Reduced spontaneous eye blink rates in recreational cocaine users: evidence for dopaminergic hypoactivity

Chronic use of cocaine is associated with a reduced density of dopaminergic D2 receptors in the striatum, with negative consequences for cognitive control processes. Increasing evidence suggests that cognitive control is also affected in recreational cocaine consumers. This study aimed at linking these observations to dopaminergic malfunction by studying the spontaneous eyeblink rate (EBR), a marker of striatal dopaminergic functioning, in adult recreational users and a cocaine-free sample that was matched on age, race, gender, and personality traits. Correlation analyses show that EBR is significantly reduced in recreational users compared to cocaine-free controls, suggesting that cocaine use induces hypoactivity in the subcortical dopamine system.     

Increases in alpha4* but not alpha3*/alpha6* nicotinic receptor sites and function in the primate striatum following chronic oral nicotine treatment

Knowledge of the effects of chronic nicotine is critical considering its widespread use in tobacco products and smoking cessation therapies. Although nicotine is well known to up-regulate alpha4* nAChR sites and function in the cortex, its actions in the striatum are uncertain because of the presence of multiple subtypes with potentially opposing effects. We therefore investigated the effect of long-term nicotine treatment on nAChR sites and function in the primate striatum, which offers the advantage of similar proportions of alpha3*/alpha6* and alpha4* nAChRs. Nicotine was given in drinking water, which resembles smoking in its intermittent but chronic delivery. Plasma nicotine and cotinine levels were similar to smokers. Chronic nicotine treatment (> 6 months) enhanced alpha4* nAChR-evoked [(3)H]dopamine release in striatal subregions, with an overall pattern of increase throughout the striatum when normalized to uptake. This increase correlated with elevated striatal alpha4* nAChRs. Under the same conditions, striatal alpha3*/alpha6* nAChR sites and function were decreased or unchanged. These divergent actions of chronic nicotine treatment on alpha4* versus alpha6* nAChRs, as well as effects on dopamine uptake, allow for a complex control of striatal activity to maintain dopaminergic function. Such knowledge is important for understanding nicotine dependence and the consequences of nicotine administration for the treatment of neurological disorders.     

Compensation in pre-synaptic dopaminergic function following nigrostriatal damage in primates

Clinical symptoms of Parkinson's disease only become evident after 70-80% reductions in striatal dopamine. To investigate the importance of pre-synaptic dopaminergic mechanisms in this compensation, we determined the effect of nigrostriatal damage on dopaminergic markers and function in primates. MPTP treatment resulted in a graded dopamine loss with moderate to severe declines in ventromedial striatum (approximately 60-95%) and the greatest reductions (approximately 95-99%) in dorsolateral striatum. A somewhat less severe pattern of loss was observed for striatal nicotinic receptor, tyrosine hydroxylase and vesicular monoamine transporter expression. Declines in striatal dopamine uptake and transporter sites were also less severe than the reduction in dopamine levels, with enhanced dopamine turnover in the dorsolateral striatum after lesioning. The greatest degree of adaptation occurred for nicotine-evoked [(3)H]dopamine release from striatal synaptosomes, which was relatively intact in ventromedial striatum after lesioning, despite > 50% declines in dopamine. This maintenance of evoked release was not due to compensatory alterations in nicotinic receptor characteristics. Rather, there appeared to be a generalized preservation of release processes in ventromedial striatum, with K(+)-evoked release also near control levels after lesioning. These combined compensatory mechanisms help explain the finding that Parkinson's disease symptomatology develops only with major losses of striatal dopamine.     

Relationship of calmodulin and dopaminergic activity in the striatum

Increasing evidence suggests a relationship between dopaminergic activity in the striatum and the content of calmodulin (CaM), an endogenous Ca2+-binding protein. The content of CaM in striatal membranes is increased by treatments that produce supersensitivity in striatal membranes is increased by treatments that produce supersensitivity of striatal dopaminergic receptors such as chronic neuroleptic treatment or injection of 6-hydroxydopamine. Concomitant with the increase in CaM is a greater sensitivity of adenylate cyclase to dopamine and an increase in Ca2+-sensitive phosphorylation in the striatal membranes. Procedures that result in dopaminergic subsensitivity, such as amphetamine treatment, increase the cytosolic content of CaM that can subsequently activate Ca2+ and CaM-dependent phosphodiesterase activity. In vitro studies have demonstrated that CaM and Ca2+ can stimulate basal adenylate cyclase activity in a striatal particulate fraction as well as increase the sensitivity of the enzyme to dopamine. Ca2+ and CaM most likely affect the dopamine-sensitive adenylate cyclase by interacting with guanyl nucleotides, which are required for dopamine sensitivity. It is concluded that a change in CaM concentration and/or location occurs during conditions of altered dopaminergic sensitivity in the striatum. These changes in CaM coupled with potential alterations in the Ca2+ concentration could modulate the sensitivity of the dopamine system and many CaM-dependent enzymes.     

Chronic corticotropin-releasing factor type 1 receptor antagonism with antalarmin regulates the dopaminergic system of Fawn-Hooded rats

Corticotropin-releasing factor is a neuropeptide associated with the integration of physiological and behavioural responses to stress and also in the modulation of affective state and drug reward. The selective, centrally acting corticotropin-releasing factor type 1 receptor antagonist, antalarmin, is a potent anxiolytic and reduces volitional ethanol consumption in Fawn-Hooded rats. The efficacy of antalarmin to reduce ethanol consumption increased with time, suggestive of adaptation to reinforcement processes and goal-directed behaviour. The aim of the present study was to examine the effects of chronic antalarmin treatment on reward-related regions of Fawn-Hooded rat brain. Bi-daily antalarmin treatment (20 mg/kg, i.p.) for 10 days increased tyrosine hydroxylase messenger RNA expression throughout the ventral mesencephalon. Following chronic antalarmin the density of dopaminergic terminals within the basal ganglia and amygdaloid complex were reduced, as was dopamine transporter binding within the striatum. Receptor autoradiography indicated an up-regulation of dopamine D2, but no change in D1, binding in striatum, and Golgi-Cox analysis of striatal medium spiny neurones indicated that chronic antalarmin treatment increased spine density. Thus, chronic antalarmin treatment modulates dopaminergic pathways and implies that chronic treatment with drugs of this class may ultimately alter postsynaptic signaling mechanisms within the basal ganglia.     

Ghrelin amplifies the nicotine-induced dopamine release in the rat striatum

The orexigenic peptide ghrelin plays a prominent role in the regulation of energy balance and in the mediation of reward mechanisms and reinforcement for addictive drugs, such as nicotine. Nicotine is the principal psychoactive component in tobacco, which is responsible for addiction and relapse of smokers. Nicotine activates the mesencephalic dopaminergic neurons via nicotinic acetylcholine receptors (nAchR). Ghrelin stimulates the dopaminergic neurons via growth hormone secretagogue receptors (GHS-R1A) in the ventral tegmental area and the substantia nigra pars compacta resulting in the release of dopamine in the ventral and dorsal striatum, respectively. In the present study an in vitro superfusion of rat striatal slices was performed, in order to investigate the direct action of ghrelin on the striatal dopamine release and the interaction of ghrelin with nicotine through this neurotransmitter release. Ghrelin increased significantly the dopamine release from the rat striatum following electrical stimulation. This stimulatory effect was reversed by both the selective nAchR antagonist mecamylamine and the selective GHS-R1A antagonist GHRP-6. Nicotine also increased significantly the dopamine release under the same conditions. This stimulatory effect was antagonized by mecamylamine, but not by GHRP-6. Ghrelin further stimulated the nicotine-induced dopamine release and this effect was abolished by mecamylamine and was partially inhibited by GHRP-6. The present results demonstrate that ghrelin stimulates directly the dopamine release and amplifies the nicotine-induced dopamine release in the rat striatum. We presume that striatal cholinergic interneurons also express GHS-R1A, through which ghrelin can amplify the nicotine-induced dopamine release in the striatum. This study provides further evidence of the impact of ghrelin on the mesolimbic and nigrostriatal dopaminergic pathways. It also suggests that ghrelin signaling may serve as a novel pharmacological target for treatment of addictive and neurodegenerative disorders.     

Effects of alloxan-induced diabetes on dopaminergic receptors in rat striatum and anterior pituitary

The binding of [3H]-spiroperidol after 4 weeks of hyperglycemia was determined in the rat striatum and anterior pituitary. Alloxan-induced diabetes increased the number of dopaminergic binding sites in the striatum but not in the anterior pituitary. The interaction of metoclopramide with striatal dopaminergic receptors was slightly modified, while that of dopamine, bromocriptine and haloperidol was unaffected. These results suggest that chronic hyperglycemia exerts selective effects on nigrostriatal dopaminergic system in the rat.     

Lack of Dopaminergic Inputs Elongates the Primary Cilia of Striatal Neurons

In the rodent brain, certain G protein-coupled receptors and adenylyl cyclase type 3 are known to localize to the neuronal primary cilium, a primitive sensory organelle protruding singly from almost all neurons. A recent chemical screening study demonstrated that many compounds targeting dopamine receptors regulate the assembly of Chlamydomonas reinhardtii flagella, structures which are analogous to vertebrate cilia. Here we investigated the effects of dopaminergic inputs loss on the architecture of neuronal primary cilia in the rodent striatum, a brain region that receives major dopaminergic projections from the midbrain. We first analyzed the lengths of neuronal cilia in the dorsolateral striatum of hemi-parkinsonian rats with unilateral lesions of the nigrostriatal dopamine pathway. In these rats, the striatal neuronal cilia were significantly longer on the lesioned side than on the non-lesioned side. In mice, the repeated injection of reserpine, a dopamine-depleting agent, elongated neuronal cilia in the striatum. The combined administration of agonists for dopamine receptor type 2 (D2) with reserpine attenuated the elongation of striatal neuronal cilia. Repeated treatment with an antagonist of D2, but not of dopamine receptor type 1 (D1), elongated the striatal neuronal cilia. In addition, D2-null mice displayed longer neuronal cilia in the striatum compared to wild-type controls. Reserpine treatment elongated the striatal neuronal cilia in D1-null mice but not in D2-null mice. Repeated treatment with a D2 agonist suppressed the elongation of striatal neuronal cilia on the lesioned side of hemi-parkinsonian rats. These results suggest that the elongation of striatal neuronal cilia following the lack of dopaminergic inputs is attributable to the absence of dopaminergic transmission via D2 receptors. Our results provide the first evidence that the length of neuronal cilia can be modified by the lack of a neurotransmitter''s input.     

Specific oxidative stress profile associated with partial striatal dopaminergic depletion by 6-hydroxydopamine as assessed by a novel multifunctional marker molecule

Abstract

Real time oxidative stress in the extracellular compartment of rat striatum was characterized by microdialysis with synthetic non-dialyzable marker molecules composed of linoleic acid, tyrosine and guanosine (N-linoleoyl tyrosine (LT) and N-linoleoyl tyrosine 2′-deoxyguanosyl ester (LTG)). Partial dopaminergic deafferentation was induced by injection of 6-hydroxydopamine (250 μg) to the left lateral ventricle, which depleted ipsilateral striatal dopamine by 46% and dopaminergic cells in left substantia nigra by 44%, 5 weeks after administration. Resting microdialysate dopamine levels in dopamine-depleted striatum were not different from sham-operated rats, although the ratio of oxidized metabolites of dopamine to free dopamine was significantly increased. Hydroperoxide and epoxy products of the linoleoyl portion of LT and LTG were detected in the striatal microdialysate by LC/MS/MS following initial separation by HPLC and were significantly increased in dopamine-depleted compared with control striatum without an increase in guanosine or tyrosine oxidation or nitration. Systemic administration of N-acetyl cysteine (350 mg/kg i.p.) decreased the increment in hydroperoxide and epoxy metabolites to levels not significantly different from control. Oxidation activity towards polyunsaturated fatty acids is present in the extracellular space of partially dopamine-denervated striatum, whereas oxidized glutathione and oxysterol levels in striatal tissue are decreased, possibly indicative of a compensatory response.     

Ventricular injection of nerve growth factor increases dopamine content in the striata of MPTP-treated mice

Experimental depletion of dopaminergic striatal neurons was induced in mice with the neurotoxin MPTP. To investigate a possible effect of nerve growth factor on the damaged neurons, we injected 4 g into the right cerebral ventricle of mice three days after the last administration of MPTP. We found a significant increase of dopamine and homovanillic acid in the striatum of MPTP treated mice after NGF administration when compared with dopamine and HVA levels in MPTP-treated control mice (p<0.001). The increase of dopamine and homovanillic acid seems to be related to a partial restorative effect of NGF on the damaged dopaminergic cells, since ventricular administration of NGF to normal mice did not increase dopamine or homovanillic acid contents above the levels measured in untreated controls. It appears that administration of nerve growth factor prcduces a beneficial effect on damaged dopaminergic neurons; this effect could be due to stimulation of neuron sprouting from neurons that survived the toxic effect of MPTP. The increase of dopamine levels was seen 8 days after injection of nerve growth factor and was maintained at least until day 25, showing a lasting persistence of the restorative effect.     

Glial cell line-derived neurotrophic factor receptor-α1 expressed in striatum in trans regulates development and injury response of dopamine neurons of the substantia nigra

Many of the cellular effects of glial cell line-derived neurotrophic factor are initiated by binding to GNDF family receptor alpha-1 (GFRα1), and mediated by diverse intracellular signaling pathways, most notably through the Ret tyrosine kinase. Ret may be activated by the cell autonomous expression of GFRα1 ('in cis'), or by its non-cell autonomous presence ('in trans'), in either a soluble or immobilized state. GFRα1 is expressed in the striatum, a target of the dopaminergic projection of the substantia nigra. To determine whether post-synaptic expression of GFRα1 in striatum in trans has effects on the development or adult responses to injury of dopamine neurons, we have created transgenic mice in which GFRα1 expression is selectively increased in striatum and other forebrain targets of the dopaminergic projection. Post-synaptic GFRα1 has profound effects on the development of dopamine neurons, resulting in a 40% increase in their adult number. This morphologic effect was associated with an augmented motor response to amphetamine. In adult mice, post-synaptic GFRα1 expression did not affect neuron survival following neurotoxic lesion, but it did increase the preservation of striatal dopaminergic innervation. We conclude that post-synaptic striatal GFRα1 expression has important effects on the biology of dopamine neurons in vivo.     

Increased sensitivity of striatal dopamine release to H2O2 upon chronic rotenone treatment

It is believed that both mitochondrial dysfunction and oxidative stress play important roles in the pathogenesis of Parkinson's disease (PD). We studied the effect of chronic systemic exposure to the mitochondrial inhibitor rotenone on the uptake, content, and release of striatal neurotransmitters upon neuronal activity and oxidative stress, the latter simulated by H(2)O(2) perfusion. The dopamine content in the rat striatum is decreased simultaneously with the progressive loss of tyrosine hydroxylase (TH) immunoreactivity in response to chronic intravenous rotenone infusion. However, surviving dopaminergic neurons take up and release only a slightly lower amount of dopamine (DA) in response to electrical stimulation. Striatal dopaminergic neurons showed increased susceptibility to oxidative stress by H(2)O(2), responding with enhanced release of DA and with formation of an unidentified metabolite, which is most likely the toxic dopamine quinone (DAQ). In contrast, the uptake of [(3)H]choline and the electrically induced release of acetylcholine increased, in coincidence with a decline in its D(2) receptor-mediated dopaminergic control. Thus, oxidative stress-induced dysregulation of DA release/uptake based on a mitochondrial deficit might underlie the selective vulnerability of dopaminergic transmission in PD, causing a self-amplifying production of reactive oxygen species, and thereby contributing to the progressive degeneration of dopaminergic neurons.     

Passive and active stabilization of dopamine in the striatum

Parkinson's disease is a neurodegenerative disorder associated with cell loss from the substantia nigra pars compacta (SNc). The dopaminergic cells of the SNc project to the striatum where the loss of dopaminergic tone is thought to be the main cause of Parkinsonism symptoms. Animal models have shown that striatal tissue content of dopamine declines proportionally to cell death in the SNc but the extracellular concentration of dopamine (EDA) in the striatum remains near normal until more than 85% of SNc neurons have died. We investigate various explanations for the remarkable homeostasis of EDA with a mathematical model that has recently been constructed for dopamine synthesis, release, and reuptake, which includes the effects of the autoreceptors. We provide evidence and explanations for the passive stabilization hypothesis and show that the autoreceptors enhance stabilization of EDA only when fewer than 25% of the SNc cells remain.     

The effect of chronic levodopa on haloperidol induced behavioral supersensitivity in the guinea pig

The effect of chronic levodopa-carbidopa administration (200 mg/kg for 21 days) on guinea pigs rendered behaviorally supersensitive by the prior administration of haloperidol (.5 mg/kg for 21 days) was examined. Animals who showed an increased behavioral response to apomorphine after chronic haloperidol administration were treated with levodopa-carbidopa and then apomorphine - induced stereotypy was reexamined. Although the chronic levodopa control groups and the chronic haloperidol control remained supersensitive to the behavioral effect of apomorphine, the haloperidol-levodopa group's behavioral response to apomorphine returned to normal. Both chronic dopaminergic antagonist and agonist administration have been demonstrated to induce heightened apomorphine-induced stereotypy and this has been interpreted as a reflection of altered striatal dopamine receptor site sensitivity. The finding that the serial administration of a chronic dopaminergic antagonist followed by a chronic dopaminergic agonist results in a return to normal of a striatal dopamine receptor-dependent behavior suggests that these chronic treatments affect dopamine receptor sites by different mechanisms of action. Since neuroleptic induced dopaminergic supersensitivity in animals is an accepted model of tardive dyskinesia, levodopa may also reverse dopaminergic supersensitivity in patients and might be a potential therapeutic agent in tardive dyskinesia.     

In vivo and in vitro synthesis, release, and uptake of [3-H]-dopamine in mouse striatal slices after in vivo exposure to chlordecone

Effects of treatment of mice with chlordecone (25 mg/kg/d) on striatal dopaminergic activities such as synthesis, turnover, uptake, and release were investigated in vivo and in vitro. In mice receiving chlordecone for five days, there were no significant changes in in vivo dopamine (DA) synthesis and turnover in striatum and in vitro [3-H]-dopamine uptake and K+-stimulated [3-H]-dopamine release in striatal slices. In mice receiving chlordecone for eight days, the in vivo synthesis of [3-H]-dopamine from [3-H]-tyrosine in striatum was slightly inhibited and the in vitro [3-H]-dopamine synthesis in striatal slices was significantly decreased. Furthermore, both uptake and K+-stimulated release of [3-H]-dopamine from striatal slices were significantly reduced. The turnover rate of newly synthesized [3-H]-dopamine from [3-H]-tyrosine in striatal slices was unchanged after eight consecutive days of chlordecone administration. These results suggest that chlordecone may cause impairments in pre- and/or postsynaptic membranes of dopaminergic neurons which modulate motor function.     

Protective action of the peroxisome proliferator-activated receptor-gamma agonist pioglitazone in a mouse model of Parkinson's disease

We examined the effect of pioglitazone, a peroxisome proliferator-activated receptor-gamma (PPARgamma) agonist of the thiazolidinedione class, on dopaminergic nerve cell death and glial activation in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse model of Parkinson's disease. The acute intoxication of C57BL/6 mice with MPTP led to nigrostriatal injury, as determined by tyrosine hydroxylase (TH) immunocytochemistry, and HPLC detection of striatal dopamine and metabolites. Damage to the nigrostriatal dopamine system was accompanied by a transient activation of microglia, as determined by macrophage antigen-1 (Mac-1) and inducible nitric oxide synthase (iNOS) immunoreactivity, and a prolonged astrocytic response. Orally administered pioglitazone (approximately 20 mg/kg/day) attenuated the MPTP-induced glial activation and prevented the dopaminergic cell loss in the substantia nigra pars compacta (SNpc). In contrast, there was little reduction of MPTP-induced dopamine depletion, with no detectable effect on loss of TH immunoreactivity and glial response in the striatum of pioglitazone-treated animals. Low levels of PPARgamma expression were detected in the ventral mesencephalon and striatum, and were unaffected by MPTP or pioglitazone treatment. Since pioglitazone affects primarily the SNpc in our model, different PPARgamma-independent mechanisms may regulate glial activation in the dopaminergic terminals compared with the dopaminergic cell bodies after acute MPTP intoxication.     

An inverse correlation between the apparent rate of dopamine clearance and tonic autoinhibition in subdomains of the rat striatum: a possible role of transporter-mediated dopamine efflux

The dopaminergic terminal field in the rat striatum is compartmentalized into sub-domains that exhibit distinct dynamics of electrically evoked dopamine release. The fast striatal domains, where dopamine release is predominantly vesicular, exhibit conventional dopaminergic activity. However, vesicular dopamine release is tonically autoinhibited in the slow domains, which suggests that dopamine reaches the autoreceptors via a non-vesicular route. Hence, it appears that the domains use distinct mechanisms to regulate the basal dopamine concentration available to activate, or not, pre-synaptic autoinhibitory receptors. However, direct detection of local variations in tonic extracellular dopamine concentrations is not yet possible. So, the present study employed voltammetry to test the hypothesis that the apparent rate of dopamine clearance from the extracellular space should be domain-dependent. The apparent rate of dopamine clearance is equal to the difference in the rates of dopamine release and uptake that determine extracellular dopamine concentrations. This study confirms that the apparent rate of dopamine clearance is slower in the slow striatal domains where vesicular dopamine release is tonically autoinhibited. These findings support the view that the basal concentration in slow domains is maintained by a non-vesicular release process, possibly transporter-mediated efflux.     

Mechanisms underlying the link between cannabis use and prospective memory

While the effects of cannabis use on retrospective memory have been extensively examined, only a limited number of studies have focused on the links between cannabis use and prospective memory. We conducted two studies to examine the links between cannabis use and both time-based and event-based prospective memory as well as potential mechanisms underlying these links. For the first study, 805 students completed an online survey designed to assess cannabis consumption, problems with cannabis use indicative of a disorder, and frequency of experiencing prospective memory failures. The results showed small to moderate sized correlations between cannabis consumption, problems with cannabis use, and prospective memory. However, a series of mediation analyses revealed that correlations between problems with cannabis use and prospective memory were driven by self-reported problems with retrospective memory. For the second study, 48 non-users (who had never used cannabis), 48 experimenters (who had used cannabis five or fewer times in their lives), and 48 chronic users (who had used cannabis at least three times a week for one year) were administered three objective prospective memory tests and three self-report measures of prospective memory. The results revealed no objective deficits in prospective memory associated with chronic cannabis use. In contrast, chronic cannabis users reported experiencing more internally-cued prospective memory failures. Subsequent analyses revealed that this effect was driven by self-reported problems with retrospective memory as well as by use of alcohol and other drugs. Although our samples were not fully characterized with respect to variables such as neurological disorders and family history of substance use disorders, leaving open the possibility that these variables may play a role in the detected relationships, the present findings indicate that cannabis use has a modest effect on self-reported problems with prospective memory, with a primary problem with retrospective memory appearing to underlie this relationship.     

Human immunodeficiency virus-1 Tat protein and methamphetamine interact synergistically to impair striatal dopaminergic function

The human immunodeficiency virus (HIV)-1 transactivating protein Tat may be pathogenically relevant in HIV-1-induced neuronal injury. The abuse of methamphetamine (MA), which is associated with behaviors that may transmit HIV-1, may damage dopaminergic afferents to the striatum. Since Tat and MA share common mechanisms of injury, we examined whether co-exposure to these toxins would lead to enhanced dopaminergic toxicity. Animals were treated with either saline, a threshold dose of MA, a threshold concentration of Tat injected directly into the striatum, or striatal injections of Tat followed by exposure to MA. Threshold was defined as the highest concentration of toxin that would not result in a significant loss of striatal dopamine levels. One week later, MA-treated animals demonstrated a 7% decline in striatal dopamine levels while Tat-treated animals showed an 8% reduction. Exposure to both MA + Tat caused an almost 65% reduction in striatal dopamine. This same treatment caused a 56% reduction in the binding capacity to the dopamine transporter. Using human fetal neurons, enhanced toxicity was also observed when cells were exposed to both Tat and MA. Mitochondrial membrane potential was disrupted and could be prevented by treatment with antioxidants. This study demonstrates that the HIV-1 'virotoxin' Tat enhances MA-induced striatal damage and suggests that HIV-1-infected individuals who abuse MA may be at increased risk of basal ganglia dysfunction.