Impulsiveness is a significant clinical problem associated with a variety of psychiatric and neuropsychiatric disorders. Clinical and experimental studies have provided evidence that individuals displaying impulsive behaviour tend to show signs of deficient functioning of the ascending 5-hydroxytryptaminergic (5-HTergic) pathways of the brain. A persistent problem in investigations of the biological basis of impulsive behaviour has been the lack of a satisfactory definition of 'impulsiveness', as distinct from other behavioural features, such as aggression, which are often apparent in 'impulsive' individuals. Research in the experimental analysis of behaviour suggests that two important characteristics of 'impulsiveness' are (i) deficient tolerance of delay of gratification and (ii) inability to inhibit or delay voluntary behaviour; both of these characteristics are amenable to study in laboratory animals. We describe some delayed reinforcement and delayed response paradigms which purport to capture these behavioural characteristics, and review recent evidence that manipulation of 5-HTergic function alters behaviour in these paradigms. It is argued that the two characteristics of 'impulsiveness' are themselves the product of disturbance of more fundamental behavioural processes; the nature of these processes is considered.

This experiment examined the effect of destroying the 5-hydroxytryptaminergic (5HTergic) pathways on rats' ability to discriminate between two durations. Rats received injections of 5,7-dihydroxytryptamine into the median and dorsal raphe nuclei or sham lesions. They were trained to press lever A following a 2-s presentation of a light and lever B following an 8-s presentation of the light. For some rats, the levers were inserted into the chamber immediately after stimulus presentation ("no-poke-requirement"); for others, the levers were not inserted until a flap covering the food tray positioned midway between the levers had been depressed ("poke-requirement"). When stable performance was attained, "probe" trials were introduced in which the light was presented for intermediate durations. Logistic functions were derived relating percent choice of lever B to log stimulus duration. Under the "no-poke-requirement" condition, the bisection point (duration yielding 50% choice of lever B) was shorter in the lesioned rats than in the control rats. Under the "poke-requirement" condition, this effect of the lesion was attenuated. There was no effect of the lesion on the Weber fraction under either condition. The levels of 5HT and 5-hydroxyindoleacetic acid were reduced in the brains of the lesioned rats, but the levels of noradrenaline and dopamine were not altered. It is proposed that rats may attain accurate timing under the interval bisection task by moving from one lever to the other during stimulus presentation; this movement may be facilitated by destruction of the 5HTergic pathways.(ABSTRACT TRUNCATED AT 250 WORDS)

This experiment examined the effect of destruction of the ascending 5-hydroxytryptaminergic (5HTergic) pathways on performance in a free-operant timing schedule. Rats received either injections of 5,7-dihydroxytryptamine into the dorsal and median raphe nuclei or sham lesions. They were trained to press levers for a sucrose reinforcer. Training sessions consisted of 40, 50-s trials in which reinforcers were available on a variable-interval 25-s schedule; in the first 25 s of each trial, reinforcers were only available for responses on lever A, whereas in the last 25 s reinforcers were available only for responses on lever B. Data were collected from probe trials (four per session) in which no reinforcers were delivered, during the last ten of 50 training sessions. Both groups showed decreasing response rates on lever A and increasing response rates on lever B as a function of time from the onset of the trial. Response rate on lever B, expressed as a percentage of overall response rate, could be described by a two-parameter logistic function; neither the indifference point (i.e. the time corresponding to 50% responding on lever B) nor the slope of the function different between the two groups. However, the lesioned group showed a higher rate of switching between response alternatives than the sham-lesioned group. The levels of 5HT and 5-hydroxyindoleacetic acid were reduced in the brains of the lesioned rats, but the levels of noradrenaline and dopamine were not significantly altered. The results confirm previous findings that behaviour in timing schedules is sensitive to destruction of the central 5HTergic pathways, and suggest that these pathways may contribute to the inhibitory regulation of switching between behavioural states.