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You are watching: A basic assumption underlying short-term memory is that it is

Siegel GJ, Agranoff BW, Albers RW, et al., editors. Basic Neurochemistry: Molecular, Cellular and Medical Aspects. 6th edition. Philadelphia: Lippincott-Raven; 1999.

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Correspondence to Bernard W. Agranoff, MentalHealth Research Institute, University of Michigan,1103 E. Huron, Ann Arbor, Michigan48104-1687.

Current hypotheses concerning the neurochemical basis of memory arebased on a number of premises, which are enumerated here.The remainder of this chapter relies on their validity.

The generally accepted basic behavioral paradigm forstudying learning and memory is the conditionedresponse

What we mean by learning and memory encompasses a widevariety of adaptive responses in greatly differentspecies. Pavlov"s characterization of conditioning<2> hasserved as a standard template for acceptablecriteria in intact animals and, by analogy, incellular and subcellular models of learning andmemory. He emphasized temporal requirements foroptimal learning: in order for learning to occur,the conditioned, or neutral, stimulus (CS), suchas a tone, must precede the unconditioned stimulus(US),such as food presentation, which results in theresponse, salivation. Similarly, a punishingelectrical shock that results in altered heartrate or a puff of air that results in an eye blinkcan serve as unconditioned stimuli to be pairedwith neutral light or sound stimuli. Thecontingency criteria, that is, that the CSprecedes the US and that there be an optimallatency, or CS—US interval, must be metin true associative learning. These criteria canbe applied to simple systems, such as theinvertebrate nervous system, where identifiedneurons appear to mediate behavior, or tomammalian brain slice preparations, in whichperformance is measured electrophysiologically, asdescribed later.

Protein synthesis is required to form long-term, butnot short-term, memory

Studies in fish, rodents, birds and invertebrates haveindicated that the formation of short-term andlong-term memory can be distinguished on the basisof susceptibility to antibiotic agents that blockbrain protein synthesis, such as puromycin,cycloheximide and anisomycin <1>.Consideration of the temporal aspects of learningand memory and knowledge of the temporal scale ofbiochemical processes have led to the predictionthat learning and short-term memory formation,which can occur within milliseconds and last forminutes to hours, are mediated bypost-translational modification at the synapse.Biochemical processes that could mediateshort-term memory are discussed below. Long-termmemory, which may take longer to form and can lasta lifetime, is predicted to be mediated by aprocess that (i) requires de novoprotein synthesis, and (ii) is therefore dependenton the neuronal genome, and (iii) thus, mustrequire that there be communication between thecell surface and nucleus, presumably by axonaltransport (see Chap. 28).

Behavioral information is ultimately stored insynaptic connections

This concept can be traced to Ramón y Cajal,who first recognized the enormous complexity ofthe neuronal networks in the brain. Although itmay seem self-evident that an organism"s mostcomplex function resides in its most complexstructures, the premise remains inferential. It issupported by many indications that synapticcomplexity increases with development andenvironmental input (see SYNAPTIC PLASTICITY,below). Alternative hypotheses, such as thatmemory resides in glia, have not been pursuedsufficiently to warrant further considerationhere. It was once proposed that memory is notbased on altered chemical states but rather onreverberating electrical circuits or chargedistributions, yet there is ample evidence thatmemory survives seizures as well as periods ofelectrical silence in the brain. Neurochemistsgenerally adopt the premise that long-livedbiological phenomena are ultimately preserved andprotected in the form of covalent chemical bonds,but this cannot lead to a simple accretionhypothesis in which new synaptic connections are“soldered,” because whilememories can last a lifetime, brain proteins turnover at measurable rates of hours, days or weeks.Altered synaptic relationships that underliestored long-term memory, which can last for years,must then depend on feed-forward loops generatedin the cell nucleus. There is ample independentevidence that the genome regulates phenotypicexpression throughout the lifetime of the cell,and in the case of neurons, this means thelifetime of the individual. It remains to bedemonstrated which of the events that occur duringlearning at the presynaptic and postsynapticmembrane surfaces are communicated to theirneuronal nuclei, as is discussed next.

These three assumptions serve as a framework forevaluating the diverse experimental neurochemicalapproaches to attaining an understanding oflearning and memory that follow in thischapter.

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