Compartmentalized dendritic plasticity during associative learning

Experience-dependent changes in behavior are mediated by long-term functional modifications in brain circuits. Activity-dependent plasticity of synaptic input is a major underlying cellular process. Although we have a detailed understanding of synaptic and dendritic plasticity in vitro, little is known about the functional and plastic properties of active dendrites in behaving animals. Using deep brain two-photon Ca2+ imaging, we investigated how sensory responses in amygdala principal neurons develop upon classical fear conditioning, a form of associative learning. Fear conditioning induced differential plasticity in dendrites and somas regulated by compartment-specific inhibition. Our results indicate that learning-induced plasticity can be uncoupled between soma and dendrites, reflecting distinct synaptic and microcircuit-level mechanisms that increase the computational capacity of amygdala circuits. Description Soma and dendrite plasticity uncoupled Although we have detailed knowledge of synaptic and dendritic plasticity in vitro, learning-induced changes in vivo are mostly investigated through unit recordings or imaging of somatic calcium activity. However, we know much less about the functional and plastic properties of dendrites in vivo. D’Aquin et al. combined deep-brain imaging with high resolution, subcellular two-photon microscopy to study the activity of dendrites and somas of identified amygdala neurons over days in awake mice undergoing classical fear conditioning. Sensory stimulation induced compartmentalized dendritic responses that were controlled by dendrite-targeting, somatostatin-positive interneurons. Fear conditioning–induced plasticity was uncoupled between soma and dendrites, possibly reflecting compartment-specific synaptic- and microcircuit-level mechanisms that may increase the computational capacity of amygdala circuits. —PRS Associative learning induces compartmentalized plasticity of sensory responses in the dendrites of amygdala principal neurons. INTRODUCTION Learning is mediated by experience-dependent plasticity of neuronal function. Although we have a detailed knowledge of synaptic and dendritic plasticity in vitro, learning-induced functional changes in vivo are mostly assessed by use of soma-centric methods such as unit recordings or imaging of somatic Ca2+ activity. However, these methods do not reveal the complexity of dendritic signaling and its regulation by local neuronal circuitry. Moreover, because dendrites are functionally independent subcellular compartments that dynamically integrate incoming information and thereby affect a neuron’s input-output function, the questions arise whether compartmentalized plasticity occurs during learning and which mechanisms ultimately control somatic output in behaving animals. RATIONALE To investigate dendritic function and plasticity in vivo, we focused on the lateral amygdala (LA), a subcortical brain structure that is central to classical auditory fear conditioning, a fast and robust form of associative learning. During auditory fear conditioning, an auditory conditioned stimulus (CS; typically a pure tone or white noise) is paired with an aversive unconditioned stimulus (US; typically a foot shock), which results in the induction of Hebbian activity-dependent synaptic potentiation at auditory synaptic inputs onto LA principal neurons (PNs). This view has recently been extended by studies reporting that similar proportions of neurons up- and down-regulate their CS response upon fear conditioning, suggesting that fear learning involves more diverse forms of plasticity. However, these studies relied on measurements of somatic activity, whereas dendritic activity and plasticity during associative learning was not explored. RESULTS To image the activity of dendrites and somas of amygdala PNs deep in the brain of awake mice undergoing classical fear conditioning, we used gradient-index lens–based high-resolution two-photon microscopy across multiple days. We show that sensory stimulation induces compartmentalized dendritic responses controlled by dendrite-targeting somatostatin-expressing (SST+) interneurons. Spontaneous inputs to PN dendrites are suppressed by SST+ interneurons, whereas salient sensory stimuli transiently alleviate SST+ interneuron–mediated inhibition of PN dendrites likely through VIP+ interneurons. In most cases, this evokes highly correlated somatic and dendritic sensory responses. However, sensory input can also lead to isolated dendritic responses without concomitant somatic output, indicating that dendrites of LA PNs can integrate auditory inputs locally. The relief of SST+ interneuron–mediated dendritic inhibition is necessary to amplify dendritic CS responses during conditioning, which is consistent with the notion that disinhibition through SST+ interneurons opens a temporal window during which CS inputs are eligible for the induction of associative dendritic plasticity upon concomitant exposure to an aversive US. Fear conditioning induces bidirectional plasticity of somatic CS responses that correlates with learning at the behavioral level. On average, dendritic CS responses increase in neurons with both up-regulated (CSup) or down-regulated (CSdown) somatic CS responses. However, fear conditioning also increases the variance of CS responses across the dendritic tree of a given neuron, indicating that not all dendrites are undergoing similar levels of plasticity during learning. Moreover, dendritic spines that show up-regulated CS responses are more likely to be located on dendrites that exhibit increased CS responses after learning. Last, even though dendritic CS responses of CSdown neurons are potentiated, their somatic responses are reduced by enhanced Parvalbumin-expressing (PV+) interneuron–mediated perisomatic inhibition, counteracting the learning-induced increased synaptic drive. CONCLUSION Our findings demonstrate that LA PNs locally integrate dendritic sensory inputs in a compartmentalized manner and that fear conditioning–induced plasticity of dendritic and somatic sensory responses can be uncoupled. Both compartmentalized dendritic integration and uncoupling of dendritic and somatic plasticity are regulated by local inhibitory circuits that specifically target dendrites or the perisomatic region, two distinct subcellular compartments of postsynaptic PNs. The regulation of compartmentalized dendritic and somatic plasticity increases the computational capacity of amygdala circuits, possibly enhancing an animal’s behavioral flexibility in the face of danger. Fear conditioning induces compartmentalized plasticity. (Left) During auditory stimuli, dendritic disinhibition through SST+ interneurons amplifies dendritic Ca2+ responses. (Right) Fear conditioning induces a correlated increase of somatic and dendritic CS responses in a population of LA PNs (CSup neurons), whereas CSdown neurons show increased dendritic but decreased somatic CS responses. This compartmentalized plasticity is mediated by soma-targeting PV+ interneurons.