Xing-Hong Jiang, Shi-Yu Guo, Shuang Xu, Qi-Zhang Yin, Yusuke Ohshita, Michiko Naitoh, Yuzo Horibe, Tadashi Hisamitsu. Neuroscience Letters (2004)
The hypothalamic-pituitary-adrenal (HPA) axis is a complex set of pathways and feedback systems that include the hypothalamus, the pituitary gland, and the adrenal glands. The HPA is a neuroendocrine system that controls reactions to stress and regulated the body. The corticotropin releasing hormone neurons in the paraventricular nucleus (PVN) are where the HPA axis starts. The locus coeruleus is an area in the pons clustered with noradrenaline containing neurons that regulates the sympathetic nervous system and the responses to stress. Previous studies have shown that a loop exists between the corticotropin releasing hormone neurons and the noradrenaline containing neurons which plays a big role in the response to stress. The goal of this study is to see how cold stress causes immunosuppression and using 6-OHDA, which is neurotoxic compound that destroys noradrenergic neurons, to cause chemical sympathectomy. The 6-OHDA is given by intracerebroventricular (ICV) injection or intraperitoneal (IP) injection to compare how cold stress effects the central noradrenergic system and the peripheral sympatho-noradrenergic system.
Adult male Wister rats weighting about 250 grams were used in this study. After allowing the rats to become acclimated to the conditions for three days, they were split into two main groups: a stress group and a non-stress group for a control. The stress group consisted of 4 subgroups: stress alone, ICV 6-OHDA and stress, IP 6-OHDA and stress, and IP propranolol and stress. To create the cold stress, the rats were placed in cages kept at 4 degrees Celsius, while the non-stress rats were kept at 22 degrees Celsius. After they were exposed to the cold, the rats were anesthetized and a blood sample was taken for the corticosterone radioimmunoassay. The spleen was then removed to be used in the natural killer cell assay. The toxicity of natural killer cells were measured by a chromium release assay. A concentration of 1E7 splenic lymphocytes/ml were used as the effector cells and chromium labeled YAC cells were used as target cells. Effector/ target cell ratios of 100:1, 50:1, 25:1, and 12.5:1 were used to titrate effector cells against target cells.
After the rats were exposed to the cold stressor of temperatures under 4 degrees Celsius, levels of plasma corticosterone levels were shown to be significantly elevated when compared to the group that did not receive the cold stressor (298.5 ± 78.03 ng/ml increased to 467.25 ± 74.8 ng/ml). When the rats were then given an injection of 6-OHDA, the plasma corticosterone levels were shown to significantly decrease (403.25 ± 54.3 ng.ml). When the 6-OHDA was injected into the group that did not receive the cold stress, there was no significant change in corticosterone levels, which suggests that the cold temperature stress leads to the hypothalamus activating the pituitary gland to release corticotropic releasing hormone.
When observing the results of natural killer cytotoxicity between the two groups, the group that received cold stress exhibited a significantly lower level than the non-stressed group at each of the effector/target cell ratios. The results also show that when 6-OHDA was injected, the activity of natural killer cells significantly increased, which decreases the suppressive effect that the cold stressor presented.
Researchers than studied the levels of Fos, which is associated with cell survival. When rats were exposed to the cold, Fos levels were increased in the PVN neruons compared to the non-stressed rats. The results showed 279.64 ± 33.53 Fos positive neurons in the stressed rats and only 9.17 ± 3.49 in the non-stressed rats. When the rats were given a intracerebroventricular injection with 6-OHDA, the stressed rats exhibited a significant decrease in Fos expression. When the same rats were given a intraperitoneal injection of 6-OHDA, there was no significant change in Fos expression, which suggests that activation of PVN neurons depend on the central noradrenergic system and not the peripheral noradrenergic system. Fos expression caused by cold stress followed a similar pattern in the LC and the effect of 6-OHDA was similar as well.
Previous studies have shown that norepinephrine in the PVN is able to stimulate the release of corticotropin releasing hormone. Since 6-OHDA caused a decrease in Fos expression and there was an increase of plasma corticosterone because of the cold stress, the researchers suggest that the activation of the HPA axis due to cold stress might be mediated by central noradrenergic system.
Recently in one of my classes we discussed how the correct temperature for rats to feel comfortable is higher than originally thought. The thought is that until recently, many years of research on rats may have been done under cold induced stress, which could completely change outcomes and results. In our lab, we do in fact keep temperatures at the correct level, but now having this knowledge has caused me to look more closely at papers I am reading. I chose this paper to just read about a few of the effects cold induced stress can have on a rat.
-Paul M
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