Affect is the fundamental neuropsychological state combining value- and arousal-related processes underpinning emotion and mood. A major goal of the emerging field of affective science is to explain the mechanisms of valuation within the brain. A core network of brain activity is seen across mammals in response to appetitive or aversive stimuli, and appears to be largely independent of stimulus modality (Bissonette et al., 2014 ; Hayes et al., 2014a ). However, the underlying mechanisms of valuation (i.e., appetitive- and aversive-related brain activity) are unclear, and there is particularly little information about how these two valuative networks interact. One candidate which is likely central to the activity of both networks is the neurotransmitter γ-aminobutyric acid (GABA). Here, I briefly discuss some of the evidence pointing to GABA as a central player in mediating appetitive and aversive activity throughout the brain. The broader implication is that the role of GABA in valuative processing may be at the heart of affective regulation, and thus also important for a wide variety of psychological phenomena, from emotion (Stan et al., 2014 ) and impulsivity (Hayes et al., 2014b ) to sense of self (Wiebking et al., 2014a , b ). Keys to understanding GABA circuitry An exploration of GABA in appetitive and aversive behavior began following its identification in the mammalian brain (Roberts and Frankel, 1950 ). Although central dopamine was discovered seven years later, many barriers to GABA-related research—including its relative ubiquity throughout the brain, the robust effects of GABAergic drugs administered systemically (e.g., which can easily lead to seizures or immobility), and little knowledge about GABAergic neurocircuitry—has led to a much greater understanding of dopamine in this context (Iversen and Iversen, 2007 ). Early advances in rodents were nonetheless made delineating key roles for GABA in consummatory behavior, stress, and anxiety (Kelly et al., 1977 ; Biggio et al., 1990 ). Studies such as these revealed that beyond dopamine, GABA was involved in mediating motivated behaviors in widespread, but regionally-selective ways (Kelly et al., 1977 ), and that dynamic cortical and subcortical changes to GABAergic microcircuits were involved (Biggio et al., 1990 ). Improved mapping of the extensive brain GABAergic circuits (illustrated in Figure 1 ), coupled with technological advances in detection and manipulation, have partly driven the recent focus of the role of GABA, and its sister excitatory neurotransmitter glutamate, in value-related processing. Moreover, structural advances have continued steadily, from the identification of key GABAergic hubs, such as the basal ganglia and nucleus accumbens (Groenewegen and Russchen, 1984 ), to more recent elaborations on the nature of inter- and intra-regional short and long-range GABAergic connections (Caputi et al., 2013 ). GABA circuits are uniquely situated as both local communicators and whole-brain integrators, given their dynamic control over excitatory and inhibitory signal conduction through axo-dendritic and astrocytic synapses (Frola et al., 2013 ) and their role in broader oscillatory and synchronistic activities (Melzer et al., 2012 ). Figure 1 Simplified schematic diagram of GABAergic circuitry underlying valuative processing . Inhibitory connections, mainly GABAergic, are indicated by black arrows; excitatory connections, which are glutamatergic unless otherwise indicated, are indicated by green arrows. Brain regions are grouped by neocortex (blue), basal ganglia (brown), thalamus (orange), midbrain/mesencephalon (gray), amygdala (purple) and brainstem (green) for illustrative purposes. Adapted from Dalley et al. ( 2011 ), Squire et al. ( 2012 ), and Nieh et al. ( 2013 ). GABAergic interneurons, particularly parvalbumin-containing, are fundamental drivers of cortical oscillations, which emerging research suggests may be a fundamental context-dependent mechanism for intra- and inter-regional communication (Sohal, 2012 ; Jadi and Sejnowski, 2015 ). For instance, beyond the hippocampus and select regions of the neocortex, where these oscillations are better studied, there is also evidence for GABA-driven oscillatory synchronization within the striatum (Sharott et al., 2009 )—a hub region connecting the cortex to the basal ganglia and heavily involved in motivation and valuative processing. Moreover, there is evidence that GABA cells are necessary for sustained reward-related signaling, as noted in a study of reversal learning in mice with decreased levels of prefrontal cortical GABAergic interneurons (Bissonette et al., 2015 ). Going forward, I briefly discuss a sample of recent studies which support the fundamental role of GABA in valuative processing and highlight future directions which will likely contribute to advances in this area. GABAergic microcircuits regulate valuative networks The present focus on GABA is not intended to ignore
展开▼
