SUMMARY Pain perception is essential for survival and can be amplified or suppressed by expectations, experiences, and context. The neural mechanisms underlying bidirectional modulation of pain remain largely unknown. Here, we demonstrate that the central nucleus of the amygdala (CeA) functions as a pain rheostat, decreasing or increasing pain-related behaviors in mice. This dual and opposing function of the CeA is encoded by opposing changes in the excitability of two distinct subpopulations of GABAergic neurons that receive excitatory inputs from the parabrachial nucleus (PB). Thus, cells expressing protein kinase C-delta (CeA-PKCδ) are sensitized by nerve injury and increase pain-related responses. In contrast, cells expressing somatostatin (CeA-Som) are inhibited by nerve injury and their activity drives antinociception. Together, these results demonstrate that the CeA can amplify or suppress pain in a cell-type-specific manner, uncovering a previously unknown mechanism underlying bidirectional control of pain in the brain.

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21Central amygdala (CeA) neurons expressing protein kinase C delta (PKCδ + ) or Somatostatin 22 (Som + ) differentially modulate diverse behaviors. The underlying features supporting cell-type-23 specific function in the CeA, however, remain unknown. Using whole-cell patch-clamp 24 electrophysiology in acute mouse brain slices and biocytin-based neuronal reconstructions, we 25 demonstrate that neuronal morphology and relative excitability are two distinguishing features 26 between Som + and PKCδ + CeLC neurons. Som + neurons, for example, are more excitable, 27 compact and with more complex dendritic arborizations than PKCδ + neurons. Cell size, intrinsic 28 membrane properties, and anatomical localization were further shown to correlate with cell-type-29 specific differences in excitability. Lastly, in the context of neuropathic pain, we show a shift in the 30 excitability equilibrium between PKCδ + and Som + neurons, suggesting that imbalances in the 31 relative output of these cells underlie maladaptive changes in behaviors. Together, our results 32 identify fundamentally important distinguishing features of PKCδ + and Som + cells that support cell-33 type-specific function in the CeA. 34 35 Keywords: central amygdala, Somatostatin, protein kinase C delta, intrinsic excitability, 36 morphology, neuropathic pain 37 2002, Chieng et al., 2006, Schiess et al., 1999, Li and Sheets, 2018. Recent studies have further 64 shown that CeLC neurons with different firing properties are topographically organized based on 65 their projection targets (Li and Sheets, 2018), suggesting that heterogeneity of function within the 66 CeLC might also be anatomically defined. 67In the present study, we contributed to the growing body of knowledge about the CeLC by 68 performing a characterization of the electrophysiological and morphological properties of PKCδ + 69and Som + neurons. Our overarching hypothesis is that these two genetically distinct populations 70 of CeLC neurons are electrophysiologically and morphologically different. We used whole-cell 71 patch-clamp electrophysiology in acute mouse brain slices in combination with biocytin-based 72 morphological reconstructions to characterize and compare the passive and active membrane 73properties, as well as the evoked repetitive firing responses, single action potential waveforms 74 and neuronal morphologies of these two subpopulations of neurons. We further evaluated 75 whether membrane properties and excitability are dependent on the anatomical localization within 76 the central amygdala, both at the subnuclei and rostro-caudal levels. 77Finally, using a mouse model of neuropathic pain, we tested whether perturbations known 78 to alter CeLC-dependent behavioral outputs would result in a shift in the relative excitability of 79 these two CeLC cell types. Using this cell-type-specific approach, we demonstrated that PKCδ + 80 and Som + neurons have distinct electrophysiological and morphological properties and that the 81 differences in the excitability of these cells are occlud...