Cajal Cells Modulate Excitatory Neuronal Inputs in the Gut
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Material below summarizes the article Excitatory Neuronal Responses of Ca2+ Transients in Interstitial Cells of Cajal in the Small Intestine, published on March 16, 2018, in eNeuro and authored by Salah A. Baker, Bernard T. Drumm, Karolina E. Skowronek, Benjamin E. Rembetski, Lauren E. Peri, Grant W. Hennig, Brian A. Perrino, and Kenton M. Sanders.
Interstitial cells of Cajal (ICC) regulate smooth muscle excitability and motility in the gastrointestinal (GI) tract.
ICC express the Ca2+ activated Cl- channel Ano1 and depolarizing current resulting from its activation can be transduced to electrically coupled smooth muscle cells (SMCs) due to the presence of gap junctions. Thus, responses of ICC can affect SMC contractility and mediate complex motility patterns.
While the role of certain classes of ICC as intestinal pacemakers is well-established, other possible roles ascribed to ICC are controversial.
There is currently debate in the literature regarding whether certain classes of ICC are innervated by enteric motor neurons and act as neuromodulators, receiving direct enteric neural input and transducing this neural signal to electrically coupled SMCs.
Innervation of intra-muscular ICC (ICC-IM) in the stomach and colon has been implied from their anatomical association with enteric motor neurons, and a loss of neural responses in mutant animals lacking ICC.
ICC in the deep muscular plexus (ICC-DMP) of the small intestine are analogous to ICC-IM in other GI organs and are also aligned closely with varicosities of enteric motor neurons and are hypothesized to transduce neural responses.
In contrast to this concept, other groups have claimed ICC do not play a role in enteric neurotransmission. Instead, they put forward a traditional view of direct neuromuscular transmission (NMT — enteric neurotransmitters acting exclusively on SMCs) is sufficient to explain experimental findings of how GI organs are innervated, pointing out direct neural innervation of ICC has not yet been demonstrated.
Until recently, a major obstacle in clarifying the role of ICC in enteric neurotransmission was that it was not possible to accurately visualize the activity (and thus responses to nerve stimulation) of ICC in situ.
However, with the advent of genetically encoded Ca2+ indicators (GECIs), it has now become possible to directly observe not only the spontaneous activity of ICC in situ but also their responses to enteric neurotransmitters and electrical field stimulation (EFS).
Recently, we using a GECI (GCaMP3), which was exclusively expressed in ICC via the Cre-Lox P system (Baker et al., 2016), we could directly observe spontaneous Ca2+ transients in ICC-DMP of the mouse small intestine.
These basal Ca2+ transients in ICC-DMP are linked to the activation of Ano1 channels, thus producing excitatory inward current. Due to the electrically coupled nature of ICC and SMCs, this would lead to increased excitability in intestinal muscles.
Using this ICC specific GECI system, we sought to clarify the role of ICC-DMP in excitatory enteric neurotransmission by observing the in situ responses of ICC-DMP to excitatory neurotransmitters and EFS.
In our study, we stimulated small intestinal enteric neurons by EFS and Ca2+ responses in ICC-DMP were monitored in situ. Our major findings demonstrate:
- Direct innervation of ICC-DMP by EFS.
- Responses of ICC-DMP to cholinergic agonists.
- Responses of ICC-DMP to neurokinins.
Neurokinins appear to be released in the absence of EFS. Binding of these peptides to NK1 receptors expressed by ICC-DMP elicits much of what had been thought to be spontaneous Ca2+ transients.
Neurokinins also appear to be the dominant excitatory neurotransmitter in the terms of eliciting post-junctional responses on ICC-DMP, and responses to EFS were attenuated by NK1 antagonists.
Previous studies showed electrophysiological responses of ICC-DMP are linked to Ca2+ release from cellular stores. Ca2+ transients in ICC-DMP couple to generation of inward currents though activation of Ano1 channels. This elicits depolarization of ICC-DMP and in coupled SMCs.
Thus, direct excitatory innervation of ICC-DMP may provide a mechanism for increasing the excitability of small intestinal SMCs via the release of neurokinins binding to NK1 receptors leading to increased Ca2+ transient firing in ICC-DMP.
Our paper shows the use of optogenetic techniques using GECI expressed exclusively in ICC provides a powerful tool to elucidate the physiological function of ICC in GI organs.
Using this approach, we have provided further evidence that certain classes of ICC in the GI tract act as neuromodulators to influence the excitability of electrically coupled SMCs.
We provide evidence of direct excitatory neural innervation of ICC-DMP in the small intestine and describe the different cholinergic and neurokinin pathways by which this innervation occurs.
These findings contribute to the concept it is highly unlikely that enteric neurotransmitters act exclusively on post junctional SMCs (NMT), and due to the electrically coupled nature of SMCs and ICC-DMP, it is more viable that excitatory neural responses are mediated at least in part by direct modulation of Ca2+ transient firing in ICC-DMP.
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Excitatory Neuronal Responses of Ca2+ Transients in Interstitial Cells of Cajal in the Small Intestine. Salah A. Baker, Bernard T. Drumm, Karolina E. Skowronek, Benjamin E. Rembetski, Lauren E. Peri, Grant W. Hennig, Brian A. Perrino, Kenton M. Sanders. eNeuro Mar 2018, 5 (2) ENEURO.0080-18.2018; DOI: https://doi.org/10.1523/ENEURO.0080-18.2018