Quantitative autoradiography of [3H]paracetamol binding in the mouse brain L. Godfrey, S.M.O. Hourani, & I. Kitchen. School of Biomedical and Life Sciences, University of Surrey, Guildford, Surrey.

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The mechanism of action of paracetamol as an analgesic has not yet been elucidated. Several groups have studied its affinity for different receptors using radioligand binding techniques. However no significant interaction between paracetamol and G-protein coupled receptors has yet been found (Pelissier et al., 1996; Raffa et al, 1996). Autoradiographic studies with [³H]paracetamol at 30nM failed to show any specific binding to murine brains (Courade et al., 2001). However this concentration is more than 1000 fold below the therapeutic plasma concentration and we have therefore carried out a new study using a more appropriate concentration (3µM).

Adjacent coronal sections (20µm) were cut at 300µm intervals from brains of 6-8 week old CD1 male mice and processed for autoradiography as previously described (Kitchen et al, 1997). Binding was carried out in 50mM Tris HCl (pH 7.4) using 3µM [³H]paracetamol (32Ci/mmol), a concentration comparable to therapeutic concentrations (Prescott, 1996), for a period of 1 hour at room temperature. Non-specific binding was determined in the presence of 3mM unlabelled paracetamol. Sections were apposed to tritium-sensitive film and analysed by video-based computerized densitometry to determine total and non specific binding in twenty-three brain regions.

Table 1 shows quantitative binding in selected regions. Two way ANOVA (for the factors binding and region) showed no significant difference between regions (P>0.05) but a significant difference between total and non specific binding (P<0.001). Post-hoc analysis (Fisher LSD test) of individual regions showed only one region where total and non-specific binding was significantly different (P<0.05 ).

Table 1: Binding of 3µM [³H]paracetamol in murine brain (n=4, mean ± s.e.m, *P<0.05).

These data suggest that paracetamol binding is homogenous and, although there is some indication of specific binding, the level of specific binding in most regions failed to reach statistical significance.

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