Intrastriatal dopamine D1 antagonism dampens neural plasticity in response to motor cortex lesion

Role of the Dopaminergic System in the Striatum and Its Association With Functional Recovery or Rehabilitation After Brain Injury

Disabilities are estimated to occur in approximately 2% of survivors of traumatic brain injury (TBI) worldwide, and disability may persist even decades after brain injury. Facilitation or modulation of functional recovery is an important goal of rehabilitation in all patients who survive severe TBI. However, this recovery tends to vary among patients because it is affected by the biological and physical characteristics of the patients; the types, doses, and application regimens of the drugs used; and clinical indications. In clinical practice, diverse dopaminergic drugs with various dosing and application procedures are used for TBI. Previous studies have shown that dopamine (DA) neurotransmission is disrupted following moderate to severe TBI and have reported beneficial effects of drugs that affect the dopaminergic system. However, the mechanisms of action of dopaminergic drugs have not been completely clarified, partly because dopaminergic receptor activation can lead to restoration of the pathway of the corticobasal ganglia after injury in brain structures with high densities of these receptors. This review aims to provide an overview of the functionality of the dopaminergic system in the striatum and its roles in functional recovery or rehabilitation after TBI.

Effects of Dopamine on Motor Recovery and Training in Adults and Children With Nonprogressive Neurological Injuries: A Systematic Review

BACKGROUND

The strong link between dopamine and motor learning has been well-established in the animal literature with similar findings reported in healthy adults and the elderly.

OBJECTIVE

We aimed to conduct the first, to our knowledge, systematic review of the literature on the evidence for the effects of dopaminergic medications or genetic variations in dopamine transmission on motor recovery or learning after a nonprogressive neurological injury.

METHODS

A PubMed search was conducted up until April 2018 for all English articles including participants with nonprogressive neurological injury such as cerebral palsy, stroke, spinal cord injury, and traumatic brain injury; quantitative motor outcomes; and assessments of the dopaminergic system or medications.

RESULTS

The search yielded 237 articles, from which we identified 26 articles meeting all inclusion/exclusion criteria. The vast majority of articles were related to the use of levodopa poststroke; however, several studies assessed the effects of different medications and/or were on individuals with traumatic brain injury, spinal cord injury or cerebral palsy.

CONCLUSIONS

The evidence suggests that a brain injury can decrease dopamine transmission and that levodopa may have a positive effect on motor outcomes poststroke, although evidence is not conclusive or consistent. Individual variations in genes related to dopamine transmission may also influence the response to motor skill training during neurorehabilitation and the extent to which dopaminergic medications or interventions can augment that response. More rigorous safety and efficacy studies of levodopa and dopaminergic medications in stroke and particularly other neurological injuries including genetic analyses are warranted.

Nonequivalent modulation of corticospinal excitability by positive and negative outcomes

OBJECTIVE

The difference between positive and negative outcomes is important in trial-and-error decision-making processes and affects corticospinal excitability. This study investigated corticospinal excitability during the performance of trial-and-error decision-making tasks with varying competing behavioral outcomes.

METHODS

Each trial began with one of five colored circles presented as a cue. Each color represented a different reward probability, ranging from 10% to 90%. The subjects were instructed to decide whether to perform wrist flexion in response to the cue. Two seconds after the presentation of the cue, a reward stimulus (picture of a coin) or penalty stimulus (mauve circle) was randomly presented to the subject. If the picture of a coin appeared, the subjects received the coin after the experiment if they had performed wrist flexion, but not if they had not performed wrist flexion. If a mauve circle appeared, a coin was deducted from the total reward if the subjects had performed wrist flexion, but not if they had not performed wrist flexion. One second after the reward or penalty stimulus, transcranial magnetic stimulation was delivered to the primary motor cortex at the midpoint between the centers of gravity of the flexor carpi radialis (agonist) and extensor carpi radialis (antagonist) muscles.

RESULTS

Cumulative wrist flexions were positively correlated with reward probabilities. Motor evoked potential (MEP) amplitudes in agonist muscles were significantly higher when wrist flexion incurred a penalty than when it incurred a reward, but there was no difference in the MEP amplitudes of antagonist muscles.

CONCLUSION

Positive and negative behavioral outcomes differentially altered behavior and corticospinal excitability, and unexpected penalties had a stronger effect on corticospinal excitability for agonist muscles.

Mirtazapine prevents induction and expression of cocaine-induced behavioral sensitization in rats

Cocaine abuse is a major health problem worldwide. Treatment based on both 5-HT2A/C and 5-HT3 receptor antagonists attenuate not only the effects of cocaine abuse but also the incentive/motivational effect related to cocaine-paired cues. Mirtazapine, an antagonist of postsynaptic α2-adrenergic, 5-HT2A/C and 5HT3 receptors and inverse agonist of the 5-HT2C receptor, has been shown to effectively modify, at the preclinical and clinical levels, various behavioral alterations induced by drugs abuse. Therefore, it is important to assess whether chronic dosing of mirtazapine alters locomotor effects of cocaine as well as induction and expression of cocaine sensitization. Our results reveal that a daily mirtazapine regimen administered for 30days effectively induces a significant attenuation of cocaine-dependent locomotor activity and as well as the induction and expression of behavioral sensitization. These results suggest that mirtazapine may be used as a potentially effective therapy to attenuate induction and expression of cocaine-induced locomotor sensitization.

Motor cortex-evoked activity in reciprocal muscles is modulated by reward probability

Horizontal intracortical projections for agonist and antagonist muscles exist in the primary motor cortex (M1), and reward may induce a reinforcement of transmission efficiency of intracortical circuits. We investigated reward-induced change in M1 excitability for agonist and antagonist muscles. Participants were 8 healthy volunteers. Probabilistic reward tasks comprised 3 conditions of 30 trials each: 30 trials contained 10% reward, 30 trials contained 50% reward, and 30 trials contained 90% reward. Each trial began with a cue (red fixation cross), followed by blue circle for 1 s. The subjects were instructed to perform wrist flexion and press a button with the dorsal aspect of middle finger phalanx as quickly as possible in response to disappearance of the blue circle without looking at their hand or the button. Two seconds after the button press, reward/non-reward stimulus was randomly presented for 2-s duration. The reward stimulus was a picture of Japanese 10-yen coin, and each subject received monetary reward at the end of experiment. Subjects were not informed of the reward probabilities. We delivered transcranial magnetic stimulation of the left M1 at the midpoint between center of gravities of agonist flexor carpi radialis (FCR) and antagonist extensor carpi radialis (ECR) muscles at 2 s after the red fixation cross and 1 s after the reward/non-reward stimuli. Relative motor evoked potential (MEP) amplitudes at 2 s after the red fixation cross were significantly higher for 10% reward probability than for 90% reward probability, whereas relative MEP amplitudes at 1 s after reward/non-reward stimuli were significantly higher for 90% reward probability than for 10% and 50% reward probabilities. These results implied that reward could affect the horizontal intracortical projections in M1 for agonist and antagonist muscles, and M1 excitability including the reward-related circuit before and after reward stimulus could be differently altered by reward probability.

Momentary reward induce changes in excitability of primary motor cortex

OBJECTIVE

To investigate the human primary motor cortex (M1) excitability changes induced by momentary reward.

METHODS

To test the changes in excitatory and inhibitory functions of M1, motor-evoked potentials (MEPs), short-interval intracortical inhibition (SICI) and short-latency afferent inhibition (SAI) were tested in the abductor pollicis brevis (APB) muscle of non-dominant hand in 14 healthy volunteers by transcranial magnetic stimulation (TMS) during a behavioral task in which subjects were pseudorandomly received either reward target or non-target stimuli in response to a cue. To control sensorimotor and attention effects, a sensorimotor control task was done replacing the reward target with non-reward target.

RESULTS

The SICI was increased, and the SAI was decreased significantly during the presentation of the reward target stimuli. Those changes were not evident during non-reward target stimuli in the sensorimotor control task, indicating that this change is specific to momentary reward.

CONCLUSIONS

Momentary rewarding is associated with change in intracortical inhibitory circuits of M1.

SIGNIFICANCE

TMS may be a useful probe to study the reward system in health and in many diseases in which its dysfunction is suspected.