Increased alcohol self-administration following exposure to the predator odor TMT in active coping female rats

Abstract

Post-traumatic stress disorder (PTSD) and alcohol use disorder (AUD) are highly comorbid. Additionally, individual differences in response to stress suggest resilient and susceptible populations. The current study exposed male and female Long Evans rats to the synthetically produced predator odor 2,5-dihydro-2,4,5-trimethylthiazoline (TMT) to examine individual differences in stress-reactive behaviors (digging and immobility) and whether these differences could predict lasting consequences of TMT and increases in alcohol drinking. Male and female Long Evans rats were trained on operant alcohol self-administration. After 9 sessions, rats underwent exposure to TMT or water (Control) in a distinct context. 6 days after TMT exposure, rats underwent re-exposure to the TMT-paired context (without TMT), and a series of behavioral assessments (acoustic startle, zero maze, light/dark box), after which rats resumed alcohol selfadministration. Rats were divided into two TMT-subgroups using a ratio of digging and immobility behavior during TMT exposure: TMT-subgroup 1 (low digging/immobility ratio) and TMT-subgroup 2 (high digging/immobility ratio). Digging/immobility ratio scores predicted elevated corticosterone levels during TMT exposure and reactivity during context re-exposure in males and females (TMT-subgroup 2), as well as elevated corticosterone levels after context re-exposure and hyperarousal behavior in females (TMT-subgroup 1). Furthermore, TMT stress reactivity predicted increases in alcohol self-administration, specifically in females. These data show that stress-reactivity can predict lasting behavioral changes which may lead to a better understanding of increases in alcohol drinking following stress in females and that individual differences .

The Role of the Nucleus Reuniens in Regulating Contextual Conditioning with the Predator Odor TMT

“…In the present work, during re-exposure to the previously TMT-paired context, rats engaged in behavioral responses, such as defensive digging and decreased immobility to a greater extent than controls, indicative of context reactivity. This finding is consistent with previous work (Ornelas et al, 2020;Shallcross et al, 2019;Tyler et al, 2020). In Experiment 1, in which bedding was provided in the context, this was evidenced by increased digging and reduced immobility behavior (i.e., greater mobility) relative to controls during the context re-exposure.…”

“…Furthermore, avoidance of the TMT-paired side of the chamber was not observed during context reexposure in either experiment. This is not altogether surprising given that it has been shown that although TMT produces significant avoidance behavior during exposure, TMT has not been shown to induce context conditioning through avoidance behavior (Ornelas et al, 2020;Rosen et al, 2015). Additionally, these data are in accordance with previous work from our lab (Ornelas et al, 2020) and others (Schwendt et al, 2018) that show TMT can produce contextual conditioning.…”

“…Immobility was operationally defined as lack of movement for more than 2 seconds as assessed using ANY-maze software. Therefore, immobility likely captures both inactivity and freezing, which is characteristic of a fear response in rodents and observed during TMT exposure (Ornelas et al, 2020). Two-way ANOVA was used to compare group differences in stress-reactive behaviors (digging, immobility, time spent on TMT side) during TMT exposure and context re-exposure.…”

“…The current study uses TMT exposure as the stressor because it activates a hardwired "learned-independent system" shown to induce innate fear and defensive behaviors (Rosen, Asok, & Chakraborty, 2015). Further, we have previously shown that an important advantage of using predator odor exposure, including TMT, as a stressor, is the ability to measure stress-reactive behaviors during stressor exposure, including defensive digging and immobility (Ornelas, Tyler, Irukulapati, Paladugu, & Besheer, 2020;Tyler et al, 2020).…”

“…Defensive digging is interpreted as a proactive response to stress (Arakawa, 2007;De Boer & Koolhaas, 2003;Fucich & Morilak, 2018), such that it reflects an active coping response (Ornelas et al, 2020) or fear-related behavior (Riittinen et al, 1986) and predator-stress responsiveness (Neal, Kent, Bardi, & Lambert, 2018). Rats also engage in immobility behavior during TMT exposure, indicative of passive coping behavior and adaptive response to stress (Ornelas et al, 2020;Tyler et al, 2020). Importantly, during re-exposure to the previously paired-TMT context, these behaviors can be quantified as an index of stress-induced contextual memory.…”