On the mechanism of enhanced release of [14C]glutamate in hippocampal long-term potentiation

Discovering long-term potentiation (LTP) - recollections and reflections on what came after

Chance events led me to a lifelong career in scientific research. They paved the way for being the first to see long-term potentiation of synaptic efficiency (LTP) in Per Andersen's laboratory in Oslo in 1966. Here I describe my way to this discovery and the experiments with Tim Bliss in 1968-1969 that led to Bliss and Lømo, 1973. Surprisingly, we later failed to reproduce these results. I discuss possible reasons for this failure, which made us both leave LTP research, in my case for good, in Tim's case for several years. After 30 years of work in a different field, I renewed my interest in the hippocampus and LTP in the early 2000s and published, for the first time, results that I had obtained 40 years earlier. Here I present my take on how interest in and research on LTP evolved after the early years. This includes a discussion of the functions of hippocampus as seen in those early days, the case of patient H.M., Donald Hebb's place in the story, the search for 'memory molecules' such as PKMζ, and the primary site for LTP expression (pre- and/or post-synaptic?). Throughout, I reflect on my life in science, how science is done and what drives it. The reflections are quite personal and I admit to mixed feelings about broadcasting them.

Effects of inhaled anesthetic isoflurane on long-term potentiation of CA3 pyramidal cell afferents in vivo

Isoflurane is a preferred anesthetic, due to its properties that allow a precise concentration to be delivered continually during in vivo experimentation. The major mechanism of action of isoflurane is modulation of the γ-amino butyric acid (GABA_A) receptor-chloride channel, mediating inhibitory synaptic transmission. Animal studies have shown that isoflurane does not cause cell death, but it does inhibit cell growth and causes long-term hippocampal learning deficits. As there are no studies characterizing the effects of isoflurane on electrophysiological aspects of long-term potentiation (LTP) in the hippocampus, it is important to determine whether isoflurane alters the characteristic responses of hippocampal afferents to cornu ammonis region 3 (CA3). We investigated the effects of isoflurane on adult male rats during in vivo induction of LTP, using the mossy fiber pathway, the lateral perforant pathway, the medial perforant pathway, and the commissural CA3 (cCA3) to CA3, with intracranial administration of Ringer’s solution, naloxone, RS-aminoindan-1, 5-dicarboxylic acid (AIDA), or 3-[(R)-2-carboxypiperazin-4-yl]-propo-2-enyl-1-phosphonic acid (CPP). Then, we compared these responses to published electrophysiological data, using sodium pentobarbital as an anesthetic, under similar experimental conditions. Our results showed that LTP was exhibited in animals anesthetized with isoflurane under vehicle conditions. With the exception of AIDA in the lateral perforant pathway, the defining characteristics of the four pathways appeared to remain intact, except for the observation that LTP was markedly reduced in animals anesthetized with isoflurane compared to those anesthetized with sodium pentobarbital. The results suggest that isoflurane may affect amplitude through activation of GABA_A receptors or mechanisms important to LTP in CA3 afferent fibers.

LTP and adaptation to inactivity: Overlapping mechanisms and implications for metaplasticity

LTP and other rapidly induced forms of synaptic modification tune individual synaptic weights, whereas slower forms of plasticity such as adaptation to inactivity are thought to keep neurons within their firing limits and preserve their capability for information processing. Here we describe progress in understanding the relationship between LTP and adaptation to inactivity. A prevailing view is that adaptation to inactivity is purely postsynaptic, scales synaptic strength uniformly across all synapses, and thus preserves relative synaptic weights without interfering with signatures of prior LTP or the relative capacity for future LTP. However, recent evidence in hippocampal neurons indicates that, like LTP, adaptation to AMPA receptor blockade can draw upon a repertoire of synaptic expression mechanisms including enhancement of presynaptic vesicular turnover and increased quantal amplitude mediated by recruitment of homomeric GluR1 AMPA receptors. These pre- and postsynaptic changes appeared coordinated and preferentially expressed at subset of synapses, thereby increasing the variability of miniature EPSCs. In contrast to the NMDA receptor-, Ca2+ entry-dependent induction of LTP, adaptation to inactivity may be mediated by attenuation of voltage-sensitive L-type Ca2+ channel function. The associated intracellular signaling involves elevation of betaCaMKII, which in turn downregulates alphaCaMKII, a key player in LTP. Thus, adaptation to inactivity and LTP are not strictly independent with regard to mechanisms of signaling and expression. Indeed, we and others have found that responses to LTP-inducing stimuli can be sharply altered by prior inactivity, suggesting that the slow adaptation changes the rules of plasticity-an interesting example of "metaplasticity".

Retrograde signaling in the development and modification of synapses

Retrograde signaling from the postsynaptic cell to the presynaptic neuron is essential for the development, maintenance, and activity-dependent modification of synaptic connections. This review covers various forms of retrograde interactions at developing and mature synapses. First, we discuss evidence for early retrograde inductive events during synaptogenesis and how maturation of presynaptic structure and function is affected by signals from the postsynaptic cell. Second, we review the evidence that retrograde interactions are involved in activity-dependent synapse competition and elimination in developing nervous systems and in long-term potentiation and depression at mature synapses. Third, we review evidence for various forms of retrograde signaling via membrane-permeant factors, secreted factors, and membrane-bound factors. Finally, we discuss the evidence and physiological implications of the long-range propagation of retrograde signals to the cell body and other parts of the presynaptic neuron.

Glutamatergically induced pattern of Ca2+ driving potential as a mechanism of postsynaptic plasticity

Simulation studies were performed in a model of neuronal dendrite with Na+ and K+ channels and with ionotropic and metabotropic glutamate receptors. The ionotropic receptors were either N-methyl-D-aspartate (NMDA)-sensitive, voltage-dependent, and permeable to Ca2+, Na+, and K+, or non-NMDA-sensitive, voltage-independent, and permeable to Na+ and K+. The metabotropic receptors provided a catalytic effect on Ca2+-induced Ca2+ release from intracellular stores. Local intracellular concentration [Ca2+]i in the cytoplasm was changed because of exchange with the stores, axial diffusion, and transmembrane inward passive and outward pump fluxes. Tonic activation of ionotropic and metabotropic receptors in a particular range of intensities triggered the formation of spatially periodic [Ca2+]i hot and cold bands arising from an initial uniform state. The period and width of the bands were smaller at higher levels of tonic NMDA activation and higher metabotropically controlled rates of Ca2+-induced Ca2+ release. The bandwidths also depended on the dendrite diameter, the specific membrane, and cytoplasm resistivity. This activity-induced pattern led to long-term, spatially inhomogeneous change in local excitatory postsynaptic potentials (EPSPs) of NMDA synapses phasically activated with the same presynaptic intensity. The phasic EPSPs were potentiated if the synapse occurred in the hot band.

Presynaptic component of long-term potentiation visualized at individual hippocampal synapses

Long-term potentiation has previously been studied with electrophysiological techniques that do not readily separate presynaptic and postsynaptic contributions. Changes in exocytotic-endocytotic cycling have now been monitored at synapses between cultured rat hippocampal neurons by measuring the differential uptake of antibodies that recognize the intraluminal domain of the synaptic vesicle protein synaptotagmin. Vesicular cycling increased markedly during glutamate-induced long-term potentiation. The degree of potentiation was heterogeneous, appearing greater at synapses at which the initial extent of vesicular turnover was low. Thus, changes in presynaptic activity were visualized directly and the spatial distribution of potentiation could be determined at the level of single synaptic boutons.

A synaptic model of memory: long-term potentiation in the hippocampus

Long-term potentiation of synaptic transmission in the hippocampus is the primary experimental model for investigating the synaptic basis of learning and memory in vertebrates. The best understood form of long-term potentiation is induced by the activation of the N-methyl-D-aspartate receptor complex. This subtype of glutamate receptor endows long-term potentiation with Hebbian characteristics, and allows electrical events at the postsynaptic membrane to be transduced into chemical signals which, in turn, are thought to activate both pre- and postsynaptic mechanisms to generate a persistent increase in synaptic strength.

Presynaptic ultrastructural correlates of long-term potentiation in the CA1 subfield of the hippocampus

To determine if long-term potentiation (LTP) is accompanied by changes in the ultrastructural distribution of calcium within presynaptic terminals, calcium was localized at the electron microscopic level using an oxalate/pyroantimonate histochemical technique. Following the induction of LTP at the Schaffer collateral/commissural synapses in the CA1 subfield of the rat hippocampal slice, there was a significant decrease (30%) in the percentage of synaptic vesicles containing calcium deposits. This effect could be accounted for by both a significant reduction in the average number of calcium deposit-bearing vesicles and a significant increase in the average number of synaptic vesicles per terminal profile in slices that displayed LTP. These changes persisted for at least one hour following the induction of LTP and were not observed in slices that received high-frequency stimulation in the presence of the N-methyl-D-aspartate (NMDA) receptor antagonist, 2-amino-5-phosphonovaleric acid (APV, 50 microM), which blocked LTP. These data suggest that LTP may be accompanied by long-term changes in intraterminal calcium homeostasis and the number of synaptic vesicles. These effects may be related to the reported increase in transmitter release following the induction of LTP.

Molecular mechanisms of neuronal plasticity during learning: the role of secondary messengers

We present published data along with our own results concerning the role of second messengers and their intracellular receptors in molecular mechanisms associated with the plasticity of neurons during learning. The participation of cyclic 3',5'-adenosine monophosphate, cyclic 3',5'-guanosine monophosphate, calcium, calmodulin, and also the metabolic products of inositol phospholipids, inositol-1,4,5-triphosphate, diacylglycerol and the protein kinase C activated by it, arachidonic acid, and the products of its lipoxygenase oxidation during the regulation of neuronal plasticity over the course of prolonged potentiation, sensitization, habituation, and classical associative training are discussed.

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.

The effects of external calcium on long-term potentiation in the rat hippocampal slice

Extracellular excitatory postsynaptic potentials (EPSPs) were recorded from the stratum radiatum of CA1 of the rat hippocampal slice. The effect of altering the external Ca concentration was studied on the amplitude of the low frequency evoked EPSP preceding and following the production of long-term potentiation (LTP). The double logarithmic plot of the relationship between the EPSP amplitude and external Ca had a maximum slope of 2.1, implying Ca cooperativity in transmitter release. The production of LTP did not alter the slope. The amplitude of LTP was found to be highly dependent on the external Ca concentration, with LTP increasing from a 5% increase in the EPSP amplitude in 0.8 mM Ca, to a 65% increase in the EPSP amplitude in 2.0 mM Ca.

Increased hydrolysis of phosphatidylinositol-4,5-bisphosphate in long-term potentiation

Inositol phospholipid hydrolysis was examined in slices and synaptosomes prepared from area CA3 of control hippocampi and hippocampi in which long-term potentiation (LTP) was induced in vivo. In both synaptosomes and slices, LTP was associated with an increase in [3H]inositol labelling of inositol phosphates but not phosphoinositides. Glutamate (10(-3) M) significantly increased labelling of inositol phosphates in slices obtained from control tissue but had no effect either in slices obtained from potentiated tissue or in synaptosomes obtained from control or potentiated tissue. The finding that glutamate had no significant effect in slices prepared from potentiated tissue suggests that glutamate-mediated stimulation of inositol phospholipid hydrolysis in a postsynaptic compartment may be saturated in LTP. The possibility that the increase in phospholipid hydrolysis associated with LTP which we report here may be linked with the previously reported increase in transmitter release is discussed.

Long-term potentiation of synaptic transmission in the hippocampus of the rat; effect of calmodulin and oleoyl-acetyl-glycerol on release of [3H]glutamate

The effect of calmodulin (CaM) and oleyl-acetyl-glycerol (OAG, a synthetic analogue of diacylglycerol, DAG) on K+-induced release of radiolabelled glutamate (Glu), was examined in synaptosomes prepared from area CA3 of control rats and rats in which long-term potentiation (LTP) had been induced in vivo by a brief train of high-frequency stimulation. Both CaM and OAG significantly enhanced release from control tissue in a dose-dependent manner. Release of preloaded L-[3H]Glu was significantly greater in synaptosomes prepared from potentiated tissue than from control tissue but neither CaM nor OAG further enhanced release in potentiated preparations. The occlusion of the two effects suggests that CaM and endogenous DAG are likely to be involved in control of Glu release and the enhancement of release associated with LTP.