Effects of chlordiazepoxide, morphine and amphetamine on responding suppressed by different levels of electric shock in the pigeon are rate dependent.

In general, chlordiazepoxide (CDP) and amphetamine reduce high rates of responding and increase low rates (rate-dependent effect). However, unlike CDP, amphetamine does not typically increase low rates resulting from suppression of responding by noxious stimuli. In the present experiment, key pecking by pigeons was reinforced under a random ratio schedule of food presentation. This responding was then suppressed by stimuli correlated with electric shocks of varying intensity (2 or 4 mA) or reduced by the omission of the food (extinction). Treatment with CDP (0.3-10.0 mg/kg) and morphine (0.3-10.0 mg/kg) increased the rate of suppressed responding: lower rates being increased to a proportionately greater extent than high rates. d-Amphetamine (0.1-1.0 mg/kg) further reduced the rate of suppressed responding: the lower rates being reduced proportionately more than the higher rates. Thus the effects of all three drugs depended upon the control rates of responding, but the effects of amphetamine were the inverse of those of CDP, and morphine. The effects of amphetamine on low, suppressed or punished response rates are therefore not an exception to the generality of rate-dependency, but a different aspect of the same principle - "inverse rate-dependency".