Pharmacologic approaches to the treatment of amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disease for which no cure or effective treatment presently exists. Many different types of drugs have been tested; most are based on various hypotheses of mechanisms for neuronal death, including oxidative damage, loss of trophic factor support, glutamate-mediated excitotoxicity, and chronic inflammation. The discovery that a small percentage of ALS cases are familial and involve mutation in a superoxide dismutase gene (SOD1) led to the development of transgenic mouse models presently widely used for testing possible drugs. Mutations in the vascular endothelial growth factor gene (VEGF) also appear to be involved. Riluzole, an inhibitor of glutamate release and the only agent presently approved for clinical use, only extends survival by a few months. A number of trophic factors, anti-inflammatory agents, and inhibitors of oxidative stress have been reported to prolong survival in mouse models and some are now in clinical trials. Gene transfer of VEGF or glial cell-line derived neurotrophic factor, anti-inflammatory COX-2 inhibitors, and minocycline have had particularly promising results in mice. No breakthrough has yet occurred and present thinking is that combinations of drugs may be required to slow the multifactorial neurodegeneration process effectively.

Distinct mechanisms of presynaptic inhibition at GABAergic synapses of the rat substantia nigra pars compacta

We investigated the mechanisms of presynaptic inhibition of GABAergic neurotransmission by group III metabotropic glutamate receptors (mGluRs) and GABA(B) receptors, in dopamine (DA) neurons of the substantia nigra pars compacta (SNc). Both the group III mGluRs agonist L-(+)-2-amino-4-phosphonobutyric acid (AP4, 100 microM) and the GABA(B) receptor agonist baclofen (10 microM) reversibly depressed the frequency of spontaneous inhibitory postsynaptic currents (sIPSCs) to 48.5 +/- 2.7 and 79.3 +/- 1.6% (means +/- SE) of control, respectively. On the contrary, the frequency of action potential-independent miniature IPSCs (mIPSCs), recorded in tetrodotoxin (TTX, 1 microM) and cadmium (100 microM) were insensitive to AP4 but were reduced by baclofen to 49.7 +/- 8.6% of control. When the contribution of voltage-dependent calcium channels (VDCCs) to synaptic transmission was boosted with external barium (1 mM), AP4 became effective in reducing TTX-resistant mIPSCs to 65.4 +/- 3.9% of control, thus confirming a mechanism of presynaptic inhibition involving modulation of VDCCs. Differently from AP4, baclofen inhibited to 58.5 +/- 6.7% of control the frequency mIPSCs recorded in TTX and the calcium ionophore ionomycin (2 microM), which promotes Ca2+-dependent, but VDCC-independent, transmitter release. Moreover, in the presence of alpha-latrotoxin (0.3 nM), to promote a Ca2+-independent vesicular release of GABA, baclofen reduced mIPSC frequency to 48.1 +/- 3.2% of control, while AP4 was ineffective. These results indicate that group III mGluRs depress GABA release to DA neurons of the SNc through inhibition of presynaptic VDCCs, while presynaptic GABA(B) receptors directly impair transmitter exocytosis.

Model of frequent, recurrent, and spontaneous seizures in the intact mouse hippocampus

This study presents a model of chronic, recurrent, spontaneous seizures in the intact isolated hippocampal preparation from mice aged P8-P25. Field activity from the CA1 pyramidal cell layer was recorded and recurrent, spontaneous seizure-like events (SLEs) were observed in the presence of low Mg2+ (0.25 mM) artificial cerebrospinal fluid (ACSF). Hippocampi also showed interictal epileptiform discharges (IEDs) of 0.9-4.2 Hz occurring between seizures. No age-specific differences were found in SLE occurrence (2 SLEs per 10 min, on average), duration, and corresponding frequencies. After long exposure to low Mg2+ ACSF (>3 h), SLEs were completely reversible within minutes with the application of normal (2 mM Mg2+) ACSF. The AMPA antagonist, CNQX, blocked all epileptiform activity, whereas the NMDA antagonist, APV, did not. The gamma-aminobutyric acid (GABA)A antagonist, bicuculline, attenuated and fragmented SLEs, implicating interneurons in SLE generation. The L-type Ca2+ blocker, nifedipine, enhanced epileptiform activity. Analysis of dual site recordings along the septotemporal hippocampus demonstrated that epileptiform activity began first in the temporal pole of the hippocampus, as illustrated by disconnection experiments. Once an SLE had been established, however, the septal hippocampus was sometimes seen to lead the epileptiform activity. The whole hippocampus with intact local circuitry, treated with low Mg2+, provides a realistic model of recurrent spontaneous seizures, which may be used, in normal and genetically modified mice, to study the dynamics of seizures and seizure evolution, as well as the mechanisms of action of anti-epileptic drugs and other therapeutic modalities.

Valproic acid inhibits histone deacetylase activity and suppresses excitotoxicity-induced GAPDH nuclear accumulation and apoptotic death in neurons

Valproic acid (VPA), used to treat bipolar mood disorder and seizures, also inhibits histone deacetylase (HDAC). Here, we found that VPA and other HDAC inhibitors, butyrate and trichostatin A, robustly protected mature cerebellar granule cell cultures from excitotoxicity induced by SYM 2081 ((2S, 4R)-4-methylglutamate), an inhibitor of excitatory amino-acid transporters and an agonist of low-affinity kainate receptors. These neuroprotective effects required protracted treatment and were correlated with enhanced acetylated histone levels, indicating HDAC inhibition. SYM-induced excitotoxicity was blocked by MK-801 ((5R,10S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine hydrogen maleate), supporting that the toxicity was largely N-methyl-D-aspartate receptor dependent. SYM excitotoxicity had apoptotic characteristics and was prevented by a caspase inhibitor. SYM-induced apoptosis was associated with a rapid and robust nuclear accumulation of glyceraldehyde-3-phosphate dehydrogenase (GAPDH), a housekeeping gene previously shown to be proapoptotic. VPA pretreatment suppressed SYM 2081-induced GAPDH nuclear accumulation, concurrent with its neuroprotective effects. Chromatin immunoprecipitation (ChIP) revealed that GAPDH is copresent with acetylated histone H3, including Lys9-acetylated histone, and that VPA treatment caused a time-dependent decrease in the levels of nuclear GAPDH with a concomitant increase in acetylated histones in the ChIP complex. Our results strongly suggest that VPA protects neurons from excitotoxicity through inhibition of HDAC activity and that this protective effect may involve suppression of excitotoxicity-induced accumulation of GAPDH protein in the nucleus.

Pronociceptive role of dynorphins in uninjured animals: N -ethylmaleimide-induced nociceptive behavior mediated through inhibition of dynorphin degradation

Intrathecal (i.t.) administration into mice of N-ethylmaleimide (NEM), a cysteine protease inhibitor, produced a characteristic behavioral response, the biting and/or licking of the hindpaw and the tail along with slight hindlimb scratching directed toward the flank. The behavior induced by NEM was inhibited by the intraperitoneal injection of morphine. We have recently reported that dynorphin A and, more potently big dynorphin, consisting of dynorphins A and B, produce the same type of nociceptive response whereas dynorphin B does not [Tan-No K, Esashi A, Nakagawasai O, Niijima F, Tadano T, Sakurada C, Sakurada T, Bakalkin G, Terenius L, Kisara K. Intrathecally administered big dynorphin, a prodynorphin-derived peptide, produces nociceptive behavior through an N-methyl-d-aspartate receptor mechanism. Brain Res 2002;952:7-14]. The NEM-induced nociceptive behavior was inhibited by pretreatment with dynorphin A- or dynorphin B-antiserum and each antiserum also reduced the nociceptive effects of i.t.-injected synthetic big dynorphin. The characteristic NEM-evoked response was not observed in prodynorphin knockout mice. Naloxone, an opioid receptor antagonist, had no effects on the NEM-induced behavior. Ifenprodil, arcaine and agmatine, antagonists at the polyamine recognition site on the N-methyl-D-aspartate (NMDA) receptor ion-channel complex, and MK-801, an NMDA ion-channel blocker inhibited the NEM-induced effects. Ro25-6981, an antagonist of the NMDA receptor subtype containing NR2B subunit was not active. NEM completely inhibited degradation of dynorphin A by soluble and particulate fractions of mouse spinal cord. Collectively, the results demonstrate that endogenous prodynorphin-derived peptides are pronociceptive in uninjured animals, and required for the NEM-induced behavior. The NEM effects may be mediated through inhibition of the degradation of endogenous dynorphins, presumably big dynorphin that in turn activates the NMDA receptor ion-channel complex by acting on the polyamine recognition site.

The neuroprotective KDI domain of γ1-laminin is a universal and potent inhibitor of ionotropic glutamate receptors

Previous work from this laboratory indicates that the KDI (Lys-Asp-Ile) domain of gamma 1-laminin promotes functional regeneration of adult rat spinal cord injuries and protects adult rat hippocampal neurons against massive neuronal death induced by intracerebral injection of the glutamate analogue kainic acid. In the present study, we used patch clamp recordings on cultured human embryonic neocortical neurons and HEK 293 cells expressing recombinant glutamate receptor subunits to study a putative interaction of the KDI with the glutamate system. We show that the KDI domain of gamma 1-laminin is a universal and potent inhibitor of AMPA, kainate, and NMDA subclasses of glutamate receptors, with a noncompetitive action on the AMPA receptor channel activity. Glutamate neurotoxicity plays a key role in both CNS trauma and neurodegenerative disorders, so this unexpected, novel function of the gamma 1-laminin-derived tripeptide may prove clinically valuable in treatment of CNS trauma and/or disease.

In the adult CNS, ethanol prevents rather than produces NMDA antagonist-induced neurotoxicity

Single doses of an NMDA antagonist cause an adult or a prepubertal form of neurodegeneration, depending on the age of the animal. Single doses of ethanol (EtOH) by blocking NMDA receptors produce apoptotic neurodegeneration in young animals. This capability could account, in part, for the ability of EtOH to produce the fetal alcohol syndrome. We investigated whether EtOH could produce NMDA antagonist-induced neurotoxicity (NAN), a different neurotoxicity that is seen only in adult animals. In spite of producing blood EtOH levels (30 to 600 mg/dl) known to block NMDA receptors, EtOH was unable to produce neurotoxicity in the adult central nervous system (CNS). Moreover, EtOH in a dose-dependent fashion (ED(50)=138 mg/dl) prevented the selective and powerful NMDA antagonist, MK-801, from producing NAN in adult animals, suggesting that activity at another site might be negating the neurotoxic effect of EtOH's inherent NMDA antagonistic activity. Because GABA(A) agonism and non-NMDA glutamate antagonism, properties which EtOH possesses, can prevent NAN, we proceeded to study whether GABA(A) antagonists (or agents capable of reversing EtOH's GABAergic effects) and non-NMDA agonists could reverse EtOH's protective effect. Bicuculline, Ro15-4513, finasteride, kainic acid or AMPA, alone or in combination, did not significantly reverse EtOH's protective effect. Given that EtOH has effects on a wide range of ion channels and receptors, determining the precise mechanism of EtOH's protective effect will take additional effort. The inability of EtOH to acutely produce NAN in the adult CNS indicates that, in contrast to fetuses, brief exposure of the adult CNS to EtOH is non-toxic for neurons.

NR2 subunit-dependence of NMDA receptor channel block by external Mg2+

The vital roles played by NMDA receptors in CNS physiology depend critically on powerful voltage-dependent channel block by external Mg(2+) (Mg(2+)(o)). NMDA receptor channel block by Mg(2+)(o) depends on receptor subunit composition: NR1/2A receptors (receptors composed of NR1 and NR2A subunits) and NR1/2B receptors are more strongly inhibited by Mg(2+)(o) than are NR1/2C or NR1/2D receptors. We investigated the effects of Mg(2+)(o) on single-channel and whole-cell currents recorded from recombinant NR1/2D and NR1/2A receptors expressed in HEK293 and 293T cells. The main conclusions are as follows: (1) Voltage-dependent inhibition by Mg(2+)(o) of whole-cell NR1/2D receptor responses was at least 4-fold weaker than inhibition of NR1/2A receptor responses at all voltages tested. (2) Channel block by Mg(2+)(o) reduced the duration of NR1/2D receptor single-channel openings; this reduction was used to estimate the apparent blocking rate of Mg(2+)(o) (k(+,app)). The k(+,app) for NR1/2D receptors was similar to but moderately slower than the k(+,app) obtained from cortical NMDA receptors composed of NR1, NR2A and NR2B subunits at all voltages tested. (3) Mg(2+)(o) blocking events induced an additional component in the closed-duration distribution; this component was used to estimate the apparent unblocking rate of Mg(2+)(o) (k(-,app)). The k(-,app) for NR1/2D receptors was much faster than the k(-,app) for cortical receptors at all voltages tested. The voltage-dependence of the k(-,app) of NR1/2D and cortical receptors differed in a manner that suggested that Mg(2+)(o) may permeate NR1/2D receptors more easily than cortical receptors. (4) Mg(2+)(o) inhibits NR1/2D receptors less effectively than cortical receptors chiefly because Mg(2+)(o) unbinds much more rapidly from NR1/2D receptors.

NMDA receptors in the anterior cingulate cortex mediate pain-related aversion

Using a rat formalin-induced conditioned place avoidance (F-CPA) model and Fos immunohistochemistry, the present study observed the effect of N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-isoxozole propionic acid/kainite (AMPA/KA) receptors on pain-related aversion. Adult Sprague-Dawley rats were implanted with cannula in the anterior cingulate cortex (ACC) or the lateral ventricle. Before (10 min) the injection of formalin into a hindpaw on days 2 and 4 of place-conditioning trials, vehicle (0.01 M PBS), the NMDA receptors antagonist, 2-amino-5-phosphonovalerate (AP5), or the AMPA/KA receptors antagonist, 6,7-dinitroquinoxaline-2,3-dione (DNQX), was injected through the cannula. F-CPA was effectively eliminated by both intracerebroventricular (icv) and intra-ACC microinjection of AP5. In contrast, the intra-ACC or icv injection of DNQX failed to alter the conditioning scores of F-CPA compared with vehicle control group (P >0.05). Intra-ACC or icv injection of AP5 or DNQX had no effect on formalin-induced acute nociceptive behaviors. Fos immunoreactivity in the ACC was activated by retrieval of pain-related aversion, and this activation was significantly suppressed by preadministration of AP5, but not DNQX (P <0.001). These results suggest that NMDA receptors in the ACC are preferentially involved in the processing of the affective dimension of pain.

Repeated application of ketamine to rats induces changes in the hippocampal expression of parvalbumin, neuronal nitric oxide synthase and cFOS similar to those found in human schizophrenia

Treatment with the phencyclidine derivative ketamine, a non-competitive N-methyl-D-aspartate receptor antagonist and a well known anesthetic, has recently been introduced to mimic schizophrenia in animals. Using rats repeatedly treated with sub-anesthetic doses we demonstrate in the hippocampal formation the cellular distribution patterns of proteins being relevant to the pathogenesis of schizophrenia. Compared with controls an increase in the density of reduced nicotinamide adenine dinucleotide phosphate diaphorase-, neuronal nitric oxide synthase- and cFOS-positive hippocampal interneurons was found, whereas the density of parvalbumin expressing cells was decreased. Our experiments show that repeated injections of sub-anesthetic doses of ketamine induce significant changes in the nitrergic and GABAergic system which, in part, resemble those described in postmortem brains of human schizophrenics indicating that sub-chronic treatment with sub-anesthetic doses of ketamine might be a useful animal model to study schizophrenia.

Opposite influence of MPEP, an mGluR5 antagonist, on the locomotor hyperactivity induced by PCP and amphetamine

Potential antipsychotic effects of a selective non-competitive antagonist of metabotropic glutamate receptor 5 (mGluR5), 2-methyl-6-phenylethynylpyridine (MPEP), was examined in two commonly used screening tests: (1) the hyperactivity induced by an NMDA receptor antagonist phencyclidine (PCP), and (2) the hyperactivity induced by an indirect dopamine agonist, D-amphetamine. PCP was administered at a dose of 2.5 mg/kg s.c. and D-amphetamine was given at a dose of 1 mg/kg s.c. MPEP (5 mg/kg i.p.) significantly enhanced the locomotor activity increased by PCP, but inhibited amphetamine-induced hyperactivity. The opposite effect of MPEP in the two above-mentioned models questions significance of the blockade of mGluR5 receptors to antipsychotic effects.

Enhancement of excitatory postsynaptic potentials by preceding application of acetylcholine in mesencephalic dopamine neurons

Previously, we reported that Ca(2+) influx through nicotinic acetylcholine (ACh) receptors (nAChRs) activates a fulfenamic acid (FFA)-sensitive inward current, presumably a Ca(2+)-activated nonselective cation current (I(CAN)), in mesencephalic dopamine (DA) neurons. This current exhibited a negative slope conductance in the voltage range between -80 and -40mV and its activation led to a dramatic change in the responses to a transient application of glutamate, from single spikes to burst discharges. In this study, to examine the effect of activation of the FFA-sensitive current on EPSPs, we applied ACh (1mM) by transient air pressure shortly before electrical stimulation to evoke EPSPs in DA neurons. Application of ACh enhanced the amplitude of EPSPs when it preceded the electrical stimulation by less than 2 s, but not when the interval was longer than 3 s. In addition, this enhancement was critically dependent on intracellular Ca(2+) and the membrane potentials of the postsynaptic cell. Furthermore, the enhancing effect of ACh on EPSPs was sensitive to FFA and phenytoin. These results suggest that Ca(2+) influx caused by cholinergic inputs enhances EPSPs via activation of the FFA- and phenytoin-sensitive current.

Effects of mitochondrial dysfunction on glutamate receptor-mediated neurotoxicity in cultured rat spinal motor neurons

Glutamate-induced excitotoxicity is implicated as playing a key role in the pathogenesis of amyotrophic lateral sclerosis (ALS), and mitochondrial dysfunction is also found in ALS patients. We investigated the relationship between glutamate excitotoxicity and mitochondrial dysfunction elicited by rotenone (a complex I inhibitor), malonate (a complex II inhibitor), or antimycin (a complex III inhibitor), in primary cultures of the embryonic rat spinal cord. Rotenone and malonate induced relatively selective toxicity against motor neurons as compared to non-motor neurons, whereas antimycin caused non-selective toxicity. The toxicity of rotenone was prevented by a non-N-methyl-D-aspartate (NMDA) receptor antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) but not by an NMDA receptor antagonist, 5-methyl-10, 11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine hydrogen maleate (MK-801). The toxicity of malonate was blocked by both CNQX and MK-801. The toxicity of antimycin was affected by neither CNQX nor MK-801. When mitochondrial complex I was mildly inhibited by a sub-lethal concentration of rotenone, AMPA-induced motor neuron death was significantly exacerbated. A sub-lethal concentration of malonate exacerbated both NMDA- and AMPA-induced motor neuron death. These data suggest that mitochondrial dysfunction predisposes motor neurons to ionotropic glutamate receptor-mediated excitotoxicity.

Effects of ibotenate pedunculopontine tegmental nucleus lesions on exploratory behaviour in the open field

The pedunculopontine tegmental nucleus (PPTg) as part of the mesencephalic locomotor region is discussed to be involved in motor activity. In this study, we examined whether the PPTg plays a role in exploratory behaviour. Therefore, we compared non-habituated motor behaviour of PPTg lesioned rats with sham lesioned rats under spontaneous, dizocilpine (MK-801) (0.1 and 0.16 mg/kg) and dl-amphetamine (1 and 2 mg/kg) conditions. In order to analyse exploratory behaviour only, session-times were limited to 5 min after placing the rats in an open field. The exploratory motor activity was compared to the motor activity obtained in rats habituated to the environment. PPTg lesions had no effect on spontaneous exploratory behaviour, but it intensified the enhanced motor activity induced by MK-801. However, PPTg lesions blocked the enhanced exploratory behaviour, i.e. horizontal activity, rearing and centre activity induced by amphetamine. These data indicate that the PPTg is involved in behaviour driven by the dopaminergic and glutamatergic systems, when the animals are in a particular motivational state, e.g. a state that increases motor activity for itself, like exploration. This is underlined by the finding that animals exploring their environment show a higher motor activity even after multiple sessions, than animals familiar to the environment.

Memory Impairment Means Less Pain for Mice

BACKGROUND

Clinical observations have reported that individuals with memory deterioration, like in Alzheimer's disease, display a lesser pain sensibility than patients with no cognitive impairment.

OBJECTIVE

To clarify the link between pain and loss of memory, we studied how memory-impaired mice behave when submitted to hotplate nociceptive tests.

METHODS

For 5 days (D1-D5), male CD1 mice were injected daily intraperitonealy with saline or scopolamine (s, an anticholinergic drug, 0.2 mg/kg) or ketamine (k, an N-methyl-D-aspartate receptor antagonist (NMDAr), 2.5 mg/kg), at doses leading to memory impairment with no analgesic effect. From D6 to D9, all received saline only. They were placed on the hotplate and removed at the first sign of discomfort, response time being recorded.

RESULTS

From D1 to D5, reaction time decreased significantly in controls only and did not change in mice with scopolamine or ketamine. From D6 to D9, response times decreased (p < 0.05 (s) and p < 0.0001 (k)) to reach the steady state of control animals. At D5, response time was significantly prolonged for scopolamine (p < 0.01) and ketamine (p < 0.05), compared to controls.

CONCLUSION

These results show that pain sensibility needs the integrity of the central cholinergic and of the NMDA systems, and that mice with memory impairment display a lesser pain sensibility than normal mice. Further research on the complex interactions of receptors and neurotransmitters involved in pain and cognition could assist in gaining a better understanding of pain and analgesia in patients with memory impairment and in demented individuals.

Initiation of electrographic seizures by neuronal networks in entorhinal and perirhinal cortices in vitro

The hippocampus is often considered to play a major role in the pathophysiology of mesial temporal lobe epilepsy. However, emerging clinical and experimental evidence suggests that parahippocampal areas may contribute to a greater extent to limbic seizure initiation, and perhaps epileptogenesis. To date, little is known about the participation of entorhinal and perirhinal networks to epileptiform synchronization. Here, we addressed this issue by using simultaneous field potential recordings in horizontal rat brain slices containing interconnected limbic structures that included the hippocampus proper. Epileptiform discharges were disclosed by bath applying the convulsant drug 4-aminopyridine (50 microM) or by superfusing Mg(2+)-free medium. In the presence of 4-aminopyridine, slow interictal- (duration=2.34+/-0.29 s; interval of occurrence=25.75+/-2.11 s, n=16) and ictal-like (duration=31.25+/-3.34 s; interval of occurrence=196.96+/-21.56 s, n=17) discharges were recorded in entorhinal and perirhinal cortices after abating the propagation of CA3-driven interictal activity to these areas following extended hippocampal knife cuts. Simultaneous recordings obtained from the medial and lateral entorhinal cortex, and from the perirhinal cortex revealed that interictal and ictal discharges could initiate from any of these areas and propagate to the neighboring structure with delays of 8-66 ms. However, slow interictal- and ictal-like events more often originated in the medial entorhinal cortex and perirhinal cortex, respectively. Cutting the connections between entorhinal and perirhinal cortices (n=10), or functional inactivation of cortical areas by local application of a glutamatergic receptor antagonist (n=11) made independent epileptiform activity occur in all areas. These procedures also shortened ictal discharge duration in the entorhinal cortices, but not in the perirhinal area. Similar results could be obtained by applying Mg(2+)-free medium (n=7). These findings indicate that parahippocampal networks provide independent epileptiform synchronization sufficient to sustain limbic seizures as well as that the perirhinal cortex plays a preferential role in in vitro ictogenesis.

Functional inhibition by methadone of N-methyl-D-aspartate receptors expressed in Xenopus oocytes: stereospecific and subunit effects

Methadone is a strong opioid analgesic that is finding increasing use in chronic pain therapeutics. We explored its reported efficacy for inhibiting N-methyl-D-aspartate (NMDA) receptors in a functional electrophysiologic assay (Xenopus laevis oocyte expression). Racemic methadone inhibited all subtypes of rat NMDA receptors with derived 50% inhibitory concentrations in the low micromolar range. These concentrations overlap with clinically achievable concentrations reported in pharmacokinetic studies. In contrast, morphine inhibited these functional ion channels only at 8-16 times larger concentrations. The NR1/2A and NR1/2B subtype combinations were in general significantly more sensitive to inhibition by methadone and morphine compared with the NR1/2C and NR1/2D subtypes. In the presence of racemic methadone, the maximum NMDA-stimulated currents were markedly decreased, but the NMDA concentration producing 50% of maximal activation was altered only slightly, indicating that methadone blocks by a noncompetitive mechanism. Although stereoisomers of methadone showed minimal stereoselectivity in most subtypes, R(-) methadone was highly selective in its inhibition of the NR1/2A combination. These results provide further functional data describing the NMDA receptor inhibitory actions of methadone and support the hypothesis that methadone acts through both opioid and NMDA receptor mechanisms.

IMPLICATIONS

At clinically achievable concentrations, methadone inhibits functional N-methyl-D-aspartate receptors. These results indicate a unique mode of action by this opioid that may enhance its ability to treat chronic pain and to limit opioid tolerance.

Creation of AMPA-silent synapses in the neonatal hippocampus

In the developing brain, many glutamate synapses have been found to transmit only NMDA receptor-mediated signaling, that is, they are AMPA-silent. This result has been taken to suggest that glutamate synapses are initially AMPA-silent when they are formed, and that AMPA signaling is acquired through activity-dependent synaptic plasticity. The present study on CA3-CA1 synapses in the hippocampus of the neonatal rat suggests that AMPA-silent synapses are created through a form of activity-dependent silencing of AMPA signaling. We found that AMPA signaling, but not NMDA signaling, could be very rapidly silenced by presynaptic electrical stimulation at frequencies commonly used to probe synaptic function (0.05-1 Hz). Although this AMPA silencing required a rise in postsynaptic Ca(2+), it did not require activation of NMDA receptors, metabotropic glutamate receptors or voltage-gated calcium channels. The AMPA silencing, possibly explained by a removal of postsynaptic AMPA receptors, could subsequently be reversed by paired presynaptic and postsynaptic activity.

Stress impairs alpha(1A) adrenoceptor-mediated noradrenergic facilitation of GABAergic transmission in the basolateral amygdala

Intense or chronic stress can produce pathophysiological alterations in the systems involved in the stress response. The amygdala is a key component of the brain's neuronal network that processes and assigns emotional value to life's experiences, consolidates the memory of emotionally significant events, and organizes the behavioral response to these events. Clinical evidence indicates that certain stress-related affective disorders are associated with changes in the amygdala's excitability, implicating a possible dysfunction of the GABAergic system. An important modulator of the GABAergic synaptic transmission, and one that is also central to the stress response is norepinephrine (NE). In the present study, we examined the hypothesis that stress impairs the noradrenergic modulation of GABAergic transmission in the basolateral amygdala (BLA). In control rats, NE (10 microM) facilitated spontaneous, evoked, and miniature IPSCs in the presence of beta and alpha(2) adrenoceptor antagonists. The effects of NE were not blocked by alpha(1D) and alpha(1B) adrenoceptor antagonists, and were mimicked by the alpha(1A) agonist, A61603 (1 microM). In restrain/tail-shock stressed rats, NE or A61603 had no significant effects on GABAergic transmission. Thus, in the BLA, NE acting via presynaptic alpha(1A) adrenoceptors facilitates GABAergic inhibition, and this effect is severely impaired by stress. This is the first direct evidence of stress-induced impairment in the modulation of GABAergic synaptic transmission. The present findings provide an insight into possible mechanisms underlying the antiepileptogenic effects of NE in temporal lobe epilepsy, the hyperexcitability and hyper-responsiveness of the amygdala in certain stress-related affective disorders, and the stress-induced exacerbation of seizure activity in epileptic patients.

The mitochondrial complex I inhibitor annonacin is toxic to mesencephalic dopaminergic neurons by impairment of energy metabolism

The death of dopaminergic neurons induced by systemic administration of mitochondrial respiratory chain complex I inhibitors such as 1-methyl-4-phenylpyridinium (MPP(+); given as the prodrug 1-methyl-1,2,3,6-tetrahydropyridine) or the pesticide rotenone have raised the question as to whether this family of compounds are the cause of some forms of Parkinsonism. We have examined the neurotoxic potential of another complex I inhibitor, annonacin, the major acetogenin of Annona muricata (soursop), a tropical plant suspected to be the cause of an atypical form of Parkinson disease in the French West Indies (Guadeloupe). When added to mesencephalic cultures for 24 h, annonacin was much more potent than MPP(+) (effective concentration [EC(50)]=0.018 versus 1.9 microM) and as effective as rotenone (EC(50)=0.034 microM) in killing dopaminergic neurons. The uptake of [(3)H]-dopamine used as an index of dopaminergic cell function was similarly reduced. Toxic effects were seen at lower concentrations when the incubation time was extended by several days whereas withdrawal of the toxin after a short-term exposure (<6 h) arrested cell demise. Unlike MPP(+) but similar to rotenone, the acetogenin also reduced the survival of non-dopaminergic neurons. Neuronal cell death was not excitotoxic and occurred independently of free radical production. Raising the concentrations of either glucose or mannose in the presence of annonacin restored to a large extent intracellular ATP synthesis and prevented neuronal cell demise. Deoxyglucose reversed the effects of both glucose and mannose. Other hexoses such as galactose and fructose were not protective. Attempts to restore oxidative phosphorylation with lactate or pyruvate failed to provide protection to dopaminergic neurons whereas idoacetate, an inhibitor of glycolysis, inhibited the survival promoting effects of glucose and mannose indicating that these two hexoses acted independently of mitochondria by stimulating glycolysis. In conclusion, our study demonstrates that annonacin promotes dopaminergic neuronal death by impairment of energy production. It also underlines the need to address its possible role in the etiology of some atypical forms of Parkinsonism in Guadeloupe.

Spinal glutamate receptor antagonists differentiate primary and secondary mechanical hyperalgesia caused by incision

Secondary mechanical hyperalgesia has been demonstrated in postoperative patients indicating that central sensitization occurs after surgery. However, the underlying mechanisms are unknown. Here, we studied the role of spinal N-methyl-D-aspartate and alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA)/kainate receptors for pain behaviors indicating secondary hyperalgesia caused by gastrocnemius incision in the rat. We further determined if Ca(2+) permeable AMPA/kainate receptors are important for secondary hyperalgesia after gastrocnemius incision and for pain behaviors indicating primary hyperalgesia and guarding behavior after plantar incision. Withdrawal thresholds (WTs) to punctate mechanical stimuli were assessed by applying calibrated monofilaments to the plantar hind paw before gastrocnemius incision. WTs were tested again 2 h after gastrocnemius incision and again after intrathecal (IT) injection of either dizocilpine maleate (MK-801), 2-amino-5-phosphonovaleric acid (AP5), 1,2,3,4-tetrahydro-6-nitro-2,3-dioxo[f]quinoxaline-7-sulfonamide (NBQX), or Joro spider toxin (JSTX). The doses used were: MK-801 (vehicle, 15, 30, 40 nmol), AP5 (vehicle, 10, 30 nmol), NBQX (vehicle, 5, 10 nmol), and JSTX (vehicle, 2, 5, 9 nmol). In the same rats, WTs were tested on postoperative day 2 before and after the same drugs were injected again. In other rats, WTs to monofilaments and response frequencies to a non-punctate mechanical stimulus or guarding behaviors were determined before, 1 h after plantar incision was made, and assessed again after JSTX (9 nmol or vehicle) was administered IT. Secondary mechanical hyperalgesia after gastrocnemius incision was dose-dependently blocked by NBQX but was only marginally affected by AP5 or MK-801. Only secondary mechanical hyperalgesia was reversed by JSTX; primary mechanical hyperalgesia and guarding behavior were unchanged. These results indicate that spinal sensitization contributing to behaviors for secondary hyperalgesia after incision requires Ca(2+) permeable AMPA/kainate receptors. The data further demonstrate that behaviors for secondary mechanical hyperalgesia after incision can be inhibited without affecting behaviors for primary mechanical hyperalgesia and guarding. Mechanisms for central sensitization causing secondary hyperalgesia in postoperative patients may therefore be separated from spontaneous pain and hyperalgesia that arises adjacent to the area of the incision.

Role of central NMDA versus non-NMDA receptor in spinal withdrawal reflex in spinal anesthetized rats under normal and hyperexcitable conditions

The present study aimed to investigate the role of central N-methyl-D-aspartate (NMDA) and non-NMDA receptors in the spinal withdrawal reflex assessed by recording single motor unit (SMU) electromyogram (EMG) response to peripheral mechanical (pressure, pinch) stimuli and repeated electrical stimuli at 3 and 20 Hz. During normal conditions, intrathecal administration of MK-801 and CNQX apparently depressed mechanically and electrically (3 Hz) evoked EMG responses in a dose-dependent manner (10, 20 and 40 nmol in 10 microl). In contrast, the after-discharges to 20 Hz electrical stimuli were suppressed only by CNQX treatment, not by MK-801 treatment. This indicates that the central mechanisms underlying the different frequencies of electrically evoked withdrawal reflex may be different. During peripheral bee venom (BV, 0.2 mg/50 microl) induced inflammation and central sensitization, the enhanced SMU EMG responses including after-discharges to pinch stimuli and 3 Hz electrical stimuli were depressed significantly by treatments with both MK-801 and CNQX. However, the enhanced SMU activities to innocuous pressure stimuli were depressed only by treatment with CNQX. Likewise, enhanced long lasting after-discharges elicited by 20 Hz electrical stimuli were also only depressed by CNQX, indicating that different central mechanisms are involved in the persistent hyperexcitability during BV-induced inflammation. The data suggest that both central NMDA and non-NMDA receptors play important roles in the transmission of nociceptive information under normal conditions. In BV-induced inflammation, however, central non-NMDA receptors, but not NMDA receptors, play a pivotal role in the generation of persistent hyperexcitability to mechanical and electrical stimuli at different frequencies (3 Hz, 20 Hz).

Excitatory GABA input directly drives seizure-like rhythmic synchronization in mature hippocampal CA1 pyramidal cells

GABA, which generally mediates inhibitory synaptic transmissions, occasionally acts as an excitatory transmitter through intense GABA(A) receptor activation even in adult animals. The excitatory effect results from alterations in the gradients of chloride, bicarbonate, and potassium ions, but its functional role still remains a mystery. Here we show that such GABAergic excitation participates in the expression of seizure-like rhythmic synchronization (afterdischarge) in the mature hippocampal CA1 region. Seizure-like afterdischarge was induced by high-frequency synaptic stimulation in the rat hippocampal CA1-isolated slice preparations. The hippocampal afterdischarge was completely blocked by selective antagonists of ionotropic glutamate receptors or of GABA(A) receptor, and also by gap-junction inhibitors. In the CA1 pyramidal cells, oscillatory depolarizing responses during the afterdischarge were largely dependent on chloride conductance, and their reversal potentials (average -38 mV) were very close to those of exogenously applied GABAergic responses. Moreover, intracellular loading of the GABA(A) receptor blocker fluoride abolished the oscillatory responses in the pyramidal cells. Finally, the GABAergic excitation-driven afterdischarge has not been inducible until the second postnatal week. Thus, excitatory GABAergic transmission seems to play an active functional role in the generation of adult hippocampal afterdischarge, in cooperation with glutamatergic transmissions and possible gap junctional communications. Our findings may elucidate the cellular mechanism of neuronal synchronization during seizure activity in temporal lobe epilepsy.