Brain Circuits Mediating Opposing Effects on Emotion and Pain

Abstract

The amygdala is important for processing emotion, including negative emotion such as anxiety and depression induced by chronic pain. Although remarkable progress has been achieved in recent years on amygdala regulation of both negative (fear) and positive (reward) behavioral responses, our current understanding is still limited regarding how the amygdala processes and integrates these negative and positive emotion responses within the amygdala circuits. In this study with optogenetic stimulation of specific brain circuits, we investigated how amygdala circuits regulate negative and positive emotion behaviors, using pain as an emotional assay in male rats. We report here that activation of the excitatory pathway from the parabrachial nucleus (PBN) that relays peripheral pain signals to the central nucleus of amygdala (CeA) is sufficient to cause behaviors of negative emotion including anxiety, depression, and aversion in normal rats. In strong contrast, activation of the excitatory pathway from basolateral amygdala (BLA) that conveys processed corticolimbic signals to CeA dramatically opposes these behaviors of negative emotion, reducing anxiety and depression, and induces behavior of reward. Surprisingly, activating the PBN-CeA pathway to simulate pain signals does not change pain sensitivity itself, but activating the BLA-CeA pathway inhibits basal and sensitized pain. These findings demonstrate that the pain signal conveyed through the PBN-CeA pathway is sufficient to drive negative emotion and that the corticolimbic signal via the BLA-CeA pathway counteracts the negative emotion, suggesting a top-down brain mechanism for cognitive control of negative emotion under stressful environmental conditions such as pain.SIGNIFICANCE STATEMENT It remains unclear how the amygdala circuits integrate both negative and positive emotional responses and the brain circuits that link peripheral pain to negative emotion are largely unknown. Using optogenetic stimulation, this study shows that the excitatory projection from the parabrachial nucleus to the central nucleus of amygdala (CeA) is sufficient to drive behaviors of negative emotion including anxiety, depression, and aversion in rats. Conversely, activation of the excitatory projection from basolateral amygdala to CeA counteracts each of these behaviors of negative emotion. Thus, this study identifies a brain pathway that mediates pain-driven negative emotion and a brain pathway that counteracts these emotion behaviors in a top-down mechanism for brain control of negative emotion.

Keywords: amygdala; brain circuits; emotion; optogenetics; pain.

Figure 1.

Excitatory PBN and BLA projections to CeA. A, A diagram illustrating bilateral delivery of a viral vector into the PBN in rats. Red dots indicate PBN. B, C, Representative immunohistochemical images of ChR2-mCherry expression in PBN (B) and in the CeA (C), but not in the BLA, 10 d after bilateral infusion of the viral vector AAV5-CaMKIIα-ChR2-mCherry into PBN of a rat. D, CeA image in higher-magnification for mCherry-expressing terminals (red) and DAPI-stained cells (blue). E, Image of ChR2-GFP expression in BLA and CeA 10 d after bilateral infusion of the viral vector AAV5-CaMKIIα-ChR2-GFP into BLA of a rat. F, CeA image in higher-magnification for GFP-expressing terminals (green) and DAPI-stained cells (blue). Scale bars: B, C, 500 μm; D, 100 μm; E, 200 μm; and F, 20 μm.

Figure 2.

Optogenetic stimulation of the PBN–CeA projection in CeA induces behaviors of negative emotion. A, Locomotion traces of rats with PBN infusion of a control vector AAV-mCherry and the AAV-ChR2 vector for real-time optical stimulation in CeA during light-off and light-on periods in the OFT. B–D, Group data of time spent (B) and distance traveled (C) in central zone and total distance traveled (D) in rats with PBN injection of the control vector (n = 5) and AAV-ChR2 vector (n = 7) in three consecutive 5 min periods of OFT. The light was on during the second period (gray areas) for optical stimulation. E, Immobility time in rats with PBN infusion of the control vector (n = 6) and AAV-ChR2 (n = 6) during optical stimulation in CeA in the FST. F, Scores of CPA in rats with PBN infusion of the control vector (n = 6) and AAV-ChR2 (n = 6) after four conditioning sessions paired with the optical stimulation in CeA. G, H, Paw-withdrawal latencies for thermal pain (G) and paw-withdrawal thresholds for mechanical pain (H) before (light-off) and after (light-on) optical stimulation in rats with PBN infusion of the control vector (n = 6) and AAV-ChR2 (n = 7). *p < 0.05, **p < 0.01.

Figure 3.

Optogenetic stimulation of the PBN–CeA projection in CeA increases number of c-fos-positive neurons in CeA. A, Immunohistochemical images of mCherry and c-fos expression in CeA 2 h after 20 min light stimulation in rats with bilateral infusion of the control vector and AAV-ChR2 into PBN. B, Summarized data of c-fos-positive neurons in randomly selected areas of CeA from the control vector-injected (n = 4) and AAV-ChR2-injected (n = 4) rats. C, Image showing the tract (arrow) of a cannula fiber targeting CeA. Scale bars: A, 100 μm; C, 200 μm. ****p < 0.0001.

Figure 4.

Optogenetic stimulation of the BLA–CeA projection in CeA inhibits behaviors of negative emotion and sensory pain. A, B, Group data of time spent in central zone (A) and total distance traveled (B) in rats with bilateral infusions of the control vector (n = 6) and AAV-ChR2 (n = 8) into BLA in three consecutive 5 min periods of OFT. The light was on during the second period. C, Immobility time in rats with BLA infusions of the control vector (n = 6) and AAV-ChR2 (n = 6) during optical stimulation in CeA in FST. D, Scores of CPP in rats with BLA infusions of the control vector (n = 6) and AAV-ChR2 (n = 6) after four conditioning sessions paired with the optical stimulation in CeA. E, F, Paw-withdrawal latencies for thermal pain (E) and paw-withdrawal thresholds for mechanical pain (F) before (light-off) and after (light-on) optical stimulation in normal rats with BLA infusions of the control vector (n = 6) and AAV-ChR2 (n = 7) in rats. G, H, Results of similar behavioral tests for thermal pain (G) and mechanical pain (H) in the vector-injected rats (AAV-mCherry, n = 8; AAV-ChR2, n = 6) 3 d after an intraplantar injection of CFA. *p < 0.05, **p < 0.01, ***p < 0.001, ****p < 0.0001.

Figure 5.

Optogenetic stimulation of the BLA–CeA projection in CeA increases number of c-fos-positive neurons in CeA. A, immunohistochemical images of mCherry and c-fos expression in CeA 2 h after 20 min light stimulation in rats 30 d after bilateral infusion of the control vector and AAV-ChR2 into BLA. B, Summarized data of c-fos-positive neurons in randomly selected areas of CeA from the control vector-injected (n = 4) and AAV-ChR2-injected (n = 4) rats. Scale bars, 100 μm. ****p < 0.0001.

Figure 6.

Simultaneous stimulation of both PBN–CeA and BLA–CeA projections in CeA does not induce behaviors of negative emotion. A, B, Group data of time spent in central zone (A) and total distance traveled (B) of OFT in rats with bilateral infusions of the control vector (n = 6) and AAV-ChR2 (n = 9) into PBN and BLA of the same rat. The light-on period is indicated by the gray columns. C, Immobility time of FST during optical stimulation in the same rats with vector infusions into both PBN and BLA. D, E, Paw-withdrawal latencies for thermal pain (D) and paw-withdrawal thresholds for mechanical pain (E) before (light-off) and after (light-on) optical stimulation in the same rats. ***p < 0.001.

Figure 7.

Differential distribution of excitatory terminals in CeA from PBN and from BLA. A–C, Representative immunohistochemical images of excitatory terminals in CeA in a rat receiving double injections of AAV-CaMKIIα-ChR2-mCherry into PBN (A, red), AAV-CaMKIIα-ChR2-GFP into BLA (B, green) in the same rat, and merged images of A and B (C). D–F, CeA images of higher-magnification in the square area marked in C for PBN–CeA projection terminals (D), BLA–CeA projection terminals (E), and their emerged image (F). Scale bars: A–C, 250 μm; D, E, 50 μm.