Steroids inhibit nicotinic acetylcholine receptors.

Application of progesterone to Xenopus oocytes expressing a cloned neuronal nicotinic acetylcholine (nAChR) revealed two effects. The first effect was a fully reversible reduction of the current induced by acetylcholine (ACh), its onset being nearly instantaneous. The second effect, which developed in a few hours, was an irreversible suppression of ACh-evoked currents. The transient inhibition had an apparent Ki of 7 microM when tested with 50 nM ACh, but the percentage of inhibition was positively correlated to the ACh concentration. A reduction of ACh-induced currents which appeared immediately upon progesterone application was also observed with muscle nAChR expressed in oocytes and with nAChR on membrane patches isolated from ciliary ganglion neurons. Thus nAChRs are modulated by progesterone and steroids may play an important role in nicotinic cholinoception.     

Behavioural, hyperthermic and neurotoxic effects of 3,4-methylenedioxymethamphetamine analogues in the Wistar rat

1. The ability of N-ethyl (MDEA) and N-butyl (MDBA) analogues of 3,4-methylenedioxymethamphetamine (MDMA, 'Ecstasy') to induce acute behavioural changes and increases in body temperature, and to cause serotonergic neurotoxicity, was assessed in young adult male Wistar rats. The in vitro ability of MDMA analogues to evoke presynaptic monoamine release from crude rat forebrain synaptosomal preparations pre-labelled with [3H]5-HT or [3H]DA was also measured. 2. In behavioural experiments, acute MDMA and MDEA (20 mg/kg, i.p.) significantly increased rat open-field locomotion scores, decreased open-field rearing, and induced stereotypy, Straub tail and head weaving. MDBA did not produce any of these behaviours. 3. After repeated dosing (8 x 20 mg/kg, i.p., twice daily for 4 days), MDMA > MDEA > MDBA > or = saline at decreasing forebrain [3H]paroxetine binding levels and concentrations of 5-HT and 5-HIAA at 14 days post-treatment. None of the analogues caused any long-term changes in dopamine or noradrenaline concentrations in the forebrain. 4. Acute MDMA and MDEA (20 mg/kg, i.p.) produced significant acute increases in rat aural temperature compared with saline-treated animals, while 20 mg/kg MDBA caused no significant effects. 5. MDA, MDMA and MDEA were equipotent at inducing [3H]5-HT release from frontal cortex/hippocampal synaptosomes, while MDBA only evoked a significant release at 100 microM concentrations. The potency order for inducing [3H]DA release from striatal synaptosomes was MDA > MDMA > MDEA = MDBA. 6. This study shows that large N-alkyl substitution decreases the ability of MDMA analogues to evoke presynaptic 5-HT and DA release, induce acute hyperthermia, hyperlocomotion and behavioural changes, and cause long-term serotonergic neurotoxicity. 7. The structure-activity relationship data presented here indicate that the neurotoxic damage caused by substituted amphetamines requires a combination of acute hyperthermia and increased neurotransmitter release. Induction of one of these effects in isolation is not sufficient to cause serotonergic nerve terminal degradation.     

D1 but not D4 Dopamine Receptors are Critical for MDMA-Induced Neurotoxicity in Mice

MDMA, an addictive psychostimulant-consumed worldwide, has the ability to induce neurotoxic effects and addiction in laboratory animals and in humans through its effects on monoaminergic systems. MDMA-induced neurotoxicity in mice occurs primarily in dopaminergic neurons and does not significantly affect the serotonergic system. As the neurotoxic effects of MDMA in mice involve excessive dopamine (DA) release, DA receptors are highly likely to play a role in MDMA neurotoxicity, but the specific dopamine receptor subtypes involved have not previously been determined definitively. In this study, dopamine D1 and D4 receptor knock-out mice (D1R^?/? and D4R^?/?) were used to determine whether these receptors are involved in MDMA neurotoxicity. D1R inactivation attenuated MDMA-induced hyperthermia, decreased the reduction of dopamine and dopamine metabolite levels, and protected against dopamine terminal loss and reactive astrogliosis as determined in the striatum, 7 days after MDMA treatment. In sharp contrast, inactivation of D4R did not prevent hyperthermia or the neurotoxic effects of MDMA. Altogether, these results indicate that D1R, but not D4R, plays a significant role in the dopaminergic striatal neurotoxicity observed after exposure to MDMA.     

Ecstasy induces apoptosis via 5-HT(2A)-receptor stimulation in cortical neurons

3,4-Methylenedioxymethamphetamine (MDMA or "Ecstasy") is a psychoactive and hallucinogenic drug of abuse. MDMA has been shown to produce neurotoxicity both in animals and humans. MDMA and other amphetamines induce serotonergic and dopaminergic terminal neurotoxicity and also neurodegeneration in areas including the cortex, hippocampus, striatum and thalamus. Herein, we investigated the mechanisms involved in MDMA-induced neurotoxicity to neuronal serum free cultures from rat cortex. The hyperthermic effect produced by MDMA has been shown to be a clinically relevant aspect for the neurotoxic events. Thus, MDMA-induced toxicity to cortical neurons was evaluated both under normothermic (36.5 degrees C) and hyperthermic (40 degrees C) conditions. Our findings showed that MDMA produced neuronal apoptosis, accompanied by activation of caspase 3, in a concentration dependent manner. MDMA neurotoxicity was completely prevented by pre-treatment with a 5-HT(2A)-receptor antibody, which acted as an "irreversible non-competitive antagonist" of this receptor. Furthermore, MDMA depleted intracellular glutathione (GSH) levels in a concentration dependent manner, an effect that was attenuated by Ketanserin, a competitive 5-HT(2A)-receptor antagonist. Accordingly, N-acetylcysteine, an antioxidant and GSH precursor, also reduced MDMA-induced toxicity. Specific inhibitors of the inducible and neuronal nitric oxide synthase (NOS) partially prevented MDMA neurotoxicity, ascertaining the involvement of reactive nitrogen species, in the toxic effect. In conclusion, direct MDMA 5-HT(2A)-receptor stimulation produces intracellular oxidative stress that leads to neuronal apoptosis accompanied by caspase 3 activation.     

In vitro regulation of serotonin transporter activity by protein kinase A and nicotinic acetylcholine receptors in the prefrontal cortex of rats

We investigated the effect of in vitro exposure to nicotinic acetylcholine receptors (nAChRs), agonists, antagonists, and protein kinase A (PKA) modulators on the activity of the serotonin transporter (SERT) in prefrontocortical (PFC) synaptosomes. The plasma membrane SERT is an active transport mechanism specific for serotonin. Receptors and second messengers capable of altering transporter activity would be expected to have profound effects on serotonergic neurotransmission and on functions involving serotonergic input, such as cognition, anxiety, and mood. Our data suggest that activation of nAChRs, quite likely via PKA, increase the activity of the SERT in the PFC and, thereby, can alter 5-HT levels in a region important in the behavioral effects of nicotine and 5-HT. Nicotine at 4 microM increased [(3)H]5-HT uptake by 75%. Because the nAChR antagonists mecamylamine and dihydro-beta-erythrodine (DHbetaE) both decreased [(3)H]5-HT uptake into synaptosomes, it appeared that the SERT might be tonically activated by acetylcholine present within our synaptosomal preparations. Blocking PKA significantly decreased [(3)H]5-HT, while stimulation of PKA activity significantly increased the uptake. A 66% decrease compared with control was produced by 100 microM Rp-cAMP, and a 41% increase in 5-HT uptake over control was observed with 30 microM Sp-cAMPs. Furthermore, the enhancement in uptake produced by 4 microM nicotine was inhibited in a time-dependent fashion by preincubation with 10 microM Rp-cAMP. A better understanding of the influence of the cholinergic system and the receptors involved in the trafficking of SERT would help clarify the important relationship between the cholinergic and serotonergic systems and the role these systems play in behavior.     

Neurochemical and Neurotoxic Effects of MDMA (Ecstasy) and Caffeine After Chronic Combined Administration in Mice

MDMA (3,4-methylenedioxymethamphetamine) is a psychostimulant popular as a recreational drug because of its effect on mood and social interactions. MDMA acts at dopamine (DA) transporter (DAT) and serotonin (5-HT) transporter (SERT) and is known to induce damage of dopamine and serotonin neurons. MDMA is often ingested with caffeine. Caffeine as a non-selective adenosine A1/A2A receptor antagonist affects dopaminergic and serotonergic transmissions. The aim of the present study was to determine the changes in DA and 5-HT release in the mouse striatum induced by MDMA and caffeine after their chronic administration. To find out whether caffeine aggravates MDMA neurotoxicity, the content of DA and 5-HT, density of brain DAT and SERT, and oxidative damage of nuclear DNA were determined. Furthermore, the effect of caffeine on MDMA-induced changes in striatal dynorphin and enkephalin and on behavior was assessed. The DA and 5-HT release was determined with in vivo microdialysis, and the monoamine contents were measured by HPLC with electrochemical detection. DNA damage was assayed with the alkaline comet assay. DAT and SERT densities were determined by immunohistochemistry, while prodynorphin (PDYN) and proenkephalin were determined by quantitative PCR reactions. The behavioral changes were measured by the open-field (OF) test and novel object recognition (NOR) test. Caffeine potentiated MDMA-induced DA release while inhibiting 5-HT release in the mouse striatum. Caffeine also exacerbated the oxidative damage of nuclear DNA induced by MDMA but diminished DAT decrease in the striatum and worsened a decrease in SERT density produced by MDMA in the frontal cortex. Neither the striatal PDYN expression, increased by MDMA, nor exploratory and locomotor activities of mice, decreased by MDMA, were affected by caffeine. The exploration of novel object in the NOR test was diminished by MDMA and caffeine. Our data provide evidence that long-term caffeine administration has a powerful influence on functions of dopaminergic and serotonergic neurons in the mouse brain and on neurotoxic effects evoked by MDMA.     

Neuroprotection by nicotine against hypoxia-induced apoptosis in cortical cultures involves activation of multiple nicotinic acetylcholine receptor subtypes

Activation of neuronal nicotinic acetylcholine receptors (nAChR) by nicotine has been suggested to protect neurons against a hypoxic insult. The objective of this study was to examine the nature of cell death induced by acute hypoxia in rat primary cortical cultures and the neuroprotective potential of nicotine in ameliorating these processes. Neuronal cell death induced by a 4-h exposure to hypoxia (0.1% O(2)) was apoptotic, as shown by TUNEL staining and assays monitoring DNA strand breaks and caspase-3/7 activity. The presence of nicotine (10 microM) during the hypoxic insult protected a subpopulation of susceptible neurones against DNA damage and apoptosis induced by oxygen deprivation. This protective effect of nicotine was prevented by a 30-min pre-incubation with either 100 nM alpha-bungarotoxin or 1 microM dihydro-beta-erythroidine, but not 1 microM atropine, suggesting that activation of at least two subtypes of nAChR, alpha7 and beta2* nAChR, is involved in mediating nicotine neuroprotection.     

Involvement of alpha7- and alpha4beta2-type postsynaptic nicotinic acetylcholine receptors in nicotine-induced excitation of dopaminergic neurons in the substantia nigra: a patch clamp and single-cell PCR study using acutely dissociated nigral neurons

The receptor subtypes, which mediate nicotine-induced excitation of dopaminergic neurons in the substantia nigra, were investigated by whole-cell patch clamp studies and single-cell RT-PCR using acutely dissociated nigral neurons. Three types of current were observed when acetylcholine (1 mM) was applied to the neurons in the presence of atropine (1 microM) by the U-tube system, which allowed the rapid application of drugs. In 50% of neurons examined, acetylcholine (1 mM) plus atropine (1 microM) evoked a current with a rapidly desensitizing decay phase (designated as type Ia current). In 14% of neurons tested, the current induced by acetylcholine plus atropine had a decay phase with slow desensitization (designated as type II current). The third type of response, which had both characteristics of type Ia and II currents, was evoked in 36% of neurons tested (designated as type Ib currents). Nicotine (1 mM) also induced three types of inward currents which were similar to those induced by acetylcholine (1 mM) plus atropine (1 microM). In all three types of current, nicotine (0.1 microM-1 mM)-evoked inward currents were dose-dependent. Type Ia and II currents were inhibited by methyllycaconitine (MLA, 0.01 microM), a selective nicotinic alpha7 receptor antagonist, and dihydro-beta-erythroidine (DHbetaE, 0.1 microM), an antagonist for alpha4beta2 receptor, respectively. In type Ib currents, a fast-decaying phase was inhibited by MLA (0.01 microM), while a slow-decaying phase was blocked by DHbetaE (0.1 microM). After recording the type Ib current, single-cell RT-PCR analysis was performed using aspirated cytoplasm as total RNA templates. The results revealed that mRNAs for alpha7 nicotinic receptor subunit and tyrosine hydroxylase were detected in the same single neuron tested, which confirms the existence of alpha7-type nicotinic acetylcholine receptor in dopaminergic neurons of this area. These results suggest that nicotine directly acts on postsynaptic alpha7- and alpha4beta2-type nicotinic acetylcholine receptors and induces inward current, which result in the excitation of dopaminergic neurons in the substantia nigra.     

3‐[(2,4‐dimethoxy)benzylidene]‐anabaseine dihydrochloride protects against 6‐hydroxydopamine‐induced parkinsonian neurodegeneration through α7 nicotinic acetylcholine receptor stimulation in rats

To explore a novel therapy against Parkinson's disease through enhancement of α7 nicotinic acetylcholine receptor (nAChR), we evaluated the neuroprotective effects of 3‐[(2,4‐dimethoxy)benzylidene]‐anabaseine dihydrochloride (DMXBA; GTS‐21), a functionally selective α7 nAChR agonist, in a rat 6‐hydroxydopamine (6‐OHDA)‐induced hemiparkinsonian model. Microinjection of 6‐OHDA into the nigrostriatal pathway of rats destroys dopaminergic neurons selectively. DMXBA dose dependently inhibited methamphetamine‐stimulated rotational behavior and dopaminergic neuronal loss induced by 6‐OHDA. The protective effects were abolished by methyllycaconitine citrate salt hydrate, an α7 nAChR antagonist. Immunohistochemical study confirmed abundant α7 nAChR expression in the cytoplasm of dopaminergic neurons. These results indicate that DMXBA prevented 6‐OHDA‐induced dopaminergic neuronal loss through stimulating α7 nAChR in dopaminergic neurons. Injection of 6‐OHDA elevated immunoreactivities to glial markers such as ionized calcium binding adaptor molecule 1, CD68, and glial fibrillary acidic protein in the substantia nigra pars compacta of rats. In contrast, these immunoreactivities were markedly inhibited by comicroinjection of DMXBA. Microglia also expressed α7 nAChR in both resting and activated states. Hence, we hypothesize that DMXBA simultaneously affects microglia and dopaminergic neurons and that both actions lead to dopaminergic neuroprotection. The findings that DMXBA attenuates 6‐OHDA‐induced dopaminergic neurodegeneration and glial activation in a rat model of Parkinson's disease raisethe possibility that DMXBA could be a novel therapeutic compound to prevent Parkinson's disease development. © 2012 Wiley Periodicals, Inc.     

Reduced nicotinic receptor-mediated antinociception following in vivo antisense knock-down in rat

Pharmacological activation of neuronal nicotinic acetylcholine receptors can produce non-opioid antinociception in rodents. However, multiple nAChR subtypes exist, the most abundant of which contain alpha4 and beta2 subunits. The purpose of the present study was to investigate the role of alpha4-containing nAChRs in mediating nicotinic antinociception using an in vivo antisense strategy. Both i.c.v. infusion and repeated bolus injections into the cerebral aqueduct of an antisense oligonucleotide against the alpha4 subunit significantly attenuated the antinociceptive effects of the nAChR agonist A-85380 in the paw withdrawal test of acute thermal pain. Rats treated with a scrambled oligonucleotide displayed a full antinociceptive response to A-85380, while discontinuing antisense treatment restored the antinociceptive effects of the nicotinic agonist. Double immunohistochemical labeling revealed near-complete overlap of expression of the serotonin marker tryptophan hydroxylase and the alpha4 nAChR subunit in the dorsal raphe nucleus. The expression of alpha4-containing nAChRs by serotonergic neurons in the dorsal raphe offered a means to address nonspecific alpha4 knock-down, i.e., oligonucleotide-induced neurotoxicity. Immunohistochemical detection of alpha4 expression was reduced by nearly 50% in the dorsal raphe of antisense-treated rats as compared to either saline or missense-treated controls. In contrast, the expression of tryptophan hydroxylase, as well as, the alpha7 nAChR subunit in antisense-infused rats was similar to that observed in saline- and missense-treated controls. The results of these studies suggest that alpha4-containing nAChRs, possibly expressed by serotonergic neurons, are involved in nicotinic-mediated analgesia. However, these data do not eliminate the possibility that other nicotinic subunit combinations may also play a role in antinociception produced by nAChR activation.     

Repeated Administration of 3,4-Methylenedioxymethamphetamine (MDMA) Elevates the Levels of Neuronal Nitric Oxide Synthase in the Nigrostriatal System: Possible Relevance to Neurotoxicity

Previous studies have consistently demonstrated that the amphetamine-related drug 3,4-methylenedioxymethamphetamine (MDMA) induces dopaminergic damage in the mouse brain, and that this effect is most marked in the nigrostriatal system. Moreover, it has been suggested that the overproduction of nitric oxide (NO) may participate in the dopaminergic damage induced by MDMA. To further elucidate this issue, we evaluated the levels of the enzyme nitric oxide synthase (nNOS), which catalyzes the production of NO, in mice treated with regimens of MDMA that induce progressive and persistent neurotoxicity in the dopaminergic nigrostriatal system. Mice received 14, 28, or 36 administrations of MDMA (10 mg/kg i.p.), twice a day/twice a week, and were sacrificed at different time-points after treatment discontinuation. Thereafter, the number of nNOS-positive neurons was quantified by immunohistochemistry in the caudate-putamen (CPu) and substantia nigra pars compacta (SNc). MDMA elevated the numbers of nNOS-positive neurons in the CPu of mice that received 28 or 36 drug administrations. This effect was still detectable at 3 months after treatment discontinuation. Moreover, MDMA elevated the numbers of nNOS-positive neurons in the SNc. However, this effect occurred only in mice that received 28 drug administrations and were sacrificed 3 days after treatment discontinuation. These results are in line with the hypothesis that activation of the NO cascade participates in the toxic effects induced by MDMA in the dopaminergic nigrostriatal system. Moreover, they suggest that activation of the NO cascade induces toxic effects that are more marked in striatal terminals, compared with nigral neurons.     

Neuroprotection by nicotine in hippocampal slices subjected to oxygen-glucose deprivation

Nicotine (NIC) is neuroprotective against glutamate and hypoxia-induced neurotoxicity, preventing neuronal death and apoptosis in primary neuronal cultures. This effect is mediated by activation of both alpha7 and alpha4beta2 subtypes of nicotinic receptors for acetylcholine (nAChR) (Kaneko et al., 1997; Hejmadi et al., 2003). Furthermore, it seems that activation of alpha7 nAChR is the mechanism by which galantamine protects against thapsigargin and beta-amyloid-induced cell death (Arias et al., 2004), as well as in neuroprotection exerted by NIC against tumor necrosis factor alpha (Gahring et al., 2003). In this context we studied possible protection produced by NIC in an oxygen-glucose deprivation (OGD) model of rat and mouse hippocampal slices. The involvement of alpha7 nAChR in neuroprotection was proved by using wild-type and alpha7 knockout (KO) mice.     

Calcium mobilization elicited by two types of nicotinic acetylcholine receptors in mouse substantia nigra pars compacta

Nicotinic acetylcholine receptors (nAChRs) are expressed in the midbrain ascending dopaminergic system, a target of many addictive drugs. Here we assessed the intracellular Ca2+ level by imaging fura-2-loaded cells in substantia nigra pars compacta in mouse brain slices, and we examined the influence on this level of prolonged exposures to nicotine using mice lacking the nAChR beta2-subunit. In control cells, superfusion with nicotine (10-100 microM) caused a long-lasting rise of intracellular Ca2+ level which depended on extracellular Ca2+. This nicotinic response was almost completely absent in beta2-/- mutant mice, leaving a small residual response to a high concentration (100 microM) of nicotine which was inhibited by the alpha7-subunit-selective antagonist, methyllycaconitine. Conversely, the alpha7-subunit-selective agonist choline (10 mM) caused a methyllycaconitine-sensitive increase in intracellular Ca2+ level both in wild-type and beta2-/- mutant mice. Nicotine-elicited Ca2+ mobilization was reduced by the Na+ channel blocker tetrodotoxin (TTX) and by T-type Ca2+ channel blocking agents, whereas the choline-elicited Ca2+ increase was insensitive to TTX. Neither nicotine nor choline produced Ca2+ increase following inhibition of the release of Ca2+ from intracellular stores by dantrolene. These results demonstrate that in nigral dopaminergic neurons, nicotine can elicit Ca2+ mobilization via activation of two distinct nAChR subtypes: that of beta2-subunit-containing nAChR followed by activation of Na+ channel and T-type Ca2+ channels, and/or activation of alpha7-subunit-containing nAChR. The Ca2+ influx due to nAChR activation is subsequently amplified by the recruitment of intracellular Ca2+ stores. This Ca2+ mobilization may possibly contribute to the long-term effects of nicotine on the dopaminergic system.     

Butyl benzyl phthalate blocks Ca2+ signaling and catecholamine secretion coupled with nicotinic acetylcholine receptors in bovine adrenal chromaffin cells

Butyl benzyl phthalate (BBP), a plasticizer and an environmental pollutant, exerts genomic estrogenic-like effects via estrogen receptors. In addition to exerting genomic effects via intracellular steroid receptors, estrogen exerts non-genomic effects through interactions with membrane ion channels to lead the rapid alteration of neuronal excitability. Estradiol is known as to have modulating role on nicotinic acetylcholine receptors (nAChR). We investigated the possibility of BBP exerting non-genomic estrogenic-like effects on nAChR in bovine adrenal chromaffin cells. Our results show that BBP inhibited Ca2+ signaling induced by the nicotinic ligands carbachol, 1,1-dimethyl-4-phenyl-piperazinium iodide (DMPP) and epibatidine (IC50 levels of 4.3, 4.1, 5.4 microM, respectively) as well as high K+ solution (IC50 50.9 microM). Additionally, in the electrophysiological observations, BBP blocked the inward current coupled with nAChR under the stimulation of carbachol. We, therefore, suggest that nAChR and voltage-gated Ca2+ channels are major and minor sites, respectively, of BBP action on the plasma membrane. The inhibitory effect of BBP on nAChR was found to be both noncompetitive and reversible, remaining unchanged as nAChR ligand concentration increased and decreased after washing. BBP was 10 times more potent than estradiol in inhibiting nAChR-coupled Ca2+ signals. We conclude that BBP exerts a novel rapidly inhibitory effect on nAChR.     

Presynaptic localisation of the nicotinic acetylcholine receptor beta2 subunit immunoreactivity in rat nigrostriatal dopaminergic neurones

Nicotinic acetylcholine receptors (nAChR) are widely distributed in the central nervous system, where they exert a modulatory influence on synaptic transmission. For the striatum, pharmacological evidence supports the presence of presynaptic alpha3beta2* and alpha4beta2* nAChR that modulate dopamine release from nigrostriatal terminals. The objective of this study was to examine the precise subcellular distribution of the nAChR beta2 subunit in these neurones and its localisation at presynaptic sites. Double immunolabelling with tyrosine hydroxylase (TH) at the confocal level revealed that the cell bodies and axon terminals (synaptosomes) of nigrostriatal neurones were also immunoreactive for the nAChR beta2 subunit. Double-preembedding electron microscopy confirmed that beta2-immunogold labelling was enriched in TH-positive terminals in the dorsal striatum. Quantitative analysis of doubly immunogold-labelled sections in postembedding electron microscopy showed that 86% of TH-positive axonal boutons are also labelled for the nAChR beta2 subunit, whereas 45% of beta2 subunit-immunolabeled boutons do not contain TH. Thus the beta2 subunit is localised within at least two populations of axon terminals in the dorsal striatum. In these structures, 15% of beta2 subunit immunoreactivity was at the plasma membrane but was rarely associated with synapses. These findings are compatible with functional presynaptic beta2-containing nAChR that may be stimulated physiologically by acetylcholine that diffuses from synaptic or nonsynaptic sites of acetylcholine release. These results demonstrate the presynaptic localisation of an nAChR subunit in nigrostriatal dopaminergic neurones, providing morphological evidence for the presynaptic nicotinic modulation of dopamine release.     

Effects of α7 Nicotinic Receptor Activation on Cell Survival in Rat Organotypic Hippocampal Slice Cultures

Glutamatergic signaling via N-methyl-D-aspartate receptors (NMDARs) is important for physiological functioning, but can also induce cell death via excitotoxic mechanisms in many neuropathological diseases, such as stroke. Altering the cellular response to excitotoxic insults by modulating the downstream effects of NMDAR activation represents a promising therapeutic approach. For example, α7 nicotinic acetylcholine receptors (α7 nAChRs) signaling has been shown to be able to change NMDA-induced neurotoxicity in some models. However, both neuroprotective and neurotoxic effects have been reported. In this study, we examined the effect of co-activation of α7 nAChRs on NMDA-mediated cell death in rat organotypic hippocampal slice cultures (OHSCs). Our results show that α7 nAChR stimulation did not significantly influence NMDA-induced excitotoxic cell damage as measured by propidium iodide uptake. However, treatment of OHSCs with the α7 nAChR agonist choline alone induced an increase in the propidium iodide signal. Both the α7 nAChR antagonist methyllycaconitine (MLA) and the NMDAR antagonist (RS)-3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP) were able to block this effect in the dentate gyrus and hippocampal subfield CA3.     

Evidence for a role of energy dysregulation in the MDMA-induced depletion of brain 5-HT

Although the exact mechanism involved in the long-term depletion of brain serotonin (5-HT) produced by substituted amphetamines is not completely known, evidence suggests that oxidative and/or bioenergetic stress may contribute to 3,4-methylenedioxymethamphetamine (MDMA)-induced 5-HT toxicity. In the present study, the effect of supplementing energy substrates was examined on the long-term depletion of striatal 5-HT and dopamine produced by the local perfusion of MDMA (100 microM) and malonate (100 mM) and the depletion of striatal and hippocampal 5-HT concentrations produced by the systemic administration of MDMA (10 mg/kg i.p. x4). The effect of systemic administration of MDMA on ATP levels in the striatum and hippocampus also was examined. Reverse dialysis of MDMA and malonate directly into the striatum resulted in a 55-70% reduction in striatal concentrations of 5-HT and dopamine, and these reductions were significantly attenuated when MDMA and malonate were co-perfused with nicotinamide (1 mM). Perfusion of nicotinamide or ubiquinone (100 microM) also attenuated the depletion of 5-HT in the striatum and hippocampus produced by the systemic administration of MDMA. Finally, the systemic administration of MDMA produced a 30% decrease in the concentration of ATP in the striatum and hippocampus. These results support the conclusion that MDMA produces a dysregulation of energy metabolism which contributes to the mechanism of MDMA-induced 5-HT neurotoxicity.     

Dopamine D2-receptor knockout mice are protected against dopaminergic neurotoxicity induced by methamphetamine or MDMA

Methamphetamine (METH) and 3,4-methylenedioxymethamphetamine (MDMA), amphetamine derivatives widely used as recreational drugs, induce similar neurotoxic effects in mice, including a marked loss of tyrosine hydroxylase (TH) and dopamine transporter (DAT) in the striatum. Although the role of dopamine in these neurotoxic effects is well established and pharmacological studies suggest involvement of a dopamine D2-like receptor, the specific dopamine receptor subtype involved has not been determined. In this study, we used dopamine D2 receptor knock-out mice (D2R(-/-)) to determine whether D2R is involved in METH- and MDMA-induced hyperthermia and neurotoxicity. In wild type animals, both drugs induced marked hyperthermia, decreased striatal dopamine content and TH- and DAT-immunoreactivity and increased striatal GFAP and Mac-1 expression as well as iNOS and interleukin 15 at 1 and 7days after drug exposure. They also caused dopaminergic cell loss in the SNpc. Inactivation of D2R blocked all these effects. Remarkably, D2R inactivation prevented METH-induced loss of dopaminergic neurons in the SNpc. In addition, striatal dopamine overflow, measured by fast scan cyclic voltammetry in the presence of METH, was significantly reduced in D2R(-/-) mice. Pre-treatment with reserpine indicated that the neuroprotective effect of D2R inactivation cannot be explained solely by its ability to prevent METH-induced hyperthermia: reserpine lowered body temperature in both genotypes, and potentiated METH toxicity in WT, but not D2R(-/-) mice. Our results demonstrate that the D2R is necessary for METH and MDMA neurotoxicity and that the neuroprotective effect of D2R inactivation is independent of its effect on body temperature.     

Mechanisms of Neuroprotective Effects of Nicotine and Acetylcholinesterase Inhibitors: Role of α4 and α7 Receptors in Neuroprotection

Neurotoxicity induced by glutamate and other excitatory amino acids has been implicated in various neurodegenerative disorders including hypoxic ischemic events, trauma, and Alzheimer’s and Parkinson’s diseases. We examined the roles of nicotinic acetylcholine receptors (nAChRs) in survival of CNS neurons during excitotoxic events. Nicotine as well as other nicotinic receptor agonists protected cortical neurons against glutamate neurotoxicity via α4 and α7 nAChRs at least partly by inhibiting the process of apoptosis in near-pure neuronal cultures obtained from the cerebral cortex of fetal rats. Donepezil, galanatamine and tacrine, therapeutic acetylcholinesterase (AChE) inhibitors currently being used for treatment of Alzheimer’s disease also protected neuronal cells from glutamate neurotoxicity. Protective effects of nicotine and the AChE inhibitors were antagonized by nAChR antagonists. Moreover, nicotine and those AChE inhibitors induced up-regulation of nAChRs. Inhibitors for a non-receptor-type tyrosine kinase, Fyn, and janus-activated kinase 2, suppressed the neuroprotective effect of donepezil and galantamine. Furthermore, a phosphatidylinositol 3-kinase (PI3K) inhibitor also suppressed the neuroprotective effect of the AChE inhibitors. The phosphorylation of Akt, an effector of PI3K, and the expression level of Bcl-2, an anti-apoptotic protein, increased with donepezil and galantamine treatments. These results suggest that nicotine as well as AChE inhibitors, donepezil and galantamine, prevent glutamate neurotoxicity through α4 and α7 nAChRs and the PI3K-Akt pathway.