The striatonigral dynorphin pathway of the rat studied with in vivo microdialysis--II. Effects of dopamine D1 and D2 receptor agonists

Naltrexone blocks alcohol-induced effects on kappa-opioid receptors in the plasma membrane

Naltrexone (NTX), a homologue of the opiate antidote naloxone, is an orally active long-acting mu-opioid receptor (MOP) antagonist used in the treatment of opiate dependence. NTX is also found to relieve craving for alcohol and is one of the few FDA-approved drugs for alcohol use disorder (AUD). Reports that NTX blocks the actions of endogenous opioids released by alcohol are not convincing, suggesting that NTX interferes with alcohol actions by affecting opioid receptors. MOP and kappa-opioid receptor (KOP) are structurally related but functionally different. MOP is mainly located in interneurons activated by enkephalins while KOP is located in longer projections activated by dynorphins. While the actions of NTX on MOP are well established, the interaction with KOP and addiction is not well understood. We used sensitive fluorescence-based methods to study the influence of alcohol on KOP and the interaction between KOP and NTX. Here we report that alcohol interacts with KOP and its environment in the plasma membrane. These interactions are affected by NTX and are exerted both on KOP directly and on the plasma membrane (lipid) structures ("off-target"). The actions of NTX are stereospecific. Selective KOP antagonists, recently in early clinical trials for major depressive disorder, block the receptor but do not show the full action profile of NTX. The therapeutic effect of NTX treatment in AUD may be due to direct actions on KOP and the receptor environment.

KOR Control over Addiction Processing: An Exploration of the Mesolimbic Dopamine Pathway

Drug addiction is a complex, persistent, and chronically relapsing neurological disorder exacerbated by acute and chronic stress. It is well known that the dynorphin/kappa opioid receptor (KOR) system regulates stress perception and responsivity, while the mesolimbic dopamine system plays a role in reward and reinforcement associated with alcohol and substance use disorders. Interestingly, the dopamine and dynorphin/KOR systems are highly integrated in mesolimbic areas, with KOR activation leading to inhibition of dopamine release, further altering the perception of reinforcing and aversive stimuli. Chronic or repeated exposure to stress or drugs potentiates KOR function ultimately contributing to a hypodopaminergic state. This hypodopaminergic state is one of the hallmarks of hyperkatifeia, defined as the hypersensitivity to emotional distress that is exacerbated during drug withdrawal and abstinence. The relationship between stress and drug addiction is bidirectional; repeated/chronic stress promotes pro-addictive behaviors, and repeated cycles of drug exposure and withdrawal, across various drug classes, produces stress. Neuroadaptations driven by this bidirectional relationship ultimately influence the perception of the reinforcing value of rewarding stimuli. In this chapter, we address the involvement of the dopamine and dynorphin/KOR systems and their interactions in shaping reinforcement value processing after drug and stress exposure, as well as a combinatorial impact of both drugs and stress.

Progress in opioid reward research: From a canonical two-neuron hypothesis to two neural circuits

Opioid abuse and related overdose deaths continue to rise in the United States, contributing to the national opioid crisis in the USA. The neural mechanisms underlying opioid abuse and addiction are still not fully understood. This review discusses recent progress in basic research dissecting receptor mechanisms and circuitries underlying opioid reward and addiction. We first review the canonical GABA-dopamine neuron hypothesis that was upheld for half a century, followed by major findings challenging this hypothesis. We then focus on recent progress in research evaluating the role of the mesolimbic and nigrostriatal dopamine circuitries in opioid reward and relapse. Based on recent findings that activation of dopamine neurons in the ventral tegmental area (VTA) and substantia nigra pars compacta (SNc) is equally rewarding and that GABA neurons in the rostromedial tegmental nucleus (RMTg) and the substantia nigra pars reticula (SNr) are rich in mu opioid receptors and directly synapse onto midbrain DA neurons, we proposed that the RTMg→VTA → ventrostriatal and SNr → SNc → dorsostriatal pathways may act as the two major neural substrates underlying opioid reward and abuse. Lastly, we discuss possible integrations of these two pathways during initial opioid use, development of opioid abuse and maintenance of compulsive opioid seeking.

Dissecting the Role of GABA Neurons in the VTA versus SNr in Opioid Reward

Opioid reward has traditionally been thought to be mediated by GABA-induced disinhibition of dopamine (DA) neurons in the VTA. However, direct behavioral evidence supporting this hypothesis is still lacking. In this study, we found that the μ opioid receptor (MOR) gene, Oprm1, is highly expressed in GABA neurons, with ∼50% of GABA neurons in the substantia nigra pars reticulata (SNr), ∼30% in the VTA, and ∼70% in the tail of the VTA (also called the rostromedial tegmental nucleus) in male rats. No Oprm1 mRNA was detected in midbrain DA neurons. We then found that optogenetic inhibition of VTA DA neurons reduced intravenous heroin self-administration, whereas activation of these neurons produced robust optical intracranial self-stimulation in DAT-Cre mice, supporting an important role of DA neurons in opioid reward. Unexpectedly, pharmacological blockade of MORs in the SNr was more effective than in the VTA in reducing heroin reward. Optogenetic activation of VTA GABA neurons caused place aversion and inhibited cocaine, but not heroin, self-administration, whereas optogenetic activation of SNr GABA neurons caused a robust increase in heroin self-administration with an extinction pattern, suggesting a compensatory response in drug intake due to reduced heroin reward. In addition, activation of SNr GABA neurons attenuated heroin-primed, but not cue-induced, reinstatement of drug-seeking behavior, whereas inhibition of SNr GABA neurons produced optical intracranial self-stimulation and place preference. Together, these findings suggest that MORs on GABA neurons in the SNr play more important roles in opioid reward and relapse than MORs on VTA GABA neurons.SIGNIFICANCE STATEMENT Opioid reward has long been believed to be mediated by inhibition of GABA interneurons in the VTA that subsequently leads to disinhibition of DA neurons. In this study, we found that more μ opioid receptors (MORs) are expressed in GABA neurons in the neighboring SNr than in the VTA, and that pharmacological blockade of MORs in the SNr is more effective in reducing heroin reward than blockade of MORs in the VTA. Furthermore, optogenetic activation of VTA GABA neurons inhibited cocaine, but not heroin, self-administration, whereas activation of SNr GABA neurons inhibited heroin reward and relapse. These findings suggest that opioid reward is more likely mediated by stimulation of MORs in GABA afferents from other brain regions than in VTA GABA neurons.

Neurometabolic profiles of the substantia nigra and striatum of MPTP-intoxicated common marmosets: An in vivo proton MRS study at 9.4 T

Given the strong coupling between the substantia nigra (SN) and striatum (STR) in the early stage of Parkinson's disease (PD), yet only a few studies reported to date that have simultaneously investigated the neurochemistry of these two brain regions in vivo, we performed longitudinal metabolic profiling in the SN and STR of 1-methyl-1,2,3,6-tetrahydropyridine (MPTP)-intoxicated common marmoset monkey models of PD (n = 10) by using proton MRS (¹ H-MRS) at 9.4 T. T₂ relaxometry was also performed in the SN by using MRI. Data were classified into control, MPTP_2weeks, and MPTP_6-10 weeks groups according to the treatment duration. In the SN, T₂ of the MPTP_6-10 weeks group was lower than that of the control group (44.33 ± 1.75 versus 47.21 ± 2.47 ms, p < 0.05). The N-acetylaspartate to total creatine ratio (NAA/tCr) and γ-aminobutyric acid to tCr ratio (GABA/tCr) of the MPTP_6-10 weeks group were lower than those of the control group (0.41 ± 0.04 versus 0.54 ± 0.08 (p < 0.01) and 0.19 ± 0.03 versus 0.30 ± 0.09 (p < 0.05), respectively). The glutathione to tCr ratio (GSH/tCr) was correlated with T₂ for the MPTP_6-10 weeks group (r = 0.83, p = 0.04). In the STR, however, GABA/tCr of the MPTP_6-10 weeks group was higher than that of the control group (0.25 ± 0.10 versus 0.16 ± 0.05, p < 0.05). These findings may be an in vivo depiction of the altered basal ganglion circuit in PD brain resulting from the degeneration of nigral dopaminergic neurons and disruption of nigrostriatal dopaminergic projections. Given the important role of non-human primates in translational studies, our findings provide better understanding of the complicated evolution of PD.

Loss of the preferential control over the striato-nigral direct pathway by striatal NMDA receptors in a rat model of Parkinson's disease

By using multi-probe microdialysis we previously demonstrated that endogenous glutamate differentially regulates the activity of the striatal output pathways in vivo, through N-methyl-d-aspartate (NMDA) receptors containing the GluN2A or GluN2B subunits. Using the same approach, we presently investigate whether reverse dialysis of NMDA in the striatum differentially affects GABA release in the striatum and in striatal target areas, i.e. globus pallidus (GP) and substantia nigra reticulata (SNr). Moreover, we ask whether this control is altered under parkinsonian conditions. Intrastriatal NMDA perfusion (10 min) evoked GABA release more potently in SNr (1-100 μM) than in other regions (10-100 μM), suggesting preferential control over striato-nigral projection neurons. Intrastriatal NMDA more potently stimulated glutamate levels in the striatum (1-100 μM) and SNr (1-10 μM) than in GP (10 μM). Striatal dopamine denervation with 6-hydroxydopamine caused a leftward shift in the NMDA concentration-response curve. Intrastriatal NMDA elevated GABA levels at 0.1 μM (all regions) and 1 μM (striatum and GP only), but not at higher concentrations, indicating that, compared to naïve animals, the GABA response in SNr was attenuated. Attenuation of the glutamate response was also observed in SNr (NMDA effective only at 0.1 μM). Conversely, the glutamate response in GP was widened (NMDA effective in the 0.1-1 μM range). We conclude that NMDA preferentially stimulates the activity of the striato-nigral direct pathway under physiological conditions. In Parkinson's disease, dopamine loss compromises the NMDA ability to stimulate striato-nigral neurons, thus shifting the NMDA control towards the striato-pallidal ones.

Acupuncture improves locomotor function by enhancing GABA receptor expression in transient focal cerebral ischemia rats

Stroke is the major cause of long-term disability among adults. Recent studies have found that GABAergic inhibitory neurotransmission plays a vital role in ameliorate locomotor damage after ischemic injury. Acupuncture has been widely used to improve locomotor function. However, the underlying mechanisms remain unclear. The present study is designed to investigate whether GABA and GABA receptors are involved in the mechanism underlying acupuncture treatment in rats with middle cerebral artery occlusion (MCAO). One week after acupuncture at JiaJi acupoint, the locomotor function and infarct volumes were tested. Then level of GABA and the expressions of GABAAγ2 and GABABR2 were assessed by high-performance liquid chromatography, immunofluorescence and immunohistochemistry, respectively. Compared with normal group, GABAAγ2 and GABABR2 expressions were decreased in striatum and spinal cord of the MCAO group. After acupuncture, the expressions of the two receptors were increased, but levels of GABA and trafficking protein, kinesin binding 1 (TRAK1), which plays a role in the intracellular transport of GABA receptors, were unchanged. The present study suggests that acupuncture could reverse locomotor function by modulating the expressions of GABA receptors in MCAO rats.

Gamma aminobutyric acidergic and neuronal structural markers in the nucleus accumbens core underlie trait-like impulsive behavior

BACKGROUND

Pathological forms of impulsivity are manifest in a number of psychiatric disorders listed in DSM-5, including attention-deficit/hyperactivity disorder and substance use disorder. However, the molecular and cellular substrates of impulsivity are poorly understood. Here, we investigated a specific form of motor impulsivity in rats, namely premature responding, on a five-choice serial reaction time task.

METHODS

We used in vivo voxel-based magnetic resonance imaging and ex vivo Western blot analyses to investigate putative structural, neuronal, and glial protein markers in low-impulsive (LI) and high-impulsive rats. We also investigated whether messenger RNA interference targeting glutamate decarboxylase 65/67 (GAD65/67) gene expression in the nucleus accumbens core (NAcbC) is sufficient to increase impulsivity in LI rats.

RESULTS

We identified structural and molecular abnormalities in the NAcbC associated with motor impulsivity in rats. We report a reduction in gray matter density in the left NAcbC of high-impulsive rats, with corresponding reductions in this region of glutamate decarboxylase (GAD65/67) and markers of dendritic spines and microtubules. We further demonstrate that the experimental reduction of de novo of GAD65/67 expression bilaterally in the NAcbC is sufficient to increase impulsivity in LI rats.

CONCLUSIONS

These results reveal a novel mechanism of impulsivity in rats involving gamma aminobutyric acidergic and structural abnormalities in the NAcbC with potential relevance to the etiology and treatment of attention-deficit/hyperactivity disorder and related disorders.

Effects of 5-HT1A receptor stimulation on D1 receptor agonist-induced striatonigral activity and dyskinesia in hemiparkinsonian rats

Accumulating evidence supports the value of 5-HT1A receptor (5-HT1AR) agonists for dyskinesias that arise with long-term L-DOPA therapy in Parkinson's disease (PD). Yet, how 5-HT1AR stimulation directly influences the dyskinetogenic D1 receptor (D1R)-expressing striatonigral pathway remains largely unknown. To directly examine this, one cohort of hemiparkinsonian rats received systemic injections of Vehicle + Vehicle, Vehicle + the D1R agonist SKF81297 (0.8 mg/kg), or the 5-HT1AR agonist ±8-OH-DPAT (1.0 mg/kg) + SKF81297. Rats were examined for changes in abnormal involuntary movements (AIMs), rotations, striatal preprodynorphin (PPD), and glutamic acid decarboxylase (GAD; 65 and 67) mRNA via RT-PCR. In the second experiment, hemiparkinsonian rats received intrastriatal pretreatments of Vehicle (aCSF), ±8-OH-DPAT (7.5 mM), or ±8-OH-DPAT + the 5-HT1AR antagonist WAY100635 (4.6 mM), followed by systemic Vehicle or SKF81297 after which AIMs, rotations, and extracellular striatal glutamate and nigral GABA efflux were measured by in vivo microdialysis. Results revealed D1R agonist-induced AIMs were reduced by systemic and intrastriatal 5-HT1AR stimulation while rotations were enhanced. Although ±8-OH-DPAT did not modify D1R agonist-induced increases in striatal PPD mRNA, the D1R/5-HT1AR agonist combination enhanced GAD65 and GAD67 mRNA. When applied locally, ±8-OH-DPAT alone diminished striatal glutamate levels while the agonist combination increased nigral GABA efflux. Thus, presynaptic 5-HT1AR stimulation may attenuate striatal glutamate levels, resulting in diminished D1R-mediated dyskinetic behaviors, but maintain or enhance striatal postsynaptic factors ultimately increasing nigral GABA levels and rotational activity. The current findings offer a novel mechanistic explanation for previous results concerning 5-HT1AR agonists for the treatment of dyskinesia.

Presynaptic regulation of extracellular dopamine levels in the medial prefrontal cortex and striatum during tyrosine depletion

RATIONALE

Available neurochemical probes that lower brain dopamine (DA) levels in man are limited by their tolerability and efficacy. For instance, the acute lowering of brain tyrosine is well tolerated, but only modestly lowers brain DA levels. Modification of tyrosine depletion to robustly lower DA levels would provide a superior research probe.

OBJECTIVES

The objective of this study was to determine whether the subthreshold stimulation of presynaptic DA receptors would potentiate tyrosine depletion-induced effects on extracellular DA levels in the medial prefrontal cortex (MPFC) and striatum of the rat.

METHODS

We administered quinpirole, a predominantly DA type 2 (D2R) receptor agonist, into the MPFC and striatum by reverse dialysis. A tyrosine- and phenylalanine-free neutral amino acid mixture [NAA(-)] IP was used to lower brain tyrosine levels. DA levels in the microdialysate were measured by HPLC with electrochemical detection.

RESULTS

Quinpirole dose-dependently lowered DA levels in MPFC as well as in the striatum. NAA(-) alone transiently lowered DA levels (80 % baseline) in the striatum, but had no effect in MPFC. The co-administration of NAA(-) and a subthreshold concentration of quinpirole (6.25 nM) lowered DA levels (50 % baseline) in both the MPFC and striatum. This effect was blocked by the mixed D2R/D3R antagonist haloperidol at IP doses that on their own did not affect DA levels (10.0 nmol/kg in the MPFC and 0.10 nmol/kg in the striatum).

CONCLUSIONS

Pharmacological stimulation of inhibitory D2R receptors during tyrosine depletion markedly lowers the extracellular DA levels in the MPFC and striatum. The data suggest that combining tyrosine depletion with a low dose of a DA agonist should robustly lower brain regional DA levels in man.

Thinking outside the cleft to understand synaptic activity: contribution of the cystine-glutamate antiporter (System xc-) to normal and pathological glutamatergic signaling

System x(c)(-) represents an intriguing target in attempts to understand the pathological states of the central nervous system. Also called a cystine-glutamate antiporter, system x(c)(-) typically functions by exchanging one molecule of extracellular cystine for one molecule of intracellular glutamate. Nonvesicular glutamate released during cystine-glutamate exchange activates extrasynaptic glutamate receptors in a manner that shapes synaptic activity and plasticity. These findings contribute to the intriguing possibility that extracellular glutamate is regulated by a complex network of release and reuptake mechanisms, many of which are unique to glutamate and rarely depicted in models of excitatory signaling. Because system x(c)(-) is often expressed on non-neuronal cells, the study of cystine-glutamate exchange may advance the emerging viewpoint that glia are active contributors to information processing in the brain. It is noteworthy that system x(c)(-) is at the interface between excitatory signaling and oxidative stress, because the uptake of cystine that results from cystine-glutamate exchange is critical in maintaining the levels of glutathione, a critical antioxidant. As a result of these dual functions, system x(c)(-) has been implicated in a wide array of central nervous system diseases ranging from addiction to neurodegenerative disorders to schizophrenia. In the current review, we briefly discuss the major cellular components that regulate glutamate homeostasis, including glutamate release by system x(c)(-). This is followed by an in-depth discussion of system x(c)(-) as it relates to glutamate release, cystine transport, and glutathione synthesis. Finally, the role of system x(c)(-) is surveyed across a number of psychiatric and neurodegenerative disorders.

Differences in basal and ethanol-induced levels of opioid peptides in Wistar rats from five different suppliers

One major cause for discrepancies in results from animal experimental studies is the use of different animal strains and suppliers. We have previously reported that Wistar rats from five different suppliers display profound differences in ethanol intake and behavior. One of the neurobiological processes that could be underlying these differences is the endogenous opioid system, which has been implicated in the rewarding and reinforcing effects of alcohol. We therefore hypothesized that the differences between the supplier groups would also be evident in the endogenous opioid system. Radioimmunoassay was used to determine the levels of the opioid peptides Met-enkephalin-Arg(6)Phe(7) and dynorphin B in several brain areas of ethanol-drinking and ethanol naïve Wistar rats from five different suppliers. In the ethanol naïve animals, differences between the supplier groups were found in the pituitary gland, hypothalamus, frontal cortex, dorsal striatum and hippocampus. In the ethanol-drinking rats, differences were found in the same structures, with the addition of medial prefrontal cortex and substantia nigra. Correlations between ethanol intake and peptide levels were also found in several of the areas examined. The structures in which differences were found have all been implicated in the transition from drug use to addiction and these differences may lead to different propensities and vulnerability to this transition. Because the endogenous opioids have been suggested to be involved in a number of neurobiological disorders the results do not only have implications for research on alcohol or drug addiction, but many other fields as well.

30 years of dynorphins--new insights on their functions in neuropsychiatric diseases

Since the first description of their opioid properties three decades ago, dynorphins have increasingly been thought to play a regulatory role in numerous functional pathways of the brain. Dynorphins are members of the opioid peptide family and preferentially bind to kappa opioid receptors. In line with their localization in the hippocampus, amygdala, hypothalamus, striatum and spinal cord, their functions are related to learning and memory, emotional control, stress response and pain. Pathophysiological mechanisms that may involve dynorphins/kappa opioid receptors include epilepsy, addiction, depression and schizophrenia. Most of these functions were proposed in the 1980s and 1990s following histochemical, pharmacological and electrophysiological experiments using kappa receptor-specific or general opioid receptor agonists and antagonists in animal models. However, at that time, we had little information on the functional relevance of endogenous dynorphins. This was mainly due to the complexity of the opioid system. Besides actions of peptides from all three classical opioid precursors (proenkephalin, prodynorphin, proopiomelanocortin) on the three classical opioid receptors (delta, mu and kappa), dynorphins were also shown to exert non-opioid effects mainly through direct effects on NMDA receptors. Moreover, discrepancies between the distribution of opioid receptor binding sites and dynorphin immunoreactivity contributed to the difficulties in interpretation. In recent years, the generation of prodynorphin- and opioid receptor-deficient mice has provided the tools to investigate open questions on network effects of endogenous dynorphins. This article examines the physiological, pathophysiological and pharmacological implications of dynorphins in the light of new insights in part obtained from genetically modified animals.

Housing conditions modulate escitalopram effects on antidepressive-like behaviour and brain neurochemistry

Despite limited understanding of the pathophysiology of depression and the underlying mechanisms mediating antidepressant effects, there are several efficient treatments. The anhedonia symptoms of depression are characterized by decreased motivation and drive and imply possible malfunctioning of the mesolimbic dopamine system, whereas cognitive deficits might reflect decreased plasticity in hippocampus. In female Flinders Sensitive Line (FSL) rats, a model of depression, we compared the effects of three long-term antidepressant treatments: voluntary running, escitalopram and the combination of both on antidepressant-like behaviour in the Porsolt swim test (PST), and on regulation of mRNA for dopamine and neuropeptides in striatal dopamine pathways and brain-derived neurotrophic factor (BDNF) in hippocampus. Escitalopram diet attenuated running behaviour in FSL rats but not in non-depressed controls rats. In the PST the running group had increased climbing activity (noradrenergic/dopaminergic response), whereas the combination of escitalopram and running-wheel access increased swimming (serotonergic response). Running elevated mRNA for dynorphin in caudate putamen and BDNF in hippocampus. The combined treatment down-regulated D1 receptor and enkephalin mRNA in accumbens. Escitalopram alone did not affect behaviour or mRNA levels. We demonstrate a novel behavioural effect of escitalopram, i.e. attenuation of running in 'depressed' rats. The antidepressant-like effect of escitalopram was dependent on the presence of a running wheel, but not actual running indicating that the environment influenced the antidepressant effect of escitalopram. Different patterns of mRNA changes in hippocampus and brain reward pathways and responses in the PST by running and escitalopram suggest that antidepressant-like responses by running and escitalopram are achieved by different mechanisms.

Amphetamine-induced locomotion and gene expression are altered in BDNF heterozygous mice

Administration of amphetamine overstimulates medium spiny neurons (MSNs) by releasing dopamine and glutamate from afferents in the striatum. However, these afferents also release brain-derived neurotrophic factor (BDNF) that protects striatal MSNs from overstimulation. Intriguingly, all three neurochemicals increase opioid gene expression in MSNs. In contrast, striatal opioid expression is less in naive BDNF heterozygous (BDNF(+/-)) vs. wild-type (WT) mice. This study was designed to determine whether partial genetic depletion of BDNF influences the behavioral and molecular response to an acute amphetamine injection. An acute injection of amphetamine [5 mg/kg, intraperitoneal (i.p.)] or saline was administered to WT and BDNF(+/-) mice. WT and BDNF(+/-) mice exhibited similar locomotor activity during habituation, whereas BDNF(+/-) mice exhibited more prolonged locomotor activation during the third hour after injection of amphetamine. Three hours after amphetamine injection, there was an increase of preprodynorphin mRNA in the caudate putamen and nucleus accumbens (Acb) and dopamine D(3) receptor mRNA levels were increased in the Acb of BDNF(+/-) and WT mice. Striatal/cortical trkB and BDNF, and mesencephalic tyrosine hydroxylase mRNA levels were only increased in WT mice. These results indicate that BDNF modifies the locomotor responses of mice to acute amphetamine and differentially regulates amphetamine-induced gene expression.

Time-course of SKF-81297-induced increase in glutamic acid decarboxylase 65 and 67 mRNA levels in striatonigral neurons and decrease in GABA(A) receptor alpha1 subunit mRNA levels in the substantia nigra, pars reticulata, in adult rats with a unilateral 6-hydroxydopamine lesion

Striatal projection neurons use GABA as their neurotransmitter and express the rate-limiting synthesizing enzyme glutamic acid decarboxylase (GAD) and the vesicular GABA transporter vGAT. The chronic systemic administration of an agonist of dopamine D1/D5-preferring receptors is known to alter GAD mRNA levels in striatonigral neurons in intact and dopamine-depleted rats. In the present study, the effects of a single or subchronic systemic administration of the dopamine D1/D5-preferring receptor agonist SKF-81297 on GAD65, GAD67, PPD and vGAT mRNA levels in the striatum and GABA(A) receptor alpha1 subunit mRNA levels in the substantia nigra, pars reticulata, were measured in rats with a unilateral 6-hydroxydopamine (6-OHDA) lesion. After a single injection of SKF-81297, striatal GAD65 mRNA levels were significantly increased at 3 but not 72 h. In contrast, striatal GAD67 mRNA levels were increased and nigral alpha1 mRNA levels were decreased at 72 but not 3 h. Single cell analysis on double-labeled sections indicated that increased GAD or vGAT mRNA levels after acute SKF-81297 occurred in striatonigral neurons identified by their lack of preproenkephalin expression. Subchronic SKF-81297 induced significant increases in striatal GAD67, GAD65, preprodynorphin and vGAT mRNA levels and decreases in nigral alpha1 mRNA levels. In the striatum contralateral to the 6-OHDA lesion, subchronic but not acute SKF-81297 induced a significant increase in GAD65 mRNA levels. The other mRNA levels were not significantly altered. Finally, striatal GAD67 mRNA levels were negatively correlated with nigral alpha1 mRNA levels in the dopamine-depleted but not dopamine-intact side. The results suggest that different signaling pathways are involved in the modulation by dopamine D1/D5 receptors of GAD65 and GAD67 mRNA levels in striatonigral neurons. They also suggest that the down-regulation of nigral GABA(A) receptors is linked to the increase in striatal GAD67 mRNA levels in the dopamine-depleted striatum.

Expression and function of CB1 receptor in the rat striatum: localization and effects on D1 and D2 dopamine receptor-mediated motor behaviors

Cannabinoid CB1 receptors are densely expressed on striatal projection neurons expressing dopamine D1 or D2 receptors. However, the specific neuronal distribution of CB1 receptors within the striatum is not known. Previous research has established that the endocannabinoid system controls facilitation of behavior by dopamine D2 receptors, but it is not clear if endocannabinoids also modulate D1 receptor-mediated motor behavior. In the present study, we show that cannabinoid CB1 receptor mRNA is present in striatonigral neurons expressing substance P and dopamine D1 receptors, as well as in striatopallidal neurons expressing enkephalin and dopamine D2 receptors. We explored the functional relevance of the interaction between dopamine D1 and D2 receptors and cannabinoid CB1 receptors with behavioral pharmacology experiments. Potentiation of endogenous cannabinoid signaling by the uptake blocker AM404 blocked dopamine D1 receptor-mediated grooming and D2 receptor-mediated oral stereotypies. In addition, contralateral turning induced by unilateral intrastriatal infusion of D1 receptor agonists is counteracted by AM404 and potentiated by the cannabinoid antagonist SR141716A. These results indicate that the endocannabinoid system negatively modulates D1 receptor-mediated behaviors in addition to its previously described effect on dopamine D2 receptor-mediated behaviors. The effect of AM404 on grooming behavior was absent in dopamine D1 receptor knockout mice, demonstrating its dependence on D1 receptors. This study indicates that the endocannabinoid system is a relevant negative modulator of both dopamine D1 and D2 receptor-mediated behaviors, a finding that may contribute to our understanding of basal ganglia motor disorders.

Dendritic transmitter release: a comparison of two model systems

Information flow through neurones was historically considered to be linear, with dendrites receiving information from incoming synaptic terminals, the soma processing the information and the axon carrying it to the terminal that synapses upon another cell or end organ. However, recent studies have shown that dendrites can release transmitters themselves, and thereby communicate with neighbouring structures, whether these are adjacent neurones or incoming synapses. Due to their anatomical features, the magnocellular vasopressin and oxytocin containing neurones of the hypothalamic supraoptic and paraventricular nuclei and the dopamine neurones of the substantia nigra have revealed important aspects of dendritic function including mechanisms of dendritic transmitter release.

Acute amphetamine exposure selectively desensitizes kappa-opioid receptors in the nucleus accumbens

In the present study, we investigated the effects of psychostimulant exposure on kappa-opioid peptide (KOP) receptor signaling in the rat mesolimbic system. A single subcutaneous (s.c.) injection of amphetamine (2.5 mg/kg) reduced the KOP receptor-mediated inhibition of glutamate release in the nucleus accumbens shell, as a consequence of KOP receptor desensitization. This effect was blocked by dopamine (DA) receptor antagonists or the nonselective opioid antagonist, naltrexone (1 mg/kg, s.c.), and mimicked by the KOP receptor agonists U69593 (0.32 mg/kg, s.c.) and dynorphin (1 microM), indicating that an amphetamine-induced release of dynorphin is producing a long-lasting desensitization of the KOP receptor. Despite the fact that amphetamine also increases dynorphin release in the ventral tegmental area (VTA), KOP receptor function in this region was not affected by amphetamine; there was no difference in the KOP receptor-mediated change in firing rate or resting membrane potential measured in VTA neurons from saline- or amphetamine-treated animals. This study demonstrates that amphetamine can produce regionally selective adaptations in KOP receptor signaling, which may, in turn, alter the effects of subsequent drug exposure.

Cotreatment with the kappa opioid agonist U69593 enhances locomotor sensitization to the D2/D3 dopamine agonist quinpirole and alters dopamine D2 receptor and prodynorphin mRNA expression in rats

RATIONALE

The repeated coadministration of the kappa opioid receptor agonist U69593 with the D2/D3 dopamine (DA) agonist quinpirole (QNP) potentiates locomotor sensitization induced by QNP. Behavioral evidence has implicated both pre- and postsynaptic changes as being involved in this augmentation.

OBJECTIVES

The objectives of this study were to obtain supporting molecular evidence of pre- and/or postsynaptic alterations in the DA system with U69593/QNP cotreatment and to examine the relationship of such changes to locomotor sensitization.

MATERIALS AND METHODS

Gene expression of D1 and D2 receptors (D1R and D2R), the DA transporter, as well as the endogenous opioid prodynorphin (DYN), in the basal ganglia was examined by in situ hybridization in rats after one or ten drug injections.

RESULTS

After one injection, changes that were specific to U69593/QNP cotreatment were decreased D1R and D2R messenger RNA (mRNA) in the nucleus accumbens (Acb) shell and increased DYN mRNA in the dorsal striatum (STR). After ten injections, U69593/QNP-specific changes were decreased D2R mRNA in substantia nigra (SN) and increased DYN mRNA in STR and Acb core. Only in U69593/QNP rats was the sensitized locomotor performance on injection ten positively correlated with DYN mRNA levels in Acb and STR.

CONCLUSIONS

Distinct alterations of D2R and DYN mRNA levels in SN and Acb/STR, respectively, strengthen the evidence implicating pre- and postsynaptic changes in augmented locomotor sensitization to U69593/QNP cotreatment. It is suggested that repeated U69593/QNP cotreatment may augment locomotor sensitization to QNP by activating D1R-expressing DYN neurons and attenuating presynaptic D2R function.

Exploring neurocircuitries of the basal ganglia by intracerebral administration of selective neurotoxins

The detailed anatomy of the monoamine pathways of the rat, first described by the students of Nils Ake Hillarp in Sweden, provided the basis for a neurocircuitry targeted pharmacology, leading to important therapeutic breakthroughs. Progress was achieved by the introduction of accurate lesion techniques based on selective neurotoxins. Systematic intracerebral injections of 6-hydroxydopamine let Urban Ungerstedt at the Karolinska Institutet, Stockholm, Sweden, to propose the first stereotaxic mapping of the monoamine pathways in the rat brain; and the 'Rotational Behaviour', as a classical model for screening drugs useful for alleviating Parkinson's disease and other neuropathologies. The direction of the rotational behaviour induced by drugs administrated to unilaterally 6-hydroxydopamine-lesioned rats reveals their mechanism of action at dopamine synapses, as demonstrated when rotational behaviour was combined with microdialysis. The model was useful for proposing a role for dopamine receptors in the gating of the flow of information integrated and/or modulated by the basal ganglia, through different efferent pathways; notably the striatopallidal system, via D(2) receptors, and the striatonigral system, via D(1) receptors. The role of other dopamine receptor subtypes on rotational behaviour has not yet been clarified.

Activation of nigral and pallidal dopamine D1-like receptors modulates basal ganglia outflow in monkeys

Studies of the effects of dopamine in the basal ganglia have focused on the striatum, whereas the functions of dopamine released in the internal pallidal segment (GPi) or in the substantia nigra pars reticulata (SNr) have received less attention. Anatomic and biochemical investigations have demonstrated the presence of dopamine D1-like receptors (D1LRs) in GPi and SNr, which are primarily located on axons and axon terminals of the GABAergic striatopallidal and striatonigral afferents. Our experiments assessed the effects of D1LR ligands in GPi and SNr on local gamma-aminobutyric acid (GABA) levels and neuronal activity in these nuclei in rhesus monkeys. Microinjections of the D1LR receptor agonist SKF82958 into GPi and SNr significantly reduced discharge rates in GPi and SNr, whereas injections of the D1LR antagonist SCH23390 increased firing in the majority of GPi neurons. D1LR activation also increased bursting and oscillations in neuronal discharge in the 3- to 15-Hz band in both structures, whereas D1LR blockade had the opposite effects in GPi. Microdialysis measurements of GABA concentrations in GPi and SNr showed that the D1LR agonist increased the level of the transmitter. Both findings are compatible with the hypothesis that D1LR activation leads to GABA release from striatopallidal or striatonigral afferents, which may secondarily reduce firing of basal ganglia output neurons. The antagonist experiments suggest that a dopaminergic "tone" exists in GPi. Our results support the finding that D1LR activation may have powerful effects on GPi and SNr neurons and may mediate some of the effects of dopamine replacement therapies in Parkinson's disease.

Striatal dysfunction increases basal ganglia output during motor cortex activation in parkinsonian rats

During movement, inhibitory neurons in the basal ganglia output nuclei show complex modulations of firing, which are presumptively driven by corticostriatal and corticosubthalamic input. Reductions in discharge should facilitate movement by disinhibiting thalamic and brain stem nuclei while increases would do the opposite. A proposal that nigrostriatal dopamine pathway degeneration disrupts trans-striatal pathways' balance resulting in sustained overactivity of basal ganglia output nuclei neurons and Parkinson's disease clinical signs is not fully supported by experimental evidence, which instead shows abnormal synchronous oscillatory activity in animal models and patients. Yet, the possibility that variation in motor cortex activity drives transient overactivity in output nuclei neurons in parkinsonism has not been explored. In Sprague-Dawley rats with 6-hydroxydopamine (6-OHDA)-induced nigrostriatal lesions, approximately 50% substantia nigra pars reticulata (SNpr) units show abnormal cortically driven slow oscillations of discharge. Moreover, these units selectively show abnormal responses to motor cortex stimulation consisting in augmented excitations of an odd latency, which overlapped that of inhibitory responses presumptively mediated by the trans-striatal direct pathway in control rats. Delivering D1 or D2 dopamine agonists into the striatum of parkinsonian rats by reverse microdialysis reduced these abnormal excitations but had no effect on pathological oscillations. The present study establishes that dopamine-deficiency related changes of striatal function contribute to producing abnormally augmented excitatory responses to motor cortex stimulation in the SNpr. If a similar transient overactivity of basal ganglia output were driven by motor cortex input during movement, it could contribute to impeding movement initiation or execution in Parkinson's disease.

Unilateral 6-hydroxydopamine lesion of dopamine neurons and subchronic L-DOPA administration in the adult rat alters the expression of the vesicular GABA transporter in different subsets of striatal neurons and in the substantia nigra, pars reticulata

The loss of dopamine neurons combined or not with the subsequent administration of L-DOPA in patients with Parkinson's disease or in experimental models of the disease results in altered GABAergic signaling throughout the basal ganglia, including the striatum and the substantia nigra, pars reticulata. However, the molecular mechanisms involved in altered GABA neurotransmission remain poorly understood. In order to be released from synaptic vesicles, newly synthesized GABA is transported from the cytosol into synaptic vesicles by a vesicular GABA transporter. The objective of this study was to examine the hypothesis that expression of the vesicular GABA transporter (vGAT) is altered in the unilateral 6-hydroxydopamine model of Parkinson's disease. Our results provide evidence that a unilateral 6-hydroxydopamine lesion results in increased and decreased vGAT mRNA levels in striatopallidal and striatonigral neurons, respectively. These two subsets of neurons were identified by the co-expression or lack of co-expression of preproenkephalin, a marker of striatopallidal neurons, using double-labeling in situ hybridization histochemistry. Such changes occurred in the striatum ipsilateral to the 6-hydroxydopamine lesion and were paralleled by decreased vGAT protein levels in the substantia nigra, pars reticulate (SNr). On the other hand, the subchronic systemic administration of L-DOPA increased vGAT mRNA levels in preproenkephalin-negative neurons on the side ipsilateral and, to a lesser extent, the side contralateral to the 6-hydroxydopamine lesion. Systemic L-DOPA also increased vGAT protein levels in the ipsi- and contralateral SNr. As a whole, the results provide original evidence that vGAT expression is altered in the 6-hydroxydopamine model of Parkinson's disease. They also suggest that the behavioral effects induced by a subchronic administration of L-DOPA to 6-hydroxydopamine-lesioned rats involve an increase in the vesicular release of GABA by striatonigral neurons.

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.

Subchronic administration of l-DOPA to adult rats with a unilateral 6-hydroxydopamine lesion of dopamine neurons results in a sensitization of enhanced GABA release in the substantia nigra, pars reticulata

L-DOPA is the most effective pharmacological agent used for the symptomatic treatment of Parkinson's disease but long-term L-DOPA treatment induces involuntary abnormal movements such as dyskinesias. The present study, using in vivo microdialysis, investigated the effects of a single or subchronic administration of L-DOPA to adult rats with a unilateral 6-OHDA lesion of dopamine neurons on GABA release in the substantia nigra, pars reticulata (SNr). The results indicate that a challenge injection of L-DOPA (50 mg/kg, i.p.) significantly increases GABA levels in the SNr of rats treated with a daily repeated administration of L-DOPA (50 mg/kg, i.p.). Further statistical analysis between groups also showed that extracellular GABA levels were significantly higher in the subchronic L-DOPA group than in the group receiving only one injection of L-DOPA. These results show that the subchronic administration of L-DOPA results in a sensitization of enhanced extracellular GABA levels in the SNr.

Food intake and reward mechanisms in patients with schizophrenia: implications for metabolic disturbances and treatment with second-generation antipsychotic agents

Obesity is highly prevalent among patients with schizophrenia and is associated with detrimental health consequences. Although excessive consumption of fast food and pharmacotherapy with such second-generation antipsychotic agents (SGAs) as clozapine and olanzapine has been implicated in the schizophrenia/obesity comorbidity, the pathophysiology of this link remains unclear. Here, we propose a mechanism based on brain reward function, a relevant etiologic factor in both schizophrenia and overeating. A comprehensive literature search on neurobiology of schizophrenia and of eating behavior was performed. The collected articles were critically reviewed and relevant data were extracted and summarized within four key areas: (1) energy homeostasis, (2) food reward and hedonics, (3) reward function in schizophrenia, and (4) metabolic effects of the SGAs. A mesolimbic hyperdopaminergic state may render motivational/incentive reward system insensitive to low salience/palatability food. This, together with poor cognitive control from hypofunctional prefrontal cortex and enhanced hedonic impact of food, owing to exaggerated opioidergic drive (clinically manifested as pain insensitivity), may underlie unhealthy eating habits in patients with schizophrenia. Treatment with SGAs purportedly improves dopamine-mediated reward aspects, but at the cost of increased appetite and worsened or at least not improved opiodergic capacity. These effects can further deteriorate eating patterns. Pathophysiological and therapeutic implications of these insights need further validation via prospective clinical trials and neuroimaging studies.

A Microdialysis Profile of Dynorphin A1-8 Release in the Rat Nucleus Accumbens Following Alcohol Administration

BACKGROUND

Pharmacological studies have implicated the endogenous opioid system in mediating alcohol intake. Other evidence has shown that alcohol administration can influence endorphinergic and enkephalinergic activity, while very few studies have examined its effect on dynorphinergic systems. The aim of the present study was to investigate the effect of alcohol administration or a mechanical stressor on extracellular levels of dynorphin A(1-8) in the rat nucleus accumbens-a brain region that plays a significant role in the processes underlying reinforcement and stress.

METHODS

Male Sprague-Dawley rats were implanted with a microdialysis probe aimed at the shell region of the nucleus accumbens. Artificial cerebrospinal fluid was pumped at a rate of 1.5 microL/min in awake and freely moving animals and the dialysate was collected at 30-minute intervals. In one experiment, following a baseline period, rats were injected intraperitoneally with either physiological saline or 1 of 3 doses of alcohol, 0.8, 1.6, or 3.2 g ethanol/kg body weight. In a second experiment, following a baseline period, rats were applied a clothespin to the base of their tail for 20 minutes. The levels of dynorphin A(1-8) in the dialysate were analyzed with solid-phase radioimmunoassay.

RESULTS

Relative to saline-treated controls, an alcohol dose of 1.6 and 3.2 g/kg caused a transient increase in the extracellular levels of dynorphin A(1-8) in the first 30 minutes of alcohol administration. However, the effect resulting from the high 3.2 g/kg dose was far more pronounced and more significant than with the moderate dose. There was no effect of tail pinch on dynorphin A(1-8) levels in the nucleus accumbens.

CONCLUSIONS

In this experiment, a very high dose of alcohol was especially capable of stimulating dynorphin A(1-8) release in the nucleus accumbens. Dynorphin release in the accumbens has been previously associated with aversive stimuli and may thus reflect a system underlying the aversive properties of high-dose alcohol administration. However, the lack of effect of tail-pinch stress in the present study suggests that dynorphin A(1-8) is not released in response to all forms of stressful/aversive stimuli.

Functional interactions between somatodendritic dopamine release, glutamate receptors and brain-derived neurotrophic factor expression in mesencephalic structures of the brain

Dopaminergic nigrostriatal neurons may be considered as bipolar functional entities since they are endowed with the ability to synthesize, store and release the transmitter dopamine (DA) at the somatodendritic level in the substantia nigra (SN). Such dendritic DA release seems to be distinct from the transmitter release occurring at the axon terminal and seems to rely preferentially on volume transmission to exert its physiological effects. An increased glutamatergic (Gluergic) transmission into the SN facilitates such dendritic DA release via activation of NMDA-receptors (NMDA-Rs) and to a lesser extent through group II metabotropic glutamate receptors (mGluRs). In addition, nigral mGluRs functionally interact with NMDA-Rs in the SN, further modulating the NMDA-R-mediated increase of DA release from dendrites in the SN. In turn, dendritically released DA may exert, via D1 receptors, a tonic inhibitory control upon nigral glutamate (Glu). Furthermore, released DA, via D2/D3 autoreceptors, produces an autoinhibitory effect upon DA cell firing and its own release process. An increased Gluergic transmission into the SN may also induce, via activation of NMDA-Rs, an augmented expression of different brain-derived neurotrophic factor (BDNF) gene transcripts in this brain area. Pharmacological evidence suggests that non-NMDA-Rs could also participate in the regulation of BDNF gene expression in the SN. Glu-mediated changes of nigral BDNF expression could regulate, in turn, the expression of important transmitter-related proteins in the SN, such as different NMDA-R subunits, mGluRs and DA-D3 receptors. In conclusion, Glu-DA-BDNF interactions in the SN may play an important role in modulating the flow of neuronal information in this brain structure under normal conditions, as well as during adaptive and plastic responses associated with various neurological and psychiatric disorders.

Expression of opioid peptides and receptors in striatum and substantia nigra during rat brain development

We have used highly sensitive in situ hybridization to determine opioid receptor and peptide expression in embryonic and postnatal rat striatum, to follow the compartmentalization into patch and matrix structures, and have examined their developmental expression in the dopaminergic cell group of the substantia nigra (SN). Furthermore, opioid receptor binding sites were characterized in adjacent sections using highly selective ligands for the opioid receptor subtypes. The major findings of the study are: (1) striatal patches were first delineated by prodynorphin mRNA followed by mu opioid receptor mRNA expression at embryonic days 19 and 21, respectively; (2) in neonates, prodynorphin, mu and kappa opioid receptor mRNAs were transiently co-distributed within patches; (3) prodynorphin mRNA was co-expressed with mu but not kappa, receptor mRNA in neonatal patch neurons; (4) in the SN, kappa receptor and prodynorphin mRNAs were detected as early as embryonic days 15 and 19, respectively; (5) kappa receptor, but not prodynorphin, mRNA was expressed in dopaminergic neurons in the SN. The anatomical results are in agreement with the hypothesis that the endogenous opioid system has a trophic role during the development of striatal patch and matrix compartments and suggest the early regulation of dopamine release by kappa opioid receptors.

The origin and neuronal function of in vivo nonsynaptic glutamate

Basal extracellular glutamate sampled in vivo is present in micromolar concentrations in the extracellular space outside the synaptic cleft, and neither the origin nor the function of this glutamate is known. This report reveals that blockade of glutamate release from the cystine-glutamate antiporter produced a significant decrease (60%) in extrasynaptic glutamate levels in the rat striatum, whereas blockade of voltage-dependent Na+ and Ca2+ channels produced relatively minimal changes (0-30%). This indicates that the primary origin of in vivo extrasynaptic glutamate in the striatum arises from nonvesicular glutamate release by the cystine-glutamate antiporter. By measuring [35S]cystine uptake, it was shown that similar to vesicular release, the activity of the cystine-glutamate antiporter is negatively regulated by group II metabotropic glutamate receptors (mGluR2/3) via a cAMP-dependent protein kinase mechanism. Extracellular glutamate derived from the antiporter was shown to regulate extracellular levels of glutamate and dopamine. Infusion of the mGluR2/3 antagonist (RS)-1-amino-5-phosphonoindan-1-carboxylic acid (APICA) increased extracellular glutamate levels, and previous blockade of the antiporter prevented the APICA-induced rise in extracellular glutamate. This suggests that glutamate released from the antiporter is a source of endogenous tone on mGluR2/3. Blockade of the antiporter also produced an increase in extracellular dopamine that was reversed by infusing the mGluR2/3 agonist (2R,4R)-4-aminopyrrolidine-2,4-dicarboxlylate, indicating that antiporter-derived glutamate can modulate dopamine transmission via mGluR2/3 heteroreceptors. These results suggest that nonvesicular release from the cystine-glutamate antiporter is the primary source of in vivo extracellular glutamate and that this glutamate can modulate both glutamate and dopamine transmission.

Effect of single and repeated methamphetamine treatment on neurotransmitter release in substantia nigra and neostriatum of the rat

The main purpose of this study was to characterize the initial neurotransmission cascade elicited by methamphetamine, analysing simultaneously with in vivo microdialysis monoamine, amino acid and neuropeptide release in substantia nigra and neostriatum of the rat. The main effect of a single systemic dose of methamphetamine (15 mg/kg, subcutaneously) was an increase in dopamine levels, both in substantia nigra ( approximately 10-fold) and neostriatum ( approximately 40-fold), accompanied by a significant, but lesser, increase in dynorphin B ( approximately two-fold, in both regions), and a decrease in monoamine metabolites. A similar effect was also observed after local administration of methamphetamine (100 microm) via the microdialysis probes, but restricted to the treated region. In other experiments, rats were repeatedly treated with methamphetamine or saline, with the last dose administered 12 h before microdialysis. Dopamine K+-stimulated release was decreased following repeated methamphetamine administration compared with that following saline, both in the substantia nigra (by approximately 65%) and neostriatum (by approximately 20%). In contrast, the effect of K+-depolarization on glutamate, aspartate and GABA levels was increased following repeated administration of methamphetamine. In conclusion, apart from an impairment of monoamine neurotransmission, repeated methamphetamine produces changes in amino acid homeostasis, probably leading to NMDA-receptor overstimulation.

A dual probe characterization of dialysate amino acid levels in the medial prefrontal cortex and ventral tegmental area of the awake freely moving rat

Dual probe microdialysis was employed to characterize the origins of dialysate glutamate, aspartate and gamma-aminobutyric acid (GABA) in the medial prefrontal cortex (mPfc) and to investigate functional interactions between the mPfc and ventral tegmental area (VTA) in awake, freely moving rats. Perfusion with elevated potassium (K(+); KCl, 100 mM, 20 min), low Ca(2+) (0.1 mM, 60 min) or tetrodotoxin (TTX, 10 microM, 100 min) was performed in the mPfc and dialysate levels of glutamate, aspartate and GABA were measured locally and in the VTA. Elevated K(+) in the mPfc rapidly increased dialysate glutamate and aspartate locally (+90+/-10 and +41+/-9% from basal, respectively) and in the VTA (+71+/-14 and +42+/-14%, respectively). MPfc GABA was also rapidly increased (+241+/-62%) while VTA GABA was not affected. Perfusion with low Ca(2+) in the mPfc decreased local glutamate, aspartate and GABA (-26+/-8; -35+/-7 and -45+/-8%, respectively) and decreased only GABA (-40+/-5%) in the VTA. Intra-mPfc TTX increased glutamate and aspartate locally (+82+/-23 and +54+/-27%, respectively) and in the VTA (+84+/-18 and +38+/-17%, respectively). In contrast, intra-mPfc TTX decreased local GABA (-33+6%) while VTA GABA levels were not affected. Taken together, these data confirm the influence of the mPfc upon the ipsilateral VTA and provide evidence for two neuronal pools which contribute to basal extracellular mPfc and VTA glutamate, aspartate and GABA levels, the first pool derived from Na(+)- and Ca(2+)-dependent release and the second derived from voltage-dependent reuptake.

NK-1 receptor blockade decreases amphetamine-induced behavior and neuropeptide mRNA expression in the striatum

The effect of intrastriatal administration of LY306740, a specific NK-1 receptor antagonist, on the behavior and changes in gene expression elicited by the psychomotor stimulant, amphetamine, was studied. Acute administration of amphetamine (2.5 mg/kg, i.p.) caused an increase in behavioral activity and preproenkephalin, preprodynorphin and substance P mRNA expression in the striatum. When amphetamine-treated rats were pretreated with LY306740 (35 and 20 nmoles per side, intrastriatally), there was a significant decrease in amphetamine-induced behavioral activity. Quantitative in situ hybridization histochemistry revealed that both concentrations of LY306740 significantly decreased amphetamine-induced mRNA expression of all three neuropeptides. These data indicate that striatal NK-1 receptors modulate amphetamine-induced behavior and mRNA expression of neuropeptides in the rat striatum.

Functional neuroanatomy of the ventral striopallidal GABA pathway. New sites of intervention in the treatment of schizophrenia

Microdialysis was employed to investigate the dopamine, cholecystokinin (CCK) and neurotensin receptor regulation of ventral striopallidal GABA transmission by intra-accumbens perfusion with selective receptor ligands and monitoring local or ipsilateral ventral pallidal GABA release. In the dual probe studies intra-accumbens perfusion with the dopamine D1 and D2 receptor agonists SKF28293 and pergolide had no effect on ventral pallidal GABA, while both the D1 and D2 receptor antagonists SCH23390 and raclopride increased ventral pallidal GABA release. In contrast, intra-accumbens CCK decreased ventral pallidal GABA release and this was reversed by local perfusion with the CCK2 receptor antagonist PD134308 but not the CCK1 receptor antagonist L-364,718. In a single probe study intra-accumbens neurotensin increased local GABA release, which was strongly potentiated when the peptidase inhibitor phosphodiepryl 08 was perfused together with neurotensin. In addition, the neurotensin receptor antagonist SR48692 counteracted this phosphodiepryl 08 induced potentiated increased in GABA release. Taken together, these findings indicate that mesolimbic dopamine and CCK exert a respective tonic and phasic inhibition of ventral pallidal GABA release while the antipsychotic activity associated with D1 and D2 receptor antagonists may be explained by their ability to increase ventral striopallidal GABA transmission. Furthermore, the findings suggest that CCK2 receptor antagonists and neurotensin endopeptidase inhibitors may be useful antipsychotics.

Prodynorphin and kappa opioid receptor mRNA expression in the cingulate and prefrontal cortices of subjects diagnosed with schizophrenia or affective disorders

The present study examined the prodynorphin and kappa opioid receptor mRNA expression levels in the anterior cingulate and dorsolateral prefrontal cortices of subjects diagnosed with schizophrenia, bipolar disorder, or major depression as compared with normal controls without a psychiatric diagnosis. Multivariate analyses failed to reveal any differences in the mRNA expression levels between the four diagnostic groups, though a group trend (non-significant) was evident for the expression of the kappa opioid receptor and prodynorphin mRNAs in the prefrontal cortex. The mRNA expression levels were not associated with lifetime history of antipsychotic treatment or with suicide as a cause of death. The results, however, suggested an influence of certain drugs of abuse on the prodynorphin cortical mRNA expression. Prodynorphin mRNA expression levels were found to be elevated in individuals with a history of marihuana or stimulant use, but not alcohol. Overall, our data do not provide strong evidence for impaired prodynorphin or kappa opioid receptor mRNA levels in the dorsolateral prefrontal or cingulate cortices of schizophrenic, bipolar disorder, or major depressed subjects.

Repeated ethanol administration induces short- and long-term changes in enkephalin and dynorphin tissue concentrations in rat brain

Recently, we have shown that rats repeatedly treated with ethanol and/or cocaine have decreased kappa-opioid receptor mRNA levels in the mesolimbic system. The aim of the present study was to investigate the short- and long-term effects of repeated ethanol administration on opioid peptide concentrations in brain tissue of male Sprague-Dawley rats. Dynorphin B (1-13) (Dyn B) and Met-enkephalinArg(6)Phe(7) (MEAP), endogenous ligands to kappa- and delta-opioid receptors, respectively, were measured using radioimmunoassays. The rats were given either ethanol [intraperitoneal (ip), twice daily, 2 g/kg bw/dose] or saline for 13 consecutive days. Thirty minutes after the last ethanol dose on Day 13, the Dyn B tissue concentration was significantly decreased in the cingulate cortex. The MEAP tissue concentration was decreased in the hippocampus 5 days after the last ethanol injection as compared to saline-treated controls. Furthermore, the Dyn B and the MEAP concentrations were increased in the periaqueductal grey area (PAG) at this time point. Of particular interest were the significant increases in Dyn B tissue concentrations found in the nucleus accumbens (NAcc) at 30 min and at 21 days after the last ethanol dose. The results suggest that repeated ethanol administration induces both short- and long-term changes in the tissue concentrations of opioids in certain brain regions associated with motivation and reward.

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.

D1 dopamine receptor activation reduces extracellular glutamate and GABA concentrations in the medial prefrontal cortex

The present study examined effect of administration of a selective D1 dopamine receptor agonist, SKF38393 on extracellular concentrations of glutamate (Glu) and gamma-aminobutyric acid (GABA) in mPFC, by using in vivo microdialysis. Perfusion with SKF38393 via a dialysis probe reduced concentrations of both Glu and GABA dose-relatedly, and these effects were prevented by co-perfusion with a D1 dopamine receptor antagonist, SCH23390 (40 microM). These results suggested that the dopaminergic hyperactivity may lead to the hypofunction of glutamatergic and GABAergic systems in mPFC via D1 dopamine receptor stimulation.

Involvement of the direct striatonigral pathway in levodopa-induced sensitization in 6-hydroxydopamine-lesioned rats

Induction of dopamine D3 receptor gene expression in 6-hydroxydopamine-lesioned rats by repeated administration of levodopa had been suggested to be responsible for behavioural sensitization developing in these animals. Using double in situ hybridization techniques, we show that D3 receptor mRNA induction after repeated administration of levodopa took place mainly in dynorphin/substance P-expressing neurons of the direct striatonigral pathway. In agreement, induction of D3 receptor binding sites was evidenced, using 7-[3H]hydroxy-N,N-di-propyl-2-aminotetralin ([3H]7-OH-DPAT), in substantia nigra pars reticulata, the projection area of the direct nigrostriatonigral pathway. Changes in D3 receptor binding and behavioural sensitization during intermittent administration of levodopa paralleled changes in prodynorphin/preprotachykinin rather than preproenkephalin/prodynorphin and preproenkephalin/preprotachykinin mRNA ratios. Behavioural sensitization, induction of D3 receptor binding and changes in prodynorphin/preprotachykinin ratio were all prevented together when levodopa was continuously delivered or intermittently delivered in combination with R-(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4, 5-tetrahydro-1H-3-benzazepine (SCH 23390), a selective D1 receptor antagonist. Our results indicate that functional changes of the direct striatal output pathway, possibly through an interaction between D1 and D3 receptors at the level of terminals in the substantia nigra pars reticulata, are important for the development of behavioural sensitization.

Concurrent modulation of anxiety and memory

We have previously shown that the ventromedial prefrontal cortex (vmPFC) is involved in spontaneous working memory and anxiety-related behaviour in CD-1 mice. Specifically, pretrial microinjection of the kappa(1) agonist, U-69,593, in the infralimbic (IL) area of the vmPFC produced a robust anxiolytic behavioural profile in the elevated plus-maze and enhanced spontaneous working memory in the Y-maze. In the present study we sought to determine whether these effects were specific to IL kappa receptors. We hypothesized that microinjection of the kappa antagonist, norBNI, in the IL cortex would influence anxiety and spontaneous memory in an opposite direction to the effects produced by the kappa(1) agonist. In week 1, transfer-latency reference memory and anxiety were tested in the elevated plus-maze in two separate trials with an intertrial interval of 24 h. In week 2, spontaneous working memory was tested in the Y-maze followed immediately by defensive/withdrawal anxiety in the open field for one half of the animals in each group, and the other half was tested in reverse order. Pretreatment with one injection of vehicle, 1, 5 or 10 nmol/0.5 microl norBNI in the IL cortex dose-dependently reduced transfer-latencies and produced an anxiogenic behavioural profile in the first elevated plus-maze trial. Following a 24 h delay, transfer-latency reference memory was not influenced, but a robust anxiogenic behavioural profile was observed in the second no-injection anxiety trial in the elevated plus-maze relative to control animals. In week 2, the same groups of mice were again pretreated with one injection of the same doses of norBNI in the IL cortex and tested in the open field and Y-maze. NorBNI pretreatment was anxiogenic in the defensive/withdrawal anxiety test and disrupted spontaneous working memory regardless of testing order. The present results show the influence of kappa receptor modulation on anxiety induction and spontaneous working memory. These results also support the hypothesis that immediate memory processing may modulate the induction of anxiety-related behaviours.

Schizophrenia: pathophysiological mechanisms--a synthesis

A Nobel Symposium, 'Schizophrenia: Pathophysiological Mechanisms' was held in Stockholm, October 1-3, 1998. The topics ranged from etiology, genetics, neuropathology, neurotransmitter, brain imaging, to integrative aspects including cognition and language. The collective amount of information is already enormous and will rapidly increase with advances in molecular pathology, gene transcript profiling and non-invasive imaging techniques. Probably reflecting the complexity is the absence of a disease model that can accommodate the data and explain its roots. In the process of moving from simplistic mechanisms involving single or a few interacting neurotransmitters or circuits and turning to more complex interactive models, there is a need for improved data storage and retrieval. Stochastic search in chemical libraries for potential new drug candidates coupled with rational approaches in their final design may improve therapeutic efficacy.

Pain modulation by release of the endogenous cannabinoid anandamide

Synthetic cannabinoids produce behavioral analgesia and suppress pain neurotransmission, raising the possibility that endogenous cannabinoids serve naturally to modulate pain. Here, the development of a sensitive method for measuring cannabinoids by atmospheric pressure-chemical ionization mass spectrometry permitted measurement of the release of the endogenous cannabinoid anandamide in the periaqueductal gray (PAG) by in vivo microdialysis in the rat. Electrical stimulation of the dorsal and lateral PAG produced CB1 cannabinoid receptor-mediated analgesia accompanied by a marked increase in the release of anandamide in the PAG, suggesting that endogenous anandamide mediates the behavioral analgesia. Furthermore, pain triggered by subcutaneous injections of the chemical irritant formalin substantially increased the release of anandamide in the PAG. These findings indicate that the endogenous cannabinoid anandamide plays an important role in a cannabinergic pain-suppression system existing within the dorsal and lateral PAG. The existence of a cannabinergic pain-modulatory system may have relevance for the treatment of pain, particularly in instances where opiates are ineffective.

Modulation of endogenous GABA release by an antagonistic adenosine A1/dopamineD1 receptor interaction in rat brain limbic regions but not basal ganglia

Behavioral and biochemical studies suggest that a negative interaction exists between adenosine A(1) and dopamine D(1) receptors in the brain and that this may contribute to the psychomotor effects of adenosine receptor agonists and antagonists. We examined the functional significance of A(1) and D(1) receptor subtypes in modulating electrically evoked endogenous GABA release from slices/punches of rat basal ganglia (striatum, globus pallidus, striatum containing globus pallidus, and substantia nigra reticulata) and limbic regions (ventral pallidum and nucleus accumbens). In basal ganglia, stimulation of A(1) receptors with the selective agonist R-PIA (1-100 nM) resulted in a concentration-dependent decrease in GABA release. The selective A(1) antagonist DPCPX (10-100 nM) increased GABA release, suggesting that endogenous adenosine tonically inhibits GABA release. However, in basal ganglia, consistent dopamine D(1) receptor modulation of GABA, release was not observed in response to either D(1) agonists or antagonists. Furthermore, the A(1) receptor-mediated inhibition of GABA release was not changed by concurrent activation of D(1) receptors, thus confirming the lack of D(1) receptor modulation under these conditions. In contrast, in ventral pallidum and nucleus accumbens, stimulation of D(1) receptors with SKF-82958 (1 microM) increased GABA release significantly. The D(1) receptor-mediated increase in GABA release was attenuated by concurrent activation of adenosine A(1) receptors. These results are consistent with the hypothesis that an antagonistic A(1)/D(1) receptor interaction may be important in modulating GABA release in limbic regions.

Endogenous dopamine increases extracellular concentrations of glutamate and GABA in striatum of the freely moving rat: involvement of D1 and D2 dopamine receptors

Interactions between endogenous dopamine, glutamate, GABA, and taurine were investigated in striatum of the freely moving rat by using microdialysis. Intrastriatal infusions of the selective dopamine uptake inhibitor nomifensine (NMF) were used to increase the endogenous extracellular dopamine. NMF produced a dose-related increase in extracellular dopamine and also increased extracellular concentrations of glutamate, GABA, and taurine. Extracellular increases of dopamine were significantly correlated with extracellular increases of glutamate and GABA, but not taurine. To investigate whether the increased extracellular dopamine produced by NMF was responsible for the concomitant increase of glutamate and GABA, D1, and D2 receptor antagonists were used. Dopamine receptor antagonists D1 (SCH23390) and D2 (sulpiride) significantly attenuated the increases of glutamate and GABA produced by NMF. These data suggest that endogenous dopamine, through both D1 and D2 dopamine receptors, plays a role in releasing glutamate and GABA in striatum of the freely moving rat.