Comparing fluctuations of synaptic responses mediated via AMPA and NMDA receptor channels--implications for synaptic plasticity

Glutamate-releasing synapses are essential in fast neuronal signalling. Plasticity at these synapses is important for learning and memory as well as for the activity-dependent control of neuronal development. We have evaluated the trial-to-trial fluctuations of excitatory postsynaptic currents mediated by glutamate receptors of the AMPA and NMDA types in CA1 pyramidal cells. By using the whole cell patch clamp technique in brain slices from young rats, we have demonstrated that the relative variability of AMPA and NMDA receptor mediated responses, expressed as the coefficient of variation, is similar for these two types of responses [Brain Res. 800 (1998) 253-259]. The present paper summarizes and discusses these results in relation to current theories on hippocampal synaptic plasticity, especially with regard to the ideas of glutamate spillover and silent synapses. Our finding of a correspondence between AMPA and NMDA responses with respect to fluctuations is compatible with our previous finding of equal relative changes of the two during activity induced synaptic plasticity. However, the results argue against the glutamate spillover model according to which the effect of glutamate--and hence the induction of plasticity--may spread unspecifically between synapses. But how can silent synapses become functional if no spread of glutamate occurs and no initial signal is present to trigger the functionalization? Is it necessary that NMDA responses are present at these synapses, which are then silent merely with respect to AMPA receptors, or do other alternatives exist? Our discussion aims to elucidate these questions.

Potentiation and depression following stimulus interruption in young rat hippocampi

We examined the effect of stimulus interruption on dual component field EPSPs in the hippocampal CA1 region. Resuming test stimulation at 0.1 Hz after 10-60 min silent periods led to an increase of the response followed by a decline, involving AMPA and NMDA components to a similar extent. Similar changes were seen when stimulation was initially applied to a naive pathway or the stimulus strength was increased during an experiment. The potentiation of the AMPA response was largely blocked by prior application of the NMDA antagonist AP5 while application of this drug immediately after the initial potentiation prevented the following decline. The results demonstrate that NMDA-dependent potentiation and depression, possibly equivalent to LTP and LTD, can both be induced by the same, very low, test stimulus frequency. Furthermore, the depression appeared to have a longer time window for its induction than the potentiation.

Variability of AMPA and NMDA receptor mediated responses in CA1 pyramidal cells of young rats

The relative variability of excitatory postsynaptic currents (EPSCs) was studied using whole cell recording in CA1 pyramidal cells of hippocampal slices from 2 to 3-week-old rats. EPSCs were evoked by stimulating the Schaffer collateral-commissural pathway and recorded at holding potentials of -75, -30 or +40 mV. The recordings were either isolated alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) or N-methyl-D-aspartate (NMDA) receptor mediated EPSCs, or composite ones. EPSC variability was quantified by coefficient of variation (CV). The inverse variability expressed as 1/CV2 was employed in comparisons. Using early (5-15 ms) and late (40-100 ms) measurements to estimate the AMPA and NMDA components of composite EPSCs showed no difference in the variability of the two components. Comparing isolated AMPA EPSCs at -75 mV with NMDA EPSCs at -30 mV also failed to reveal a difference. However, in accord with previous studies by others, NMDA EPSCs recorded at +40 mV were less variable than AMPA EPSCs at -75 mV, the ratio of 1/CV2 for NMDA vs. AMPA being around 1.7. A comparison between isolated AMPA EPSCs revealed a similar pattern of dependency of CV on membrane potential, the EPSCs at +40 mV being less variable than those at -30 or -75 mV (1/CV2 ratios of 1.5-1.6). In conclusion, our results did not demonstrate any inherent difference in CV between AMPA and NMDA receptor mediated EPSCs and the observed differences in CV could be accounted for by a dependency on membrane potential, the mechanism of which remains to be resolved. The present results have implications for the interpretation of CV changes as observed, for instance, during synaptic plasticity.

Activity-dependent decay of early LTP revealed by dual EPSP recording in hippocampal slices from young rats

The early maintenance of long-term potentiation (LTP) was studied in the CA1 region of hippocampal slices from 12- to 18-day-old rats in a low-magnesium solution (0.1 mM). The alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) and N-methyl-D-aspartate (NMDA) receptor-mediated components of the field excitatory postsynaptic potential were estimated in parallel using early and late measurements of the composite potential. At the normal test stimulus frequency of 0.1 Hz, LTP was seen initially as a predominant increase in the AMPA component, but converted, via a substantial decay of this component and a gradual growth of the NMDA component, into nearly equal changes of the two components. Interrupting the test stimulation for 10 min, changing the test stimulus frequency to 1/60 Hz after LTP induction, or using a test stimulus frequency of 1/60 Hz during the entire experiment significantly reduced the decay of the potentiation of the AMPA component while enhancing the potentiation of the NMDA one. The ratio between the magnitudes of the two excitatory postsynaptic potential (EPSP) components showed a decaying time course that was independent of the manipulations used. Application of the NMDA antagonist D(-)-2-amino-5-phosphonopentanoic acid (50 microM) after LTP induction stabilized the LTP of the AMPA component until washout was started. On the other hand, the phosphatase inhibitor okadaic acid (1 microM) resulted in decay of the potentiation of both EPSP components back to around baseline and altered the time course of the ratio between the components. Our results show that the early maintenance of LTP is controlled in an activity-dependent and NMDA-dependent manner. This process accelerates the decay of LTP of both AMPA and NMDA components in parallel, suggesting that it is similar to homosynaptic long-term depression, although it operates at the normal test stimulus frequency. The data support a scenario in which LTP ensues as a selective AMPA receptor modification and subsequently converts to another modification, possibly a presynaptic one.

The complementary nature of long-term depression and potentiation revealed by dual component excitatory postsynaptic potentials in hippocampal slices from young rats

Homosynaptic long-term depression and long-term potentiation were studied in hippocampal slices from 12-18-day-old rats using field excitatory postsynaptic potentials recorded in the CA1 subfield (stratum radiatum). Independent estimates of the alpha-amino-3-hydroxy-5-methylisoxazolepropionic acid (AMPA) and the N-methyl-D-aspartate receptor-mediated components of the field excitatory postsynaptic potential were obtained in parallel using early and late measurements of a dual component excitatory postsynaptic potential in a solution containing low (0.1 mM) magnesium and 1 microM of the AMPA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). Long-term depression, induced by 2 Hz stimulation for 10 min, was observed as an equal relative depression of the AMPA and N-methyl-D-aspartate receptor-mediated components of the field excitatory postsynaptic potential, whereas long-term potentiation induced by single or repeated high-frequency stimulation, was seen initially as a predominant potentiation of the AMPA receptor-mediated component. Within the first 30-60 min, long-term potentiation gradually changed to more equal increases of the two components of the excitatory postsynaptic potential. During alternating induction of long-term depression and long-term potentiation, the AMPA and N-methyl-D-aspartate receptor-mediated components could both be repeatedly regulated up and down. Long-term depression and long-term potentiation also showed several signs of interaction with each other during such experiments; e.g., long-term depression removed the occlusive effect of large long-term potentiation on a subsequent long-term potentiation, and long-term potentiation applied after the induction of long-term depression was found to be more stable than otherwise. The results support the notion that long-term depression and long-term potentiation employ changes in a common synaptic property. A tentative mechanism for this modification, expressed as equal changes of AMPA and N-methyl-D-aspartate receptor-mediated components of the excitatory postsynaptic potential, is an alteration in transmitter release, while the initial asymmetric part of long-term potentiation indicates involvement of an additional short-term modification.

On the linkage between AMPA and NMDA receptor-mediated EPSPs in homosynaptic long-term depression in the hippocampal CA1 region of young rats

Homosynaptic long-term depression (LTD) was studied in hippocampal slices from 12-18-d-old rats using field EPSP recording in the apical dendritic layer of CA1 pyramidal cells. Independent estimates of the alpha-amino-3-hydroxy-5-methylisoxazolepropionic acid (AMPA) and the N-methyl-D-aspartic acid (NMDA) receptor-mediated components of the field EPSP were obtained in parallel using early and late measurements of a dual-component EPSP in a low-magnesium solution. LTD was induced by low-frequency stimulation (LFS; 2 Hz for 10 min), resulting in equal relative changes of the AMPA and NMDA receptor-mediated components. Under conditions when the AMPA receptor-mediated component was fully blocked, a similarly sized LTD was observed for the pure NMDA receptor-mediated EPSP (measured as initial slope or peak amplitude). Equal changes in AMPA and NMDA receptor-mediated components occurred also upon application of the adenosine agonist N6-cyclohexyladenosine (CHA), known to act by decreasing transmitter release. On the other hand, LTD was found to interact in a multiplicative manner with the presynaptic release changes induced by CHA and by paired-pulse facilitation. The induction of the LTDs of both AMPA and NMDA receptor-mediated EPSPs was blocked by the NMDA receptor antagonist D(-)-2-amino-5-phosphonopentanoic acid and by the phosphatase inhibitor okadaic acid. Partial blockade of LTD by okadaic acid resulted in equal partial blockade of the LTDs of the AMPA and NMDA receptor-mediated components. On the other hand, the L-type calcium channel blocker nifedipine, the metabotropic glutamate receptor antagonist (RS)-alpha-methyl-4-carboxyphenylglycine, the nitric oxide synthase inhibitor N omega-nitro-L-arginine, and the heme oxygenase inhibitor protoporphyrin IX zinc(II) had no effect on LTD of either the AMPA or the NMDA receptor-mediated component. These results of equal changes of AMPA and NMDA receptor-mediated components of the field EPSP in association with LTD, and the consistent parallelism of effects or noneffects on these components by various receptor antagonists and enzyme inhibitors, seem more easily explained by a presynaptic locus for LTD than by a postsynaptic one.

Effect of adenosine-induced changes in presynaptic release probability on long-term potentiation in the hippocampal CA1 region

In the present study some characteristics of long-term potentiation (LTP) in the hippocampal CA1 region were examined under different conditions of transmitter release. Adenosine A1 agonist/antagonists, or in some instances changes in the extracellular calcium/magnesium ratio, were used to alter release probability. The overall LTP time course (onset latency, growth phase, and subsequent decay for both the non-NMDA and NMDA receptor-mediated EPSPs) following a brief tetanus was essentially the same over an almost 10-fold variation in release probability (measured as change in field EPSP magnitude). The major difference observed was a faster initial decay of LTP evoked at low levels of release probability, possibly related to impaired induction conditions. It was also observed that LTP induced at one level of release probability occluded that induced at a lower (or higher) level, and that changes in release probability induced by adenosine agonist/antagonists affected potentiated and "naive" EPSPs to an equal extent. Taken together, these data do not provide support for the notion of different locations for LTP expression at different conditions of release probability. The results are also more compatible with the notion of a single, rather than several, expression mechanism(s) within the first hour of LTP in the hippocampal CA1 region.

Long-term depression in the hippocampal CA1 region is associated with equal changes in AMPA and NMDA receptor-mediated synaptic potentials

In the CA1 hippocampal region low-frequency (1-2 Hz) afferent activation leads to a long-term depression of excitatory synaptic potentials that is induced by calcium influx through postsynaptic N-methyl-D-aspartate receptor channels. In the present experiments using 2- to 3-week-old rats, long-term depressions of field excitatory postsynaptic potentials mediated by amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and by N-methyl-D-aspartate receptor channels were examined in parallel, using a perfusion solution containing low concentrations of an AMPA receptor antagonist and of magnesium (0.1 mM). These experiments revealed that long-term depression was associated with equal relative changes in the two components of the field potential, compatible with a presynaptic location of the expression mechanism for the long-term depression.

[Memory at the synaptic level]

Ever since the discovery of the synapse at the end of the last century it has been surmised that elementary neural changes underlying learning and memory are located at the junctions between nerve cells. Experimental studies during the past 20 years have demonstrated the existence of several synaptic modification processes, the most prominent being long-term potentiation (LTP) in the hippocampus. Several links between LTP and learning/memory have been established. For example, memory impairment in older rats is well correlated, with increasing decline of LTP, and N-methyl-D-aspartate (NMDA) receptor antagonists give rise to a parallel blockade of LTP and of spatial task learning. Studies on rats in a natural learning situation have also demonstrated 'spontaneous' occurrence of LTP. LTP is induced as a consequence of coincident pre- and post-synaptic activity, and thus in conformity with a basic principle of learning theory known as Hebb's rule. Responsible for this associative induction is the NMDA-subtype of glutamate receptor channel with its unique property of being both transmitter and voltage controlled. Its opening allows calcium ions to enter the postsynaptic cell and to initiate biochemical processes leading to a lasting synaptic modification. The nature of the critical processes involved in establishing the modification(s) is uncertain, although the participation of calcium-activated protein kinases seems likely. There is still considerable controversy whether the actual change occurs postsynaptically, or presynaptically, triggered via a retrograde signal from the postsynaptic cell. LTP similar to that in the hippocampus has recently been described for various neocortical regions.

Synaptic potentiation in the hippocampal CA1 region induced by application of N-methyl-D-aspartate

The effect of local pressure application of N-methyl-D-aspartate (NMDA) in the synaptic layer of CA1 pyramidal cells was investigated in the guinea pig hippocampal slice preparation using extracellular recording technique. Application of NMDA produced a transient depression and a subsequent 30-60 min potentiation of the field excitatory postsynaptic potential (EPSP) seen as an increase of the initial slope and amplitude of the EPSP. The increase in amplitude was consistently greater than that of the initial slope. Prior tetanization that caused saturation of long-term potentiation prevented the generation of an NMDA-induced potentiation of the initial slope for more than 1-2 h, but not the generation of an increase of the amplitude.

Long-lasting potentiations evoked by a brief heterosynaptic tetanus in the guinea pig dentate gyrus in vitro

The heterosynaptic effects induced by a brief afferent tetanization in the molecular layer of the dentate gyrus were investigated in the guinea pig hippocampal slice preparation using extracellular recording technique. At a brief interval (5 ms) between a single stimulation of the test afferents and the tetanus evoked in the conditioning afferents, a long-lasting (greater than 1 h) potentiation of the test field excitatory postsynaptic potential (EPSP) initial slope and amplitude was observed. This potentiation was occluded by prior homosynaptic tetanization of the test afferents, suggesting that it represents long-term potentiation (LTP). Thus, in the dentate gyrus, a single activation of a single test EPSP suffices to induce LTP when coinciding in time with a brief tetanus to other afferents. When not temporally paired with the test stimulation, i.e. at longer test-conditioning intervals (greater than 50 ms), the conditioning tetanus also elicited a long-lasting potentiation of the test field EPSP. This potentiation was, however, seen as a prolongation of the rising phase with no change in the field EPSP initial slope, and may represent a potentiation distinct from LTP.

Decreased nerve conduction velocity in optic nerve following early post-natal low-dose lead exposure

A study was made on nerve conduction velocity of the optic nerve in rats subjected to lead exposure during the first 2 weeks of post-natal life. The rats were given intraperitoneal injections with a calculated daily exposure of 7.6 micrograms (low-dose) or 15.8 micrograms (high-dose) lead g-1 body weight. Growth retardation at 30 days was seen only with the higher dose. Littermates of low-dose exposed rat were injected with vehicle only and served as controls. Lead concentrations in blood and brain were measured in rats of 20 days of age in order to ascertain that exposure was adequate in the present litters. Nerve conduction velocity of the optic nerve was examined in 14 rats of 30 days of age taken from 10 different litters. The optic nerve(s) was prepared in anaesthetized rats and placed in a flow-through incubation chamber. One stimulating and two recording tungsten electrodes were used. In all rats, three positive-negative waves, regarded as representing three functional groups of optic nerve axons, could be recorded. The last one often had a long duration and a small amplitude without distinct peaks and hence was omitted from further analysis. The mean conduction velocities for the two faster axonal groups were 16.8 and 5.4 m s-1 in control rats, 10.3 and 5.8 m s-1 in low-dose rats and 9.4 and 5.2 m s-1 in high-dose rats. The difference in conduction velocity for the fastest axons was significant for controls versus low-dose and high-dose but not for low-dose versus high-dose.(ABSTRACT TRUNCATED AT 250 WORDS)

Long-term potentiation in the hippocampal CA1 region: its induction and early temporal development

Long-term potentiation (LTP) is a process that due to its prolonged time course and associative nature of induction is believed to be involved in learning and memory in the mammalian brain. In this chapter the experimental evidence for the view that LTP is initiated by an influx of calcium ions through synaptically controlled N-methyl-D-aspartate (NMDA) receptor channels is discussed. It will also be described how LTP develops following its induction. It will be shown that there is a considerable delay, about 2-3 s, between a tetanus and the initiation of LTP, and that additional 20-30 s are needed for the potentiation to reach peak levels. The potentiation subsequently decays to a degree which depends primarily on tetanus length. It will be argued that this early phase of tetanus-induced LTP is of the same nature as that present a few hours later.

Onset Characteristics of Long-Term Potentiation in the Guinea-Pig Hippocampal CA1 Region in Vitro

The temporal development of long-term potentiation (LTP) was examined in the CA1 region of the hippocampal slice preparation (bath temperature 30 degrees C). LTP was evoked by a single brief afferent tetanus (3 - 40 impulses at 50 Hz) given in the presence of picrotoxin (to facilitate LTP induction). Short-lasting potentiation processes unrelated to LTP were excluded by comparing the potentiation obtained in picrotoxin solution with that obtained in normal solution or in the presence of the N-methyl-D-aspartate receptor antagonist 2-amino-5-phosphonovalerate. LTP was also evoked by pairing single test volleys with brief (2 - 3 impulses) heterosynaptic tetani in picrotoxin solution. Both methods showed no significant rise of LTP until about 3 s after the induction event. LTP thereafter developed almost linearly towards a peak within 20 - 25 s after the tetanus, the time course being practially independent of the induction method and of the relative amount of LTP evoked. The latency and rise time of LTP depended on bath temperature, being about twice as long at 25 degrees C as at 30 degrees C. Following the peak, LTP rapidly decayed to less than half its peak value in 8 min, the decay tending to be less with longer trains. The LTP component reaching its peak 20 - 25 s after a tetanus was practically occluded after a saturating homosynaptic tetanization, and was only partially recovered 1 h afterwards. The latency to the onset of LTP suggests an indirect coupling between the calcium influx, presumed to trigger the potentiation, and the expression of LTP. The independence of the early time course with respect to the induction strength indicates that the intervening system(s) operates in a linear manner.

Maintained changes in motoneuronal excitability by short-lasting synaptic inputs in the decerebrate cat

1. During investigation of the tonic stretch reflex in the unanaesthetized decerebrate cat we observed that a short train of impulses in Ia afferents from the soleus muscle (or its synergists) may cause a prolonged activity in the soleus muscle as judged by EMG and tension recordings. This excitability increase, which outlasted the stimulus train, could stay virtually constant during long periods (even minutes), but could be terminated at any time by a train of impulses in, for example, the peroneal nerve. 2. Gradation of the strength of stimulation and the duration of the train of impulses show that the amount of maintained excitability increase depends-within some limits-on the total amount of Ia impulses. 3. In paralysed preparations a short train of impulses in Ia afferents from any part of the triceps surae, caused a maintained increase of the efferent activity in the nerves to triceps surae and a maintained increase of the triceps surae monosynaptic test reflex. These experiments demonstrate the existence of a central mechanism (in the spinal cord and/or the brain stem), which is responsible for the maintained excitability increase seen in motoneurones to the homonymous and synergic muscles. 4. In acute spinal preparations it was not possible to demonstrate any long-lasting excitability increase by a train of Ia impulses. Following intravenous administration of the serotonin precursor 5-hydroxytryptophan, mimicking the tonic activity of these pathways in the decerebrate state, it was again possible to elicit the long-lasting excitability increase by a train of impulses in Ia afferents. A subsequent I.V. injection of methysergide (a serotonin receptor blocker) abolished the long-lasting excitability increase. This set of experiments demonstrates that the basic mechanism responsible for the maintained excitability increase is located at segmental level, and involves serotonergic systems. 5. It was demonstrated that activation of several ipsilateral and crossed reflex pathways by trains of impulses in cutaneous or high-threshold muscle afferents could trigger a maintained excitability increase of those motoneurone pools which were activated by the stimulation. Trains of stimuli to facilitatory regions in the brain stem could also cause a long-lasting excitability increase of motoneurones. Furthermore, activation of all reflex pathways which mediate postsynaptic inhibition to a motor nucleus (including recurrent inhibition via Renshaw cells) could terminate the prolonged excitability increase of that particular motor nucleus.(ABSTRACT TRUNCATED AT 400 WORDS)

Distribution of recurrent inhibition within a motor nucleus. I. Contribution from slow and fast motor units to the excitation of Renshaw cells

The relation between the size of a monosynaptic reflex (MSR) to triceps surae and the resulting Renshaw cell discharge was used to evaluate the contribution from slow and fast motor units to the excitation of Renshaw cells. It is, however, difficult to interpret these results in terms of excitation contributed by slow and fast motor units because of the following reasons. First, the size of the MSR recorded in ventral roots is not linearly related to the number of recruited motor units, since larger motor axons contribute more to the size of the MSR than smaller ones. Second, the number of spikes evoked in a Renshaw cell burst is not linearly related to the excitatory input because Renshaw cell discharge saturates in the case of large responses. The contribution of small, early-recruited motoneurones to Renshaw cell excitation is consequently overestimated. Procedures were introduced to deal with these problems. It is concluded that the last-recruited motor units (probably 'fast twitch, fast fatiguing') on average contribute four times as much excitation to Renshaw cells as the first recruited ('slow twitch') motor units.

Facilitation of hippocampal long-term potentiation in slices perfused with high concentrations of calcium

The effect of increased extracellular calcium on long-term potentiation (LTP) of synaptic transmission has been examined in the CA1 region of guinea pig hippocampal slice preparation using extracellular recordings from the dendritic layer. The application of high calcium (4 mM) led to an increase in the initial slope of the field potential that reversed following return to control (2 mM calcium) solution. The magnitude of the field potential change was unaffected by prior induction of LTP, and inputs tetanized after return to control solution showed the same amount of LTP as those tetanized before the high calcium application. These results suggest that the calcium application by itself did not induce LTP. Inputs tetanized in the high calcium solution showed a greater amount of potentiation than in control solution, any given train producing about twice as much potentiation. However, using long trains (40 impulses) at high strength (2 x test strength) gave similar LTP values in the two solutions. The facilitatory effect of high calcium on LTP was completely blocked by raising extracellular magnesium from 2 to 4 mM. As in control solution. LTP evoked in the high calcium solution was blocked by 2-amino-5-phosphono-valerate. The results support the view that calcium influx through postsynaptic N-methyl-D-aspartate receptor channels is directly involved in the induction of LTP.

Phorbol ester-induced synaptic potentiation differs from long-term potentiation in the guinea pig hippocampus in vitro

The relationship between the synaptic potentiations evoked by the protein kinase C activator phorbol-12,13-diacetate and by afferent tetanization has been examined in the CA1 region of the hippocampal slice preparation using extracellular recording. It has been found that the potentiation of the field excitatory postsynaptic potential produced by 1 microM phorbol ester does not affect the amount of long-term potentiation (LTP) that can be evoked by afferent tetanization, and vice versa. A dissociation between phorbol ester-induced and tetanus-induced potentiation is also indicated by the fact that only the former was associated with changes in paired-pulse facilitation. On the other hand, as previously described, higher concentrations (10 microM) of phorbol ester blocked the tetanus-induced potentiation. Since the total potentiation given by 10 microM phorbol ester and tetanization depended on the order of presentation of the potentiation-inducing stimuli, it appears that the blockade of LTP is, at least partly, independent of the phorbol ester-induced potentiation.

Long-term potentiation involves enhanced synaptic excitation relative to synaptic inhibition in guinea-pig hippocampus

1. Tetanization of hippocampal pyramidal cell afferents travelling in stratum radiatum of area CA1 induces both long-term potentiation (l.t.p.) of extracellularly recorded excitatory postsynaptic potentials (e.p.s.p.s), and an increase in the number of cells firing, as measured by the extracellular population spike, for a given sized field e.p.s.p. The mechanism of this latter change, known as e.p.s.p.-spike (E-S) potentiation, was investigated in the guinea-pig hippocampal slice preparation. 2. Plots of the E-S relation before and after tetanization were constructed from measures taken over a series of stimulus strengths. Tetanization of afferents in stratum radiatum decreased the spike threshold by 24%, while the gamma-aminobutyric acid antagonist picrotoxin (PTX) decreased spike threshold by 72%. Sequential administration of PTX and tetanization, in either order, resulted in no more change in the E-S threshold than did PTX application alone. 3. Extracellular synaptic potentials, matched for initial slope before and after tetanization by adjusting the stimulus strength, showed an increased peak amplitude and increased peak latency following tetanization. PTX produced similar but larger percentage changes. Tetanization in the presence of PTX, however, did not alter the field potential wave shape. 4. Intracellular postsynaptic potentials (p.s.p.s) were also matched for initial slope before and after tetanization. Tetanization induced p.s.p. shape changes similar to those observed extracellularly, i.e. in the direction of less inhibition. Such changes did not occur in the presence of PTX. 5. Inhibitory p.s.p.s (i.p.s.p.s) were studied in depolarized pyramidal cells with microelectrodes filled with QX-314. Tetanization of afferents in stratum radiatum produced i.p.s.p. increases in eight of nineteen cells. These increases were generally attributable to an increased activity in the recurrent inhibitory pathway. Tetanization of the alveus failed to produce any lasting increases in the i.p.s.p. amplitude. 6. Tetanization of afferents in stratum radiatum decreased the ratio of the intracellular i.p.s.p. to field e.p.s.p. over stimulus strengths below population spike threshold. Above population spike threshold, the ratio tended towards its pretetanization level. 7. The results indicate that E-S potentiation results from an increase in the level of depolarization reached by a synaptic potential of given initial slope. These findings support the hypothesis that tetanization induces greater l.t.p. of excitatory inputs onto pyramidal cells than of inputs onto feed-forward inhibitory interneurones.

Facilitated induction of hippocampal long-term potentiation in slices perfused with low concentrations of magnesium

The generation of long-term potentiation of synaptic transmission in area CA1 of hippocampal slices of the guinea-pig has been examined in solutions containing low concentrations of magnesium ions. It was found that the induction of long-term potentiation is greatly facilitated in slices perfused with 0.1 mM magnesium but much less so with 0.5 mM magnesium solution. The long-term potentiation evoked by brief tetanization in 0.1 mM magnesium was prevented following application of the N-methyl-D-aspartate receptor antagonist 2-amino-5-phosphonovalerate. Moreover, the response to tetanization, recorded in the dendritic layer, contained a much greater than normal component blocked by 2-amino-5-phosphonovalerate. The latter represents current through postsynaptic N-methyl-D-aspartate receptor channels, suggesting that the facilitation of long-term potentiation is related to a facilitated opening of these channels. The results support the notion that the generation of long-term potentiation is related to current through N-methyl-D-aspartate receptor channels which is under the control of extracellular magnesium ions.

Long-term potentiation in the hippocampus using depolarizing current pulses as the conditioning stimulus to single volley synaptic potentials

The conditions responsible for the associative properties of long-term potentiation (LTP) were examined in the CA1 region of the hippocampal slice preparation. Intracellularly recorded EPSPs resulting from single-volley stimulation at low frequency (0.15-0.1 Hz) in the stratum radiatum or oriens were paired with depolarizing current pulses (50-100 msec) injected through the recording microelectrode. It is shown that these EPSPs, when paired with pulses of sufficient magnitude, become potentiated. This potentiation generally reached a peak after 20-30 pairing events and could outlast the conditioning period by more than 1 hr. It was specific to the paired input, was blocked by 2-amino-5-phosphonovalerate (APV) and was largely blocked by prior homosynaptic tetanization (and vice versa). In experiments performed with picrotoxin (PTX) in the bath, EPSPs were potentiated using 2-4 nA current pulses, with somewhat higher values in normal solution. The effective current pulses, in both normal and PTX solution, produced a repetitive spike discharge of 7-11 spikes (per 100 msec), and within this range, higher frequencies were associated with larger potentiations. However, since similar degrees of EPSP potentiation were observed following blockade of spike activity by intracellular QX-314, spike activity was not the primary conditioning factor. For the potentiation to appear, the EPSP had to occur together with the current pulse or precede it by less than about 100 msec. No potentiation was observed when the EPSP immediately succeeded the pulse. The results suggest that the cooperativity aspect of LTP is related to a need for sufficient postsynaptic depolarization.(ABSTRACT TRUNCATED AT 250 WORDS)

Single high strength afferent volleys can produce long-term potentiation in the hippocampus in vitro

Although hippocampal long-term potentiation (LTP) is normally elicited by tetanization of an afferent input, it may also be induced by pairing afferent volleys with strong depolarizing conditioning stimuli. In extracellular recordings made from area CA1 of hippocampal slices bathed with a picrotoxin-containing solution, long-lasting potentiation was produced by high strength single volleys alone. Potentiation occurred with intervals between high strength stimuli as great as one per minute. Tetanization-induced LTP was no greater than tetanization-induced LTP plus high strength single volley potentiation, given in either order. These data suggest that single afferent volleys can induce LTP under conditions of reduced inhibition.

The effect of axotomy on posttetanic potentiation of group Ia synapses in the cat

Posttetanic potentiation (PTP) of composite Ia excitatory postsynaptic potentials (EPSPs) has been studied in normal cat alpha-motoneurons and in motoneurons axotomized 2-3 wk earlier by ventral root section. The maximal amount of PTP of EPSP amplitude (expressed relative to unpotentiated amplitude) was considerably less in the axotomized population compared with the normal population. The decrease in PTP provoked by axotomy occurs in association with a postaxotomy increase of input resistance, the net effect being that PTP in axotomized cells was much the same as that observed by others in normal motoneurons possessing similarly high input resistance. In agreement with previous results, EPSP peak amplitudes were decreased after axotomy. This decrease seemed to be largely related to an absence of the largest EPSPs, since otherwise the EPSP distributions of normal and axotomized motoneurons showed considerable overlap. It is suggested that the observed decrease in PTP after axotomy is related to a change in synaptic release properties and not secondary to changes in the electrical properties of motoneurons. A previous analysis has suggested that axotomy causes an alteration of the distribution of passive electrical properties among motoneurons such that axotomized cells resemble normal high-resistance motoneurons. The present results suggest that axotomy may affect the distribution of Ia synaptic release properties in a similar manner, since PTP in axotomized motoneurons resembles that observed in normal high-resistance motoneurons.

Hippocampal long-lasting potentiation produced by pairing single volleys and brief conditioning tetani evoked in separate afferents

Cooperative effects between afferents on the induction of long-lasting potentiation (LLP) of synaptic transmission were examined in the CA1 region of the hippocampal slice preparation. Synaptic activity was recorded extracellularly in the dendritic layer (stratum radiatum) as a field EPSP, and the amount of LLP produced was measured from the change in slope of the rising phase of this potential. Experiments were performed with the GABA antagonist picrotoxin in the bath solution in order to facilitate the induction of LLP. It is shown that under these conditions a test input evoked by single volleys (at 0.2 Hz) is potentiated when paired with brief tetani (2-15 impulses at 50 Hz) to a separate population of fibers in the stratum radiatum. For this potentiation to appear, the test input had to occur during the train or precede it by less than 40 msec. Maximal effects were observed with the test volley positioned in the early part of the tetanus, and were largely independent of train duration. The potentiation obtained in this manner reached a peak level after some 20 conjunction events, and its magnitude measured 10 min after conjunction was about half that which could be induced by homosynaptic tetanization. Prior homosynaptic potentiation occluded the potentiation induced by conjunction, suggesting an identity of their underlying mechanisms. A test input to the apical dendritic layer was potentiated also by inputs to the basal dendritic layer, although greater effects were observed with both inputs in the same dendritic layer. It is suggested that the conditioning effect is related to the postsynaptic depolarization created by the tetanus.(ABSTRACT TRUNCATED AT 250 WORDS)

Mode of action of excitatory amino acid receptor antagonists on hippocampal long-lasting potentiation

The effects of the N-methyl-D-aspartate receptor antagonists 2-amino-5-phosphonovalerate and gamma-D-glutamylglycine on the induction of long-lasting potentiation in the CAl and dentate areas of the hippocampal slice preparation have been examined. Synaptic activity was recorded extracellularly in the dendritic layer as a field excitatory postsynaptic potential, and the amount of long-lasting potentiation produced was measured from the change in slope of the rising phase of this potential. Experiments were generally performed with the gamma-aminobutyric acid antagonist picrotoxin in the solution. It is shown that 2-amino-5-phosphonovalerate prevents the induction of long-lasting potentiation following afferent tetanization of an input, without any effect on other inputs projecting to the same postsynaptic neurons. This result makes it unlikely that the preventive action of 2-amino-5-phosphonovalerate is related to any unspecific depressive action. Instead, 2-amino-5-phosphonovalerate was observed to block a postsynaptic depolarizing process appearing during the tetanus, likely related to current through synaptically activated N-methyl-D-aspartate receptor channels. It is suggested that 2-amino-5-phosphonovalerate prevents the induction of long-lasting potentiation by blockade of these currents through its antagonistic action on the N-methyl-D-aspartate receptors. Application of gamma-D-glutamylglycine similarly prevented the induction of long-lasting potentiation. No potentiation appeared following wash-out of the drug. The results exclude the possibility that the preventive action of this drug is related to a mere masking action on long-lasting potentiation induced in presynaptic terminals. It is suggested that gamma-D-glutamylglycine blocks the induction of long-lasting potentiation by its antagonistic action on the N-methyl-D-aspartate receptors, i.e. in a manner similar to that of 2-amino-5-phosphonovalerate.

Functional development of the visual system in normal and protein deprived rats. III: Recordings from adult optic nerve in vitro

A persistent increase in the latencies of the visual evoked response recorded from the cortical surface of protein deprived adult rats was described recently (Sjöström et al. 1984). The morphological correlate to this alteration is unknown. Previous studies on malnourished rats have shown a reduction of axonal diameters and in the number of myelin lamellae in relation to axonal circumference, and hence the possibility of a decrease in fibre conduction velocity must be considered. In parallel study, we have established that changes in diameters and myelination of optic nerve fibres similar to those previously reported in malnourished rats are present in adult protein deprived (PD) rats (Conradi et al. 1985). In the present paper, recordings of optic nerves in vitro from adult normal (C) and protein deprived (PD) rats are described. The compound action potentials were very similar in the two groups. Three positive peaks were easily defined, probably corresponding to three functional groups of optic nerve fibers. No significant differences in amplitudes or conduction velocities for the three peaks were found between the C and PD rats. It is concluded that the increased latencies of the evoked response are not caused by a decrease in conduction velocity.

Facilitation of hippocampal long-lasting potentiation by GABA antagonists

Long-lasting potentiation (LLP) of synaptic transmission in the CAI region of the hippocampal slice preparation has been examined. The effects of reduced postsynaptic inhibition given by application of gamma-aminobutyric acid (GABA) antagonists (mainly picrotoxin) on the generation of LLP were investigated. It was first demonstrated that picrotoxin had little effect on excitatory synaptic transmission itself as judged by the rising phase of the field EPSP. Moreover, there were largely no actions on short-lasting synaptic effects such as paired pulse facilitation and frequency potentiation. On the other hand, following drug application, much fewer afferent volleys were needed to generate a given amount of LLP. Long-lasting potentiation could be produced by trains containing as few as 2-5 impulses, trains that normally give rise to only short-lasting effects. There was no apparent difference in the maximal amount of LLP that could be produced for a given input, suggesting that the GABA antagonists do not operate by enhancing the capacity for LLP production but by facilitating its induction. As in normal solution, the LLP in the presence of the drugs was confined to the tetanized pathway. Tetanization in the treated slices was associated with enhanced somatic firing as well as an increase of the negative extracellular potential recorded in the dendritic layer. It is proposed that part of this increased negativity represents current through synaptically opened N-methyl-D-aspartate (NMDA) receptor channels. Furthermore, it is suggested that the facilitated induction of LLP in the presence of GABA antagonists is related to a facilitated activation of these NMDA receptor channels which is secondary to the higher levels of dendritic depolarization attained during tetanization under conditions of reduced postsynaptic inhibition.

A possible correlate of the postsynaptic condition for long-lasting potentiation in the guinea pig hippocampus in vitro

Picrotoxin and 2-amino-5-phosphonovalerate (APV) have previously been reported to influence the production of long-lasting potentiation in the hippocampus (facilitation and depression, respectively). We have examined how these drugs modify the postsynaptic responses to tetanic afferent activation used to elicit long-lasting potentiation. The experiments were performed in the CA1 area of transverse hippocampal slices maintained in vitro. A slow extracellular dendritic negativity-interpreted as a sign of a dendritic depolarizing process-was enhanced by picrotoxin and depressed by APV. This dendritic potential may represent a postsynaptic event involved in the production of long-lasting potentiation.

Hyperpolarization following long-lasting tetanic activation of hippocampal pyramidal cells

Long-lasting spike activation of CA3 hippocampal pyramidal neurons is shown to cause the development of a large and long-lasting (greater than 50 s) membrane hyperpolarization (PTH). Under normal conditions this PTH is mainly given by a relatively potential-independent process, presumably an electrogenic sodium pump. Following reduction in pump activity (cooling, ouabain), the PTH remains but is mainly produced by a conductance process, presumably a K conductance increase resulting from a sodium-induced calcium release from intracellular stores.

Large long-lasting potentiation in the dentate gyrus in vitro during blockade of inhibition

The generation of long-lasting synaptic potentiation was studied in the dentate gyrus of hippocampal slices before and after blockade of inhibition by application of GABA antagonists. Field potential analysis showed that inhibition in the dentate gyrus is to a large extent dendritically located. During blockade of inhibition, induction of long-lasting potentiation was facilitated, together with a large increase in the maximal potentiation that could be produced. It is suggested that the enhanced production of long-lasting potentiation in inhibition-free slices is related to an increased postsynaptic depolarization in granule cell dendrites.

Long-term synaptic enhancement in hippocampus is not regulated by postsynaptic membrane potential

The contributions of membrane potential and postsynaptic discharge during afferent fibre tetanization to the generation of long-lasting synaptic enhancement was studied by intra- and extracellular recording in the CAl region of hippocampal slices. Neither parameter affected the magnitude of intracellular synaptic enhancement. Thus, if postsynaptic integration is involved in the control of the enhancement process, the integration must be chemically rather than electrically mediated.

Increased excitability of hippocampal unmyelinated fibres following conditioning stimulation

Unmyelinated fibres in the stratum radiatum (area CA1) were electrically stimulated in hippocampal slices from guinea pigs. The size of the evoked fibre volley was increased by a preceding conditioning stimulation to the same fibres, provided that the strength of the conditioning stimulus was larger than that of the test pulse. This facilitation was maximal for 20-30 ms interstimulus intervals and had a duration of about 200 ms. The results are compatible with an increased excitability (reduction in threshold) specific for fibres activated by the conditioning stimulation.

Shape of frequency-current curves in CAI pyramidal cells in the hippocampus

Frequency-current curves for CA1 hippocampal pyramidal neurons are shown to have basically the same shape as those for spinal motoneurones, with a region of shallow slope at low frequencies, preceding a sleep linear or upward convex region at higher frequencies. The frequency range is, however, displaced towards lower frequencies. The results suggest, in a qualitative sense, that the firing behaviour of CA1 pyramidal cells is regulated by the afterhyperpolarization, at least in the low frequency range.

Two types of synaptic facilitation recorded in pyramidal cells of in vitro hippocampal slices from guinea pigs

PyramidaL cells in CA1 hippocampus show two types of increase in the excitatory postsynaptic potential (EPSP) during paired pulse stimulation of the afferent fibres in stratum radiatum. For 15-300 ms interstimulus intervals, both the initial slope and the peak amplitude of the EPSP are increased, which is mainly due to an increased excitatory synaptic current. For 300 ms-2 s intervals, the predominant change is an increase in the peak amplitude. The latter type of facilitation is caused by a decrease in a superimposed inhibitory postsynaptic potential (IPSP).

Strontium supports synaptic transmission and long-lasting potentiation in the hippocampus

(1) Synaptic transmission was studied in isolated transverse hippocampal slices from guinea pigs. Extracellular evoked potentials were recorded in the region CA1. (2) Changing the normal perfusion solution (containing 2 mM Ca2+) to calcium-free Ringer abolished synaptic transmission which was again restored by adding strontium. A synaptic efficacy of 25--50% ofn normal was obtained for 10 mM Sr2+. (3) Two different synaptic inputs to CA1 pyramidal cells were tested with respect to their ability to produce long-lasting synaptic potentiation after tetanization in strontium Ringer. Following a brief tetanus the field EPSP and, especially, the population spike were greatly enhanced. (4) The potentiation so produced was similar to the long-lasting potentiation seen in the normal slice, because it (i) had a very long duration (hours), (ii) was specific for the tetanized pathway, (iii) showed potentiation of both 'volley-EPSP' and 'EPSP-spike' relations, and (iv) was accompanied by short-lasting (less than 5 min) generalized depression.

Possible mechanisms for long-lasting potentiation of synaptic transmission in hippocampal slices from guinea-pigs

1. Long-lasting potentiation of synaptic transmission was studied in the CA1 region of guinea-pig hippocampal slices maintained in vitro. 2. Stimulating pulses were delivered alternately to two independent afferent pathways, stratum radiatum and stratum oriens. The presynaptic volleys and field e.p.s.p.s. were recorded from the same two layers, while an electrode in the pyramidal cell body layer recorded the population spike or in other experiments the extra- or intracellular potentials from a single pyramidal cell. 3. A short tetanus to either of the two input pathways produced a long-lasting enhancement of the field e.p.s.p. as well as an increased size and a reduced latency of the population spike. This long-lasting potentiation was observed for up to 110 min after tetanization. Extracellular unit recordings showed that this potentiation is accompanied by an increased probability of firing and a reduced firing latency. Intracellular recordings showed an increased e.p.s.p., through the increase was smaller and less regular than for the extracellular field e.p.s.p. 4. No corresponding changes were seen in the field potential responses to stimulation of the untetanized input path, or in the intracellularly measured soma membrane potential, resistance, or excitability. The latter two properties were measured by intracellular injection of current pulses. It is concluded that long-lasting potentiation is specific to the pathway which has received the tetanization. 5. Following tetanization there was also a short-lasting (usually 2-4 min) depression, most often seen for the control pathway but sometimes visible on the tetanized side as well, superimposed on the potentiation. It is concluded that the short-lasting depression is not confined to any particular pathway but is a generalized (unspecific) phenomenon.

Recurrent inhibition and afterhyperpolarization following motoneuronal discharge in the cat

1. The relation between the size of a monosynaptic reflex (varying from the smallest values to the maximal motor response) and the output from Renshaw cells was investigated. This relation was extremely variable from one Renshaw cell to another. However, a linear relation between the reflex size and the early discharge emerged when the responses of all the Renshaw cells were averaged or when the summed activity of a pool of Renshaw cells was estimated by recording the recurrent inhibition in their target motoneurones. It was concluded that the lowest threshold motoneurones were efficient in producing recurrent inhibition. 2. In motoneurones, recorded intracellularly, the size of the depression caused by the afterhyperpolarization was compared to the maximum autogenetic recurrent inhibition. Under the particular experimental conditions used to mimic human experiments (Hultborn & Pierrot-Deseilligny, 1979a), it was found that recurrent inhibition had the same order of magnitude as the depression caused by afterhyperpolarization. 3. The additional depression caused by the summation of afterhyperpolarizations of two consecutive spikes was measured. It was shown that a summation of importance equal to the maximum autogenic recurrent inhibition required a mean interspike interval of 25 msec.

On the function of recurrent inhibition in the spinal cord

Recurrent inhibition of alpha-motoneurons, via motor axon collaterals and Renshaw cells, obviously reduces the response (output) from a motor nucleus to a given synaptic input. It is proposed that the supraspinal covergence on Renshaw cells allows recurrent inhibition to serve as a variable gain regulator at motoneuronal level. This would allow for an optimal resolution in the force control during weak as well as strong contractions. Renshaw cells are not only inhibiting alpha-motoneurons but also gamma-motoneurons and IA inhibitory interneurons. It is argued that this distribution is meaningful since all these receptive neurons act together as a functional unit, forming an "output stage" of the motor system.

Functional characteristics of unmyelinated fibres in the hippocampal cortex

(1) In transverse hippocampal slices (350 micrometer thick), taken from guinea pigs initially anaesthetized with ether, intracortical afferent fibres were activated by small current pulses delivered through tungsten microelectrodes. Extracellular potentials were recorded from the zone of activated fibres in dendritic layers while intracellular recordings were made from the soma of CA1 pyramidal cells. (2) When recording was made from the same level as the stimulating cathode, the extracellular potential consisted of a diphasic deflection followed by a larger negative wave with a superimposed population spike. The negative wave corresponded to an intracellularly recorded EPSP, and is called an extracellular EPSP, whereas the initial diphasic deflection had no intracellular counterpart. (3) The initial diphasic deflection was linearly related to the size of both the intracellular and extracellular EPSP. It was not changed by removal of calcium ions from the bathing fluid, whereas all postsynaptic activity disappeared. The diphasic deflection was propagated along fibres lying parallel to the pyramidal layer with a velocity of 0.3 m/sec. It could follow short bursts of stimulation at 300 Hz. The absolute refractory period was 2.0 msec. (4) The initial diphasic deflection is interpreted as the compound action potential of the largely unmyelinated afferent fibres to the CA1 neurones.