Localizing the mechanism of action of Paracetamol in the mouse brain

Based on the evidence that the antinociceptive effects of paracetamol (acetaminophen) could be mediated centrally, distribution of the cellular activity of the drug after oral administration was aimed to be determined in mouse. This study comprises four groups of mice which were categorized into saline group, paracetamol group, aspirin group and paracetamol group without hot plate test. First, the mice were orally injected with drugs and 30min later they were treated in hot plate to induce pain. Two hours later, mice were perfused with 4% paraformaldehyde. The brains were removed and paraffin embedded. Each brain was sectioned into 6μm and fixed on coated slides. C-fos polyclonal antibody (with dilution 1:100 in phosphate buffer solution (PBS)) was used as a marker of pain. Brown colour was developed with the aid of Diaminobenzidine (DAB) chromagen. Stained slides were observed under light microscope and digital photographs were taken. Significant of Fos expression of paracetamol group at the posterior ventral medial region of brain was observed. Many nociceptive neurons were expressed in the periqueductal grey (PAG), cuneiform nuclei (CnF), pedunculopontine tegmental nuclei (PPTg) and parabrachial internal lateral nucleus (PBil). C-fos expressions of the above regions were not seen in aspirin group. These results suggest that aspirin and paracetamol administrated orally may be mediated by different nociceptive processing in the mice brain. INTRODUCTION Paracetamol and Aspirin are widely used analgesic drugs and act as inhibitor of cyclooxygenase to inhibit brain prostaglandin synthetase. Paracetamol is considered to be a nonsteroidal anti-inflammatory drug (NSAID). Two cyclooxygenase isoenzyme, COX1 and COX-2 are known to catalyze the rate-limiting step of prostaglandin synthesis and are targets of NSAID. Recently, COX-3, derived from COX-1 was sequenced and it was found that it was inhibited by paracetamol preferentially. It was significantly more sensitive to paracetamol than either COX-1 or COX-2 at the lower substrate concentration. However, aspirin was tested to be a COX-1 preferential inhibitor. Thus, inhibition of COX-3 in brain and the spinal cord may be the long sought-after mechanism of action of paracetamol. (N.V. Chandrasekharan et al, 2002) The aim of this study is to find out the mechanism of action of paracetamol in mouse brain. METHODS Mice were habituate to handling for one week and to oral admistration of saline for three days. Dosage of drugs was given according to mouse weight. (With standard dose 0.1ml/20g mouse). The hot is performed after 30min after oral administration. The animals were perfused and processed for paraffin embedding. Brain were sectioned into