Effect of Intracerebral Injection of Substance P on Pain Threshold,Electroacupuncture Analgesia and Met-enkephalin Level in Rat Brain

Radioimmunoassay was employed to dctermine the immunorcactivc met-enkephalin(ir-MEK)contents of several brain regions .The pain threshold(PT)andir-MEK content of striatum and hypothalamus increased markedly afterintraoerebroventricular injection of substance P(SP)in the dosage of lμgg/10μl ,but nosignificant change of ir-MEK content in hippocampus was observed.When theperiaqueductal gray(PAG)was destroyed with electrolysis,the increase of PT andir-MEK content in the two brain regions after SP injection was obviouslysuppressed(P【0.05-0.01).The PT and ir-MEK content of hypothalamus andstriatum would also increase after intra-PAG injection of SP(0.4μg/4μl),electroacupuncture or intraperitoneal injection of morphine.The ir-MEK content ofhippocampus could also increase after intra-PAG injection of SP or intraperitoncal injec-tion of morphine.The combination of intra-PAG injection of SP andelectroacupuncture caused a more marked elevation of PT and ir-MEK content of the 3brain regions than either of the two used singly could(P【0.01 ).The combined effect ofintra-PAG injection of SP and intraperitoneal injection of morphine was similar to thatof either of the two used singly(P】0.05).These facts indicate that thc analgesic effect ofSP activated by PAG was mediated through the increase of cerebral met-cnkcphalin con-tent,and there was a synergic effect between SP and elcctroacupuncturc.

Levels of three opioid peptides in goats' CNS induced by different electroacupuncture frequencies

Acupuncture is a traditional therapeutic technique in Oriental medicine,which has a long history of 4000 years. As a modern version of hand acupuncture,electroacupuncture (EA) can provide a valid analgesic effect and has little physiological interference. It was successfully used to ameliorate pain not only in varieties of painful diseases,but in various operations,such as cesarean section,gastrectomy,enterectomy and castration,in animals during the 1970s. Since then,analgesia-regulating mechanism of EA has been extensively investigated. Previous studies found that electroacupuncture analgesia (EAA) was involved in modulations of neurotransmitters or neuromodulators in the central nerve system (CNS),and most early studies focused on the role of neurotransmitters such as serotonin,noradrenaline, dopamine and acetylcholine. Later,it was certified that some endogenous opioid peptides (EOPs),mainly including enkephalin,β-endorphin and dynorphin,played a more important role in EAA. EA of different frequencies can promote the release of different EOPs in the CNS. Studies showed that EAA induced by 2 Hz (low frequency) was mediated by the release of met-enkephalin (M-ENK) and β-endorphin (β-EP),while EAA by 100 Hz (high frequency) was mediated by the release of dynorphin-A (DYN-A) in the CNS in rats. Although these results in rats above are extrapolated to give reasonable explanations for acupuncture analgesia phenomenon and its treatment of related diseases in human,there are still some unknown mechanisms to be investigated. It has been proved that analgesia induced by EA varies in animal species. In order to quantitatively estimate the degree of acupuncture-induced analgesia,some researchers used an anesthetic to ensure a complete analgesia and to assess the reduction of the amount of the anesthetics consumed in the EA plus anesthetic group as compared to the anesthetic group without acupuncture. Studies showed EA in combination with anesthetics resulted in the reduction of the dosage of the anesthetics in human,rat and goat by 45%-55%,50%-60% and over 75%,respectively. It is clear that the analgesic effect induced by EA in goats (ruminants) is superior to that in rats or human. Therefore,the modalities of EOP release elicited by different frequencies in ruminants could be different from those in rats. In the present study,goats were stimulated with EA of different frequencies to determine the analgesic efficacy and the release levels of M-ENK,β-EP and DYN-A in the CNS in order to probe into the mechanisms of EA-induced analgesia in ruminants. Goats were stimulated by electroacupuncture of different frequencies: 0,2,40,60,80 or 100 Hz at a set of Baihui,Santai,Ergen,and Sanyangluo points for 30 min. The pain threshold was measured using the method of potassium iontophoresis. The levels of met-enkephalin,β-endorphin and dynorphin-A were determined with SABC immunohistochemisty. The results showed that 60 Hz increased pain threshold by 91%; its increasing rate was higher (P < 0. 01) than that by any other frequency used. The three EOPs were distributed in most analgesia-related nuclei and areas in the CNS. 2 Hz and 100 Hz induced met-enkephalin immunoactivities to increase (P < 0. 05) in nucleus accumbens,septal area,caudate nucleus,amygdala,paraventricular nucleus of hypothalamus,periaqueductal gray,dorsal raphe nucleus and locus ceruleus. These two frequencies elicited β-endorphin immunoactivities to increase (P < 0. 05) in nucleus accumbens,septal area,supraoptic nucleus,ventromedial nucleus of hypothalamus,periaqueductal gray,dorsal raphe nucleus,locus ceruleus,solitary nucleus,amygdale. 60 Hz increased (P < 0. 05) the release of met-enkephalin or β-endorphin in the nuclei and areas mentioned above,and habenular nucleus,substantia nigra,parabrachial nucleus and nucleus raphe magnus. High frequencies induced dynorphin-A immunoactivities to increase (P < 0. 05) in spinal cord dorsal horn and most analgesia-related nuclei in the CNS. It suggested that 60 Hz induced the simultaneous release of the three EOPs in extensive analgesia-related nuclei and areas of the CNS,which may be contributive to optimal analgesic effects and species variation.