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The Journal of Neuroscience, October 8, 2003, 23(27):9171-9184

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Behavioral/Systems/Cognitive
Dorsal Horn Neurons Firing at High Frequency, But Not Primary Afferents, Release Opioid Peptides that Produce µ-Opioid Receptor Internalization in the Rat Spinal Cord

Bingbing Song and Juan Carlos G. Marvizón

Center for Neurovisceral Sciences and Women's Health, Division of Digestive Diseases, Department of Medicine, David Geffen School of Medicine at University of California Los Angeles, Los Angeles, California 90073

To determine what neural pathways trigger opioid release in the dorsal horn, we stimulated the dorsal root, the dorsal horn, or the dorsolateral funiculus (DLF) in spinal cord slices while superfusing them with peptidase inhibitors to prevent opioid degradation. Internalization of µ-opioid receptors (MOR) and neurokinin 1 receptors (NK1R) was measured to assess opioid and neurokinin release, respectively. Dorsal root stimulation at low, high, or mixed frequencies produced abundant NK1R internalization but no MOR internalization, indicating that primary afferents do not release opioids. Moreover, capsaicin and NMDA also failed to produce MOR internalization. In contrast, dorsal horn stimulation elicited MOR internalization that increased with the frequency, being negligible at <10 Hz and maximal at 500 Hz. The internalization was abolished by the MOR antagonist D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH₂ (CTAP), in the presence of low Ca²⁺ and by the Na⁺ channel blocker lidocaine, confirming that it was caused by opioid release and neuronal firing. DLF stimulation in "oblique" slices (encompassing the DLF and the dorsal horn of T11-L4) produced MOR internalization, but only in areas near the stimulation site. Moreover, cutting oblique slices across the dorsal horn (but not across the DLF) eliminated MOR internalization in areas distal to the cut, indicating that it was produced by signals traveling in the dorsal horn and not via the DLF. These findings demonstrate that some dorsal horn neurons release opioids when they fire at high frequencies, perhaps by integrating signals from the rostral ventromedial medulla, primary afferents, and other areas of the spinal cord.

Key words: C-fiber; dynorphin; endocytosis; endomorphin; enkephalin; dorsal horn; dorsolateral funiculus; internalization; neurokinin; neurokinin 1 receptor; µ-opioid receptor; opioid; primary afferent; rat; release; spinal cord; substance P; tachykinin

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Received July 17, 2003; revised August 21, 2003; accepted August 26, 2003.

This article has been cited by other articles:

				Y. Guan, J. Borzan, R. A. Meyer, and S. N. Raja Windup in dorsal horn neurons is modulated by endogenous spinal mu-opioid mechanisms. J. Neurosci., April 19, 2006; 26(16): 4298 - 4307. [Abstract] [Full Text] [PDF]

				I. Kondo, J. C. G. Marvizon, B. Song, F. Salgado, S. Codeluppi, X.-Y. Hua, and T. L. Yaksh Inhibition by Spinal {micro}- and {delta}-Opioid Agonists of Afferent-Evoked Substance P Release J. Neurosci., April 6, 2005; 25(14): 3651 - 3660. [Abstract] [Full Text] [PDF]

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