Excitatory amino acid receptor antagonists decrease hypoxia induced increase in extracellular dopamine in striatum of newborn piglets

Role of ionotropic glutamate receptors in the regulation of hippocampal norepinephrine output in vivo

In vitro evidence indicates that norepinephrine release in the mammalian hippocampus is modulated by glutamate receptors. With the use of microdialysis, we have now evaluated the role of ionotropic glutamate receptors in the regulation of hippocampal norepinephrine output in vivo. Stimulation of N-methyl-D-aspartate (NMDA)-sensitive glutamate receptors by local administration of NMDA (1-100μM) resulted in a concentration-dependent decrease in the extracellular concentration of norepinephrine in the hippocampus of freely moving rats, whereas the blockade of these receptors with MK801 (1-100μM) induced a concentration-dependent increase in norepinephrine output. Activation of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)-sensitive glutamate receptors with AMPA (1-100μM) resulted in a biphasic effect on the extracellular norepinephrine concentration, with a decrease in this parameter apparent at 10μM and an increase at 100μM. The AMPA receptor antagonist 6,7-dinitroquinoxaline-2,3-dione had no effect on norepinephrine output. The GABA(A) receptor antagonist bicuculline (10μM) prevented the decrease in hippocampal norepinephrine output induced by either NMDA or 10μM AMPA. Our results thus implicate ionotropic glutamate receptors as key regulators of norepinephrine release in the hippocampus and may therefore provide a basis for the development of new drugs for stress-related disorders.

Acute hydrocephalus upregulates monoamine oxidase mRNA in neonatal rat brain

Metabolic derangement of the dopamine system in hydrocephalic brain has been observed, but the change of monoamine oxidase (MAO), the major enzyme to metabolize dopamine, is not known. The metabolic changes of dopamine and MAO mRNA in the striatum were examined in acute hydrocephalic rats whose ventricular size and intracranial pressure were controlled to a similar degree. The tissue levels of dopamine and its metabolites as well as MAO-A and MAO-B mRNA elevated significantly in hydrocephalus. Cerebrospinal fluid (CSF) diversion reversed these changes and induced an initial decline, followed by an elevation of these substances in extracellular fluid. In summary, the metabolism of dopamine system in the striatum was up-regulated in acute hydrocephalus and CSF diversion reversed this metabolic derangement.

Effect of hypoxia/hypercapnia on metabolism of 6-[(18)F]fluoro-L-DOPA in newborn piglets

There is evidence that the dopaminergic system is sensitive to altered p(O(2)) in the immature brain. However, the respective enzyme activities have not been measured in the living neonatal brain together with brain oxidative metabolism. Therefore 18F-labelled 6-fluoro-L-3,4-dihydroxyphenylalanine (FDOPA) together with positron emission tomography (PET) was used to estimate the activity of the aromatic amino acid decarboxylase (AADC) in the brain of fifteen newborn piglets (2-5 days old). Two PET scans were performed in each piglet. Eleven animals underwent a period of normoxia and moderate hypoxia/hypercapnia (H/H). The remaining four animals were used as an untreated control group. Simultaneously, the brain tissue p(O(2)) was recorded, the regional cerebral blood flow (CBF) was measured with colored microspheres and the cerebral metabolic rate of oxygen (CMRO(2)) was determined. In addition, in four untreated and six H/H treated piglets the relative amounts of fluorodopamine and the respective metabolites were determined in brain tissue samples using HPLC analysis. H/H conditions were induced by lowering the inspired fraction of oxygen from 0.35 to 0.10 and adding CO(2) to the inspired gas resulting in an arterial p(CO(2)) between 74 and 79 mmHg. H/H elicited a more than 3-fold increase of the CBF (P<0.05) so that the CMRO(2) remained unchanged throughout the H/H period. Despite this, the brain tissue p(O(2)) was reduced from 19+/-4 to 6+/-3 mmHg (P<0.05). The permeability-surface area product of FDOPA (PS(FDOPA)) was unchanged. However, the transfer rate of FDOPA (k(3)(FDOPA)) of the nigrostriatal dopaminergic system and the relative amounts of fluorodopamine and the respective metabolites were significantly increased (P<0.05). It is suggested that H/H induces an increase of AADC activity. However, an H/H-induced CBF increase maintains bulk O(2) delivery and preserves CMRO(2).

Peripheral NMDA and non-NMDA receptors contribute to nociception: an electrophysiological study

The present study investigated the effects of peripheral administration of N-methy-D-aspartate (NMDA) and non-NMDA receptor antagonists on C-fiber evoked responses of the spinal dorsal horn neurons in the spinalized rats. When DL-2-amino-5-phosphonovaleric acid (AP5) (10 mM, 1 mM, 0.1 mM, 20 microl) or 6, 7-dinitroquinoxaline-2, 3-dione (DNQX) (1 mM, 0.1 mM, 0.01 mM, 20 microl) was subcutaneously injected into the receptive field on the hindplantar region, C-fiber evoked responses of the dorsal horn neurons were profoundly inhibited in a dose-dependent manner. Three hours after subcutaneous injection of carrageenan into the ipsilateral hindpaw, NMDA and non-NMDA antagonist-induced inhibition of C-fiber evoked responses was more potent than that in the normal rat (Student's t-test, p < 0.05). In the carragenan-treated rats, DNQX-induced inhibition was stronger than AP-5-induced one (Student's t-test, p < 0.05). The results suggest that peripheral NMDA and non-NMDA receptors are involved in mediating excitation of nociceptors.

CPP antagonizes hypoxia-induced changes in dopamine metabolism in the striatum of newborn rat

Treatment with 3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP), a potent and competitive N-methyl-D-aspartate antagonist, is able to reduce the hypoxia-induced increase in striatal dopamine level by 26% even after the hypoxic insult has occurred. The hypoxia-induced decrease of the striatal 3,4-dihydroxyphenylacetic acid level can also be reversed by CPP. This study demonstrates that CPP can antagonize the hypoxia-induced changes in the dopamine metabolism in the striatum of the newborn rat.

Upregulation of the aromatic amino acid decarboxylase under neonatal asphyxia

Perinatal hypoxic-ischemic cerebral injury is a major determinant of neurologic morbidity and mortality in the neonatal period and later in childhood. There is evidence that the dopaminergic system is sensitive to asphyxia. However, the respective enzyme activities have not yet been measured in the living neonatal brain. In this study, we have used 18F-labeled 6-fluoro-L-3,4-dihydroxyphenylalanine (FDOPA) together with positron-emission tomography (PET) to estimate the activity of the aromatic amino acid decarboxylase (AADC), the ultimate enzyme in the synthesis of dopamine (DA), in the brain of newborn piglets. Simultaneously, the cerebral blood flow (CBF) was measured with colored microspheres. Asphyxia elicited an up to threefold increase of the CBF. Despite this, the blood-brain transfer of FDOPA as well as the clearance rate constants from brain were unchanged. However, the synthesis rate of FDA from FDOPA was significantly increased in frontal cortex, striatum, and midbrain. This increase of the AADC activity and the decrease of monoamine oxidase activity may contribute to the increase of extracellular DA during asphyxia which is expected to be involved in severe disturbances of neuronal metabolism, e.g., by generating free radicals.

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.