Small Changes in Ambient Temperature Cause Large Changes in
3,4-Methylenedioxymethamphetamine (MDMA)-induced Serotonin Neurotoxicity and Core Body Temperature in the Rat
Malberg, J. E. and Seiden, L. S.
The Journal of Neuroscience, July 1998, 18(13): 5086-5094
In this study, researchers further investigated the effects of ambient and core temperatures on serotonin(5-HT) neurotoxicity in the rat brain. In previous studies, researchers had show that ambient temperature effects the core temperature of methamphetamine treated mice (Miller and O'Callaghan, 1994). It has also been shown that there is a positive correlation between core temperature and MDMA-induced neurotoxicity (Nash et al., 1988). Based on these past findings, Malberg and Seiden proposed that an increase in ambient temperature during the administration of MDMA would lead to an increase in core temperature, which would mitigate an increase in serotonin neurotoxicity.
Methods
For this study, researchers used one hundred forty six male Holtzman rats, each with a weight between 250 and 300 grams. Rats were housed four or five to a cage in an ambient temperature between 22 and 24 degrees Celsius, except on drug administration days. The rats had unlimited access to food and water and were kept on a 12 hour light/dark cycle. The MDMA was obtained from the National Institute on Drug Abuse, and ketamine and xylazine were obtained from Abbott Labs (IL). Drug dosages were dissolved in 0.9% Sodium Chloride(NaCl).
Researchers developed a novel temperature environment which consisted of a series of thermometers and computers which together performed the function of measuring and maintaining a constant ambient temperature between 20 and 30 degrees Celsius. The chambers themselves were modified refrigerators which has a 1-inch thick Plexiglass window in the door to allow observation. Ambient temperature was measured using a thermistor mounted in the chamber. Core temperature was measured using temperature-sensitive transmitters that emits an AM radio signal proportional to the core temperature of the rat. The transmitters were surgically implanted in the peritoneal cavity, and they were anesthetized with .6ml/kg of ketamine and 22mg/kg of xylazine. Rats were given a minimum of three days to recover from the surgery before they were subjected to the experiment.
On injection days, rats were first placed in an ambient temperature of 24 degrees Celsius, the normal temperature for the laboratory, and a baseline core temperature was taken. After one hour the temperature was adjusted according to the experimental group, 20, 22, 24, 26, 28 or 30 degrees Celsius. The rats were given either 20mg/kg or 40mg/kg of MDMA, or 1ml/kg of saline(NaCl). They remained at their respective ambient temperatures for 24 hours after injection. After 24 hours, rats were returned to their group housing. Two weeks after injection they were sacrificed by decapitation and the frontal cortex, somatosensory cortex, hippocampus and striatum were dissected. The tissue samples were stored in liquid nitrogen. Neurochemical assays were performed on 5-HT, 5-HIAA, dopamine (DA), HVA, DOPAC, and norepinephrine in the various regions dissected.
Results
For the control group, rats treated with 1ml/kg of saline, their were no decreases in any of the aforementioned regions of the brain, or of any neurochemicals. Additionally, the core temperatures of the saline-treated rats was not affected by the ambient temperature in the range of 20-30 degrees Celsius. However, rats treated with MDMA showed marked differences in core temperatures measured at all 6 of the tested ambient temperatures. Rats treated with MDMA in an ambient temperature of 20 or 22 degrees Celsius (68-74F) exhibited signs of hypothermia against the control, while rats in ambient temperatures of 28-30 degrees Celsius (82-86F) exhibited signs of hyperthermia against the control. Both the control and condition groups core temperatures returned to normal after 11 hours, so only the first 11 hours of data are used.
At ambient temperatures between 20 and 24 degrees Celsius, no damage was observed in any of the brain regions examined. Above temperatures of 24 degrees Celsius, depletions were seen in all regions of the brain examined. These results indicate that at ambient temperatures below 24 degrees Celsius, serotonin neurotoxicity is protected against by hypothermia induced by the MDMA, while at temperatures above 24 degrees Celsius, MDMA-induced hyperthermia is the main cause of serotonin neurotoxicity. These results support the researchers hypothesis of a causal relationship between ambient and core temperatures and MDMA-induced neurotoxicity to 5-HT neurons.
Relevance
Before this study, it was unclear of the exact mechanism of action by which MDMA-induced neurotoxicity occurs at 5-HT neurons, although it was know that core temperatures were a mitigating factor. The researchers propose at the end of their study that there is “break-point” ambient temperature of 24 degrees Celsius, beyond which cellular destruction occurs at 5-HT neurons. While this study indicates that cellular destruction occurs beyond a certain temperature, it also shows that below the “break-point”, neurotoxicity is protected against by MDMA. Further studies can now be free of a seemingly meaningless confounding variable that could greatly influence the results of a study.
Sources
Miller, D. B. and O'Callaghan, J. P. (1994). Neurotoxicity profiles of substituted amphetamines in the C57BL/6J mouse. Journal of Pharmacology and Experimental Therapeutics, 270(2): 741-751.
Nash et al. (1988). Elevation of serum prolactin and corticosterone concentrations in the rat brain after the administration of 3,4-Methylenedioxymethamphetamine. Journal of Pharmacology and Experimental Therapeutics, 245(3): 873-879.
3,4-Methylenedioxymethamphetamine (MDMA)-induced Serotonin Neurotoxicity and Core Body Temperature in the Rat
Malberg, J. E. and Seiden, L. S.
The Journal of Neuroscience, July 1998, 18(13): 5086-5094
In this study, researchers further investigated the effects of ambient and core temperatures on serotonin(5-HT) neurotoxicity in the rat brain. In previous studies, researchers had show that ambient temperature effects the core temperature of methamphetamine treated mice (Miller and O'Callaghan, 1994). It has also been shown that there is a positive correlation between core temperature and MDMA-induced neurotoxicity (Nash et al., 1988). Based on these past findings, Malberg and Seiden proposed that an increase in ambient temperature during the administration of MDMA would lead to an increase in core temperature, which would mitigate an increase in serotonin neurotoxicity.
Methods
For this study, researchers used one hundred forty six male Holtzman rats, each with a weight between 250 and 300 grams. Rats were housed four or five to a cage in an ambient temperature between 22 and 24 degrees Celsius, except on drug administration days. The rats had unlimited access to food and water and were kept on a 12 hour light/dark cycle. The MDMA was obtained from the National Institute on Drug Abuse, and ketamine and xylazine were obtained from Abbott Labs (IL). Drug dosages were dissolved in 0.9% Sodium Chloride(NaCl).
Researchers developed a novel temperature environment which consisted of a series of thermometers and computers which together performed the function of measuring and maintaining a constant ambient temperature between 20 and 30 degrees Celsius. The chambers themselves were modified refrigerators which has a 1-inch thick Plexiglass window in the door to allow observation. Ambient temperature was measured using a thermistor mounted in the chamber. Core temperature was measured using temperature-sensitive transmitters that emits an AM radio signal proportional to the core temperature of the rat. The transmitters were surgically implanted in the peritoneal cavity, and they were anesthetized with .6ml/kg of ketamine and 22mg/kg of xylazine. Rats were given a minimum of three days to recover from the surgery before they were subjected to the experiment.
On injection days, rats were first placed in an ambient temperature of 24 degrees Celsius, the normal temperature for the laboratory, and a baseline core temperature was taken. After one hour the temperature was adjusted according to the experimental group, 20, 22, 24, 26, 28 or 30 degrees Celsius. The rats were given either 20mg/kg or 40mg/kg of MDMA, or 1ml/kg of saline(NaCl). They remained at their respective ambient temperatures for 24 hours after injection. After 24 hours, rats were returned to their group housing. Two weeks after injection they were sacrificed by decapitation and the frontal cortex, somatosensory cortex, hippocampus and striatum were dissected. The tissue samples were stored in liquid nitrogen. Neurochemical assays were performed on 5-HT, 5-HIAA, dopamine (DA), HVA, DOPAC, and norepinephrine in the various regions dissected.
Results
For the control group, rats treated with 1ml/kg of saline, their were no decreases in any of the aforementioned regions of the brain, or of any neurochemicals. Additionally, the core temperatures of the saline-treated rats was not affected by the ambient temperature in the range of 20-30 degrees Celsius. However, rats treated with MDMA showed marked differences in core temperatures measured at all 6 of the tested ambient temperatures. Rats treated with MDMA in an ambient temperature of 20 or 22 degrees Celsius (68-74F) exhibited signs of hypothermia against the control, while rats in ambient temperatures of 28-30 degrees Celsius (82-86F) exhibited signs of hyperthermia against the control. Both the control and condition groups core temperatures returned to normal after 11 hours, so only the first 11 hours of data are used.
At ambient temperatures between 20 and 24 degrees Celsius, no damage was observed in any of the brain regions examined. Above temperatures of 24 degrees Celsius, depletions were seen in all regions of the brain examined. These results indicate that at ambient temperatures below 24 degrees Celsius, serotonin neurotoxicity is protected against by hypothermia induced by the MDMA, while at temperatures above 24 degrees Celsius, MDMA-induced hyperthermia is the main cause of serotonin neurotoxicity. These results support the researchers hypothesis of a causal relationship between ambient and core temperatures and MDMA-induced neurotoxicity to 5-HT neurons.
Relevance
Before this study, it was unclear of the exact mechanism of action by which MDMA-induced neurotoxicity occurs at 5-HT neurons, although it was know that core temperatures were a mitigating factor. The researchers propose at the end of their study that there is “break-point” ambient temperature of 24 degrees Celsius, beyond which cellular destruction occurs at 5-HT neurons. While this study indicates that cellular destruction occurs beyond a certain temperature, it also shows that below the “break-point”, neurotoxicity is protected against by MDMA. Further studies can now be free of a seemingly meaningless confounding variable that could greatly influence the results of a study.
Sources
Miller, D. B. and O'Callaghan, J. P. (1994). Neurotoxicity profiles of substituted amphetamines in the C57BL/6J mouse. Journal of Pharmacology and Experimental Therapeutics, 270(2): 741-751.
Nash et al. (1988). Elevation of serum prolactin and corticosterone concentrations in the rat brain after the administration of 3,4-Methylenedioxymethamphetamine. Journal of Pharmacology and Experimental Therapeutics, 245(3): 873-879.
