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

Abstract

Single-unit recordings were made in the intact anesthetized rat of the responses of dorsal horn neurons to C-, Adelta-, and Abeta-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 microg) were observed on these neuronal responses. The C- and Adelta-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 microg bicuculline, stimulation at suprathreshold Abeta-fiber-evoked activity caused enhanced firing, mainly at later latencies corresponding to Adelta-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 microg bicuculline, as were the effects of morphine and 7-chlorokynurenate (7-CK). N6-CPA inhibited prod, C- and Adelta-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 Adelta-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 Adelta-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, Adelta-fiber-evoked activity, increased C-fiber input onto these cells along with the appearance of responses at latencies normally associated with Adelta fibers, but evoked by suprathreshold Abeta-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.

Effect of Morphine on Deep Dorsal Horn Projection Neurons Depends on Spinal GABAergic and Glycinergic Tone: Implications for Reduced Opioid Effect in Neuropathic Pain

“…These data suggest that morphine most likely inhibits the response of dorsal horn projection neurons by augmentation of the GABAergic and glycinergic inputs. Consistent with our finding, it has been shown that the effects of spinally applied morphine on lumbar wide-dynamic-range neurons are largely attenuated by spinal bicuculline (Reeve et al, 1998). Notably, the inhibitory effect of morphine on dorsal horn projection neurons was attenuated to a greater extent by bicuculline than strychnine.…”

“…Previous studies have shown that 10 M morphine produces a near maximal inhibition of voltage-gated calcium channels in dorsal root ganglion neurons (Wu et al, 2004) and hyperpolarization of lamina II neurons in spinal cord slices (Yoshimura and North, 1983). The similar dose of morphine also inhibits wide-dynamic-range neurons in the lumbar spinal cord in vivo (Reeve et al, 1998). Furthermore, the inhibitory effect on dorsal horn projection neurons by topical 10 M morphine (see Results) is similar to the inhibition achieved by intravenous 2.5 mg/kg morphine Chen et al, 2005), which produces analgesia in conscious rats (Chen and Pan, 2001).…”

Glycine Inhibitory Dysfunction Induces a Selectively Dynamic, Morphine-Resistant, and Neurokinin 1 Receptor- Independent Mechanical Allodynia

“…In line with such idea, we found that blockade of GABA A or glycine receptors induced selective static or dynamic mechanical allodynia, respectively. Accordingly, in vivo electrophysiological studies have consistently demonstrated that bicuculline has no effect on low-threshold responses of dorsal horn neurons, but increases A␦ fibermediated mechanical responses (Reeve et al, 1998;Kontinen et al, 2001;Seagrove et al, 2004). Finally, our results are also consistent with findings in a rat model of spinal nerve ligation in which selective chemical ablation of superficial dorsal horn neurons expressing the NK1 receptor did not alter electrophysiological response of dorsal horn neurons to brushing of the skin (Suzuki et al, 2005).…”

Functional GABAA-Receptor–Mediated Inhibition in the Neonatal Dorsal Horn

“…Spinal application of the GABA A -receptor (GABA A R) antagonist bicuculline leads to a robust disinhibition of adult rat dorsal horn neurons and a reduction in cutaneous mechanical thresholds (Cronin et al 2004;Ishikawa et al 2000;Reeve et al 1998;Sivilotti and Woolf 1994). Newborn dorsal horn cells have lower mechanical thresholds and larger receptive fields than those in the adult (Andrews and Fitzgerald 1994;Fitzgerald 1985;Holmberg and Schouenborg 1996;Torsney and Fitzgerald 2002), which could result from a delayed maturation of inhibitory circuits in the neonatal dorsal horn.…”