Involvement of a Descending Pathway from the A7 Region in Nociceptive Processing under Neuropathic Conditions in Rats

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  • TAMAKI Junichiro
    Department of Oral Physiology, Showa University School of Dentistry
  • TSURUOKA Masayoshi
    Department of Oral Physiology, Showa University School of Dentistry Department of Physical Therapy, Teikyo Heisei University Faculty of Community Health Care
  • MAEDA Masako
    Department of Oral Physiology, Showa University School of Dentistry College of Arts and Sciences, Showa University
  • HAYASHI Bunsho
    Department of Oral Physiology, Showa University School of Dentistry
  • INOUE Tomio
    Department of Oral Physiology, Showa University School of Dentistry

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The A7 cell group in the dorsolateral pons provides noradrenergic innervation of the spinal cord. Activation of this descending pathway (the A7 descending system) produces bi-directional effects on nociceptive processing in the dorsal horn, which are facilitation mediated by α1-adrenoceptor and inhibition via α2-adrenoceptor. Peripheral nerve injury sometimes results in neuropathic pain. Hypersensitivity of dorsal horn neurons under neuropathic conditions is linked to activity in descending pathways from the brain. The aim of this study was to examine the involvement of the A7 descending system under neuropathic conditions. Experiments were performed on male Sprague-Dawley rats (n=35). Bilateral lesions of the A7 area were performed by microinjection of kainic acid. The tibial and common peroneal the nerves were sectioned produce neuropathic conditions (spared nerve injury, SNI). For estimating mechanical allodynia, mechanical hypoalgesia and cold allodynia, paw withdrawal threshold (PWT), paw withdrawal latency (PWL) and paw withdrawal frequency (PWF) were measured. PWTs significantly decreased following A7 lesions. After SNI, PWTs significantly decreased in A7-lesioned and sham-lesioned rats. However, no significant difference was observed between the decreased rates of PWTs in A7-lesioned and sham-lesioned rats. PWLs significantly increased in sham-lesioned rats compared with A7-lesioned group. PWFs significantly increased in the A7-lesioned and sham-lesioned rats. Intrathecal injection of prazosin, an α1-adrenoceptor antagonist, failed to change the PWT, PWL and PWF in non-A7-lesioned neuropathic rats. These results suggest that (1) the A7 descending inhibitory system has tonic activity under normal conditions, and (2) this system functions in a complex manner during the neuropathic pain state.

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