H. McLennan

The action of six antagonists of the excitatory amino acids on neurones of the rat spinal cord

SummaryThe differential susceptibility to blockade by six antagonists of the excitatory actions produced by a number of acidic amino acids has been determined. Four of these, DL-α-aminoadipate, 2-amino-5-phosphonovalerate, 2-amino-6-phosphonocaproate and γ-D-glutamylglycine block the action of N-methyl-D-aspartate (NMDA) most effectively and those of L-glutamate, trans-1-amino-1,3-dicarboxy-cyclopentane (trans-ADCP) and kainate the least. Cis-2,3-piperidine dicarboxylate resembles the other four, save in that kainate excitations are also powerfully blocked. L-glutamate diethylester was most effective against L-glutamate and least against NMDA: kainate and trans-ADCP excitations were also relatively unaffected.The data support the hypothesis of the existence of at least three classes of amino acid receptor (NMDA-, L-glutamate- and kainate-excited), and the possibility of a fourth category activated by trans-ADCP must be considered.

Pre- and postsynaptic actions of baclofen: blockade of the late synaptically-evoked hyperpolarization of CA1 hippocampal neurones

SummaryUsing intracellular recording techniques, the effects of β-p-chlorophenyl-GABA (baclofen) on passive membrane properties and postsynaptic potentials of CA1 pyramidal neurones were investigated. In experiments where only the hyperpolarizing action of baclofen was precluded by conventional current clamp techniques, 20 μM (±) baclofen blocked the early GABA-mediated IPSP and also a late hyperpolarization which, since it could be evoked by orthodromic stimulation subthreshold for spike firing, would not be expected to be produced by a Ca2+-activated increase in potassium conductance (AHP), but to be a transmitter-mediated event. In addition the conductance increase associated with this late IPSP evoked by subthreshold stimulation and also that associated with the AHP produced by spike activation were abolished. Baclofen also appeared to increase the duration of EPSPs, an event possibly related to loss of IPSPs. The hyperpolarization produced by baclofen was associated with an increased conductance of the resting membrane, an event possibly associated with an elevated potassium flux. To preclude this postsynaptic effect as a cause of reduced synaptic responses, tetraethylammonium chloride (TEA), a compound which decreases conductance and depolarizes the membrane of CA1 pyramidal neurones by a reduction of a ‘leak’ or resting potassium conductance (gK), was added to the bathing medium. A comparison of the effect of TEA on the hyperpolarizations with that of baclofen was undertaken since TEA also interferes with the increased gK evoked by Ca2+ inflow during spike activation. Whereas TEA reduced only an early phase of the postspike hyperpolarization possibly related to the AHP, baclofen abolished the remaining late IPSP. While loss of the AHP or IPSPs individually did not provoke additional spike activity, the abolition of both components promoted extra action potentials in response to synaptic excitation. Baclofen also increased the reduced conductance evoked by TEA towards control levels and caused membrane hyperpolarization. Thus baclofen is considered to evoke its postsynaptic effects through an increased membrane potassium conductance which TEA may also affect to reduce membrane conductance. The resultant uncontrolled hyperpolarization (even in the presence of TEA) occurring in inhibitory interneurones might contribute to the disinhibition recorded in this study.

Effects of kainic and other amino acids on synaptic excitation in rat hippocampal slices: 1. Extracellular analysis

SummaryThe actions of kainic acid on excitatory synaptic responses in rat hippocampal slices have been investigated and compared with the effects of other excitatory amino acids. Kainate administered iontophoretically or via the superfusate produced a large and long lasting potentiation of the population spike evoked in the CA1 region by Schaffer collateral-commissural stimulation. This potentation was associated with a reduction in the field EPSP recorded in the dendritic region (stratum radiatum) but with no change in the presynaptic fibre volley or with any long lasting alteration in the antidromic population spike. The results suggest that one effect of kainate may be to produce dendritic depolarisation in CA1 pyramidal neurones.Kainate in equivalent amounts elicited similar potentiations of the population spike recorded in the dentate gyrus in response to either lateral or medial perforant path stimulation. Smaller amounts of kainate than those required to affect either CA1 or dentate pathways were able to potentiate the mossy fibre-evoked population spike in the CA3 region.Folic acid, which shares kainates ability to produce seizures and distant brain damage when injected into the brain, elicited similar potentiations of synaptic excitation. Higher doses of folate than of kainate were required which is consistent with its weaker epileptogenic actions in vivo. In contrast, N-methyl-aspartate, ibotenate, L-glutamate and L-aspartate were unable to mimic kainates potentiating action. In higher doses the substances depressed the population spike for long periods. These data suggest that potentiation of synaptic events may underlie the ability of kainate (and folate) to elicit seizures and distant brain damage.

Effects of folic and kainic acids on synaptic responses of hippocampal neurones.

The actions of the neurotoxic amino acids folate and kainate have been compared on ortho-and antidromic responses evoked in CA1, CA3 and the dentate gyrus of slices of rat hippocampus maintained in vitro. Both in CA1 and the dentate gyrus superfusion of these acids caused an increase in amplitude of the population spike discharging from an excitatory postsynaptic potential which either remained unaffected or was reduced. In the CA3 region kainate and folate had broadly similar actions to enhance the probability of cell firing to synaptic excitation, and also caused epileptiform discharges to occur spontaneously or in response to electrical stimulation. Spontaneous and evoked population bursts in CA3 did not persist in low calcium/high magnesium medium indicating their dependence on intact synaptic transmission; spontaneously occurring bursts in CA1 were eliminated with the latter treatment or when the axonal connections between it and CA3 were cut. Following folate superfusion the commissural-evoked response in CA3 showed large and variable shifts of the latency which were dependent on the stimulus intensity and its timing after a spontaneous population discharge. Although all of the effects of folate were reproduced by bicuculline, no evidence for a decreased recurrent inhibition in CA1 was obtained although this was observed with kainate. The finding that folate and kainate produced their effects in the absence of a detectable effect on the antidromic population spike suggests a mechanism of action other than neuronal depolarization. The implications of these data for the neurotoxic mechanism(s) and the receptor homologies of folate and kainate are discussed.

The N-methyl-d-aspartate receptor and burst firing of ca1 hippocampal pyramidal neurons

Previous intracellular investigations in the rat hippocampus have demonstrated that N-methyl-D-aspartate, ibotenate and 2,3-pyridine dicarboxylate (quinolinate) all evoke burst firing of CA1 pyramidal neurons, whereas kainate and quisqualate, which are thought to react with different receptors, do not. The purpose of the present study has been to investigate the ability of a series of compounds either to trigger burst firing or to antagonize this pattern of excitation. We report here that N-methyl-L-aspartate, 1,2-benzene dicarboxylate (phthalate) and methylene succinate (itaconate) are also capable of evoking burst firing. The results of this investigation suggest that since both quinolinate and phthalate are rigid planar molecules and only the 2 and 3 positioning of the carboxylates of pyridine was active, a cis configuration of the carboxyls with respect to the 2,3 carbon bond appears to be necessary for excitation. While a nitrogen atom is not necessary for activity (this is absent in phthalate and itaconate) a third functional group, bearing at least a partial positive charge, and in a position alpha to one of the carboxyl groups is required. The requirements for pyridine derivatives to trigger burst firing is similar to that reported as necessary for evoking convulsions and neurotoxicity after intrahippocampal infusion and a correlation between N-methyl-D-aspartate-like burst firing and depolarization and this neuropathology is considered. An important observation has been that the addition of a benzene ring to either quinolinate or phthalate to yield 2,3-quinoline dicarboxylate and 2,3-napthalene dicarboxylate, respectively, converted these excitants into antagonists of burst firing.(ABSTRACT TRUNCATED AT 250 WORDS)

2-amino-5-phosphonovalerate and CO2+ selectively block depolarization and burst firing of rat hippocampal CA1 pyramidal neurones by N-methyl-d-aspartate

Intracellular recordings from pyramidal neurones during microiontophoretic ejection of N-methyl-D-aspartate and quisqualate into the pyramidal cell layer of the CA1 region of the rat hippocampal slice showed that both amino acids caused depolarization and evoked spike activity. Whereas quisqualate evoked tetrodotoxin-sensitive spikes, those produced by N-methyl-D-aspartate consisted of bursts of tetrodotoxin-sensitive action potentials superimposed on an underlying depolarizing shift of membrane potential. Both membrane depolarization and the superimposed depolarizing shifts associated with N-methyl-D-aspartate excitation were selectively and reversibly antagonized by the D(-) isomer of 2-amino-5-phosphonovalerate and Co2+. Both amino acids caused an increase in membrane conductance when small ejection currents were used, and the depolarizing response to these compounds was prevented by current injection. However, only the increase by N-methyl-D-aspartate was blocked by 2-amino-5-phosphonovalerate and Co2+. These results provide evidence to support the suggestion that different mechanisms underlie the excitatory response to N-methyl-D-aspartate and quisqualate in CA1 pyramidal neurones.

The synaptic activation of neurones of the feline ventrolateral thalamic nucleus: Possible cholinergic mechanisms

SummaryThe responses of individual neurones of nucleus ventrolateralis thalami (VL) have been recorded extracellularly following stimulation of the brachium conjunctivum (BC), nucleus entopeduncularis (EN) and precruciate cortex. In anaesthetized cats stimulation of these structures produced either short latency single spike responses or brief bursts of action potentials with somewhat longer latency: the latter responses could be converted to single spikes by the electrophoretic application of acetylcholine or an excitatory amino acid to the neurone. Atropine attenuated the effect of BC stimulation but did not alter excitations from the cortex or EN. Acetylcholine was found to depress the excitation of VL neurones from EN. Collateral fibres of the EN neurones were shown to innervate neurones in the lateral parts of the centrum medianum — parafascicular complex and in VL.It was concluded that VL neurones receive monosynaptic inputs from cortex, EN and the cerebellar nuclei, but that only-the last may have a significant cholinergic component.

An electrophysiological characterization of inhibitions and postsynaptic potentials in rat hippocampal CA3 neurones in vitro

SummaryInhibitory processes in the CA3 region of the rat hippocampal slice were studied extracellularly using paired stimuli and with intracellular impalements of pyramidal neurones. As with mossy fibre (MF) or commissural (COMM) conditioning stimuli (Kehl and McLennan 1983), activation of the perforant path (PP) input caused a long-lasting inhibition of test orthodromic population spikes (PSs) evoked by shocks delivered to the fimbria. That at least a portion of this orthodromically-evoked inhibition reflected postsynaptic events was shown by the reduction both of the amplitude of antidromic PSs and the firing rate of spontaneously active single units. Experiments in which the extracellular concentration of chloride was reduced indicated that only an early component of the inhibition was due to a conductance for that anion. The existence of two inhibitory mechanisms distinguishable extracellularly by their sensitivity to bicuculline and manipulation of extracellular ion concentrations was correlated intracellularly with two hyperpolarising peaks occurring approximately 20 and 150 ms following MF, COMM or PP stimuli. The later hyperpolarisation had an equilibrium potential 20–25 mV more negative than the early IPSP, was unaffected by manipulations of extra- or intracellular concentrations of chloride and was associated with a decrease of membrane resistance suggesting that a potassium conductance was involved in its generation. The fact that it was recorded in the absence of any preceding depolarisation, was blocked by drugs acting presynaptically to cause disinhibition (Kehl and McLennan 1985) and, like the early inhibition, was reversibly reduced by hypoxia suggested that the late inhibition/ hyperpolarisation was a synaptic phenomenon rather than an intrinsic membrane event. Because the late inhibition/IPSP could be shown to have a lower threshold for activation vis-à-vis the chloride-dependent early inhibition, it is possible that two distinct populations of interneurones mediate these two synaptic events.

A pharmacological characterization of chloride- and potassium-dependent inhibitions in the CA3 region of the rat hippocampus in vitro

SummaryPopulation spikes evoked in CA3 pyramidal cells of rat hippocampal slices by stimulation of the fimbria are subject to an early and a late inhibition following activation of the perforant path or the mossy fibres. The early inhibition is known to be GABA-mediated, and is blocked by addition of bicuculline to the superfusing medium; however the late inhibition is bicuculline-insensitive. Both inhibitions are reduced by the addition of (±)-baclofen or noradrenaline to the medium; the early inhibition only is blocked by D-Ala-D-Leu-enkephalinamide while the late inhibition is preferentially reduced by kainate. These data together with the results in the preceding paper suggest that both inhibitions are synaptically mediated, possibly by two distinct types of interneurone, one GABAergic and a second which may release an unidentified transmitter.

Inhibitions of long duration in the cerebral cortex. A quantitative difference between excitatory amino acids

Summary1.Repetitive electrical stimulation of the surface of the cerebral cortex of cats, with current intensities less than those required to produce epileptiform after-discharges, elicited a suppression of discharge of many neurones within the same gyrus which may last many seconds. Such suppression was only rarely preceded by excitation of the cells.2.Such inhibitions were observed in neurones tiring spontaneously, or in those activated by the iontophoretic application of glutamate, homocysteate or acetylcholine.3.No alterations in the pattern of inhibition in individual neurones were observed after the iontophoretic administration of atropine or of strychnine to them; nor did the systemic administration of these drugs affect the production of the inhibition.4.The inhibition produced by a given stimulation was generally considerably longer in neurones activated by glutamate than in the same cells excited by homocysteate. Quantitative examination suggested that this difference was most marked in cells lying in layers II and V of the cortex.