We recently described a critical role for adrenergic signaling in the hippocampus during contextual and spatial memory retrieval. To determine which neurons are activated by contextual memory retrieval and its sequelae in the presence and absence of adrenergic signaling, transcriptional imaging for the immediate-early gene Arc was used in control and mutant mice lacking norepinephrine and epinephrine. This imaging approach permits the identification of neuronal genomic activation specific to one of two behavioral epochs in the same animal. Analysis revealed several brain regions that were more greatly activated by re-exposure to a salient versus neutral context 1 d after training in control mice (e.g., hippocampal CA3 and CA1, the amygdala, the dorsolateral caudate/putamen, the primary motor cortex, and parts of the rhinal cortices). In mice lacking norepinephrine and epinephrine, many of these regions exhibited significantly reduced activation (e.g., hippocampal CA1), while other regions did not (e.g., hippocampal CA3). In consideration with previous results, the current findings suggest a hypothesis in which adrenergic signaling may be critical for the transfer of retrieved contextual information from CA3 to CA1, where it would be compared to online sensory information coming directly from the cortex.One goal for understanding memory is to define the neurons activated during memory storage and retrieval. Electrophysiologic recordings and brain lesions have been mainstays in this pursuit. More recently, imaging techniques have been used. These include functional brain imaging studies in humans using positron emission tomography (PET) and functional magnetic resonance imaging (fMRI), which are noninvasive and permit repeated measurements; however, they cannot resolve activity at the cellular level (Schacter and Wagner 1999;Mayes and Montaldi 2001). For this, induction of immediate-early gene (IEG) expression ("genomic activation") has been used in animals, offering high sensitivity and cellular resolution that can be analyzed throughout the brain (Clayton 2000). A disadvantage, however, is that each animal is assessed under a single condition.A technique that overcomes this disadvantage uses fluorescence in situ hybridization (FISH) to the IEG Arc. Arc (Lyford et al. 1995), also termed Arg3.1 (Link et al. 1995), is an activityregulated, cytoskeleton-associated protein implicated in synaptic plasticity and memory (Steward et al. 1998;Guzowski et al. 2000). Basal expression of Arc is very low in most neurons. However, transcription is rapidly induced by neuronal activation, and FISH to Arc reveals two small, intense intranuclear foci within 2 min of activation (Guzowski et al. 1999). By ∼20 min the nuclear signal has disappeared and perinuclear cytoplasmic labeling is transiently observed before disappearing due to dendritic transport of Arc mRNA (Steward et al. 1998). These properties allow one to determine the recent activation history of a neuron at two distinct times, referred to as cellular compartment analys...