Effect of intrathecal glycine and related amino acids on the allodynia and hyperalgesic action of strychnine or bicuculline in mice

Abstract

Background: The intrathecal (IT) administration of glycine or GABA(A) receptor antagonist result in a touch evoked allodynia through disinhibition in the spinal cord. Glycine is an inhibitory neurotransmitter that appears to be important in sensory processing in the spinal cord. This study was aimed to evaluate the effect of glycine-related amino acids on antagonizing the effects of IT strychnine (STR) or bicuculline (BIC) when each amino acid was administered in combination with STR or BIC.

Methods: A total of 174 male ICR mice were randomized to receive an IT injection of equimolar dose of glycine, betaine, beta-alanine, or taurine in combination with STR or BIC. Agitation in response to innocuous stimulation with a von Frey filament after IT injection was assessed. The pain index in hot-plate test were observed after IT injection. The effect of IT muscimol in combination with STR or BIC were also observed.

Results: The allodynia induced by STR was relieved by high dose of glycine or betaine. But, allodynia induced by BIC was not relieved by any amino acid. Whereas the STR-induced thermal hyperalgesia was only relieved by high dose of taurine at 120 min after IT injection, the BIC-induced one was relieved by not only high dose of taurine at 120 min but also low dose of glycine or betaine at 60 min after IT injection. The BIC-induced allodynia and thermal hyperalgesia was relieved by IT muscimol.

Conclusions: This study suggests that IT glycine and related amino acids can reduce the allodynic and hyperalgesic action of STR or BIC in mice.

Keywords: Allodynia; Bicuculline; Glycine; Neuropathic pain model; Strychnine; Thermal hyperalgesia.

Figures

Fig. 1

Effect of the intrathecal (IT) injection of strychnine (STR) or bicuculline (BIC) on the agitation score induced by von Frey filament stimulation (A) and on the pain index in hot-plate test (HPPI) (B). Assessment of agitation score and HPPI were made as described in materials and methods. IT administration of STR or BIC in mice increased a sum of agitation score (A) and lowered on the HPPI at 60 min after IT injection (B) compared with artificial cerebrospinal fluid (aCSF) group. Data are expressed as the mean ± SEM. *P < 0.05 compared with aCSF group. TEA: touch evoked agitation, SA: spontaneous agitation, TA: total agitation.

Fig. 2

Effect of the intrathecal (IT) injection of equimolar doses (33 mM or 100 mM) of glycine G, betaine B, β-alanine A or taurine T on the agitation score induced by von Frey filament stimulation (A, B) and on the pain index in hot-plate test (HPPI) (C, D). Assessment of agitation score and HPPI were made as described in materials and methods. A sum of agitation score and the HPPI were not different by IT administration of G, B, A or T in mice compared with artificial cerebrospinal fluid (aCSF) group. Data are expressed as the mean ± SEM. TEA: touch evoked agitation, SA: spontaneous agitation, TA: total agitation.

Fig. 3

Effect of the combined intrathecal (IT) injection of strychnine (STR) with equimolar doses (33 mM or 100 mM) of glycine G, betaine B, β-alanine A or taurine T on the agitation score induced by von Frey filament stimulation (A, B) and on the pain index in hot-plate test (HPPI) (C, D). Assessment of agitation score and HPPI were made as described in materials and methods. Combined IT STR with high dose of G or B in mice decreased a sum of agitation score compared with STR group (B). Combined IT STR with high dose of T increased on the HPPI at 120 min after IT injection compared with STR group (D). Data are expressed as the mean ± SEM. *P < 0.05 compared with STR group. TEA: touch evoked agitation, SA: spontaneous agitation, TA: total agitation.

Fig. 4

Effect of the combined intrathecal (IT) injection of bicuculline (BIC) with equimolar doses (33 mM or 100 mM) of glycine G, betaine B, β-alanine A or taurine T on the agitation score induced by von Frey filament stimulation (A, B) and on the pain index in hot-plate test (HPPI) (C, D). Assessment of agitation score and HPPI were made as described in materials and methods. A sum of agitation score were not different by combined IT BIC with G, B, A or T compared with BIC group (A, B). Combined IT BIC with low dose of G or B in mice increased on the HPPI at 60 min after IT injection compared with BIC group (C). Combined IT BIC with high dose of T also increased on the HPPI at 120 min after IT injection (D). Data are expressed as the mean ± SEM. *P < 0.05 compared with BIC group. TEA: touch evoked agitation, SA: spontaneous agitation, TA: total agitation.

Fig. 5

Effect of the combined intrathecal (IT) injection of muscimol (M) with either strychnine (STR) or bicuculline (BIC) on the agitation score induced by von Frey filament stimulation (A) and on the pain index in hot-plate test (HPPI) (B, C). Assessment of agitation score and HPPI were made as described in materials and methods. A sum of agitation score and the HPPI were not different by combined IT M with STR in mice compared with STR group(A, B). Combined IT M with BIC decreased a sum of agitation score (A) and increased on the HPPI at 60 min after IT injection compared with BIC group (C). Data are expressed as the mean ± SEM. *P < 0.05 compared with BIC group. TEA: touch evoked agitation, SA: spontaneous agitation, TA: total agitation.