Long-term potentiation and depression of synaptic responses in the rat hippocampus: localization and frequency dependency

Stable hippocampal long-term potentiation elicited by 'theta' pattern stimulation

The stability of long-term potentiation (LTP) elicited by a stimulation paradigm in which short high-frequency bursts of pulses were given in a 'theta' pattern (i.e. 5 bursts/s) was tested in a chronic recording study. Stimulation electrodes were implanted bilaterally in the Schaffer-commissural system while the recording electrode was placed in the apical dendritic field of the Ca1 zone of the hippocampus. Following 4 days of baseline testing, 'theta' stimulation was applied to one electrode for a total of 2 s (ten 30 ms bursts), after which testing was continued for 3 weeks or until the responses fell to below baseline levels. Data were collected from 25 animals and 3 types of results were obtained: (1) no LTP (n = 4), (2) LTP that decreased steadily from 24 h after high frequency stimulation onward (n = 4), and (3) LTP that was stable until recording was terminated or until the responses began a precipitous decrease to below baseline values (n = 17). The mean of the slopes of the curves relating degree of potentiation to days after 'theta' stimulation was less than 1%/day with a mean correlation coefficient of only 0.1 prior to the point at which the responses began their rapid decline. Control responses were unaffected by the induction of LTP in neighboring CA1 afferents and did not exhibit a reliable relationship with time. These results suggest that, for most rats, LTP elicited by theta pattern stimulation is stable until such time that stimulation-recording arrangements begin to deteriorate.(ABSTRACT TRUNCATED AT 250 WORDS)

Bremazocine differentially antagonizes responses to selective mu and delta opioid receptor agonists in rat hippocampus

The effects of mu, delta and kappa opioid receptor agonists were examined on evoked field potentials in brain slices prepared from rat hippocampus. The effects of the mu-selective opioid peptide [D-Ala2, NMe-Phe4, Met(O)5ol]enkephalin (FK 33-824) and the delta-selective peptide [D-Pen2, D-Pen5]enkephalin (DPDPE) were qualitatively and quantitatively similar. Both increased the amplitude of evoked population spike responses when perfused in low nanomolar concentrations in a fashion consistent with what has been previously reported for other opiate agonists such as morphine. The kappa-selective agonists bremazocine and U-50, 488H were without effect upon evoked responses at concentrations as high as 10 microM. Bremazocine, but not U-50, 488H, proved to be an extremely potent antagonist of responses to both mu- and delta- selective agonists. Moreover, bremazocine was considerably more potent in antagonizing responses to FK 33-824 than DPDPE, which supports the hypothesis that FK 33-824 and DPDPE act via different receptors. Thus, although bremazocine is an agonist at kappa receptors, it appears to act as an antagonist at other opioid receptor sites.

Induction of long-term depression and potentiation by low- and high-frequency stimulation in the dentate area of the anesthetized rat: magnitude, time course and EEG

We investigated the possible importance of stimulus train frequency for the induction and magnitude of long-term synaptic plasticity in the perforant path-granule cell pathway. Under the same experimental conditions, low- (15 Hz) or high-frequency (400 Hz) stimulation could elicit a profound long-term depression (LTD), or typical long-term potentiation (LTP), of the population spike amplitude, excitatory postsynaptic potential (EPSP) amplitude and spike onset latency. In addition, changes in the relationship between the EPSP and population spike amplitude indicated that granule cell excitability was enhanced during LTP and reduced during LTD. LTD occurred primarily after low-frequency stimulation (5 of 6 cases), and was always accompanied by striking changes in the EEG, most notably a biphasic slow potential. While the EEG changes were confined to the first 5 min after the tetanus, LTD lasted from 1 to 4 h. The nature of the EEG events is still unclear, it is suggested that they may represent a spreading depression-like episode. Finally, we found that LTP evoked by high-frequency stimulation was larger and generally reached peak magnitude faster than when it followed low-frequency stimulation. A possible mechanism and role for hippocampal LTD is proposed.

Evoked potential changes in rat hippocampal slices under helium pressure

High pressures of helium affect the physiology of the central nervous system in animals and humans. We examined these effects in rat hippocampal slices. The in vitro preparation displayed a reversible reduction in postsynaptic and antidromic field potentials of CA1 pyramidal cells, but no significant change in the amplitude of the afferent volley. Although the subliminal synaptic response of CA1 neurons was depressed, the ability of these cells to produce population spikes was enhanced. These changes resembled those previously found in vivo in the rat hippocampus. The present results support the hypothesis of a helium pressure-induced depolarization of hippocampal neurons. Other possible mechanisms are discussed.

Induction of hippocampal long-term potentiation using physiologically patterned stimulation

Lasting increases in the population spike recorded in area CA1 of hippocampal slices may be evoked by the patterned presentation of as few as 5 stimulus pulses delivered to the commissural/associational afferents. The most effective pattern of stimulus presentation was a single priming pulse followed 170 ms later by 4 pulses at 100 Hz; control trains of 5 pulses at 100 Hz had no significant enduring effect. This pattern-dependent phenomenon, termed primed burst potentiation, is of lesser magnitude than is the long-term potentiation induced by 100 Hz/l s stimulation, but appears to show a similar time course and duration. In addition, the two phenomena are not additive, suggesting that they may share a common mechanism.

The magnitude of long-term potentiation of field potentials induced monosynaptically in region CA3 of guinea pig hippocampus

The magnitude of long-term potentiation (LTP) was re-examined for monosynaptic transmission between mossy fibers and CA3 neurons in thin sections of the hippocampus of the guinea pig. Although an initial negative deflection in the field potential showed apparent short-term potentiation, this deflection was concluded to reflect non-synaptic activation of CA3 neurons. Accordingly, the magnitude of LTP of monosynaptic transmission was re-calculated at 3.4 +/- 0.9.

Adenosine analogs mediating depressant effects on synaptic transmission in rat hippocampus: structure-activity relationships for the N6 subregion

Previous studies have shown that analogs of adenosine with substituents upon the N6-nitrogen (e.g., N6-[1-phenyl-2(R)-propyl]adenosine; R-PIA) are often much more potent than the parent compound in activating adenosine receptors, particularly those of the A1 subtype. The present investigation characterized the potencies of a number of N6-substituted adenosine analogs in depressing excitatory synaptic transmission in slices of rat hippocampus, an electrophysiological response mediated by receptors of the A1 subtype. These potencies correlated well with previously reported affinities of these analogs for A1 receptor sites in brain, but not with coronary vasodilation in the dog heart, an A2 receptor mediated response. Analogs with alkyl or aryl substituents at the N6 position were generally more potent than adenosine, although analogs with a tertiary carbon attached directly to the N6-nitrogen were usually only weakly active. Although it has been suggested that there may be a subregion of the A1 receptor with some specificity for aryl groups, these experiments did not suggest that this was the case. Analogs with chiral centers attached to the N6-nitrogen usually displayed stereoselectivity, with R-isomers more potent than the S-isomers. The mechanism underlying this selectivity appeared to be both a facilitating effect of alkyl substituents in the propyl C1 position of R-PIA, and a hindering effect of substituents in the position normally occupied by the hydrogen attached to propyl C2 of R-PIA. These results indicate that although there are some similarities in terms of requirements for activity at A1 and A2 receptors, differences between the N6 sub-regions of these receptors are sufficient to permit the development of selective analogs for these two receptor sites.

Separation of components of hippocampal field potentials by digital filtering on a PC-based microcomputer

A versatile digital filtering system is described which enables separation of wave components of evoked potentials based on their frequencies. The utility of this technique is exemplified by application of digital filtering to hippocampal field potentials. Separation of population spikes from lower frequency synaptic potentials provides less ambiguous interpretation of the compound slope of the field potential during the first 5 milliseconds of the response. The entire procedure of stimulation, data acquisition, and digital filtering was implemented on a PC-compatible computer.

Induction of synaptic potentiation in hippocampus by patterned stimulation involves two events

Electrical stimulation of axons in the hippocampus with short high-frequency bursts that resemble in vivo activity patterns produces stable potentiation of postsynaptic responses when the bursts occur at intervals of 200 milliseconds but not 2 seconds. When a burst was applied to one input and a second burst applied to a different input to the same target neuron 200 milliseconds later, only the synapses activated by the second burst showed stable potentiation. This effect was observed even when the two inputs innervated completely different regions of the postsynaptic cells; but did not occur when the inputs were stimulated simultaneously or when the second burst was delayed by 2 seconds. Intracellular recordings indicated that the first burst extended the decay phase of excitatory postsynaptic potentials evoked 200 milliseconds later. These results suggest that a single burst of axonal stimulation produces a transient, spatially diffuse "priming" effect that prolongs responses to subsequent bursts, and that these altered responses trigger spatially restricted synaptic modifications. The similarity of the temporal parameters of the priming effect and the theta rhythm that dominates the hippocampal electroencephalogram (EEG) during learning episodes suggests that this priming may be involved in behaviorally induced synaptic plasticity.

Postsynaptic hyperpolarization during conditioning reversibly blocks induction of long-term potentiation

Activity-induced changes in the efficacy of synaptic transmission between neurones are central to several prominent theories of learning. In both in vivo and in vitro preparations of the hippocampus, a conditioning high-frequency stimulus delivered to afferent fibres results in a long-term potentiation of synaptic transmission at those inputs. Evidence has been provided supporting both presynaptic and postsynaptic sites as loci where critical events occur in the development of potentiation. In this study we report that long-term potentiation is reversibly blocked by intracellular injection of hyperpolarizing current in the postsynaptic cell during the conditioning high-frequency stimulus, suggesting the involvement of a voltage-dependent postsynaptic mechanism.

The acute effects of 6-hydroxydopamine treatment on noradrenergic function in the rat hippocampus in vitro

The electrophysiological consequences of in vitro treatment with 6-hydroxydopamine (6-OHDA) were examined in the CA1 region of the rat hippocampal slice. In control slices, norepinephrine (NE) increased the amplitude of the population spike response elicited by synaptic stimulation of hippocampal pyramidal neurons with a threshold of approximately 5 microM. When hippocampal slices were pretreated with 500 microM 6-OHDA for 10 min, perfusion with a subthreshold concentration of NE (0.5 microM) produced responses similar to those observed with a 10-fold higher concentration of NE in untreated slices. Baseline electrophysiological responses were unchanged following the 6-OHDA exposure. The potentiation of the response to NE by in vitro pretreatment with 6-OHDA was accompanied by a greater than 40% decrease in NE content and greater than 90% decrease in [3H]NE accumulation. In vivo treatment with 6-OHDA or N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine hydrochloride (DSP4) also potentiated the electrophysiological response to NE in a manner similar to that observed with acute in vitro 6-OHDA pretreatment. This action does not appear to be due to development of beta-adrenergic receptor supersensitivity, because the apparent potency of isoproterenol in increasing the population spike amplitude was unaffected. These data suggest that the increase in the potency of NE in slices pretreated with 6-OHDA is due to the rapid disruption of the high-affinity NE uptake mechanism characteristic of noradrenergic nerve terminals.

Long-term potentiation and depression in hippocampal slices

Antidromic stimulation of hippocampal CA 1 cells, in the presence of Mg2+ to eliminate synaptic transmission and with a pattern of impulses which when applied via a synaptic pathway produced long-term potentiation, was found to decrease the response of the CA 1 cells to subsequent synaptic activation. It was found that stimulation via synapses with the same pattern of stimuli caused long-term potentiation in normal conditions but not in the presence of 25 mM Mg2+.

Influence of a 14-day regimen of electrical stimulation involving large portions of the cerebral mass on later habit acquisition in the rat

Four groups of rats, one of which served as a normal, nonoperated group, were used. The other three groups were operated and fitted with an electrode carrier of eight electrodes of varying lengths arranged in a 7.5 mm X 5 mm rectangle. About 10 or 12 days after surgery, the following groups of animals underwent a 14-day regimen wherein each animal spent 15 min in a deep, metal, electrically insulated cylinder: Group A, normal, nonoperated group, with no intervention; Group B, stimulated each day with 45 trains of rectangular biphasic pulses (3 trains per min) at the rate of 20 pulses per second (pps); Group C, the same but at the rate of 100 pps; Group D, the same but 180 pps. After a 48-hr fast and without benefit of shaping, each animal was placed for 60 min in a lever-pressing device and was rewarded with a pellet after each three to four presses. The resulting performance curves showed no improvement in pressing rate for Group A; marginal improvement for Group B; definite improvement for Group C; and the greatest improvement for Group D, the terminal rate being almost twice that of Group C.

Tetanic stimulation-induced changes in [3H]glutamate binding and uptake in rat hippocampus

Tetanus-induced (400 Hz, 200 pulses) long-lasting potentiation of the stratum radiatum-evoked CA1 population spike in hippocampal slices is not accompanied by any change in Na+-independent [3H]glutamate binding sites. Homosynaptic depression that occurs subsequent to either a low frequency tetanus (20 Hz, 600 pulses) or a transient exposure to Cl(-)-free (containing NO3-) medium is associated with an elevation in the amino acid binding. [3H]Glutamate uptake into slices was decreased following a high frequency (400 Hz, 200 pulses) tetanus but in the majority of cases was increased following a low frequency (20 Hz, 600 pulses) tetanus to stratum radiatum. When the high frequency tetanus was given in the absence of extracellular Ca2+, there was a further reduction in [3H]glutamate uptake.

Low frequency perforant path stimulation as a conditioned stimulus demonstrates correlations between long-term synaptic potentiation and learning

Stimulation of the perforant path with impulse trains of 15 cps and 670 msec duration was used as a conditioned stimulus in a two-way shuttle box avoidance on rats. Field potentials in the dentate area evoked by test stimuli were measured after the training sessions until the 7th day. Foot-shock and unconditioned escape elicited only a transient slight depression of the population spike amplitude (P) and increased also slightly the slope function (SF) of the population EPSP of the evoked test potentials. The control stimulation of the perforant path without pairing with foot-shock as in conditioning did only slightly increase SF of test potentials, but produced a strong transient inhibition followed by a long lasting moderate depression of P. After conditioning, all animals exhibited the same initial inhibition of P as shown in control stimulation of the perforant path. However during the following 4 hours, good learners with a relearning index greater than 30% developed a significant potentiation of P lasting until the second training session 24 hours later, which resulted in a further enhancement. SF of the evoked test potentials increased in good learners with a similar time course after conditioning but without initial depression. After 7 days P showed still enhanced but non-significant values. Poor learners with a relearning index less than 10% did not develop a potentiation of P after conditioning and initial inhibition, but a long-term depression. Also SF of test potentials decreased in poor learners during 4 hours after conditioning and returned almost to baseline until the following day. After 7 days, P and SF did not differ from baseline. The analysis of the observed synaptic changes by E-S curves demonstrated the post-tetanic LTP seems to differ in some ways from post-conditioning LTP in good learners. The latter exhibits a clear tendency of a right shift contrary to the left shift commonly occurring after tetanization. Furthermore poor learners do not only fail to produce long-term potentiation, but fail to show a change in the opposite direction with a left shift of the E-S curves. The observed correlation of LTP in the conditioning pathway with the learning ability suggests an involvement of LTP at least in the acquisition and early retention of this learned behavior. The results do however not finally clarify the role of LTP in long-term retention.

Effect of olfactory bulb kindling on evoked potentials in the pyriform cortex

The potential evoked in the pyriform cortex by single-pulse stimulation of the olfactory bulb was examined before and after single and repeated elicitation of an epileptiform afterdischarge produced by stimulation of the olfactory bulb. A single afterdischarge (AD) produced a rapid (i.e. within 5 min) increase in the amplitude of an early surface-negative wave and duration of a later surface-positive wave. These effects persisted at least 48-72 h. Repeated elicitation of ADs resulted in kindling. A large increase in the amplitude of a later surface-negative wave (approximately 25 ms latency) occurred during kindling. This wave remained significantly elevated for at least 72 h after the last AD. Long-term potentiation of the early surface-negative wave was produced by kindling or two focal ADs. A short-term effect which was consistently observed following a focal or generalized AD was a prolongation of a late surface-positive wave. These effects are discussed in relation to long-term potentiation, postseizure inhibition, and kindling development.

Functional plasticity in two afferent systems of the granule cells in the rat dentate area: frequency-related changes, long-term potentiation and heterosynaptic depression

Monosynaptic evoked field potentials (MEFP) were recorded in the dentate gyrus of male Wistar rats upon stimulation of either the perforant path or the commissural system. While the perforant path potential exhibited in acute experiments a clear reversal point of the field excitatory postsynaptic potential (EPSP) and population spike when protruding the registration electrode from the hippocampal fissura to the hilus of the dentate gyrus, the simultaneously registered commissural potential elicited by stimulation of the contralateral hilus showed no reversal of the negative monophasic wave but merely an amplitude maximum 40 microns above the reversal point of the perforant path potential. Frequency-related changes of the MEFPs during short tetanic stimulation with 15 Hz both in acute and chronic experiments, revealed differences in the properties of the input systems in that the commissural potential exhibited a clear frequency potentiation whereas the perforant path potential showed frequency depression. Pronounced long-term potentiation of the perforant path potential induced by 4 trains of tetanizing stimuli and lasting up to 72 h was accompanied by a long-term heterosynaptic depression of the commissural potential for up to 7 days after tetanization. Both the different frequency-related changes of the inputs and the extremely long duration of the heterosynaptic depression are discussed with respect to their proposed functions in the mechanisms of functional plasticity.

Two components of long-term potentiation in mossy fiber-induced excitation in hippocampus

Experiments were made in thin transverse sections of the guinea pig hippocampus to clarify whether tetani to a heterosynaptic input can induce long-term potentiation (LTP) in the absence of seizure discharges and whether the heterosynaptic LTP occurs in a monosynaptic or polysynaptic pathway. Early and late responses were differentiated in field potentials elicited in region CA3 by mossy fiber stimulation. The LTP of the early response was specific to a tetanized pathway, whereas heterosynaptic LTP was observed in the late response in the absence of seizure discharges. The magnitude of LTP of the early response was significantly smaller than that of the late response. LTPs of the early and late responses were not induced at a low Ca2+ concentration. A brief exposure to a high Ca2+ solution resulted in a long-lasting potentiation of both the early and late responses. Counterparts to the early and late responses were recorded from the distal dendritic layer in reversed polarities. The intracellular counterpart to the late response was a slow depolarization superimposed on the peak and falling phase of excitatory postsynaptic potentials. These results suggest that LTP of the early response reflects enhanced transmission through synapses between mossy fibers and CA3 neurons and LTP of the late response reflects modified interaction among postsynaptic neurons.

Adenosine increases synaptic facilitation in the in vitro rat hippocampus: evidence for a presynaptic site of action

The effect of adenosine on paired synaptic responses was characterized in the CA1 region of the rat hippocampus in vitro. Adenosine increased the degree of synaptic facilitation at a 40 ms conditioning-testing interval under all conditions tested. Even when the stimulation intensity was increased so as to counteract the direct depressant effect of adenosine on synaptic transmission, its effect on facilitation was maintained. The ability of adenosine to increase synaptic facilitation was a complex function of several variables. The effect was enhanced by increasing the calcium concentration of the medium, and was most pronounced at short conditioning-testing intervals and at low response amplitudes. Adenosine was particularly efficacious in blocking the depression of synaptic responses observed in high-calcium medium at short conditioning-testing intervals. Because this depression most probably reflects depletion of the available store of releasable transmitter, one mechanism by which adenosine could reverse this effect would be by blocking the depletion of transmitter. These results suggest that adenosine diminishes transmitter release via an action at the presynaptic terminal. The reduction in the release of neurotransmitter, particularly at excitatory synapses, may be responsible for the depressant effects of adenosine upon the central nervous system.

The duration of long-term potentiation in the CA1 region of the hippocampal slice preparation

The duration of long-term potentiation (LTP) of the monosynaptic excitatory Schaffer collateral-commissural input to hippocampal neurons of the CA1 region was examined in the in vitro slice. Relatively stable evoked potentials were obtained under conventional perfusion conditions at least for 10 hours. Tetanic stimulation (100 Hz, 1 sec) increased the population spike (pop-spike) amplitude by about 150% and the slope of the field-EPSP by about 30% over the pre-LTP baseline, whereas the latency and peak latency of the pop-spike decreased. In comparison to control experiments (same number of stimuli at 0.2 Hz) the differences were statistically significant for 2 hr (field-EPSP) and for greater than or equal to 10 hr (pop-spike), respectively. Repeated tetanization (3 X 100 Hz/1 sec), however, substantially prolongs EPSP-LTP (greater than or equal to 10 hr) and doubles the approximated half-life of pop-spike LTP. The threshold current intensity to elicit pop-spike responses decreased after the induction of LTP. Furthermore, the smaller field-EPSP values necessary to evoke near-threshold pop-spikes demonstrate an E-S potentiation (left-shift) at least in the low-intensity range. While the total duration of potentiation of the different parameters has not been determined, all the above mentioned effects could be observed at least 10 hr following the repeated tetanization. It is proposed that the slice preparation is suitable for the investigation of mechanisms of a postulated late phase of LTP if appropriate conditions are used.

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.

Inhibition of Purkinje cell firing by systemic administration of phenylisopropyl adenosine: effect of central noradrenaline depletion by DSP4

The effect of the metabolically stable adenosine analog (-)-N6(R-phenyl-isopropyl)-adenosine (PIA) on the rate of spontaneous Purkinje cell firing was studied in anesthetized rats. In control animals, systemically administered PIA elicited only small and inconsistent changes in firing rate. However, in animals previously treated with DSP4 (50 mg/kg i.p.), which selectively lesions central noradrenergic afferents, or with the adrenergic antagonist sotalol (15 mg/kg), PIA elicited consistent decreases in firing rate. These effects were antagonized by the systemic administration of the adenosine receptor antagonist aminophylline (50-150 mumol/kg). Local administration of adenosine by pressure ejection caused a dose-dependent depression of Purkinje cell firing that was likewise inhibited by the methylxanthine. In DSP4 treated rats the depression of synaptic transmission by adenosine in rat hippocampus in vitro was unaltered, and theophylline did not cause any marked rise in Purkinje cell firing, suggesting that DSP4 does not sensitize neurons to the depressant effects of adenosine derivatives. PIA also caused a dose-dependent decrease in arterial blood pressure and a decrease in heart rate that was of equal magnitude in control and DSP4 treated rats. The results show that the central effects of systemically administered adenosine analogs are altered by procedures that disrupt the normal depressant effect of tonic noradrenergic input.

Heterosynaptic changes accompany long-term but not short-term potentiation of the perforant path in the anaesthetized rat

Brief high-frequency trains of electrical stimulation delivered to the perforant path result in long-term potentiation (l.t.p.) of field potentials recorded extracellularly from granule cells of the dentate gyrus. L.t.p. of the population spike is often disproportionately greater than l.t.p. of the population excitatory post-synaptic potential (e.p.s.p.). We have investigated the basis of this effect in rats anaesthetized with sodium pentobarbitone. A series of graded stimuli were given before and after tetanization of the perforant path. From data obtained in this way, we plotted stimulus-response curves, and the relation (E-S curve) between the slope of the population e.p.s.p. (E) and the amplitude of the population spike (S). Curves relating spike onset latency to the slope of the e.p.s.p. were also constructed. Tetanization of the combined medial and lateral components of the perforant path led to long-term changes in the relation between the e.p.s.p. and the population spike. For a given e.p.s.p., the corresponding population spike was of greater amplitude and earlier onset. This E-S potentiation was marked by a shift to the left of the E-S amplitude curve and a downward displacement of the E-S latency curve. Tetanization of the lateral component of the perforant path had two long-term effects on responses evoked by test stimuli to the untetanized medial component: (1) long-term depression of the medial e.p.s.p. and (2) long-term E-S potentiation. The net result of these two heterosynaptically induced effects was to leave unaltered information transfer across medial perforant path-granule cell synapses; for a given test volley the e.p.s.p. was smaller, but because of E-S potentiation the population spike remained relatively unaffected. Short-term potentiation, which has a time course of only a few minutes and is presumed to be mediated by presynaptic mechanisms, was not accompanied by E-S potentiation or by corresponding changes in spike latency. Possible mechanisms of long-term heterosynaptic depression of the e.p.s.p. and of homo- and heterosynaptic E-S potentiation, are discussed. We conclude that although these effects probably reflect a generalized post-synaptic change, this change is unlikely to be a prolonged reduction in the membrane potential of granule cells.

Perforant pathway-evoked long-term potentiation of CA1 neurons in the hippocampal slice preparation

Previously, we have presented electrophysiological evidence reaffirming the existence of a controversial hippocampal pathway. These fibers are part of the perforant pathway and terminate directly on the CA1 cells. We now report that, in the hippocampal slice preparation, tetanic stimulation of the perforant pathway produces long-term potentiation (LTP) of CA1 cell responses. LTP of population spikes varied from 150% to 500%. The results were of interest because these axons synapse at distal sites on the apical dendrite. This location is usually thought to be a difficult site to evoke action potentials.

The adenosine receptor activity of EMD 28422, a purine derivative with reported actions on benzodiazepine receptors

The effects of a novel purine derivative, N6-[2-(4-chlorophenyl)-bicyclo-2.2.2.octyl-(3)]-adenosine (EMD 28422), that has been found to influence central benzodiazepine receptors, has been compared to those of other adenosine analogues such as L-phenylisopropyladenosine (L-PIA), cyclohexyladenosine (CHA) and adenosine-5'-N-ethyl-carboxamide (NECA). EMD 28422 was about 30 times less potent than CHA and 4 times less potent than NECA in displacing bound [3H]-L-PIA from specific binding sites in the rat brain, presumably reflecting adenosine A1-receptors. A similar relative potency was found using depression of field e.p.s.p. in the hippocampal slice in vitro. In isolated fat cells EMD 28422 was antilipolytic, but some 1000 times less potent than L-PIA. In rat isolated hippocampal slices, which have adenosine A2-receptors, EMD 28422 was more than 300 times less potent than NECA and in guinea-pig thymocytes, which similarly have A2-receptors, EMD 28422 was about 60 times less potent. The results are compatible with the opinion that EMD 28422 is a rather weak agonist at adenosine receptors, with limited selectivity for A1- or A2-receptors. The compound is highly lipophilic, which plays a role in determining its potency in a given biological system. The results are discussed in relation to reported adenosine modulation of benzodiazepine receptors.

Influences of sinusoidal electric fields on excitability in the rat hippocampal slice

The influence of extracellular sinusoidal electric fields on the amplitude of population spikes evoked by single test pulses in excitatory pathways to CA1 pyramidal neurons was studied in rat hippocampal slices. The fields in the tissue were of the order of EEG gradients. Stimulation at 5 Hz, a frequency representative of hippocampal theta activity, was compared with 60 Hz, which is often used in kindling procedures. Brief stimulation (5-30 s) with both 5 and 60 Hz fields (20-70 mV/cmp-p in the perfusing solution) often produced a long-term increase (longer than 10 min) of the population spike. Fields at 60 Hz, but not at 5 Hz, also induced short-term depression (1-6 min) or transient post-field excitation (15-30 s). Prolonged stimulation (3 min) emphasized this frequency dependent response: fields at 5 Hz induced long-lasting potentiation while fields at 60 Hz always resulted in progressive depression persisting for a few minutes after the end of stimulation. These effects appeared as a global response of CA1 neurons. Antidromic responses studied during blockade of synaptic transmission (0.2 mM Ca2+, 4 mM Mg2+) were depressed during and following 3 min field stimulation at either frequency, which could reflect failing calcium mechanisms in the tissue. The field influence on the potential evoked by synaptic or antidromic stimulation was independent of the phase of the sine wave at which the test pulse was delivered, arguing against a direct polarization of the cell membrane by the fields. The experimental evidence suggests a functional role for EEG-like fields in hippocampal excitability.

Chronically elevating plasma Mg2+ improves hippocampal frequency potentiation and reversal learning in aged and young rats

One prediction of the hypothesis that the capacity for hippocampal frequency potentiation is relevant to behavioral plasticity is tested in this study, by chronically elevating magnesium in intact aged and young rats. Elevated extracellular Mg2+ specifically improves frequency potentiation in hippocampal slices, and chronic alterations in plasma Mg2+ can increase brain Mg2+ in intact animals. Aged and young rats on a diet that elevated plasma Mg2+ exhibited stronger frequency potentiation under urethane anesthesia, and showed improved maze reversal learning.

Modulation of long-term potentiation by peripherally administered amphetamine and epinephrine

Long-term potentiation (LTP) has received considerable attention as a neurophysiological model for studying the biology of memory. The present experiments examined the susceptibility of LTP in the dentate gyrus to modification by peripheral injections of amphetamine and epinephrine. Both drugs enhanced the development of LTP in a dose-related manner comparable to that seen previously in behavioral studies. Such results suggest that the development of this long-lasting electrophysiological change can be regulated by peripheral catecholamine levels in a manner analogous to that seen in behavioral studies of memory.

Anisomycin blocks the late phase of long-term potentiation in the dentate gyrus of freely moving rats

Freely moving rats, chronically implanted with stimulation electrodes in the medial entorhinal cortex and recording electrodes in the dentate gyrus, received two 400 micrograms intraventricular injections of anisomycin during a tetanization procedure that induced a long-lasting potentiation (72 hours) of the monosynaptic field potential. Inhibition of protein synthesis during the tetanization procedure did not immediately influence the induction of long-term potentiation (LTP). However, 3-4 hours after the beginning of tetanization the potentiation effect decayed progressively and was abolished totally during the remaining 7 day observation period. In control experiments anisomycin did not affect the slope of field EPSP's and produced a reversible depression of the population spike amplitude. These data indicate a relatively specific effect of the protein synthesis inhibitor on mechanisms involved in a late phase of LTP stabilization.

Adenosine receptors mediating inhibitory electrophysiological responses in rat hippocampus are different from receptors mediating cyclic AMP accumulation

Electrophysiological and biochemical techniques were used to characterize adenosine receptors in rat hippocampus. The site which mediates the inhibitory action of adenosine on excitatory synaptic transmission and on spontaneous interictal spiking had properties similar to the adenosine A1 receptor. Thus, the relative order of potency for adenosine analogs was L-PIA greater than or equal to CHA greater than NECA greater than 2CA (L-PIA = N6-phenylisopropyladenosine; CHA = N6-cyclohexyl-adenosine; NECA = adenosine 5'-ethylcarboxamide; 2CA = 2-chloroadenosine), with EC50 values for the most potent analogs between 10-30 nM. The effect of the stable adenosine analog, particularly CHA and L-PIA, was slow in onset and very slowly reversible. This is suggested to be due both to a slow dissociation of these compounds from the receptors but particularly to the slow equilibrium between the concentration of the drug in the medium surrounding the slices and the biophase within the slices. Adenosine analogs bound specifically to membrane preparations of the rat hippocampus with the order of potency 3H-CHA greater than or equal to 3H-L-PIA greater than 3H-NECA. Eadie-Hofstee plots of the binding data were curvilinear for each ligand, but only for 3H-L-PIA could the existence of two binding sites with different apparent Kd-values (0.27 and 11.8 nM) be confirmed by curve-fitting. The estimated Kd-values for CHA and NECA were 1.5 and 20 nM, respectively. The adenosine analogs also enhanced 3H-cyclic AMP accumulation in 3H-adenine-labelled hippocampal slices.(ABSTRACT TRUNCATED AT 250 WORDS)

The biochemistry of memory: a new and specific hypothesis

Recent studies have uncovered a synaptic process with properties required for an intermediate step in memory storage. Calcium rapidly and irreversibly increases the number of receptors for glutamate (a probable neurotransmitter) in forebrain synaptic membranes by activating a proteinase (calpain) that degrades fodrin, a spectrin-like protein. This process provides a means through which physiological activity could produce long-lasting changes in synaptic chemistry and ultrastructure. Since the process is only poorly represented in the brain stem, it is hypothesized to be responsible for those forms of memory localized in the telencephalon.

Long-term potentiation in the hippocampal slice: possible involvement of pyruvate dehydrogenase

Tetanic stimulation of fibers in the hippocampal slice preparation produces long-term potentiation (LTP) and also decreases the in vitro incorporation of phosphate into the alpha subunit of pyruvate dehydrogenase (alpha PDH). This paper describes 6 experiments that were undertaken to replicate this observation. Hippocampal slices were incubated in a specially designed chamber and stimulated with a tungsten wire electrode in the stratum radiatum for 1 s at 100 Hz. Two minutes after the tetanus, the stimulated slices were removed alternately with control (not tetanized) slices and each group was pooled for subcellular fractionation and labeling of the fractions with [32P]ATP. Proteins were separated by electrophoresis and relative 32P contents of 41K and 50K protein bands were studied. Tetanic stimulation of the stratum radiatum did not affect subsequent phosphorylation of a 50K Mr protein that has been reported to be altered by perforant path activation. Stimulation also had no effect on pyruvate dehydrogenase enzyme activity or on the ratio of active (dephosphorylated) to inactive enzyme. In most cases tetanic stimulation produced no significant change in the in vitro phosphorylation of this enzyme. Only under one set of conditions, labeling with 250 microM [gamma-32P]ATP for 10 s, was a decrease in the in vitro labeling of alpha PDH shown to be statistically significant. These findings suggest that LTP is not necessarily accompanied by an initial change in PDH phosphorylation level or activity but may be associated with a decrease in the kinase activity directed toward this protein.

Long-term potentiation of hippocampal synaptic transmission affects rate of behavioral learning

Electrical stimulation techniques were used to produce a long-lasting potentiation of synaptic transmission in the hippocampus of naïve rabbits. Animals were then classically conditioned. Long-term potentiation of the hippocampus before training increased the rate at which animals subsequently learned the conditioning task. This result has significance for potential cellular mechanisms of associative learning.

Interactions between the effects of adenosine and calcium on synaptic responses in rat hippocampus in vitro

The effect of adenosine on synaptic responses in the in vitro rat hippocampus was examined. As has been previously described, adenosine had a profound depressant effect on synaptic transmission at excitatory synapses on the CA1 pyramidal cells. Although adenosine also produced small decreases in the amplitude of the presynaptic fibre spike, this was unable to account for the relatively much greater decrease in the amplitude of the extracellularly recorded field excitatory post-synaptic potential (e.p.s.p.). The effect of the calcium concentration of the bathing medium on responses to adenosine was also examined. Adenosine generally had a greater depressant effect on field e.p.s.p.s from slices maintained in low concentrations of calcium (1 mM) than in high (10 mM-calcium). However, at low response amplitudes, the effect of adenosine in the two kinds of medium was quite similar. When corrections were made for non-linear summation of e.p.s.p.s, it was found that the effects of adenosine appeared to be independent of the calcium concentration of the medium. These data suggest that adenosine inhibits synaptic transmission by a calcium-independent regulatory mechanism, and that adenosine does not interfere with calcium influx or calcium levels in the presynaptic terminal. Comparisons of dose-response relationships in media containing different concentrations of calcium suggest that a maximal drug response can occur with less than maximal receptor occupancy at physiological calcium concentrations (less than 2.5 mM).

Hippocampal field potential: a microcomputer aided comparison of amplitude and integral

A microcomputer based system is described for the analysis of various aspects of hippocampal field potential response recorded in vitro, either in the dendritic or somatic region. A comparison is made between the average amplitude and the time integral of synaptically evoked population spike at the cell body layer, following various intensities of stimuli delivered to the stratum radiatum. There was a good correlation between the two measures at low and intermediate intensities of stimulation. With stimulus intensities beyond those eliciting the population spike of maximal integral, the amplitude of the population spike further increased while its duration decreased. It is concluded that the integral provides a more accurate estimate of the extent of neuronal firing than the amplitude.

Acidic amino acid binding sites in mammalian neuronal membranes: their characteristics and relationship to synaptic receptors

This review summarizes studies designed to label and characterize mammalian synaptic receptors for glutamate, aspartate and related acidic amino acids using in vitro ligand binding techniques. The binding properties of the 3 major ligands employed--L-[3H]glutamate, L-[3H]aspartate and [3H]kainate--are described in terms of their kinetics, the influence of ions, pharmacology, molecular nature, localization and physiological/pharmacological function. In addition, the binding characteristics are described of some new radioligands--[3H]AMPA, L-[3H]cysteine sulphinate, L-[35S]cysteate, D-[3H]aspartate, D,L-[3H]APB, D-[3H]APV and D,L-[3H]APH. Special emphasis is placed on recent findings which allow a unification of the existing binding data, and detailed comparisons are made between binding site characteristics and the known properties of the physiological/pharmacological receptors for acidic amino acids. Through these considerations, a binding site classification is suggested which differentiates 5 different sites. Four of the binding site subtypes are proposed to correspond to the individual receptor classes identified in electrophysiological experiments; thus, A1 = NMDA receptors; A2 = quisqualate receptors; A3 = kainate receptors; A4 = L-APB receptors; the fifth site is proposed to be the recognition site for a Na+-dependent acidic amino acid membrane transport process. An evaluation of investigations designed to elucidate regulatory mechanisms at acidic amino acid binding sites is made; hypotheses such as the Ca2+-activated protease hypothesis of long-term potentiation are assessed in terms of the new binding site/receptor classification scheme, and experiments are suggested which will clarify and expand this exciting area in the future.

Norepinephrine elicits both excitatory and inhibitory responses from Purkinje cells in the in vitro rat cerebellar slice

Superfusion of Purkinje neurons in the in vitro rat cerebellar slice with norepinephrine caused increases and decreases of spontaneous Purkinje cell firing. Excitations were evoked by low concentrations of norepinephrine (0.5-10 microM) and by the beta receptor agonist isoproterenol (0.1-5 microM). These excitations were reduced by timolol (1-2 microM), a beta receptor antagonist. Perfusion with higher concentrations of norepinephrine (greater than 16 microM), caused a depression of Purkinje neuron spontaneous activity. This inhibitory response was blocked by the alpha receptor antagonist phentolamine. The alpha 1 selective agonist phenylephrine had no effect on spontaneous activity at concentrations up to 100 microM, but the alpha 2 selective agonist clonidine (1-50 microM) elicited decreases in firing rate. These responses appeared to be due to a direct action on Purkinje cells, because neither the excitation nor the depression of Purkinje neuron activity elicited by norepinephrine was substantially altered when tested in a medium which substantially blocked synaptic transmission within the slice. Under these in vitro conditions, norepinephrine appears to increase the firing rate of Purkinje neurons via an interaction with beta adrenergic receptors, while norepinephrine induced depressions may be linked to alpha adrenergic receptor interactions; both receptors appear to be located directly on the Purkinje neurons.

Electrophysiological effects of cocaine in the rat hippocampus in vitro

The effects of cocaine on electrophysiological activity were examined in the CA1 region of the in vitro hippocampal slice. Low concentrations of cocaine (0.25-2.5 microM) had no direct effect on evoked potentials, but potentiated responses to threshold concentrations of norepinephrine (NE; 0.5 microM); this effect was also seen with other inhibitors of catecholamine uptake such as desipramine. The ability of low doses of cocaine to augment the changes in population spike amplitude elicited by NE appeared to be related to the ability of cocaine to block the uptake of NE into the noradrenergic nerve terminals in the rat hippocampus. Higher concentrations of cocaine (5-200 microM) elicited responses consistent with local anesthetic actions on neuronal membranes.

Verapamil counteracts depression but not long-lasting potentiation of the hippocampal population spike

In transversely sectioned rat hippocampal slices, population spikes and population "EPSPs" were recorded from CA1 neurones in response to the stimulation of Schaffer collateral and commissural inputs. High frequency tetanic stimulation (400 Hz, 200 pulses) of an input induced LLP of the homosynaptic response without significantly changing the heterosynaptic response. This LLP was not interrupted by either a 400 Hz tetanus given to the heterosynaptic input or by verapamil (0.33 microM) which blocks Ca++ channels, but not transmitter release. A low frequency tetanus (20 Hz, 200 pulses) given to an input induces co-occurring homosynaptic and heterosynaptic depressions of about 20 min duration. This tetanus could also mask an established LLP in homosynaptic or heterosynaptic pathway. Verapamil counteracts homo- and heterosynaptic depressions. The population spike as well as the population "EPSP" were depressed following iontophoretic application of Ca++ (2-100 nA) at the CA1 cell body area. These results indicate that homosynaptic and heterosynaptic depressions are at least partly due to an accumulation of Ca++ into CA1 neurones. An established LLP is not interrupted by LLP of another input. Homo- and heterosynaptic depressions mask, but not reverse, LLP.

Long-term potentiation as a candidate mnemonic device

One approach to studying the neurophysiological correlates of long-term memory is to search for, and study properties of the nervous system that impart plasticity of synaptic efficacy. Within this context, we argue that long-term potentiation is, currently, the most plausible device for subserving or initiating long-term information storage in the mammalian brain. This argument is derived from examining features of LTP with respect to the constraints posed by our current concepts of learning and memory. In addition, we examine evidence for a role by LTP in behavioral learning. We conclude that studying LTP within the context of behavioral learning and memory may provide new insights into the neurophysiological bases of learning and memory.

Electrophysiological responses to adenosine analogs in rat hippocampus and cerebellum: evidence for mediation by adenosine receptors of the A1 subtype

Adenosine has profound depressant effects upon the electrophysiological activity of the brain, but the adenosine receptor subtypes which mediate these responses are uncertain. In order to resolve this question, we have characterized the effects of two adenosine analogs which differ in their relative potencies at adenosine A1 and A2 receptors. The effects of these adenosine analogs were examined on spontaneous firing rate of Purkinje neurons in the rat cerebellum in situ, in cerebellar brain slices in vitro, and on synaptic transmission in the rat hippocampus in vitro. Although the A2 agonist appeared to be more potent with local drug application techniques in situ, our in vitro results suggest that the A1 receptor subtype is involved in the electrophysiological actions of these drugs in both rat cerebellum and hippocampus. Furthermore, these data indicate that the physical properties of some adenosine analogs may reduce apparent drug potencies when they are studied with local application techniques.

Developmental onset of long-term potentiation in area CA1 of the rat hippocampus

Long-term potentiation (l.t.p.) was studied in area CA1 of rat hippocampal slices during development at post-natal days 1-8, 15 and 60. Tetanic stimulation at 100 Hz for 1 s was delivered to the fibres in stratum radiatum and the time course of potentiation was recorded in stratum pyramidale for 20 min after tetanus. L.t.p. was measured at 20 min post-tetanus as an increase in the amplitude of the population spike. The time course and magnitude of post-tetanic potentiation (p.t.p.) differed with age. For 60-day-old animals p.t.p. was seen as a maximally potentiated response immediately post-tetanus that declined to a smaller potentiated response by 5 min post-tetanus. For animals younger than 15 days the response was also maximally potentiated immediately post-tetanus with subsequent decline. However, the duration of maximal potentiation was shorter and the magnitude was less. A different time course of p.t.p. was observed at 15 days. The maximal potentiation was approximately equal to that seen at 60 days, but instead of declining, the response remained maximally potentiated throughout the entire post-tetanus monitoring period. L.t.p. was first observed at post-natal day 5, and by post-natal days 7 and 8 substantial levels of l.t.p. were seen consistently. The greatest magnitude of l.t.p. was found at 15 days, and was considerably more than that produced at 60 days. When the duration of l.t.p. was monitored for longer than 20 min the response declined back to pretetanus levels by 1-1 1/2 h for animals younger than 15 days. In 15-day-old rats the response remained maximally potentiated for the full 72 min that it was monitored, with no decline. In control experiments of low-frequency stimulation (l.f.s.) at 1/15 s for 100 stimuli, hippocampal slices from 60-day-old animals showed response elevation. In contrast, l.f.s. resulted in response decrement over time for slices from 5-15-day-old animals. Three measures of pretetanus excitability in area CA1 suggested an increase with age. The stimulus intensity required for field excitatory post-synaptic potential (e.p.s.p.) threshold declined, the magnitude of the maximal population spike amplitude increased, and the population spike latency decreased. These results suggest that the magnitude of l.t.p. is not strictly related to the pretetanus excitability of CA1 cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Long-term potentiation in the hippocampus

Long-term potentiation of field and single neuronal responses recorded in various hippocampal fields is described on the basis of author's and literary data. Most of intrahippocampal and extrinsic connections in both in vivo and in vitro hippocampal preparations show this phenomenon after one or several conditioning trains of comparatively short duration (20 s or less) at various frequencies (from 10 to 400 Hz). Properties of hippocampal potentiation are described. The properties include long term persistence (hours and days) of the potentiated response, its low frequency depression, self-restoration after the depression, specificity of the potentiation for the tetanized pathway, necessity of activation of a sufficient number of neuronal elements ('cooperativity') to produce the potentiation, possible involvement of 'reinforcing' brain structures during conditioning tetanization. These properties are distinct from those of 'usual' short-term post-tetanic potentiation and lead to the suggestion that the neuronal mechanisms underlying long-term post-tetanic are similar to those underlying memory and behavioral-conditioned reflex. Neurophysiological mechanisms of long-term potentiation are discussed. The main mechanism consists in an increase in efficacy of excitatory synapses as shown by various methods including intracellular recording and quantal analysis. The latter favours presynaptic localization of changes of synaptic efficacy showing increase in the number of transmitter quanta released per presynaptic impulse. However, changes in the number of subsynaptic receptors or localized changes in dendritic postsynaptic membrane are not excluded. Biochemical studies indicate the increase in transmitter release and calcium-dependent phosphorylation of pyruvate dehydrogenase after tetanization. Instances of persistent response facilitations at other levels of the vertebrate central nervous system (especially at neocortical level) are considered and compared with hippocampal long-term potentiation. It is suggested that modifiable excitatory synapses necessary for learning have been identified in studies of long-term potentiation. These synapses are presumably modified as a result of close sequential activation of the following three structures: excitatory presynaptic fibers, the postsynaptic neuron and a 'reinforcing' brain system.

Associative long-term potentiation in hippocampal slices

Interactions between two excitatory monosynaptic inputs to hippocampal neurons of the CA1 region were examined in the in vitro slice. By adjusting the strengths of the electrical stimuli delivered to the two input pathways, one was made to generate a weak and the other a strong synaptic response. Simultaneous tetanic stimulation of both input pathways resulted in a subsequent long-term enhanced synaptic efficacy in the weak input under conditions in which the same tetanic stimulation of either input alone failed to have this effect. This form of long-term synaptic potentiation (LTP), known as associative LTP, was shown in some cases to last hours without decrement. The plastic changes were localized within the CA1 region and appear to reside in the pre- or postsynaptic elements of the monosynaptic excitatory input to the pyramidal neurons. The increased synaptic efficacy could not be accounted for by any of several measured postsynaptic passive membrane properties.

Calcium in long-term potentiation as a model for memory

The granule, CA1 and CA3 cells of the hippocampus have been much investigated during the last decade because there is superimposed on the standard features of synaptic transmission a very prolonged potentiation lasting for weeks that is called long-term potentiation. Evidently long-term potentiation is a promising candidate in the construction of a model for memory. The thesis here developed is that the influx of calcium ions across the membrane of the granule pyramidal cells plays the key role in the generation of long-term potentiation. The proposal makes it possible to account for the necessity of strong repetitive synaptic stimulation, preferably in bursts so as to optimize the conditions for the calcium influx. Studies on hippocampal slices with variations in the synaptic inputs in the granule cells give evidence of cooperativity, which is interpreted in relation to the threshold membrane depolarization for calcium influx. It is conjectured that the large increase of calcium in the granule and pyramidal cells results in the combination with the specific protein, calmodulin, to form a second messenger system, which produces metabolic changes leading to an increase in receptors of the postsynaptic membrane of the spine synapses, i.e. the postsynaptic densities, to the synaptic transmitter, glutamate. For example, Ca2+ could activate calcium-dependent kinases in the postsynaptic density resulting in the modification of protein components by phosphorylation. Other postsynaptic factors contributing to long-term potentiation are presumed to be protein synthesis with spine swelling and increased transport up the dendritic microtubules. There is discussion of the evidence for the alternative hypothesis that long-term potentiation is primarily presynaptic, being due to an increased output of transmitter. A unifying hypothesis is formulated, namely, that the primary event in long-term potentiation is in the increased sensitivity of the postsynaptic densities to the transmitter, and that, secondarily, this induces an increased output of transmitter from the presynaptic terminals by a trophic action across the synaptic cleft. It is shown how the proposed combination of calcium with calmodulin will account for the hypothesis of Marr that cognitive memory is due to conjunction potentiation. Furthermore, the Marr-Albus hypothesis for cerebellar learning is accounted for if the calcium-calmodulin messenger system causes the observed depression of the transmitter sensitivity of the spine synapses on Purkynĕ cells.

Quantitative relationship between post-tetanic biochemical and electrophysiological changes in rat hippocampal slices

Slices of rat brain hippocampus were tetanized in the perforant path fibers. In individual slices long-term changes of the electrophysiological parameters were determined simultaneously with post hoc endogeneous phosphorylation of proteins in a lyzed crude mitochondrial/synaptosomal fraction. A quantitative relation between the electrophysiological parameters and the degree of phosphorylation of a 52K protein was found to follow a non-linear function.

Asymmetric relationships between homosynaptic long-term potentiation and heterosynaptic long-term depression

All synaptically-based neuropsychological theories of learning postulate that there are changes resulting from neural activity which are long-lasting and confined to specific sets of synapses. In the past decade a form of synaptic strengthening known as long-term potentiation (LTP) has been found which results from high-frequency neural activity and is of sufficient duration to model as a learning mechanism. Some early tests of the synaptic specificity of LTP in area CA1 of the hippocampus indicated that although LTP was specific to the tetanized pathway, in a converging untetanized pathway it was associated with depression of synaptic transmission lasting for at least 30 min. However, others have found that this heterosynaptic depression more usually decays within 5-15 min post-tetanus despite the maintenance of LTP in the tetanized pathway. Similarly, in the dentate gyrus (DG), LTP of either the lateral (LPP) or medial (MPP) components of the perforant path afferents has been associated with only short-lasting reciprocal heterosynaptic depression. Here, using more detailed measurement of stimulus intensity curves, we report that tetanization of either MPP or LPP reliably depresses synaptic transmission in the other pathway for at least 3 h. This heterosynaptic depression, considerably smaller than the usual magnitude of LTP, was obtained regardless of whether LTP had been produced in the tetanized homosynaptic pathway. Heterosynaptic long-term depression was not observed if the test pathway had been previously tetanized.