Presynaptic adenosine A2 and N-methyl-D-aspartate receptors regulate dopamine synthesis in rat striatal synaptosomes

Functional interaction between pre-synaptic α6β2-containing nicotinic and adenosine A2A receptors in the control of dopamine release in the rat striatum

BACKGROUND AND PURPOSE

Pre-synaptic nicotinic ACh receptors (nAChRs) and adenosine A2A receptors (A2A Rs) are involved in the control of dopamine release and are putative therapeutic targets in Parkinson's disease and addiction. Since A2A Rs have been reported to interact with nAChRs, here we aimed at mapping the possible functional interaction between A2A Rs and nAChRs in rat striatal dopaminergic terminals.

EXPERIMENTAL APPROACH

We pharmacologically characterized the release of dopamine and defined the localization of nAChR subunits in rat striatal nerve terminals in vitro and carried out locomotor behavioural sensitization in rats in vivo.

KEY RESULTS

In striatal nerve terminals, the selective A2A R agonist CGS21680 inhibited, while the A2A R antagonist ZM241385 potentiated the nicotine-stimulated [(3) H]dopamine ([(3) H]DA) release. Upon blockade of the α6 subunit-containing nAChRs, the remaining nicotine-stimulated [(3) H]DA release was no longer modulated by A2A R ligands. In the locomotor sensitization experiments, nicotine enhanced the locomotor activity on day 7 of repeated nicotine injection, an effect that no longer persisted after 1 week of drug withdrawal. Notably, ZM241385-injected rats developed locomotor sensitization to nicotine already on day 2, which remained persistent upon nicotine withdrawal.

CONCLUSIONS AND IMPLICATIONS

These results provide the first evidence for a functional interaction between nicotinic and adenosine A2A R in striatal dopaminergic terminals, with likely therapeutic consequences for smoking, Parkinson's disease and other dopaminergic disorders.

Memantine block depends on agonist presentation at the NMDA receptor in substantia nigra pars compacta dopamine neurones

NMDA glutamate receptors (NMDARs) have critical functional roles in the nervous system but NMDAR over-activity can contribute to neuronal damage. The open channel NMDAR blocker, memantine is used to treat certain neurodegenerative diseases, including Parkinson's disease (PD) and is well tolerated clinically. We have investigated memantine block of NMDARs in substantia nigra pars compacta (SNc) dopamine neurones, which show severe pathology in PD. Memantine (10 μM) caused robust inhibition of whole-cell (synaptic and extrasynaptic) NMDARs activated by NMDA at a high concentration or a long duration, low concentration. Less memantine block of NMDAR-EPSCs was seen in response to low frequency synaptic stimulation, while responses to high frequency synaptic stimulation were robustly inhibited by memantine; thus memantine inhibition of NMDAR-EPSCs showed frequency-dependence. By contrast, MK-801 (10 μM) inhibition of NMDAR-EPSCs was not significantly different at low versus high frequencies of synaptic stimulation. Using immunohistochemistry, confocal imaging and stereological analysis, NMDA was found to reduce the density of cells expressing tyrosine hydroxylase, a marker of viable dopamine neurones; memantine prevented the NMDA-evoked decrease. In conclusion, memantine blocked NMDAR populations in different subcellular locations in SNc dopamine neurones but the degree of block depended on the intensity of agonist presentation at the NMDAR. This profile may contribute to the beneficial effects of memantine in PD, as glutamatergic activity is reported to increase, and memantine could preferentially reduce over-activity while leaving some physiological signalling intact.

Caffeine regulates frontocorticostriatal dopamine transporter density and improves attention and cognitive deficits in an animal model of attention deficit hyperactivity disorder

Attention deficit hyperactivity disorder (ADHD) likely involves dopaminergic dysfunction in the frontal cortex and striatum, resulting in cognitive and motor abnormalities. Since both adenosine and dopamine modulation systems are tightly intertwined, we tested if caffeine (a non-selective adenosine receptor antagonist) attenuated the behavioral and neurochemical changes in adolescent spontaneously hypertensive rats (SHR, a validated ADHD animal model) compared to their control strain (Wistar Kyoto rats, WKY). SHR were hyperactive and had poorer performance in the attentional set-shifting and Y-maze paradigms and also displayed increased dopamine transporter (DAT) density and increased dopamine uptake in frontocortical and striatal terminals compared with WKY rats. Chronic caffeine treatment was devoid of effects in WKY rats while it improved memory and attention deficits and also normalized dopaminergic function in SHR. Additionally, we provide the first direct demonstration for the presence of adenosine A2A receptors (A2AR) in frontocortical nerve terminals, whose density was increased in SHR. These findings underscore the potential for caffeine treatment to normalize frontocortical dopaminergic function and to abrogate attention and cognitive changes characteristic of ADHD.

Adenosine receptors and neurological disease: neuroprotection and neurodegeneration

Adenosine receptors modulate neuronal and synaptic function in a range of ways that may make them relevant to the occurrence, development and treatment of brain ischemic damage and degenerative disorders. A(1) adenosine receptors tend to suppress neural activity by a predominantly presynaptic action, while A(2A) adenosine receptors are more likely to promote transmitter release and postsynaptic depolarization. A variety of interactions have also been described in which adenosine A(1) or A(2) adenosine receptors can modify cellular responses to conventional neurotransmitters or receptor agonists such as glutamate, NMDA, nitric oxide and P2 purine receptors. Part of the role of adenosine receptors seems to be in the regulation of inflammatory processes that often occur in the aftermath of a major insult or disease process. All of the adenosine receptors can modulate the release of cytokines such as interleukins and tumor necrosis factor-alpha from immune-competent leukocytes and glia. When examined directly as modifiers of brain damage, A(1) adenosine receptor (AR) agonists, A(2A)AR agonists and antagonists, as well as A(3)AR antagonists, can protect against a range of insults, both in vitro and in vivo. Intriguingly, acute and chronic treatments with these ligands can often produce diametrically opposite effects on damage outcome, probably resulting from adaptational changes in receptor number or properties. In some cases molecular approaches have identified the involvement of ERK and GSK-3beta pathways in the protection from damage. Much evidence argues for a role of adenosine receptors in neurological disease. Receptor densities are altered in patients with Alzheimer's disease, while many studies have demonstrated effects of adenosine and its antagonists on synaptic plasticity in vitro, or on learning adequacy in vivo. The combined effects of adenosine on neuronal viability and inflammatory processes have also led to considerations of their roles in Lesch-Nyhan syndrome, Creutzfeldt-Jakob disease, Huntington's disease and multiple sclerosis, as well as the brain damage associated with stroke. In addition to the potential pathological relevance of adenosine receptors, there are earnest attempts in progress to generate ligands that will target adenosine receptors as therapeutic agents to treat some of these disorders.

Physiology and pathophysiology of purinergic neurotransmission

This review is focused on purinergic neurotransmission, i.e., ATP released from nerves as a transmitter or cotransmitter to act as an extracellular signaling molecule on both pre- and postjunctional membranes at neuroeffector junctions and synapses, as well as acting as a trophic factor during development and regeneration. Emphasis is placed on the physiology and pathophysiology of ATP, but extracellular roles of its breakdown product, adenosine, are also considered because of their intimate interactions. The early history of the involvement of ATP in autonomic and skeletal neuromuscular transmission and in activities in the central nervous system and ganglia is reviewed. Brief background information is given about the identification of receptor subtypes for purines and pyrimidines and about ATP storage, release, and ectoenzymatic breakdown. Evidence that ATP is a cotransmitter in most, if not all, peripheral and central neurons is presented, as well as full accounts of neurotransmission and neuromodulation in autonomic and sensory ganglia and in the brain and spinal cord. There is coverage of neuron-glia interactions and of purinergic neuroeffector transmission to nonmuscular cells. To establish the primitive and widespread nature of purinergic neurotransmission, both the ontogeny and phylogeny of purinergic signaling are considered. Finally, the pathophysiology of purinergic neurotransmission in both peripheral and central nervous systems is reviewed, and speculations are made about future developments.

Overlapping intracellular and differential synaptic distributions of dopamine D1 and glutamate N-methyl-D-aspartate receptors in rat nucleus accumbens

The dopamine D1 receptor (D1R) in the nucleus accumbens (Acb) shell is highly implicated in psychostimulant-evoked locomotor activity and reward, whereas the D1R in the Acb core is more crucial for appetitive instrumental learning. These behavioral effects depend in part on interactions involving glutamatergic N-methyl-D-aspartate (NMDA) receptors, whose essential NR1 subunit has physical associations with the D1R. To determine the relevant sites for D1R activation and interactions involving NMDA receptors, we examined the electron microscopic immunolabeling of D1R and NR1 C-terminal peptides in rat Acb shell and core. In each Acb subdivision, the D1Rs were located principally on extrasynaptic plasma membranes of dendritic shafts and spines and more rarely were associated with cytoplasmic endomembranes. Many D1R-labeled somata and dendrites also contained NR1 immunoreactivity. In comparison with D1R, NR1 immunoreactivity was more often seen in the cytoplasm and near asymmetric synapses on somatodendritic profiles. In these profiles, notable overlapping distributions of D1R and NR1 occurred near endomembranes. The exclusively D1R- or D1R- and NR1-containing dendrites were most prevalent in the Acb shell, but were also present in the Acb core. In each region, NR1 was also detected in axon terminals without D1R, which formed excitatory-type synapses with D1R-labeled dendrites. These results provide ultrastructural evidence that D1Rs in the Acb have subcellular distributions supporting, 1) intracellular cotrafficking with NR1 and 2) modulation of the postsynaptic excitability in spiny neurons affected by presynaptic NMDA receptor activation. The region-specific differences in receptor distributions suggest a major, but not exclusive, involvement of Acb D1R in reward-related processing.

Acute ethanol-induced adenosine diphosphate ribosylation regulates the functional activity of rat striatal pertussis toxin-sensitive g proteins

BACKGROUND

We demonstrated previously that striatal adenosine modulates ethanol-induced motor incoordination (EIMI) via adenosine A1 receptors coupled to pertussis toxin (PT)-sensitive G protein and adenylyl cyclase-cyclic adenosine monophosphate (cAMP). Additionally, intrastriatal (IST) PT antagonizes EIMI and its potentiation by the adenosine A1 agonist N-cyclohexyladenosine; it also inhibits cAMP concentration.

METHODS

Guide cannulas were stereotaxically implanted for IST pretreatment with PT followed 5 days later by IST of N-cyclohexyladenosine and intraperitoneal ethanol. The adenosine diphosphate (ADP) ribosylation reaction involved PT-catalyzed [P]nicotinamide adenine dinucleotide (NAD) labeling of rat striatal membranes. Antagonism of EIMI (Rotorod method) after IST microinfusion of PT was investigated to determine whether it was due to a decrease in the functional activity of G proteins due to ADP ribosylation of the Gialpha subunit caused it.

RESULTS

Striatal membranes from IST PT (0.5 microg)-treated animals exhibited significantly attenuated (up to 90%) in vitro ADP ribosylation with [P]NAD. Striatal membranes from animals injected with ethanol (1.5 g/kg intraperitoneally) exhibited statistically significant increase (11%) in in vitro ADP ribosylation. Similarly, ethanol (50 mM) added to striatal membranes from untreated animals produced significant stimulation of in vitro ADP ribosylation. The decrease in the functional activity of G proteins due to ADP ribosylation of the Gialpha subunit after IST PT was functionally correlated with marked attenuation in EIMI, as observed previously. This finding suggests a blockade of functional activity of PT-sensitive striatal Gi/Go proteins (i.e., fewer available sites for labeled NAD incorporation). The in vivo ethanol results indicate that it must have caused an increase in the ribosylation capacity of Gialpha in vivo (i.e., increased Gi activity). Increased ADP ribosylation by in vitro ethanol increases Gi/Go activity, consistent with EIMI, as previously reported.

CONCLUSIONS

The results provide biochemical evidence of an ethanol-induced increase in ADP ribosylation of Gialpha causing a decrease in the functional activity of G proteins coupled via Gi/Go to adenylyl cyclase-cAMP. These results confirm the previously observed antagonism of EIMI by PT (IST).

Striatal adenosine A(2A) receptor blockade increases extracellular dopamine release following l-DOPA administration in intact and dopamine-denervated rats

The influence of the selective adenosine A(2A) receptor antagonist ZM 241385 on exogenous l-DOPA-derived dopamine (DA) release in intact and dopamine-denervated rats was studied using an in vivo microdialysis in freely moving animals. Local infusion of l-DOPA (2.5 microM) produced a marked increase in striatal extracellular DA level in intact and malonate-lesioned rats. Intrastriatal perfusion of ZM 241385 (50-100 microM) had no effect on basal extracellular DA level, but enhanced dose-dependently the l-DOPA-induced DA release in intact and malonate-lesioned animals. A non-selective adenosine A(2A) receptor antagonist DMPX (100 microM), similarly to ZM 241385, accelerated conversion of l-DOPA in intact and malonate-denervated rats. This effect was not produced by the adenosine A(1) receptor antagonist, CPX (10-50 microM). However, ZM 241385 did not affect the l-DOPA-induced DA release in rats pretreated with reserpine (5 mg/kg i.p.) and alpha-methyl-p-tyrosine (AMPT, 300 mg/kg i.p.). Obtained results indicate that blockade of striatal adenosine A(2A) receptors increases the l-DOPA-derived DA release possibly by indirect mechanism exerted on DA terminals, an effect dependent on striatal tyrosine hydroxylase activity. Selective antagonists of adenosine A(2A) receptors may exert a beneficial effect at early stages of Parkinson's disease by enhancing the therapeutic efficacy of l-DOPA applied exogenously.

Halothane anesthesia affects NMDA-stimulated cholinergic and GABAergic modulation of striatal dopamine efflux and metabolism in the rat in vivo

Microdialysis of the striatum of halothane-anesthetized rats was used to study the participation of local cholinergic and GABAergic neurotransmission in NMDA receptor-modulated striatal dopamine release and metabolism. Reverse dialysis.of NMDA (1 mM) evoked a 10-fold increase in dopamine efflux and reduced DOPAC and HVA to > 20% of basal values. The effect of NMDA on dopamine efflux was abolished by atropine (10 microM) but unaffected by (+)-bicuculline (50 microM). NMDA-induced decrease in DOPAC (but not HVA) efflux was potentiated by atropine, whereas (+)-bicuculline attenuated the decrease in DOPAC and HVA. Compared to our previous studies in unanesthetised rats, our data suggest that halothane anesthesia alters the balance between NMDA-stimulated cholinergic and GABAergic influences on striatal dopamine release and metabolism. Differential sensitivity to halothane of NMDA receptors expressed by the neurones mediating these modulatory influences, or loss of specific NMDA receptor populations through voltage-dependent Mg2+ block under anesthesia, could underlie these observations.

Effect of the adenosine A2A receptor antagonist 8-(3-chlorostyryl)caffeine on l-DOPA biotransformation in rat striatum

In the present study, we investigated effects of the new selective adenosine A2A receptor antagonist 8-(3-chlorostyryl)caffeine (CSC) on L-DOPA-induced dopamine (DA) release in the striatum of intact and reserpine-treated rats. CSC given in a pharmacologically effective dose of 5 mg/kg i.p. significantly increased striatal DA release after joint administration of L-DOPA (100 mg/kg, i.p.) and benserazide (50 mg/kg, i.p.) to intact and reserpine (2.5 mg/kg, s.c.)-injected rats. CSC did not change the elevated level of 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in intact rats, but raised it in DA-depleted animals. The availability of exogenous L-DOPA in the extracellular space was similar and equally increased by CSC in both intact and reserpinized rats. Our results suggest that the observed effects may be mediated by striatal adenosine A2A receptors, and are probably related to the CSC action on DA metabolism and the increased transport of exogenous L-DOPA into the brain. These observations might be of relevance, considering the use of selective A2A antagonists as potential supplements to L-DOPA therapy of Parkinson's disease.

Purines and neuroprotection

The activation of adenosine A1, A2 andA3 receptors can protect neurones against damage generated by mechanical or hypoxic/ischaemic insults as well as excitotoxins. A1 receptors are probably effective by suppressing transmitter release and producing neuronal hyperpolarisation. They are less likely to be of therapeutic importance due to the plethora of side effects resulting from A1 agonism, although the existence of receptor subtypes and recent synthetic chemistry efforts to increase ligand selectivity, may yet yield clinically viable compounds. Activation of A2A receptors can protect neurons, although there is much uncertainty as to whether agonists are acting centrally or via a peripheral mechanism such as altering blood flow or immune cell function. Selective antagonists at the A2A receptor, such as 4-(2-[7-amino-2-(2-furyl)(1,2,4)triazolo(2,3-a)(1,3,5)triazin-5-yl-amino]ethyl)phenol (ZM 241385) and 7-(2-phenylethyl)-5-amino-2-(2-furyl)-pyrazolo-[4,3e]-1,2,4-triazolo[1,5-c]pyrimidine (SCH 58261), can also protect against neuronal death produced by ischaemia or excitotoxicity. In addition, A2A receptor antagonists can reduce damage produced by combinations of subthreshold doses of the endogenous excitotoxin quinolinic acid and free radicals. Since the A2A receptors do not seem to be activated by normal endogenous levels of adenosine, their blockade should not generate significant side effects, so that A2A receptor antagonists appear to be promising candidates as new drugs for the prevention of neuronal damage. Adenosine A3 receptors have received less attention to date, but agonists are clearly able to afford protection against damage when administered chronically. Given the disappointing lack of success of NMDA receptor antagonists in human stroke patients, despite their early promise in animal models, it is possible that A2A receptor antagonists could have a far greater clinical utility.

Involvement of intrinsic cholinergic and GABAergic innervation in the effect of NMDA on striatal dopamine efflux and metabolism as assessed by microdialysis studies in freely moving rats

Microdialysis perfusion was used to study the participation of striatal cholinergic and gamma-aminobutyric acid-ergic (GABAergic) neurotransmission in basal and N-methyl-D-aspartate (NMDA) receptor-modulated dopamine release and metabolism in the striatum of the freely moving rat. Reverse dialysis of atropine (1-50 microM) induced a concentration-related increase in dopamine efflux and decrease in 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) efflux. (+)-Bicuculline (10-100 microM) similarly increased dopamine efflux, but was without consistent effect on metabolite efflux. Reverse dialysis of NMDA (1 mM) evoked an approximately twofold increase in dopamine efflux and decreased DOPAC and HVA efflux to 30-40% of basal levels. The effect of NMDA on dopamine efflux was completely abolished by coadministration of tetrodotoxin (TTX; 1 microM) or atropine (10 microM), and markedly potentiated (approximately fourfold) by coadministration of (+)-bicuculline (50 microM). The NMDA-induced decrease in dopamine metabolite efflux was inhibited by coadministration of TTX or (+)-bicuculline, but was unaffected by atropine. Our data suggest that dopamine release in the striatum is subject to both cholinergic and GABAergic tonic inhibitory mechanisms mediated through muscarinic and GABAA receptors, respectively. Furthermore, NMDA-stimulated dopamine release also involves obligatory cholinergic facilitation and an inhibitory GABAergic component mediated through these respective receptors.

Modulation of ethanol-induced motor incoordination by mouse striatal A(1) adenosinergic receptor

We have demonstrated that ethanol-induced motor incoordination is modulated by cerebellar adenosine A(1) receptor. This study represents an extension into another important brain motor area, the striatum that, unlike cerebellum, has high density of both A(1) and A(2A) receptors. Direct intra-striatal micro-infusion of Ro15-4513 (0.05, 0.5, 1 ng), a partial inverse-agonist of benzodiazepine, significantly and nearly dose-dependently attenuated ethanol-induced motor incoordination indicating mediation of ethanol's motor incoordination by striatum. Intra-striatal A(1)-selective agonist N(6)-cyclohexyladenosine (CHA; 1, 2, 4 ng), A(1) = A(2A) non-selective agonist, 5'-N-ethylcarboxamidoadenosine (NECA; 1.5, 3, 6 ng), and A(1)-selective antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX; 25, 50, 100 ng) dose-dependently accentuated and attenuated, respectively, ethanol-induced motor incoordination, strongly suggesting modulation by striatal adenosine A(1) receptor. Intra-striatal DPCPX significantly antagonized not only ethanol-induced motor incoordination but also its potentiation by intra-striatal CHA, R-(+)-N(6)-(2-phenylisopropyladenosine) (R-PIA), or NECA. No change in motor coordination occurred after the highest dose of CHA, R-PIA, or NECA followed by saline. Similarly, the highest intra-striatal dose of Ro15-4513 or DPCPX neither altered motor coordination or locomotor activity indicating relative selectivity of interaction with ethanol. Nearly 25-fold higher dose of A(2A)-selective agonist, CGS-21680, compared to CHA was necessary to produce a comparable potentiation of ethanol's motor incoordination perhaps suggesting a lack of or less significant striatal A(2A) involvement. Intra-striatal pertussis toxin (0.5 microg) pre-treatment markedly attenuated ethanol-induced motor incoordination as well as its potentiation by intra-striatal CHA. These results support that striatum is one of the brain motor areas mediating the motor impairing effects of acute ethanol and that the latter's modulation occurs via A(1)-selective receptors coupled to pertussis toxin-sensitive G proteins.

MK801 influences L-DOPA-induced dopamine release in intact and hemi-parkinson rats

In vivo microdialysis was used to investigate the influence of dizocilpine (MK801) on basal and levodopa (L-DOPA)-induced extracellular dopamine levels in striatum and substantia nigra of intact and 6-hydroxydopamine-lesioned rats. In lesioned rats, extracellular dopamine was decreased in striatum but not in substantia nigra. L-DOPA (25 mg/kg i.p. after benserazide 10 mg/kg i. p.) increased the dopamine levels in striatum and substantia nigra of intact and dopamine-depleted rats. This increase was significantly higher in dopamine-depleted compared to intact striatum. Pretreatment with MK801 (0.1 and 1.0 mg/kg i.p.) dose-dependently attenuated the L-DOPA-induced dopamine release in substantia nigra of intact rats. In dopamine-depleted striatum, MK801 enhanced L-DOPA-induced dopamine release. The present results indicate that the influence of MK801 on L-DOPA-induced dopamine release in striatum and substantia nigra depends on the integrity of the nigrostriatal pathway. In Parkinson's disease, NMDA receptor antagonists could be beneficial by enhancing the therapeutic efficacy of L-DOPA at the level of the striatum.

Purine-induced alterations of dopamine metabolism in rat pheochromocytoma PC12 cells

Studies with cerebrospinal fluid from subjects with Parkinson's disease suggest that purine abnormalities may be present in this disorder. The effects of purines on dopamine metabolism have not been characterized, though adenosine is known to inhibit dopaminergic neurotransmission. In this study, dopamine, its precursor 3,4-dihydroxyphenylalanine (DOPA), and its degradation products 3,4-dihydroxyphenylacetic acid (DOPAC), and homovanillic acid (HVA) were measured in rat pheochromocytoma PC12 cells following 24-h incubation with 5, 50, and 500 microM adenosine, adenine, guanosine, guanine, hypoxanthine, xanthine, and uric acid. Incubation with adenosine increased DOPA, DOPAC, and HVA, while adenine treatment decreased DOPA. Guanosine (500 microM) decreased DOPA, dopamine, and DOPAC, while lower concentrations increased DOPAC and HVA. Incubation with guanine decreased dopamine, and xanthine decreased dopamine and DOPAC. Hypoxanthine and uric acid exerted minimal effects. These results indicate that purines exert a variety of effects on dopamine metabolism. The influence of purine metabolism on the dopaminergic deficit in the Parkinsonian brain merits further investigation.

Differential expression of a(2a), A(1)-adenosine and D(2)-dopamine receptor genes in rat peripheral arterial chemoreceptors during postnatal development

The sensitivity of peripheral arterial chemoreceptors in the carotid body to hypoxia increases with postnatal maturation. Carotid sinus nerve activity is augmented by adenosine binding to A(2a)-adenosine receptors and attenuated by dopamine binding to D(2)-dopamine receptors. In this study, we used in situ hybridization histochemistry to determine the change in the levels of mRNA expression for A(2a) and A(1)-adenosine receptors and D(2)-dopamine receptors in the rat carotid body. We also investigated the cellular distribution and possible colocalization of these receptor mRNAs and tyrosine hydroxylase (TH) mRNAs during the first 2 weeks of postnatal development. By using immunohistocytochemistry, we detected A(2a)-adenosine receptor protein in the carotid body and petrosal ganglion. We found that A(2a)-adenosine receptor mRNA and protein are expressed in the carotid body in animals at 0, 3, 6 and 14 postnatal days. The level of A(2a)-adenosine receptor mRNA expression significantly decreased by 14 postnatal days (P<0.02 vs. day 0) while D(2)-dopamine receptor mRNA levels significantly increased by day 3 and remained greater than D(2)-dopamine receptor mRNA levels at day 0 (P<0.001 all ages vs. day 0). TH mRNA was colocalized in cells in the carotid body with A(2a) adenosine receptor and D(2)-dopamine receptor mRNAs. A(1)-adenosine receptor mRNA was not expressed in the carotid body at any of the ages examined. In the petrosal ganglion, A(1)-adenosine receptor mRNA was abundantly expressed in numerous cells, A(2a)-adenosine receptor mRNA was expressed in a moderate number of cells while D(2)-dopamine receptor mRNA was seen in a few cells in the rostral petrosal ganglion. In conclusion, using in situ hybridization histochemistry, we have shown that mRNA for both the excitatory, A(2a)-adenosine receptor, and the inhibitory, D(2)-dopamine receptor, is developmentally regulated in presumably type I cells in the carotid body which may contribute to the maturation of hypoxic chemosensitivity. Furthermore, the presence A(1)-adenosine receptor mRNAs in cell bodies of the petrosal ganglion suggests that adenosine might also have an inhibitory role in hypoxic chemotransmission.

Regulation of tyrosine hydroxylase activity and phosphorylation at Ser(19) and Ser(40) via activation of glutamate NMDA receptors in rat striatum

The activity of tyrosine hydroxylase, the rate-limiting enzyme in the biosynthesis of dopamine, is stimulated by phosphorylation. In this study, we examined the effects of activation of NMDA receptors on the state of phosphorylation and activity of tyrosine hydroxylase in rat striatal slices. NMDA produced a time-and concentration-dependent increase in the levels of phospho-Ser(19)-tyrosine hydroxylase in nigrostriatal nerve terminals. This increase was not associated with any changes in the basal activity of tyrosine hydroxylase, measured as DOPA accumulation. Forskolin, an activator of adenylyl cyclase, stimulated tyrosine hydroxylase phosphorylation at Ser(40) and caused a significant increase in DOPA accumulation. NMDA reduced forskolin-mediated increases in both Ser(40) phosphorylation and DOPA accumulation. In addition, NMDA reduced the increase in phospho-Ser(40)-tyrosine hydroxylase produced by okadaic acid, an inhibitor of protein phosphatase 1 and 2A, but not by a cyclic AMP analogue, 8-bromo-cyclic AMP. These results indicate that, in the striatum, glutamate decreases tyrosine hydroxylase phosphorylation at Ser(40) via activation of NMDA receptors by reducing cyclic AMP production. They also provide a mechanism for the demonstrated ability of NMDA to decrease tyrosine hydroxylase activity and dopamine synthesis.

N-methyl-D-aspartate receptor 1 in the caudate-putamen nucleus: ultrastructural localization and co-expression with sorcin, a 22,000 mol. wt calcium binding protein

Entry of calcium through N-methyl-D-aspartate-type glutamate receptors in the caudate-putamen nucleus is essential for normal motor activity, but can produce cytotoxicity with continued stimulation and subsequent release of intracellular calcium. To determine potential functional sites for N-methyl-D-aspartate receptor activation in this region, we examined the ultrastructural localization of the R1 subunit of the N-methyl-D-aspartate receptor (NMDAR1) in rat brain. In addition, we comparatively examined the localization of NMDAR1 and sorcin, a 22,000 mol. wt calcium binding protein present in certain striatal neurons and involved in calcium-induced calcium release. NMDAR1-like immunoreactivity was seen at synaptic and non-synaptic sites on neuronal plasma membranes. Of 1514 NMDAR1-labeled profiles, 62% were dendrites and dendritic spines and the remainder were mainly unmyelinated axons and axon terminals. Sorcin-like immunoreactivity was present in 39% of the profiles that contained NMDAR1 labeling, most (533/595) of which were dendrites and dendritic spines. Of 1807 sorcin-labeled profiles, 42% were identified, however, as small processes including spine necks and unmyelinated axons or axon terminals. These profiles also occasionally contained NMDAR1 or showed synaptic or appositional contacts with other NMDAR1-immunoreactive neurons. The results of this study suggest that in the caudate-putamen nucleus, activation of NMDA receptors permits calcium influx at plasmalemmal sites mainly on dendrites where sorcin may play a role in calcium-induced calcium release. The presence of sorcin in some, but not all NMDA-containing neurons in the caudate-putamen nucleus has potential implications for the known differential vulnerability of certain striatal neurons to excitotoxins.

Protection by an adenosine analogue against kainate-induced extrahippocampal neuropathology

1. The glutamate analogue kainic acid produces neuronal damage in the central nervous system. We have reported that analogues of adenosine, such as R-N6-phenylisopropyladenosine (R-PIA) can, at doses as low as 10 microg/kg IP, prevent the hippocampal damage that follows the systemic administration of kainate. The present work was designed to examine purine protection against kainate in extrahippocampal regions by using histological methods. 2. The results show that R-PIA, at a dose of 25 microg/kg IP in rats, can protect against the neuronal damage caused by kainate in the basolateral amygdaloid nuclei, the pyriform cortex and around the rhinal fissure. This protection could be prevented by the simultaneous administration of the A1 adenosine receptor antagonist 1,3-dipropyl-8-cyclopentylxanthine, confirming that the protection involved adenosine A1 receptors. No protection was observed in the posterior amygdaloid nuclei or the entorhinal cortex, suggesting the absence of relevant adenosine receptors or a different mechanism of excitotoxicity.

Ultrastructural localization of sorcin, a 22 kDa calcium binding protein, in the rat caudate-putamen nucleus: association with ryanodine receptors and intracellular calcium release

Sorcin is a 22 kDa calcium binding protein that is widely distributed in mammalian tissues, including brain, and is associated with the ryanodine receptor (RyR) family of intracellular calcium-release channels in the heart. To determine the cellular sites for potential central functions of sorcin, we examined the electron microscopic immunocytochemical localization of antipeptide antisera against sorcin and against cardiac and brain RyR in the rat caudate-putamen nucleus (CPN), one of the few regions expressing high levels of brain RyR. Sorcin-like immunoreactivity (S-LI) was detected in both neurons and glia by using immunoperoxidase and immunogold methods. Of 1,735 profiles containing immunogold-silver labeling for sorcin, almost 50% were dendrites and many of these dendrites were spiny. The remainder were mainly small axons, axon terminals, and, more rarely, glia. Furthermore, analysis of dually labeled tissue sections showed the presence of sorcin in many of the dendrites and some of the axonal and glial processes containing RyR. In dendrites, gold-silver deposits showing S-LI were prominently localized to saccules of smooth endoplasmic reticulum and mitochondria, both of which are known to store calcium. These labeled structures were located near the plasma membrane at sites postsynaptic to excitatory-type asymmetric junctions, as well as non-synaptic portions of the plasma membrane. In axons, S-LI was also often seen at extrasynaptic sites on, or near, the plasma membrane. We conclude that in the rat CPN, sorcin may act independently or, in conjunction with RyR, to modulate cytoplasmic release of calcium, mainly from smooth endoplasmic reticulum and/or mitochondria in neurons.

Dopamine and glutamate control each other's release in the basal ganglia: a microdialysis study of the entopeduncular nucleus and substantia nigra

This study utilized microdialysis in conscious rats to investigate dopaminergic control of excitatory amino acid release in the entopeduncular nucleus (EPN), and glutamatergic control of dopamine release in the substantia nigra pars reticulata (SNr). EPN dialysates contained both glutamate and aspartate, which were elevated by dopamine depletion with reserpine and 6-hydroxydopamine (6-OHDA), reduced by the D2/3 agonist LY 171555 and unaffected by the D1 agonist SKF 38393, in line with current theory. The D2/3 agonist RU 24213 was behaviourally active but paradoxically increased glutamate and aspartate release in EPN, possibly via kappa opioid receptor blockade. 6-OHDA-hemilesioned rats also showed a significant increase in glutamate and aspartate contralaterally, suggesting that nigrostriatal dopamine affects EPN neurotransmission bilaterally. In reserpine-treated rats, basal levels of dopamine in the SNr were greatly reduced, and were further lowered by focal application of NMDA antagonists, suggestive of the removal of a high glutamatergic tone. A threshold amount of L-DOPA applied to the SNr elevated dopamine output about two-fold and 5-HT output about 13-fold, indicating L-DOPA effects the release of monoamines other than dopamine. Concomitant addition of the NMDA antagonists potentiated these releases synergistically, suggesting that this could be how they facilitate the antiparkinsonian action of L-DOPA.

Dual ultrastructural localization of mu-opioid receptors and NMDA-type glutamate receptors in the shell of the rat nucleus accumbens

The effectiveness of NMDA antagonists in modulating the motor and motivational effects of opiates is attributed, in part, to functional associations involving NMDA receptors and micro-opioid receptors (MORs) in the shell of the nucleus accumbens (Acb). To determine the subcellular sites for potential functional interactions between opiate ligands and NMDA receptors in this region, we examined the ultrastructural localization of antipeptide antisera against MOR and the R1 subunit of the NMDA receptor in the Acb shell of the adult rat brain. MOR-like immunoreactivity (MOR-LI) was seen primarily in dendrites, whereas NMDAR1-like immunoreactivity (NMDAR1-LI) was detected more often in axon terminals forming asymmetric synapses. In these profiles, MOR labeling was localized mainly to extrasynaptic plasma membranes, whereas NMDAR1-LI was associated with both synaptic and extrasynaptic sites. Of 307 MOR-labeled processes, 17.9% of the dendrites and 9.4% of the axon terminals also contained NMDAR1-LI. In addition, 24.7% of the dendrites containing only MOR-LI were apposed to NMDAR1-labeled axons or terminals. We conclude that in the shell of the Acb, the output of single neurons can be dually modulated by (1) activation of MOR and NMDA receptors in the same dendrites or (2) combined activation of presynaptic NMDA receptors in afferents contacting dendrites containing MOR. In addition, the colocalization of MOR and NMDAR1 in certain axon terminals in the Acb suggests their dual involvement in the presynaptic release of neurotransmitters in this region.

Ultrastructural localization and comparative distribution of nitric oxide synthase and N-methyl-D-aspartate receptors in the shell of the rat nucleus accumbens

Nitric oxide (NO), the diffusible gas formed by nitric oxide synthase (NOS) has been implicated in the enhanced locomotor activity attributed mainly to increased dopamine release in the shell of the nucleus accumbens (Acb). Furthermore, the release of both NO and dopamine are known to be altered by agonists of N-methyl-D-aspartate (NMDA) type glutamate receptors in this region. We examined the cellular sites of NO synthesis and the sites of potential relevancy for functional associations between neurons containing NOS and the NMDA receptor in the shell of the Acb. This was achieved by dual ultrastructural immunogold and immunoperoxidase labeling of antisera raised against the brain form of NOS and the NMDAR1 subunit of the NMDA receptor in this region of rat brain. NOS-like immunoreactivity (NOS-LI) was seen throughout the cytoplasm of isolated medium-large somata, aspiny dendrites and axon terminals. In 217 NOS-labeled profiles, NMDAR1-like immunoreactivity (NMDAR1-LI) was colocalized in 17% of somata and dendrites. Additionally, 35% of NOS-labeled dendrites apposed glial processes containing NMDAR1-LI, and 29% apposed axon terminals containing NMDARI-LI. NOS-labeled terminals more rarely colocalized NMDAR1 or apposed NMDAR1-labeled glial processes or dendrites. These results provide anatomical evidence that, in the shell of the Acb, NMDA receptors are localized so as to directly modulate the output of neurons producing NO as well as to influence other neurons and glia having the greatest access to the released gas.

NMDA and kainate-evoked release of nitric oxide and classical transmitters in the rat striatum: in vivo evidence that nitric oxide may play a neuroprotective role

The effects of N-methyl-D-aspartate (NMDA), kainate, S-alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) and KCl on striatal nitric oxide (NO), acetylcholine (ACh), dopamine (DA), serotonin (5-HT), aspartate (ASP), glutamate (GLU) and gamma-aminobutyric acid (GABA) release were measured in anaesthetized rats in vivo by microdialysis and in vitro in organotypic slice cultures. Local NMDA (1-100 microM) infusion by retrodialysis dose-dependently increased levels of classical transmitters, NO2-, NO3-, citrulline and arginine at similar thresholds (10 microM). Similar patterns of NMDA-evoked (50 microM) release were seen in striatal cultures. NMDA-evoked changes were all calcium-dependent and blocked by NMDA (APV or MK-801) but not AMPA/kainate (DNQX) receptor antagonists, excepting DA which could be prevented by both. In vivo, kainate increased NO2-, NO3-, CIT and ARG levels at 50 and 100 microM but was less potent than NMDA. Kainate also evoked significant ACh, DA and GLU release dose-dependently starting at 1-10 microM whereas 5-HT, ASP and GABA required 50 or 100 microM doses. Kainate effects were inhibited by DNQX, but not by APV, and were calcium-dependent, AMPA failed to alter NO2-, NO3-, CIT or ARG levels at 50 or 100 microM doses but dose-dependently increased ACh and DA. Similar results were seen with kainate (50 microM) and AMPA (50 microM) in vitro. KCl evoked NO2-, NO3-, CIT and ARG release as well as that of the classical transmitters in vivo and in vitro. In vivo administration of the NO synthase inhibitor L-nitroarginine (L-NARG; 100 microM) significantly reduced NO2-, NO3- and CIT levels and prevented NMDA, kainate or KCl-evoked increases. It also potentiated ACh, ASP, GLU and GABA release and reduced that of DA in response to 50 microM NMDA whereas treatment with an NO-donor (SNAP; 10 microM) significantly reduced evoked ACh, ASP and GLU release. The NO synthase inhibitor L-NARG potentiated kainate-evoked ACh release and reduced that of DA, although less potently than NMDA, but it had no effect on KCl-evoked transmitter release. Overall, these results show that both NMDA and kainate increase striatal NO release at similar dose-thresholds as for classical transmitter release suggesting that NO is dynamically released under physiological and not just pathological conditions. Reductions of striatal NO levels also potentiates calcium-dependent transmitter release in response to NMDA and, to a lesser extent, kainate, whereas increasing them reduces it. This is consistent with a role for NO as a neuroprotective agent in this region acting to desensitize NMDA receptors.

Ultrastructural immunocytochemical localization of the N-methyl-D-aspartate receptor and tyrosine hydroxylase in the shell of the rat nucleus accumbens

The N-methyl-D-aspartate (NMDA)-type glutamate receptors in the shell region of the nucleus accumbens (ACB) have been implicated in the modulation of dopamine release and in amphetamine-induced neurotoxicity. We used electron microscopic immunocyto-chemistry to determine the anatomical sites for NMDA-mediated effects of glutamate and for their potential interactions with dopaminergic afferents identified by the presence of tyrosine hydroxylase (TH) in this region of the rat brain. Immunogold and immunoperoxidase methods were used to localize antisera against the R1 subunit of the NMDA receptor (NMDAR1) alone or combined with TH. In single labeling experiments, approximately half of the NMDAR1-like immunoreactivity (NMDAR1-LI) was localized to extrasynaptic plasma membranes of neuronal processes, many (92 out of 215) of which were dendrites, and only 33 out of 215 were unmyelinated axons or terminals. Surprisingly, the neuronal labeling of NMDAR1 was almost equaled by that seen in astrocytic processes (88 out of 215). Dual labeling for TH and NMDAR1 was rarely observed and was only seen in axons. However, in favorable planes of section, NMDAR1 was noted along intervaricose segments of axons in which TH was more readily seen in the varicosity. This differential intra-axonal distribution suggests an underestimation of dual labeling in single coronal sections through unmyelinated axons and terminals. The TH-immunoreactive terminals were more often seen apposed to NMDA-immunoreactive astrocytic processes and dendrites. These results provide the first ultrastructural evidence for presynaptic modulation of dopamine release by NMDA receptors in the shell of the nucleus accumbens. They also indicate that NMDA receptors modulate postsynaptic neurons receiving input from the dopaminergic afferents and suggest a previously unsuspected functional association involving glial NMDA receptors and dopaminergic afferents in this brain region.

NMDA receptor antagonists increase the release of dopamine in the substantia nigra of reserpine-treated rats

Microdialysis of the substantia nigra pars reticulata in freely moving rats disclosed a steady release of dopamine and its metabolites which was greatly reduced after reserpine (4 mg/kg s.c.) and alpha-methyl-p-tyrosine (200 mg/kg i.p.) pretreatments. Local infusion of high K+ (100 mM) or L-3,4-dihydroxyphenylalanine (L-DOPA, 10 microM) significantly increased dialysate levels of dopamine and 3,4-dihydroxyphenylacetic acid (DOPAC), but not homovanillic acid (HVA) in this model. Intranigral application of the non-competitive NMDA receptor antagonist dizocilpine (150 nM), or the competitive NMDA receptor antagonist R-DL-(E)-2-amino-4-methyl-5-phosphono-3-pentanoate (CGP 40116, 10 microM), via the dialysis probe, did not affect the release of dopamine or its metabolites in intact rats, but further suppressed these releases in reserpine plus alpha-methyl-p-tyrosine-treated animals. When the same amounts of dizocilpine or CGP 40116 were coinfused with L-DOPA, however, they potentiated the recovery of dopamine 12-24 times, and of DOPAC 5-10 times (but not HVA), as well as producing detectable behavioural arousal. The facilitation of dopamine formation from L-DOPA by NMDA receptor antagonists in the substantia nigra pars reticulata could explain the enhancement of L-DOPA's antiparkinsonian activity by these compounds in behavioural experiments.

Differential regulation, by MK-801, of dopamine receptor gene expression in rat nigrostriatal and mesocorticolimbic systems

Glutamate agonists have been shown to stimulate striatal dopamine release, but less is known about dopamine-glutamate interactions at the receptor level. We treated rats with 0.3, 1.0, or 3.0 mg/kg of MK-801, an NMDA antagonist, daily for 1 week and, using in situ hybridization, measured dopamine receptor mRNA levels in cortical and subcortical structures. MK-801 caused a significant increase of D1 and D2 mRNA in the dorsal and ventral striatum, a significant decrease of D3 mRNA in the nucleus accumbens, and a significant decrease of D1 mRNA in the limbic cortex. Dopamine autoreceptor expression, reflected by D2 mRNA in the midbrain, was increased in the ventral tegmental area, but not in the substantia nigra. Thus, MK-801 appears to differentially regulate the mesocorticolimbic and nigrostriatal dopamine systems.

Concentration-dependent dual action of locally applied N-methyl-D-aspartate on extracellular dopamine in the rat prefrontal cortex in vivo

Using microdialysis, the glutamate agonist N-methyl-D-aspartate (NMDA) was perfused for 20 min through the medial prefrontal cortex of freely moving rats, and its effects on extracellular concentrations of dopamine (DA) were determined. NMDA (1 mM) increased DA to 170-1500%, depending on the intensity and duration of the clonic forelimb jerks and convulsions that were induced. NMDA (0.1 mM), however, decreased DA to 61%. Metabolites of DA were decreased after both concentrations of NMDA. The effects of both 0.1 mM and 1 mM NMDA were blocked by 0.5 mM of the competitive NMDA-antagonist D-AP-5. The NMDA-induced decrease in release and metabolism possibly results from an indirect action via an inhibitory local interneuron or polysynaptic circuit.

Comparative ultrastructural localization of the NMDAR1 glutamate receptor in the rat basolateral amygdala and bed nucleus of the stria terminalis

The N-methyl-D-aspartate (NMDA)-type glutamate receptor in the basolateral amygdala (BLA) has been implicated in activity-dependent plasticity important for cortically evoked acquisition of fear-potentiated startle response. We examined the ultrastructural immunoperoxidase labeling of the R1 subunit of the NMDA receptor in the BLA of adult rats to determine the potential cellular and subcellular sites mediating the effects generated by NMDA activation. The localization was compared with that seen in the bed nucleus of the stria terminalis (BNST), the major efferent pathway from the central nucleus of the amygdala, which has a more pronounced involvement in autonomic function. Electron microscopy established that in the BLA, 68.4% (n = 177) of the profiles showing NMDAR1-like immunoreactivity (NMDAR1-LI) were dendrites, and 19.8% were distal tips of astrocytic processes. In contrast, profiles containing NMDAR1-LI (n = 262) in the BNST were more equally distributed between dendrites (37.4%) and axons (38.2%). The subcellular localization of NMDAR1 immunoreactivity was, however, similar in both regions. Our findings provide the first ultrastructural evidence that glutamate may prominently act through NMDAR1 receptors to elicit postsynaptic actions on intrinsic neurons in the BLA and BNST. The results also indicate that, in the BLA, the NMDAR1 receptor plays an important role in astrocytic function, whereas the receptor is more preferentially a presynaptic modulator in axons which terminate in or pass through the BNST.

Direct in vivo comparison of two mechanisms releasing dopamine in the rat striatum

A push-pull cannula supplied with artificial CSF was implanted in the striatum of anaesthetized rats, and the basal extracellular DA and DOPAC was assayed in the superfusates using HPLC and electrochemical detection. Simultaneously, a carbon fibre electrode was implanted in close proximity of the cannula and the evoked DA release was detected by differential pulse amperometry during stimulation of the DA axons. Local treatments with cadmium (100 microM) blocked the evoked DA release (-90%), but substantially increased the basal extracellular DA (+125%). The effects of glutamate agonists NMDA (1 mM) and kainate (0.1 mM), known to increase basal extracellular DA were confirmed (+150% and +60% respectively). It was, however, simultaneously observed that the evoked DA release was inhibited (-80% and -50%, respectively). Amphetamine (1 microM) released DA (+150%) and produced also an increase (+100%) of the evoked DA release. These results, apparently conflicting, show that the two mechanisms releasing dopamine (firing-dependent and not) can be directly and simultaneously observed. These two releasing processes appear to be not strictly antagonist. They are also differently and independently modulated by calcium and by local influences such those conveyed by glutamate.

Possible role of striatal adenosine in the modulation of acute ethanol-induced motor incoordination in rats

Several reports from our laboratory have suggested the involvement of the brain adenosinergic system in ethanol-induced motor incoordination (EIMI). This study is an extension of the previous work and pertains to the evaluation of the role of the striatal adenosine in EIMI in male Sprague-Dawley rats. Using the motor incoordination induced by 1.5 g/kg of ethanol (ip) as a test response, the possible behavioral interactions between ethanol and adenosine agonists and antagonists in the striatum were investigated. Intrastriatal (IST) administration of adenosine A1-, A1 = A2-, and As-selective agonists, R(-)N6-(2-phenylisopropyl)adenosine (R-PIA), 5'-N-ethylcarboxamido-adenosine (NECA), and 5'-(N-cyclopropyl)-carboxamidoadenosine, respectively, significantly and dose-dependently accentuated EIMI when evaluated by rotorod test, suggesting the striatal adenosinergic modulation of EIMI. No significant change in normal motor coordination was noted, even when the highest IST doses of adenosine agonists were followed by saline instead of ethanol, suggesting that the observed behavioral interactions of these drugs were selective to ethanol. Hippocampus, which is known not to be involved in the normal motor functions, was selected as a control brain area because of the presence of high density of adenosine receptors, as well as the high levels of adenosine. Intrahippocampal NECA failed to alter EIMI, indicating the specific role of striatal and not hippocampal adenosinergic system in the modulation of EIMI. The potentiating effects of adenosine agonists N6-cyclohexyladenosine (CHA) and CGS-21680 on EIMI were blocked by adenosine A1- and A2-selective antagonists, 8-cyclopentyl-1,3-dipropylxanthine and 3,7-dimethyl-1-propargylxanthine, respectively, suggesting the participation of specific adenosine receptors in this functional interaction. A role for the adenosine A1 receptor in the striatal adenosinergic modulation of EIMI was favored based on the rank-order potency of adenosine agonists. IST pretreatment with pertussis toxin (PT), but not with PT beta-oligomer, nearly completely eliminated the accentuation of EIMI by CHA, further supporting the favored role of adenosine A1 receptors in EIMI. Histological and IST [3H]R-PIA distribution data confirmed that the observed behavioral effects were caused by exclusive striatal distribution of intrastriatally microinjected drugs. Data obtained suggested modulation of acute EIMI by striatal adenosine receptor-mediated mechanism(s) and the coupling of these adenosine receptor to the PT-sensitive Gi protein.

The role of N-methyl-D-aspartate receptors in the regulation of physiologically released dopamine

In vivo voltammetry was used to measure changes in ascorbate, which are an index of changes in the release of glutamate, and microdialysis was used to measure changes in dopamine in the striatum of freely moving rats. A 5 min tail pinch produced a rapid rise in striatal ascorbate paralleled by an increase in motor activity and a slower, more prolonged rise in dopamine. Systemic administration of ketamine or dizocilpine maleate, non-competitive antagonists of the N-methyl-D-aspartate glutamate receptor, produced an increase in the basal level of ascorbate but not dopamine; however, the tail pinch-evoked rise in both ascorbate and dopamine was completely abolished by these drugs. The rise in dopamine was also abolished by local infusion of dizocilpine maleate into the striatum. Local application of N-methyl-D-aspartate produced a dose-dependent increase in dopamine, which was partially reduced in the presence of tetrodotoxin. The results show that the tail pinch-evoked increase in motor activity involves an increase in the release of striatal dopamine which requires the activation of N-methyl-D-aspartate receptors in the striatum. This suggests that phasic increases in striatal dopamine release are triggered by the action of glutamate on dopaminergic nerve terminals.

Mechanism of the hippocampal loss of adenosine sensitivity in calcium-free media

Adenosine depresses the excitability of pyramidal neurones in the hippocampus. This effect is lost in calcium-free media and we have now investigated the mechanism of this. Extracellular recordings were made of antidromically and orthodromically evoked population potentials from CA1 region of rat hippocampal slices. It was observed that the activity of adenosine can be restored in the presence of procaine or carbamazepine, known inhibitors of sodium channels. The GABAB agonist baclofen was able to depress potential size but did not restore sensitivity to adenosine. It is concluded that the loss of postsynaptic sensitivity to adenosine in calcium-free solution results from the increased sodium conductances.

Presynaptic control of dopamine synthesis and release by excitatory amino acids in rat striatal synaptosomes

Purified striatal synaptosomes were continuously superfused with L,3,5[3H]tyrosine in order to estimate the synthesis ([3H]water) and release of newly formed [3H]dopamine. In the presence of magnesium, L-glutamate, D,L-alpha-amino-3-hydroxy-5-methyl-4-isoxazole-4-propionate (AMPA) and kainate, but not N-methyl-D-aspartate (NMDA) and 1-aminocyclopentane-1S,3R-dicarboxylate (t-ACPD), stimulated the release of [3H]dopamine, in a dose-dependent manner. When magnesium was omitted or in the presence of AMPA, NMDA also increased the release of [3H]dopamine. The effects of AMPA and kainate were competitively inhibited by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) or 6,7-dinitro-quinoxaline-2,3-dione (DNQX), whereas those of NMDA were reduced by 2-amino-5-phosphonovalerate (APV) or (+)-5-methyl-10,11-dihydro-5-H-dibenzo(a,d)cyclo-hepten-5,10-imine maleate (MK801). The stimulation of [3H]dopamine release by a high concentration of glutamate resulted from the concomitant activation of AMPA and NMDA receptors since this effect was potentiated by glycine and reduced by 2-amino-5-phosphonovalerate or MK801. This reduction was almost complete in the combined presence of DNQX and MK801. Surprisingly, glutamate and NMDA (in the absence of magnesium) reduced the efflux of [3H]water. The reduction of [3H]dopamine synthesis was blocked by 2-amino-5-phosphonovalerate indicating the involvement of NMDA receptors. Neither AMPA nor kainate affected dopamine synthesis. The inhibition of [3H]dopamine synthesis resulting from the stimulation of NMDA receptors was prevented when synaptosomes were continuously superfused with adenosine deaminase and quinpirole, a combined treatment known to markedly reduce the phosphorylation of tyrosine hydroxylase by cAMP-dependent protein kinase. The opposite effects of a high concentration of glutamate on [3H]dopamine synthesis and release were mimicked by ionomycin. As a working hypothesis, it is proposed that the NMDA-triggered calcium influx could lead to a reduction of tyrosine hydroxylase phosphorylation, possibly through an activation of calcineurin.

The effect of theophylline on parkinsonian symptoms

Adenosine is known to inhibit the release of dopamine from central synaptic terminals. The present open trial was therefore conducted to determine whether the adenosine receptor-antagonist theophylline would be of value in Parkinson's disease. Fifteen parkinsonian patients were treated for up to 12 weeks with a slow release oral theophylline preparation (150 mg day-1), yielding serum theophylline levels of 4.44 mg L-1 after one week. The patients exhibited significant improvements in mean objective disability scores and 11 reported moderate or marked subjective improvement. It is suggested that theophylline might be a useful adjunct to the routine therapy of parkinsonian patients.

Release and actions of adenosine in the central nervous system

Adenosine is released from active neurons into the extracellular fluid at a concentration of about 1 mumol/l. Neither the precise cellular origin nor the biochemical form of release has been firmly established, though the nucleotide is probably released partly directly, as a result of raised intracellular levels, and partly as nucleotides, which are subsequently hydrolysed. Once in the extracellular medium, adenosine markedly inhibits the release of excitatory neurotransmitters and modulatory peptides and has direct inhibitory effects on postsynaptic excitability via A1 receptors. A population of A2 receptors may mediate depolarization and enhanced transmitter release. Adenosine also modulates neuronal sensitivity to acetylcholine and catecholamines, all these effects probably contributing to the behavioural changes observed in conscious animals. As a result of their many actions, adenosine analogues are being intensively investigated for use as anticonvulsant, anxiolytic, and neuroprotective agents.

Opposite presynaptic regulations by glutamate through NMDA receptors of dopamine synthesis and release in rat striatal synaptosomes

Purified striatal synaptosomes were superfused continuously with L-[3,5-3H]tyrosine to measure simultaneously the synthesis ([3H]water formed during the conversion of [3H]tyrosine into [3H]DOPA) and the release of [3H]dopamine ([3H]DA). Glutamate (10(-3) M) and NMDA (10(-3) M, in the absence of Mg2+) stimulated the release of [3H]DA, but they reduced the efflux of [3H]water. This reduction of [3H]DA synthesis was blocked by 2-amino-5-phosphonovalerate indicating the involvement of NMDA receptors. Although D,L-alpha-amino-3-hydroxy-5-methyl-4-isoxazole-4-propionate (AMPA) and kainate stimulated the release of [3H]DA, they did not affect its synthesis. The glutamate-evoked inhibition of [3H]DA synthesis was prevented when synaptosomes were superfused continuously with adenosine deaminase plus quinpirole, a treatment which markedly reduces the phosphorylation of tyrosine hydroxylase by cAMP dependent protein kinase. The opposite effects of glutamate on [3H]DA synthesis and release were mimicked by ionomycin (10(-6) M). It is proposed that both an activation of a cyclic nucleotide phosphodiesterase and a dephosphorylation of tyrosine hydroxylase linked to the influx of calcium through NMDA receptors is responsible for the inhibition of dopamine synthesis by glutamate and that calcineurin could play a critical role in these processes.

Dopamine receptors: molecular biology, biochemistry and behavioural aspects

The description of new dopamine (DA) receptor subtypes, D1-(D1 and D5) and D2-like (D2A, D2B, D3, D4), has given an impetus to DA research. While selective agonists and antagonists are not generally available yet, the receptor distribution in the brain suggests that they could be new targets for drug development. Binding characteristics and second messenger coupling has been explored in cell lines expressing the new cloned receptors. The absence of selective ligands has meant that in vivo studies have lagged behind. However, progress has been made in understanding the function of DA-containing discrete brain nuclei and the functional consequence of the DA's interaction with other neurotransmitters. This review explores some of the latest advances in these various areas.

The subcellular distribution of [3H]-CGS 21680 binding sites in the rat striatum: copurification with cholinergic nerve terminals

The subcellular distribution of the adenosine A2a receptor in rat striatum has been investigated using specific binding of the A2a-selective ligand [3H]-CGS 21680. After subcellular fractionation, the distribution of [3H]-CGS 21680 binding was similar to that of the cholinergic nerve terminal marker acetylcholinesterase rather than the more general membrane marker 5'-nucleotidase, with 42% of binding associated with the synaptosomal sub-fraction and 19% with a light membrane fraction. Binding of [3H]-CGS 21680 was also found to co-purify with the cholinergic nerve terminal marker choline acetyltransferase during immunoaffinity purification of striatal cholinergic nerve terminals. These results demonstrate that some adenosine A2a receptors are present on cholinergic nerve terminals in rat striatum.

Influences of different adenosine receptor subtypes on catalepsy in mice

The effects of adenosine A1 and A2 receptors on catalepsy were studied in mice. The adenosine agonists 5-N'-ethylcarboxamide-adenosine (NECA), N6-phenylisopropyladenosine (PIA) and N6-cyclohexyladenosine (CHA) induced dose dependent catalepsy. The A1 adenosine antagonist 8-phenyltheophylline (8-PT) potentiated catalepsy induced by NECA, R-PIA and CHA. However, theophylline did not potentiate but inhibited the responses induced by NECA, R-PIA and CHA. Neither 8-PT nor theophylline alone has any effect on catalepsy in mice. It is concluded that catalepsy induced by the adenosine agonists may be due to A2 receptor stimulation and that the A1 antagonism may potentiate the response.

Release of endogenous adenosine and its metabolites by the activation of NMDA receptors in the rat hippocampus in vivo

1. The effects of N-methyl-D-aspartate (NMDA), KCl, and veratridine on the release of endogenous adenosine and its metabolites, inosine and hypoxanthine, from the rat hippocampus have been studied by in vivo microdialysis. 2. In the hippocampus of rats anaesthetized with urethane the adenosine level reached a stable state estimated at 0.93 microM during the first 2 h after the implantation of the dialysis probe. NMDA (50 microM to 25 mM) in the perfusate evoked a concentration-dependent release of adenosine, inosine and hypoxanthine with an EC50 of 180 microM. The release was reduced by 93% by the specific NMDA receptor antagonist 2-amino-5-phosphonopentanoic acid (2-AP5) at 200 microM, indicating an NMDA receptor-mediated process. In addition, the 100 mM KCl-evoked release of adenosine was also substantially reduced by 77% by 2-AP5, suggesting that a large component of the K(+)-evoked release is NMDA-receptor-mediated. 3. Perfusion with zero-Ca2+ artificial cerebrospinal fluid attenuated the NMDA-evoked release of adenosine only by 16% (not significant) but depressed the K(+)-evoked release by 62%, indicating that most of the NMDA-evoked release is directly receptor-mediated, whereas a large component of the K(+)-evoked release could be via the release of an excitatory amino acid acting at the NMDA receptors.