Delayed unpaired extinction as a treatment for hyperarousal of the rabbit nictitating membrane response and its implications for treating PTSD

Abstract

Treatment for PTSD (Post-traumatic stress disorder) is rarely available immediately after trauma and often delayed for weeks or months after an event. In a rabbit eyeblink conditioning model of PTSD, we have previously shown that presentations of a tone conditioned stimulus (CS) and shock unconditioned stimulus (US) in an explicitly unpaired manner known as unpaired extinction is effective in reducing CS responding and US hyperarousal even if shock intensity is reduced eight-fold and elicits only minimal responding. Here we determined if delayed delivery of unpaired extinction would still be effective in extinguishing hyperarousal. Rabbits were tested for sensitivity to shock before CS-US pairings and after six days of unpaired extinction presented a day, a week or a month after CS-US pairings. Hyperarousal was extinguished a day and a week after conditioning but not after a month suggesting a significant delay in "treatment" can make hyperarousal persist. We next assessed if this persistence of hyperarousal was associative by comparing rabbits given CS-US pairings to those given explicitly unpaired CS and US presentations, measuring hyperarousal a day and a month later, followed by unpaired extinction and hyperarousal assessment. After four weeks, there was an increase in responding for all rabbits but only rabbits receiving CS-US pairings showed a significant increase in associatively-mediated hyperarousal. Importantly, both paired and unpaired groups showed increased levels of responding after unpaired extinction suggesting treatment delayed for too long may no longer be effective and could cause generalized hyperarousal.

Inactivation of the interpositus nucleus during unpaired extinction does not prevent extinction of conditioned eyeblink responses or conditioning-specific reflex modification.

“…Of note, in this study we did not find a significant reduction in the size of CRM following extinction treatment in muscimol rabbits nor in saline controls, despite several previous publications demonstrating that six days of unpaired extinction with weak shock induces significant decreases in amplitude and area of CRM (Burhans et al, 2015, 2018; Schreurs et al, 2011, 2018). Because we typically see a fair degree of variability in terms of individual responsiveness to extinction treatment, it may be that by chance, we had more extinction-resistant subjects in this cohort.…”

“…The current study has confirmed, however, that IP is not needed to extinguish CRM timing, and it's important to note that all aspects of CRM, including the latency increase, do not simply fade with the passage of time, persisting for at least a month following conditioning (Schreurs et al, 2018). Therefore, the extinction of CRM is likely an active process with distinct neural substrates.…”

“…The US posttest following extinction (Post2) was a test for the effects of extinction treatment under IP inactivation on extinction of CRM. Previous studies had established that 6 days of unpaired extinction with weak shock, as utilized in this study, produced significant decreases in the latency and size of CRM (Burhans et al, 2015, 2018; Schreurs et al, 2011, 2018). Examination of the UR topographies in Figure 7, however, suggested that increased size of the UR at intermediate intensities at Post1 was maintained and possibly potentiated at Post2 regardless of the state of the IP during extinction treatment.…”

“…CRM is characterized by increases in the frequency, latency, amplitude, and area of URs following delay eyeblink conditioning in rabbits, particularly to USs that are four- to eightfold smaller than the US utilized during CS–US pairings (Schreurs et al, 1995). CRM is deemed conditioning-specific because the same changes in the UR are not found in subjects receiving explicitly unpaired CS and US presentations (Buck, Seager, & Schreurs, 2001; Schreurs et al, 1995; Schreurs, Shi, Pineda, & Buck, 2000; Schreurs, Smith-Bell, & Burhans, 2018). CRM-like changes have also been reported by others in rabbits (Gruart & Yeo, 1995; Wikgren & Korhonen, 2001; Wikgren, Ruusuvirta, & Korhonen, 2002) and rodents (Servatius, Brennan, Beck, Beldowicz, & Coyle-DiNorica, 2001).…”