Superficial Dorsal Horn Neurons Identified by Intracutaneous Histamine: Chemonociceptive Responses and Modulation by Morphine

• Published in: Journal of neurophysiology 2000-08, Vol.84 (2), p.616-627
• Main Authors: Jinks, Steven L, Carstens, E
• Format: Article
• Language: English
• Subjects: Analgesics, Opioid - pharmacology; Animals; Capsaicin - pharmacology; Electrophysiology; Ganglionic Stimulants - pharmacology; Histamine - pharmacology; Hot Temperature; Injections, Intradermal; Irritants - pharmacology; Male; Morphine - pharmacology; Nicotine - pharmacology; Nociceptors - drug effects; Posterior Horn Cells - drug effects; Posterior Horn Cells - physiology; pruritis; Pruritus - chemically induced; Rats; Rats, Sprague-Dawley; Reaction Time - drug effects; Stimulation, Chemical; Tachyphylaxis - physiology
• ISSN: 0022-3077; 1522-1598
• DOI: 10.1152/jn.2000.84.2.616

Section of Neurobiology, Physiology and Behavior, University of California, Davis, California 95616 Jinks, Steven L. and E. Carstens. Superficial Dorsal Horn Neurons Identified by Intracutaneous Histamine: Chemonociceptive Responses and Modulation by Morphine. J. Neurophysiol. 84: 616-627, 2000. We have investigated whether neurons in superficial laminae of the spinal dorsal horn respond to intracutaneous (ic) delivery of histamine and other irritant chemicals, and thus might be involved in signaling sensations of itch or chemogenic pain. Single-unit recordings were made from superficial lumbar dorsal horn neurons in pentobarbital sodium-anesthetized rats. Chemoresponsive units were identified using ic microinjection of histamine (3%, 1 µl) into the hindpaw as a search stimulus. All superficial units so identified [9 nociceptive-specific (NS), 26 wide-dynamic-range (WDR)] responded to subsequent ic histamine. A comparison group of histamine-responsive deep dorsal horn neurons ( n  = 16) was similarly identified. The mean histamine-evoked discharge decayed to 50% of the maximal rate significantly more slowly for the superficial (92.2 s ± 65.5, mean ± SD) compared with deep dorsal horn neurons (28.2 s ± 11.6). In addition to responding to histamine, most superficial dorsal horn neurons were also excited by ic nicotine (22/25 units), capsaicin (21/22), topical mustard oil (5/6), noxious heat (26/30), and noxious and/or innocuous mechanical stimuli (except for 1 unit that did not have a mechanosensitive receptive field). Application of a brief noxious heat stimulus during the response to ic histamine evoked an additive response in all but two cases, followed by transient depression of firing in 11/20 units. Intrathecal (IT) administration of morphine had mixed effects on superficial dorsal horn neuronal responses to ic histamine and noxious heat. Low morphine concentrations (100 nM to 1 µM) facilitated histamine-evoked responses (to >130% of control) in 9/24 units, depressed the responses (by >70%) in 11/24, and had no effect in 4. Naloxone reversed morphine-induced effects in some but not all cases. A higher morphine concentration (10 µM) had a largely depressant, naloxone-reversible effect on histamine responses. Responses of the same superficial neurons to noxious heat were facilitated (15/25), reduced (8/25), or unaffected (2/25) by low morphine concentrations and were depressed by the higher morphine concentration. In contrast, deep dorsal horn neuronal r