Molecular and Cellular Approaches to Memory Allocation in Neural Circuits Alcino J. Silva*, Yu Zhou, Thomas Rogerson, Justin Shobe, and J. Balaji Departments of Neurobiology, Psychiatry & Biobehavioral Sciences, Psychology and the Brain Research Institute, University of California, Los Angeles, 695 Charles Young Drive South, Los Angeles, CA 90095-1761, USA Abstract Although memory allocation is a subject of active research in computer science, little is known about how the brain allocates information within neural circuits. There is an extensive literature on how specific types of memory engage different parts of the brain, and how neurons in these regions process and store information. Until recently, however, the mechanisms that determine how specific cells and synapses within a neural circuit (and not their neighbors) are recruited during learning have received little attention. Recent findings suggest that memory allocation is not random, but rather specific mechanisms regulate where information is stored within a neural circuit. Novel methods that allow tagging, imaging, activation and inactivation of neurons in behaving animals, promise to revolutionize studies of brain circuits, including memory allocation. Results from these studies are likely to have a considerable impact on both computer science as well as on the understanding of memory and its disorders. Introduction How and where specific items are stored has a lot to do with how easily they can be retrieved and used. Organized storage saves space (for example superfluous or duplicate items can be eliminated), it minimizes search times and reduces errors during retrieval. Memory allocation refers to a set of processes that determine where information is specifically stored in a neural circuit. Are there neurobiological processes that determine which cells and synapses within a given circuit are engaged during learning, or is this random? Does memory allocation involve competition between different cells (or synapses) activated during learning? Do memory allocation processes take place at different time scales? The allocation of information is an especially important problem for the brain because of the enormous number of related memories stored through out a lifetime. Without a mechanism that appropriately groups and separates memories, how would the brain store so many complex memories of both similar and discrete events? One possibility is that the brain stores related memories in overlapping populations of neurons and in synapses within the same dendritic branch, so that activation of one component of the memory increases the likelihood of retrieval of other related components. This strategy would require mechanisms that regulate where memory is stored in neural networks. The principles and mechanisms of memory allocation are not only fascinating because of the insights they provide into how the brain stores information, but they also have far reaching implications for the design of artificial intelligence systems and for cognitive disorders. The central question that we will address in this review is whether memory allocation is random or *To whom correspondence should be addressed: silvaa@ucla.edu. NIH Public Access Author Manuscript Science. Author manuscript available in PMC 2010 March 24. Published in final edited form as: Science. 2009 October 16 326(5951): 391���395. doi:10.1126/science.1174519. NIH-PA Author Manuscript NIH-PA Author Manuscript NIH-PA Author Manuscript

whether specific mechanisms determine where information is stored within a neural network. Recent findings suggest that there are mechanisms that regulate allocation of memory to specific neurons in a network and modulate the synaptic selection processes that determine where these memories are stored (Fig. 1). CREB and memory allocation The cAMP Responsive Element Binding protein (CREB) regulates the transcription of other genes and has a well-known role in the stability of synaptic potentiation and memory (1,2). A recent series of papers (3���5) provided compelling evidence that there are molecular and cellular processes that determine which cells are recruited to store information within a neural circuit. In particular, this work suggested the hypothesis that CREB activated during learning triggers changes in the cell (i.e. increase in excitability) that then affect whether that cell participates in subsequent memories. This idea was tested by artificially increasing the levels of CREB in amygdala neurons using a replication defective Herpes viral vector. The initial results showed that higher CREB levels increase the probability (~3 fold) that amygdala neurons participate in memory for tone fear conditioning (3). In this form of Pavlovian conditioning, animals learn to associate a tone with an aversive event, such as a mild foot shock. The amygdala is a subcortical brain structure with a well-known role in emotional memories such as fear memory (6,7). ARC (Activity-Regulated Cytoskeletal-associated protein) RNA, a gene required for synaptic function and memory, was used to identify the neurons encoding a tone conditioning memory. ARC has been extensively used to determine which neuronal populations are activated by specific behavioral stimuli (8),(9). Immediately after a memory test, amygdala cells transfected with the viral CREB (identified through its fluorescent tag) were three times more likely to express ARC RNA (i.e., be involved in memory) than neighboring cells (3). Thus, CREB levels seem to bias which neurons encoded tone conditioning in the amygdala. It is conceivable that an interaction between the effects of the viral vectors used and the genes transfected could have affected the results of these early studies. To address this and other possible confounding interpretations of the results, cell lesion and inactivation strategies were also used to probe the role of CREB in memory allocation (4,5). A targeted cell lesion strategy was used to explore further the possibility that CREB biases the allocation of memory for tone conditioning in the amygdala (4). The authors took advantage of a transgenic mouse with a silenced diphtheria toxin receptor to specifically kill the cells with the virally-delivered CREB in mice. A replication defective Herpes viral vector carried CREB and a recombinase that could activate the silenced diphtheria receptor gene by deleting a RNA translation STOP sequence. CREB biased memory allocation since killing the cells transfected with the viral CREB disrupted memory for tone conditioning, while ablating the cells transfected with the control virus had no effect. A series of elegant control experiments showed that ablating the CREB cells did not prevent the animals from making new amygdala-dependent memories, and nor did it affect memories acquired before viral infection. Instead, killing the cells with the viral CREB only affected the memory for tone conditioning acquired after viral transfection. Targeted killing of nearly 20% of cells in a neural circuit may have had unintended effects that could confound the interpretation of behavioral experiments. However, this does not seem to have been the case since similar results (5) to those just described were obtained with another approach (10) that allows for reversible neuronal inactivation. This strategy takes advantage of a Drosophila receptor (the allatostatin receptor) that can be functionally linked to potassium channels in mouse neurons. When activated by the allatostatin receptor, these channels silence neurons (keep them from firing action potentials). Since mice lack the ligand for the allatostatin receptor, only neurons that have the exogenously expressed receptor can be inactivated by treatments with the allatostatin peptide. The results (5) show that inactivating amygdala cells Silva et al. Page 2 Science. Author manuscript available in PMC 2010 March 24. NIH-PA Author Manuscript NIH-PA Author Manuscript NIH-PA Author Manuscript