Rats that have received electric shocks after a warning signal develop --if the experience is repeated several times --even at the beginning of the signal a state of emotional tension which manifests itself in a cowering attitude, pile-erection and defaeeation.
This emotional state can be evaluated objectively and quantitatively if it is found to inhibit in conditioned anhnals a reeordable activity which they have been trained to perform. Thus, when the warning signal is given, rats that have learned to obtain water or food by pressing a lever remain anxiously where they are and only rarely or never push the reward-lever. This behaviour, or process of conditioning and deeonditiouing, was described for the first time by EST~S and SKI~NV.~. The suspension of lever-pressing activity has been called "conditioned suppression" (CER) by StaMeN and "conditioned emotional response" by HUNT (1956).
The experimental procedure devised by EsT~s and SKINNER was first used by B~ADY for studying the activity of psychopharmacological drugs. He showed that reserpine can largely abolish the CEI~ in the rat. SID~A~ demonstrated the same effect in the monkey. However, ST]~IN, YA~A-~I~O et al. and KIN~A~D et al. did not confirm B~ADY'S findings. HU~T (1956, 1957) failed to abolish the CER in rats with either chlorpromazine or meprobamate, whereas HILL et al. were successful after high doses of morphine.
With a somewhat modified experimental design, MILLE~ (1960) produced with sodium amytal a distinct but not total abolition of the CEI~ in rats. In our laboratory CAVI~ZEL found that in meprobamatetreated rats the CER, as measured by the total number of lever-presses, is less than in the controls.
The aim of our investigation was (a) to re-evMuate these promising findings using a modified time pattern and worldng with a more suitable apparatus and (b) to test and possibly to discriminate different types of central nervous depressants.