A role for N-methyl-D-aspartate receptors in norepinephrine-induced long-lasting potentiation in the dentate gyrus

Mechanisms of action of norepinephrine (NE) on dentate gyrus granule cells were studied in rat hippocampal slices using extra- and intracellular recordings and measurements of stimulus and amino acid-induced changes in extracellular Ca2+ and K+ concentration. Bath application of NE (10-50 microM) induced long-lasting potentiation of perforant path evoked potentials, and markedly enhanced high-frequency stimulus-induced Ca2+ influx and K+ efflux, actions blocked by beta-receptor antagonists and mimicked by beta agonists. Enhanced Ca2+ influx was primarily postsynaptic, since presynaptic delta [Ca2+]o in the stratum moleculare synaptic field was not altered by NE. Interestingly, the potentiation of both ionic fluxes and evoked population potentials were antagonized by the N-methyl-D-aspartate (NMDA) receptor antagonist 2-amino-5-phosphonovalerate (APV). Furthermore, NE selectively enhanced the delta [Ca2+]o delta [K+]o and extracellular slow negative field potentials elicited by iontophoretically applied NMDA, but not those induced by the excitatory amino acid quisqualate. These results suggest that granule cell influx of Ca2+ through NMDA ionophores is enhanced by NE via beta-receptor activation. In intracellular recordings, NE depolarized granule cells (4.8 +/- 1.1 mV), and increased input resistance (RN) by 34 +/- 6.5%. These actions were also blocked by either the beta-antagonist propranolol or specific beta 1-blocker metoprolol. Moreover, the depolarization and RN increase persisted for long periods (93 +/- 12 min) after NE washout. In contrast, while NE, in the presence of APV, still depolarized granule cells and increased RN, APV made these actions quickly reversible upon NE washout (16 +/- 9 min). This suggested that NE induction of long-term, but not short-term, plasticity in the dentate gyrus requires NMDA receptor activation. NE may be enhancing granule cell firing by some combination of blockade on the late Ca2+-activated K+ conductance and depolarization of granule cells, both actions that can bring granule cells into a voltage range where NMDA receptors are more easily activated. Furthermore, NE also elicited activity-independent long-lasting depolarization and RN increases, which required functional NMDA receptors to persist.

Spontaneous activity mediated by NMDA receptors in immature rat entorhinal cortex in vitro

Intracellular recordings were made from cells in layer II of entorhinal cortex slices from 9 to 13-day-old rats. The majority of these cells showed pronounced spontaneous synaptic activity which could summate into large depolarizing events and give rise to bursts of spikes. These events could be associated with an apparent decrease in membrane conductance. They were reduced in amplitude by hyperpolarization of the cell and tended to increase on depolarization. Perfusion with 2-amino-5-phosphonovalerate abolished the spontaneous activity. Thus, in layer II of the entorhinal cortex at this stage of development there seems to be a functional enhancement of ongoing synaptic activity mediated via activation of N-methyl-D-aspartate receptors.

Evidence for an excitatory amino acid pathway in the brainstem and for its involvement in cardiovascular control

The source and possible role of excitatory amino acid projections to areas of the ventrolateral medulla (VLM) involved in cardiovascular control were studied. Following the injection of [3H]D-aspartate ([3H]D-Asp), a selective tracer for excitatory amino acid pathways, into vasopressor or vasodepressor areas of the VLM in rats, more than 90% of retrogradely labelled neurones were found in the nucleus of the solitary tract (NTS). Very few of the [3H]D-Asp-labelled cells were immunoreactive for tyrosine hydroxylase, none for phenylethanolamine-N-methyltransferase or gamma-aminobutyric acid. The density of labelled cells in the NTS was similar to that obtained with the non-selective tracers wheat germ agglutinin-horseradish peroxidase (WGA-HRP) and WGA-colloidal gold, but these tracers also labelled other cell groups in the medulla. Furthermore, the decrease in blood pressure, caused by pharmacological activation of neurones in the NTS of rats, or by electrical stimulation of the aortic depressor nerve in rabbits could be blocked by the selective N-methyl-D-aspartate (NMDA) receptor antagonist 2-amino-5-phosphonovalerate injected into the caudal vasodepressor area of the VLM. This area corresponds to the termination of [3H]D-Asp transporting NTS neurones. These results provide evidence that a population of NTS neurones projecting to the VLM use excitatory amino acids as transmitters. Among other possible functions, this pathway may mediate tonic and reflex control of blood pressure via NMDA receptors in the VLM.

Baroreceptor-vasomotor reflex after N-methyl-D-aspartate receptor blockade in rabbit caudal ventrolateral medulla

1. Experiments were performed in anaesthetized rabbits to determine whether blockade of N-methyl-D-aspartate receptors in the caudal ventrolateral medulla oblongata prevents the changes in renal sympathetic vasomotor activity which normally occur in response to increases or decreases in arterial pressure. 2. N-Methyl-D-aspartic acid receptor blockade using bilateral injections of either kynurenic acid (5 nmol) or DL-amino-5-phosphonovaleric acid (5 nmol) caused a rise in arterial pressure with a variable change in renal sympathetic nerve activity. 3. The depressor and renal sympathoinhibitory responses normally seen after intramedullary injection of N-methyl-D-aspartic acid (50 pmol) into the caudal ventrolateral medulla were entirely prevented by prior receptor blockade. The corresponding responses to L-glutamate (10 nmol) were not affected. 4. The depressor and renal sympathoinhibitory responses normally seen with electrical stimulation of the aortic depressor nerve were abolished by the receptor blockade. 5. The reflex inhibition of renal sympathetic nerve activity normally seen in response to increasing arterial pressure using a cuff around the descending aorta was not affected by kynurenic acid. The response was reduced by DL-amino-5-phosphonovaleric acid but a significant degree of inhibition was preserved. The reflex increase in renal sympathetic nerve activity normally seen in response to reducing arterial pressure using a cuff occluder around the inferior vena cava was unchanged after injection of DL-amino-5-phosphonovaleric acid and was increased in magnitude after injection of kynurenic acid. 6. The results indicate that blockade of excitatory amino acid receptors in the caudal ventrolateral medulla of the rabbit abolishes the depressor response evoked by electrical stimulation of the aortic nerve but leaves intact the normal baroreceptor-vasomotor responses elicited by raising or lowering arterial pressure.

Excitatory amino acid receptor-mediated activation of solitarial deglutitive loci

The deglutitive actions of glutamate were investigated in urethane-anaesthetised rats in order to determine whether different excitatory amino acid receptors mediate activation of pattern generator elements contained within the nucleus tractus solitarii. When applied by micropneumophoresis (0.01-10 pmol) from multibarrelled glass micropipettes (tip diameter 2-5 microns), the excitatory amino acid-receptor agonists, N-methyl-D, L-aspartate (NMA), N-methyl-D-aspartate (NMDA), quisqualate and kainate displayed a rank order of potency at glutamate-responsive pharyngeal sites, in the subnuclei ventralis and intermedialis, where KA greater than NMA/NMDA greater than QA; however, the potency followed the order NMA/NMDA greater than KA greater than QA at oesophageal sites within the subnucleus centralis. The NMDA-receptor blockers, 2-amino-5-phosphonovaleric acid (APV) and 2-amino-7-phosphonoheptanoic acid (AP7), selectively and reversibly inhibited the glutamate-evoked oesophageal responses, but had no corresponding effect on rhythmic oesophageal responses elicited by muscarine. At loci in the nucleus tractus solitarius, where glutamate elicited a complete swallowing sequence, APV/AP7 spared the pharyngeal component but selectively blocked the oesophageal component. The nonselective glutamate-receptor antagonist, gamma-D-glutamylglycine suppressed both pharyngeal and oesophageal responses elicited by glutamate. It is concluded that different types of excitatory amino acid receptors are associated with the deglutitive premotor subnuclei of the nucleus tractus solitarii; kainate receptors predominate within the subnuclei ventralis and intermedialis and NMDA receptors within the subnucleus centralis. Both kainate- and NMDA-mediated mechanisms can operate under physiological conditions.

Enflurane-induced burst discharge of hippocampal CA1 neurones is blocked by the NMDA receptor antagonist APV

The basis for the hyperexcitability and seizure activity associated with enflurane anaesthesia was investigated using extracellular and intracellular recording in rat hippocampal brain slices. Enflurane produced seizure-like burst discharges in CA1 pyramidal neurones, accompanied by depressed field potential amplitudes and a reduced threshold for synaptically evoked population spikes. However, threshold for action potentials evoked by intracellular current injection did not change, nor did action potential amplitude, duration or spike frequency accommodation in single neurones. Enflurane 2.0 vol% hyperpolarized CA1 neurones (3.1 (SD 1.3)mV), decreased membrane conductance (12 (6)% below control), and depressed EPSP amplitudes (34% of control) (P less than 0.01). Enflurane appeared to enhance both intrinsic and synaptically mediated inhibitory potentials. The N-methyl-D-aspartate (NMDA) receptor antagonist amino-phosphonovalerate (APV) 5-20 mumol litre-1 completely blocked seizure-like burst discharge of CA1 neurones in the presence of enflurane, and the enflurane-induced reduction of population spike threshold; it did not alter anaesthetic depression of EPSP amplitude. Thus enflurane-induced burst discharge of CA1 neurones appeared to involve an enhancement of excitatory synaptic transmission rather than depression of intrinsic or synaptic inhibition.

Giant synaptic potentials in immature rat CA3 hippocampal neurones

1. Intracellular recordings were made from rat CA3 hippocampal neurones in vitro during the first eighteen days of postnatal life. The cells had resting membrane potentials more negative than -51 mV, action potentials greater than 55 mV and membrane input resistances of 117 +/- 12 M omega. An unusual characteristic of these cells was the presence of spontaneous giant depolarizing potentials (GDPs) which were observed during the first eight postnatal (P) days in over 85% of neurones. They were less frequent between P9 and P12 (48%) and disappeared after P12. 2. The GDPs were synchronously generated by a population of neurones; they reversed polarity at -27 mV when recorded with KCl-containing electrodes and at -51 mV with potassium acetate- or potassium methylsulphate-filled electrodes. 3. The GDPs were blocked by bath application of bicuculline (10 microM) or picrotoxin (100-200 microM). Exogenously applied gamma-aminobutyric acid (GABA; 0.2-1 mM) induced at resting membrane potential a bicuculline-sensitive membrane depolarization which reversed polarity at -25 and -51 mV when recorded with KCl- or potassium methylsulphate-filled electrodes respectively. 4. The GDPs were reduced in frequency or blocked by the N-methyl-D-aspartate (NMDA) receptor antagonists DL-2-amino-7-phosphonoheptanoate (AP-7; 50 microM), D(-)2-amino-5-phosphonovalerate (AP-5, 10-50 microM) and (+-)3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP, 10-50 microM) or NMDA channel blockers phencyclidine (2 microM) and ketamine (20 microM). 5. Stimulation of the hilus during the first week of life evoked a GDP followed by a hyperpolarization. The GDPs were generated by a population of synchronized neurones and reversed polarity at -27 mV with KCl-filled electrodes and at -52 mV with potassium acetate- or potassium methylsulphate-containing electrodes. 6. Bath application of bicuculline (1-10 microM) or picrotoxin (100-200 microM) reversibly blocked the evoked GDPs in the majority of cells. The NMDA receptor antagonists AP-5 (50 microM), AP-7 (50 microM) and CPP (30 microM) usually reduced the amplitude and the duration of the evoked GDPs. In neurones in which evoked GDPs were blocked by bicuculline, a NMDA-mediated component was revealed by increasing the strength or the frequency of stimulation. 7. During the second week of postnatal life, when spontaneous GDPs were extremely rare or absent, superfusion with bicuculline (10 microM) induced, as in adult slices, interictal discharges. These reversed polarity near 0 mV with KCl- or potassium acetate-containing electrodes and were reduced in amplitude and duration by AP-5 (50 microM).(ABSTRACT TRUNCATED AT 400 WORDS)

Pharmacology of calcium-induced long-term potentiation in rat hippocampal slices

A transient increase (10 min) in extracellular calcium concentration (4 mM) causes a long-lasting (greater than 2 hr) enhancement of population spike responses evoked by radiatum fibers to CA1 pyramidal neurons in rat hippocampal slices. This phenomenon is similar to tetanic long-term potentiation (LTP), and is also related to memory processes. The influence of various drugs was investigated on calcium-induced LTP. The NMDA antagonist 2 amino-5-phosphonopentanoic acid (AP5; 100 microM) was able to prevent the calcium-induced LTP, while atropine sulphate (10 microM), propranolol hydrochloride (10 microM) and verapamil hydrochloride (100 microM) were ineffective. The results suggest an involvement of the NMDA receptor in the development of calcium-induced LTP.

Long-term potentiation and N-methyl-D-aspartate receptors in the visual cortex of young rats

1. Long-term potentiation (LTP) of synaptic transmission following tetanic stimulation of the white matter was studied by recording extracellular field potentials and intracellular synaptic potentials from layer II/III of visual cortical slices from young rats ranging in age from 21 to 40 days. 2. Single shocks applied to the white matter at 0.1 Hz, used as test stimuli, elicited field potentials that consisted of primary and secondary components. The removal of Ca2+ ions from the perfusate allowed identification of the secondary component as originating postsynaptically and the primary one as reflecting a mixture of antidromic and postsynaptic potentials. 3. Tetanic stimulation at 5 Hz for 60 s was delivered to the white matter and field potentials were observed for 20 min to 9 h after the tetanus. LTP was defined as being present when the response displayed more than a 20% increase in amplitude of the Ca2+-sensitive components 20 min after the tetanus. LTP was induced in twelve of twenty-three slices tested, and this potentiation lasted throughout the period of observation. The average magnitude of potentiation was 147.8 +/- 28.4% of the control value for the twelve slices. 4. Administration of D,L-2-amino-5-phosphonovalerate (APV), an antagonist selective for N-methyl-D-aspartate (NMDA)-preferring receptors, slightly reduced the amplitudes of Ca2+-sensitive components of the field potentials. The average magnitude of reduction was 80.2 +/- 15.3% of the pre-drug control values. In the presence of APV, LTP was induced in only one slice of twelve tested. 5. Stable intracellular recordings were obtained from twenty-three cells from layer II/III. Excitatory postsynaptic potentials (EPSPs) evoked by white matter stimulation had mean onset and peak latencies of 4.1 and 11.3 ms, respectively. In some cells these fast EPSPs were followed by another slow EPSP with a much longer latency and higher amplitude. Administration of APV revealed further that the fast EPSPs consisted of two components, i.e. early and late components. 6. Tetanization of the white matter induced long-lasting enhancement of EPSPs in eight of twelve cells tested. In five of these eight cells, fast EPSPs were enhanced in amplitude and in the remaining three cells, slow EPSPs appeared de novo after the tetanus. 7. APV reduced the amplitudes of the fast EPSPs and abolished the slow EPSPs if present. The average magnitude of reduction for the fast EPSPs was 65.6 +/- 15.1% and this reduction was due mainly to an elimination of the late component.(ABSTRACT TRUNCATED AT 400 WORDS)

Traumatic neuronal injury in vitro is attenuated by NMDA antagonists

Pure traumatic neuronal injury was modeled in dispersed neocortical cell cultures derived from fetal mice. A plastic stylet was used to tear the neuronal and glial cell layer; medium oxygen content, pH, and glucose remained unchanged. Adjacent to this local disruption, many neurons developed acute swelling and went on to degenerate over the next day, but glia were relatively spared. If the same mechanical insult was delivered in the presence of the N-methyl-D-aspartate (NMDA) antagonists dextrorphan or D-2-amino-5-phosphonovalerate, resultant neuronal degeneration was markedly reduced. The protective effect of these NMDA antagonists was concentration-dependent between 1 and 100 microM, with EC50 near 10 microM for both compounds. Present findings suggest that endogenous excitatory amino acids may participate significantly in the propagation of central neuronal cell loss in response to a purely mechanical insult.

Excitatory amino acids mediate responses elicited in vitro by stimulation of cortical afferents to reticularis thalami neurons of the rat

The effects of the excitatory amino acids on the nucleus reticularis thalami were examined by intracellular recordings from rat thalamic slices. Non-N-methyl-D-aspartate receptor agonists and glutamate induced a membrane depolarization and a reduction in input resistance, while N-methyl-D-aspartate and aspartate induced a prolonged discharge, which in some neurons took the form of a burst firing associated with an apparent increase in membrane input resistance. Both the N-methyl-D-aspartate and the aspartate effects were blocked by D-2-amino-5-phosphonovalerate, while the effects of glutamate, kainate and quisqualate were not. The excitatory postsynaptic potential evoked by corticothalamic fiber stimulation shows two components: an early, short-lasting, 2-amino-5-phosphonovalerate-insensitive portion, and a late, 2-amino-5-phosphonovalerate-sensitive decay phase. It is suggested that glutamate acts in nucleus reticularis thalami cells preferentially on the non-N-methyl-D-aspartate receptors, while aspartate shows an N-methyl-D-aspartate-like effect. The two excitatory amino acids glutamate and aspartate play a determinant role in the modulation of thalamic activity driven by corticothalamic projection.

N-methyl-D-aspartate receptors in the spinal cord mediate pressor responses to stimulation of the rostral ventrolateral medulla in the rat

1. Activation of bulbospinal neurons projecting from the C1 area of the rostral ventrolateral medulla evokes a pressor response. The nature of the neurotransmitters involved in mediating this response at spinal cord level has not been established. 2. Amino acid antagonists were introduced into the spinal subarachnoid space to investigate the role of spinal amino acid receptors in mediating this pressor response in the anaesthetized rat. 3. Intrathecal administration of the amino acid receptor antagonists 2-amino-phosphono valeric acid (2APV), gamma-glutamyl glycine, kynurenate or glutamic acid diethylester (GDEE) attenuated the pressor responses to stimulation of the C1 area. These compounds have been shown to antagonize N-methyl-D-aspartate (NMDA) sensitive receptors. 4. Intrathecal administration of 2APV significantly attenuated the increase in blood pressure produced by injection of NMDA into the spinal subarachnoid space. 5. These results suggest that the pressor response produced by stimulation in the area of the C1 neurons in the rostral ventrolateral medulla of the rat is mediated at least in part by NMDA receptors in the spinal cord.