Glutamate and dopamine in the VTA participate differently in the acute and chronic effect of methylphenidate

Abstract

Psychostimulants such as methylphenidate (MPD) have long been the treatment of choice in behavioral disorders such as attention deficit/hyperactivity disorder (ADHD) and narcolepsy in both children and adults. However, its abuse by healthy children and adults for academic enhancement or recreation is on the rise. This raises concern for brain chemistry alteration leading to dependence during a period of neuroplasticity and brain development. Psychostimulants such as MPD are indirect dopamine antagonists and are known to act on the dopaminergic system of the brain to produce their effects. The ventral tegmental area (VTA) is one of the primary sources of dopamine in the CNS and is a part of the reward circuits affected by MPD. In order to elucidate the role of the VTA in MPD exposure, five groups of rats were used: VTA intact control, sham VTA surgery, nonspecific electrolytic VTA lesion, glutamatergic specific VTA chemical lesion, and dopaminergic specific VTA chemical lesion. Baseline locomotor activity was established, then the surgeries were performed followed by several days of recovery and establishment of post-surgical baseline. Following the recovery period, the rats were challenged with 6 days of MPD exposure, followed by 3 washout days, then a re-challenge of MPD to assess chronic MPD exposure on animals behavior. Locomotive activity was recorded for 120 min after each injection by a computerized animal activity monitor system. The results indicate that glutamatergic synapses in the ventral tegmental area are critical for acute and chronic MPD response, while dopaminergic synapses contribute to tonic inhibition of the ventral tegmental area on rat locomotor excitation.

Introduction

Methylphenidate (MPD) is a psychostimulant that is used as a primary treatment for behavioral disorders such as attention deficit hyperactivity disorder (ADHD) [[1], [2], [3], [4], [5]]. However, the use of MPD has expanded beyond its clinical target and is increasingly being used as an academic enhancement agent or for its euphoric properties [[6], [7], [8]]. While MPD is an appropriate medication for patients with ADHD, there has been a tremendous increase in MPD use by healthy young adults for these non-prescription purposes [9,10]. Further investigation of the effects of MPD is needed for ordinary subjects.

When ingested, MPD acts by binding to the dopamine transport in the synaptic cleft, preventing the reuptake of dopamine from the synaptic cleft back to the presynaptic terminal. This results in an increased dopamine concentration in the synaptic cleft that continues to activate the post-synaptic terminal [[11], [12], [13], [14], [15], [16], [17]]. The psychostimulants cocaine and amphetamine act via a similar mechanism [18]. All three agents have previously been reported to produce behavioral tolerance, withdrawal, and sensitization, the key experimental biomarkers that indicate whether a drug has the potential for abuse. Tolerance refers to the phenomena of needing an increased amount of substance needed to elicit the same response or eliciting a reduced response with the same dose. Sensitization is the functional inverse of tolerance in which use of the same dose augments, or increases, the effect. These behaviors are elicited following repetitive (chronic) exposure to a drug of abuse and are used as experimental biomarkers to indicate whether a substance in question has potential to cause dependence.

These experimental biomarkers are known to occur in substance use disorders. Drugs of abuse activate the brain’s reward circuit which develops reward-seeking behaviors involved in substance abuse disorders. This circuit is made up of several central nervous system (CNS) nuclei that work in concert to facilitate communication between the limbic and motor systems to ultimately produce the behavior of an organism [13,20,21]. This circuit includes the nucleus accumbens, the pre-frontal cortex, the caudate nucleus, and the ventral tegmental area (VTA). The VTA is also part of the mesolimbic system, which is a major dopaminergic pathway in the brain that is involved in the regulation of motivation [[11], [12], [13], [14], [15], [16], [17]]. This system is critical in the expression of behavioral sensitization following chronic exposure to psychostimulants [17,[22], [23], [24]]. Previous work has shown that the VTA participates in the induction of sensitization in response to chronic exposure to MPD [24,25] suggesting that the VTA plays a key role in the underlying mechanism of psychostimulant dependence, relapse, and craving [24,26,27].

The VTA has input from numerous cortical and subcortical structures [28,29]. This relay of information through the VTA is thought to be critical to the formation of patterns of behavior and sensitization [17,[22], [23], [24]]. The main afferent pathways through the VTA are primarily composed of dopaminergic, glutamatergic, and GABAergic neurons [30,31]. There is significant evidence that differing sections of the VTA induce different behavioral outcomes via distinct efferent pathways [[32], [33], [34]]. The mixed medial VTA has been shown to participate in an inhibitory, or aversion pathway, while the predominately dopaminergic lateral VTA participates in the reward circuitry [[32], [33], [34]].

Other studies, however, have attempted to characterize the effect that the VTA has on behavior. Lesion studies of intracranial structures have previously been used as an experimental tool to understand the mechanistic role of the VTA in perception and sucrose consumption [28]. Non-specific electrical and specific chemical ablation of the dopaminergic system in the VTA has been used to study the addictive effects of N-allylnormetazocine, phencyclidine, morphine, cocaine, amphetamine, and more recently of MPD [[51], [52], [53], [54]]. While the lesion studies surrounding MPD use have focused on complete electrolytic ablation and specific dopaminergic ablation with 6 hydroxydopamine (6−OHDA), the glutamatergic system of the VTA remains uncharacterized.

The response of the glutamatergic synapses in other reward circuit nuclei have been studied previously, and found to be critical in the modulation of the long-term response MPD exposure, including the development of habit formation and addiction [17,24,33,[35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48], [49], [50]]. These studies showed that glutamatergic signaling in other areas of the reward circuit are necessary for acute and chronic behavioral response to MPD [34,35,39]. Glutamate in the prefrontal cortex was shown to be necessary for both acute response to MPD as well as sustained chronic effects of MPD (expression phase) [34]. Glutamate in the caudate nucleus, however, was shown to be necessary for chronic response to MPD, but not necessary for acute response [39] Little work has examined the role of glutamatergic signaling in the VTA and its role remains poorly understood.

This study seeks to assess the contribution of the glutamatergic and dopaminergic systems of the VTA on behavioral activity in response to acute and chronic MPD administration in rats using three specific and non-specific VTA lesions. Adult male Sprague-Dawley rats underwent non-specific bilateral electrolytic ablation, specific glutamatergic ablation using the neurotoxins ibotenic acid, or specific dopaminergic ablation using the neurotoxin 6−OHDA. The open field assay was used to assess rat locomotor response to MPD administration. We hypothesize that the glutamatergic and dopaminergic system in the VTA exert different roles on behavioral expression to MPD.