An NTS-expressing inhibitory circuit from interpeduncular nucleus to dorsal raphe nucleus controls pain and comorbid depression in male mice

Chronic pain often co-occurs with depression; however, the underlying neural mechanisms remain unclear. This study explored a specific inhibitory pathway-interpeduncular nucleus (IPN) neurotensin (NTS)-expressing neurons projecting to the dorsal raphe nucleus (DRN)-in mediating pain and depression comorbidity in male mice. We demonstrate that complete Freund's adjuvant (CFA) mice show increased IPN^NTS neuronal activity. Optogenetically activating IPN^NTS neurons or their projections to DRN in naive mice reliably induces pain- and depressive-like behaviors. Conversely, optogenetic inhibition of IPN^NTS neurons or their DRN projections reverses CFA-induced pain behaviors and comorbid depression. Notably, this optogenetic activation evokes GABA and NTS co-release in DRN. Surprisingly, activating IPN^NTS-DRN projection induced behavioral outcomes are prevented by intra-DRN infusion of GABA_A receptor antagonist, whereas antagonizing DRN NTS receptors induces pain but does not affect depressive-like behaviors. Our findings indicate a neural mechanism by which IPN^NTS neurons and their DRN projection regulate pain and depression comorbidity.