Spontaneous release from mossy fiber terminals inhibits Ni2+-sensitive T-type Ca2+ channels of CA3 pyramidal neurons in the rat organotypic hippocampal slice

Mossy fibers (axons arising from dentate granule cells) form large synaptic contacts exclusively onto the proximal apical dendrites of CA3 pyramidal neurons. They can generate large synaptic currents that occur in close proximity to the soma. These properties mean that active conductance in the proximal apical dendrite could have a disproportionate influence on CA3 pyramidal neuron excitability. Ni(2+)-sensitive T-type Ca(2+) channels are important modulators of dendritic excitability. Here, we use an optical approach to determine the contribution of Ni(2+) (100 microM)-sensitive Ca(2+) channels to action potential (AP) elicited Ca(2+) flux in the soma, proximal apical and distal apical dendrites. At resting membrane potentials Ni(2+)-sensitive Ca(2+) channels do not contribute to the Ca(2+) signal in the proximal apical dendrite, but do contribute in the other cell regions. Spontaneous release from mossy fiber terminals acting on 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX)-sensitive postsynaptic channels underlies a tonic inhibition of Ni(2+)-sensitive channels. Chelating Zn(2+) with CaEDTA blocks CNQX-sensitive changes in Ca(2+) flux implicating a mechanistic role of this ion in T-type Ca(2+) channel block. To test if this inhibition influenced excitability, progressively larger depolarizing pulses were delivered to CA3 pyramidal neurons. CNQX significantly reduced the size of the depolarizing step required to generate APs and increased the absolute number of APs per depolarizing step. This change in AP firing was completely reversed by the addition of Ni(2+). This mechanism may reduce the impact of T-type Ca(2+) channels in a region where large synaptic events are common.     

AMPA- and kainate-receptors differentially mediate excitatory amino acid-induced dopamine and acetylcholine release from rat striatal slices

Rat striatal slices, preincubated with [³H]dopamine (DA) and [¹⁴C]choline, were superfused continuously. Detection of radioactivity was used to monitor the release of the neurotransmitters DA and acetylcholine (ACh). 6-Cyano-7-nitroquinoxaline-2,3-dione (CNQX) and 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(f) quinoxaline (NBQX) caused a concentration-dependent decrease in 100 μM α-amino-3-hydroxy-5-methylisoxazol-4-propionate (AMPA)-, 100 μM kainate- or 100 mM glutamate-induced release of DA and ACh. The ₅₀ of NBQX is 3-fold higher (for ACh release) and is 2-fold lower (for DA release) than that of CNQX. This is in agreement with the ₅₀ ratio of NBQX and CNQX on kainate- and AMPA-receptor binding. These two antagonists, at doses that produce an equivalent blockade of kainate-receptor binding (5 μM for NBQX and 1.56 μM for CNQX), caused an approximately equal decrease in ACh- but not DA-release induced by 100 μM kainate or AMPA. At doses that produce an equivalent blockade of AMPA-receptor binding (5 μM for NBQX and 10 μM for CNQX), they caused an approximately equal decrease in DA but not ACh release induced by 100 μM AMPA or kainate. Moreover, concanavalin A (0.3 and 0.5 mg/ml), which selectively potentiates kainate-receptor responses, markedly enhanced 100 μM kainate-induced release of ACh but not DA. Cyclothiazide (10 μM), which selectively potentiates AMPA-receptor responses, significantly increased 100 μM AMPA- or kainate-induced release of DA but not ACh. In summary, these results indicate that AMPA- and kainate-receptor activation, respectively, are predominantly involved in excitatory amino acid (EAA)-induced DA and ACh release in the striatum.     

Enhanced NMDA receptor NR1 phosphorylation and neuronal activity in the arcuate nucleus of hypothalamus following peripheral inflammation

AbstractAim:To investigate the role of glutamate and N-methyl-D-aspartate (NMDA) receptors in central sensitization following peripheral inflammation in the arcuate nucleus (ARC) of the mediobasal hypothalamus.Methods:Mediobasal hypothalamic slices were prepared from rats undergoing peripheral inflammation, which was induced by a unilateral injection of complete Freund's adjuvant (CFA) into hind paw. Neuronal activation levels in the ARC were monitored by recording extracellular unit discharges. The NMDA receptor NR1 subunit (NR1) was measured using Western blot analysis.Results:Enhanced NR1 phosphorylation was observed in the ARC of CFA-inflamed rats. Compared with the control rats, the firing rate of spontaneous discharges in ARC neurons of inflamed rats was significantly higher, and it was significantly reduced both by an NMDA receptor antagonist (MK-801, 300 μmol/L) and by a non-NMDA receptor antagonist (CNQX, 30 μmol/L). Application of exogenous glutamate (200 μmol/L) or NMDA (25 μmol/L) resulted in increased neuronal discharges for ARC neurons, which was enhanced to a greater extent in inflamed rats than in control rats.Conclusion:Glutamate receptor activation in the hypothalamic ARC plays a crucial role in central sensitization associated with peripheral inflammation.     

The lathyrus toxin, beta-N-oxalyl-L-alpha,beta-diaminopropionic acid (ODAP), and homocysteic acid sensitize CA1 pyramidal neurons to cystine and L-2-amino-6-phosphonohexanoic acid

A brief exposure of hippocampal slices to L-quisqualic acid (QUIS) sensitizes CA1 pyramidal neurons 30- to 250-fold to depolarization by certain excitatory amino acids analogues, e.g., L-2-amino-6-phosphonohexanoic acid (L-AP6), and by the endogenous compound, L-cystine. This phenomenon has been termed QUIS sensitization. A mechanism similar to that previously described for QUIS neurotoxicity has been proposed to describe QUIS sensitization. Specifically, QUIS has been shown to be sequestered into GABAergic interneurons by the System x(c)(-) and subsequently released by heteroexchange with cystine or L-AP6, resulting in activation of non-NMDA receptors. We now report two additional neurotoxins, the Lathyrus excitotoxin, beta-N-oxalyl-L-alpha,beta-diaminopropionic acid (ODAP), and the endogenous compound, L-homocysteic acid (HCA), sensitize CA1 hippocampal neurons >50-fold to L-AP6 and >10-fold to cystine in a manner similar to QUIS. While the cystine- or L-AP6-mediated depolarization can be inhibited by the non-NMDA receptor antagonist CNQX in ODAP- or QUIS-sensitized slices, the NMDA antagonist D-AP5 inhibits depolarization by cystine or L-AP6 in HCA-sensitized slices. Thus, HCA is the first identified NMDA agonist that induces phosphonate or cystine sensitization. Like QUIS sensitization, the sensitization evoked by either ODAP or HCA can be reversed by a subsequent exposure to 2 mM alpha-aminoadipic acid. Finally, we have demonstrated that there is a correlation between the potency of inducers for triggering phosphonate or cystine sensitivity and their affinities for System x(c)(-) and either the non-NMDA or NMDA receptor. Thus, the results of this study support our previous model of QUIS sensitization and have important implications for the mechanisms of neurotoxicity, neurolathyrism and hyperhomocystinemia.     

Roles of periventricular neurons in retinotectal transmission in the optic tectum

The midbrain roof is a retinorecipient region referred to as the optic tectum in lower vertebrates, and the superior colliculus in mammals. The retinal fibers projecting to the tectum transmit visual information to tectal retinorecipient neurons. Periventricular neurons are a subtype of these neurons that have their somata in the deepest layer of the teleostean tectum and apical dendrites ramifying at more superficial layers consisting of retinal fibers. The retinotectal synapses between the retinal fibers and periventricular neurons are glutamatergic, and ionotropic glutamate receptors mediate the transmission in these synapses. This transmission involves long-term potentiation, and is modulated by hormone action. Visual information processed in the periventricular neurons is transmitted to adjacent tectal cells and target nuclei of periventricular neuron axonal branches, some of which relay the visual information to other brain areas controlling behavior. We demonstrated that periventricular neurons play a principal role in visual information processing in the teleostean optic tectum; the effects of tectal output on behavior is discussed also in the present review.     

Noradrenaline involvement in basic and higher integrated REM sleep processes

There has been an abundance of literature devoted to the involvement of noradrenaline in basic rapid eye movement (REM) sleep processes since the subject was first investigated in 1964. Nowadays, the great majority of studies highlight the need for silence in the locus coeruleus noradrenergic neurons as a condition for the occurrence and maintenance of REM sleep. However, throughout the successive years of work on this topic, few researchers have consistently claimed that some amount of noradrenaline is essential for the appearance of this sleep stage. In the first part of this review, each of the papers published in this field is analyzed. Then, in the discussion, arguments supporting the requirement for a given level of noradrenaline for REM sleep occurrence are presented. This second part also examines, based on waking noradrenergic influences on higher integrated brain processes, the major consequences of noradrenergic neuron silence during REM sleep for mental functioning.     

The non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) impairs late reconsolidation of passive avoidance learning in the day-old chick

Both NMDA and non-NMDA receptors participate in the consolidation of passive avoidance learning (PAL) in the day-old chick. NMDA antagonists have also been implicated in reconsolidation processes following reminder-trials. In this study, we examined the effect of administering 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), a non-NMDA receptor antagonist, on reconsolidation following memory reactivation. New HampshirexWhite leghorn cockerels were trained using a modified version of the PAL task. When CNQX was administered 20min following a reminder trial, a retention deficit was detected at 90min, but this had resolved by 24h following the reminder. The parameters of the reconsolidation deficit were similar to those induced by CNQX injections post-training with the exception of their transience. This finding suggests that the action of non-NMDA receptors may perform a similar role in both consolidation and reconsolidation processes.     

Simultaneous detection of intracellular target and off-target binding of small molecule cancer drugs at nanomolar concentrations

Background and purpose:

Current single drug treatments for rheumatoid arthritis have problems of limited efficacy and/or high toxicity. This study investigates the benefits of individual and combined treatments with dexamethasone and substance P and glutamate receptor antagonists in a rat model of arthritis.

Experimental approach:

Arthritis was induced in rats by unilateral intra-articular injection of Freund''s complete adjuvant. Separate groups of rats were subjected to the following treatments 15 min before induction of arthritis: (i) control with no drug treatment; (ii) single intra-articular injection of a NK₁ receptor antagonist RP67580; (iii) single intra-articular injection of a NMDA receptor antagonist AP7 plus a non-NMDA receptor antagonist CNQX; (iv) daily oral dexamethasone; and (v) combined treatment with dexamethasone and all of the above receptor antagonists. Knee joint allodynia, swelling, hyperaemia and histological changes were examined over a period of 7 days.

Key results:

Treatment with dexamethasone suppressed joint swelling, hyperaemia and histological changes that include polymorphonuclear cell infiltration, synovial tissue proliferation and cartilage erosion in the arthritic rat knees. Treatment with RP67580 or AP7 plus CNQX did not attenuate hyperaemia or histological changes, but reduced joint allodynia and swelling. Co-administration of dexamethasone with these receptor antagonists produced greater inhibition on joint allodynia and swelling than their individual effects.

Conclusions and implications:

The data suggest substance P and glutamate contribute to arthritic pain and joint swelling. The efficacy of dexamethasone in reducing arthritic pain and joint swelling can be improved by co-administration of substance P and glutamate receptor antagonists.     

Ionic storm in hypoxic/ischemic stress: Can opioid receptors subside it?

Neurons in the mammalian central nervous system are extremely vulnerable to oxygen deprivation and blood supply insufficiency. Indeed, hypoxic/ischemic stress triggers multiple pathophysiological changes in the brain, forming the basis of hypoxic/ischemic encephalopathy. One of the initial and crucial events induced by hypoxia/ischemia is the disruption of ionic homeostasis characterized by enhanced K⁺ efflux and Na⁺-, Ca²⁺- and Cl⁻-influx, which causes neuronal injury or even death. Recent data from our laboratory and those of others have shown that activation of opioid receptors, particularly δ-opioid receptors (DOR), is neuroprotective against hypoxic/ischemic insult. This protective mechanism may be one of the key factors that determine neuronal survival under hypoxic/ischemic condition. An important aspect of the DOR-mediated neuroprotection is its action against hypoxic/ischemic disruption of ionic homeostasis. Specially, DOR signal inhibits Na⁺ influx through the membrane and reduces the increase in intracellular Ca²⁺, thus decreasing the excessive leakage of intracellular K⁺. Such protection is dependent on a PKC-dependent and PKA-independent signaling pathway. Furthermore, our novel exploration shows that DOR attenuates hypoxic/ischemic disruption of ionic homeostasis through the inhibitory regulation of Na⁺ channels. In this review, we will first update current information regarding the process and features of hypoxic/ischemic disruption of ionic homeostasis and then discuss the opioid-mediated regulation of ionic homeostasis, especially in hypoxic/ischemic condition, and the underlying mechanisms.     

Involvement of other neurotransmitters in behaviors induced by the cannabinoid CB1 receptor antagonist SR 141716A in naive mice

Summary. The receptor mechanisms by which the selective cannabinoid CB₁ receptor antagonist/inverse agonist, SR 141716A [N-piperidino-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-3-pyrazole-carboxamide] produces scratching and head-twitch response (HTR) in naive mice were examined. Acute intraperitoneal administration of varying doses of SR 141716A produced both scratchings (ED₅₀ = 3.9 mg/kg) and head-twitches (ED₅₀ = 4.6 mg/kg) in a dose-dependent manner. A dose of 10 mg/kg SR 141716A was used to induce the cited behaviors for drug interaction studies. The selective 5-HT_2A/C receptor antagonist, SR 46349B [trans-4-[(3Z)3-(2-dimethylaminoethyl) oxyimino-3-(2-fluorophenyl) propen-1-yl] phenol] potently and completely blocked the head-twitches produced by SR 141716A (ID₅₀ = 0.08 mg/kg). The induced scratching behavior was partially (68%) and less potently (ID₅₀ = 0.6 mg/kg) blocked by SR 46349B pretreatment. The AMPA/kainate receptor antagonist, CNQX [6-cyano-7-nitroquinoxaline-2,3-dione), partially attenuated (68–78%) the induced scratching and head-twitching behaviors. On the contrary, the selective NMDA antagonist, AP-3 [(±)-2-amino-3-phosphonopropionic acid), had no significant effect on these behaviors. The selective tachykinin NK₁ antagonist, CP 94, 994 [(±)-(2S, 3S)-3-(2-methoxybenzylamino)-2-phenylpiperidine], also partially attenuated both the scratching (64%) and the head-twitching (76%) symptoms produced by SR 141716A. Since SR 141716A lacks affinity for the discussed receptors, it appears that the induction of the cited behaviors probably involve indirect activation of their respective neurotransmitter systems. Received September 9, 1999; accepted December 16, 1999