Effect of central 5-hydroxytryptamine depletion on performance in the "time-left" procedure: further evidence for a role of the 5-hydroxytryptaminergic pathways in behavioural "switching"

This experiment examined the effect of destruction of the ascending 5-hydroxytryptaminergic (5HTergic) pathways on performance in a free-operant timing schedule: the "time-left" procedure. Rats received either injections of 5,7-dihydroxytryptamine into the dorsal and median raphe nuclei or sham lesions. They were trained in a discrete trials schedule in which reinforcers were provided for responding on either of two levers, A and B. At a random time point, t s after the start of each trial, a response on A resulted in the delivery of one food pellet after dA s, whereas a response on B resulted in the delivery of two pellets after 60-t s. The value of dA was varied between 1 and 8 s in different phases of the experiment. Both groups showed decreasing response rates on lever A and increasing response rates on lever B as a function of time within the trial. An index of timing (T75: the time within the trial at which relative response rate on B attained a value of 75%) was systematically related to the value of dA, but did not differ significantly between lesioned and control rats. However, the lesioned group showed significantly higher rates of switching between response alternatives than the sham-lesioned group at all values of dA. 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 provide further evidence that the ascending 5HTergic pathways may contribute to the inhibitory regulation of switching between behavioural states.

Destruction of central noradrenergic neurones with DSP4 impairs the acquisition of temporal discrimination but does not affect memory for duration in a delayed conditional discrimination task

This experiment examined the effect of destroying central noradrenergic neurones using the selective neurotoxin N-(2-chloroethyl)-n-ethyl-2-bromobenzylamine (DSP4) on the acquisition of a temporal discrimination and on memory for duration, using a delayed conditional discrimination task. In phase I, rats that had received systemic treatment with DSP4 and vehicle-treated control rats were trained in a series of discrete trials to press lever A following a 2-s presentation of a light stimulus, and lever B following an 8-s presentation of the same stimulus. Following stimulus offset, a response on a panel placed midway between the two levers was required to initiate lever presentation; a single response on either lever resulted in withdrawal of both levers and, in the case of a "correct" response, reinforcer delivery. Both groups acquired accurate discrimination, achieving 90% correct choices within 50 sessions; the DSP4-treated group acquired accurate performance more slowly than the control group. In phase II, delays were interposed between stimulus offset and lever presentation in 50% of the trials. In the absence of a delay, discriminative accuracy was lower in the DSP4-treated group than in the control group. Accuracy declined as a function of post-stimulus delay in both groups; both groups showed a delay-dependent bias towards responding on lever A ("choose-short" bias). Neither of these effects differed significantly between the two groups. The concentrations of noradrenaline in the parietal cortex and hippocampus were reduced by 90% and 89% in the DSP4-treated group, compared to the levels in the control group, but the levels of dopamine, 5-hydroxytryptamine and 5-hydroxyindoleacetic acid did not differ significantly between the groups. The results confirm the deleterious effect of DSP4 on the acquisition of temporal discrimination, but do not provide evidence for a role of the noradrenergic innervation of the hippocampus and neocortex in temporal working memory.

Effect of destruction of the 5-hydroxytryptaminergic pathways on temporal memory: quantitative analysis with a delayed interval bisection task

This experiment examined the effect of destruction of the ascending 5-hydroxytryptaminergic (5HTergic) pathways on memory for duration, using a delayed interval bisection task. Rats that had received injections of 5,7-dihydroxytryptamine into the dorsal and median raphe nuclei, and sham-lesioned control rats, were trained in a series of discrete trials to press lever A following a 2-s presentation of a light stimulus, and lever B following an 8-s presentation of the same stimulus. Following stimulus offset a response on a panel placed midway between the two levers was required in order to initiate lever presentation; a single response on either lever resulted in withdrawal of both levers and, in the case of a "correct' response, reinforcer delivery. When > 90% correct choices had been attained, an 8-s (phase I) or a 12-s (phase II) delay was interposed between stimulus offset and lever presentation in 50% of the trials, and probe trials (10% of both non-delay and delay trials) were introduced in which the light was presented for intermediate durations. Logistic functions were derived relating percent choice of lever B to stimulus duration. In both groups, the imposition of post-stimulus delays displaced the bisection point (duration yielding 50% choice of lever B) towards longer durations; this effect was significantly greater in the lesioned group than in the control group. Imposition of post-stimulus delays resulted in increases in the Weber fraction, which did not differ significantly between the two groups. 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.

Effect of destruction of the 5-hydroxytryptaminergic pathways on behavioural timing and "switching" in a free-operant psychophysical procedure

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.

Effects of desipramine and fluvoxamine on timing behavior investigated with the fixed-interval peak procedure and the interval bisection task

Acute treatment with antidepressant drugs is known to increase the mean interresponse time (IRT) in the IRT > 72-s schedule of reinforcement. In order to examine the possibility that this effect may reflect an action of the antidepressants on timing processes, we tested the effects of two antidepressants, desipramine and fluvoxamine, on behaviour maintained under two other timing schedules in rats. In the fixed-interval peak procedure (fixed-interval 30-s), acute treatment with desipramine (8 mg kg-1) reduced response rate, whereas acute treatment with fluvoxamine (8 mg kg-1) increased it. Neither drug significantly altered the time to attainment of peak response rate or the Weber fraction. In the interval bisection task (standard durations 2 s and 8 s), the bisection point was not significantly altered by acute treatment with either drug. Chronic treatment with desipramine (8 mg kg-1 b.d.) had no effect on any of the indices of timing under either schedule. Chronic treatment with fluvoxamine (8 mg kg-1 b.d.) reduced the time to attainment of peak response rate but had no effect on the Weber fraction under the fixed-interval peak procedure, and did not alter the bisection point or Weber fraction under the interval bisection procedure. The failure of desipramine and fluvoxamine to increase the time to peak response rate or the bisection point at doses that significantly altered operant response rate suggests that the effect of these drugs on IRT schedule performance is unlikely to reflect an interaction with timing processes.

The role of the ascending 5-hydroxytryptaminergic pathways in timing behaviour: further observations with the interval bisection task

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)

Retarded acquisition of a temporal discrimination following destruction of noradrenergic neurones by systemic treatment with DSP4

This experiment examined the effect of destroying central noradrenergic neurones, using the selective neurotoxin DSP4 (N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine) on the acquisition and performance of discrimination between two time intervals. Rats that had received systemic treatment with DSP4 and vehicle-treated control rats were trained in a series of discrete trials to press lever A following a 2-s presentation of a light stimulus and lever B following an 8-s presentation of the same stimulus. Both groups acquired the discrimination (> 90% correct choices) within 15 sessions; however, the DSP4-treated group showed significantly slower acquisition than the control group. When stable performance had been attained, 'probe' trials were introduced in which the light was presented for intermediate durations. Both groups showed sigmoid functions relating percent choice of lever B to log stimulus duration. Neither the bisection point (duration corresponding to 50% choice of lever B) nor the Weber fraction differed significantly between the DSP4-treated and control groups. The levels of noradrenaline were markedly reduced in the neocortex and hippocampus of the DSP4-treated group, but the levels of dopamine and 5-hydroxytryptamine were not altered. The results indicate that noradrenaline depletion induced by DSP4 retarded the acquisition of temporal discrimination, but did not impair steady-state discriminative precision.