Corticolimbic circuits in learning, memory, and disease

Changing environmental circumstances demand a high degree of flexibility in behavioral responsivity. Corticolimbic brain networks orchestrate flexible responding, especially when stimuli elicit defense or approach behaviors. The corticolimbic system processes a broad range of behavioral and cognitive functionality that includes motor programming and control, decision making, mnemonic function, and emotional regulation. Environmental factors, including stressand fearinducing stimuli, all have the potential to modify the corticolimbic system. The prefrontal cortices, amygdala, and hippocampus, which make up the corticolimbic system, are highly interconnected with the hypothalamicpituitary-adrenal axis, rendering them particularly susceptible to the chronic effects of stress (Vyas et al., 2002; Radley et al., 2004). In addition, alterations in corticolimbic circuitry are associated with various neuropsychiatric and neurological disorders. For example, the corticolimbic system processes traumatic fear memories, which have the potential to persist and become dysregulated, representing a central symptomatic dimension of fearregulatory disorders such as posttraumatic stress disorder (PTSD) (Bergstrom, 2016). Likewise, addictive drugs preferentially target the corticolimbic system, strengthening patterns of compulsive and habitual behaviors that define addiction (Robbins and Everitt, 2002). Various components of the corticolimbic system can degenerate, resulting in devastating neurological conditions such as Alzheimer and Parkinson disease and profound learning and memory deficits. Despite significant progress unraveling the neuroanatomy and functionality of the corticolimbic system, much remains unknown. This In Focus issue of The Journal of Neuroscience Research aims to highlight new perspectives and data on the anatomical organization and functional interactions of corticolimbic circuits in learning, memory, and disease. A special emphasis is placed on corticolimbic networks involved in processing emotional learning and memory. We brought together a diverse group of leading neuroscientists studying various aspects of the corticolimbic system, including amygdalohippocampal interconnectivity and long-range GABAergic projection neurons (McDonald and Mott, 2017), the temporal dynamics of systems-level memory consolidation processes in hippocampocortical circuits (Jasnow et al., 2017), cholinergic influence on corticolimbic circuitry during Pavlovian conditioning (Wilson and Fadel, 2017), and medial prefrontal control of Pavlovianinstrumental learning (Halladay and Blair, 2017). McDonald and Mott (2017) review the complex neuroanatomy of amygdalohippocampal interconnections, including those involving GABAergic projection neurons. The involvement of reciprocal connections between discrete amygdala nuclei and distinct layers of the hippocampal/parahippocampal regions in the temporal lobe memory system is well established. Less understood is how longrange GABAergic neurons are uniquely positioned to enhance the emotional contribution to declarative memory. The potential role of GABAergic amygdalohippocampal projections in both coupling synchronized oscillations and facilitating the synaptic plasticity involved in mnemonic function is highlighted. Memory disorders in relation to altered amygdalohippocampal circuitry are discussed, including PTSD, temporal lobe epilepsy, and Alzheimer disease. In their review, McDonald and Mott set the stage for designing new electrophysiological and behavioral experiments that selectively target different components of the amygdalohippocampal circuit. Rarely do circumstances in the environment reoccur. For organisms to respond to changing environmental circumstances in an adaptive way, Jasnow et al (2017) discuss how the nervous system has adapted mechanisms to