Spinal Effects of Bicuculline: Modulation of an Allodynia-Like State by an A1-Receptor Agonist, Morphine, and an NMDA-Receptor Antagonist

Abstract

Reeve, Alison J., Anthony H. Dickenson, and Nicola C. Kerr. Spinal effects of bicuculline: modulation of an allodynia-like state by an A1-receptor agonist, morphine, and an NMDA-receptor antagonist. J. Neurophysiol. 79: 1494–1507, 1998. Single-unit recordings were made in the intact anesthetized rat of the responses of dorsal horn neurons to C-, Aδ-, and Aβ-fiber stimulation. The postdischarge and windup responses of the same cells along with responses to innocuous stimuli, prod and brush, also were measured. The effects of (−)-bicuculline-methobromide (0.5, 5, 50, and 250 μg) were observed on these neuronal responses. The C- and Aδ-fiber–evoked responses were facilitated significantly in a dose-dependent manner. The input was facilitated, but as the final overall response was not increased by the same factor, windup appeared to be reduced. However, postdischarge, resulting from the increase in the excitability produced by windup, tended to be facilitated. After doses of ≥5 μg bicuculline, stimulation at suprathreshold Aβ-fiber–evoked activity caused enhanced firing, mainly at later latencies corresponding to Aδ-fiber–evoked activity in normal animals. Few cells responded consistently to brush and so no significant change was observed. Responses evoked by innocuous pressure (prod) always were observed in cells that concurrently responded to electrical stimulation with a C-fiber response. This tactile response was facilitated significantly by bicuculline. The effects of N6-cyclopentyladenosine (N6-CPA), an adenosine A1-receptor agonist, was observed after pretreatment with 50 μg bicuculline, as were the effects of morphine and 7-chlorokynurenate (7-CK). N6-CPA inhibited prod, C- and Aδ-fiber–evoked responses as well as the initial and overall final response to the train of C-fiber strength stimuli. Inhibitions were reversed with 8(p-sulphophenyl) theophylline. Morphine, the mu-receptor agonist, also inhibited the postbicuculline responses to prod, C-, and Aδ-fiber responses and initial and final responses to a train of stimuli. Inhibitory effects of morphine were reversed partly by naloxone. 7-CK, an antagonist at the glycine site on the N-methyl-d-aspartate-receptor complex, inhibited the responses to C- and Aδ-fiber–evoked activity as well as prod. The postdischarges were inhibited by this drug. Again both the initial and overall responses of the cell were inhibited. To conclude, bicuculline caused an increase in the responses of deep dorsal horn cells to prod, Aδ-fiber–evoked activity, increased C-fiber input onto these cells along with the appearance of responses at latencies normally associated with Aδ fibers, but evoked by suprathreshold Aβ-fiber stimulation. These alterations may be responsible for some aspects of the clinical phenomenon of allodynia and hyperalgesia. These altered and enhanced responses were modulated by the three separate classes of drugs, the order of effectiveness being 7-CK, N6-CPA, and then morphine.