Sex similarities and dopaminergic differences in interval timing

Rodent behavioral studies have largely focused on male animals, which has limited the generalizability and conclusions of neuroscience research. Working with humans and rodents, we studied sex effects during interval timing that requires participants to estimate an interval of several seconds by making motor responses. Interval timing requires attention to the passage of time and working memory for temporal rules. We found no differences between human females and males in interval timing response times (timing accuracy) or the coefficient of variance of response times (timing precision). Consistent with prior work, we also found no differences between female and male rodents in timing accuracy or precision. In female rodents, there was no difference in interval timing between estrus and diestrus cycle stages. Because dopamine powerfully affects interval timing, we also examined sex differences with drugs targeting dopaminergic receptors. In both female and male rodents, interval timing was delayed after administration of sulpiride (D2-receptor antagonist), quinpirole (D2-receptor agonist), and SCH-23390 (D1-receptor antagonist). By contrast, after administration of SKF-81297 (D1-receptor agonist), interval timing shifted earlier only in male rodents. These data illuminate sex similarities and differences in interval timing. Our results have relevance for rodent models of both cognitive function and brain disease by increasing representation in behavioral neuroscience. (PsycInfo Database Record (c) 2024 APA, all rights reserved).

The effects of microdose LSD on time perception: a randomised, double-blind, placebo-controlled trial

RATIONALE

Previous research demonstrating that lysergic acid diethylamide (LSD) produces alterations in time perception has implications for its impact on conscious states and a range of psychological functions that necessitate precise interval timing. However, interpretation of this research is hindered by methodological limitations and an inability to dissociate direct neurochemical effects on interval timing from indirect effects attributable to altered states of consciousness.

METHODS

We conducted a randomised, double-blind, placebo-controlled study contrasting oral administration of placebo with three microdoses of LSD (5, 10, and 20 μg) in older adults. Subjective drug effects were regularly recorded and interval timing was assessed using a temporal reproduction task spanning subsecond and suprasecond intervals.

RESULTS

LSD conditions were not associated with any robust changes in self-report indices of perception, mentation, or concentration. LSD reliably produced over-reproduction of temporal intervals of 2000 ms and longer with these effects most pronounced in the 10 μg dose condition. Hierarchical regression analyses indicated that LSD-mediated over-reproduction was independent of marginal differences in self-reported drug effects across conditions.

CONCLUSIONS

These results suggest that microdose LSD produces temporal dilation of suprasecond intervals in the absence of subjective alterations of consciousness.

The Effect of Attention-Deficit/Hyperactivity Disorder and Methylphenidate Treatment on the Adult Auditory Temporal Order Judgment Threshold

PURPOSE

The Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition notes that attention-deficit/hyperactivity disorder (ADHD) diagnosed in childhood will persist into adulthood among at least some individuals. There is a paucity of evidence, however, regarding whether other difficulties that often accompany childhood ADHD will also continue into adulthood, specifically auditory processing deficits. The aim of this study was to examine the effect of ADHD and the stimulant medication methylphenidate on auditory perception performance among adults.

METHOD

A total of 33 adults diagnosed with ADHD according to Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition criteria (ADHD group) and 48 adults without ADHD (non-ADHD group) performed an auditory temporal order judgment task. Participants with ADHD performed the task twice: with and without taking methylphenidate (Ritalin), in random order.

RESULTS

Temporal order judgment thresholds of the ADHD group were significantly higher than those of the non-ADHD group. Methylphenidate significantly decreased temporal order judgment thresholds within the ADHD group, making their performance similar to the non-ADHD participants.

CONCLUSIONS

Auditory processing difficulties of those diagnosed with ADHD seem to persist into adulthood. Similar to findings with children, methylphenidate treatment improves performance on tasks requiring this ability among adults. Therefore, given the association between auditory temporal processing and linguistic skills, the beneficial effect of methylphenidate on adults' academic achievement may be accomplished by positively affecting auditory temporal processing. Further studies in this line of research are needed.

Time on timing: Dissociating premature responding from interval sensitivity in Parkinson's disease

BACKGROUND

Parkinson's disease (PD) can cause impulsivity with premature responses, but there are several potential mechanisms. We proposed a distinction between poor decision-making and the distortion of temporal perception. Both effects may be present and interact, but with different clinical and pharmacological correlates.

OBJECTIVES

This study assessed premature responding during time perception in PD.

METHODS

In this study, 18 PD patients and 19 age-matched controls completed 2 temporal discrimination tasks (bisection and trisection) and a baseline reaction-time task. Timing sensitivity and decision-making processes were quantified by response and response time. An extended version of the modified difference model was used to examine the precision of time representation and the modulation of response time by stimulus ambiguity.

RESULTS

In the bisection task, patients had a lower bisection point (P < .05) and reduced timing sensitivity when compared with controls (P < .001). In the trisection task, patients showed lower sensitivity in discriminating between short and medium standards (P < .05). The impairment in timing sensitivity correlated positively with patients' levodopa dose equivalent (P < .05). Critically, patients had disproportionately faster response times when compared with controls in more ambiguous conditions, and the degree of acceleration of response time increased with disease severity (P < .05). Computational modeling indicated that patients had poorer precision in time representation and stronger modulation of response time by task ambiguity, leading to smaller scaling of the decision latency (P < .05).

CONCLUSIONS

These findings suggest that timing deficits in PD cannot be solely attributed to perceptual distortions, but are also associated with impulsive decision strategies that bias patients toward premature responses. © 2016 The Authors. Movement Disorders published by Wiley Periodicals, Inc. on behalf of International Parkinson and Movement Disorder Society.

Revisiting the effect of nicotine on interval timing

This paper reviews the evidence for nicotine-induced acceleration of the internal clock when timing in the seconds-to-minutes timescale, and proposes an alternative explanation to this evidence: that nicotine reduces the threshold for responses that result in more reinforcement. These two hypotheses were tested in male Wistar rats using a novel timing task. In this task, rats were trained to seek food at one location after 8s since trial onset and at a different location after 16s. Some rats received the same reward at both times (group SAME); some received a larger reward at 16s (group DIFF). Steady baseline performance was followed by 3 days of subcutaneous nicotine administration (0.3mg/kg), baseline recovery, and an antagonist challenge (mecamylamine, 1.0mg/kg). Nicotine induced a larger, immediate reduction in latencies to switch (LTS) in group DIFF than in group SAME. This effect was sustained throughout nicotine administration. Mecamylamine pretreatment and nicotine discontinuation rapidly recovered baseline performance. These results support a response-threshold account of nicotinic disruption of timing performance, possibly mediated by nicotinic acetylcholine receptors. A detailed analysis of the distribution of LTSs suggests that anomalous effects of nicotine on LTS dispersion may be due to loss of temporal control of behavior.

Partial inactivation of nucleus accumbens core decreases delay discounting in rats without affecting sensitivity to delay or magnitude

Increased preference for smaller, sooner rewards (delay discounting) is associated with several behavioral disorders, including ADHD and substance use disorders. However, delay discounting is a complex cognitive process and the relationship is unclear between the pathophysiology of the disorders and the component processes underlying delay discounting, including sensitivity to reinforcer delay and sensitivity to reinforcer magnitude. To investigate these processes, male Long Evans rats were trained in one of three tasks measuring sensitivity to delay, sensitivity to magnitude, or both (typical delay discounting task). After learning the task, animals were implanted with bilateral cannulae into either the nucleus accumbens core (AcbC) or the lateral orbitofrontal cortex (lOFC), both of which have been implicated in delay discounting. Upon recovering from the surgery, a baclofen/muscimol cocktail was infused to temporarily inactivate each of these two regions and task performance was assessed. Unlike previous studies showing that lesions of the AcbC increased delay discounting, partial inactivation of the AcbC decreased delay discounting, although it had no effects on the tasks independently assessing either sensitivity to delay or magnitude. The effects of AcbC inactivation were larger in animals that had low levels of delay discounting at baseline. Inactivation of the lOFC had no effects on behavior in any task. These findings suggest that the AcbC may act to promote impulsive choice in individuals with low impulsivity. Furthermore, the data suggest that the AcbC is able to modulate delay and magnitude sensitivity together, but not either of the two in isolation.

Dissociations between interval timing and intertemporal choice following administration of fluoxetine, cocaine, or methamphetamine

The goal of our study was to characterize the relationship between intertemporal choice and interval timing, including determining how drugs that modulate brain serotonin and dopamine levels influence these two processes. In Experiment 1, rats were tested on a standard 40-s peak-interval procedure following administration of fluoxetine (3, 5, or 8 mg/kg) or vehicle to assess basic effects on interval timing. In Experiment 2, rats were tested in a novel behavioral paradigm intended to simultaneously examine interval timing and impulsivity. Rats performed a variant of the bi-peak procedure using 10-s and 40-s target durations with an additional "defection" lever that provided the possibility of a small, immediate reward. Timing functions remained relatively intact, and 'patience' across subjects correlated with peak times, indicating a negative relationship between 'patience' and clock speed. We next examined the effects of fluoxetine (5 mg/kg), cocaine (15 mg/kg), or methamphetamine (1 mg/kg) on task performance. Fluoxetine reduced impulsivity as measured by defection time without corresponding changes in clock speed. In contrast, cocaine and methamphetamine both increased impulsivity and clock speed. Thus, variations in timing may mediate intertemporal choice via dopaminergic inputs. However, a separate, serotonergic system can affect intertemporal choice without affecting interval timing directly. This article is part of a Special Issue entitled: Associative and Temporal Learning.

Effects of dopaminergic and subthalamic stimulation on musical performance

Although subthalamic-deep brain stimulation (STN-DBS) is an efficient treatment for Parkinson's disease (PD), its effects on fine motor functions are not clear. We present the case of a professional violinist with PD treated with STN-DBS. DBS improved musical articulation, intonation and emotional expression and worsened timing relative to a timekeeper (metronome). The same effects were found for dopaminergic treatment. These results suggest that STN-DBS, mimicking the effects of dopaminergic stimulation, improves fine-tuned motor behaviour whilst impairing timing precision.

Comparison of alcohol impairment of behavioral and attentional inhibition

BACKGROUND

Despite the wealth of studies demonstrating the impairing effects of alcohol on behavioral inhibition, less is known regarding effects of the drug on attentional inhibition (i.e., the ability to ignore distracting stimuli in the environment in order to focus attention on relevant information). The current study examined alcohol impairment of both behavioral and attentional inhibition, as well as potential associations between the two mechanisms of inhibitory control.

METHODS

Men (n=27) and women (n=21) performed a measure of behavioral inhibition (cued go/no-go task) and a measure of attentional inhibition (delayed ocular return task) following three doses of alcohol: 0.65 g/kg, 0.45 g/kg, and 0.0 g/kg (placebo).

RESULTS

Alcohol impaired both behavioral and attentional inhibition relative to placebo; however, correlational analyses revealed no associations between measures of behavioral and attentional inhibition following any dose. Additionally, men committed more inhibitory failures on the behavioral inhibition task, whereas women committed more inhibitory failures on the attentional inhibition task.

CONCLUSIONS

These findings suggest that behavioral and attentional inhibition are equally sensitive to the impairing effects of alcohol, yet represent distinct components of inhibitory control. Additionally, the observed gender differences in control of behavior and attention could have important implications regarding negative consequences associated with alcohol-induced disinhibition in men and women.

The effect of alcohol administration on human timing: a comparison of prospective timing, retrospective timing and passage of time judgements

Previous research suggests that human timing may be affected by alcohol administration. The current study aimed to expand on previous research by examining the effect of alcohol on prospective timing, retrospective timing and passage of time judgements. A blind between-subjects design was employed in which participants were either administered 0 g of alcohol per kilogramme of body weight (placebo), 0.4 g/kg (low dose) or 0.6g/kg (high dose). Participants completed four types of temporal task; verbal estimation and temporal generalisation, a retrospective timing task and a passage of time judgement task. A high dose of alcohol resulted in overestimations of duration relative to the low dose and placebo group in the verbal estimation task. A high dose of alcohol was also associated with time passing more quickly than normal. Alcohol had no effect on retrospective judgements. The results suggest that a high dose of alcohol increases internal clock speed leading to over-estimations of duration on prospective timing tasks, and the sensation of time passing more quickly than normal. The absence of an effect of alcohol on retrospective timing supports the suggestion that retrospective judgements are not based on the output of an internal clock.

Baseline training history and effects of methamphetamine on performance of pigeons on an interval-bisection task

Length of baseline training influences how methamphetamine disrupts temporal performance under a peak interval schedule. Acute methamphetamine produces overestimation of time following relatively brief training, but following extended training, methamphetamine produces more general loss of stimulus control. The current study extends the study of training length on the effects of methamphetamine to an interval-bisection procedure. Six pigeons responded under a psychophysical choice procedure in which responses to one key color were correct after presentation of four shorter sample durations and responses to another key color were correct after presentation of four longer sample durations. One group of three pigeons received briefer baseline training (45 sessions), while another group received more extended training (223 sessions) prior to methamphetamine administration. There was no evidence of overestimation of time or generalized loss of stimulus control in either group. Sensitivity (precision of timing) was higher in the group with more extensive training and was disrupted by methamphetamine.

[Different effect of eleuterococcus on various psychophysiological parameters of healthy humans depending on their chronotype and the day time]

The administration of a liquid extract of eleuterococcus as a psychotonic drug by young healthy humans leads to changes in some psychophysiological parameters, including an increase in the aural memory volume, decrease in reactive anxiety, and shortening of individual minute. These effects were dependent on the daytime (morning versus evening) and the individual chronotype (circadian features) of each volunteer.

Prenatal choline supplementation increases sensitivity to contextual processing of temporal information

The effects of prenatal choline availability on contextual processing in a 30-s peak-interval (PI) procedure with gaps (1, 5, 10, and 15 s) were assessed in adult male rats. Neither supplementation nor deprivation of prenatal choline affected baseline timing performance in the PI procedure. However, prenatal choline availability significantly altered the contextual processing of gaps inserted into the to-be-timed signal (light on). Choline-supplemented rats displayed a high degree of context sensitivity as indicated by clock resetting when presented with a gap in the signal (light off). In contrast, choline-deficient rats showed no such effect and stopped their clocks during the gap. Control rats exhibited an intermediate level of contextual processing in between stop and full reset. When switched to a reversed gap condition in which rats timed the absence of the light and the presence of the light served as a gap, all groups reset their clocks following a gap. Furthermore, when filling the intertrial interval (ITI) with a distinctive stimulus (e.g., sound), both choline-supplemented and control rats rightward shifted their PI functions less on trials with gaps than choline-deficient rats, indicating greater contextual sensitivity and reduced clock resetting under these conditions. Overall, these data support the view that prenatal choline availability affects the sensitivity to the context in which gaps are inserted in the to-be-timed signal, thereby influencing whether rats run, stop, or reset their clocks.

Effects of mGlu1 receptor blockade on working memory, time estimation, and impulsivity in rats

RATIONALE

Metabotropic glutamate 1 (mGlu1) receptor antagonists were reported to induce cognitive deficits in several animal models using aversive learning procedures.

OBJECTIVE

The present study aimed to further characterize behavioral effects of mGlu1 receptor antagonists using appetitively motivated tasks that evaluate working memory, timing, and impulsivity functions.

MATERIALS AND METHODS

Separate groups of adult male Wistar rats were trained to perform four food-reinforced operant tasks: delayed non-matching to position (DNMTP), differential reinforcement of low rates of responding 18 s (DRL 18-s), signal duration discrimination (2-s vs 8-s bisection), and tolerance to delay of reward. Before the tests, rats were pretreated with (3-ethyl-2-methyl-quinolin-6-yl)-(4-methoxy-cyclohexyl)-methanone methanesulfonate (EMQMCM; 2.5-10 mg/kg, i.p.; JNJ16567083).

RESULTS

In DNMTP task, EMQMCM produced delay-dependent increases in performance accuracy so that, at 10 mg/kg dose level, percentage of correct lever choices was enhanced at 8- and 16-s delays. In DRL task, at all three tested doses, response rates were higher, and reinforcement rates were lower than under control conditions. In signal duration discrimination tasks, EMQMCM did not have any specific effects on temporal control. In tolerance to delay of reward, EMQMCM (5 and 10 mg/kg) facilitated choice of the lever associated with large reward at longer delay levels.

CONCLUSIONS

Blockade of mGlu1 receptors improves working memory and reduces impulsive choice at the doses that have no effects on time perception but appear to facilitate impulsive action.

Dopaminergic modulation of timing control and variability in the gait of Parkinson's disease

The basal ganglia have been implicated in timing control, yet the nature of timing disturbances in Parkinson's disease (PD) is poorly understood. We evaluated the influence of timing cues on spatiotemporal aspects of gait control and its variability, and the impact of dopaminergic treatment on timing. Three separate groups: 19 PD (OFF state); 24 PD (ON state); and 30 control participants were tested. Participants walked on a computerized carpet at four randomized and metronome-controlled rates: self-paced, 60, 80, or 100 steps/min. To our knowledge, this is the first study to demonstrate that medicated PD patients had poorer timing control than patients withdrawn from medication and healthy participants when modulating timing to an external stimulus. Increased step-to-step timing variability and deficits in mean temporal gait characteristics revealed that the medicated PD group (in contrast to nonmedicated PD group) performed least like healthy participants. This was observable in externally-cued conditions, but not during self-paced gait. Similar to previous research, step length contributed to overall slowness in PD, while temporal characteristics of gait did not. Interestingly, healthy participants increased stride length with each increase in cue rate, whereas both PD groups locked their step length regardless of temporal demand. Step-to-step variability differences between PD and healthy (e.g. step and double-support time measurements) may be indicative of specific basal ganglia involvement in temporal control of gait.

Evidence for the sensitivity of operant timing behaviour to stimulation of D1 dopamine receptors

RATIONALE

Temporal differentiation of operant behaviour is sensitive to dopaminergic manipulations. Previous studies using the fixed-interval peak procedure implicated D(2)-like dopamine receptors in these effects. However, recent findings suggest that d-amphetamine alters timing performance on the free-operant psychophysical procedure via D(1)-like receptors. It is not known whether this effect of d-amphetamine is mimicked by direct D(1)-like receptor stimulation.

OBJECTIVE

The effects of a D(1)-like receptor agonist 6-chloro-2,3,4,5-tetrahydro-1-phenyl-1H-3-benzazepine (SKF-81297) on performance on the free-operant psychophysical procedure and the interaction between SKF-81297 and a D(1)-like receptor antagonist 8-bromo-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-3-benzazepin-7-ol (SKF-83566) and a D(2)-like receptor antagonist haloperidol, were examined.

MATERIALS AND METHODS

Rats were trained to respond on two levers (A and B) under a free-operant psychophysical schedule, in which sucrose reinforcement was provided intermittently for responding on A during the first half and on B during the second half of 50-s trials. Logistic psychometric functions were fitted to the relative response rate data (percent responding on B [%B] vs time from trial onset [t]) under each treatment condition, and quantitative indices of timing (T(50) [value of t corresponding to %B = 50] and the Weber fraction [(T(75)-T(25))/2T(50); T(25) and T(75) are values of t corresponding to %B = 25 and %B = 75] were compared among treatments.

RESULTS

SKF-81297 (0.8 mg kg(-1)) reduced T(50); this effect was antagonized by SKF-83566 (0.03 mg kg(-1)) but not by haloperidol (0.05, 0.1 mg kg(-1)).

CONCLUSIONS

Stimulation of D(1)-like dopamine receptors affects performance in the free-operant psychophysical procedure.

Interval timing and Parkinson’s disease: heterogeneity in temporal performance

Interval timing deficiencies in Parkinson's disease (PD) patients have been a matter of debate. Here we test the possibility of PD heterogeneity as a source for this discrepancy. Temporal performance of PD patients and control subjects was assessed during two interval tapping tasks and during a categorization task of time intervals. These tasks involved temporal processing of intervals in the hundreds of milliseconds range; however, they also covered a wide range of behavioral contexts, differing in their perceptual, decision-making, memory, and execution requirements. The results showed the following significant findings. First, there were two clearly segregated subgroups of PD patients: one with high temporal variability in the three timing tasks, and another with a temporal variability that did not differ substantially from control subjects. In contrast, PD patients with high and low temporal variability showed similar perceptual, decision-making, memory, and execution performance in a set of control tasks. Second, a slope analysis, designed to dissociate time-dependent from time-independent sources of variation, revealed that the increase in variability in this group of PD patients was mainly due to an increment in the variability associated with the timing mechanism. Third, while the control subjects showed significant correlations in performance variability across tasks, PD patients, and particularly those with high temporal variability, did not show such task correlations. Finally, the results showed that dopaminergic treatment restored the correlation effect in PD patients, producing a highly significant correlation between the inter-task variability. Altogether, these results indicate that a subpopulation of PD patients shows a strong disruption in temporal processing in the hundreds of milliseconds range. These findings are discussed in terms of the role of dopamine as a tuning element for the synchronization of temporal processing across different behavioral contexts in PD patients.

Ketamine “unlocks” the reduced clock-speed effects of cocaine following extended training: Evidence for dopamine–glutamate interactions in timing and time perception

The present study examined the clock-speed modulating effects of acute cocaine administration in groups of male rats that received different amounts of baseline training on a 36-s peak-interval procedure prior to initial drug injection. After injection of cocaine (10, 15, or 20mg/kg, ip), rats that had received a minimal amount of training (e.g., <or=30 sessions) prior to drug administration displayed a horizontal leftward shift in their timing functions indicating that the speed of the internal clock was increased. In contrast, rats that had received an extended amount of training (e.g., >or=180 sessions) prior to cocaine (15 mg/kg, ip) administration did not produce this "classic" curve-shift effect, but instead displayed a general disruption of temporal control following drug administration. Importantly, when co-administered with a behaviorally ineffective dose of ketamine (10mg/kg, ip) the ability of cocaine to modulate clock speed in rats receiving extended training was restored. A glutamate "lock/unlock" hypothesis is used to explain the observed dopamine-glutamate interactions as a function of timing behaviors becoming learned habits.

NMDA receptor modulation by D-cycloserine promotes episodic-like memory in mice

RATIONALE

NMDA-R (N-methyl-D-aspartate receptors) have been implicated in synaptic plasticity underlying one-trial learning of event-place associations. In rodents, episodic-like memory (ELM) of personally experienced events can be inferred from behavior that reflects the remembrance of the content (what kind of object was presented), place (where was this object placed), and temporal context (when was the object presented). We have previously shown that that D-cycloserine (DCS), an NMDA-R agonist, ameliorates stress-induced deficits in ELM.

OBJECTIVES

In this study, we used an experimental protocol designed to detect promnestic drug effects and investigated whether DCS, which is known to enhance learning and memory, can induce ELM under conditions where mice normally do not show ELM.

RESULTS

Mice that have been treated i.p. with DCS (20 mg/kg) both remembered the temporal order in which two different objects had been encountered during two consecutive sample trials, as well as their spatial position during the sample trials. Most importantly, the test trial performance of these mice is compatible with ELM in terms of an integrated memory for unique experiences comprising "what", "where", and "when" information. In contrast, mice that have received either a saline injection or lower doses of DCS (0.2 and 2.0 mg/kg) did not show such an integrated ELM.

CONCLUSIONS

To our knowledge, this is the first report showing that DCS can promote ELM in mice.

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.

Evidence for separate neural mechanisms for the timing of discrete and sustained responses

Methamphetamine (MAP), an indirect dopamine agonist, has been shown to produce a leftward shift in the time of responding under operant response protocols that encourage repetitive responding (e.g., lever pressing). Given the involvement of striatal dopamine activity in the control of discrete motor behavior, as well as in the timing of these responses, an important question arises as to whether a dissociation is possible between changes in the timing of discrete responding and timing of other behaviors. Rats were trained on a modified peak-interval (PI) procedure such that reward was contingent upon the presence of the animal's snout in a nosepoke apparatus at the target time, as an alternative to the typical requirement of a discrete head entry response. Thus spatial selection, but not necessarily motor behavior, at the appropriate time was required to receive a reward. Rats were given MAP in one of 3 doses (0.5, 1.0, or 1.5 mg/kg), or a saline control injection before PI sessions to determine whether the drug elicits a dose-dependent effect on timing of spatial position, as it has been shown to do for discrete behaviors. Following administration of MAP, the peak time of the proportion of time spent in the nosepoke did not change, while the peak time of the rate of response shifted to the left. Single-trial analysis revealed a similar pattern: Position of response step functions defined by being in the nosepoke did not shift, but step functions based on response rate changed with increasing doses of MAP. These data support a model of multiple timing processes controlling different behaviors, at least one of which is specific to discrete motor behavior and is modifiable by dopamine.

Tolerance to the effect of 2,5-dimethoxy-4-iodoamphetamine (DOI) on free-operant timing behaviour: interaction between behavioural and pharmacological mechanisms

RATIONALE

The psychostimulant d-amphetamine, the D(2/3) dopamine receptor agonist quinpirole and the 5-HT(2) receptor agonist 2,5-dimethoxy-4-iodoamphetamine (DOI) have similar effects on free-operant timing behaviour. There is evidence that tolerance develops to the effects of psychostimulants on timing performance during chronic treatment; this tolerance is generally attributed to behavioural adaptation rather than to pharmacological desensitisation. There have been no previous investigations of tolerance to the effect of DOI on free-operant timing behaviour.

OBJECTIVE

To demonstrate tolerance to DOI's effect on timing performance and to examine the nature of this tolerance.

MATERIALS AND METHODS

Rats were trained under the free-operant psychophysical procedure to press two levers (A and B) in 80-s trials in which reinforcement was provided intermittently for responding on A in the first half and B in the second half of the trial. Percent responding on B (%B) was recorded in successive 8-s epochs of the trials; logistic functions were fitted to the data from each rat for the derivation of timing indices (T (50) [time corresponding to %B = 50]; Weber fraction).

RESULTS

In experiment 1, DOI (0.25 mg kg(-1)) reduced T (50) compared to vehicle; tolerance to this effect was seen after repeated daily treatments with DOI if the rats were exposed to behavioural training during the period of treatment but not if the repeated treatments took place during a 'holiday' from behavioural training. In experiment 2, repeated treatment with DOI resulted in tolerance to the effect of DOI on T (50) and cross-tolerance to the effect of d-amphetamine (0.4 mg kg(-1)), but no cross-tolerance was seen to the effect of quinpirole (0.08 mg kg(-1)).

CONCLUSIONS

The results indicate that behavioural adaptation is involved in the development of tolerance to DOI's effect on timing. The finding of cross-tolerance to d-amphetamine but not to quinpirole suggests that the reduction of T (50) in the free-operant psychophysical procedure may be brought about by two distinct pharmacological mechanisms, one activated by DOI and d-amphetamine, and the other by quinpirole.

Characterizing the effects of d-amphetamine on temporal discrimination

Although the effects of dopamine agonists on temporal discrimination have been widely studied, conclusions as to their behavioral and neurophysiological effects are difficult due to a number of discrepant findings in the literature. This study examined whether a previously unexplored procedural difference could account for some of these discrepancies. In three experiments, pigeons categorized the duration of temporal samples during two variants of the interval-bisection procedure. In the position variant, responses to a side key that corresponded to the sample duration produced food. In the color-matching variant, responses to the key color that corresponded to the sample duration produced food. Across experiments, d-amphetamine produced a general disruption of temporal discrimination not accompanied by over or underestimation of time in both procedures. This effect occurred whether pigeons were exposed to both procedures within session (Experiment 1) or across conditions (Experiments 2 and 3) and whether the pigeons were drug experienced (Experiments 1 and 2) or naïve (Experiment 3). Analyses of the cumulative normal functions fitted to the proportion-long response data indicated that d-amphetamine produced its effects by selectively decreasing stimulus control, rather than by affecting timing.

Effects of methylphenidate on response to oral levodopa: a double-blind clinical trial

OBJECTIVE

To determine if repeated dosing with methylphenidate hydrochloride (MPD) (Ritalin; Novartis Pharmaceuticals, East Hanover, NJ), an inhibitor of the dopamine transporter, would augment the effects of oral levodopa in patients with Parkinson disease.

DESIGN

The study was a double-blind, randomized, placebo-controlled crossover trial.

SETTING

The trial was conducted at the General Clinical Research Center (GCRC) as an inpatient study. Subjects Thirteen people with idiopathic Parkinson disease and a fluctuating motor response to levodopa were recruited from movement disorder clinics as a convenience sample. One subject was excluded because he did not have a 10% increase in tapping speed in response to levodopa. The remaining 12 subjects completed the protocol.

INTERVENTIONS

A 0.4-mg/kg dose of MPD was administered orally at 8 am, noon, and 4 pm in conjunction with the subjects' normal oral antiparkinsonian medications. Oral levodopa dosage was decreased as clinically feasible during the first 4 days in the GCRC during open-label administration of MPD and hourly monitoring of parkinsonism and vital signs between 7 am and 8 pm. Subjects were discharged taking their usual antiparkinsonian medications without MPD. They returned 1 and 2 weeks later to the GCRC for 1 day of hourly monitoring of their response to the medication regimen derived during the 4 days in the GCRC, once with MPD and once with identical-appearing placebo, in a randomized sequence and double-blind conditions.

MAIN OUTCOME MEASURES

The main outcome measure was the duration of "on" time between 9 am and 8 pm measured by an increase in tapping speed by 10% over the average of the 7 am to 8 am predosing tapping speed measurements. Secondary measures were estimates of "on" time obtained with the timed walking task, tremor scores, and dyskinesia scores. In addition, averages of hourly tapping speeds, walking speed, tremor scores, dyskinesia scores, vital signs, and analog scale scores for mood, anxiety, and fatigue between 9 am and 8 pm were examined. Adverse events on the double-blinded days were compared.

RESULTS

Methylphenidate tended to increase the time "on" as measured by tapping (P = .09) but not by walking time or dyskinesia scores (P = .40 and .42, respectively). Methylphenidate tended to increase average tapping speed, decrease time to perform walking task, decrease tremor, and increase dyskinesia score but only the decrease in tremor reached significance. Neither the investigators nor the subjects could reliably identify active drug. Methylphenidate was well tolerated.

CONCLUSIONS

The effects of 0.4 mg/kg of MPD 3 times per day on the motor response to levodopa were small and variable and judged to be clinically insignificant. Trial Registration clinicaltrials.gov Identifier: NCT00359723.

Amphetamine affects the start of responding in the peak interval timing task

In this paper we investigate how amphetamine affects performance in a PI task by comparing two analyses of responding during peak trials. After training on 24 s fixed interval (FI-24) with 96 s peak trials, rats were given amphetamine for 4 consecutive days at doses of .5 and 1.0 mg/kg. Responses during peak trials were fitted with a Gaussian distribution to estimate the expected time of reinforcement from the peak time. A single trials analysis was also performed to determine the start time and stop time of the transition into and out of a high rate of responding on each peak trial. Amphetamine significantly decreased peak times as measured with the Gaussian curve fitting. However, in the single trials analysis, animals initiated responding significantly earlier, but did not stop responding earlier. Thus, fitting a Gaussian to the average performance across trials sometimes provides a different characterization of the timing process than does analyzing the start and stop of responding on individual trials. In the current experiment, the latter approach provided a more precise characterization of the effects of amphetamine on response timing.

Effects of psilocybin on time perception and temporal control of behaviour in humans

Hallucinogenic psilocybin is known to alter the subjective experience of time. However, there is no study that systematically investigated objective measures of time perception under psilocybin. Therefore, we studied dose-dependent effects of the serotonin (5-HT)2A/1A receptor agonist psilocybin (4-phosphoryloxy-N, N-dimethyltryptamine) on temporal processing, employing tasks of temporal reproduction, sensorimotor synchronization and tapping tempo. To control for cognitive and subjective changes, we assessed spatial working memory and conscious experience. Twelve healthy human volunteers were tested under placebo, medium (115 microg/kg), and high (250 microg/kg) dose conditions, in a double-blind experimental design. Psilocybin was found to significantly impair subjects' ability to (1) reproduce interval durations longer than 2.5 sec, (2) to synchronize to inter-beat intervals longer than 2 sec and (3) caused subjects to be slower in their preferred tapping rate. These objective effects on timing performance were accompanied by working-memory deficits and subjective changes in conscious state, namely increased reports of 'depersonalization' and 'derealization' phenomena including disturbances in subjective 'time sense.' Our study is the first to systematically assess the impact of psilocybin on timing performance on standardized measures of temporal processing. Results indicate that the serotonin system is selectively involved in duration processing of intervals longer than 2 to 3 seconds and in the voluntary control of the speed of movement. We speculate that psilocybin's selective disruption of longer intervals is likely to be a product of interactions with cognitive dimensions of temporal processing -presumably via 5-HT2A receptor stimulation.

Modeling the effects of the NMDA receptor antagonist MK-801 on timing in rats

The NMDA receptor antagonist MK-801 produces different effects on timing tasks. In particular, MK-801 produces an underestimation of duration when animals are tested with the differential reinforcement of low rate of responding (DRL) schedule and an overestimation of duration when animals are tested with the peak-interval (PI) procedure. The goal of this study was to develop a model-based explanation for this discrepancy. Two computer simulations were conducted via an implementation of scalar expectancy theory (SET). In Simulation 1, SET was used to provide a quantitative account of PI timing data. Simulation 2 used parameter estimates from Simulation 1 to predict effects of MK-801 on the DRL task. DRL predictions provided a close match to previous empirical data. Results of the simulations suggest that differences in the literature are likely due to inherent differences between PI and DRL tasks, rather than fundamental differences in timing. Overall, the role of NMDA receptors in timing appears to be multifaceted, impacting perception, memory, and decision processes.

Effects of d-amphetamine and DOI (2,5-dimethoxy-4-iodoamphetamine) on timing behavior: interaction between D1 and 5-HT2A receptors

RATIONALE

The dopamine-releasing agent d-amphetamine and the 5-HT(2) receptor agonist 2,5-dimethoxy-4-iodoamphetamine (DOI) have similar effects on free-operant timing behavior. The selective D(1) dopamine receptor antagonist 8-bromo-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-3-benzazepin-7-ol (SKF-83566), but not the D(2) dopamine receptor antagonist haloperidol, can antagonize the effect of d-amphetamine, and the selective 5-HT(2A) receptor antagonist (+/-)2,3-dimethoxyphenyl-1-(2-(4-piperidine)-methanol (MDL-100907) can antagonize the effect of DOI. However, it is not known whether the effect of d-amphetamine can be reversed by MDL-100907 and the effect of DOI by dopamine receptor antagonists.

OBJECTIVE

The objective of this work is to examine the interactions of d-amphetamine and DOI with MDL-100907, SKF-83566, and haloperidol on timing performance.

MATERIALS AND METHODS

Rats (n = 12-15 per experiment) were trained under the free-operant psychophysical procedure to press two levers (A and B) in 50-s trials in which reinforcement was provided intermittently for responding on A in the first half, and B in the second half of the trial. Percent responding on B (%B) was recorded in successive 5-s epochs of the trials; logistic functions were fitted to the data from each rat for the derivation of timing indices [T (50) (time corresponding to %B = 50); Weber fraction]. Rats were treated systemically with d-amphetamine or DOI, alone and in combination with haloperidol, SKF-83566, or MDL-100907.

RESULTS

d-Amphetamine (0.4 mg kg(-1)) reduced T (50) compared to vehicle; this effect was antagonized by SKF-83566 (0.03 mg kg(-1)) and MDL-100907 (0.5 mg kg(-1)), but not by haloperidol (0.05, 0.1 mg kg(-1)). DOI (0.25 mg kg(-1)) also reduced T (50); this effect was reversed by MDL-100907 (0.5 mg kg(-1)), but not by SKF-83566 (0.03 mg kg(-1)) or haloperidol (0.05 mg kg(-1)).

CONCLUSIONS

The results suggest that both 5-HT(2A) and D(1) receptors, but not D(2) receptors, are involved in d-amphetamine's effect on timing behavior in the free-operant psychophysical procedure. DOI's effect on timing is mediated by 5-HT(2A) receptors, but neither D(1) nor D(2) receptors are involved in this effect.

Interaction of raclopride and preparatory interval effects on simple reaction time performance

In a series of three experiments, simple reaction time (RT) was characterized with respect to a variable preparatory interval (PI) in order to investigate the relationship between interval timing and RT. In Experiment 1, it was shown that RT decreases as a function of PI and that this effect varies with amount of training. In Experiment 2, RT was shown to increase during probe trials that used a novel 6.25s PI, suggesting that the specific durations of the PIs encoded during initial training contribute to the PI effect on RT. In Experiment 3, 100 microg/kg i.p. of raclopride proportionally slowed RT as a function of the PI. These results are discussed within the context of neuropsychological models of interval timing and support an underlying role for cortico-striatal dopaminergic function in temporal processing and simple RT measurements.

Cognitive and subjective acute dose effects of intramuscular ketamine in healthy adults

Ketamine is a noncompetitive N-methyl-D-aspartate (NMDA) antagonist. Given the purported role of the NMDA receptor in long-term potentiation, the primary purpose of the present study was to further understand the dose-related effects of ketamine on memory. The study was also designed to provide information about the relative effects of ketamine on memory versus nonmemory effects and to more fully characterize ketamine's overall pattern and time course of effects. Single intramuscular injections of ketamine (0.2 mg/kg, 0.4 mg/kg) were administered to 18 healthy adult volunteers using a double-blind, placebo-controlled, crossover design. Word lists were used to evaluate episodic memory (free recall, recognition memory, source memory) and metamemory. Working memory, time estimation, psychomotor performance, and subjective effects were assessed repeatedly for 5 hours after drug administration. Ketamine selectively impaired encoding (as measured by free recall) while sparing retrieval, working memory while sparing attention, and digit symbol substitution task speed while sparing accuracy. Ketamine did not significantly impair recognition or source memory, metamemory, or time estimation. There were no hallucinations or increases in mystical experiences with ketamine. Memory measures were less sensitive to ketamine effects than subjective or psychomotor measures. Subjective effects lasted longer than memory and most psychomotor impairments. Ketamine produces selective, transient, dose- and time-related effects. In conjunction with previous studies of drugs with different mechanisms of actions, the observed selectivity of effects enhances the understanding of the pharmacological mechanisms underlying memory, attention, psychomotor performance, and subjective experience.

Effects of 5-HT2A receptor stimulation on the discrimination of durations by rats

We recently found that rats' ability to discriminate durations of exteroceptive stimuli is disrupted by the non-selective 5-HT receptor agonist quipazine. Ketanserin reversed this effect, suggesting that the effect may be mediated by 5-HT2A receptors. Here, we report that the 5-HT2A/2C receptor agonist 2,5-dimethoxy-4-iodoamphetamine (DOI) also disrupts temporal discrimination, and that this effect can be reversed by ketanserin and the highly selective 5-HT2A receptor antagonist (+/-)2,3-dimethoxyphenyl-1-[2-(4-piperidine)-methanol] (MDL-100907). Twenty rats were trained to discriminate durations in a discrete-trials psychophysical procedure. In each 50-s trial, a light was presented for t seconds, following which two levers (A and B) were presented. A response on A was reinforced if t < 25 s, and a response on B if t > 25 s. Logistic psychometric curves were fitted to the proportional choice of B (%B) for derivation of timing indices [T50: time corresponding to %B = 50; Weber fraction: (T75-T25)/2T50, where T75 and T25 are times corresponding to %B = 75 and 25, respectively]. DOI 0.25 mg kg (subcutaneous) significantly increased the Weber fraction and tended to increase T50. Ketanserin 2 mg kg (subcutaneous) did not alter either parameter, but completely antagonized the effects of DOI. Similarly, MDL-100907 0.5 and 1 mg kg (intraperitoneal) did not affect performance, but completely antagonized the effects of DOI. The results indicate that the mixed 5-HT2A/2C receptor agonist DOI disrupts temporal discrimination via stimulation of 5-HT2A receptors.

Effect of clozapine on interval timing and working memory for time in the peak-interval procedure with gaps

Previous research indicates that dopamine controls both the speed of an internal clock [Maricq, A.V., Church, R.M., 1983. The differential effects of haloperidol and methamphetamine on time estimation in the rat. Psychopharmacology 79, 10-15] and sharing of resources between the timer and other cognitive processes [Buhusi, C.V., 2003. Dopaminergic mechanisms of interval timing and attention. In: Meck, W.H. (Ed.), Functional and Neural Mechanisms of Interval Timing. CRC Press, Boca Raton, FL, pp. 317-338]. For example, dopamine agonist methamphetamine increases the speed of an internal clock and resets timing after a gap, while dopamine antagonist haloperidol decreases the speed of an internal clock and stops timing during a gap [Buhusi, C.V., Meck, W.H., 2002. Differential effects of methamphetamine and haloperidol on the control of an internal clock. Behav. Neurosci. 116, 291-297]. Using a 20-s peak-interval procedure with gaps we examined the acute effects of clozapine (2.0mg/kg i.p.), which exerts differential effects on dopamine and serotonin in the cortex and striatum, two brain areas involved in interval timing and working memory. Relative to saline, clozapine injections shifted the response functions leftward both in trials with and without gaps, suggesting that clozapine increased the speed of an internal clock and facilitated the maintenance of the pre-gap interval in working memory. These results suggest that clozapine exerts effects in different brain areas in a manner that allows for the pharmacological separation of clock speed and working memory as a function of peak trials without and with gaps.

Single-trials analyses demonstrate that increases in clock speed contribute to the methamphetamine-induced horizontal shifts in peak-interval timing functions

INTRODUCTION

Drugs that increase dopamine (DA) transmission have been shown to produce an overestimation of time in duration production procedures as exhibited by horizontal leftward shifts of the psychophysical functions. However, the generality of these results has been inconsistent in the literature.

MATERIALS AND METHODS

The present report evaluates the effects of five doses of methamphetamine (MAP) (0.5-1.5 mg/kg, i.p.) on two duration production procedures, the single duration peak-interval (PI) procedure and the multiduration tri-peak procedure in rats.

RESULTS

We replicated and extended prior results by showing a dose-dependent proportional overestimation of time that was equivalent on both procedures (i.e., subjects behaved as though they expected reinforcement to be available earlier in real time). Single-trials analyses demonstrated that the reduction in peak rate that is often observed after MAP administration is due to an increase in the proportion of trials in which responding occurred at very low rates and without temporal control. However, these low-rate trials were not the source of the leftward shift in the temporal estimates. Rather, we found that the leftward shift of the PI functions was due to proportional changes in the placement of temporally controlled high-rate responding, which is consistent with a DA-mediated alteration in clock speed.

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.

Pharmacological manipulations of food protection behavior in rats: Evidence for dopaminergic contributions to time perception during a natural behavior

Operant procedures combined with pharmacological manipulations have implicated a role for the dopaminergic system in the perception and production of temporal intervals. Because studies have suggested that animals use temporal information to organize food protection behavior, the current study investigates whether dopaminergic systems are involved in timing during this natural behavior. The experiment examined the influence of a dopaminergic agonist (amphetamine) and an antagonist (haloperidol) on food protection behavior initiated to avoid theft by a conspecific. Amphetamine increased the time spent dodging and decreased the time spent bracing during the consumption of a hazelnut. On the other hand, haloperidol decreased the time spent dodging while showing no systematic changes in bracing. Topographic and kinematic analyses of rat movement conflicted with motivational, motoric, and social accounts of drug-induced changes in food protection behavior organization. These observations provide evidence that rats use temporal information to organize movements in the natural behavior of protecting food from theft by a conspecific, and this organization is influenced by both a dopaminergic agonist and an antagonist.

Pharmacological dissociation of trace and long-delay fear conditioning in young rats

In most studies comparing trace and delay conditioning, CS duration is kept constant across training conditions but the interstimulus interval (ISI), the time from CS onset to US onset, is confounded. In the infrequently used long-delay condition, however, ISI is kept constant across the trace and delay conditions but CS duration varies. A recent study reported that trace and long-delay fear conditioning have the same developmental trajectory, with both emerging later in development than standard-delay conditioning (). Past studies have shown that trace conditioning is mediated by the cholinergic system; given the parallel developmental emergence of trace and long-delay conditioning, the present study examined whether the cholinergic system also mediates long-delay conditioning. Two experiments, both involving Sprague-Dawley-derived rats and using freezing as a measure of learned fear, showed that the cholinergic system is critically involved in trace conditioning but is not involved in long-delay conditioning. Specifically, pre-training injections of the muscarinic receptor antagonist scopolamine impaired acquisition of a CS-US association in 32-day-old rats trained with a trace procedure but had no effect on rats this age trained with a long-delay procedure (Experiment 1). Similarly, pre-training injections of physostigmine, a cholinesterase inhibitor, enhanced acquisition of trace conditioning in 25-day-old rats but had no effect on long-delay conditioning in rats this age (Experiment 2). Taken together, the results indicate that despite the similarities between trace and long-delay conditioning in terms of developmental emergence and level of conditioned responding, they are mediated by different physiological systems.

Duration selectivity organization in the inferior colliculus of the big brown bat, Eptesicus fuscus

Duration selectivity of auditory neurons plays an important role in sound recognition. Previous studies show that GABA-mediated duration selectivity of neurons in the central nucleus of the inferior colliculus (IC) of many animal species behave as band-, short-, long- and all-pass filters to sound duration. The present study examines the organization of duration selectivity of IC neurons of the big brown bat, Eptesicus fuscus, in relation to graded spatial distribution of GABA(A) receptors, which are mostly distributed in the dorsomedial region of the IC but are sparsely distributed in the ventrolateral region. Duration selectivity of IC neuron is studied before and during iontophoretic application of GABA and its antagonist, bicuculline. Bicuculline application decreases and GABA application increases duration selectivity of IC neurons. Bicuculline application produces more pronounced broadening of the duration tuning curves of neurons at upper IC than at deeper IC but the opposite is observed during GABA application. The best duration of IC neurons progressively lengthens and duration selectivity decreases with recording depth both before and during drug application. As such, low best frequency neurons at upper IC have shorter best duration and sharper duration selectivity than high best frequency neurons in the deeper IC have. These data suggest that duration selectivity of IC neurons systematically varies with GABA(A) receptor distribution gradient within the IC.

Single-dose levodopa administration and aging independently disrupt time production

We tested the hypothesis that age-related time production deficits are dopamine-mediated. The experiment was conducted double-blind, and with random assignment of 32 healthy aged and 32 healthy young participants to either inert placebo or levodopa (200 mg) groups. The procedure included training participants to produce two target time intervals (6 and 17 sec) in separate blocks, drug/placebo administration, a 1-hr delay, and then delayed free-recall time production retesting without feedback. Participants also performed a speeded choice reaction time (RT) task, as a control for potential dopaminergic and aging effects on attention and psychomotor speed. Results indicate that during retesting, aged participants show duration-dependent timing errors that are larger than those shown by the young participants. Levodopa administration yielded lengthened time production of both target intervals. The aging and levodopa effects did not interact. Also, aging slowed RT and increased RT variability, but levodopa had no effect on the RT. These results suggest that at this dosage and under these specific conditions, timing is dopamine-mediated but the effect of aging on time production is not. Moreover, the levodopa timing effect cannot be attributed to the effects of dopaminergic function on psychomotor speed.

Evidence for a role of D1 dopamine receptors in d-amphetamine's effect on timing behaviour in the free-operant psychophysical procedure

RATIONALE

Temporal differentiation of operant behaviour is sensitive to dopaminergic manipulations. Studies using the fixed-interval peak procedure implicated D2 dopamine receptors in these effects. Less is known about the effects of dopaminergic manipulations on temporal differentiation in other timing schedules.

OBJECTIVE

To examine the effects of a D1 antagonist,8-bromo-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-3-benzazepin-7-ol (SKF-83566), and a D2 antagonist, haloperidol, on performance on the free-operant psychophysical procedure, and the ability of these antagonists to reverse the effects of the catecholamine-releasing agent, d-amphetamine on performance. The antagonists' ability to reverse d-amphetamine-induced hyperlocomotion was also examined.

MATERIALS AND METHODS

Rats responded on two levers (A and B) under a free-operant psychophysical schedule, in which reinforcement was provided intermittently for responding on A during the first half, and B during the second half, of 50-s trials. Logistic functions were fitted to the relative response rate data (percent responding on B [%B] vs time [t]) in each treatment condition, and quantitative timing indices [T50 (value of t corresponding to %B=50) and Weber fraction] were compared among treatments. Effects of the treatments on locomotion were measured in a separate experiment.

RESULTS

SKF-83566 (0.015, 0.03, 0.06 mg kg(-1)) did not affect timing performance. Haloperidol (0.025, 0.05 mg kg(-1)) had no effect; a higher dose (0.1 mg kg(-1)) reduced T (50). d-Amphetamine (0.4 mg kg(-1)) reduced T50; this effect was antagonised by SKF-83566 but not by haloperidol. Both antagonists reduced d-amphetamine-induced hyperlocomotion.

CONCLUSIONS

The results suggest that d-amphetamine's effect on performance in the free-operant psychophysical procedure is mediated by D1 rather than D2 receptors.

alpha7 Nicotinic acetylcholine receptors and temporal memory: synergistic effects of combining prenatal choline and nicotine on reinforcement-induced resetting of an interval clock

We previously showed that prenatal choline supplementation could increase the precision of timing and temporal memory and facilitate simultaneous temporal processing in mature and aged rats. In the present study, we investigated the ability of adult rats to selectively control the reinforcement-induced resetting of an internal clock as a function of prenatal drug treatments designed to affect the alpha7 nicotinic acetylcholine receptor (alpha7 nAChR). Male Sprague-Dawley rats were exposed to prenatal choline (CHO), nicotine (NIC), methyllycaconitine (MLA), choline + nicotine (CHO + NIC), choline + nicotine + methyllycaconitine (CHO + NIC + MLA), or a control treatment (CON). Beginning at 4-mo-of-age, rats were trained on a peak-interval timing procedure in which food was available at 10-, 30-, and 90-sec criterion durations. At steady-state performance there were no differences in timing accuracy, precision, or resetting among the CON, MLA, and CHO + NIC + MLA treatments. It was observed that the CHO and NIC treatments produced a small, but significant increase in timing precision, but no change in accuracy or resetting. In contrast, the CHO + NIC prenatal treatment produced a dramatic increase in timing precision and selective control of the resetting mechanism with no change in overall timing accuracy. The synergistic effect of combining prenatal CHO and NIC treatments suggests an organizational change in alpha7 nAChR function that is dependent upon a combination of selective and nonselective nAChR stimulation during early development.

Effects of marijuana on temporal discriminations in humans

Marijuana has been reported to alter the discrimination of time. The present study used a psychophysical approach to examine the effects of marijuana on temporal discrimination in humans. Research participants were required to push one of two buttons depending on the duration of a conditional stimulus (a blue square on a computer monitor). Correct choices ('C' button after a 2-s stimulus; 'A' button after a 4-s stimulus) resulted in an increase in session earnings of 0.12 dollars. Intermediate durations (probe stimuli between 2 and 4 s) were also presented. Psychophysical functions relating the probability of judging a duration as 'long' (4 s) as a function of actual stimulus durations were characterized by a logistic function fitted to the data. Administration of both low (1/2 placebo and 1/2 2.2% Delta (9)-tetrahydrocannabinol cigarette) and high (3.89% Delta(9)-tetrahydrocannabinol) potency marijuana cigarettes produced a bias of judging intervals as long, consistent with an interpretation that subjective time passes more quickly when an individual is intoxicated by marijuana. Deliberation time, operationally defined as response latency, peaked on trials with sample durations that corresponded to the measure of central tendency, and shifted in a similar manner after marijuana administration. The data are consistent with other studies on the effects of marijuana on time estimation.

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.

Effects of d-amphetamine on the behavior of pigeons exposed to the peak procedure

The effects of d-amphetamine on pigeons' key-pecking under the peak interval (PI) procedure were investigated in two experiments. In experiment I the effects of doses of d-amphetamine from 0.75 to 3.0 mg/kg on responding under PI 30 and 45 s were studied for 10 successive days. Reductions in peak time and wait time were observed at both PI values and an increase in the width was found at PI 30 s. There was no evidence of tolerance. In experiment II, pigeons exposed to a PI 45 s schedule were administered doses of D-amphetamine of 1.5 and 3.0 mg/kg for 30 successive days. Reductions in peak time and wait time were found here. Evidence of tolerance was found in wait time, peak time and width of the distribution at the higher dose. In both experiments a rate-dependent effect of the drug was found in the portion of each peak trial before the time that food was delivered on reinforced trials; this effect was weaker after the customary time of food delivery. The rate-dependent effect for responses before food time, combined with little effect of the drug on responses after food time, is shown by simulation to be sufficient to account for the reduction in peak time, without the need to appeal to an internal clock mechanism.

Effects of pharmacologically induced changes in NMDA receptor activity on human timing and sensorimotor performance

Unlike processing of time intervals in the range of seconds or more, processing of brief durations in the subsecond range appears to be beyond cognitive control and based on subcortical mechanisms located in the basal ganglia. The present study was designed to evaluate the effects of NMDA receptor activity on temporal processing in the second and subsecond range. In a double-blind crossover design, either 30 mg of the noncompetitive NMDA receptor antagonist memantine or placebo was administered to 32 healthy male volunteers. While memantine induced a marked impairment in temporal processing of intervals in the range of seconds, temporal processing of intervals in the range of milliseconds was not affected. Furthermore, no effect of memantine on speed of information processing could be observed. Speed of motor response execution, however, was significantly enhanced by memantine compared to placebo. The overall pattern of results provides converging evidence for the notion that temporal processing of longer intervals involves higher order working memory functions such as working memory capacity. The absence of an effect on temporal processing of very brief intervals in combination with the beneficial effect on movement time suggests that NMDA receptor activity in the basal ganglia is not directly related to the timing of intervals in the subsecond range.

Within-subject comparison of the subjective and psychomotor effects of a gaseous anesthetic and two volatile anesthetics in healthy volunteers

Inhalant abuse is a serious public health problem throughout the world. The present study compared the states of intoxication produced by three inhaled anesthetics that represent two of the three major classes of abused inhalants, as classified by [Balster, R.L., 1998. Neural basis of inhalant abuse. Drug Alcohol Depend 51, 207-214.]. Isoflurane and sevoflurane represent the class of volatile substances, and nitrous oxide (N2O) comprises a class of its own. Fourteen healthy volunteers inhaled the vehicle (100% O2) and two concentrations each of isoflurane (0.1 and 0.2%), sevoflurane (0.2 and 0.4%), and N2O (15 and 30%) for 40 min each, across seven separate sessions. Drug concentrations were chosen to produce similar ratings of drug effect strength and similar impairment on a psychomotor test, the digit-symbol substitution test (DSST). Ratings of drug effect strength and performance on the DSST were similar across drugs; however, the volatile anesthetics produced greater sedation and greater impairment on three other psychomotor tests than N2O, whereas N2O produced a greater magnitude of putatively pleasant and psychedelic-like subjective effects. These results are consistent with the drugs' putative receptor mechanisms of action and confirm Balster's classification of the volatile anesthetics into a class distinct from N2O.

[Effect of melatonin on memory, individual time perception, and anxiety in young people of different chronotype groups]

Chronic administration of the pineal hormone melatonin in a low dose (0.75 mg) improved memory and optimized individual time perception in a group of young volunteers. These changes were pronounced even two weeks after termination of the drug administration. The expression of the melatonin effect was depended on the chronotype of humans tested.

Age and medication effects on rhythmic responses in ADHD: Possible oscillatory mechanisms?

Voluntary motor responses in children with attention-deficit hyperactivity disorder (ADHD) may reflect underlying neuronal oscillatory mechanisms. The aims of this study were: (1) to corroborate the presence of rhythmic motor abnormalities on tapping test in children with attention-deficit hyperactivity disorder, shown in previous studies; (2) to delineate the characteristics of ADHD children demonstrating these rhythmic abnormalities; (3) to assess the impact of methylphenidate (Ritalin) on this abnormal motor phenomenon. The study was designed in a double blind manner. Sixty-four ADHD children aged 6-12 years and 60 matched controls underwent a finger tapping test (1-5 Hz). We measured the abnormal rhythmic tapping with and without methylphenidate treatment in a double blind design. Conventional statistical analysis was used to assess the correlation of the presence of motor abnormality with various clinical characteristics (such as degree of hyperactivity/impulsivity). Fifty-six percent of children with ADHD compared to 8% of controls demonstrated abnormal rhythmic responses at a mean frequency of 3.1+/-0.9 Hz. These erroneous voluntary movements were more common in children with a greater degree of hyperactivity, in younger children but not significantly altered by methylphenidate treatment. Surprisingly, abnormal rhythmic responses showed less variability than found for responses for which the tapping was adequately paced, suggesting different motor control processes for normal and abnormal paced tapping. We speculate that rhythmic tapping responses reflect abnormal oscillatory mechanisms involved in ADHD, exacerbated in younger children with ADHD and in those in whom the hyperactivity/impulsivity is more pronounced.

Evidence that the effect of 5-HT2 receptor stimulation on temporal differentiation is not mediated by receptors in the dorsal striatum

5-HT2 receptor stimulation alters temporal differentiation in free-operant timing schedules. The anatomical location of the receptor population responsible for this effect is unknown. We examined the effect of a 5-HT2 receptor agonist and antagonists, injected systemically and into the dorsal striatum, a region that is believed to play a major role in interval timing. Rats were trained under the free-operant psychophysical procedure to press levers A and B in 50s trials in which reinforcement was provided intermittently for responding on A in the first half, and B in the second half of the trial. Percent responding on B (%B) was recorded in successive 5s epochs of the trials; logistic functions were fitted to the data from each rat to derive timing indices (T50: time corresponding to %B = 50; Weber fraction: [T75-T25]/2T50, where T75 and T25 are the times corresponding to %B = 75 and %B = 25). Systemic treatment with the 5-HT(2A/2C) receptor agonist 2,5,-dimethoxy-4-iodo-amphetamine (DOI) (0.25 mg/kg, s.c.) reduced T50; the 5-HT2A receptor antagonist MDL-100907 (0.5 mg/kg, i.p.) did not affect performance, but completely blocked the effect of DOI. DOI (1 and 3 microg) injected bilaterally into the dorsal striatum did not alter T50. The effect of systemic treatment with DOI (0.25 mg/kg, s.c.) was not altered by intra-striatal injection of MDL-100907 (0.3 microg) or the 5-HT2C receptor antagonist RS-102221 (0.15 microg). The ability of systemically administered MDL-100907 to reverse DOI's effect on T50 confirms the sensitivity of temporal differentiation to 5-HT2A receptor stimulation. The failure of intra-striatal MDL-100907 to antagonize the effects of DOI suggests that 5-HT2A receptors in the dorsal striatum are unlikely to be primarily responsible for DOI's effects on timing. Furthermore, the results provide no evidence for a role of striatal 5-HT2C receptors in DOI's effect on timing.