Differential effects of N-methyl-D-aspartic acid and L-homocysteic acid on cerebellar Purkinje neurons

Hormone disorder and vitamin deficiency in attention deficit hyperactivity disorder (ADHD) and autism spectrum disorders (ASDs)

BACKGROUND

The aim of this study was to analyze thyroid hormones and antibodies, ferritin, vitamins B12 and D, adrenal and gonadal steroid levels, and celiac antibodies in children diagnosed with attention deficit hyperactivity disorder (ADHD) and autism spectrum disorder (ASD).

METHODS

Between February 2014 and July 2014, a total of 77 children and adolescents (31 girls, 46 boys) who were admitted to the Van Training and Research Hospital were included in the study. The study population was divided into three groups including ADHD (n=34), ASD (n=16), and age- and sex-matched healthy controls (n=27). The diagnosis of ADHD was made on the basis of Diagnostic and Statistical Manual of Mental Disorders - Fifth Edition (DSM-5) and DSM-4 Turkish version with the diagnostic interview and Disruptive Behavior Disorder Rating Scale (DBDRS). The diagnosis of ASD was based on the DSM-4 and DSM-5 Turkish version with the diagnostic interview and the Childhood Autism Rating Scale (CARS). The blood samples were obtained between 8:00 and 9:00 A.M.

RESULTS

There was a statistically significant difference in vitamin B12 and D levels and ferritin values among the three groups. The ASD group had the highest ferritin and the lowest vitamins B12 and D levels. Vitamin D levels of the ADHD group were significantly lower compared to the healthy controls.

CONCLUSIONS

Our study results highlight the importance of supplementation of vitamins B12 and D in the ASD and ADHD patients.

Homocysteine potentiates beta-amyloid neurotoxicity: role of oxidative stress

The cause of neuronal degeneration in Alzheimer's disease (AD) has not been completely clarified, but has been variously attributed to increases in cytosolic calcium and increased generation of reactive oxygen species (ROS). The beta-amyloid fragment (Abeta) of the amyloid precursor protein induces calcium influx, ROS and apoptosis. Homocysteine (HC), a neurotoxic amino acid that accumulates in neurological disorders including AD, also induces calcium influx and oxidative stress, which has been shown to enhance neuronal excitotoxicity, leading to apoptosis. We examined the possibility that HC may augment Abeta neurotoxicity. HC potentiated the Abeta-induced increase in cytosolic calcium and apoptosis in differentiated SH-SY-5Y human neuroblastoma cells. The antioxidant vitamin E and the glutathione precursor N-acetyl-L-cysteine blocked apoptosis following cotreatment with HC and Abeta, indicating that apoptosis is associated with oxidative stress. These findings underscore that moderate accumulation of excitotoxins at concentrations that alone do not appear to initiate adverse events may enhance the effects of other factors known to cause neurodegeneration such as Abeta.

Characterization of L-homocysteate-induced currents in Purkinje cells from wild-type and NMDA receptor knockout mice

L-Homocysteic acid (HCA), an endogenous excitatory amino acid in the mammalian CNS, potently activates N-methyl-D-aspartate (NMDA) receptors in hippocampal neurons. However, the responses to HCA in Purkinje cells, which lack functional NMDA receptors, have been largely unexplored: HCA may activate conventional non-NMDA receptors by its mixed agonistic action on both NMDA and non-NMDA receptors, or it may activate a novel non-NMDA receptor that has high affinity for HCA. To test these possibilities, we compared the responses to HCA in cultured Purkinje cells with those in hippocampal neurons by using the whole cell patch-clamp technique. To clearly isolate HCA responses mediated by non-NMDA receptors, we complemented pharmacological methods by using neurons from mutant mice (NR(-/-)) that lack functional NMDA receptors. A moderate dose of HCA (100 microM) induced substantial responses in Purkinje cells. These responses were blocked by non-NMDA receptor antagonists but were insensitive to NMDA receptor antagonists. HCA also activated responses mediated by non-NMDA receptors in both wild-type and NR1(-/-) hippocampal neurons. HCA responses in Purkinje cells had a pharmacological profile (EC(50) and Hill coefficient) very similar to that of non-NMDA receptor components of HCA responses in hippocampal neurons. Moreover, the amplitude of the non-NMDA receptor component of HCA responses was directly correlated with that of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)- and kainate-induced responses in both types of neurons. Finally, in both types of neurons, HCA currents mediated by non-NMDA receptors were potently blocked by the AMPA receptor antagonist GYKI52466. These findings indicate that HCA-activated AMPA receptors in Purkinje cells are similar to those in hippocampal neurons and that there is no distinct HCA-preferring receptor in Purkinje cells. We also found that in hippocampal neurons, the EC(50)s of HCA for non-NMDA receptors and for NMDA receptors were more similar than originally reported; this finding indicates that HCA is similar to other mixed agonists, such as glutamate. HCA responses may appear to be selective at NMDA receptors in cells that express these receptors, such as hippocampal neurons; in cells that express few functional NMDA receptors, such as Purkinje cells, HCA may appear to be selective at non-NMDA receptors.

Functional NMDA receptors are transiently active and support the survival of Purkinje cells in culture

Conflicting evidence exists concerning the activity of NMDA receptors (NMDARs) in cerebellar Purkinje cells and their possible functions. To investigate the activity of NMDARS, we used whole-cell recording on immunocytochemically identified Purkinje cells in primary culture. In addition, we used mice with a disrupted NMDAR1 gene that lack functional NMDARs (NR1-/-) to assess the physiological role of NMDARs. In cultures from normal mice, NMDA-medicated currents were detected in all identified Purkinje cells at 4 d in vitro (div). After 14 d, however, NMDA responses were reduced in amplitude, whereas the responses to kainate and glutamate increased steadily in amplitude. In addition, the NMDA-induced current displayed a pronounced desensitization at these later stages; peak current declined to zero during steady application of NMDA. At 7 div, the number of surviving Purkinje cells was less in cultures treated with NMDA antagonists, and their survival was dose-dependent. Purkinje cell survival was correspondingly poorer in cultures from the NR1-/- mice than in wild-type controls, suggesting that NMDAR activity enhances the survival of Purkinje cells in vitro. The addition of moderate doses of NMDA promoted the survival of wild-type Purkinje cells in the presence of tetrodotoxin. Feeder layers of cerebellar granule cells derived from wild-type or NR1-/- mice promoted survival of Purkinje cells to a similar degree, suggesting that the NMDAR in Purkinje cells, but not in other cells, is directly involved in Purkinje cell viability. The results demonstrate that NMDARs transiently produce membrane current in Purkinje cells and may serve as one of the epigenetic factors that support the survival of Purkinje cells in vitro.

Electrophysiological and electrochemical responses of NMDA in the cerebellum: interactions with nonadrenergic pathway

In the present experiments, we measured N-methyl-D-aspartate (NMDA)-induced norepinephrine (NE) release and extracellular action potentials in the cerebellar cortex of urethane-anesthetized rats. The overflow of NE was measured using a Nafion coated-carbon fiber electrode and in vivo chronoamperometry. We found that both NMDA and quisqualate evoked NE release. Our previous work demonstrated that the electrophysiological activity of cerebellar Purkinje neurons could be either excited or inhibited by local NMDA application. It was reported that bicuculline antagonized NMDA-induced inhibition in Purkinje neurons, suggesting that a GABAergic mechanism was activated during NMDA application. We and others previously found that GABA-mediated electrophysiological depressions were enhanced by NE acting via beta-adrenergic receptors while these responses were decreased by activation of alpha-adrenergic receptors. Since NMDA evokes overflow of endogenous NE, the electrophysiological depression induced by NMDA may contain an NE-mediated modulatory component. In this study, we first examined the interaction of NMDA with beta-adrenergic agonist. We found that local application of isoproterenol facilitated NMDA- or GABA-mediated electrophysiological depressions of the Purkinje neurons. Applications of phenoxybenzamine, which antagonized the alpha-adrenergic response of synaptically released NE, also facilitated NMDA-elicited depression. In contrast, the depression induced by GABA, which did not induce NE overflow, was not potentiated by phenoxybenzamine. The facilitation of NMDA-induced depression by phenoxybenzamine was antagonized by the beta-adrenergic antagonist timolol. Taken together, these data suggested that the nonadrenergic pathway is involved in NMDA-induced electrophysiological responses in the cerebellum. NMDA may induce neuronal depression through modulation of GABAergic inhibition via NMDA-evoked NE release onto cerebellar Purkinje neurons.

The neurotransmitter candidature of sulphur-containing excitatory amino acids in the mammalian central nervous system

While L-glutamate (L-Glu) is considered to be the predominant excitatory amino acid transmitter in the mammalian CNS, other amino acids have come under scrutiny as possible rivals for such a role. These include four sulphur-containing analogues of L-Glu and L-aspartate known as the SAAs. The L-Glu analogues are L-homocysteic acid and L-homocysteine sulphinic acid, while the L-aspartate analogues are L-cysteic acid and L-cysteine sulphinic acid. They are mixed agonists of excitatory amino acid receptors on a variety of neurones and are reported to be present in and released from mammalian CNS tissue. This review serves to summarize the current state of research into the possibility that one or more of these compounds is indeed a transmitter within the mammalian CNS.

Methamphetamine facilitates ethanol-induced depressions in cerebellar Purkinje neurons of prazocin- or DSP4-treated rats

Methamphetamine (MA) and ethanol (EtOH) are two commonly abused drugs. Previous behavioral studies indicated that MA may synergistically alter EtOH responses. In the present study, we found that local application of MA did not potentiate ethanol-induced depressions of the spontaneous activity of Purkinje neurons in urethane-anesthetized rats. We and others previously found that, in cerebellar Purkinje neurons, EtOH and gamma-amino-butyric acid (GABA)-mediated depressions can be enhanced by norepinephrine (NE) acting via beta-adrenergic receptors while these responses are decreased by activation of alpha-adrenergic receptors. In the present experiment, after blocking alpha-adrenergic receptors with prazocin, MA significantly enhanced EtOH responses in most of neurons studied. It has been reported that MA may directly and indirectly enhance alpha-adrenergic and beta-adrenergic receptor-mediated responses. The present study may suggest that MA can negatively modulate (antagonize) the depressant effects of ethanol via the alpha-adrenergic receptor, which oppose the positive modulatory mechanism (potentiation of EtOH depression) via actions of the beta-adrenergic receptors. We found that lesioning NE neurons with N-chloroethyl-N-ethyl-2-bromobenzylamine hydrochloride (DSP4), a selective noradrenergic neurotoxin, enhance the MA-facilitated ethanol responses, suggesting that this action of MA may not require NE. Since it has been reported that MA increases serotonin (5-HT) and catecholamine release from their nerve terminals, MA may potentiate EtOH depressions by facilitating the release of NE and 5-HT. Taken together, our data suggested that MA may modulate EtOH responses via catecholaminergic and serotonergic mechanisms in cerebellar Purkinje neurons.

Serotonin depresses excitatory amino acid-induced excitation of cerebellar Purkinje cells in the adult rat in vivo

The modulatory effects of serotonin (5-HT) on excitatory amino acid (EAA)-induced excitations of Purkinje cells (PCs) were examined in urethane-anesthetized adult male rats using microiontophoresis and extracellular recordings. Application of 5-HT had minimal effects on the spontaneous firing rates of PCs but depressed excitations elicited by glutamate (Glu), aspartate (Asp), kainate (KA), and quisqualate (QA), and to a lesser extent those of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA). Excitations induced by the metabotropic EAA agonist, (+-)-1-aminocyclopentane-trans-1,3-dicarboxylate (t-ACPD), were unaffected by 5-HT. In summary, 5-HT depressed EAA-mediated excitations with the following rank order of effectiveness: Glu = Asp = KA = QA > AMPA >> t-ACPD. These findings suggest that 5-HT shows some selectivity in its modulation of EAA-mediated excitations of PCs and thus may serve an important neuromodulatory role in the cerebellum.

l-Homocysteic acid selectively activates receptors of rat retinal ganglion cells

To explore the possible role of L-homocysteic acid (HCA) as a retinal transmitter, whole-cell recordings with patch electrodes were performed on isolated rat retinal ganglion cells (RGCs) in culture. HCA elicited an inward current at -60 mV. Similar to currents evoked at this potential in RGCs by N-methyl-D-aspartate (NMDA), HCA-activated currents were of relatively small amplitude (10-150 pA), and the noise level increased dramatically during the response. When HCA was co-applied with concentrations of NMDA that elicited a maximal response, the current was not increased over that of NMDA alone. HCA-evoked currents were almost completely blocked by the NMDA antagonists D-2-amino-5-phosphonovalerate (D-AP5), Mg2+, or 7-chlorokynurenate (7-Cl KYN). Unlike its effects on other preparations, even millimolar concentrations of HCA did not activate kainate-like currents. These observations suggest that HCA specifically activates the NMDA receptor-channel complex of rat RGCs.

Multiple ionic mechanisms are activated by the potent agonist quisqualate in cultured cerebellar Purkinje neurons

Current clamp recordings were used to analyze responses of cultured cerebellar Purkinje neurons to quisqualate and several other selective non-N-methyl- D-aspertate (NMDA) agonists. Quisqualate, a potent agonist in the cerebellar Purkinje neuron, evoked both short- and long-term changes in excitability, that activated within seconds and lasted for several minutes. Two components of the response were activated differentially by subtype selective agonists, and differed in their mechanism of expression and time course. The initial component of the response was activated by ionotropic agonists ((RS)-d-amino-3-hydroxyl-5-methyl-4-isoxazolepropionic acid (AMPA) domoate), and by quisqualate and glutamate which are effective at both the ionotropic and metabotropic quisqualate receptor subtypes, but not by the metabotropic agonist trans (+/-)-1-amino-1,3-cyclopentanedicarboxylic acid (ACPD). This component was dependent on extracellular Na+, and characterized by a rapid depolarization with a short latency (less than 1-2 s) and a decrease in membrane resistance as expected for an ionotropic reponse. The rapid depolarization extended into an agonist-dependent plateau phase, which could not be evoked by depolarization alone. The second ('late') phase of the response was a slowly-activating, long-lasting change in membrane excitability, accompanied by little or no change in the membrane potential. The late phase, marked by an increase in voltage-dependent bursting spike activity, was induced by the metabotropic agonist, ACPD, and by quisqualate and glutamate, but not by ionotropic selective agonists such as AMPA. Little or no bursting was evoked by AMPA, domoate, kainate or homocysteate. This late phase was also accompanied by increases in the magnitude and duration of the complex spikes and in the afterhyperpolarization following brief current-driven depolarizations. The slower time course of the late component is consistent with a pathway involving second messenger systems. Our results support the hypothesis that coregulation of both ionotropic and metabotropic mechanisms produces the complex and prolonged excitatory response characteristic of the Purkinje neuron.

Excitatory amino acid receptor-channels in Purkinje cells in thin cerebellar slices

Glutamate receptors of the N-methyl-D-aspartate (NMDA) and non-NMDA type serve different functions during excitatory synaptic transmission. Although many central neurons bear both types of receptor, the evidence concerning the sensitivity of cerebellar Purkinje cells to NMDA is contradictory. To investigate the receptor types present in Purkinje cells, we have used whole-cell and outside-out patch-clamp methods to record from cells in thin cerebellar slices from young rats. At a holding potential of -70 mV (in nominally Mg(2+)-free medium, with added glycine) NMDA caused a whole-cell current response which consisted of a dramatic increase in the frequency of synaptic currents. In the presence of tetrodotoxin (TTX) and the gamma-aminobutyric acidA (GABAA) receptor antagonist bicuculline, spontaneous synaptic currents and responses to NMDA were inhibited. In a proportion of cells a small polysynaptic response to NMDA persisted, which was further reduced by the non-NMDA receptor antagonist 6-cyano-2,3-dihydro-7-nitroquinoxalinedione (CNQX). The non-NMDA glutamate receptor agonists kainate (KA), quisqualate (QA) and s-alpha-amino-3-hydroxy-5-methyl-4-isoazolepropionic acid (s-AMPA), evoked large inward currents due to the direct activation of receptors in Purkinje cells. NMDA applied to excised membrane patches failed to evoke any single-channel currents, whereas s-AMPA and QA caused small inward currents accompanied by marked increases in current noise. Spectral analysis of the s-AMPA noise in patches gave an estimated mean channel conductance of approximately 4 pS.(ABSTRACT TRUNCATED AT 250 WORDS)

Responses to excitatory amino acids of Purkinje cells' and neurones of the deep nuclei in cerebellar slice cultures

1. The actions of the endogenous excitatory amino acids (EAAS) glutamate (Glu), aspartate (Asp) and homocysteate (HCA) on Purkinje cells and neurones of the deep nuclei in cerebellar slice cultures were investigated using intracellular recordings in the single-electrode voltage-clamp mode and the whole-cell configuration of the patch-clamp technique. 2. Purkinje cells and neurones of deep cerebellar nuclei were identified according to their localization in the living cultures, their morphology as revealed by intracellular injections of Lucifer Yellow and their immunoreactivity to antibodies to the 28 kDa Ca2(+)-binding protein. 3. When Purkinje cells were voltage-clamped near their resting membrane potential in a TTX-containing salt solution, Glu, Asp and HCA induced inward currents which were abolished by 6-cyano-7-nitroxaline-2,3-dione (CNQX), a selective antagonist of the non-N-methyl-D-aspartate (NMDA) subtype of EAA receptors. The selective antagonist of NMDA receptors, D-(-)-2-amino-5-phosphonovaleric acid (D-APV), was ineffective in blocking the responses induced by these three amino acids. NMDA, even at high concentrations and in magnesium-free bathing solution, had no detectable effect on membrane properties of Purkinje cells grown in culture during 11-34 days. 4. In magnesium-containing saline, the amplitude of the responses induced by Glu, Asp and HCA was a linear function of the membrane potential. 5. In contrast, neurones of the deep cerebellar nuclei were responsive to NMDA and the inward currents induced by Glu, Asp and HCA were partially blocked both by CNQX and by D-APV. 6. In magnesium-containing saline, the amplitude of the currents induced by NMDA as well as by the three endogenous EAAs decreased at hyperpolarizing holding potentials whereas the current-voltage relation of the responses induced by quisqualate (QA) was strictly linear. 7. It is concluded that Purkinje cells in cerebellar slice cultures do not express NMDA receptors and that excitation of these neurones by the endogenous amino acids Glu, Asp and HCA is mediated exclusively through the activation of non-NMDA receptors. In the same preparation, neurones of the deep cerebellar nuclei possess NMDA and non-NMDA receptors which can be both activated by the three endogenous excitatory amino acids.

Effect of climbing fiber deprivation on release of endogenous aspartate, glutamate, and homocysteate in slices of rat cerebellar hemispheres and vermis

Aspartate (Asp) and/or glutamate (Glu) have been proposed as putative excitatory transmitters released from synaptic terminals of the olivo-cerebellar climbing fiber afferents to the Purkinje cells. Investigations of the climbing fiber transmitter(s) separately for hemispheres and vermis were performed to examine whether the current controversy over the role of Asp as a neurotransmitter in the climbing fibers may be due to topographic differences. K(+)-induced Ca2(+)-dependent release of endogenous substances was investigated in slices of cerebellar hemisphere and vermis of control rats and those deprived of climbing fibers by 3-acetylpyridine (3-AP) treatment. A release of Asp and Glu, as well as a small but significant release of homocysteic acid (HCA) was confirmed in control rats. Climbing fiber deprivation by 3-AP treatment reduced the stimulated release of Asp by 48% in slices of cerebellar hemispheres, but not in vermis. Climbing fiber deprivation completely abolished the release of HCA in both hemispheres and vermis. The release of HCA, Asp, and Glu from slices of control and climbing fiber-deprived rats evoked by 50 mM K+ was greater than 90% Ca2(+)-dependent. These results support the hypothesis that Asp is a transmitter candidate of the climbing fibers projecting to the cerebellar hemispheres, but not to the vermis, and provide the first evidence that HCA can be linked to a specific pathway.

L-homocysteate stimulates [3H]MK-801 binding to the phencyclidine recognition site and is thus an agonist for the N-methyl-D-aspartate-operated cation channel

Rat brain synaptosomal membranes that are depleted of endogenous excitatory amino acids cannot bind [(+)-5-methyl-10, 11-dihydro-5H-dibenzo(a,d]cyclohept-5,10-imine maleate] ([3H]MK-801). However, they do so upon the restoration of excitatory amino acid agonists such as L-glutamate. [3H]MK-801 provides a molecular probe which is specific for a binding site located within the ionophore of the N-methyl-D-aspartate-type excitatory amino acid receptor, [3H]MK-801 does not bind to non-N-methyl-D-aspartate excitatory amino acid receptors. Exploiting [3H]MK-801 binding as a quantitative measure of agonist activity with respect to ability of inducing the open channel conformation, the present study demonstrates that L-homocysteate is an agonist almost equivalent to L-glutamate in terms of efficacy (maximal N-methyl-D-aspartate response) as well as potency (EC50). The effect of L-homocysteate was dose-dependent, stereospecific (L-homocysteate greater than DL-homocysteate greater than D-homocysteate), suppressible by the N-methyl-D-aspartate-selective competitive antagonist (+/-)-3(2-carboxy-piperazine-4-yl)propyl-l-phosphonate, and potentiated by the N-methyl-D-aspartate-selective "allosteric" modulator glycine. The demonstrated inactivity of L-homocysteine (and virtually all naturally occurring, non-acidic amino acids) implies that the omega-sulphonic acid moiety is an acceptable substitute for the omega carboxyl group for activating the N-methyl-D-aspartate receptor. While the potency of L-homocysteate at N-methyl-D-aspartate receptors was by a factor of only 1.6 smaller than that of L-glutamate, the affinity of L-homocysteate for kainate-type excitatory amino acid receptors was approximately four-fold lower than that of L-glutamate.(ABSTRACT TRUNCATED AT 250 WORDS)

Excitatory amino acids and Alzheimer's disease: idle thoughts on an exciting subject

The reported loss of cortical glutaminase activity in Alzheimer's disease is another possible indicator of loss of glutamate neurons. A diversity of excitatory amino acids (EAA) and NMDA receptor subtypes might explain selective neuronal losses of neurons in various diseases. Weaknesses in the arguments presented are the multiple actions of THA and that 2-amino-3-(methylamino)propionate is probably not an EAA.