Effect of lesions of the ascending 5-hydroxytryptaminergic pathways on timing behaviour investigated with the fixed-interval peak procedure

A behavioral timing intervention upregulates striatal serotonergic markers and reduces impulsive action in adult male mice

Many studies support the hypothesis that time-based interventions reduce impulsive behavior in rodents. However, few studies have directly assessed 1) how such interventions affect impulsive action rather than impulsive choice, 2) if intervention effects differ by sex, and 3) how time-based interventions affect neurochemistry in regions mediating decision-making and reward. Thus, we assessed how a fixed-interval (FI) intervention initiated during late adolescence and extending into adulthood affected dopaminergic and serotonergic analytes in the frontal cortex and striatum and subsequent impulsive action in adult male and female mice. Beginning on postnatal day (PND) 45, mice were either trained on a progressive series of FI schedules (FI 20, 40, & 60 s) or remained in the home cage. Following the intervention, increases in striatal serotonergic analytes were found in FI-exposed males and females (n = 8/sex/group) with few changes found in the frontal cortex. Impulsive action was assessed in the remaining mice (n = 10/sex/group) using a fixed-ratio waiting-for-reward (FR-wait) task in which completion of an FR-25 component initiated a "free" pellet component in which pellets were delivered at increasing intervals according to a fixed delay increment that varied across sessions. Responses reset the additive delay and initiated a new FR-25 component. FI-exposed males, but not females, showed fewer delay resets and no-wait resets relative to control mice. Importantly, FI-exposure did not affect discrimination reversal performance in either sex. These data suggest that time-based interventions may reduce impulsive action in addition to impulsive choice perhaps with increased male sensitivity. Additionally, time-based interventions appear to operate through striatal serotonergic augmentation.

Age-related differences in appetitive trace conditioning and novel object recognition procedures

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Longitudinal study of middle age in the rat with matched younger control cohort.

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Appetitive trace conditioning and novel object recognition tests of working memory.

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Transient between-groups working memory impairments aged 12 compared with 2 months.

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Object exploration reduced with age but working memory recovered.

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Object exploration and ITI nosepoking showed some correlation with 5-HIAA/5-HT.

Appetitive trace conditioning (TC) was examined over 6 months in younger-adult (2–8 months) and middle-aged (12–18 months) male Wistar RccHan rats, to test for early age-related impairment in working memory. Novel object recognition (NOR) was included as a comparison task, to provide a positive control in the event that the expected impairment in TC was not demonstrated. The results showed that TC improved at both ages at the 2 s but not at the 10 s trace interval. There was, however, evidence for reduced improvement from one day to the next in the middle-aged cohort tested with the 2 s trace conditioned stimulus. Moreover, within the 10 s trace, responding progressively distributed later in the trace interval, in the younger-adult but not the middle-aged cohort. Middle-aged rats showed NOR discriminative impairment at a 24 h but not at a 10 min retention interval. Object exploration was overall reduced in middle-aged rats and further reduced longitudinally. At the end of the study, assessing neurochemistry by HPLC-ED showed reduced 5-HIAA/5-HT in the dorsal striatum of the middle-aged rats and some correlations between striatal 5-HIAA/5-HT and activity parameters. Overall the results suggest that, taken in isolation, age-related impairments may be overcome by experience. This recovery in performance was seen despite the drop in activity levels in older animals, which might be expected to contribute to cognitive decline.

Dorsal hippocampal lesions disrupt Pavlovian delay conditioning and conditioned-response timing

The involvement of the rat dorsal hippocampus (dhpc) in Pavlovian conditioning and timing of conditioned responding was examined in an appetitive preparation in which presentation of a relatively long, 40-s auditory conditioned stimulus (CS) was followed immediately by food delivery. Dorsal hippocampal lesions impaired Pavlovian conditioning in this task. They also produced a deficit in interval timing, replicating previous findings with short CSs. The conditioning and timing deficits observed are consistent with the findings from single-unit recording studies in other species, and suggest that the involvement of the dhpc in Pavlovian processes could be more general than is assumed by many of the current theories of hippocampal function.

Neonatal serotonin depletion alters behavioral responses to spatial change and novelty

Multiple brain disorders that show serotonergic imbalances have a developmental onset. Experimental models indicate a role for serotonin as a morphogen in brain development. To selectively study the effects of serotonin depletions on cortical structural development and subsequent behavior, we developed a mouse model in which a serotonin neurotoxin, 5,7-dihydroxytryptamine (5,7-DHT), is injected into the medial forebrain bundle (mfb) on the day of birth. Littermates with saline injections into the mfb and age matched mice served as controls. This study characterized the extent and duration of serotonergic denervation after the selective neonatal lesion and investigated effects on exploratory behavior, spatial learning and anxiety in mice of both sexes. We report significant decreases in the serotonergic (5-HT) innervation to cortex and hippocampus, but not to subcortical forebrain structures in 5,7-DHT-lesioned mice. The depletion of 5-HT fibers in cortical areas was long lasting in lesioned mice but autoradiographic binding to high affinity 5-HT transporters was only transiently reduced. Male but not female lesioned mice reduced their exploration significantly in response to spatial rearrangement and object novelty, suggesting increased anxiety in response to change but normal spatial cognition. Our data show that developmental disruptions in the serotonergic innervation of cortex and hippocampus are sufficient to induce permanent, sex specific, behavioral alterations. These results may have significant implications for understanding brain disorders presenting with cortical morphogenetic abnormalities and altered serotonin neurotransmission, such as autism, schizophrenia and affective disorders.

Methylphenidate administration to adolescent rats determines plastic changes on reward-related behavior and striatal gene expression

Administration of methylphenidate (MPH, Ritalin) to children with attention deficit hyperactivity disorder (ADHD) is an elective therapy, but raises concerns for public health, due to possible persistent neurobehavioral alterations. Wistar adolescent rats (30 to 46 day old) were administered MPH or saline (SAL) for 16 days, and tested for reward-related and motivational-choice behaviors. When tested in adulthood in a drug-free state, MPH-pretreated animals showed increased choice flexibility and economical efficiency, as well as a dissociation between dampened place conditioning and more marked locomotor sensitization induced by cocaine, compared to SAL-pretreated controls. The striatal complex, a core component of the natural reward system, was collected both at the end of the MPH treatment and in adulthood. Genome-wide expression profiling, followed by RT-PCR validation on independent samples, showed that three members of the postsynaptic-density family and five neurotransmitter receptors were upregulated in the adolescent striatum after subchronic MPH administration. Interestingly, only genes for the kainate 2 subunit of ionotropic glutamate receptor (Grik2, also known as KA2) and the 5-hydroxytryptamine (serotonin) receptor 7 (Htr7) (but not GABA(A) subunits and adrenergic receptor alpha1b) were still upregulated in adulthood. cAMP responsive element-binding protein and Homer 1a transcripts were modulated only as a long-term effect. In summary, our data indicate short-term changes in neural plasticity, suggested by modulation of expression of key genes, and functional changes in striatal circuits. These modifications might in turn trigger enduring changes responsible for the adult neurobehavioral profile, that is, altered processing of incentive values and a modified flexibility/habit balance.

Differential effects of clozapine and haloperidol on interval timing in the supraseconds range

The effects of clozapine (0.6, 1.2, and 2.4 mg/kg) and haloperidol (0.03, 0.06, and 0.12 mg/kg) on the timing of 10, 30, and 90-s intervals were characterized in rats. Each drug's effect on timing behavior was assessed following intraperitoneal injections using a variant of the peak-interval procedure. Although haloperidol proportionately shifted peak times rightward in a manner consistent with a decrease in clock speed, clozapine exerted the opposite effect and proportionately shifted peak times leftward in a manner consistent with an increase in clock speed. These results support the proposal that typical antipsychotic drugs such as haloperidol and atypical antipsychotic drugs such as clozapine exert differential effects on dopaminergic, serotonergic, and glutamatergic systems within the cortex and striatum, two brain regions shown to be crucial for interval timing.

Effect of 8-OH-DPAT on temporal discrimination following central 5-hydroxytryptamine depletion

The 5-hydroxytryptamine (5-HT)(1A) receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) alters performance in discrete-trials timing schedules. 5-HT(1A) receptors occur both presynaptically and postsynaptically, but it is not known which receptor population mediates the effects of 8-OH-DPAT on timing. Rats received intra-raphe injections of 5,7-dihydroxytryptamine (n=16) or sham lesions (n=14). They were trained in a discrete-trials psychophysical procedure in which levers were presented at a predetermined time after the onset of each trial (2.5, 7.5,., 47.5 s). A response on lever A was reinforced if lever presentation occurred < 25 s after trial onset; a response on lever B was reinforced if lever presentation occurred >25 s after trial onset. After 70 preliminary sessions, the rats received 8-OH-DPAT (25, 50, 100, 200 microg kg(-1) sc) and saline vehicle. The percentage of responses on lever B (%B) increased as a function of time from trial onset. Under the baseline (vehicle-treatment) condition, performance did not differ between the two groups. 8-OH-DPAT did not alter the indifference point (time corresponding to %B=50%), but dose-dependently increased the Weber fraction in both groups. Forebrain concentrations of 5-HT and 5-HIAA in the lesioned group were approximately 10% of control levels. The results suggest that the effect of 8-OH-DPAT on performance on discrete-trials timing schedules is mediated by postsynaptic 5-HT(1A) receptors.

5-Hydroxytryptamine and interval timing behaviour

Interval timing behaviour is revealed by prospective, immediate and retrospective timing schedules. Prospective timing tasks are used to study intertemporal choice (choice between outcomes occurring after different delays), immediate timing tasks to study temporal differentiation (temporal regulation of the animal's behaviour) and retrospective timing tasks to study temporal discrimination (discrimination between the durations of external events). Central 5-hydroxytryptamine (5-HT) depletion promotes preference for small early reinforcers over large delayed reinforcers, possibly by facilitating the time-dependent degradation of reinforcer value. Central 5-HT depletion retards the learning of temporal differentiation, and increases the variability of timing in some immediate timing tasks; however, it does not impede (in some cases it facilitates) the acquisition of temporal discrimination. Attempts to ascribe all the effects of 5-HT depletion on timing to a single behavioural process have been unsuccessful, although disinhibition of switching between operant responses may account for some of the findings. Acute treatment with drugs affecting 5-HTergic mechanisms alters timing behaviour in qualitatively different ways in different timing schedules, casting doubt on the idea that the effects of these drugs are mediated by interaction with a unitary timing process. The receptors that mediate 5-HT's putative involvement in interval timing behaviour remain to be identified.

Forebrain serotonin depletion facilitates the acquisition and performance of a conditional visual discrimination task in rats

Three experiments examined the effects of depleting forebrain 5-hydroxytryptamine (5-HT) on the acquisition and performance of an operant conditional discrimination in the visual modality. In the first experiment, rats with 5-HT lesions induced by infusing the neurotoxin 5,7-dihydroxytryptamine intracerebroventricularly acquired the conditional visual discrimination more rapidly than the sham-operated controls. Following acquisition, a series of manipulations of the task parameters tested the effects of the lesion on cognitive, sensory and motivational aspects of performance. In experiment two, the performance of rats that had acquired the task to asymptote before receiving lesions was assessed. The performance of this second group of serotonin-lesioned rats was similar to that of the pre-acquisition lesioned group following all but one manipulation of the task parameters. When the rate of stimulus presentations was increased, rats with forebrain 5-HT depletions were protected from the disruptive effects on performance seen in the sham-operated controls. This latter finding was also observed in a third experiment, in which the infusion of the 5-HT1A receptor partial agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OHDPAT), directly into the dorsal raphe nucleus improved the performance of unlesioned rats following an increase in the rate of stimulus presentations. The results are discussed in terms of the behavioural, neurochemical and neuroanatomical specificity of serotonin function in appetitive learning and the implications for general theories of the function of serotoninergic processes in cognition.

5-hydroxytryptamine and impulse control: prospects for a behavioural analysis

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.

Insights into the neurobiology of impulsive behavior from serotonin receptor knockout mice

Although the concept of impulsivity has proved useful in human and animal studies of addiction, violent aggression, and violent suicide, and has been recognized as an important component of human behavior, little research has been done to understand the underlying psychobiological mechanisms. We explore the concept of impulsivity and its relation with the neurotransmitter serotonin in the context of aggressive behavior and behavior associated with positive reinforcement using a knockout mouse that lacks one of the serotonin receptors, the 5-HT1B receptor. This knockout mouse shows more impulsive aggression, acquires cocaine self-administration faster, and drinks more alcohol than the corresponding wild-type control. We show that the impulsive characteristics of these mice are not due to change in cognitive functions: in a cognitive task involving a choice between a small immediate one and a larger more delayed reward, knockout mice showed intact choice and timing capabilities and good discrimination of reward amounts. Thus, this mouse may prove an animal model of addiction and motor impulsivity.

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.

Effect of destruction of the 5-hydroxytryptaminergic pathways on the acquisition of temporal discrimination and memory for duration in a delayed conditional discrimination task

This experiment examined the effect of destruction of the ascending 5-hydroxytryptaminergic (5HTergic) pathways 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 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. Both groups gradually acquired accurate discrimination, achieving > 90% correct choices within 20-30 sessions; the lesioned group acquired accurate performance significantly faster than the control group. In phase II, delays were interposed between stimulus offset and lever presentation in 50% of the trials (2, 4, 8, 16 and 32 s; 10% of trials in each case). Accuracy declined as a function of post-stimulus delay in both groups, and there was no significant difference between the performances of the two groups. Both groups showed an increasing tendency to respond on lever A following longer post-stimulus delays ("choose-short" effect); this effect was somewhat enhanced in the 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 altered.

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)

Effects of combined or separate 5,7-dihydroxytryptamine lesions of the dorsal and median raphe nuclei on responding maintained by a DRL 20s schedule of food reinforcement

Previous studies have shown that long-term 5-hydroxytryptamine (5-HT) depletion induced by combined dorsal and median raphe injections of 5,7-dihydroxytryptamine (5,7-DHT) leads to impairments in the acquisition and performance of behaviour maintained under a differential-reinforcement-of-low-rate (DRL) schedule of reinforcement. The present studies examined the relative importance of dorsal versus median raphe 5-HT projections in mediating these effects by investigating the impact of separate versus combined infusions of 5,7-DHT into these sites on DRL responding. Adult male rats received injections of 3 micrograms 5,7-DHT into both dorsal raphe and median raphe, dorsal raphe only, or median raphe only. Sham-operated controls received vehicle injections. Animals were then trained to respond on a DRL 20s schedule for 36 days. Combined dorsal raphe and median raphe 5,7-DHT infusions depleted striatal and hippocampal 5-HT by > 90%, increased responding, decreased the number of reinforcers earned, lowered the mean inter-response time (IRT), and shifted the frequency distribution of IRTs to the left. Median raphe 5,7-DHT infusions induced similar behavioural effects and reduced hippocampal 5-HT by 64%. Dorsal raphe 5,7-DHT lesions, that depleted striatal 5-HT by 56%, and hippocampal 5-HT by 30%, had no effect on behaviour. In a final experiment it was shown that providing an external cue light to signal food availability, and that removed the need to rely on internal timing processes, resulted in rapid acquisition of responding, and prevented the behavioural deficits in 5,7-DHT-lesioned rats. Following removal of this cue, responding deteriorated, and the 5,7-DHT treated animals performed worse than controls. These results suggest that destruction of 5-HT neurons arising from the median raphe is sufficient to disrupt behaviour maintained by a DRL 20s schedule, and that the behavioural changes observed may involve a deficit in timing.

Facilitated acquisition of a temporal discrimination following destruction of the ascending 5-hydroxytryptaminergic pathways

This experiment examined the effect of destroying the 5-hydroxytryptaminergic (5HTergic) pathways on the acquisition and performance of discrimination between two brief time intervals. 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 200-ms presentation of a light stimulus and lever B following an 800-ms presentation of the same stimulus. Both groups gradually acquired accurate performance, attaining 80%-85% accuracy by the end of 40 sessions. The lesioned group learnt the task significantly faster 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. The bisection point (duration corresponding to 50% choice of lever B) did not differ significantly between the two groups; however, the Weber fraction was significantly smaller in the lesioned group than in the control group. The levels of 5HT and 5-hydroxy-indole-acetic acid were markedly reduced in the brains of the lesioned rats, but the levels of noradrenaline and dopamine were not altered. The results indicate that destruction of the 5HTergic pathways facilitates acquisition of a temporal discrimination. The lack of an effect of the lesion on the bisection point contrasts with our previous finding using longer stimulus durations; it is suggested that different behavioural processes may underlie millisecond-range and second-range temporal discrimination, and that these may be differently affected by 5HT depletion.