Pharmacology Biochemisto, & Behavior, Vol. 9, lap. 287-295. Printedin the U.S.A. Role of the Periaqueductal Gray Substance in the Antianxiety Action of Benzodiazepines L. C. SCHENBERG" AND F. G. GRAEFF Department of Pharmacology, Medical School, 14.100 Ribeir6o Preto, S.P., Brazil (Received 10 March 1978) SCHENBERG0 L. C. AND F. G. GRAEFF. Role of the periaqueductal gray substance in the antianxiety action of benzodiazepines. PHARMAC. BIOCHEM. BEHAV. 9(3) 287-295, 1978.--In order to study the interactions between serotonergic mechanism and electrical stimulation of the mesencephalic cent:al gray substance, rats were trained to lever-press for terminating aversive electric stimuli applied at the Periaqueductal gray and adjoining tectum of the mesencephalon. Experimental sessions consisted of 40 discrete escape trials .~f a maximum of 30 sec duration, separated by 30 sec intervals. Dose-effect curves of two tryptamine antagonists, cyproheptadine and methysergide, as well as of the benzodiazepine minor tranquilizer, chlordiazepoxide, on average escape latencies and on frequency distribution of indi- vidual latencies were determined. Doses of 3 to 10 mg/kg of cyproheptadine decreased average latencies of escape responding in six of eight rats studied. Doses of 10 and 30 mg/kg of methysergide also facilitated escape responding in one of three rats. In contrast, doses from 1 to 10 mg/kg of chlordiazepoxide, that cause little sedation or ataxia, produced dose-dependent increases in escape latencies. Furthermore, doses of 5.6 and I0 mg/kg of chlordiazepoxide partially blocked escape responding. The facilitatory effects of the tryptamine antagonists suggest that escape behavior is inhibited by brain tryptaminergic mechanisms, whereas the specific depressant effect of chlordiazepoxide on escape from Periaqueductal gray electrical stimulation suggest that this region may be involved in the antianxiety action of ben- zodiazepines. Periaqueductal gray substance Electrical stimulation Chlordiazepoxide Behavior inhibition Anxiety Escape behavior Tryptamine antagonists THE TRYPTAMINE antagonists, bromolysergic acid (BOLL methysergide and cyproheptadine have been shown to increase response rate of positively reinforced operant behavior, whether responding was simultaneously punished by response-contingent electric shock or not 12, 7, 8, 9, 10, 39, 45]. In the same way, inhibition of serotonin (5-HT) syn- thesis by para-chlorophenylalanine (PCPA), as well as selective destruction of brain serotonergic neurons by intraventricular injection of 5,6-dihydroxytryptamine have been reported to facilitate punished responding in rats I6, 30, 47]. More specifically, Tye et al. [411 have recently shown that injection of 5-7-dihydroxytryptamine into the ventral tegmentum of the rat mesencephalon, destroying most of the ascending serotonin system 151, desinhibits punished re- sponding. On the other hand, directly acting tryptamine agonists, such as a-methyltryptamine and N,N-di- methyltryptamine or the 5-HT precursor, 5-hydroxy- tryptophan, suppress punished and non-punished responding [1, 7,10]. These results suggest the existence of serotonergic neurons in the brain causing behavior inhibition. The serotonergic behavior-inhibitory system may be re- lated to the antianxiety action of minor tranquilizers. In the same way as tryptamine antagonists, minor tranquilizers can enhance punished operant responding and this facilitatory action on punished behavior has good predictive value of their clinical efficacy in reducing anxiety I2, 19, 24]. Since the increases in punished responding caused by the ben- zodiazepine, oxazepam, on rat punished behavior correlated with the drug-induced decrease in 5-HT turnover in the midbrain-hindbrain region, Wise et al. [46] suggested that the antianxiety as well as the antipunishment action of the ben- zodiazepines is due to a decrease in functioning of the behavior-inhibitory serotonin system. More recently, Stein et al. [38] have also suggested that this action of the ben- zodiazepines is indirect, due to a facilitation of y-aminobutyric acid-mediated presynaptic inhibition of serotonin nerve terminals. In more general terms, a relation- ship between behavioral inhibition and fear or anxiety has also been suggested by Gray 1121. Nevertheless, in addition to the brain serotonergic neurons inhibiting behavior, the central gray matter of the mesencephalon may also be involved in anxiety. Electrical stimulation of the periaqueductal gray has been reported to cause flight behavior or defensive aggression in cats and rats 118, 20, 31, 32, 33, 48] and these effects have been associated either with pain [33, 36, 421 or fear and anxiety I20,271. In- deed feelings of fear, fright, and sometimes diffuse pain sen- sations have been reported by human patients, following the electrical stimulation of the central gray matter of the mesencephalon 1261. These results indicate that this area ~'Fhis work was supported by Grant 75-0079 from the Fundaqgto de Amparo h Pesquisa do Estado de S~io Paulo (FAPESPL Brazil. ~Work done under tenure of Master Fellowship of FAPESP. Copyright "" 1978 ANKHO International Inc.--0091-3057/78/090287-09501.40/0