Na_V-igating excitement in the enteric nervous system

The enteric nervous system (ENS) resides in the gut wall and is the division of the autonomic nervous system that controls gastrointestinal motility, secretomotor function and local blood flow. The ENS performs most of these functions independently of the central nervous system because the ENS contains the elements required for integrative control of gastrointestinal behaviours. The ENS contains ‘sensory’ neurons that detect chemical and mechanical stimuli. The sensory neurons synapse with interneurons making synaptic connections with motor neurons which release neurotransmitters that contract or relax gut muscle and blood vessels or cause water and electrolyte secretion by enterocytes. The ENS is composed of the myenteric plexus which controls motility and the submucosal plexus which controls local blood flow and secretion (Furness, 2006).

A key player in enteric neural circuitry is the intrinsic primary afferent neuron (IPAN), which is a sensory neuron (Clerc et al. 2002). The IPAN has a process terminating in the mucosa, a cell body in submucosal or myenteric ganglia and a second process that makes connections with other IPANS or interneurons. Chemical or mechanical stimulation of the mucosa activate IPANs directly or indirectly via activation of enterochromaffin cells which release 5-HT and perhaps ATP. 5-HT and/or ATP act at their receptors on the mucosal terminals of the IPAN causing action potentials to propagate back through the nerve cell body (Clerc et al. 2002; Bertrand, 2003). This sequence of events initiates gut reflexes. The IPAN has electrophysiological properties that allow modulation of the output from these cells. IPANs are also known as AH neurons because their action potentials are followed by a long-lasting (1–10 s) afterhyperpolarization that limits action potential firing. The afterhyperpolarization is mediated by an intermediate conductance calcium-activated potassium current (I_KCa) that is activated by calcium entering the neuron during action potentials (Clerc et al. 2002). IPANs/AH neurons also express a hyperpolarization-activated cation current (I_H) that is carried by a cyclic nucleotide-gated cation channel (Clerc et al. 2002). Activation of I_H increases the excitability of the IPAN/AH neuron by causing depolarization and by shortening the afterhyperpolarization.

I_KCa is reduced by treatments which activate protein kinase A (PKA) and protein kinase C (PKC). Inhibition of I_KCa increases IPAN/AH neuron excitability. I_H is activated by treatments which increase intracellular cAMP. Inhibition of I_KCa and activation of I_H in IPANs/AH neurons enhances gut reflexes. This is important because inflammatory mediators activate signalling pathways leading to inhibition of I_KCa or I_H activation. IPANs/AH neurons also exhibit slow excitatory postsynaptic potentials (sEPSPs) that are mediated by substance P acting at neurokinin 3 (NK₃) receptors. NK₃-mediated sEPSPs are due to inhibition of I_KCa (Clerc et al. 2002). Slow synaptic excitation and acute inflammation both enhance gut reflexes.

The pathways described above help to explain plasticity in enteric reflexes. However, the paper by Copel and co-workers (Copel et al. 2009) in this issue of The Journal of Physiology provides evidence for a new mechanism that modulates the excitability of the IPAN/AH neuron and therefore might contribute to plasticity in enteric reflexes. Copel et al. used whole-cell patch clamp techniques to record from myenteric neurons in the acutely isolated guinea pig duodenum longitudinal muscle myenteric plexus preparation. The investigators isolated Na_v1.9 by adjusting the intracellular and extracellular ion concentrations and by adding TTX to the extracellular solution. In myenteric neurons, Na_v1.9 carries a window current (the current between the steady-state activation and inactivation curves) where there is persistent channel activation in the membrane potential range between 0 and −40 mV. Copel et al. go on to show that Na_v1.9 activity is enhanced by application of senktide, a NK₃ receptor agonist, or substance P. This enhancement occurs because there is a negative shift in the activation curve of about 10 mV while the inactivation curve shifts in the negative direction by only 4 mV. This moves the range of potentials in which Na_v1.9 exhibits persistent activation to between −10 and −50 mV. In addition, the negative shift in the activation curve decreases the threshold for activation of Na_v1.9 and this will decrease action potential threshold in IPANs/AH neurons. Both of these effects will increase IPAN/AH neuron excitability. Finally, Copel et al. show that the effects of NK₃ receptor activation on Na_v1.9 function can be mimicked by phorbol esters which activate PKC. This is important for several reasons. Firstly, NK₃ receptors couple to PKC-ɛ activation in myenteric neurons (Poole et al. 2008). Secondly, other enteric neurotransmitters will have an opportunity to modulate Na_v1.9 function via PKC activation. Finally inflammatory mediators that signal through PKC-dependent pathways can modulate Na_v1.9 function.

Neural control of gastrointestinal motility requires finely tuned interactions between nerves, muscle and a variety of other cell types in the gut wall. Modulation of IPAN/AH neuron excitability is one point where the gain of the neural circuitry can be controlled. Na_v1.9 is a newly identified target for treatments that can modulate the gain of the neural circuitry and it may also prove useful as a target for drugs that could be used to treat motility disorders associated with IPAN/AH neuron hyperexcitability such as inflammation (Linden et al. 2003).