Anatomical specificity within rat striatum for the dopaminergic modulation of DRL responding and activity

Resource Allocation in the Noise-Free Striatal Beat Frequency Model of Interval Timing

The Striatal Beat Frequency (SBF) model of interval timing uses many neural oscillators, presumably located in the frontal cortex (FC), to produce beats at a specific criterion time Tc. The coincidence detection produces the beats in the basal ganglia spiny neurons by comparing the current state of the FC neural oscillators against the long-term memory values stored at reinforcement time Tc. The neurobiologically realistic SBF model has been previously used for producing precise and scalar timing in the presence of noise. Here we simplified the SBF model to gain insight into the resource allocation problem in interval timing networks. Specifically, we used a noise-free SBF model to explore the lower limits of the number of neural oscillators required for producing accurate timing. Using abstract sine-wave neural oscillators in the SBF-sin model, we found that the lower limit of the number of oscillators needed is proportional to the criterion time Tc and the frequency span (fmax − fmin) of the FC neural oscillators. Using biophysically realistic Morris–Lecar model neurons in the SBF-ML model, the lower bound increased by one to two orders of magnitude compared to the SBF-sin model.

Impaired Decision Making and Loss of Inhibitory-Control in a Rat Model of Huntington Disease

Cognitive deficits associated with Huntington disease (HD) are generally dominated by executive function disorders often associated with disinhibition and impulsivity/compulsivity. Few studies have directly examined symptoms and consequences of behavioral disinhibition in HD and its relation with decision-making. To assess the different forms of impulsivity in a transgenic model of HD (tgHD rats), two tasks assessing cognitive/choice impulsivity were used: risky decision-making with a rat gambling task (RGT) and intertemporal choices with a delay discounting task (DD). To assess waiting or action impulsivity the differential reinforcement of low rate of responding task (DRL) was used. In parallel, the volume as well as cellular activity of the amygdala was analyzed. In contrast to WT rats, 15 months old tgHD rats exhibited a poor efficiency in the RGT task with difficulties to choose advantageous options, a steep DD curve as delays increased in the DD task and a high rate of premature and bursts responses in the DRL task. tgHD rats also demonstrated a concomitant and correlated presence of both action and cognitive/choice impulsivity in contrast to wild type (WT) animals. Moreover, a reduced volume associated with an increased basal cellular activity of the central nucleus of amygdala indicated a dysfunctional amygdala in tgHD rats, which could underlie inhibitory dyscontrol. In conclusion, tgHD rats are a good model for impulsivity disorder that could be used more widely to identify potential pharmacotherapies to treat these invasive symptoms in HD.

What is all the noise about in interval timing?

Cognitive processes such as decision-making, rate calculation and planning require an accurate estimation of durations in the supra-second range-interval timing. In addition to being accurate, interval timing is scale invariant: the time-estimation errors are proportional to the estimated duration. The origin and mechanisms of this fundamental property are unknown. We discuss the computational properties of a circuit consisting of a large number of (input) neural oscillators projecting on a small number of (output) coincidence detector neurons, which allows time to be coded by the pattern of coincidental activation of its inputs. We showed analytically and checked numerically that time-scale invariance emerges from the neural noise. In particular, we found that errors or noise during storing or retrieving information regarding the memorized criterion time produce symmetric, Gaussian-like output whose width increases linearly with the criterion time. In contrast, frequency variability produces an asymmetric, long-tailed Gaussian-like output, that also obeys scale invariant property. In this architecture, time-scale invariance depends neither on the details of the input population, nor on the distribution probability of noise.

The effects of clinically relevant doses of amphetamine and methylphenidate on signal detection and DRL in rats

Low dose amphetamine (AMPH) and methylphenidate (MPH, Ritalin(®)) are the most widely prescribed and most effective pharmacotherapy for attention-deficit/hyperactivity disorder (ADHD). Certain low, clinically relevant doses of MPH improve sustained attention and working memory in normal rats, in contrast to higher doses that impair cognitive ability and induce locomotor activity. However, the effects of AMPH of MPH on sustained attention and behavioral inhibition remain poorly characterized. The present experiments examined the actions of AMPH (0.1 and 0.25 mg/kg) and MPH (0.5 and 1.0 mg/kg) in a rat model of 1) sustained attention, where signal and blank trials were interspersed randomly and occurred at unpredictable times, and 2) behavioral inhibition, using a differential reinforcement of low rate (DRL) schedule. In a signal detection paradigm, both 0.5 mg/kg and 1.0 mg/kg MPH and 0.25 mg/kg AMPH improve sustained attention, however neither AMPH nor MPH improve behavioral inhibition on DRL. Taken together with other recent studies, it appears that clinically-relevant doses of AMPH and MPH may preferentially improve attention-related behavior while having little effect on behavioral inhibition. These observations provide additional insight into the basic behavioral actions of low-dose psychostimulants and further suggest that the use of sustained attention tasks may be important in the development of novel pharmacological treatments for ADHD.

Endocannabinoid-dependent modulation of phasic dopamine signaling encodes external and internal reward-predictive cues

The mesolimbic dopamine (DA) system plays an integral role in incentive motivation and reward seeking and a growing body of evidence identifies signal transduction at cannabinoid receptors as a critical modulator of this system. Indeed, administration of exogenous cannabinoids results in burst firing of DA neurons of the ventral tegmental area and increases extracellular DA in the nucleus accumbens (NAcc). Implementation of fast-scan cyclic voltammetry (FSCV) confirms the ability of cannabinoids to augment DA within the NAcc on a subsecond timescale. The use of FSCV along with newly developed highly selective pharmacological compounds advances our understanding of how cannabinoids influence DA transmission and highlights a role for endocannabinoid-modulated subsecond DAergic activation in the incentive motivational properties of not only external, but also internal reward-predictive cues. For example, our laboratory has recently demonstrated that in mice responding under a fixed-interval (FI) schedule for food reinforcement, fluctuations in NAcc DA signal the principal cue predictive of reinforcer availability - time. That is, as the interval progresses, NAcc DA levels decline leading to accelerated food seeking and the resulting characteristic FI scallop pattern of responding. Importantly, administration of WIN 55,212-2, a synthetic cannabinoid agonist, or JZL184, an indirect cannabinoid agonist, increases DA levels during the interval and disrupts this pattern of responding. Along with a wealth of other reports, these results illustrate the role of cannabinoid receptor activation in the regulation of DA transmission and the control of temporally guided reward seeking. The current review will explore the striatal beat frequency model of interval timing as it pertains to cannabinoid signaling and propose a neurocircuitry through which this system modulates interoceptive time cues.

Phase resetting and its implications for interval timing with intruders

Time perception in the second-to-minutes range is crucial for fundamental cognitive processes like decision making, rate calculation, and planning. We used a striatal beat frequency (SBF) computational model to predict the response of an interval timing network to intruders, such as gaps in conditioning stimulus (CS), or distracters presented during the uninterrupted CS. We found that, depending on the strength of the input provided to neural oscillators by the intruder, the SBF model can either ignore it or reset timing. The significant delays in timing produced by emotionally charged distracters were numerically simulated by a strong phase resetting of all neural oscillators involved in the SBF network for the entire duration of the evoked response. The combined effect of emotional distracter and pharmacological manipulations was modeled in our SBF model by modulating the firing frequencies of neural oscillators after they are released from inhibition due to emotional distracters. This article is part of a Special Issue entitled: SI: Associative and Temporal Learning.

Why noise is useful in functional and neural mechanisms of interval timing?

BACKGROUND

The ability to estimate durations in the seconds-to-minutes range - interval timing - is essential for survival, adaptation and its impairment leads to severe cognitive and/or motor dysfunctions. The response rate near a memorized duration has a Gaussian shape centered on the to-be-timed interval (criterion time). The width of the Gaussian-like distribution of responses increases linearly with the criterion time, i.e., interval timing obeys the scalar property.

RESULTS

We presented analytical and numerical results based on the striatal beat frequency (SBF) model showing that parameter variability (noise) mimics behavioral data. A key functional block of the SBF model is the set of oscillators that provide the time base for the entire timing network. The implementation of the oscillators block as simplified phase (cosine) oscillators has the additional advantage that is analytically tractable. We also checked numerically that the scalar property emerges in the presence of memory variability by using biophysically realistic Morris-Lecar oscillators. First, we predicted analytically and tested numerically that in a noise-free SBF model the output function could be approximated by a Gaussian. However, in a noise-free SBF model the width of the Gaussian envelope is independent of the criterion time, which violates the scalar property. We showed analytically and verified numerically that small fluctuations of the memorized criterion time leads to scalar property of interval timing.

CONCLUSIONS

Noise is ubiquitous in the form of small fluctuations of intrinsic frequencies of the neural oscillators, the errors in recording/retrieving stored information related to criterion time, fluctuation in neurotransmitters' concentration, etc. Our model suggests that the biological noise plays an essential functional role in the SBF interval timing.

How noise contributes to time-scale invariance of interval timing

Time perception in the suprasecond range is crucial for fundamental cognitive processes such as decision making, rate calculation, and planning. In the vast majority of species, behavioral manipulations, and neurophysiological manipulations, interval timing is scale invariant: the time-estimation errors are proportional to the estimated duration. The origin and mechanisms of this fundamental property are unknown. We discuss the computational properties of a circuit consisting of a large number of (input) neural oscillators projecting on a small number of (output) coincidence detector neurons, which allows time to be coded by the pattern of coincidental activation of its inputs. We show that time-scale invariance emerges from the neural noise, such as small fluctuations in the firing patterns of its input neurons and in the errors with which information is encoded and retrieved by its output neurons. In this architecture, time-scale invariance is resistant to manipulations as it depends neither on the details of the input population nor on the distribution probability of noise.

Prefrontal cortical-striatal dopamine receptor mRNA expression predicts distinct forms of impulsivity

Variation in dopamine receptor levels has been associated with different facets of impulsivity. To further delineate the neural substrates underlying impulsive action (inability to withhold a prepotent motor response) and impulsive choice (delay aversion), we characterised rats in the Differential Reinforcement of Low Rates of Responding task and a delay discounting task. We also measured performance on an effort-based discounting task. We then assessed D1 and D2 dopamine receptor mRNA expression in subregions of the prefrontal cortex and nucleus accumbens using in situ hybridisation, and compared these data with behavioral performance. Expression of D1 and D2 receptor mRNA in distinct brain regions was predictive of impulsive action. A dissociation within the nucleus accumbens was observed between subregions and receptor subtypes; higher D1 mRNA expression in the shell predicted greater impulsive action, whereas lower D2 mRNA expression in the core predicted greater impulsive action. We also observed a negative correlation between impulsive action and D2 mRNA expression in the prelimbic cortex. Interestingly, a similar relationship was present between impulsive choice and prelimbic cortex D2 mRNA, despite the fact that behavioral indices of impulsive action and impulsive choice were uncorrelated. Finally, we found that both high D1 mRNA expression in the insular cortex and low D2 mRNA expression in the infralimbic cortex were associated with willingness to exert effort for rewards. Notably, dopamine receptor mRNA in these regions was not associated with either facet of impulsivity. The data presented here provide novel molecular and neuroanatomical distinctions between different forms of impulsivity, as well as effort-based decision-making.

Time-scale invariance as an emergent property in a perceptron with realistic, noisy neurons

In most species, interval timing is time-scale invariant: errors in time estimation scale up linearly with the estimated duration. In mammals, time-scale invariance is ubiquitous over behavioral, lesion, and pharmacological manipulations. For example, dopaminergic drugs induce an immediate, whereas cholinergic drugs induce a gradual, scalar change in timing. Behavioral theories posit that time-scale invariance derives from particular computations, rules, or coding schemes. In contrast, we discuss a simple neural circuit, the perceptron, whose output neurons fire in a clockwise fashion based on the pattern of coincidental activation of its input neurons. We show numerically that time-scale invariance emerges spontaneously in a perceptron with realistic neurons, in the presence of noise. Under the assumption that dopaminergic drugs modulate the firing of input neurons, and that cholinergic drugs modulate the memory representation of the criterion time, we show that a perceptron with realistic neurons reproduces the pharmacological clock and memory patterns, and their time-scale invariance, in the presence of noise. These results suggest that rather than being a signature of higher order cognitive processes or specific computations related to timing, time-scale invariance may spontaneously emerge in a massively connected brain from the intrinsic noise of neurons and circuits, thus providing the simplest explanation for the ubiquity of scale invariance of interval timing.

An examination of the effects of subthalamic nucleus inhibition or μ-opioid receptor stimulation on food-directed motivation in the non-deprived rat

The subthalamic nucleus (STN) serves important functions in regulating movement, cognition, and motivation and is connected with cortical and basal ganglia circuits that process reward and reinforcement. In order to further examine the role of the STN on motivation toward food in non-deprived rats, these experiments studied the effects of pharmacological inhibition or μ-opioid receptor stimulation of the STN on the 2-h intake of a sweetened fat diet, the amount of work exerted to earn sucrose on a progressive ratio 2 (PR-2) schedule of reinforcement, and performance on a differential reinforcement of low-rate responding (DRL) schedule for sucrose reward. Separate behavioral groups (N=6-9) were tested following bilateral inhibition of the STN with the GABA(A) receptor agonist muscimol (at 0-5 ng/0.5 μl/side) or following μ-opioid receptor stimulation with the agonist D-Ala², N-MePhe⁴, Gly-ol-enkephalin (DAMGO; at 0, 0.025 or 0.25 μg/0.5 μl/side). Although STN inhibition increased ambulatory behavior during 2-h feeding sessions, it did not significantly alter intake of the sweetened fat diet. STN inhibition also did not affect the breakpoint for sucrose pellets during a 1-h PR-2 reinforcement schedule or impact the number of reinforcers earned on a 1-h DRL-20s reinforcement schedule in non-deprived rats. In contrast, STN μ-opioid receptor stimulation significantly increased feeding on the palatable diet and reduced the reinforcers earned on a DRL-20 schedule, although DAMGO microinfusions had no effect on PR-2 performance. These data suggest that STN inhibition does not enhance incentive motivation for food in the absence of food restriction and that STN μ-opioid receptors play an important and unique role in motivational processes.

Rapid and acute effects of estrogen on time perception in male and female rats

Sex differences in the rapid and acute effects of estradiol on time perception were investigated in adult male and female Sprague-Dawley rats. Because estradiol has been shown to increase striatal dopamine release, it may be able to modify time perception and timed performance by increasing the speed of an internal clock in a manner similar to indirect dopamine agonists such as amphetamine and cocaine. Two groups of females (neonatally estradiol-treated/adult ovariectomized and neonatally oil-treated/adult ovariectomized) and two groups of males (neonatally castrated and adult castrated) were trained in a 2 vs. 8-s duration bisection procedure and tested using intermediate signal durations. After obtaining oil-injected baseline psychometric functions over several days, rats were administered 5 μg of estradiol for 4 days and behaviorally evaluated 30 min following each injection. This oil-estradiol administration cycle was subsequently repeated three times following the re-establishment of baseline training. Results revealed significant sex differences in the initial baseline functions that were not modifiable by organizational hormones, with males' duration bisection functions shifted horizontally to the left of females'. Upon the first administration of estradiol, females, but not males, showed a significant, transient leftward shift in their bisection functions, indicative of an increase in clock speed. After extensive retraining in the duration bisection procedure, rats that were exposed to gonadal hormones during the first week of life showed a significant rightward shift in their bisection functions on the fourth day of estradiol administration during each cycle, suggesting a decrease in clock speed. Taken together, our results support the view that there are multiple mechanisms of estrogens' action in the striatum that modulate dopaminergic activity and are differentially organized by gonadal steroids during early brain development.

Modeling pharmacological clock and memory patterns of interval timing in a striatal beat-frequency model with realistic, noisy neurons

In most species, the capability of perceiving and using the passage of time in the seconds-to-minutes range (interval timing) is not only accurate but also scalar: errors in time estimation are linearly related to the estimated duration. The ubiquity of scalar timing extends over behavioral, lesion, and pharmacological manipulations. For example, in mammals, dopaminergic drugs induce an immediate, scalar change in the perceived time (clock pattern), whereas cholinergic drugs induce a gradual, scalar change in perceived time (memory pattern). How do these properties emerge from unreliable, noisy neurons firing in the milliseconds range? Neurobiological information relative to the brain circuits involved in interval timing provide support for an striatal beat frequency (SBF) model, in which time is coded by the coincidental activation of striatal spiny neurons by cortical neural oscillators. While biologically plausible, the impracticality of perfect oscillators, or their lack thereof, questions this mechanism in a brain with noisy neurons. We explored the computational mechanisms required for the clock and memory patterns in an SBF model with biophysically realistic and noisy Morris–Lecar neurons (SBF–ML). Under the assumption that dopaminergic drugs modulate the firing frequency of cortical oscillators, and that cholinergic drugs modulate the memory representation of the criterion time, we show that our SBF–ML model can reproduce the pharmacological clock and memory patterns observed in the literature. Numerical results also indicate that parameter variability (noise) – which is ubiquitous in the form of small fluctuations in the intrinsic frequencies of neural oscillators within and between trials, and in the errors in recording/retrieving stored information related to criterion time – seems to be critical for the time-scale invariance of the clock and memory patterns.

The effect of changes in criterion value on differential reinforcement of low rate schedule performance

The differential reinforcement of low rate (DRL) schedule is commonly used to assess impulsivity, hyperactivity, and the cognitive effects of pharmacological treatments on performance. A DRL schedule requires subjects to wait a certain minimum amount of time between successive responses to receive reinforcement. The DRL criterion value, which specifies the minimum wait time between responses, is often shifted towards increasingly longer values over the course of training. However, the process invoked by shifting DRL values is poorly understood. Experiment 1 compared performance on a DRL 30-s schedule versus a DRL 15-s schedule that was later shifted to a DRL 30-s schedule. Dependent measures assessing interresponse time (IRT) production and reward-earning efficiency showed significant detrimental effects following a DRL schedule transition in comparison with the performance on a maintained DRL 30-s schedule. Experiments 2a and 2b assessed the effects of small incremental changes vs. a sudden large shift in the DRL criterion on performance. The incremental changes produced little to no disruption in performance compared to a sudden large shift. The results indicate that the common practice of incrementing the DRL criterion over sessions may be an inefficient means of training stable DRL performance.

Behavioural effects of manipulation of basal ganglia neurotransmitters

Topographically organized dopaminergic projections from the extrapyramidal structures of the ventral mesencephalon (substantia nigra and ventral tegmental area) to the dorsal (body of caudate-putamen) and ventral (anterior-ventral caudate, nucleus accumbens, tuberculum olfactorium) striatum subserve sensorimotor integration in the rat. Selective depletion of DA impairs the animal's ability to integrate sensory input with motor output; in the dorsal striatum the exteroceptive sensory input and in the ventral or limbic striatum the interoceptive input principally related to motivation and affect. Grafts of fetal DA neurons to the damaged dorsal striatum reverse sensorimotor asymmetry and sensory neglect. A large number of other excitatory and inhibitory neurotransmitters, including recently discovered neuropeptides, contribute to the functional balance afforded by the DA neurons. This chemical heterogeneity of the basal ganglia offers the possibility that novel therapeutic approaches with drugs could be used to control the chemical imbalances in basal ganglia that are associated with a number of neurological and psychiatric conditions.

Effort-related functions of nucleus accumbens dopamine and associated forebrain circuits

BACKGROUND

Over the last several years, it has become apparent that there are critical problems with the hypothesis that brain dopamine (DA) systems, particularly in the nucleus accumbens, directly mediate the rewarding or primary motivational characteristics of natural stimuli such as food. Hypotheses related to DA function are undergoing a substantial restructuring, such that the classic emphasis on hedonia and primary reward is giving way to diverse lines of research that focus on aspects of instrumental learning, reward prediction, incentive motivation, and behavioral activation.

OBJECTIVE

The present review discusses dopaminergic involvement in behavioral activation and, in particular, emphasizes the effort-related functions of nucleus accumbens DA and associated forebrain circuitry.

RESULTS

The effects of accumbens DA depletions on food-seeking behavior are critically dependent upon the work requirements of the task. Lever pressing schedules that have minimal work requirements are largely unaffected by accumbens DA depletions, whereas reinforcement schedules that have high work (e.g., ratio) requirements are substantially impaired by accumbens DA depletions. Moreover, interference with accumbens DA transmission exerts a powerful influence over effort-related decision making. Rats with accumbens DA depletions reallocate their instrumental behavior away from food-reinforced tasks that have high response requirements, and instead, these rats select a less-effortful type of food-seeking behavior.

CONCLUSIONS

Along with prefrontal cortex and the amygdala, nucleus accumbens is a component of the brain circuitry regulating effort-related functions. Studies of the brain systems regulating effort-based processes may have implications for understanding drug abuse, as well as energy-related disorders such as psychomotor slowing, fatigue, or anergia in depression.

Impulsive responding on the peak-interval procedure

The pattern of responding on a peak-interval timing task allows one to make inferences regarding the sources of variation that contribute to interval timing behavior. Non-temporal factors such as impulsivity may impact the validity of these inferences. Rats were trained on a 15s peak-interval procedure (PI) or a mixed 15s behaviorally dependent variable-interval, 15s peak-interval procedure (bdVIPI) for an extended number of sessions. Extended training on the PI revealed a bi-modal distribution in the times at which subjects started responding for temporally predictable reinforcement, suggesting that multiple processes contribute to the behavioral pattern obtained in this procedure. Training on the bdVIPI eliminated the early mode of this bi-modal distribution, thereby decreasing the variation in start times. These results suggest that alternative response options can modulate the influence of impulsivity in timing tasks.

Neuroanatomical localization of an internal clock: A functional link between mesolimbic, nigrostriatal, and mesocortical dopaminergic systems

The effects of selective dopamine (DA) depleting lesions with 6-hydroxydopamine microinjection into the SN, CPu, and NAS, as well as radiofrequency lesions of the CPu on the performance characteristics of rats trained on a single-valued 20-s peak-interval (PI) timing procedure or a double-valued 10-s and 60-s PI procedure were evaluated. A double dissociation in the performance of duration discriminations was found. Rats with CPu lesions were unable to exhibit temporal control of their behavior suggesting complete insensitivity to signal duration but were able to show discrimination of the relative reward value of a signal by differentially modifying their response rates appropriately. In contrast, rats with NAS lesions were able to exhibit temporal control of their behavior by differentially modifying their response rates as a function of signal duration(s), suggesting no impairment of sensitivity to signal duration, but were unable to show discrimination of the relative reward value of a signal.

Cortico-striatal circuits and interval timing: coincidence detection of oscillatory processes

Humans and other animals demonstrate the ability to perceive and respond to temporally relevant information with characteristic behavioral properties. For example, the response time distributions in peak-interval timing tasks are well described by Gaussian functions, and superimpose when scaled by the criterion duration. This superimposition has been referred to as the scalar property and results from the fact that the standard deviation of a temporal estimate is proportional to the duration being timed. Various psychological models have been proposed to account for such responding. These models vary in their success in predicting the temporal control of behavior as well as in the neurobiological feasibility of the mechanisms they postulate. A review of the major interval timing models reveals that no current model is successful on both counts. The neurobiological properties of the basal ganglia, an area known to be necessary for interval timing and motor control, suggests that this set of structures act as a coincidence detector of cortical and thalamic input. The hypothesized functioning of the basal ganglia is similar to the mechanisms proposed in the beat frequency timing model [R.C. Miall, Neural Computation 1 (1989) 359-371], leading to a reevaluation of its capabilities in terms of behavioral prediction. By implementing a probabilistic firing rule, a dynamic response threshold, and adding variance to a number of its components, simulations of the striatal beat frequency model were able to produce output that is functionally equivalent to the expected behavioral response form of peak-interval timing procedures.

Differential Modulation of Clock Speed by the Administration of Intermittent Versus Continuous Cocaine

The roles that psychostimulant sensitization and tolerance play in temporal perception in the seconds-to-minutes range were assessed in rats. Cocaine (20 mg/kg/day) was administered for 2 weeks either intermittently via daily injections (induces sensitization) or continuously via an osmotic minipump (induces tolerance). Interval timing was evaluated throughout administration and withdrawal. Injections of cocaine caused immediate, proportional, leftward shifts in peak times, indicating an increase in the speed of an internal clock. These shifts grew progressively larger with repeated administration, indicating that stimulant-induced increases in clock speed can be sensitized. Continuous cocaine administration produced no reliable effects. These results suggest that the mechanisms of sensitization may play a considerable role in drug-induced alterations of the perception of time.

Interval timing and the encoding of signal duration by ensembles of cortical and striatal neurons

This study investigated the firing patterns of striatal and cortical neurons in rats in a temporal generalization task. Striatal and cortical ensembles were recorded in rats trained to lever press at 2 possible criterion durations (10 s or 40 s from tone onset). Twenty-two percent of striatal and 15% of cortical cells had temporally specific modulations in their firing rate, firing at a significantly different rate around 10 s compared with 40 s. On 80% of trials, a post hoc analysis of the trial-by-trial consistency of the firing rates of an ensemble of neurons predicted whether a spike train came from a time window around 10 s versus around 40 s. Results suggest that striatal and cortical neurons encode specific durations in their firing rate and thereby serve as components of a neural circuit used to represent duration.

Impaired DRL 30 performance during amphetamine withdrawal

Repeated administration of psychomotor stimulants may produce an impulsive state that could contribute to the cycle of drug abstinence and relapse seen in human drug addicts. We have previously reported that the inhibitory effects of dopamine (DA) on the firing rate of medial prefrontal cortex (mPFC) neurons were reduced in rats after repeated amphetamine treatment suggesting impaired mPFC DA function. Here, we used a differential reinforcement of low rates of responding (DRL) operant conditioning task, which is dependent on mPFC DA, to test impulsivity and inhibitory control. Food-restricted rats were trained to inhibit a nose poke response for 30s before a subsequent nose poke would result in a food reward (DRL 30). Once training was completed, rats received 5 days of no treatment, daily i.p. saline injections or daily i.p. injections of 5mg/kg amphetamine. Nine days of DRL 30 test performance began following a 3-day withdrawal from treatment. The percent of training active hole nose pokes was significantly increased and the percent of training efficiency was significantly decreased in rats withdrawn from repeated amphetamine administration as compared to saline or nai;ve rats. This suggests that impulsivity is increased during amphetamine withdrawal, which we hypothesize is associated with disrupted DA function in the mPFC.

Regional infusions of serotonin into the striatum and memory consolidation

Lesions, temporal inactivation, electrical stimulation and administration of drugs that antagonize synaptic activity of the striatum lead to significant deficits of memory. Also, it has been shown that interruption of dopaminergic, GABAergic, or cholinergic activity in discrete areas of this structure is sufficient to disrupt cognitive functions. In spite of the known interactions among dopamine, GABA, acetylcholine, and serotonin, there is a notable scarcity of data germane to the participation of striatal serotonin in learning and memory. It was important, therefore, to investigate the possible involvement of serotonin in cognition. In light of the differential distribution of serotonergic elements within the striatum, a prediction was made that focal injections of serotonin into distinctive regions would produce dissimilar effects on memory. Rats were trained in a one-trial step-through inhibitory avoidance task and a retention test was carried out 24 h later. Posttraining injections of serotonin into the dorsal and ventral aspects of the posterior region produced strong amnesia compared to similar injections into the dorsal and ventral aspects of the anterior region. The present findings support the hypothesis that striatal serotonergic activity is involved in memory functions and also provide further evidence of neurochemical heterogeneity within the striatum regarding memory consolidation.

Complex deficits on reaction time performance following bilateral intrastriatal 6-OHDA infusion in the rat

The present study examined the ability of rats subjected to bilateral 6-hydroxydopamine lesions of the terminal area of the nigrostriatal dopamine system to perform a prelearned reaction time task. This lesion model, the induction of a partial dopamine denervation of the striatum (74% depletion of dopamine striatal tissue content) with a retrograde degeneration of dopamine cell bodies in the substantia nigra, sparing the mesolimbic dopaminergic pathway, closely approximates the neuronal degeneration observed in human idiopathic Parkinson's disease. Rats were trained previously to release a lever, within a reaction time limit, after the presentation of a visual cue through reinforcement with food pellets. The onset of the light stimulus varied randomly after an unpredictable delay period of 0.25-1.0 s. Rats with dopaminergic lesions showed moderate to extensive performance deficits which were not compensated for the five postoperative weeks. More than half of the lesioned animals (64%) showed severe deficits, characterized by a concomitant increase in the number of anticipated (premature release of the lever before the visual cue) and delayed responses (lever release after the reaction time limit) with shortened reaction times in some cases. A smaller proportion (36%) of lesioned animals exhibited mild impairment of performance with a large increase in delayed responses and lengthening of reaction times but with no change in the number of anticipated responses. Asymmetric lesions had no effect on the reaction time performance. Examination of tyrosine hydroxylase immunostaining revealed that in the most impaired animals dopamine depletion was extensive in the medial striatum, whereas it was restricted to the dorsolateral striatum in the least impaired animals.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of excitotoxic lesions of the nucleus accumbens on a matching to position task

The effects of bilateral ibotenic acid lesions of the nucleus accumbens in the rat were examined on a delayed matching to position task. The lesion induced a stable delay-dependent performance deficit suggestive of a short-term memory problem. Following analysis of the impaired performance on trials with long delays, using measures akin to signal detection indices, the deficit was interpreted as being largely due to the intrusion of a side-dependent response bias. Low-dose amphetamine (0.75 mg/kg) produced a similar disruption in the sham-operated rats, both in terms of accuracy impairment and degree of strategical bias. As well as mimicking the effects of low-dose amphetamine in sham-operated rats, the lesion also protected against the disruptive effects of the drug at low doses. Whilst exhibiting no performance deficit when the matching schedule lacked a delay component, the lesioned rats were very significantly impaired in switching their response strategies when exposed to a series of reversals. In addition, the lesioned rats were remarkably resistant to an extinction procedure. Both these findings indicate that the lesioned rats were unable to exhibit flexibility in their response patterns. These results, taken together with those of the delayed matching procedure, imply that one of the functions of the intact nucleus accumbens is to inhibit habitual responding under conditions of non-reward and low stimulus control, possibly via its connections to the dorsal striatal system.

Dopamine and conditioned reinforcement. I. Differential effects of amphetamine microinjections into striatal subregions

In the conditioned reinforcement paradigm, animals learn a new instrumental response reinforced solely by conditioned reward (a stimulus that has previously been associated with primary reward). It has been shown that psychostimulants potentiate responding for conditioned reward and there is evidence that the nucleus accumbens is involved in this effect. The present experiments extend this work and examine the roles of various striatal subregions in the enhancement of responding for conditioned reward. In the conditioning phase, hungry rats were trained to associate a light/click stimulus with food delivery, with no levers present in the operant chamber. In the test phase, two levers were present and responding on one provided conditioned reward (presentation of the compound stimulus but no food). During this phase, microinjections of d-amphetamine (0, 0.2, 2.0, 20.0 micrograms/0.5 microliters) were made into seven striatal subregions in separate groups of rats. Injection of amphetamine into the nucleus accumbens elicited a dose-dependent, selective increase in responding for CR. Injections into posterior regions of the striatum had no effect. Significant and selective increases in CR responding were noted after injections into two regions neighboring the nucleus accumbens, the anterior dorsal and the ventromedial striatum, although the magnitude of these effects was considerably less than that following accumbens injections. Injections into ventrolateral regions increased responding in some rats, but this effect was very variable and not selective for the CR lever. These results are interpreted as evidence for functional heterogeneity of the striatum with regard to enhancement of conditioned reinforcement. The findings are discussed in relation to the theory that increased dopaminergic activity in the nucleus accumbens results in amplification of the response to a previously learned reward-related signal.

Antagonism of marihuana effects by indomethacin in humans

To investigate whether the nonsteroidal anti-inflammatory drug (NSAID) indomethacin antagonized the effects of marihuana, an exploratory single-blind, placebo-controlled study was conducted. Subjects (n = 4) smoked marihuana after pre-treatment with placebo and indomethacin. The subjective rating of marihuana "high", heart rate, word recall, time estimation/production, and plasma concentrations of THC and PGE2 were measured. It was found that: 1) indomethacin pre-treatment decreased the elevation of prostaglandins induced by THC; 2) indomethacin significantly attenuated the subjective "high" and the heart rate accelerating effects of THC, although the magnitude of this effect was modest; 3) indomethacin abolished the profound effect of THC on time estimation and production; and 4) indomethacin pretreatment did not affect the decremental effects of THC on word recall. We conclude that prostaglandins are involved in the neurophysiologic mechanisms that mediate some of the typical clinical effects of THC, particularly the distortion of time perception.

Behavior, striatal and nucleus accumbens field potential patterns and dopamine levels in rats given amphetamine continuously

Behavioral, electrophysiological (field potential recordings, analyzed by computer) and biochemical techniques were employed before, during and after 7 days of continuous administration of amphetamine to rats. Significant changes were observed using all three protocols. Behavioral alterations were greatest on the second day of treatment and progressively normalized during the remainder of treatment. Electrophysiological changes in the striatum were significant only on the second day of treatment, while electrophysiological alterations in the nucleus accumbens were significant on all treatment and recording days, and the magnitude of the changes paralleled the pattern of overt behavioral changes. Levels of DA in the striatum progressively decreased from normal throughout the treatment, declining to less than half of the control level by the sixth day of treatment. In contrast, DA levels in the nucleus accumbens were augmented on the second day of treatment and progressively approached the control level as treatment continued, again paralleling behavior and changes in the electrophysiology of the nucleus accumbens. These results indicate that continuous administration of amphetamine in rats differentially affects electrical activity in the striatum and nucleus accumbens as well as concentrations of DA. In addition, these results have implications for the study of paranoid schizophrenia.

Intracranial self-administration methodologies

Intracranial drug self-administration (ICSA) offers a relatively new approach for investigating the neurobiological mechanisms involved in brain reinforcement processes. Discrete brain regions responsible for the initiation of neuronal activity associated with the response-contingent delivery of a drug reinforcer can be identified using these procedures since the drug is infused directly into a specific brain locus. In the last decade, several papers have appeared in the literature reporting the self-administration of various substances into a number of brain regions. However, different laboratories often employ diverse methodological procedures to demonstrate ICSA, and this can lead to erroneous conclusions when comparing data from different investigations. This review presents a critical evaluation of the current status of research in this area and suggests behavioral as well as methodological guidelines for future investigations to follow.

Functional neuroanatomic mapping of the rat striatum: regional differences in glucose utilization in normal controls and after treatment with apomorphine

Glucose utilization was measured in 5 regions of rat striatum within four anterior-posterior levels. Areas of minimal and maximal glucose utilization were identified. Density-window image analysis was used to quantify areas of minimal and maximal glucose utilization, which were also apparent upon close visual inspection. In a normal control group, there were substantial regional variations in striatal glucose utilization (e.g. dorsal vs ventral; dorsomedial vs dorsolateral), revealing a detailed pattern, previously unavailable, which served as a baseline to study the effect of systemic apomorphine. The highest levels of glucose utilization were in small islands 0.01-0.25 mm2, and in what appeared to be dense clusters of islands that formed larger zones. Apomorphine treatment decreased glucose utilization in dorsomedial regions, increased it in a ventromedial region, and did not change it in others. The findings emphasize the importance of regional analysis of striatum in functional and physiological studies, and provide a new baseline for analyses of striatal glucose utilization in studies of development and aging, drug effects, external stimuli, and rat models of movement disorders.

Striatal dopamine activity and unilateral barpressing in rats

A micro-punch tissue assay was used to measure changes in dopamine content at twenty-six sites within the striatum of rats trained to barpress exclusively with one forepaw for food pellets. Analysis of dopamine (DA) and its metabolite, 3,4-dihydroxyphenylacetic acid (DOPAC), was carried out using HPLC with electrochemical detection. The barpress group had a significantly higher DOPAC/DA ratio in both the contralateral and ipsilateral hemispheres when compared to feeding and homecage controls. The DOPAC/DA ratio is considered to be a measure of dopaminergic neuronal activity, thus suggesting a bilateral activation of the neostriatal dopaminergic afferents as a result of the motor performance. Topographical analysis within the barpress group revealed an anterior-posterior, medial-lateral gradient of dopaminergic activity with the posterior and lateral sites showing the greatest increases over the controls. The results of this experiment indicate that localized changes in neuronal activity can be monitored with the micro-punch assay-HPLC/EC technique and that voluntary motor behavior produces an activation of the striatal dopamine system.

Dopamine depletion in a striatal subregion disrupts performance of a skilled motor task in the rat

Unilateral 6-hydroxydopamine injections into the lateral neostriatum disrupted the ability of rats to retrieve small food pellets with the contralateral forepaw. Similar injections into the medial striatum did not interfere with performance accuracy. Dopamine assays indicated that of the 5 striatal subregions analyzed, the lateral striatum at the level of the anterior commissure was most directly related to the behavioral deficit. The behavioral results are interpreted as providing evidence in support of a sensory role for the lateral striatal dopamine system.

New aspects on the role of dopamine, acetylcholine, and GABA in the development of tardive dyskinesia

In this paper various new findings on the possible anatomical substrates of tardive dyskinesia will be presented. The results show that the striatum is heterogeneously organized, and the syndromes of biting, gnawing, and licking activities in the rat model involve a complex balance between various dopamine (DA), cholinergic, and GABAergic systems within the striatum and the mesolimbic and mesocortical systems.

The role of noradrenaline in tuning and dopamine in switching between signals in the CNS

Neuronal catecholaminergic activity modulates central nervous function. Specifically noradrenaline can exert a tuning or biassing function whereby the signal to noise ratio is altered. Dopamine activity may promote switching between inputs and outputs of information to specific brain regions. It has been ten years since evidence for a tuning function was advanced for noradrenaline and in the last 5 years the switching hypothesis for dopamine has been tentatively put forward. Recent studies are reviewed to show that while catecholamine activity contributes to neural interactions in separate brain regions that give rise to the organization of different functions, their working principles may be common between species and independent of the nucleus of origin. Behavioral examples are discussed and an attempt is made to integrate this with evidence from intracellular recording studies. It is suggested that the tuning principle in noradrenergic systems is particularly important for the formation of associations and neural plasticity (interference control) and that the switching principle of dopaminergic systems modulates the timing, time-sharing and initiation of responses (program-control).

Passive avoidance in rats: disruption by dopamine applied to the nucleus accumbens

The acquisition of a one-trial step-through passive avoidance task was examined in rats following the administration of nialamide IP and dopamine (DA) or saline into the nucleus accumbens. DA-treated rats displayed impaired learning of the task as evidenced by their lower step-through latencies on a retest trial 7 days later. The specificity of this impairment was studied in a 2 x 2 design involving intracerebral injections prior to both training and testing trials. It was found that DA treatment prior to the training trial disrupted learning or memorization of the task but that DA did not affect performance or retrieval and did not induce state-dependent learning. These findings suggest that DA applied to the nucleus accumbens does not facilitate learning per se.

Neurotoxic lesions of ventrolateral but not anteromedial neostriatum in rats impair differential reinforcement of low rates (DRL) performance

In two separate experiments, rats received either kainic acid (KA), 6-hydroxydopamine (6-OHDA) or control lesions bilaterally into either the anteromedial or ventrolateral neostriatum. Both ventrolateral neurotoxic lesions disrupted post-operative body weight regulation and the acquisition of a DRL-20 sec operant schedule of reinforcement. The pattern of operant responding suggested that these animals had an impairment in response sequencing or switching. Rats with anteromedial lesions manifested neither body-weight nor DRL impairments.

Sensorimotor impairments following localized kainic acid and 6-hydroxydopamine lesions of the neostriatum

Rats received either control saline, 6-hydroxydopamine (6-OHDA) or kainic acid (KA) unilateral lesions of 5 different foci in the neostriatum. Both KA and 6-OHDA lesions in the mid-ventral focus resulted in a substantial and prolonged impairment in sensorimotor orientation to contralateral stimuli. At all other placements 6-OHDA induced a temporary mild impairment, and KA had no detectable biasing effects. The results suggest that the neostriatum is topographically heterogeneous in the mediation of this particular aspect of sensorimotor performance.

The amygdalostriatal projection in the rat--an anatomical study by anterograde and retrograde tracing methods

Tritiated leucine and proline injected into the amygdaloid complex was found to label a voluminous amygdalostriatal fiber system which is distributed to all parts of the striatum except an antero-dorsolateral striatal sector. The connection is established by way of the longitudinal association bundle as well as the stria terminalis, and includes a modest (10-15%), symmetrically distributed contralateral component conveyed by the anterior commissure. Both autoradiographic findings and subsequent observations in retrograde cell-labelling (horseradish peroxidase) material indicate that the amygdalostriatal projection originates mainly from the nucleus basalis lateralis amygdalae, in much lesser volume from the nucleus basalis medialis, and minimally from the nucleus lateralis amygdalae; no other contributing amygdaloid cell group could be identified. A comparison of the present findings with earlier reports indicates that the amygdalostriatal projection widely overlaps the striatal projections from the ventral tegmental area, the mesencephalic raphe nuclei and the prefrontal cortex. Like the amygdalostriatal projection, these striatal afferents largely or entirely avoid the antero-dorsolateral striatal quadrant, which thus appears to be the striatal region most sparsely innervated by afferents originating from structures within the circuitry of the limbic system. Findings in additional autoradiographic material identify this relatively non-limbic striatal quadrant as the main region of distribution of the corticostriatal projection from the sensorimotor cortex.

Monosynaptic input from the nucleus accumbens--ventral striatum region to retrogradely labelled nigrostriatal neurones

After placement of lesions (either electrolytic or by injection of kainic acid) in an area including the nucleus accumbens and part of the ventral striatum in the rat, the ipsilateral substantia nigra was studied in the electron microscope. Degenerating axons and nerve terminals were found mainly in the zona reticulata and in the ventral layer of the zona compacta. Degenerating synaptic boutons were found in contact with cell bodies (symmetric synapses) and dendrites (mainly symmetric, but a few asymmetric). The postsynaptic target of some of the afferent fibres from the accumbens-ventral striatum was established by demonstrating degenerating synaptic boutons of the above types in contact with nigrostriatal neurones which had been identified by the retrograde transport of horseradish peroxidase (HRP) from the main body of the striatum. Some of the HRP-labelled cells were also impregnated by the Golgi stain and degenerating boutons were found in contact with their distal dendrites. We also observed two types of HRP-containing boutons (presumably labelled anterogradely) in the subsantia nigra after injection of HRP into the main body of the striatum : type 1 boutons contained large spherical vesicles, and formed symmetrical synapses mainly on dendritic shafts in the zona reticulata and in one case the dendrite was from a nigrostriatal neurone; type 2 boutons had pleomorphic and flattened vesicles and formed symmetrical synapses with perikarya and proximal dendrites, especially in the zona compacta. The latter type of HRP-labelled bouton was frequently found in synaptic contact with the cell bodies of nigrostriatal neurones and the same neurones sometimes also received degenerating boutons originating from neurones in the nucleus accumbens-ventral striatum. It is concluded that part of the striato-nigro-striatal circuit includes a monosynaptic link between neurones in the ventral striatum-accumbens and some nigrostriatal neurones. The possible convergence of input from different regions of the striatum on to single nigrostriatal neurones is also suggested.

Reinnervation of the denervated striatum by substantia nigra transplants: functional consequences as revealed by pharmacological and sensorimotor testing

Embryonic substantia nigra (SN) was transplanted to the neostriatum in adult rats subjected to a unilateral or serial bilateral destruction of the nigrostriatal dopamine (DA) pathway. The survival of the graft and the growth of DA-containing fibers from the graft into the host brain was studied by fluorescence histochemistry and micro-fluorometry. The motor asymmetry, and the contralateral 'sensory neglect' induced by a unilateral destruction of the nigrostriatal DA pathway, were monitored in the transplanted rats and in non-transplanted controls through repeated measurements of the amphetamine-and apomorphine-induced rotational behaviour, and through analysis of the rats performance in a number of sensorimotor tests. Finally, the development of aphagia and adipsia after a serial bilateral destruction of both nigrostriatal DA pathways were followed in rats bearing bilateral SN transplants and in lesioned non-transplanted controls. The results show that large parts of the dorsal neostriatum can be reinnervated by DA-containing axons from the intracortical transplant and that this new DA input can fully compensate for the amphetamine-induced motor assymmetry that resulted from the initial destruction of the innate nigrostriatal DA pathway. The fluorescence microscopical observations provide strong evidence that the compensation of the amphetamine-induced rotational response was specifically related to the re-establishment of a new DA input to the denervated neostriatum, and that the degree of rotational compensation was well correlated to the magnitude of ingrowth into the neostriatum. Subsequent surgical removal of the SN transplant reinstated the initial rotational behaviour. In sharp contrast to the marked compensation in motor asymmetry, the transplanted rats showed no tendency to recover in their sensorimotor performance. Thus, while the sensorimotor deficit had recovered in the control group a marked contralateral 'sensory neglect' remained in the transplanted animals. Furthermore, removal of the transplant produced a significant improvement in their sensorimotor performance within 3 days. In the bilaterally transplanted animals the presence of the transplants did not prevent the development of severe adipsia, aphagia and akinesia following the destruction of the remaining contralateral nigrostriatal pathway. In fact, the recovery from the consummatory deficits tended to be better in the lesioned control rats than in the transplanted ones. It is concluded that SN transplants reinnervation the dorsal part of the neostriatum are able to replace the innate SN in normalizing some aspects of the rats motor behaviour, while their sensorimotor deficits and deficits in consummatory behaviour were unaffected. It is suggested that this dissociation of transplant-induced recovery is due to the failure of the SN transplants to reinnervate those parts of the neostriatum which are most directly implicated in sensorimotor and consummatory behaviour.

Identification of a subregion within rat neostriatum for the dopaminergic modulation of lateral hypothalamic self-stimulation

Experiments were conducted to test the hypothesis that the involvement of neostriatal dopaminergic transmission in lateral hypothalamic self-stimulation might be specific to a striatal subregion. Crystalline application of dopamine or D-amphetamine increased self-stimulation rate only when made to ventral anterior striatum (VAS); more dorsal or posterior applications were ineffective. A comparison of dose-response functions for dopamine using solution injections in VAS and posterior striatum (PS) confirmed that only VAS was responsive. Injections or applications of 6-hydroxydopamine suppressed responding only when made into VAS. Haloperidol injections decreased responding only for VAS and not PS injection sites. Applications or injections of scopolamine often increased responding when made into VAS, but this effect was unreliable. Applications or injections of scopolamine to more posterior sites consistently suppressed responding. It was concluded that dopaminergic transmission in VAS, alone among the striatal sites tested, is facilitatory on hypothalamic self-stimulation. The effects of drug applications to nucleus accumbens were generally similar to VAS, and it was suggested that these areas may be functionally similar. An examination of the known afferents to VAS indicated that this area of neostriatum, like n. accumbens, may be influenced by activity in limbic structures. This anatomy may help provide an understanding of how neostriatum, traditionally considered to have a motor function, might be involved in central reward processes.