Effects of nucleus raphe magnus and locus coeruleus in descending modulation of habenula on pain threshold and acupuncture analgesia

The tail-flick method was used to measure the pain threshold as an index of efficacy of habenula in modulating the pain threshold and acupuncture analgesia. The pain threshold of rats were decreased after microinjection of L-glutamic acid (6.8 nmol) into habenula. Meanwhile, the decreases of pain threshold and acupuncture analgesia by L-glutamic acid were abolished after nucleus raphe magnus was blocked by microinjection of lidocaine (2%, 0.5 pLI) (p<0.01). However. blockade of locus coeruleus cancelled the opposed effect on acupuncture analgesia by excited habenula, but a weaker action on the efficacy of decreasing pain threshold by excited habenula was seen. It is concluded that nucleus raphe magnus is an essential relay in descending pathway originated from habenula for modulation of pain threshold and acupuncture analgesia.