Residual Chemosensory Capabilities in Double P2X2/P2X3 Purinergic Receptor Null Mice: Intraoral or Postingestive Detection?

Mice lacking the purinergic receptors, P2X2 and P2X3 (P2X2/P2X3(Dbl-/-)), exhibit essentially no tastant-evoked activity in the chorda tympani and glossopharyngeal nerves and substantial loss of tastant-evoked behavior as measured in long-term intake experiments. To assess whether the residual chemically driven behaviors in these P2X2/P2X3(Dbl-/-) mice were attributable to postingestive detection or oropharyngeal detection of the compounds, we used brief access lickometer tests to assess the behavioral capabilities of the P2X2/P2X3(Dbl-/-) animals. The P2X2/P2X3(Dbl-/-) mice showed avoidance to high levels (10 mM quinine and 10-30 mM denatonium benzoate) of classical "bitter"-tasting stimuli in 24-h, 2-bottle preference tests but minimal avoidance of these substances in the lickometer tests, suggesting that the strong avoidance in the intake tests was largely mediated by post-oral chemosensors. Similarly, increases in consumption of 1 M sucrose by P2X2/P2X3(Dbl-/-) mice in long-term intake tests were not mirrored by increases in consumption of sucrose in lickometer tests, suggesting that sucrose detection in these mice is mediated by postingestive consequences. In contrast, in brief access tests, P2X2/P2X3(Dbl-/-) mice avoided citric acid and hydrochloric acid at the same concentrations as their wild-type counterparts, indicating that these weak acids activate oropharyngeal chemoreceptors.