(1 Department of Anatomy and K.K.Leung Brain Research Centre，Fourth Military Medical University，Xi'an710033，China;

    2 Department of Physiology，Norman Bethune University of Medical Sciences，Changchun130021，China)

    Keywords:receptors，GABA-B;serotonin;cerebral aque-duct;periaqueductal gray;raphe nuclei;rats CLC number:

    Abstract:AIM To investigate the co-localization of GABA B receptor(GABA B R)and serotonin(5-HT)in neurons of the midbrain periaqueductal gray(PAG)，raphe nuclei and gigan-tocellular reticular nucleus parsα(GiA)of the rat.METH┐ODS Immunofluorescence histochemical double-or triple-staining technique was used with specific antibodies against GABA B R and5-HT or GABA B R，5-HT and GABA.RE┐SULTS Densely stained GABA B R-and5-HT-like im-munoreactive double-labelled neuronal cell bodies were ob-served in the dorsal raphe nucleus(DR)，median raphe nucle-us(MnR)，pontine raphe nucleus(PnR)，nucleus raphe magnus(NRM)，nucleus raphe obscurus(ROb)and nucleus raphe pallidus(RPa).Additionally，GABA

    B R/5-HT double-labelled neuronal cell bodies were also found in the PAG and GiA.GABA-like immunoreactive terminals were found to be in close appositions with GABA B R/5-HT double-labelled neuronal cell bodies andproximalendrites.CONCLUSION GABA B R and5-HT co-exist in majority of the serotonergic neurons in the PAG ......