An intact dopaminergic system is required for context-conditioned release of 5-HT in the nucleus accumbens of postweaning isolation-reared rats

We investigated the effect of the tyrosine hydroxylase inhibitor, alpha-methyl-para-tyrosine (AMPT) on extracellular dopamine and 5-HT levels in the nucleus accumbens of group- and isolation-reared rats. Microdialysis with high-performance liquid chromatography-electrochemical detection was used to quantify dopamine and 5-HT efflux in the nucleus accumbens following foot shock and in association with a conditioned emotional response (CER). Isolation- and group-reared rats received i.p. injections of either saline (0.9%) or AMPT (200 mg/kg) 15 h and 2 h prior to sampling. There was no significant difference between saline-treated isolation- or group-reared rats for basal efflux of dopamine or 5-HT, however as expected, AMPT-treatment significantly reduced dopamine efflux in both groups to an equivalent level (50-55% saline-treated controls). Exposure to mild foot shock stimulated basal dopamine efflux in saline-treated groups only, although the effect was significantly greater in isolation-reared rats. In AMPT-treated rats, foot shock did not affect basal dopamine efflux in either rearing group. Foot shock evoked a prolonged increase in 5-HT efflux in both isolation- and group-reared saline-treated rats but had no effect on 5-HT efflux in AMPT-treated rats. In response to CER, isolation-rearing was associated with significantly greater efflux of both dopamine and 5-HT in saline-treated rats, compared to saline-treated, group-reared controls. However in AMPT-treated rats, efflux of dopamine or 5-HT did not change in response to CER. These data suggest that unconditioned or conditioned stress-induced changes in 5-HT release of the nucleus accumbens are dependent upon intact catecholaminergic neurotransmission. Furthermore, as the contribution of noradrenaline to catecholamine efflux in the nucleus accumbens is relatively minor compared to dopamine, our findings suggest that dopamine efflux in the nucleus accumbens is important for the local regulation of 5-HT release in this region. Finally, these findings implicate the isolation-enhanced presynaptic dopamine function in the accumbens with the augmented ventral striatal 5-HT neurotransmission characterized by isolation-reared rats.     

Nicotinamide reduces dopamine in postnatal hypothalamus and causes dopamine-deficient phenotype

Dopamine is an important neurotransmitter in the human central nervous system and also plays a key role in the development of postnatal brains. We previously reported that nicotinamide, a SIRT1 inhibitor, regulates tyrosine hydroxylase (TH) expression in vitro. To investigate the effect of nicotinamide-mediated TH regulation in vivo, nicotinamide was chronically injected into neonatal mice. Interestingly, nicotinamide-treated mice were smaller in size, and their locomotor activity was reduced. L-DOPA treatment caused hypersensitive locomotor activity that indicates a dopamine-depleted state. These changes seemed to be associated with dopamine metabolism in hypothalamus, since dopamine in hypothalamus was reduced but not in striatum. The present study suggests that the regulation of dopamine metabolism during the postnatal development is important and the underlying molecular mechanisms may be associated with SIRT1 signaling.     

Non-Dopaminergic Neurons Expressing Dopamine Synthesis Enzymes: Differentiation and Functional Significance

The development and functional significance of neurons in the arcuate nucleus expressing tyrosine hydroxylase and/or aromatic L-amino acid decarboxylase were studied in rat fetuses, neonates, and adults using immunocytochemical (single and double immunolabeling of tyrosine hydroxylase and aromatic L-amino acid decarboxylase) methods with a confocal microscope and computerized image analysis, HPLC with electrochemical detection, and radioimmunological analysis. Single-enzyme neurons containing tyrosine hydroxylase were first seen on day 18 of embryonic development in the ventrolateral part of the arcuate nucleus. Neurons expressing only aromatic L-amino acid decarboxylase or both enzymes of the dopamine synthesis pathway were first seen on day 20 of embryonic development, in the dorsomedial part of the nucleus. On days 20–21 of embryonic development, dopaminergic (containing both enzymes) neurons amounted to less than 1% of all neurons expressing tyrosine hydroxylase and/or aromatic L-amino acid decarboxylase. Nonetheless, in the ex vivo arcuate nucleus and in primary neuron cultures from this structure, there were relatively high levels of dopamine and L-dihydroxyphenylalanine (L-DOPA), and these substances were secreted spontaneously and in response to stimulation. In addition, dopamine levels in the arcuate nucleus in fetuses were sufficient to support the inhibitory regulation of prolactin secretion by the hypophysis, which is typical of adult animals. During development, the proportion of dopaminergic neurons increased, reaching 38% in adult rats. Specialized contacts between single-enzyme tyrosine hydroxylase-containing and aromatic L-amino acid decarboxylase-containing neurons were present by day 21 of embryonic development; these were probably involved in transporting L-DOPA from the former neurons to the latter. It was also demonstrated that the axons of single-enzyme decarboxylase-containing neurons projected into the median eminence, supporting the secretion of dopamine into the hypophyseal portal circulation. Thus, dopamine is probably synthesized in the arcuate nucleus not only by dopaminergic neurons, but also by neurons expressing only tyrosine hydroxylase or aromatic L-amino acid decarboxylase.     

A microdialysis study of striatal dopamine release in the circling rat, a genetic animal model with spontaneous lateralized rotational behavior

The circling rat is an autosomal recessive mutant (homozygous ci2/ci2) that displays lateralized circling behavior, locomotor hyperactivity, hyperexcitability, ataxia, and stereotypic head-movement. These abnormal behaviors are induced or intensified by stress. Heterozygous (ci2/+) littermates display normal spontaneous behaviors. We have previously found that ci2 rats of both genders have a lower tissue content of dopamine in the striatum ipsilateral to the preferred direction of rotation, indicating that the rats turn away from the brain hemisphere with higher striatal dopaminergic activity. For further evaluation as to whether the spontaneous turning behavior of the mutant rats results from specific disturbances within the nigrostriatal circuitry, we used microdialysis in freely moving mutant rats and their unaffected littermates to measure extracellular levels of dopamine and its metabolites in the striatum of both hemispheres. Rats were habituated to the experimental procedures, so that mutant animals behaved as normal during a first phase of microdialysis ("rest phase"), which was followed by a "stress phase" with induction of lateralized circling by handling-stress. During rest, no significant imbalance in striatal dopamine release was observed in mutant rats, their unaffected littermates, and a second control group consisting of normal, unaffected rats from the same Lewis/Ztm strain. Stress induced a significant increase in dopamine release in the contralateral striatum of mutant rats of both genders, whereas no significant alterations in dopamine release were seen in either the left or right striatum of control groups. When amphetamine (100 or 500 microM) was added to the perfusion medium, the evoked dopamine release in the contralateral striatum of female mutant rats was significantly higher than that in the ipsilateral striatum, whereas no such asymmetry was observed in male mutants or unaffected female and male controls.The data further substantiate that mutant circling rats possess a genetically mediated dysfunction of the central dopaminergic system, but it remains to be determined whether neurochemical disturbances in other regions contribute to the behavioral phenotype of the ci2 rat. The continued study of this mutant may provide important new insights into the anatomical, neurochemical and molecular basis of hyperkinetic motor syndromes and other disorders related to dopaminergic dysfunction.     

Glutamate regulates the spontaneous and evoked release of dopamine in the rat striatum

Resting and evoked extracellular dopamine levels in the striatum of the anesthetized rat were measured by fast-scan cyclic voltammetry in conjunction with carbon fiber microelectrodes. Identification of the substance detected in vivo was achieved by inspection of background-subtracted voltammograms. Intrastriatal microinfusion of kynurenate, a broad-spectrum antagonist of ionotropic glutamate receptors, caused a decrease in the resting extracellular level of dopamine. The kynurenate-induced decrease was unaffected by systemic pretreatment with pargyline, an inhibitor of monoamine oxidase, but was significantly attenuated by systemic pretreatment with alpha-methyl-p-tyrosine, an inhibitor of tyrosine hydroxylase. Although glutamate by itself did not affect resting extracellular dopamine levels, glutamate did attenuate the kynurenate-induced decrease. Kynurenate decreased dopamine release in response to electrical stimulation of the medial forebrain bundle, an effect that was also attenuated by glutamate. These results suggest that both spontaneous and evoked dopamine release in the rat striatum are under the local tonic excitatory influence of glutamate. Interactions between central dopamine and glutamate systems that have been implicated in the etiologies of Parkinson's disease, schizophrenia, stress, and substance abuse. The precise nature of those interactions, however, remains a matter of some controversy.     

Nociceptin/orphanin FQ modulation of rat midbrain dopamine neurons in primary culture

Previous microdialysis studies have identified a suppressive effect of the novel opioid peptide nociceptin (also known as orphanin FQ) on dopamine release from mesolimbic neurons. In order to further evaluate the locus of this action, we investigated nociceptin's action in an in vitro model system, namely midbrain dopamine neurons in primary culture. Immunohistochemical analysis revealed abundant tyrosine hydroxylase- and GABA-immunoreactive neurons, with a strong correlation between tyrosine hydroxylase content and basal endogenous dopamine release. Nociceptin (0.01-100 nM) suppressed basal dopamine release by up to 84% (EC50=0.65 nM). This action was reversible by drug removal and attenuated by co-application of the non-peptidergic ORL1 antagonist, Compound B. Nociceptin had no significant effect on dopamine release evoked by direct depolarization of the terminals with elevated extracellular K+, suggesting that nociceptin suppresses dopamine release by modulating the firing rate of the dopamine neurons. Nociceptin also suppressed GABA release from the cultures (45% maximal inhibition; EC50=1.63 nM). Application of the GABA-A antagonist, bicuculline, elevated extracellular dopamine concentrations but the dopamine release inhibiting property of nociceptin persisted in the presence of bicuculline. The NMDA receptor antagonist, D(-)-2-amino-5-phosphononpentanoic acid (AP-5) had no effect on basal dopamine release and failed to modify nociceptin's inhibitory effects. Thus, nociceptin potently modulates dopamine release from midbrain neurons most likely as a result of a direct suppression of dopamine neuronal activity.     

Age-Related Neostriatal Alterations in the Rat: Failure of L-DOPA to Alter Behavior

Age differences in rotational behavior were examined in young (6 mo) and old (24 mo) Wistar rats lesioned in the left substantia nigra with 6-OHDA. Young animals showed a 50% increase in rotational behavior with L-DOPA pretreatment and a 15–20% increase following L-tyrosine pretreatment. However, neither L-DOPA nor L-tyrosine pretreatment potentiated amphetamine-induced rotational behavior of senescent animals. Pretreatment with tranylcypromine, an MAO inhibitor, did not enhance rotational behavior in either group. After assessing rotational responses to amphetamine, half of each age group was given L-DOPA 1 hr prior to sacrifice, and right (RS) and left striatal (LS) levels of dopamine (DA) were examined in all groups. Comparable LS depletion was found in both age groups. L-DOPA significantly raised DA levels in the RS of the young animals while causing no change effect in old animals although the amount of L-DOPA entering the striatum was even higher in the senescent animals. Striatal tyrosine hydroxylase showed only a small decrease (15%) in activity, while DOPA decarboxylase activity showed no significant age-related decline. Despite the lack of substantial decrease in enzyme activity, the results indicate an age-dependent decrease in the capacity for L-DOPA potentiation of rotational behavior. Defects may exist at the level of elevation of the functional pool of DA, the release of DA, or the interaction of DA with a decreasing number of a class of DA receptors involved in motor control.     

Age-Related Neostriatal Alterations in the Rat: Failure of L-DOPA to Alter Behavior

Age differences in rotational behavior were examined in young (6 mo) and old (24 mo) Wistar rats lesioned in the left substantia nigra with 6-OHDA. Young animals showed a 50% increase in rotational behavior with L-DOPA pretreatment and a 15–20% increase following L-tyrosine pretreatment. However, neither L-DOPA nor L-tyrosine pretreatment potentiated amphetamine-induced rotational behavior of senescent animals. Pretreatment with tranylcypromine, an MAO inhibitor, did not enhance rotational behavior in either group. After assessing rotational responses to amphetamine, half of each age group was given L-DOPA 1 hr prior to sacrifice, and right (RS) and left striatal (LS) levels of dopamine (DA) were examined in all groups. Comparable LS depletion was found in both age groups. L-DOPA significantly raised DA levels in the RS of the young animals while causing no change effect in old animals although the amount of L-DOPA entering the striatum was even higher in the senescent animals. Striatal tyrosine hydroxylase showed only a small decrease (15%) in activity, while DOPA decarboxylase activity showed no significant age-related decline. Despite the lack of substantial decrease in enzyme activity, the results indicate an age-dependent decrease in the capacity for L-DOPA potentiation of rotational behavior. Defects may exist at the level of elevation of the functional pool of DA, the release of DA, or the interaction of DA with a decreasing number of a class of DA receptors involved in motor control.     

L-DOPA reverses motor deficits associated with normal aging in mice

We wished to determine whether L-DOPA, a common treatment for the motor deficits in Parkinson's disease, could also reverse the motor deficits that occur during aging. We assessed motor performance in young (2–3 months) and old (20–21 months) male C57BL/6 mice using the challenge beam and cylinder tests. Prior to testing, mice were treated with L-DOPA or vehicle. Following testing, striatal tissue was analyzed for phenotypic markers of dopamine neurons: dopamine, dopamine transporter, and tyrosine hydroxylase. Although the dopaminergic markers were unchanged with age or L-DOPA treatment, L-DOPA reversed the motor deficits in the old animals such that their motor coordination was that of a young mice. These findings suggest that some of the locomotor deficits that accompany normal aging are responsive to L-DOPA treatment and may be due to subtle alterations in dopaminergic signaling.     

L-dopaergic components in the caudal ventrolateral medulla in baroreflex neurotransmission.

L-3,4-Dihydroxyphenylalanine (L-DOPA) is probably a transmitter of the primary baroreceptor afferents terminating in the nucleus tractus solitarii; L-DOPA functions tonically to activate depressor sites of the caudal ventrolateral medulla, which receives input from the nucleus tractus solitarii [Misu Y. et al. (1996) Prog. Neurobiol. 49, 415-454]. We have attempted to clarify whether or not L-DOPAergic components within the caudal ventrolateral medulla are involved in baroreflex neurotransmission in anesthetized rats. Electrolytic lesions of the right nucleus tractus solitarii (1 mA d.c. for 10 s, 10 days before measurement) selectively decreased by 45% the tissue content of L-DOPA in the dissected ipsilateral caudal ventrolateral medulla. Electrolytic lesions did not decrease dopamine, norepinephrine and epinephrine levels. During microdialysis of the right caudal ventrolateral medulla, extracellular levels of L-DOPA, norepinephrine, epinephrine and 3,4-dihydroxyphenylacetic acid were consistently detectable using high-performance liquid chromatography with electrochemical detection. However, extracellular dopamine levels were lower than the assay limit. Baroreceptor activation by i.v. phenylephrine selectively evoked L-DOPA without increasing the levels of norepinephrine, epinephrine and 3,4-dihydroxyphenylacetic acid. This L-DOPA release was suppressed by acute lesion in the ipsilateral nucleus tractus solitarii. Intermittent stimulation of the right aortic depressor nerve (20 Hz, 3 V, 0.3 ms duration, for 30 min) repetitively and constantly caused L-DOPA release, hypotension and bradycardia, without increases in levels of norepinephrine, epinephrine and 3,4-dihydroxyphenylacetic acid. Local inhibition of L-DOPA synthesis with alpha-methyl-p-tyrosine (30 microM) infused into the ipsilateral caudal ventrolateral medulla gradually decreased basal levels of L-DOPA and 3,4-dihydroxyphenylacetic acid without decreasing norepinephrine and epinephrine. The inhibition of L-DOPA synthesis interrupted L-DOPA release and decreased by 65% depressor responses elicited by aortic nerve stimulation; however, it produced no effect on bradycardic responses. CoCl2 (119 ng), a mainly presynaptic inhibitory transmission marker, and L-DOPA methyl ester (1 microg), a competitive L-DOPA antagonist, when microinjected into depressor sites of the right caudal ventrolateral medulla, reduced by 60% depressor responses to transient ipsilateral stimulation of the aortic nerve (20 Hz, 3 V, 0.1 ms duration, for 10 s). No changes in bradycardic responses were observed. There may exist an L-DOPAergic relay from the nucleus tractus solitarii to the caudal ventrolateral medulla. L-DOPAergic components in the caudal ventrolateral medulla are involved in baroreflex neurotransmission via a baroreceptor-aortic depressor nerve-nucleus tractus solitarii-caudal ventrolateral medulla relay in the rat.     

An L-dopaergic relay from the posterior hypothalamic nucleus to the rostral ventrolateral medulla and its cardiovascular function in anesthetized rats.

We have proposed that L-3,4-dihydroxyphenylalanine (L-DOPA) is a neurotransmitter in the central nervous system [Misu Y. et al. (1996) Prog. Neurobiol. 49, 415-454]. Herein, we attempt to clarify whether lesions in the posterior hypothalamic nucleus decrease the tissue content of L-DOPA in the rostral ventrolateral medulla. We also attempt to clarify whether or not endogenous L-DOPA is evoked by electrical stimulation of the posterior hypothalamic nucleus. It is possible that evoked L-DOPA functions as a transmitter candidate to activate pressor sites of the rostral ventrolateral medulla in anesthetized rats. Electrolytic lesions were made in the bilateral posterior hypothalamic nucleus by a monopolar direct current of 2 mA for 10 s, 10 days before measurements. The effect of the lesions was to selectively decrease the tissue content of L-DOPA by one-half in the right rostral ventrolateral medulla. Decreases in the amounts of dopamine, noradrenaline or adrenaline were not observed. Decreases were also not evident in the right caudal ventrolateral medulla. During microdialysis of the right rostral ventrolateral medulla, extracellular basal levels of L-DOPA and three types of catecholamine were consistently detectable by high-performance liquid chromatography with electrochemical detection. Tetrodotoxin (1 microM) perfused into the right rostral ventrolateral medulla gradually decreased basal levels of L-DOPA by 25%; it decreased basal levels of noradrenaline and adrenaline by 25-30% and dopamine levels by 40%. Intensive electrical stimulation of the ipsilateral posterior hypothalamic nucleus (50 Hz, 0.3 mA, 0.1 ms duration, twice for 5 min at an interval of 5 min) selectively caused the release of L-DOPA in a repetitive and constant manner. The stimulation was accompanied by hypertension and tachycardia. However, catecholamines were not released. Tetrodotoxin suppressed the release of L-DOPA, but partially inhibited hypertension with only a slight inhibition of tachycardia evoked by stimulation of the posterior hypothalamic nucleus. L-DOPA methyl ester, a competitive L-DOPA antagonist, was bilaterally microinjected into pressor sites of the rostral ventrolateral medulla at 1.5 microg x 2 and 3 microg x 2. The antagonist dose-dependently and consistently antagonized pressor and tachycardiac responses to mild transient stimulation of the unilateral posterior hypothalamic nucleus (33 Hz, 0.2 mA, 0.1 ms duration, for 10 s). In addition, the antagonist alone (3 microg x 2) elicited hypotension and bradycardia. These results show that an L-DOPAergic relay may project from the posterior hypothalamic nucleus directly to pressor sites of the rostral ventrolateral medulla and/or indirectly to certain neurons near pressor sites in microcircuits of the same region. When released, L-DOPA appears to function tonically to activate pressor sites; it also appears to be involved in the maintenance and regulation of blood pressure and heart rate.     

Electroacupuncture-related changes of NADPH-diaphorase and neuronal nitric oxide synthase in the brainstem of spontaneously hypertensive rats

The effects of chronic alcohol use on the mesostriatal dopamine (DA) system remain relatively unknown. The aim of the present study was to assess multiple measures of the status of the mesostriatal DA system in rats chronically fed an alcohol diet for approximately 1 year. Tissue levels of DA and its metabolite, 3,4-dihydroxyphenylacetic acid, were significantly decreased in both the dorsal striatum (34 and 33%, respectively) and ventral striatum (33 and 36%, respectively) in alcohol-fed rats compared to pair-fed matched controls. Western blotting revealed a mean 20% decrease in tyrosine hydroxylase protein levels in the dorsal and ventral striatum of alcohol-fed animals while dopamine transporter protein levels from the same animals were significantly increased compared to controls (mean 60% increase for the dorsal and ventral striatum). The present results demonstrate significant alterations in the mesostriatal DA system after 1 year of chronic alcohol use. It is possible that the observed changes in DA synthesis and re-uptake measures result in altered intracellular and extracellular DA levels, perhaps contributing to the addictive properties of alcohol.     

Melatonin attenuates the amphetamine-induced decrease in vesicular monoamine transporter-2 expression in postnatal rat striatum

The vesicular monoamine transporter-2 (VMAT-2) is responsible for packaging intraneuronal dopamine into synaptic vesicles in preparation for synaptic release and is a critical regulator of cytoplasmic dopamine levels and dopaminergic function. It has long been recognized that VMAT-2 is also a critical mediator of amphetamine-induced dopamine release. Amphetamine-induced lesions during development have the potential to produce numerous permanent abnormalities in neural circuitry and function. Therefore, in the present study, we investigated the effects of amphetamine on the levels of VMAT-2, α-synuclein and phosphorylated tyrosine hydroxylase in the striatum of neonatal rats. We found that chronic amphetamine administration in postnatal rats produces dopaminergic deficits in the striatum, including decreases in the levels of VMAT-2 and phosphorylated tyrosine hydroxylase. In addition, an increase in α-synuclein expression was observed in the striatum of postnatal rats following chronic amphetamine treatment. Furthermore, we identified a role of (10mg/kg) melatonin, a methoxyindole released from the pineal gland, in attenuating the detrimental effects of amphetamine on dopaminergic neurons.     

Differential effects of transection of the spinal cord and splanchnic nerve on adrenal tyrosine hydroxylase and catecholamines

The role of spinal pathways in the regulation of adrenal medullary tyrosine hydroxylase and catecholamines was studied in adult rats subjected to spinal cord transection at the third thoracic level. In these animals the sympathoadrenal preganglionic neurons were isolated from their supraspinal afferents. This treatment led after three days to a progressive reduction of tyrosine hydroxylase activity and dopamine content (as compared to unoperated controls) until at least the 10th day. The results of the administration of dexamethasone or adrenocorticotropic hormone to spinalized rats suggest that in these animals glucocorticoid hypersecretion is not involved in the decline of adrenal tyrosine hydroxylase and that in fact adrenocorticotropic hormone supplementation can prevent it. A neurogenic origin for the depression of adrenomedullary function is favoured because unilateral splanchnicotomy (which by itself does not affect adrenal tyrosine hydroxylase), prior to cord section, prevented the diminution of tyrosine hydroxylase activity in the denervated gland.The decline of adrenal tyrosine hydroxylase and dopamine after spinal section may result from a decrease of modulatory impulses to the adrenal from decentralized sympathoadrenal preganglionic neurons in the isolated cord, following the loss of a descending facilitation of these neurons and/or the release of a segmental interneuronal inhibition of these neurons from a descending inhibitory influence. Such descending pathways may decussate partially below the low cervical level because rats with hemisection of the cord at C₆-C₇ exhibited no decline of adrenal tyrosine hydroxylase or of dopamine measured on either side seven days postoperatively.     

Dopamine Synthesis by Non-Dopaminergic Neurons in the Arcuate Nucleus of Rat Fetuses

The aim of the present work was to verify the hypothesis that non-dopaminergic neurons expressing individual complementary dopamine synthesis enzymes can perform the co-located synthesis of dopamine. According to this hypothesis, neurons expressing tyrosine hydroxylase use L-tyrosine for the synthesis of L-dihydroxyphenylalanine (L-DOPA), which then enters neurons expressing aromatic amino acid decarboxylase, which converts L-DOPA to dopamine. Experiments were performed using the mediobasal hypothalamus of rat fetuses, which mostly contains single-enzyme neurons (>99%) and occasional double-enzyme neurons (<1%). Controls were obtained from the fetal substantia nigra, which is enriched with dopaminergic neurons. High-performance liquid chromatography was used to measure levels of dopamine and L-DOPA in cell extracts and the incubation medium after incubation in the presence and absence of exogenous L-tyrosine. Addition of L-tyrosine to the medium led to increases in the level of synthesis and release of L-DOPA in the mediobasal hypothalamus and substantia nigra. In addition, L-tyrosine increased dopamine synthesis in the substantia nigra and decreased dopamine synthesis in the mediobasal hypothalamus. This regional difference in levels of dopamine synthesis is probably due to inhibition of the uptake of L-DOPA from the intercellular medium by neurons in the mediobasal hypothalamus containing aromatic amino acid decarboxylase, due to the competitive binding of the L-DOPA transporter by L-tyrosine. Thus, these results provide the first evidence for the co-located synthesis of dopamine by non-dopaminergic neurons expressing single complementary enzymes involved in the synthesis of this neurotransmitter.__________Translated from Rossiiskii Fiziologicheskii Zhurnal imeni I. M. Sechenova, Vol. 90, No. 7, pp. 825–832, July, 2004.     

Opposite tonic modulation of dopamine and adenosine on c-fos gene expression in striatopallidal neurons

The impulse flow-dependent dopamine release in the striatum was acutely blocked by unilateral lesion of the medial forebrain bundle with 6-hydroxydopamine. Within 45 min this disruption reduced the striatal extracellular dopamine levels by 80% as determined by in vivo voltammetry. A strong induction of c-fos messenger RNA was detected in the ipsilateral dorsolateral striatum 75 min after 6-hydroxydopamine injection by in situ hybridization. Double labelling demonstrates that this induction was confined to neurons expressing the dopamine D2 receptor messenger RNA. At this time-point, there were no changes in the striatal levels of either tyrosine hydroxylase immunoreactivity or dopamine D2 receptor messenger RNA. The c-fos messenger RNA expression induced by acute 6-hydroxydopamine injection was abolished by intraperitoneal pretreatment with the dopamine D2 receptor agonist, quinelorane (2 mg/kg) and strongly reduced by administration of the selective adenosine A2A receptor antagonist SCH-58261 (5 mg/kg). The results reported here show, by using a novel methodological approach, that an acute decrease of dopamine release causes an induction of c-fos messenger RNA in dopamine D2 receptor-containing striatopallidal neurons. This, together with previous findings, demonstrates that the c-fos gene expression is tonically inhibited by the impulse flow-dependent dopamine release via D2 receptors. In addition, this study provides evidence that endogenous adenosine, acting via adenosine A2A receptors, induces striatal c-fos messenger RNA when extracellular dopamine levels are strongly reduced. Thus endogenous dopamine and adenosine exert opposite effects on the activity of the D2-containing striatopallidal neurons.     

Effects of chronic environmental and social stimuli during adolescence on mesolimbic dopaminergic circuitry markers

Previously, we have shown that chronic exposure to environmental and social stimuli (ESS) during adolescence prevents the development of behavioral sensitization to amphetamine in adult rats. At the onset of the peripubertal-juvenile period (28-d) male rats were subjected to a 28-d long intermittent ESS protocol or handled as controls (NO-ESS). Twenty-four hours after the last session of ESS or NO-ESS, all rats started a regimen of behavioral sensitization to amphetamine (1mg/kg, i.p.), in which rats were injected every third day with amphetamine or saline on four occasions. Then following one week abstinence all rats were challenged with a lower dose of amphetamine (0.5mg/kg, i.p.) and their locomotor activity monitored for 2h. Our results showed that while NO-ESS rats developed behavioral sensitization to amphetamine, ESS rats did not develop this behavior. All rats were then sacrificed 3 days following the challenge to allow for amphetamine clearance. Since mesolimbic dopamine has been implicated in behavioral sensitization to amphetamine we compared messenger RNA (mRNA) expression of key dopamine-related molecules in the mesolimbic circuitry in ESS and NO-ESS rats. A decrease in dopaminergic D1 receptor (D1R) gene expression in the caudate-putamen (CPu) was associated with amphetamine sensitization in the controls, possibly as a result of a chronic increase in DA release. In contrast, amphetamine treatment did not modulate D1R mRNA levels in ESS rats. No change has been detected in any other dopaminergic markers [D2R, D3R, tyrosine hydroxylase (TH) or dopamine transporter (DAT) mRNAs]. Consequently, we conclude that ESS may inhibit the development of behavioral sensitization to amphetamine through preventing the decrease in CPu D1R mRNA levels.     

Nicotine induces calcium spikes in single nerve terminal varicosities: a role for intracellular calcium stores

We have previously shown that a large part of the D-amphetamine-induced release of dopamine in the nucleus accumbens is not associated with an increase in locomotor activity, and that "functional" dopamine release (i.e. release of dopamine associated with locomotor activity) requires the distal facilitation of noradrenergic transmission through alpha1-adrenergic receptors in the prefrontal cortex. To determine the role of monosynaptic or polysynaptic projections from the prefrontal cortex to the nucleus accumbens in these amphetamine responses, either AMPA/kainate (6-cyano-7-nitroquinoxaline-2,3-dione, CNQX, 300microM), N-methyl-D-aspartate (D(-)-2-amino-5-phosphono-pentanoic acid, APV, 500microM) or metabotropic [(+)-alpha-methyl-4-carboxy-phenylglycine, MCPG, 10mM] glutamate receptor antagonists were infused through a dialysis probe in the rat nucleus accumbens. CNQX and MCPG but not APV reduced the "non-functional" release of dopamine evoked by local (3microM) and systemic D-amphetamine (2mg/kg i.p.) treatments. However, the locomotor hyperactivity and functional dopamine release induced by systemic D-amphetamine were abolished by MCPG, but neither by CNQX nor by APV. MCPG treatment also abolished the hyperlocomotor activity and functional dopamine release evoked by bilateral morphine injection into the ventral tegmental area. The dopamine release evoked by this morphine treatment was 16-fold lower than that induced by the systemic D-amphetamine injection, although similar behavioral activations were observed. Altogether, our results further aid the discrimination of functional and non-functional release of dopamine. We suggest that the activation of metabotropic glutamate receptors in the nucleus accumbens is required for functional dopamine release following systemic D-amphetamine injection.     

Reduced extracellular dopamine and increased responsiveness to novelty: neurochemical and behavioral sequelae of intermittent hypoxia

STUDY OBJECTIVES: Vesicular monoamine transporter and dopamine D1-receptor protein expression are upregulated within the striatum of adults rats exposed to intermittent hypoxic insults as neonates. These observations prompted us to test the hypothesis that intermittent hypoxic insults, occurring during this period of critical brain development, lead to persistent reductions in extracellular levels of dopamine within the striatum. We also tested the hypothesis that post-hypoxic rats exhibit increased novelty-induced behavioral activation and increased basal levels of locomotor activity, two indexes of impaired dopaminergic functioning. STUDY DESIGN: Extracellular levels of dopamine were measured over a 50-hour interval via in-vivo microdialysis. Responsivity to novelty and basal levels of locomotor activity were measured via cubicles equipped with infrared transmitters and photosensors. INTERVENTIONS: Between postnatal days 7 and 11, rat pups were exposed to intermittent hypoxia (20-second bursts of hypoxic gas consisting of 10% oxygen balance nitrogen; 60 events/hour) or compressed air for 6 hours during their major sleep period. On postnatal day 35, locomotor activity was continuously recorded over a 72-hour period. On postnatal day 42, extracellular levels of dopamine were determined in microdialysis samples collected at 30-minute intervals continuously for 50 hours. MEASUREMENTS AND RESULTS: Post-hypoxic rats exhibited heightened behavioral activation when placed into the novel environment of locomotor-recording cubicles. Following 24 hours of adaptation to the cubicles, post-hypoxic rats exhibited locomotor hyperactivity during each dark phase of the circadian cycle, their typical waking period. These same rats also demonstrated reduced levels of extracellular dopamine during both the dark and light phases of the circadian cycle. CONCLUSIONS: We demonstrate increased responsivity to novelty, locomotor hyperactivity, and reduced levels of extracellular dopamine within the striata of juvenile rats exposed to intermittent hypoxic insults between postnatal days 7 and 11. These data, in conjunction with our previous observations, support our hypothesis that intermittent hypoxic insults occurring during a period of critical brain development lead to sequestration of dopamine presynaptically within nigrostriatal axons. We postulate that neonatally occurring hypoxic insults are one potential pathogenic mechanism underlying disorders of minimal brain dysfunction, such as attention-deficit/hyperactivity disorder, characterized by executive dysfunction and hyper responsiveness to novel stimuli, which is responsive to agents promoting enhanced extracellular levels of synaptic dopamine.