Interstitial cells of Cajal in the deep muscular plexus mediate enteric motor neurotransmission in the mouse small intestine.

Interstitial cells of Cajal (ICC) provide important regulatory functions in the motor activity of the gastrointestinal tract. In the small intestine, ICC in the myenteric region (ICC-MY), between the circular and longitudinal muscle layers, generate and propagate electrical slow waves. Another population of ICC lies in the plane of the deep muscular plexus (ICC-DMP), and these cells are closely associated with varicose nerve terminals of enteric motor neurons. Here we tested the hypothesis that ICC-DMP mediate excitatory and inhibitory neural inputs in the small bowel. ICC-DMP develop largely after birth. ICC-DMP, with receptor tyrosine kinase Kit-like immunoreactivity, appear first in the jejunum and then in the ileum. We performed electrophysiological experiments on mice immediately after birth (P0) or at 10 days post partum (P10) to determine whether neural responses follow development of ICC-DMP. At P0, slow-wave activity was present in the jejunum, but neural responses were poorly developed. By P10, after ICC-DMP developed, both cholinergic excitatory and nitrergic inhibitory neural responses were intact. Muscles of P0 mice were also put into organotypic cultures and treated with a neutralizing Kit antibody. Neural responses developed in culture within 3-6 days in control muscles, but blocking Kit caused loss of ICC and loss of cholinergic and nitrergic neural responses. Non-cholinergic excitatory responses remained after loss of ICC-DMP. Our observations are consistent with the idea that cholinergic and nitrergic motor neural inputs are mediated, to a large extent, via ICC-DMP. Thus, ICC-DMP appear to serve a function in the small intestine that is similar to the role of the intramuscular ICC in the stomach.

Development of Ca2+-channel and BK-channel expression in embryos and larvae of Xenopus laevis.

The patterns and density of channels expressed in neurons critically determine their electrical properties. We have examined developmental regulation of Ca2+-channel expression during the maturation of the spinal motor circuits in Xenopus as it develops from an embryo to a larva. In embryonic neurons approximately 60% of the current is carried by N-type channels, 8% by l-type channels and the remainder by an unidentified channel. As the embryo matures, omega-agatoxin-sensitive P/Q channels are gradually expressed and replace the unidentified HVA channel such that at stage 42 approximately 25% of the current is carried by P/Q channels. We have used fluorescent labelling of selective channel toxins to directly observe the distribution of P/Q, N and BK channels. The P/Q channel distribution was most prevalent on the cell surface proximal to the areas of the soma where processes emerge. Both N and BK channels were distributed throughout the soma but still exhibited concentration around the areas adjacent to the emergence of processes from the soma. The patterns of fluorescence labelling during development mirrored the development of the respective ionic currents. Both N and P/Q channels contribute roughly equally to activation of the BK current, suggesting that overlap in the distribution of the N, P/Q and BK channels is important in their functional interdependence. The newly expressed P/Q channels play a role in spike initiation and repetitive firing in larval spinal neurons and contribute to burst generation during swimming in the larva.