Peripheral nerve injury-induced rearrangement of neural circuit in the spinal dorsal horn revealed by cross-correlation analysis.

Peripheral nerve injury often induces abnormal pain states, such as hyperalgesia and allodynia. In this study, we attempted to elucidate how neurons are synaptically integrated into the neuronal circuitry in the spinal dorsal horn and how synaptic connectivity patterns among dorsal horn neurons are altered by peripheral nerve injury. Experiments were performed on 6-8-week-old ICR mice. Partial sciatic nerve ligation was performed. Transverse slices of the lumbar spinal cord were prepared. Spike activities were simultaneously recorded from multiple neurons in the superficial dorsal horn (SDH) using a multi-electrode array system, and cross-correlograms between spike trains of neuron pairs were constructed. In sham-operated control mice, except for the flat cross-correlogram, the most common pattern was a cross-correlogram suggestive of common excitatory synaptic inputs to neuronal pairs. Peripheral nerve ligation increased the incidences of cross-correlograms suggestive of common excitatory synaptic inputs and excitatory synaptic connections, and decreased that of inhibitory synaptic connections. Additionally, bath-applied capsaicin, an agonist for transient receptor potential vanilloid 1 receptor, increased the frequency of action potentials. The effects of capsaicin stimulation on the incidence of cross-correlograms with various patterns were significantly different between sham-operated control and sciatic nerve-ligated mice. The present observations seem to indicate that neurons in the SDH form excitatory and/or inhibitory synapses with nearby neurons, and that synaptic connections among neurons in the SDH may remarkably change after the development of neuropathic pain.

Effects of pregabalin on spinal d-serine content and NMDA receptor-mediated synaptic transmission in mice with neuropathic pain.

Pregabalin (PGB) is a chemical derivative of the inhibitory neurotransmitter γ-aminobutyric acid, and is successfully used for the treatment of neuropathic pain. Substantial evidence suggests that d-serine, an endogenous co-agonist at the strychnine-insensitive glycine site of the NMDA receptor, counteracts the antinociceptive actions of PGB at the level of the spinal cord. In the present study, we examined the impact of PGB treatment on spinal d-serine content and NMDA receptor-mediated synaptic transmission in the superficial dorsal horn of peripheral nerve-ligated neuropathic mice. Mechanical allodynia was assessed using von Frey filaments. On post-surgical day 9 (after 5days of treatment with PGB [50mg/kg] or saline vehicle), the lumbar spinal cord was removed, homogenized, and ultrafiltrated. Supernatant samples were treated with Marfey's reagent and analyzed with liquid chromatography-mass spectrometry to measure d-serine content. In the electrophysiological experiments, tight-seal whole-cell recording was performed on neurons in the superficial dorsal horn of spinal cord slices. Partial sciatic nerve ligation increased spinal d-serine content, increased the NMDA/non-NMDA ratio of EPSC amplitudes, and slowed the decay phase of the NMDA component of EPSCs (NMDA-EPSCs). PGB treatment attenuated mechanical allodynia and reduced spinal d-serine content, decreased the NMDA/non-NMDA ratio, and shortened the decay time of NMDA-EPSCs. Furthermore, bath-applied d-serine attenuated the effects of PGB treatment. Although the precise mechanism for the effect of PGB on d-serine metabolism and abundance is unknown, the antinociceptive action of PGB likely involves the reduction of spinal d-serine content and subsequent attenuation of NMDA receptor-mediated synaptic transmission in the superficial dorsal horn.

The herpes simplex virus type 1 BgKL variant, unlike the BgOL variant, shows a higher association with orolabial infection than with infections at other sites, supporting the variant-dispersion-replacement hypothesis.

The identification and geographic distribution of the herpes simplex virus type 1 (HSV-1) BglII restriction fragment length polymorphism (RFLP) variants named BgK(L) and BgO(L) in clinical isolates from orolabial and cutaneous sites were described in our previous reports, in which the dispersion and replacement of HSV-1 variants were proposed. The base substitution sites deduced from the BgK(L) multiple RFLP variations were mapped to the U(L)12 (DNase), R(L)2 (alpha0 transactivator), and latency-associated transcript genes in the present study. The results show that the relative frequencies (RFs) of BgK(L) are significantly higher in orolabial and cutaneous HSV-1 infections than in ocular infections. For the BgO(L) variant, the opposite was found; i.e., the RF of BgO(L) was significantly lower in orolabial and cutaneous infections than in ocular infections. No significant differences in the RFs of non-BgK(L):non-BgO(L) isolates were observed. The ratio of the BgK(L) RF to the BgO(L) RF was much higher for the orolabial and cutaneous infection groups than for the ocular infection group, whereas the BgK(L) RF-to-non-BgK(L):non-BgO(L) RF ratios for the former groups were slightly higher than those for the latter group. The higher efficiency of orolabial and cutaneous infections caused by BgK(L) compared to the efficiency of infections caused by BgO(L) allows BgK(L) to spread more efficiently in human populations and to displace BgO(L), because the mouth and lips are the most common HSV-1 infection sites in children. The present study supports our HSV-1 dispersion-and-replacement hypothesis and suggests that HSV-1, the latency-reactivation of which allows variants to accumulate in human populations, has evolved under competitive conditions, providing a new perspective on the polymorphism or variation of HSV-1.