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Key-pecking behavior in the pigeon was maintained under second-order schedules in which food was presented after a variable number of 2-min fixed-interval components were completed. When either the same stimulus (Exp. I) or different stimuli (Exp. II) appeared on the key during consecutive components, and a stimulus that was occasionally paired with food was presented briefly at completion of each component, (1) patterns of positively accelerated responding were maintained during the components, and, (2) mean response rates were generally as high during the initial components of a sequence as during the later components. In both experiments, when the food-paired stimulus was omitted and either no stimulus or a stimulus never paired with food was presented at completion of each component, mean rates of responding increased, but patterns of positively accelerated responding were not maintained during individual components. When a food-paired stimulus was not presented at completion of the components, mean response rates in Exp. I were low during the initial components of a sequence and gradually increased during subsequent components; in Exp. II mean response rates were variable, and pauses and abrupt changes in response rates were typical.

Key-pressing responses in the cat were maintained under conditions in which brief electric shock was first postponed by responses (avoidance), then periodically presented independently of responses, and finally produced by responses on a fixed-interval schedule of 15 min . A steady rate of responding occurred under shock avoidance and under responseindependent shock; positively accelerated responding was engendered by the FT 15-min schedule. A second experiment studied responding under second-order schedules composed of three FI 5-min components. Responding was suppressed when a stimulus was presented briefly at completion of each FI 5-min component and a shock followed the brief stimulus at completion of the third component. Responding was maintained when each of the first two components was completed either with or without presentation of a brief stimulus and a shock alone was presented at completion of the third FI 5-min component.The effects of electric shock upon operant responding have traditionally been investigated under conditions in which responding is maintained by terminating or postponing shock (Sidman, 1953), or suppressed by presenting shock (Azrin, 1956). Yet, under certain conditions, electric shock can enhance responding. Sidman, Herrnstein, and Conrad (1957) reported that under a schedule of continuous shock avoidance, rates of responding in the rhesus monkey increased in the presence of a stimulus (pre-shock stimulus) which terminated when a response-independent shock was presented. The enhancing effect was temporary, however; with continued training, higher response rates in the presence of the pre-shock stimulus gradually decreased and approached the rates in the absence of the stimulus. Frequency of responding declined when all shocks, except those delivered when the pre-shock stimulus terminated, were omitted, but the decline was slower in the 1 presence of the stimulus than in its absence. Later, Herrnstein and Sidman (1958) showed in the rhesus monkey that after training under a schedule of continuous shock avoidance, response rates were higher in the presence of a pre-shock stimulus than in its absence when responding was maintained under a schedule of food presentation.More-recent investigations have shown that increased rates of responding in the presence of a pre-shock stimulus can persist in other species under different conditions. Waller and Waller (1963), studying responding in the dog under a multiple schedule of food presentation, shock avoidance, and extinction, found that rates of responding in the presence of a pre-shock stimulus consistently increased during the extinction periods and during the avoidance periods. Moderately high response rates in the presence of the pre-shock stimulus persisted even after avoidance responding was extinguished.In a series of experiments, Kelleher, Riddle, and Cook (1963) found that responding in the squirrel monkey increased in the presence of a pre-shock stimulus presented during the food component of a multiple schedule of food present...

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The behavioral effects of GBR 12909, a selective dopamine uptake inhibitor, were determined in squirrel monkeys trained to respond under a fixed-interval (FI) schedule of stimulus termination and a second-order schedule of IV drug self-administration. Intermediate doses of GBR 12909 increased FI response rate markedly, and the highest dose decreased response rate below control values. The 5HT uptake inhibitors, alaproclate and fluoxetine, and the 5HT agonist, quipazine, attenuated the behavioral-stimulant effects of GBR 12909, whereas the 5HT2A/2C antagonist, ritanserin, enhanced the behavioral-stimulant effects of the lowest dose. GBR 12909 reliably maintained self-administration, and ritanserin increased response rate maintained by the highest dose. The dopamine agonist, quinpirole, increased FI response rate in only one of three subjects, and ritanserin enhanced the behavioral-stimulant effects of quinpirole in that subject. The dopamine agonist, apomorphine, only decreased FI response rate, and ritanserin did not alter its behavioral effects. The pharmacological profile of GBR 12909 administered alone and in combination with selective 5HT drugs in the present study was similar to that obtained previously with cocaine, further demonstrating that 5HT can reliably modulate the behavioral effects of psychomotor stimulants with prominent dopaminergic actions.