The A1 Adenosine Receptor Agonist N6 Cyclohexyl Adenosine Suppresses the Cold-Defense Threshold to a Surface Temperature Below 4°C in Sprague Dawley Rats

Abstract

A1 adenosine receptor (A1AR) agonists have applications in targeted temperature management (TTM) for neurogenic fever, cardiac arrest and neonatal ischemic encephalopathy. By mitigating the cold-defense response the A1AR agonist N6 cyclohexyl adenosine (CHA) blocks shivering and cold-defense metabolic stress that limits the application and benefit of TTM. Stimulating CNS A1AR inhibits cold-induced shivering and nonshivering thermogenesis in rats and induces a torpor-like response in rats and hibernating species [1,2]. Dynamic control of surface temperature (Ts) in rats, designed to mimic conductive cooling used clinically, is an effective means to regulate body temperature after CHA [3]. In prior studies 17°C was the minimum Ts tested while in the clinic a fast rate of cooling is facilitated by setting cooling devices to 4°C. Unknown is if CHA suppresses the cold-defense response at a Ts of 4°C. The present study tests the hypothesis that the cold-defense threshold of CHA (1.0mg/kg, IP) is less than a Ts of 4°C, and we ask if the threshold differs between sexes. To define the cold-defense threshold, male and female Sprague-Dawley rats fed ad lib and housed at 20°C and 12:12 L:D were held overnight in a metabolic cage with a temperature-controlled surface set to 29°C and a video camera. The following day after 4h at 29°C with lights on, rats were injected with vehicle (week 1) or CHA (week 2) and the Ts was decreased to 28, 20, 16, 12, 8 and 4°C for 30min each. Cold-defense was assessed by an increase in the rate of oxygen consumption (VO2) and by behaviors suggestive of escape. Based on prior work showing a bimodal distribution of rats that respond to CHA and those that do not respond [4], we a priori chose to assess the proportion of nonresponders in the study population and to analyze responders and nonresponders separately. Of 9 rats (5 M, 4 F), one male did not respond to CHA. In rats that responded to CHA, vehicle (veh) treatment increased VO2 at Ts of 16°C to 4°C in both sexes (p<0.0001 drug x Ts, 2-way RM ANOVA; p<0.01, veh-29°C (lights on) vs veh-16, 12, 8, and 4°C, Tukey, n=8,8). CHA treatment decreased VO2 at all Ts tested (p<0.05 CHA vs Veh, Tukey, n=8,8). Females trended towards higher VO2 at 29°C than males at all Ts, but sex differences decreased after treatment with CHA. Blinded analysis of behavior is in progress. Results show that CHA lowers the threshold of Ts induced cold-defense from 16°C to below 4°C. Suppressed VO2 at Ts above 16°C illustrates CHA-induced metabolic suppression unrelated to thermogenesis. Results inform optimal Ts to minimize time to reach target temperature without a cold-defense response. 1. Jinka et al., 2011, J Neurosci 31 (30):10752-10758. 2. Tupone et al., 2013, J Neurosci 33 (36):14512-14525. 3. Laughlin et al., 2018, Ther Hypothermia Temp Manag 8 (2):108-116. 4. Bailey et., 2017, J Pharm Exp Ther 362 (3):424-430. NNX13AB28A, 22-22EPSCoR-0018, P20GM130443, P20GM103395. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.