Figure 1: Obese mice of the ob/ob strain.
Many health issues have been correlated with the rise in obesity during the last few decades. Chronic pain is one of those health problems and it is treated with pain killers, often of the opiate class. These medications can cause breathing to slow, not be as deep, or stop completely. These signs are called respiratory depression. Obesity itself can also cause respiratory depression thus obese patients needing opiates for pain are particularly at risk for this dangerous side effect. Chelsea Angel, at the University of Tennessee is working in the laboratories of Dr. Helen Baghdoyan and Dr. Ralph Lydic to find out how buprenorphine-induced respiratory depression is changed in obese animals. Chelsea presented the results of this study at the recent Experimental Biology meeting in San Diego, CA.
Buprenorphine has previously been studied in humans and mice of a normal weight. To better mimic the effects of buprenorphine on respiratory depression in obese patients, these investigators used normal weight C57Bl/6J mice and three different mouse models of obesity. One group, the ob/ob mice, has a spontaneous gene mutation that prevents production of the satiety hormone leptin, a molecule that signals when to stop feeding. Another group of mice has a mutation in the db/db gene, which codes for the leptin receptor. Without the receptor, the leptin in the mice cannot exert its effects to inhibit hunger so the mice eat and eat and become obese. A third group was comprised of C57Bl/6J mice that have diet induced obesity (DIO) produced by being fed a diet of 60% fat (yum!). These NIH-funded studies were designed to test the hypothesis that buprenorphine-induced respiratory depression was greater in DIO mice, mice lacking leptin (ob/ob), or leptin receptors (db/db) than in non-obese mice (Figure 1, ob/ob mice).
Figure 2: Mice in plethysmograph chambers.
To measure respiration, Chelsea used whole body plethysmography (a really cool word to say). Theplethysmograph is a small well-sealed chamber (Figure 2) into which the mouse was placed for 15 min to get used to it, removed and given a dose of buprenorphine known to relieve pain (anti-nociceptive), and then monitored for 1 hr. The software calculated tidal volume (how much air was inhaled) and respiratory frequency (breaths per minute) among other parameters. The results showed that the DIO, ob/ob, and db/db mice had impaired respiration when compared with non-obese mice.
Those findings raised the question of the extent to which the respiratory depression was due to obesity or to disruptions in leptin or leptin receptors. To address this question the study used the mice fed a high fat diet to induce obesity. These DIO mice were the same weight as the mice with leptin mutations but do secrete leptin and have receptors that respond to leptin. The DIO mice also had buprenorphine-induced respiratory depression but not to the extent of mice containing leptin-related mutations. These results were consistent with the interpretation that leptin, not just obesity, somehow promotes buprenorphine-induced respiratory depression.
A final set of studies used Poincaré analysis to quantify the effects of buprenorphine on the variability in respiratory rate. Variability in biological rhythms such as heart rate or rate of breathing is essential for an organism to mount an adaptive response to an environmental challenge. An example includes the ability of a healthy person to increase heart rate and respiratory frequency when running or climbing a long flight of stairs. The results of the Poincaré analyses revealed that the obese mice had diminished respiratory variability compared to the non-obese mice and that buprenorphine caused a greater depression in respiratory variability among obese mice with altered leptin than in non-obese mice.
Chelsea will start medical school during the summer of 2016 and Dr. Baghdoyan and Dr. Lydic will continue these investigations
by focusing on the brain regions that may contribute to
leptin enhancement of opiate-induced respiratory depression. Identifying
brain regions and receptor systems are an essential first step toward the
development of counter measures aiming to prevent or diminish opiate-induced