Carstens EE, Iodi-Carstens M, Simons CT (2003) Reduced scratching in rats receiving intracisternal substance-saporin to ablate cervical superficial dorsal horn neurons that express NK-1 receptors. Neuroscience 2003 Abstracts 908.2. Society for Neuroscience, New Orleans, LA.
Summary: While glutamate and the neuropeptide substance P (SP) are involved in the spinal neurotransmission of nociceptive signals, little is known about transmitters involved in itch. We investigated a role for SP in itch by determining if scratching behavior is affected by selective neurotoxic destruction of cervical superficial dorsal neurons that express NK-1 receptors for SP. Sprague-Dawley rats received intracisternal microinjection of SP conjugated to saporin (SP-SAP; 2.27 μM/ 20 μl). Controls received saporin (SAP) only. At least 2 wk post-surgery, rats were tested for dose-related hindlimb scratching directed toward the site of intradermal microinjection of serotonin (5-HT; 50, 100 or 200 μg/10 μl) or saline (control) into the nape of the neck, with at least 1 wk between sessions for each dose. After the intradermal injection, rats were videotaped for 44 min. The numbers and durations of individual scratching bouts were counted and averaged for each dose. Rats receiving SAP exhibited a dose-related increase in scratching bouts similar to naïve rats. Rats receiving SP-SAP exhibited significantly reduced scratching (to ~38%) at all 5-HT doses. Individual bout durations (~2 s) did not vary significantly between groups or by dose of 5-HT. After behavioral testing, rats were perfused with fixative and caudal medullary and cervical spinal cord sections processed immunohistochemically for NK-1 receptors. Tissue from SAP-treated rats exhibited a normal distribution of pronounced NK-1 immunoreactivity in superficial layers of the dorsal horn at caudal medullary through C5 levels, while in SP-SAP-treated rats there was a complete absence of NK-1 immunoreactivity at these levels. These results indicate that SP plays an important role in neurotransmission from itch-signaling primary afferent fibers to second-order neurons in the superficial dorsal horn.
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