Continued administration of GM1 ganglioside is required to maintain recovery from neuroleptic-induced sensorimotor deficits in MPTP-treated mice

Injection of a dose of haloperidol that has no obvious behavioral effects in normal mice, produces akinesia, catalepsy, and sensory neglect in MPTP-treated mice. Chronic GM1 ganglioside administration improves the behavioral impairments, partially restores striatal dopamine (DA) content and prevents DA D-2 receptor up-regulation. Discontinuation of GM1 ganglioside treatment results in a time-dependent decline of striatal DA content to pretreatment pathological levels, return of haloperidol-induced sensorimotor deficits and a rise of DA D-2 receptor density in the striatum. Apparently, continuous administration of GM1 ganglioside is necessary to maintain the biochemical and behavioral recovery in the MPTP-treated mouse. These observations may provide useful cues for understanding the mechanism of action of GM1 ganglioside.     

The effect of subchronic, intermittent L-DOPA treatment on neuronal nitric oxide synthase and soluble guanylyl cyclase expression and activity in the striatum and midbrain of normal and MPTP-treated mice

We have investigated the effects of low (10 mg/kg) and high (100 mg/kg) doses of L-DOPA on the expression and activity of neuronal nitric oxide synthase (nNOS) and guanylyl cyclase (GC) in the striatum and midbrain of mice. L-DOPA was administered subchronically for 11 days (beginning 3 days after last MPTP/NaCl injection) or for 14 days (with dosing started immediately following the last MPTP/NaCl injection). Adult mice received three intraperitoneal (i.p.) injections of physiological saline or MPTP at 2h intervals (total dose of 40 mg/kg). Normal and MPTP-injected mice were treated twice a day for 11 or 14 days with low (10/2.5 mg/kg bw) or high (100/25mg/kg bw) doses of L-DOPA/benserazide. The present study indicates that several days of treatment with L-DOPA does not affect MPTP-activation of the nNOS/sGC/cGMP pathway or the neurodegenerative processes that occur in the striatum and midbrain of mice. In normal mice, L-DOPA upregulates the expression and activity of nNOS and GC to levels found in MPTP-injected mice. Due to upregulation of nNOS and GC, cGMP levels in the mouse striatum and midbrain are also elevated, however, significantly lower in mice administrated with low dose of L-DOPA. In both investigated brain regions of normal mice cGMP-dependent PDEs activities were elevated after low dose administration of L-DOPA, but no change in PDEs activities has been detected in MPTP and high L-DOPA-injected mice as compared to control values. The enhancement of nNOS mRNA and GCbeta1 mRNA levels were generated by both doses of L-DOPA, given in a time-dependent fashion. L-DOPA-injected for 11 or 14 days caused a decrease in TH protein levels in the striatum and midbrain, respectively; this result was noted irrespective of dose. L-DOPA therapy did not prevent the MPTP-induced decrease in TH protein levels in either investigated brain region.     

Aromatic L-Amino Acid Decarboxylase Is Modulated by D1 Dopamine Receptors in Rat Retina

Aromatic L-amino acid decarboxylase (AAAD) activity of rat retina increases when animals are placed in a lighted environment from the dark. The increase of activity can be inhibited by administering the selective dopamine D1 receptor agonist SKF 38393, but not the selective D2 agonist quinpirole, or apomorphine. Conversely, in the dark, enzyme activity can be enhanced by administering the selective D1 antagonist SCH 23390 or haloperidol, but not the selective D2 antagonist (-)-sulpiride. Furthermore, in animals exposed to room light for 3 h, the D1 agonist SKF 38393 reduced retinal AAAD activity, and this effect was prevented by prior administration of SCH 23390. In contrast, quinpirole had little or no effect when administered to animals in the light. Kinetic analysis indicated that the apparent Vmax for the enzyme increases with little change in the apparent Km for the substrate 3,4-dihydroxyphenylalanine or the cofactor pyridoxal-5'-phosphate. We suggest that dopamine released in the dark tonically occupies D1 receptors and suppresses AAAD activity. When the room light is turned on, D1 receptors are vacated and selective D1 agonists can either prevent the rise of AAAD or reverse light-enhanced AAAD activity.     

Evidence for Cyclic AMP-Mediated Increase of Aromatic L-Amino Acid Decarboxylase Activity in the Striatum and Midbrain

Abstract: Aromatic L-amino acid decarboxylase (AAAD) activity of mouse striatum and midbrain increased after an intracerebro-ventricular injection of either forskolin or 8-bromo-cyclic AMP. The increase was transient, peaking between 15 and 30 min and returning to baseline by ˜90 min. The increase of AAAD activity after forskolin was not affected by pretreatment with cycloheximide. Kinetic studies indicated an apparent increase of V _max with little change of the K _m for L-DOPA or pyridoxal 5'-phosphate. We conclude that AAAD activity of striatum and midbrain can be modulated by a cyclic AMP-dependent process.     

Systemic Blockade of Dopamine D2-Like Receptors Increases High-Voltage Spindles in the Globus Pallidus and Motor Cortex of Freely Moving Rats

High-voltage spindles (HVSs) have been reported to appear spontaneously and widely in the cortical–basal ganglia networks of rats. Our previous study showed that dopamine depletion can significantly increase the power and coherence of HVSs in the globus pallidus (GP) and motor cortex of freely moving rats. However, it is unclear whether dopamine regulates HVS activity by acting on dopamine D₁-like receptors or D₂-like receptors. We employed local-field potential and electrocorticogram methods to simultaneously record the oscillatory activities in the GP and primary motor cortex (M1) in freely moving rats following systemic administration of dopamine receptor antagonists or saline. The results showed that the dopamine D₂-like receptor antagonists, raclopride and haloperidol, significantly increased the number and duration of HVSs, and the relative power associated with HVS activity in the GP and M1 cortex. Coherence values for HVS activity between the GP and M1 cortex area were also significantly increased by dopamine D₂-like receptor antagonists. On the contrary, the selective dopamine D₁-like receptor antagonist, {"type":"entrez-protein","attrs":{"text":"SCH23390","term_id":"1052733334","term_text":"SCH23390"}}SCH23390, had no significant effect on the number, duration, or relative power of HVSs, or HVS-related coherence between M1 and GP. In conclusion, dopamine D₂-like receptors, but not D₁-like receptors, were involved in HVS regulation. This supports the important role of dopamine D₂-like receptors in the regulation of HVSs. An siRNA knock-down experiment on the striatum confirmed our conclusion.     

The role of tissue plasminogen activator in methamphetamine-related reward and sensitization

In the central nervous system, tissue plasminogen activator (tPA) plays a role in synaptic plasticity and remodeling. Our recent study has suggested that tPA participates in the rewarding effects of morphine by regulating dopamine release. In this study, we investigated the role of tPA in methamphetamine (METH)-related reward and sensitization. Repeated METH treatment dose-dependently induced tPA mRNA expression in the frontal cortex, nucleus accumbens, striatum and hippocampus, whereas single METH treatment did not affect tPA mRNA expression in these brain areas. The METH-induced increase in tPA mRNA expression in the nucleus accumbens was completely inhibited by pre-treatment with R(+)-SCH23390 and raclopride, dopamine D1 and D2 receptor antagonists, respectively. In addition, repeated METH treatment increased tPA activity in the nucleus accumbens. There was no difference in METH-induced hyperlocomotion between wild-type and tPA-deficient (tPA-/-) mice. On the other hand, METH-induced conditioned place preference and behavioral sensitization after repeated METH treatment were significantly reduced in tPA-/- mice compared with wild-type mice. The defect of behavioral sensitization in tPA-/- mice was reversed by microinjections of exogenous tPA into the nucleus accumbens. Our findings suggest that tPA is involved in the rewarding effects as well as the sensitization of the locomotor-stimulating effect of METH.     

Cardiac,respiratory, and motor activity in norm and after activation of catecholaminergic systems in newborn rat pups

Study of parameters of the cardiac, respiratory, and motor activity (MA) was carried out on newborn rat pups for the first day after birth (P0) and at the 14th day of postnatal development (P14) after change of the activity levels of dopaminergic and noradrenergic systems. To provide an excessive level of catecholamines, the animals were administered individually with L-DOPA (25–100 mg/kg) and with the indirect adrenomimetic isoamine (3 and 10 mg/kg). Additionally there were studied effects of L-DOPA and isoamine after blockade of D1 and D2 dopamine receptors (antagonists SCH-23390 and sulpirid). The L-DOPA administration produced a dose-dependent MA enhancement with its subsequent possible conversion into the continuous generalized activity. At P0 the release of monoamines was accompanied by development of weak bradycardia. There was noted a tendency for acceleration of respiration at administration of the low dose both of L-DOPA and of isoamine and for its deceleration at high doses. At P14 the L-DOPA administration was accompanied by deceleration of the heart rate (HR) by 8% and by acceleration of respiration rate (RR) by 26%. The isoamine administration produced an insignificant decrease of HR and an increase of RR by 8% at the low dose and by 21% at the high dose of the agent. At the blockade of D1 receptors, RR remained close to the background values, while at the blockade of D2 receptors it decreased insignificantly. Blockade of D1 and D2 receptors did not cause significant HR changes. Analysis of the HR variability has shown that both after L-DOPA administration and at blockade of dopamine receptors no unidirectional reaction was observed: in 80% of rat pups the portion of nerve mechanisms of HR regulation increased, while in the rest-of sympathetic and humoral factors at a decrease of parasympathetic effects. In all rat pups the isoamine administration was accompanied by a shift of the specter power into the higher frequency area; in 60% of the animals there were enhanced sympathetic effects. At P14 in rat pups after administration both of L-DOPA and of isoamine, the sympathetic nervous effects were predominant. Thus, at P0 both at release of endogenous catecholamines and at their excessive concentration in rat pups there occurs a qualitative change of character of the catecholaminergic effects on functional activity of excitable structures, particularly of those connected with regulation of respiration.     

L-DOPA does not enhance hydroxyl radical formation in the nigrostriatal dopamine system of rats with a unilateral 6-hydroxydopamine lesion

The debate about the toxicity of L-DOPA to dopaminergic neurons has not been resolved. Even though enzymatic and nonenzymatic metabolism of L-DOPA can produce hydrogen peroxide and oxygen free radicals, there has been controversy as to whether L-DOPA generates an oxidant stress in vivo. This study determined whether acute or repeated administration of L-DOPA caused in vivo production of hydroxyl radicals in striatum and other brain regions in rats with a unilateral 6-hydroxydopamine lesion of the dopaminergic nigrostriatal projections. Salicylate trapping combined with in vivo microdialysis provided measurements of extracellular 2,3-dihydroxybenzoic acid (2,3-DHBA) in striatum following L-DOPA administration systemically (100 mg/kg, i.p.) or by intrastriatal perfusion (1 mM, via the microdialysis probe). Tissue concentrations of 2,3-DHBA and salicylate were also measured in striatum, ventral midbrain, and cerebellum following repeated administration of L-DOPA (50 mg/kg, i.p., once daily for 16 days). In each instance, treatment with L-DOPA did not increase 2,3-DHBA concentrations, regardless of the nigrostriatal dopamine system's integrity. When added to the microdialysis perfusion medium, L-DOPA resulted in a significant decrease in the striatal extracellular concentration of 2,3-DHBA. These results suggest that administration of L-DOPA, even at high doses, does not induce hydroxyl radical formation in vivo and under some conditions may actually diminish hydroxyl radical activity. Furthermore, prior damage to the nigrostriatal dopamine system does not appear to predispose surviving dopaminergic neurons to increased hydroxyl radical formation following L-DOPA administration. Unlike L-DOPA, systemic administration of methamphetamine (10 mg/kg, s.c.) produced a significant increase in the concentration of 2,3-DHBA in striatal dialysate, suggesting that increased formation of hydroxyl radicals may contribute to methamphetamine neurotoxicity.     

Restorative effects of glutamate antagonists in experimental parkinsonism

Several compounds with antagonistic actions on N-methyl-D-aspartate (NMDA) receptors were tested for an antiakinesic action in hypoactive MPTP-treated C57 BL/6 mice rendered tolerant to the motor activity enhancing effects of the 20 mg/kg, s.c., dose of L-Dopa; each compound was administered 60 min before the administration of the dopamine precursor. The classes of compounds studied included the noncompetitive NMDA antagonists, memantine, amantadine and MK-801, the competitive NMDA antagonist, CGP 40116, the anticonvulsive and putative anticonvulsive agents, lamotrigine and FCE 26743, with a partial glutamatergic antagonistic action. All six compounds elevated locomotor, rearing and total activity counts of L-Dopa-tolerant mice in co-administration with L-Dopa in dose-specific or dose-dependent manners but only memantine and MK-801 affected motor activity in the control mice, that also received chronic L-Dopa treatment. Thus, the restorative actions of those compounds in suprathreshold L-Dopa-tolerant MPTP-treated mice subjected to "wearing-off" of L-Dopa efficacy were assessed in a series of experiments. Within each class of potentially therapeutic agents a differential restorative efficacy of the motor activity-stimulating effects of hypoactive MPTP mice was obtained, confirming the putative antiparkinsonian applications of compounds with glutamate antagonistic actions.     

Carbidopa-Based Modulation of the Functional Effect of the AAV2-hAADC Gene Therapy in 6-OHDA Lesioned Rats

Progressively blunted response to L-DOPA in Parkinson’s disease (PD) is a critical factor that complicates long-term pharmacotherapy in view of the central importance of this drug in management of the PD-related motor disturbance. This phenomenon is likely due to progressive loss of one of the key enzymes involved in the biosynthetic pathway for dopamine in the basal ganglia: aromatic L-amino acid decarboxylase (AADC). We have developed a gene therapy based on an adeno-associated virus encoding human AADC (AAV2-hAADC) infused into the Parkinsonian striatum. Although no adverse clinical effects of the AAV2-hAADC gene therapy have been observed so far, the ability to more precisely regulate transgene expression or transgene product activity could be an important long-term safety feature. The present study was designed to define pharmacological regulation of the functional activity of AAV2-hAADC transgene product by manipulating L-DOPA and carbidopa (AADC inhibitor) administration in hemi-parkinsonian rats. Thirty days after unilateral striatal infusion of AAV2-hAADC, animals displayed circling behavior and acceleration of dopamine metabolism in the lesioned striatum after administration of a low dose of L-DOPA (5 mg/kg) co-administered with 1.25 mg/kg of carbidopa. This phenomenon was not observed in control AAV2-GFP-treated rats. Withdrawal of carbidopa from a daily L-DOPA regimen decreased the peripheral L-DOPA pool, resulting in almost total loss of L-DOPA-induced behavioral response in AAV2-hAADC rats and a significant decline in striatal dopamine turnover. The serum L-DOPA level correlated with the magnitude of circling behavior in AAV2-hAADC rats. Additionally, AADC activity in homogenates of lesioned striata transduced by AAV2-AADC was 10-fold higher when compared with AAV2-GFP-treated control striata, confirming functional transduction. Our data suggests that the pharmacological regulation of circulating L-DOPA might be effective in the controlling of function of AAV2-hAADC transgene product in PD gene therapy.     

Preproenkephalin mRNA and Methionine-Enkephalin Content Are Increased in Mouse Striatum After Treatment with Nicotine

Abstract: A single dose of nicotine increased methionine-enkephalin (Met-Enk) immunoreactivity in the striatum of mice in a time-dependent manner. Met-Enk content reached a maximum by ∼1 h after nicotine and returned to control values by 6 h. The response to nicotine was blocked by pretreating animals with the nicotinic receptor antagonist mecamylamine. In contrast, pretreating mice with the muscarinic receptor antagonist atropine or the dopamine receptor antagonist haloperidol did not block the response. A single dose of nicotine also increased mRNA for the precursor peptide preproenkephalin (PPE). The increase of PPE mRNA preceded that of Met-Enk and reached a maximum by ∼30 min after nicotine. PPE mRNA levels returned to near normal by ∼3 h and increased again by 6 h after nicotine. Daily administration of nicotine for 14 days increased Met-Enk content and PPE mRNA in the striatum of mice as well. Taken together, our results suggest that nicotinic receptors modulate Met-Enk content and PPE mRNA in the mouse striatum.     

Long-Term L-DOPA Treatment Causes Indiscriminate Increase in Dopamine Levels at the Cost of Serotonin Synthesis in Discrete Brain Regions of Rats

(1) The treatment of choice for Parkinson’s disease (PD) is 3,4-dihydroxyphenylalanine (L-DOPA) with peripheral decarboxylase inhibitor, but long-term therapy leads to motor and psychiatric complications. In the present study we investigated 5-hydroxytryptamine (5-HT) and dopamine concentrations in serotonergic and dopaminergic nuclei following chronic administration of L-DOPA to find whether the neurotransmitter synthesis in these brain areas are compensated. (2) Rats were administered L-DOPA (250 mg/kg) and carbidopa (25 mg/kg) daily for 59 and 60 days, and killed on the 60th day, respectively at 24 h and 30 min after the last dose. L-DOPA, norepinephrine, 5-HT, 5-hydroxyindoleacetic acid (5-HIAA), dopamine, homovanillic acid (HVA), and 3,4-dihydroxyphenylacetic acid (DOPAC) were measured in striatum, nucleus raphe dorsalis (NRD), nucleus accumbens (NAc), substantia nigra, cerebellum, and cortex employing HPLC-electrochemical procedure. (3) Prolonged treatment of L-DOPA caused depression in the animals as revealed in a forced swim test. Serotonin content was significantly decreased in all brain regions studied 30 min after long-term L-DOPA, except in NAc. The cortex and striatum showed lowered levels of this indoleamine 24 h after 59 doses of L-DOPA. Dopamine, HVA, and DOPAC concentrations were significantly higher in all the regions studied after 30 min, and in the cerebellum after 24 h of L-DOPA. The levels of DOPAC were elevated in all the brain areas studied 24 h after prolonged L-DOPA treatment. (4) The present results suggest that long-term L-DOPA treatment results in significant loss of 5-HT in serotonergic and dopaminergic regions of the brain. Furthermore, while L-DOPA metabolism per se was uninfluenced, dopamine synthesis was severely impaired in all the regions. The imbalance of serotonin and dopamine formation may be the cause of overt cognitive, motor, and psychological functional aberrations seen in parkinsonian patients following prolonged L-DOPA treatment.     

Effects of acute and long-term treatment with neuroleptics on regional telencephalic neurotensin levels in the male rat

By means of radioimmunoassay measurements of regional neurotensin (NT) levels in the forebrain of the male rat it was shown that selective D2 DA receptor antagonists, such as haloperidol and sulpiride, and unselective D1 and D2 antagonists such as thioridazine, flupenthixol clozapine and fluperlapine, can acutely increase NT levels in the striatum and the nucleus accumbens without affecting NT levels in the amygdaloid or anteromedial frontal cortex. Conversely, acute treatment with the D1 DA receptor antagonist Schering 23390 (SCH 23390) produced a selective reduction of striatal NT levels. After long-term treatment clozapine, fluperlapine or SCH 23390, tolerance developed with regard to their ability to modulate striatal and accumbens levels. No tolerance occurred after chronic haloperidol, chlorpromazine and sulpiride. The results indicate that the acute administration of D1 and D2 DA receptor antagonists differentially modifies NT levels in the striatum and nuc. accumbens, and that antipsychotic drugs showing a relative lack of extrapyramidal side effects may be characterised by a failure to maintain increased NT levels in the basal ganglia upon long-term treatment.     

Exposure to Far Infrared Ray Protects Methamphetamine-Induced Behavioral Sensitization in Glutathione Peroxidase-1 Knockout Mice via Attenuating Mitochondrial Burdens and Dopamine D1 Receptor Activation

Evidence indicates that stress conditions might lead to drug dependence. Recently, we have demonstrated that exposure to far infrared ray (FIR) attenuates acute restraint stress via induction of glutathione peroxidase-1 (GPx-1) gene. We investigated whether FIR affects methamphetamine (MA)-induced behavioral sensitization and whether FIR-mediated pharmacological activity requires interaction between dopamine receptor and GPx-1 gene. We observed that MA treatment significantly increased GPx-1 expression in the striatum of wild-type (WT) mice. Interestingly, exposure to FIR potentiated MA-induced increase in GPx-1 expression. This phenomenon was also observed in animals receiving MA with dopamine D1 receptor antagonist SCH23390. However, dopamine D2 receptor antagonist sulpiride did not affect MA-induced GPx-1 expression. FIR exposure or SCH23390, but not sulpiride, significantly attenuated MA-induced behavioral sensitization. Exposure to FIR significantly attenuated MA-induced dopamine D1 receptor expression, c-Fos induction and oxidative burdens. FIR-mediated antioxidant effects were also more pronounced in mitochondrial- than cytosolic-fraction. In addition, FIR significantly attenuated against MA-induced changes in mitochondrial superoxide dismutase and mitochondrial GPx activities, mitochondrial transmembrane potential, intramitochondrial Ca²⁺ level, mitochondrial complex-I activity, and mitochondrial oxidative burdens. The attenuation by FIR was paralleled that by SCH23390. Effects of FIR or SCH23390 were more sensitive to GPx-1 KO than WT mice, while SCH23390 treatment did not exhibit any additive effects on the protective activity mediated by FIR, indicating that dopamine D1 receptor constitutes a molecular target of FIR. Our result suggests that exposure to FIR ameliorates MA-induced behavioral sensitization via possible interaction between dopamine D1 receptor and GPx-1 gene.     

Involvement of Dopamine D1 and D2 Receptors in the Regulation of Proenkephalin mRNA Abundance in the Striatum and Accumbens of the Rat Brain

The effects of short-term treatment (6 h) with selective D1 or D2 agonists and antagonists on the mRNA for proenkephalin in the medial and anterior aspects of the caudate-putamen and the nucleus accumbens were assessed by in situ hybridization histochemistry. Proenkephalin mRNA abundance was significantly changed in the striatum and accumbens in response to D2 receptor manipulation. D2 blockade with haloperidol or raclopride increased, whereas D2 stimulation with LY-171555 (D2 agonist) decreased, striatal and accumbens proenkephalin mRNA abundance. Antagonism of D1 receptor activity with SCH-23390 significantly decreased proenkephalin mRNA abundance in all brain regions. Concurrent administration of the D1 agonist SKF-38393 prevented the SCH-23390 effect in all brain areas. The data demonstrate that acute treatment with dopaminergic D2 agonists and antagonists affects proenkephalin mRNA abundance in the striatum and accumbens via a D2 receptor mechanism, consistent with the concept that D2 receptor function inhibits the synthesis of the mRNA encoding the enkephalin peptides. Moreover, D1 receptor activity, directly or indirectly, exerts modulatory effects on proenkephalin mRNA abundance in the striatum and nucleus accumbens.     

Plasticity of glutamatergic control of striatal acetylcholine release in experimental parkinsonism: opposite changes at group-II metabotropic and NMDA receptors

To investigate whether adaptive changes of glutamatergic transmission underlie dysfunction of the cholinergic system in experimental parkinsonism, the effects of group-II metabotropic glutamate and NMDA receptor ligands on acetylcholine release was studied in striatal slices and synaptosomes obtained from naive rats, 6-hydroxydopamine hemi-lesioned rats and 6-hydroxydopamine hemi-lesioned rats chronically treated with levodopa (L-DOPA) plus benserazide (non-dyskinetic). Group-II metabotropic glutamate receptor agonists LY354740, DCG-IV and L-CCG-I inhibited the electrically-evoked endogenous acetylcholine release from slices, while NMDA facilitated it. LY354740 also inhibited K+-evoked acetylcholine release from synaptosomes. LY354740-induced inhibition was prevented by the group-II metabotropic glutamate receptor antagonist LY341495. In hemi-parkinsonian rats, sensitivity towards LY354740 was reduced while that to NMDA was enhanced in the lesioned (denervated) compared with unlesioned striatum. Moreover, dizocilpine inhibited acetylcholine release in the lesioned compared with unlesioned striatum. Chronic treatment with L-DOPA normalized sensitivity towards glutamatergic agonists. We conclude that striatal dopamine denervation results in plastic changes at group-II metabotropic glutamate and NMDA receptors that may shift glutamatergic control of acetylcholine release towards facilitation. From a clinical perspective, L-DOPA and NMDA antagonists appear effective in counteracting overactivity of striatal cholinergic interneurones associated with Parkinson's disease.     

Biotransformation of l-DOPA to Dopamine in the Substantia Nigra of Freely Moving Rats: Effect of Dopamine Receptor Agonists and Antagonists

Abstract: We investigated the effects of continuous intranigral perfusion of dopamine D1 and D2 receptor agonists and antagonists on the biotransformation of locally applied l -DOPA to dopamine in the substantia nigra of freely moving rats by means of in vivo microdialysis. The "dual-probe" mode was used to monitor simultaneously changes in extracellular dopamine levels in the substantia nigra and the ipsilateral striatum. Intranigral perfusion of 10 µ M l -DOPA for 20 min induced a significant 180-fold increase in extracellular nigral dopamine level. No effect of the intranigral l -DOPA administration was observed on dopamine levels in the ipsilateral striatum, suggesting a tight control of extracellular dopamine in the striatum after enhanced nigral dopamine levels. Continuous nigral infusion with the D1 receptor agonist CY 208243 (10 µ M ) and with the D2 receptor agonist quinpirole at 10 µ M (a nonselective concentration) attenuated the l -DOPA-induced increase in dopamine in the substantia nigra by 85 and 75%, respectively. However, perfusion of the substantia nigra with a lower concentration of quinpirole (1 µ M ) and the D1 antagonist SCH 23390 (10 µ M ) did not affect the nigral l -DOPA biotransformation. The D2 antagonist (−)-sulpiride (10 µ M ) also attenuated the l -DOPA-induced dopamine release in the substantia nigra to ∼10% of that of the control experiments. We confirm that there is an important biotransformation of l -DOPA to dopamine in the substantia nigra. The high concentrations of dopamine formed after l -DOPA administration may be the cause of dyskinesias or further oxidative stress in Parkinson's disease. Simultaneous administration of D1 receptor agonists with l -DOPA attenuates the biotransformation of l -DOPA to dopamine in the substantia nigra. The observed effects could occur via changes in nigral GABA release that in turn influence the firing rate of the nigral dopaminergic neurons.     

Attenuation of D-1 antagonist-induced D-1 receptor upregulation by concomitant D-2 receptor blockade

The effect of chronic selective D-1 and/or D-2 dopamine receptor blockade on regional D-1 receptor binding was studied in rat brain following chronic treatment with the specific D-1 antagonist SCH 23390 and/or the predominantly D-2 antagonist haloperidol. D-1 receptor density and affinity were evaluated by quantitative autoradiography using 125I-SCH 23982. Chronic SCH 23390 treatment increased D-1 receptor density by 30 to 40% in the striatum, accumbens and tuberculum olfactorium; receptor affinity remained unchanged. Haloperidol had no effect on D-1 receptor Bmax or Kd values, although, when administered with SCH 23390, reduced the D-1 receptor upregulation induced by the D-1 antagonist in striatum and tuberculum olfactorium, but not in nucleus accumbens. These results may be attributable to D-1/D-2 dopamine receptor interactions occurring in the striatum and tuberculum olfactorium and may have implications for the prevention and treatment of drug-induced extrapyramidal disorders.     

Stimulatory role of dopamine on fibroblast growth factor-2 expression in rat striatum

We have previously shown that systemic injection of (-)nicotine produces a selective up-regulation of fibroblast growth factor (FGF)-2 mRNA levels in rat striatum. Because (-)nicotine can increase striatal release of dopamine and glutamate, in the present study we have investigated the contribution of these neurotransmitters in the modulation of FGF-2 expression. We found that coinjection of dopaminergic D1 (SCH23390) or D2 (haloperidol) receptor antagonists prevents nicotine-induced elevation of FGF-2 expression. However, injection of the NMDA receptor antagonist MK-801 produced a significant increment of FGF-2 mRNA and protein levels in rat striatum similar to the effect produced by (-)nicotine alone. Interestingly this effect of MK-801 could also be prevented by D1 or D2 receptor antagonists, suggesting that an elevation of dopamine levels may be required for the regulation of the trophic molecule. Accordingly we found that the non-selective dopaminergic agonist apomorphine can similarly increase striatal FGF-2 mRNA levels. Despite the observation that both D1 and D2 receptors appear to contribute to the modulation of FGF-2 expression, only a direct activation of D2 receptors, through quinpirole administration, was able to mimic the effect of apomorphine. On the basis of FGF-2 neurotrophic activity, these results suggest that direct or indirect activation of dopaminergic system can be neuroprotective and might reduce cell vulnerability in degenerative disorders.     

β-Lactotensin derived from bovine β-lactoglobulin exhibits anxiolytic-like activity as an agonist for neurotensin NTS(2) receptor via activation of dopamine D(1) receptor in mice

β-Lactotensin (His-Ile-Arg-Leu) is a bioactive peptide derived from bovine milk β-lactoglobulin, acting as a natural agonist for neurotensin receptors. We found that β-lactotensin exhibited anxiolytic-like activity in an elevated plus-maze test after its intraperitoneal (i.p.) administration in mice. β-Lactotensin was also orally active. The anxiolytic-like activity of β-lactotensin after i.p. administration was blocked by levocabastine, an antagonist for the neurotensin NTS(2) receptor. β-Lactotensin had anxiolytic-like activity in wild-type but not Ntsr2-knockout mice. β-Lactotensin increased intracellular Ca(2+) flux in glial cells derived from wild-type mice but not Ntsr2 knockout mice. These results suggest that β-lactotensin acts as an NTS(2) receptor agonist having anxiolytic-like activity. The anxiolytic-like activity of β-lactotensin was also blocked by SCH23390 and SKF83566, antagonists for dopamine D(1) receptor, but not by raclopride, an antagonist for D(2) receptor. Taken together, β-lactotensin may exhibit anxiolytic-like activity via NTS(2) receptor followed by D(1) receptor.