Unilateral dopamine depletion paradoxically enhances amphetamine-induced Fos expression in basal ganglia output structures

The Basal Ganglia as a Substrate for the Multiple Actions of Amphetamines

Amphetamines are psychostimulant drugs with high abuse potential. Acute and chronic doses of amphetamines affect dopamine (DA) neurotransmission in the basal ganglia. The basal ganglia are a group of subcortical nuclei that are anatomically positioned to integrate cognitive, motor and sensorimotor inputs from the cortex. Amphetamines can differentially alter the functioning of specific BG circuits to produce neurochemical changes that affect cognition, movement, and drug seeking behavior through their effects on DA neurotransmission. This review focuses on how alterations in dopaminergic neurotransmission within distinct basal ganglia pathways can modify their functional output to predict and explain the acute and long term behavioral consequences of amphetamine exposure.

Intrapallidal metabotropic glutamate receptor activation in a rat model of Parkinson's disease: behavioral and histological analyses

Metabotropic glutamate receptors (mGluRs) have been recently implicated as robust therapeutic targets for Parkinson's disease (PD). Here, we explored how activation of mGluRs in globus pallidus (GP) affected the amphetamine-induced rotational behavior in the unilateral 6-hydroxydopamine (6-OHDA) lesion rat model of PD. The amphetamine-induced rotations were completely suppressed by the ipsilateral intrapallidal injection of the non-selective mGluR agonist, 1-aminocyclopentane-1S,3R-dicarboxylic acid (ACPD) and the selective group I mGluR agonist, (R,S)-3,5-dihydroxyphenylglycine (DHPG), but not the selective group III mGluR agonist, l-2-amino-4-phosphonobutyric acid (l-AP4). The suppressive effects were detected at 2, 4, 6, 8, and 12 h after ACPD injection, but returned to the control level at 24 h. A remarkable c-fos expression was found in the lesioned side of GP, subthalamic nucleus (STN), and substantia nigra pars reticulata (SNr) of rats that received the ACPD or DHPG injection, compared to rats treated with L-AP-4 or phosphate buffer-injection. The results indicate that the blockade of amphetamine-induced rotations might be at least partially mediated by group I mGluR activation. This study advances the use of selective group I mGluRs directed toward the GP for PD treatment.

Amphetamine-induced c-fos mRNA expression in the caudate-putamen and subthalamic nucleus: interactions between dose, environment, and neuronal phenotype

When administered in a novel environment relatively low doses of amphetamine induce c-fos mRNA in the subthalamic nucleus (STN) and in preproenkephalin mRNA-containing (ENK+) neurons in the caudate-putamen (CPu). When administered at home, however, low doses of amphetamine do not produce these effects. Environmental novelty also facilitates the behavioral effects of acute and repeated amphetamine, but this is dose-dependent. The purpose of the present experiment therefore was to determine if the effect of context on amphetamine-induced c-fos expression is also dose-dependent. It was found that: (i) No dose of amphetamine tested (1-10 mg/kg) induced c-fos in many ENK+ cells when given at home. (ii) When given in a novel environment low to moderate doses of amphetamine (1-5 mg/kg) induced c-fos in substantial numbers of ENK+ cells, but the highest dose examined (10 mg/kg) did not. (iii) Environmental novelty enhanced the ability of low to moderate doses of amphetamine to induce c-fos in the STN, but the highest dose of amphetamine induced robust c-fos mRNA expression in the STN regardless of context. The results do not support the idea that engaging ENK+ cells, at least as indicated by c-fos mRNA expression, is critical to produce robust behavioral sensitization, but do suggest a possible role for the STN. Furthermore, the results highlight the importance of drug-environment interactions on the neurobiological effects of drugs, and have implications for thinking about the circuits by which context modulates the acute and long-lasting consequences of amphetamine treatment.

Dopamine D1/5 receptor stimulation induces c-fos expression in the subthalamic nucleus: possible involvement of local D5 receptors

The activity of neurons in the subthalamic nucleus controls various aspects of movement. The present study examined the action of dopamine receptor agonists on c-fos gene expression in the subthalamic nucleus in normal rats. We found that systemic administration of the dopamine D1/5 receptor agonist, SKF 82958 (1 mg/kg), induces c-fos expression in the subthalamic nucleus. In contrast, systemic administration of the dopamine D2/3 receptor agonist, quinelorane (2 mg/kg) had no effect. When combined, SKF 82958 and quinelorane induced c-fos expression in subthalamic neurons that was similar to that found following administration of SKF 82958 alone. We also examined c-fos expression in the substantia nigra pars reticulata, the major projection area for subthalamic neurons, and found that SKF 82958, but not quinelorane, caused an induction of c-fos expression in this area. In order to clarify the mechanisms underlying the SKF 82958-mediated induction of c-fos expression in the subthalamic nucleus and substantia nigra pars reticulata, in situ hybridization for the dopamine D1, D2, D3 and D5 receptor mRNAs was performed. The only significant observation was that D5 receptor mRNA is expressed in subthalamic neurons. The present data show that dopamine, via D1/D5 receptors, upregulates c-fos expression in subthalamic neurons, and that the high expression of D5 receptors in this area might be involved. Taken together, these data suggest that dopamine D1/5 receptors are more important for the action of dopamine in the so-called indirect pathway of the basal ganglia circuitry than what is recognized in current models of basal ganglia organization.

Amphetamine and cocaine induce different patterns of c-fos mRNA expression in the striatum and subthalamic nucleus depending on environmental context

In the dorsal striatum, there are two major populations of medium spiny projection neurons. One population is positive for dynorphin mRNA (DYN+), and these cells project preferentially to the substantia nigra, forming the so-called 'direct pathway'. A second population is positive for enkephalin mRNA (ENK+), and these cells influence the substantia nigra indirectly, via the globus pallidus and subthalamic nucleus. Psychostimulant drugs, such as amphetamine and cocaine, are reported to induce immediate early genes (IEGs) in only one subpopulation of dorsal striatal projection neurons, DYN+ cells. However, this apparent selectivity appears to be a function of environmental context. We found that when given in the animal's home cage, amphetamine and cocaine increased expression of the IEG, c-fos, almost exclusively in DYN+ cells. However, when given in a novel environment, amphetamine and cocaine increased c-fos mRNA in both DYN+ and ENK+ cells. Furthermore, amphetamine and cocaine increased c-fos mRNA expression in the subthalamic nucleus when administered in the novel environment, but not when given at home. We conclude that the neural circuitry engaged by psychostimulant drugs, and their ability to induce specific patterns of gene expression, are determined by the environmental context in which they are experienced. This may be related to the ability of environmental novelty to facilitate psychostimulant drug-induced neuroplasticity.