C Peter Bengtson & Hilmar Bading
Department of Neurobiology, Interdisciplinary Center for Neurosciences (IZN), University of Heidelberg, Germany

https://doi.org/10.36866/pn.60.29

Textbook descriptions of the physiology of memory consistently refer to Hebb’s postulate that, when one neuron repeatedly takes part in activating another, the efficiency of this activation will increase (Hebb, 1949). Long term potentiation (LTP) was regarded as the manifestation of Hebb’s postulate following its discovery over 30 years ago (Bliss & Lomo, 1973), and has remained the primary experimental platform for investigating the physiology of synaptic plasticity and memory. LTP studies typically use the classic brain slice experimental preparation, with single (usually extracellular) recording electrodes and the electrical stimulation of a subset of synaptic inputs. Applications of these techniques to various brain regions have revealed a surprising diversity in the conditions, which facilitate plasticity. However, the cellular mechanisms underlying the maintenance (beyond 4 hours) of changes in synaptic efficacy, that may be critical for long-term memory, remain poorly understood. While stimulus-induced gene transcription mediated by the cAMP response element (CRE) appears to play a critical role, the multitude of genes whose regulation is affected by LTP appears endless. Studies of mice with targeted disruption of genes has done little to clarify the sequence of molecular events critical for late phase LTP. Also, the brief (<8 hours) lifespan of slice preparations, and the low tissue sample volumes of single electrode work, impede or complicate most molecular analyses. Our use of a cell culture system, and the assessment of network properties as an indicator of plasticity (Arnold et al. 2005), must seem a radical departure from the traditional LTP assay. The advantages are obvious, however: signalling events and the expression of genes are easily monitored and manipulated in cultured neurons. The idea is that network features such as activity patterns may undergo stimulus-induced changes, perhaps long-lasting changes that may require the same set of genes responsible for late LTP or even learning and memory. Assessment of network properties can be done non-invasively, and is straightforward using an array of 60 micro-electrodes embedded into each culture dish. The observed changes were dramatic: upon a brief application of a GABAA receptor antagonist (that triggered trains of action potentials and generated robust calcium signals both in the cytoplasm and the nucleus), the network activity pattern changes from random firing to periodic, synchronized bursting (Fig. 1). This highly organized activity pattern was maintained for well over 24 hours and, most excitingly, required gene transcription taking place in a critical period of 2 hours after induction (Fig. 1).

The synchronous nature of bursting across the network implies that activity in individual neurons successfully contributes to eliciting activity in other neurons in alignment with Hebb’s postulate. On a synaptic level, miniature excitatory postsynaptic currents (mEPSCs) mediated by AMPA receptors are increased in frequency, and to a lesser extent in amplitude, following induction of network plasticity. This indicates an increase in synaptic efficacy. This and several other features of LTP at synapses between hippocampal pyramidal neurons in slice preparations are also features of our new plasticity model. The persistence of bursting, like LTP, requires the activation of NMDA receptors (Morris et al. 1986), extracellular signal regulated kinases (ERK1/2) (English & Sweatt, 1997) as well as protein synthesis and gene transcription (Nguyen et al. 1994). Furthermore, activation of cytoplasmic and nuclear calcium/calmodulin dependent protein kinases and CREB­ mediated gene expression are features of both models (Malenka et al. 1989; Bading, 2000; Hardingham et al. 2001). Thus, the new system may shape up as a promising experimental platform for plasticity that, given its simplicity, may facilitate the analysis of key signals such as nuclear calcium (Bading, 2000) and the search for genes important for learning and memory.

References

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