Desensitization of NMDA receptors does not proceed in the presence of kynurenate

Major dorsoventral differences in the modulation of the local CA1 hippocampal network by NMDA, mGlu5, adenosine A2A and cannabinoid CB1 receptors

Recent research points to diversification in the local neuronal circuitry between dorsal (DH) and ventral (VH) hippocampus that may be involved in the large-scale functional segregation along the long axis of the hippocampus. Here, using CA1 field recordings from rat hippocampal slices, we show that activation of N-methyl-d-aspartate receptors (NMDARs) reduced excitatory transmission more in VH than in DH, with an adenosine A1 receptor-independent mechanism, and reduced inhibition and enhanced postsynaptic excitability only in DH. Strikingly, co-activation of metabotropic glutamate receptor-5 (mGluR5) with NMDAR, by CHPG and NMDA respectively, strongly potentiated the effects of NMDAR in DH but had not any potentiating effect in VH. Furthermore, the synergistic actions in DH were occluded by blockade of adenosine A2A receptors (A2ARs) by their antagonist ZM 241385 demonstrating a tonic action of these receptors in DH. Exogenous activation of A2ARs by 4-[2-[[6-amino-9-(N-ethyl-β-D-ribofuranuronamidosyl)-9H-purin-2-yl]amino]ethyl]benzenepropanoic acid hydrochloride (CGS 21680) did not change the effects of mGluR5-NMDAR co-activation in either hippocampal pole. Importantly, blockade of cannabinoid CB1 receptors (CB1Rs) by their antagonist 1-(2,4-dichlorophenyl)-5-(4-iodophenyl)-4-methyl-N-4-morpholinyl-1H-pyrazole-3-carboxamide (AM 281) restricted the synergistic actions of mGluR5-NMDARs on excitatory synaptic transmission and postsynaptic excitability and abolished their effect on inhibition. Furthermore, AM 281 increased the excitatory transmission only in DH indicating that CB1Rs were tonically active in DH but not VH. Removing the magnesium ions from the perfusion medium neither stimulated the interaction between mGluR5 and NMDAR in VH nor augmented the synergy of the two receptors in DH. These findings show that the NMDAR-dependent modulation of fundamental parameters of the local neuronal network, by mGluR5, A2AR and CB1R, markedly differs between DH and VH. We propose that the higher modulatory role of A2AR and mGluR5, in combination with the role of CB1Rs, provide DH with higher functional flexibility of its NMDARs, compared with VH.

Whole cell and single channel analysis of the kinetics of glycine-sensitive N-methyl-D-aspartate receptor desensitization

1. The kinetics of glycine-sensitive, N-methyl-D-aspartate (NMDA) receptor desensitization were investigated in cultured neurones with the patch clamp technique. 2. The degree of fast NMDA-receptor desensitization was inversely related to glycine concentration. Thus, increasing concentrations of glycine from 30 nM to 2.5 microM potentiated desensitized NMDA responses (873% +/- 101%) to a greater degree than peak responses (260% +/- 27%). 3. The desensitization was due to a decrease in the affinity of glycine for the strychnine-insensitive, glycine modulatory site (glycineB site) following activation of the NMDA-receptor complex. Thus, the A50 for glycine in potentiating peak responses (77 nM, 95% confidence limited 58-104 nM) was five fold lower than that for plateau responses (399 nM, 340-468 nM). 4. The rate of desensitization was related to glycine concentration such that a reciprocal plot of desensitization rate (1/tau S-1) against glycine concentration had a slope of 9.5* 10(6) M-1 S-1. 5. Recovery from desensitization following step increases in glycine or L-alanine concentration in the continuous presence of NMDA (200 microM) reflected the association kinetics of the glycineB agonist used. 6. The rate and degree of NMDA receptor desensitization was independent of holding potential. 7. NMDA receptor desensitization was also evident at the single channel level. 8. The glycineB antagonist 7-chlorokynurenic acid (7-Chl-Kyn 3 and 10 microM) concentration-dependently induced an identical form of desensitization in the presence of 1 microM glycine. 9. In contrast, the competitive NMDA antagonist (+/-)-amino-phosphonovaleric acid (APV 30 to 300 microM) concentration-dependently antagonized and slowed the onset kinetics of NMDA responses.

Activation and desensitization of N-methyl-D-aspartate receptors in nucleated outside-out patches from mouse neurones

1. Activation and desensitization of N-methyl-D-aspartate (NMDA) receptors were studied in large outside-out patches excised from cultured embryonic neurones dissociated from mouse forebrain. The patches were exposed to rapid changes of NMDA or L-glutamate concentrations in the presence of glycine at concentrations (10-20 microM) saturating the modulatory site of the NMDA receptor. 2. Immediately after formation of the patch the responses to NMDA and L-glutamate showed a slow and small desensitization, even with high concentrations of agonist. During the following hour, the peak response either decreased or remained relatively stable, but in all cases the desensitization increased and accelerated until it stabilized. In this 'stabilized' state, the desensitization produced by high concentrations of NMDA (1 mM) or L-glutamate (300 microM) had an exponential time course, with a time constant of about 30 ms. The ratio of the peak over the steady-state current was in the order of 40 for NMDA and about 30 for L-glutamate. 3. Concentration-response curves were built for the peak and the plateau responses, for NMDA and for L-glutamate. The comparison of these curves indicated that (i) the EC50 of the peak (K(app) was always higher than the EC50 of the plateau (Kss); (ii) the two EC50 values for NMDA (K(app) and Kss) were higher than those for L-glutamate; (iii) the Hill coefficient was close to 1.4 for each of the four curves. 4. The application of NMDA or L-glutamate at a low concentration for 3 s periods reduced the response to a subsequent application of the same agonist at a saturating concentration. The IC50 of this 'predesensitization', termed Kpre, was lower than the EC50 of the steady-state response, Kss. 5. The onset rates of desensitization increased with the concentration of agonist. The EC50 of this relation was close to the value of K(app). 6. The decay of the currents at the end of a 3 s application of agonist was usually well described by the sum of two exponentials both of which were faster for NMDA than for L-glutamate. 7. The recovery from desensitization after a long (3 s) pulse of agonist was approximately exponential, with a time constant of about 0.5 s for NMDA and about 3.5 s for L-glutamate.(ABSTRACT TRUNCATED AT 400 WORDS)

Properties of vertebrate glutamate receptors: calcium mobilization and desensitization

Glutamate is now recognized as a major excitatory neurotransmitter in the vertebrate CNS, participating in a number of physiological and pathological processes. The importance of glutamate in the mobilization of intracellular Ca2+ as well as the relationship between excitatory and toxic properties has made it important to understand factors that regulate the responsivity of glutamate receptors. In recent years considerable insight has been gained about regulatory sites on NMDA receptors, with the recognition that these receptors are modulated by multiple endogenous and exogenous agents. Less is known about the regulation of responses mediated by AMPA, kainate, ACPD or APB receptors. Desensitization represents a potentially powerful means by which glutamate responses may be regulated. Indeed, two agents closely linked to the physiology of NMDA receptors, glycine and Ca2+, appear to modulate different types of desensitization. In the case of glycine, alteration of a rapid form of desensitization may be important in the role of this amino acid as a necessary cofactor for NMDA receptor activation. Additionally, changes in the affinity of the receptor complex for glycine may underlie the use-dependent decline in NMDA responses under certain conditions. Likewise, Ca2+ is a crucial player in the synaptic and toxic effects mediated by NMDA receptors, and is involved in a slower form of desensitization, in effect helping to regulate its own influx into neurons. The site and mechanism of the Ca2+ regulatory effects remain uncertain with evidence supporting both intracellular and ion channel sites of action. A clear role for Ca(2+)-dependent desensitization in the function of NMDA receptors under physiological conditions has not yet been demonstrated. AMPA receptor desensitization has been an area of intense investigation in recent years. The rapidity and degree of this process, coupled with its apparent rapid recovery, has suggested that desensitization is a key mechanism for the short-term regulation of responses mediated by these receptors. Furthermore, rapid desensitization appears to be one factor determining the time course and efficacy of fast excitatory synaptic transmission mediated by AMPA receptors, highlighting the physiological relevance of the process. The molecular mechanisms underlying desensitization remain uncertain. Traditionally, desensitization, like inactivation of voltage-gated channels, has been thought to represent a conformational change in the ion channel complex (Ochoa et al., 1989). However, it is unknown to what extent desensitization, in particular rapid AMPA receptor desensitization, has mechanistic features in common with inactivation. In voltage-gated channels, conformational changes in the channel protein restrict ion flow through the channel (Stuhmer, 1991).(ABSTRACT TRUNCATED AT 400 WORDS)

Two types of steady-state desensitization of N-methyl-D-aspartate receptor in isolated hippocampal neurones of rat

1. Whole-cell voltage-clamp recordings were made from rat isolated hippocampal neurones. Aspartate (Asp) and/or glycine (Gly) were applied by a method in which the external solution could be changed within 30 ms and thereafter held constant. 2. Asp and Gly applied together at maximal concentrations (5 mM and 10 microM, respectively) evoked an inward current due to activation of N-methyl-D-aspartate (NMDA) receptors. The current peaked and then declined to a steady state during the application. The time constant of desensitization (tau) was about 1 s when the agonists were applied soon after the onset of whole-cell recording. The desensitization became more rapid (tau = 0.3 s) and more complete during the first 15 min of recording, and thereafter remained stable; the amplitude of the peak response did not change throughout. In solutions containing 10 microM-Gly, Asp had an apparent Kd of 51 microM at the peak of response and 20 microM measured at the steady state. The steady-state current was 14% of the peak current. 3. Asp was applied after a conditioning exposure of the cell of Gly (from 1 to 50 microM), together with the same Gly concentration. The maximum current evoked by the application of Asp was increased while increasing Gly in the conditioning solution, with no change in the apparent Kd for Asp at the peak of Asp-activated response. 4. Various concentrations of Asp (plus 10 microM-Gly) were applied after a conditioning exposure to Asp (which alone was without effect). The maximum current induced by Asp applications was only 28% of that observed without conditioning Asp application, but the apparent Kd was unchanged (about 57 microM). 5. Test solution containing maximal concentrations of Asp and Gly was applied after conditioning exposure to both Asp (varying concentrations) and Gly (10 microM). Complete desensitization was caused by 200 microM-Asp. The apparent Kd for Asp to induce desensitization (8.7 microM) was less than the Kd as an agonist (51 microM). 6. Test solution containing maximal concentrations of Asp and Gly was applied after conditioning exposure to both Gly (varying concentrations) and Asp (5 mM). Complete desensitization was caused by 1 microM-Gly. The apparent Kd for Gly to induce desensitization (120 nM) was less than the Kd as a co-agonist (about 1 microM).(ABSTRACT TRUNCATED AT 400 WORDS)

Neurodegenerative Effects Induced by Chronic Infusion of Quinolinic Acid in Rat Striatum and Hippocampus

In this study we examined whether the potency of quinolinic acid (Quin) in inducing neurodegeneration in vivo was dependent on the exposure time of the tissue to the excitotoxin. The effect of chronic infusion of Quin into rat striatum and hippocampus was examined at the light microscopic level and by cell count on 40 microm Cresyl violet stained brain sections. Continuous infusion was at a constant speed (0.5 microl/h) for various times (15 h - 2 weeks) by osmotic minipumps (Alzet 2002). No build up of [3H]Quin occurred in the tissue during infusion; this was assessed by measuring the radioactivity 3 - 14 days after minipump placement. Intrastriatal infusion of 6 and 10 nmol/h Quin, but not of nicotinic acid, for 1 week induced a dose-dependent neurodegeneration (70 and 90% loss of neurons, respectively, compared to the contralateral striatum) extending 1.2 - 2 mm from the centre of the injection. The onset of the neurotoxicity caused by 10 nmol/h Quin was >24 h. One week's infusion of 4 nmol/h Quin did not induce neurotoxicity, but a 40% drop of neurons, compared to the contralateral side, occurred after 2 weeks. One week's intrahippocampal infusion of 2.4 and 6 nmol/h Quin, but not of nicotinic acid, caused a dose-dependent neurodegeneration with a radius of approximately 1 - 1.5 mm around the injection track. The onset of the neurotoxicity induced by 2.4 nmol/h Quin was < 15 h. The pattern of nerve cell loss induced by 1.2 nmol/h Quin after 1 week (CA4 cells lost in 50% of the rats) did not differ from that observed after 2 weeks of infusion. Nerve cell loss caused by Quin in the striatum and in the hippocampus was restricted to the injected area and antagonized by coinfusion with d(-)-2-amino-7-phosphonoheptanoic and kynurenic acids in molar ratios of 1:0.1 and 1:3, respectively. These data show that Quin's potency in inducing neurodegeneration in the striatum, but not in the hippocampus, depends on the exposure time of the tissue to the excitotoxin. In addition, neurodegeneration is induced faster by Quin in the hippocampus than in the striatum. The usefulness of this model to study the sequelae of the neurotoxic process in vivo will be discussed.

Modulatory action of cholinergic drugs on N-methyl-D-aspartate response in dissociated rat nucleus basalis of Meynert neurons

The effect of cholinergic drugs on N-methyl-D-aspartate (NMDA)-activated responses in neurons dissociated freshly from the nucleus basalis of Meynert (nBM) of immature and mature rats were investigated with the whole-cell patch-clamp technique. The NMDA (10(-4) M)-evoked inward current (INMDA) consisted of transient peak and successive steady-state components. In mature neurons, the peak component was suppressed by pretreatment with acetylcholine (ACh, 10(-12)-10(-5) M) but facilitated by higher concentrations (greater than 10(-5) M). Regardless of the concentration, ACh had no effect on INMDA of immature neurons. The results suggest that ACh behaves as either inhibitory or excitatory modulator of glutaminergic transmission in nBM neurons of adult mammals depending on ACh concentration.