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

Derivation of Encoding Characteristics of Layer II Cerebral Cortex

Computer simulations of layers I and II of pirifonn (olfactory) cortex indicate that this biological network can generate a series of distinct output responses to individual stimuli, such that different responses encode different levels of information about a stimulus. In particular, after learning a set of stimuli modeled after distinct groups of odors, the simulated network's initial response to a cue indicates only its group or category, whereas subsequent responses to the same stimulus successively subdivide the group into increasingly specific encoding of the individual cue. These sequences of responses amount to an automated organization of perceptual memories according to both their similarities and differences, facilitating transfer of learned information to novel stimuli without loss of specific information about exceptions. Human recognition performance robustly exhibits such multiple levels: a given object can be identified as a vehicle, as an automobile, or as a Mustang. The findings reported here suggest that a function as apparently complex as hierarchical recognition memory, which seems suggestive of higher ‘cognitive’ processes, may be a fundamental intrinsic property of the operation of this single cortical cell layer in response to naturally-occurring inputs to the structure. We offer the hypothesis that the network function of superficial cerebral conical layers may simultaneously acquire and hierarchically organize information about the similarities and differences among perceived stimuli. Experimental manipulation of the simulation has generated hypotheses of direct links between the values of specific biological features and particular attributes of behavior, generating testable physiological and behavioral predictions.

Rate-limiting step for transmission at excitatory synapses in hippocampus

To examine mechanisms that might be responsible for limiting transmission at excitatory synapses in hippocampus, we analyzed the relationship between extracellular calcium concentrations (1-6 mM) and postsynaptic responses in field CA1 of hippocampal slices using low stimulation intensities and a paired-pulse paradigm. Three effects were observed: One, the relationship between calcium levels and the slope (or amplitude) of the postsynaptic response was described by a sigmoidal function with an asymptote at about 4 mM. Double reciprocal pilots relating calcium concentration to the initial slope of EPSPs provided evidence for the cooperativity expected between calcium ions and transmitter release. Two, both the rise time and half-decay time of the postsynaptic responses were reduced with increasing calcium concentrations. These effects of calcium were more pronounced on the first response elicited by paired-pulse stimulation and were considerably attenuated by 2 microM bicuculline, indicating that feed-forward inhibition was positively related to calcium concentration and differentially activated by repetitive stimulation. However, inhibition was not responsible for the asymptotic relationship observed between calcium and response size. Three, while increasing the calcium concentration beyond 4 mM did not further affect the initial slope of excitatory postsynaptic potentials (EPSPs), paired-pulse facilitation and 4-aminopyridine were still effective in increasing response size. These results suggest 1) that neither the number of postsynaptic receptors nor the number of transmitter quanta available for release were limiting transmission as a function of the calcium concentration; and 2) that calcium entry into presynaptic terminals was likely to represent the limiting step under the conditions used.

Long-term potentiation: persisting problems and recent results

In this paper we discuss recent experimental results pertinent to three unresolved issues regarding the long-term potentiation (LTP) effect: the nature of its enduring substrates, the biochemical mechanisms that produce it, and its potential role in memory. LTP appears to be triggered by a postsynaptic influx of calcium and is associated with alterations in the shape of dendritic spines and probably the formation of new synapses. We discuss the possibility that morphological reorganization also modifies membrane surface chemistry of synaptic elements. Evidence is presented that LTP is not associated with changes in presynaptic calcium currents. Activation of protein kinase C is shown to be insufficient for the induction of LTP, although it may play a modulatory role. The hypothesis that activation of a calcium-sensitive protease (calpain) is pivotal to the establishment of LTP is supported by experiments showing that a calpain inhibitor, leupeptin, blocks LTP. Furthermore, activation of NMDA receptors, an event implicated in LTP induction, is accompanied by calcium-sensitive proteolysis of spectrin, a major dendritic cytoskeletal protein. The finding that stimulation patterns designed to mimic naturally-occurring cell discharge patterns are highly effective for LTP induction greatly strengthens the hypothesis that LTP actually occurs during the encoding of information in cortical systems. Potential contributions of LTP to learning are explored using computer simulations of a simple cortical network.

Phorbol ester-induced synaptic facilitation is different than long-term potentiation

The studies described here tested the hypothesis that the changes in synaptic efficacy produced by phorbol esters in hippocampal slices are equivalent to the long-term potentiation (LTP) induced by high-frequency stimulation. In contrast to the extremely stable synaptic potentiation induced by electrical stimulation, the facilitatory effects of phorbol 12,13-diacetate and phorbol 12,13-dibutyrate were transient: washout of the drugs restored normal responses in approximately 1-2 and 2-4 hr for phorbol diacetate and phorbol dibutyrate, respectively. It is noteworthy that the more liposoluble of the phorbol esters required longer washout periods. Robust LTP still occurred in response to high-frequency stimulation after washout of phorbol esters and to a lesser degree during their application. Treatment of slices with H-7, an inhibitor of protein kinase C, did not prevent LTP induction although it significantly affected neuronal excitability and produced effects opposite to those of phorbol esters. Finally, phorbol esters altered responses to repetitive stimulation in a way that could account for the reduced LTP elicited in their presence. These results indicate that the increases in synaptic responses caused by phorbol esters and high-frequency electrical stimulation are quite different and thus do not support the hypothesis that activation of protein kinase C, the presumed target of the phorbol esters, triggers LTP.

Chronic administration of a thiol-proteinase inhibitor blocks long-term potentiation of synaptic responses

It has been proposed that activation of a calcium-sensitive protease (calpain) is a crucial step in the induction of long-term potentiation (LTP). To test this hypothesis, we used chronic recording techniques to measure the effects of intraventricular infusion of leupeptin, a calpain inhibitor, on LTP in the hippocampus. Rats implanted bilaterally with stimulating electrodes in the Schaffer-commissural system and one recording electrode in the apical dendrites of field CA1 were fitted with osmotic mini-pumps delivering either leupeptin (20 mg/ml) or saline at a rate of 0.5 microliter/h into the lateral ventricle. Short bursts of high-frequency stimulation with the bursts delivered at 5/s were used to induce LTP in those animals which had stable responses for several days. Rats in the saline group (n = 11) exhibited an immediate LTP effect that remained in place over successive days of testing, while only 3 of 13 leupeptin treated animals showed evidence of LTP 24 h after high-frequency stimulation, and in only one of those was a sizeable effect recorded over several days. The average change in responses at the 24-h test point was +33% for the controls and +4% for the leupeptin group (P less than 0.01). The block of LTP induction was reversible, since high-frequency stimulation applied after disconnecting the pumps led to a robust LTP effect that lasted for several days in 6 of 7 animals tested. There were no detectable differences in baseline responses in the presence and absence of leupeptin.