Long-term enhancement of dopamine release by high frequency tetanic stimulation via a N-methyl-D-aspartate-receptor-mediated pathway in rat striatum

Cyclic AMP and afferent activity govern bidirectional synaptic plasticity in striatopallidal neurons

Recent experimental evidence suggests that the low dopamine conditions in Parkinson's disease (PD) cause motor impairment through aberrant motor learning. Those data, along with computational models, suggest that this aberrant learning results from maladaptive corticostriatal plasticity and learned motor inhibition. Dopaminergic modulation of both corticostriatal long-term depression (LTD) and long-term potentiation (LTP) is proposed to be critical for these processes; however, the regulatory mechanisms underlying bidirectional corticostriatal plasticity are not fully understood. Previously, we demonstrated a key role for cAMP signaling in corticostriatal LTD. In this study, mouse brain slices were used to perform a parametric experiment that tested the impact of varying both intracellular cAMP levels and the strength of excitatory inputs on corticostriatal plasticity. Using slice electrophysiology in the dorsolateral striatum, we demonstrate that both LTP and LTD can be sequentially induced in the same D2-expressing neuron and that LTP was strongest with high intracellular cAMP and LFS, whereas LTD required low intracellular cAMP and high-frequency stimulation. Our results provide a molecular and cellular basis for regulating bidirectional corticostriatal synaptic plasticity and may help to identify novel therapeutic targets for blocking or reversing the aberrant synaptic plasticity that likely contributes to motor deficits in PD.

Neurotoxicity of endocrine disruptors: possible involvement in brain development and neurodegeneration

Environmental chemicals that act as endocrine disruptors do not appear to pose a risk to human reproduction; however, their effects on the central nervous systems are less well understood. Animal studies suggested that maternal exposure to endocrine-disrupting chemicals (EDC) produced changes in rearing behavior, locomotion, anxiety, and learning/memory in offspring, as well as neuronal abnormalities. Some investigations suggested that EDC exert effects on central monoaminergic neurons, especially dopaminergic neurons. Our data demonstrated that EDC attenuate the development of dopaminergic neurons, which might be involved in developmental disorders. Perinatal exposure to EDC might affect neuronal plasticity in the hippocampus, thereby potentially modulating neuronal development, leading to impaired cognitive and memory functions. Endocrine disruptors also attenuate gender differences in brain development. For example, the locus ceruleus is larger in female rats than in males, but treatments with bisphenol-A (BPA) enlarge this region in males. Some reports indicated that EDC induce hypothyroidism, which might be evidenced as abnormal brain development. Endocrine disruptors might also affect mature neurons, resulting in neurodegenerative disorders such as Parkinson's disease. The current review focused on alterations in the brain induced by EDC, specifically on the possible involvement of EDC in brain development and neurodegeneration.

A Role for Adenosine A1 Receptors in GABA and NMDA-Receptor Mediated Modulation of Dopamine Release: Studies Using Fast Cyclic Voltammetry

In the striatum many neurotransmitters including GABA, glutamate, acetylcholine, dopamine, nitric oxide and adenosine interact to regulate synaptic transmission. Dopamine release in the striatum is regulated by a number of pre- and post-synaptic receptors including adenosine. We have recently shown using isolated rat striatal slices, and the technique of fast cyclic voltammetry, that adenosine A₁ receptor-mediated inhibition of dopamine release is modulated by dopamine D₁ receptors. In the present study we have investigated the influence of NMDA and GABA receptor activation on the modulation of electrically stimulated dopamine release by adenosine. Application of the adenosine A₁ receptor agonist, N⁶-cyclopentyladenosine (CPA), concentration-dependently inhibited dopamine release to a maxiumum of 50%. Perfusion of the glutamate receptor agonist, NMDA, in low magnesium, caused a rapid and concentration-dependent inhibition of dopamine release. Prior perfusion with the adenosine A₁ receptor antagonist, DPCPX, significantly reduced the effect of 5 μM and 10 μM NMDA on dopamine release. The GABA_A receptor agonist, isoguvacine, had a significant concentration-dependent inhibitory effect on dopamine release which was reversed by prior application of the GABA_A receptor antagonist, picrotoxin, but not DPCPX. Finally inhibition of dopamine release by CPA (1μM) was significantly enhanced by prior perfusion with picrotoxin. These data demonstrate an important role for GABA, NMDA and adenosine in the modulation of dopamine release.

Behavioural characteristics and gene expression in the hyperactive wiggling (Wig) rat

Recently, congenic wiggling (Wig) rats were described as a good model for attention-deficit hyperactivity disorder; 12- to 14-week-old animals demonstrated hyperactivity, impulsive behaviour and an impaired working memory. Here, we show that 4- to 5-week-old Wig rats displayed significantly greater spontaneous motor activity than control rats during a period of darkness. Subcutaneous injection of 4 mg/kg methamphetamine exacerbated hyperactivity, the reverse of its effect in rats with neonatally induced 6-hydroxydopamine lesions. Immunohistochemistry showed low levels of tyrosine hydroxylase in the ventral midbrain, similar to 6-hydroxydopamine-treated rats. In cDNA macroarrays, 4-week-old Wig rats showed increased expression of the adenosine A2a receptor in the dorsal striatum, macrophage migration inhibitory factor in the frontal cortex, ventral striatum and midbrain, and calbindin 2 in the dorsal and ventral midbrain. Expression of the gamma-aminobutyric acid (GABA) transporter and sterol carrier protein 2 genes was reduced in all regions. Dopamine transporter gene expression was increased in the dorsal midbrain but decreased in the ventral midbrain, a pattern distinct from that induced by 6-hydroxydopamine. Although abnormal development of dopaminergic neurons may underlie motor hyperactivity, other mechanisms may control responsiveness to methamphetamine. Wig rats may provide a model of attention-deficit hyperactivity disorder in which treatment with psychostimulants accelerate the hyperactivity.

Dopamine-glutamate reciprocal modulation of release and motor responses in the rat caudate-putamen and nucleus accumbens of "intact" animals

Functional interactions between dopaminergic neurotransmission and glutamatergic neurotransmission are well known to play a crucial integrative role in the striatum, the major input structure of the basal ganglia now widely recognized to contribute to the control of motor activity and movements but also to the processing of cognitive and limbic functions. However, the nature of these interactions is still a matter of debate and controversy. This review (1) summarizes anatomical data on the distribution of dopaminergic and glutamatergic receptors in the striatum-accumbens complex, (2) focuses on the dopamine-glutamate interactions in the modulation of each other's release in the striatum-accumbens complex, and (3) examines the dopamine-glutamate interactions in the entire striatum involved in the control of locomotor activity. The effects of dopaminergic and glutamatergic receptor selective agonists and antagonists on dopamine and glutamate release as well on motor responses are analyzed in the entire striatum, by reviewing both in vitro and in vivo data. Regarding in vivo data, only findings from focal injections studies in the nucleus accumbens or the caudate-putamen of "intact" animals are reviewed. Altogether, the available data demonstrate that dopamine and glutamate do not uniformly interact to modulate each others' release and postsynaptic modulation of striatal output neurons. Depending on the receptor subtypes involved, interactions between dopaminergic and glutamatergic transmission vary as a multiple and complex combination of tonic, phasic, facilitatory, and inhibitory properties.

Motor hyperactivity caused by a deficit in dopaminergic neurons and the effects of endocrine disruptors: a study inspired by the physiological roles of PACAP in the brain

Recent studies have revealed that the pituitary adenylate cyclase-activating polypeptide (PACAP) might act as a psychostimulant. Here we investigated the mechanisms underlying motor hyperactivity in patients with pervasive developmental disorders, such as autism, and attention-deficit hyperactivity disorder (ADHD). We studied the effects of intracisternal administration of 6-hydroxydopamine (6-OHDA) or endocrine disruptors (EDs) on spontaneous motor activity (SMA) and multiple gene expression in neonatal rats. Treatment with 6-OHDA caused significant hyperactivity during the dark phase in rats aged 4-5 weeks. Motor hyperactivities also were observed after treatment with endocrine disruptors, such as bisphenol A, nonylphenol, diethylhexyl phthalate and dibutyl phthalate, during both dark and light phases. Gene-expression profiles produced using cDNA macroarrays of 8-week-old rats with 6-OHDA lesions revealed the altered expression of several classes of gene, including the N-methyl-D-aspartate (NMDA) receptor 1, glutamate/aspartate transporter, gamma-aminobutyric-acid transporter, dopamine transporter 1, D4 receptor, and peptidergic elements such as the galanin receptor, arginine vasopressin receptor, neuropeptide Y and tachykinin 2. The changes in gene expression caused by treatment with endocrine disruptors differed from those induced by 6-OHDA. These results suggest that the mechanisms underlying the induction of motor hyperactivity and/or compensatory changes in young adult rats might differ between 6-OHDA and endocrine disruptors.

Motor activity and gene expression in rats with neonatal 6-hydroxydopamine lesions

A rat model of a hyperkinetic disorder was used to investigate the mechanisms underlying motor hyperactivity. Rats received an intracisternal injection of 6-hydroxydopamine on post-natal day 5. At 4 weeks of age, the animals showed significant motor hyperactivity during the dark phase, which was attenuated by methamphetamine injection. Gene expression profiling was carried out in the striatum and midbrain using a DNA macroarray. In the striatum at 4 weeks, there was increased gene expression of the NMDA receptor 1 and tachykinins, and decreased expression of a GABA transporter. At 8 weeks, expression of the NMDA receptor 1 in the striatum was attenuated, with enhanced expression of the glial glutamate/aspartate transporter. In the midbrain, a number of genes, including the GABA transporter gene, showed decreased expression at 4 weeks. At 8 weeks, gene expression was augmented for the dopamine transporter, D4 receptor, and several genes encoding peptides, such as tachykinins and their receptors. These results suggest that in the striatum the neurotransmitters glutamate, GABA and tachykinin may play crucial roles in motor hyperactivity during the juvenile period. Several classes of neurotransmitters, including dopamine and peptides, may be involved in compensatory mechanisms during early adulthood. These data may prompt further neurochemical investigations in hyperkinetic disorders.

Age-related differences in NMDA/metabotropic glutamate receptor binding in rat substantia nigra

Both N-methyl-D-aspartate (NMDA) and quisqualate/AMPA-insensitive metabotropic glutamate (mGlu) receptors mediate glutamate neurotransmission in substantia nigra (SN). In this work, NMDA and mGlu receptor sites in substantia nigra pars compacta (SNC) and pars reticulata were autoradiographically mapped in rat brains using specific binding of (+)3H-MK801 or 3H-glutamate, with saturating concentrations of NMDA, AMPA and quisqualate. In brains of both adult and postnatal day 15 (PN15) male rats, prepared at subjective mid-day of a 12h light/12h dark (12h L/12h D) cycle, specific binding at NMDA and mGlu sites in substantia nigra was pronounced when compared with control binding. The (+)3H-MK801 binding in adults was spatially heterogeneous. Overall binding density in pars compacta was higher relative to binding density in pars reticulata with a mean percent change (Deltaxmacr;%) of 32%. Within the pars reticulata but not pars compacta, there were rostro-caudal differences with considerably denser binding in the posterior compared with the anterior pars reticulata (Deltaxmacr;%=108%). PN15 rats showed a less pronounced heterogeneity in pars compacta versus pars reticulata binding, (Deltaxmacr;%=27%), and less rostro-caudal differentiation in (+)3H-MK801 binding density throughout pars reticulata (Deltaxmacr;%=46%). 3H-glutamate binding in both adult and PN15 rats was less dense overall than (+)3H-MK801 binding. In adults, there was no difference in binding density between pars compacta and pars reticulata (Deltaxmacr;%=0.4%), but there were marked heterogeneities when binding was compared between anterior versus posterior pars compacta (Deltaxmacr;%=29%), and anterior versus posterior pars reticulata (Deltaxmacr;%=25%). This rostro-caudal heterogeneity in 3H-glutamate binding density was also present in PN15 pars compacta (Deltaxmacr;%=45%) but not in pars reticulata. Our findings mirror similar anterior/posterior heterogeneities in the GABAergic system in adult and PN15 male rats and may reflect a developmental change in both the structure and anticonvulsant/proconvulsant properties of substantia nigra pars reticulata (SNR) with age.

Dopamine-dependent plasticity of corticostriatal synapses

Knowledge of the effect of dopamine on corticostriatal synaptic plasticity has advanced rapidly over the last 5 years. We consider this new knowledge in relation to three factors proposed earlier to describe the rules for synaptic plasticity in the corticostriatal pathway. These factors are a phasic increase in dopamine release, presynaptic activity and postsynaptic depolarisation. A function is proposed which relates the amount of dopamine release in the striatum to the modulation of corticostriatal synaptic efficacy. It is argued that this function, and the experimental data from which it arises, are compatible with existing models which associate the reward-related firing of dopamine neurons with changes in corticostriatal synaptic efficacy.

Both metabotropic glutamate I and II receptors mediate augmentation of dopamine release from the striatum in methamphetamine-sensitized rats

The role of metabotropic glutamate receptor (mGluR) on dopamine overflow from the striatum was studied in methamphetamine (MAP)-sensitized rats. The increase of dopamine release by MAP was significantly inhibited by perfusion of a mGluR antagonist R,S-alpha-methyl-4-carboxyphenylglycine. The perfused mGluR agonist [S,3R-1-aminocyclopentane-1,3-dicarboxylic acid enhanced the dopamine level. The enhancement was significantly attenuated by co-perfusion of a mGluR group I antagonist (S)-4-carboxy-3-hydroxyphenylglycine or a mGluR group II antagonist R,S-a-methyl-4-tetrazolylphenylglycine. These suggest that both mGluR group I and II mediate augmentation of dopamine release in MAP-sensitized rats.

Mu and kappa opioid agonists modulate ventral tegmental area input to the ventral pallidum

The ventral pallidum (VP) is situated at the convergence of midbrain dopamine and accumbal opioid efferent projections. Using in vivo electrophysiological procedures in chloral hydrate-anaesthetized rats, we examined whether discrete application of mu- [D-Ala2,N-Me-Phe4,Gly-ol5 (DAMGO)] or kappa- (U50488) opioid receptor agonists could alter VP responses to electrical stimulation of ventral tegmental area. Rate suppressions occurred frequently following ventral tegmental area stimulation. Consistent with an involvement of dopamine in this effect, none of the 12 spontaneously active ventral pallidal neurons recorded in rats that had monoamines depleted by reserpine responded to electrical stimulation of ventral tegmental area. Moreover, in intact rats, the dopamine antagonist flupenthixol attenuated evoked suppression in 100% of the neurons tested; however, the GABAA antagonist bicuculline was able to slightly attenuate the response in 50% of the neurons tested. These observations concur with our previous studies in indicating that ventral tegmental area stimulation releases dopamine (and sometimes GABA) onto ventral pallidal neurons. Both DAMGO and U50488 decreased the inhibitory effects of ventral tegmental area stimulation. These effects on the endogenously released transmitter differed from those seen with exogenously applied dopamine, for DAMGO did not alter the efficacy or potency of microiontophoretically applied dopamine. Taken together, these observations suggest that the interaction between DAMGO and dopamine does not occur at a site that is immediately postsynaptic to the dopaminergic input within the VP, but rather that opioid modulation involves mechanisms governing presynaptically released dopamine. These modulatory processes would enable ventral pallidal opioids to gate the influence of ventral tegmental area dopamine transmission on limbic system outputs at the level of the VP.

Effects of adenosine A1- and A2A-receptor agonists on enhancement of dopamine release from the striatum in methamphetamine-sensitized rats

We report here both adenosine A1- and A2A-receptor agonists inhibit the expression of methamphetamine (MAP)-induced behavioral sensitization in rats. Animals were treated with MAP (1.0 mg/kg, i.p.) every 3 days with a total of 5 administrations. The augmentation of dopamine release from the striatum was demonstrated by MAP re-administration (0.5 mg/kg, i.p.) after 7-day withdrawal by microdialysis. The augmentation of dopamine release was inhibited by pre-treatment not with N6-cyclohexyladenosine (0.01 mg/kg, i.p.) but by with 2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxy-amide adenosine (0.1 mg/kg, i.p.). These results suggested that adenosine A1 and A2A receptors play an inhibitory role in sensitization via different mechanisms.

Involvement of ryanodine receptor type 3 in dopamine release from the striatum: evidence from mutant mice lacking this receptor

Although it is known that ryanodine receptor type 3 is expressed in the striatum, the function of this receptor has not been elucidated. Therefore, we examined whether caffeine- and ryanodine-induced dopamine release in striatal slices is affected in mice lacking ryanodine receptor type 3. Pretreatment with thapsigargin, an inhibitor of the Ca(2+) ATPase pump of the endoplasmic reticulum, abolished caffeine- or ryanodine-induced dopamine release in slices from normal mice. Dopamine concentration in the striatum and KCl-induced dopamine release were unaffected by a ryanodine receptor type 3 deficiency. Ryanodine-induced dopamine release was significantly attenuated in mice lacking ryanodine receptor type 3, whereas caffeine-induced dopamine release was partially attenuated. Caffeine produced a similar hyper-motor activity in both wild and homozygous mice. The present results suggest the involvement of ryanodine receptor type 3 in dopamine release from the striatum.

Expression of N-methyl-D-aspartate receptor-dependent long-term potentiation in the neostriatal neurons in an in vitro slice after ethanol withdrawal of the rat

To examine changes in corticostriatal synaptic transmission in rats with ethanol withdrawal syndrome, intracellular and extracellular responses to subcortical white matter stimulation were recorded in neostriatal slice preparations. The resting membrane potential, input resistance and depolarizing postsynaptic potentials to single cortical white matter stimulation were similar in the neostriatum of naive and ethanol withdrawal rats. Repetitive stimulation of the white matter induced more pronounced N-methyl-D-aspartate receptor-mediated postsynaptic potentials in ethanol withdrawal than naive rat neostriatum. In intracellular recording, tetanic stimulation (50 Hz, 20 s) induced more pronounced post-tetanic potentiation of depolarizing postsynaptic potentials in the neostriatum of ethanol withdrawal than naive rats. However, in extracellular recording, tetanic stimulation induced smaller post-tetanic depression of population spikes in the neostriatum of ethanol withdrawal than naive rats. Tetanic stimulation of the subcortical white matter induced long-term potentiation of postsynaptic potentials and population spikes in the ethanol withdrawal rat neostriatum, while long-term depression was evoked in the naive rat neostriatum. The induction of long-term potentiation was blocked by D-2-amino-5-phosphonovaleric acid or 7-chlorokynurenic acid, N-methyl-D-aspartate receptor antagonists, but not by (RS)-methyl-4-carboxyphenyl-glycine, a metabotropic glutamate receptor antagonist. Dopamine also significantly depressed the induction of long-term potentiation in ethanol withdrawal rat neostriatum and this depressant effect was antagonized by the D2 antagonist L-sulpiride but not by the D1 antagonist SCH23390. These results indicate that the N-methyl-D-aspartate component of the corticostriatal glutamatergic responses, which might be necessary for induction of long-term potentiation, was enhanced in ethanol withdrawal rats. The depression of long-term potentiation induction by activation of D2 receptor suggests that corticostriatal N-methyl-D-aspartate response or intracellular mechanisms involving in the induction of the long-term potentiation can be suppressed by D2 activation and that the D2 effects are inhibited in the neostriatum of ethanol withdrawal rats.

Plasticity of first-order sensory synapses: interactions between homosynaptic long-term potentiation and heterosynaptically evoked dopaminergic potentiation

Persistent potentiations of the chemical and electrotonic components of the eighth nerve (NVIII) EPSP recorded in vivo in the goldfish reticulospinal neuron, the Mauthner cell, can be evoked by afferent tetanization or local dendritic application of an endogenous transmitter, dopamine (3-hydroxytyramine). These modifications are attributable to the activation of distinct intracellular kinase cascades. Although dopamine-evoked potentiation (DEP) is mediated by the cAMP-dependent protein kinase (PKA), tetanization most likely activates a Ca2+-dependent protein kinase via an increased intracellular Ca2+ concentration. We present evidence that the eighth nerve tetanus that induces LTP does not act by triggering dopamine release, because it is evoked in the presence of a broad spectrum of dopamine antagonists. To test for interactions between these pathways, we applied the potentiating paradigms sequentially. When dopamine was applied first, tetanization produced additional potentiation of the mixed synaptic response, but when the sequence was reversed, DEP was occluded, indicating that the synapses potentiated by the two procedures belong to the same or overlapping populations. Experiments were conducted to determine interactions between the underlying regulatory mechanisms and the level of their convergence. Inhibiting PKA does not impede tetanus-induced LTP, and chelating postsynaptic Ca2+ with BAPTA does not block DEP, indicating that the initial steps of the induction processes are independent. Pharmacological and voltage-clamp analyses indicate that the two pathways converge on functional AMPA/kainate receptors for the chemically mediated EPSP and gap junctions for the electrotonic component or at intermediaries common to both pathways. A cellular model incorporating these interactions is proposed on the basis of differential modulation of synaptic responses via receptor-protein phosphorylation.

NMDA induced glutamate release from the suprachiasmatic nucleus: an in vitro study in the rat

The suprachiasmatic nucleus (SCN) has been identified as a pacemaker for mammalian circadian rhythms. Excitatory amino acid receptors, especially N-methyl-D-aspartate (NMDA) receptors, have been considered to play an important role in the transmission of light information from the retina to the circadian clocks in the SCN. In the present study, we showed that application of NMDA at circadian time (CT) 12-15 induced significant glutamate release from the SCN region in vitro. The NMDA-induced glutamate release was blocked by co-application of the NMDA receptor antagonist MK-801, but not by that of tetrodotoxin. These results suggested that glutamate stimulated its own release by activating NMDA receptors. This NMDA-induced glutamate release through NMDA receptor-mediated mechanisms might be involved in NMDA-induced potent phase shifts.

Reciprocal dopamine-glutamate modulation of release in the basal ganglia

Dopaminergic and glutamatergic transmissions have long been known to interact at multiple levels in the basal ganglia to modulate motor and cognitive functions. One important aspect of their interactions is represented by the reciprocal modulation of release. This topic has been the object of interest since the late 70's, particularly in the striatum and in midbrain dopaminergic areas (substantia nigra and ventral tegmental area). Analysis of glutamate-dopamine interactions in the control of each other's release is complicated by the fact that both glutamate and dopamine act on multiple receptor subtypes which can exert different effects. Therefore, glutamatergic modulation of dopamine release has been reviewed by analyzing the effects of glutamatergic selective receptor agonists and antagonists in the striatum (both motor and limbic portions) and in midbrain dopaminergic areas, as revealed by in vitro (slices, cell cultures, synaptosomes) and in vivo (push-pull, microdialysis and voltammetry techniques) experimental approaches. The same approach has been followed for dopaminergic modulation of glutamate release. The facilitatory nature of glutamate modulating both presynaptic and dendritic dopamine release has clearly emerged from in vitro studies. However, evidence is presented that, at least in the striatum and in the nucleus accumbens of awake rats, glutamate-mediated inhibitory effects may also occur. In vitro and in vivo experiments in the striatum and midbrain dopaminergic areas mainly depict dopamine as an inhibitory modulator of glutamate release. However, in vivo studies reporting dopamine D1 receptor mediated facilitatory effects are also considered. Therefore, the general notion that glutamate and dopamine act oppositely to regulate each other's release, is only partly supported by the available data. Conversely, the nature of the interaction between the two neurotransmitters seems to vary depending on the experimental approach, the brain area considered and the subtype of receptor involved.

Vulnerability of synaptic plasticity in the striatum of methamphetamine-sensitized rats

We examined the influence of ischemia on methamphetamine (MAP)-induced behavioral sensitization and enhancement of dopamine (DA) release. After the recovery period of the ischemia operation, rats were treated with MAP (1 mg/kg, i.p.) once daily for 6 consecutive days. Re-administration of MAP (0.5 mg/kg, i.p.) potentiated the increase of locomotor activity after a 3-day withdrawal and the enhancement of DA release from striatal slices after a 6-day withdrawal. The MAP-induced sensitization was impaired by 5 min ischemia. On the other hand, the increase of locomotor activity induced by single MAP (1 mg/kg, i.p.) administration was impaired by 20 min of ischemia. Moreover, in saline-treated rats the increase of DA release from striatal slices induced by MAP (10 microM) application was also impaired by 20 min of ischemia. These results indicate that the neuronal plastic change may be very vulnerable to ischemia in MAP-induced sensitization.

Methamphetamine-induced sensitization of dopamine release via a metabotropic glutamate receptor mediated pathway in rat striatal slices

We studied the roles of metabotropic glutamate receptors in methamphetamine (MAP)-induced sensitization of dopamine (DA) release from striatal slices. Rats were first treated with MAP (1 mg/kg, i.p.) once daily for 6 consecutive days. After a 6-day withdrawal, DA release from striatal slices evoked by +/- (-)1-aminocyclopentane-trans-1,3-dicarboxylic acid (trans-ACPD) was measured, trans-ACPD-induced DA release was significantly enhanced in MAP-sensitized rats, but the inactive form of trans-ACPD (1R,3S-ACPD) did not enhance DA release. The active form of trans-ACPD (1S,3R-ACPD) (0.1 mM)-evoked DA release was attenuated by treatment with 0.4 mM RS-alpha-methyl-4-carboxyphenylglycine, a metabotropic glutamate receptor antagonist. The present results suggest that metabotropic glutamate receptors play an important role in expression of MAP-induced sensitization.

Release of [3H]dopamine from striatal and cerebral cortical slices from rats with thioacetamide-induced hepatic encephalopathy: different responses to stimulation by potassium ions and agonists of ionotropic glutamate receptors

The effects of depolarizing stimuli; high (50 mM) potassium ions and the glutamate receptor agonists N-methyl-D-aspartate, kainate and 2-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) on the release of newly-loaded [3H]dopamine were studied in frontal cortical and striatal slices from control rats and from rats with acute hepatic encephalopathy induced with a hepatotoxin, thioacetamide. Hepatic encephalopathy enhanced the stimulatory effect of potassium ions by 20% in striatal slices and by 34% in frontal cortical slices. In striatal slices the stimulatory effects of N-methyl-D-aspartate and kainate were depressed in hepatic encephalopathy by 46% and 21%, respectively, which may be taken to reflect impaired modulation of striatal dopamine release by glutamate acting at N-methyl-D-aspartate or kainate receptors. In frontal cortical slices, the stimulatory effect of kainate was enhanced by 35% in hepatic encephalopathy but N-methyl-D-aspartate-stimulated release was not affected. The release evoked by 2-amino-3-hydroxy-5-methyl-4-isoxazolepropionate was not affected in hepatic encephalopathy in either brain region. Stimulation of dopamine release in the frontal cortex by depolarization or glutamate acting at kainate receptors could inhibit the activity of descending corticostriatal glutamatergic pathways, further impairing regulation of dopamine release by glutamate in the striatum.

N-methyl-D-aspartate receptors mediate a slow excitatory postsynaptic potential in the rat midbrain dopaminergic neurons

Repetitive local application of a short train of stimuli to the rat substantia nigra and ventral tegmental area elicited a predominant depolarizing, slow, long-lasting synaptic response in the dopaminergic cells intracellularly recorded in vitro. This slow excitatory postsynaptic potential ranged between 13 and 27 mV at holding potentials of about-75 mV and lasted for 0.2-6 s. It was not greatly affected by the perfusion of 6-cyano-7-nitroquinoxaline-2,3-dione (10-20 microM), while it was potentiated in the presence of bicuculline methiodide (30 microM) or picrotoxin (50-100 microM) and 2-hydroxysaclofen (100-300 microM). In contrast, a substantial component of the slow excitatory postsynaptic potential was reversibly depressed, in a concentration-dependent manner, by the application of the N-methyl-D-aspartate receptor antagonists D,1-2-amino-5-phosphonovalerate (10-100 microM). Furthermore, the slow excitatory postsynaptic potential was reversibly increased by the superfusion of nominally magnesium-free solution. It was graded, increasing in amplitude with increased stimulus intensity, and was blocked by tetrodotoxin (0.5 microM). We suggest that a sustained activation of synaptic terminals containing excitatory amino acids mediates a slow excitatory postsynaptic potential in the dopaminergic cells of the midbrain. N-Methyl-D-aspartate receptors participate in the generation of this slow potential, while the alpha-amino-3-hydroxy-5-methylisoxazole-4-proprionate/kainate receptors do not seem to contribute substantially to this potential. This N-methyl-D-aspartate-mediated synaptic event could be implicated in the release of dopamine as well as in the excitotoxic injury of the dopaminergic neurons.

Regulation of synaptic strength at mixed synapses: effects of dopamine receptor blockade and protein kinase C activation

Previous studies of the mixed excitatory synapses between eighth nerve afferents and the lateral dendrite of the goldfish Mauthner (M-) cell have shown that synaptic strength is enhanced for an hour or longer following either repeated brief tetanizations or local extracellular applications of dopamine. Both the initial electrotonic coupling potential, mediated via current flow through gap junctions, and the subsequent chemically mediated excitatory postsynaptic potentials (EPSPs) are potentiated. Different second messenger pathways are implicated in the postsynaptic induction of these potentiations, with a Ca2+ influx presumably triggering the activity dependent long-term potentiations (LTP) and dopamine acting via a cAMP dependent pathway. Experiments performed to determine whether the LTP involves a stimulus-induced release of dopamine or requires a background level of dopamine receptor activation suggest neither is the case, as tetanization in the presence of a D1 receptor antagonist, which blocks the dopamine effects, produced an LTP comparable to that in the absence of the blocker. The effects of Ca2+ are presumably not due to protein kinase C (PKC) activation, since phorbol esters had no effect on the mixed excitatory synaptic responses, although they did enhance the frequency of spontaneously occurring inhibitory PSPs.