Pituitary adenylate cyclase-activating polypeptide as a mediator of depression following repetitive mild traumatic brain injury - PROJECT SUMMARY Adolescents are prone to mild traumatic brain injury (mTBI) due to sports-related concussions, which often occur repetitively. Evidence suggests that, compared to boys, girls are more likely to sustain mTBI, take longer to recover, and report depression more often following TBI. Depression after mTBI is delayed in onset, exacerbated after repetitive mTBI, and persists for years following injury. Importantly, very little is known about the mechanisms underlying post-traumatic depression. Although not yet been evaluated in the context of mTBI, there is strong evidence that the paraventricular nucleus of the thalamus (PVT) and its major excitatory projection region, the nucleus accumbens (NAc), regulates depression-like behaviors. The PVT has dense expression of the excitatory neuropeptide pituitary adenylate cyclase-activating polypeptide (PACAP). Using a preclinical model, the Raghupathi lab has recapitulated the clinical TBI findings by showing that, beginning at 5 weeks following repetitive mTBI in adolescence, both male and female rats display impaired cognition, but only females exhibit depression-like behaviors, demonstrating increased immobility in a forced swim test as well as increased latency to groom in a sucrose splash test. The Barson lab has shown that increased levels of PACAP-27 in cells of the PVT of female rats promote depression-like behavior in a forced swim test. Our collective preliminary data show that, following repetitive mTBI in adolescence, (1) there is increased PACAP mRNA in the PVT of females but not males, and (2) knockdown of PACAP mRNA in the PVT normalizes the depression-like behavior, that (3) injection of PACAP into the NAc promotes depression-like behavior, that (4) there is a specific increase in levels of choline acetyltransferase (ChAT) (a marker for acetylcholine interneurons) in the NAc in females after repetitive mTBI, and (5) the muscarinic acetylcholine receptor antagonist, scopolamine, reduces depression-like behavior in females after repetitive mTBI. The hypothesis of this proposal is that after repetitive mTBI during adolescence, chronic endogenous overexpression of PACAP in PVT neurons that project to the NAc promotes depression-like behaviors in female but not male rats, and this is associated with changes in activity of cholinergic interneurons in the NAc. Aim 1 is to determine the role of PACAP in the PVT in post-injury depression-like behavior. Female and male brain-injured or control rats will be tested for depression-like behavior after shRNA knockdown of PACAP in the PVT, and the activity of their NAc cholinergic neurons will be investigated during these behaviors with fiber photometry. Aim 2 is to determine if PVT PACAP affects post-injury affective behavior via the NAc. Female and male brain-injured or control rats will be examined for their PACAP+ PVT projections to the NAc after repetitive injury using immunohistochemistry, and they will be tested for depression-like behavior after shRNA knockdown of PACAP in the PVT-NAc pathway. These experiments will, for the first time, point to the PACAP within the PVT and acetylcholine within the NAc as possible targets for pharmacologic strategies for the treatment of mood disorders after repeated mTBI in adolescence.
