional demonstration of purinergic neuromuscular transmission has not been correlated with structural studies. Labelling of purinergic neurons is still experimental and is not performed in routine pathology studies from human samples, even when possible neuromuscular impairment is suspected. Accordingly, the contribution of purinergic neurotransmission in neuromuscular diseases affecting GI motility is not known. In this review, we have focused on the physiological mechanisms responsible for nerve-mediated purinergic relaxation providing the functional basis for possible future clinical and pharmacological studies on GI motility targeting purine receptors.

Figure 1

Single pulses or short trains elicit an IJPf in different areas of the GI tract. Note the absence of an IJPf in oesophageal tissues and the presence of spontaneous IJP in some tracings.

Figure 2

Pulses of 5 Hz for 5 s elicit a fast followed by a sustained hyperpolarization in different areas of the GI tract. Note the absence of an IJPf in the human oesophagus (oesophageal body and lower oesophageal sphincter).

Figure 3

Inhibitory junction potentials are concentration-dependently inhibited by P2Y₁ receptor antagonists. Tracings are from human jejunum and data from human jejunum and colon.

Figure 4

Representative tracings summarizing studies in knockout mice. MRS2500-sensitive IJPf recorded in the colon (A), caecum (B) and gastric antrum (C) in wild-type animals are absent in P2Y₁−/− mice. In P2Y₁−/− animals, the IJPs is totally L-NNA-sensitive.

Figure 5

Pulse of 1 Hz for 5 s reveal the presence of an IJPf rundown. The first IJPf has a bigger amplitude compared with the following responses.

Figure 6

Spontaneous IJP are MRS2500 sensitive in wild-type mice and completely absent in P2Y₁ KO mice. Frequency distribution of the RMP fully supports these results. In the frequency distribution of recordings from wild-type animals, ongoing sIJP create a tail towards the most negative values. This tail does not appear in the frequency distribution obtained from tissue incubated with MRS2500 or from P2Y₁ KO mice. L-NNA-treated tissue (not shown) depolarizes smooth muscle cells without changing the internal frequency distribution, which is consistent with the presence of spontaneous IJP.

Figure 7

Both L-NNA and MRS2179 are necessary to inhibit (5 Hz, supramaximal voltage) EFS-induced relaxation in the human small and large intestine.

Figure 8

Purinergic fast and spontaneous IJP are recorded in deficient mouse (W^sh/sh) with impaired ICC development (unpublished data).

Figure 9

Smooth muscle relaxation is accomplished by enteric inhibitory motor neurons in the gastrointestinal tract. NO and a purine (ATP/ADP/β-NAD) are possibly co-released by inhibitory motor neurons. GC (ODQ sensitive) mediates the nitrergic slow component of the IJP. P2Y₁ receptors (MRS2179-, MRS2279- and MRS2500-sensitive) mediate the purinergic fast component of the IJP. Smooth muscle can transduce both nitrergic and purinergic signals through a direct communication. ICC and PDGFRα+ cells are potential intercalated cells that might transduce nitrergic and purinergic inputs to smooth muscle cells respectively. Due to the electrophysiological profile of the response, nitrergic IJP is tonic since it can be time-sustained, whereas purinergic IJP is phasic because the response runs down. The combination of both mechanisms is responsible for relaxation in different regions of the gastrointestinal tract that might be potentially impaired in primary and secondary disorders affecting the neuromuscular junction. [agonists (neurotransmitters) are depicted in green and antagonists (blocking these pathways at different levels) are depicted in red].