REVIEW ARTICLE | FOCUS https://doi.org/10.1038/s41593-019-0503-3REVIEW ARTICLE | FOCUS https://doi.org/10.1038/s41593-019-0503-3 Neuromodulation in circuits of aversive emotional learning Ekaterina Likhtik! !1,2* and Joshua P. Johansen! !3,4* Emotional learning and memory are functionally and dysfunctionally regulated by the neuromodulatory state of the brain. While the role of excitatory and inhibitory neural circuits mediating emotional learning and its control have been the focus of much research, we are only now beginning to understand the more diffuse role of neuromodulation in these processes. Recent experimental studies of the acetylcholine, noradrenaline and dopamine systems in fear learning and extinction of fear respond- ing provide surprising answers to key questions in neuromodulation. One area of research has revealed how modular organiza- tion, coupled with context-dependent coding modes, allows for flexible brain-wide or targeted neuromodulation. Other work has shown how these neuromodulators act in downstream targets to enhance signal-to-noise ratios and gain, as well as to bind distributed circuits through neuronal oscillations. These studies elucidate how different neuromodulatory systems regulate aversive emotional processing and reveal fundamental principles of neuromodulatory function. euromodulation shapes emotional learning in distributed of signal-to-noise dynamics and context-dependent coding coor- circuits across the brain. Studies using tracing, lesions, dinated by modular organization of neuromodulatory nuclei. We Nrecordings and pharmacology have provided important describe how these neuromodulatory functions are fundamentally information about projection pathways and neuromodulator con- different from neuro-regulation: the latter acts via feedback loops to tributions to shifting behavioral state during learning1–14. This field maintain homeostasis, whereas neuromodulators are geared more continues to benefit from genetic and molecular developments in toward shifting circuits to a new baseline during learning. cell-specific circuit techniques that build on the existing foundation to create a more detailed analysis of how neuromodulatory systems Roles of Ach in emotional learning connect with and influence different cell types at target structures Acetylcholine modulates amygdala, prefrontal cortex and hip- during learning. Although a wide array of neurotransmitters and pocampus in emotional learning. Ach is a critical neurotransmit- neuropeptides can act as neuromodulators, here we focus on ace- ter for several functions in the central nervous system, including tylcholine (Ach), noradrenaline (NA) and dopamine (DA) in setting sleep–wake rhythms, attention, cue detection, working and mammals, and we highlight their actions in sculpting circuit-level spatial memory1,20–23. Ach signals through nicotinic (ionotropic) processing to change fear learning, extinction and fear-discrimina- and muscarinic (Gq- or Gi-coupled) receptors. There are two main tion learning. Although not discussed here, other neuromodulatory cholinergic centers that innervate the brain: the basal forebrain systems, such as serotonin, also exhibit some degree of projection (BF) and the pedunculopontine and laterodorsal tegmental nuclei specificity and regulate aversive emotional learning15,16. We high- (Fig. 1)3,23–26. The role of BF cholinergic projections in process- light recent findings in fear-related functions (for example, learned ing emotional learning and memory has been mostly extensively passive Pavlovian defensive responses, as opposed to other types of studied. The BF is an umbrella term for a collection of telencepha- aversive learning or conscious emotional processes17), as a great deal lon nuclei that include the septum, the nucleus basalis of Meynert of progress has been made in understanding general principles of (NBM), the substantia innominata (SI), the ventral pallidum (VP) neuromodulation using these behavioral approaches, and previous and the diagonal band2,3. Notably, the BF contains more GABAergic reviews have covered classical lesion and pharmacological find- and glutamatergic neurons than cholinergic neurons27,28. All three ings18–20. This emerging literature on neuromodulation of fear learn- cell types project to the cortical mantle, including medial prefron- ing and extinction reveals fundamental features of Ach, NA and DA tal cortex (mPFC), and to subcortical regions that partake in fear that are applicable to how they generally regulate brain function. and extinction learning5,29–33. BF innervation is roughly organized We first discuss how Ach sharpens the signal-to-noise ratio, cue along the anterior–posterior axis34,35 and has modular connectiv- processing and communication in the amygdala, cortex and hip- ity: anatomically distributed cholinergic BF cell groups innervate pocampus during emotional learning. Next, we examine how NA targets that contribute to different aspects of emotional learning. exerts its effects on target microcircuitry and molecular signaling During fear conditioning, neural spiking in the dorsal hippocampus during fear and extinction learning. We then discuss recent work (dHPC) coincides with context encoding, in the lateral and basolat- demonstrating how the anatomical organization of the locus coe- eral amygdala (LA/B) it coincides with encoding the conditioned ruleus–NA system allows it to flexibly control the balance between cue, and in the mPFC it coincides with both cue and context36,37. these opposing states at anatomically distributed sites. Finally, we The modular pattern of BF cholinergic innervation of these regions discuss the role of the DA system in fear learning and examine how allows for its multipronged impact on emotional learning, whereby it acts as a detector of when fear responding is no longer adaptive in the amygdala, cholinergic input modulates cue-based aversive to switch behavioral strategies to extinction. Overall, we identify learning; in the dHPC, spatial processing; and in the mPFC, cue several mechanisms by which neuromodulators adjust circuit-level encoding and consolidation of extinction (Fig. 2). Accordingly, opto- communication during emotional learning, including synchro- genetic activation of cholinergic BF afferents to the BLA enhances nization of distal areas, gain control of sensory stimuli, shifting acquisition and retention of cued fear38, whereas inactivation 1Biology Department, Hunter College, City University of New York, New York, NY, USA. 2The Graduate Center, City University of New York, New York, NY, USA. 3RIKEN Center for Brain Science, Laboratory for Neural Circuitry of Learning & Memory, Wako, Japan. 4Department of Life Sciences, Graduate School of Arts and Sciences, University of Tokyo, Tokyo, Japan. *e-mail: [email protected]; [email protected] 1586 NATURE NEUROSCIENCE | VOL 22 | OCTOBER 2019 | 1586–1597 | www.nature.com/natureneuroscience NATURE NEUROSCIENCE FOCUS | REVIEW ARTICLE Glu PV+ Ach Ctx HPC Basal forebrain mPFC PAG Str ppn Ch1/2 module θ HPC LT (MS/DBB) LC Context encoding VTA and SN BF NTS in fear conditioning and extinction Amy PT Ch4 module θ? (VP/SI, NBM) Cue discrimination; NA DA Ach cue encoding in LA/B fear conditioning ? Fig. 1 | Neuromodulatory projections overlap at their target sites. θ , Regions containing cell bodies that synthesize the neuromodulatory θ γ θ–γ neurotransmitters NA (solid orange), DA (solid red), and Ach (solid yellow) Wake are shown in a sagittal cross-section of the adult mouse brain. Regions that partake in aversive emotional learning and are innervated by these Ach Glu neuromodulatory neurotransmitter systems are shown in the ovals, which mPFC denote the strength of innervation by the corresponding neuromodulatory PV+ system. Multiple systems innervate each region with different strength. Amy, amygdala; Ctx, cortex (sensory); HPC, hippocampus; LT, laterodorsal θ, theta oscillation tegmental nucleus; ppn, pedunculopontine nucleus; PT, pontine γ, gamma oscillation – , theta–gamma coupling tegmentum; Str, striatum. θ γ Fig. 2 | Basal forebrain connectivity with the aversive emotional learning network. Cholinergic (Ach, yellow), GABAergic from PV+ cells (blue) of muscarinic cholinergic receptors in the dHPC before contextual and glutamatergic (Glu, green) cells in the medial septum–diagonal fear conditioning impairs recall of the fear-associated context but limb of the diagonal band (MS/DBB)—also known as the cholinergic does not affect freezing to the cue (Fig. 3)39. group 1/2 (Ch1/2) region of the BF—innervate the hippocampus (HPC) Cholinergic signaling also plays an important role in encoding and contribute to the generation and pacing of HPC theta oscillations. spatial information during fear extinction, and lesions of BF cho- Cholinergic and GABAergic projections from the MS/DBB to the HPC linergic cells impair extinction acquisition40. Furthermore, block- also contribute to contextual fear conditioning and extinction. There is ing Ach muscarinic receptors before extinction training eliminated a separate module of cholinergic, glutamatergic and GABAergic cells in fear renewal in a nonextinguished context, suggesting that down- the cholinergic group 4 region (Ch4) of BF, which is comprised of the regulating cholinergic signaling during extinction training blocks ventral pallidum, substantia innominata (VP/SI) and the nucleus basalis hippocampus-dependent encoding of contextual information, of Meynert (NBM). The Ch4 region projects to the LA/B and mPFC. rendering