Characterization of an N-methyl-D-aspartate receptor component of synaptic transmission in rat hippocampal slices.

The involvement of N-methyl-D-aspartate receptors in synaptic transmission from Schaffer collateral-commissural fibres to CA1 neurons has been investigated in rat hippocampal slices. When the perfusion medium was changed from one containing 1 mM Mg2+ to one with no added Mg2+ there was a pronounced increase in the amplitude of the population spike, the appearance of secondary population spikes and in some slices spontaneous epileptiform discharges developed. The secondary and spontaneous population spikes were abolished by the selective N-methyl-D-aspartate antagonist, D-2-amino-5-phosphonovalerate. The effects on the primary population spike depended on the strength of synaptic activation. At low intensities, the N-methyl-D-aspartate antagonist reduced or abolished this response whereas at high intensities the primary population spike was slightly increased in amplitude by this compound. Mg2+ had dose-dependent (20-500 microM) effects on synaptic responses which were identical to those of D-2-amino-5-phosphonovalerate. Increasing the Ca2+ concentration over a range of 1-3 mM also reduced or abolished secondary population spikes and, at low stimulus intensities, the primary population spike. At higher stimulus intensities, however, the primary population spike was insensitive to the Ca2+ concentration over this range. These results demonstrate the major extent to which N-methyl-D-aspartate receptors can contribute to synaptic transmission and epileptiform activity in the CA1 region of the hippocampus. They also show that an important role of Mg2+ in this region is to prevent significant activation of this receptor system during low-frequency synaptic transmission.

Effects of phencyclidine, SKF 10,047 and related psychotomimetic agents on N-methyl-D-aspartate receptor mediated synaptic responses in rat hippocampal slices.

The effects of representative drugs from three classes of psychotomimetic compounds (arylcyclohexylamines, benzomorphan opioids and dioxolanes) have been examined on synaptic transmission at an identified monosynaptic pathway in rat hippocampal slices. The compounds tested were phencyclidine (PCP) and ketamine, the racemate and isomers of SKF 10,047 (N-allylnormetazocine), and the isomers of dioxadrol (dexoxadrol and levoxadrol). In the absence of added magnesium ions (Mg) in the perfusion medium low frequency stimulation of the Schaffer collateral-commissural pathway evoked a burst of population spikes in the CA1 cell body region. The secondary components of this response could be abolished by the selective N-methyl-D-aspartate (NMDA) antagonist D-2-amino-5-phosphonovalerate (APV). PCP (1 microM) or ketamine (10 microM) selectively blocked the secondary components of the synaptic response. The effect of PCP was neither mimicked nor prevented by hexamethonium and atropine, phentolamine and propranolol, or clonidine and was therefore unlikely to involve cholinergic or adrenergic neurotransmitter systems. The sigma opiate, (+/-)-SKF 10,047 (10 microM) also abolished selectively the secondary components of the synaptic response. There was no apparent difference between the potency of the stereoisomers of this compound. The action of (+/-)-SKF 10,047 was not affected by either naloxone or haloperidol, indicating that this effect did not involve opioid receptors or the haloperidol-sensitive sigma site. Dexoxadrol (10 microM), but not levoxadrol (10 microM), also selectively blocked the secondary components of the synaptic response. It is concluded that these psychotomimetic agents can block an NMDA receptor-mediated component of synaptic transmission in the hippocampus and that this effect is mediated by a specific PCP/sigma site.

Increased expression of voltage-activated calcium channels in cultured hippocampal neurons from mouse trisomy 16, a model for Down syndrome.

Calcium is an important second messenger that affects metabolic and physiological activities of developing and mature neurons. It has been reported that electrical activity is abnormal in cultured hippocampal and DRG neurons from the trisomy 16 (Ts16) mouse, a model for Down syndrome (trisomy 21-Ts21 in human). Whole-cell voltage-clamp, radiolabeled ligand binding techniques and mRNA measurements were used to study the effect of Ts16 on voltage-dependent calcium currents in cultured fetal hippocampal neurons from the Ts16 mouse. In neither Ts16 nor control diploid neurons were low-voltage-activated calcium currents detected. However, a high-voltage-activated (HVA) calcium current was identified and shown to be dihydropyridine sensitive. The density of this HVA calcium current was 80% greater in Ts16 neurons than in control. This difference correlated with a 70% increase in binding of radiolabeled dihydropyridine, PN200-110, a marker of L-type calcium channels. However, mRNA levels encoding the alpha1C and alpha1D subunits were unchanged in the Ts16 neurons. In contrast, mRNA level of the myo-inositol transporter, the gene for which is located on mouse chromosome 16, was elevated in Ts16 neurons due to a gene-dosage effect. Therefore, it is likely that posttranscriptional regulation of dihydropyridine-sensitive voltage-dependent calcium channels is abnormal in Ts16. As dihydropyridine sensitive HVA Ca channels are implicated in heterosynaptic long-term depression and long-term potentiation, the differences reported here, if also present in the Down syndrome brain, may contribute to mental retardation in that disorder.

Effects of nerve growth factor on whole-cell currents and other electrical membrane properties in cultured dorsal root ganglion neurons from normal and trisomy 16 mice.

The trisomy 16 mouse is considered to be a model of human trisomy 21 (Down syndrome). Dorsal root ganglia (DRG) from trisomy 16 and diploid control fetuses were cultured in the presence of nerve growth factor (NGF) for 10 days, after which NGF was withdrawn from 50% of the dishes. Withdrawing NGF at 10 days did not affect the survival rate of either trisomy 16 or control neurons. With or without NGF, trisomy 16 neurons had a significantly larger inward current (171%, 163%) and larger inward conductance (156%, 166%), a faster rate of depolarization (219%, 149%), and a shorter duration of the action potential (83%, 81%) than control neurons, indicating that these parameters are determined solely by the trisomic state. In the absence of NGF, the outward conductance was significantly larger (143%), and the rate of repolarization was faster (131%), in trisomy cells compared to controls. Withdrawing NGF resulted in a smaller outward conductance (86%) in control neurons and a larger outward conductance (132%) and faster rate of repolarization (118%) in trisomy neurons, indicating that these parameters are NGF-dependent, and that trisomy and control neurons exhibit a differential sensitivity to NGF. This is the first report of a differential sensitivity of trisomic and control neurons to NGF, and demonstrates significant abnormalities in active electrical membrane properties of trisomic DRG neurons.

Magnesium ions block an N-methyl-D-aspartate receptor-mediated component of synaptic transmission in rat hippocampus.

The involvement of N-methyl-D-aspartate (NMDA) receptors in synaptic transmission in the Schaffer collateral-commissural pathway of rat hippocampal slices has been examined in the presence and absence of Mg2+. Superfusion of slices with Mg2+ -free medium resulted in an increase in the amplitude of the population spike and the appearance of multiple population spikes. An NMDA antagonist, which had no effect on the synaptic response in the presence of Mg2+, completely prevented the appearance of multiple responses. If added to slices already bathed in Mg2+ -free medium, the antagonist eliminated the multiple responses and partly reduced the amplitude of the initial population spike. These data suggest that the extent to which NMDA receptors participate in synaptic transmission in rat hippocampus depends on the extracellular Mg2+ concentration.

MK-801 blocks NMDA receptor-mediated synaptic transmission and long term potentiation in rat hippocampal slices.

The effect of the anticonvulsant and neuroprotective agent (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclo-hepten-5,10-imine maleate (MK-801) has been studied on synaptic events in the CA1 region of rat hippocampal slices. MK-801 blocked selectively the N-methyl-D-aspartate receptor-mediated component of synaptic transmission, which can be recorded in response to single shock stimulation of the Schaffer collateral-commissural pathway in the absence of added Mg2+ to the perfusate. MK-801 also prevented the induction of long term potentiation, which is normally produced in this pathway by high frequency stimulation in the presence of Mg2+.

Low-frequency activation of the NMDA receptor system can prevent the induction of LTP.

In rat hippocampal slices bathed in Mg2(+)-free medium tetanic stimulation generally failed to elicit long-term potentiation (LTP) in the Schaffer collateral-commissural pathway. However, LTP could be induced in the same slices following the re-introduction of 1 mM Mg2+. In Mg2(+)-free medium, LTP could be induced in the presence of 20 microM D-2-amino-5-phosphonovalerate (APV) but was blocked by increasing the APV concentration to 200 microM. The lower concentration of APV is sufficient to prevent N-methyl-D-aspartate (NMDA) receptor activation during low-frequency transmission but not during the tetanus. We suggest therefore, that certain types of activation of the NMDA receptor system can disable the LTP induction mechanism.

Responses to NMDA in cultured hippocampal neurons from trisomy 16 embryonic mice.

The trisomy 16 (Ts16) mouse is regarded as a model of human trisomy 21 (Ts21), or Down syndrome. The ionic current evoked by the glutamate receptor agonist N-methyl-D-aspartate (NMDA) was investigated in cultured hippocampal neurons from embryonic Ts16 and control diploid mice. In both Ts16 and control neurons, NMDA- (6-150 microM) evoked a similar inward current. The reversal potential, the minimum current, the dose response plot of the conductance, the effect of Mg2+ on the current-voltage plot and the inhibition by D-2-amino-5-phosphonovaleric acid (AP5; 50 microM) showed no significant difference between Ts16 and control neurons. These data suggest that, although voltage-dependent ion channels are reported to have altered active properties in Ts16 neurons, NMDA-evoked currents are not altered.

Ketamine blocks an NMDA receptor-mediated component of synaptic transmission in rat hippocampus in a voltage-dependent manner.

We have examined the voltage dependence of the effects of ketamine on synaptic currents in hippocampal CA1 neurons in vitro under conditions where there is a large N-methyl-D-aspartate (NMDA) receptor mediated component of the response. Ketamine reduced inward currents to a greater extent than outward currents of a corresponding size. D-2-Amino-5-phosphonovalerate (APV) substantially reduced the residual outward currents recorded in ketamine, but had only a small effect on the residual inward ones. It is concluded that in this system the action of ketamine in blocking synaptically evoked NMDA receptor-mediated currents shows some voltage dependence.

Intracellular demonstration of an N-methyl-D-aspartate receptor mediated component of synaptic transmission in the rat hippocampus.

Rat hippocampal CA1 pyramidal neurones were monosynaptically activated via stimulation of the Schaffer collateral-commissural pathway. On changing from a 1 mM Mg2+-containing to a Mg2+-free medium there was a pronounced prolongation of the intracellularly recorded excitatory postsynaptic potential. This effect was reversibly abolished by the selective N-methyl-D-aspartate (NMDA) antagonist, D-2-amino-5-phosphonovalerate (APV). We propose that Mg2+ normally prevents expression of NMDA receptor-mediated responses during low-frequency stimulation. During a period of tetanic stimulation, however, cells may depolarize sufficiently to allow a significant NMDA component of the response to be manifest. This could then initiate long-term potentiation.

Frequency-dependent involvement of NMDA receptors in the hippocampus: a novel synaptic mechanism.

Acidic amino acids, such as l-glutamate, are believed to be excitatory neurotransmitters in the mammalian brain and exert effects on several different receptors named after the selective agonists kainate, quisqualate and N-methyl-D-aspartate (NMDA). The first two receptors collectively termed non-NMDA receptors, have been implicated in the mediation of synaptic transmission in many excitatory pathways in the central nervous system (CNS), whereas NMDA receptors, with few exceptions do not appear to be involved; this is typified in the hippocampus where there is a high density of NMDA receptors yet selective NMDA receptor antagonists, such as D-2-amino-5-phosphonovalerate (APV), do not affect synaptic potentials. NMDA receptors have, however, been shown to be involved in long-term potentiation (LTP) in the hippocampus, a form of synaptic plasticity which may be involved in learning and memory. NMDA receptors have also been found to contribute to epileptiform activity in this region. We now describe how NMDA receptors can participate during high-frequency synaptic transmission in the hippocampus, their involvement during low-frequency transmission being greatly suppressed by Mg2+. A frequency dependent alleviation of this blockade provides a novel synaptic mechanism whereby a single neurotransmitter can transmit very different information depending on the temporal nature of the input. This mechanism could account for the involvement of NMDA receptors in the initiation of LPT and their contribution, in part, to epileptic activity.