Early intervention with a PACAP lactoside following repetitive mild traumatic brain injury prevents persistent LHb hyperactivity and motivational deficits in male mice
Mild traumatic brain injury (mTBI) often leads to long-lasting mood disorders and motivational deficits, significantly decreasing quality of life. The neuropeptide pituitary adenylate cyclase-activating polypeptide (PACAP) has known neuroprotective effects and is often deficient in neurological conditions, but its role in preventing mTBI's long-term effects on mood circuits remains unclear. Our previous studies of repetitive closed-head mTBI in murine models found that the lateral habenula (LHb) is a key area for post-injury impairments. These impairments are characterized by mTBI-induced LHb hyperactivity and mood-related behavioral deficits. Here we investigated if a novel PACAP type I receptor (PAC1R) agonist, TES2320, could prevent or treat these long-term mTBI effects. The drug is based on a truncated glycoside analogue of PACAP that is stable and brain penetrant. Using RNAscope, we first found that mTBI caused a persistent and widespread reduction in PACAP mRNA within the LHb, suggesting a possible mTBI-related PACAP signaling deficiency in LHb circuits. We then used two intervention strategies in young adult male mice: an early intervention administered immediately after injury and a late intervention given as a single injection one-month post-injury. One month after injury, we measured LHb activity and self-care grooming motivation. Both early and late PAC1 agonist interventions almost completely normalized the mTBI-induced increases in LHb spontaneous tonic activity and hyperexcitability. However, only the early intervention improved the delayed initiation of grooming seen in mTBI mice. Unfortunately, a single late PAC1R agonist injection not only failed to reverse the grooming delay, but also significantly reduced total grooming behavior 24 hours after treatment. This finding points to a potential behavioral side effect of late PAC1R agonist administration on grooming. Overall, our findings confirm that an early PAC1R agonist intervention effectively prevents mTBI-induced LHb hyperactivity and associated motivational deficits, likely by restoring the persistent PACAP signaling deficiency in the LHb and its related circuits. This preclinical study provides strong evidence that novel PAC1R agonists could be a valuable preventive therapy for mTBI-related depression and anti-reward circuit dysfunction.