Early intervention with a PACAP lactoside following repetitive mild traumatic brain injury prevents persistent LHb hyperactivity and motivational deficits in male mice

Technical Summary: Early Intervention with a PACAP Lactoside Following Repetitive Mild Traumatic Brain Injury

Problem Statement
Mild traumatic brain injury (mTBI), particularly when repetitive, frequently leads to chronic psychiatric sequelae, including depression, anxiety, and motivational deficits. These conditions often resist standard treatments like SSRIs, suggesting distinct underlying circuit mechanisms. The lateral habenula (LHb), a brain region functioning as an "anti-reward" hub, has been identified as a critical site of dysfunction following mTBI, characterized by persistent hyperactivity and hyperexcitability. While the neuropeptide Pituitary Adenylate Cyclase-Activating Polypeptide (PACAP) possesses known neuroprotective properties, its specific role in post-mTBI circuit dysfunction and the feasibility of targeting its receptor (PAC1R) for therapeutic intervention remain poorly understood. Furthermore, native PACAP is limited by rapid peripheral metabolism and poor blood-brain barrier (BBB) penetration.

Methodology
The study utilized a murine model of repetitive closed-head mTBI in young adult male C57BL/6 mice. The experimental design involved:

• Drug Development: The authors employed TES2320, a novel, truncated, glycosylated PACAP1–23 analogue (a lactoside-bearing serine-amide) designed for enhanced stability and BBB penetration. Its binding affinity (Kd = 4.50 pM) and structural interaction with the PAC1 receptor were validated via Plasmon Waveguide Resonance (PWR) spectroscopy and in silico modeling.
• Pharmacokinetics: Simultaneous microdialysis (striatum) and blood sampling were used to quantify BBB penetration and serum half-life of TES2320 compared to its unglycosylated counterpart (TES2300).
• Intervention Timing: Two strategies were tested:
  1. Early Intervention: Repeated intraperitoneal (IP) injections of TES2320 (10 mg/kg) immediately following each of the five daily injury sessions.
  2. Late Intervention: A single IP injection of TES2320 administered one month post-injury.
• Molecular Analysis: RNAscope in situ hybridization was used to quantify PACAP (Adcyap1) and PAC1R (Adcyap1r1) mRNA expression in the medial and lateral subregions of the LHb.
• Electrophysiology: Whole-cell and cell-attached patch-clamp recordings were performed on LHb neurons approximately four weeks post-injury to assess spontaneous firing patterns, neuronal excitability (I-V curves), intrinsic membrane properties, and specific ion currents (Ih and M-currents).
• Behavioral Assessment: The sucrose splash test was utilized to measure self-care grooming motivation, specifically analyzing latency to groom and total grooming duration.

Key Results

• Molecular Changes: mTBI induced a persistent, widespread reduction in PACAP mRNA expression within the LHb, accompanied by a compensatory upregulation of PAC1R mRNA, indicating a local signaling deficit.
• Pharmacokinetics: Glycosylation significantly improved the pharmacokinetic profile. TES2320 demonstrated a 1.43-fold increase in brain dialysate exposure (AUC) and a 4.73-fold increase in plasma exposure compared to the unglycosylated TES2300, confirming enhanced BBB penetration and stability.
• Electrophysiological Effects:
  • mTBI caused persistent LHb tonic hyperactivity and hyperexcitability (increased action potential firing).
  • Both early and late TES2320 interventions successfully normalized this spontaneous hyperactivity and hyperexcitability.
  • However, regarding specific currents, mTBI enhanced Ih currents and reduced M-currents. Early intervention did not prevent the Ih enhancement, while late intervention exacerbated it. Neither intervention normalized the reduced M-currents.
  • Crucially, only early intervention normalized the mTBI-induced increase in hyperpolarization-induced rebound bursting. Late intervention failed to reverse this specific phenotype.
• Behavioral Outcomes:
  • mTBI increased the latency to initiate grooming (motivational deficit).
  • Early intervention completely prevented the prolonged grooming latency, restoring motivation to sham levels.
  • Late intervention failed to reverse the increased grooming latency. Furthermore, a single late dose acutely reduced total grooming time 24 hours post-treatment, suggesting a transient adverse effect on behavior.

Significance and Claims
The paper claims that early intervention with the PAC1R agonist TES2320 prevents both the physiological (LHb hyperactivity and rebound bursting) and behavioral (motivational deficits) consequences of repetitive mTBI in male mice. The authors posit that this efficacy is likely mediated by the restoration of persistent PACAP signaling deficits within LHb circuits, which are disrupted by the injury.

The study highlights the critical importance of timing in therapeutic intervention. While late administration could normalize tonic firing rates, it was insufficient to reverse specific circuit-level pathologies (rebound bursting) or behavioral deficits, and it introduced acute side effects. The findings support the development of stable, brain-penetrant PAC1R agonists as a preventive strategy for mTBI-related affective disorders and anti-reward circuit dysfunction. The authors explicitly note that their findings are limited to male mice and that future work is required to address sexual dimorphism in PACAP signaling and to expand the behavioral battery to fully map the therapeutic scope.