Mechanisms underlying domoic acid-induced dopamine release from striatum: an in vivo microdialysis study

Systems Toxicology Approach for Assessing Developmental Neurotoxicity in Larval Zebrafish

Adverse outcomes that result from chemical toxicity are rarely caused by dysregulation of individual proteins; rather, they are often caused by system-level perturbations in networks of molecular events. To fully understand the mechanisms of toxicity, it is necessary to recognize the interactions of molecules, pathways, and biological processes within these networks. The developing brain is a prime example of an extremely complex network, which makes developmental neurotoxicity one of the most challenging areas in toxicology. We have developed a systems toxicology method that uses a computable biological network to represent molecular interactions in the developing brain of zebrafish larvae. The network is curated from scientific literature and describes interactions between biological processes, signaling pathways, and adverse outcomes associated with neurotoxicity. This allows us to identify important signaling hubs, pathway interactions, and emergent adverse outcomes, providing a more complete understanding of neurotoxicity. Here, we describe the construction of a zebrafish developmental neurotoxicity network and its validation by integration with publicly available neurotoxicity-related transcriptomic datasets. Our network analysis identified consistent regulation of tumor suppressors p53 and retinoblastoma 1 (Rb1) as well as the oncogene Krüppel-like factor (Klf8) in response to chemically induced developmental neurotoxicity. The developed network can be used to interpret transcriptomic data in a neurotoxicological context.

Participation of glutamatergic and nitrergic systems in the striatal dopamine release induced by isatin, a MAO inhibitor

Isatin is a biofactor with different biochemical and pharmacological properties whose effects attract much attention because it is an endogenous inhibitor of the monoamine oxidase in the brain. When exogenously administrated, isatin increases dopamine levels in intact and denervated striatum of rats, an effect that could indicate its potential as a therapeutic agent in Parkinson disease. However, the neurochemical mechanisms by which isatin increases dopamine in the striatum are poorly understood. In the present study, we evaluate the role of the glutamatergic and nitrergic systems in the isatin-induced dopamine release from rat striatum. Our findings show that the intrastriatal administration of 10 mM isatin significantly increases the in vivo release of dopamine (1,104.7% ± 97.1%), and the amino acids glutamate (428.7% ± 127%) and taurine (221% ± 22%) from rat striatum measured by brain microdialysis. The pretreatment with MK-801 (500 µM) or AP5 (650 µM) (glutamatergic NMDA receptors antagonists) significantly reduces the effect of isatin on dopamine release by 52% and 70.5%, respectively. The administration of the nitric oxide synthase inhibitors, L-NAME (100 µM) or 7-NI (100 µM) also decreases the isatin-induced dopamine release by 77% and 42%, respectively. These results show that isatin, in addition to increasing dopamine release, also increases glutamate levels, and possibly activates NMDA receptors and nitric oxide production, which can promote a further increase in the dopamine release.

In vivo neurochemical characterization of clothianidin induced striatal dopamine release

Clothianidin (CLO) is a neonicotinoid insecticide with selective action on nicotinic acetylcholine receptors. The aim of this study was to determine the neurochemical basis for CLO-induced striatal dopamine release using the microdialysis technique in freely moving and conscious rats. Intrastriatal administration of CLO (3.5mM), produced an increase in both spontaneous (2462 ± 627% with respect to basal values) and KCl-evoked (4672 ± 706% with respect to basal values) dopamine release. This effect was attenuated in Ca(2+)-free medium, and was prevented in reserpine pre-treated animals or in presence of tetrodotoxin (TTX). To investigate the involvement of dopamine transporter (DAT), the effect of CLO was observed in presence of nomifensine. The coadministration of CLO and nomifensine produced an additive effect on striatal dopamine release. The results suggest that the effect of CLO on striatal dopamine release is predominantly mediated by an exocytotic mechanism, Ca(2+), vesicular and TTX-dependent and not by a mechanism mediated by dopamine transporter.

Subclinical effects of saxitoxin and domoic acid on aggressive behaviour and monoaminergic turnover in rainbow trout (Oncorhynchus mykiss)

The algal produced neurotoxins saxitoxin and domoic acid may have serious effects on marine life and can be responsible for the intoxication of for instance sea mammals, sea birds and fish. Given that farmed fish cannot escape algal blooms, they may be more susceptible to intoxication than wild stocks. In the present study, subclinical effects of saxitoxin and domoic on aggressive behaviour and monoaminergic systems in the brain of the rainbow trout (Oncorhynchus mykiss) were investigated. The resident-intruder test was used to measure aggression where only the resident fish were subjected to the toxins and analysed for monoamines and their metabolites. The resident-intruder test was carried out on two consecutive days. On day one basal aggression was measured in the four groups. On day two three of the groups were injected with subclinical doses of one of the following: saxitoxin (1.752 microg/kg bw), domoic (0.75 mg/kg bw) or 0.9% saline solution. This was performed 30 min prior to the aggression test. Handling stress and injection affected aggressive behaviour, cortisol and the serotonergic system in telencephalic brain regions. Cortisol levels were elevated in all of the injected groups when compared to the control group. An increase in serotonergic turnover was evident when all injected groups were pooled and compared to the control group. All together this suggests that the handling stress in connection with the injection was similar in all of the three injected groups. In contrast to both the undisturbed control group and the toxin-injected groups, the saline-injected group displayed a reduction in aggressive behaviour which was evident in increased attack latency. Furthermore the domoic injected group displayed more aggressive attacks towards their conspecifics than the saline-injected group. Consequently the two toxins appear to mask the stress induced alteration in aggressive behaviour. Monoamine levels and monoaminergic turnover could not be demonstrated to be directly affected by the two toxins at the given doses in the investigated brain regions (dorsal and ventral parts of telencephalon, optic tectum, locus coeruleus, raphe nucleus, molecular and granular layer of cerebellum). This could indicate that the toxins mediate aggressive behaviour either through other systems than the monoaminergic systems, such as neuroactive amino acids, or that the mediation occurs in other brain regions.

Transporter mediated GABA release in the retina: Role of excitatory amino acids and dopamine

In general, the release of neurotransmitters in the central nervous system is accomplished by a calcium-dependent process which constitutes a common feature of exocytosis, a conserved mechanism for transmitter release in all species. However, neurotransmitters can also be released by the reversal of their transporters. In the retina, a large portion of GABA is released by this mechanism, which is under the control of neuroactive agents, such as excitatory amino acids and dopamine. In this review, we will focus on the transporter mediated GABA release and the role played by excitatory amino acids and dopamine in this process. First, we will discuss the works that used radiolabeled GABA to study the outflow of the neurotransmitter and then the works that took into consideration the endogenous pool of GABA and the topography of GABAergic circuits influenced by excitatory amino acids and dopamine.

Mediation of glutamatergic receptors and nitric oxide on striatal dopamine release evoked by anatoxin-a. An in vivo microdialysis study

In this work, the involvement of ionotropic glutamatergic receptors and nitric oxide on striatal dopamine release induced by anatoxin-a was investigated in conscious and freely-moving rats. To study the participation of glutamatergic receptors, the effects of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)/kainate receptors antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), and N-methyl-D-aspartate (NMDA) receptor antagonists, dizocilpine (MK-801) and d(-)-2-amino-5-phosphonopentanoic acid (APV), were examined. The perfusion of 3.5 mM anatoxin-a increased the extracellular dopamine levels to 701% relative to the basal. When CNQX was administered with 3.5 mM anatoxin-a, the increase of dopamine levels was 29% smaller than that observed with anatoxin-a alone. When MK-801 and APV were administered, the effect of anatoxin-a was attenuated 26% and 25% respectively in terms of that observed with anatoxin-a alone. And with CNQX plus MK-801, the effect of anatoxin-a was 53% inhibited in terms of the effect of anatoxin-a alone. These results suggest that the striatal dopamine release induced by anatoxin-a is partly mediated by activation of both ionotropic glutamatergic receptors. Since the neuronal form of nitric oxide synthase (nNOS) produces nitric oxide (NO) primarily in response to activation of NMDA receptors, it was tested if NO could play any role in the effect of anatoxin-a. Treatment with NOS inhibitors, L-nitro-arginine methyl ester (L-NAME) and d(-)-2-amino-5-phosphonopentanoic acid (7-NI), induced decreased anatoxin-a effects of 22% and 26% respectively. In conclusion, the present in vivo results demonstrate that anatoxin-a induced an indirect activation of ionotropic glutamatergic receptors (NMDA and AMPA/kainite receptors), which stimulate striatal dopamine release. On the other hand, activation of NMDA receptors may elicit NO increased levels enhancing dopamine release.

In vivo neurochemical characterization of Anatoxin-a evoked dopamine release from striatum

Anatoxin-a (AnTx) is a natural neurotoxin, which acts as a potent and stereoselective agonist at the nicotinic acetylcholine receptors. The in vivo actions of the AnTx on dopamine (DA) release are scarcely characterized. The aim of this study was to determine the neurochemical bases for AnTx-induced striatal DA release, using the brain microdialysis technique, in freely moving rats. Local application of AnTx (3.5 mM) through the microdialysis probe produced an increase in striatal DA levels (701 +/- 51% with respect to basal values). The effect of infusion of AnTx in Ca(2+)-free Ringer medium, in Na(+)-free Ringer medium and with TTX in the medium, was inhibited. Also, reserpine pre-treatment blocked the action of AnTx on striatal DA levels. To investigate the involvement of the DA transporter, the effects of AnTx were observed in the presence of nomifensine. The coadministration of AnTx and nomifensine evoked an additive effect on striatal DA levels. The latter results show that the DA release is not mediated by a decreased DA uptake. Taken as a whole, these results suggest that the effects of AnTx are predominantly mediated by an exocytotic mechanism, Ca(2+)-, Na(+)- and TTX-dependent, and not by a mechanism mediated by the DA transporter.

In vivo Effects of the Anatoxin-a on Striatal Dopamine Release

Anatoxin-a is an important neurotoxin that acts a potent nicotinic acetylcholine receptor agonist. This characteristic makes anatoxin-a an important tool for the study of nicotinic receptors. Anatoxin-a has been used extensively in vitro experiments, however anatoxin-a has never been studied by in vivo microdialysis studies. This study test the effect of anatoxin-a on striatal in vivo dopamine release by microdialysis.The results of this work show that anatoxin-a evoked dopamine release in a concentration-dependent way. Atropine had not any effect on dopamine release evoked by 3.5 mM anatoxin-a. However, perfusion of nicotinic antagonists mecamylamine and alpha-bungarotoxin induced a total inhibition of the striatal dopamine release. Perfusion of alpha7*-receptors antagonists, metillycaconitine or alpha-bungarotoxin, partially inhibits the release of dopamine stimulated by anatoxin-a. These results show that anatoxin-a can be used as an important nicotinic agonist in the study of nicotinic receptor by in vivo microdialysis technique and also support further in vivo evidences that alpha7*nicotinic AChRs are implicated in the regulation of striatal dopamine release.

Determination of dopamine in rat striatum by microdialysis and high-performance liquid chromatography with electrochemical detection on a functionalized multi-wall carbon nanotube electrode

High performance liquid chromatography coupled with microdialysis sampling and electrochemical detection (HPLC-ECD) has been used to determine dopamine (DA). In the HPLC-ECD a multi-wall carbon nanotube electrode chemically modified with carboxyl groups (MWNT-COOH CME) was used as the working electrode for determination of DA. The results indicated that the MWNT-COOH CME enabled efficient electrocatalytic oxidation of DA with relatively high sensitivity and stability and long life. Peak currents for dopamine were linearly dependent on concentration in the range 5.0 x 10(-9) to 5.0 x 10(-5) mol L(-1) and the calculated detection limit (S/N=3) was 2.5 x 10(-9) mol L(-1). The method had been successfully used to measure dopamine in rat striatal microdialysate. To study the physiological effect of nitric oxide (NO) on striatal release of DA, 0.5 mmol L(-1) sodium nitroprusside (SNP) was a continuously perfused into rat striatum. This resulted in a 46% increase in DA basal level.