Enkephalin regulates acute D2 dopamine receptor antagonist-induced immediate-early gene expression in striatal neurons

DNA methylation and hydroxymethylation characterize the identity of D1 and D2 striatal projection neurons

Neuronal DNA modifications differ from those in other cells, including methylation outside CpG context and abundant 5-hydroxymethylation whose relevance for neuronal identities are unclear. Striatal projection neurons expressing D1 or D2 dopamine receptors allow addressing this question, as they share many characteristics but differ in their gene expression profiles, connections, and functional roles. We compare translating mRNAs and DNA modifications in these two populations. DNA methylation differences occur predominantly in large genomic clusters including differentially expressed genes, potentially important for D1 and D2 neurons. Decreased gene body methylation is associated with higher gene expression. Hydroxymethylation differences are more scattered and affect transcription factor binding sites, which can influence gene expression. We also find a strong genome-wide hydroxymethylation asymmetry between the two DNA strands, particularly pronounced at expressed genes and retrotransposons. These results identify novel properties of neuronal DNA modifications and unveil epigenetic characteristics of striatal projection neurons heterogeneity.

The Role of the Endogenous Opioid System in the Vocal Behavior of Songbirds and Its Possible Role in Vocal Learning

The opioid system in the brain is responsible for processing affective states such as pain, pleasure, and reward. It consists of three main receptors, mu- (μ-ORs), delta- (δ-ORs), and kappa- (κ-ORs), and their ligands – the endogenous opioid peptides. Despite their involvement in the reward pathway, and a signaling mechanism operating in synergy with the dopaminergic system, fewer reports focus on the role of these receptors in higher cognitive processes. Whereas research on opioids is predominated by studies on their addictive properties and role in pain pathways, recent studies suggest that these receptors may be involved in learning. Rodents deficient in δ-ORs were poor at recognizing the location of novel objects in their surroundings. Furthermore, in chicken, learning to avoid beads coated with a bitter chemical from those without the coating was modulated by δ-ORs. Similarly, μ-ORs facilitate long term potentiation in hippocampal CA3 neurons in mammals, thereby having a positive impact on spatial learning. Whereas these studies have explored the role of opioid receptors on learning using reward/punishment-based paradigms, the role of these receptors in natural learning processes, such as vocal learning, are yet unexplored. In this review, we explore studies that have established the expression pattern of these receptors in different brain regions of birds, with an emphasis on songbirds which are model systems for vocal learning. We also review the role of opioid receptors in modulating the cognitive processes associated with vocalizations in birds. Finally, we discuss the role of these receptors in regulating the motivation to vocalize, and a possible role in modulating vocal learning.

Blockade of adenosine A2A receptors inhibits Tremulous Jaw Movements as well as expression of zif-268 and GAD65 mRNAs in brain motor structures

Tremor is one of the motor symptoms of Parkinson's disease (PD), present also in neuroleptic-induced parkinsonism. Tremulous Jaw Movements (TJMs) are suggested to be a well-validated rodent model of PD resting tremor. TJMs can be induced by typical antipsychotics and are known to be reduced by different drugs, including adenosine A_2A receptor antagonists. The aim of the present study was to search for brain structures involved in the tremorolytic action of SCH58261, a selective A_2A receptor antagonist, in TJMs induced by subchronic pimozide. Besides TJMs, we evaluated in the same animals the expression of zif-268 mRNA (neuronal responsiveness marker), and mRNA levels for glutamic acid decarboxylase 65-kDa isoform (GAD65) and vesicular glutamate transporters 1 and 2 (vGluT1/2) in selected brain structures, as markers of GABAergic and glutamatergic neurons, respectively. We found that SCH58261 reduced the pimozide-induced TJMs. Pimozide increased the zif-268 mRNA level in the striatum, nucleus accumbens (NAc) core, and substantia nigra pars reticulata (SNr). Additionally, it increased GAD65 mRNA in the striatum and SNr, and vGluT2 mRNA levels in the subthalamic nucleus (STN). A positive correlation between zif-268, GAD65 and vGluT2 mRNAs and TJMs was found. SCH58261 reversed the pimozide-increased zif-268 mRNA in the striatum and NAc core and GAD65 mRNA in the striatum and SNr. In contrast, SCH58261 did not influence vGluT2 mRNA in STN. The present study suggests an importance of the striato-subthalamo-nigro-thalamic circuit in neuroleptic-induced TJMs. The tremorolytic effect of A_2A receptor blockade seems to involve this circuit bypassing, however, STN.

Sedative drugs modulate the neuronal activity in the subthalamic nucleus of parkinsonian patients

Microelectrode recording (MER) is often used to identify electrode location which is critical for the success of deep brain stimulation (DBS) treatment of Parkinson’s disease. The usage of anesthesia and its’ impact on MER quality and electrode placement is controversial. We recorded neuronal activity at a single depth inside the Subthalamic Nucleus (STN) before, during, and after remifentanil infusion. The root mean square (RMS) of the 250–6000 Hz band-passed signal was used to evaluate the regional spiking activity, the power spectrum to evaluate the oscillatory activity and the coherence to evaluate synchrony between two microelectrodes. We compare those to new frequency domain (spectral) analysis of previously obtained data during propofol sedation. Results showed Remifentanil decreased the normalized RMS by 9% (P < 0.001), a smaller decrease compared to propofol. Regarding the beta range oscillatory activity, remifentanil depressed oscillations (drop from 25 to 5% of oscillatory electrodes), while propofol did not (increase from 33.3 to 41.7% of oscillatory electrodes). In the cases of simultaneously recorded oscillatory electrodes, propofol did not change the synchronization while remifentanil depressed it. In conclusion, remifentanil interferes with the identification of the dorsolateral oscillatory region, whereas propofol interferes with RMS identification of the STN borders. Thus, both have undesired effect during the MER procedure.

Trial registration: NCT00355927 and NCT00588926.

Neuropathic Pain Dysregulates Gene Expression of the Forebrain Opioid and Dopamine Systems

Disturbances in the function of the mesostriatal dopamine system may contribute to the development and maintenance of chronic pain, including its sensory and emotional/cognitive aspects. In the present study, we assessed the influence of chronic constriction injury (CCI) of the sciatic nerve on the expression of genes coding for dopamine and opioid receptors as well as opioid propeptides in the mouse mesostriatal system, particularly in the nucleus accumbens. We demonstrated bilateral increases in mRNA levels of the dopamine D1 and D2 receptors (the latter accompanied by elevated protein level), opioid propeptides proenkephalin and prodynorphin, as well as delta and kappa (but not mu) opioid receptors in the nucleus accumbens at 7 to 14 days after CCI. These results show that CCI-induced neuropathic pain is accompanied by a major transcriptional dysregulation of molecules involved in dopaminergic and opioidergic signaling in the striatum/nucleus accumbens. Possible functional consequences of these changes include opposite effects of upregulated enkephalin/delta opioid receptor signaling vs. dynorphin/kappa opioid receptor signaling, with the former most likely having an analgesic effect and the latter exacerbating pain and contributing to pain-related negative emotional states.

Nanostructure Endows Neurotherapeutic Potential in Optogenetics: Current Development and Future Prospects

Optogenetics have evolved as a promising tool to control the processes at a cellular level via photons. Specially, it confers a specific control over cellular function through real-time cytomodulation even in freely moving animals. Neuronal stimulation is prerequisite for deep tissue light penetration or insertion of optrode for light illumination to the neurons that have been proven to be compromised due to poor light penetration and invasiveness of the procedure, respectively. In this review, the application of nanotechnology is being elaborated by the use of metal nanoparticles (AuNPs), upconversion nanocrystals (UCNPs), and quantum dots (CdSe) for targeting particular organs or tissues, and their potential to emit a specific light on excitation to overcome the limitations associated with earlier methods has been elucidated. The optothermal and magnetothermal properties, photoluminescence, and higher photostability of nanomaterials are explored in context of therapeutic applicability of optogenetics. The nanostructure characteristics and specific ion channel targeting have shown promising therapeutic potential against neurodegenerative disorders (Alzheimer's, Parkinson's, Huntington's), epilepsy, and blindness. This review compiles mechanical and optical characteristics of nanomaterials that endow superior optogenetic therapeutic potentials to cure immedicable infirmities.

Insight Into the Emerging Role of Striatal Neurotransmitters in the Pathophysiology of Parkinson's Disease and Huntington's Disease: A Review

Alteration in neurotransmitters signaling in basal ganglia has been consistently shown to significantly contribute to the pathophysiological basis of Parkinson's disease and Huntington's disease. Dopamine is an important neurotransmitter which plays a critical role in coordinated body movements. Alteration in the level of brain dopamine and receptor radically contributes to irregular movements, glutamate mediated excitotoxic neuronal death and further leads to imbalance in the levels of other neurotransmitters viz. GABA, adenosine, acetylcholine and endocannabinoids. This review is based upon the data from clinical and preclinical studies to characterize the role of various striatal neurotransmitters in the pathogenesis of Parkinson's disease and Huntington's disease. Further, we have collected data of altered level of various neurotransmitters and their metabolites and receptor density in basal ganglia region. Although the exact mechanisms underlying neuropathology of movement disorders are not fully understood, but several mechanisms related to neurotransmitters alteration, excitotoxic neuronal death, oxidative stress, mitochondrial dysfunction, neuroinflammation are being put forward. Restoring neurotransmitters level and downstream signaling has been considered to be beneficial in the treatment of Parkinson's disease and Huntington's disease. Therefore, there is an urgent need to identify more specific drugs and drug targets that can restore the altered neurotransmitters level in brain and prevent/delay neurodegeneration.

Differential electrophysiological and morphological alterations of thalamostriatal and corticostriatal projections in the R6/2 mouse model of Huntington's disease

Huntington's disease (HD) is a fatal genetic disorder characterized by cell death of medium-sized spiny neurons (MSNs) in the striatum, traditionally attributed to excessive glutamate inputs and/or receptor sensitivity. While changes in corticostriatal projections have typically been studied in mouse models of HD, morphological and functional alterations in thalamostriatal projections have received less attention. In this study, an adeno-associated virus expressing channelrhodopsin-2 under the calcium/calmodulin-dependent protein kinase IIα promoter was injected into the sensorimotor cortex or the thalamic centromedian-parafascicular nuclear complex in the R6/2 mouse model of HD, to permit selective activation of corticostriatal or thalamostriatal projections, respectively. In symptomatic R6/2 mice, peak amplitudes and areas of corticostriatal glutamate AMPA and NMDA receptor-mediated responses were reduced. In contrast, although peak amplitudes of AMPA and NMDA receptor-mediated thalamostriatal responses also were reduced, the areas remained unchanged due to an increase in response decay times. Blockade of glutamate reuptake further increased response areas and slowed rise and decay times of NMDA responses. These effects appeared more pronounced at thalamostriatal synapses of R6/2 mice, suggesting increased activation of extrasynaptic NMDA receptors. In addition, the probability of glutamate release was higher at thalamostriatal than corticostriatal synapses, particularly in R6/2 mice. Morphological studies indicated that the density of all excitatory synaptic contacts onto MSNs was reduced, which matches the basic electrophysiological findings of reduced amplitudes. There was a consistent reduction in the area of spines but little change in presynaptic terminal size, indicating that the postsynaptic spine may be more significantly affected than presynaptic terminals. These results highlight the significant and differential contribution of the thalamostriatal projection to glutamate excitotoxicity in HD.

Functional Differences Between Direct and Indirect Striatal Output Pathways in Huntington's Disease

There is morphological evidence for differential alterations in striatal medium-sized spiny neurons (MSNs) giving rise to the direct and indirect output pathways in Huntington's disease (HD). MSNs of the indirect pathway appear to be particularly vulnerable and markers for these neurons are lost early in postmortem brains and in genetic mouse models. In contrast, MSNs of the direct pathway appear to be relatively spared in the early stages. Because of the great morphological and electrophysiological similarities between MSNs of these pathways, until recently it was difficult to tease apart their functional alterations in HD models. The recent use of the enhanced green fluorescent protein gene as a reporter to identify dopamine D1 (direct pathway) and D2 (indirect pathway) receptor-expressing MSNs has made it possible to examine synaptic function in each pathway. The outcomes of such studies demonstrate significant time-dependent changes in the balance of excitatory and inhibitory inputs to both direct and indirect pathway MSNs in HD and emphasize early increases in both excitatory and inhibitory inputs to direct pathway MSNs. There also is a strong influence of alterations in dopamine modulation that possibly cause some of the changes in excitatory and inhibitory synaptic transmission in the HD models. These changes will markedly alter the output structures, the GPi and the SNr. In the future, the use of combined optogenetics with identified neurons in each pathway will help unravel the next set of questions about how the output nuclei are affected in HD.

Neuroimaging studies of striatum in cognition part II: Parkinson's disease

In recent years a gradual shift in the definition of Parkinson's disease (PD) has been established, from a classical akinetic-rigid movement disorder to a multi-system neurodegenerative disease. While the pathophysiology of PD is complex and goes much beyond the nigro-striatal degeneration, the striatum has been shown to be responsible for many cognitive functions. Patients with PD develop impairments in multiple cognitive domains and the PD model is probably the most extensively studied regarding striatum dysfunction and its influence on cognition. Up to 40% of PD patients present cognitive impairment even in the early stages of disease development. Thus, understanding the key patterns of striatum and connecting regions' influence on cognition will help develop more specific approaches to alleviate cognitive impairment and slow down its decline. This review focuses on the contribution of neuroimaging studies in understanding how striatum impairment affects cognition in PD.

Fluoxetine potentiation of methylphenidate-induced gene regulation in striatal output pathways: potential role for 5-HT1B receptor

Drug combinations that include the psychostimulant methylphenidate plus a selective serotonin reuptake inhibitor (SSRI) such as fluoxetine are increasingly used in children and adolescents. For example, this combination is indicated in the treatment of attention-deficit/hyperactivity disorder and depression comorbidity and other mental disorders. Such co-exposure also occurs in patients on SSRIs who use methylphenidate as a cognitive enhancer. The neurobiological consequences of these drug combinations are poorly understood. Methylphenidate alone can produce gene regulation effects that mimic addiction-related gene regulation by cocaine, consistent with its moderate addiction liability. We have previously shown that combining SSRIs with methylphenidate potentiates methylphenidate-induced gene regulation in the striatum. The present study investigated which striatal output pathways are affected by the methylphenidate + fluoxetine combination, by assessing effects on pathway-specific neuropeptide markers, and which serotonin receptor subtypes may mediate these effects. Our results demonstrate that a 5-day repeated treatment with fluoxetine (5 mg/kg) potentiates methylphenidate (5 mg/kg)-induced expression of both dynorphin (direct pathway marker) and enkephalin (indirect pathway). These changes were accompanied by correlated increases in the expression of the 5-HT1B, but not 5-HT2C, serotonin receptor in the same striatal regions. A further study showed that the 5-HT1B receptor agonist CP94253 (3-10 mg/kg) mimics the fluoxetine potentiation of methylphenidate-induced gene regulation. These findings suggest a role for the 5-HT1B receptor in the fluoxetine effects on striatal gene regulation. Given that 5-HT1B receptors are known to facilitate addiction-related gene regulation and behavior, our results suggest that SSRIs may enhance the addiction liability of methylphenidate by increasing 5-HT1B receptor signaling.

Dopamine imbalance in Huntington's disease: a mechanism for the lack of behavioral flexibility

Dopamine (DA) plays an essential role in the control of coordinated movements. Alterations in DA balance in the striatum lead to pathological conditions such as Parkinson's and Huntington's diseases (HD). HD is a progressive, invariably fatal neurodegenerative disease caused by a genetic mutation producing an expansion of glutamine repeats and is characterized by abnormal dance-like movements (chorea). The principal pathology is the loss of striatal and cortical projection neurons. Changes in brain DA content and receptor number contribute to abnormal movements and cognitive deficits in HD. In particular, during the early hyperkinetic stage of HD, DA levels are increased whereas expression of DA receptors is reduced. In contrast, in the late akinetic stage, DA levels are significantly decreased and resemble those of a Parkinsonian state. Time-dependent changes in DA transmission parallel biphasic changes in glutamate synaptic transmission and may enhance alterations in glutamate receptor-mediated synaptic activity. In this review, we focus on neuronal electrophysiological mechanisms that may lead to some of the motor and cognitive symptoms of HD and how they relate to dysfunction in DA neurotransmission. Based on clinical and experimental findings, we propose that some of the behavioral alterations in HD, including reduced behavioral flexibility, may be caused by altered DA modulatory function. Thus, restoring DA balance alone or in conjunction with glutamate receptor antagonists could be a viable therapeutic approach.

Addiction-related gene regulation: risks of exposure to cognitive enhancers vs. other psychostimulants

The psychostimulants methylphenidate (Ritalin, Concerta), amphetamine (Adderall), and modafinil (Provigil) are widely used in the treatment of medical conditions such as attention-deficit hyperactivity disorder and narcolepsy and, increasingly, as "cognitive enhancers" by healthy people. The long-term neuronal effects of these drugs, however, are poorly understood. A substantial amount of research over the past two decades has investigated the effects of psychostimulants such as cocaine and amphetamines on gene regulation in the brain because these molecular changes are considered critical for psychostimulant addiction. This work has determined in some detail the neurochemical and cellular mechanisms that mediate psychostimulant-induced gene regulation and has also identified the neuronal systems altered by these drugs. Among the most affected brain systems are corticostriatal circuits, which are part of cortico-basal ganglia-cortical loops that mediate motivated behavior. The neurotransmitters critical for such gene regulation are dopamine in interaction with glutamate, while other neurotransmitters (e.g., serotonin) play modulatory roles. This review presents (1) an overview of the main findings on cocaine- and amphetamine-induced gene regulation in corticostriatal circuits in an effort to provide a cellular framework for (2) an assessment of the molecular changes produced by methylphenidate, medical amphetamine (Adderall), and modafinil. The findings lead to the conclusion that protracted exposure to these cognitive enhancers can induce gene regulation effects in corticostriatal circuits that are qualitatively similar to those of cocaine and other amphetamines. These neuronal changes may contribute to the addiction liability of the psychostimulant cognitive enhancers.

Endogenous opioid release in the human brain reward system induced by acute amphetamine administration

BACKGROUND

We aimed to demonstrate a pharmacologically stimulated endogenous opioid release in the living human brain by evaluating the effects of amphetamine administration on [(11)C]carfentanil binding with positron emission tomography (PET).

METHODS

Twelve healthy male volunteers underwent [(11)C]carfentanil PET before and 3 hours after a single oral dose of d-amphetamine (either a "high" dose, .5 mg/kg, or a sub-pharmacological "ultra-low" dose, 1.25 mg total dose or approximately .017 mg/kg). Reductions in [(11)C]carfentanil binding from baseline to post-amphetamine scans (ΔBP(ND)) after the "high" and "ultra-low" amphetamine doses were assessed in 10 regions of interest.

RESULTS

[(11)C]carfentanil binding was reduced after the "high" but not the "ultra-low" amphetamine dose in the frontal cortex, putamen, caudate, thalamus, anterior cingulate, and insula.

CONCLUSIONS

Our findings indicate that oral amphetamine administration induces endogenous opioid release in different areas of human brain, including basal ganglia, frontal cortex areas, and thalamus. The combination of an amphetamine challenge and [(11)C]carfentanil PET is a practical and robust method to probe the opioid system in the living human brain.

Fluoxetine potentiation of methylphenidate-induced neuropeptide expression in the striatum occurs selectively in direct pathway (striatonigral) neurons

Concomitant therapies combining psychostimulants such as methylphenidate and selective serotonin reuptake inhibitors (SSRIs) are used to treat several mental disorders, including attention-deficit hyperactivity disorder/depression comorbidity. The neurobiological consequences of these drug combinations are poorly understood. Methylphenidate alone induces gene regulation that mimics partly effects of cocaine, consistent with some addiction liability. We previously showed that the SSRI fluoxetine potentiates methylphenidate-induced gene regulation in the striatum. The present study investigated which striatal output pathways are affected by the methylphenidate + fluoxetine combination, by assessing effects on pathway-specific neuropeptide markers. Results demonstrate that fluoxetine (5 mg/kg) potentiates methylphenidate (5 mg/kg)-induced expression of substance P and dynorphin, markers for direct pathway neurons. In contrast, no drug effects on the indirect pathway marker enkephalin were found. Because methylphenidate alone has minimal effects on dynorphin, the potentiation of dynorphin induction represents a more cocaine-like effect for the drug combination. On the other hand, the lack of an effect on enkephalin suggests a greater selectivity for the direct pathway compared with psychostimulants such as cocaine. Overall, the fluoxetine potentiation of gene regulation by methylphenidate occurs preferentially in sensorimotor striatal circuits, similar to other addictive psychostimulants. These results suggest that SSRIs may enhance the addiction liability of methylphenidate.

Prefrontal dopaminergic and enkephalinergic synaptic accommodation in HIV-associated neurocognitive disorders and encephalitis

Changes in synapse structure occur in frontal neocortex with HIV encephalitis (HIVE) and may contribute to HIV-associated neurocognitive disorders (HAND). A postmortem survey was conducted to determine if mRNAs involved in synaptic transmission are perturbed in dorsolateral prefrontal cortex (DLPFC) in subjects with HIVE or HAND. Expression of the opioid neurotransmitter preproenkephalin mRNA (PENK) was significantly decreased in a sampling of 446 brain specimens from HIV-1 infected people compared to 67 HIV negative subjects. Decreased DLPFC PENK was most evident in subjects with HIVE and/or increased expression of interferon regulatory factor 1 mRNA (IRF1). Type 2 dopamine receptor mRNA (DRD2L) was decreased significantly, but not in the same set of subjects with PENK dysregulation. DRD2L downregulation occurred primarily in the subjects without HIVE or neurocognitive impairment. Subjects with neurocognitive impairment often failed to significantly downregulate DRD2L and had abnormally high IRF1 expression. Conclusion: Dysregulation of synaptic preproenkephalin and DRD2L in frontal neocortex can occur with and without neurocognitive impairment in HIV-infected people. Downregulation of DRD2L in the prefrontal cortex was associated with more favorable neuropsychological and neuropathological outcomes; the failure to downregulate DRD2L was significantly less favorable. PENK downregulation was related neuropathologically to HIVE, but was not related to neuropsychological outcome independently. Emulating endogenous synaptic plasticity pharmacodynamically could enhance synaptic accommodation and improve neuropsychological and neuropathological outcomes in HIV/AIDS.

COMT (Val158Met) polymorphism is not associated to neuropathic pain in a Spanish population

It is well known that the response to painful stimuli varies between individuals and this could be consequence of individual differences to pain sensitivity that may be related to genetic factors. Catechol-O-methyltransferase (COMT) is one of the enzymes that metabolize catecholamine neurotransmitters. Differences in the activity of COMT influence the functions of these neurotransmitters. A single nucleotide polymorphism (Val158Met) of COMT leads to a three to four fold reduction in the activity of the enzyme and has been associated to modifications in the response to a pain stressor. Neuropathic pain is a progressive nervous system disease due to an alteration of the peripheral or central nervous system. To elucidate the possible role of COMT polymorphism in the susceptibility to neuropathic pain, we have performed a case-control study in a Spanish population. Analysis of the (Val158Met) COMT polymorphism was performed by PCR amplification and DNA digestion with restriction enzymes. Our study concludes that functional Val158Met polymorphism of COMT gene is not associated to increased susceptibility to neuropathic pain.

Pathophysiology of Huntington's disease: time-dependent alterations in synaptic and receptor function

Huntington's disease (HD) is a progressive, fatal neurological condition caused by an expansion of CAG (glutamine) repeats in the coding region of the Huntington gene. To date, there is no cure but great strides have been made to understand pathophysiological mechanisms. In particular, genetic animal models of HD have been instrumental in elucidating the progression of behavioral and physiological alterations, which had not been possible using classic neurotoxin models. Our groups have pioneered the use of transgenic HD mice to examine the excitotoxicity hypothesis of striatal neuronal dysfunction and degeneration, as well as alterations in excitation and inhibition in striatum and cerebral cortex. In this review, we focus on synaptic and receptor alterations of striatal medium-sized spiny (MSNs) and cortical pyramidal neurons in genetic HD mouse models. We demonstrate a complex series of alterations that are region-specific and time-dependent. In particular, many changes are bidirectional depending on the degree of disease progression, that is, early vs. late, and also on the region examined. Early synaptic dysfunction is manifested by dysregulated glutamate release in striatum followed by progressive disconnection between cortex and striatum. The differential effects of altered glutamate release on MSNs originating the direct and indirect pathways is also elucidated, with the unexpected finding that cells of the direct striatal pathway are involved early in the course of the disease. In addition, we review evidence for early N-methyl-D-aspartate receptor (NMDAR) dysfunction leading to enhanced sensitivity of extrasynaptic receptors and a critical role of GluN2B subunits. Some of the alterations in late HD could be compensatory mechanisms designed to cope with early synaptic and receptor dysfunctions. The main findings indicate that HD treatments need to be designed according to the stage of disease progression and should consider regional differences.

Ultrastructural relationship between the mu opioid receptor and its interacting protein, GPR177, in striatal neurons

GPR177, the mammalian ortholog of Drosophila Wntless/Evi/Sprinter, was recently identified as a novel mu-opioid receptor (MOR) interacting protein. GPR177 is a trans-membrane protein pivotal to mediating the secretion of Wnt signaling proteins. Wnt proteins, in turn, are essential in regulating neuronal development, a phenomenon inhibited upon chronic exposure to MOR agonists such as morphine and heroin. We previously showed that GPR177 and MOR are co-localized in the mouse dorsolateral striatum; however, the nature of this interaction was not fully elucidated. Therefore, in the present study, we examined cellular substrates for interactions between GPR177 and MOR using a combined immunogold-silver and peroxidase detection approach in coronal sections in the dorsolateral segment of the striatum. Semi-quantitative analysis of the ultrastructural distribution of GPR177 and MOR in striatal somata and in dendritic processes showed that, of the somata and dendritic processes exhibiting GPR177, 32% contained MOR immunolabeling while for profiles exhibiting MOR, 37% also contained GPR177 immunoreactivity. GPR177-labeled particles were localized predominantly along both the plasma membrane and within the cytoplasm of MOR-labeled dendrites. Somata and dendritic processes that contained both GPR177 and MOR more often received symmetric (inhibitory-type) synapses from unlabeled axon terminals. To further define the phenotype of GPR177 and MOR-containing cellular profiles, triple immunofluorescence detection showed that GPR177 and MOR are localized in neurons containing the opioid peptide, enkephalin, within the dorsolateral striatum. The results provide an anatomical substrate for interactions between MOR and its interacting protein, GPR177, in striatal opioid-containing neurons that may underlie the morphological alterations produced in neurons by chronic opiate use.

Does the difference between physically active and couch potato lie in the dopamine system?

Obesity and other inactivity related diseases are increasing at an alarming rate especially in Western societies. Because of this, it is important to understand the regulating mechanisms involved in physical activity behavior. Much research has been done in regard to the psychological determinants of physical activity behavior; however, little is known about the underlying genetic and biological factors that may contribute to regulation of this complex trait. It is true that a significant portion of any trait is regulated by genetic and biological factors. In the case of voluntary physical activity behavior, these regulating mechanisms appear to be concentrated in the central nervous system. In particular, the dopamine system has been shown to regulate motor movement, as well as motivation and reward behavior. The pattern of regulation of voluntary physical activity by the dopamine system is yet to be fully elucidated. This review will summarize what is known about the dopamine system and regulation of physical activity, and will present a hypothesis of how this signaling pathway is mechanistically involved in regulating voluntary physical activity behavior. Future research in this area will aid in developing personalized strategies to prevent inactivity related diseases.

Beta-endorphin and drug-induced reward and reinforcement

Although drugs of abuse have different acute mechanisms of action, their brain pathways of reward exhibit common functional effects upon both acute and chronic administration. Long known for its analgesic effect, the opioid beta-endorphin is now shown to induce euphoria, and to have rewarding and reinforcing properties. In this review, we will summarize the present neurobiological and behavioral evidences that support involvement of beta-endorphin in drug-induced reward and reinforcement. Currently, evidence supports a prominent role for beta-endorphin in the reward pathways of cocaine and alcohol. The existing information indicating the importance of beta-endorphin neurotransmission in mediating the reward pathways of nicotine and THC, is thus far circumstantial. The studies described herein employed diverse techniques, such as biochemical measurements of beta-endorphin in various brain sites and plasma, and behavioral measurements, conducted following elimination (via administration of anti-beta-endorphin antibodies or using mutant mice) or augmentation (by intracerebral administration) of beta-endorphin. We suggest that the reward pathways for different addictive drugs converge to a common pathway in which beta-endorphin is a modulating element. Beta-endorphin is involved also with distress. However, reviewing the data collected so far implies a discrete role, beyond that of a stress response, for beta-endorphin in mediating the substance of abuse reward pathway. This may occur via interacting with the mesolimbic dopaminergic system and also by its interesting effects on learning and memory. The functional meaning of beta-endorphin in the process of drug-seeking behavior is discussed.

Mild postnatal manipulation reduces proenkephalin mRNA in the striatum in developing mice and increases morphine conditioned place preference in adulthood

Stressful events during certain neonatal periods may increase the vulnerability of an individual to develop psychopathology and/or drug dependence later in life. Therefore, in the present study, we assessed activity levels, emotionality, sensitivity to the effects of morphine, as well as expression of proenkephalin and prodynorphin in several brain regions in 35 and 90-day-old male mice, subjected to postnatal manipulation consisting in brief exposures to clean bedding (CB). In comparison with controls, CB mice showed reduced emotionality expressed as percentage of time in open arms of the elevated plus maze both at 35 days of life and in adulthood. Increased nociceptive threshold was also present in both time points measured. Conversely, higher locomotor activity was recorded in 35 days of life but not in adulthood. Analysis of film autoradiograms revealed no changes in prodynorphin mRNA level, but statistically significant decrease in the level of proenkephalin mRNA in striatum in young CB mice in comparison with young controls; no difference was observed between adult CB and control animals. CB adult mice also showed hypersensitivity to the rewarding effect of morphine in comparison with controls in the place preference test. In conclusion, our results revealed that in the critical period of development the effects of manipulation were evident, not only on behavioral responses but also on the neurochemical markers considered in the present research. Postnatal manipulation could induce changes in the dynamic neuronal processes occurring during development with long-term behavioral effects.

Food-associated cues alter forebrain functional connectivity as assessed with immediate early gene and proenkephalin expression

Background

Cues predictive of food availability are powerful modulators of appetite as well as food-seeking and ingestive behaviors. The neurobiological underpinnings of these conditioned responses are not well understood. Monitoring regional immediate early gene expression is a method used to assess alterations in neuronal metabolism resulting from upstream intracellular and extracellular signaling. Furthermore, assessing the expression of multiple immediate early genes offers a window onto the possible sequelae of exposure to food cues, since the function of each gene differs. We used immediate early gene and proenkephalin expression as a means of assessing food cue-elicited regional activation and alterations in functional connectivity within the forebrain.

Results

Contextual cues associated with palatable food elicited conditioned motor activation and corticosterone release in rats. This motivational state was associated with increased transcription of the activity-regulated genes homer1a, arc, zif268, ngfi-b and c-fos in corticolimbic, thalamic and hypothalamic areas and of proenkephalin within striatal regions. Furthermore, the functional connectivity elicited by food cues, as assessed by an inter-regional multigene-expression correlation method, differed substantially from that elicited by neutral cues. Specifically, food cues increased cortical engagement of the striatum, and within the nucleus accumbens, shifted correlations away from the shell towards the core. Exposure to the food-associated context also induced correlated gene expression between corticostriatal networks and the basolateral amygdala, an area critical for learning and responding to the incentive value of sensory stimuli. This increased corticostriatal-amygdalar functional connectivity was absent in the control group exposed to innocuous cues.

Conclusion

The results implicate correlated activity between the cortex and the striatum, especially the nucleus accumbens core and the basolateral amygdala, in the generation of a conditioned motivated state that may promote excessive food intake. The upregulation of a number of genes in unique patterns within corticostriatal, thalamic, and hypothalamic networks suggests that food cues are capable of powerfully altering neuronal processing in areas mediating the integration of emotion, cognition, arousal, and the regulation of energy balance. As many of these genes play a role in plasticity, their upregulation within these circuits may also indicate the neuroanatomic and transcriptional correlates of extinction learning.

Delta opioids reduce the neurotransmitter release probability by enhancing transient (KV4) K+-currents in corticostriatal synapses as evaluated by the paired pulse protocol

Field recordings were used to determine the influence of delta-opioid receptor activation on corticostriatal synaptic transmission. Application of the selective delta-opioid receptor agonist, [Tyr-D-Pen-Gly-Phe-D-Pen]-enkephalin (DPDPE, 1 microM), decreased the amplitude of the field-excitatory synaptic potential and at the same time increased the paired pulse ratio (PPR) suggesting a presynaptic site of action. This response reversed rapidly when DPDPE was washed and blocked by 1 nM of the selective delta-receptor antagonist naltrindole. Neither omega-conotoxin GVIA (1 microM) nor omega-agatoxin TK (400 nM), blockers of N- and P/Q-type Ca2+-channels, respectively, nor TEA (1 mM), blocker of some classes of K+-channels, occluded the effects of DPDPE. Instead, 1 mM 4-AP or 400 microM Ba2+ occluded completely the effects of DPDPE. Therefore, the results suggest that the modulation by delta opioids at corticostriatal terminals is mediated by transient (KV4) K+-conductances.

Co-localization of GABA with other neuroactive substances in the basal ganglia

The dorsal striatum (caudate putamen) contains two types of GABAergic medium spiny neurons (MSNs) that are distinguished by the expression of either the opioid peptide, enkephalin, or the opioid peptide, dynorphin, as well as the tachykinin substance P. Pharmacological studies suggest that these peptides modulate local neurotransmission in the striatum in response to direct and indirect dopamine agonists. In contrast, GABA appears to have minimal impact within the striatum under these conditions. The actions of the peptide cocktail are dependent on the cellular distribution of their receptors in the striatal network. The net result of their actions is a homeostatic response that regulates striatal output and balances dopamine and glutamate receptor stimulation.

Comparison of ethanol preference and neurochemical measures of mesolimbic dopamine and adenosine systems across different strains of mice

BACKGROUND

To extend the known phenotype of strains commonly used in the development of mutant mice, ethanol, saccharin, and caffeine preferences were examined in C57Bl/6J, CD-1, and hybrid C57Bl/6J x CD-1 mice. As dopaminergic mechanisms are inherently involved in the neuronal processing of many drugs of abuse (including ethanol), and an important role for adenosine-dopamine interactions has also been reported, the dopaminergic and purinergic neurochemical profiles of mice were compared against the consummatory phenotype observed.

METHODS

Ethanol (5% v/v), saccharin (0.1% w/v), and caffeine (0.1% w/v) consumption and preference were examined using a 2-bottle free-choice paradigm. Dopamine and adenosine receptor and transporter mRNA and protein density were quantified using in situ hybridization histochemistry and in vitro autoradiography, respectively.

RESULTS

C57Bl/6J and hybrid C57Bl/6J x CD-1 mice demonstrated a clear ethanol preference, voluntarily consuming large quantities of ethanol when given the choice between drinking vessels containing either ethanol or water. Conversely, CD-1 mice were characterized as ethanol-avoiding under the present paradigm. Differences in D(1) receptor mRNA between the strains were consistent with the observed behavioral differences in ethanol preference. The high ethanol-preferring phenotype of C57Bl/6J mice could not be directly linked to alterations in dopamine transporter neurochemistry and/or enkephalin levels as proposed by earlier researchers. Ethanol-seeking behavior appeared to correlate with D2 receptor expression, however, with evidence that ethanol-preferring mice also exhibit an increased density of D2 receptors within limbic dopaminergic projection nuclei. Interestingly, strain differences in the expression of the ethanol-sensitive nucleoside transporter paralleled differences in ethanol consumption, a novel finding consonant with purinergic involvement in dopamine-related behaviors.

CONCLUSIONS

This study has highlighted the relevance of alterations in dopamine receptor expression and purinergic modulation within the mesolimbic pathway and predisposition toward the development of ethanol-seeking behavior.

Imaging brain mu-opioid receptors in abstinent cocaine users: time course and relation to cocaine craving

BACKGROUND

Cocaine treatment upregulates brain mu-opioid receptors (mOR) in animals. Human data regarding this phenomenon are limited. We previously used positron emission tomography (PET) with [11C]-carfentanil to show increased mOR binding in brain regions of 10 cocaine-dependent men after 1 and 28 days of abstinence.

METHODS

Regional brain mOR binding potential (BP) was measured with [11C]carfentanil PET scanning in 17 cocaine users over 12 weeks of abstinence on a research ward and in 16 healthy control subjects.

RESULTS

Mu-opioid receptor BP was increased in the frontal, anterior cingulate, and lateral temporal cortex after 1 day of abstinence. Mu-opioid receptor BP remained elevated in the first two regions after 1 week and in the anterior cingulate and anterior frontal cortex after 12 weeks. Increased binding in some regions at 1 day and 1 week was positively correlated with self-reported cocaine craving. Mu-opioid receptor BP was significantly correlated with percentage of days with cocaine use and amount of cocaine used per day of use during the 2 weeks before admission and with urine benzoylecgonine concentration at the first PET scan.

CONCLUSIONS

These results suggest that chronic cocaine use influences endogenous opioid systems in the human brain and might explain mechanisms of cocaine craving and reinforcement.

Topography of methylphenidate (ritalin)-induced gene regulation in the striatum: differential effects on c-fos, substance P and opioid peptides

Dopamine action alters gene regulation in striatal neurons. Methylphenidate increases extracellular levels of dopamine. We investigated the effects of acute methylphenidate treatment on gene expression in the striatum of adult rats. Molecular changes were mapped in 23 striatal sectors mostly defined by their predominant cortical inputs in order to determine the functional domains affected. Acute administration of 5 and 10 mg/kg (i.p.) of methylphenidate produced robust increases in the expression of the transcription factor c-fos and the neuropeptide substance P. Borderline effects were found with 2 mg/kg, but not with 0.5 mg/kg. For 5 mg/kg, c-fos mRNA levels peaked at 40 min and returned to baseline by 3 h after injection, while substance P mRNA levels peaked at 40-60 min and were back near control levels by 24 h. These molecular changes occurred in most sectors of the caudate-putamen, but were maximal in dorsal sectors that receive sensorimotor and medial agranular cortical inputs, on middle to caudal levels. In rostral and ventral striatal sectors, changes in c-fos and substance P expression were weaker or absent. No effects were seen in the nucleus accumbens, with the exception of c-fos induction in the lateral part of the shell. In contrast to c-fos and substance P, acute methylphenidate treatment had minimal effects on the opioid peptides dynorphin and enkephalin. These results demonstrate that acute methylphenidate alters the expression of c-fos and substance P preferentially in the sensorimotor striatum. These molecular changes are similar, but not identical, to those produced by other psychostimulants.

Activity-regulated cytoskeleton-associated protein Arc is targeted to dendrites and coexpressed with mu-opioid receptors in postnatal rat caudate-putamen nucleus

Dendritic expression of the activity-regulated cytoskeleton-associated protein (Arc) is dramatically enhanced by increased synaptic activity in adult brain. We used immunocytochemical electron microscopy to determine whether the subcellular localization of Arc in developing dendrites corresponds to the peak period of synaptogenesis in the postnatal rat caudate-putamen nucleus (CPN). The distribution was compared with that of mu-opioid receptors (MORs), whose localization in dendritic spines closely parallels excitatory synapse formation during postnatal development (Wang et al. [2003] Neuroscience 118:695-708). Sections were processed for immunocytochemical detection of antisera against Arc or MORs at the beginning (postnatal day 15; P15) and the end (P30) of the peak period of synaptogenesis in rat CPN. At P15, immunolabeling for Arc showed a punctate distribution in the cytoplasm of dendritic shafts, some of which was associated with polyribosomes. In some spiny dendrites, Arc immunoreactivity was more intensely localized in putative spines than in their parental dendrites, whereas, in other spiny dendrites, Arc labeling was restricted in the shafts. Many dendritic shafts and spines also showed immunoreactivity for MORs, although dually labeled spines were less numerous than the shafts. At P30, the proportion of singly and dually labeled spines significantly increased from 2.0% to 7.5% and from 9.5% to 21%, respectively. Arc labeling in spines was more detectable beneath the postsynaptic density or at extrasynaptic sites on the plasma membrane. Our results suggest a correlation between Arc expression in dendritic spines during postnatal development and the onset of synaptogenesis in opioid-responsive neurons in the rat CPN.

Opiate-like effects of sugar on gene expression in reward areas of the rat brain

Drugs abused by humans are thought to activate areas in the ventral striatum of the brain that engage the organism in important adaptive behaviors, such as eating. In support of this, we report here that striatal regions of sugar-dependent rats show alterations in dopamine and opioid mRNA levels similar to morphine-dependent rats. Specifically, after a chronic schedule of intermittent bingeing on a sucrose solution, mRNA levels for the D2 dopamine receptor, and the preproenkephalin and preprotachykinin genes were decreased in dopamine-receptive regions of the forebrain, while D3 dopamine receptor mRNA was increased. While morphine affects gene expression across the entire dopamine-receptive striatum, significant differences were detected in the effects of sugar on the nucleus accumbens and adjacent caudate-putamen. The effects of sugar on mRNA levels were of greater magnitude in the nucleus accumbens than in the caudate-putamen. These areas also showed clear differences in the interactions among the genes, especially between D3R and the other genes. This was revealed by a novel multivariate analysis method that identified cooperative interactions among genes, specifically in the nucleus accumbens but not the caudate-putamen. Finally, a role for these cooperative interactions in a load-sharing response to perturbations caused by sugar was supported by the finding of a different pattern of correlations between the genes in the two striatal regions. These findings support a major role for the nucleus accumbens in mediating the effects of naturally rewarding substances and extend an animal model for studying the common substrates of drug addiction and eating disorders.

Taking stock: from chasing occlusal contacts to vulnerability alleles

Over the past 70 years, temporomandibular disorders (TMDs) have been subject to shifts in conceptual understanding. Unable to account for disease patterns, the mismatch between case assignment and treatment need, and very different interventions producing similar treatment outcomes (except for the risk to patients), emerging theories make persuasive arguments in support of alternative explanations.

Altered GABAergic neurotransmission in mice lacking dopamine D2 receptors

The levels of glutamic acid decarboxylase (GAD) were strongly increased in the cortex and the striatum in dopamine D2 receptors null (D2R-/-) mice, which show a significant locomotor impairment. In this study, the effects of different GABAergic drugs on locomotor activity were analyzed in D2R-/- mice. After administering muscimol (1 mg/kg), a GABA(A) receptor agonist, the D2R-/- mice showed increased locomotor activity up to 200%. When the muscimol dose was increased (4-6 mg/kg), the D2R-/- mice exhibited seizure-like behavior, and the electroencephalographic (EEG) recordings during these behaviors showed a high amplitude rhythmic epileptiform activity in these mice. In situ hybridization showed that after injecting muscimol in the D2R-/- mice, the expression of enkephalin and immediate early gene, NGFI-A, was closely regulated with the locomotor activity regulated by GABAergic stimulation. These results suggest that the absence of D2R alters the GABAergic neurotransmission, specifically on GABA(A)-receptor mediated signaling, and stimulating the GABA(A) receptor can reverse the dysfunction of GABAergic inhibition in the motor circuits in the basal ganglia.

Ropinirole versus l-DOPA effects on striatal opioid peptide precursors in a rodent model of Parkinson's disease: implications for dyskinesia

The dopamine precursor, L-3,4-dihydroxyphenylalanine (L-DOPA), remains the most common treatment for Parkinson's disease. However, following long-term treatment, disabling side effects, particularly L-DOPA-induced dyskinesias, are encountered. Conversely, D2/D3 dopamine receptor agonists, such as ropinirole, exert an anti-parkinsonian effect while eliciting less dyskinesia when administered de novo in Parkinson's disease patients. Parkinson's disease and L-DOPA-induced dyskinesia are both associated with changes in mRNA and peptide levels of the opioid peptide precursors preproenkephalin-A (PPE-A) and preproenkephalin-B (PPE-B). Furthermore, a potential role of abnormal opioid peptide transmission in dyskinesia is suggested due to the ability of opioid receptor antagonists to reduce the L-DOPA-induced dyskinesia in animal models of Parkinson's disease. In this study, the behavioural response, striatal topography and levels of expression of the opioid peptide precursors PPE-A and PPE-B were assessed, following repeated vehicle, ropinirole, or L-DOPA administration in the 6-OHDA-lesioned rat model of Parkinson's disease. While repeated administration of L-DOPA significantly elevated PPE-B mRNA levels (313% cf. vehicle, 6-OHDA-lesioned rostral striatum; 189% cf. vehicle, 6-OHDA-lesioned caudal striatum) in the unilaterally 6-OHDA-lesioned rat model of Parkinson's disease, ropinirole did not. These data and previous studies suggest the involvement of enhanced opioid transmission in L-DOPA-induced dyskinesia and that part of the reason why D2/D3 dopamine receptor agonists have a reduced propensity to elicit dyskinesia may reside in their reduced ability to elevate opioid transmission.

Glutamatergic Agents for Cocaine Dependence

Effective medications for cocaine dependence are needed to improve outcome in this chronic, relapsing disorder. Medications affecting glutamate function are reasonable candidates for investigation, given the involvement of glutamate circuits in reward-related brain regions and evidence of cocaine-induced glutamatergic dysregulation. In addition, it is increasingly apparent that glutamatergic mechanisms underlie several clinical aspects of cocaine dependence, including euphoria, withdrawal, craving, and hedonic dysfunction. Even denial, traditionally viewed as purely psychological, may result, in part, from dysfunctional glutamate-rich cortical regions. We review the involvement of glutamate in reward-related circuits, the acute and chronic effects of cocaine on these pathways, and glutamatergic mechanisms that contribute to the neurobiology of cocaine dependence. We also present preliminary data from our research of modafinil, a glutamate-enhancing agent with promise in the treatment of cocaine-addicted individuals.

Further characterization of preproenkephalin mRNA-containing cells in the rodent globus pallidus

The globus pallidus (external pallidum of primates) is an essential nucleus within basal ganglia circuitry, in part because it receives at least one-half of striatal efferent projections. Neurons of the globus pallidus can be divided into subpopulations based on anatomical, physiological, and chemical features. Globus pallidus neurons project to several structures (the striatum, subthalamic nucleus, entopeduncular nucleus, and substantia nigra pars reticulata), have one of two alternative waveforms (positive/negative versus negative/positive), contain either the calcium binding protein parvalbumin or the neuropeptide precursor preproenkephalin mRNA and show differential immediate early gene responses to dopamine receptor agonists and antagonists. The objective of the present study was to characterize in greater detail the preproenkephalin mRNA-containing pallidal neurons using Sprague-Dawley rats. In situ hybridization for preproenkephalin mRNA was combined with immunocytochemical detection of: (i) the neuron-specific nuclear protein, NeuN, (ii) FluoroGold-labeled pallidostriatal and pallidosubthalamic cells, or (iii) Fos induced by either systemic combined D1-class/D2-class dopamine receptor agonists or a D2-class receptor antagonist. These experiments demonstrated that a substantial population (42%) of globus pallidus neurons contains preproenkephalin mRNA, and that globus pallidus neurons retrogradely labeled after FluoroGold injections into the striatum are more frequently preproenkephalinergic, compared to the population of pallidosubthalamic neurons. Furthermore, systemic administration of a D2 receptor antagonist, eticlopride, induced Fos immunoreactivity predominantly in globus pallidus neurons expressing preproenkephalin mRNA, while combined administration of D1 and D2 receptor agonists induced Fos predominantly in pallidal neurons lacking preproenkephalin mRNA.These results support the conclusion that preproenkephalin mRNA identifies one of the two major subpopulations of pallidal neurons. This preproenkephalin mRNA-expressing pallidal subpopulation preferentially targets the striatum and is more readily activated in its immediate early gene expression by D2 receptor antagonists than by dopamine receptor agonists. This projection provides a pallidal substrate for the dopaminergic regulation of striatal information processing.

Intrinsic GABA neurons inhibit proenkephalin gene expression in slice cultures of rat neostriatum

In the neostriatum, the proenkephalin gene is expressed in medium spiny GABA neurons, which project to the globus pallidus. The expression is activated by glutamatergic projections from the neocortex via NMDA receptors. In these experiments we have used slice cultures of rat neostriatum to study the role of GABA in proenkephalin gene expression. Our results show that GABA is released from neostriatal neurons and negatively regulates the proenkephalin gene expression induced by NMDA receptor stimulation. The GABAA receptors involved seem to be colocalized with NMDA receptors on the projection neurons, which express the proenkephalin gene. In further experiments, we have found that the proenkephalin gene expression is not only activated by neocortical projection neurons but also by intrinsic striatal neurons as well as by projections from the thalamus. All these glutamatergic afferents enhance the proenkephalin gene expression via NMDA receptors. Their efficacy is regulated by endogenous GABA.

Unilateral striatal dopamine depletion: time-dependent effects on cortical function and behavioural correlates

Previously, we showed that unilateral blockade of D1 dopamine receptors in the striatum inhibits immediate-early gene expression bilaterally throughout large parts of the cortex, including sensory-evoked expression in the barrel cortex. To further investigate this dopamine regulation of cortical function, we examined the effects of dopamine depletion on cortical gene regulation and behavioural correlates. Two days after unilateral infusion of 6-hydroxydopamine into the midbrain, rats displayed a (to some degree) bilateral reduction in cortical zif 268 expression that was more pronounced on the lesioned side. This decrease was found across motor, somatosensory, insular and piriform, but not cingulate, cortex, similar to the effects of blockade of striatal D1 receptors. Furthermore, whisker stimulation-evoked c-fos and zif 268 expression in the barrel cortex ipsilateral to the lesion was also attenuated by acute dopamine depletion. These cortical deficits were accompanied by a breakdown of spontaneous behaviours in an open-field test. In contrast, 21 days after dopamine depletion, both basal and sensory-evoked gene expression in the cortex were near-normal. This cortical recovery was paralleled by recovery in locomotion and in sensory-guided behaviour (scanning) related to the hemisphere contralateral to the lesion, but not in scanning by the dopamine-depleted hemisphere. Our results suggest that striatal dopamine exerts a widespread facilitatory influence on cortical function that is necessary, but not sufficient, for normal behaviour. Moreover, the mechanisms mediating this cortical facilitation appear to be subject to substantial neuroplasticity after dopamine perturbation.

Unique responses of limbic met-enkephalin systems to low and high doses of methamphetamine

A single administration of a low (0.5 mg/kg) or high (10 mg/kg) dose of methamphetamine (METH) significantly altered the met-enkephalin (M-Enk) systems associated with some, but not all, limbic structures examined. Neither treatment influenced M-Enk levels 3 h after drug exposure in any limbic region studied; however, 12 h after drug administration, 0.5 mg/kg of METH reduced the tissue content of this peptide in both the nucleus accumbens shell (NAs) and the frontal cortex (FrCx). This was similar to the effect of this treatment on the anterior striatal region. In contrast, the high dose of METH increased M-Enk content in the frontal cortex and anterior striatum (AS), but had no effect in the nucleus accumbens shell. By 24 h, the effects of METH in the anterior striatum subsided, but decreases in M-Enk levels were still observed after both the low- and the high-dose METH treatments in the nucleus accumbens shell. The levels of M-Enk were not changed at any of the time points examined in the core of the nucleus accumbens (NAc). In general, treatment with a low or high dose of METH causes distinct and regional selective changes in the tissue levels of M-Enk in the limbic system. These changes appear to be mediated by dopamine (DA) D(2) and D(1) receptor activation.

Contrasting responses by basal ganglia met-enkephalin systems to low and high doses of methamphetamine in a rat model

The influence of methamphetamine (METH) on basal ganglia met-enkephalin (Menk) was studied by determining levels of this peptide in striatal, pallidal and nigral regions after administering a single low (0.5 mg/kg) or high (10 mg/kg) dose of this stimulant. The Menk levels in the striatal and pallidal areas were reduced and increased after the low- and high-dose METH treatments, respectively, 12 h after drug administration in all striatal and pallidal regions examined. The low-dose effect appeared to be principally influenced by increased activation of the dopamine D2-like receptor, while the high-dose effect seemed to result from dominance of D1-like receptor activation. However, both effects required coactivation of D1- and D2-like receptors. For the most part, both low- and high-dose METH-induced changes in Menk tissue content were fully recovered by 24 h. The Menk levels were not significantly altered in the substantia nigra 3-24 h after either METH treatment. Results reported herein indicated that striatal and pallidal Menk pathways respond differently after acute treatment with low or high doses of METH.

Regulation of rat cortex function by D1 dopamine receptors in the striatum

Interactions between the basal ganglia and the cerebral cortex are critical for normal goal-directed behavior. In the present study, we used immediate-early genes (c-fos, zif 268) as functional markers to investigated how basal ganglia output altered by stimulation/blockade of D1 dopamine receptors in the striatum affects cortical function. Systemic administration of the mixed D1/D2 receptor agonist apomorphine (3 mg/kg) increased immediate-early gene expression in the striatum and throughout most of the cortex. Unilateral intrastriatal infusion of the selective D1 receptor antagonist SCH-23390 (0.5-10 microg) blocked this response bilaterally in striatum and cortex in a dose-dependent manner. Even apparently regionally restricted blockade of striatal D1 receptors attenuated gene expression throughout striatum and cortex in both hemispheres. Intrastriatal administration of the D1 antagonist inhibited apomorphine-induced sniffing/whisking, whereas other motor behaviors were unaffected. To determine whether such changes in cortical gene expression could reflect altered cortical function, we examined the effects of blocking striatal D1 receptors on whisker stimulation-evoked immediate-early gene expression in the sensorimotor cortex. Apomorphine increased sensory stimulation-evoked gene expression in the barrel cortex, and intrastriatal infusion of SCH-23390 attenuated this effect. These results suggest that stimulation of D1 dopamine receptors in the striatum exerts a widespread facilitatory effect on cortical function.