The effects of acute or repeated cocaine administration on nerve terminal glutamate within the rat mesolimbic system

A single dose of cocaine raises SV2A density in hippocampus of adolescent rats

OBJECTIVE

Cocaine is a highly addictive psychostimulant that affects synaptic activity with structural and functional adaptations of neurons. The transmembrane synaptic vesicle glycoprotein 2A (SV2A) of pre-synaptic vesicles is commonly used to measure synaptic density, as a novel approach to the detection of synaptic changes. We do not know if a single dose of cocaine suffices to affect pre-synaptic SV2A density, especially during adolescence when synapses undergo intense maturation. Here, we explored potential changes of pre-synaptic SV2A density in target brain areas associated with the cocaine-induced boost of dopaminergic neurotransmission, specifically testing if the effects would last after the return of dopamine levels to baseline.

METHODS

We administered cocaine (20 mg/kg i.p.) or saline to rats in early adolescence, tested their activity levels and removed the brains 1 hour and 7 days after injection. To evaluate immediate and lasting effects, we did autoradiography with [3H]UCB-J, a specific tracer for SV2A, in medial prefrontal cortex, striatum, nucleus accumbens, amygdala, and dorsal and ventral areas of hippocampus. We also measured the striatal binding of [3H]GBR-12935 to test cocaine's occupancy of the dopamine transporter at both times of study.

RESULTS

We found a significant increase of [3H]UCB-J binding in the dorsal and ventral sections of hippocampus 7 days after the cocaine administration compared to saline-injected rats, but no differences 1 hour after the injection. The [3H]GBR-12935 binding remained unchanged at both times.

CONCLUSION

Cocaine provoked lasting changes of hippocampal synaptic SV2A density after a single exposure during adolescence.

Alpha-1 Adrenergic Receptors Modulate Glutamate and GABA Neurotransmission onto Ventral Tegmental Dopamine Neurons during Cocaine Sensitization

The ventral tegmental area (VTA) plays an important role in the reward and motivational processes that facilitate the development of drug addiction. Presynaptic α1-AR activation modulates glutamate and Gamma-aminobutyric acid (GABA) release. This work elucidates the role of VTA presynaptic α1-ARs and their modulation on glutamatergic and GABAergic neurotransmission during cocaine sensitization. Excitatory and inhibitory currents (EPSCs and IPSCs) measured by a whole cell voltage clamp show that α1-ARs activation increases EPSCs amplitude after 1 day of cocaine treatment but not after 5 days of cocaine injections. The absence of a pharmacological response to an α1-ARs agonist highlights the desensitization of the receptor after repeated cocaine administration. The desensitization of α1-ARs persists after a 7-day withdrawal period. In contrast, the modulation of α1-ARs on GABA neurotransmission, shown by decreases in IPSCs' amplitude, is not affected by acute or chronic cocaine injections. Taken together, these data suggest that α1-ARs may enhance DA neuronal excitability after repeated cocaine administration through the reduction of GABA inhibition onto VTA dopamine (DA) neurons even in the absence of α1-ARs' function on glutamate release and protein kinase C (PKC) activation. α1-AR modulatory changes in cocaine sensitization increase our knowledge of the role of the noradrenergic system in cocaine addiction and may provide possible avenues for therapeutics.

Repeated Administration of Cigarette Smoke Condensate Increases Glutamate Levels and Behavioral Sensitization

Nicotine, a nicotinic acetylcholine receptor agonist, produces the reinforcing effects of tobacco dependence by potentiating dopaminergic and glutamatergic neurotransmission. Non-nicotine alkaloids in tobacco also contribute to dependence by activating the cholinergic system. However, glutamatergic neurotransmission in the dorsal striatum associated with behavioral changes in response to cigarette smoking has not been investigated. In this study, the authors investigated alterations in glutamate levels in the rat dorsal striatum related to behavioral alterations after repeated administration of cigarette smoke condensate (CSC) using the real-time glutamate biosensing and an open-field behavioral assessment. Repeated administration of CSC including 0.4 mg nicotine (1.0 mL/kg/day, subcutaneous) for 14 days significantly increased extracellular glutamate concentrations more than repeated nicotine administration. In parallel with the hyperactivation of glutamate levels, repeated administration of CSC-evoked prolonged hypersensitization of psychomotor activity, including locomotor and rearing activities. These findings suggest that the CSC-induced psychomotor activities are closely associated with the elevation of glutamate concentrations in the rat dorsal striatum.

Behavioral changes after nicotine challenge are associated with α7 nicotinic acetylcholine receptor-stimulated glutamate release in the rat dorsal striatum

Neurochemical alterations associated with behavioral responses induced by re-exposure to nicotine have not been sufficiently characterized in the dorsal striatum. Herein, we report on changes in glutamate concentrations in the rat dorsal striatum associated with behavioral alterations after nicotine challenge. Nicotine challenge (0.4 mg/kg/day, subcutaneous) significantly increased extracellular glutamate concentrations up to the level observed with repeated nicotine administration. This increase occurred in parallel with an increase in behavioral changes in locomotor and rearing activities. In contrast, acute nicotine administration and nicotine withdrawal on days 1 and 6 did not alter glutamate levels or behavioral changes. Blockade of α7 nicotinic acetylcholine receptors (nAChRs) significantly decreased the nicotine challenge-induced increases in extracellular glutamate concentrations and locomotor and rearing activities. These findings suggest that behavioral changes in locomotor and rearing activities after re-exposure to nicotine are closely associated with hyperactivation of the glutamate response by stimulating α7 nAChRs in the rat dorsal striatum.

VTA dopamine neuron plasticity - the unusual suspects

Dopamine neurons in the ventral tegmental area (VTA) are involved in a variety of physiological and pathological conditions, ranging from motivated behaviours to substance use disorders. While many studies have shown that these neurons can express plasticity at excitatory and inhibitory synapses, little is known about how inhibitory inputs and glial activity shape the output of DA neurons and therefore, merit greater discussion. In this review, we will attempt to fill in a bit more of the puzzle, with a focus on inhibitory transmission and astrocyte function. We summarize the findings within the VTA as well as observations made in other brain regions that have important implications for plasticity in general and should be considered in the context of DA neuron plasticity.

Diversity of Dopaminergic Neural Circuits in Response to Drug Exposure

Addictive substances are known to increase dopaminergic signaling in the mesocorticolimbic system. The origin of this dopamine (DA) signaling originates in the ventral tegmental area (VTA), which sends afferents to various targets, including the nucleus accumbens, the medial prefrontal cortex, and the basolateral amygdala. VTA DA neurons mediate stimuli saliency and goal-directed behaviors. These neurons undergo robust drug-induced intrinsic and extrinsic synaptic mechanisms following acute and chronic drug exposure, which are part of brain-wide adaptations that ultimately lead to the transition into a drug-dependent state. Interestingly, recent investigations of the differential subpopulations of VTA DA neurons have revealed projection-specific functional roles in mediating reward, aversion, and stress. It is now critical to view drug-induced neuroadaptations from a circuit-level perspective to gain insight into how differential dopaminergic adaptations and signaling to targets of the mesocorticolimbic system mediates drug reward. This review hopes to describe the projection-specific intrinsic characteristics of these subpopulations, the differential afferent inputs onto these VTA DA neuron subpopulations, and consolidate findings of drug-induced plasticity of VTA DA neurons and highlight the importance of future projection-based studies of this system.

Continuous exposure to dizocilpine facilitates the acquisition and escalation of cocaine consumption in male Sprague-Dawley rats

BACKGROUND

Blocking N-methyl-d-aspartate (NMDA) glutamate receptors (NMDARs) prevents cocaine locomotor sensitization, but facilitates escalation of cocaine self-administration and produces ambiguous effects on acquisition of cocaine self-administration. This study used a recently described model of acquisition and escalation to test the hypothesis that continuous NMDAR antagonism functionally increases the effects of a given dose of cocaine.

METHODS

We assessed acquisition of cocaine self-administration (0.6 mg/kg/infusion) in rats treated continuously with either vehicle or the NMDAR antagonist dizocilpine (0.4 mg/kg/day) for 14 consecutive 2h fixed ratio 1 (FR1) sessions. In a separate experiment that assessed the effect of dizocilpine treatment on escalation of cocaine self-administration, rats acquired cocaine self-administration (0.6 mg/kg/infusion) prior to vehicle or dizocilpine treatment. Then, immediately post-acquisition, rats were treated continuously with either vehicle or dizocilpine and allowed to self-administer either 0.6 or 1.2mg/kg/infusion cocaine for an additional seven consecutive 2h FR1 sessions.

RESULTS

Relative to vehicle-treated rats, a significantly greater percentage of dizocilpine-treated rats acquired cocaine self-administration. During the escalation experiment, both vehicle- and dizocilpine-treated rats escalated intake of 1.2mg/kg/infusion cocaine. Whereas vehicle-treated rats exhibited stable intake of 0.6 mg/kg/infusion cocaine, dizocilpine-treated rats escalated intake of this moderate cocaine dose to levels indistinguishable from intake levels produced by self-administration of the high cocaine dose (i.e., 1.2mg/kg/infusion).

CONCLUSIONS

These findings suggest that chronic NMDAR blockade potentiates, rather than attenuates, cocaine's effects and argue for reconsideration of the role of NMDARs in cocaine "addiction-like" behavior.

Effects of context-drug learning on synaptic connectivity in the basolateral nucleus of the amygdala in rats

Context-drug learning produces structural and functional synaptic changes in the circuitry of the basolateral nucleus of the amygdala (BLA). However, how the synaptic changes translated to the neuronal targets was not established. Thus, in the present study, immunohistochemistry with a cell-specific marker and the stereological quantification of synapses was used to determine if context-drug learning increases the number of excitatory and inhibitory/modulatory synapses contacting the gamma-aminobutyric acid (GABA) interneurons and/or the pyramidal neurons in the BLA circuitry. Amphetamine-conditioned place preference increased the number of asymmetric (excitatory) synapses contacting the spines and dendrites of pyramidal neurons and the number of multisynaptic boutons contacting pyramidal neurons and GABA interneurons. Context-drug learning increased asymmetric (excitatory) synapses onto dendrites of GABA interneurons and increased symmetric (inhibitory or modulatory) synapses onto dendrites but not perikarya of these same interneurons. The formation of context-drug associations alters the synaptic connectivity in the BLA circuitry, findings that have important implications for drug-seeking behavior.

Cocaine-induced adaptations in metabotropic inhibitory signaling in the mesocorticolimbic system

The addictive properties of psychostimulants such as cocaine are rooted in their ability to activate the mesocorticolimbic dopamine (DA) system. This system consists primarily of dopaminergic projections arising from the ventral tegmental area (VTA) and projecting to the limbic and cortical brain regions, such as the nucleus accumbens (NAc) and prefrontal cortex (PFC). While the basic anatomy and functional relevance of the mesocorticolimbic DA system is relatively well-established, a key challenge remaining in addiction research is to understand where and how molecular adaptations and corresponding changes in function of this system facilitate a pathological desire to seek and take drugs. Several lines of evidence indicate that inhibitory signaling, particularly signaling mediated by the Gi/o class of heterotrimeric GTP-binding proteins (G proteins), plays a key role in the acute and persistent effects of drugs of abuse. Moreover, recent evidence argues that these signaling pathways are targets of drug-induced adaptations. In this review we discuss inhibitory signaling pathways involving DA and the inhibitory neurotransmitter GABA in two brain regions - the VTA and PFC - that are central to the effects of acute and repeated cocaine exposure and represent sites of adaptations linked to addiction-related behaviors including sensitization, craving, and relapse.

Retrieval of contextual memories increases activity-regulated cytoskeleton-associated protein in the amygdala and hippocampus

Activity-regulated cytoskeleton-associated protein (Arc) integrates information from multiple intracellular signaling cascades and, in turn, regulates cytoskeletal proteins involved in structural synaptic modifications. The purposes of the present study were: (1) to determine if the retrieval of contextual memories would induce Arc in hippocampal and amygdalar neurons; (2) use unbiased stereology at the ultrastructural level to quantify synapses contacting Arc-labeled (Arc+) and unlabeled (Arc-) postsynaptic structures in brain regions in which the amount of Arc integrated density (ID) correlated strongly with the degree of amphetamine conditioned place preference (AMPH CPP). The retrieval of contextual memories increased the Arc ID in the dentate gyrus, cornu ammonis (CA)1, and CA3 fields of the hippocampus and the basolateral, lateral, and central nuclei of the amygdala but not the primary auditory cortex, a control region. Stereological quantification of Arc+ and Arc- synapses in the basolateral nucleus of the amygdala (BLA) was undertaken because the strongest relationship between the amount of Arc ID and AMPH CPP was observed in the BLA. The retrieval of contextual memories increased the number and density of asymmetric (presumed excitatory) synapses contacting Arc+ spines and dendrites of BLA neurons, symmetric (presumed inhibitory or modulatory) synapses contacting Arc+ dendrites of BLA neurons, and multisynaptic boutons contacting Arc+ postsynaptic structures. Thus, the retrieval of contextual memories increases Arc in the amygdala and hippocampus, an effect that could be important for approach behavior to a drug-associated context.

Repeated exposure to cocaine alters medial prefrontal cortex dopamine D₂-like receptor modulation of glutamate and dopamine neurotransmission within the mesocorticolimbic system

Repeated exposure to cocaine progressively increases drug-induced locomotor activity, which is termed behavioral sensitization. Previous studies have demonstrated that sensitization to cocaine is associated with a decrease in dopamine D₂ receptor function in the medial prefrontal cortex. The present report tested the hypothesis that reduced medial prefrontal cortex D₂ receptor function as a result of repeated cocaine exposure results in augmented excitatory transmission to the nucleus accumbens and ventral tegmental area, possibly as a partial result of enhanced inhibition of local dopamine release. Dual probe microdialysis experiments were conducted in male Sprague-Dawley rats 1, 7 or 30 days following the last of four daily injections of saline (1.0 mL/kg) or cocaine (15 mg/kg). Infusion of quinpirole (0.01, 1.0 and 100 μM), a D₂-like receptor agonist, into the medial prefrontal cortex produced a dose-dependent decrease in cortical, nucleus accumbens and ventral tegmental area extracellular glutamate levels in control but not sensitized animals. Quinpirole also reduced basal dopamine levels in the medial prefrontal cortex in sensitized animals following 1 day of withdrawal from cocaine. Following 30 days of withdrawal, quinpirole also reduced dopamine levels in sensitized animals relative to saline controls, but not relative to baseline levels. These findings indicate that the expression of sensitization to cocaine is associated with altered modulation of mesocorticolimbic glutamatergic transmission at the level of the medial prefrontal cortex.

Cocaine-induced neuroadaptations in glutamate transmission: potential therapeutic targets for craving and addiction

A growing body of evidence indicates that repeated exposure to cocaine leads to profound changes in glutamate transmission in limbic nuclei, particularly the nucleus accumbens. This review focuses on preclinical studies of cocaine-induced behavioral plasticity, including behavioral sensitization, self-administration, and the reinstatement of cocaine seeking. Behavioral, pharmacological, neurochemical, electrophysiological, biochemical, and molecular biological changes associated with cocaine-induced plasticity in glutamate systems are reviewed. The ultimate goal of these lines of research is to identify novel targets for the development of therapies for cocaine craving and addiction. Therefore, we also outline the progress and prospects of glutamate modulators for the treatment of cocaine addiction.

Hypothesis-driven medication discovery for the treatment of psychostimulant addiction

Psychostimulant abuse is a serious social and health problem, for which no effective treatments currently exist. A number of review articles have described predominantly 'clinic'-based pharmacotherapies for the treatment of psychostimulant addiction, but none have yet been shown to be definitively effective for use in humans. In the present article, we review various 'hypothesis'- or 'mechanism'-based pharmacological agents that have been studied at the preclinical level and evaluate their potential use in the treatment of psychostimulant addiction in humans. These compounds target brain neurotransmitter or neuromodulator systems, including dopamine (DA), gamma-aminobutyric acid (GABA), endocannabinoid, glutamate, opioid and serotonin, which have been shown to be critically involved in drug reward and addiction. For drugs in each category, we first briefly review the role of each neurotransmitter system in psychostimulant actions, and then discuss the mechanistic rationale for each drug's potential anti-addiction efficacy, major findings with each drug in animal models of psychostimulant addiction, abuse liability and potential problems, and future research directions. We conclude that hypothesis-based medication development strategies could significantly promote medication discovery for the effective treatment of psychostimulant addiction.

Continuous exposure to the competitive N-methyl-D: -aspartate receptor antagonist, LY235959, facilitates escalation of cocaine consumption in Sprague-Dawley rats

RATIONALE

Chronic high dose consumption of cocaine is associated with significant negative effects to individual users and society. Nevertheless, the precise mechanisms that mediate increases in cocaine consumption in a drug-using individual are not fully understood.

OBJECTIVES

This study used a long access version of the drug self-administration procedure to determine whether escalation of cocaine consumption is mediated by increased activity through N-methyl-D: -aspartate (NMDA) receptors.

MATERIALS AND METHODS

Male Sprague-Dawley rats (n = 63) were first trained to self-administer cocaine (0.33 mg/infusion, i.v.) under a fixed-ratio 1 schedule of reinforcement. After training, some rats were implanted with subcutaneous osmotic minipumps filled with vehicle or the competitive NMDA receptor antagonist, LY235959, and subsequently allowed to self-administer cocaine in short (2 h) or long (6 h) access self-administration sessions.

RESULTS

Vehicle-treated rats escalated cocaine self-administration across 14 long-access self-administration sessions. Rats treated with LY235959 via osmotic minipump, but not twice daily injections, escalated cocaine self-administration at a greater rate and to a greater degree than vehicle-treated rats. In post-escalation cocaine dose-infusion tests, rats treated continuously with LY235959 self-administered more cocaine (0.08-1.32 mg/infusion) than vehicle-treated rats, regardless of access condition, shifting the dose-infusion curves upward. During extinction sessions, which were conducted after the escalation phase of the study, rats that had long (6 h) access to cocaine stopped responding sooner than rats that had short (2 h) access to cocaine, independent of LY235959 treatment.

CONCLUSIONS

These data are consistent with hypo-glutamatergic consequences of repeated cocaine exposure.

The group II metabotropic glutamate receptor agonist, LY379268, inhibits both cocaine- and food-seeking behavior in rats

RATIONALE

Group II metabotropic glutamate receptor (mGluR2/3) agonists are proposed to serve as potential treatment for addiction.

OBJECTIVES

The present study examined the hypothesis that mGluR2/3 agonists exert inhibitory effects on cocaine-induced reinstatement of cocaine-seeking.

METHODS

Rats were trained to self-administer either cocaine or control reinforcer (food), then responding on the reinforcer-paired lever was extinguished. Reinstatement of responding was induced by a noncontingent presentation of the self-administered reinforcer (10 mg/kg cocaine, i.p. or 765 mg of food). In one experiment, rats were systemically pretreated with vehicle (Veh) or the mGluR2/3 agonist LY379268 (0.3, 1, or 3 mg/kg, i.p.) 30 min before the reinstatement test session. In a second experiment, Veh or LY379268 (0.05, 0.5, or 5 nmol/side) was microinjected into the nucleus accumbens core (NAc core) 5 min before the reinstatement test session. The effects of LY379268 on cocaine- and food-induced reinstatement on reward seeking were assessed.

RESULTS

Both systemic and intra-NAc core pretreatment with LY379268 inhibited both cocaine- and food-seeking behavior. However, the effect of LY379268 appeared somewhat more effective for cocaine-seeking than food-seeking.

CONCLUSIONS

These results support a potential therapeutic role for mGluR2/3 agonists on relapse of cocaine-seeking. However, doses that inhibited cocaine-seeking were only threefold lower than those inhibiting food-seeking, indicating possible unacceptable nonspecific effects. In addition, the NAc core is one site of action where the mGluR2/3 agonists elicit effects on reward-seeking behavior.

Regulation of Synaptic Transmission by CRF Receptors

Corticotropin-releasing factor (CRF or CRH) and its family of related peptides have long been recognized as hypothalamic-pituitary-adrenal (HPA) axis peptides that function to regulate the release of other hormones, e.g., ACTH. In addition, CRF acts outside the HPA axis not as a hormone, but as a regulator of synaptic transmission, pre- and post-synaptically, within specific CNS neuronal circuits. Synaptic transmission within the nervous system is today understood to be a more complex process compared to the concepts associated with the term 'synapse' introduced by Sherrington in 1897. Based on more than a century of progress with modern cellular and molecular experimental techniques, prior definitions and functions of synaptic molecules and their receptors need to be reconsidered (see Glossary and Fig. 1), especially in light of the important roles for CRF, its family of peptides and other potential endogenous regulators of neurotransmission, e.g., vasopressin, NPY, etc. (see Glossary). In addition, the property of 'constitutive activity' which is associated with G-protein coupled receptors (GPCRs) provides a persistent tonic mechanism to fine-tune synaptic transmission during both acute and chronic information transfer. We have applied the term 'regulator', adapted from the hormone literature, to CRF, as an example of a specific endogenous substance that functions to facilitate or depress the actions of neuromodulators on fast and slow synaptic responses. As such, synaptic neuroregulators provide a basic substrate to prime or initiate silently plastic processes underlying neurotransmitter-mediated information transfer at CNS synapses. Here we review the role of CRF to regulate CNS synaptic transmission and also suggest how under a variety of allostatic changes, e.g., associated with normal plasticity, or adaptations resulting from mental disorders, the synaptic regulatory role for CRF may be 'switched' in its polarity and/or magnitude in order to provide a coping mechanism to deal with daily and life-long stressors. Thus, a prominent role we assign to non-HPA axis CRF, its family of peptides, and their receptors, is to maintain both acute and chronic synaptic stability.

Time-dependent changes in extracellular glutamate in the rat dorsolateral striatum following a single cocaine injection

Acute cocaine administration has been shown to alter dorsal striatal plasticity [Proc Natl Acad Sci USA 87 (1990) 6912; Brain Res Bull 30 (1993) 173] and produce long-term neurochemical changes [Pharmacol Biochem Behav 27 (1987) 533]. To date, the effects of acute cocaine on extracellular glutamate and nerve terminal glutamate immunolabeling in the rat dorsolateral striatum have not been reported. To investigate cocaine-induced changes in extracellular glutamate, in vivo microdialysis was carried out in the dorsolateral striatum of rats 1-14 days after receiving a single injection of either vehicle or 15 mg/kg cocaine. There was an increase in the group injected with cocaine 1 day prior to measuring extracellular glutamate as compared with the control group. The group injected with cocaine 3 days prior to the microdialysis session had decreased extracellular glutamate levels. Furthermore, extracellular glutamate remained attenuated 14 days after acute cocaine treatment. Striatal glutamate decreased in the cocaine-treated rats after calcium removal, suggesting that cocaine-induced changes in extracellular glutamate were partially calcium-dependent. The density of nerve terminal glutamate immunolabeling was measured using immunogold electron microscopy in the contralateral striatum of the same rats that had been acutely treated with cocaine or vehicle. There were no changes in the density of glutamate immunolabeling within identified nerve terminals making an asymmetrical (excitatory) synaptic contact 1, 2, 3, or 14 days after acute cocaine exposure as compared with the control groups. Hence, these alterations in extracellular glutamate did not result from changes in glutamate immunolabeling within the synaptic vesicle pool. In addition, no changes in glutamate immunolabeling were found in rats that received cocaine 2 h previously or were withdrawn after 1 week of cocaine administration. The results demonstrate that a single injection of cocaine produces biphasic, time-dependent changes in extracellular glutamate in the rat dorsolateral striatum.

High and low responders to novelty show differential effects in striatal glutamate

The goal of this study was to determine whether there was a difference in glutamate within the dorsolateral striatum in mice exhibiting either a high (HR) or low (LR) locomotor response to a novel environment. The number of line crossings over a 30-min-period when the mice were placed in a novel environment was determined, and those mice for which the values were above the mean were in the HR group and those with the values below the mean were in the LR group. In vivo microdialysis was carried out to determine the basal extracellular level of striatal glutamate, and the contralateral striatum was taken to measure the density of glutamate immunolabeling within nerve terminals making an asymmetrical (excitatory) synaptic contact using quantitative immuno-gold electron microscopy. There was a statistically significant difference (35%) in the basal extracellular level of striatal glutamate between the HR and LR groups, with the HR group having a lower level, compared with that of the LR group. There was a 25% difference in the density of nerve terminal glutamate immuno-gold labeling associated with the synaptic vesicle pool in the HR, compared with that in the LR group, but this difference was not statistically significant. There was no change in the basal extracellular level of striatal dopamine between the two groups, but there was a statistically significant difference (73%) in the basal turnover ratio of striatal dopamine and its metabolites in the HR, compared with that in the LR group. The data suggests that the difference in extracellular striatal glutamate between the HR and LR groups is not due to an alteration in basal extracellular dopamine but could be due to an increase in dopamine turnover.

Nerve terminal glutamate immunoreactivity in the rat nucleus accumbens and ventral tegmental area after a short withdrawal from cocaine

Cocaine administration has been shown to alter glutamate transmission in numerous studies. Using quantitative electron microscopic immunogold labeling, our laboratory has previously reported that nerve terminal glutamate immunoreactivity is transiently altered following cocaine administration. The present study was undertaken to examine presynaptic nerve terminal glutamate immunoreactivity at shorter time points after withdrawal from cocaine. Animals received saline or cocaine for 7 days followed 3 days later by a cocaine or saline challenge. Most (>75%) cocaine-challenged animals had a heightened locomotor response to cocaine compared to the first day of cocaine and were considered behaviorally sensitized. One day after the challenge, glutamate immunogold-labeling was quantified in nerve terminals making asymmetrical synaptic contacts within the core and shell of the nucleus accumbens and ventral tegmental area. A single dose of cocaine did not alter the density of presynaptic nerve terminal glutamate immunoreactivity in the nucleus accumbens (NAc) or ventral tegmental area (VTA). The density of nerve terminal glutamate immunoreactivity in the shell, but not the core, was significantly increased in the animals receiving repeated cocaine. In the VTA the density of nerve terminal glutamate immunoreactivity did not change in the cocaine-sensitized group, but was significantly increased in the nonsensitized group. The finding that repeated cocaine treatment increased glutamate nerve terminal immunolabeling within the nucleus accumbens shell, but not the core, supports the hypothesis that glutamate synapses in the core and shell are differentially sensitive to repeated cocaine administration. Overall, our study does not support a role for changes in presynaptic glutamate in the development of behavioral sensitization.

Acute and subchronic MPTP administration differentially affects striatal glutamate synaptic function

We previously reported that 1 month following unilateral loss (>95%) of striatal dopamine, there is an increase in striatal glutamate function as measured by in vivo microdialysis and quantitative immuno-gold electron microscopy, Neuroscience 88, 1-16). The goal of this study was to determine the effect of bilateral loss of striatal dopamine on striatal glutamate function following acute or subchronic administration of the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) to C57/B6J mice. Animals were administered either single injections (ip) of 30 mg/kg/day for 7 days (subchronically treated group) or 20 mg/kg x 4 doses every 2 h (acutely treated group) of the toxin or saline. One month following the first injection, there was a 44 and 65% loss in the relative density of tyrosine hydroxylase (TH) immunolabeling within the dorsolateral striatum in the subchronically and acutely MPTP-treated groups compared to the saline group, respectively. There was a decrease in the basal level of extracellular glutamate within the striatum in the subchronically MPTP-treated animals compared to an increase in the acutely treated group in relationship to the saline group. Ultrastructurally, only in the acutely MPTP-treated group was there a decrease in the density of glutamate immunolabeling within nerve terminals associated with an asymmetrical synaptic contact in the dorsolateral striatum compared to either the subchronic or saline groups. In addition, there was a decrease in the relative density of GluR-2/3 subunit immunolabeling within the dorsolateral striatum in the acute MPTP compared to the saline group. These data indicate that differences in striatal glutamate function appear to be associated with the dosing interval of MPTP administration and the variable loss of striatal TH immunolabeling.

Lesion of subthalamic or motor thalamic nucleus in 6-hydroxydopamine-treated rats: Effects on striatal glutamate and apomorphine-induced contralateral rotations

A unilateral lesion of the rat nigrostriatal pathway with 6-hydroxydopamine (6-OHDA) results in a decrease in the basal extracellular level of striatal glutamate, a nearly complete loss of tyrosine hydroxylase (TH) immunolabeling, an increase in the density of glutamate immunogold labeling within nerve terminals making an asymmetrical synaptic contact, and an increase in the number of apomorphine-induced contralateral rotations. [Meshul et al. (1999) Neuroscience 88:1-16; Meshul and Allen (2000) Synapse 36:129-142]. In Parkinson's disease, a lesion of either the subthalamic nucleus (STN) or the motor thalamic nucleus relieves the patient of some of the motor difficulties associated with this disorder. In this rodent model, either the STN or motor thalamic nucleus was electrolytically destroyed 2 months following a unilateral 6-OHDA lesions. Following a lesion of either the STN or motor thalamic nucleus in 6-OHDA-treated rats, there was a significant decrease (40-60%) in the number of apomorphine-induced contralateral rotations compared to the 6-OHDA group. There was a significant decrease (<30%) in the basal extracellular level of striatal glutamate in all of the experimental groups compared to the sham group. Following an STN and/or 6-OHDA lesion, the decrease in striatal extracellular levels was inversely associated with an increase in the density of nerve terminal glutamate immunolabeling. There was no change in nerve terminal glutamate immunogold labeling in either the motor thalamic or motor thalamic plus 6-OHDA lesion groups compared to the sham group. The decrease in the number of apomorphine-induced rotations was not due to an increase in TH immunolabeling (i.e., sprouting) within the denervated striatum. This suggests that alterations in striatal glutamate appear not to be directly involved in the STN or motor thalamic lesion-induced reduction in contralateral rotations.