Effects of the dopamine D2 agonist, quinpirole, on time and number processing in rats

A proxy measure of striatal dopamine predicts individual differences in temporal precision

The perception of time is characterized by pronounced variability across individuals, with implications for a diverse array of psychological functions. The neurocognitive sources of this variability are poorly understood, but accumulating evidence suggests a role for inter-individual differences in striatal dopamine levels. Here we present a pre-registered study that tested the predictions that spontaneous eyeblink rates, which provide a proxy measure of striatal dopamine availability, would be associated with aberrant interval timing (lower temporal precision or overestimation bias). Neurotypical adults (N = 69) underwent resting state eye tracking and completed visual psychophysical interval timing and control tasks. Elevated spontaneous eyeblink rates were associated with poorer temporal precision but not with inter-individual differences in perceived duration or performance on the control task. These results signify a role for striatal dopamine in variability in human time perception and can help explain deficient temporal precision in psychiatric populations characterized by elevated dopamine levels.

The scalar property during isochronous tapping is disrupted by a D2-like agonist in the nonhuman primate

Dopamine, and specifically the D2 system, has been implicated in timing tasks where the absolute duration of individual time intervals is encoded discretely, yet the role of D2 during beat perception and entrainment remains largely unknown. In this type of timing, a beat is perceived as the pulse that marks equally spaced points in time and, once extracted, produces the tendency in humans to entrain or synchronize their movements to it. Hence, beat-based timing is crucial for musical execution. In this study we investigated the effects of systemic injections of quinpirole (0.005–0.05 mg/kg), a D2-like agonist, on the isochronous rhythmic tapping of rhesus monkeys, a classical task for the study of beat entrainment. We compared the rhythmic timing accuracy, precision, and the asynchronies of the monkeys with or without the effects of quinpirole, as well as their reaction times in a control serial reaction time task (SRTT). The results showed a dose-dependent disruption in the scalar property of rhythmic timing due to quinpirole administration. Specifically, we found similar temporal variabilities as a function of the metronome tempo at the largest dose, instead of the increase in variability across durations that is characteristic of the timing Weber law. Notably, these effects were not due to alterations in the basic sensorimotor mechanism for tapping to a sequence of flashing stimuli, because quinpirole did not change the reaction time of the monkeys during SRTT. These findings support the notion of a key role of the D2 system in the rhythmic timing mechanism, especially in the control of temporal precision.

NEW & NOTEWORTHY Perceiving and moving to the beat of music is a fundamental trait of musical cognition. We measured the effect of quinpirole, a D2-like agonist, on the precision and accuracy of rhythmic tapping to a metronome in two rhesus monkeys. Quinpirole produced a flattening of the temporal variability as a function of tempo duration, instead of the increase in variability across durations that is characteristic of the scalar property, a hallmark property of timing.

Novel Inversions in Auditory Sequences Provide Evidence for Spontaneous Subtraction of Time and Number

Animals, including fish, birds, rodents, non-human primates, and pre-verbal infants are able to discriminate the duration and number of events without the use of language. In this paper, we present the results of six experiments exploring the capability of adult rats to count 2–6 sequentially presented white-noise stimuli. The investigation focuses on the animal’s ability to exhibit spontaneous subtraction following the presentation of novel stimulus inversions in the auditory signals being counted. Results suggest that a subtraction operation between two opposite sensory representations may be a general processing strategy used for the comparison of stimulus magnitudes. These findings are discussed within the context of a mode-control model of timing and counting that relies on an analog temporal-integration process for the addition and subtraction of sequential events.

Temporal sensitivity changes with extended training in a bisection task in a transgenic rat model

The present study investigated temporal perception in a Huntington disease transgenic rat model using a temporal bisection procedure. After initial discrimination training in which animals learned to press one lever after a 2-s tone duration, and the other lever after a 8-s tone duration for food reward, the bisection procedure was implemented in which intermediate durations with no available reinforcement were interspersed with trials with the anchor durations. Bisection tests were repeated in a longitudinal design from 4 to 8 months of age. The results showed that response latencies evolved from a monotonic step-function to an inverted U-shaped function with repeated testing, a precursor of non-responding on trials with intermediate durations. We inferred that temporal sensitivity and incentive motivation combined to control the transformation of the bisection task from a two-choice task at the outset of testing to a three-choice task with repeated testing. Changes in the structure of the task and/or continued training were accompanied by improvement in temporal sensitivity. In sum, the present data highlight the possible joint roles of temporal and non-temporal factors in the temporal bisection task, and suggested that non-temporal factors may compensate for deficits in temporal processing.

Disruptive effects of stimulus intensity on two variations of a temporal discrimination procedure

Previous reports using stimulus intensity changes to disrupt temporal discrimination have shown shifts in the psychophysical curve for time, while studies using other disruptors have shown a flattening of the curve. The current study investigated the impact of increases and decreases in stimulus intensity on temporal discrimination in pigeons, to determine if a flattening of the curve could be extended to this disruptor. The brightness of the sample to be timed was manipulated under two procedural variations, in which the response alternatives were differentiated by color or location. Results showed that all subjects in the color procedure, and one in the location procedure, showed a flattening of the psychophysical curve when they experienced increased stimulus intensity in descending order. No subjects exposed to an ascending order of stimulus intensities, and none of the other subjects in the location procedure, showed any impact of changed stimulus intensity. Minimal disruption was found when test sessions presented decreased stimulus intensity levels in a second series. These results, together with those using other types of disruptors, add to the evidence of a flattening of the psychophysical curve when temporal discrimination is disrupted.

ABA chronic dosing of D-amphetamine produces differential drug effects in two variants of a temporal discrimination procedure in pigeons

The impact of two procedural variants of the matching to sample of duration procedure on the effects of chronically administered D-amphetamine on temporal discrimination was assessed. The task consisted of subjects classifying durations as short or long to produce a psychophysical curve for time. Procedural variations included response alternatives being defined by either the location of the response keys (Location), or by their color (Color). During Phase 1, two groups of pigeons experienced one of these procedures while being administered with D-amphetamine in an ABA chronic dosing regimen. Phase 2 was a reversal condition in which select birds from Phase 1 were trained on the other procedural variation that they had not experienced and were again given D-amphetamine in an ABA chronic regimen. Results showed that during Phase 1, subjects in the Color group showed a flattening of the psychophysical curves during amphetamine administration, whereas those in the Location group showed a shift of the psychophysical curve to the left and right. Phase 2 revealed that subjects who had experienced the Location variant first were less disrupted by the drug, even when experiencing the Color condition. The procedural variation, dosing regimen, and form of data analysis all provide more information regarding the cause of discrepancies in the timing literature regarding differential effects of D-amphetamine on timing behavior.

Effects of acute and chronic d-amphetamine on two variations of a temporal discrimination procedure

This study assessed temporal discrimination using a procedure in which pigeons classified temporal intervals as short or long based on different responses following visually presented stimulus durations. For the Location group, response alternatives were defined by the location of response keys, whereas for the Color group response alternatives were defined by key color. Temporal parameters were derived from psychophysical curves for time, and the impact of d-amphetamine was determined during acute and chronic dosing. Results showed that classification of temporal intervals was disrupted dose dependently during the acute drug assessment, regardless of the procedural variation. During the chronic drug assessment, differences in one of the psychophysical parameters, Range, were found between groups, as well as a significant effect of Dose on Standard Deviation values in the Color group. These data suggest that procedural variation does not lead to differential effects of d-amphetamine when administered in an acute regimen; however, differences found during chronic administration suggest that procedural variations may influence the development of tolerance as evidenced by differing Range values between groups and marked disruption of Standard Deviation values at the low dose in the Color group. The influence exerted on temporal behavior and tolerance by procedural variations during acute and chronic regimens of d-amphetamine should be explored further.

Interval timing in genetically modified mice: a simple paradigm

We describe a behavioral screen for the quantitative study of interval timing and interval memory in mice. Mice learn to switch from a short-latency feeding station to a long-latency station when the short latency has passed without a feeding. The psychometric function is the cumulative distribution of switch latencies. Its median measures timing accuracy and its interquartile interval measures timing precision. Next, using this behavioral paradigm, we have examined mice with a gene knockout of the receptor for gastrin-releasing peptide that show enhanced (i.e. prolonged) freezing in fear conditioning. We have tested the hypothesis that the mutants freeze longer because they are more uncertain than wild types about when to expect the electric shock. The knockouts however show normal accuracy and precision in timing, so we have rejected this alternative hypothesis. Last, we conduct the pharmacological validation of our behavioral screen using d-amphetamine and methamphetamine. We suggest including the analysis of interval timing and temporal memory in tests of genetically modified mice for learning and memory and argue that our paradigm allows this to be done simply and efficiently.

Habit formation and the loss of control of an internal clock: inverse relationship between the level of baseline training and the clock-speed enhancing effects of methamphetamine

INTRODUCTION

Drugs that modulate the effective level of dopamine (DA) in cortico-striatal circuits have been shown to alter the perception of time in the seconds-to-minutes range. How this relationship changes as a function of repeated experience with the reinforcement contingencies and the gradual adaptation of the underlying neural circuits remains unclear.

MATERIALS AND METHODS

The present study examined the clock-speed enhancing effects of methamphetamine (MAP 0.5 or 1.0 mg/kg, ip) in groups of rats that received different levels of baseline training (20, 60, or 120 sessions) on a 50-s peak-interval (PI) procedure before initial drug administration.

RESULTS

A curvilinear relationship was observed such that rats that received either minimal or intermediate levels of training (<or=60 sessions) displayed dose- x training-related horizontal leftward shifts in their timing functions, suggesting that the speed of the internal clock was increased. In contrast, rats that had received an extended level of training (>or=120 sessions) did not show this "classic" DA agonist curve-shift effect, but instead displayed a dose-dependent disruption of temporal control after MAP administration. A transition from DA-sensitive to DA-insensitive mechanisms is proposed to account for the loss of control of clock speed, as timing behaviors associated with the PI procedure gradually become learned habits through the strengthening of DA-glutamate connections.

Effects of morphine on temporal discrimination and color matching: general disruption of stimulus control or selective effects on timing?

Discrepant effects of drugs on behavior maintained by temporal-discrimination procedures make conclusive statements about the neuropharmacological bases of timing difficult. The current experiment examined the possible contribution of a general, drug-induced disruption of stimulus control. Four pigeons responded on a three-component multiple schedule that included a fixed-interval 2-min, temporal discrimination, and color-matching component. Under control conditions, response rates and choice responses during the first two components showed evidence of control by time, and accuracy for color matching was high in the third component. Morphine administration flattened the distribution of fixed-interval responding and produced a general disruption of accuracy in the temporal-discrimination component, whereas accuracy in the color-matching component was relatively unaffected. Analysis of the psychophysical functions from the temporal-discrimination component indicated that morphine decreased accuracy of temporal discrimination by decreasing overall stimulus control, rather than by selectively affecting timing. These results suggest the importance of determining the neurophysiological bases of stimulus control as it relates to temporal discrimination.

Dopamine agonists and antagonists can produce an attenuation of response bias in a temporal discrimination task depending on discriminability of target duration

The current study examined the effects of the D2 agonist (quinpirole) and D2 antagonist (eticlopride) on temporal discrimination performance in a conditional discrimination task (Experiment I) and a delayed conditional discrimination task (Experiment II). In both experiments rats discriminated between a scheduled stimulus duration of 3 s versus 9 s. Consistent with previous reports, overall discrimination performance decreased in a dose-dependent manner with both drugs. Changes in response bias (the tendency to choose-short or choose-long irrespective of actual stimulus duration), however, were best characterized in terms of both drugs tending to attenuate the bias effects normally observed during baseline drug-free performance. Specifically, the 'choose-short' bias observed in Experiment I and at a relatively short, 0.1 s, delay in Experiment II became less extreme with increasing doses. In addition, the 'choose-long' bias observed at a relatively long, 6 s, delay in Experiment II also became less extreme with increasing doses. Thus, whether there was an apparent shift from a short response bias to long, or vice versa, was the product of the delay interval between stimulus presentation and choice rather than whether the drug in question was a D2 agonist or antagonist. Such an attenuation of bias may have arisen because of subjects confounding the delay interval with the actual discriminative stimulus duration.

The effect of the adenosine A(2A) antagonist KW-6002 on motor and motivational processes in the rat

RATIONALE

It is well established that humans and rats respond to an imperative stimulus more rapidly as a function of the foreperiod preceding the target, and with this decrease in mean response time, there is also an increase in anticipatory (prior to the signal) responses. These changes reflect enhanced motor readiness. Also, reaction times are quicker when the cost of reward (amount of work required) is minimum. Antagonism of the adenosine A(2A) receptor has been shown to effect motor-related processes.

OBJECTIVE

This study examined the behavioural effects of systemic administration of the adenosine A(2A) antagonist KW-6002 in a cued reaction time task in the rat. The purpose of this study is to ascertain whether KW-6002 would enhance motor readiness and effect performance as a function of reward cost.

METHODS

Rats were trained on a visually cued reaction time task with variable foreperiods, and the effects of three doses of KW-6002 (0.3, 1.0 and 3.0 mg/kg systemically, compared to vehicle) were examined.

RESULTS

Increasing doses of KW-6002 resulted in faster reaction times and an increase in the number of anticipatory responses. KW-6002 enhanced the foreperiod effect on reaction time distributions and anticipatory responses. In addition, KW-6002 had differential effects on performance between rewarded and unrewarded trials.

CONCLUSION

Antagonism of adenosine A(2A) receptors by systemic KW-6002 speeds reaction time and enhanced motor preparatory processes as well as interacting with motivational processes.

Effects of D-amphetamine on temporal discrimination in pigeons

Two experiments used a matching to sample of durations procedure to examine changes in temporal discrimination evoked by D-amphetamine (0.75, 1.50, 2.25 and 3.0 mg/kg i.m.) in pigeons. Responses to one key color were reinforced after long (8-s) stimuli: responses to the alternative key color were reinforced after short (2-s) stimuli. Stimuli of intermediate duration were reinforced in Experiment 1 and were not reinforced in Experiment 2. Sigmoid functions relating percent of choices of the key reinforced after long-duration stimuli to the duration of stimulus presented were fit by a cumulative normal function. In addition to a parameter giving the left/right position of the sigmoid curves (which indexes temporal perception), this function also provided estimates of the range and slope of the curve (measures of stimulus control). The results showed that, contrary to many published reports, amphetamine had no effect on the left/right positioning of the sigmoid curve. There was, however, an effect of amphetamine on stimulus control, as shown by the general flattening of the psychophysical functions. Little evidence of tolerance to the drug's behavioral effects was found after 30 consecutive days of behavioral testing under amphetamine.

Systems-level integration of interval timing and reaction time

Reaction time (RT) procedures are a prominent tool for the study of information processing by humans and other animals. The interpretation of how RT changes after manipulating the appropriate experimental variables has contributed to the contemporary understanding of a variety of cognitive constructs, including attention and memory. With the use of properly designed tasks, evaluating how RT is modified in response to various neural perturbations has become common within the realms of behavioral and cognitive neuroscience. One interesting observation made during both human and animal RT experiments is that the RT to a signal often speeds-up as more time is allotted to prepare for the signal's onset-referred to as the preparatory interval (PI) effect. In the human RT literature, the PI effect has been used as evidence for time estimation playing a fundamental role in the determination of RT. On the other hand, our theoretical understanding of time estimation remains largely divorced from the RT findings in the animal cognition literature. In order to bridge these different perspectives, we provide here a review of the behavioral parallels between RT and interval-timing experiments. Moreover, both the PI effect and interval timing are shown to be jointly influenced by neuropathologies such as Parkinson's disease in humans or dopamine-depleting brain lesions in experimental animals. The primary goal of this review is to consider human and animal RT experiments within the broader context of interval timing. This is accomplished by first integrating human RT theory with scalar timing theory-the leading model of interval timing. Following this, both RT and interval timing are discussed at a brain systems level insofar as these two processes share common neural substrates. Our conclusion is that interval timing and RT processes are in fact two sides of the same coin.

Cannabinoid modulation of sensitivity to time

The present studies used a psychophysical approach to examine the effect of cannabinoids on temporal processing. Rats trained to discriminate 2- and 8-s (Experiment 1, n=72) and 4- and 16-s (Experiment 2, n=60) intervals were tested with intermediate durations. Psychophysical functions for time, relating the probability of judging a duration as "long" as a function of the actual stimulus durations, were characterized by measures of central tendency (point of subjective equality, PSE) and variability (Weber fraction, WF). The potent cannabinoid agonist, WIN55,212-2 (1-3 mg/kg), produced a dose-related decrease in sensitivity to time (i.e. increase in WF) without systematically affecting PSE (Experiments 1 and 2). The central cannabinoid CB1 antagonist, SR141716A (1-3 mg/kg), did not alter either the WF or PSE (Experiments 1 and 2). Coadministration of SR141716A with WIN55,212-2 blocked the effect of the agonist on WF (Experiment 2), suggesting that the WF effect is mediated by actions at cannabinoid CB1 receptors. Computational modeling with an information processing theory of timing suggests that the reduction in sensitivity to time can be attributed to a disorder of attention.