As the 45 year anniversary for long-term potentiation (LTP) is just around the corner, I thought it would be interesting to review the rich history of this field and where we are. Some of the most fascinating questions of our time involve how we learn and how our brain stores information. It is now accepted that memory is not a unitary process and can be broadly divided into declarative and nondeclarative forms (). Declarative memory is what we ordinarily mean by the term memory and involves the conscious recollection of facts and events. Nondeclarative memory underlies the changes in skilled behavior and its improvement with practice. The cellular changes that underline these two forms of memory differ considerably, but both are thought to involve changes in the strength of neuronal connections as proposed by Cajal () more than a century ago. This Review will focus on declarative memories. Based to a considerable degree on the profound memory loss observed in the patient H.M. (), who had a bilateral resection of the medial structures of the temporal lobe, attention was focused on the temporal lobes and particularly the hippocampus in its role in declarative memory. The notion that synaptic strength changes during learning and memory was refined into an elegant concrete model by Hebb in 1949 (), in which he postulated a synaptic modification for learning and memory that occurs as a consequence of coincidence between pre- and postsynaptic activity. However, as discussed below, experimental evidence that synapses are plastic in the mammalian brain had to wait almost 20 years, until the discovery of LTP (), in which brief, high-frequency stimulation, typically referred to as a tetanus, of hippocampal excitatory synapses produced a rapid and long-lasting increase in the strength of these synapses that could persist for many days (). LTP, which has been described at synapses throughout the brain, remains to this day one of the most attractive cellular models for learning and memory. Some of the confusion that has plagued the field of LTP may be due, in part, to the existence of multiple forms of LTP. The variables include the type of synapse, the stimulation parameters, the time analyzed after LTP induction, and the developmental age. Perhaps the most dramatic example of different forms is a comparison of LTP at CA1 hippocampal synapses and that at mossy fiber synapses onto CA3 pyramidal cells. LTP at CA1 synapses, which is broadly representative of LTP at excitatory synapses, is dependent on NMDA receptor activation and primarily involves a postsynaptic modification (see below). Mossy fiber LTP is independent of NMDA receptors and is entirely expressed presynaptically (). There is even evidence that under certain conditions CA1 synapses can express an NMDA receptor-independent component to LTP (). Finally, there are reports that there are mechanistic differences between neonatal animals (

Long-lasting potentiation of synaptic transmission in the dentate area of the unanaestetized rabbit following stimulation of the perforant path.

Long-lasting potentiation of synaptic transmission in the dentate area of the anaesthetized rabbit following stimulation of the perforant path.

The field began with Terje Lomo’s publication of a single author abstract (), when he was a student in Per Andersen’s lab. In this abstract Lomo concluded, “This represents an example of a plastic change in a neuronal chain, expressing itself as a long-lasting increase of synaptic efficacy. The effect, which may last for hours, is dependent on repeated use of the system.” Lomo was occupied with finishing his thesis and did not pursue his finding. It is my understanding that Tim Bliss, who majored in psychology at McGill, talked to Andersen about his interest in learning and memory and Andersen said he should talk to his student Lomo “who has something that will interest you” (). Thus began the collaboration that resulted in the landmark paper by Bliss and Lomo (), entitled Long-lasting potentiation of synaptic transmission in the dentate area of the anaesthetized rabbit following stimulation of the perforant path. This paper launched the field of LTP. It is impossible to overstate the importance of this paper; it is truly a landmark in the field of neuroscience and should be required reading for any student in the neurosciences. It elegantly outlines the logic for carrying out the experiments, which are remarkably well controlled and impeccably address virtually all of the possible artifacts that could confound their interpretation. They conclude that the long-lasting change they recorded is due to an increase in the strength of synaptic transmission. Furthermore, they make two additional fundamental discoveries, showing that LTP is saturable and that there is also an increase in the coupling between the synaptic response and the firing of postsynaptic neurons. There is not a single controversial finding in this paper, which is a very remarkable thing in this field. They end their paper with a most prescient summary of where the field will go. First, they outline that the increase in strength could be either due to an increase in transmitter release (importantly, the transmitter was not known and would have to await the pharmacological tools that identified glutamate as the transmitter, see below) or to an increase in the sensitivity of the postsynaptic cell to the transmitter. Second, they raise the question of whether this cellular phenomenon has anything to do with learning and memory. Figure 1 shows a photograph of Tim Bliss, Per Andersen, and Terje Lomo taken in 2003 at the Royal Society in London for the 30anniversary of the discovery of LTP.

From left to right: Timothy Bliss, Per Andersen, and Terje Lomo. This picture was taken by John Lisman in 2003 at a Royal Society meeting celebrating the 30 year anniversary of the discovery of LTP.

Long-lasting potentiation of synaptic transmission in the dentate area of the anaesthetized rabbit following stimulation of the perforant path.

With the discovery of LTP, three key questions immediately came to the forefront. (1) What occurs during the brief tetanus (∼1 s) that initiates LTP? This process is referred to as “induction.” (2) In what way are the synapses altered following LTP induction? This process is referred to as “expression.” (3) Is LTP involved in learning and memory? I have provided a timeline for what I consider to be some of the key discoveries that have propelled the field forward ( Figure 2 ). I have purposely stopped the timeline at 2005, because I believe that considerable time is required to accurately weigh the importance of new discoveries.

Lomo (). Bliss and Lomo (). LTP in slice (). Associativity (). LTP requires NMDARs (). EGTA blocks LTP (). Push-pull cannula: glutamate increase during LTP (). Mgblocks NMDARs (). LTP requires postsynaptic depolarization (). MNDARs are Capermeable (). APV impairs learning (). CaMKII: a Catrigger switch (). CaMKII as a memory storage devise (). LTP increases AMPAR-relative to NMDAR-EPSPs (). First glutamate receptor cloned (). Two-photon microscopy (). “Quantal analysis” of LTP (). CaMKII KO has impaired LTP (). “Silent” synapses and LTP (). CaMKII mimics and occludes LTP (). LTP is blocked by inhibiting exocytosis (). LTP of electrophysiologically tagged AMPARs (). Single AMPAR tracking (). Structural LTP ().

Long-lasting potentiation of synaptic transmission in the dentate area of the anaesthetized rabbit following stimulation of the perforant path.

Within a year the answer to this conundrum jumped out with a most amazing and simple discovery by Ascher () and independently by Mayer and Westbrook (). The NMDA receptor is profoundly voltage dependent and thus conducts little current at resting membrane potentials. Ascher and Mayer/Westbrook showed that this was due to a voltage-dependent block of extracellular Mgand that as the cell was depolarized this block was relieved. These studies involved the application of glutamate agonists onto dissociated neurons. The findings were quickly linked to excitatory synapses by Dale and Roberts (), who showed that unitary EPSPs in Xenopus embryos were composed of a fast nonNMDA receptor component and a slow NMDA receptor component. Combining the voltage sensitivity of the NMDA receptor with the requirement of postsynaptic depolarization, which occurs during a tetanus, not only explained the previously published baffling pharmacological results (), but also suggested that the Hebbian mechanism underlying LTP resides in the NMDA receptor itself, laying a mechanistic foundation for LTP as a compelling logical link to associative learning. A seemingly complex phenomenon turns out to be extraordinarily simple. So the NMDA receptor is critical for LTP, but in what way? The answer was not long in coming and it came, again, from the same players: Ascher () and Mayer/Westbrook (). Again, the answer was disarmingly simple. Unlike the AMPA receptor, the NMDA receptor is highly permeable to Ca. This finding immediately explained a previous finding by Lynch () reporting that chelating postsynaptic Caprevents LTP. So within a matter of a few years, the answer to what occurs during the 1 s tetanus, i.e., induction, was solved ( Figure 4 ). These were heady times for the LTP field. We were well on our way to providing the biophysical basis for learning and memory.

(B) The events occurring when the postsynaptic membrane is depolarized, as would occur during a high-frequency tetanus. The depolarization relieves the Mgblock of the NMDA receptor channel, allowing Na, K, and most importantly Cato flow through the channel. The primary target of the rise in Cain dendritic spines is the calcium-calmodulin-dependent kinase II (CaMKII) (modified from).

(A) The events occurring during low-frequency synaptic transmission. Glutamate is released from the presynaptic terminal and binds to both NMDA and AMPA receptors. Na + and K + flow through the AMPA receptor channel but not through the NMDA receptor channel, due to Mg 2+ block of this channel.

Model for the Induction of LTP in the CA1 Region of the Hippocampus

Figure 4 Model for the Induction of LTP in the CA1 Region of the Hippocampus

Making sense of these early observations had to await a deeper understanding of excitatory synaptic transmission. Due in large part to the lifetime work of Jeff Watkins, the pharmacology of these synapses was revealed (). This involved the design of a number of highly selective glutamate receptor agonists and antagonists. With these tools in hand, the transmitter was established to be glutamate, which acts primarily on NMDA receptors and non-NMDA receptors (AMPA receptors). AMPA receptors are responsible for the reliable moment-to-moment transmission. The NMDA receptors were an enigma. They were clearly expressed on neurons because the application of NMDA evoked strong responses. However, when the selective NMDA receptor antagonist APV was applied it had no effect on excitatory postsynaptic potentials (EPSPs), leading Collingridge et al. () to conclude that, “The firmest conclusion that can be drawn from the antagonist studies is that the NMA (now referred to as NMDA) receptor is not involved in mediating synaptic excitation in the Schaffer collateral-commissural pathway.” In the same series of experiments, and adding to the confusion, Collingridge et al. found that application of APV blocked LTP. They conclude the paper as follows: “the present study has shown that the NMA receptor plays no role in the mediation of synaptic transmission but may be involved in the generation of l.t.p.” What could be the explanation for the seeming incompatibility of these two observations?

It is interesting to note that very little attention (∼50 citations) was given to LTP for a decade after its discovery. The reason for this was primarily 2-fold. First, and most importantly, neither the neurotransmitter nor the receptors were known for these excitatory synapses that expressed this remarkable plasticity. Second, the development of the in vitro hippocampal slice preparation was essential for rigorous pharmacological and biophysical studies. Nevertheless, some fundamental discoveries were made during this time, which set the foundation for this field. McNaughton et al. () and shortly thereafter, Levy and Stewart (), reported that LTP had the property of “cooperativity” and “associativity.” A weak input, in which only a few excitatory synapses were tetanized, failed to induce LTP, whereas a strong input reliably induced LTP (cooperativity), although only in the tetanized pathway (“input specificity”). In addition, the simultaneous activation of two separate inputs, one of which is weak and fails to undergo LTP on its own, exhibits robust LTP when tetanized together with a strong input (associativity). These findings established LTP as a Hebbian process and raised a fundamental question central to LTP: how does the strong input communicate to the weak input? Clues to the answer came with two further observations. First, injecting depolarizing current into the postsynaptic cell could substitute for a strong tetanus () and, second, preventing depolarization during a strong tetanus by hyperpolarizing () or voltage clamping () the cell prevented LTP. These findings indicate that there are only two requirements for LTP: postsynaptic depolarization coupled with synaptic stimulation. There is no need to stimulate the synapses at high frequency. This is shown experimentally in Figure 3

The top series of diagrams (A1 and B1) illustrates schematically, at an expanded timescale, the stimulation of the excitatory synapses (Stim) and the control of the membrane potential (MP). The graphs below (A2 and B2) plot the maximal initial slope of the EPSP. In (A), synaptic stimulation was stopped and the cells were depolarized to 0 mV for 2 min. In (B), synaptic stimulation continued throughout the experiment and the cells were depolarized to 0 mV for 2 min. The recording electrode contained cesium to allow depolarization of the membrane. Each graph averages the results from 8–12 cells. Each slope measurement in an individual experiment was normalized to the average value of all points on the baseline (at least 10 min prior to each manipulation) for that experiment. Experiments were then divided into 20 s bins, each of which was averaged. Data are shown as mean ± SE (modified from).

In What Way Are the Synapses Altered following LTP Induction: Expression?