Postsynaptic dorsal column neurons express NK1 receptors following colon inflammation

Recent clinical and experimental studies have suggested that the dorsal column pathway and specifically postsynaptic dorsal column neurons play an important role in the transmission of visceral pain. In our study we have mapped the distribution of postsynaptic dorsal column neurons in thoracic, lumbar and sacral spinal cord segments. The presence of immunoreactivity for neurokinin 1 receptors on these postsynaptic dorsal column neurons was examined under control conditions and after colon inflammation. The largest number of postsynaptic dorsal column neurons was found in the lumbar enlargement. They were mostly located in laminae III-IV, but depending on the spinal segment, about 7-15% of them were in the deep medial dorsal horn and in the central canal area. Under control conditions none of the 1438 postsynaptic dorsal column neurons examined expressed neurokinin 1 receptors. However, after induction of colon inflammation about 1.4% of the 2015 postsynaptic dorsal column neurons observed in the experimental group showed immunoreactivity for neurokinin 1 receptors. These neurons were preferentially found in the lower thoracic and lumbosacral spinal segments where they represented about 3-4% of the total population of postsynaptic dorsal column neurons examined. The de novo expression of neurokinin1 receptors on postsynaptic dorsal column neurons after colon inflammation suggests that substance P released from visceral primary afferents under inflammatory conditions could help produce central sensitization of these neurons.