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 IPNNTS neuronal activity. Optogenetically activating IPNNTS neurons or their projections to DRN in naive mice reliably induces pain- and depressive-like behaviors. Conversely, optogenetic inhibition of IPNNTS 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 IPNNTS-DRN projection induced behavioral outcomes are prevented by intra-DRN infusion of GABAA receptor antagonist, whereas antagonizing DRN NTS receptors induces pain but does not affect depressive-like behaviors. Our findings indicate a neural mechanism by which IPNNTS neurons and their DRN projection regulate pain and depression comorbidity.