Paracetamol is a potent analgesic and antipyretic with limited side effects compared to the nonsteroidal anti-inflammatory drugs (NSAIDs) and opiates. Worldwide paracetamol is commonly used to treat pain and fever in both children and adults. Although, this drug has been in clinical use for more than a century, the mechanisms of action are not fully understood. Historically some of the actions of paracetamol were attributed to the inhibition of central cyclooxygenase (COX-1 and COX-2) enzymes however given the weak inhibitory effects on COX-1 and COX-2 enzymes, alternative targets have been suggested including a possible novel COX-3.
The inhibition of COX-2 is accepted as the mechanism by which paracetamol reduces core temperature (Tc) in febrile animals. However, in non-febrile animals where COX-2 is not induced, paracetamol has also been shown to cause hypothermia by a mechanism that is not fully understood. Both the reduction of pyresis and induction of hypothermia can only occur when peripheral metabolic rate decreases and/or heat loss increases. In terms of antipyresis and hypothermia, the inhibition of lipolysis, fatty acid oxidation and mitochondria function are obvious alternative targets. Studies were undertaken to identify and characterise the putative COX-3 at protein and mRNA level using western blot analysis and reverse transcription polymerase chain reaction (RT-PCR) in mouse brain endothelial cells (b.End3) and whole brain tissues isolated from male C57BL/6 mice. Additional studies were also undertaken to assess if the hypothermic properties of paracetamol could be attributed to direct inhibition of thermogenic pathways in both 3T3-L1 adipocytes and primary brown adipocytes isolated from male Wistar rats. Adipocytes and isolated mitochondria were exposed to paracetamol and lipolysis, fatty acid oxidation (FAO), mitochondrial electron transport chain (ETC), assessed by measuring oxygen consumption rate (OCR).
In these studies no expression of the COX-3 protein could be detected in brain endothelial cells and homogenates and no evidence of a COX-3 was detected at mRNA level. However, paracetamol caused a significant decrease (upto 70%; P<0.01, from control) in both basal and stimulated lipolysis at 1, 3 and 24 hours without affecting cell viability. Paracetamol (10 mM) and its metabolite N-acetyl-p-benzoquinone imine (NAPQI) at 50 µM also significantly (P<0.01, from control), reduced endogenous and exogenous FAO by 50% and 70% respectively. NAPQI (50 µM) had limited effect on mitochondrial uncoupling. Finally, paracetamol and other antipyretic compounds also significantly reduced ETC activity (upto 90%; P<0.01, from control). Both the maintenance of normal body temperature (Tb) and the induction of pyresis require increased mitochondrial ETC activity normally initiated centrally and driven peripherally by reduction of substrates such as fatty acids and glucose. The failure to identify the COX-3 protein and the direct inhibition of lipolysis, FAO and ETC activity indicate that antipyretic actions of paracetamol could partly be attributed to it actions on peripheral energy generation systems and provide new drug targets for reducing fever and chemically inducing hypothermia.