L~fe Sciences, Vol. 28, pp. 2257-2263 Pergamon Press Prmnted in the U.SoA. NORHARMAN INHIBITION OF [JH]-DIAZEPAN BIN~ING IN MOUSE BRAIN Anne M. Morln, Irls A. Tanaka and Claude G. Wasterlaln Neurology and Epllepsy Research Laboratorles Veterans Admlnlstratlon Medlcal Center Sepulveda, Callfornla and Department of Neurology and Braln Research Instltute, School of Medlclne, bnlverslty of Californla, Los Angeles, Callfornla (Recelved In f!nal form February 26, 1981) Summary Norharman competltlveiy inhlblts speclflc blndlng of [BH]- dlazepam in mouse braln homogenates. In vlvo thls B-carbollne produces a striking rlgld catatonlc-liKe appearance whlch is abolished by diazepam. It also causes a rapld tremor but has llttle antlconvulsant effect. Measurement of in vivo concentra- tlons and receptor occupancy demonstrate that these biological effects occur at doses whlch occupy a large proportion of benzo- dlazeplne receptors. It may represent a llgand of the benzo- dlazepine receptors WhOSe effects are opposlte those of dlazepam. Norharman is one of a group of harmala alkaloids, some of whlch are re- sponslble for halluclnatlons, tremors, inhlbltion of monoamlne oxldase, and convulslons (see revlews 1,2). We report here that nornarman inhlblts benzo- dlazeplne bindlng in vitro In mouse bramn mn a competltlve manner and produces tremors and a catatonlc-llke rlgldlty whlch are reversed by dlazepam. Norhar- man contalns an indole rlng structure as do some other llgands of the benzo- dlazeplne receptor such as tryptophan (3) and ethyl B-carbollne-3-carboxyllc acld (4,5). Recent reports have descrlbed the blnding afflnltles of a number of nltrogen contalnlng ringed structures, such as hypoxanthlne and Inosine (6), nlcotlnamlde (8), along with synthetlc compounds such as irazepine (9) dlphenyl- hydan~oln (I0) and trlazolopyrldazines (3) and pentylenetetrazol (8). Our studie indlcate that norharman and dlazepam share a binding site and that diazenam may reverse the In vlvo effects of norharman. Such a dlverslty of actlons by com- pounds which blnd to a con=non slte may be explalned in terms of types of receptor llgand interactlons. Methods Preparatlon of Mouse Braln Homo senates: Adult male C57 black mlce (20-30 gm) were k111ed by decapltat~on, their bralns removed and placed on a chllled surface. Speclflc braln areas were re- moved, welghed and homogenlzed in 20 volumes of 0.05 M Tris HCI buffer pH 7.5. Whole homogenates (50-200 ug proteln) were used in all assays. Protein deter- mlnatlon was accordlng to Lowry et al. (12). 0024-3205181/202257-07502.00/0