中脳中心灰白質刺激による脊髄後角侵害受容ニューロン活動の抑制

INHIBITION OF NOCICEPTIVE RESPONSES OF SPINAL DORSAL HORN NEURONS BY PERIAQUEDUCTAL GRAY STIMULATION

I．はじめに

　ラットの中脳中心灰白質の電気刺激によつて，麻酔薬を使わずに開腹手術を行うことができたというRey—nolds （1969）の報告以来，脳の電気刺激による鎮痛作用の研究が盛んになり，間脳の第III脳室周囲部，中脳の背側縫線核とその周囲の中心灰白質，橋—延髄縫線核などの電気刺激によるラット（Mayer,Wolfle,Akil,Carder　＆　Liebeskind,1971；Balagura　＆　Ralph,1973；Mayer　＆　Liebeskind,1974；Morrow　＆　Casey,1976；Rhodes　＆　Lielbeskind,1978），ネコ（Liebeskind,Guilbaud,Besson　＆　Oliveras,1973；Melzack　＆　Melinkoff,1974；Olive—ras,Redjemi,Guilband　＆　Besson,1975），サル（Goodman　＆　Holcombe,1976）の侵害防御反応や，ヒトの痛覚（Adams,1976；Richardson　＆　Akil,1977 a,b；Hoso—buchi,Adams　＆　Linchitz,1977）の抑制が報告されている。

　他方，モルフィンの作用点が第III脳室や中脳水道の周囲であろうという考えがすでにあつて（Tsou　＆　Jang，1964；Herz,Albus,Metys,Schubert　＆　Teschemacher，1970），上記の部位，とくに中脳背側縫線とその周囲の中心灰白質の電気刺激とモルフィンが共通な作用機序によつて鎮痛をもたらすとする仮説（Akil＆Mayer，1972）が提唱された。また，鍼麻酔の作用機序を中脳中心灰白質ないし背側縫線核の賦活によつて説明しようとする試みもある（Kaada,1976）。

In order to analyze analgesic effects of peri-aqueductal gray stimulation, experiments were carried out in nitrous oxide anesthetized cats.

Spinal dorsal horn neurons receiving C-fiber inputs from the posterior tibial and/or superficial peroneal nerves were excited by noxious heat stimulation of the ipsilateral hindpaw. The evoked spike discharges were inhibited by repetitive stimulation of the periaqueductal gray at currents 300-900 μA. Effective sites for inhibition were distributed throughout the ventral half of the peri-aqueductal gray bilaterally.

A monotonous relationship was found between stimulus intensity (skin temperature) and numberof spike discharges; As predicted by Stevens'law, the number of spike discharges Φ grew as a power function:

ψ=κ(φ-φ₀)^β

where Φ is the stimulus intensity, Φ₀ is the effective threshold, and κ is a slope constant of the spike discharges versus stimulus intensity curve depending upon neurons. The exponent β was approximately 1.O.The periaqueductal gray stimulation resulted in a decrease in the slope constant κ, without alteration either in the threshold constant Φ₀ or in the exponent β.